Zircon U-Pb chronology, geochemistry and their petrogenesis of Early Paleoproterozoic granitoid gneisses in Ulashan area, North China Craton.

Early Paleoproterozoic granitoid gneisses are widely distributed in Ulashan area in the khondalite belt, North China Craton, they record continent-crustal growth and tectonic evolution in the study area. The LA-ICP-MS U-Pb analyses of zircons from four representative granitoid gneiss samples collected from the study area unraveled that they formed at～2.45Ga, and experienced polyphase metamorphic events at ～2.45Ga and 1.89～1.95Ga. The geochemical analyses of the Early Paleoproterozoic granitoid gneisses show they are characterized by high REEs contents (∑REE=301×10-6～909×10-6), enrichment in light REE and depletion in heavy REE with (La/Yb)N ratios of 13～56. They have weakly positive and/or negative Eu anomalies (Eu/Eu*=0.76～1.81,average 1.18), high Sr content (495×10-6～895×10-6) and low Yb content (0.5×10-6～2.61×10-6). Their primitive mantle normalized trace element spider diagrams show they are strongly depleted in high field-strength element Nb, Ta, Th, U, and Ti, and are enrich in large ion lithophile element Ba, K, Rb and Sr. They are magnesian, high-K calc-alkalic and metalumous-weakly peraluminous, indicating that they originated from partial melting of basaltic lower crust under island arc on margin of active continent. It is suggested that there existed arc-continent accretionary process during Early Paleoproterozoic, and experienced tectonic evolutional history of arc-continent accretion, continent-continent collision and extensions post-orogen in Ulashan area.