First Petrogenetical Conclusions on Megacrystic S-Type Granitoids from the Eastern Ghats Belt (India)

O. Krause Mineralogisch-Petrologisches Inst. der Universität, Poppelsdorfer Schloß, 53115 Bonn, Germany,

and Max-Planck-Institut f. Chemie, Abt. Geochemie, Postfach 3060, 55020 Mainz, Germany


K. Mezger Max-Planck-Institut f. Chemie, Abt. Geochemie, Postfach 3060, 55020 Mainz, Germany

M. Raith Mineralogisch-Petrologisches Inst. der Universität, Poppelsdorfer Schloß, 53115 Bonn, Germany

A. Bhattacharya Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur, India

The Proterozoic Eastern Ghats Belt (EGB) is made up of an association of inter-banded sedimentary and igneous rocks, metamorphosed at extreme granulite facies conditions (900-1000°C, 8-10 kbar, peak metamorphism at 1Ga) and affected by two subsequent phases of ductile deformation (D1 and D2).

The important lithologies of the EGB comprise khondalites (garnet-quartz-perthite-sillimanite gneiss), calc-granulites (diopside-wollastonite-scapolite rocks), sapphirine-spinel granulites, two pyroxene basic granulites, enderbitic to charnockitic granulites (garnet-perthite-orthopyroxene-plagioclase gneiss), and leptynites (leucocratic quartz-feldspar-garnet gneiss).

Voluminous porphyritic granitoids intruded this association at mid-crustal levels before D2. These intrusions form a mojor magmatic component in the high grade crustal terrane of the EGB and represent one of the largest exposures of S-type granitoids in the world. These granitoids are exposed in numerous intrusive bodies predominantly along a c. 150 km wide zone extending parallel to the coastline and over almost the entire length of the EGB.

These granitoids have a uniform mineralogy including quartz, megacrystic alkalifeldspar (Or×86-92, Ab×13-8, An¾1), plagioclase (An×35-61, Ab×63-38, Or×2-1), biotite, garnet (pyrope-almandine), and hypersthene with apatite and zircon as accessories. Two major petrographic types can be distinguished: an Akf-megecryst rich and a megacryst poor variety. The contacts between the two types can be sharp to diffuse. Both types have high Al2O3 contents (13-17 wt.%; Al2O3/(Na2O+K2O+CaO)=1.4-1.8) and high K2O/Na2O ratios (2-4). This indicates strong S-type characteristics, consistent with the d18O values which all lie above 10”. The eNd values are varible and range from eNd(1Ga)×-10 to eNd(1Ga)×-5. The Akf-rich variety has eNd(1Ga) ×-9 and the Akf-poor variety has eNd(1Ga) ×-6 to -5. Lead isotope ratios of leached Akf megacrysts are uniform in 207Pb/204Pb×15.8±0.1 as well as in their 206Pb/204Pb×17.9±0.1, but they are remarkably heterogeneous in their 208Pb/204Pb ratios which range from 35 to 41.

The 207Pb/204Pb and 208Pb/204Pb ratios of leached Akf-megacrysts lie all above the Stacey&Kramers growth curves. This demands a component which has at least an early Proterozoic or even late Archean age. This is consistent with the Nd-Model ages calculated for the granitoid rocks which range from 1.8 to 2.2Ga. The strong S-type caracteristics as well as the isotope data indicate that these granitoids are dominatly derived from crustal precursors.

Based on the close spacial association it is likely that the khondalites play an important role as a source for the porphyritic granitoids. This is supported by the occurrence of sillimanite bearing relict garnet in the porphyritic granitoids which is typical for the khondalites.