Late Carboniferous gabbro-granite suite from West Ujimqin of central Inner Mongolia: Petrogenesis and geodynamic implication

Abstract

辉长岩和花岗岩组成的双峰式火成岩系列通常产出于威尔逊板块构造旋回的诸多伸展周期。基于其岩石地球化学特征示踪伸展地球动力学过程与相关壳幔相互作用的独特视角，本文报道了中亚造山带东段内蒙古中部西乌旗地区新近识别的晚石炭世猴头庙辉长岩-花岗岩系列。SIMS锆石U-Pb定年揭示辉长岩和花岗岩的侵位年龄分别为~307Ma和~305Ma。辉长岩主要由斜长石、辉石和角闪石组成；SiO₂含量介于50.4%~53.4%之间，呈现镁质、准铝质和钙碱性特征；富集大离子亲石元素，亏损高场强元素；并具有OIB型全岩Sr-Nd （I_Sr（t）=0.704249~0.704330，ε_Nd（t）=+1.73~+3.81）和锆石Hf （ε_Hf（t）=+4.58~+15.16）同位素组成，其锆石δ¹⁸O介于5.75‰~6.61‰之间。这些特征指示辉长岩可能源于俯冲板片流体交代的亏损地幔楔部分熔融。同期花岗岩主要由斜长石、钾长石、石英和黑云母组成；其SiO₂含量变化于76.3%~77.6%，高硅碱、贫铁镁、弱过铝、富钍铀、亏锶钡、Eu负异常，契合典型高分异I型花岗岩的元素地球化学特征；花岗岩具有中等放射性成因的全岩I_Sr（t）（0.704923~0.704981）、ε_Nd（t）值（-0.15~+0.57）和锆石ε_Hf（t）值（-2.63~+7.40），以及高的锆石δ¹⁸O （6.44‰~8.06‰）。鉴于元素和同位素特征指示的高分异特征和高演化源区，猴头庙高硅花岗岩可能衍生自中元古代和古生代多期增生物质所组成的复合中基性地壳的部分熔融及其后的结晶分异作用。综合区域石炭纪-早二叠世蛇绿混杂岩和岛弧岩浆岩时空展布格局，晚石炭世猴头庙辉长岩-花岗岩系列记录了内蒙古中部其时因洋脊俯冲引发的弥散性区域伸展事件。一方面，该事件既有别于建造西乌旗早-中石炭世弧-盆体系的板片回退阶段，又迥异于早二叠世大规模后碰撞伸展过程；另一方面，这些周期性伸展事件共同见证了中亚造山带东段晚古生代威尔逊造山旋回的完整历程。;Coeval bimodal igneous suites of gabbro and granite commonly typify frequent extensional episodes in the Wilson cycle of plate tectonics. With a unique view to tracing extensional geodynamic processes and related crust-mantle interactions through the petro-geochemical characteristics of such suites, this paper reports the newly identified Late Carboniferous Houtoumiao gabbro-granite suite from West Ujimqin of central Inner Mongolia, eastern Central Asian Orogenic Belt (CAOB). SIMS zircon U-Pb dating unravels the emplacement age of gabbro at ca. 307Ma and that of granite at ca. 305Ma. The gabbros mainly consist of plagioclase, pyroxene and amphibole. They range in SiO₂ from 50.4% to 53.4%, exhibiting magnesian, metaluminous and calc-alkalic affinities, with enrichment in large ion lithophile elements (LILEs) and depletion in high field strength elements (HFSEs). They possess OIB-type whole-rock Sr-Nd (I_Sr(t)=0.704249~0.704330, ε_Nd(t)=+1.73~+3.81) and zircon Hf (ε_Hf(t)=+4.58~+15.16) isotope compositions, and zircon δ¹⁸O values from 5.75‰ to 6.61‰. These features suggest that the gabbros might be derived from the partial melting of a depleted metasomatised mantle wedge from subduction-derived fluids. The coeval granites mainly comprise plagioclase, K- feldspar, quartz and biotite. They span in SiO₂ from 76.3% to 77.6%, with high alkalis, thorium and uranium but low iron, magnesium, strontium and barium. They are weakly peraluminous with negative Eu anomalies. These features are typical of highly fractionated I-type felsic magmas. With respect to isotopic compositions, the granites are characterized with medium radiogenic whole-rock I_Sr(t) (0.704923~0.704981), ε_Nd(t) (-0.15~+0.57) and zircon ε_Hf(t) (-2.63~+7.40), and high zircon δ¹⁸O (6.44‰~8.06‰). Given their highly fractionated character and highly evolved sources, the Houtoumiao granites tend to have been derived from the partial melts of the composite mafic to intermediate lower crust composed of multi-stage accretionary materials in the Mesoproterozoic and Paleozoic times, with subsequent fractional crystallization yielding the high-silica granites. Combined with the spatial and temporal distribution patterns of regional Carboniferous-Early Permian ophiolitic mélange and island arc magmatic complexes, the Late Carboniferous Houtoumiao gabbro-granite suite seems to record diffuse regional extension event due to ridge subduction in central Inner Mongolia. On one hand, this event is not only distinct from slab rollback episode of the Early-Middle Carboniferous arc-basin system in West Ujimqin, but also in contrast with large-scale post-collisional extension during Early Permian time. On the other hand, when taken together, these episodic extensional events tend to witness the complete course of the Late Paleozoic Wilson cycle in the eastern CAOB.