新疆中天山北缘望峰地区花岗岩的地球化学、锆石U-Pb年代学及Hf同位素组成研究

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, A12O3 and ALK with K20/Na20 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 (SEu =0.42 -0.49),which are in relation to magmatic fractionation. Wangfeng granite intruded at 439.9 ?2.2 Ma based on the dating results of zircon U-Pbrngeochronology. Zircon Lu-Hf isotopic compositions indicate that εHf(t) = -5.0~ -1.3 with an average of -2. 72,two-stage modelrnagesC 'die ) range from 1294Ma tol482Ma, which yields Wangfeng granite derived from partial melting of Mid-Mesoprolerozoic 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.%新疆中天山北缘望峰地区花岗岩的岩石学、地球化学及锆石U-Pb-Hf同位素研究表明:(1)望峰地区花岗岩岩石类型主要为二云母二长花岗岩和黑云母二长花岗岩,岩体高SiO2 、Al2O3,中等富碱且相对富钾,A/CNK≥1.1,稀土含量中等(∑REE=162.6×10-6～ 211.8×10-6),轻重稀土分馏明显且富集轻稀土,具中等Eu负异常(8Eu=0.42 ～0.49),相对富集LILE和LREE,亏损Ba、Sr、P、Ti、Nb、Ta、HREE等,属高钾钙碱性过铝质S型花岗岩;(2)锆石LA-ICP-MS U-Pb定年获得岩浆结晶年龄为439.9±2.2Ma,成岩时代为早志留世;(3)锆石Hf同位素组成较均一,εHf(t)=-5.0～ -1.3,平均-2.72,二阶段模式年龄tDM2变化范围在1294～1482Ma之间,岩浆主要来源于中元古代中期地壳结晶基底岩系的部分熔融,源岩为经历过风化旋回、成熟度较低的硬砂岩,形成于中天山北缘早古生代碰撞造山期同碰撞环境.