Compositional variations of chromiferous spinel in Mg-rich rocks of the Deccan Traps, India

Abstract

Composition of chromiferous spinel included in olivines of Mg-rich basalts and gabbros of the Deccan Traps (Gujarat and Western Ghats) are reported here. They vary from Al-rich compositions [Al 2 O 3 = 53 wt.%; Cr#, 100Cr/(Cr + Al) = 12] to Cr-rich compositions [Cr 2 O 3 = 51 wt.%; Cr# = 84], and from Cr-Al rich compositions towards Cr-rich Ti-magnetite (TiO 2 up to 23 wt.%, ulvöspinel up to 67 mol.%). The Mg# [100Mg/(Mg + Fe 2+ )] of spinel decreases from 81 to nearly zero. The highest Cr# has been found in the Bushe Fm., Thakurvadi Fm., and some high-Ti basalts of the Pavagadh section, whereas some of the low-Ti basalts of Saurashtra have Al-rich compositions typical of spinels found in mid-ocean ridge basalts. The chemical composition of the Deccan Trap spinels is completely different compared to that observed in mantle spinel suites, with very few exceptions. The decreasing Al and increasing Fe and Ti of spinel seems to be mainly the result of decrease of Mg in the locally coexisting melts and favourable cationic substitutions in the lattice. There is barely any evidence of general relationships between the composition of the Deccan spinels and inferred mantle sources of the host magmas. Pyroxene inclusions in spinels may witness a high-pressure stage of crystallization, but the possibility of non-equilibrium crystallization, or even magma mixing, cannot be ruled out. Overall, the compositional ranges of chromiferous spinel in the Deccan Traps closely match those observed in the other Large Igneous Provinces having mafic/ultramafic intrusions and mafic magma compositions (e.g., Siberian Traps, Karoo, Emeishan).

Evidence of crystallization in residual, Cl–F-rich, agpaitic, trachyphonolitic magmas and primitive Mg-rich basalt–trachyphonolite interaction in the lava domes of the Phlegrean Fields (Italy)

“…2k). The Ti concentration is low (0.5-1.5 wt % TiO 2 ; Table S1), as typical of spinels crystallized in primitive basalts and found in olivines with high forsterite content (see Melluso, de' Gennaro & Rocco, 2010; S. Conticelli & L. Melluso, unpub. 8).…”

“…Major and trace elements were analysed by ICP-MS (inductively coupled plasma mass spectrometry) at ACTLABS, Ancaster, Ontario (see www.actlabs.com for details of the analytical procedures and Table 1 for analyses of reference standards). Backscattered electron (BSE) images were obtained with the same instrument (see de'Gennaro, 2010 andMelluso, de'Gennaro &Rocco, 2010 for details). Some 950 mineral compositions were obtained with Energy Dispersive Spectrometry at CISAG, University of Napoli Federico II, utilizing an Oxford Instruments Microanalysis Unit, equipped with an INCA X-act detector and a JEOL JSM-5310 microscope operating at a 15 kV primary beam voltage, 50-100 μA filament current, variable spot size and 50 s net acquisition time.…”

The crystallization of shoshonitic to peralkaline trachyphonolitic magmas in a H2O–Cl–F-rich environment at Ischia (Italy), with implications for the feeder system of the Campania Plain volcanoes

“…Magmatic Crspinels rich in both Al and Fe 3+ are unknown worldwide, due to a miscibility gap (cf. Barnes and Roeder, 2001;Melluso et al, 2010b), and, indeed, there is no evidence that the Ischian spinels crystallized in a highly oxidized magma.…”

“…The Ischian spinel compositions plot in the field of N-MORB (normal mid-ocean ridge basalts) in the TiO 2 -Al 2 O 3 diagram, used for discrimination of the chromiferous spinels in various tectonic settings (Fig. 5d;Kamenetsky et al, 2001; see also Melluso et al, 2010b). No composition plots in the OIB (ocean island basalt) field, as defined by Kamenetsky et al (2001).…”

Geology, petrochemistry, and genesis of the bimodal lavas of Osham Hill, Saurashtra, northwestern Deccan Traps

“…This has no obvious relationship to the Osham Hill, from which it is separated by soil cover. Major and trace element data and spinel compositions for this gabbro are available (sample D30 of Melluso et al, 2010a). From the northeasterly trend of this olivine gabbro body we consider it as one of the many arcuate dykes surrounding the main intrusive focus of the Girnar complex (Auden, 1949).…”