REE fractionation by accessory minerals in epidote ...

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the accessory minerals are Ep+Aln, Ttn, Ap and. Zrn. The GPCH unit is coarse grained, character- ized by the presence of alkali feldspar megacrysts. (4 cm) and ...
Mineralogica l Magazine, August 2001, Vol. 65(4), pp. 463–475

REE fractionation by accessory minerals in epidote-bearing metaluminous granitoids from the Sierras Pampeanas, Argentina J. A. DAHLQUIST1 , *, { 1

Centro de Investigacione s Geolo´gicas, CONICET-U.N.L.P., calle 1 N8 644, La Plata (1900), Argentina

ABSTR ACT

A study of the distribution of REE in epidote-bearing metaluminous granitoids from Sierra de Chepes, Sierras Pampeanas, Argentina, reveals that a large proportion of the REE reside in the accessory minerals (allanite, epidote, titanite, apatite and zircon), and therefore these minerals control the behaviour of REE in granitic magmas. Well-developed chemical zonation in titanite indicates that the REE content decreases in the melt during crystallization of this mineral. The textural and chemical characteristics of euhedral epidote suggest a magmatic origin, and in that case it may have played an important role in the fractionation of the REE. The amount of silica and any other geochemical parameter indicative of fractionation progress in the dominant granodioritic-tonalitic facies (gtf) do not correlate with observed variations in the REE patterns. When many accessory minerals are involved, as in the gtf, the differentiated melts (e.g. aplites) are REE poor. Thus, the presence/absence of accessory minerals in granitoids can be indicative of the generation of differentiated melt enriched or poor in REE and other trace elements. This may have an economic significance, as it may allow us to predict the probable geochemistry of the differentiated melts (i.e. those that tend to develop mineralization) from the textural analysis of the ‘regional’ granitic rock. Finally, the type and abundance of accessory minerals in the granitic suite can also help us to define the geotectonic environment where magmas were generated. K EY WORDS : accessory minerals, REE, trace elements, epidote-bearing granitoids, Sierra de Chepes, Argentina.

Introduction VARIOUS studies of granitoid rocks indicate that accessory minerals control the distribution of the REE, as well as other trace elements such as Y, Th, Zr, Hf (Exley, 1980; Fourcade and Allegre, 1981; Gromet and Silver, 1983; Sawka 1988; Bea, 1996; Liu et al., 1999). Thus, the accessory minerals housed in granitic rocks can provide relevant information on crystallization processes and the history of magma cooling. This paper focuses on the study of accessory minerals (allanite, epidote, titanite, apatite and

* E-mail: dahlquis@ci g.museo.unlp.edu.a r { Present address: Centro de Investigacione s Cientõ´ficas y Transferencia Tecnolo´gica La Rioja, Dpto. de Geociencias , Petrologõ´a I´gnea y Geoquõ´mica, Entre Rõ´os y Mendoza, Anillaco (5301), La Rioja, Argentina

# 2001 The Mineralogical Society

zircon), integrating mineral and whole-rock chemistry with textural characteristics in epidote-bearing granitic rocks. The rocks analysed, from the Sierra de Chepes, were formed during the Lower Ordovician Famatinian orogeny along the proto-Andean margin of Gondwana, that developed an extensive granitic province in central west Argentina (see Fig. 1 in Pankhurst et al., 1998). In this work it is concluded that there is direct control of the REE by the accessory minerals, and that rocks with different SiO2 contents can show similar REE patterns depending on the type and abundance of these minerals. The presence or absence of accessory minerals, as well as their relative modal abundance in the analysed rocks, is re ected in the whole-rock behaviour of the REE. Previous studies indicate that Famatinian tonalite to granodiorite intermediate primary magmas originated from melting of an old crust

J. A. DAHLQUIST

and were differentiated by fractional crystallization in a closed magmatic system (Pankhurst et al., 1998; Dahlquist, 2000). Some tonalites and granodiorites may have large modal proportions of accessory minerals due to their effective segregation during crystallization, producing total REE-enriched patterns. On the other hand, similar rocks but with scarce accessory minerals have low total REE patterns that are indistinguishable from those of the differentiated monzogranites and aplites. This observation may be economically useful as it allows us to predict the probable geochemistry of the differentiated melts (i.e. those that tend to develop mineralization) from the textural analysis of ‘regional’ granitic rocks. The textural characteristics, the REE contents and pistacite in the epidotes analysed suggest a magmatic origin. Hence the epidotes must have participated together with the other accessory minerals in the fractionation of incompatible elements in the magma. Finally, the type and abundance of accessory minerals in the granitic suites of the Sierra de Chepes may help to identify granitic rocks that underwent similar petrogenetic processes, and thus help to deŽ ne the geotectonic environment where magmas were generated. Geological setting and depth of emplacement The Sierra de Chepes (approximate area 2000 km2 ) is set within the Famatinian orogenic belt, developed during the Lower Palaeozoic on the proto-Andean margin of Gondwana (Pankhurst et al., 1998; Rapela et al., 1999). It is composed mainly of Lower Ordovician, dominantly metaluminous, calc-alkaline granitoid suites. Petrological, geochemical and isotopic studies indicate that the observed compositional range of the granitic rocks (60 75% SiO2 ) was produced by fractionation of primary magmas generated in turn by partial melting of an old lower crust (Pankhurst et al., 1998; Dahlquist, 2000). There is a general isotopic homogeneity in initial 8 7 Sr/8 6 Sr and 1 43 Sm/1 4 4 Nd ratios, which has been interpreted as the result of a closed crystallization system, without signiŽ cant contamination of the magma during its evolution (Pankhurst et al., 1998; Dahlquist, 2000). Fractional crystallization is considered to have been the main differentiation process (Dahlquist and Rapela, 1997; Pankhurst et al. , 1998; Dahlquist, 2000). Two main magmatic units, the Chepes granodiorite (GCH) and the Chepes porphyritic 464

