The origin and significance of crustal minerals in ophiolitic chromitites and peridotites

Abstract

Various combinations of zircon, quartz, corundum, K-feldspar, plagioclase, apatite, amphibole, rutile, titanite, almandine garnet, kyanite, andalusite, and coesite have been recovered from podiform chromitites of the Luobusa and Dongqiao ophiolites in Tibet, the Oman ophiolite and the Ray–Iz ophiolite in the Polar Urals, Russia. Chromitites in all four ophiolites also contain moissanite and the Luobusa and Ray–Iz ophiolites contain in-situ diamonds. Most of the recovered zircons are sub-rounded grains with complex internal structures indicating polyphase growth. Trace element contents and a low-pressure inclusion assemblage (quartz, muscovite, K-feldspar, apatite, ilmenite, rutile) indicate a continental crustal origin for the zircons. They have SIMS U–Pb ages that are generally much older than the host body (total range: 90 to 2500 Ma). The presence of numerous crustal minerals, particularly zircon, suggests derivation from metasedimentary rocks subducted into the mantle. Their preservation in chromitites and peridotites implies effective isolation from the mafic melts that formed the ophiolites. We suggest that most of these minerals were derived from the crustal parts of subducted slabs and were encapsulated into chromite grains precipitated from rising asthenospheric and suprasubduction magmas. The chromite grains were carried in melt channels to shallow levels in suprasubduction mantle wedges and then deposited as podiform chromitites near the Moho. The rise of asthenospheric peridotites into suprasubduction zones was facilitated by subduction initiation and possibly by slab tear allowing mixing of the UHP, highly reduced minerals and crustal minerals now found in ophiolitic chromitites and peridotites.