| Early Paleozoic granitoid rocks are widespread in the Lajishan tectonic belt,northern margin of the South Qilian Belt.The Machang pluton,located in the southern edge of the Lajishan tectonic belt,is mainly composed of biotite granite,granodiorite,and mafic microgranular enclaves(MMEs).LA-ICP-MS zircon U-Pb dating yields the middle Ordovician crystallization ages of 468 Ma to 466 Ma for those rocks.The biotite granites exhibit low K2O/Na2O(0.28-0.37)and high Sr/Y(125-168)and(La/Yb)N ratios,typical features of adakitic rocks.They are enriched in LREEs,LILEs,Zr,and Hf,but depleted in HREEs and HFSEs(Nb,Ta,Ti)with positive Eu anomalies.In addition,they also have relatively low contents of Mg O,Cr,and Ni and Mg#values,and slightly depleted Sr-Nd isotopic composition of whole-rocks and depleted Hf isotopic composition of zircons((87Sr/86Sr)t=0.7044-0.7046,?Nd(t)=+2.05-+2.21),?Hf(t)=+8.2-+10.2),overlapping with those of the late Cambrian to early Ordovician arc-related mafic igneous rocks of the Lajishan tectonic belt,hinting an derivation from anatexis of juvenile crustal materials.Most of the MMEs are sharp contact with their host granodiorites,but a few show transitional contacts.The MMEs and host granodiorites are characterized by enrichment of LILEs(Cs,K,Pb,Sr)and depletion of HFSEs(Nb,Ta,Zr,Hf),and have similar Sr-Nd-Hf isotopic composition,which are slightly enriched relative to those of biotite granites.However,the MMEs have lower Si O2,higher Mg O(6.15%-9.12%),Mg#(57-60),Cr(271×10-6-424×10-6),and Ni(54.7×10-6-86.6×10-6)contents than the host rocks.Moreover,the MMEs are featured by disequilibrium textures,such as back veining,poikilitic hornblende,disequilibrium-textured plagioclase,mafic clot,blade-shaped biotite,and acicular apatite.It is therefore inferred that the MMEs and granodiorites are generated by the mixing of subduction-related metasomatized mantle-derived mafic magmas and felsic magmas originating from partial melting of the juvenile crust.In combination with evidence of regional geology,it can be concluded that the Machang pluton were formed during the subduction of the South Qilian Ocean and related to partial melting of the juvenile crust by the upwelling mantle and magma mixing. |
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