Early Mesozoic retrograded eclogite and mafic granulite from the Badu Complex of the Cathaysia Block, South China: Petrology and tectonic implications

Abstract

The high-grade metamorphic terrane in the Badu region along the northeastern Cathaysia Block in South China preserves retrograded eclogites and mafic granulites. Here we present the petrology, mineral phase equilibria and P-T conditions based on pseudosection computations, as well as zircon U-Pb ages of these rocks. Mineral textures and reaction relationships suggest four metamorphic stages for the retrograded eclogite as follows: (1) eclogite facies stage (M1), (2) clinopyroxene retrograde stage (M2), (3) amphibole retrograde stage (M3), and (4) chlorite retrograde stage (M4). For the mafic granulite, three stages are identified as: (1) plagioclase-absent stage (M1), (2) granulite facies stage (M2) and (3) amphibolite facies stage (M3). Metamorphic evolution of both of the rock types follows clockwise P-T path. Conventional geothermometers and geobarometers in combination with phase equilibria modelling yield metamorphic P-T conditions for each metamorphic stage for the eclogite as 500–560°C, 23–24kbar (M1), 640–660°C, 14–16kbar (M2), 730–750°C, and 11–13kbar (M3). The chlorite retrograde stage (M4) is inferred to have occurred at lower amphibolite to greenschist facies conditions. Phase equilibria modelling of the mafic granulite shows P-T conditions for each metamorphic stage as 600–720°C, >13kbar (M1) and 860–890°C, 5–6kbar (M2) and M3 at amphibolite facies conditions. LA-ICPMS zircon U-Pb dating and trace element analysis show that the high pressure metamorphism occurred at 245–251Ma. Protolith age of the mafic granulite is 997Ma, similar to that of the mafic to ultramafic rocks widely distributed in the Cathaysia Block and also along the Jiangnan belt. Subduction of ancient oceanic lithospheric materials (or crustal thickening) during Mesozoic and formation of eclogites suggest that the Cathaysia Block was perhaps in the Tethyan oceanic domain at this time. The granulite formation might have been aided by Mesozoic mafic magma underplating associated with lithospheric delamination, heating and retrogression of the eclogite accompanied by rapid uplift.