西南天山阔克萨彦岭巴雷公镁铁质岩石的地球化学特征、LA-ICP-MS U-Pb年龄及其大地构造意义

Geochronology,Petrogenesis and Significance of Baleigong Mafic Rocks in Kokshal Segment,Southwestern Tianshan Mountains

巴雷公镁铁—超镁铁质岩套出露于南天山阔克萨彦岭地区，本文对其中的镁铁质岩石进行了详细的地球化学和锆石U-Pb年代学研究。岩石学和地球化学特征表明：巴雷公镁铁质岩石主要为洋岛拉斑岩石系列，P2O5（0.33%-0.75%）、TiO2（2.49%-3.70%）、TFe2O3（12.69%-15.63%）含量高，富集轻稀土，LREE/HREE分异明显，（La/Yb）N介于3.66-6.54之间。Cr（28.93×10^-6-123.99×10^-6）、Co（44.04×10^-6-52.40×10^-6）和Ni（25.61×10^-6-63.04×10^-6）含量低，且Ni、Cr与MgO呈正相关关系指示其母岩浆在岩浆房中或上升途中经受了橄榄石和尖晶石分离结晶作用。Zr/Nb（7.06-7.99）和Zr/Hf（35.98-37.53）比值低，推测其可能来自于含石榴子石的富集地幔的深部熔融。Zr/Y—Nb/Y和Nb/Th—Zr/Nb图解显示，其源区具有EM1—EM2型地幔端元组分混入，可能主要有再循环的发生了交代熔融作用的深部大洋岩石圈的参与。锆石LA-ICP-MS U-Pb定年结果表明，巴雷公洋岛火山岩的结晶年龄为450±2Ma。结合新获得的该岩套夹层灰岩中的牙行刺化石资料，指示该岩体在早石炭世以后成为蛇绿混杂岩的一部分。综合区域年代学和地球化学研究资料，推测南天山古洋盆在晚奥陶世—早志留世期间已演化成为成熟的多岛洋盆。

Southern Tianshan Mountains is a prominent intercontinental collision orogenic belt and key to understand the tectonic evolution of central Asia. This paper mainly focused on the mafic--ultramafic rocks located in the southwest of Kokshal, western margin of the Southern Tianshan Mountains, China. Petrological, geochronological, and geochemical studies were performed in order to reveal the petrogenesis and geochemical evolution of the mafic--ultramafic rocks, as well as the tectonic evolution of the Southern Tianshan Mountains. Geochemically, the Baleigong basic volcanic rocks are plotted as tholeiitic series, with high Ti, P, Fe contents and low Cr, Co and Ni concentrations; and also characterized by OIB-type trace element patterns, displaying significant enrichment of LILE, HFSE, LREE and MREE, and slight depletion of HREE, when normalized to N-MORB. In addition, the positive correlation between Ni,Cr and MgO have implied that the fractionation of olivine and chromium spinel had occurred in the parent magmas. Lower Zr/Nb, Zr/Hf ratios suggest that these rocks were most likely to derive from partial melting of Garnet-bearing enriched mantle. Ratios of high field strength element also indicate a hotspot or plume tail sources originated from the mixing of EM1 and EM2 components with a significant contribution from the melting metasomatised subducted oceanic lithosphere to the Early Paleozoic mantle. Zircon U-Pb dating of the diabase which, together with basalt, formed the interlayers in the ophiolitic mélange of Baleigong after early Carboniferous, yielded the forming age of 450±2Ma. Combined with the recently published geochronological data of the region, we postulate that the multi-island oceanic basin in the southern Tianshan Mountains have been formed in Early Paleozoic during Late Ordovician to Early Silurian.