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A new metallogenic model of the Panzhihua giantV–Ti–iron oxide deposit (Emeishan Large IgneousProvince) based on high-Mg olivine-bearing wehrliteand new field evidenceTong Hou a , Zhaochong Zhang a & Franco Pirajno ba State Key Laboratory of Geological Process and Mineral Resources, China University ofGeosciences, Beijing, 100083, PR Chinab School of Earth and Environment, Centre for Exploration Targeting, University of WesternAustralia, Crawley, WA, 6009, AustraliaVersion of record first published: 01 Mar 2012.

To cite this article: Tong Hou, Zhaochong Zhang & Franco Pirajno (2012): A new metallogenic model of the Panzhihua giantV–Ti–iron oxide deposit (Emeishan Large Igneous Province) based on high-Mg olivine-bearing wehrlite and new field evidence,International Geology Review, 54:15, 1721-1745

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International Geology ReviewVol. 54, No. 15, November 2012, 1721–1745

A new metallogenic model of the Panzhihua giant V–Ti–iron oxide deposit (Emeishan LargeIgneous Province) based on high-Mg olivine-bearing wehrlite and new field evidence

Tong Houa, Zhaochong Zhanga* and Franco Pirajnob

aState Key Laboratory of Geological Process and Mineral Resources, China University of Geosciences, Beijing 100083, PR China;bSchool of Earth and Environment, Centre for Exploration Targeting, University of Western Australia, Crawley, WA 6009, Australia

(Accepted 6 February 2012)

The Panzhihua layered intrusion hosts a giant V–Ti–iron oxide deposit with ore reserves estimated at 1333 Mt. Laser ablationinductively coupled plasma mass spectrometry (LA–ICP–MS) U–Pb zircon dating of comagmatic anorthosite yields a crys-tallization age of 259.77 ± 0.79 million years, coeval with the Emeishan flood basalts. Recently, we identified a small wehrlitedike in microgabbroic rocks and marbles. The wehrlite consists of high-Mg olivine phenocrysts with up to 90.44 wt.% Fo.Incompatible element-normalized patterns between bulk wehrlite and clinopyroxenes in gabbro suggest that they are coge-netic. The Panzhihua parental magma is estimated to have been picritic (∼10 wt.% FeO and ∼16 wt.% MgO), produced bypartial fusion of garnet peridotite. Much of the melting occurred in garnet-facies mantle at an initial melting temperature ofabout 1530◦C and pressure of ∼3.4 GPa, suggesting involvement of a mantle plume. The degree of partial melting was rathermodest and could have been generated by plume–lithosphere interaction or ascending plume-derived melting contaminatedby lithospheric mantle. Field relationships show sharp contacts between the massive ores and gabbro, between wehrlite andfine-grained gabbro, and between disseminated ores and gabbro. Considering the entire intrusion, which is locally cut bydikes or veins of anorthosite, together with the occurrence of a breccia made up of gabbro clasts cemented by disseminatedores, we suggest that different types of magmas were generated by liquid differentiation in a deeper-level chamber. This dif-ferentiation could have resulted from double-diffusive convection cells, with melt later intruding into a higher-level chamber,rather than by crystal settling or in situ growth on the floor of the intrusion. However, rhythmic layering produced by in situcrystallization only occurs in the middle of the Panzhihua intrusion and was caused by periodic fluctuation in water pressure.

Keywords: Panzhihua layered intrusion; Emeishan Large Igneous Province; wehrlite; V–Ti–iron oxide ore metallogenicmodel; Panzhihua field evidence

Introduction

Globally, magmatic V–Ti–iron oxide ores are commonlyassociated with or hosted in layered mafic intrusions orProterozoic anorthosite complexes (Bateman 1951; Lister1966; Force 1991; Cawthorn 1996). The Panxi (Panzhihua-Xichang) area, Southwest China, is the most importantV–Ti–iron ore district in China (e.g. Zhang et al. 2009),where several mafic–ultramafic intrusions host some of theworld’s largest V–Ti–iron oxide deposits. These includePanzhihua, Hongge, Baima, and Taihe, which have been amajor source of V, Ti, and Fe since the 1960s, making Chinaa major producer of these metals (Zhou et al. 2005 and ref-erences therein). Although many research efforts have beenmade on these intrusions and associated ore deposits, twoimportant issues remain poorly understood. (1) Why are somany V–Ti–iron oxide deposits clustered in the Panxi area?(2) Why are such large V–Ti–iron oxide ores hosted in asingle intrusion? Because of the similar ages between these

*Corresponding author. Email: [email protected]

intrusions and the Emeishan flood basalts, the V–Ti–ironoxide deposits have been generally considered to be genet-ically related to a ∼260 Ma Emeishan plume event (e.g.Zhou et al. 2005; Pang et al. 2010). However, convinc-ing evidence, such as high-temperature primitive magma,is still lacking in the published literature to support suchan inference. Moreover, our recent field observations showthat some anorthosites cut the gabbros, so it is not veryclear that they were formed by the same magmatic event,even though they are spatially associated. Thus, it is nec-essary to determine the precise age of the anorthosites toassess the petrogenesis of the intrusions and in turn the pet-rogenic link with the flood basalts. For the second question,since the giant V–Ti–iron oxide ore deposits are hosted bydistinctive V–Ti–iron-rich gabbroic intrusions (e.g. Panget al. 2010), many authors have proposed that the forma-tion of V–Ti–iron oxide ores in mafic intrusions can beattributed to the accumulation of V–Ti–iron oxide grains

ISSN 0020-6814 print/ISSN 1938-2839 online© 2012 Taylor & Francishttp://dx.doi.org/10.1080/00206814.2012.665211http://www.tandfonline.com

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1722 T. Hou et al.

from a parental magma originally enriched in Fe and Ti(e.g. Zhou et al. 2005, 2008; Zhang et al. 2009). However,such a parental magma can be formed as a melt that wasalready rich in Fe and Ti, generated by partial melting ofmantle, or from a normal mantle-derived tholeiitic magmathat became enriched in Fe and Ti as a result of differ-entiation prior to final emplacement (Cawthorn 1996), ora combination thereof (Pang et al. 2008). Therefore, thekey to answer the above questions is a clearer understand-ing of the nature of the parental magmas that formed theore-bearing intrusions.

The Panzhihua intrusion is a typical layered maficintrusion hosting V–Ti–iron oxides ores. It is an exception-ally well-studied igneous complex of highly differentiatedrocks, thanks to the long history of mining and the vastamount of data compiled by at least three generationsof petrologists. We recently discovered a small high-Mgolivine-bearing wehrlite dike intruding marble and micro-gabbroic rocks, which provides a rare opportunity to con-strain the nature and composition of the parental magmasof the Panzhihua intrusion and its differentiation. In addi-tion, exposure of fresh rocks by mining, allowed us to findmore field evidence not recognized before. Our new obser-vations cannot be readily explained by the current mostpopular model proposed by Pang et al. (2008), namelythat the stratiform oxide ores in the Panzhihua intrusionformed by accumulation of V–Ti–iron oxide grains. Hence,in this article, we report a highly precise LA–ICP–MSzircon age on the anorthosite in the Panzhihua intrusion andthe chemistry of high-Mg olivine-bearing wehrlite, com-bined with the new field and petrographic observations inthe new mining pit. We then advance a new and, in ourview, robust metallogenetic model which can well explainthe field relations that we so far observed.

Geological setting

Regional geology

The Yangtze Block consists of Mesoproterozoic graniticgneisses and metasedimentary rocks, which have beenintruded by the Neoproterozoic Kangdian (∼800 Ma) gran-ites (Zhou et al. 2002b), which are overlain by marine andterrestrial strata from the late Neoproterozoic (∼600 Ma) tothe Late Permian (Yan et al. 2003). Permian rocks includecarbonate-rich rocks and the Emeishan continental floodbasalts (Emeishan Large Igneous Province – ELIP; Aliet al. 2010). Triassic strata include both continental andmarine sedimentary rocks, whereas Jurassic to Cretaceousstrata are entirely terrigenous clastic. Neoproterozoicarc plutonic-metamorphic assemblages occur along thewestern and northern margins of the Yangtze Block, whichare believed to have been related to subduction of oceaniclithosphere towards the Yangtze Block during a period from860 to 760 Ma (Zhou et al. 2002a). A late Palaeozoic

to early Mesozoic (∼280–230 Ma) rifting event has alsobeen recognized (Cong 1988). The region was furtherdeformed during the Palaeogene India–Asia collision (Yinand Harrison 2000).

Emeishen province and associated V–Ti–iron oxidedeposits

The province is dominated by the Emeishan flood basalts,ranging in thickness from a few hundred metres to a max-imum of ∼5 km. Erosional remnants of the flood basaltscover an area of at least 2.5 × 105 km2. In contrast tothe Siberian flood basalts, which formed at a relativelyhigh northern latitude, emplacement of the Emeishan floodbasalts occurred near the Equator (Enkin et al. 1992).Overall, the province appears to be slightly older thanthe ∼251 Ma Siberian Traps: 40Ar–39Ar ages of 254 ±5 million years (Boven et al. 2002), 255 million years,and 251–253 million years (Lo et al. 2002). More recentSHRIMP U–Pb dating for mafic intrusions, dikes, andvolcanic rocks of the ELIP have indicated emplacementbetween 257 and 263 Ma (e.g. He et al. 2007; Shellnuttet al. 2008), which suggests a link between the ELIP andthe end-Guadalupian mass extinction (middle Permian; Heet al. 2010 and references therein).

The Panxi region lies in the central-western part of theELIP where flood basalts are variably deformed, uplifted,and eroded due to strong tectonic activity in the Cenozoicbecause of the India–Eurasian collision. The layered intru-sions, including Panzhihua, Hongge, Baima, Taihe, andXinjie, generally host V–Ti–iron oxide ores, locally asso-ciated with platinum group element (PGE). The intrusionshave been dated by the U–Pb zircon method at ∼260 Ma(Figure 1; Pang et al. 2010 and references therein). GiantV–Ti–iron oxide deposits are hosted in several relativelylarge layered intrusions, which are controlled by N–S-trending faults and spatially associated with contempora-neous flood basalts and many granitoids (Figure 1; Zhonget al. 2005). The total oxide ore reserves of four largeV–Ti–iron oxide deposits are estimated to be 7544 Mt withan average ore grade of 36 wt.% Fetotal, 0.28 wt.% V2O5,and 12.6 wt.% TiO2 (Ma et al. 2003).

