GEOCHRONOLOGY, GEOCHEMICAL CHARACTERISTICS AND TECTONIC IMPLICATIONS OF THE GRANITOIDS FROM LIUYUAN AREA, BEISHAN, GANSU PROVINCE, NORTHWEST CHINA

Abstract

Large amounts of granitoids, including granodiorite, monzogranite and potassium granite which divided into HT-potassium granite and HN- potassium granite on the base of where they locate, were emplaced in Liuyuan area, Beishan, Gansu province. The SHRIMP zircon U-Pb dating indicates that the emplaced age of granodiorite, monzogranite and HT- potassium granite are 423 ± 8 Ma, 397 ± 7 Ma and 436 ± 9 Ma respectively. Four kinds of granitic rocks all have enriched large ion lithophile elements relative to high field strength elements, enriched light rare earth element relative to heavy rare earth element (HREE), negative anomalies in K, Ni, Ta, P and Ti characteristics, and are metaluminous to peraluminous and high-K post-collisinal granite. In addition, granodiorite has a lack of negative anomalies in Sr and Eu, εNd(t) = -2.5 - -0.8, and its magmas resulted from partial melting of subcontinental mantle or admixture between partial melting of subcontinental mantle and partial melting of asthenospheric mantle, and contaminated by juvenile crust including orogenic components. HN-potassium granite, monzogranite and HT- potassium granite all have negative anomalies in Sr and Eu, and εNd(t) = +1.4, -4.0 - -2.0 and -2.7 - -0.3 respectely. All the geochemlical characteristics indicate the magmas of HT- potassium granite were derived from partial melting of juvenile crust induced by subcontinental mantle. The magmas of HN- potassium granite were from partial melting of asthenospheric mantle, and likely contaminated by juvenile crust. And the magmas of monzogranite were from partial melting of juvenile crust. The four kinds of granitoid rocks are all outcomes of emplaced magmas under the post-collisional tectonic setting and genesis of magmas are likely related to slab break-off.