中国阿尔泰北部山区早泥盆世花岗岩的年龄、成因及构造意义

Ages and origin of the early Devonian granites from the north part of Chinese Altai Mountains and its tectonic implications.

对阿尔泰北部山区喀纳斯及琼库尔花岗岩体进行锆石 LA-ICPMS U—Pb 定年分别获得398±5Ma(MSWD=2.3)和399±4Ma(MSWD=1.2)的生成年龄,确定其形成于早泥盆世。结合近期研究成果,进一步表明,阿尔泰造山带花岗岩主要形成于早—中古生代,峰期在400Ma 左右。这两个花岗岩同阿尔泰其它400Ma 左右的花岗岩具有相同或相似的野外产状、岩石学、地球化学特征和同位素组成,如高硅、富钾、铝,轻稀土富集,具有 Eu 负异常,富集大离子亲石元素(Rh、Ba、K),具有明显的 Nb、Ta、Sr、P、Ti 负异常,具有负的ε_(Nd)(t)值(-1.5～-0.1)和相对于区内其它花岗岩较老的 Nd 同位素模式年龄 t_(DM)(1.15～1.35Ga),表明这些花岗岩具有钙碱性-高钾钙碱性特征,其物源源除了地幔物质的贡献,可能有更多地壳物质的加入。推断这些花岗岩可能是在洋壳俯冲过程中,可能是熔自深部地壳(可能是弧物质)的岩浆与来源于亏损地幔的镁铁质岩浆发生混合形成的。结合区域地质背景分析,显示喀纳斯、琼库尔岩体形成于陆弧环境,与其它同期花岗岩一起揭示了阿尔泰造山带最为强烈的一次俯冲、裂解和拼合事件。为我们已经提出的构造演化模式提供了新的证据。

Abstract Zircon U-Pb （LA-ICPMS） dating yields the ^206Pb/^238U ages of 398 ± 5Ma（ MSWD=2.3 ） and 399 ±4 （MSWD=1.2 ） for the Kanasi and Qiongkur granite plutons from the north part of the Chinese Ahai Mountains, respectively. The results, combing with previously published data, show that the 400Ma ca. granitoids distribute widely in the Chinese Ahai, and this period is a very important granitoids formation stage. The Kanasi, Qiongkur and other 400Ma ca. granites are enriched in K, Al, REE and LILE （Rb, K, Ba） with negative anomaly in Nb, Ta, Sr, Ti and P, and have negative БNd（t） values （-1.5～-0.5） and older tDM （1.15～1.35Ga） in relation to other granitoids in this area, which suggest that the granitoids mainly follow calc-alkaline and high-K calc- alkaline trends, form in continental arc settings, and the sources of the granitoids contain both old continental and mantle-derived rocks. These features are the same as those of 400Ma ca. granitoids recognized from the central and southern Chinese Altai. All of these provide the new evidences for the seduction and accretion processes of the Chinese Ahai in early-middle Paleozoic.