内蒙古乌兰敖包图早古生代侵入岩年代学、地球化学特征及地质意义

对造山带中岩浆作用的研究可以为构造演化提供约束.兴蒙造山带中段乌兰敖包图一带出露一套早古生代早期的闪长质侵入岩(石英闪长岩和闪长岩)，具有俯冲带型侵入岩的岩石组合和地球化学特征，标志着早古生代古亚洲洋的俯冲事件.通过锆石LA-MC-ICPMS U-Pb定年，在石英闪长岩和闪长岩中分别获得了474.0±1.8 Ma和487.8 ±1.9 Ma的年龄，属于晚寒武世－早奥陶世.锆石原位Hf同位素分析表明，乌兰敖包图侵入岩具有富集的Hf同位素组成，176Hf/177Hf范围为0.282 123~0.282 304，ε_Hf(t)均为负值(两件样品的平均值分别为－10.84和－8.53)，二阶段模式年龄为古元古代(范围为1 851~2 250 Ma)，表明这套侵入岩源于古元古代中期再循环地壳物质.岩石地球化学分析显示，石英闪长岩和闪长岩具有中等SiO₂含量(56.52%~66.06%)，较高的Na₂O(2.60%~6.81%)、Al₂O₃(14.69%~18.35%)、CaO(0.57%~6.58%，平均值为3.97%)含量，低的K₂O(0.73%~2.94%)、TFeO、MgO含量，Na₂O/K₂O比值较高(0.92~5.66，平均值为3.19)；微量元素具有Rb、Ba、Th、U、Sr、K等大离子亲石元素的富集以及Nb、Ta、P、Ti等高场强元素的亏损；稀土总量较低，轻稀土富集，具有较弱的Eu负异常(δEu范围为0.76~0.99).地球化学特征以及锆石Hf同位素组成表明，乌兰敖包图早古生代侵入岩在主动大陆边缘的构造背景之下，由古老地壳物质熔融形成的，代表了兴蒙造山带中段早古生代早期的俯冲事件，将兴蒙造山带中段早古生代北部岩浆岩带向东延伸，并且佐证了锡林浩特地区存在前寒武纪基底. 

Geochronology,Geochemistry and Geological Significance of Early Paleozoic Wulanaobaotu Intrusive Rocks,Inner Mongolia

The study of magmatism can reveal the orogenic tectonic evolution. In this paper, zircon LA-MC-ICPMS U-Pb dating, Hf isotopic compositions and whole rock geochemical data of the Early Paleozoic dioritic rocks from Wulanaobaotu region, central part of the Xing-Meng Orogenic belt are present. The rock suite consists of two major rock types, i.e., quartz-diorite and diorite, with zircon U-Pb ages of 474.0±1.8 Ma and 487.8±1.9 Ma, respectively. In-situ zircon Hf isotopic analysis shows enriched compositions with 176Hf/177Hf ratios varying from 0.282 123 to 0.282 304, negative ε_Hf(t) values and Hf two stage model ages(T_DMC) from 1 851 to 2 250 Ma. The results indicate that the intrusive rocks are likely derived from an ancient basement material. Geochemically, these rocks show moderate SiO₂ contents (56.52%-66.06%), high and variable Na₂O (2.60%-6.81%), Al₂O₃ (14.69%-18.35%), and CaO (0.57%-6.58%, average 3.97%) contents, but low K₂O (0.73%-2.94%), TFeO and MgO contents. Most Na₂O/K₂O ratios are all higher (0.92-5.66, average 3.19). The trace elemental data of these rocks display enrichment of Rb, Ba, Th, U, K and Sr, but depletion of Nb, Ta, P and Ti. The total REE contents are low with moderate fractionation between LREE and HREE. Meanwhile, weakly negative Eu anormalyies (δEu ranges from 0.76 to 0.99) are observed in all samples. Thus, we propose that the Wulanaobaotu dioritic intrusive rocks were likely formed in an active continental margin setting during Late Cambrian to Early Ordovician, consistent with the existence of Early Paleozoic subduction-related tectonic-magmatic belt in Xing-Meng orogenic belt. The enriched zircon Hf isotopic characteristics further northern suggest the magma originated from a Precambrian basement source.