Microchemistry and magnesium isotope composition of the Purang ophiolitic chromitites (SW Tibet): New genetic inferences

Abstract

New petrographic and microanalytical studies of mineral inclusions in the Purang ophiolitic chromitites (SW Tibet) are used to scrutinize the evolution of the associated Cretaceous sub-oceanic lithospheric mantle section. Silicate inclusions in the chromite grains include composite and single-phase orthopyroxene, clinopyroxene, amphibole, and uvarovite. Most inclusions are sub-rounded or globular, whereas a few inclusions exhibit cubic/octahedral crystal morphologies. The latter are randomly distributed in the large chromite grains, though discrete aggregates are consistently confined to the grain centers. Abundant micron-scale, clinopyroxene inclusions are topotaxially aligned along crystallographic planes. Less abundant sulfide, wüstite, apatite and uvarovite inclusions are observed in some samples.The trace element geochemistry of the Purang chromitite evoke parental MORB-and boninite-like melts, consistent with the supra-subduction zone setting. The δ 26 Mg values of the high-Cr and high-Al chromitites range from -0.25 to -0.29‰ and -0.05 to -0.32‰, respectively. The associated harzburgite has nearly overlapping δ 26 Mg values of -0.13 to -0.37‰, but pyroxenite sills show a rather wider range of the δ 26 Mg values (-0.61 to -0.67‰).The variable Mg isotope signatures, combined with abundant exotic, ultra-high pressure and super reduced (UHP-SuR) mineral inclusions in the chromite grains, suggest that recycling and recrystallization under different mantle conditions played an important role in the genesis and evolution of these rocks. Further, discrete silicate, sulfide, and metal alloy inclusions in the Purang chromitites are comparable to those reported in other Tethyan ophiolites, and collectively suggest a common geodynamic evolution.

Genesis of the Sartohay Podiform Chromitite Based on Microinclusions in Chromite

“…(1) trapped droplets of influx liquids/melts, enriched with volatile components and incompatible elements resulting from melt-peridotite interactions and melt mixing, rapidly crystallized upon closed-system cooling within liquidus chromite [62,63]; (2) the entrapment of exotic fluids [64] or volatile-rich self-differentiated melts [65] reacting with anhydrous silicates; (3) the entrapment of previously crystallized minerals as xenoliths or xenocrysts at the early magmatic stage, in presence of hydrothermal fluids during subsolidus annealing of chromite [16,46,57]. However, neither of these interpretations can explain the formation of all inclusion types [66].…”