Iberian Group of Petrology, Geochemistry and Geochronology...

1

Iberian Group of Petrology, Geochemistry and Geochronology (IGPGG)

Interactions between intrusive granites and host rocksCentral System batholith (Ávila batholith)

A field trip to the Gredos massif in the area close to Plataforma and Refugio del Rey

June 2014

Antonio Castro and Carlos Fernández, Universidad de Huelva

Important no+ce:This is an internal document for exclusive use of the members of this group. The reported data and illustra8ons were already published elsewhere and the copyrights were transferred to interna8onal editors (Elsevier, Oxford, etc.). No part of this document can be reproduced without permission of the publishers. To refer data and interpreta8ons contained in this document, please use the published papers listed below. Do not use this guide as a source of data in further publica8ons.

Source papers:

Castro, A., 2013. Tonalite-Granodiorite suites as cotectic systems: A review of experimental studies with applications to granitoid petrogenesis. Earth Science Reviews, 124, 68-95.

Díaz-Alvarado, J. Fernández, C., Castro, A., Moreno Ventas, I., 2013. SHRIMP U–Pb zircon geochronology and thermal modeling of multilayer granitoid intrusions: Implications for the building and thermal evolution of the Central System batholith, Iberian Massif, Spain. LITHOS, 175-176, 104-123.

Díaz-Alvarado, J., Fernández, C., Díaz-Azpiroz, M., Castro, A., Moreno-Ventas, I. 2012. Fabric evidence for granodiorite emplacement with extensional shear zones in the Variscan Gredos massif (Spanish Central System). Journal of Structural Geology (http://www.sciencedirect.com/science/article/pii/S0191814112001502?v=s5).

Díaz-Alvarado, J., Castro, A., Fernández, C. and Moreno-Ventas, I. 2011. Assessing Bulk Assimilation in Cordierite-bearing Granitoids from the Central System Batholith, Spain; Experimental, Geochemical and Geochronological Constraints. Journal of Petrology, 52, 223-256. doi:10.1093/petrology/egq078

martes, 10 de junio de 14

http://www.sciencedirect.com/science/article/pii/S0191814112001502?v=s5




2

The Gredos massif is part of a huge granite-granodiorite batholith of more than 300 km length and 60 km width outcropping in central Spain, from Madrid region to South Salamanca province and North Extremadura (Fig. 1a). Continuous granitic exposures form a great part of the Alpine mountains of the Spanish Central System, so the batholith is normally referred to as the Central System batholith (Castro et al., 2002). As in many other batholiths of similar compositional features (e.g. Newer Caledonian granites in Scotland: Stephens & Halliday, 1984; French Massif Central: Didier & Lameyre, 1969; Pin & Duthou, 1990; Western Australia: Collins, 1996), these calc-alkaline batholiths are always late orogenic, postdating by more than 20 Ma the major orogenic phases.

One of the most salient features of the Central System batholith is the presence of low-P metamorphic complexes that form large (>20 km length) roof-pendants in the centre and margins of the batholith (Fig. 1a). Low-grade metasediments are metamorphosed into migmatitic hornfelses locally around the intrusion of granitoids. High-grade complexes are characterized by the presence of upper crustal migmatites with associated anatectic granites (S-type). Most of these migmatites are derived from pelitic and semipelitic metasediments that form part of the several km thick Neo-Proterozoic turbiditic series, and Ordovician ferrosilicic metavolcanics (Fernández et al., 2008), which are widely represented in the Iberian massif in Spain and Portugal. In some places, as the case of the study area, both migmatitic hornfelses and regional, high-grade migmatites are close together. Thus, the intrusion took place in a previously structured host in which large extensional detachments put into contact low-grade metasediments and anatectic complexes. There, the processes of interaction between intruding sheets of granodiorites and metasediments are common, making this area one of special interest for the study of assimilation processes (Ugidos & Recio, 1993; Ugidos et al., 2008). Though local, the complex geometry of contacts and the multiple pulses of magma intrusion in km-sized layered structures, make the “local” interaction a common process in this batholith.

Large homogeneous volumes of coarse grained, porphyritic granitic rocks of granodiorite to monzogranite composition constitute more than 90 vol% of the intrusive rocks. Minor amounts of more basic rocks ranging from gabbro to Qtz diorite are present elsewhere in the batholith, some of them being locally dominant over km-sized irregular areas, but dominantly concentrated close to the contacts with the country rocks. In general, all plutonic rocks from gabbros to granodiorites form a typical K-rich calc-alkaline association (Moreno-Ventas et al., 1995) having close similarities with typical Caledonian I-type batholiths (Chappell & Stephens, 1988), and belonging to the large group of Variscan intrusives

catalogued as “calc-alkaline series granitoids” in regional syntheses and classifications of the Variscan magmatism (Capdevila et al., 1973; Castro et al., 2002). According to the Frost et al. (2001) classification scheme, the intrusive units are magnesian-type and evolve from typical calc-alkalic (CA) to alkali-calcic (AC) series (Fig. 2b). Hornblende-bearing rocks, Bt tonalites and part of the Bt granodiorites are calc alkalic and the Hbl-diorites are metaluminous (ASI<1). These will be referred to as calc-alkalic intrusive unit (CAIU) in this paper. Most of Bt granodiorites and monzogranites are transitional between calc-alkalic and alkali-calcic series (Frost et al., 2001) and have moderate peraluminosity (1<ASI<1.2) (Fig. 2a). These rocks will be grouped together in a single unit called transitional intrusive unit (TIU). The Crd-bearing unit is characteristically composed of monzogranites (Fig. 2c). They are more peraluminous (ASI>1.2) and they plot in the alkali-calcic region (Fig. 2b). These will be called alkali-calcic unit (ACU) in this study. Other magmatic rocks, not included in this study are mafic microgranular enclaves that are widespread in all rock types of the CAIU and TIU. They are also present, but less abundant, in the Crd monzogranites of the ACU. Many of these enclaves are true autoliths that represent fragments from early pulses that were quenched against the country rocks and were dragged and dismembered by successive magma pulses by magmatic erosion (Paterson & Janousek, 2008).

Field relations of granitoids and country rocks

Figure 1b shows a detailed geological map of a region corresponding to the central part of the batholith (Sierra de Gredos, location at Fig. 1a). Far from being massive and homogeneous, the granitoids of the TIU and ACU are well structured in parallel layers of variable thickness, commonly approaching 1 km. Alternating layers of migmatites and granitoids are affected by at least two generations of folds in the studied area, which originates a complex fold interference pattern (Figs. 1b, c). Scarce structures indicative of solid-state deformation can be seen in the granitoid layers, although ductile shear zones have been occasionally observed in some contacts between granitoids and migmatites.

References are given in the paper:

Díaz-Alvarado, J., Castro, A., Fernández, C. and Moreno-Ventas, I. 2011. Assessing Bulk Assimilation in Cordierite-bearing Granitoids from the Central System Batholith, Spain; Experimental, Geochemical and Geochronological Constraints. Journal of Petrology, 52, 223-256.

A brief introduc+on


3

Navacepeda

de Tormes

Sierra de Gredos

El Arenal

Hoyos del

Espino

Post-Variscan sedimentary cover

Metasedimentary rocks

Migmatites

Leucogranites

Gabbros and cortlandtites

Calc-alkalic unit (quartz-diorites)

Transitional and alkali-calcic units

(granodiorites and monzogranites)

N

Arenas de

San Pedro

Pedro

Bernardo

Mombeltrán

Navarredonda

de Gredos

San Martín

del Pimpollar

5 km

NPost-Variscan cover

Metasediments

Orthogneisses

Migmatites

Basic dykes

Acid dykes

Iberian

Massif

25 km

Ávila

Segovia

40 N40 N

41 N41 N

4 W

4 W

5 W

5 W

6 W

6 W

Arenas de

San Pedro

200 km

Transitional and alkali-calcic units

(mostly Bt-granodiorites and Cord-monzogranites)

Leucogranites

Calc-alkalic unit (mostly quartz-diorites)

Variscan Intrusive Rocks

Plasencia

Barco-Béjar Complex

J707-16

J806-3

J707-7

Fig. 1. (a) Geological map of the Spanish C e n t r a l S y s t e m batholith. (b) Detailed geological map of the Barbellido-Las Pozas area (Sierra de Gredos, C e n t r a l S y s t e m batholith). Location in Fig. 1a. (c) WNW-E S E - t r e n d i n g g e o l o g i c a l c r o s s -section showing the m a i n s t r u c t u r a l features of the mapped area.

