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Xiaoxia Wang, Chinese Academy of Geological Sciences (China)
Tao Wang, Chinese Academy of Geological Sciences (China)
Xinxiang Lu, Henan Institute of Geology (China)
Qinghui Xiao, Information Center, the Ministry of Land and Resources of China (China)
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The Qinling in central China, one of the major orogens in East Asia, was finally formed by collision between the North China Block (NCB) and South China Block (SCB) during Triassic times. The exact timing, however, is still controversial. Voluminous Triassic intrusions occur in the orogen, but interpretations of their setting disagree. Our studies recognized rapakivi-textured granitoids from the Triassic granitoids and recently identified coeval associated mafic intrusions including lamprophyre dykes.
The rapakivi-textured granitoids (212-217Ma, zironc ages) show rapakivi texture. They are magnesian, calc-alkalic and metaluminous (A/CNK=0.71-0.95) and have moderately enriched light rare earth element and flat heavy rare earth element patterns with weak negative or no Eu anomalies. They show depleted in Nb, Ta, P and Ti, enriched in Sr, F, and REE, and slightly enriched in Na2O, Rb, Ba, Th, La, Ce, Zr and Hf , revealing some transitional features of I-to A-type granites. The εNd values of the granitoids are from -0.3 to-7 and initial Sr isotope values from 0.70508 to 0.70600.
The enclaves have been dated by zircon U-Pb method as 209-218Ma. They are also magnesian, calc-alkalic and metaluminous (A/CNK = 0.71-0.95) and have fractionated REE patterns with LaN >100 and LuN < 10 times chondrite. (La/ Yb)N ratios range from 5 to 24 and (Gd/ Yb)N from 1.8 to 2.2. Most enclaves show small negative or positive Eu anomalies (Eu/Eu* =.0.82-1.45). They have high Ba/Nb, La/Nb, Sm/Zr and Eu/Ti values. But Nb/Ta and Zr/Hf ratios differ only a little from primitive mantle. εNd values range from -0.3 to C6.5 and initial Sr isotope values from 0.70444 to 0.70590. The lamprophyre dykes, dated as ca.219Ma by biotite Ar-Ar method, are calc-alkaline, and rich in large ion lithophile elements (e.g. Ba, K), but depleted in Nb, Ta and Ti. They show highly fractionated REE patterns with LaN > 100 and HREE < 10 times chondrite abundances. εNd values range from -0.5 to -3.3 and initial Sr isotope values from 0.7036 to 0.7058.
The petrology, geochemistry, isotope and geochronology of the Qinling rapakivi-textured granitoids and mafic intrusions indicate that they are of bimodal magmatism, and magma mixing is the major reason for the origin of the rapakivi-textured granitoids. Thus, the rapakivi-textured granitoids show both similarities and differences compared with the typical Proterozoic non-orogenic rapakivi granitoids in cratons. However, they are similar to some rapakivi granitoids in the orogenic belt and can be regarded as orogen-type rapakivi granite. The bimodal magmatism association and lithology and geochemistry features of the Triassic plutons, together with their passive emplacement, suggest that the Qinling orogen went into a post-collision setting during late Triassic time.
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