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Petrogenesis of the Ultrapotassic Fanshan
Intrusion in the North China Craton:
Implications for Lithospheric Mantle
Metasomatism and the Origin of Apatite Ores
Tong Hou1,2,3, Zhaochong Zhang1*, Jakob K. Keiding4 and
Ilya V. Veksler4,5
1State Key Laboratory of Geological Process and Mineral Resources, China University of Geosciences, Beijing
100083, China, 2Institut fur Mineralogie, Leibniz Universitat Hannover, Callinstrasse 3, Hannover, D-30167, Germany,3Department of Petrology and Economic Geology, Geological Survey of Denmark and Greenland (GEUS), Øster
Voldgade 10, DK-1350 Copenhagen K, Denmark, 4Helmholtz Centre Potsdam GFZ German Research Centre for
Geosciences, Section 4.2, Telegrafenberg, Potsdam, D-14473, Germany and 5Department of Mineralogy, Technical
University Berlin, Ackerstrasse 71–76, Berlin 13555, Germany
*Corresponding author. Telephone: þ86 10 82322195. Fax: þ86 10 82323419.
E-mail: zczhang@cugb.edu.cn.
Received February 23, 2014; Accepted April 8, 2015
ABSTRACT
The Fanshan intrusion in the North China Craton (NCC) is concentrically zoned with syenite in the
core (Unit 1), surrounded by ultramafic rocks (clinopyroxenite and biotite clinopyroxenite; Unit 2),
and an outer rim of garnet-rich clinopyroxenite and orthoclase clinopyroxenite and syenite (Unit 3).
The intrusive rocks are composed of variable amounts of Ca-rich augite, biotite, orthoclase, melanite,
garnet, magnetite and apatite, with minor primary calcite. Monomineralic apatite rocks, nelsonite
and glimmerite exclusively occur in Unit 2. Geochemically, the Fanshan rocks are highly enriched in
light rare earth elements (LREE) and large ion lithophile elements (LILE), moderately depleted in highfield strength elements (HFSE), and have a limited range of Sr–Nd–O isotopic compositions. The
similar mineralogy, mineral compositions, and trace element characteristics of the three units sug-
gest that all the rocks are co-magmatic. The parental magma is ultrapotassic and is akin to kamafu-
gite. Very low-degree partial melting of metasomatized lithospheric mantle best explains the geo-
chemistry and petrogenesis of the parental magmas of the Fanshan intrusion. We propose that the
mantle source may have been metasomatized by a hydrous carbonate-bearing melt, which has im-printed the enriched Sr–Nd isotopic signature and incompatible element enrichment with conspicu-
ous negative Nb–Ta–Zr–Hf–Ti anomalies and LREE enrichments. The mantle source enrichment may
be correlated with oceanic sediment recycling during southward subduction of the Paleo-Asian oce-
anic plate during the Carboniferous and Permian. We propose that crystal settling and mechanical
sorting combined with repeated primitive magma replenishment and mixing with previously fractio-
nated magma is the predominant process responsible for the formation of the apatite ores.
Key words: apatite; ultrapotassic; Fanshan Intrusion; North China Craton; mantle metasomatism
INTRODUCTION
The study of ultrapotassic rocks, which are rare and
volumetrically minor, has been justified by their geneticlink with terrestrial mantle evolution and specific
tectonic settings (e.g. Foley et al., 1987; Miller et al.,
1999; Conticelli et al., 2007, 2009, 2013), as well as their
role in the generation of economic mineral deposits
(e.g. Dill, 2010). Although they can provide valuable
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J O U R N A L O F
P E T R O L O G Y
Journal of Petrology, 2015, 1–26
doi: 10.1093/petrology/egv021
Original Article
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information about their mantle source owing to their
extreme trace element characteristics, the origin of
ultrapotassic rocks has been a matter of substantial de-
bate (see Conticelli et al., 2013; Prelevic et al., 2014, and
references therein). Apatite deposits associated withultrapotassic intrusions are even rarer, with an equally
controversial origin; for example, Fanshan in China
(Jiang et al., 2004; Niu et al., 2012). Compared with
other occurrences of apatite-rich rocks (e.g. phoscorite)
much less information is available to constrain the ori-
gin and formation of monomineralic apatite ores.
The Fanshan alkaline intrusion in the North ChinaCraton (NCC; Fig. 1) hosts orebodies composed of alter-
nating layers of nelsonite and nearly monomineralic
apatite rocks (�38�7 wt % P2O5; Cheng & Sun, 2003).
One of its notable characteristics is the ultrapotassic na-
ture of bulk-rock compositions, with whole-rock K2O
concentrations up to 14% (Jiang et al., 2004). Its potas-sic nature contrasts with the Na-rich nepheline syenites
that host economic apatite deposits elsewhere in the
world, such as the Lovozero and Khibina intrusions,
Kola Peninsula, Russia (Downes et al., 2005; Zaitsev
et al., 2014). Thus, the Fanshan apatite deposits repre-
sent a unique type of magmatic apatite accumulationformed from an unusual type of parental magma. To
understand the origin of the deposits it is necessary to
constrain the composition of the parental magma and
the geochemical characteristics of its source. Although
some work has been done on the Fanshan intrusion
(e.g. Jiang et al., 2004; Niu et al., 2012), the origin of the
intrusion and its apatite deposits is still open to debate.For example, on the basis of Os isotopes Niu et al.
(2012) suggested that the Fanshan parental magma had
been contaminated by crustal rocks during magma em-
placement at crustal levels. However, the low Os con-
centrations (<300 ppt) in all the Fanshan rocks cast
doubts on such a conclusion.
In this contribution, we present comprehensive min-eral, whole-rock major and trace element and Sr–Nd–O
isotopic data for the Fanshan intrusion. The oxygen iso-
tope data are of special interest because they are prob-
ably a more robust indicator of crustal contamination
than Os isotopes (James, 1981). These new data are
used to constrain the composition of the parental
magma and to shed new light on the origin of phos-phorus mineralization in ultrapotassic intrusions.
GEOLOGICAL BACKGROUND
The Fanshan intrusion is located �100 km NW of
Beijing (Fig. 1) at the northern margin of the NCC. The
up to c. 3�85 Ga NCC (Liu et al., 1992) is bounded by
the Central Asian Orogenic Belt to the north and theQinling–Dabie and Sulu Orogens to the south (Fig. 1)
and consists of Paleoarchean to Paleoproterozoic
crystalline basement rocks that are mainly composed
of mafic granulites and amphibolites and tonalite–
trondhjemite–granodiorite (TTG) gneisses, overlain un-
conformably by Mesoproterozoic to Cenozoic sediment-
ary strata (Zhao et al., 2001).The northern margin of the NCC was strongly influ-
enced by the southward subduction of the Paleo-Asian
oceanic plate during Carboniferous to Permian times
(Xiao et al., 2003) with development of an Andean-style
continental margin during the Late Carboniferous–Early
Permian (Zhang et al., 2009). The final closure of the
North China Craton
Qilianshan Orogen
Qinling-Dabie OrogenYangtze Craton
Su-Lu Orogen240-210Ma
LA-ICP-MS
224±4MaSHRIMP
234±2MaLA-ICP-MS
Fanshan218±2MaSHRIMP
220±2MaSHRIMP
225-209MaLA-ICP-MS
220±5MaSHRIMP
Beijing
223±2Ma;222±4MaLA-ICP-MS300km
N40°
N30°
E100° E110° E120° E130°
N40°
N30°
Late Triassicalkaline intrusion
231±1MaSHRIMP
221±5MaSHRIMP
224±2MaSHRIMP
Solonker sutureCentral Asian Orogenic Belt
EAST CHINA SEA
Fault
Datong
Yaojiazhuang
Fig. 1. Simplified tectonic map of North China showing the locations of Late Triassic alkaline intrusions [modified from Ren et al.(2009)]. Age data and analytical methods are compiled from Zhang et al. (2012) and references therein.
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Paleo-Asian Ocean and amalgamation of Mongolian arc
terranes with the NCC along the Solonker suture
occurred between the Late Permian and earliest Triassic
(Zhang et al., 2012). Following this closure, post-colli-
sional Triassic (250–200 Ma) alkaline intrusions wereemplaced along an east–west-trending belt parallel to
the northern margin of the NCC (Fig. 1). The Early
Triassic alkaline rocks consist mainly of monzogranite,
K-feldspar granite and minor monzonite, whereas the
Middle–Late Triassic alkaline rocks include syenite and
peralkaline granite, as well as associated mafic–ultra-
mafic intrusions such as the Fanshan and Yaojiazhuangintrusions, which host magmatic apatite ores (Fig. 1).
Coeval lamprophyre and carbonatite dyke swarms have
been recognized in the Datong region of Shanxi prov-
ince, close to the northern margin of the NCC (Shao
et al., 2003; Fig. 1). In contrast to the Fanshan intrusion,
which contains nearly monomineralic apatite layers, apa-
tite is present throughout the Yaojiazhuang intrusion, lo-
cally in modal proportions up to 15% (Chen et al., 2013).
FANSHAN INTRUSION
GeologyThe Fanshan intrusion was emplaced at c. 218 Ma (Ren
et al., 2009; Niu et al., 2012), at the intersection between
pre-existing NNW–SSE- and east–west-trending faults,
into Meso- to Neoproterozoic limestones and clastic
rocks of the Wumishan Formation. It is an �6� 5 km
oval-shaped body in plan (Fig. 2). However, much of the
pluton is covered by over 100 m of Quaternary sedi-ments, and thus the present geometry of the intrusion
Syenite dykeClinopyroxenesyenite
Fe-P orebodies
Clinopyroxenitedominated rocks
Orthoclaseclinopyroxenite
Garnet-richclinopyroxenite
Late Cretaceous granodiorite
Z2w
Z2w
N
Z2wMesoproterozoic limestones
Z2w
BA
-200
200
600(m)
Height
(a)
Quaternary sediments
Drill hole
of samples from units 2-3
Samples from unit 1(b)
LENDEND
B
A
0 1km
Unit 1
Unit 2
Unit 3
Unit 1
Unit 2
Unit 3
Fig. 2. Geological map (a) and cross-section (b) of the Fanshan intrusion. Modified from Jiang et al. (2004). Most of the intrusion iscovered by Quaternary sediments, and the geological map is largely inferred from borehole and geophysical data. The dashed linein (a) indicates the approximate near-surface locations of samples collected from Units 2 and 3. A–B indicates the line of section in(b). The position of the borehole from which the clinopyroxene syenite samples of Unit 1 were obtained is indicated.