(a)

(b)


4

Two main granitoid layers (called here Las Pozas and Barbellido; (Fig. 2) have been identified in the mapped area. The Las Pozas Crd monzogranite constitutes a tabular body, around 800 m thick, sandwiched between two migmatite layers. Several septa of migmatite hornfels are included within Las Pozas Crd monzogranite, particularly near its top. In the vicinity of the contacts with pelitic metasediments, the intruding granitoid contains abundant xenoliths of partially digested country rocks. Although pelitic xenoliths of any size from cm to tens of m are the most abundant, also metaquartzites, calc-silicates and fragments of mafic rocks are present. All these exotic xenoliths represent true “resisters” that survived digestion by the intruding magma. The Barbellido Bt granodiorite overly the migmatite-leucogranite layer located at the top of Las Pozas granitoid. Magma-magma contacts can be seen between the Refugio del Rey leucogranite and the Barbellido Bt granodiorite.

For the purposes of this work it is highly illustrative to analyze the content distribution of cordierite and K feldspar in granitoids, measured at a large number of stations across the mapped area and represented in the form of isocontour lines in Fig. 3. High cordierite volume fractions (8-12 %) can be observed near the contacts between granitoids and either hornfelses or nebulites. By contrast, low volume fractions (<1 %) have been measured near the Refugio del Rey leucogranite and its associated leucocratic nebulites. As a whole, Las Pozas Crd monzogranite shows average volume fractions of cordierite between 4 and 8 %, whereas the Crd content in the Barbellido Bt granodiorite is less than 1 vol%, excepting its eastern and northern contacts. The contents in Kfs megacrysts show similar patterns to those of cordierite (Fig. 3), evidencing the general observation that cordierite-rich granitoids are in turn richer in Kfs megacrysts. The sill-like geometry of the intrusive bodies entails large surface areas of mutual contacts between intruding magma and host rocks. They mainly intruded sub-parallel to the main foliation in the hornfelses, although local disruption of the host-rock layering can be commonly observed. The result is an intricate geometrical pattern with abundant fragments of hornfels rock partially or totally incorporated into the intruded granitoid mass.

1400

1600

1800

2000

2200

2400

I

(WNW)

II

(ESE)

1 km

vertical

scale (m)

A

II

2100 m

2100 m

2300 m

2300 m

1800 m

1900 m

2000 m

2100 m

2000 m

1900 m

1800 m

1700 m

1700 m1800 m

1900 m

2000 m

1900 m

1800 m

1900 m2000 m

2100 m

2200 m

2000 m

1900 m

Hoyos del Espino

N

25

37

56

68

32

70

58

50

77

50

34

30

59

5252

7064 27

30

58

57

63

77

50

40

78

45

70

69

88

4050

50

68

58

72

65

30

4160

7043

38

48

58

40

3552

55

5858

60

46

35

7350

52 11

80

65

43

56

73

32

27

50

50

55

54

44

68

70

68

34

20

45

52

45

45

3063

45

66

50

65

64

3650

70 31

4869

67

6561

84

43

45

46

44

30

50

45

72 68

40

72

74

46

37

48

50

56

46

52

42

51

51

60

73

75

60

56

39

70

60

47

5455 48

60

66

70

66

48

45

5 15 18 W

5 15 18 W

5 13 30 W

5 13 30 W

40 16 30 N 40 16 30 N

40 15 18 N 40 15 18 N

**

Quaternary cover

Cord-monzogranite

(Las Pozas)

Basic rocks

Leucogranite

(Refugio del Rey)

Nebulite

Hornfelses

Late-Variscan ductile

shear zone

Magmatic foliation

Host-rock foliations

*Bt-granodiorite

(Barbellido) *

AV-931

I

Autholiths

Ferrosilicic migmatite

Layered migmatite

1 km0

B

Fig. 2


5

B

Equal-area

Lower-hemisphere

projections

Magmatic

foliation

Host-rock

foliations

n = 64

n = 47

N

N

Crd-monzogranite

Bt-granodiorite

N

Fold axial

traces

Foliation

trajectories

First stage

Second stage

Antiform

Synform

Magmatic rocks

Host rocks

?

Basic rocks

A

C D

1 km0

Migmatite and

leucogranite

Hornfelses


Cordierite

?

8-12 %

4-8

1-4

<1 %

K Feldspar

?

> 25 %

< 10 %

20-25

15-20

10-15

(a) Structural sketch of the mapped area (b) Equal area, lower hemisphere projections of poles to the magmatic foliation and host-rock foliations in the mapped area. (c) and (d) Isocontour maps representing volume fractions of cordierite and K feldspar, respectively, in the granitoids of the mapped area.

Fig. 3


0.6

1.0

1.4

1.8

2.2

2.6

3.0

Metaluminous

Peraluminous

AS

I=A

l/(C

a-1.

67*P

+Na+

K)

0,4

0,5

0,6

0,7

0,8

0,9

Magnesian

Ferroan

FeO

*/(F

eO*+

MgO

)M

ALI

(Na

O+K

O-C

aO)

-8,0

-4,0

0,0

4,0

8,0

50 55 60 65 70 75 80

Calcic

Alkalic

A-C

C-A

wt% SiO2

22

AbAn

Or

Trondhjemites Tonalites

Granit

es s.

s.

Granodiorites

Monzogranitesand Qtz-monzonites

Hbl diorites

Bt granodiorites and monzogranites

Cord monzogranitesAutoliths

NebulitesXenoliths and migmatitic hornfelsesPeraluminous leucogranites (mostly intrusive)

Bt-tonalitesCalc-alkalic intrusive unit (CAIU)

Transitional intrusive unit (TIU)

Alkali-calcic unit (ACU)

}LEGEND

}

6

Series classification diagrams (Frost et al., 2001) and O’Connor rock classification triangle plotting the granitoids of the Gredos Massif. Calc-alkalic and transitional intrusive units (CAIU and TIU) are defined according to the MALI-silica diagram. Most Crd monzogranites define an alkali-calcic unit (ACU). With exception of the Hbl-diorites of the CAIU, all the remaining granitoids are peraluminousFig. 4


7

Cation plots showing the varied geochemical trend displayed by granitoids of the Gredos massif and their related country rocks. Arrows are fractionation vectors calculated by assuming constant partitioning over the T range of 800-1100 °C. This is the range for which the major element abundances have been modelled by using the MELTS code. Vectors for joint fractionation of two phases (e.g. Cpx+Pl) are calculated by assuming an average D value with equal and constant proportions of the two minerals. Large grey arrows mark the trends displayed by the intrusive granitoids of the CAIU and TIU groups. In most diagrams, these fit fairly well the direction of ideal fractionation vectors implying the coupled subtraction of Cpx+Pl. Note that the trends defined by the Crd monzogranites (ACU) are transversal to the fractionation vectors. They point to the composition of host xenoliths and nebulites.

