Mineralogical variations in the Delakhari sill, Deccan trap intrusion, central India

Abstract

The Delakhari sill (maximum thickness cf. 200 m) is the most extensive Deccan Trap instrusion which occurs in central India, between longitutdes 78~ to 78~ and latitudes 22~ ' and 22~ ". Based on petrographic examination, the sill is divided, from bottom to top, into (1) the Lower Chilled Zone (LCZ), up to 8 m thick, marked by abundant interstitial glass and an overall fine grain size, (2) the Olivine-Rich Zone (ORZ), 27 m thick, enriched in olivine (relative to the other zones in the sill), (3) the Central Zone (CZ), 70 m thick, marked by depletion in olivine and overall coarse grain size, (4) the Upper Zone (UZ), 55 m thick, marked by the presence of two chemically and morphologically distinct olivine types and abundant interstitial grano- phyre, and (5) the Upper Chilled Zone (UCZ), 10- 25 m thick, marked by abundant interstitial glass. Compositions of the pyroxenes and olivines show an overall increase in Fe/Mg with crystallization, but extensive interzonal and intrazonal variations and overlaps exist. Olivine ranges from Fa24 (ORZ) to Fa95 (UZ). In the UZ and inner UCZ, an equant (Fa44- 50, called type-A olivine) and interstitial skele- tal olivine (Fa70-95, called type-B olivine) occur to- gether. Compositions of the Ca-rich and Ca-poor pyr- oxenes fall in the range Wo38En34Fsz8 to Wo33EnsFs59 and Wo14En4~Fs45 to Wo16Ent9Fs65, respectively. Overall, the two pyroxene trends con- verge with Fe-enrichment except for one anomalous sample from the UZ which contains a Ca-rich (Wo34EnsFs58) and a Ca-poor (WoloEnlsFs72) pyr- oxene well within the 'Forbidden Zone' of Smith (1972). Compositions of coexisting oxide minerals indi- cate that the sill crystallized at oxygen fugacities from 10 -l~ atm (ORZ) to 10 -13 (UZ). The magma prior to intrusion appears to have been derived from a