Alkali Exchange as a Possible Mechanism for Genesis of Low-K Granite: Evidence from Ajitgarh Pluton, Proterozoic Delhi Fold Belt, NW India

We discuss a geochemically atypical, leucocratic low-K granite that occurs as a volumetrically subordinate component of the Ajitgarh pluton (NW India), which is predominantly composed of a 'normal' K-rich alkali granite. The pluton has been described to intrude the metasedimentary rocks of the NE-trending Meso- Neoproterozoic Delhi Supergroup in its northern domain. The alkali granite mineralogy includes K-feldspar, quartz and sodic plagiodase with minor biotite and hornblende while low-K granite comprises of equivalent proportions of sodic plagioclase and quartz, minor K-feldspar and accessory hornblende, titanite and zircon. In contrast to other syn-tectonically ernplaced granitoids of the,North Delhi Fold Belt (NDFB) that are variably deformed, the Ajitgarh granites are undeformed and do not show any evidence of metamorphism andfor post-crystallization alteration. Apart from the significantly low K₂O content (∼1.5%) and a high Na₂O/K₂O ratio (-3), other geochemical parameters such as high silica content (∼75%), high FeO/MgO ratio, enrichment in LREE and HFSE, and lower abundances of CaO (0.54 - 1.51 %), MgO (0.01 - 0.54%) and Sr (21 - 56 ppm) of the low-K granite are consistent with A-type affinity. A close similarity in HFSE behaviour and REE patterns between low-K granite and alkali granite underlines a genetic link between the two granite types and their derivation from a common igneous protolith. It is suggested that both low-K granite and alkali granite were derived from a high-temperature melt, produced by low degree of partial melting of an amphibolite source under low-pressure conditions. Alkali exchange across the interface between granitic melt and basic magma effected the removal of K from the granitic melt, which subsequently crystallized as low-K granite.