Himalayan Geology, Vol. 24 (2), 2003, pp. 63-76, Printed in India
Geochemistry, Petrogenesis and Tectonic setting of Akpa-Rakcham Granites ofSutlej Valley, Himachal Pradesh, India
R. ISLAM & N. S. GURURAJAN Wadia Institute of Himalayan Geology, Dehra Dun-248 001, India
Integrated field, petrography and geochemical studies have been carried out on the Early Paleozoic (500 Ma) Akpa-Rakcham granite of Sutlej Valley. This pluton is also intruded by fine grained Tertiary leucogranites, that occur as dykes and small bodies with intricate network of pegmatite. The mineralogical composition and whole-rock chemistry of the Early Paleozoic granites exhibit minor variation from contact to most evolved core sample. These granites are enriched in SiO2, with high A/CNK value, and the presence of normative corundum suggests that they are crustally derived peraluminous S- type granites. They have higher Fe2O3 (t), MgO, CaO, TiO2, Sr, Ba, Zr, Ni, Y, Th and total REE contents and lower in Na2O, K2O, P2O5, Rb contents and Rb/Sr ratios than Tertiry leucogranites. Marked inverse relationship with Fe, Ca, Mg, Ti, Sr, Ba, Zr and Ni is interpreted in terms of crystal fractionation. Occurrence of large numbers of plutons around 500+ Ma ages with identical field setting, petrography, geochemistry and geochronology in various sectors of the Himalaya suggests that there was a widespread tectonothermal event during the Early Paleozoic time. An orogenic event may be responsible for generation of the Early Paleozoic magma in the Himalaya. Coupled with Early Paleozoic magmatic event and sedimentological data from the Late Proterozoic to Early Paleozoic sequence of the Himalaya it points towards a late Pan- African Orogenic event close to the Cambro- Ordovician boundary.
The Tertiary leucogranites are peraluminus and alkali rich and classified as muscovite-biotite and tourmaline- muscovite varieties. These rocks are enriched in SiO2. K2O, Na2O, P2O5, Rb content and Rb/Sr ratios and are depleted in Sr, Ba, Zr and ∑REE content compared to Early Paleozoic granites. Lower content of Sr and Ba and high Rb/ Sr ratio in leucogranite is well explained by fractional crystallization processes. Decrease in phyllosilicate minerals with decreasing Ca as well as depletion of Ti and Fe in granites leading to the biotite fractionation and depletion of Sr is in accordance with plagioclase fractionation. Low ∑REE, moderate LREE/HREE ratios and more pronounced negative Eu anomalies are also in accord with fractional crystallization processes. The field setting indicates that the pelitic gneisses of the Vaikrita Crystallines occurring below the Haimanta must have been a source component of melting to produce the leucocratic melt during anatexis. In addition, the heat refraction effect at the sloping interface between the Haimantas and the Vaikrita Crystallines further enhanced the melting of older granitoids and the associated metamorphic rocks to produce leucogranitic magma.