Panzhihua intrusion and new field and petrographicobservations

The Panzhihua layered igneous complex is 19 km long and2 km thick, with an exposed area of ∼30 km2 near thetown of Panzhihua, and intrudes Neoproterozoic dolomiticlimestones (Dengying Formation). The V–Ti–iron oxidedeposit hosted in the Panzhihua intrusion was discoveredbetween 1936 and 1940 (Ma et al. 2003); mining activitycommenced in 1967 and is still ongoing today. Orereserves have been estimated at 1333 Mt with an average

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International Geology Review 1723

30 km0 100 km

Chengdu

PanzhihuaLimahe263 ± 3 Ma

Xichang

Baima262 ± 2 Ma

0 800 km

Yongren

Yuanmou

Xinjie259 ± 3 Ma

Distribution of the EmeishanFlood Basalts

Hongge

263 ± 3 Ma

261 ± 2 Ma

259.3 ± 1.3 Ma

Zhubu

Map area

28°40′N

24°40′N

26°40′N

102°00′E

104°00′E

27°20′N

26°40′N

103°00′E

Basalt

V–Ti–Fe-oxide-bearing intrusion

Cu–Ni–(PGE)Sulfide-bearing intrusion

Syenite

Jurassic--Quaternarystrata

Triassic strata

Palaeozoic strata

Mesozoic granite

Faults

Neoproterozoic igneous

metamorphic complex-

PIL

na

hsi

em

E

S

YG

800 km

CHINA

Figure 1. Map showing the main outcrops of the Emeishan flood basalts and related mafic–ultramafic intrusions in the central part of theEmeishan Large Igneous Province, Southwest China (modified from Zhang et al. 2009).

grade of ∼33% total Fe, ∼12% TiO2, and ∼0.3% V2O5

(Ma et al. 2003).Despite extensive regional tectonic activity associ-

ated with the Cenozoic Indian–Eurasian collision, thePanzhihua intrusion has undergone little deformation ormetamorphism after its solidification, except locally alongshear zones and marginal zones. Throughout the intru-sion, the frequency of V–Ti–iron oxide layers decreasesupwards. Mineral compositions also show regular upwardvariations. For example, forsterite (Fo) contents of olivine

and anorthite (An) contents of plagioclase decreaseupwards (82–63% for Fo and 68–40% for An; Zhang et al.1988; Pang et al. 2009). The compositions of clinopyrox-enes are less variable, that is, En41–46Fs10–17Wo37–48 (e.g.Pang et al. 2009). Based on differences in internal structureand the extent of oxide mineralization, four major zoneshave been identified (Figure 2): a marginal zone at thebase, followed successively upwards by a V–Ti–iron oxide-bearing gabbro zone, a layered gabbro zone with someoxide orebodies, and leucogabbro zones.

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1724 T. Hou et al.

ColumnarsectionZone

Leucogabbro

Layered gabbro

V–Ti–Fe–

bearing gabbro

Marginal

Neoproterozoic

Thick-ness(m)

500–1500

10–20

166–600

30–240

5–110

6–60

0–50

0–60

0–40

Marble

Fine grained gabbrointercalated olivine gabbro

Massive ores withdisseminated ores

Gabbro intercalated bandedores

Banded ores

Ores alternating withlayered gabbro

Layered gabbro intercalateddisseminated ores

Anor thosite at the upper par t,gabbro intercalated thin-layerores

5% apatite-bearing gabbrowith disseminated ores

Leucogabbro

Lithology

Quaternary Triassic sandstone Proterozoic marble Proterozoic metamorphic complex Syenite

Olivine gabbro Leucogabbro Melanogabbro Boundary Fault

0 3 km

Silicate(vol%)

Fe–Ti oxides

Mineral vol% End member

Ol+Cpx

(vol%)

Mt+Ilm

(×100 Mt)

Pl

(An)

Ol

(Fo)

Cpx

Ol

Amp

Pl

Ilm

Mt

43

49

3

22 69

9497

8353 51

5218

10

1

30 71

788

5 48 57

57

4049555659

59

6768

5968 82

80

78

78

77

7676726363

(A)

(B)

V–Ti magnetite orebody

Figure 2. (A) Geological map of the Panzhihua intrusion, Southwest China; (B) magmatic lithostratigraphy showing the vertical variationof silicate and metal minerals and mineral compositions (modified from Zhang et al. 1988).

Marginal zone

The marginal zone is 0–40 m thick, is very heteroge-neous in terms of its composition, and consists of fine-grained hornblende-bearing gabbro (microgabbro) inter-calated with olivine gabbro. The microgabbro tends tocontain an increasing amount of hornblende towards thecountry rocks and grade into rocks composed essentiallyof hornblende adjacent to the footwall contact (Figure 3A).Moreover, a thick (∼60 m) V–Ti–iron oxide orebodyoccurs in the marginal zone and has a sharp contact with themicrogabbro (Figure 4A) which predominantly consists offine-grained anhedral clinopyroxene and plagioclase with agranulitic texture (Figure 4F).

One of the notable discoveries of our study is the localoccurrence of small pods and dikes of wehrlites withinmicrogabbroic rocks and marbles (Figure 3). They intrudeda little later than the gabbroic intrusion and solidifiednear the base, along the southern edge of the gabbroicrocks, and have sharp contact with the country rocks, i.e.marble (Figure 3C) and microgabbroic rocks (Figure 3B).The wehrlites are mainly composed of olivine phenocrysts(50–60 modal%; Figure 3D)) of various sizes (0.5 upto 5 mm in length) and interstitial phases (see below).Olivine phenocryst is euhedral to subhedral or round,though it also displays elongated rod-like or harrisitictextures. Individual olivine grains are generally unzoned.

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MarbleMassiveFe–Ti–Voxide ore

Fine-grained olivinegabbro in marginal zone

Pods and dykes ofWehrlite

Wehrlite

Marble

Ol

Wehrlite

Fine-grainedolivine gabbro

(D)(C)

(B)(A)

Road bed

Ol

1 mm

Figure 3. (A) View of the mine towards the south, showing the occurrence of V–Ti–iron oxide ore and small pods and dikes of wehrlitesin the marginal zone in the lower part of the intrusion adjacent to the marble country rocks. (B) Photograph showing the sharp contactrelation between fine-grained olivine gabbro and the wehrlites. (C) Photograph showing the occurrence of a dike of wehrlite that intrudedin the marble. (D) Porphyritic texture displaying the wehrlite-containing olivine crystals in interstitial phases of fine-textured crystalsunder crossed polars, which consist of predominantly olivine and augite with minor accessory anhedral iron oxides.

Small (0.2–0.5 mm) crystals of olivine (∼20 modal %)and clinopyroxene (∼20 modal %) occur as interstitialphases between olivine. Minor anhedral crystals of V–Ti–iron oxide (up to 3 modal% and 0.8 mm in size) areobserved as inclusions in olivine or at the contact betweensilicates. These features suggest the wehrlite was emplacedas a crystal mush, and as discussed below, these rocks arenow known to have unambiguous petrologic relation tothe Panzhihua magma and played a crucial role during itsevolution.

V–Ti–iron oxide-bearing gabbro zone

The gabbro zone is between 5 and 100 m thick and iscomposed of layered melagabbros with major V–Ti–ironlayers (the orebodies) up to 50 m thick. Both massiveand disseminated ores are common. The contacts betweenthe massive orebodies and the host silicate rocks, suchas melagabbro and gabbro, are always sharp (Figure 4Band 4C). Many V–Ti–iron oxide layers form intercalationsin the mainbody of the intrusion. In places, dikes andveins of anorthosite cut the oxide ore layers. Additionally,brecciated ores consisting of angular fragments of the

gabbro cemented by massive ores have been observed dur-ing this study (Figure 4D). Massive ores typically contain>80% titanomagnetite with variable amounts of clinopy-roxene, plagioclase, and olivine. The sparse euhedral sil-icate minerals are completely surrounded by abundantFe–V–Ti oxides (Figure 4E).

In general, the massive ores grade upward into dissemi-nated ores, which in turn grade into unmineralized gabbros(Zhou et al. 2005). However, our observations show thatmost disseminated ores also have sharp boundaries withgabbros. They range from massive rocks (Figures 5A and5C), to chaotic intrusive breccias (Figures 5B and 5D).Disseminated ores are generally coarse-grained and consistof ∼50% titanomagnetite, ∼20% clinopyroxene, ∼20%plagioclase, ∼10% ilmenite, and small amounts of olivine(Figure 5E). In addition, the host gabbros are homoge-nous and composed of coarse-grained clinopyroxene andplagioclase with eutectic texture (Figure 5F). Fe–Ti oxideminerals in these gabbroic rocks occur as irregular aggre-gates of titanomagnetite and ilmenite surrounding clinopy-roxene and plagioclase. These aggregates in many placesshow curved boundaries with coexisting silicates, a texturepresumably related to annealing (Duchesne 1999).

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Massive ore

Fine-grainedgabbro

Massive ore

Melagabbro

(A) (B)

Massive ore

Gabbro

Cpx

200 µm

OpaCpx

Pl200 µm

Brecciatedgabbro

Massive ore

(D)

(F)(E)

(C)

Figure 4. (A)–(C) Sharp contact relations between the massiveores and gabbros in the marginal zone and V–Ti–iron oxide-bearing gabbro zone, respectively; (D) brecciated fresh fragmentsof gabbro, cemented by the massive ores; (E) clinopyroxene inthe massive ore; (F) equigranular clinopyroxene and plagioclasein microgabbro in the marginal zone under crossed polars.

Note: Cpx, clinopyroxenes; Opa, opaque oxides; Pl, plagioclase.

Layered gabbro zone

All but the middle sector of the intrusion is well lay-ered with features similar to those described in theMuskox intrusion (Mackie et al. 2009), Skaergaardintrusion (McBirney 2009), Stillwater complex (Godeland Barnes 2008), and Bushveld Igneous Complex (Ealesand Cawthorn 1996; Clarke et al. 2009). The layeredgabbro zone, 200–800 m thick (Figure 6A), consistsof pronounced layering occasionally with intercalationsof several thin V–Ti–iron oxide ore layers. Layering isprevalent in all parts of this zone up to the leucogabbrozone. Here igneous layering makes a spectacular display inthe repetition of complete or incomplete titanomagnetite-clinopyroxenites-gabbro-anorthosite cycles, ranging inthickness from <1 cm to several metres (Figures 6A and6B). All the layered gabbros are medium to coarse-grainedwith local pegmatitic facies, but they are confined almostentirely to modal rather than textural variations, andmanifested in the modal proportions of light and darkminerals, i.e. plagioclase and clinopyroxenes. Although,the rocks become progressively more felsic upwardon average, the modal proportions of plagioclase andmafic minerals range between those of anorthosite andpyroxenite. Even seemingly homogeneous rocks vary bothvertically and along strike. The dark layers are composedof clinopyroxene phenocrysts with eutectic clinopyroxeneand plagioclase in the groundmass (Figure 6C). In contrast,

the light layers are characterized by the occurrence ofplagioclase phenocrysts with eutectic clinopyroxenes andplagioclase in the groundmass (Figure 6D). Disseminatedtitanomagnetite in the thin V–Ti–iron oxide ore layers isconsistently present in the layered gabbro zone silicatecumulates as an accessory phase.

