内蒙古阿尔山地区中生代A型花岗岩LA-ICP-MS锆石U-Pb年龄、地球化学特征及其构造意义

LA-ICP-MS zircon U-Pb age, geochemistry and tectonic significance of Mesozoic A-type granitesin Aershan area, Inner Mongolia

在岩石学研究的基础上，运用LA-ICP-MS锆石U-Pb定年和地球化学方法，研究了阿尔山地区不同时代花岗岩体的成因和构造意义。研究表明，三广山中粒碱长花岗岩形成于印支期（228.5±3.0Ma）；南兴安似斑状碱长花岗岩形成于燕山期（141.7±2.6Ma），捕获锆石年龄为1847±21Ma，反映该区可能具有前寒武纪结晶基底。两岩体岩石化学以富Si、高K、低Mg、贫Ca为特点，A/CNK值为1.00～1.05；富集高场强元素和轻稀土元素，亏损重稀土元素和大离子亲石元素，稀土元素配分模式为典型的右倾海鸥型。岩相学和地球化学特征显示，三广山和南兴安花岗岩均为A型花岗岩。其中，三广山花岗岩为A2型花岗岩，南兴安花岗岩为A1型花岗岩。全岩Sr-Nd同位素组成（εNd（t）值为2.26～5.58）表明，阿尔山地区花岗岩的源区可能为显生宙-新元古代期间从亏损地幔中新增生的地壳物质，但南兴安和三广山2个岩体的形成机制不同。三广山A2型花岗岩可能是古亚洲洋在三叠纪闭合后进入造山后阶段岩石圈伸展体制下的产物，而南兴安A1型花岗岩可能形成于早白垩世大兴安岭地区板内伸展作用下的拉张减薄环境。

Zircon U-Pb dating results by LA-ICP-MS technique and geochemical data of Aershan granitoid plutons are reported in this paper. The results of LA-ICP-MS zircon U-Pb dating are 228.5±3.0Ma for Sanguangshan medium-grained alkali feldspar gran-ite and 141.7 ± 2.6Ma for Nanxing＆#39;an porphyraceous alkali feldspar granite. The age of captured zircons in Nanxing＇an granite is 1847 ± 21Ma, suggesting that Precambrian basement may exist in this region. Petrochemically, the rocks are Si-rich, K-rich, Mg-low, and Ca-poor with A/CNK in the range of 1.00～1.05. These granites are enriched in HFSE and LREE, and depleted in LILE and HREE. They show typical＆quot;right-inclined seagull wing-shaped＆quot;REE patterns, suggesting A-type granites. The whole rock Sr-Nd isotopic data （εNd（t）=2.26～5.58） show that the source of the granites in Aershan area was probably the new accreted crustal mate- rial which was derived from the depleted mantle. Late Triassic Sanguangshan A2-type granite resulted from the post-orogenic litho-spheric extension setting after the closure of the Paleo-Asian Ocean. Early Cretaceous Nanxing＆#39;an A 1-type granite might have been formed in a thinned continental intraplate extensional environment.