Geochemistry and Petrogenesis of a Supracrustal Granite from Dalhousie, Himachal Himlaya

A two mica granite body occurs in and around Dalhousie (Himachal Himalaya) as an intrusion into the core of an antiform at the southern flank of the Dhauladhar range. The rocks near the contact are enriched in biotite (bio>mus), whereas those of the core are more enriched in muscovite (mus>bio). Otherwise the major mineralogical make-up and their whole rock 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 mean felsic S-type trend.

The low normative as well as calculated modal quartz and high Na_zO content suggest their origin from a less mature sedimentary source. The ubiquitous Eu anomaly, depleted Sr with enriched Rb and non-involvement of plagioclase or alkali feldspar in the fractionation model further implies that there was abundant feldspar in the 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 consisting of biotite and muscovite with minor amounts of feldspar are required to account for the major elemental mass balance requirement between the compositional differences. Whereas the depletion of REEs as well as other trace element abundances in the more . fractionated samples 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 similar way involving such REE sink accessory phases. The haplogranitic phase relations suggest that the emplacement of the granitic magma took place under water-sufficient condition at a shallower depth (-2-3kb.) whereas magma generated at relatively deeper level but restricted to a depth where plagioclase is stable rather than garnet. The 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 higher population of Eu³⁺ as compared to Eu²⁺ that might have behaved much similar to other adjacent MREEs (Sm and Gd) which occurs in 3+ state.