Granitoid Rocks of Wangtu Gneissic Complex, Himachal Pradesh: An Example of In situ Fractional Crystallisation and Volatile Action

The Wangtu Gneissic Complex (WGC) of Himachal Himalaya represents the basement rocks which remained more or less in its original realm. The unique feature of this complex is that the various phases of deformed and undeformed granitoids are exposed in the area. The complex has a peripheral zone of augen gneisses and core of non-foliated granitoids comprising of Coarse Porphyritic Granites (CPG) and Fine Grained Granites (FGG).

In the field the FGG rocks have sharp intrusive contacts with CPG rocks. At places they are entrapped or have protrusions into CPG rocks. Compositionally they represent syenogranites. They are peraluminous to meta-aluminous with Sr initial ratio of 0.705. The major and trace elements of these rocks show unimodal distribution pattern on different variation diagrams. They along with rare earth elements show marked fractionation trends, with a significant negative Eu anomaly. The rocks more or less define an isochron, representing Rb-Sr age of 1895±64 Ma.

The petrochemical studies favour a fractionated rather than multiple intrusions of magma. The studies suggest that fractional crystallisation of parental magma gave rise to early formed CPG rocks and the residual melts resulted in the crystallisation of FGG rocks. The last formed granitoids, the FGG rocks are interpreted as the final residue which crystallised rapidly after build-up and loss of fluids and volatiles which migrated to higher levels to form pegmatite phase in the aureol zone. The Rb-Sr systematics in the present case indicate that the granite have primary crystallisation melts derived from the deep crustal levels around 1895±64 Ma.