Zircon U-Pb Dating, Geochemical Characteristics of the Gneissic Granite from North Qinling Terrain in Tongbai Area and their Geological Significance

The zircon geochronology and geochemistry of two gneissic granite masses intruded into Qinling group-complex in North Qinling terrain, Tongbai area have been studied in this paper. The main lithology of Xinzhuang granite mass is gneissic talc monzonitic granite, while the ones of Baoshuisi granite mass is monzonitic granite. Both of them have experienced mylonitization. The LA-MC-ICP-MS Zircon U-Pb dating has obtained the ages of (429±3) Ma (MSWD=0.12, n=9) and (445±3) Ma (MSWD=1.02, n=16), which represent the emplacement ages of these two rock mass, that is they were formed in Silurian period. The geochemistry analysis show that both granites are characterized by high Si and Al, low Mg and Fe, and they belong to high-K calc-alkaline series. Their REEs are relatively low, with relatively enriched LREE, depleted HREE and medium-negative Eu anomaly. In addition, these rocks are enriched in LILE (such as Rb, Ba, U), but depleted in HFSE (such as Nb, Ta), P and Ti. The Xinzhuang gneissic granite belongs to strongly peraluminous S-type granite, while the Baoshuisi gneissic granite belongs to weak peraluminous I type granite, with obviously depleted HREE (Y, Yb, Lu). Based on the formation age of related rock mass and the contact relation with wall rock, it's limited that the granulite-facies metamorphism in Qinling group-complex took place at ca. 498~488 Ma, the amphibolite-facies retrogressive metamorphism occurred at ca. 488~428 Ma, and the regional mylonitization take place after 428 Ma. The Baoshuisi granite mass was formed in island arc environment, which is related to the northward subduction of Paleo-Tethys Ocean (Shangdan Ocean). The Xinzhuang granite mass was formed in impact environment, which is related to the arc-continent collision event at the end of the Early Paleozoic in Tongbai area.