川西义敦岛弧中生代典型花岗岩体矿物学、地球化学特征及岩浆来源探讨

Mineralogical,Geochemical Features of Typical Mesozoic Granites in the Yidun Arc,Western Sichuan and a Discussion on the Magma Origin

义敦岛弧是位于松潘—甘孜褶皱带和羌塘地体之间的三叠纪火山岛弧。稻城岩体和海子山岩体分别为义敦岛弧上出露的晚三叠世和白垩纪花岗质岩体。结合岩石地球化学,以及偏光显微镜和电子探针(EPMA)、扫描电镜(SEM)对上述岩体进行了系统的矿物学研究,结果表明:两个岩体的主要造岩矿物为斜长石(中长石—更长石),钾长石(正长石)、石英和黑云母(铁叶云母—铁质黑云母),副矿物为锆石、榍石、磁铁矿和磷灰石等;黑云母的成分表明两个岩体的源区均为壳幔混源。稻城岩体属高钾钙碱性系列,具过铝质特征的花岗岩,海子山岩体为高钾钙碱性—钾玄岩系列,同样具过铝质特征。根据锆元素饱和浓度温度计和稀土元素饱和浓度温度计对两个岩体进行了温度限定,稻城岩体岩浆形成的平均上限温度为783℃,海子山岩体岩浆形成时的平均上限温度为844℃。结合前人研究结果,笔者等认为稻城岩体为高分异I型花岗岩,可能为与扬子克拉通有关的晚古元古代至早中元古代的下地壳物质,在甘孜—理塘洋向西俯冲和闭合之后的弧—陆同碰撞背景下,因幔源岩浆的底侵作用而发生部分熔融,同时伴有少量的亏损地幔成分加入,因密度上升至中上—上地壳深度侵位,并且侵位后经历了快速的冷却过程,在侏罗纪之初冷却至300℃以下;海子山岩体为与俯冲有关的造山后伸展环境下形成的白垩纪A2型花岗岩,岩浆来源同样为地壳物质伴有少量地幔物质混合而成,且在地壳中侵位深度较浅,之后经历快速的冷却过程。

Objectives：The Yidun Arc is a Triassic volcanic arc located between the Songpan Garzê Fold Belt and the Qiangtang Block, southwestern China. Daocheng Pluton and Haizishan Pluton which outcroped in the Yidun Arc are late Triassic and Cretaceous granitic plutons, respectively. Methods： In this paper, we combine petrogeochemistry with systematic mineralogical characteristics of the plutons mentioned above by means of petrological observation, electron microprobe analysis and scanned eletron microscope. Results：The results show that： the two granitic plutons are mainly composed of plagioclase （andesine—oligoclase）, K feldspar（orthoclase）,quartz and biotite（siderophyllite—iron biotite）,and the accessory minerals include zircon, sphene, magnetite and apatite; the chemical composition of biotites shows that the parental magma for the two granitic plutons was a hybrid melt possessed depleted mantle and typical crustal source. Daocheng Pluton shows the features of high-K calc-alkaline with peraluminous characteristics, whereas Haizishan Pluton belongs to high-K calc-alkaline to shoshonite series with peraluminous feature. The temperature of the two plutons were limited according to two geothermometers. The average upper limit temperature of Daocheng Pluton were 783℃, and the average upper limit formed temperature of magma of Haizishan Pluton was 844℃. Conclusions：In combination with previous studies on magmatism in the Yidun Arc, we think that Daocheng pluton belongs to high fractionated I-type granite, and we propose that the source for the Daocheng granite which probably derived from a Late Paleoproterozoic to Early Mesoproterozoic lower crust material related to Yangtze Craton, underwent the partial melting of the lower crust triggered by the underplating of large-scale mantle-derived magma under the syn collisional tectonic setting following the westward subduction and closure of the Garz —Litang paleo ocean. The generated hybrid melts which including lower crustal magma and minor depleted mantle-derived magma intruded in middle—upper crust and gave rise to the Daocheng granite because of the density. Daocheng granite then underwent rapid cooling and had cooled to below 300℃ by the beginning of the Jurassic. Haizishan pluton was an A2 type granitic pluton and likely to form under post collisional extensional tectonic setting related to arc—continent collision during the Early Cretaceous. As same as Daocheng granite, the source melt of Haizishan granite also included crustal magma and minor mantle derived magma, these hybrid melts thereafter were continuously injected into a shallow level chamber and experienced rapid cooling, suggesting they were emplaced at shallow crustal depths.