新疆可可托海近3号脉花岗岩成岩时代及地球化学特征研究

Chronology and geochemistry of the granite near the Keketuohai No. 3 pegmatite in Xinjiang

可可托海伟晶岩田内花岗岩类发育。本文研究的花岗岩体位于3号伟晶岩脉矿坑的东部,岩性为似斑状黑云母二长花岗岩,野外特征与矿区北部的阿拉尔花岗岩非常相似。花岗岩中锆石LA-MC-ICP-MS U-Pb测年结果表明,19个测点206Pb/238U年龄集中于399.6～409.0Ma,加权平均年龄为405.4±1.4Ma(MSDW=0.98),属于早泥盆世,比阿拉尔花岗岩的侵入时期早得多,与3号伟晶岩脉没有成因上的联系,应是区域上岩浆活动最为强烈时期的产物。岩石具有富硅(SiO2=70.69%～73.81%)、富铝(Al2O3=14.00%～15.74%)、总体髙钾(K2O/Na2O≥1),贫Fe、Mg、Ti、P特点,铝饱和指数较高(A/CNK≥1.1),属于髙钾钙碱性强过铝质花岗岩;微量元素表现出Sr、Ti、Ba、P、Nb、Ta明显的负异常和Th、U、La、Zr、Hf的正异常特征。稀土配分型式显示LREE的相对弱富集,HREE比较平坦以及Eu中等负异常。样品中87Sr/86Sr(0.72259～0.72810)、143Nd/144Nd(0.51235～0.51237)接近于陆源沉积物,Sr初始值(0.70155～0.70341)较低,f Sm/Nd(-0.32～-0.26)、Sm/Nd(0.222～0.240)比值显示分异小、较均一的Sm/Nd同位素体系;εNd(t)值变化于-3.07～-2.16,模式年龄t2DM集中在1.35Ga左右,属于中元古代。综合研究岩石中主量、微量和稀土元素、同位素特征,同时对比区域上同期岩浆活动特征,认为本文研究的花岗岩可能是在活动大陆边缘环境下,强烈的俯冲-碰撞作用导致的地壳加厚引发了深熔作用,使阿尔泰微古陆边缘内部中元古代基底部分熔融,同时有幔源物质参与;随后岩浆经过结晶分异,最终上升侵位,反映了陆缘弧花岗岩的特点。

There are very strong granitic activities in the Keketuohai pegmatite field. The biotite monzogranite bearing megaporphyric crystal in this paper outcrops near the east of No. 3 pegmatite. Its feature in field is very similar to the Aral granite in north. The result of LA-MC-ICP-MS U-Pb dating for the granite indicates that206Pb /238U ages range from 399. 6Ma to 409. 0Ma（ from 19 spot analyses）,and the weighted mean age is 405. 4 ± 1. 4Ma（ MSDW = 0. 98）. As an Early-Devonian pluton,it formed during strongest magma intrusion in region,much earlier than the Aral granite and genetically no relationship with No. 3 pegmatite. Geochemically,the granite is relatively enriched in Si（ SiO 2 = 70. 69% ～ 73. 81%）,Al（ Al 2 O 3 = 14. 00% ～ 15. 74%）,high K（ in general,K 2 O / Na 2 O≥1）,relatively poor in Fe,Mg,Ti and P,and its A / CNK ratios range from 1. 09 to 1. 12,suggesting high K calc-alkaline and strong peraluminous granite. Trace elements in the granite show markedly negative anomalies for Sr,Ti,Ba,P,Nb, Ta and positive anomalies for Th,U,La,Zr,Hf. LREE is relatively markedly fractionated,showing weak enrichment,HREE is weakly fractionated,and Eu has a moderate negative anomaly.87Sr /86Sr ratios of samples range from 0. 72259 to 0. 72810 and143Nd / 144Nd from 0. 51235 to 0. 51237,closing to that of terrigenous deposits. I Sr range from 0. 70155 to 0. 70341,f Sm / Nd from- 0. 32 to- 0. 26 and Sm / Nd from 0. 222 to 0. 240,implying a weak fractionated and homogeneous isotopic system for Sm / Nd. ε Nd（ t） range from- 3. 07 to- 2. 16 and t 2DM concentrates on 1. 35Ga,meaning Mid-Proterozoic basement. Based on studying the granite for its major,trace and REE elements,isotopic composition,related ratios,and contrasting with synchronous granite in the region,we suggest that granite in the paper formed in an active continental margin arc setting related to the subduction of plate,where thickening of crust resulted in partial melting of Mid-Proterozoic basement in the margin of Altay micro-continent and the added mantle-derived composition. Finally,biotite monzogranite formed during the fractionation of melting magma and following arising.