Leucogabbro zone

This zone has a thickness of between 500 and 1500 m andconsists mainly of unmineralized leucogabbro and dior-ite (Figure 6E). This latter lacks any V–Ti–iron oxidelayers. Leucogabbro is relatively fine-grained and con-sists of coarse-grained plagioclase crystals up to 5 mmlong and moderate amounts of clinopyroxenes with minorhornblende and magnetite (Figure 6F).

Anorthosite

Small amounts of anorthosite occur as single and dendriticdikes or lenses within the intrusion, from place to place,and cut through the entire intrusion, including the mainorebody (Figures 7A–7C), suggesting that these dikes andlenses of anorthosite postdate the main body of the intru-sion. Former studies have shown that the anorthosite iscomagmatic with the main body of the intrusion (Zhouet al. 2005). The anorthosites are medium- to coarse-grained rocks, consisting of >90% plagioclase and minorclinopyroxene with spotted apatite and zircon (Figure 7D).

Analytical methods

Zircon U–Pb dating

Zircons were separated using heavy liquid and magnetictechniques and then handpicked under a binocular micro-scope. Zircon grains were mounted on adhesive tape thenenclosed in epoxy resin and polished to about half oftheir diameter. In order to observe textures of the pol-ished zircons, cathodoluminescence (CL) imaging wascarried out using a Hitachi S3000-N scanning electronmicroscope (SEM; Hitachi, Tokyo, Japan) with a MonoCL3 Cathodoluminescence System for high-resolutionimaging and spectroscopy at the Institute of Geology,Chinese Academy of Geological Sciences.

Zircon U–Pb dating work for samples was performedby a Finnigan Neptune multi-collector ICP-MS with aNewwave UP213 laser-ablation system at the Instituteof Mineral Resources, Chinese Academy of GeologicalSciences, Beijing. Helium was used as the carrier gas toenhance the transport efficiency of the ablated material.The analyses were conducted with a beam diameter of25 lm with a 10 Hz repetition rate and a laser powerof 2.5 J/cm2 (Hou et al. 2009). The masses of 206Pb,

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Gabbro

Disseminated ore

(A) (B)

Disseminated oreAnor thosite

GabbroGabbro

gabbro

Disseminated ore

Cpx Pl

200 µm

Opa

Disseminated ore

Gabbro

Cpx

Pl

Fe–Ti oxide

Disseminated ore

gabbro

200 µm

(D)

(F)(E)

(C)

Figure 5. (A) Small pods of disseminated ores in the gabbros cut by a dike of anorthosite. (B) An irregular vesicle of disseminatedore occurs in the leucogabbro. (C) The sharp contact relation between disseminated ore and gabbro. (D) Brecciated fresh fragment ofgabbro, cemented by disseminated ores. (E) Photomicrograph showing the Cpx and Pl in the disseminated ore under crossed polars.(F) Cumulus clinopyroxene, plagioclase, V–Ti–iron oxides, and apatite with interlocking boundaries under crossed polars, in the fragmentof leucogabbro cemented by disseminated ore.

207Pb, 204(Pb + Hg), and 202Hg were measured by multi-ion counters, while the masses of 208Pb, 232Th, 235U, and238U were collected by a Faraday cup. Zircon GJ1 wasused as the standard and zircon Plesovice was used tooptimize the machine. U, Th, and Pb concentrations werecalibrated using 29Si as the internal standard and zirconM127 (U: 923 ppm; Th: 439 ppm; Th/U: 0.475; Nasdala

et al. 2008) as the external standard. 207Pb/206Pb and206Pb/238U ratios were calculated using the in-house soft-ware program, ICPMSDataCal, produced by Liu et al.(2008), for off-line selection and integration of backgroundand analysis signals, time-drift correction and quantita-tive calibration for the trace element analyses and U-Pbdating. The common Pb was not corrected because of the

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1728 T. Hou et al.

(A)

Rhythmic layered gabbro

200 µm

Cpx

200 µm

Pl

PlCpx

Ol

1 m 200 µm

Pl

Cpx

(B)

(D)

(F)(E)

(C)

Figure 6. (A) Overview of the Lanjiahuoshan open-cast mine of the Panzhihua intrusion, showing the rhythmic layered gabbro.(B) Rhythmic layering of a leucogabbro. (C) Dark minerals, primarily clinopyroxene, dominate in the dark bands of each layer. (D)Lighter minerals, primarily plagioclase, dominante in the light bands of each layer. Note that the layering is also manifested in the varia-tions in grain size. (E) Photograph of leucogabbro in the upper zone of the Panzhihua intrusion. (F) Photomicrograph of the leucogabbroin the upper zone of the Panzhihua intrusion under crossed polars.

high 206Pb/204Pb ratios (>1000). Data with abnormallyhigh 204Pb counts were deleted. The zircon Plesovice isdated as unknown samples and yielded a weighted mean206Pb/238U age of 337 ± 2 million years (2SD, n =12), which is in good agreement with the recommended206Pb/238U age of 337.13 ± 0.37 million years (2SD)(Sláma et al. 2008). The age calculation and plottingof concordia diagrams was performed using Isoplot/Ex3.0, the most-recent, Excel-based version of Isoplot,designed by Ludwig (2003). The results are presented inTable 1.

Electron microprobe analyses

Electron microprobe analyses were determined forsome olivine in the wehrlites using a JEOL JXA-8230 Superprobe (JEOL Ltd., Tokyo, Japan) at the EMPALaboratory of Analysis Center of Mineral and Rocksof Institute of Mineral Resources, Chinese Academy ofGeological Sciences. Operating conditions were set at15 kV at 10 nA beam current. Natural minerals and

synthetic pure oxides from SPI Supplies Inc. (West Chester,PA, USA) were used as standards. For pyroxene, the cali-bration standards used were hornblende (for Si, Ti, Al, Fe,Ca, Mg, Na, and K), fayalite (for Mn), and Cr2O3 (for Cr).For plagioclase, the standards used were hornblende (forSi, Ti, Al, Fe, Ca, and Mg), albite (for Na), orthoclase (forK), and fayalite (for Mn). Precision is better than 1% forelement oxides.

Major and trace element analyses

After screening under the microscope, relatively freshsamples were selected and sawed into slabs and the cen-tral parts were used for whole-rock analyses. Specimenswere crushed in a steel mortar and grounded in a steelmill to powders of ∼200 mesh. Major elements wereacquired through the analysis of fused glass discs using ascanning wavelength dispersion X-ray fluorescence (XRF)spectrometer at the Centre of Modern Analysis, NanjingUniversity. The analytical uncertainties are less than1%, estimated from repeated analyses of two standards

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Anorthosite

Ore-bearinggabbro

Melagabbro

Anorthosite

(D)

(B)(A)

200 µm

Pl

Pl

Anorthosite

Ore-bearinggabbro

Massive ore1 m

(C)

Figure 7. (A)–(C) Field photograph of anorthosite cut ore-bearing gabbro, melagabbro, and massive ores. (D) Photomicrograph of theanorthosite under crossed polars.

(andesite GSR-2 and basalt GSR-3). Loss on ignition wasdetermined gravimetrically after heating the samples at980◦C for 30 min.

Trace elements were determined by solution ICP-MSperformed at the ICP-MS Laboratory at the NationalResearch Centre for Geoanalysis, Beijing. After completedissolution, powders (∼40 mg) were dissolved in dis-tilled HF + HClO4 in 15 ml Savillex Teflon screw-cap

breakers. Precision for most elements was typically bet-ter than 5% RSD (relative standard deviation), and themeasured values for Zr, Hf, Nb, and Ta were within 10%of the certified values. The detailed sample preparations,instrument operating conditions, and calibration proce-dures follow those established by Qi and Grégoire (2000).Two standards (granite GSR-1 and basalt GSR-3) wereused to monitor the analytical quality.

Table 1. Representative results of in situ U–Pb LA–MC–ICP–MS zircons analyses in Panzhihua gabbroic intrusion, ELIP.

Isotopic ratios Apparent ages (million years)

Sample 207Pb/206Pb 1σ 206Pb/235U 1σ 207Pb/238U 1σ 207Pb/206Pb 1σ 206Pb/235U 1σ 207Pb/238U 1σ ρ

Sample PZHPZH-1 0.0497 0.0004 0.2829 0.0034 0.0412 0.0003 183.42 23.14 252.95 2.68 260.01 1.65 97%PZH-2 0.0521 0.0003 0.2961 0.0025 0.0412 0.0003 300.06 11.11 263.33 1.94 260.05 1.7 98%PZH-4 0.0504 0.0003 0.286 0.0026 0.0412 0.0003 213.04 17.59 255.41 2.03 259.97 1.66 98%PZH-6 0.0509 0.0002 0.2886 0.0022 0.0411 0.0003 235.25 11.11 257.48 1.7 259.87 1.57 99%PZH-7 0.0523 0.0013 0.2984 0.0087 0.0411 0.0002 298.21 53.7 265.17 6.77 259.92 1.47 98%PZH-8 0.0497 0.0003 0.281 0.0025 0.041 0.0002 188.97 16.67 251.43 1.98 258.88 1.47 97%PZH-9 0.0504 0.0015 0.2856 0.0106 0.0407 0.0004 213.04 68.5 255.11 8.39 257.26 2.52 99%PZH-10 0.0512 0.0005 0.2892 0.0033 0.0409 0.0002 250.07 22.22 257.95 2.61 258.63 1.25 99%PZH-11 0.0498 0.0008 0.283 0.0048 0.0412 0.0003 183.42 37.03 253.06 3.79 260.51 1.64 97%PZH-15 0.0486 0.0003 0.2759 0.0029 0.0412 0.0003 127.87 8.33 247.42 2.34 260.08 2.13 95%PZH-17 0.0495 0.0002 0.2812 0.0022 0.0412 0.0002 172.31 11.11 251.63 1.76 260.06 1.23 96%PZH-18 0.0508 0.0005 0.2897 0.0043 0.0412 0.0003 231.55 22.22 258.34 3.37 260.29 1.8 99%PZH-19 0.0502 0.0003 0.2851 0.0021 0.0412 0.0002 205.63 14.81 254.74 1.7 260.08 1.01 97%PZH-20 0.0521 0.0003 0.2969 0.0033 0.0412 0.0003 300.06 12.96 263.96 2.56 260.31 1.72 98%

Notes: 207Pb/235U calculated using 207Pb/206Pb/(238U/206Pb×1/137.88). ρ is the error correlation defined as err206Pb/238U/err207Pb/235U.ELIP, Emeishan Large Igneous Province.

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1730 T. Hou et al.