granodiorite (GPCH) have been recognized in the Sierra de Chepes, within a suite that is dominated by granodiorites and tonalites, with less abundant monzogranites and leucogranites (Pankhurst et al., 1998; Dahlquist, 2000). Samples from these two main units host the accessory minerals analysed in this paper. The GCH unit is the most abundant. It is equigranular, medium grained (1.5 cm), with typical microgranular maŽ c enclaves. The magmatic paragenesis (mineral abbreviations from Kretz, 1983) is Mc, Pl, Bt and Hbl, and the accessory minerals are Ep+Aln, Ttn, Ap and Zrn. The GPCH unit is coarse grained, characterized by the presence of alkali feldspar megacrysts (4 cm) and the rare occurrence or absence of maŽ c microgranular enclaves. The mineral paragenesis is similar to that of the GCH, except for a notable increase in some felsic mineral phases such as Mc and Qtz and also accessory minerals such as Aln. Major element geochemistry indicates that both units are metaluminous (ASI = 0.92 1.1), displaying a ‘normal’ K2 O content of 1.5 3.5%, and a Peacock index of 63.5, indicating a calcic association. The GCH unit shows a silica range of 60.34 67. 13% (average = 64.47%) while the GPCH unit shows a silica range between 66. 70 and 70. 31% (average 68%). The granitoid units were emplaced at shallow levels as estimated from paragenesis and thermobarometric calculations in the metamorphic envelope (2.5+0. 5 kbar) and igneous rocks (3.1 kbar according to the geobarometer of Johnson and Rutherford, 1989) (Dahlquist and Baldo, 1996; Dahlquist, 2000). Schmidt (1992) concluded that the Al-in-hornblende geobarometer of Johnson and Rutherford (1989) is valid only for Hbl that crystallized at high temperatures (7608C). Using the geothermometer of Holland and Blundy (1994) for Hbl-Pl pairs, initial emplacement temperatures (see similar determination in Blundy and Holland, 1990, Ghent et al., 1991; Vhynal et al., 1991; Schmidt, 1992; Holland and Blundy, 1994; Anderson and Smith 1995; Sial et al., 1999) of 818 8258C were obtained for the Sierra de Chepes (Dahlquist, 2000), and 700 7508C for the adjacent comagmatic rocks of the Sierra de Ulapes (Murra and Baldo, 2000). The ratios [Feto ta l /(Feto ta l +Mg) = 0.44 0.48] and [Fe3 + /(Fe3 + +Mg) = 0.20 0.33] for Hbl in granodiorites of the GCH and GPCH units are in the same range as those of Hbl used in most experimental calibrations (Anderson and

REE FRACTIONATION IN METALUMINOUS GRANITOIDS

Smith, 1995). They suggest crystallization at high fO 2 , which is consistent with the occurrence of magmatic Ep at low pressure, according to the experimental work of Schmidt and Thompson (1996). Detailed petrological and geochemical information on the rocks of the Sierra de Chepes is reported in Pankhurst et al. (1998) and Dahlquist (2000).

inclusion in the Mca. The maŽ c minerals (Bt and Hbl) occur in two grain sizes: (1) coarse-grained ( 2 3 6 0. 8 1 m m , B t a a n d H b l a ); an d (2) medium/Ž ne-grained (1 0.560.7 0.4 mm, Btb and Hblb ). The Bt and Hbl (1) variety is dispersed uniformly in the crystalline groundmass, while the Bt and Hbl variety (2) is included in the Mca . Both varieties of Hbl and Bt are subhedral. Chlorite was observed as a secondary mineral associated with Bt and/or Hbl.

