Zircon U–Pb ages and Hf isotopic analyses of migmatite from the ‘paired metamorphic belt’ in Chinese SW Tianshan: Constraints on partial melting associated with orogeny

Abstract

A paired metamorphic belt with ubiquitous migmatites and rare granulites in its high dT/dP part formed due to oceanic subduction in Chinese southwestern (SW) Tianshan. Although several geochronological studies have addressed it in recent years, the exact timing of the high-grade metamorphism and partial melting is still controversial. For this study, we selected nine samples obtained from three valleys transecting the SW Tianshan Migmatite Complex for U–Pb geochronology and Lu–Hf isotope analysis to provide constraints on partial melting and the mid-crustal evolution during oceanic subduction and subsequent continental collision.On the basis of internal morphology, Th/U ratios and REE patterns, three types of zircons are discriminated. Dating them reveals five age groups: (1) Inherited zircons either are detrital, with ages scattering from 432Ma to 3261Ma, or yield ages concentrated at 406–410Ma reflecting Early Devonian magmatism; (2) Recrystallized zircon has been transformed from pre-existing zircon under subsolidus conditions at ~400Ma and ~360Ma; (3) Metamorphic rims have grown on zircons at ~400Ma, ~290Ma and ~270Ma, corresponding to three phases of anatexis. The partial melting phase at ~400Ma is probably related to the emplacement of voluminous mafic to felsic magmas in a continental arc region above the subduction zone consuming the South Tianshan Paleo-Ocean that triggered the most extensive regional-scale thermal event. However, the two subsequent partial melting phases at ~290Ma and ~270Ma representing post-collisional stages were not penetrative. P–T conditions of the studied migmatites estimated by Hb–Pl thermobarometry and Ti-in-zircon thermometry are 682–763°C, 4.7–8.0kbar; together with composite Kfs+Q+Pl, Kfs+Q and Kfs+Q+Ab inclusions representing entrapped melt and Kfs inclusions with Ab exsolution lamellae in zircon rims, the P–T estimates support the occurrence of anatexis. Zircon Lu–Hf isotope analysis indicates that zircon rims formed under high-grade metamorphic conditions and during partial melting by dissolution–precipitation of pre-existing zircons in a close system. Zircon two-stage Hf model ages constrain the development of the protoliths of the migmatites at four major crustal formation periods (~1871Ma, 1368–1426Ma, 1099–1276Ma and 743Ma). Combined with previous researches in this area, we conclude that the main crustal formation of the Yili and Central Tianshan Plates has mostly occurred in Proterozoic times, but not or only subordinately during the Archean.