Fig. 5


8

A

C F

Pl

Crd

EA-8 sil850 ºC400 MPa+ Qtz+ Kfs+ Ab+ Ilm+ H2O+ FeMg-1+ FeMn-1

Crd area melts

Granodiorite melts

Peraluminous

Metaluminous

Bt

Bt (Bt-granodiorite)

Sil

EA-8 Sil Bt-granodiorite+Sil (20 wt.%)

A

C F

Pl

Crd

EA-5850 ºC400 MPa+ Qtz+ Kfs+ Ab+ Ilm+ H2O+ FeMg-1+ FeMn-1

Crd area melts

Granodiorite melts

Peraluminous

Metaluminous

Bt


EA-5 Bt-granodiorite+Crd (20 wt.%)(a)

0 0.2 0.4 0.6 0.8 1

15

20

10

30

35

25

2 2.4 2.8 3.2 3.62.6

2.7

2.8

2.9

AlIV

Mg(pfu)

Mg#=Mol MgO/(MgO+FeO)

Mol

Al2O

3*10

0

Crd

Bt

Bt

coresrims

Bt

Pl

core

rim

EA-5 Bt-granodiorite+Crd (20 wt.%)

(b)

(c)

A

C F

Pl

Crd

EA-8 Xen850 ºC400 MPa+ Qtz+ Kfs+ Ab+ Ilm+ H2O+ FeMg-1+ FeMn-1

Xenolith area melts

Granodiorite melts

Peraluminous

Metaluminous

Bt granodiorite area


Bt xenolith area

Sil

Spl

EA-8 XenBt-granodiorite+Xenolith (20 wt.%)

(d)

Baseline xenolith melt 850 ºC

Possible reactive subsystem

Bt Granodiorite (starting material)

Baseline Bt-granodiorite melt 850 ºC

Bt-granodioritesCrd-monzogranites

Xenoliths (starting material)

LEGEND

Crd core EA-5Crd rim EA-5Bt core EA-5Bt rim EA-5

Bt 850 ºC (Bt-granodiorite)Pl 850 ºC (Bt-granodiorite)

ACF projected diagrams (A=Al2O3; C=CaO; F=FeO) plotting the coexisting assemblages and melts of composite experimental runs with addition of Crd (a), Sil-rich spot (b), and xenolith (c) to the Bt granodiorite. Molar proportions of ACF are projected by algebraic inversion of the compositional space defined by the components A, C, F, Qtz, Kfs, Ab, Ilm, H2O and the exchange vectors FeMg-1 and FeMn-1. The area of reactive subsystems has been traced tentatively according to qualitative observations of textures in the runs. The thick line with tick marks is the bulk mixing line of the two systems represented by the Bt granodiorite and the contaminant. Tick marks represent 10 wt% proportions of one of the two end-members. The composition of Crd monzogranites is close to the mixing array using xenoliths as contaminants (c). (d) represents the constant slope of the Mg# partitioning between Crd and Bt supporting the shift in composition of the local system with time. Note that new-formed Bt, both in the granodiorite layer and the Crd-rich layer, is richer in Al with respect to the baseline represented by the Bt composition in the granodiorite alone. The four phase assemblage depicted in (a) and (c) indicates the peritectic reaction that that forms Crd at expense of Bt. The remaining melt in the granodiorite system alone at the same PT conditions (red circle) is very close in aluminosity to the melts formed in the composite capsules (green circles). These relations point to a potential equilibrium of Crd in the residual melt of a partially crystallized granodiorite.Fig. 6


9

An

F Or

Pl

Hbl

Opx Bt

5

3

4

2

1

Projection of granitoids of the Gredos massif onto the pseudoternary projection defined by Opx-Or-An. Lines of multisaturation (cotectic like) are traced from experimental data with calc-alkaline systems. These represent liquid trajectories by either crystal fractionation or partial melting. Arrows mark the direction of decreasing T. Bt granodiorites of Gredos (grey circles) fairly follow the cotectic trend corresponding to liquids produced either by fractionation of an andesitic parental magma or by partial melting of a diorite solid source. Crd monzogranites follow a trend transversal to the general attitude of cotectic lines. The main transversal trend points to the composition of country rock pelitic xenoliths. These relations report that Crd monzogranites are resulting by bulk assimilation of pelitic metasediments by Bt granodiorite magma. Cotectic lines (1) and (3) correspond to experiments by Sisson et al. (2005) at P=700 MPa and 2.3 and 1.7 wt% H2O respectively; (2) Castro et al. (2010) at P=1.5 GPa and 0.9 wt% H2O; (4) Carrol & Wyllie (1990) at P=1.5 GPa and 2.5 wt% H2O ; (5) Skjerlie & Patiño Douce (2002) at P=2.1 GPa and <2 wt% H2O.

Fig. 7


10

A summary of the geochronology results (SHRIMP U-‐Pb on zircons) is summarized here in the next 5 pages


11

N

1 km0

Quaternary coverCord-monzogranite(Las Pozas)

Basic rocksLeucogranite(Refugio del Rey)

Nebulite

Hornfelses

Late-Variscan ductile-fragile shear zone

Magmatic foliation


*Bt-granodiorite(Barbellido-Plataforma)*Autholiths

Ferrosilicic migmatiteLayered migmatite

Bt-granodiorite(Circo de Gredos)

Early ductile shear zone

2100 m

2100 m

2000 m

1700 m

1700 mHoyos del Espino

25

56

6832

70

58

773430

52

7027

30

58

77

40

45

70

69

88

40

50

68

586541

7043

38

48

58

40

3555

585860

35

50

11

65

43

56

73

32

5050

55

44

70

68

34

20

45

52

45

3063

45

66

50

65

64

36

31

4869

6584

43

45

4650

45

72

40

72

74

46

37

48

5646

42

51

60

73

75

6039

70

60

47

5455 48

6066

70

48

45

5 15 18 W

5 15 18 W

5 13 30 W

5 13 30 W

40 16 30 N 40 16 30 N

40 15 18 N 40 15 18 N

**

AV-931

I

PradoBarbellido

2000 m

60

58

52

34

82

54

56

56

64

5527

45

54

60

40

22

73

4466

636178

43

Prado delas Pozas

Plataformade Gredos

2100 m

1900 m1800 m

1900 m

1900 m

1800 m

1800 m1900 m

2000 m1900 m

1800 m

1900 m2000 m

1600 m

1700 m

2100 m

2200 m

2000 m

2300 m

2300 m

II

64

Circo de Gredos

J707-8

Las Pozas Hornfels

J806-4

R. Rey Leucogranite

J806-3

Barbellido Crd-monzogranite

J809-1

Plataforma Bt-granodiorite

J806-1

Las Pozas

Crd-monzogranite

J706-47

Circo de Gredos

Bt-granodiorite

II

I

299.7 ±2.8304.1 ±3.4

294.6 ±3

286.5 ±3.8303 ±2

295.6 ±1306.9 ±1.5

298.1 ±3.1305.4 ±1.6

308.2 ±3.4

Rela

tive

prob

abili

ty

Zircon 206Pb/238U age (Ma)

0

0.2

0.4

0.6Bt-granodiorite

J707-16n=11

0

0.2

0.4Crd-monzogranite

J806-3n=11

280 360 440 520 600 6800

0.2

0.4XenolithJ707-7n=34

320 Ma

Fig. 8


12Age (Ma)

Rel

ativ

e pr

obab

ility

280 288 296 304 312 320 328

0

0.1

0.2

0.3

0.4

0.5 Anatecticmelts

Circo de GredosBt-granodiorites

Las PozasCrd-monzogranites

Barbellido-Plataformagranodiorites andmonzogranites

Age (Ma)

Rel

ativ

e pr

obab

ility

Las Pozas migmatitichornfelses

Refugio del Reyleucogranites


280 288 296 304 312 320 328

0

0.1

0.2

0.3

0.4

0.5 Intrusivegranitoids


Las Pozas hornfelses

Age (Ma)R

elat

ive

prob

abili

ty Barbellido-PlataformaCrd-monzogranites andBt-granodiorites



280 288 296 304 312 320 328

0

0.1

0.2

0.3

280 288 296 304 312 320 328

0

0.1

0.2

0.3

Barbellido-PlataformaCrd-monzogranites andBt-granodiorites


280 288 296 304 312 320 328

0

0.1

0.2

0.3

Las Pozas migmatitichornfelses

280 288 296 304 312 320 328

0

0.1

0.2

0.3


280 288 296 304 312 320 328

0

0.1

0.2

0.3

0.4

a)b)

c)

Relative probability histograms showing the main populations of U-Pb ages analyzed in the studied samples. Histograms are referred to the igneous or intrusive stage, so that the inherited ages are not included. (a) Probability plots. Photographs of selected zircon crystals for the distinct probability maxima are included. (b) Probability density distribution of the three intrusive granodioritic and monzogranitic bodies showing the characteristics of the main age groups analyzed and described in each sample. Maxima of the age density distribution of anatectic melts are added for comparison. (c) Probability density distribution in the two samples of anatectic leucogranites. The main maxima of the age density distribution of the three intrusive granitoids are also depicted to reveal the age relation between granodioritic intrusion and melt generation in the metasedimentary host rocks at the emplacement level.

