内蒙古阿钦楚鲁二长花岗岩锆石SHRIMPU-Pb年龄及地球化学特征

阿钦楚鲁二长花岗岩体位于西伯利亚板块东南缘查干敖包—奥尤特—朝不楞早古生代构造-岩浆岩带中段,主要岩石类型为中细粒二长花岗岩和中粗粒二长花岗岩。SHRIMP锆石U-Pb同位素定年结果表明,阿钦楚鲁二长花岗岩的成岩年龄为(296.3±3.8)Ma,为华力西晚期。岩石地球化学分析结果表明:阿钦楚鲁二长花岗岩富硅,w(SiO2)为73.48%~74.22%,过铝质,w(Al2O3)为13.63%~14.01%,A/CNK值为1.04~1.10,碱质含量较高,w(K2O)+w(Na2O)为8.08%~8.54%,里特曼指数(σ)为2.13~2.46,相对富钾,K2O/Na2O值为1.31~1.54,属高钾钙碱性系列。该岩石富集大离子亲石元素Rb、Sr、Ba和轻稀土元素(LREE),相对亏损Ta、Nb、Ti等高场强元素,稀土元素总量为(112.05~130.16)×10-6,中等Eu负异常(δEu=0.52~0.65),稀土元素配分曲线呈现出略微右倾型,轻稀土较陡,重稀土较缓,具有向A型花岗岩过渡的后碰撞高钾花岗岩特征;岩石具有较低的87Sr/86Sr初始值(0.703 849~0.704 236)和正的εNd(t)值(4.2~4.3),反映其物质来源可能主要为幔源岩浆底侵作用形成的新生大陆地壳。基于上述分析研究和构造环境判别,结合区域对比,推测阿钦楚鲁二长花岗岩为在岩石圈由挤压增厚向伸展体制转换的动力学背景下,由于俯冲板片的断离,造成软流圈上涌和岩石圈地幔的部分熔融,而部分幔源岩浆底侵到地壳的下部或者呈基性侵入体的形式侵入地壳,引起上部地壳的熔融而形成后碰撞高钾钙碱性花岗岩。

SHRIMP Zircon U Pb Age and Geochemical Characteristics of the Achieng Qulu Monzogranite in Inner Mongolia

The Achieng Qulu monzogranite complex is located in the middle of Early Paleozoic Chaganaobao-Aoyoute-Chaobuleng tectonic-magmatic belt which belongs to the southeast margin of the Siberian plate.The main rock types of the complex are moderate-and fine-grained monzogranite as well as moderate-and coarse-grained monzogranite.SHRIMP zircon U Pb isotopic dating result indicates that the emplacement age of Achieng Qulu monzogranite is (296.3±3.8)Ma,namely Late Hercynian. Rock geochemical analysis shows silicon-rich of the monzogranite,as peraluminous with SiO2 content of 73.48% to 74.22%;with Al2 O3 content of 13.63% to 14.01% and A/CNK ratio of 1.04 to 1.10;alkali-rich,(K2 O+ Na2 O)content of 8.08% to 8.54%,with Rittmann index of (σ)2.13 to 2.46, relatively rich in potassium,K2 O/Na2 O ratio of 1.31 to 1.54,belonging to high-K calc-alkaline series;enriched in such large ion lithophile elements as Rb,Sr,Ba and light rare earth elements (LREE), relatively depleted in high field strength elements such as Ta,Nb,Ti;total REE to 1 12.05 × 10 -6 130.1 6×10 -6 ,moderate negative Eu anomality (δEu = 0.52 to 0.65),showing a slightly rightward with steep LREE line and slow HREE line,containing geochemical characteristics of from post-collision high-K granite to A-type granites.Relatively low initial 87 Sr/86 Sr ratios (0.703 849 to 0.704 236)and positiveεNd (t)values (4.2 to 4.3),reflect that the material source of the complex may be mainly derived from the new continental crust formed by underplating of mantle-derived magma.Based on the data mentioned above and discrimination on tectonic setting, combined with regional comparison, we conclude that the Achieng Qulu monzogranite belongs to post-collision high-K calc-alkaline granite derived from the partial melting of the upper crust,which is caused by sub-ducted slab break-off, asthenosphere upwelling and partial melting of the lithosphere mantle,meanwhile,part of the mantle-derived magma under-plated to the lower part of the crust or formed mafic intrusions invading crust. These events took place under the dynamics background of lithosphere transforming from compressive thickening to extension system.This recognition provides petrology evidence and age restricting for the Late Paleozoic tectonic magmatic evolution of East Wuzhu Muqinqi and for post-orogenic extensional tectonic mechanism transformation from geochemical combination after the collision against Siberian plate and the North China plate.