Zircon U-Pb age, geochemical characteristics and geological significance of the Caledonian strongly peraluminous granites in the Nanshan area, Qinghai Province

The Nanshan area of Qinghai is located in the junction of the South Qilian belt and the West Qinling orogenic belt. A garnet-bearing muscovite monzogranite intruding into the Proterozoic metamorphic strata was newly recognized in Langriniang, Nanshan area of Qinghai. This paper reports the LA-ICP-MS zircon U-Pb dating result as well as petrology and geochemistry of this monzogranite. The crystallization age of the Langriniang garnet-bearing muscovite monzogranite was dated at 438.7±4.2Ma. The Langriniang garnet-bearing muscovite monzogranites are characterized by high SiO₂, Al₂O₃ and ASI (A/CNK=1.09~1.28) values. In addition, they have aluminous mineral assemblages of muscovite, tourmaline and garnet, showing typical features of strongly peraluminous S-type granites. Geochemically, the garnet-bearing muscovite monzogranites display enrichment of LREE and LILEs (e.g., Cs, Rb, K, U and Pb) and depletion of HREE and Nb, Ta, P, Zr, Ti, Ba and Sr, with weak to moderate negative Eu anomaly. Besides, the Langriniang monzogranites have high Rb/Sr ratios (1.83~3.95) and low CaO/Na₂O ratios (0.11~0.19). These features indicate that the Langriniang monzogranites were derived from muscovite dehydration melting of metapelites under the water-absent condition. Combined with regional geology and age of the Langriniang monzogranites, the authors hold that the Langriniang monzogranites were formed during the closure of the Tethys Ocean and collision orogeny.