Analytical results

LA–ICP–MS U–Pb dating on anorthosite

Zircons in samples of anorthosite are generally transpar-ent, euhedral, and prismatic, with clear oscillatory zoning(Figure 8A) and high Th/U ratios (0.58–3.37, Table 1),typical of igneous zircons (Hanchar and Rundnick 1995;Corfu et al. 2003). A group of 20 zircon grains defines aconcordia age of 259.77 ± 0.79 million years (Figure 8B)interpreted as the crystallization age for the anorthosite inthe Panzhihua intrusion.

Mineral chemistry of olivine and clinopyroxene ofwehrlite

Olivine (Table 2) and clinopyroxene (Table 3) were anal-ysed in the wehrlite samples where these minerals occur asa phenocryst and interstitial phase, respectively. The majorelement composition of the olivine is homogeneous withinthe sample, with forsterite content (Fo) varying within a rel-atively wide range of values (77.42–90.44). These valuesoverlap the Fo values in the main body of the Panzhihuagabbroic intrusion, but the highest value is much higher

than those reported previously for the marginal zone of thePanzhihua intrusion (∼82; e.g. Pang et al. 2009; Zhanget al. 2009). Ni, Co, and Mn are compatible in olivine andmost olivine crystals contain chromite and melt inclusions;hence, all contain significant amounts of NiO, CoO, MnO2,Cr2O3, and CaO varying within narrow ranges.

The clinopyroxenes analysed in this study occur asinterstitial phases between olivine and consist mainly ofaugite with minor diopiside (Table 3). The Mg# of clinopy-roxene varies from 0.67 to 0.76, which is slightly lowerthan the abundant published data for clinopyroxene fromthe marginal, lower, and middle zones of the Panzhihua(Figure 9). The TiO2 contents of these clinopyroxene crys-tals vary from 0.37 to 1.64 wt.%, similar to data reportedfor clinopyroxene from the main body of the Panzhihuaintrusion (e.g. Zhou et al. 2005). In contrast, Lijiang picritecontains Ti-rich diopiside (TiO2 content up to 2.52 wt.%).

Whole-rock geochemistry of wehrlite

As expected, the wehrlite shows high concentrations ofcompatible elements (FeOTotal, MnO, MgO, Ni, Cr, Co,

1

2 46

7

8 9 10 11

15

17 1819

20

100 µm

266

262

258

254

2500.0394

0.0398

0.0402

0.0406

0.0410

0.0414

0.0418

0.0422

0.25 0.27 0.29 0.31 0.33

207Pb/235U

206P

b/2

38U

Data-point error ellipses are 68.3% confidence

253

255

257

259

261

263Data-point error symbols are 1 sigma

Mean = 259.77 ± 0.79 Ma

MSWD = 0.22

95% confidence

(A)

(B)

Figure 8. (A) CL images of typical zircon populations. Most grains display CL oscillatory zoning typical for magmatic origin.(B) LA–ICP–MS U–Pb zircon concordia diagrams for anorthosite in the Panzhihua intrusion.

Note: CL, cathodoluminescence.

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International Geology Review 1731Ta

ble

2.C

hem

ical

com

post

ion

ofth

eol

ivin

ein

the

Panz

hihu

aw

ehrl

ite.

Com

men

tP

ZH

05-0

1P

ZH

05-0

2P

ZH

05-0

3P

ZH

05-0

4P

ZH

05-0

5P

ZH

05-0

6P

ZH

05-0

7P

ZH

05-0

8P

ZH

05-0

9P

ZH

05-1

0P

ZH

05-1

1P

ZH

05-1

2P

ZH

05-1

3P

ZH

05-1

4

SiO

240

.86

40.0

039

.86

39.9

639

.91

40.1

740

.67

40.3

840

.03

39.4

440

.15

38.1

739

.46

39.2

8T

iO2

0.00

0.03

0.00

0.00

0.05

0.00

0.00

0.00

0.15

0.07

0.01

0.00

0.00

0.00

Al2

O3

0.07

0.00

0.19

0.12

0.00

0.00

0.00

0.20

0.00

0.10

0.04

0.00

0.00

0.00

Cr2

O3

0.00

0.00

0.08

0.00

0.21

0.05

0.00

0.00

0.04

0.10

0.11

0.00

0.00

0.00

FeO

14.2

313

.62

15.3

815

.24

15.8

514

.16

14.0

314

.31

16.1

115

.87

15.0

219

.62

20.5

117

.17

MnO

0.43

0.47

0.34

0.28

0.18

0.31

0.49

0.38

0.14

0.29

0.51

0.24

0.27

0.53

MgO

44.8

344

.42

43.2

743

.61

43.4

844

.66

45.0

944

.52

43.3

042

.15

43.2

539

.29

39.5

941

.84

NiO

0.00

0.00

0.00

0.00

0.20

0.15

0.22

0.00

0.00

0.00

0.42

0.73

0.35

0.34

CaO

0.11

0.00

0.12

0.13

0.11

0.07

0.00

0.41

0.04

0.73

0.45

0.08

0.00

0.00

V2O

30.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

00C

oO0.

000.

220.

000.

240.

130.

250.

270.

120.

330.

250.

000.

250.

190.

29

Tota

l10

0.53

98.7

699

.24

99.5

810

0.12

99.8

210

0.77

100.

3210

0.14

99.0

099

.96

98.3

810

0.37

99.4

5

Fo84

.89

85.3

383

.38

83.6

183

.02

84.9

085

.14

84.7

382

.73

82.5

683

.70

78.1

277

.48

81.2

9

Com

men

tP

ZH

05-1

5P

ZH

05-1

6P

ZH

05-1

7P

ZH

05-1

8P

ZH

05-1

9P

ZH

05-2

0P

ZH

05-2

1P

ZH

05-2

2P

ZH

05-2

3P

ZH

05-2

4P

ZH

06-1

PZ

H06

-2P

ZH

06-3

PZ

H06

-4

SiO

240

.87

41.5

240

.47

41.2

041

.32

41.4

341

.43

40.3

941

.23

41.2

740

.82

40.6

840

.29

39.7

9T

iO2

0.00

0.00

0.16

0.15

0.06

0.22

0.00

0.00

0.00

0.00

0.01

0.03

0.00

0.00

Al2

O3

0.00

0.00

0.00

0.01

0.13

0.01

0.00

0.21

0.00

0.00

0.00

0.02

0.00

0.00

Cr2

O3

0.01

0.00

0.00

0.27

0.00

0.09

0.14

0.00

0.00

0.00

0.00

0.00

0.00

0.03

FeO

10.4

69.

179.

609.

589.

629.

2710

.26

9.34

9.35

9.00

10.9

010

.73

11.9

312

.11

MnO

0.36

0.27

0.42

0.09

0.31

0.55

0.44

0.45

0.34

0.29

0.26

0.23

0.22

0.30

MgO

47.7

447

.61

47.0

347

.46

47.7

748

.34

48.1

746

.97

47.8

847

.75

46.8

446

.49

46.3

646

.14

NiO

0.24

0.51

0.69

0.18

0.51

0.06

0.51

0.99

0.46

0.59

0.13

0.18

0.20

0.20

CaO

0.16

0.14

0.00

0.07

0.17

0.12

0.20

0.10

0.05

0.00

0.08

0.08

0.08

0.09

V2O

30.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

010.

000.

050.

02C

oO0.

000.

000.

010.

210.

030.

000.

000.

000.

040.

180.

030.

030.

040.

00

Tota

l99

.84

99.2

298

.38

99.2

299

.92

100.

0910

1.15

98.4

599

.35

99.0

899

.07

98.4

599

.18

98.6

8

Fo89

.06

90.2

589

.73

89.8

389

.85

90.2

989

.33

89.9

790

.13

90.4

488

.45

88.5

487

.38

87.1

7

Com

men

tP

ZH

06-5

PZ

H06

-6P

ZH

06-7

PZ

H06

-8P

ZH

06-9

PZ

H06

-10

PZ

H06

-11

PZ

H06

-12

PZ

H06

-13

PZ

H06

-14

PZ

H06

-15

PZ

H06

-16

PZ

H06

-17

PZ

H06

-18

SiO

240

.64

40.1

541

.34

41.3

340

.51

39.9

040

.49

41.0

641

.71

40.6

340

.34

41.6

440

.37

40.6

2T

iO2

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.01

0.07

0.08

0.01

0.04

0.02

Al2

O3

0.01

0.02

0.00

0.01

0.00

0.00

0.00

0.01

0.00

0.00

0.00

0.00

0.00

0.00

Cr2

O3

0.00

0.05

0.00

0.01

0.00

0.00

0.01

0.06

0.00

0.00

0.00

0.00

0.00

0.00

FeO

11.0

311

.03

10.6

810

.45

10.9

711

.07

11.0

811

.01

11.2

110

.96

10.9

811

.56

11.3

711

.13

MnO

0.27

0.28

0.21

0.22

0.30

0.29

0.26

0.24

0.29

0.32

0.28

0.25

0.28

0.28

MgO

47.5

546

.62

47.2

348

.18

47.1

246

.08

46.7

247

.20

48.6

247

.63

47.2

047

.29

45.2

747

.70

NiO

0.19

0.22

0.21

0.17

0.21

0.23

0.21

0.25

0.26

0.18

0.27

0.18

0.20

0.19

CaO

0.02

0.07

0.08

0.14

0.07

0.11

0.10

0.07

0.11

0.06

0.10

0.03

0.12

0.07

V2O

30.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

00C

oO0.

030.

020.

000.

030.

030.

020.

050.

010.

020.

010.

030.

050.

030.

04

Tota

l99

.73

98.4

699

.74

100.

5499

.20

97.7

098

.92

99.9

010

2.22

99.8

599

.29

100.

9997

.66

100.

04

Fo88

.49

88.2

888

.75

89.1

688

.45

88.1

388

.26

88.4

388

.54

88.5

788

.46

87.9

587

.66

88.4

3

(Con

tinu

ed)

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1732 T. Hou et al.Ta

ble

2.(C

onti

nued

).

Com

men

tP

ZH

06-1

9P

ZH

06-2

0P

ZH

07-0

1P

ZH

07-0

2P

ZH

07-0

3P

ZH

07-0

4P

ZH

07-0

5P

ZH

07-0

6P

ZH

07-0

7P

ZH

07-0

8P

ZH

07-0

9P

ZH

07-1

0P

ZH

07-1

1P

ZH

07-1

2

SiO

240

.79

40.8

740

.61

40.8

240

.40

40.5

041

.25

40.5

538

.50

39.1

539

.37

39.8

039

.03

39.5

0T

iO2

0.00

0.01

0.00

0.00

0.03

0.01

0.04

0.10

0.02

0.00

0.00

0.00

0.06

0.01

Al2

O3

0.04

0.01

0.00

0.00

0.01

0.01

0.02

0.04

0.00

0.02

0.00

0.00

0.00

0.01

Cr2

O3

0.00

0.01

0.00

0.01

0.00

0.00

0.04

0.03

0.00

0.01

0.00

0.00

0.00

0.00

FeO

11.3

711

.41

11.0

510

.66

10.7

210

.82

11.0

111

.86

20.1

920

.14

19.9

719

.96

20.8

620

.38

MnO

0.29

0.30

0.30

0.32

0.29

0.28

0.27

0.12

0.20

0.31

0.27

0.29

0.24

0.30

MgO

46.8

047

.16

47.3

747

.85

47.6

447

.87

47.6

446

.48

40.6

641

.18

40.9

240

.57

40.6

340

.54

NiO

0.16

0.20

0.19

0.07

0.10

0.18

0.19

0.17

0.26

0.29

0.30

0.30

0.27

0.30

CaO

0.07

0.24

0.10

0.11

0.15

0.14

0.07

0.23

0.00

0.00

0.00

0.01

0.00

0.00

V2O

30.