GCH and GPCH textures and rock-forming minerals The typical magmatic assemblage for the GCH unit is: 7. 1% Mc (Or 9 0 Or 9 7 ), 35. 0% Pl (An 5 2 ( c o r e ) An 3 9 ), 32. 7% Qtz, 15. 8% Bt [Fe 2 + /Fe 2 + +Mg) = 0.48 on average], 3.3% magnesian Hbl, and 6.1% accessories (Ap, Ttn, Ep, Aln, Mag and Zrn). The magmatic association for the GPCH consists of: 14% Mc (Or8 7- 9 5 ), 34% Qtz, 33.7% Pl (An5 4 ( c o re ) An3 9 ), 12.5% Bt [Fe2 + /(Fe 2 + +Mg) = 0.50 on average)], 0.9% magnesian Hbl, and 4.9% accessories (Ap, Ttn, Ep, Aln, Mag and Zrn, albeit with a higher quantity of Aln than in the GCH unit). Both units show myrmekite and albitic exsolution associated with Pl and Mc. The alkaline feldspar is Mc with cross-hatched twinning, perthitic crystals of this mineral with similar textural characteristics being common. The Mc appears both as poikilitic phenocrysts (Mca ) and Ž ne-grained Mc (Mcb ). The Mca is approximately square and anhedral, with inclusions of Pl, Bt, Qtz and Hbl. Similar characteristics are shown by the Mc in the GPCH unit, although here it is coarser and contains rare i n c l u s i o n s. T h e M c b i s Ž n e- g r a i n e d (0.760.4 mm), generally amorphous, anhedral and without inclusions. The Pl is abundant and was identiŽ ed as (1) coa rse- grained Pl (2. 561.2 mm, Pl a ), (2) medium-grained Pl (1.260.5 mm, Plb ), and (3) Ž ne-grained Pl (0.2560.5 mm, Pl c ). The Pl a is rectangular with rim overgrowths, polysynthetic twinning, and normal zonation and/or ‘patchy’ zonation. The Pl b and Pl c are subhedral, rectangular with polysynthetic twinning and/or normal zonation. The Pla is not abundant but is distributed homogeneously in thin-section. The Plb and Plc , dominant varieties, are spread homogeneously in the crystalline groundmass and occur as inclusions in the Mca . The Qtz shows two varieties, a coarse-grained Qtz (3 2 mm in diameter, Qtza ) and a Ž ne-grained Qtz (0.4 8 mm in diameter, Qtzb ). The Qtza occurs as an individual mineral phase, whilst the Qtzb appears essentially as an

Optical and analytical methodology Gromet and Silver (1983) determined the abundance of accessory minerals using pointcounting in ten thin sheets of the same rock, demonstrating the tendency to over-estimate (20 40%) the mineral contribution to the concentration of REE in the bulk rock. On the other hand, the normative mineral estimation of the accessory minerals such as those analysed in this paper is complex. Examples in the literature of estimating accessory minerals percentages through normative mineral models are subject to considerable uncertainty (e. g. Sawka and Chappell, 1988). Nevertheless, although it is difŽ cult to quantify the real contribution of accessory minerals to the concentration of REE in the bulk rock, relevant qualitative observations are worthwhile. TABLE 1. Estimated modal proportions of principal and accessory minerals. Igneous unit Sample No. of points counted1 Wt.% Qtz Kfs Pl Bt Hbl Ap Zrn Aln Ep Mag Ttn 1

465

GPCH CAS-59

GCH AMBI-28

GCH RLC-236

2294

2000

1885

22 8 53 12 1 1 0.4 0.5 0.9 0.6 0.7

21.4 0 54 9.8 6.5 0.7 0.5 0.1 3.3 0.7 3

38 6.1 35 9 8.4 0.6 0.2 0 0.1 2.1 0.5

Single thin-sectio n

J. A. DAHLQUIST

The modal content of accessory minerals has been determined with a Swift point-counter, with a total count per sample (10 cm2 ) of between 2,200 and 1,800 points (Table 1). The mode was only used to provide a rough estimation of accessory mineral abundance in ‘rich/enriched’ and ‘poor/impoverished’ rocks. It was not

possible to use these results to quantify the contributions made by accessory minerals to the whole-rock concentration of REE. Figure 1 shows the different textural contexts in which the accessory mineral phases were analysed and the approximate areas where the speciŽ c electron microprobe analysis was made. It

a

Pl Aln

Mag

Aln

Aln

52

c

b

Bt

Aln Ttn

Ep

Bt

e

d Hbl

Ep

f Ttn

Ep

Hbl

43

Tt n

45

Ep

45

Bt Hbl

Bt Hbl

Bt

h

g

i Ep Bt

47

Hbl

Bt

46

FIG. 1. Plane-polarize d light photomicrograph s of the minerals analysed with analysis locations and associate d mineral context shown (see Table 2 for analytica l results ). The scale bar represent s 1 mm in a, b, c and d and 0.5 mm in e, f, g, h and i. (a) Euhedral Aln with oscillatory zoning surrounde d by Ep and rimmed by Bt. (b) Two euhedral Aln crystals with remarkable growth associate d with Ttn, Bt and Hbl. Both Aln display oscillator y zoning. (c) Oscillatory zoning in Aln. (d) Textural context of the Ep and Ttn analysed . Small inclusions of Hbl in Ep, and euhedral-subhedra l contact between Bt and Ep can be seen in detail in e and f. (g) Euhedral Ttn from c. (h) Small inclusion s of Ap (hexagonal ) in Bt and in the Ep-Bt contact (see black and white frames). The Ap analysed is that included in Bt, marked in a white frame. Zrn with ovoid habit is shown with a white ellipse. (i) Euhedral and prismatic zircon generatin g the typical pleochroic halo in Bt (see black frame).

