Multiple metamorphic events revealed by zircons from the Diancang Shan−Ailao Shan metamorphic complex, southeastern Tibetan Plateau

Abstract

Analyses of mineral inclusions within complexly zoned zircons, combined with SIMS UPb ages for various domains within the zircons, provide evidence of the origin and multistage metamorphic evolution of the Diancang Shan−Ailao Shan metamorphic complex, southeastern Tibetan Plateau. Zircons separated from paragneisses, amphibolites, garnet pyroxenites, and marbles were subdivided into inherited (magmatic or detrital) and metamorphic zircons based on cathodoluminescence (CL) and Laser Raman spectroscopy. Weakly zoned, inherited detrital zircons from paragneisses yield ages of 700−540Ma, 1000−800Ma, ~1800Ma, and 2550−2450Ma, and contain inclusion assemblages of Qtz+Kfs+Pl+Ap, recording widespread geological events throughout the ancient Gondwana continent. In contrast, clearly zoned, inherited magmatic zircons in amphibolites and garnet pyroxenites yield ages of ~778−773Ma and contain Cpx+Pl±Qtz+Ap inclusions, representing the crystallization age of the mafic protolith. Metamorphic zircons also record discrete Early−Middle Triassic and the Eocene−Oligocene thermal events. Homogeneous metamorphic zircons with low Th/U ratios (<0.1) contain high-pressure (HP)-diagnostic Grt+Ky+Phe/Mus+Kfs+Qtz+Ap, and Grt+Cpx+Pl±Qtz inclusions in paragneisses, and amphibolites and garnet pyroxenites, respectively, suggesting their formation at 650−720°C and ~14kb. These HP zircons yield 206Pb/238U ages of 249−230Ma, indicating an Early−Middle Triassic HP metamorphic event in the Diancang Shan−Ailao Shan complex belt. Two groups of Eocene−Oligocene metamorphic zircons are also present. One of the groups comprises homogenous zircons with low Th/U ratios (<0.1) that contain medium-pressure (MP) amphibolite–granulite facies Grt+Ky+St+Bt+Kfs+Qtz, Grt+Cpx+Hb+Pl, and Dol+Cal+Ap in paragneisses, amphibolites, and marbles. These assemblages are stable at peak pressure−temperature (P−T) conditions of 720°C−760°C and 8.0−9.6kb, and yield consistent Eocene ages (44−36 Ma). The second group is homogeneous and all zircons have similar Th/U ratios, but contain distinct low-pressure (LP) Grt+Sil+Bt+Kfs+Qtz+Ap, Opx±Cpx±Grt+Hb+Pl, Cal±Cpx+Ap inclusions in paragneisses, amphibolites and garnet pyroxenites, and marbles, recording post-peak P−T conditions of 700°C−750°C and 5.0−6.5kb, and yielding younger Oligocene metamorphic ages (32−25Ma). 40Ar/39Ar analyses of biotite, muscovite, and amphibole yield the youngest ages (25−14Ma, Miocene) related to late cooling during retrogressive metamorphism under conditions of 520°C−620°C and 4.0−4.5kb.These new data suggest a clockwise P−T−t path for the Diancang Shan−Ailao Shan metamorphic complex belt, typical of continent−continent collision, indicating collision−subduction tectonism prior to the strong, left-lateral, ductile deformation along the Ailao Shan−Red River (ASRR) shear zone that started at ~32Ma and lasted until 25Ma, causing mid-crustal (18−25km depth) high-temperature metamorphic conditions. Continued uplift, and a slowing of left-lateral ductile shearing occurred at ~25−14Ma at a depth of 10−15km and under relatively low-temperature conditions. This temporal and kinematic link between left-lateral shearing along the ASRR and the opening of the South China Sea supports the occurrence of the block extrusion of Indochina from Eurasia along lithospheric-scale strike-up faults.