050.

000.

000.

000.

050.

000.

060.

000.

000.

040.

000.

010.

000.

01C

oO0.

050.

050.

020.

010.

030.

030.

040.

050.

060.

040.

010.

030.

060.

07

Tota

l99

.62

100.

2599

.64

99.8

499

.41

99.8

210

0.60

99.6

499

.89

101.

1710

0.83

100.

9810

1.15

101.

13

Fo88

.01

88.0

588

.43

88.8

988

.79

88.7

588

.53

87.4

878

.21

78.4

878

.51

78.3

777

.64

78.0

1

Com

men

tP

ZH

07-1

3P

ZH

07-1

4P

ZH

07-1

5P

ZH

07-1

6P

ZH

07-1

7P

ZH

07-1

8P

ZH

07-1

9P

ZH

07-2

0P

ZH

09-0

1P

ZH

09-0

2P

ZH

09-0

3P

ZH

09-0

4P

ZH

09-0

5P

ZH

09-0

6

SiO

242

.74

39.3

738

.79

39.1

338

.93

39.3

338

.76

39.3

939

.16

39.1

638

.98

39.8

139

.20

39.1

3T

iO2

0.05

0.06

0.00

0.02

0.04

0.00

0.06

0.05

0.04

0.12

0.01

0.01

0.00

0.00

Al2

O3

0.19

0.02

0.02

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.02

0.00

0.00

Cr2

O3

0.05

0.00

0.04

0.03

0.00

0.00

0.01

0.03

0.03

0.01

0.02

0.03

0.00

0.00

FeO

19.3

020

.65

20.4

520

.41

20.7

020

.64

19.3

619

.36

19.1

919

.22

19.4

418

.52

19.3

619

.17

MnO

0.22

0.30

0.27

0.05

0.29

0.31

0.27

0.24

0.17

0.22

0.33

0.30

0.32

0.31

MgO

38.2

239

.89

41.0

840

.98

40.0

840

.29

41.0

941

.18

41.2

741

.13

40.8

541

.04

41.1

841

.15

NiO

0.24

0.31

0.31

0.34

0.32

0.30

0.29

0.28

0.20

0.16

0.18

0.29

0.25

0.29

CaO

0.07

0.01

0.00

0.01

0.00

0.00

0.02

0.00

0.01

0.01

0.00

0.02

0.02

0.04

V2O

30.

060.

000.

040.

020.

000.

000.

020.

030.

000.

000.

030.

000.

050.

03C

oO0.

040.

010.

080.

050.

040.

070.

060.

070.

050.

030.

070.

080.

050.

07

Tota

l10

1.18

100.

6210

1.09

101.

0410

0.38

100.

9599

.93

100.

6410

0.11

100.

0699

.91

100.

1110

0.42

100.

17

Fo77

.92

77.5

078

.17

78.1

777

.54

77.6

879

.10

79.1

379

.31

79.2

378

.94

79.8

079

.13

79.2

8

Com

men

tP

ZH

09-0

7P

ZH

09-0

8P

ZH

09-0

9P

ZH

09-1

0P

ZH

09-1

1P

ZH

09-1

2P

ZH

09-1

3P

ZH

09-1

4P

ZH

09-1

5P

ZH

09-1

6P

ZH

09-1

7P

ZH

09-1

8P

ZH

09-1

9P

ZH

09-2

0

SiO

239

.61

38.9

339

.50

39.2

338

.85

39.4

339

.35

39.5

639

.90

39.7

240

.08

39.4

839

.88

40.1

6T

iO2

0.00

0.04

0.07

0.00

0.00

0.00

0.01

0.12

0.00

0.10

0.00

0.10

0.10

0.00

Al2

O3

0.00

0.02

0.00

0.00

0.00

0.02

0.00

0.00

0.04

0.10

0.03

0.03

0.13

0.00

Cr2

O3

0.00

0.03

0.04

0.00

0.01

0.01

0.01

0.00

0.00

0.00

0.00

0.03

0.22

0.00

FeO

19.3

119

.40

19.0

819

.16

19.3

119

.25

19.1

917

.90

16.8

116

.67

15.3

617

.82

16.4

914

.07

MnO

0.28

0.20

0.22

0.34

0.20

0.24

0.19

0.60

0.41

0.28

0.22

0.24

0.00

0.34

MgO

40.8

241

.27

41.1

341

.62

41.1

240

.85

41.4

042

.01

42.0

043

.03

43.2

942

.03

42.4

844

.31

NiO

0.27

0.30

0.26

0.28

0.33

0.27

0.26

0.35

0.01

0.41

0.00

0.41

0.00

0.03

CaO

0.04

0.01

0.02

0.03

0.00

0.00

0.03

0.00

0.19

0.00

0.16

0.00

0.11

0.00

V2O

30.

000.

050.

040.

000.

030.

000.

050.

000.

000.

000.

000.

000.

000.

00C

oO0.

050.

060.

040.

010.

040.

060.

070.

000.

190.

000.

000.

000.

570.

16

Tota

l10

0.37

100.

3210

0.39

100.

6799

.88

100.

1310

0.57

100.

5499

.55

100.

3199

.14

100.

1499

.98

99.0

7

Fo79

.03

79.1

379

.35

79.4

879

.15

79.1

079

.36

80.7

181

.67

82.1

583

.40

80.7

982

.12

84.8

8

(Con

tinu

ed)

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Tabl

e2.

(Con

tinu

ed).

Com

men

tP

ZH

10-0

1P

ZH

10-0

2P

ZH

10-0

3P

ZH

10-0

4P

ZH

10-0

5P

ZH

10-0

6P

ZH

10-0

7P

ZH

10-0

8P

ZH

10-0

9P

ZH

10-1

0P

ZH

10-1

1P

ZH

10-1

2P

ZH

10-1

3P

ZH

10-1

4

SiO

239

.57

40.3

940

.85

40.7

938

.47

38.5

038

.87

38.3

238

.91

38.8

139

.56

39.4

040

.57

40.3

1T

iO2

0.12

0.00

0.00

0.00

0.00

0.00

0.06

0.00

0.16

0.00

0.00

0.04

0.00

0.04

Al2

O3

0.05

0.00

0.00

0.00

0.00

0.04

0.00

0.05

0.02

0.00

0.00

0.00

0.03

0.00

Cr2

O3

0.00

0.00

0.00

0.13

0.00

0.00

0.24

0.14

0.00

0.00

0.00

0.00

0.10

0.00

FeO

16.2

316

.23

14.7

814

.55

19.5

520

.48

17.3

719

.16

19.4

120

.24

19.0

619

.76

13.2

513

.42

MnO

0.19

0.66

0.23

0.19

0.42

0.47

0.48

0.24

0.24

0.68

0.46

0.41

0.25

0.39

MgO

42.7

342

.70

44.1

843

.91

39.5

338

.92

40.8

540

.70

39.7

438

.92

40.7

340

.23

45.2

144

.47

NiO

0.18

0.00

0.00

0.34

0.02

0.20

0.51

0.00

0.30

0.39

0.35

0.18

0.27

0.24

CaO

0.00

0.00

0.06

0.00

0.08

0.08

0.03

0.03

0.00

0.26

0.00

0.06

0.00

0.05

V2O

30.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

00C

oO0.

000.

000.

000.

330.

050.

190.

000.

300.

000.

000.

410.

210.

000.

00

Tota

l99

.07

99.9

810

0.10

100.

2498

.12

98.8

898

.41

98.9

498

.78

99.3

010

0.57

100.

2999

.68

98.9

2

Fo82

.44

82.4

384

.20

84.3

378

.28

77.2

180

.74

79.1

178

.50

77.4

279

.21

78.4

085

.88

85.5

2

Com

men

tP

ZH

10-1

5P

ZH

10-1

6P

ZH

10-1

7P

ZH

10-1

8P

ZH

10-1

9P

ZH

10-2

0

SiO

240

.64

40.1

240

.72

40.0

840

.25

39.9

2T

iO2

0.14

0.06

0.00

0.00

0.00

0.00

Al2

O3

0.00

0.00

0.00

0.00

0.00

0.00

Cr2

O3

0.00

0.00

0.22

0.00

0.18

0.32

FeO

13.3

714

.14

12.9

014

.09

15.0

314

.39

MnO

0.38

0.50

0.37

0.39

0.57

0.29

MgO

45.3

444

.01

45.0

944

.55

43.7

543

.67

NiO

0.24

0.34

0.00

0.33

0.24

0.21

CaO

0.12

0.03

0.00

0.06

0.03

0.42

V2O

30.

000.

000.

000.

000.

000.

00C

oO0.

000.

400.

000.

110.

000.

00

Tota

l10

0.23

99.6

099

.30

99.6

110

0.05

99.2

2

Fo85

.81

84.7

386

.17

84.9

383

.84

84.4

0

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1734 T. Hou et al.

Tabl

e3.

Rep

rese

ntat

ive

anal

yses

ofcl

inop

yrox

ene

inth

ePa

nzhi

hua

weh

rlit

e.

No.

05C

px01

05C

px02

05C

px03

05C

px04

05C

px05

07C

px06

07C

px07

07C

px08

07C

px09

07C

px10

07C

px11

07C

px12

07C

px13

Cpx

m08

-1C

pxm

08-2

SiO

251

.36

49.8

850

.63

54.3

452

.00

52.2

852

.56

51.0

051

.73

50.9

050

.91

48.1

650

.84

49.5

748

.84

TiO

20.

711.

131.

520.

370.

911.

110.

831.

050.

910.

820.

921.

641.

181.

401.

38A

l 2O

32.

753.

984.

371.

583.

323.

273.

633.

453.

143.

623.

466.

004.

236.

154.

92Fe

O9.

5710

.77

9.05

9.05

8.91

7.81

8.81

8.80

9.02

8.53

8.64

10.1

49.

548.

459.

82M

nO0.

240.

420.

080.

380.

300.

150.

000.

530.

100.

400.

370.

170.

370.

190.

