兴蒙造山带中段锡林浩特跃进地区石炭纪岛弧型侵入岩:年代学、地球化学、Sr-Nd-Hf同位素特征及其地质意义

兴蒙造山带中段锡林浩特跃进地区出露一套石炭纪侵入岩，具有类似岛弧侵入岩的岩石组合（花岗闪长岩、英云闪长岩、石英闪长岩）和地球化学特征.锆石LA-MC-ICP-MS U-Pb测年结果表明，这套侵入岩形成时代为310~330 Ma，属于石炭纪中晚期.岩石地球化学分析表明，跃进侵入岩属于钠质钙碱性系列岩石；A/CNK范围为0.86~1.17，大多属于准铝-弱过铝质系列；样品均具有较高的Mg#值，范围为47.5~55.8；微量元素分析表明，跃进侵入岩具有Rb、Ba、Th、U、K等大离子亲石元素的富集，Nb、Ta、P、Ti等高场强元素的亏损；稀土总量较低（3.62×10-6~172.33×10-6，平均值为118.33×10-6），具有轻稀土富集的配分模式，（La/Yb）_N大于1，范围为2.48~9.44，重稀土分馏较弱，（Gd/Yb）_N范围为1.19~1.96，具有弱的Eu负异常（δEu值0.70~1.04）.跃进侵入岩均具有亏损的Hf同位素组成，176Hf/177Hf范围为0.282 651~0.282 870，ε_Hf（t）均为正值，范围为+2.0~+9.9，地壳存留模式年龄T_DMC范围为829~1 536 Ma，集中于900~1 200 Ma；（87Sr/86Sr）_i范围为0.703 7~0.704 8，（143Nd/144Nd）_i范围为0.512 320~0.512 401，ε_Nd（t）均为正值（+2.1~+3.6），二阶段Nd模式年龄T_DM2较集中，为新元古代（793~918 Ma）.跃进侵入岩是在石炭纪活动大陆边缘的背景下，由中元古代晚期-新元古代早期的新生基性下地壳物质，在俯冲流体及岛弧基性岩浆的共同作用下部分熔融形成的，表明石炭纪中晚期兴蒙造山带中段仍然存在板块俯冲，古亚洲洋并未关闭. 

Geochronology,Geochemistry, Sr-Nd-Hf Isotopic Characteristics and Geological Significance of Carboniferous Yuejin Arc Intrusive Rocks of Xilinhot,Inner Mongolia

The study of magmatism can reveal the structure and tectonic evolution of the orogeny. In this paper, we present zircon LA-MC-ICP-MS U-Pb dating, Hf-Sr-Nd isotopic compositions and whole rock geochemical data of the Carboniferous Arc intrusive rocks from Yuejin region of Xilinhot, central part of the Xingmeng Orogenic Belt. The rock suit consists of three major rock types, i.e., granodiorite, tonalite and quartz-diorite, with zircon U-Pb ages ranging from 310±1 to 330±1 Ma. Geochemically, these rocks show moderate SiO₂, high and variable Na₂O, Al₂O₃, and CaO contents, but low K₂O, FeO_t and MgO contents.The Na₂O/K₂O ratios are all higher than 1(1.05-2.68, average of 1.95), while these rocks are of Calc-Alkaline series. The A/CNK ratios range from 0.86 to 1.17, indicating most of samples are meta-to weak per-aluminous series. The Mg# are moderately high, from 47.5-55.8. The trace elemental data of these rocks display enrichment of LILEs, such as Rb, Ba, Th, U, K, but depletion of HFSE, i.e., Nb, Ta, P and Ti. The total REE contents are low (3.62×10-6-172.33×10-6, average of 118.33×10-6) with moderate fractionation between LREE and HREE ((La/Yb)_N range from 2.48-9.44), while weak negative Eu abnormity are observed(δEu range from 0.70 to 1.04). The in-situ zircon Hf isotopic analysis show depleted compositions with 176Hf/177Hf ratios varying from 0.282 723 to 0.282 863, ε_Hf(t) values from +2 to +9.9 and Hf two stage model ages(T_DMC) from 829 to 1 536 Ma(concentrating from 900 to 1 200 Ma). The whole rock Sr-Nd isotopic analysis of (87Sr/86Sr)_i range from 0.703 7 to 0.704 8, (143Nd/144Nd)_i range from 0.512 320 to 0.512 401, with positive ε_Nd(t) (+2.1 to +3.6), indicating depleted source, and the Nd two stage model ages(T_DM2) are of Neoproterozoic(793-918 Ma). The rock suit, whole rock geochemistry, Hf-Sr-Nd isotopic characteristics indicate that Yuejin Carboniferous intrusive rocks were formed in active continental margin, by the partial melting of basic juvenile crust (formed in Late Mesoproterozoic to Early Neoproterozoic) triggered by subduction fluid and arc basic magma. This tectonic setting demonstrates that there was still subduction in Carboniferous in Xingmeng orogeny and the ocean was not closed.