天山石炭—二叠纪后碰撞花岗岩的Nd、Sr、Pb同位素源区示踪

Discussion of the sources and characteristics on Sr、Nd、Pb isotopes of the Carboniferous to Permian post-collision granites from Tianshan

天山造山带不仅是中亚巨型造山带的组成部分,也是我国西部晚古生代岩浆活动的集中区,蕴含着丰富的古亚洲洋演化的复合地质信息。笔者以探索天山晚古生代花岗岩浆活动的物质来源为目标,采用Sr、Nd、Pb同位素地球化学方法,对该区不同构造单元中具有代表性的花岗岩体进行了全面系统的研究,发现天山不同构造单元中花岗岩Sr、Nd、Pb同位素特征具有明显差异。在与其围岩的同位素特征进行对比研究的基础上,提出了产于不同构造单元石炭—二叠纪花岗岩的可能源区。觉罗塔格石炭—二叠纪裂谷带的晚古生代花岗岩,具有最高的εNd(t)值和最低的(87Sr/86Sr)t值,如,以康古尔石英闪长岩为代表的二叠纪花岗岩的源岩可能是早期泥盆纪大南湖岛弧火山岩(新生地壳);博格达地区的晚石炭—二叠纪花岗岩是早期基性火山岩,亦可能是底侵基性岩部分熔融的产物。中新元古代基底晚石炭—二叠纪裂谷带中花岗岩的源岩亦为同期基性火山岩,但在岩浆演化过程中受到古老基底地壳的混染,导致其εNd(t)值低于石炭—二叠纪裂谷带中花岗岩,而(87Sr/86Sr)t高于石炭—二叠纪裂谷带花岗岩。南天山东段和前寒武系微地块及其边缘中的晚古生代(含个别早古生代花岗岩)花岗岩分别是类似于库米什斜长角闪岩的古老地壳和西天山元古代古老地壳部分熔融的产物,因此它们均具有负的εNd(t)值和异常高的(87Sr/86Sr)t值。上述认识对于了解天山晚古生代区域岩石圈演化及深部过程具有重要意义。

Aiming at the magmatic sources of Late-Palaeozoic granites from Tianshan, this paper comprehensively and systemically discusses the representative granites from various tectonic units of Tianshan according their Sr、Nd、Pb isotopic characteristics, and reveals many obvious differences in these granites. On the basis of comparison with the isotopic characteristics of their wall rocks, the potential sources of these granites are indicated. The Late-Palaeozoic granites in Carboniferous to Permian rift belt of Jueluotage have the highest εNd(t) values and the lowest (87Sr/86Sr)t values, and the island-arc volcanic rocks (juvenile crust) of Devonian Dananhu Formation may be the potential source of Permian granites represented by the quartz diorites in Kangguer; the Late-Carboniferous to Permian granites of Bogda area are probably the products of the partial-melting of the contemporaneous basic volcanic or underplating basic magma. The source rock of the granites in the Carboniferous to Permian rift belts that have mesoproterozoic-neoproterozoic basements is the contemporaneous basic volcanic too, but may be contaminated by the basement crust during the magma generation, which resulted in the lower εNd(t) values and higher (87Sr/86Sr)t values of these granites than those of the granites of Carboniferous to Permian rift belts without Proterozoic basements. The Late-Palaeozoic granites (including some Early Palaeozoic granites) in east section of the Southern Tianshan and some Precambrain microcontinents or on their marginal are the partial melting products of the age-old crust similar to plagihornblendite of Kumishi and the Proterozoic crust of Western Tianshan respectively. These granites have the negative εNd(t) values and outstandingly high (87Sr/86Sr)t values. The results above are significant to the study of Late-Palaeozoic lithosphere evolution and deep process of Tianshan.