胶-辽-吉造山带北辽河变基性岩的成因、地球化学属性及其构造意义

大规模出露于胶-辽-吉造山带的北辽河变基性岩(NLHmetabasicrocks)是恢复造山带构造演化的关键之一,然而,研究者们对这些变基性岩的成因和构造环境有很大争议,制约了对胶-辽-吉造山带早期构造演化的深入研究。基于详细的野外地质调查,本文对北辽河变基性岩进行了系统的岩石学研究并提供了新的全岩地球化学数据,结合前人的测试结果,我们对这些变基性岩的成因和构造环境进行了相关探讨。变基性岩侵入于北辽河群的大理岩和碎屑岩中,经历了一定程度的蚀变以及绿片岩相-角闪岩相变质。岩石类型包括变质辉长岩、变质辉绿岩、斜长角闪岩以及石榴斜长角闪岩;前两者具有明显的变余辉长或者辉绿结构,可见自形斜长石,少见原生矿物单斜辉石的残留;后两者主要由斜长石和角闪石组成,部分含有石榴子石变斑晶;此外,岩石中出现大量Fe-Ti氧化物(磁铁矿和钛铁矿)。地球化学研究表明,北辽河变基性岩均属于拉斑玄武岩系列(SiO_2=44.55%~54.11%,Nb/Y=0.16~0.31),以低TiO_2(0.69%~1.99%)和低MgO(多数5.47%~7.97%)为特征,轻稀土轻微富集((La/Sm)_N=1.22~2.66;(La/Yb)_N=1.66~4.56),并具有明显的Nb、Ta、P以及轻微的Zr、Ti亏损,岩石经历了橄榄石、单斜辉石和斜长石等矿物的三相结晶分异以及Fe-Ti氧化物(磁铁矿和钛铁矿)的堆晶;它们的微量元素配分型式类似于E-MORB和地壳岩石,并且地球化学组成与MORB相近,而不同于弧玄武岩和板内玄武岩;这些变基性岩的Ce/Pb(3.07~26.58)、(Ta/La)PM(0.47~1.45)以及(Hf/Sm)PM(0.91~1.15)显示出有限的俯冲相关流体的交代作用;此外,大多数不相容元素比值(例如Th/Nb、La/Nb及Th/La等)的变化趋势则表明北辽河变基性岩的岩浆源区不可能是大陆岩石圈地幔,并且在形成过程中经历了明显的地壳物质混染。基于这些野外地质调查和地球化学研究,我们认为北辽河变基性岩应形成于相对成熟的弧后盆地,这样一个形成环境也表明胶-辽-吉造山带在早期存在~2.1Ga的洋壳俯冲过程,该构造带应该是~1.9Ga岛弧与大陆碰撞形成的一条古元古代造山带。

Petrogenesis and geochemical characteristics of the North Liaohe metabasic rocks, Jiao-Liao-Ji orogenic belt and their tectonic significance

The metabasic rocks, which widely occur in the North Liaohe Group (NLH metabasic rocks) of the Jiao-Liao-Ji orogenic belt (JLJB), can provide important information for understanding the tectonic evolution of this belt. However, the petrogenesis and tectonic setting of the NLH metabasic rocks remain the subject of intense debate, and this prevents the study on the early tectonic evolution of the JLJB. Based on the detailed field investigation, we make a systematic study on their petrology and report new whole-rock geochemical data. The study aimed to investigate their petrogenesis and tectonic setting. These metabasic rocks were emplaced in marble and clastic rocks of the North Liaohe Group, and underwent varying degrees of alteration and greenschist to amphibolite facies metamorphism. They consist of meta-gabbro, meta-diabase, amphibolite and garnet amphibolite. The two former rocks containing idiomorphic plagioclase and minor relict clinopyroxene preserve original gabbro and ophitic textures, and the two later rocks are mainly composed of hornblende and plagioclase, and partly contain garnet as porphyroblast. In addition, Fe-Ti oxides (magnetite and ilmenite) widely exist in the NLH metabasic rocks. Whole-rock geochemistry shows that the NLH metabasic rocks with tholeiitic compositions (SiO₂=44.55%~54.11%,Nb/Y=0.16~0.31) are characterized by low TiO₂ (0.69%~1.99%) and MgO (mostly 5.47%~7.97%), and are lightly enriched in light rare earth elements ((La/Sm)_N=1.22~2.66;(La/Yb)_N=1.66~4.56) but depleted in Nb, Ta and P strongly and in Zr and Ti lightly. The rocks have experienced fractional crystallization of a three-phase assemblage of olivine, clinopyroxene and plagioclase and accumulation of Fe-Ti oxides (magnetite and ilmenite). Their trace element patterns are comparable to those of E-MORB and total crust (TC), and their geochemical compositions are similar to those of MORB but different from those of arc basalts and within-plate basalts. The NLH metabasic rocks have relatively wide ranges of Ce/Pb (3.07~26.58) and (Ta/La)_PM (0.47~1.45) and a narrow range of (Hf/Sm)_PM (0.91~1.15), indicating limited fluid-related metasomatism. In addition, the continental lithospheric mantle (CLM) and TC clearly deviates from and follows the geochemical trends of mostly incompatible trace element ratios, respectively, implying that the NLH metabasic rocks were unlikely derived from a metasomatized CLM source, and that they have experienced important crustal contamination. The detailed field and geochemical investigations allow us to speculate that the NLH metabasic rocks formed in a relatively matured back-arc basin setting, suggesting a~2.1Ga oceanic subduction in the JLJB. The JLJB is a paleoproterozoic orogenic belt formed from the~1.9Ga arc-continent collisional event.