塔里木二叠纪石英正长斑岩岩墙的发现及其构造意义  

Discovery of a Permian quartz syenitic porphyritic dyke from the Tarim basin and its tectonic implications.

作　　者：杨树锋[1] 厉子龙[1] 陈汉林[1] 肖文交[2] 余星[1] 林秀斌[1] 施锡桂[1] 

机构地区：[1]浙江大学地球科学系,杭州310027 [2]中国科学院地质与地球物理研究所岩石圈演化国家重点实验室,北京100029

出　　处：《岩石学报》2006年第5期1405-1412,共8页Acta Petrologica Sinica

基　　金：国家重点基础研究发展规划项目(2001CB409801);国家自然科学基金(40472120和40472040);高等学校优秀青年教师教学科研奖励计划的资助;中科院广州地球化学研究所同位素年代学和地球化学重点实验室的部分资助.

摘　　要：新疆塔里木盆地内部发育了大量二叠纪(260～292 Ma)玄武岩类为主的岩浆岩,主要由玄武岩、辉绿岩、玄武安山岩、超镁铁质岩石和正长岩等组成。作者野外地质调查过程中在巴楚水工团附近发现石英正长斑岩岩墙的出现。这些石英正长斑岩属准铝质(A/CNK＜1),其中SiO_2含量为66～67%,并以富K_2O+Na_2O(10～11%)、K_2O/Na_2O(0．8～0．9)和较低的Mg/(Mg+Fe)比值为特征,且具有富集大离子亲石元素(Ba、Rb)和高场强元素含量(Zr、Nb、Y)、Ga/Al比值、高的稀土总量(631～734×10^(-6))、高度富集的LREE/HREE比值以及具Eu的负异常。这些特征并结合构造判别图解显示其具有典型的A型花岗岩的特征。低的Y/Nb(0．4)＜1．2以及微量元素蜘蛛网图中呈现大离子亲石元素富集以及Nb平坦或稍凸的图谱指示其源区来自于地幔,且在典型板内环境下形成。与小海子水库出露的正长岩的成分特征比较接近并很可能具有相同的源区成分。笔者对小海子正长岩体中粗粒正长岩采用精确的SHRIMP锆石U-Pb同位素定年,获得可靠的一致性年龄值为277±4 Ma,暗示小海子正长岩体侵位时代为277 Ma。根据野外观察、地球化学特征和构造环境的初步研究和对比,石英正长斑岩岩墙与小海子正长岩体一起很可能都是在早二叠世晚期形成,约在277Ma左右,它们代表了二叠纪陆内稳定环境下形成的产物,并可能作为塔里木地区最后一次大的岩浆热事件结束的标志。Huge Permian basalts and igneous rocks related （260 -292 Ma）, occurred in the Tarim Basin, Xinjiang are mainly composed of basalts, diabases, basaltic andesites, ultramafic rocks and syenites. This study is the first report of the quartz syenitic porphyritic dyke occurred in Shuigongtuan of Bachu County, Tarim Basin. The quartz syenitic porphyry in this dyke is metaluminous （ A/CNK 〈 1 ） , and has SiO2 of 66 - 67% , enriched in K2O ＋ Na2O（ 10 - 11% ） and K2O/Na2O （0.8 - 0.9 ）, and low Mg/（ Mg ＋ Fe） ratios. The rocks are enriched in LILE（Ba,Rb）and HSFE（ Zr,Nb,Y）, Ga/Al ratios and total rare earth elements（631 -734 × 10^-6 ）, high LREE/HREE ratios, and having negative Eu anomalies. The chemical characteristics and tectonic discriminative diagrams show that the rocks have geochemical affinity with A -type granites. Low Y/Nb（0.4） 〈 1.2, and enriched in LILE and Nb evenness - enrichment in the spider diagram indicates typical within - plate environment and a source from mantle, which may be derived from hot basic magma by fractional crystallization. Compared with the Xiaohaizi syenites, the quartz syenitic porphyry has almost the same geochemical characteristics, and they may be derived from the same source. The concordia age of 277Ma 4 Ma was obtained from the Xiaohaizi syenitic body by the SHRIMP U - Pb zircon age dating in this study, indicating that the intrusive age of the Xiaohaizi syenitic body is approximately 277Ma. Based on the present study, the Shuigongtuan quartz syenitic porphyritic dyke and Xiaohaizi syenitic body probably formed during Early Permian（277Ma）, and represented the products of within- plate environments, indicating the end of the last large magmatic thermal event happened in the Tarim Basin.

关 键 词：A型石英正长斑岩 板内环境 地球化学特征 SHRIMP定年 塔里木盆地 

分 类 号：P588.133[天文地球—岩石学] P597.3[天文地球—地质学]