Petrogenesis and tectonic implications of Early Cretaceous S- and A-type granites in the northwest of the Gan-Hang rift, SE China

摘要

The Gan-Hang rift, trending at least 450 km in a NE-SW direction, is a part of a Mesozoic Basin and Range Province in southeastern China. Detailed SHRIMP zircon U-Pb chronology, major and trace element, and Sr-Nd-Hf isotope data of three granitic plutons and a diabasic dike in the northwest of the Gan-Hang rift, are used to explore the origin of these granites and their relationship to the evolution of the Gan-Hang rift. SHRIMP zircon U-Pb dating shows that the granitic plutons and diabasic dike were emplaced in the Early Cretaceous (122-129 Ma). The Tongshan and Damaoshan plutons, close to the Gan-Hang rift, consist mainly of weakly peraluminous granitic rocks, which show A2 subtype affinity. These granites have initial ~(87)Sr/~(86)Sr ratios of 0.7080-0.7103, ε_(Nd) (T) values of-1.4 to-5.6 and ε_(Hf) (T) (in-situ zircon) values of -3.8 to +1.2. Detailed elemental and isotopic data suggest that they were formed by partial melting of granulitized Mesoproterozoic metamorphic basement (including metasedimentary and metaigneous rocks) in the shallow (<30 km) crust at a high temperature (～823 °C). These pure crustal melts underwent crystal fractionation of biotite, plagioclase and K-feldspar, forming the more felsic end-member of granitic rocks. The Ehu pluton, more distant from the Gan-Hang rift, is composed of two-mica granites, which are strongly peraluminous and are S-type granites. These granites have higher initial ~(87)Sr/~(86)Sr ratios (0.7162-0.7191) and lower ε_(Nd)(T) ( — 7.1 to —7.4) and ε_(Hf) (T) ( — 6.8; in-situ zircon) values than the Tongshan and Damaoshan granites. Detailed elemental and isotopic data suggest that the Ehu granites were formed by partial melting of Paleoproterozoic metasedimentary basement in the deep (～40-50 km) crust at a relatively low temperature (-757 °C), with no fractional crystallization and mantle-derived magma mixing in their evolution. The coeval diabasic dike is calc-alkaline and has initial ~(87)Sr/~(86)Sr ratio of 0.7085, ε_(Nd) (T) value of + 0.9 and ε_(Hf) (T) (in-situ zircon) value of — 7.1, suggesting that it was derived from the asthenospheric mantle wedge that had been metasomatised via addition of subducted sediment-derived melt with monazite in the residual phase. The association of Early Cretaceous (122-129 Ma) S- and A-type granites in the northwest of the Gan-Hang rift marks the onset of back-arc extension or intra-arc rift With ongoing extension the crust and lithospheric mantle became progressively thinned. The upwelling of asthenosphere triggered partial melting of both metasedimentary and metaigneous rocks in the more thinned crust close to the Gan-Hang rift, forming the A-type granitic magmas such as Tongshan and Damaoshan, whereas partial melting of metasedimentary rocks in the less thinned crust farther from the Gan-Hang rift formed the S-type granitic magmas such as Ehu. The red sediments with the total thickness more than 10,000 m have been successively deposited in the Gan-Hang rift valley since the late Early Cretaceous (～105 Ma), suggesting that this region experienced the most back-arc extension.