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is largely deduced from geophysical studies and
boreholes. The intrusion consists of three main litholo-
gical units (Fig. 2): an inner, irregular, central syenitic
core (Unit 1), followed by rings of ultramafic layered
rocks (Unit 2) and garnet–orthoclase-clinopyroxenite(syenite) (Unit 3). Minor syenite dykes and veins occur
within the intrusion and cut through the entire succes-
sion; some of these may have been displaced by later
faulting.
The contact relationships between the three units are
sharp. At the contact between Units 2 and 3, a breccia
of Unit 2 clinopyroxenite can be seen in drill core, indi-cating that Unit 2 was the earlier intrusive phase (Mu
et al., 1988). Unit 1 cuts both Units 3 and 2 (Fig. 2) and
contains breccias of Units 2 and 3, and is thus the latest
intrusive phase. No country-rock xenoliths have been
found, but the country-rocks are locally changed into
marble by contact metamorphism.Unit 1 consists predominantly of syenitic rocks, which
are relatively homogeneous and show no layering, al-
though the mafic mineral content varies locally. Unit 2 is
the most complex and is the only unit containing Fe–P
ore, including monomineralic apatite rocks and nelson-
ite. It is composed predominantly of coarse-grained cli-nopyroxenite and locally shows well-developed
rhythmic layering. The clinopyroxenite locally grades
into biotite-clinopyroxenite, orthoclase clinopyroxenite
and glimmerite, and is intercalated with nine layers of
Fe–P ore (Fig. 3). Each rhythmic layer contains a clinopyr-
oxenite layer in the lower part and a biotite clinopyroxen-
ite layer in the upper part (Fig. 3). Although the rhythmiclayers have variable thickness, from a few centimetres to
several tens of meters, the clinopyroxenite and biotite cli-
nopyroxenite layers are typically several meters to 90 m
thick. The monomineralic apatite rocks and nelsonites lo-
cally show prominent rhythmic layering (Fig. 4a) and are
present as paired lithologies. They are recognized only inthe southeastern part of the intrusion; their absence in
the northwestern part could be attributed to removal by
the intrusion of the younger syenitic core (Unit 1). The
intercalated monomineralic apatite rocks and nelsonite
layers generally have a constant thickness and steep in-
ward concentric dips (Fig. 2b). Six phosphorus orebodies
with >4�5 wt % P2O5 have been identified in Unit 2 (Muet al., 1988); however, the cut-off grade for economic
mining activity is 7 wt % P2O5, and therefore only two
orebodies have been exploited. The glimmerite layers
are always intercalated with the Fe–P ore (Fig. 4b) and
are typically less than 1 m thick. The modal mineralogy
varies considerably in the glimmerites; apatite or Ca-richaugite locally reaches more than 25 modal %.
Unit 3 predominantly comprises orthoclase clinopyr-
oxenite and garnet-rich clinopyroxenite and syenite.
The garnet-rich clinopyroxenites dominate the rim
of the intrusion (Unit 3; Figs 2 and 3), and are character-
ized by the occurrence of melanite garnet, which locally
can be up to 35 modal % (Mu et al., 1988) in the outerpart of this unit. The contacts of these rock types are
gradual.
PetrographyCa-rich augite, biotite, magnetite, apatite and K-feldspar
(orthoclase) are the five main minerals in each unit.
Accessory minerals may include melanite, garnet, titan-
ite, pyrite, chalcopyrite, calcite, and rutile.
Unit 1The syenite is grayish white, and consists of euhedral–
subhedral K-feldspar (>70 modal %), Ca-rich augite(�20 modal %) and minor anhedral garnet, biotite, apa-
tite, titanite and magnetite. Biotite and magnetite typic-
ally account for less than 5 modal %, and apatite
accounts for less than 2 modal %. The syenite is rela-
tively homogeneous and has a cumulate texture evi-
denced by the orientation of euhedral to subhedral,
medium- to coarse-grained K-feldspar and Ca-rich aug-ite (Fig. 5a). The syenitic dykes are porphyritic and con-
sist of K-feldspar phenocrysts (up to 30 modal %) set in
a matrix of fine-grained K-feldspar.
Unit 2The modal mineralogy of Unit 2 is summarized in
Table 1 and illustrated in Fig. 6.
The clinopyroxenite in this unit exhibits a cumulatetexture, except locally within the ore horizons (Fig. 6a);
in most cases the crystals of Ca-rich augite have a pre-
ferred orientation, which is probably caused by magma
flow. These rocks are coarse-grained, composed essen-
tially of euhedral Ca-rich augite (up to 5 mm in length;
>80 modal %) and variable amounts of euhedral apatite(up to 10 modal %), with minor interstitial magnetite,
biotite and calcite (Fig. 5b and c). Euhedral apatite can
occur as inclusions in the Ca-rich augite. The calcite is
believed to be primary and magmatic in origin, as evi-
denced by textures (Fig. 5c) similar to those described
from alkaline intrusions associated with carbonatites
(e.g. Le Roex & Lanyon, 1998; Veksler et al., 1998;Zaitsev et al., 2014).
Biotite clinopyroxenite contains less Ca-rich augite
and magnetite but more apatite and biotite than clino-
pyroxenite. In addition, it displays a somewhat different
texture from the clinopyroxenite in which intercumulus
biotite occurs as subhedral grains but locally biotitecrystallized earlier than clinopyroxene (Fig. 6). Both bio-
tite and Ca-rich augite exhibit the same preferred orien-
tation. In some samples, biotite commonly forms
coarse poikilitic plates enclosing Ca-rich augite (Fig. 5d).
Glimmerites are medium- to coarse-grained cumu-
late rocks composed mainly of oriented, euhedral
brown biotite (>70 modal %; Fig. 6c) and small amountsof Ca-rich augite, apatite, and interstitial magnetite
(Fig. 5e). However, the modal proportions in this rock
type vary considerably, and either apatite or Ca-rich
augite locally reaches more than 25 modal % (Fig. 6).
Ca-rich augite is subhedral and occurs as granular ag-
gregates. Biotite grains are also subhedral and can con-tain small euhedral apatite grains, but no Ca-rich augite
inclusions were found. Biotite grains also exhibit a
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FS01FS02
FS03FS04FS05FS06
FS07FS08FS09
FS10FS11
FS12
FS13
FS15
FS16FS17
FS19
FS22
FS14
FS18
FS20
FS23FS24
FS-1-01
FS-1-02
FS-1-03FS-1-04FS-1-05
FS-1-06FS-1-07FS-1-08FS-1-09FS-1-10
FS-1-11
FS-1-12FS-1-13
SFS01
SFS05
SFS06
SFS07SFS08
Layer
9
8
7
6
5
4
3
2
1
Thickness (m)
Sampling
42
24
45
49
63
72.7
36
94
827
48
59
34
11
33
14
63
25
90 Clinopyroxenite
Clinopyroxenite
Clinopyroxenite
Clinopyroxenite
clinopyroxenite
Clinopyroxenite
clinopyroxenite
Clinopyroxenite
clinopyroxenite andglimmerite)
clinopyroxenite
Nelsonite
Clinopyroxenite(Interlayered orthoclase clinopyroxenite)
Orthoclaseclinopyroxenite
Clinopyroxenite
clinopyroxenite
Clinopyroxenite
clinopyroxenite
Clinopyroxenite
LithofaciesColumnarCentre of
the complex
Mesoproterozoiclimestones
0m
~900m
~2200m
~3100m
Syenite vein
ClinopyroxenesyeniteFe-P orebodies
Clinopyroxenitedominated rocks
Orthoclaseclinopyroxenite
Garnet-richclinopyroxenite
FS-2-27FS-2-25FS-2-19FS-2-09FS-2-08
Unit 2
Unit 3
Unit 1
Fig. 3. Schematic stratigraphy of the Fanshan intrusion showing the stratigraphic position of the samples of Unit 2 and garnet clino-pyroxenite in Unit 3 [modified from Cheng & Sun (2003)]. The orthoclase clinopyroxenite samples of Unit 3 were collected from atunnel in which a complete lithological succession is exposed through mining, whereas for the syenite in Unit 1, four core samples15–25 cm long were collected randomly from a recently drilled borehole. The 0 m reference level corresponds to the contact zonebetween garnet clinopyroxenite and Mesoproterozoic limestones of the Wumishan Formation.
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preferred orientation, but are usually bent and show
undulose extinction. Magnetite is rare.
Monomineralic apatite rocks and nelsonite dominate
the ore horizons (Fig. 6) and are composed mainly ofapatite and magnetite, with minor Ca-rich augite and
biotite. The monomineralic apatite rocks are light yel-
low to green in colour and very friable. They are me-
dium grained, granular, with a subhedral texture;
apatite contents can be up to 95 modal % (Fig. 6b). In
some of these rocks apatite displays 120� triple junc-tions (Fig. 5f). In those rocks containing biotite, apatite
typically occurs between euhedral or subhedral biotite
grains, and both show a preferred orientation. Most bio-
tite grains are bent and have undulose extinction. Ca-
rich augite is a minor interstitial phase and occurs as
subhedral or anhedral grains that crystallized later than
the biotite and apatite. Some inclusions composed ofbiotite and Ca-rich augite (Fig. 5g) with minor carbonate
(dolomite and calcite) have been observed within
apatite.
Nelsonite is also an important component of the ore
horizons; it is gray–black in colour (Fig. 4a), and con-
tains less apatite and biotite but more magnetite (up to40 modal %; Fig. 6d) and Ca-rich augite (up to 10 modal
%) than the monomineralic apatite rocks. Apatite is eu-
hedral and ranges from 0�5 to 5 mm in cross-section,
and up to 10 mm in length. Ca-rich augite occurs as eu-
hedral to subhedral aggregates and locally contains eu-
hedral apatite grains. In some samples biotite forms
coarse poikilitic plates enclosing small Ca-rich augitegrains. K-feldspar is absent in the nelsonite.
In most samples, Ca-rich augite occurs as a primoc-
ryst phase suggesting that it crystallized first, followed
by apatite, biotite and K-feldspar. Magnetite and calcite
are interstitial to the main rock-forming minerals in theultramafic rocks, and crystallized at a late stage. Apatite
accumulates together with magnetite in Units 2 and 3
(Fig. 5h), forming the layered Fe–P-rich lithologies
(Figs 3, 4a and 6).
