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

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±1.8Ma和487.8±1.9Ma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0.282123~0.282304,εHf(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 1851~2250Ma),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( δE u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.
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/176Hf ratios varying from 0.282123 to 0.282304, negative εHf(t) values and Hf two stage model ages(TDM-C) from 1851 to 2250Ma. The results indicate that the intrusive rocks are likely derived from an ancient basement material. Geochemically, these rocks show moderate SiO2 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 (δEu 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