Diverse P– T paths of the northern Dabie complex in central China and its reworking in the early Cretaceous

Abstract

The northern Dabie complex (NDC) in the Dabie orogen, central China, is a high-temperature migmatite terrane. It is composed mainly of banded gneiss, migmatite, and minor amphibolite, granulite, retro-eclogite, marble and spinel peridotite, and intruded by the early Cretaceous massive granitoid plutons and subordinate mafic–ultramafic rocks. Whether or not the NDC experienced a high-pressure or ultrahigh-pressure (HP/UHP) metamorphic history has been a long standing issue. However, in the last 10 years, an increasing number of retrograde UHP eclogite relics and marbles and HP granulites have been identified throughout the NDC. This, together with the discovery of microdiamond preserved as inclusions in zircons within eclogite relics and gneisses and numerous age dates of 220–240 Ma and ∼130 Ma from the NDC, supports the notion that at least part of the NDC was subjected to Triassic continental subduction and HP/UHP metamorphism, the youngest ages being related to an intense overprinting of ultrahigh-temperature (UHT) granulite facies metamorphism in the early Cretaceous. Distinct peak UHP conditions (⩾2.5 GPa and ∼800 °C; ∼3.5 GPa and ⩾750–800 °C; ⩾4 GPa and ⩾900 °C or ∼750 °C) have been inferred from oriented Qtz or Ab + Kfs + Qtz needles in Ca–Na clinopyroxene of retrograde eclogite, Ti-Chu-bearing assemblage preserved in marble, rod-like Cpx–Rt–Ap inclusion in garnet of retrograde eclogite, and microdiamond inclusions in zircon of tonalitic gneiss and retrograde eclogite from the NDC. The overprinted UHT (>900 °C or ∼1000 °C) metamorphism is indicated by Osm growth in garnet, Pgt exsolution in clinopyroxene of retrograde eclogite, and Spl + Crd + Qtz symplectite in felsic granulites. Diverse P– T paths of an overall clockwise evolution are suggested with post-peak decompression or heating followed by decompression-cooling, and consist of two distinct paths. Opx + Pl inclusions in garnet and zircon in retrograde eclogite relics indicate that the lower continental crust in the NDC underwent Triassic subduction. The NDC shows overprinted UHT granulite facies metamorphism, thus consists of two metamorphic events in contrast to the single event of the central Dabie UHP eclogite belt (CDB). Distinct peak P– T conditions may be attributed to the lack of suitable mineral assemblages for geobarometry. The overall clockwise P– T paths consist of a peak HP/UHP event with overprinted UHT event suggesting that the NDC experienced Triassic subduction and intense intracontinental reworking in the early Cretaceous. The latter stage may be associated with a heat pulse due to asthenospheric upwelling and lithospheric thinning as a result of Cretaceous mantle superwelling and the westward subduction of the Pacific plate.