Unit 3Unit 3 consists predominantly of orthoclase- and gar-
net-clinopyroxenite and syenite. Compared with the cli-
nopyroxenites in Unit 2, the orthoclase-clinopyroxenite
in this unit contains more interstitial K-feldspar, al-
though the contents of K-feldspar and Ca-rich augite
vary considerably. Locally, K-feldspar is the most abun-
dant mineral in lithologies such as Ca-rich augite syen-ite. The garnet-rich clinopyroxenites typically contain
�5 modal % melanite garnet, which locally reaches as
much as 35 modal %. They are composed of subhedral
garnet, Ca-rich augite, biotite, orthoclase and magnet-
ite. Euhedral apatite is common and titanite is also
present.
SAMPLE PREPARATION AND ANALYTICALMETHODS
Owing to the different conditions in the mine workings,we sampled the Fanshan intrusion using different meth-
ods. A total of 41 samples of Unit 2, 24 samples of Unit
Apatite rock
Nelsonite
(a) (b)
Glimmerite
Apatite rock
Syenite
Clinopyroxenite
(c) (d)
Garnet-richclinopyroxenite
Syenite
Fig. 4. Field photographs of Fanshan rocks from underground excavations. (a) Rhythmic layered rocks comprising alternatinglayers of monomineralic apatite rock and nelsonite, Unit 2. (b) Monomineralic apatite rocks occurring as enclaves in glimmerite,Unit 2. (c) Clinopyroxenite intruded by a syenite vein, Unit 2. (d) Garnet-rich clinopyroxenite intruded by a syenite vein, Unit 3.
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Cpx
Mt
Ap
0.2mm
Cpx
Bt
0.2mm
Ap
Bt
Cpx
0.2mm
Ap
ApAp
0.1mm
0.2mm
Mt
Ap
0.1mmBt
Cc
Cpx
Ap
Bt
50µm
0.2mm
Kfs
KfsCpx
(b)
(d)
(e) (f)
(g)
(c)
(h)
(a)
Cc
Kfs Cc
Fig. 5. Representative photomicrographs of Fanshan rocks. (a) Clinopyroxene syenite, Unit 1; plane-polarized light. (b)Clinopyroxenite, Unit 2; plane-polarized light. (c) Occurrence of calcite, Unit 2; plane-polarized light. (d) Biotite-clinopyroxenite, Unit2; cross-polarized light. (e) Glimmerite, Unit 2; cross-polarized light. (f) Monomineralic apatite rock, Unit 2; cross-polarized light. (g)Back-scattered electron image of clinopyroxene and biotite forming a crystallized melt inclusion in apatite, Unit 2. (h) Nelsonite,Unit 2; plane-polarized light. Ap, apatite; Bt, biotite; Cpx, clinopyroxene; Kfs, K-feldspar; Mt, magnetite; Cc, calcite.
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Tab
le1
:M
od
alm
ine
ralo
gy
of
the
Fa
nsh
an
intr
usi
on
.
Sa
mp
le:
FS
01
FS
02
FS
-1-0
1F
S0
3F
S-1
-02
FS
04
FS
05
FS
06
FS
-1-0
3F
S-1
-04
FS
-1-0
5F
S0
7F
S0
8F
S0
9F
S-1
-06
FS
-1-0
7F
S-1
-08
FS
-1-0
9U
nit
:2
22
22
22
22
22
22
22
22
2R
ock
typ
e:
CC
CC
NC
NN
AA
CG
CA
CA
CC
Dis
tan
ce(m
):1
49
01
47
01
46
01
42
01
40
51
40
41
40
21
40
11
39
91
39
81
39
71
39
51
39
01
38
71
38
51
38
21
38
01
37
7
Cli
no
py
rox
en
e6
67
27
15
88
60
85
0�5
3�5
59
14
76
17
61
67
69�5
Ap
ati
te7
98
5�4
41�5
12
40
36
94�5
90
18
25
10
90
58
81
76
Bio
tite
11�5
5�5
61
51
05
51
53�5
45
54
15
89�9
6�5
10
Ma
gn
eti
te7
98
5�6
38
16
44�5
40
0�5
0�5
16
11
21
90�1
61
1�5
K-f
eld
spa
r2
35
15
2�5
72
30�5
2tr
ace
51
31
0�9
32
Ga
rne
t0�1
0�2
Ca
lcit
e0�5
10�5
11
0�5
0�5
Tit
an
ite
10�5
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C,cl
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ite
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lim
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ase
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8 Journal of Petrology, 2015, Vol. 0, No. 0
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ay 26, 2015http://petrology.oxfordjournals.org/
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nloaded from
3 and four samples of the syenitic dykes were collected
from a tunnel in which a complete lithological succes-
sion is exposed (Figs 2 and 3). For the syenite in Unit 1,four core samples 15–25 cm long were collected from a
recently drilled borehole.
Weathered or altered surfaces were removed from
the samples before jaw-crushing. Fresh chips were then
selected for analysis using a binocular microscope and
pulverized into powders using agate mortars. Augite
grains for O isotope analysis were crushed to �0�05 mmto avoid oxide inclusions, washed and separated by
standard gravimetric and magnetic methods, and final
hand-picking under a binocular microscope.
Mineral chemistryElectron microprobe analyses of Ca-rich augite, mag-
netite, biotite, K-feldspar, garnet and apatite were ob-
tained using the JEOL Superprobe JXA-8200 electron
microprobe at Washington University in St. Louis, USA,
and the JXA 8230 electron microprobe at the German
Research Centre for Geosciences, GFZ, Potsdam,
Germany. A focused beam of 2mm was employed, ex-cept for K-feldspar where a 5 mm beam was used.
Accelerating voltage was 15 kV and beam current
was 15 nA. Elements were analysed with wavelength-
dispersive spectrometers and were calibrated by
reference to oxide and mineral standards using the PAP
correction routine. The precision for oxide concentra-tions is better than 1%. The counting times were 20 s on
the peak and 10 s on the background.
Trace element concentrations in clinopyroxene (Ca-
rich augite) were determined on thin sections by laser
ablation sector-field inductively coupled plasma massspectrometry (LA-SF-ICP-MS) at the Geological Survey
of Denmark and Greenland. A UP213 frequency-quin-
tupled Nd:YAG solid-state laser system from New Wave
Research employing two-volume cell technology was
coupled to an Element2 double-focusing single-collec-
tor magnetic sector-field ICP-MS system from Thermo-
Fisher Scientific. The mass spectrometer was equippedwith a Fassel-type quartz torch shielded with a
grounded Pt electrode and a quartz bonnet. Operating
conditions and data acquisition parameters are listed
in the Supplementary Data (supplementary data are
available for downloading at http://www.petrology.
oxfordjournals.org). Standards used were the BCR-2and NIST-614 glass reference standards, and for the
60mm square-spot laser analyses also the BB-2 cpx
(Norman et al., 1996), during the analytical sequence,
yielding internal 2SE precision and accuracy of <10%
for all elements measured. Data were acquired from
single round-spot analyses 40 mm in size or by 60 mm
square ablation patterns, using nominal laser fluence of10–12 J cm–2 and a pulse rate of 10 Hz. Total acquisition
times for single analyses were c. 120 s, including 50 s
gas blank followed by laser ablation for 30–50 s and
washout for 30 s. Factory-supplied software from
Thermo-Fisher Scientific was used for the acquisition of
the transient data, obtained through pre-set spot loca-tions. Data reduction was performed off-line using the
1500m
1400m
1300m
1200m
1100m
1000m
0 20 40 60 80 100
Ca-rich augite(wt.%) (wt.%) (wt.%) (wt.%)
K-feldspar(wt.%)
20 40 60 80 100 20 40 60 80 10 20 30 40 50 5 10 15 20
DIST
ANCE
(m)
Cpx Ap Bt Mt Kfs
cumulus/intercumulus
(a) (b) (c) (d) (e)
Fig. 6. Mineral modes (see Supplementary Data) of (a) Ca-rich augite, (b) apatite, (c) biotite, (d) magnetite and (e) K-feldspar in Unit2 of the Fanshan intrusion as a function of stratigraphic height and stratigraphy of cumulus (grey) and intercumulus (white) phasesin Unit 2. The grey bands in (a)–(e) indicate the ore horizons.
Journal of Petrology, 2015, Vol. 0, No. 0 9
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ay 26, 2015http://petrology.oxfordjournals.org/
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nloaded from
software Iolite version 2.5 (Hellstrom et al., 2008; Paton
et al., 2011), the Trace_Elements_IS routine and 29Si as
the internal standard.
Bulk-rock major and trace elementsSelected chips were crushed in an agate mortar and
ground in an agate mill to powders of �200 mesh.
Major elements were analysed on fused glass discsusing a scanning wavelength dispersion X-ray fluores-
cence spectrometer at the ICP-MS Laboratory of the
National Research Centre for Geoanalysis, Beijing and
at the Department of Earth Sciences, Nanjing
University, Nanjing, China. The analytical uncertainties
are less than 1%, estimated from repeat analyses of twostandards (andesite GSR-2 and basalt GSR-3). Loss on
ignition (LOI) was determined gravimetrically after heat-
ing the samples at 980�C for 30 min. The ferrous oxide
content was determined by wet chemical methods.
Bulk-rock trace elements were analysed by solution
ICP-MS at the National Research Centre for
Geoanalysis, Beijing and the Department of EarthSciences, Nanjing University, Nanjing, China. For the
analyses, rock powders (�40 mg) were dissolved in dis-
tilled HFþHClO4 in 15 ml Savillex Teflon screw-cap
breakers. The precision for most elements was typically
better than 5% RSD (relative standard deviation), and
the measured values for Zr, Hf, Nb and Ta were within10% of the certified values of the two employed stand-
ards (granite GSR-1 and basalt GSR-3).
Sr, Nd and O isotopesSr and Nd isotopes were analysed at the Department of
Earth Sciences, Nanjing University, China, using a
Finnigan Triton TI mass spectrometer. The samples
were analysed for Rb, Sr, Sm and Nd concentrations,
and 87Sr/86Sr and 143Nd/144Nd isotope ratios. Rb, Sr, Nd
and Sm concentrations were determined by isotope di-lution using 85Rb–84Sr and 150Nd–149Sm spikes. The ac-
curacy is 60�5% for Rb and Sr, 61�1% for 87Rb/86Sr and
60�01% for 87Sr/86Sr (2r), 60�5% for Sm and Nd, 60�5%
for 147Sm/144Nd and 60�005% for 143Nd/144Nd (2r). Sr
and Nd isotopic ratios were normalized against86Sr/88Sr¼ 0�1194 and 146Nd/144Nd¼ 0�7219, respect-ively. 87Sr/86Sr for the NIST987 Sr standard was
0�710268 6 0�000007 (2r, n¼ 8), and 143Nd/144Nd for the
La Jolla Nd standard was 0�511842 6 0�000006 (2r,
n¼ 6). Total blanks were 100 pg for Sr and 80 pg for Nd,
and negligible for the determination of isotopic com-
positions. eNd(t) values were calculated using the pre-
sent-day values for a chondritic uniform reservoir(CHUR) 143Nd/144Nd¼0�512638 and 147Sm/144Nd¼0�1967 (Jacobsen & Wasserburg, 1980).
