西准噶尔宝贝岩体的LA-ICP-MS锆石U-Pb年代学与地球化学

Petrogenesis of the Baobei pluton in West Junggar： Constraints from LA-ICP-MS zircon U-Pb geochronology and geochemistry

宝贝岩体位于新疆西准噶尔南部的晚古生代达尔布特中酸性岩浆岩带中，其主要岩性为花岗斑岩。通过LA—ICP—MS锆石U—Pb定年，结合锆石阴极发光图像（CL）和U、Th元素特征，获得宝贝岩体的年龄为297-＋10Ma（MSWD=14），时代属晚石炭世晚期。花岗斑岩为典型的斑状结构，斑晶以斜长石、条纹长石和石英为主；岩石中SiO2含量为74．59％-76．26％、全碱变化于8．13％～8．63％；铝饱和指数A／CNK为1．00-1．02，属弱过铝质；岩石轻稀土富集且轻重稀土元素分馏明显，（La／Yb）。为1．54～3．05，负Eu异常（8Eu为0．25-0．53）；富集Rb、Th、U、K和LREE等大离子亲石元素，而贫Ba、Nb、Sr、P、Ti等元素，属高钾钙碱性I型花岗岩系列。根据岩体的成因类型并结合区域构造环境演化，分析认为西准噶尔不仅在晚石炭世早期存在与俯冲作用相关的岛弧花岗岩，在晚石炭世晚期一早二叠纪早期也存在持续俯冲作用，在此环境下形成了与俯冲作用相关的岛弧型宝贝花岗斑岩。

The Baobei pluton is located in the intermediate acidic magmatic rocks belt of the West Junggar area of Xinjiang. The pluton is composed mainly of granite porphyry. Combined with the cathodolumine scence （CL） images and element U and Th features of zircon, this study yielded an age of 297±10 Ma （MSWD = 14） for the pluton using the LA-ICP-MS zircon U-Pb dating, indicating the Late Carboniferous. The plagioclase, perthite and quartz phenocrystals are dominant in granite porphyry with a typical porphyrotopic texture. The geochemical analysis show that the pluton is characterized by SiO2 = 74.59%- 76.26%, total alkali （Na2O＋ K2O） = 8.13% -8.63%, and aluminum index A/CNK = 1.00- 1.02, indicating a transitional composition between metaluminous and peraluminous. The pluton is enriched in LREE, depleted in HREE （LREE/HREE = 2.38 - 4.03, （La/Yb）N = 1.54 - 3.05 ）, with negative Eu anomaly （δEu = 0.25 - 0.53）. While the trace elements are characterized by Rb, Th, U, K, LREE enriched, and Ba, Nb, Sr, P, Ti depleted. All the features suggest a high K calcic- alkaline I -type granite. Based on comprehensive analysis of the genetic type of the pluton and evolution of regional tectonic environment, we suggested that when the Junggar Ocean perpetually continued its deep subduction toward north during from the early Late Carboniferous to the Early Permian, and the Baobei granite porphyry was formed just during this tectonic evolution.