Petrogenesis, P-T-t path, and tectonic significance of high-pressure mafic granulites from the Jiaobei terrane, North China Craton

摘要

High-pressure (HP) mafic granulites in the Jiaobei terrane are composed predominantly of garnet mafic granulites, garnet-hypersthene granulites, and garnet amphibolites, and they are found as irregular lenses or deformed dike swarms within tonalitic-trondhjemitic-granodioritic gneisses and granitic gneisses. The HP mafic granulites contain four distinct metamorphic assemblages, of which the early prograde assemblage (M_1) is represented by the cores of garnets, together with mineral inclusions of clinopyxene+plagioclase±quartz, and it formed at 740-770°C and 0.90-1.00GPa. In contrast, the peak assemblage (M_2) consists of high-Ca cores in garnet, high-Al cores in clinopyroxene, and high-Na cores in plagioclase in the matrix, which formed under P-T conditions of 850-880°C and 1.45-1.65GPa. The peak metamorphism was followed by near-isothermal decompression (M_3), which resulted in the development of orthopyroxene+clinopyxene+plagioclase±quartz±amphibole±magnetitie symplectites or coronas surrounding some garnet grains, with P-T conditions of 780-830°C and 0.65-0.85GPa. Surrounding some garnet grains are symplectites of amphibole+plagioclase+quartz±magnetitie, which formed during a cooling retrograde stage (M_4) with P-T conditions of 590-650°C and 0.62-0.82GPa. An integrated study involving laser Raman analysis of mineral inclusions, cathodoluminescence imaging, and in situ U-Pb dating of zircons shows that the protolith ages of the HP mafic granulites are mainly 2550-2500Ma, and that the timing of the peak metamorphism of the HP mafic granulites ranges from 1900 to 1860 Ma, as recorded by the cores of metamorphic zircons. On the other hand, the medium- to low-pressure granulite-amphibolite facies retrogression occurred mainly at 1860-1820Ma, as recorded by the rims of some single zircon grains and the zircon grains that contain inclusions of garnet+orthopyroxene+plagioclase+sphene. The combination of petrography, mineral compositions, metamorphic reaction history, thermobarometry, and geochronology defines a near-isothermal decompressional clockwise P-T-t path for the Jiaobei HP mafic granulites, suggesting that the Jiaobei terrane underwent initial crustal thickening during 1950-1860Ma, followed by relatively rapid exhumation, cooling, and retrogression in the period 1860-1820Ma. This tectonothermal path was probably generated by subduction and collision-related tectonic processes.