No genetic link between the two main types of chromitite, stratiform and podiform chromitites, has ever been discussed. These two types of chromitite have very different geological contexts; the stratiform one is a member of layered intrusions, representing fossil magma chambers, in the crust, and the podiform one forms pod-like bodies, representing fossil magma conduits, in the upper mantle. Chromite grains contain peculiar polymineralic inclusions derived from Na-bearing hydrous melts, whose features are so similar between the two types that they may form in a similar fashion. The origin of the chromite-hosted inclusions in chromitites has been controversial but left unclear. The chromite-hosted inclusions also characterize the products of the peridotite–melt reaction or melt-assisted partial melting, such as dunites, troctolites and even mantle harzburgites. I propose a common origin for the inclusion-bearing chromites, i.e., a reaction between the mantle peridotite and magma. Some of the chromite grains in the stratiform chromitite originally formed in the mantle through the peridotite–magma reaction, possibly as loose-packed young podiform chromitites, and were subsequently disintegrated and transported to a crustal magma chamber as suspended grains. It is noted, however, that the podiform chromitites left in the mantle beneath the layered intrusions are different from most of the podiform chromitites now exposed in the ophiolites.
Ultra-depleted dunites from the Sibuyan Island, Romblon (Central Philippines) are primarily composed of olivine, orthopyroxene (<5%), trace clinopyroxene and chromian spinel and are totally free of hydrous minerals and plagioclase. Mineral chemistry shows very refractory compositions (olivine Fo = 92-94; spinel Cr# > 0.75). Rare clinopyroxene preserved in the ultra-depleted dunites is anhedral and interstitial to olivine grains. They are neither subsolidus exsolution products from orthopyroxene nor metasomatic in origin. Clinopyroxene has high Mg# = 0.94-0.97 and low Al2O3 contents (<0.85 wt%). They are also very depleted in trace elements with only selected heavy rare earth elements detected during the LA-ICP-MS analysis. Light to middle REEs and LILEs were hardly detected, which is indicative of the very depleted character of the melt that precipitated the clinopyroxene. The calculated melts in equilibrium with the clinopyroxene have heavy REE contents similar to olivine-hosted ultra-depleted melts of MORB affinity from the East Pacific Rise and the Mid-Atlantic Ridge. The formation of the Sibuyan ultra-depleted dunites is being attributed to extremely high degree of dry melting below a mid-oceanic ridge.
We investigate in this paper mineral compositions and geochemical evolution of the mantle wedge peridotites preserved in the Late Cretaceous Zagros ophiolites of SW Iran. Mantle peridotites above subduction zones commonly experience distinct melting, depletion and refertilization processes as a result of the circulation of fluids derived from subducting slabs and flux melting. Our results reveal that the mantle wedge peridotites in the Zagros ophiolites are characterized mainly by residual and impregnated types. Residual peridotites resulted from early depletion and later refertilization processes, whereas impregnated peridotites developed due to episodic melt impregnations within and across the mantle. Mg#s and NiO contents, spinel Cr#, Mg#, and TiO 2 in olivines, Mg# and Al 2 O 3 contents of orthopyroxenes, and Mg#, TiO 2 and Al 2 O 3 contents in clinopyroxenes of dunites, harzburgites and lherzolites indicate the significant role of re-equilibration processes among different mineral phases and interactions with basaltic melts percolating within the host peridotites. The observed geochemical variations in the mineral chemistry of the Zagros peridotites reflect changes in magma chemistry and fluctuations in the degree of melt extraction and melt–rock interactions within the mantle peridotites. However, our data suggest that Mg–Fe distribution in the spinels of some dunites and harzburgites might also have resulted from subsolidus redistribution and exchange with surrounding olivine grains. Spinel and clinopyroxene phases in gabbroic rocks and ultramafic cumulates within the Zagros ophiolites also show significant variations in their compositions, suggesting that their magmas evolved from MORB-like to IAT, calc–alkaline and boninite suites, typical of subduction initiation-generated melts. Hence, the Zagros ophiolites present a case study of time-progressive melt evolution of the forearc oceanic lithosphere. Supplementary material: Datasets of the mineral chemistry and melt evolution of the mantle wedge peridotites in the Late Cretaceous Zagros Belt ophiolites (Iran) are available in tabular and figurative form at https://doi.org/10.6084/m9.figshare.c.7093528 . Thematic collection: This article is part of the Ophiolites, melanges and blueschists collection available at: https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists
The Mersin ophiolite, Turkey, is of typical arc type based on geochemistry of crustal rocks without any signs of mid-ocean ridge (MOR) affinity. We examined its ultramafic rocks to reveal sub-arc mantle processes. Mantle peridotites, poor in clinopyroxene (<1.0 vol.%), show high Fo content of olivine (90–92) and Cr# [=Cr/(Cr + Al) atomic ratio] (=0.62–0.77) of chromian spinel. NiO content of olivine is occasionally high (up to 0.5 wt.%) in the harzburgite. Moho-transition zone (MTZ) dunite is also highly depleted, i.e., spinel is high Cr# (0.78–0.89), clinopyroxene is poor in HREE, and olivine is high Fo (up to 92), but relatively low in NiO (0.1–0.4 wt.%). The harzburgite is residue after high-degree mantle melting, possibly assisted by slab-derived fluid. The high-Ni character of olivine suggests secondary metasomatic formation of olivine-replacing orthopyroxene although replacement textures are unclear. The MTZ dunite is of replacive origin, resulted from interaction between Mg-rich melt released from harzburgite diapir and another harzburgite at the diapir roof. The MTZ dunite is the very place that produced the boninitic and replacive dunite. The MTZ is thicker (>1 km) in Mersin than in MOR-related ophiolite (mostly < 500 m), and this is one of the features of arc-type ophiolite.
