小兴安岭-张广才岭构造带早中生代镁铁质侵入岩岩石成因及地质意义

The Lesser Xing’an Range-Zhangguangcai Range is located in the central of the Xingmeng orogenic belt (XMOB). The region underwent not only the evolution of the Paleo-Asian oceanic tectonic system, but was later overprinted and modified by events related to the circum-Pacific tectonic system and the Mongol-Okhotsk tectonic system in the early-Mesozoic. In recent years, extensive attention has been paid to the XMOB because it was the world’s most important juvenile crust production in the Phanerozoic era. Previous studies mostly focused on the granite and volcanic rocks, and little attention has been paid to the widely spread mantle-derived mafic intrusions. The present study focuses on the early-Mesozoic mafic intrusions from the Lesser Xing’an Range-Zhangguangcai Range, based on the investigation of geochronology (zircon U-Pb and amphibole Ar-Ar dating) and geochemistry (mineral compositions, major and trace elements), aims to constrain the time and stages of the lithosphere extension, investigate the magma evolution and mantle metasomatism, discuss the nature of the mantle source ,the petrogenesis and the geodynamic process of these rocks. The research will be of significance in understanding the early-Mesozoic tectonic-magmatic evolution processes and dynamical mechanism, and also will provide evidence for the discussion of the mechanism and patterns involved in the crustal growth.

小兴安岭-张广才岭构造带位于兴蒙造山带的中部，该区早中生代处于古亚洲洋闭合及闭合后伸展拉张阶段，同时又叠加了古太平洋构造体系以及蒙古-鄂霍茨克大洋的影响，构造背景复杂且具争议，而且存在显著的显生宙地壳增生。以往的研究主要集中于花岗岩和火山岩等方面，但对该区广泛分布的镁铁质侵入岩没有引起足够的重视。本项目拟以小兴安岭-张广才岭构造带早中生代镁铁质侵入岩为研究对象，通过详细的年代学（锆石U-Pb定年和单矿物40Ar/39Ar定年）和地球化学研究（矿物化学、元素和Sr-Nd-Hf-O同位素），查明镁铁质侵入岩的时空分布、岩浆演化过程、地幔源区性质、地幔交代作用、岩石成因及构造属性。该研究对深入认识兴蒙造山带早中生代构造演化过程及动力学背景具有重要意义，同时可为探讨地壳增生的机制和方式提供重要依据。

兴蒙造山带是一个在古生代到中生代期间由多个微陆块拼合而形成的构造堆叠区，先后受到古亚洲洋闭合以及古太平洋俯冲影响，具有复杂的构造演化背景，且在显生宙存在显著的地壳增生。小兴安岭-张广才岭构造带位于兴蒙造山带的中部，本项目以小兴安岭-张广才岭构造带早中生代镁铁质侵入岩为研究对象，通过详细的矿物化学、年代学和地球化学研究（元素和Sr-Nd-Hf-O同位素），查明镁铁质侵入岩的时空分布规律、岩浆演化过程、地幔源区性质、地幔交代作用、岩石成因及构造属性。取得了以下认识和成果：（1）早中生代研究区共存在两期主要的镁铁质岩浆事件，时间分别集中在209-202 Ma和188-182 Ma；（2）209-202 Ma镁铁质岩浆事件空间上沿牡丹江断裂带西缘呈南北向分布，同牡丹江断裂带的空间展布高度耦合，岩浆源区较为富集且富水，受到充分的流体交代，并混入了陆间洋盆的远洋沉积物，该事件形成于牡丹江洋西向俯冲于松嫩-张广才岭地块导致的陆缘弧环境，该研究限定209-202 Ma牡丹江洋仍然处于持续俯冲阶段，且俯冲带之上存在弧岩浆岩；（3）188-182 Ma镁铁质岩浆事件空间范围较广，大体呈NNE向贯穿小兴安岭-张广才岭以及南部的延吉地区，岩体分为弧岩浆岩和OIB型岩浆岩两种，弧岩浆岩受到明显的流体交代作用的影响，而OIB型岩浆岩没有流体交代的痕迹，两者兼存说明此时为古太平洋俯冲形成的弧后伸展环境的初始阶段，板片后撤导致软流圈侧向上涌，在石榴子石二辉橄榄岩和尖晶石二辉橄榄岩过渡区域形成了OIB型岩浆岩，而俯冲板片熔流体交代上覆地幔楔部分熔融则形成了弧形镁铁质岩浆岩。（4）早侏罗世晚期（~173 Ma）花岗闪长岩中幔源暗色包体的形成则表明东北地区此时仍处于古太平洋俯冲导致的弧后伸展环境，幔源岩浆底侵导致下地壳部分熔融形成了寄主花岗闪长岩，同时也证明早中生代东北地区仍然存在地壳增生，而且是垂向增生。