R. Dhana Raju

Petrogenetic Model of A-Type Granitoids of the Kullampatti Area, Salem District, Tamil Nadu, India

Abstract The late Proterozoic granitoids at Kullampatti are of A-type and comprise three phases: (1) barren phase I (granite-adamellite), (2) fertile phase II (trondjhemite) and (3) rarely-mineralized phase III (pegmatitic granite). They were emplaced within the Archean cratonic granulite terrane of southern India, close to the Moyar-Bhavani-Attur shear zone, and form a consanguinous association with the Suriyamalai or Sankari granite-adamellite batholithic dome. On the basis of their field, tectonic, petromineragraphic and geochemical aspects, a petrogenetic model involving a two-stage melting of an undepleted middle to lower crustal granulitic protolith is envisaged. In this, the first-stage melting led to the formation of an LCT-rich hydrous melt that was removed from the site of its generation, followed by second-stage partial melting of the depleted granulite during post-orogenic or anorogenic environment, resulting in the HCD- and HFS-rich, NYF- and A-type melt that on fractional crystallization, together with hydrothermal activity, led to the formation of these granitoids.

Chemical distinction between replacement and magmatic granitic rocks

Utilising about 170 chemical analyses of granitic rocks of proved origin from different parts of the world, a new field is suggested in the Ca-Na-K (ionic weight per cent) trilinear diagram to distinguish the replacement from the magmatic type.

Feldspars from the granitic rocks of chipurupalle-razam area, srikakulam district, andhra pradesh

Potassium feldspars from the granitic rocks of Chipurupalle-Razam area of Srikakulam District, Andhra Pradesh were studied by optical, chemical and X-ray diffraction methods. Most of the potassium feldspars occur both as porphy roblasts and in the groundmass in both orthopyroxene-bearing and garnetiferous granitic rocks. Potassium feldspars of the opx bearing granitic rocks are more or less homogeneous, while those of the garnetiferous granitic rocks and pink granite are mostly perthitic. The perthites are classified as stringlets, strings, rods and beads, and it is suggested that the perthite formation was due to exsolution.Chemical analysis and X-ray study showed that some of the feldspars are ferriferous orthoclasses. Qualitative spectrographic analysis indicates that Ba, Sr, Cr, Ga, Cu, Pb and Mn are present, while other elements such as Rb, Li, V, Be, Co, Ni, Ag, Ge and Sn were not detected. X-ray diffraction studies have shown the microcline to be of intermediate type with Δ=0·59. The structural equivalents of the feldspars studied are P50–56F, SH 1070 and Spencer ‘U’ of Wright and Stewart (1968). It is claimed that ferriferous orthoclase represents high temperature and pressure form, while intermediate microcline represents low temperature and pressure form in this series. Twin law studies on plagioclase feldspars indicated a metamorphic origin for the granitic rocks in the area.Potassium feldspars from the granitic rocks of Chipurupalle-Razam area of Srikakulam District, Andhra Pradesh were studied by optical, chemical and X-ray diffraction methods. Most of the potassium feldspars occur both as porphy roblasts and in the groundmass in both orthopyroxene-bearing and garnetiferous granitic rocks. Potassium feldspars of the opx bearing granitic rocks are more or less homogeneous, while those of the garnetiferous granitic rocks and pink granite are mostly perthitic. The perthites are classified as stringlets, strings, rods and beads, and it is suggested that the perthite formation was due to exsolution.