Paleoproterozoic Multistage Metamorphic Ages Registered in the Precambrian Basement Rocks at the Southeastern Margin of the North China Craton and Their Geological Implications

The Precambrian basement rocks in the Bengbu and neighboring areas, located at the southeastern margin of the North China Craton, occur as granulite terrains and xenoliths in the Mesozoic dioritic porphyry.

A Sm－Nd mineral isochron of mafic granulite from the Sφstrene Island， East Antarctica

Redetedrination of the metamorphic age of the gsrnet-bearing mafic wholeulite from the Sstrene Island, Esat Antarctica has been made and an isochron of wholerock-garnet-pyroxene-plagioclase is obtained, giving an age of [604±28(2σ)] Ma. Itis pointed out that this age represents the time of peak granulite facies metamorphism ofthe area. As to the geological relation of the Sstrene Island with the Bolingen Islandsand the Larsemann Hills to the east, it is deduced from the rock assemblages, metamorphic superposition, spatial changes and metamorphic ages between the above regions that the degree of the superposition of the later low pressure metamorphism is becoming stronger from west to east. While the evolution of the earlier medium pressure to later low pressure implies probably the substages of the same metamorphic cycle.

LA-ICP-MS U-Pb Zircon Dating for Felsic Granulite, Huangtuling Area, North Dabieshan: Constraints on Timing of Its Protolith and Granulite-Facies Metamorphism, and Thermal Events in Its Provenance

Information about the protolith of the Huangtuling granulite in North Dabieshan has been unavailable. The complex evolution history of the rock and its host basement must be further discussed. LA-ICP-MS U-Pb dating was conducted on three textural domains in zircon from a high-temperature, high-pressure felsic granulite in the Huangtuling area, North Dabieshan, Central China. The metamorphic growth-derived detrital zircon domain yields a 207^ pb/206^Pb age in the range of （2 49±54 ） -- （2 500±180） Ma. The magmatic genesis-derived detrltal zircon domain gives a 207^pb/ 206^Pb age ranging from 2 628 Ma to 2 690 Ma, with an oldest 206^ pb/ 238^U age of （2 790 ± 150） Ma. The metamorphic overgrowth or metamorphic recrystallization zircon domain yields a diesordia with an upper intercept age of （2 044. 7 ± 29.3 ） Ma. Compositions of the mineral assemblage, major element geochemistry, and especially the complex interior texture of the zircon suggest that the prololith of the felsic granulite is of sedimentary origin. Results show that the protolith material of the granulite came from a provenance with a complex thermal history, i.e. -2.8 Ga magmatlsm and -2.5 Ga metamorphism, and was deposited in a basin not earlier than 2.5 Ga. The high-temperature and high-pressure granulite-facies metamorphic age was precisely constrained at （2.04±0.03） Ga, which indicates the granulite in Huangtuling area should be a relict of a Paleoproterozoic UHT （ultrahigh temperature） metamorphosed slab.

Early Paleozoic granulite-facies metamorphism and anatexis in the northern West Qinling orogen： Monazite and zircon U-Pb geochronological constraints

In the northern West Qinling orogen(WQO), granulite-facies metamorphic rocks are recognized within the Qinling Complex. These rocks are composed of amphibole-bearing two-pyroxene granulite and garnet-sillimanite gneiss with widespread migmatitization. We investigate three granulite-facies samples and one leucosome sample from the Qinling Complex, which are suitable for U-Pb analyses of zircon and monazite. SHRIMP and LA-ICPMS U-Pb age dating of zircon and monazite from two pelitic granulites provides weighted mean ages of 430±4 Ma(MSWD=0.88) and 433±4 Ma(MSWD=0.27), respectively.Based on the petrographic characteristics and zircon CL imagery, we postulated a ca. 430 Ma metamorphic timing for the pelitic granulites. LA-ICPMS zircon U-Pb data from an amphibole two-pyroxene granulite sample reports two weighted mean age groups:424±3 Ma(MSWD=0.45) and 402±3 Ma(MSWD=1.4), which were interpreted as granulite-facies metamorphic and retrograde ages, respectively. LA-ICPMS U-Pb dating of zircons from the leucosome sample yields a weighted mean age of 426±2 Ma(MSWD=0.3), which is interpreted as the crystallization age of the leucosome. These data indicate that the West QOB experienced early Paleozoic granulite-facies metamorphism and anatexis similar to the East QOB. However, it remains unclear whether the early Paleozoic granulite facies metamorphism resulted from an arc setting created by the northward subduction of the Shangdan ocean or from a continental collisional orogenic event.