Fig. 9


13


U-Pb Concordia diagrams and relative probability histograms for the three studied samples. Data-point error ellipses in Concordia diagrams are 68.3% conf., including error from standard, and include the 2σ error bars diagram for ages between 280 and 350 Ma. Grey colored rhombs are coherent groups of ages used to obtain mean Concordia age. (a) U-Pb Concordia diagram for Bt granodiorite (sample J707-16). (b) and (c) are Concordia diagrams for the analyzed samples of Crd monzogranite (sample J806-3) and xenolith (sample J707-7), respectively. An enlarged area plot for Variscan ages is also shown. (d) Relative probability histogram for 206Pb/238U ages (listed in Table 3). Grey areas mark main ages of the host sediments to the Central System batholith

Fig. 10


15

a

320

330

310

300

290

bc

d e

0.048

0.056

0.060

0.064

20

6P

b/2

38U

0.044

0.052

0.30 0.34 0.38 0.42

207Pb/235U

0.46

320

340

360

380

400320

330

310

300

290

303 ±2 Ma

N=11 MSWD=0.9

286.5 ±3.8 Ma

N=3 MSWD=2.3

Metamorphic

ages

J707-8 Las Pozas Hornfelses (leucosome)

330

340

0.044

0.046

0.048

0.050

0.052

0.054

0.056

20

6P

b/2

38U

0.31 0.33 0.35 0.37 0.39 0.41

207Pb/235U

320

320

310

300

290

303.5 ±2.8 Ma

N=4 MSWD=2.2

294.6 ± 3 Ma

J806-1 Las Pozas Crd monzogranite

290

320

340

0.044

0.046

0.048

0.050

0.052

0.054

0.056

0.31 0.33 0.35 0.37 0.39 0.41

207Pb/235U

20

6P

b/2

38U

280

J706-47 Circo de Gredos Bt granodiorite

304.1 ±3.4 Ma

N=3 MSWD=1.8

299.7 ±2.8 Ma

N=3 MSWD=1.5

312.6 ±2.8 Ma

N=8 MSWD=1.3

0.044

0.046

0.048

0.050

0.052

0.054

0.056

0.31 0.33 0.35 0.37 0.39 0.41

207Pb/235U

20

6P

b/2

38U

290

320

330

340

300

320

330

310

300

290

J806-3 and J809-1

Barbellido-Plataforma granodiorite

and monzogranite

315.5 ±1.6 Ma

N=3 MSWD=3.6

295.6 ±1.1 Ma

N=4 MSWD=1

285.9 ± 2.9 Ma

0.045

0.047

0.049

0.051

0.27 0.31 0.35 0.39

20

6P

b/2

38U

207Pb/235U

320

305.4 ±1.6 Ma

N=9 MSWD=1.9

J806-4 Refugio del Rey Leucogranites

300

322 ±3.5 Ma

317 ±3.2 Ma

310

290

310

306.9 ±1.5 Ma

N=7 MSWD=1.01

280

High U (>2000 ppm)

analyses

320

330

310

300

290

298.1 ±3.1 Ma

N=3 MSWD=1.9

314 ±3.1 Ma

290

U-Pb Concordia diagrams of the studied samples. Error ellipses show distinct colors for each group of zircons exhibiting specific morphological features, and concordia ages are indicated. Error ellipses in Concordia diagrams represent a 68.3% conf., including the standard error. Insets show error-bar diagrams (2). (a) Concordia diagram of the Circo de Gredos Bt-granodiorite, sample J706-47. (b) Concordia diagram of the Las Pozas Crd-monzogranite, sample J806-1. (c) Concordia diagram of the Barbellido-Plataforma monzogranite and granodiorite, samples J806-3 and J809-1, respectively. (d) Concordia diagrams of zircons from the leucosome of the Las Pozas migmatic hornfelses, sample J707-7. (e) Concordia diagram of the Refugio del Rey leucogranite, sample J806-4.

Fig. 11


16

The general structure of the batholith in this sector is summarized here in the next 6 pages


Magmatic and solid-state structuresThe granitoids of the Gredos massif occur in parallel layers of variable thickness, commonly approaching 1 km, and extending laterally for several km. Three main granitoid bodies were mapped in the study area. The Circo Bt-granodiorite is the floor layer and its total extent is not covered by the detailed map. The Las Pozas Crd-monzogranite constitutes about 800 m thick sheet, intruded between two migmatite layers and includes several septa of migmatite hornfels, particularly near its top. The Barbellido Bt-granodiorite overlies the migmatite and related leucogranite body located at the top of Las Pozas Crd-monzogranite.

The granitoid layers show a variably developed magmatic foliation, mostly defined by the preferred orientation of Kfs megacrysts, and also by the parallel arrangement of enclave corridors, schlieren, xenolith septa and igneous leucocratic veins. The volume fractions of Kfs megacrysts vary from less than 10% to more than 25%, with the greatest contents observed near the contacts between granitoids and host rocks. Scarce structures indicative of solid-state deformation are visible in the granitoid layers, although shear zones with predominant magmatic flow and weak ductile deformation are occasionally observed in some contacts between granitoids and migmatites. The whole system is pervasively invaded by several sets of leucocratic veins mostly resulting from the partial melting of the country rocks. The alternating layers of migmatites and granitoids as well as their country rock bodies are affected by at least two generations of folds, which involve a complex fold interference pattern. This pattern is defined by the contacts between migmatites, granitoids and hornfels septa, as well as by the foliation inside each layer, which is mostly sub-parallel to the external contacts. The axial traces of the folds of the first generation are NNE-SSW to ENE-WSW trending. Folds of this generation are upright or steeply inclined, polyharmonic, with wavelengths ranging from less than 1 m to more than 1 km. Fold hinges are curved due to the interference with the second set of folds, and periclinal closures are frequently observed. In profile, folds of the first generation are open to tight and sometimes are box folds. The axial traces of the folds of the second stage are NNW-SSE trending. The fold interference pattern, although rather irregular, tends to be intermediate between types 1 and 2 of Ramsay (1967). Offsets of the fold traces are locally observed due to several late Variscan, WNW-ESE-trending, strike-slip shear zones. These shear zones are sub-vertical, showing decimetric to metric thickness and metric to kilometric displacement. Deformation within these shear zones is ductile-brittle, with a sub-vertical foliation and a sub-horizontal stretching lineation and slickenside striation. The abundant kinematic criteria shown by the ductile-brittle shear zones (S/C composite fabrics, asymmetric folds, extensional crenulation cleavage and many others) consistently indicate sinistral displacement. The use of these kinematic criteria together with the offset contacts enables the determination of the displacement magnitudes for the zones. Figure X1 shows a geometric reconstruction of the mapped area after removing the displacement due to the shear zones.

Fabric evidence for granodiorite emplacement in the central part of the Gredos massif

1 km

N

Cord-monzogranite(Las Pozas)

Basic rocks

Migmatite andleucogranite Hornfelses

Bt-granodiorite(Barbellido) Ferrosilicic migmatite

Bt-granodiorite(Circo)

Magmatic shear zone

station ofSPO analysis

N

n = 47

Host-rockfoliations

N

n = 64

Magmaticfoliations

N

n = 29

Fold axes

Equal-areaLower hemisphereprojections

Fold axial tracesFirst stage

Antiform

Synform

Second stage

Foliation trajectories

Magmatic rocks

Host rocks

2

1

3

4

5

6 7

8

9

10

11

12

13

14

161718

1920 21

22

23

2425

15

Fig. X1. Structural map of the study zone showing an interpretation after removing younger rocks and sediments and the displacements of the late ductile shear zones. Equal-area, lower hemisphere projection of fold axes (black: first generation; blue: second generation) and poles to the magmatic and host-rock foliations.


