老挝-越南长山成矿带北西段高分异I型花岗岩LA-ICP-MS锆石U-Pb年龄及其地质意义

Highly fractionated I-type granites in the northwestern part of Truong Son metallogenic belt, Laos-Vietnam: LA-ICP-MS zircon U-Pb age and its geological implications

关键词:  锆石U-Pb年龄  高分异I型花岗岩  长山成矿带  老挝  越南

Key words:  zircon U-Pb age  highly fractionated I-type granite  Truong Son metallogenic belt  Laos  Vietnam

摘要:

马江结合带俯冲时代一直是地质研究的热点问题,老挝-越南长山成矿带花岗岩的构造属性是对马江洋闭合时间的重要制约。本文报道了老挝-越南长山成矿带北西段的中细粒花岗岩,其LA-ICP-MS锆石U-Pb年龄为261.34 ±0.69 Ma (MSWD=1.4),为晚二叠世。岩石具有高SiO₂(77.01%~78.07%)、高K₂O (4.32%~4.57%)、低钙镁(CaO=0.32%~0.34%;MgO=0.04%~0.06%)和低P₂O₅含量特征,富集Rb、U、Th,明显亏损Ba、Sr、P、Ti和Eu等,属高分异I型花岗岩。该岩体可能是马江洋岩石圈南向俯冲的背景下,由俯冲海洋岩石圈诱导的幔源岩浆侵入古老地壳,形成初生地壳,在后期持续的热事件的影响下,提供足够的热引起初生地壳和古老地壳的混合地壳部分熔融,形成母岩浆,再经历高度分离结晶作用形成。

Abstract:

The subduction age of the Song Ma suture zone has long been the hot spot for geological research in this region. The tectonic attributes of the granites from the Truong Son metallogenic belt in Laos-Vietnam zone are an important clue to the timing of the closure of the Song Ma Ocean. The present paper reports the LA-ICP-MS zircon U-Pb age of 261.34±0.69 Ma (MSWD=1.4) for the medium-to fine-grained granites from the northwestern part of Truong Son metallogenic belt, Laos-Vietnam zone. This age shows that the emplacement of the granites may be traced back to the Late Permian. The granites are characterized by high SiO₂(77.01%-78.07%), high K₂O (4.32%-4.57%), low Ca and Mg (CaO=0.32%-0.34%; MgO=0.04%-0.06%) and low P₂O₅, enrichment of Rb, U and Th, and depletion of Ba, Sr, P, Ti and Eu, suggesting the highly fractionated I-type granites. During the southward subduction of the Song Ma Ocean lithosphere, the mantle-derived magmas above the subduction zone firstly penetrated into the ancient crust giving rise to the generation of the juvenile crust, followed by the formation of the parent magmas caused by the mixed partial melting of the juvenile crust and ancient crust. Finally the highly fractional crystallization resulted in the formation of the granites in the study area.