466

REE FRACTIONATION IN METALUMINOUS GRANITOIDS

is worth mentioning that in general the minerals are unzoned. Two exceptions were Aln, which has a clear optical zonation showing cores and rims of different compositions, and Ttn, for which the probe analyses revealed compositional differences between core and rim. The chemical analyses obtained on the Aln rims have not been considered, as low totals reduced our conŽ dence in the precision and accuracy of the data. Low totals (90%) are a problem in Aln analyses (e.g. Gromet and Silver, 1983; Sawka and Chapell, 1988). The former authors attributed this to the presence of volatiles and Fe3 + in the mineral, exacerbated by errors associated with the REE and Th standards used, and incorrect absorption factors for these elements. In this study all the minerals have been analysed with a JEOL-JXA-8900M electron microprobe in the Luis Bru´ Electron Microscopy Centre, Complutense University, Madrid, Spain. The accelerating voltage was 20 kV and the beam current was between 15 and 50 nA, while the beam diameter was between 2 and 5 mm. The counting interval ranged from 10 to 100 s, depending on the element being measured. The standards selected are those of the Smithsonian Institute, Washington, described by Jarosewich and Boatner (1991). Interelemental interferences were suppressed by peakoverlap corrections (Roeder, 1985). More details of the measurement conditions can be found in Gonza´lez del Ta´nago (1997). Representative analyses of the accessory minerals considered in this paper, as well as the whole-rock chemical composition of the host rocks, are given in Tables 2 3, respectively. The minerals were analysed for REE (La, Ce, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu), but not for Eu. Europium has not been determined, as it is usually below the electron microprobe detection limit in most accessory minerals analysed in this study (the Eu concentration is essentially controlled by the principal mineral phases, e.g. Pl). Analyses for Y, Th, Zr and Hf were also performed. Chemical analyses of major elements in wholerocks were performed by XRF on fused pellets, while the REE were determined by inactive neutron activation analyses (INAA) and plasma mass spectrometry (ICP-MS), at ACTLABS Canada, as shown in Table 3. The U-Pb dating studies of these rocks, both SHRIMP and conventional (Pankhurst et al., 1998; Rapela et al., 1999; Pankhurst et al., 2001), show the occurrence of magmatic zircons

with euhedral prismatic habit, as well as ancient inherited zircons. Electron microprobe analyses were performed only on zircons with euhedral prismatic habit. Figure 2 shows the different REE patterns for the accessory minerals analysed in this paper, as well as some determined by other authors for rocks with granodioritic composition (Gromet and Silver, 1983; Sawka, 1988; Dawes and Evans, 1991; Bea, 1996; Keane and Morrison, 1997). Gromet and Silver (1983) and Sawka (1988) analysed the minerals following separation and concentration, while Dawes and Evans (1991), Bea (1996) and Keane and Morrison (1997) used the electron microprobe. By and large, the REE patterns determined in this paper are similar to those obtained by other workers with only slight differences, discussed below. Petrography and mineral chemistry of accessory minerals Allanite Allanite (Aln) is found in both the GCH and GPCH units, although it is more common in the latter. Its modal occurrence is very variable. If two thin-sections of the same rock are considered, it can be present in one and absent in the other. Consequently, it is very difŽ cult to quantify the Aln modal presence in any plutonic unit of the Sierra de Chepes. Modal contents in the thinsections studied are shown in Table 1. The Aln is euhedral to subhedral, generally showing oscillatory zoning and rims of Ep and, more externally, Bt (Fig. 1a). Apart from this, it may be associated with coarse Ttn (Fig. 1b). A general characteristic of the Aln is a clear oscillatory zoning as can be seen in Fig. 1c. The Aln analysed in this paper (Fig. 1a), shows an almost rectangular shape (160.5 mm), with irregular subhedral contacts with the Pl and the Ep. The Ep surrounds and is included within the Aln, possibly having grown from it. Optically, the Aln displays oscillatory zoning, with a small included crystal of euhedral Bt that in turn has a very small core of Aln. Figure 1a shows the locations on the minerals where the analyses were performed and the associated textural context, while the data obtained are given in Table 2. Figure 2a indicates that the Aln is enriched in REE, in particular the light REE, (La, Pr and Nd). The REE pattern is similar to that determined by Gromet and Silver (1983) and Sawka (1988), though the Aln analysed is comparatively more enriched in medium and

467

J. A. DAHLQUIST

TABLE 2. Representatives electron microprobe1 ,2 analyses of accessory minerals. Igneous unit Sample Mineral Wt.% SiO2 TiO2 Al2 O3 FeO Fe2 O3 MnO MgO CaO P2 O5 3 REEt Total Ps ppm La Ce Pr Nd Sm Gd Tb Dy Ho Er Tm Yb Lu Th Y Zr Hf

GPCH CAS-59 Aln-52

GCH AMBI-28 Ep-43

GCH AMBI-28 Ep-45

GCH AMBI-28 Ttn-46 C

GCH AMBI-28 Ttn-47 R

GCH AMBI-28 Ap-48

GPCH CAS-59 Zrn-59

30.39 1.33 14.9 7.8 5.3 0.42 2.35 11.07 0.05 24.42 98.03

35.13 0.08 23.77

35.26 0.3 22.7

28.03 38.17 1.28 0.9

28.12 37.69 1.5 1.4

0.46 0.02 0.02 0.34

32.28 0.01 0.01 0.21

54320 98683 9371 28583 1379 6930 bdl 697 bdl 613 bdl 439 bdl 6503 bdl bdl bdl

.

