Petrology and zircon U-Pb dating of well-preserved eclogites from the Thongmon area in central Himalaya and their tectonic implications

Abstract

The discovery of eclogites is reported within the Great Himalayan Crystalline Complexin the Thongmön area, central Himalaya, and their metamorphic evolution is deci-phered by petrographic studies, pseudosection modelling, and zircon dating. For thefirst time, omphacite has been found in the matrix of eclogites taken from a metamor-phic mafic lens. Two groups of garnet have been identified in the Thongmön eclogiteson the basis of major and rare earth elements and mineral inclusions. Core and interme-diate sections of garnet represent Grt I, in which the major elements (Ca, Mg, and Fe)show a nearly homogenous distribution with little or weak zonation. This Grt I displaysan almost flat chondrite‐normalized HREE pattern, and the main inclusions are amphi-bole, apatite, quartz, and abundant omphacite. Grt II, forms thin rims on large garnetgrains, and is characterized by rim‐ward Ca decrease and Mg increase and MREEenrichment relative to HREE and LREE. No amphibole inclusions are found in Grt II,indicating the decomposition of amphibole contributed to its MREE enrichment. Twometamorphic stages, recorded by matrix minerals and inclusions in garnet and zircon,outline the burial of the Thongmön eclogites and progressive metamorphic processesto the pressure peak: (a) the assemblage of amphibole–garnet–omphacite–phengite–ru-tile–quartz, with the phengite interpreted as having been replaced by Bt+Pl symplec-tites, represents the prograde amphibole eclogite facies stage M1(1), (b) in the peakeclogite facies [stage M1(2)], amphibole was lost and melting started. Based on thecompositions of garnet and omphacite inclusions, M1(1)is constrained to 19–20 kbarand 640–660°C and M1(2)occurred at >21 kbar, >750°C, with appearance of meltand its entrapment in metamorphic zircon. SHRIMP U–Pb dating of zircon from twoeclogite samples yielded consistent metamorphic ages of 16.7 ± 0.6 Ma and17.1 ± 0.4 Ma respectively. The metamorphic zircon grew concurrently with Grt II inthe peak eclogite facies. Thongmön eclogites characterized by the prograde metamor-phism from amphibolite facies to eclogite facies were formed by the continuing conti-nental subduction of Indian plate beneath the Euro‐Asian continent in the Miocene. Show more