Anatectic Leucogranites of Molinos Range, Easthern Cordillera, NW Argentina: some issues concerning Nd-isotopic system

SOLA, A, BECCHIO, R. A, PIMENTEL, M. M

Brasilia

Simposio; VII SSAGI South American Symposium on Isotope Geology; 2010

Universidad de Brasília

INTRODUCTION Two compositional groups were distinguished within migmatites leucosomes and leucogranite from Molinos igneous/metamorphic complex. The first one is syenogranitic (K-rich, high K/Na ratio) and the second one displays a trondhjemitic trend (Na-rich and low K/Na ratio). A detailed geochemical and isotopic study for these rocks reveals heterogeneities among the same rock type, even within the same pluton, the Pumayaco leucogranite. These heterogeneities may have potential origin in the nature of the melt-producing reactions and the stability of accessory minerals during the anatexis. The role of crustal anatexis at various crustal levels in generating geochemical and isotopic heterogeneities is poorly understood. Thus, characterization of the geochemical and isotopic nature of anatectic melts in high grade metamorphic terrains would help to better understand the sources of such heterogeneity and their potential magnitude Zeng et al. (2005c). GEOLOGICAL FRAMEWORK Migmatites and leucogranites are widely distributed along the transition of Eastern Cordillera and the Northern Sierras Pampeanas, NW Argentina cropping out as high grade metamorphic complexes. Specifically in the Molinos range, basement units of low to high metamorphic grade (Puncoviscana metaturbidites and La Paya Formations) are intimately related with leucogranite plutons. The granitoids belong to Cachi Formation with ages ranging between 460-480 Ma which overlap with the age of migmatization (Sola et al, 2010). In this area, can be observed the relationship between different components of an anatectic system developed in upper amphibolite facies conditions (HTLP) allowing the direct observations on the origin of leucosomes and leucogranites by fusion of paragneisses. The granites are intimately linked with migmatitic rocks (metatexites and diatexites) and characterized by the presence of accessory minerals such as garnet, cordierite, sillimanite and tourmaline. All transitions were found in the field between granite veins in migmatite complexes to thick sheets and plutons. The plutonic units are: Pumayaco leucogranite, La Angostura granite and the Molinos trondjhemite (Sola et al., 2010). All belong broadly to the leucogranitic suite ranging from granodiorite to syenogranite and trondhjemites. The rocks display magmatic-flow structures, defined by the preferred orientation of biotite, suggesting syn-tectonic emplacement. The field relationships point to low rheological contrast between the granitic liquids and the host metamorphic rocks. The La Angostura pluton is a biotite granite located at the eastern flank of the range, at the margin of the Calchaquí river. The Molinos trondjhemite is a small elliptical body consisting mostly of plagioclase and quartz. The Pumayaco leucogranite is the main pluton, exposed along the center of the range; the width of the pluton seems to be duplicated by folding with the axial plane oriented approximately parallel to the N-S main foliation. Several facies can be recognized based on the occurrence of accessory minerals as garnet, cordierite, tourmaline and sillimanite (see Sola et al., 2010). Trondhjemites have particular interest since they have controversial origin. Previous work suggested depleted mantle rocks and lower crust rocks as possible sources with no conclusive results or related with a TTG (trondjhemite-tonalite-granodiorite) magmatism extending along the Sierras Pampeanas. The conditions required to form trondjhemitic melts from depleted grabbroid source (5-10% batch melting) are estimated at 10-12 Kbar/900ºC. An alternative to these proposals is to explain its genesis by H2O-fluxed melting at conditions equivalent to that expected for the leucogranites and migmatites (-5 Kbar/700-750ºC) see Sola and Becchio 2010. GEOCHEMISTRY The leucogranites similarly to the leucosomes, have a narrow range of silica (72-74%), are slightly peraluminous (ASI≥ 1.1) and belong to the calc-alkaline series. The K/Na ratio vary broadly so the CaO values. The migmatite leucosomes and leucogranites may be divided into two groups: those with trondhjemitic composition trend and those with syeno-granitic composition. The first group can be distinguished from the second one chiefly by the higher Na2O and CaO and lower K2O contents. DISCUSSION Isotopic data were acquired including metasediments, granitoids and migmatites from Molinos area. The initial isotopic ratios were calculated for 470 Ma considered as the age of migmatization. The samples including metasediments of different grades, migmatites and granitoids are all characterized by continental crust isotopic signature. The low grade metasediments exhibit narrow εNdt (-6.7; -7.2) variations, similarly to migmatic gneisses and migmatites. The leucogranites show more varied εNdt values, from -7.81 going up to -3.02, when compared with their metasedimentary protoliths. Leucogranite samples with trondhjemitic tendency display higher εNdt values in positive correlation with the Na/K ratios (Fig. 3A). The heterogeneity of the 143Nd/144Nd ratios among samples of granitoids, could be related to Nd-isotope disequilibrium during crustal anatexis as mentioned by Harris (1996) and more recently by Zeng et al. (2005c). The possibility that granitic melt do not inherit the 143Nd/144Nd signature of its protolith as a result of accessory-phase dissolution has been recognized by various authors (see Ayres and Harris, 1997 and references therein). This phenomenon has been observed specially in those anatectic granites where the low temperature and the rapid melt extraction avoided isotopic equilibration with the restite component. For this reason careful should be taken interpreting the geologic meaning of Nd model ages of anatectic granites. Model age calculation generally assumes that no Sm/Nd fractionation occurs during melt formation, and that the melt reach the equilibrium with its protolith. However, it is well established that Sm/Nd ratios are affected by crustal anatexis or by late-stage fractional crystallization where dissolution of accessory phases such as monazite and apatite may strongly influence the REE chemistry of the melt (Pimentel and Charnley, 1991; Ayres and Harris, 1997 and references therein). Because Sm/Nd fractionation renders model Nd ages of high silica melts virtually meaningless, such calculations are better applied to the source rocks from which the granites are derived than to the granites themselves (see also Zeng et al., 2005c; Fig. 3B). Unlike the results obtained by Zeng (2005c) our samples do not present a clear connection between the P2O3 content (the relative apatite content) and the increasing of Sm/Nd ratios and εNdt values. Nevertheless, the preliminary studies carried out at the time and the complexity of the mechanisms involved in anatexis does not permit us to arrive to a simple explanation of this behavior. CONCLUSIONS The isotopic data is consistent with all these rocks being genetically related, with the granitoids, leucogranites and trondhjemites, having been generated by anatexis of its high-grade host rocks and not as a result of a mixed source or mantle component. The various degrees of parent/daughter fractionations in the Sm?Nd isotopic systems could be consequence of several factors such as the non-modal nature of partial melting reactions, nature of source or protolith, H2O activity and the stability and equilibrium of the trace elements-bearing minerals entering the melt. During crustal anatexis these factors operating simultaneously would render melt products with distinct isotopic signatures, which could profoundly influence the products of subsequent mixing events. This is not only important for geochemical patterns of intracrustal differentiation, but also potentially important processes in generating crustal-scale as well as individual pluton-scale isotopic heterogeneities Zeng (2005c). Consequently, disequilibrium processes, as the mentioned, must be considered at the moment of interpreting the geological meaning of isotopic data of Famatinian granitic magmatism. REFERENCES Ayres, M., Harris, N., 1997. 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