13.08 0.44 0.03 23.42 0.04 0.32 96.31 26 170 939 bdl bdl bdl bdl bdl bdl bdl 438 bdl 351 bdl bdl 157 229 bdl

.

14 0.15 0.02 24.09 0.02 0.28 96.82 28

.

0.11 0.01 28.5 0.13 1.76 98.89

bdl 854 bdl bdl bdl bdl bdl bdl bdl 350 bdl 176 bdl bdl bdl bdl bdl

bdl 6317 774 2667 603 434 bdl bdl bdl 788 bdl 351 bdl 527 1968 bdl bdl

.

0.1 0.02 28.62 0.07 1.01 98.53

bdl 4439 bdl 1833 431 347 bdl bdl bdl 350 bdl 439 bdl 267 1574 370 bdl

.

0.05 0 52 38.4 0.83 92.12

409 2535 585 1125 207 bdl bdl bdl bdl bdl bdl 167 132 114 953 bdl 211

.

0 0.01 0.01 0.17 68.15 100.85

bdl 1195 bdl bdl bdl bdl bdl 475 bdl 1576 613 966 615 bdl 1338 491434 11144

1

Na2 O, K2 O, F, Eu2 O3 and Nb2 O3 not determined Total Fe measured as FeO. All Fe recalculate d to be ferric for epidote, except for allanite where the following formulae were used: Fe2+ = Zrn+Ti+2Th+STR+Y Mg Mn, and SFe-Fe2+ = Fe3+. All Mn assumed to be divalent for Aln and trivalent for Ep. Normalization is based on 8 cations. H2 O calculate d by stoichiometr y 3 plus Th, Y, Zr and Hf. bdl = below detection limit; C = core and R = rim 2

heavy REE (with a notable peak at Gd). The Y and Zr contents are low while that of Th is high, the latter being the most abundant element together with the light REE (Fig. 2a). Epidote This mineral occurs in both granodioritic units and shows variable modal proportions (Table 1). 468

The textural evidence allows discrimination of two main epidote groups: Group I magmatic epidote (GI) and Group II secondary epidote (GII). Details of the Ep textural characteristics can be found in Dahlquist (1999). Figure 1a, d, e, f illustrates the textural context of the analysed Ep. Figure 1e shows the areas analysed, the results of which are detailed in Table 2. Two representative analyses have been

REE FRACTIONATION IN METALUMINOUS GRANITOIDS

TABLE 3. Analyses of igneous rocks from the Sierra de Chepes. Sample Igneous unit Method of analysis Wt.% SiO2 TiO2 Al2 O3 Fe2 O3 FeO MnO MgO CaO Na2 O K2 O P2 O5 H2 O+ H2 O Total ppm La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu U Th Y Nb Zr Hf

AMBI-28 GCH ICP-MS

RLC-236 GCH INAA

CAS-59 GPCH ICP-MS

RLC-241 GA INAA

PUL-245 APL INAA

60.34 0.82 16.4 2.89 3.88 0.14 2.64 5.7 2.76 1.96 0.22 0.8 0.38 98.93

66.93 0.57 14.8 2.93 2.26 0.09 2.29 4.83 2.89 1.98 0.12 0.8 0.11 100.6

66.7 0.56 15.3 1.56 2.8 0.1 1.6 3.99 2.59 2.31 0.13 0.63 0.18 98.45

75.02 0.16 13.51 0.87 0.51 0.08 0.28 1.33 3.38 4.05 0.05 0.61 0.14 99.99

75.65 0.05 14.03 0.08 0.56 0.06 0.14 0.67 3.59 5.32 0.09 0.42 0.18 100.84

55.4 106 n.d. 51.9 10.2 2.02 8.16 1.34 7.31 1.45 4.2 0.6 3.78 0.56 1.44 10.9 42 15 375 9.6

28 57 n.d. 24 4.48 0.95 n.d. 0.7 n.d. n.d. n.d. n.d. 2.79 0.43 1.1 11.6 28 4 165 4.5

70.7 127 n.d. 57.6 11.1 1.7 8.48 1.59 9.12 1.8 5.36 0.79 4.57 0.64 2.45 18.2 54 20 238 6.5

28.2 58 n.d. 21 4 0.7 n.d. 0.5 n.d. n.d. n.d. n.d. 2.9 0.42 3.5 12 34 13 118 3.3

5.7 14 n.d. 5 1.22 0.24 n.d. 0.4 n.d. n.d. n.d. n.d. 2.58 0.4 1.8 5.3 23 4 36 1.2

n.d. = no determined GA = Aspereza Granites = monzogranite s APL = Aplites

chosen in order to illustrate better the general Ep composition. Figure 1d f shows a rather rectangular Ep form with euhedral to subhedral rims in contact with the Bt, Hbl and Ttn. It has a relatively large size (161.3 mm) with inclusions of Hbl and occasionally Ap. 469

The Ep has high contents of some light REE, such as La and Ce and relatively high contents of heavy REE such as Er and Yb (Fig. 2b, Table 2). In the Sierra de Chepes the Ep shows lower Y and Zr contents (Fig. 2b). The pistacite content (Ps%) [Ps = (Fe3 + /( Fe3 + /Al))6100] for the analysed Ep varies from Ps2 5 to Ps2 9 , with an

