Mineral chemistry of zeolites from the Deccan basalts
The 500,000 km2 of Deccan basalts are situated in Maharastra, India. The basalt flows are predominantly tholeiitic and were penetrated by post-depositional fluids from which zeolites, and associated secondary minerals, crystallised. The frequency of occurrence and distribution of the zeolite species in an area 15,000 km2 has been determined and does not support the zonal scheme proposed by earlier workers. The zeolites occur in vesicles and veins which are often lined by celadonite. Heulandite, stilbite, mordenite and apophyllite are the most common secondary minerals with other species occurring at a frequency of less than one identification per locality studied. Evidence is discussed which shows that the observed zeolite distribution was produced by temperatures of 70°C at the top and 110°C at the base of the basalt pile. This temperature range is low in comparison with other zeolitised basalt domains and zeolite zones which are characteristic of similar regions elsewhere have not developed. Chemical analysis of the secondary minerals for major and trace elements indicates considerable variation in the principle cations within a species, and a marked absence of trace constituents, especially the rare earth elements. The trace element compositions of the secondary minerals cannot be used as petrogenetic indicators. The sequences of zeolite crystallisation have been determined and these together with the stability relationships are examined by thermodynamic analysis. Chemical and oxygen isotopic compositions of the basalt indicate that interaction occurred between the zeolitising fluid and the basalt and that this interaction was more intensive along fluid pathways e.g. at flow boundaries. Major elements, Ba, Rb and Sr were leached from the basalt in contrast to other trace elements including the rare earth elements. The leached elements were incorporated by the secondary minerals to varying degrees. Zeolitisation may therefore have considerable implications in petrogenetic studies of basaltic rocks.