The oxygen isotopic compositions of Ca-rich augite
were measured at the Institute of Mineral Resources,
Chinese Academy of Sciences. Ca-rich augite was
chosen for study because the mineral is present in allthe rocks of the intrusion. Oxygen isotope measure-
ments were performed using the bromine pentafluoride
method of Vennemann & Smith (1990). The analyses
were compared with those of an internal standard, cali-
brated relative to NBS-28 (d18OSMOW¼þ9�6%), and no
data correction was needed. Almost all samples have
been duplicated with analytical precision 6 0�2% (1r) orbetter.
RESULTS
Mineral chemistryRepresentative chemical compositions and structural
formulae of Ca-rich augite, K-feldspar, biotite, garnet,
magnetite and apatite from Fanshan are listed in the
Supplementary Data. Those of Ca-rich augite in Unit 2
are reported in Table 2.
Ca-rich augiteCa-rich augites in the three units of the Fanshan intru-
sion show similar compositions with a limited range of
Wo46�42–50�09En29�72–42�02Fs8�26–19�49 (Table 2; Figs 7a, b
and 8a). These compositions are comparable with those
reported for clinopyroxene from ultrapotassic volcanic
rocks of Central Italy that have been considered to becontaminated by carbonate sediments (Cellai et al.,
1994; Gaeta et al., 2006; Mollo & Vona, 2014; and refer-
ences therein). The Mg-number [Mg-number (Mg#) is
defined as Mg/(MgþFetot), in atoms per formula unit]
varies from 0�70 to 0�92, and exhibits several gradual re-
versals from the base of Unit 2 upwards (Fig. 7a). TiO2
concentrations (0�07–1�76 wt %) generally correlate withthe change of Mg-number (Fig. 7b). The relatively low
SiO2 and high Al2O3 contents of the Ca-rich augites
from the Fanshan intrusion probably reflect the SiO2-
undersaturated nature of the parental magma (see
below).
Ca-rich augite has total rare earth element (REE) con-tents ranging from 108 to 124 ppm (Table 3) and shows
‘hump-shaped’ light REE (LREE)-enriched chrondrite-
normalized REE patterns (Fig. 9a), coupled with Hf en-
richment and negative Nb–Ta anomalies in primitive
mantle-normalized trace element patterns (Fig. 9b).
K-feldsparK-feldspar in the Fanshan intrusion shows only a
small range of compositional variation through the in-
trusion (Fig. 8b). It is almost pure orthoclase Or90�6–100
Ab0�1–9�6An0–0�3.
BiotiteBiotite exhibits much larger compositional variations inFe and Mg compared with Ca-rich augite, especially in
Unit 2; our data compare well with those previously re-
ported for the Fanshan intrusion (Niu et al., 2012). Total
Fe2O3 ranges from 8�5 to 24�4 wt %, and MgO from 8�8to 19�5 wt %. The TiO2 contents of biotite in Unit 2 range
from 1�2 to 5�3 wt %. The MgO, K2O and TiO2 contentsexhibit several gradual reversals from the base of Unit 2
upwards (Fig. 7c–e); MgO contents are notably higher in
10 Journal of Petrology, 2015, Vol. 0, No. 0
at Georgetow
n University on M
ay 26, 2015http://petrology.oxfordjournals.org/
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nloaded from
Tab
le2
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Ca
0�9
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65
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0�0
28
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26
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29
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21
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28
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41
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37
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tal
4�0
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e2þ
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01
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82
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94
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3þ
/(F
e3þþ
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2þ
)0�4
49
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67
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87
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99
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66
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06
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51
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58
0�5
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Mg
#0�8
50�8
40�8
40�8
20�8
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80�7
90�8
50�7
0�7
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o4
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n3
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93
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72
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33
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13
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s1
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51
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De
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nd
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.
Journal of Petrology, 2015, Vol. 0, No. 0 11
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nloaded from
the ore horizons (Fig. 7c). On an Fe3þ–Fe2þ–Mg diagram
most of the Fanshan biotites suggest crystallization
under relatively normal redox conditions for plutonic
rocks close to the fayalite-magnetite–quartz–magnetite
oxygen buffer (FMQ; Fig. 8c).
Melanite, magnetite and apatiteAll the analysed garnets are Ti-rich (1�8–12�3 wt %) and
classified as melanite (i.e. Fe3þ>Ti in the octahedral
position; Deer et al., 1992) with Ti¼ 0�142–0�778, Ca¼2�898–3�018 and Fe3þ¼ 1�193–1�620 (a.p.f.u.). Magnetite
shows a wide compositional range with TiO2 contents
ranging from 0�1 to 10�9 wt %, 0�0–3�6 wt % MgO and0�2–1�6 wt % MnO. However, no systematic trend was
found between the units and within the layered rocks
(Fig. 7f). Apatites in Units 2 and 3 are predominantly flu-
orapatite characterized by high F (1�8–2�1 wt %) and low
Cl contents (0�04–0�06 wt %); the F/Cl ratios are constant
(35–53) throughout Unit 2 (Fig. 7g).
Bulk-rock major and trace element compositionsAll analysed samples have low or negligible LOI values
except those containing considerable amounts of apatite
and biotite (Table 4). As one would expect from stronglymodally layered cumulates, the bulk-rocks exhibit large
compositional variations (Table 4). Specifically, the
(biotite-)clinopyroxenites in Unit 2 have low SiO2 and
high CaO, consistent with the high content of apatite in
these rocks. They also have variable Mg-numbers
(Table 4). The Al2O3, K2O and Na2O contents are lowerthan those of other rock types in the Fanshan intrusion.
The two glimmerite samples (FS-07 and FS-1-10) are
characterized by high MgO contents and relatively low
contents of total Fe2O3 compared with the clinopyroxen-
ites. Generally, the compositional variations can be
attributed to the varying proportions of Ca-rich augite
and biotite and to the variable amounts of intercumulus
phases (mainly orthoclase, apatite, and magnetite).Compared with the clinopyroxenite and glimmerite in
Unit 2, the syenites in Unit 1 have much higher SiO2,
Al2O3, K2O and Na2O contents, but lower CaO, MgO and
total Fe2O3, consistent with the dominance of orthoclase
in these rocks. The garnet-rich clinopyroxenites and
orthoclase-clinopyroxenites in Unit 3 show transitional
chemical compositions between the ultramafic rocks [i.e.(biotite-)clinopyroxenite and glimmerite] and the syenites
(Table 4), but the former exhibit dispersion of the data for
Na2O and TiO2, possibly owing to the presence of melan-
ite. In particular, the garnet-rich clinopyroxenites have
low Na2O contents and are characterized by higher TiO2
contents, with limited variation in SiO2, Al2O3, K2O andMgO, but variable CaO and total Fe2O3. The monominer-
alic apatite rocks have the highest P2O5 contents, corres-
ponding to almost pure apatite; the nelsonites also have
high Fe2O3 and P2O5 contents (Table 4).
Representative trace element compositions of the
Fanshan intrusive rocks are given in Table 4 and illus-
trated in chondrite-normalized and mantle-normalizeddiagrams in Figs 10 and 11. All the rock types are char-
acterized by significant enrichment in large ion litho-
phile elements (LILE), such as Sr, Ba and Rb, and LREE,
and display prominent troughs in Nb, Ta, Zr, Hf and Ti.
The garnet-clinopyroxenite and syenite samples
(Fig. 11e and f) show convex-upward REE patterns with
1500m
1400m
1300m
1200m
1100m
1000m
0.6 0.7 0.8 0.9 1.0Mg# in Cpx
0 0.5 1.0 1.5 2.0TiO2(wt.%) in Cpx
10 20 300MgO(wt.%)in Bt
K2O(wt.%)in Bt
9 9.5 10 10.5TiO2(wt.%)in Bt
0 2 4 6 0 5 10 15TiO2(wt.%)in Mt
0 20 40 60F/Cl in Ap
(a) (b) (c) (d) (e) (f) (g)
DIST
ANCE
Fig. 7. Major element compositional variations of (a) Ca-rich augite (Mg#), (b) Ca-rich augite (TiO2), (c) biotite (MgO), (d) biotite(K2O), (e) biotite (TiO2), (f) magnetite (TiO2), and (g) apatite (F/Cl ratio) with stratigraphic position in Unit 2 of the Fanshan intrusion.The grey bands indicate the ore horizons.
12 Journal of Petrology, 2015, Vol. 0, No. 0
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nloaded from
relatively flat heavy REE (HREE), which is probably due
to the presence of melanite garnet.
Sr–Nd–O isotope dataThe analysed Fanshan rocks show limited variations in
Sr, Nd and O isotopic composition (Table 4). The age-
corrected 87Sr/86Sr ratios (t¼ 218 Ma, Ren et al., 2009;
Niu et al., 2012), range from 0�70513 to 0�70601, andeNd(t) values vary from –6�8 to –5�5; d18O values range
from þ7�5 to þ8�1%. Our data (Table 3) are consistent
with those of previous isotopic studies on the rocks
from the three units (Mu et al., 1988; Niu et al., 2012).
The data for our samples overlap the field of ultrapo-
tassic rocks from the Roman Magmatic Province
(Fig. 12), and plot above the fields of the Late Paleozoicto Early Mesozoic carbonatite intrusions in the NCC.
The Yaojiazhuang intrusion and the Precambrian base-
ment rocks in the NCC have much lower eNd(t) and
higher initial (87Sr/86Sr) ratios. Notably, the Fanshan
samples have a slightly more enriched Nd–Sr isotopic
signature compared with the Palaeozoic lithosphericmantle of the NCC represented by kimberlite-hosted
xenoliths (Zheng & Lu, 1997; Xu et al., 2004).
