Petrology, geochemistry and U-Pb zircon geochronology of alkali granites of Jhalda, eastern India and their possible linkage to Rodinia Supercontinent

The present study deals with the petrogenesis and age implication of anorogenic peralkaline granitoid rocks exposed along the North Puruliya Shear Zone (NPSZ) in Jhalda area of Puruliya district, West Bengal. Alkali granite consists of quartz, alkali feldspar, aegirine, riebeckite, arfvedsonite and biotite. These granitoid rocks have a high range of silica, very high total alkali content and are poor in CaO, Al2O3, FeO and MgO content. Geochemically, they are ferroan, alkalic, reduced and peralkaline granitoid rocks and have many similarities with A-type granites. Crystallisation temperatures of these granitoid rocks are greater than 900 degrees C. U-Pb isotopic ages of zircon indicate a major age cluster similar to 966.7 +/- 7.0 Ma. The oldest lower crustal rocks in and around Jhalda area are charnockite, khondalite, garnetiferous granite gneiss, which might have acted as source rocks. Trace element model indicates that a moderate degree partial melting (5-20%) of charnockite + khondalite source rock followed by similar to 30% fractional crystallisation of plagioclase feldspar is responsible to generate parent magma of alkali granite. Similar and overlapping crystallisation ages of 966.7 +/- 7.0 Ma of the per-alkaline anorogenic/post-orogenic granites of present study with already reported orogenic I-type granites from Jhalda and S-type granites from nearby Raghunathpur area (age 1000 Ma) may indicate origin and emplacement of post-orogenic granites of Jhalda during orogeny-anorogeny transition at the time of waning stage of orogenic activity. Mantle upwelling in late to post-orogenic stage provides additional heat to initiate partial melting of lower crustal source rocks.