Petrogenesis of a high‐temperature metamorphic terrane: a new tectonic interpretation for the north Dabieshan, central China

Abstract

ABSTRACT The northern Dabie terrane consists of a variety of metamorphic rocks with minor mafic‐ultramafic blocks, and abundant Jurassic‐Cretaceous granitic plutons. The metamorphic rocks include orthogneisses, amphibolite, migmatitic gneiss with minor granulite and metasediments; no eclogite or other high‐pressure metamorphic rocks have been found. Granulites of various compositions occur either as lenses, blocks or layers within clinopyroxene‐bearing amphibolite or gneiss. The palaeosomes of most migmatitic gneisses contain clinopyroxene; melanosomes and leucosomes are intimately intermingled, tightly folded and may have formed in situ. The granulites formed at about 800–830 °C and 10–14 kbar and display near‐isothermal decompression P–T paths that may have resulted from crust thickened by collision. Plagioclase‐amphibole coronae around garnets and matrix PI + Hbl assemblages from mafic and ultramafic granulites formed at about 750–800 °C. Partial replacement of clinopyroxene by amphibole in gneiss marks amphibolite facies retrograde metamorphism. Amphibolite facies orthogneisses and interlayered amphibolites formed at 680–750 °C and c. 6 kbar. Formation of oligoclase + orthoclase antiperthite after plagioclase took place in migmatitic gneisses at T ≤ 490°C in response to a final stage of retrograde recrystallization. These P–T estimates indicate that the northern Dabie metamorphic granulite‐amphibolite facies terrane formed in a metamorphic field gradient of 20–35 °C km ‐1 at intermediate to low pressures, and may represent the Sino‐Korean hangingwall during Triassic subduction for formation of the ultrahigh‐ and high‐P units to the south. Post‐collisional intrusion of a mafic‐ultramafic cumulate complex occurred due to breakoff of the subducting slab.