SPO fabric of Kfs megacrystsEven though the origin of Kfs megacrysts is still controversial (Vernon and Paterson, 2008; Johnson and Glazner, 2010), we infer a magmatic origin, given the euhedral habits, large sizes, or the concentric distribution of inclusions (Bt, Pl and Qtz). Additionally, in the study area, the foliation defined by the megacrysts is parallel to the main contacts between igneous and metamorphic units in the batholith, and we believe that the solid-state deformation mostly occurs in the late ductile-brittle shear zones. So, the Kfs megacryst fabric is assumed to be a magmatic fabric and Kfs megacrysts (phenocrysts) are used as suitable markers to estimate shape preferred orientation (SPO) fabrics. Magmatic foliation was measured in more than 150 sites. SPO ellipsoids were determined at 25 stations. Three perpendicular field sections were analysed at each station (Fig. X2). High-resolution digital images were taken at the studied sections and edited with Photoshop® and ImageJ editor software to extract megacrysts from the matrix, reduce image noise and to create a grey scale image file. These final images were analysed via software SPO (Robin, 2002; Launeau and Robin, 2005) to obtain the shape ratio and long axis orientation of the sectional fabric ellipses. With this information, the three-dimensional ellipsoid data were determined for each station using Ellipsoid 2003 software (Launeau and Robin, 2005). The software output includes the orientation and normalised length of each principal axis. Results are shown in Figures X3 (orientation and shape of the SPO ellipsoid) and X4 (spatial distribution of shape and intensity of the SPO ellipsoid).

20 cm20 cm2D ellipse 2D ellipse

SPO analysed image

20 cm2D ellipse

STATION 15

N 3F 5.8%

A B CnLenght 1.371 1.218 0.599

strike 121.4 217.4 326.4dip 28.2 10.9 59.4

Foliation/Dip 56.4/30.6 L Rake 68.2

A/C 2.289 Flinn 0.122 A/B 1.126 P' 2.479 B/C 2.034 T 0.714

3D ellipsoid stereogram

c d

SPO analysed image

SPO analysed image

a b

1 km

N

Equal area, Lower hemispheren = 25

X

N

Y

N

Z

N

Basic rocks

Migmatite andleucogranite Hornfelses


Granodioritesand monzogranites


0.0

0.2

0.4

0.6

0.8

1.0

υ

+1.0

0+0.5

-1.0

-0.5

s

Foliation trajectories

Magmatic rocksHostrocks

X-Y planes

X axes

0 dip 30

30 dip 60

60 dip 90

90

0 plunge 30

30 plunge 60

60 plunge 90

90

a

b

c d

e

1

1.4

1.8

2.2

2.6

1 1.4 1.8 2.2

K=1

Apparentflattening

Apparentconstriction

Y/Z

X/Y

Fig. X2. Results of the SPO determination at one selected station. The fabric defined by the Kfs megacrysts was analysed from three differently oriented surfaces (a, b, c) with the SPO 2003 software (Launeau and Robin, 2005). The orientation and shape of the 3D fabric ellipsoid (d) was obtained using Ellipsoid 2003 software (Launeau and Robin, 2005).

Fig. X3. Results of the SPO fabric measurement at the 25 selected sites. Plot of the orientation of the XY planes (a) and X axes (b) on a geological sketch map for the study area. (c) Spherical projection of the orientation of the X (top), Y (centre) and Z (bottom) axes of the SPO ellipsoid. (d) Flinn (1962) diagram showing the shape of the measured SPO ellipsoids. (e) Hossack (1968) diagram. The thick grey arrows in Fig. d, e show the change in the shape of the SPO ellipsoids with the increase in the fabric intensity (distance from the origin of Flinn diagram).


N

400 m

I

II

I

II

I III II

a b

Shape parameter (Lode) Intensity parameter (Nadai)c d

e

1 km0

N

n = 18

X - Y - Z(measured)

Equal-areaLower hemisphereprojections

N

n = 6

X restored N

n = 6

Y restored N

n = 6

Z restored

XYZFold axis

1516

1718

2023

15

16

1718

20

23

15

16

17

18

20

23

1516

17

18

20

2315

15

15

16

1616

17

17

1718

18

18

20

20

2023

23

23

1516

1718

2023

I

II

I

II

-0.8-0.6-0.4-0.2

00.20.40.60.8

0.10.20.30.40.50.60.70.8

16

1718

2023

1516

1718

2023

15

Flattening ellipsoids

Constriction ellipsoids

0.0 / 0.20.2 / 0.40.4 / 0.60.6 / 0.8

0.0 / -0.2-0.2 / -0.4-0.4 / -0.6-0.6 / -0.8 < -0.8

NADAI parameter0.1 - 0.20.2 - 0.30.3 - 0.5 > 0.5

Fig. X4. Spatial distribution of (a) the shape (Lode’s parameter) and (b) intensity (Nadai’s parameter) of the SPO fabric ellipsoid for Kfs megacrysts. Isocontours were traced using the kriging contouring method. (c) and (d) show the variation across cross section I-II of the Lode’s and Nadai’s parameters, respectively. (e) Projection of the X, Y and Z principal axes of the SPO fabric ellipsoids found at 6 measurement places (15, 16, 17, 18, 20, 23) near cross section I-II. Left diagram: Present-day attitude of the principal axes. Remaining diagrams: Restored orientation of the X, Y and Z axes after eliminating the late folding episode. The primitive coincides with the boundaries of the granitoid body at the diagrams with the restored position of the principal axes of the SPO ellipsoid.

Isocontour maps of shape and intensity parameters reveal that apparent flattening ellipsoids are found near granitoids contacts, so that the closer to metasedimentary bodies the greater positive values of shape parameter. Instead, ellipsoids of apparent constriction are found internally within the large granitoid bodies. Intensity values are greater at the roof of the tabular granitoid layers. The X and Y axes are located along major circles in their inferred old attitude, whereas most of the restored Z axes are sub-vertical.


The sheared top of the Las Pozas Crd-monzograniteThe top of the Las Pozas monzogranite in contact with its migmatitic host rock is affected by a sub-parallel shear zone about 15 m thick. The average orientation of the shear zone is N45ºE, dipping around 60ºSE. The shear zone is characterised by the presence of alternating bands, 5-30 cm thick, composed of Crd-monzogranite and migmatite, and arranged parallel to the boundaries of the zone. The banding is interpreted to have formed by successive intrusion of sill-like bodies of granitoid magma inside the migmatitic host rocks, at the boundary of the main magma batch of Las Pozas with stretching of the alternating bodies (magma and host-rock septa) within the shear zone (Díaz Alvarado et al., 2006, 2007). The lineation defined by the preferred orientation of the Kfs megacrysts is sub-horizontal. The foliation observed inside the Crd-monzogranite bands, marked by the strong preferred orientation of the Kfs megacrysts in the Crd-monzogranite, is locally oblique to the lithological banding, consistently indicating a dextral shear sense. Other kinematic indicators of dextral displacement are best seen on surfaces normal to the foliation and parallel to the lineation. These indicators include C’-type shear bands and restite blocks or mafic enclaves with asymmetrically arranged, adjacent host foliations, yielding geometries akin to delta-type and sigma-type mantled porphyroclasts (Passchier and Trouw, 1996).

In this work, a methodology for the analysis of interacting megacrysts (i.e., tiling and non-parallel interactions) that expands the approach originally proposed by Mulchrone et al. (2005) was applied. One sample located at the shear zone at the top of the Las Pozas Crd-monzogranite was analysed (Fig. X5). The section is approximately orthogonal to the foliation defined by the megacrysts and parallel to the lineation marked by their preferred alignment. All interaction groups identified in the sample (501) were analysed, which implies the inspection of a minimum of 1002 individual Kfs megacrysts. 138 groups produced ambiguous sense of interaction, because of mutually inconsistent senses within pairs and/or because distinct senses of interaction deduced from pairs of the same group. This gives a total number of 363 consistent senses of interaction. From these, 213 (59%) were dextral and 150 (41%) were sinistral. Therefore, the analysis of interacting megacrysts in the granite affected by the Las Pozas shear zone suggests that this shear zone was affected by a dextral simple-shearing component accompanied by a pure-shearing component yielding a kinematic vorticity of around 0.85 (Fig. X6).

