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Mineralogy, geochemistry and petrogenesis of the Khopoli mafic intrusion, Deccan Traps, India
 
Ciro Cucciniello1, Ashwini Kumar Choudhary2, Alberto Zanetti3, Hetu C. Sheth4, Shreyas Vichare4 and Rohan Pereira4
 
1 Dipartimento di Scienze della Terra, dell’Ambiente e delle Risorse, Università degli Studi di Napoli Federico II, Largo San Marcellino 10, 80138 Napoli, Italy
2 Institute Instrumentation Center, Indian Institute of Technology Roorkee (IITR), Roorkee 247 667, India
3 CNR-Istituto di Geoscienze e Georisorse (IGG), Sezione di Pavia, Pavia 27100, Italy
4 Department of Earth Sciences, Indian Institute of Technology Bombay (IITB), Powai, Mumbai 400 076, India
 
Mineralogy and Petrology, 108, 333-351, 2014

Doi: 10.1007/s00710-013-0309-z

 Abstract

Khopoli FiguraThe Khopoli intrusion, exposed at the base of the Thakurvadi Formation of the Deccan Traps in the Western Ghats, India, is composed of olivine gabbro with 50-55% modal olivine, 20-25% plagioclase, 10-15% clinopyroxene, 5-10% low-Ca pyroxene, and <5% Fe-Ti oxides. It represents a cumulate rock from which trapped interstitial liquid was almost completely expelled. The Khopoli olivine gabbros have high MgO (23.5-26.9 wt.%), Ni (733-883 ppm) and Cr (1432-1048 ppm), and low concentrations of incompatible elements including the rare earth elements (REE). The compositions of the most primitive cumulus olivine and clinopyroxene indicate that the parental magma of the Khopoli intrusion was an evolved basaltic melt (Mg# 49-58). Calculated parental melt compositions in equilibrium with clinopyroxene are moderately enriched in the light REE and show many similarities with Deccan tholeiitic basalts of the Bushe, Khandala and Thakurvadi Formations. Nd-Sr isotopic compositions of Khopoli olivine gabbros (εNdt = –9.0 to –12.7; 87Sr/86Sr = 0.7088-0.7285) indicate crustal contamination. AFC modelling suggests that the Khopoli olivine gabbros were derived from a Thakurvadi or Khandala-like basaltic melt with variable degrees of crustal contamination. Unlike the commonly alkalic, pre- and post-volcanic intrusions known in the Deccan Traps, the Khopoli intrusion provides a window to the shallow subvolcanic architecture and magmatic processes associated with the main tholeiitic flood basalt sequence. Measured true density values of the Khopoli olivine gabbros are as high as 3.06 g/cm3, and such high-level olivine-rich intrusions in flood basalt provinces can also explain geophysical observations such as high gravity anomalies and high seismic velocity crustal horizons.

 

Legenda Figura:

Photomicrographs (left column, crossed nicols) and back- scattered electron (BSE) images (right column) of the Khopoli olivine gabbros. (a–h) Poikilitic textures of the olivine gabbros with many grains of olivine and clinopyroxene enclosed within large plagioclase grains, examples being (e) and (f). Orthopyroxene and pigeonite occur either at the boundary of olivine and clinopyroxene grains (d, h) or are enclosed in large plagioclase grains (f, g). Areas marked “vs” are vesicle fillings. Abbreviations: ol, olivine; pl, plagioclase; cpx, clinopyroxene; pg, pigeonite; opx, orthopyroxene; cr, chromiferous spinel; mt, magnetite

 

Key-words: Volcanism, Deccan Traps, flood basalt, gabbro, cumulate