GEOCHEMISTRY AND PETROGENESIS OF A SUPRACRUSTAL GRANITE FROM DALHOUSIE, HIMACHAL HIMALAYA

A two mica granite body occurs in and around Dalhousie (Himachal Himal aya) as an intrusion into the core of an antiform at the southern flan k of the Dhauladhar range. The rocks near the contact are enriched in biotite (bio>mus), whereas those of the core are more enriched in musc ovite (mus>bio). Otherwise the major mineralogical make-up and their w hole lock chemistry are monotonous. The rocks are weakly to moderately peraluminous in nature. The trace elemental characteristics are very similar to the average upper crust and follow more closely with the me an felsic S-type trend. The low normative as well as calculated modal quartz and high Na2O content suggest their origin from a less mature s edimentary source. The ubiquitous Eu anomaly, depleted Sr with enriche d Rb and non-involvement of plagioclase or alkali feldspar in the frac tionation model further implies that there was abundant feldspar in th e source and more calcic plagioclase were left out in the residue that selectively retained Eu and Si and released Rb in its early fraction of melts. It was noted that only -10% fractionation of solids mainly c onsisting of biotite and muscovite with minor amounts of feldspar are required to account for the major elemental mass balance requirement b etween the compositional differences. Whereas the depletion of REEs as well as other trace element abundances in the more fractionated sampl es from the central part of the intrusion were modeled and attributed due to fractionation of minor quantity of REE enriched phases (-1.5%) like monazite, apatite and zircon. However, the Zr requirement in this model is insufficient. Nevertheless, the observed nearly three times depletion in REEs, Zr, Th and P may be required to be explained in a s imilar way involving such REE sink accessory phases. The haplogranitic phase relations suggest that the emplacement of the granitic magma to ok place under water-sufficient condition at a shallower depth (simila r to 2-3kb.) whereas magma generated at relatively deeper level but re stricted to a depth where plagioclase is stable rather than garnet. Th e water requirement was homogeneously made available internally by the breakdown of hydrous minerals like micas and a(H2O) was sufficiently high. This also led to high oxidizing condition of the magma causing h igher population of Eu3+ as compared to Eu2+ that might have behaved m uch similar to other adjacent MREEs (Sm and Gd) which occurs in 3+ sta te.