80

60

40

20

00 0.2 0.4 0.6 0.8 1.0

Wk

% d

extra

l int

erac

tions

Fig. X5. Field examples illustrating determination of the rotation sense from interacting groups of Kfs megacrysts. (a) Dextral pair. (b) Sinistral pair (note antithetical geometry). (c) Inconsistent pair. (d) Unambiguous dextral group. (e) Ambiguous group. From these, only groups (a), (b) and (d) should be used to deduce the sense of shear and kinematic vorticity.

Fig. X6. Relation between proportion of interactions with dextral sense and kinematic vorticity Wk (thin solid line). The thick line corresponds to the proportion of dextral tiling pairs mathematically predicted for aspect ratio R=3 and spacing p=200 (from Fig. 14c of Mulchrone et al., 2005). Dashed lines represent lower and upper limits for dextral interactions (confidence interval) and their respective Wk values.

top (SE)

0.611.001.632.664.3411.55

max = 18.85NE

0.131.001.302.007.009.00

max = 32.00NE

top (SE)

a b

S

L L

S

Some evidences of solid-state deformation can be observed at the base of the shear zone. All the kinematic criteria show dextral sense of shear for the solid-state deformed band of the shear zone. Acontinuous transition from magmatic to solid-state flow is inferred. The shape of the observed Qtz c-axis fabrics (Fig. X7) suggests a rotational component with a dextral sense of shear. The absence of clear asymmetric crossed girdles can be attributed to sub-vertical shortening, i.e. orthogonal to the original attitude of migmatitic layering.

After restoring the effects of the late folding, we infer that the shear zone was sub-horizontal and the restored shear-sense is top-to-the-SW. Our results are consistent with deformation involving a combination of simple and pure shearing in a sheared top to the granitic body, because of vorticity values of around 0.85 determined from the analysis of interaction between Kfs megacrysts, quartz c-axis fabric data with a strong maximum close to the pole of the foliation, and flattening SPO fabric ellipsoids.

Fig. X7. Quartz c-axis fabrics measured by SEM/EBSD from two samples from the shear zone at the top of the Las Pozas Crd-monzogranite body. Lower-hemisphere, equal area projections. Contour intervals are multiples of uniform distribution (mud). S: foliation plane; L: lineation. (a) Qtz-rich level inside a migmatite band (b) Crd-monzogranite deformed in the solid state.


Kinematic interpretation of the emplacement and shearing of the studied granitoids (Fig. X8)In the Gredos massif, magmatic and solid-state foliations measured in more than 100 sites reveal a structural continuity between the metasedimentary host and granitic bodies during, at least, the post-emplacement folding stages. The same applies to the measured Kfs SPO fabrics. Geometrical and kinematic identity between the magmatic and solid-state structures and fabrics analysed at the sheared top of the Las Pozas Crd-monzogranite further strengthen this argument. Continuity between the magmatic fabrics in the granitoid bodies and the country-rock structures suggests that all fabric patterns were probably strongly influenced by regional deformation during D3 Variscan deformation phase.

NE SW?

NE SW?

a

b

Fig. X8. Conceptual model of emplacement of magma batches at extensional detachments. (a) First stage: growth of the pluton as a sub-horizontal sheet complex exploiting an extensional detachment, possibly at a dilation jog, releasing bend or similar structure. (b) Second stage: Reworking of the roof of the pluton as a consequence of simple shearing (thick blue line) and, possibly, flattening due to the extensional collapse of the crust (vertical arrow) and/or gravity ascent of magma inside the pluton.


Structural evolution (Fig. X9) and emplacement sequence (Fig. X10)

Fig. X9. Evolution model of the Spanish Central batholith. (a) Schematic cross section of the central part of the Spanish Central System during extensional detachments characteristic of the Variscan second deformation phase (D2). Low-grade metamorphic rocks (green) were tectonically juxtaposed above high-grade domains (yellow), which became partially molten creating a mid-crustal migmatitic belt. The hangingwall (green) shows D1 folds reoriented by the D2 extensional detachments. The rectangle indicates the approximate location of the area enlarged in (b) and (c). (b) Initial D3, with ductile extensional structures (bold black lines) and sequential emplacement of granitoid batches (pink). (c) Later D3 deformation with two generations of upright folds and ductile-brittle, sub-vertical, discrete shear zones (not shown) that deformed the entire system.

aNW SE0

10

20

30(km)

D3 ( 300-320 Ma)

c 1km

1km

D3folding

Low-grade metasediments

Migmatites

D3 shearzone

Granodioriteand monzogranite

Old D2extensionaldetachment

NW SE

Basicrocks

b

D1+D2 ( 320 Ma)

First stage: Extensional detachments

Second stage: Folding

337-320 Ma

Effects of D1+D2deformation phases

(a) 312 Ma

Intrusion of the Circode Gredos Bt-granodiorite

(b)

Intrusion of theBarbellido-Plataforma granitoids

307 MaR. Reyleucogranites

(c) 303 Ma

Intrusion of theLas Pozas Crd-monzogranite

Hornfelses

(d)D1+D2shear zones

High grademetasedimentsLow-medium grademetasediments

Leucogranites

Fig. X10. Idealized 2D sketch depicting the time evolution of the distinct granitoid sheets of the studied area. The pictures approximately represent the present-day plan-view surface.

The Variscan deformation of the study region started with the D1 contractional phase, followed by D2 extensional detachments. At the end of this extensional period (≈ 320 Ma), low-grade metamorphic rocks became tectonically juxtaposed onto high-grade domains. The structures of the D1 and D2 Variscan deformation phases are crosscut by the granitoids at the northern boundary of the Central System batholith (e.g., Doblas, 1991; Fernández and Castro, 1999), which agrees with the geochronological data. The D2 phase was followed by a period (≈320-300 Ma) of strongly heterogeneous and complex deformation (e.g., Martínez Catalán et al., 2009), which is collectively assigned to the D3 deformation phase. D3 began with an extensional phase, coincident with the period of late-orogenic collapse identified in other regions of the Variscan Orogenic Belt like the French Massif Central (Faure, 1995; Talbot et al., 2005; Joly et al., 2009). Granites intruded coevally with this extensional episode of the D3 deformation phase. In the study area, the successive magma batches intruded sequentially along a rather prolonged time period spanning near 10 Ma, from 312 to 303 Ma (Díaz-Alvarado et al., 2011b). The D3 extensional episode was followed by the generation of upright folds. Similar complex structural evolutions have been described elsewhere for the D3 phase at the Central Iberian zone (Días et al., 1998; Escuder Viruete et al., 1994; Valle Aguado et al., 2005).


ReferencesDías, G., Leterrier, J., Mendes, A., Simoes, P.P., Bertrand, J.M., 1998. U-Pb zircon and monazite geochronology of post-collisional Hercynian granitoids

from the Central Iberian Zone (Northern Portugal). Lithos 45, 349-369.Díaz-Alvarado, J., Fernández, C., Moreno-Ventas, I., Castro, A., 2006. Estructura de los cuerpos plutónicos y migmatíticos en la parte central del Macizo

de Gredos. Geogaceta 39, 27-30.Díaz-Alvarado, J., Fernández, C., Díaz Azpiroz, M., Martínez-Rico, J., Moreno-Ventas, I., Castro, A., 2007. Análisis de la microfábrica de cuarzo en los

resister cuarcíticos de la zona de cizalla dúctil de la Garganta de las Pozas, parte central del Macizo de Gredos. Geogaceta 43, 31-34.Díaz-Alvarado, J., Castro, A., Fernández, C., Moreno-Ventas, I., Armstrong, R., 2011b. SHRIMP U-Pb zircon geochronology of intrusive granitoids and

anatectic leucogranites in the Gredos Massif, Central System batholith, Spain. Implications of the building of the batholith. Geophysical Research Abstracts 13, EGU2011-6231.

