内蒙古晚石炭世高镁玄武岩的成因和构造背景  

Petrogenesis and tectonic settings of the Late Carboniferous high Mg basalts of Inner Mongolia

作　　者：邵济安[1,2] 田伟[1,2] 唐克东[3] 王友 

机构地区：[1]造山带与地壳演化教育部重点实验室 [2]北京大学地球与空间科学学院,北京100871 [3]沈阳地质矿产研究所,辽宁沈阳110034 [4]内蒙古地勘局第十勘察开发院,内蒙古赤峰024005

出　　处：《地学前缘》2015年第5期171-181,共11页Earth Science Frontiers

基　　金：国家自然科学基金项目(41172196)

摘　　要：近年来，人们对古亚洲洋何时消亡存在争论，其中一种主流的观点认为直至晚古生代末大洋才闭合，导致大量的石炭纪一二叠纪岛弧火山岩的出现。本文围绕这一问题对内蒙古西乌珠穆沁旗达青牧场晚石炭世高镁玄武岩（318．4~314Ma）的形成环境进行讨论，分析和讨论是从以下4方面展开的：（1）玄武岩的基础地质特征：分布、产状、岩石一地球化学组成；（2）玄武岩成因和岩浆来源；（3）双峰式火山岩形成环境；（4）晚石炭世和早二叠世火山岩的对比研究。通过研究，得到以下认识：本区玄武岩的时、空分布是稳定的；玄武岩具有高镁（〉8％）、低钾（〈1％）、LREE亏损（La／Yb=0．82）、铪同位素元素高（εHf（t）=14．4～23．9）等特征。据此认为，玄武岩不存在地壳物质混染的现象，推测在压力不大的伸展背景下，来源于新生岩石圈地幔橄榄岩部分熔融的高温岩浆，快速通过上覆陆壳到达地表。对玄武岩同时存在的Nb、Ta及其它HFSE亏损现象，推论与地幔楔长期受到俯冲流体交代的亏损地幔源区有关。通过对本区及邻区晚石炭世双峰式火山岩的讨论，提出了本区火山岩属于大陆拉张减薄类型火山岩的观点。同时，从构造一岩相古地理研究的角度，对晚石炭世本区所处的伸展环境进行了讨论，支持了上述观点。最后，通过晚石炭世和旱二叠世两期双峰式火山岩的对比，从演化角度进一步阐明中亚造山带年轻陆壳成熟的过程。中亚造山带晚石炭世一早三叠世裂谷型岩浆岩带呈数千km大规模带状分布的事实，进一步肯定了晚石炭世以来中亚造山带已经作为板内的年轻陆壳，承受了全球性大规模伸展事件。Recently a debate has arisen about when the paleo-Asiatic ocean was consumed. One of the prominent view-points suggests that it was until the end of the Late Paleozoic when the ocean was closed, leading to large amounts of Carboniferous-Permian island-arc volcanic rocks occurring. Respect to this issue, this study is aiming at the formative environment of the Late Carboniferous high-Mg basalt （318. 4 - 314 Ma） in Daqing pasture, Xiwuzhumuqinqi, Inner Mongolia. The study was carried out in four aspects. （1） the basic geological characteristics of the basalt., its distribution, occurrence, and petrological and geochemical compositions, （2） the genesis and magma source of the basalt, （3） the formative environment of the Late Carboniferous bimodal volcanic rocks, and （4） the comparison of the bimodal volcanic rocks of the Late Carboniferous with those of the Early Permian. It is concluded from our study that the basalt in the concerned area has a stable spatial-temporal distribution and is characterized by higher MgO （〉8%）, lower K2O （〈 1%）, depleted LREE （La/Yb=0. 82）, and positive εHf （t） = 14. 4 - 23. 9. It is suggested that the basalt had not been hybridized by crustal materials and the partially-melted high-temperature magma had come under a relatively low pressure from a newly formed lithospheric-mantle peridotite, quickly passed through the overlying continental crust, and arrived at the surface. The depletion of Nb, Ta, and other HFSE-s in the basalt should be related to the depleted mantle-source region where the mantle wedge had been replaced by the subducting fluids over a long period of time. Comparison of the bimodal volcanic formations in this area and in adjacent regions indicates that the volcanic rocks in this area belong to the continental extensional thinning type. Such a conclusion is also supported by the extensional tectonic settings of the area in the Late Carboniferous, obtained by the tectono-palaeogeographic studies on the basis of elemental geochemis

关 键 词：晚石炭世 高镁玄武岩 双峰式火山岩 陆壳 伸展背景 

分 类 号：P588.145[天文地球—岩石学]