45M

gO13

.33

12.2

612

.81

14.6

013

.49

13.9

514

.11

14.0

013

.95

13.4

313

.87

12.4

312

.69

12.6

212

.29

CaO

20.3

121

.15

20.5

819

.89

20.8

520

.63

20.3

819

.88

20.5

020

.63

20.5

520

.63

20.5

020

.76

20.5

8N

a 2O

0.37

0.41

0.37

0.51

0.41

0.24

0.33

0.23

0.24

0.51

0.77

0.55

0.42

0.26

0.42

K2O

0.04

0.01

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.05

0.00

0.00

0.00

0.12

Tota

l98

.69

100.

0199

.40

100.

7210

0.19

99.4

510

0.64

98.9

399

.60

98.8

399

.55

99.7

399

.77

99.4

198

.83

Mg#

0.71

0.67

0.72

0.74

0.73

0.76

0.74

0.74

0.74

0.74

0.74

0.69

0.71

0.00

0.00

Wo

43.0

344

.38

44.5

140

.98

43.8

244

.14

42.8

842

.22

43.1

743

.69

42.6

043

.97

43.9

245

.56

44.3

2E

n39

.30

35.8

038

.55

41.8

639

.46

41.5

341

.32

41.3

840

.88

39.5

840

.01

36.8

637

.83

38.5

436

.83

Fs

16.2

518

.27

15.4

915

.25

15.1

613

.40

14.5

415

.52

15.0

414

.77

14.5

117

.05

16.6

214

.86

17.2

2A

c1.

421.

561.

451.

901.

560.

931.

260.

880.

911.

952.

892.

121.

631.

031.

64

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Wo

FsEn

Clinopyroxene composition in

the main body of Panzhihua intrusion

Clinopyroxene composition in the wehrlite

Figure 9. Chemical variations of clinopyroxene from thewehrlite. The shaded field is the compositions of clinopyroxenein the main body of the Panzhihua intrusion.

and V) and low concentrations of incompatible elements(Table 4) consistent with the petrographic observations thatthe rocks consist largely of olivine phenocrysts with minorclinopyroxenes and V–Ti–iron oxide (Table 3). All of oursamples have similar, primitive mantle-normalized traceelement patterns, which are enriched (up to 30 times mantlevalues) in large-ion lithophile elements (LILEs), coupledwith positive Th, Nb, and Ta anomalies (Figure 10A).Moreover, they are enriched in light rare earth elements(LREEs) relative to the middle and heavy rare earthelements (MREEs and HREEs, Figure 10B). Overall,the primitive mantle-normalized patterns of the sam-ples are parallel to average ocean island basalt (OIB;Sun and McDonough 1989) and those of Lijiang picrite(Figure 10A). Moreover, these data are undoubtedly consis-tent with those of the main body of the Panzhihua intrusion(e.g. Zhou et al. 2005; Zhang et al. 2009).

Discussion

Petrogenetic linkage between wehrlite and the Panzhihuagabbro intrusion

There are several small dikes and pods of wehrlite whichare clearly intrusive into the fine-grained olivine gabbroin the marginal zone and country rocks, i.e. marble ofDengying Formation (Figures 3A and 3C). Before we dis-cuss the petrogenetic linkage between the wehrlite and themain body of Panzhihua gabbro intrusion, we must assesswhether or not they are related. For example, blocks ofwehrlite and olivine gabbro near the base of Skaergaardintrusion are now known to have come from an older bodyand have no petrologic relation to the Skaergaard magmaand played no role in its evolution (Kays and McBirney1982). In the case of Panzhihua, three lines of evidencesuggest that the wehrlite and gabbros are cogenetic. First,

they have similar trace element patterns and Nd-Sr isotopiccompositions (Figures 10 and 12). Second, according tothe regional geology, no magmatic activity has been rec-ognized after the late Permian, during which the ELIP wasemplaced in the vicinity of Panzhihua (SBGMR 1991).Third, the Fo content in olivine in wehrlite varies con-tinuously and the relatively low Fo values are consistentwith those in the mainbody of the Panzhihua intrusion(Figure 11), combined with the similar Cpx composition(Figure 9). Moreover, the wehrlites cut across the fine-grained olivine gabbro in the marginal zone (Figure 3B),implying that these intrusive wehrlites emplaced into thepre-existing rocks in the marginal zone. That the cogeneticintrusive wehrlite consists of high-Mg olivine phenocrysts(the highest fosterite content is 90.44 wt.%) is sugges-tive that the magma which formed the main body of thegabbro intrusion had differentiated at depth rather than insitu. Therefore, the gabbroic magma was possibly formedby moderate amounts of fractional crystallization of high-Mg olivine in the residual magma from the commonparent.

Primitive magma composition

Layered intrusions are important for understanding thegenesis and chemical evolution of mafic–ultramafic mag-mas and the processes associated with the formation of Cr,Fe–Ti–(V), and PGE deposits (e.g. Wager and Brown 1968;Irvine 1975; Parsons 1987; Cawthorn 1996; Keays et al.1999; Cabri 2002; Maier and Groves 2011). One of thekey parameters in the modelling of the crystallization his-tory of an intrusion is the estimation of the composition ofparental magma(s) (Wager and Brown 1968). Traditionally,the composition of the parental liquid of a layered intru-sion has been estimated by using two different approaches:(1) by analysing the whole-rock composition of the fine-grained rocks found at the contact of the intrusion as chilledmargins, or as dikes or sills spatially associated with theintrusion (e.g. Barnes et al. 2010 and reference therein);or (2) by using the major and trace element compositionsof cumulate minerals to back-calculate the parental magmawith which they were in equilibrium (e.g. Duchesne andCharlier 2005).

Due to the fact that the marginal zone has been exten-sively modified by the interaction of the magmas with thewall rocks at Panzhihua, and the absence of cumulate min-erals, Zhang et al. (2009) suggested that the Panzhihuaintrusion crystallized from a ferropicrite with high Feand Ti but low SiO2 contents on the basis of bulk sum-mation. The compositions of the parental melts of theintrusion have been estimated using the method of Chaiand Naldrett (1992). The results show that the samples plotbelow the equilibrium line (Figure 11). Assuming that thewehrlites represent mixtures of olivine Fo90.4 with a coex-isting liquid, the FeO and MgO contents in the liquid can

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Table 4. Major (wt.%) and trace elements (ppm) compositions of the wehrlite.

PZH05 PZH06 PZH07 PZH09 PZH10

SiO2 39.63 39.43 41.34 42.24 38.6Al2O3 6.87 5.93 9.36 8.34 8.4TFe2O3 9.77 9.52 9.95 9.51 7.74CaO 4.91 4.32 5.94 7.91 15.16MgO 31.91 33.51 25.87 24.52 25.45K2O 0.47 0.3 1.01 0.44 0.02Na2O 1.36 0.99 1.92 1.77 0.11MnO 0.138 0.132 0.131 0.135 0.092TiO2 0.899 0.748 1.29 1.39 1.26P2O5 0.151 0.129 0.22 0.192 0.708LOI 3.27 4.39 2.41 2.89 1.95Total 99.378 99.399 99.441 99.337 99.49Mg# 0.87 0.88 0.84 0.84 0.87Y 9.15 9.23 13.9 14 17La 7.68 6.95 13.8 13.8 13.7Ce 17.7 15.7 31.6 32.2 31.3Pr 2.28 2.08 4.09 4.15 4.12Nd 9.64 8.85 17.3 17.4 17.1Sm 2.3 2.23 3.72 3.9 4.1Eu 0.73 0.73 1.23 1.26 1.4Gd 2.52 2.32 3.82 3.76 4.37Tb 0.35 0.35 0.54 0.56 0.64Dy 1.91 1.85 2.93 2.93 3.43Ho 0.37 0.33 0.52 0.54 0.62Er 1.01 0.93 1.48 1.46 1.8Tm 0.12 0.12 0.17 0.17 0.22Yb 0.7 0.72 1.05 1.09 1.3Lu 0.12 0.1 0.16 0.15 0.2Rb 38.8 7.1 42 6.58 0.55Sr 147 82.7 260 205 91Cs 1.71 0.28 1.55 0.16 0.05Ba 238 80.7 254 129 21.6Th 1.05 0.97 1.9 1.83 1.5U 0.26 0.24 0.46 0.38 1.73Pb 94.6 31.2 3.96 49.3 3.33Zr 59.5 56.9 107 102 84.1Hf 1.81 1.69 3.28 3.29 2.89Nb 9.91 8.25 13.2 12.8 12.3Ta 0.64 0.55 0.88 0.82 1.05Cr 2150 2980 1849 1981 1452Co 124 151 136 109 108Ni 1594 1892 1338 1155 1108Sc 18 20.6 25.4 34.3 27.6V 174 180 266 292 339Cu 169 21 121 229 174Ga 9.96 9.74 14 13.9 15.2

be estimated by extrapolation of line A to B to interceptline C to D at point E, which gives ∼10 wt.% FeO and∼16 wt.% MgO in the liquid. Such a parental magma com-position is virtually normal picrite, suggesting that a mantleplume was involved in the petrogenesis of the Panzhihuaintrusion. Moreover, this inference is also evidenced bythe patterns of alteration-resistant incompatible elementssimilar to OIB (Figure 10). Hence, this conclusion updatesthe former understanding, which was mainly based on theestimation of major elements that the parental magma ofthe Panzhihua intrusion is ferropicritic (Zhang et al. 2009)

or ferrobasaltic (Zhou et al. 2005). The crystallization ofMg-rich olivine could cause the relatively high Fe contentin the residual magma as a Fenner trend (e.g. Fenner 1929).

Plume-related origin

It has been suggested that the magmas that formed thePanzhihua ore-bearing intrusion were derived from aFe-rich and fertile plume source (Zhou et al. 2008).However, no direct evidence has been produced, exceptfor the crystallization age of 263 ± 3 million years,

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Th Nb Ta La Ce Pr Nd Zr Hf Sm Eu Gd Tb Dy Y Ho Er Tm Yb Lu

Rock/p

rim

itiv

e m

antle

Rock/c

hondrite

100

10

1

Avg.OIB

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

(B)

(A)

100

1000

10

1

Avg.OIB

Lijiang picrite

Lijiang picrite

Figure 10. (A) Primitive mantle-normalized trace element diagrams of the wehrlite from the Panzhihua gabbroic intrusion. The diagramsfor those of Lijiang picrite (Zhang et al. 2006) are shown for comparison. (B) Chondrite-normalized REE patterns of the wehrlite fromthe Panzhihua gabbroic intrusion. Primitive mantle-normalizing values and average OIB patterns are from Sun and McDonough (1989).

which is close to the eruptive age of the Emeishan basalts(∼260 Ma; Zhou et al. 2005). Therefore, whether theEmeishan plume has been involved in the derivation ofprimitive magma that supplied the Panzhihua intrusionremains unclear, since no other strong evidence has beenpresented.

