Petrogenesis of the Kangjinla peridotite in the Luobusa ophiolite, Southern Tibet

The Kangjinla peridotite in the eastern part of the Luobusa ophiolite consists mainly of fresh harzburgite with less abundant Iherzolite and dunite. The generally depleted nature of the rocks and minerals suggests that they are the residue of partial melting of MOR-like mantle. However, all of the peridotites show some degree of LREE enrichment which is attributed to modification by subduction-related fluids. The principal minerals in the peridotites typically show two stages of mineral growth. Early stage olivine and pyroxene typically form large grains with kink banding and wavy extinction; the pyroxene also shows kinking of twin lamellae. Later stage minerals are normally smaller in size and occur along fractures and cleavage planes within or between the early stage minerals. The late stage minerals show no evidence of deformation. Early stage orthopyroxene and clinopyroxene have higher Al2O3 and Cr2O3 than the later varieties. Fo values of olivine are 90-92 in harzburgite and Iherzolite and 92-94 in dunite. The late-stage olivine has higher Cr2O3 and NiO than the early stage grains. The Cr numbers (Cr# = (Cr x 100)/(Cr + Al)) of chrome spinel in the mantle peridotites are between 30 and 77, being lowest in the Iherzolites and highest in the dunites. On the Cr# vs. Mg# (=(Mg x 100)/(Mg + Fe2+)) diagram the Iherzolites and clinopyroxene harzburgites plot in the abyssal peridotite field, whereas the harzburgites and dunites plot in the island-arc peridotite field. Our preferred explanation for these compositional features is that the mantle peridotite formed in a MOR environment, then was modified altered by later-stage melts and fluids in a suprasubduction zone (SSZ) setting. The Kangjinla peridotites host a number of small podiform chromitites with a wide range of textures, including massive, disseminated, layered, nodular and antinodular. The chromitites have uniformly high Cr# (75.6-82.7) and moderately high Mg numbers (56.4-74.1), similar to that elsewhere in the Luobusa ophiolite. Many of the textures suggest precipitation from mafic magma but the chromitites contain a variety of ultrahigh pressure (UHP) minerals that indicate crystallization at depths > 120 km. (C) 2011 Elsevier Ltd. All rights reserved.