DISCUSSION
Parental magma compositionThe similar mineralogy, mineral compositions, chon-
drite-normalized REE patterns (Figs 10 and 11) and
Sr–Nd isotopic compositions (Fig. 12) observed in the
three units suggest that all the rocks were derived from
a common parent magma by similar processes of mag-matic differentiation. Unfortunately, neither chilled mar-
gins nor melt inclusions in primitive cumulates have
been found that could help to constrain the parent
magma composition. Previous attempts to estimate the
parental magma composition have been mainly based
on the area of surface outcrops and data from several
boreholes in the part of the intrusion buried underQuaternary sediments (Mu et al., 1988). Those recon-
structions and associated calculations suggest low SiO2
(35�26 wt %) and Al2O3 (5�68 wt %), but high CaO
(19�46 wt %) and K2O (2�01 wt %) contents and Fe2O3/
FeO ratios (1�2) in the parental magma (Mu et al., 1988).
Such chemical characteristics correspond to those ofGroup II ultrapotassic rocks as defined by Foley et al.
(1987), including kamafugites, which have consistently
low SiO2 (<46%) and Al2O3, but high CaO contents.
On the basis of the partition coefficients of trace
elements between clinopyroxene and silicate melt
(Bedard, 2014), we calculated the trace element com-
position of the melt that would be in equilibrium withthe early stage Ca-rich augite of Unit 2 (see
Supplementary Data). The calculated trace element pat-
terns (Fig. 9c) are similar to those of a representative
Group II kamafugite from the Western Qinling province,
NCC (Guo et al., 2014). Considering that previous esti-
mates of the parental magma also belong to Group II
Table 3: Trace element content of clinopyroxene and calculated equilibrated melts in Unit 2 of the Fanshanintrusion
Sample: SFS-7 Calculated SFS-6 Calculated SFS-5 Calculated Kamafugite(n¼5) melt-sfs-7 (n¼5) melt-sfs-6 (n¼5) melt-sfs-5
V 114 35 134 41 196 60 161Cr 6�18 1�48 21�42 5�13 1�79 0�43 661Zn 25�44 58�54 28�26 65�03 68�30 157�16 117Ga 7�43 24�30 9�22 30�15 15�93 52�09 15�9Rb 0�01 0�63 0�02 1�91 0�19 17�51 30�5Sr 528 6290 554 6608 1142 13617 1367Y 12�40 25�05 13�53 27�34 13�48 27�23 32�8Nb 0�169 21�030 0�255 31�757 0�292 36�361 136Ba 0�302 157 0�302 157 0�462 240 729La 9�328 157 9�880 166 14�948 251 122Ce 38�72 392 45�32 459 42�63 432 237Pr 6�8 44�9 7�8 51�2 7�72 50�7 26�8Nd 32�16 147 35�54 162 34�36 157 102Sm 7�11 20�72 9�34 27�23 8�06 23�49 18Eu 2�53 5�92 2�62 6�14 2�28 5�34 5�17Gd 4�66 10�48 5�48 12�33 6�18 13�91 13�4Tb 0�77 1�62 0�92 1�94 0�75 1�57 1�74Dy 3�37 6�88 3�60 7�35 3�64 7�44 8�05Ho 0�44 0�90 0�54 1�10 0�55 1�12 1�22Er 1�05 2�15 1�35 2�78 1�22 2�51 2�82Tm 0�16 0�34 0�13 0�28 0�20 0�41 0�32Yb 0�65 1�34 0�98 2�02 1�15 2�38 1�74Lu 0�17 0�36 0�17 0�34 0�22 0�46 0�22Hf 6�48 38�47 9�20 54�59 16�71 99�15 8�09Ta 0�09 4�02 0�08 3�85 0�06 2�58 6�19Th 0�05 3�50 0�07 5�64 0�11 8�60 16U 0�01 0�64 0�01 0�76 0�01 0�65 3�41Total REE 108 124 124
n, number of analysed points that are adjacent to the core of the crystals. Partition coefficient values are fromBedard (2014); the composition of a kamafugite from West Qinling, North China Craton is from Guo et al. (2014).
Journal of Petrology, 2015, Vol. 0, No. 0 13
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nloaded from
ultrapotassic rocks, we propose that the parental
magma of the Fanshan intrusion is compositionally
similar to that of Group II kamafugites. However, the
notable differences in Nb–Ta–Hf concentration between
kamafugites and our calculated parent magma may in-
dicate a different magma source or degree of crustalcontamination as discussed below.
Crustal contaminationThe Fanshan rocks are characterized by crustal-like
trace element and isotopic compositions such as the
relative depletion in high field strength elements (HFSE)(Figs 10 and 11) and enriched Sr–Nd isotopic compos-
itions [87Sr/86Srt¼ 0�70513–0�70601, eNd(t)¼ –6�8 to
–5�5]. Niu et al. (2012) proposed that the Fanshan parent
magma experienced contamination by mafic lower
crust, on the basis of Os isotope data. The relatively
high d18O values (þ7 to þ9%; Table 4) also indicate thepossibility of crustal contamination. Because the intru-
sion is emplaced into limestones of the Wumishan
Formation, contamination by limestone seems inev-
itable. Assimilation of limestone will drive the
crystallization of Ca-rich clinopyroxene, resulting in
desilication of the melt and an increase of Si-under-
saturation (Gaeta et al., 2006; Mollo & Vona, 2014), as
supported by several experimental studies (e.g. Iacono-Marziano et al., 2007, 2008, 2009; Freda et al., 2008;
Conte et al., 2009). However, the assimilation of carbon-
ate will have no significant effect on the 87Sr/86Sr and
LREE/HREE ratios (Conticelli et al., 2002; Perini et al.,
2004), and may be visible only in terms of oxygen
isotope ratios (Gaeta et al., 2006). Assimilation andfractional crystallization (AFC) modelling of the
Wo
En Fs
Wo(a)
(c)
Or60
80
30Ab
An(b)
Syenite, unit 1Clinopyroxenite, unit 2(Garnet-)clinopyroxenite, unit 3
Alm+Sp
from potassic and ultrapotassicrocks in central Italy
MH
NNOFMQ
Fe2+
Mg
Fe3+ Fe3+
Fig. 8. Compositional variations of minerals from the Fanshanintrusion. (a) Clinopyroxene (Ca-rich augite); field of clinopyr-oxene from potassic and ultrapotassic rocks in central Italy isfrom Cellai et al. (1994), Gaeta et al. (2006), Melluso et al.(2008) and Mollo & Vona (2014). (b) K-feldspar. (c) Biotite(Wones & Eugster 1965). Data for rocks from Unit 3 from Jianget al. (2004) and Niu et al. (2012) are also plotted.
100
1000
Th Nb Ta La Ce Pr Nd Hf Sm Eu Gd Tb Dy Y Ho Er Tm Yb Lu
10
Calculated parent
Kamafugitefrom Western Qinling
1
Th Nb Ta La Ce Pr Nd Hf Sm Eu Gd Tb Dy Y Ho Er Tm Yb Lu
(a)
(c)
Ca-rich augite from unit 2
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Sam
ple/
Chro
ndrit
e
Ca-rich augite from unit 2
(b)1000
100
10
1
0.1
100
10
1
Fig. 9. Chondrite-normalized REE patterns (a) and primitivemantle-normalized trace element patterns (b) for the Ca-richaugite in Unit 2. (c) Calculated parent magma compositionscompared with that of kamafugite from Western Qinling in theNCC (Guo et al., 2014).
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Table 4: Representative bulk-rock major (wt %), trace element (ppm) and Sr–Nd–O isotopic compositions for the Fanshan intrusion
