SHRIMP zircon U-Pb ages and REE partition for high-grade metamorphic rocks in the North Dabie complex: Insight into crustal evolution with respect to Triassic UHP metamorphism in east-central China

摘要

We present SHRIMP U-Pb ages and REE data for zircon of two granulites, a garnet pyroxenite and a clinopyroxene amphibolite from the North Dabie complex. The North Dabie complex is a major fault-bounded petrotectonic unit in the Dabie continental collisional orogen between the South and North China Blocks. Zircon REE data are applied to link ages to specific petrological processes. In addition to SHRIMP zircon U-Pb dating, measurements were made on monazites from a HP-UHT metapsammitic granulite. Metamorphic zircon grains and coeval overgrowths on detrital cores from one granulite yielded a weighted mean ~(207)Pb/ ~(206)Pb age of 2042±7Ma, corresponding to peak HP/UHT metamorphism. The zircon cores have well-preserved oscillatory, magmatic growth zoning and generated a range of ~(207)Pb/ ~(206)Pb ages from 2766±8Ma to 3089±3Ma, representing the detrital source. Concordant analyses of monazites from the same granulite sample yielded a weighted mean ~(207)Pb/ ~(206)Pb age of 2001±6Ma, which we interpret as the time of exhumation to the middle crustal level. A magnetite-orthopyroxene-garnet quartzite associated with a banded iron formation has a metamorphic zircon age of 2293±7Ma (granulite-facies) with a detrital core age of 2435±28Ma. These results are interpreted to indicate a composite affinity to the South China and North China Blocks for the ancient Archean-Paleoproterozoic crust in the North Dabie metamorphic rocks. The garnet pyroxenite, known as a typical HP-UHP granulite, contains a multiple, latest Permian to Triassic metamorphic zircon population with ~(206)Pb/ ~(238)U ages of ca. 254-253, ca. 242-226, ca. 210, and ca. 203Ma. Along with published zircon and titanite U-Pb age data and mineralogical evidence, we suggest that the HP-UHP granulite evolved broadly synchronously with UHP eclogites during continental collision in the latest Permian to Triassic. In the garnet pyroxenite, there is also zircon inheritance of variable ages (ca. 291-2477Ma), indicative of a probable sedimentary origin of the protolith; there also occur young, mid-Jurassic to Cretaceous zircons ranging in age between 165 and 112Ma, signifying considerable crustal reworking in mid-Jurassic and Cretaceous times. Zircons from the clinopyroxene amphibolite have a weighted mean ~(206)Pb/ ~(238)U age of 780±6Ma representing the minimum time of protolith emplacement, and 133±2Ma for an anatectic event in the post-collisional stage. The age data of this amphibolite are similar to those of granitic gneisses/migmatites in the North Dabie complex. The new U-Pb zircon ages for the North Dabie rocks provide constraints on the affinity of collided crustal blocks, the metamorphic history of HP-UHP granulites, and post-collisional crustal reworking.