Mineralogy, geochemistry, and melt evolution of the Kalaymyo peridotite massif in the Indo-Myanmar Ranges (western Myanmar), and tectonic implications

We present new whole-rock major, trace, and platinum group element (PGE) and mineral chemistry data from the Kalaymyo peridotite massif in the central part of the Indo-Myanmar Ranges (western Myanmar) and discuss its mantle melt evolution. The Kalaymyo peridotites consist mainly of harzburgites, which show typical porphyroclastic or coarse-grained equigranular textures. They are composed of olivine (forsterite, Fo = 89.8-90.5), orthopyroxene (enstatite, En(86-91), wollastonite, Wo(1-4), ferrosilite, Fs(8-10); Mg# = 89.6-91.9), clinopyroxene (En(46-49) Wo(47-50) Fs(3-5); Mg# = 90.9-93.6), and spinel (Mg# = 67.1-78.9; Cr# = 13.5-31.5), and have relatively homogeneous whole-rock compositions with Mg#s of 90.1-90.8 and SiO2 (41.5-43.65 wt%), Al2O3 (1.66-2.66 wt%), and CaO (1.45-2.67 wt%) contents. They display light rare earth element (LREE)-depleted chondrite-normalized (CN) REE patterns with (La/Yb)(CN) = 0.04-0.21 and (Gd/Yb)(CN) = 0.40-0.84, and show a slight enrichment from Pr to La with (La/Pr)(CN) in the range of 0.98-2.36. The Kalaymyo peridotites are characterized by Pd-enriched chondrite-normalized PGE patterns with superchondritic (Pd/Ir)(CN) ratios (1.15-2.36). Their calculated oxygen fugacities range between the quartz-fayalite-magnetite (QFM) oxygen buffers, QFM-0.57 and QFM+0.90. These mineralogical and geochemical features collectively suggest that the Kalaymyo peridotites represent residual upper mantle rocks after low to moderate degrees (5%-15%) of partial melting at a mid-oceanic ridge environment. The observed enrichment in LREE and Pd was a result of their reactions with enriched mid-oceanic ridge basalt-like melts percolating through these already depleted residual peridotites. The Kalaymyo and other ophiolites in the Indo-Myanmar Ranges therefore represent mid-oceanic ridge-type Tethyan oceanic lithosphere derived from a downgoing plate and accreted into a westward-migrating subduction-accretion system along the eastern margin of India.