松潘—甘孜造山带北部达日泽龙花岗岩体地球化学、年代学及构造意义

Geochemistry, Chronology and Tectonic Significances of the Darizelong Granite Intrusion in the Northern Songpan-Garze Orogenic Belt

达日泽龙岩体位于松潘—甘孜造山带北部,主要岩石类型有花岗闪长岩及二长花岗斑岩,讨论其岩石成因和形成的构造环境,有助于揭示整个松潘—甘孜造山带复杂的构造演化历史。本文通过LA-ICP-MS锆石U-Pb定年分析表明,花岗闪长岩年龄为(203.8±1.9) Ma,二长花岗斑岩年龄为(201.6±2.1) Ma,属晚三叠世末期岩浆活动的产物。全岩地球化学显示,岩石具有高SiO_(2)、Al_(2)O_(3)、Na_(2)O、K_(2)O含量和低FeO、TiO_(2)、MnO、MgO特征,微量元素富集Rb、Th、U、Sm、Hf,相对亏损Ba、Nb、Ta、Sr、P元素,稀土元素配分模式为右倾海鸥型,具中等负铕异常,均表明该岩体为弱过铝质-强过铝质S型花岗岩,是上地壳部分熔融的产物,但不排除有地幔物质的混染,其中花岗闪长岩源岩为杂砂岩类,二长花岗斑岩源岩为泥质岩类。微量元素特征显示岩体形成于后碰撞环境。结合区域构造背景,认为松潘—甘孜造山带在印支末期-燕山早期地壳增厚达到顶峰,岩石圈拆沉作用引发的软流圈上涌已扩升至中上地壳,诱发了变质沉积岩的重熔,达日泽龙岩体的出现正是这一构造机制下的响应。

The Darizelong granitic intrusion is located in the northern part of the Songpan-Garze orogenic belt,and it mainly consists of granodiorite and monzogranitic porphyry.To discuss its genesis and its forming tectonic environment will be helpful for revealing the complicated tectonic evolution history of the Songpan-Garze orogenic belt.Zircon LA-ICP-MS U-Pb dating results show that the age of granodiorite is 203.8±1.9 Ma,and the age of monzogranitic porphyry is 201.6±2.1 Ma.Both granodiorite and monzogranitic porphyry are products of the magmatic activity at the end of Late Triassic.Geochemical results of bulk rocks show that these rocks are characterized with high contents of SiO_(2),Al_(2)O_(3),Na_(2)O and K_(2)O,and low contents of FeO,TiO_(2),MnO and MgO,with enriched trace elements of Rb,Th,U,Sm and Hf,relatively depleted elements of Ba,Nb,Ta,Sr and P,right-declined REE distribution patterns,and moderately negative Eu anomaly.All kinds of characteristics show that the intrusion belongs to the weakly peraluminous to strongly peraluminous S-type granite.Its magma is a product of partial melting of the upper crust,though the contamination of mantle materials could not be ruled out.The provenance rock of granodiorite could be greywacke,while the provenance rock of monzogranitic porphyry could be argillaceous rock.Based on characteristics of trace elements,the granitic intrusion was formed in the post-collisional environment.Combined with the regional tectonic setting,it is believed that the crust of the Songpan-Garze orogenic belt was thickened to its maximum thickness during the period from Late Indosinian to Early Yanshanian,and then the asthenosphere upwelling caused by the lithospheric delamination reached to the upper and middle crusts where the remelting of meta-sedimentary rocks was triggered to form granitic magma for the occurrence of the Darizelong granitic intrusion as a response of tectonic movement under this tectonic mechanism.