J. A. DAHLQUIST

Minerals/Chondrites

a

References Analysed minerals in this work

Allanite CAS-59

Analysed minerals in other works and host rocks

5

10

PAL-22, Granodiorite SiO2 = 66.9% Sawka (1988) 4

10

Granodiorite Vierskisets ASI = 1.02, Bea (1996) Volcanic rock SiO2 = 69.4% Dawes and Evans (1991) Hbl-rich quartz monzonite Keane and Morrison (1997) Granitoid I-type (KB22), Lachland Fold Belt, Australia, Sha and Chappell (1999)

Whole-rocks

2

10

Granodiorite, SiO2 = 65 % Gromet and Silver (1983)

5

3

10

1

10

Epidote AMBI-28 45 43

b 4

10

3

10

Titanite AMBI-28 46 (core) 47 (rim)

c

5

7

2

10

1

10

d

e

Apatite AMBI-28

10

4

10

5

10

4

Zircon CAS-59

3

Tm Lu Er Yb

3

10

10 10

2

1

Ce La

Nd Pr

Sm Pm

Eu

Gd

Dy Tb

Er Ho

Yb Tm

Y Lu

Hf

Ce

Th

Zr

La

Nd Pr

Sm Pm

Eu

Gd

Dy Tb

Er Ho

Y

Yb Tm

Lu

Hf Th

Zr

FIG. 2. Chondrite-normalized trace element patterns of minerals analysed in this and other works. Normalized values for REE after Nakamura (1974) and for Y, Th, Hf and Zr after Thompson (1982 ). The lower shaded Ž elds represen t the whole-rock trace element chondrite-normalize d patterns for the granitic rocks of the Sierra de Chepes (see details in Fig. 3). The upper shaded Ž elds represent the mineral trace element chondrite-normalize d patterns for allanite, epidote, titanite, apatite and zircon. The numbers over the shaded Ž elds indicate the number of analyses. Apatite has only one analysis. (e) The Tm and Lu contents in xenotime, determined by Gonza´lez del Ta´nago (1997 ) (diamond symbol), are compared with the Tm and Lu contents in zircon.

average value of Ps2 7 (Table 2). The REE contents are similar to those reported by Dawes and Evans (1991) and Keane and Morrison (1997) for primary Ep, though with higher heavy REE values (Fig. 2b).

Titanite Titanite is also present in both the GCH and GPCH units (Table 1). It generally occurs as large euhedral to subhedral crystals usually associated with maŽ c minerals such as Bt or Hbl, sometimes

470

REE FRACTIONATION IN METALUMINOUS GRANITOIDS

with Ep, Mag and occasionally with Aln (Fig. 1b, d). Inclusions of Mag, Zrn and Ap are common. The large (1.361.2 mm) and euhedral crystal illustrated in Fig. 1d, g occurs in contact with Hbl, Bt and Ep, and has the greatest REE content. The host rock is a tonalite with SiO2 = 60.34% (AMBI-28, Table 3). The areas analysed are shown in Fig. 1g and the chemical data given in Table 2. This mineral displays a compositional zonation with a remarkable compositional difference in Ce and Nd contents between core and rim, a difference which is also conspicuous for other REE elements such as Pr, Sm, Gd, Er, Th and Y (see Table 2 and electron microprobe spot locations in Fig. 1g). Small subhedral, rectangular crystals of Ttn (~130640 mm), e.g. in Fig. 1a have been found enclosed in epidote. High Ti contents in Ep have been attributed by Tulloch (1979) as being due to possible submicroscopic Ttn inclusions. The REE pattern for Ttn shows relatively high Ti contents (Fig. 2c). The high Th content and the presence of P are notable (Table 2). The REE patterns are similar to those obtained by Gromet and Silver (1983) and Bea (1996), although the heavy REE contents (especially Er) are greater (Fig. 2c). Apatite Apatite (Ap) is a ubiquitous accessory mineral in all the rocks analysed; estimated modal proportions are shown in Table 1. It is generally euhedral and hexagonal in shape, though sometimes ovoid or prismatic, depending on the observed section. It generally occurs included in Bt, though may also occur in the crystalline framework. Figure 1e, h shows an approximately hexagonal basal section, 0.7 mm in diameter. Major element determinations indicate that it is a  uorapatite, with a F content of 3.46 3.77%. A single trace element analysis (Table 2) shows similar REE contents to those reported for Ap by Sawka (1988) and Sha and Chappell (1999). This analysis reveals high Y, Th Hf and REE values, with a peak at Pr (also seen in the Ap data of Sha and Chappell, 1999) which is quite remarkable (Fig. 2d). Zircon Zircon (Zrn) appears in both the GCH and GPCH units (Table 1), typically included in Bt with euhedral and prismatic habit, but occasionally with ovoid shape. The Zrn illustrated in Fig. 1h is 471

