Mineralogy and geochemistry of fenitized nephelinites of the Amba Dongar complex, Gujarat

Abstract

In the Amba Dongar sub-volcanic complex, nephelinite plugs and dikes of phonolites were emplaced before the carbonatites. The fenitizing fluids released from carbonatite magma caused extensive fenitization in nephelinites with intensity varying from mild carbonatization (CO₂ metasomatism accompanied by hematitization) in the distal outcrops, to a high degree of K-fenitization (feldspathization, phlogopitization) in outcrops closer to the carbonatite bodies. The intensely fenitized rocks are the xenoliths of nephelinites within the sövite. The primary texture of the nephelinites is mostly preserved, but the highly feldspathised nephelinites look more like nepheline syenite with water clear poikilitic K-feldspar in the groundmass. Invariably, nepheline is largely replaced by hydromuscovite and to lesser extent by zeolite (mainly natrolite and phillipsite). Cancrinite is also noticed in some thin sections. Pyroxene has been partially or totally replaced by phlogopite while the groundmass has been feldspathised with formation of poikilitic Kfeldspar (Or>90%). On the basis of mineralogical study, back scattered electron images and elemental mapping on microprobe, three processes of fenitization, namely, K-metasomatism (formation of hydromuscovite, phlogopite and Kfeldspar), zeolitization and CO2-metasomatism along with hematitization are identified. K-feldspar in the norms of fenitized nephelinite varies from 9% to 36%. Fenitized nephelinites show enrichment in trace and rare earth elements over the unfenitized nephelinites.