Abstract:

The Karamay plutons from the west Junggar are mainly made up of dioritic to granitic rocks. UPb dating of zircons from a quartz diorite suggests that the pluton was emplaced at 3155±28 Ma (late Carboniferous)，based on the SHRIMP method. The Karamay plutons show geochemical features reminiscent of arc magmas: Narich，A/CNK=057-084，enrichment of LREE((La/Yb)N=47-61) and LILEs with minor negative Eu anomalies (δEu=076-091)，depletion of HFSEs such as Nb and Ta，and being typical of Itype granitoids. Linear trends of SiO2 versus other major elements were observed，suggesting that mixing between mantlederived magma and crustal melts and/or fractional crystallization have played an important role in forming different rock types of the plutons. Microgranular mafic enclaves (MMEs) are common in the plutons. Compositional and textural disequilibrium of plagioclase and hornblende in the MMEs and host rocks was observed，suggesting a mechanical and chemical exchange between the two rock units and also a mixing origin for the MMEs. The host rocks and MMEs are similar in terms of Nd and Sr isotopic compositions，with ISr(t)=070352-070388，εNd(t)=69-74，typical of that of depleted mantle. Isotopic modeling suggests no signs of old continental crust beneath the west Junggar. The petrogenesis of the Karamay plutons can be summarized as below: postcollisional lithospheric extension caused the upwelling of asthenosphere and partial fusion of lithospheric mantle，producing voluminous basaltic magmas，which underplated in，and thus caused the partial melting of the juvenile lower crust(Paleozoic arc series and oceanic crust) and resulted in the formation of granitic magmas;subsequent mixing between the granitic and basaltic magmas gave birth to a hybrid magma，from which the Karamay plutons were formed through significant fractionation of calcic plagioclase and ferromagnesian phases such as pyroxene and hornblende.

Key words:  , Karamay plutons, mafic enclaves, magma mixing, NdSr isotopes, west Junggar