Sample: FS01 FS03 FS07 FS08 FS19 FS22 FS23 SFS-08 FS-2-01 FS-2-07 FS-2-10Unit: 2 2 2 2 2 2 2 2 3 3 3Rock type: C C G C C C C C OC OC OC
SiO2 33�91 41�59 32�03 28�5 34�58 35�76 37�28 38�68 37�31 38�71 33�96TiO2 2�27 1�49 2�84 2�19 2�72 2�27 2�41 2�09 2�71 2�1 3�06Al2O3 5�06 10�78 7�68 4�98 8�18 6�86 8�04 8�37 7�22 7�14 7�5Fe2O3-t 17�6 15�85 10�91 15�33 12�21 16�94 12�27 20�2 20�06 14�51 21�14CaO 18�7 12�61 18�3 24�73 15�05 16�45 15�75 14�78 17�25 18�97 15�49MgO 10�65 6�13 14�87 10�89 15�04 13�19 14�68 6�9 7�48 7�59 8�68MnO 0�17 0�12 0�1 0�14 0�12 0�21 0�12 0�23 0�25 0�21 0�25K2O 1�98 5�54 4�72 2�22 5�06 3�22 4�75 4�42 1�81 2�88 2�94Na2O 0�25 1�03 0�17 0�23 0�11 0�39 0�18 0�29 1�33 1�05 0�86P2O5 3�16 1�68 6�74 9�11 4�12 2�25 2�93 1�52 1�55 2�66 2�91CO2 (%) 0�94 1�29 0�77H2Oþ (%) 1�38 1�02 0�94LOI 0�8 2�03 1�06 0�78 1�88 1�71 1�04 1�48 1�45 2�28 0�78Total 94�55 98�85 99�44 99�1 99�07 99�27 99�44 98�96 98�42 98�1 97�57Mg# 56�16 45�01 74�27 60�07 72�3 62�25 71�69 41�97 44�12 52�56 46�51
Trace elements (ppm)Sc 13�71 7�17 12�54 43�57 19�81 8�99 17�95 29�2 30�9 32�8 31�3Ti 12239 8552 15481 11454 14008 12212 12420 12360 16260 12600 18360V 419�07 331�39 265�02 395�6 227�35 374�33 241�08 520 506 377 539Cr 33�12 33�46 151�05 32�2 139�63 77�42 21�63 4�26 4�98 34�7 4�85Co 69�23 123�73 56�09 76�13 75�99 71�85 58�54 60�2 69�1 56�6 67Ni 51�61 31�56 81�2 51�63 139�46 52�63 76�69 17�5 20�9 30�3 39�2Cu 436�46 490�5 36�8 421�76 1246�38 170�24 693�48 149 230 331 723Ga 16�27 16 15�39 16�56 13�34 17�08 14�41 17�4 16�9 16�7 18Rb 68�12 547�87 218�46 87�89 134�06 106�81 190�53 257 101 102 115Sr 1380 745 1765�09 2184 1152 1754 1103�07 1700 1362 2212 1933Y 36�39 19�73 48�48 63�09 26�88 19�95 25 28�9 33�9 43�1 34�6Zr 254�69 163�17 112�02 171�78 131�08 206�14 170�49 258 396 451 244Nb 4�05 1�77 5�31 3�63 5�99 11�34 6�41 5�3 17�1 10�8 16�5Ba 594 396 3728 1211 3216 3517 2443 911 497 1175 3486La 103�86 58�47 180�38 220�78 96�4 89�02 80�98 87�7 102 145 122Ce 276�29 156�13 447�56 557�78 249�42 218�75 189�73 197 237 326 282Pr 36�24 20�06 58�01 65�98 31�89 26�31 25�14 26�4 31�7 41�8 36�3Nd 159�68 89�01 249�82 309�9 137�28 107�31 112 112 148 194 169Sm 30�1 17�18 44�59 55�02 26�31 18�42 21�58 20�9 24�8 31 26�6Eu 7�87 4�43 11�55 14�15 6�84 5�09 5�71 5�68 6�69 8�15 6�88Gd 20�96 11�82 31�64 38�65 18�01 13�14 15�47 15�8 17�9 21�6 18�2Tb 2�01 1�16 2�91 3�52 1�69 1�22 1�45 1�89 2�15 2�64 2�15Dy 9�97 5�69 13�82 17�12 7�88 6�01 7�23 7�75 9�4 11�4 9�18Ho 1�58 0�9 2�11 2�63 1�26 0�95 1�13 1�21 1�27 1�52 1�21Er 3�6 1�93 4�56 5�65 2�78 2�1 2�49 2�89 3�23 3�96 3�11Tm 0�37 0�2 0�43 0�55 0�27 0�22 0�26 0�3 0�32 0�4 0�3Yb 1�91 1�04 2�1 2�65 1�4 1�15 1�35 1�73 2�05 2�38 1�74Lu 0�29 0�15 0�29 0�38 0�2 0�17 0�2 0�24 0�28 0�34 0�25Hf 10�27 6�19 4�1 6�32 4�97 7�26 6�25 10�9 14�2 14�6 8�83Ta 0�29 0�14 0�32 0�22 0�35 0�46 0�34 0�21 0�84 0�78 0�81Pb 9�83 0�74 0�92 8�21 54�33 4�71 19�46 12�4 6�3 4�49 45�4Th 6�47 3�79 8�74 13�36 5�37 5�71 5�46 5�53 6�78 9�53 7�72U 1�12 0�58 1�22 1�59 0�93 1�71 1�02 0�69 0�99 1�93 1�0187Sr/86Sr 0�70559 0�71267 0�70651 0�70636 0�70693 0�70573 0�70703 0�70576 0�70583 0�70577 0�705842r 3 8 3 4 5 5 4 3 3 3 4143Nd/144Nd 0�51222 0�51223 0�51222 0�51221 0�51222 0�51221 0�51224 0�51222 0�51222 0�5122 0�51222r 3 4 2 2 3 4 3 3 2 3 387Rb/86Sr 0�1428 2�1279 0�3581 0�1165 0�3368 0�1762 0�4998 0�1762 0�2146 0�1334 0�1721147Sm/144Nd 0�1139 0�1166 0�1078 0�1073 0�1158 0�1037 0�1164 0�1127 0�1012 0�0965 0�0951(87Sr/86Sr)t 0�70514 0�70601 0�70539 0�706 0�70587 0�70517 0�70546 0�70522 0�70517 0�70536 0�70531eNd(t) –5�8 –5�8 –5�7 –5�8 –5�8 –5�7 –5�5 –5�7 –5�6 –5�8 –5�7(143Nd/144Nd)t 0�51206 0�51206 0�51206 0�51206 0�51206 0�51206 0�51207 0�51206 0�51207 0�51206 0�5120718O (%) 7�5 7�9 7�6 7�9 7�7 7�6 7�5 7�8 7�6 7�8 7�6
Sample: FS-2-13 FS-2-26 FS-2-09 FS-2-27 ST01 ST02 ST03 FS05 FS09 FS11Unit: 3 3 3 3 1 1 1 2 2 2Rock type: OC OC GC-ST GC-ST ST ST ST N A A
SiO2 38�45 40�12 43�48 42�5 50�81 50�87 50�52 7�7 5�46 10�75TiO2 2�4 1�47 2�21 4�17 1�35 1�33 1�42 4�31 0�34 1�35Al2O3 7�4 13�38 15�02 12�7 14�66 14�76 14�64 1�68 0�77 3�4Fe2O3-t 17�98 11�77 9�69 11�79 7�89 7�89 8�14 41�34 4�16 4�96CaO 17�57 10�38 7�62 13�97 7�41 7�46 7�75 24�15 55�4 44�31MgO 7�8 4�56 2�31 2�85 5�25 4�96 5 4�46 1�76 5�81MnO 0�23 0�22 0�13 0�22 0�14 0�14 0�14 0�22 0�05 0�05K2O 3�34 8�92 10�54 6�21 5�58 5�59 5�74 0�17 0�38 2�26Na2O 0�49 0�37 0�01 1�13 3�73 3�61 3�37 0�12 0�13 0�15P2O5 1�41 0�81 0�27 0�28 1 1 1�02 14�81 29 24�03CO2 (%) 1�46 4�63 5�57 1�2H2Oþ (%) 0�7 1�52 2�22 1�64LOI 1�9 5�89 7�36 2�81 1�6 1�73 1�61 0 0�82 1�24Total 98�97 97�89 98�64 98�63 99�43 99�34 99�35 98�96 98�27 98�31Mg# 47�88 45�07 33�55 33�86 58�49 57�08 56�55 18�58 47�26 71�24
Trace elements (ppm)Sc 28�9 9�58 8�35 7�96 11�21 11�47 11�72 12�73 6�78 5Ti 14400 8820 13260 25020 7475 8206 8104 24103 1830 6474V 427 324 284 480 210�61 219�26 218�21 1078�19 237�71 237�29Cr 3�47 3�86 1�52 2�1 82�43 89�15 85�78 23�08 15�67 29�14Co 57�3 44�9 27�1 31�7 21�83 24�75 26�47 122�49 24�3 23�36
(continued)
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Alban Hills (Roman Province) clinopyroxene d18O and87Sr/86Sr isotopic compositions is able to reproduce the
observed variation in terms of fractionation of high-Caclinopyroxeneþ leucite 6 apatite 6 magnetite coupled
with assimilation of 5–10% of limestone (Gaeta et al.,
2006). This may also be the case for Fanshan, as evi-
denced by the similarities in crystallizing phases and
the clinopyroxene composition between Fanshan and
Central Italy (Fig. 8a).Nevertheless, a limited degree of crustal contamin-
ation (5–10%) at source is suggested by the combined
Sr–O isotopic characteristics of the Fanshan samples
(Fig. 13a), which show relatively higher initial Sr iso-
topic values compared with O isotopic values. This vari-
ability of Sr–O isotopic compositions could be
indicative of differing amounts of crustal components inthe mantle source in addition to shallow-level assimila-
tion. This is similar to the model proposed for some of
the ultrapotassic rocks in Western Qinling that also
show a crustal signature in their mantle source (Guo
et al., 2014). The addition of crustal materials to the
mantle source could also be used to explain the differ-ence in HFSE between inferred and calculated parental
magmas (Fig. 9c) because crustal materials usually
have lower Nb–Ta–Hf relative to LILE and REE (Rudnick
& Gao, 2003).
