太行山-大兴安岭构造岩浆带中生代侵入岩Nd、Sr、Pb同位素特征及物质来源探讨

Nd, Sr, Pb isotopic characteristics of the Mesozoic intrusive rocks in the Taihang-Da Hinggan Mountains Tectonomagmatic Belt and their source region.

太行山-大兴安岭构造岩浆带是中国东部中生代大规模岩浆活动的集中区,它纵跨华北克拉通和兴蒙海西造山带两个大地构造单元,既是我国东部规模最大的北北东向深大断裂带之一,又是我国东部北北东向延伸的地壳厚度陡变带与重力异常高梯度带,因而具有十分重大的地质意义。本文针对研究区在中生代岩浆活动物质来源认识方面的不足,首次对太行山-大兴安岭整个构造岩浆带40余个代表性岩体的Nd、Sr、Pb同位素地球化学特征进行了全面系统的研究,发现不同区段(南、北太行山地区、大兴安岭中-南地区)、不同侵入期次(早、中、晚)岩石具有明显不同的Nd、Sr、Ph同位素特征,可以推断其岩浆源区性质各不相同:南太行山地区基-中性岩浆均来源于富集地幔储库;北太行山地区早期基-中性岩浆主要来源于富集地幔的部分熔融;中期中-酸性岩浆来源与下地壳关系密切;晚期富碱质中-酸性岩浆来源可能与下地壳、甚至中地壳物质有关;大兴安岭地区中-酸性岩浆来源与亏损地幔关系密切,可能源自与亏损地幔有联系的年轻的下地壳。由此表明,中生代华北克拉通地区岩石圈地幔为富集地幔,而兴蒙造山带地区岩石圈地幔则为亏损地幔。此外,根据岩石的Nd模式年龄,初步认为2543～1485Ma可能指示华北克拉通岩石圈地幔发生富集作用的时间,而983～540Ma则可能为大兴安岭地区地壳生长的一个重要时期。

The Taihang - Da Hinggan Mountains Tectonomagmatic Belt is the Mesozoic magmaeric activities concentrating area in eastern China, which passes through the two tectonic units of the North China Craton and the Inn Mongolian Orogenic Zone. In this paper a systematic geochemical study on Nd, Sr and Ph isotopic characteristics from more than 40 typical complexes along the tectonomagmatic belt is presented to indicate that the Mesozoic intrusives in different sections of this belt ( mainly the north, south Taihang Mountains and the middle-south Da Hinggan Mountains), even those of different stages ( mainly three stages) in the same section have entirely different Nd, Sr and Pb isotopic characteristics, which reveals that their source regions are different. The source rocks in the southern section of Taihang Mountains are related to materials from the enriched mantle reservoir. In the northern section the early stage basic-intermediate rocks are mainly formed by the partial melting of materials from the enriched mantle, the main stage intermediate-acid ones have a close connection to materials from the lower crust, while the late stage alkali-rich ones are even derived from those from the lower-middle crust. And the intermediate-acid magma in the middle-south Da Hinggan Mountains is mainly originated from depleted mantle materials. These also reveal that the lithospheric mantle below the North China Craton is enriched and that below the Inner Mongolia Orogenic Zone is depleted. In addition, implicated by Nd depleted mantle model ages (T-DM), 2543 similar to 1485Ma probably represents the time for mantle enrichment beneath the North China Craton, and 983 similar to 540Ma ( Proterozoic to early Palaeozoic period) suggests one of the main stages for the crust growth in Da Hinggan Mountains area.