Doblas, M., 1991. Late Hercynian extensional and transcurrent tectonics in central Iberia. Tectonophysics 191, 325-334.Escuder Viruete, J., Arenas, R., Martínez Catalán, J.R., 1994. Tectonothermal evolution associated with Variscan crustal extension in the Tormes Gneiss

Dome (NW Salamanca, Iberian Massif, Spain). Tectonophysics 238, 1-22.Faure, M., 1995. Late Carboniferous extensions in the Variscan French Massif Central. Tectonics 14, 132-153.Fernández, C., Castro, A., 1999. Brittle behaviour of granitic magma: the example of Puente del Congosto, Iberian Massif, Spain. In: Castro, A.,

Fernández, C., Vigneresse, J.L. (Eds.) Understanding Granites: Integrating New and Classical Techniques. Geological Society, London, Special Publications 168, pp. 191-206.

Flinn, D., 1962. On folding during three dimensional progressive deformation. Quarterly Journal of the Geological Society, London 118, 385-428.Hossack, J.R., 1968. Pebble deformation and thrusting in the Bygdin area (S. Norway). Tectonophysics 5, 315-339.Johnson, B. R., Glazner, A. F., 2010. Formation of K-feldspar megacrysts in granodioritic plutons by thermal cycling and late-stage textural coarsening.

Contributions to Mineralogy and Petrology 159, 599-619.Joly, A., Faure, M., Martelet, G., Chen, Y., 2009. Gravity inversion, AMS and geochronological investigacions of syntectonic granitic plutons in the

southern part of the Variscan French Massif Central. Journal of Structural Geology 31, 421-443.Launeau, P., Robin, P-Y.F., 2005. Determination of fabric and strain ellipsoids from measured sectional ellipses – Implementation and applications.

Journal of Structural Geology 27, 2223-2233.Martínez Catalán, J.R., and 16 others, 2009. A rootless suture and the loss of the roots of a mountain chain: The Variscan belt of NW Iberia. Comptes

Rendus Geoscience 341, 114-126.Mulchrone, K.F., Grogan, S., De, P., 2005. The relationship between magmatic tiling, fluid flow and crystal fraction. Journal of Structural Geology 27,

179-197.Passchier, C.W., Trouw, R.A.J., 1996. Microtectonics. Springer, Berlin.Ramsay, J.G., 1967. Folding and Fracturing of Rocks. McGraw-Hill, New York.Robin, P–Y.F., 2002. Determination of fabric and strain ellipsoids from measured sectional ellipses – theory. Journal of Structural Geology 24, 531-544.Talbot, J.Y., Faure, M., Chen, Y., Martelet, G., 2005. Pull-apart emplacement of the Margeride granitic complex (French Massif Central). Implications for

the late evolution of the Variscan orogen. Journal of Structural Geology 27, 1610-1629.Valle Aguado, B., Azevedo, M.R., Schaltegger, U., Martínez Catalán, J.R., Nolan, J., 2005. U-Pb zircon and monazite geochronology of Variscan

magmatism related to syn-convergence extension in Central Northern Portugal. Lithos 82, 169-184.Vernon, R. H., Paterson, S.R., 2008. How late are K-feldspar megacrysts in granites?. Lithos 104, 327–336.


24

Areas to make observa-ons and discussions

1. Anatec8c granites of second genera8on. Refugio del Rey leucogranite

General features: This is a homogeneous leucogranite with intrusive rela+ons against the host (?) granodiorite and transi+ons to migma+tes. The granite is coeval with the granodiorite (zircon agers are almost iden+cal). The age (ca.300 Ma) is 40 Ma younger with respect to the age of Variscan migma+za+on. We interpret this granite as formed in situ by local effects of the intrusion of the calc-‐alkaline batholith (Granodiorites). In fact the granite is the host of the batholith, but they were magmas at the same +me.Points of interest for discussion: Conatacts with the granodiorite. Textures: heterogeneous in patches (fluid reten+on and no thermal gradient. Enclaves: autoliths are common. Enclaves of granodiorite.

2. Migma8tes

These are the migma+tes associates to the former granite. They contain abundant quartzite (calc-‐silicates) resisters, typical of the Neoproterozoic forma+on in the region. Other migma+tes in the area are derived from the Ordovician Ollo de Sapo forma+on (see ferrosilicic migma+tes in the map).

3. Migma88c hornfelses

These are migma+tes derived from local metamorphism of low-‐grade schists and greywackes of the Neoproterozoic regional country rock. They have agma++c structures and abundant cordierite in melanosomes. Xenoliths from these migma+tes appear isolated within the granodiorite. These are responsible of bulk assimila+on processes. The presence of Crd in the intrusive granodiorite is the consequence of xenolith assimila+on. In place it is possible to observe transi+onal contacts between granodiorite and migma+te host, through a zone of par+al assimila+on and diges+on. Resisters (Calc-‐silicate rocks) appear widespread within the contaminated granodiorite, locally transformed into a monzogranite. Some ques+ons to discuss:1. Why resisters are so randomly distributes and isolated when they were exo+c and local bodies in the host?2. How to understand that Crd is euhedral and coarse grained (similar to grain size of the granite) being the by-‐product of assimila+on?3. Why Crd is not uniformed distributed in the magma+c rock as they are Bt or Pl.

Con$nued next page

The path is shown on next page. Acer one hour and a half through a foot path we will reach the first outcrop to visit and discuss. The other stops are on the way down back to Plataforma. Total dura+on of the trip is about 6 hours. A quick lunch will be held in the field. The following points are the main stops labeled in the geological map with blue circles


25

4. Synplutonic intrusions of mafic magmas (Prado de las Pozas complex)

This is a classical area ocen visited in Gredos. Superb outcrops of mafic rocks with complex rela+ons with the surrounding granodiorites. The outcrop can be seen as a magma+c breccia. However, a long history can be deciphered by examina+on of contacts, grain size varia+ons, enclaves, xenocrysts, etc. The last pages of this guide offer details on this outcrop. A detailed map that was carried out by students from Huelva is included.This is a good outcrop to discuss the meaning of basic magmas in the origin of the batholith.

5. Magma8c structures in granodiorites and origin of cordierite

Beside the basic intrusion, it is possible to observe interes+ng structures in the host granodiorites. These are related with crystal-‐liquid frac+ona+on by filter-‐pressing mechanisms. Carmen Rodríguez (PhD student of Huelva) is working on these structures as related to processes observed in experiments with thermal gradients.

The area is plenty of superb outcrops. So, we are open to discuss on any observa+ons proposed by any of the par+cipants.


26

Plataforma

Garganta de las Pozas

Prado Barbellido

Refugio del Rey

Prado de las Pozas

Laguna grande

Crico de Gredos

Google view of the area from the north. In dashed light green the trip from Plataforma de Gredos to Prado de las Pozas y Refugio del Rey.


27

The field trip

2100 m

2100 m

2000 m

1700 m


25

56

6832

70

58

773430

59

52

7027

30

58

77

40

45

70

69

88

4050

50

68

586541

7043

38

48

58

40

3555

585860

35

50

11

80

65

43

56

73

32

5050

55

44

68

70

68

34

20

45

52

45

3063

45

66

50

65

64

36

31

4869

656184

43

45

4650

45

72

40

72

74

46

37

48

5646

52

42

51

60

73

75

6039

70

60

47

5455 48

6066

70

48

45

5 15 18 W

5 15 18 W

5 13 30 W

5 13 30 W

40 16 30 N 40 16 30 N

40 15 18 N 40 15 18 N

**

AV-931

PradoBarbellido

2000 m

60

58

52

34

82

54

56

56

64

5527

45

54

60

40

22

73

44

636178

43 0

Prado delas Pozas

Plataformade Gredos

2100 m

1900 m1800 m

1900 m

1900 m

1800 m

1800 m1900 m

2000 m1900 m

1800 m

1900 m2000 m

1600 m

1700 m

2100 m

2200 m

2000 m

2300 m

2300 m

64

Circo de Gredos

II

I

66

N

Quaternary coverCord-monzogranite(Las Pozas)

Basic rocksLeucogranite(Refugio del Rey)Nebulite

Hornfelses

Late-Variscan ductile-brittle shear zone

Magmatic foliation


*Bt-granodiorite(Barbellido) *Autholiths


Layered migmatite

Bt-granodiorite(Circo)