ovoid with a notable pleochroic halo and is included in the same Bt grain as the analysed Ap. Figure 1i shows a typical prismatic Zrn, included in Bt with an appreciable pleochroic halo. A representative analysis of this euhedral zircon is shown in Table 2. The REE pattern shows heavy REE enrichment, together with the expected high values of Hf and Th. The REE pattern is similar to that determined by Bea (1996), but with greater light REE, such as Ce (Fig. 2e). The Zrn shows two peaks at Tm and Lu, which are notable. Very similar peaks at Tm and Lu are determined in xenotime from electron microprobe analyses by Gonza´lez del Ta´nago (1997) (Fig. 2e). (Xenotime is isostructural with zircon and appears to form a solid solution series with this mineral). in whole-rock compositions It has already been mentioned that it is very difŽ cult to quantify accurately the accessory mineral contribution to the whole-rock REE budget. Relevant qualitative observations can be made however, using approximate modal proportions and individual analyses of the accessory minerals (Table 1). Although it is feasible to estimate the approximate modal content of an accessory mineral in a single thin-section (e.g. 0.5% Aln in sample CAS-59, Table 1), it is not usually possible to infer its accurate whole-rock abundance due to the very inhomogeneous distribution. Representative whole-rock REE patterns for the granitic rocks of Sierra de Chepes are shown in Fig. 3 (see individual analyses in Table 3). For the purpose of this study we have selected: (1) granodiorite samples enriched in accessory minerals (AMBI-28, GCH unit and CAS-59, GPCH unit, see modal percentage Table 1); (2) a granodiorite with normal accessory minerals content (RLC-236, GCH unit); and (3) a monzogranite (RLC-241) and an aplite (PUL245). The high silica samples RLC-241 and PUL245 have Zrn as their sole accessory mineral. Granodiorite samples with greater contents of accessories are clearly enriched in light and middle REE in comparison with granodiorites with a normal accessory minerals content, i.e. monzogranite and aplite (Fig. 3, Table 3). Both enriched granodiorites contain Ep and Ttn as well as Ap and Zrn. Sample CAS-59 contains abundant Aln (0.5%), while sample AMBI-28 has abundant Ttn (3%). Figure 3 shows that the REE pattern for AMBI-28 (SiO2 = 60.34%) is very similar to that

J. A. DAHLQUIST

1000

Rock/Chondrite s

Accessory minerals-rich granodiorites, AMBI-28 and CAS-59 (modal proportions in Table 1) Lower total REE in RLC-236 and RLC-241, compared to the AMBI-28 and CAS-59

100

10 RLC-236 (GCH) 66.93% SiO2 RLC-241 (GA) 75.02% SiO2

AMBI-28 (GCH) 60.34% SiO 2

PUL-245 (APL) 75.65% SiO2

CAS-59 (GPCH) 66.7% SiO 2

1

Ce La

Nd Pr

Sm

Pm

Gd

Eu

Dy Tb

Er Ho

Yb Tm

Y Lu

Hf Th

Zr

FIG. 3. Whole-roc k trace element chondrite-normalize d patterns. Normalized values for REE after Nakamura (1974 ) and for Y, Th, Hf and Zr after Thompson (1982 ).

of CAS-59 (SiO 2 = 66. 70%). Despite the remarkable difference in the silica content between these rocks, the REE pattern is similar due to the high and comparable content of REErich accessory minerals. Thus the abundance of accessory minerals is a key factor controlling the whole-rock REE pattern, and granitic rocks with different major element composition can achieve similar REE patterns (Fig. 3). Granodiorites with normal accessory minerals content (e.g. RLC236) contain lower total REE, compared to the accessor y- rich gr ano dior iti c equ ivalent s, displaying a REE pattern similar to, or slightly greater than that of the monzogranite (Fig. 3). There is little difference between the heavy REE patterns of the aplites and monzogranites, and those of the granodiorites enriched in accessory minerals, indicating very little fractionation of the heavy REE during magma differentiation. This suggests that though the heavy REE are preferentially partitioned in the Zrn, the replacement is limited and was not efŽ cient enough to deplete the melt in Yb or Lu. The aplites are, however, very depleted in light REE and also in the middle REE, suggesting effective segregation of accessory minerals such as Aln and Ttn (Fig. 3). The granodiorites with large accessories contents are also enriched in Th, Zr and Hf. The Zr and Hf enrichment in AMBI-28 is clearly related to the abundance of Zrn in the rock. By contrast, sample CAS-59 is the richest in

Th, which might be explained by the abundance of Aln in this rock. (Table 1). Thus, the wholerock compositional behaviour of elements like the REE, Th, Zr and Hf matches the type and abundance of accessory minerals. Discussion and conclusions

472

The integral study of the accessory mineral chemistry, whole-rock geochemistry and textural analysis in epidote-bearing metaluminous granitoids suggests the geochemistry of trace elements such as the REE, Y, Th, Zr and Hf are entirely controlled by the presence/absence and modal abundance of the accessory minerals that concentrate them. The amount of silica and any other geochemical parameter indicative of fractionation progress in the dominant granodiorite-tonalite facies do not correlate with observed variations in the REE patterns. The well developed chemical zonation in Ttn indicates that the REE content decreases in the melt during the crystallization of the accessory minerals, as has been reported in other studies (e.g. Gromet and Silver, 1983). As a result of this, effective fractionation of Aln and Ttn (in addition to other accessory minerals, e.g. Ep, Ap and Zrn) should lead to a lack of REE in highly differentiated melts. Textural and chemical characteristics of euhedral Ep, which has Ps and REE contents similar to those of the Ep considered primary by Tulloch

