PETROGENESIS OF THE BANDED GNEISSIC COMPLEX AROUND MASUDA, RAJASTHAN : IMPLICATIONS FOR THE PRECAMBRIAN CRUSTAL EVOLUTION OF THE ARAVALLI CRATON, NW INDIA

Abstract

The Precambrian Banded Gneissic Complex in the northern part of Aravalli Craton, Rajasthan, is lithologically heterogeneous. Both meta-igneous and meta-sedimentary lithologies constitute the Banded Gneissic Complex. The meta-igneous rocks are classified as amphibolites, diorite-monzodiorite-quartz diorite gneisses and granodiorite gneisses with small volumes of granophyres and aplites. The meta-sedimentary components are defined by cherts, calc-silicate rocks, garnet-biotite-schist and migmatized mica schist. Mineral chemistry data on amphibolites indicate that the rocks of Banded Gneissic Complex had attained amphibolite grade of metamorphism. The rocks had been subjected to four phases of folding and two episodes of shearing. They are inter banded and co-folded in all scales. Contact relationships between the different litho-units appear to be tectonic. The amphibolites are silica undersaturated Fe-enriched tholeiites and they have higher K2O and lower Ni than Archaean tholeiites. They exhibit a flat to slightly light REE enriched chondrite normalized REE patterns. The REE and other less mobile trace element characteristics of the amphibolites are very much comparable to the marginal sea basalts. Quantitative geochemical modelling suggests that they are derived by 5-20% partial melting of flat to slightly light REE depleted mantle sources, but had higher [Fe]/(Mg] ratio than garnet Iherzolite. The diorite-monzodiorite-quartz diorite rocks occur both as volcanic flows and plutonic bodies in the field. On the basis of geochemistry, they are classified as sanukitoids. Further, on the basis ofTi02/MgO ratio they are classified as low-Ti and high-Ti sanukitoids. The sanukitoids have light REE and large ion lithophile elements enriched characteristics. The low-Ti sanukitoids have higher mg#, lower Ba, Sr, Zr, Nband light REE abundances than the high-Ti sanukitoids. On the basis of geochemical modelling, it is suggested that magmas represented by the sanukitoids could not have derived by partial melting of tholeiitic amphibolites occurring in the Masuda area. Further, neither fractional crystallization nor assimilation cum fractional fu crystallization of tholeiitic parental magmas could produce the observed elemental abundances in the sanukitoids. [MgHFe] modelling suggests that magmas represented by two sanukitoid suites could have been derived from mantle sources with high but variable [Fe]/[Mg] ratios and at different pressure and temperature conditions. High-Ti sanukitoids could not be related to low-Tl sanukitoids either by fractional crystallization or by assimilation cum fractional crystallization. It is suggested that the sanukitoid magmas were derived from enriched sources. Such enrichment could haveoccurred by addition offluids or melts derived from deeper sources prior to melting. The sources for the high-Ti sanukitoid magmas were relatively more enriched in LILEs, LREEs and Nb compared tothe sources for the low-Ti sanukitoid magma. The granodiorite suite of rocks form chemical continuum with sanukitoids. On the basis of REE and other trace element abundances, they are classified into two different suites. The suite-l has higher LREE, HREE and large negative Eu anomalies compared to the second suite. The granodiorite suite-l might have been derived by partial melting of a source similar to the sanukitoids. The parental magma thus derived could have undergone variable extents of fractional crystallization of plagioclase, alkali-feldspar and allanite which gave rise to residual magmas representing samples ofthis suite. The suite-ll of granodiorites have chondrite normalized REE patterns sub-parallel to that of the low-Ti sanukitoids, but have lower REEs and large ion lithophile elements compared to the latter. Furthermore, granodiorites and sanukitoids have overlapping biotite compositions. The magmas represented by the granodiorite suite-ll might have fractionated from parental sanukitoid magmas by processes including magma unmixing. Granophyres and aplite veins, occupy a small volume proportion of the study area. The foliated granophyres could have crystallized from residual fluids formed during last stages of crystallization of sanukiotids and granodiorite magmas. The post-tectonic aplite veins could have crystallized from hydrothermal fluids formed during high grade metamorphism. Petrogenetic considerations of sanukitoids and granodiorite suites indicate that they were emplaced in an arc related tectonic setting. Whereas, the amphibolites are suggested to represent magmas emplaced in marginal basin environment. These geochemically diverse suite of rocks with distinct petrogenetic histories which were emplaced in different tectonic settings are closely associated in space and make up the Banded Gneissic Complex in the study area. They must have been brought together by accretionary processes. Subsequently, the amphibolites and granodiorite gneisses were co-folded and sheared to give rise to present - day character of the Banded Gneissic Complex in the northern Aravalli Craton.