1 km

1

2

3

45


Magma-c breccia; Prado de las Pozas, Gredos

Prado de las Pozas is a high and flat area at 2000 m al+tude in the Gredos Mountains, Avila province. The outcrops of basic rocks extend over a small area about 600 m across. It is a large breccia formed by irregular, angular and globose bodies of massive diorite invaded by the host monzogranite and aplite-‐pegma+te veins

N

2100 m

2100 m

2000 m

1700 m


25

563270

3430

59

52

7027

30

58

77

40

45

70

69

88

5841

70

48

58

40

3555

585860

35

50

11

80

65

43

5632

5050

55

44

68

70

68

45

45

3063

45

66

50

64

4869

656184

43

45

4650

45

72

40

72

74

46

5646

52

42

51

60

73

75

6039

70

60

47

5455 48

6066

70

48

45

5 15 18 W

5 15 18 W

5 13 30 W

5 13 30 W

40 16 30 N 40 16 30 N

40 15 18 N 40 15 18 N

*

AV-931

PradoBarbellido

2000 m

60

58

52

34

82

54

56

56

64

5527

45

54

60

40

22

73

44

636178

43 1 km0

Prado delas Pozas

Plataformade Gredos

2100 m

1900 m1800 m

1900 m

1900 m

1800 m

1800 m1900 m

2000 m1900 m

1800 m

1900 m2000 m

1600 m

1700 m

2100 m

2200 m

2000 m

2300 m

2300 m

64

Circo de Gredos

66

Pra

do d

e la

s P

ozas


3 m

Detailed map of one of the outcrops of las Pozas showing lobate contacts and chilled margins, together with brecciated parts and back-‐veining of pegma+te


Important observa+ons on the map

Diorites form fragmented globular bodies that were dismembered from a major globular intrusionFine-‐grained zones are irregularly distributed near the contacts with the host monzograniteAplite and pegma+te veins are concentrated in the fine-‐grained zonesFlow structures (orienta+ons of K-‐feldspar megacrysts) are embracing the diorite globular bodiesFine-‐grained enclaves (autoliths) from the margin facies are present in the medium-‐grained dioritesAplites and pegma+tes are transi+onal to the host monzograniteOcelar quartz crystals, showing a corona of mafic minerals, are common

Several pictures on these observa+ons

Quartz ocelli surrounded by a corona of mafic minerals (Hornblende and bio+te). These are interpreted as xenocrysts that were captured by the intruding diorite into a crystal rich granite. The interpreta+on of these ocelli as xenocrysts is the most widely acepted. However, some of them are bigger than normal crystals in the host granite. The elongated ocelli on the right hand photo is polycrystalline and 2 cm length. Another possible origin is fragmented quartz veins... ? These ocelli are enigma+c, and present in many other similar complexes (e.g. in Gerena in Seville).


Enclaves of fine-‐grained marginal facies enclosed by medium-‐grained diorites. These are interpreted as autoliths, i.e. fragments from the margins that were eroded and captured by the intruding magma

The fine-grained zones close to the contacts against the host granite are interpreted as chilled margins. The diorite magma is frozen against the colder granite magma. In the right hand photo, we may observe the veins of aplite and pegmatite intruding back to the fragmented diorite. These are filled with fluids (water-rich magma) coming from the residual melt in the host granite magma.


Different aspects of the breccia-like relations observed in the complex. Some veins contain turmaline (left hand photo)

Flow structures defined by K-feldspar megacrysts in the host monzogranite


The map of the outcrops at Prado de las Pozas represent one of the lobes of a large (>500 m diameter) body of diorite magma that was intruding into a monzogranite magma. During cooling of the whole granite intrusive complex (granite and diorite), the diorite becomes solid before the host granite. For instance, at the temperature of 800 °C the diorite may be a solid rock but the host granite may have a 50 vol% liquid. The rigid parts of the diorite lobes (the chilled margins) behave as a rigid body (rock) and are fragmented and intruded by residual liquid of the host granite magma. These are the veins of aplite and pegmatite. Before to reach the level of emplacement, the diorite traversed a high crystalline zone of the monzogranite, where xenocrysts of quartz were trapped (also plagioclase). Quartz xenocrysts are a proof about the presence of a crystal-rich zone in the pluton. This is the expected solidification front, from which the pluton is crystallizing from the walls inwards. A near-solid region in the granite magma is a condition needed to allocate the intrusion of a more dense magma by pressure gradient imposed by fractures opened in the near-solid margin of the pluton. This is drafted in the cartoon below. On the implications of the quartz ocelli in the diorites of the Gredos massif, Scarrow et al. (2009), pag 63 say:(...The presence of coarse quartz xenocrysts derived from the granodiorite in the appinite stock border facies (Fig. 2D) indicate that the granodiorite must have been partially solidified with a significant crystal fraction when the basic magmas were emplaced....).J.H. Scarrow, J.F. Molina, F. Bea, P. Montero (2009) Lithos 110 (2009) 50–64

/...Synplutonic intrusions of intermediate magma are common feature in calc-alkaline batholiths. They may be fragmented to form mafic microgranular enclaves (Vernon, 1984; Pitcher, 1987; Didier and Barbarin, 1991). Occasionally, they may appear as irregular bodies or magma blobs with lobate and crenulated contacts with the host granite (Fig. 7a). They have unequivocal features of coexistence as magmas. An intriguing feature is the presence of rounded Qtz crystals rimed by a corona of magmatic Hbl (Fig. 7b). These are interpreted as xenocrysts trapped by the mafic magma during intrusion into the felsic host magma. Paradoxically, Qtz is a late phase in the host granite, his presence implying a high crystal content that can be close to the rheological threshold (60-70 vol% crystals; (Vigneresse et al., 1996). However, the felsic magma hosting the synplutonic dikes behaved as crystal-poor fluid, which is evidenced by the liquid-liquid contacts (Fig. 7a). The inference is that Qtz crystal entrapment was previous to the arrival of the mafic magma to the final level of emplacement. The passage of the mafic magma through a crystal-rich zone of the felsic chamber or pluton is a necessary condition to account for the presence of Qtz xenocrysts. Xenocryst entrapment at the contact of synplutonic dikes (Fig. 7c) and crystal mingling zones in areas of high crystal contents (Fig. 7d) are common features in calc-alkaline plutons, giving account of mechanical interaction between a crystal-poor mafic magma and a crystal-rich granitic host. This crystal-rich zone may correspond to the outer part of the pluton, supporting the existence of a solidification front. Accordingly, the mafic magmas crossed the solidification front and were finally emplaced into a low-crystalline zone of the magma chamber (Fig. 7e). In parallel with this observation is the need for a rigid host able to accommodate by brittle fracturing mafic dikes of magmas that are denser than the granite host. Once the mafic magmas have crossed the rigid carapace of the pluton, where magma was almost completely crystallized, they are emplaced as magma blobs in a crystal-poor area inwards of the pluton. In summary, the presence of Qtz xenocrysts in synplutonic intrusions of mafic magmas are indicating the existence of a more crystalline zone, possibly close to the walls of the host silicic pluton.../Castro, A., 2013. Tonalite-Granodiorite suites as cotectic systems: A review of experimental studies with applications to granitoid petrogenesis. Earth Science Reviews, 124, 68-95 DOI: http://dx.doi.org/10.1016/j.earscirev.2013.05.006.

Crystallization front

Liquid

Mush

Intrusion of mafic bodies into a crystallizing magma chamber or pluton in whicha crystallization front was developed

Cartoon showing a hypothetical magma chamber in which a solidification front is formed. The crystal mush of the margins is intruded in a near-solid state by dikes of mafic magma dragging by magmatic erosion crystals from the host that are incorporated as xenocrysts. Hypothetical locations of pictures are labeled in boxes.


http://dx.doi.org/10.1016/j.earscirev.2013.05.006




Iberian Group of Petrology, Geochemistry and Geochronology...

Documents

Transcript of Iberian Group of Petrology, Geochemistry and Geochronology...