摘要
内蒙中部晚石炭世岩浆活动的成因及动力学背景对于揭示古亚洲洋俯冲和兴蒙造山带演化的具有重要意义。本文选取锡林浩特南部晚石炭世辉长质侵入体为研究对象,开展高精度SIMS锆石U-Pb定年、全岩主微量和Sr-Nd-Hf同位素分析,旨在讨论其岩石成因,为制约兴蒙造山带的演化和理解深部流体循环过程提供更多信息。辉长质岩体样品的锆石CL图像呈弱环状结构、条带状或岩浆生长带(Th/U=0. 3～2. 5)。SIMS锆石U-Pb定年结果表明,锡林浩特辉长质岩体的侵位年龄为316. 9±2. 2Ma,为兴蒙造山带晚石炭世区域性岩浆活动的产物。该岩体经历了橄榄石和单斜辉石的分离结晶作用,但受堆晶作用和地壳物质混染作用的影响较小;同时具有较低的87Sr/86Sr初始比值(0. 7034～0. 7041),但相对高的εNd(t)(+5. 58～+6. 88)和εHf(t)值(+12. 07～+13. 44),并显示Rb、Sr、U和Pb等流体活动性元素相对富集,但Nb和Ta等非流体活动性元素相对亏损的特征,暗示其起源于遭受含水流体交代富集的地幔源区。流体交代富集作用很可能发生在源区部分熔融过程中或者在部分熔融之前较短的时间内,即反映较为年轻的交代事件。锡林浩特辉长闪长岩具有相对较高的Si O_2(51. 7%～53. 2%)、Cr (138. 4×10^(-6)～757. 2×10^(-6))和Ni (50. 4×10-6～141. 1×10^(-6))含量以及Zn/Fe比值(10. 8～11. 5),但相对较低的Al2O3(13. 1%～16. 8%)含量,暗示源区可能富含斜方辉石,由橄榄岩+辉石岩脉组成。通过对比总结兴蒙造山带晚石炭世岩浆岩岩石组合的地球化学特征以及其他地质资料,本文认为古亚洲洋在晚石炭世之前已经闭合,而锡林浩特辉长质岩体形成于陆内伸展的构造背景。结合同期玄武岩母岩浆的高含水量特征(高达4. 41%),本文认为锡林浩特及其邻区晚石炭世岩浆活动是遭受地幔转换带滞留俯冲板块析出富水流体交代的地幔源区部分熔融的产物,从而建立了深部流体循环过程与造山后伸展背景下岩浆活动的成因联系。
The petr0genesis and geodynamic setting of Late Carboniferous magmatism in Inner Mongolia,China,hold a key to understanding the final closure of the Paleo-Asian Ocean and formation of the Xing＇an-Inner Mongolia Orogenic Belt（ XMOB）. This study carried out secondary ion mass spectrometer（ SIMS） zircon U-Pb geochronological,and bulk-rock geochemical and Sr-Nd-Hf isotopic analyses of gabbroic diorites sampled in the Xilinhot region with the aim of investigating its petrogenesis and unravelling the geodynamic setting of its emplacement. Cathodoluminescence（ CL） images show that the analyzed zircon grains are euhedral to subhedral in shape,with paint zoning or oscillatory zoning（ Th/U = 0. 3 - 2. 5）. SIMS U-Pb zircon dating shows that Xilinhot gabbroic diorites were intruded at 316. 9 ± 2. 2 Ma,suggesting a Late Carboniferous magmatic event in the XMOB. The effects of crystal accumulation and crustal contamination on the whole-rock composition of the gabbroic rocks are insignificant,whereas fractional crystallization of olivine and clinopyroxene played an important role in magma differentiation. The source of the rocks display depleted signatures as evidenced by presence of low initial87 Sr/86 Sr ratios（ 0. 7034 - 0. 7041） and MORB-mantle source-like positive εNd（ t）（ ＋ 5. 58 - ＋ 6. 88） and εHf（ t） values（ ＋ 12. 07 - ＋ 13. 44）,and they also have enriched features as characterized by enrichment of fluid-mobile elements（ Rb,U,Sr,Pb）,but depletion of fluid-immobile elements（ Nb,Ta）. Such co-existence of depleted Sr-Nd-Hf isotopes and enriched trace element signatures suggests the enrichment of mantle source by water-rich fluids may occur during or shortly prior to the melting event,i. e. a recent metasomatic event. The gabbroic rocks are also characterized by relatively high Si O2（ 51. 7%- 53. 2%）,Cr（ 138. 4 × 10-6- 757. 2 × 10^-6）,Ni（ 50. 4 × 10-6- 141. 1 × 10^-6） and Zn/Fe ratios（ 10. 8 - 11. 5）,but relatively low Al_2 O_3（ 13. 1% - 16. 8%） contents,indicating that they derived from a mixed source composed of an orthopyroxene-rich pyroxenite vein-plus-peridotite source. A range of geological evidence indicates an intracontinental extensional origin for the Late Carboniferous gabbroic intrusions in Xilinhot area,rather than a subduction setting. Together with the high water contents（ up to 4. 41%） of contemporaneous basalts,the geochemical and isotopic characteristics of the studied gabbroic rocks indicate that the Late Carboniferous magmatism in Xilinhot and adjacent areas was generated by partial melting of mantle sources hydrated by water-rich fluids released from subducted slabs stagnated in mantle transition zone. Therefore,we proposed a deep-Earth water cycling process to account for mantle hydration and subsequent Late Carboniferous magmatism in XMOB,supporting a geodynamic link between deep-Earth water cycling,and post-orogenic magmatism and lithospheric extension.
作者
庞崇进
王选策
温淑女
Krapez Bryan
王炎阳
廖闻
PANG ChongJin;WANG XuanCe;WEN ShuNv;KRAPEZ Bryan;WANG YanYang;LIAO Wen(Collaborative Innovation Center for Exploration of Hidden Nonferrous Metal Deposits and Development of New Materials in Guangxi,Guangxi Key Laboratory of Hidden Metallic Ore Deposits Exploration,College of Earth Sciences,Guilin University of Technology,Guilin 541004,China;The Inztitute for Geoscience Research（TIGER）,School of Earth and Planetary Sciences,Curtin University,GPO Box U1987,Perth,WA 6845,Australia;School of Earth Science and Resources,Chang＇an University,Xi＇an 710054,China;MOE Key Laboratory of Orogenic Belts and Crustal Evolution;School of Earth and Space Sciences,Peking University,Beijing 100871,China)
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2018年第10期2956-2972,共17页
Acta Petrologica Sinica
基金
科技部国家重点研发计划(2017YFC0601302)
国家自然科学基金项目(41703039、41672214)
广西自然科学基金项目(2015GXNSFCA139016、2015GXNSFBA139182)
广西隐伏金属矿产勘查重点实验室项目(16-140-27-07)
同位素地球化学国家重点实验室开放基金项目(SKLabIG-KF-16-01)联合资助
关键词
晚石炭世侵入岩
流体活动性元素
辉长岩
深部流体循环
兴蒙造山带
Late Carboniferous intrusive rocks
Fluid-mobile elements
Gabbroic rock
Deep-Earth fluid cycling
Xing＇ an-InnerMongolia Orogenic Belt
