阿尔泰造山带的铅同位素地质及其构造意义

阿尔泰遣山带中各构造带混合岩、片麻状花岗岩、钾长花岗岩的17个单矿物长石铅同位素测试分析表明:阿尔泰造山带中普遍存在前海西基底,模式年龄l_1年龄段为:1286~2483Ma,对应着陆壳的增生事件及初始成岩年龄.模式年龄l_2年龄段为:389~412Ma,311~346Ma,166~196Ma,分别对应着晚志留-海西早期大陆裂谷作用;海西中晚期强烈挤压遣山和斜向逆冲及印支期造山期后花岗岩的侵入作用.t_2中各时间段与各构造环境的判别相吻合.

LEAD ISOTOPE GEOLOGY AND ITSTECTONIC IMPLICATIONS IN ALTAIDES, CHINA

:The studies show that the resources and genesis of rocks, the composition of upper mantle, tectonic environments and evolutions, and the model ages can be obtained through the analyses and calculation of feldspar lead isotope. The measured results of seventeen samples of feldspar lead isotopic composition of migmatitic-gnesisic granite and gnesis granite indicate that the values of 206Pb/204Pb vary from 17.973 to 19.581, mostly from 18.200 to 18.600, the values of 207Pb/204Pb vary from 15.422 to 15.748, and the values of 208Pb/204Pb vary from 37.800 to 38.920. The resources of lead isotope of Altaides originate from erogenic belts mostly, and the upper mantle as well as upper crust lessly compared with the composition of lead isotope in different tectonic environments of Doe and Zartman. The tectonic divisions of Altaides, China will be divided into five tectonic belts by the detailed studies on the structural geology and sedimentary formations, they are: the North Altai Belt, the South Altai Belt, the Qiongkuer-Abagong Belt, the Irtys Belt, the Buerjing-Ertai Belt. Some principles between the lead isotopic composition and tectonic environments in Altaides will be discovered when we drop the measured data of seventeen samples to the evolutionary diagram of lead isotope and the diagram of lead isotopic composition in different geological and tectonic environments: a. Some measured samples originate from mantle and mix with the crustal rocks when they transit to the upper crust determined by the distribution of measured samples projected along the mantle evolutionary line and the field occurences of magmatic rocks. b. The early staged covered rocks (preDevonian) of crust become cratonic crust finally by the cratonization. c. Different originated rocks of the upper crust, the lower crust and the mantle undergo the strong mixing and have a tendancy to result in the homogenization of lead isotopic composition during the continental rifting and the orogeny, the intense tectono-thermal events of Hercynian are the controlling factors for the lead isotope. So it is concluded that: 1). The early staged upper crustal rocks indicate that the preDevonian covered rocks develop in Altaides. 2). The rocks originated from mantle resource mixed with the various crustal rocks uplift to the upper crust during the continental rifting stage of the late Silurian to the early Hercynian in Altaides. 3). The rocks with the different crustal composition mix strongly with the mantle intrusions and have a tendancy to result in the homogenization lead isotopic composition during the intense erogenic and oblique thrusting stage of the middle to the late Hercynian in Altaides. The calculations of two stage model ages of seventeen measured samples by the computer indicate that the model ages of formation of primary rocks t, can be divided into three periods: 248.3 Ma (one data), 2048-1786 Ma (nine data), 1551-1286Ma (five data), which indicate that preDevonian rocks may be distributed in Altaides and may be represented the crustal increasing events of the Wutai orogeny, the Zhongtiao orogeny and the Wuling orogeny.The model ages t2 also can be divided into three periods: 389-412 Ma (eight data), 311-346 Ma (five data), 166-196 Ma (two data), which probably correspond to the tectono-thermal events of continental rifting (389-412 Ma), intense orogeny and oblique thrusting (311-346 Ma), and the intrusions of feldspar granite after orogeny in the Indo-China movement. These model ages t2 are in concordance with the results from the chronological data measured by the K-Ar, Rb-Sr methods and geological formations. The analyses of the tectonic environments are consistent with the analyses of the feldspar lead model ages.