东昆仑大格勒地区碱性杂岩体中辉石角闪石岩的年代学、地球化学特征及地质意义

Chronology,Geochemistry,and Geological Significance of Pyroxene Hornblendites in the Alkaline Complex in the Dagele area,East Kunlun

东昆仑大格勒地区发现了以铌为主的稀有和稀土矿化碱性杂岩体,辉石角闪石岩构成了杂岩体的主体且有弱矿化显示。为了查明辉石角闪石岩的成岩时代和岩石成因、了解稀有和稀土矿的成矿背景,指导区域找矿突破,本文对其进行了详细野外地质调查、年代学、矿物学、全岩元素地球化学及Sr-Nd同位素研究。辉石角闪石岩由角闪石、黑云母、少量辉石以及矿石矿物(易解石、褐帘石、磷灰石、含铌钛铁矿)等组成,其中角闪石主要为钙角闪石组中的镁绿钙闪石,少量为浅闪石和阳起石;黑云母为镁质黑云母;辉石为透辉石和普通辉石。辉石角闪石岩成岩年龄为418 Ma,岩石具有低硅(SiO_(2)=35.49%~42.62%)、高MgO(7.27%~16.04%)、CaO(11.94%~13.44%)、TiO_(2)(1.80%~6.13%)、P_(2)O_(5)(0.52%~2.79%)含量特征,轻稀土元素强烈富集,在微量元素蛛网图中显示Nb、Ta正异常以及Pb负异常,与OIB特征类似。岩石具有富集的Sr-Nd同位素组成((87Sr/86Sr)i=0.700444~0.705404,ε_(Nd)(t)=-0.4~0.9),落入OIB型富集地幔的范围。综合研究表明大格勒辉石角闪石岩可能源于OIB型软流圈地幔,源区Nb-Ta的富集及Pb和K的亏损主要与再循环脱水洋壳有关。大格勒辉石角闪石岩的形成时代稍晚于东昆仑高压变质带榴辉岩相的峰期变质时代,因此其产出的构造环境可能与陆-陆碰撞期间俯冲大洋岩石圈断离软流圈地幔上涌熔融有关。

A rare(niobium-dominated)and rare earth mineralized alkaline complex has been found in the Dagele area of East Kunlun.The main body of the complex is composed of pyroxene hornblendites,showing weak mineralization.On the basis of detailed field geological survey,a comprehensive geochronological,mineralogical,whole-rock geochemical and Sr-Nd isotopic investigations were conducted in order to determine the formation age,tectonic setting,and petrogenesis of the pyroxene hornblendites.The pyroxene hornblendites consist of hornblende,biotite,a small amount of pyroxene,and ore minerals such as epidote,actinolite,apatite,and niobium-bearing ilmenite.The amphibole is mainly magnesia green calcium amphibole of calcium amphibole group,with minor edenite and actinolite.The biotite is magnesium biotite,and the pyroxene is diopside and augite.Zircon U-Pb dating indicates the pyroxene hornblendites were generated at about 418 Ma.The pyroxene hornblendites have low SiO_(2)(35.49%-42.62%)and high MgO(7.27%-16.04%),CaO(11.94%-13.44%),TiO_(2)(1.80%-6.13%),and P_(2)O_(5)(0.52%-2.79%)contents,and are strongly enriched in light rare earth elements.Primitive mantle-normalized trace element spider diagrams for the pyroxene hornblendites show positive anomalies of Nb and Ta and negative anomalies of Pb,exhibiting elemental characteristics similar to rocks derived from OIB-type mantle sources.The Sr-Nd isotopic characteristics of the pyroxene hornblendites show a moderately enriched mantle and falling within the range of OIB-type mantle.Our comprehensive petrological and geochemical study suggests that the pyroxene hornblendites in the Dagele area likely originated from an OIB(oceanic island basalt)-type asthenospheric mantle,with the enrichment of Nb-Ta and depletion of Pb mainly related to the recycling of dehydrated oceanic crust.The formation age of the pyroxene hornblendites in the Dagele area is slightly later than the peak metamorphic period of eclogites in the high-pressure metamorphic belt of the East Kunlun area,suggesting that its tectonic environment may be related to the break-off of subducted oceanic lithosphere during the continental collision period.