Generally, in areas where a mantle plume rises, toform LIPs, the degree of melting would be greater andthe parental melt can be picritic (Chung and Jahn 1995).The inference that the parental magma of Panzhihua intru-sion is normal picrite provides direct evidence for con-tribution from the plume head in the source. Such high-magnesian liquids can not only provide constraints ondefining geochemical characteristics of the plume sourceregion (Campbell and Griffiths 1990), but also be used toevaluate the role of plume–lithosphere interaction (Ellamet al. 1992; Saunders et al. 1992).

We use the parental magma composition to estimate theconditions under which the primary picritic magmas weregenerated. The model of adiabatic pressure–temperaturepaths for primary ultramafic magmas (Herzberg andO’Hara 2002) suggests an initial melting temperature ofabout 1530◦C and pressure of ∼3.4 GPa (Figure 13A).The estimated temperature and pressure are moderatelylower than that of Lijiang picritic lavas (∼1630–1680◦Cand 4.2–5.0 GPa, respectively; Zhang et al. 2006). Recentestimates of mantle potential temperature for the sourcesof Hawaii and Iceland are in the same range as above,whereas those for normal asthenosphere are 150–230◦Clower (Putirka 2005). Thus, the source temperature esti-mated for the Panzhihua intrusion appears to be consistentwith some type of mantle thermal plume, whether ‘standardtype’ (e.g. Griffiths and Campbell 1991), non-Newtonian(e.g. Larsen and Yuen 1997), or thermochemical (e.g.

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MgO(wt.%)

Liquids in eq with

Fo = 90.4

Wehrlite

Fo(mol.%) in olivine

D

0

5

10

15

20

25

30

0 10 20 30 40 50

E

AB

C

FeO

(wt.%

)

35

40

4545

Fo(mol.%) in olivine

from the main body

of Panzhihua intrusion

Figure 11. MgO versus FeO for olivine and bulk rocks of thewehrlite from the Panzhihua intrusion. The line indicates liquidcompositions in equilibrium with olivine of Mg number (Fo =90.4), assuming the ratio of FeO/FeOtotal to be 0.9. A detailedexplanation can be found in the text. ‘A’ represents the composi-tions of the highest Fo content in olivine. The shaded field is thecomposition of olivine in the main body of Panzhihua intrusion(Zhang et al. 1988; Pang et al. 2009).

Davaille et al. 2005). In addition, our new U–Pb zircondating on the anorthosite, which is close to the Emeishanbasalts, also supports such an inference. The slightly lowertemperature and pressure relative to Lijiang picrite canbe caused by two possibilities: (1) the primary magma ofPanzhihua intrusion is derived from the plume peripherywhere the temperature is relatively low (Campbell 2005),according to the model proposed by Campbell and Griffiths(1990); and (2) varying degrees of contribution from litho-spheric mantle in the plume source (Molzahn et al. 1996).Since the Panzhihua intrusion is located at the inner zoneof ELIP where the rising mantle plume head impinged atthe base of the lithosphere (Thompson et al. 2001), anevent which may or may not be accompanied by litho-spheric extension, the possibility that the primary magmaof Panzhihua intrusion is derived from the plume periph-ery can be easily excluded. Hence, we hereby proposethat the parental melt of the Panzhihua intrusion couldhave been generated by plume–lithosphere interaction orascending plume-derived melts contaminated by the litho-spheric mantle, a model that is consistent with the previousstudy (Zhang et al. 2009). This model has been advocatedto account for the petrogenesis of basalts and picrites inELIP by many workers (e.g. Chung and Jahn 1995; Xuet al. 2001; Zhang et al. 2006).

Lijiang picrite

Emeishan

basalts

Siberian traps

Panzhihua wehrlite

ε Nd (

t)

(87Sr/86Sr)t

0.702 0.703 0.704 0.705 0.706 0.707 0.708

+12

+8

+4

0

–4

–8

–12

Main body of

Panzhihua intrusion

Figure 12. εNd(t) versus (87Sr/86Sr)t for the Panzhihua wehrlite(our unpublished data). The data for Lijiang picrite are fromZhang et al. (2006). The data for main body of Panzhihua intru-sion are from Zhang et al. (2009). The Emeishan basalts dataare from Xu et al. (2001), Zhang and Wang (2002), and Xiaoet al. (2004). Siberian Traps data are from Sharma et al. (1992),Lightfoot et al. (1993), and Wooden et al. (1993).

Constraints on the mantle source

In order to understand the petrogenesis and metallogene-sis, it is necessary to confirm whether the parental magmasof the Panzhihua ore-bearing intrusion were derived froma garnet peridotite source or a spinel peridotite source.In the (Tb/Yb)P versus (Yb/Sm)P diagram, melting of gar-net peridotite produces a markedly different trajectory frommelting of spinel peridotite (Figure 13B). The Panzhihuadata lie closer to a melt path for garnet peridotite thanto one for spinel peridotite. With the assumptions usedto construct Figure 13B, the data are consistent with a∼80% contribution to the melt from melting in the pres-ence of garnet, and with rather small amounts of partialmelting, ∼5–8%. We caution that these values should notbe taken literally, because the positions of the curves inFigure 13B can vary with a different choice of melt-ing model, distribution coefficients, source composition,and/or mineral proportions. However, the conclusion thatmuch of the melting occurred in garnet-facies mantle andthat the amount of partial melting was rather modest isdifficult to avoid.

Origin of rhythmic layering

The rhythmic layering in layered intrusions is enigmatic.Layer-forming processes are numerous and include bothinternal processes, such as gravity subsidence (Wager andBrown 1968), in situ crystallization (Jackson 1970), dis-continuous to continuous nucleation (Goode 1976), doublediffusive convection (DDC; Sparks et al. 1984), self-organization (Ortoleva 1984), and recrystallization (Zingg1996), and external processes such as magma-recharge

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All liquid

0 200100

Gorgona K

W.Greenland

Hawaii iceland Gergona

MORB

Fer tile peridotite

Depth(km)

Tem

pera

ture

(°C

)

Solidus

Pressure(GPa)

10

0 2 4 6 81000

1200

1400

1600

1800

2000

15%

10%

5%

1%

Gar 0

Gar 50

Gar 100

Model sourcePanzhihua wehrlitesLijiang picritic lavas

1.0 1.5 2.0 2.5 3.0 3.5 4.0

(Tb/Yb)P

(Tb/S

m) P

(B)(A)

20

30

38

Figure 13. (A) Adiabatic temperature–pressure paths for primary magmas produced by melting of fertile peridotite, after Herzberg andO’Hara (2002). The black circle is the estimated 1 atm temperature of the modelled primary magma; the square symbol represents arough estimate of temperature and pressure at the start of melting. Numbers on paths to the left of the solidus indicate wt.% MgO ofliquids. Ranges of estimated mantle pressure and temperature for Gorgona komatiites (K) and picritic basalts, West Greenland picriticlavas, modern Hawaii and Iceland, and MORB (mid-ocean ridge basalt) are from Herzberg and O’Hara (2002). (B) (Tb/Yb)P versus(Yb/Sm)P for the wehrlite from the Panzhihua intrusion. The grid indicates the range of model melt compositions produced by 1%, 5%,10%, and 15% of aggregated fractional melting (Shaw 1970) of peridotite in which the amount of melting that occurs in the presenceof garnet varies from 0% to 100%. The light lines indicate the percentage of melt contribution from garnet-facies mantle (Gar); forexample, Gar 0 corresponds to melt from spinel peridotite. Curves of constant melt fraction are shown by bold lines. The curves are fora source consisting of a 1:1 mix of estimated average-depleted mantle (Workman and Hart 2005) and model-enriched mantle peridotite(Ito and Mahoney 2005); such a source is consistent with the Nd isotope values of our highest εNd samples. Partition coefficients aretaken or interpolated from Salters and Stracke (2004). The unmelted peridotite is assumed to be 53% olivine, 30% orthopyroxene, 10%clinopyroxene, and 7% garnet or spinel, and melting of these minerals is assumed to occur in proportions of 10%, 10%, 40%, and 40%,respectively, after Janney et al. (2000).

(Shervais et al. 2006), magma-extraction, or magma-mixing (Naslund and McBirney 1996). Mafic magmachambers may operate as a closed system or as an open sys-tem (Ferré et al. 2009). According to our field observations,only the middle of the intrusion’s magmatic stratigraphy iswell layered, similar to those described in the Skaergaardintrusion (e.g. McBirney and Noyes 1979; McBirney 1995)and Bushveld Igneous Complex (see reviews by Eales andCawthorn 1996; Cawthorn and Spies 2003), and consists ofpronounced layered gabbro occasionally interlayered withseveral thin V–Ti–iron oxide ore layers. As stated above,all the layered gabbros are medium to coarse-grainedwith local pegmatitic facies, but they are confined almostentirely to modal rather than textural variations, and mani-fested in the modal proportions of dark and light minerals,i.e. plagioclase and clinopyroxenes. The dark layers consistof clinopyroxene phenocrysts and eutectic clinopyroxenesand plagioclase assemblage in the groundmass (Figure 6C).In contrast, the light layers are characterized by the occur-rence of porphyritic plagioclase and a similar mineralassemblage with dark layers (Figure 6D). These character-istics indicate that in the more mafic layer, clinopyroxenescrystallized prior to plagioclase, thereafter the clinopyrox-ene and plagioclase crystallized together in the ‘ground-mass’. In the light layers, the plagioclase crystallized early,and then similar assemblage in the ‘groundmass’ formed.

Thus, we are inclined to advocate the model of in situcrystallization to explain the rhythmic layering.

On the basis of estimation by combining the totalthickness of Neoproterozoic to Late Permian (∼260 Ma)rocks, the emplacement depth of the Panzhihua intrusionis ∼6 km (SBGMR 1991). In the Di–An phase diagram(Figure 14A), this depth is in accordance with a pressureof ∼2000 bars. In the middle magmatic stratigraphy of theintrusion where the rhythmic layering extensively occurred,the ratio of plagioclase (Pl)/clinopyroxene (Cpx) calcu-lated by CIPW norms range from 60.5:39.5 to 65.25:34.75(Song et al. 1999). Therefore, the isopleth defined by such acomposition (isopleth A) is approximately coincident withthat defined by the eutectic point. As temperature decreases(from point 1 to point 2: Figure 14B), the Cpx and Pl wouldcrystallize together, thus forming the typical gabbroic tex-ture. When the water pressure decreases, the liquidus wouldmove up and the eutectic point would move to the Di-richdirection. Hence, on the same isopleth, Pl would crys-tallize first as magma cools (point 3: Figure 14B) andwhen the temperature decreases to the eutectic point, Cpxwould crystallize together with Pl, forming the light lay-ers. In contrast, if the water pressure increases, the liquiduswould descend and the eutectic point would move to theAn-rich direction. When the temperature decreases on theisopleth, Cpx would crystallize first (point 4: Figure 14B)

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L+Di L+An

50 60 70 80 90 1001000

1100

1200

1300

Di+An

An(wt.%)

Tem

pera

ture

(°C

) PH 2O = 2 kbar

PH 2O < 2 kbar

PH 2O = 2 kbar

PH 2O > 2 kbar

PH 2O = 5 kbar

1

2

3

4

1

nAiD

2

4

3

a

b

c

(B)(A)

B A C

Figure 14. (A) Phase diagram for the Di–An system at 5 kbar (Yoder and Tilley 1962) and the estimated at 2 kbar. (B) The enlargementof the area shown in (A) (the detailed explainations are seen in the text).

followed by Pl until the eutectic point at which the Cpx andPl crystallize together, forming dark layers. Consequently,the alternating crystallization sequence of clinopyroxeneand plagioclase could be in response to the periodicallychanged water pressure, thereby resulting in the rhythmiclayering. However, it is noteworthy that only those hav-ing compositions close to the eutectic point (the shadedarea: Figure 14B) can faciliate the forming of the rhythmiclayering under changing water pressure. Those that are dis-tant from this area (isopleth B and C: Figure 14B) firstcrystallize Cpx or Pl no matter how the water pressurechanged at such a depth. For example, the crystallizationalong isopleth B (Di>>An) only forms melagabbro in thelower part whereas that along isopleth C (An>>Di) formsleucogabbro in the upper part of the intrusion, respectively.However, it is still unclear why the water pressure changedperiodically, but this inference closely meets our observa-tions and could best explain why the rhythmic layering isonly pronounced in the middle part of the intrusion.