P–T conditions and MELTS modellingThe parental magmas of the Fanshan intrusion may
have experienced variable degrees of fractional crystal-
lization, en route from source to surface. The morph-
ology of the Ca-rich augite crystals, which are up to5 mm in length, strongly indicates that they crystallized
at depth under higher pressure rather than crystallized
in situ. Using the thermobarometer proposed by Putirka
(2008), most of the Ca-rich augite crystallized at pres-
sures of 7�4–16�4 kbar, corresponding to depths of 24–
54 km, at moderately high temperatures (1200–1300�C;
Table 2). On the basis of the total thickness ofMesoproterozoic to Early Triassic (�218 Ma) sequences
in the area, the final emplacement depth of the Fanshan
intrusion is estimated to be �5 km (�2 kbar; Mu et al.,
1988). Thus, the Fanshan magmas probably experi-
enced variable amounts of crystallization at different
crustal levels.We use the geochemical modeling program MELTS
(Ghiorso & Sack, 1995) to test if a parent magma com-
position similar to a kamafugite from Western Qinling
(sample LN10-001-8 of Guo et al., 2014; Supplementary
Data) could produce the cumulus phases present in the
Fanshan intrusion by crystal fractionation. The result of
the MELTS modelling is shown in Fig. 14a. In the model,we use fO2¼FMQ, a starting temperature of 1250�C, a
Table 4. Continued
Sample: FS-2-13 FS-2-26 FS-2-09 FS-2-27 ST01 ST02 ST03 FS05 FS09 FS11Unit: 3 3 3 3 1 1 1 2 2 2Rock type: OC OC GC-ST GC-ST ST ST ST N A A
Ni 17�9 5�53 1�65 3�49 35�19 42�34 38�98 64�26 17�5 29�45Cu 400 39�8 65�5 102 28�25 110�02 74�33 382�32 380�88 69�22Ga 17�2 21 17�2 19�1 23�95 24�63 23�94 25�65 16�49 16�56Rb 190 285 472 326 86�27 99�87 91�36 8�08 16�53 84�87Sr 1270 1993 1015 1212 3004�69 3757�17 3515�16 2646 6038 4945Y 28�5 27�6 90�6 166 24�13 25�22 26�1 81�79 120�6 129�99Zr 417 368 836 2145 523�53 407�84 421�72 74�98 35�12 43�59Nb 10�2 15�4 13�1 28�1 45�32 31�24 34�18 2�27 0�78 2�56Ba 858 2720 1484 1241 2227 2993 2876 98 152 1579La 83�9 120 49�8 74 111�59 113�92 121�96 320�5 729�3 601�33Ce 194 229 147 258 235�27 249�83 268�49 806�26 1739�35 1472�92Pr 26�6 25�9 23�5 46�3 24�4 28�09 29�08 97�02 222�93 189�05Nd 130 108 137 285 95�03 101�96 105�82 416�8 956�54 769�81Sm 21�4 15�1 34�3 65�4 14�71 15�68 16�3 78�83 169�91 148�41Eu 5�8 3�64 10�6 19�7 4�05 4�59 4�83 19�82 42�61 37�25Gd 15�5 9�94 29�9 52 10�23 11�05 11�77 54�91 114�32 101�69Tb 1�85 1�18 4�39 7�48 1�08 1�15 1�24 4�91 10�16 8�99Dy 7�93 5�49 21�9 36�4 6�09 6�57 6�37 23�1 47�21 42�31Ho 1�11 0�79 3�59 5�83 1�04 1�14 1�16 3�38 7�01 6�55Er 2�7 2�13 8�95 15�2 2�67 2�78 2�95 7�41 14�66 13�5Tm 0�27 0�22 1�14 1�93 0�34 0�35 0�37 0�7 1�3 1�31Yb 1�85 1�37 7�24 12�8 2 2�02 2�08 3�32 5�92 6�05Lu 0�28 0�21 1�06 1�84 0�29 0�31 0�32 0�43 0�77 0�78Hf 15�8 11�5 27�4 60�7 12�15 10�75 11�53 2�71 1�01 1�09Ta 0�44 0�58 1�21 2�87 2�41 1�74 1�9 0�21 0�06 0�16Pb 23�5 6�21 2�71 3�62 30�24 26�9 27�24 9�37 16�81 5�15Th 7�75 11�8 8�59 15 21�35 13�27 15�3 17�82 12�48 21�6U 1�78 3�18 1�89 5�32 8�25 3�94 4�19 1�82 4�39 3�5487Sr/86Sr 0�70661 0�70663 0�70949 0�70773 0�7054 0�70544 0�70545 0�70518 0�70515 0�705332r 6 4 4 3 3 4 4 3 4 3143Nd/144Nd 0�51221 0�51218 0�51228 0�51226 0�51214 0�51214 0�51214 0�51222 0�51221 0�512212r 4 4 3 3 3 7 4 2 2 287Rb/86Sr 0�4329 0�4138 1�3456 0�7783 0�0831 0�0769 0�0752 0�0088 0�0079 0�0497147Sm/144Nd 0�0995 0�0845 0�1513 0�1386 0�0935 0�0929 0�093 0�1143 0�1073 0�1165(87Sr/86Sr)t 0�70529 0�70537 0�70538 0�70535 0�70515 0�70521 0�70522 0�70515 0�70513 0�70518eNd(t) –5�6 –5�8 –5�7 –5�8 –6�8 –6�8 –6�8 –5�9 –5�8 –6�1(143Nd/144Nd)t 0�51207 0�51206 0�51206 0�51206 0�512 0�51201 0�51201 0�51205 0�51206 0�5120418O (%) 7�5 7�6 7�7 7�5 8 8�1 7�9 7�8 7�7 8
LOI, weight loss on ignition at 1000�C. Total iron oxide expressed as Fe2O3 (Fe2O3-t); Mg#¼ [molar Mg/(MgþFe2þ)]�100, assum-ing 15% of total iron is ferric. Chondritic uniform reservoir (CHUR) values [(143Sm/144Nd)CHUR
0¼0�512638, (143Nd/144Nd)CHUR0¼0
�1967] are used for the calculation. kRb¼1�42�10�11 a–1 (Steiger & Jager, 1977), kSm¼6�5�10�12 a–1 (Lugmair & Harti, 1978).(87Sr/86Sr)t and eNd(t) were calculated at 218 Ma. Rock type abbreviations as in Table 1.
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final temperature of 760�C, H2O¼ 4 wt % and a pressure
of 10 kbar. The MELTS modelling shows that the
sequence of mineral crystallization is augite–biotite–
apatite–magnetite–garnet. This is consistent with petro-
graphic observations from Unit 2 and some of the rocks
in Unit 3. However, K-feldspar, which is an interstitialphases in these two units, is not present in the model
assemblage. It is probable that owing to the high water
content in the model parent magma most of the potas-
sium entered biotite. To test this, we conducted another
model run using the same starting composition with an
elevated K2O content (3 wt %) but low water content
(0�5 wt %) under lower pressure (2 kbar). The resultsshow that sanidine (K-feldspar) crystallizes after augite
and biotite, consistent with the mineral assemblage in
Unit 1. Therefore, our modelling suggests that the par-
ental magma generating the Fanshan intrusion may be
composionally similar to the kamafugite from Western
Qinling but with higher K2O and lower H2O contents.
Magma generation and nature of the mantlesourceLithospheric mantlePartial melting of metasomatized subcontinental litho-
spheric mantle is widely regarded as the most likelyprocess to explain the origin of Group II potassic and
ultrapotassic rocks (Peccerillo, 2005). Indeed, the Sr and
Nd isotope compositions of the investigated samples
(Fig. 12) fall far outside the ranges for oceanic basalts
[mid-ocean ridge basalt (MORB) and ocean-island bas-
alt (OIB)]. This argues against exclusively astheno-
spheric or mantle plume sources. The enrichment of
LREE and LILE and depletion of HFSE (Figs 10 and 11)support an origin from the lithospheric mantle. The
Fanshan rocks have La/Yb and Nb/La ratios consistent
with an origin from the lithospheric mantle (Fig. 13b;
Condie, 1997).
Subduction-related metasomatismThe enrichment of LILE (Rb, K, Th, U, Sr, and Pb) and
depletion of the high-field strength elements (HFSE; Nb
and Ti) and the HREE (Yb) are characteristic features of
magmas generated in suprasubduction-zone settings
(e.g. Wilson, 1989; Castillo & Newhall, 2004). The high
Th/Yb ratios above the MORB–OIB array (Fig. 15a) are
presumed to reflect the influence of subduction-zonefluids or melts enriched in Th in their petrogenesis. The
Fanshan intrusive rocks plot in the fields of arc volcanic
rocks in Fig. 15a and b. Thus, a likely scenario for the
petrogenesis of the ultrapotassic magmas is that a fluid
or melt derived from subducted pelagic or terrigeneous
sediments was channelled in the overlying lithosphericmantle, forming a zone of hybrid veined mantle
(Conticelli et al., 2013). As stated above, the northern
Fig. 10. Primitive mantle-normalized trace element and chondrite-normalized rare earth element (REE) patterns for the clinopyrox-ene syenite of Unit 1, and the monomineralic apatite rocks and nelsonites of Unit 2 of the Fanshan intrusion. The primitive mantleand chondrite normalizing values are from Sun & McDonough (1989).
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glimmeriteglimmerite
Fig. 11. Primitive mantle-normalized trace element and chondrite-normalized rare earth element (REE) patterns of the (biotite-)clino-pyroxenite and glimmerite of Unit 2, the orthoclase clinopyroxenite, garnet-rich clinopyroxenite and syenite of Unit 3 and syeniticdykes from the Fanshan intrusion. The primitive mantle and chondrite normalizing values are from Sun & McDonough (1989).
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margin of the NCC was strongly influenced by the
southward subduction of the Paleo-Asian (Mongolian)
oceanic plate during Carboniferous to Permian times
(Zhang et al., 2009). This is evidenced by the occurrence
of Late Paleozoic Andean-type continental arc magma-
tism on the northern margin of the craton (390–280 Ma;Zhang et al., 2007, 2009). The termination of the sub-
duction of the Paleo-Asian Ocean beneath the north-
ern margin of the NCC was at c. 280 Ma. The
218 Ma Fanshan intrusion and many other contem-
poraneous Late Triassic alkaline intrusions are re-
stricted to the northern margin of the NCC, forming an
east–west-trending alkaline–ultramafic magmatic belt,
suggesting a genetic link to the Palaeozoic subduction
of the Paleo-Asian oceanic slab. Furthermore, an exten-
sional tectonic regime probably developed in the north-
ern margin of the NCC during the late Triassic following
the final collision of the Mongolian oceanic arc terraneswith the NCC. We thus propose that the parental mag-
mas of the Fanshan intrusion were generated by
decompression melting of enriched mantle peridotite in
a post-collision, extensional tectonic setting. The en-
riched lithospheric mantle source was probably meta-
somatized by infiltration of subduction zone fluids
Fig. 12. Variation of eNd(t) vs (87Sr/86Sr)t for the Fanshan intrusion. Plotted for comparison are Sr–Nd isotopic compositions ofPrecambrian basement rocks from the northern North China Craton (calculated at 218 Ma) and those of Late Paleozoic–earlyMesozoic intrusions (calculated at 218 Ma) in the northern margin of the NCC from Jiang (2005) and Zhang et al. (2009, 2012). Thecomposition of the lower continental crust is after Jahn et al. (1999). The composition of Paleozoic lithospheric mantle in the NCC(represented by kimberlite-hosted xenoliths; Zheng & Lu, 1997; Xu et al., 2004), the Roman Magmatic Province (Prelevic et al., 2008,2013; Boari et al., 2009) and Yaojiazhuang intrusion (Chen et al., 2013) are shown for comparison. The compositions of igneousrocks of the Devonian Kola Alkaline Carbonatite Province in NW Russia and eastern Finland are compiled from Downes et al. (2005)and Lee et al. (2006).
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and/or melts during the late Palaeozoic when the Paleo-
Asian oceanic slab was subducted beneath the northern
margin of the NCC.
Mantle mineralogy and melting processesConsidering the ultrapotassic composition of the
Fanshan rocks, we believe that a K-bearing phase, such
as amphibole (K-richterite) or phlogopite, was present
in the mantle source. These minerals have long beenrecognized as important reservoirs in the mantle for K,
Rb and Ba as well as volatiles (e.g. Wilson, 1989;
Rudnick et al., 1993). As stated above, both melts and
aqueous fluid phases were probably responsible for the
enrichment of the mantle source in LILE and REE (Baker
& Wyllie, 1992).The elevated [Tb/Yb]N ratios (Fig. 15d) indicate that
the magmas parental to the Fanshan intrusion were
probably derived from a garnet-facies peridotite source
region rather than a spinel-bearing source (Xu, 2001).
