Studies on Geochemistry, Zircon U-Pb Geochronology and Hf isotopes of Granite in Wangfeng Area at the Northern Margin of Middle Tianshan, Xinjiang
摘要
Rock samples in Wangfeng area consisting of two-mica monzonitic granite and biotite monzonitic granite locate in the northern margin of Middle Tianshan in Xinjiang,exhibiting as the direct gold-bearing country rocks,which synthetically display characteristics of peraluminous S-type granite or MPG belonging to high-K calcalkaline series: Allitic mineral assemblages of granitic rocks with medium-fine grained granitic texture and weakly gneissic structure comprise primary muscovite and biotite.Geochemically,Wangfeng granite is characterized by high SiO2,Al2O3 and ALK with K2O/Na2O ratios changing from 1.07 to 1.25 and 1.15 on average,A/CNK ratios range from 1.10 to 1.12 and 1.11 on average.Granite samples are rich in LREE and LILE(K,Rb,Th,U etc.),infertile in HREE and Ba,Sr,P,Ti,Nb,Ta etc.,coupling with moderate Eu negative anomaly(δEu=0.42～0.49),which are in relation to magmatic fractionation.Wangfeng granite intruded at 439.9±2.2Ma based on the dating results of zircon U-Pb geochronology.Zircon Lu-Hf isotopic compositions indicate that εHf(t)=-5.0～-1.3 with an average of -2.72,two-stage model ages(tDM2)range from 1294Ma to1482Ma,which yields Wangfeng granite derived from partial melting of Mid-Mesoproterozoic crystallized basements formed during the process of metamorphism of greywacke combining with the plotting results of experimental petrology.In terms of both previous research results on regional tectonic setting and tectonic discrimination diagrams,Wangfeng granite formed at the syn-collisional stage of Caledonian Orogeny,there is a quite significant conclusion that Middle Tianshan may be compound,which is composed of orogenic belts with distinct inner frameworks.