Magma evolution and mineralization

There is little doubt about the magmatic origin of thePanzhihua V–Ti–iron oxide deposits, although consider-able debate still surrounds the processes that led to theaccumulation of the oxides. One of the most popular viewsfirst proposed by Wager (1960) is that very extensive frac-tionation causes the formation of Fe-rich melts, whicheventually become FeO saturated, and as a result form theV–Ti–iron-oxide-rich rocks in the upper part of layeredmafic intrusions, such as in the Bushveld Igneous Complex(e.g. Cawthorn and Spies 2003). However, the massive ore-bodies at the base of the Panzhihua intrusion cannot beinterpreted by this mechanism. Alternatively, the accumu-lation of Ti–V oxides at the base of the magmatic stratigra-phy can be interpreted by immiscibility of V–Ti–iron oxideliquid (Zhou et al. 2005) or settling of V–Ti–iron oxidecrystals (Ganino et al. 2008; Pang et al. 2008). Previousexperiments have shown that high FeO content and strong

enrichment in P, Ti, rare earth element (REE), and highfield strength elements (HFSEs) are the geochemical hall-marks of Fe-rich immiscible liquids (Veksler et al. 2006).But as stated above, the ores in the Panzhihua intrusion donot exhibit such features (e.g. Zhou et al. 2005; Zhang et al.2009). Besides, Zhou et al. (2005) found that the massiveV–Ti–iron oxide ore horizons in the Panzhihua intrusionalso contain silicate phases such as clinopyroxene andplagioclase. These features cannot be plausibly interpretedby an immiscible V–Ti–iron oxide liquid. Additionally,Pang et al. (2008) and Ganino et al. (2008) conclude thatthe V–Ti–iron oxides had crystallized at an early stageof the solidification of Panzhihua intrusion, because ofoxidizing conditions in response to an interaction of therelatively evolved basaltic magma with sedimentary wallrocks (Ganino et al. 2008) or the combination of moderateH2O content, high Fe and Ti contents, and low SiO2 contentof the parental magmas (Pang et al. 2008). As indicated bytheir Sr, Nd, Pb, and O isotopic compositions (Zhang et al.2009), the magmas that produced the Panzhihua intrusionwere not significantly contaminated by crustal materials,and it is unlikely that fluids were introduced into thesystem during crystallization through magma–wall-rockinteraction.

According to our field observations, five types of rocksare considered in this study: (1) wehrlite; (2) massiveore; (3) disseminated ore; (4) anorthosite; and (5) layeredgabbro (melagabbro and leucogabbro). As stated above,these rocks were formed by differentiation from a commonparental magma. The current debate is focused on whetherthey were generated by fractional settling or in situ growthon the floor of the intrusion (Cawthorn and Spies 2003)or liquid differentiation in a deeper-level magma chamber,as for example in the Skaergaard intrusion (Brooks et al.1991). In the Panzhihua mine, we have observed sharp con-tacts between the massive ores and gabbro, wehrlite andfine-grained gabbro, disseminated ores and gabbro, andanorthosite and gabbro. In addition, the fragments of gab-bro are cemented by disseminated V–Ti–iron oxide ores.

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All these features lead us to conclude that all these dif-ferent types of magmas differentiated in the chamber atdepth, and then intruded into the higher lever chamber,rather than crystallized in situ. Since most rocks in the mainbody of the intrusion show equigranular or fine-grainedtexture, they probably ascended as melt or crystal mushfrom a deep-level magma chamber. These characteristicscould be explained by two possible models: (1) a deep-level chamber is a one-component system, in which crystalsettling is a crucial factor influencing the composition ofthe residual magmas. Hence, the different types of rocks inthe intrusion are derived from the residual magma formedat different stages during evolution in the deep chamber.(2) A deep-level chamber is a multi-component system, andconvective motions and crystal settling may have played animportant role in the dynamics of deep magma chambers(e.g. Blanchette et al. 2004). Thus, these different typesof rocks should be derived from different compositionalstratifications in the deep-level chamber.

In the first model, a continuously decreasing bulk-rockMg# value should be observed from less evolved to moreevolved igneous layers or zones. However, according toprevious studies this has not been observed (e.g. Zhouet al. 2005; Pang et al. 2008, 2009; Zhang et al. 2009).

Therefore, bearing in mind that a deep-level chamber gen-erally cools from above, so that heat- and mass-transferprocesses may be present, the second option seems rea-sonable. This is known as DDC (Huppert and Sparks1984), which can play an important role in the differenti-ation of magmas (e.g. Liang 2010 and reference therein).Besides, some of the important effects, such as gravitysubsidence, in the deep-level chamber are due to the crys-tallization induced by cooling (e.g. Wilson 1993); it canbe envisaged that the compositional stratification proba-bly was formed by the combination of DDC and gravitysubsidence. Specifically, from bottom to top, according tothe density, stratification of heavy oxide melts, ultramaficmelts containing olivine phenocrysts, and ferrogabbroic togabbroic and anorthositic melts are developed.

On this basis, we propose a new petrogenetic/metallogenetic model, as follows: (1) a primitive plume-derived magma (picritic) ascends into the relatively deepcrustal-level chamber (Figure 15A); (2) a Fe-rich residualmelt was produced by the crystallization of V–Ti–iron-poorminerals, e.g. high-Mg olivine, in the early stage of differ-entiation (Figure 15B); (3) as magma cools, DDC operatedin the deep chamber, combined with gravity subsidence,forming several compositional stratifications (convection

Anorthositic melt

oxide meltsMelts contain olivine crystals

Gabbroic to ferrogabbroic melt

Anorthositic melt

Oxide meltsMelts contain olivine crystals

Plume-derived

picritic melt

Sedimentation of olivine

crystals, formed by fractional

crystallization

Residual Fe-rich melt


Anorthositic melt

1

3

Massive and disseminated orebodies

Rhythmic layered gabbro

Dikes of anorthosite Dikes of wehrlite

Compositional stratification, formed by

the combination of double-diffusive

convection and gravity subsidence

4

Plagioclase-rich gabbro

Clinopyroxene-rich gabbro

Trapped liquid

Melts contain olivine crystals

Oxide melts

Melts contain olivine crystals


2

(A)

(B)

(C) (D)

(E)

Figure 15. Proposed petrogenetic model for the Panzhihua intrusion. The panels (A), (B) and (C) illustrate the magma evolution in thedeep-level chamber. On basis of the field observations, the intrusive sequence is illustrated in panels (D) and (E), as shown by the number:1, gabbroic melt; 2, ferrogabbroic and oxide melts; 3, anorthositic melt; 4, melt contains olivine phenocrysts. See text for details.

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cells) from bottom upwards – oxide melts, melts contain-ing high-Mg olivine crystals, ferrogabbroic, gabbroic, andanorthositic melts (Figure 15C); (4) after the major bodyof gabbroic melts emplaced in the upper level chamber, theV–Ti–iron oxide melts and ferrogabbroic melts intrudedor cut the mainbody of the intrusion, and the rhythmiclayering occurred in the gabbroic intrusion by in situ crys-tallization (Figure 15D). Then, the anorthositic melts andultramafic melts containing olivine phenocrysts ascendedto the high level (Figure 15E).

This model differs from the V–Ti–iron oxides miner-alization in the Bushveld intrusion where the V–Ti–ironoxides are only concentrated in the upper part, which can beexplained by fractional crystallization of V–Ti–iron oxidesat a late stage (Cawthorn and Spies 2003), and is alsoinconsistent with the previous model of immiscible sepa-ration (Zhou et al. 2005), or the V–Ti–iron ores are formedby in situ fractional crystallization (Pang et al. 2008).

Conclusions

(1) The Panzhihua anorthosite body was emplacedin the late Permian (259.77 ± 0.79 Ma), coevalwith the eruption of the Emeishan basalts. Theregional geology and mineralogy and geochemicalcharacteristics of the wehrlite dike suggest thatthe anorthosite body and the gabbros were coge-netic. The wehrlite consists of high-Mg olivinephenocrysts (the highest fosterite content is90.44 wt.%), and thus the primitive magma of thePanzhihua intrusion is estimated to have been nor-mal picritic (∼10 wt.% FeO and ∼16 wt.% MgO)and was produced by partial melting of garnet peri-dotite. Much of the fusion occurred in garnet-faciesmantle at an initial melting temperature of about1530◦C and a pressure of ∼3.4 GPa, suggestingthe involvement of a mantle plume in the petrogen-esis. The amount of partial melt was modest andcould have been generated by plume–lithosphereinteraction or by ascending plume-derived meltscontaminated by lithospheric mantle.

(2) Our new field observations document sharp con-tact relations between the massive ores and gab-bro, between wehrlite and fine-grained gabbro,as well as between disseminated ores and gab-bro. In addition, we describe some occurrence ofintrusive anorthosites and brecciated fragments ofgabbro cemented by disseminated ores. These geo-logic relationships suggest that the different magmatypes differentiated in a chamber at depth andwere intruded into a higher-lever chamber, ratherthan having crystallized in situ. However, rhythmiclayering only present in the middle of Panzhihuaintrusion could have been formed by in situ crys-tallization.

AcknowledgementsAspects of this work were supported by the 973 Project (Grant No.2012CB416800), National Natural Science Foundation of China(Grant No. 40925006), Special Fund for Scientific Research inthe Public Interest (200911007-25), ‘the Fundamental ResearchFunds for the Central Universities’, 111 Project (B07011), andPCSIRT.

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