Thus, the Fanshan magmas could have been derived
from an amphibole- or phlogopite-bearing garnet-facies
5101520
30354045
SiO2
25H2O
Al2O3
MgO
CaO
FeO
Na2O
P2O5
Fe2O3
K2O
TiO2
7008009001000110012001300024681012141618
wt.%
Mineral phase in the systemP=10kbar, fO2=FMQ, H2O(wt.%)=3%
Augite
Garnet
7008009001000110012001300
(a)
(b)
5101520
30354045
25
wt.%
SiO2
Al2O3
MgOCaO FeO
0
2
4
6
8
10
12
Fe2O3
TiO2
K2OP2O5
Na2O
70080090010001100 0060021Mineral phase in the system
P=2kbar, fO2=FMQ, H2O(wt.%)=0.5%,elevated K2O content(3wt.%)Augite
Sanidine
70080090010001100 0060021
Fig. 14. Result of MELTS modelling (Ghiorso & Sack, 1995)using a starting magma composition based on a kamafugitefrom Western Qinling, NCC (Guo et al., 2014) assumingfO2¼FMQ, a starting temperature of 1250�C and final tempera-ture of 730�C: (a) H2O¼3 wt %, pressure of 10 kbar (�30 km);(b) H2O¼0�5 wt %, pressure of 2 kbar. The sequence of appear-ance of mineral phases in each of the MELTS runs is displayedby the grey bars. In (b) K2O content of the starting magma com-position has been increased to 3 wt %.
Fig. 13. (a) Theoretical two-component mixing curves for d18Ovs initial 87Sr/86Sr. Ratios shown on each curve denote the pro-portion of Sr in the mantle or mantle-derived end-member tothe proportion of Sr in the crustal contaminant or slab-derivedfluid (after James, 1981). (b) Nb/La vs La/Yb variation inthe Fanshan samples. The black lines separating fields of as-thenospheric, lithospheric and mixed lithospheric–astheno-spheric mantle are from Abdel-Rahman (2002). Symbols are asin Fig. 12.
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lithospheric mantle source that was metasomatized by
subduction-related melts or fluids prior to magmageneration.
Experimental studies suggest that small degrees of
melting (<10%) of mantle peridotite can yield alkali-rich
primary magmas (Hirschmann et al., 1998) with LREE-
enriched REE patterns (Henderson, 1984). During partial
melting, apatite and hydrous phases are rapidly con-sumed over a small temperature interval close to the
solidus (Wilson, 1989; Hammouda et al., 2010).
Experimental melting studies of phlogopite-bearing
harzburgite or lherzolite indicate that under F-rich con-
ditions and elevated pressure (>12 kbar for harzburgite
or >18 kbar for lherzolite) melt compositions change
from silica-saturated to silica-undersaturated (Melzer &
Foley, 2000, and references therein). A decrease in the
degree of silica-saturation of potassic melts has alsobeen observed experimentally under F-poor, H2O-rich
conditions at elevated pressure (Foley, 1992, 1993). The
degree of silica-saturation of primary potassic melts,
however, is also controlled by the fluid composition
during partial melting. A predominance of CO2 over
H2O during magma generation will suppress the stabil-ity field of olivine, favouring the formation of silica-
undersaturated melts (Wendlandt & Eggler, 1980).
Therefore, low-degree melting of such a mantle source
under high-pressure conditions could well explain the
geochemical characteristics of the Fanshan parental
magmas, analagous to models proposed for the origin
of Group II ultrapotassic rocks in Central Italy and
glimmerite,
Fig. 15. Variation of (a) Th/Yb vs Nb/Yb, (b) Ba/Nb vs La/Nb, and (c) [Ta/La]N vs [Hf/Sm]N for the Fanshan intrusion. The trends ofsubduction- and carbonatite-related metasomatism are from LaFleche et al. (1998) and references therein. (d) Variation of Tb/Yband La/Sm normalized to primitive mantle values (Sun & McDonough, 1989). The boundary between products of spinel- and gar-net-dominated melting is from Wang et al. (2002) and references therein; OIB from Sun & McDonough (1989).
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Western Qinling in the NCC (e.g. Conticelli et al., 2013;
Guo et al., 2014).
Origin of the apatite oresThe Fanshan intrusion includes a distinctive type of
magmatic apatite-rich rocks comprising layers of mono-
mineralic apatite. Cumulates with very high apatitemodes approaching monomineralic facies have been
reported from other alkaline intrusions such as the
Kihibina Complex in the Kola Peninsula (e.g. Notholt,
1979; Veksler et al., 1998; Zaitsev et al., 2014) but the
Fanshan apatite-rich rocks are to our knowledge unpar-
alleled in terms of size and the unique cumulus assem-blage characterized by the coexistence of glimmerite
with apatite 6 magnetite rocks. Like silica-poor phoscor-
ites (e.g. Zaitsev et al., 2014), the almost silica-free char-
acteristics of the Fanshan ores (monomineralic apatite
rocks and nelsonite) probably preclude the possibility
that these rocks crystallized from immiscible P-rich con-jugates (e.g. Veksler et al., 1998, 2006, 2008).
Fractional crystallization and magmareplenishment?It is notable that the monomineralic apatite rocks andnelsonites contain biotite and are often associated with
glimmerite. According to our studies, the Fanshan rocks
exhibit a large range of major element compositional
variation (Table 4), yet all have similar Sr–Nd isotopic
and trace element characteristics (Figs 10–12), suggest-
ing that the parental magmas have experienced varyingdegrees of fractional crystallization and crystal accumu-
lation after emplacement. The order of crystallization
can be deduced from the field relations, petrographic
observations and MELTS modelling. Except for melan-
ite, the following crystallization sequence is proposed:
Ca-rich augite is the earliest phase on the liquidus,
followed by biotite, apatite, magnetite and finallyK-feldspar. The absence of apatite in the early formed
clinopyroxenite suggests undersaturation of apatite
during the early stage. Further fractionation of silicate
minerals after magma emplacement may have driven
the elevated concentrations of phosphorus until the
monomineralic apatite rocks and nelsonites started toform (Tollari et al., 2006). However, MELTS modelling
suggests that apatite crystallized simultaneously with
several other silicate phases including Ca-rich augite
and biotite. Therefore, simple fractional crystallization is
incapable of explaining the formation of monomineralic
rocks.
The combination of large variations in modal min-eralogy and evidence of fractionation suggest that even
Unit 2 does not exhibit closed-system behaviour. The
first and most important line of evidence is the presence
of several Mg-number reversals in Ca-rich augite com-
position from the base of Unit 2 upwards. At these
stratigraphic levels, the concentration of Ti in Ca-richaugite shifts to higher values (Fig. 7b), consistent with
abrupt increases of these elements in the crystallizing
magma. Thus, these intervals probably record the re-
charge and mixing of more primitive magmas with the
fractionating magma in the chamber. Such open-
system behaviour has previously been proposed for
other phoscorite intrusions (e.g. alkaline plutons of theKola Peninsula, Russia; Verhulst et al., 2000).
In the high-level Fanshan magma chamber the dens-
ity of the evolved magma crystallizing Ca-rich augite,
biotite, apatite and magnetite is likely to be lower than
that of the primitive magma replenished from below.
Thus, when a new pulse of primitive magma arrived it
would have formed a layer at the base of the magmachamber (Campbell & Turner, 1989; Snyder & Tait,
1995). The widespread planar foliation and lineation
shown by the main minerals in the rocks of Unit 2 may
indicate that magmatic currents and a laminar flow re-
gime may have resulted from the replenishment of
magma (e.g. Wager & Brown, 1967; Irvine, 1987;Conrad & Naslund, 1989). Thus the thick monomineralic
apatite rocks could be produced by apatite crystalliza-
tion from fractionated P-rich magmas frequently
recharged with more primitive magma from a deeper
crustal magma chamber. If this is the case, the Fanshan
magma is required to be low in viscosity to facilitatemagma flow. Addition of H2O is known to reduce melt
viscosities (e.g. Baker & Vaillancourt, 1995; Giordano
et al., 2008), and fluorine also acts to significantly de-
crease melt viscosity (e.g. Dingwell & Hess, 1998;
Zimova & Webb, 2006). Compaction and subsolidus
growth of minerals probably also contributed to the de-
velopment of mineral foliation (McBirney & Hunter,1995) as reflected by the presence of 120� triple junc-
tions of apatite in the monomineralic apatite rocks and
deformed biotite in the glimmerite.
In conclusion, we consider that crystal settling and
mechanical sorting, combined with repeated magma re-
plenishment and mixing with the fractionated chambermagma, is the predominant process responsible for the
formation of the apatite ores. However, we admit that
the exact mechanism of apatite and biotite concentra-
tion in monomineralic layers is still unclear.
SUMMARY AND CONCLUSIONS
New data presented in this study show that the
Fanshan intrusion reflects open-system magma cham-
ber processes. The parental magmas are deduced to be
kamafugitic in composition (Group II ultrapotassic)
but with relatively high K2O and low water contents;
these evolved in a deep-seated magma chamber via
fractional crystallization and assimilation of wall-rocklimestone. Low degrees of melting of an enriched litho-
spheric mantle source (apatite–carbonate–amphibole–
phlogopite-bearing garnet lherzolite) could explain the
petrogenesis of the Fanshan primary magma. Mantle
enrichment probably resulted from metasomatism
associated with oceanic sediment recycling duringsouthward subduction of the Paleo-Asian oceanic plate
in Carboniferous to Permian times. The P-rich Fanshan
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rocks are clearly cumulates and we consider that crystal
settling and mechanical sorting is the predominant pro-
cess responsible for their formation. The presence of
several Mg# and TiO2 reversals in Ca-rich augite com-
position from the base of Unit 2 upwards indicates anopen system. We favour a model in which the Fanshan
intrusion marks the periodic injection of primitive mag-
mas derived from a subduction-modified lithospheric
mantle source into an upper crustal magma chamber
where they mixed with previously fractionated magma,
leading to the production of exotic and enigmatic
monomineralic lihologies (monomineralic apatite rocksand glimmerite).
ACKNOWLEDGEMENTS
We are grateful to B. Ronald Frost, Dejan Prelevic, and
Editor Marjorie Wilson for their thoughtful and con-structive comments. Tonny Bernt Thomsen at the
Geological Survey of Denmark and Greenland, Paul
Carpenter of Washington University in St. Louis and
Oona Appelt at the Helmholtz Centre GFZ Potsdam are
thanked for their assistance with laser ablation and elec-
tron microprobe analysis; Ziliang Jin and Liu Han arethanked for their help in the field. Zhenhui Bian is
acknowledged for provision of logistical support in the
Fanshan Phosphorus Mine.
FUNDING
Parts of this work were supported by 973 Program(2012CB416806), the National Natural Science Founda-
tion of China (Nos 40925006 and 40821061), the ‘Funda-
mental Research Funds for the Central Universities’, the
111 Project (B07011), and PCSIRT, and DFG grant VE 619/
2-1. I.V.V. also acknowledges support by the Russian Sci-
ence Foundation grant No. 14-17-00200.
SUPPLEMENTARY DATA
Supplementary data for this paper are available at
Journal of Petrology online.
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