内蒙古锡林浩特早石炭世构造环境:来自变质基性火山岩的年代学和地球化学证据

Late Paleozoic Subduction of the Paleo-Asian Ocean:Geochronological and Geochemical Evidence from the Meta-basic Volcanics of Xilinhot,Inner Mongolia

内蒙古锡林浩特地区变质基性火山岩位于锡林浩特市东南部,大地构造上隶属于兴蒙造山带东段,其野外产出状态为锡林郭勒杂岩中的斜长角闪岩。应用LA-MC-ICP-MS锆石U-Pb测年方法,测得两组年龄为334.5±3.5 Ma和323.4±2.4 Ma,表明岩石的形成时代为早石炭世。地球化学特征显示研究区变质基性火山岩具有高Fe(TFeO=6.07%~10.55%)、高Mg(MgO=3.90%~11.29%)、高Al(Al_2O_3=12.68%~17.22%)、贫Ti(TiO_2=0.56%~1.69%)、富Na贫K(K_2O+Na_2O=2.12%~7.37%,K_2O/Na_2O=0.02~0.59)的特征,属于低钾—中钾低钛高镁铝钙碱性玄武岩或者安山岩,部分变质基性火山岩表现出高镁安山岩的特征。变质基性火山岩的稀土元素总量较低(∑REE=15.97×10^(-6)~176.0×10^(-6)),轻、重稀土分异不太显著(LREE/HREE=0.77~6.26),无明显Eu异常(δEu=0.48~1.13)。多数样品微量元素表现出强烈富集大离子亲石元素K、Rb等,亏损高场强元素Nb、Ta、Zr、Hf。以上地球化学特征表明研究区变质基性火山岩的原岩可能为玄武岩或安山岩,形成于岛弧环境。岩石Sr-Nd同位素分析结果显示研究区变质基性岩的(^(87)Sr/^(86)Sr)_i较低,为0.701828~0.704096,ε_(Nd)(t)为+2.83^+6.61,表明其直接来源于亏损的岩石圈地幔。综上所述,基本可以判定锡林浩特地区变质基性火山岩的原始岩浆应起源于受俯冲板片脱水交代的亏损地幔楔,是岛弧环境岩浆活动的产物。结合区域上前人的研究成果,认为该基性火山岩应该是在古亚洲洋洋壳向北俯冲,俯冲板片脱水形成的富集大离子亲石元素的流体进入上覆地幔楔,使地幔楔橄榄岩发生部分熔融而形成的,表明在早石炭世,古亚洲洋在研究区并没有关闭,而是正处于俯冲消减阶段。

The recta-basic volcanic Hinggan-Mongolian orogenic s, lying at the Southeast of Xilinhot area, distributed in the east of the belt in Inner Mongolia, are the amphibolite part of Xilin Gol complex. From the amphibolite, by LA-MC-ICP-MS Zircon U-Pb Dating technique, this paper gets two formation ages： （334.5±3.5） Ma and （323.4±2.4）Ma, which belong to late Carboniferous epoch. Geochemical ananlysis shows that the recta-basic volcanics are characterized by relatively high Fe （TFeO= 6.07%-10.55%）, high Mg （MgO=3.90%-11.29%）, high A1 （A1203 =12.68%-17. 22%）, low Ti （TiO2 =0. 56%-1.69%）, high Na low K （K2O＋Na2O=2.12%-7.37%, K2O/Na2O=0.02-0.59）, belonging to calc- alkaline basalt or andesite. These metabasic volcanics are also characterized by relatively low in REE content（∑REE= 15. 97 × 10.6 - 176.0 × 10-6 ）, relatively light fractionation between LREE and HREE （LREE/HREE=0.77-6.26）, no obvious Eu negative anomaly（SEu=0.48-1.13） and particular strong enrichment in large-ion lithophile elements （Rb, Th, U and K） and depletion in high-field-strength elements （Zr, Hf, Nb and Ta）, showing an affinity to MORB, suggesting that their original rock mostly are island-arc basalt or andesite. Further, The Isr and 8Nd（t） values are 0. 701828-0. 704096 and ＋2.83- ＋ 6. 61, which implies that they could be derived from the depleted lithospheric mantle. In a word, it is presumedly determined that their primary magma may originate from depleted mantle wedge which was metasomatosed due to the subduction of plate. They may result from island arc magmatism. Based on these results, it is inferred that these volcanics were melting products of the metasomatized mantle wedge formed during the Paleo-Asian Ocean subduction northward. And therefore in late Carboniferous the Paleo-Asian Ocean was not closed in study area but was in its subduction stage.