REE FRACTIONATION IN METALUMINOUS GRANITOIDS

(1986), Dawes and Evans (1991) and Keane and Morrison (1997), suggest a magmatic origin for this mineral. In that case it should play an important role in the fractionation of the light REE and, to a lesser extent, middle REE. A magmatic origin for the Ep would not imply deep magma emplacement, as indicated by the geobarometric calculations in igneous and metamorphic rocks. This seems to be the case for most igneous rocks generated during the Famatinian orogeny, which were emplaced at shallow depths (Murra and Baldo, 2000). When many accessory mineral phases are involved, as in the case of the granodioritic-tonalitic rocks of the Sierra de Chepes, the differentiated melts (monzogranites, aplites and/or pegmatites) are REE-poor. The accessory mineral assemblage works as a Ž lter that retains most of the REE. If effectively retained, the resultant differentiated melts are unable to generate REE-related mineralizations like those associated with some granitic pegmatites (e.g. pegmatites of La Cabrera, Central Iberian Massif, Gonza´lez del Ta´nago, 1997). Thus, the presence/absence of accessory minerals in the rocks of intermediate composition can be indicative of the probable generation of differentiated melt rich or poor in REE and other trace elements. This may have a potential economic signiŽ cance, as it may allow the prediction of the probable geochemistry of the differentiated melts from the textural analysis of the ‘regional’ granitic rock. The widespread barren metaluminous magmatism developed during the Lower Ordovician in the proto-Andean margin of Gondwana shows completely different isotopic signatures compared to that of the coastal Andean batholiths (Rapela, 2000), the latter associated with important economic deposits. The Ordovician granites were derived from melting of an old lithospheric middle or lower crust (1700 Ma) and subcontinental mantle, while the Mesozoic Andean metaluminous granites derive mainly from at least two stages of partial melting of an asthenospheric, incompatible element-depleted mantle (Pankhurst et al., 1998; Rapela, 2000 ). The widespread occurrence of primary Ep is a remarkable characteristic of the Ordovician granites (Saavedra et al., 1987) that is not observed in the Andean granites with otherwise very similar major and trace element abundances. Mineralization associated with the Andean granites is probably related to the composition

of the source, which in turn determined the low abundance of accessory minerals controlling the REE and other trace element enrichment in the residual magmas. Despite the lack of isotopic and/or geochemical data, many sectors of the extensive early Palaeozoic granitic belts of the southern Andes can therefore be assigned provisionally to the Famatinian orogeny based only on the characteristic assemblage of accessory minerals obtained from detailed petrography. This is therefore, a useful diagnostic tool that might precede the application of more sophisticated techniques. Acknowledgements Funds were provided by U.N.L.P. grant, Exp. N8 100-49.781/99 and CONICET grant PIP 4148. The author is grateful to C. Casquet, J. Gonza´lez del Ta´nago and A. Ferna´ndez, for their help with the electron microprobe analyses performed at the L ui s B ru´ El ect ron M i croscopy C ent re, Complutense University, Madrid, Spain. Thanks are also due to C.W. Rapela (especially), R.J. Pankhurst, J. Saavedra and E. Baldo for their constructive reviews of the manuscript. H. Stosch made numerous helpful comments and suggestions on an early draft of this paper. Technical support by CRILAR in the preparation of the Ž nal version of the manuscript is acknowledged. References Anderson, J.L. and Smith, D.R. (1995 ) The effects of temperatur e and f O 2 on the Al-in hornblend e barometer. Amer. Mineral., 80, 549 59. Bea, F. (1996) Residence of REE, Y, Th and U in granites and crustal protoliths ; implications for the chemistry of crustal melts. J. Petrol. , 37, 521 52. Blundy, J. D. and Holland, T.J. B. (1990 ) Calcic amphibole equilibri a and a new amphibole-plagioclase geothermometer. Contrib. Mineral. Petrol., 104, 208 24. Dahlquist, J.A. (1999) SigniŽ cado petrogene´tico del epidoto magma´tico en las granodiorita s famatiniana s de la Sierra de Chepes, Argentina. Bol. Soc. Espan~ ola Mineral., 22a, 33 4. Dahlquist, J. A. (2000 ) Geologõ´ a, petrologõ´ a y geoquõ´mica de las rocas de la Sierra de Chepes, La Rioja, Argentina. PhD thesis, Univ. Co´rdoba, Spain. Dahlquist, J.A. and Baldo, G.E. (1996 ) MetamorŽ smo y deformacio´n Famatinianos en la Sierra de Chepes, La Rioja, Argentina. XIII Congreso Geolo´gico

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Tulloch, A.J. (1986) Comment on ‘‘Implication s of magmatic epidote-bearin g plutons on crustal evolution in the accreted terranes of north-wester n North America’’ and ‘‘Magmatic epidote and its petrologi c signiŽ cance’’. Geology, 14, 186 7. Vhynal, C.R., McSween, H.Y. Jr. and Speer, J.A. (1991 ) Hornblend e chemistry in souther n Appalachian granitoids : implication s for aluminum hornblend e thermobarometr y and magmatic epidote stability. Amer. Mineral., 76, 176 88. [Manuscript received 21 January 2001: revised 30 April 2001]

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