西天山阿吾拉勒西段群吉萨依花岗斑岩地球化学特征及其成因

Geochemistry and genesis of the Qunjisayi granite porphyry in the west of Awulale area,Western Tianshan Mountains

西天山阿吾拉勒西段群吉萨依花岗斑岩侵入于群吉萨依背斜北西侧伏端下二叠统乌朗组火山岩中,具似斑状结构,斑晶为斜长石、钾长石、角闪石、黑云母,基质具花岗质半自形微晶粒状结构,主要由斜长石、钾长石、石英等组成。群吉萨依花岗斑岩具有高SiO2(71.17%～72.59%)、富Na2O(8.86%～9.39%)、贫K2O(0.07%～0.28%)的特点,铝饱和指数(A/CNK)为0.87～0.92,A/NK为0.97～1.01,富集Th、U和Pb,亏损Nb、Ta、Sr、P、Ti,相对于Th和U亏损Rb、Ba和K等,属过碱质-弱准铝质的分异I型花岗岩类。锆石SHRIMPU-Pb法测得花岗斑岩成岩年龄302±4Ma,即形成于石炭纪末—二叠纪初。岩石具有低的ISr(0.70512～0.70536)和正的εNd(t)值(+2.1～+2.7),样品的fSm/Nd值为-0.43～-0.56,单阶段模式年龄tDM为713～863Ma,和该区早石炭世分布的玄武质岩石具有相似的Sr-Nd同位素特征。结合区域构造背景,推测该岩体形成于后碰撞伸展阶段,是由底侵的玄武质岩石在角闪石-石榴石相的条件下发生部分熔融形成的。

The Qunjisayi porphyry granite is located in Awulale area of Western Tianshan Mountains. It intrud- ed into the volcanic rocks of lower Permian Wulang Formation located at the northwestern end of the Qunjisayi anticline. All samples from the pluton are characterized by porphyraceous texture, the phenocrysts are composed of plagioclase, potassium feldspar, hornblende and biotite, and the matrix consisting mainly of plagioclase, potassium feldspar and quartz is of granitic hypidiomorphic microcrystalline-granular texture. The porphyry granite is geochemically characterized by high SiO2 （71.17 % -72.59 % ）and Na20（8.86 % --9.39 % ）, and low K20（0.07% --0.28% ）, with A/CNK being 0.87--0.92 and A/NK 0.97--1.01. It is enriched in Th, U, Pb, but depleted in Nb, Ta, Sr, P and Ti. It is also depleted in Rb, Ba and K relative to Th and U. The porphyry granite belongs to peralkaline-mataluminous fractionated I-type granite. The zircon SHRIMP U-Pb age of the porphyry granite 302 ＋ 4 Ma indicates its generation in the Late Carboniferous to Early Permian period. It is suggested that the porphyry granite is similar to the lower Carboniferous basalts in the same area, as evidenced by the following data： low initial Sr isotope ratio （Isr= 0.705 12-0.705 36）, positive s Nd（t） = ＋ 2.1- ＋ 2.7, fSm/Nd = - 0.43- - 0.56 and the Nd model age（ tDM = 713- 863 Ma）. In association with the tectonic background, it is inferred that the primary magma of Qunjisayi porphyry granite was most likely derived from the partial melting of the underplating basaltic rocks under the conditions of amphibolite- or garnet-phase after collision.