摘要:
在内蒙古牙克石地区晚古生代岩浆岩中发现了岛弧火山岩和埃达克质花岗闪长岩。火山岩由玄武岩、安山岩、英安岩及凝灰岩组成,属于钙碱性系列,它们富集大离子亲石元素(LILE)、轻稀土元素(LREE),亏损高场强元素(HFSE)。玄武岩的SHPIMP锆石U-Pb年龄表明其形成于373.2±5.3Ma。锆石εHf(t)值为+14.67~+18.67,类似大洋中脊玄武岩(MORB)和现代俯冲带玄武岩的Hf同位素特征。上述特征说明,玄武岩很可能起源于受俯冲板片流体改造的亏损地幔楔,形成于活动大陆边缘。花岗闪长岩的结晶年龄为331.2±3.7Ma。岩石高SiO2、Sr,高Sr/Y比值,富集轻稀土元素,亏损重稀土元素和Y及高场强元素,铕异常基本不明显,具备埃达克岩的地球化学特征。锆石εHf(t)值较高(+12.78~+14.54),说明源区可能为亏损地幔或新生的玄武质下地壳。根据微量元素组成认为花岗闪长岩主要起源于晚古生代底侵的岛弧玄武-安山质下地壳。弧火山岩和埃达克质花岗闪长岩的发现表明,区内晚泥盆世-早石炭世发生过大洋板片的俯冲作用,可能与额尔古纳-兴安地块与松嫩地块的碰撞拼合密切相关。
Abstract:
Arc volcanic rocks and adakitic granodiorites are identified to occur within the Late Paleozoic magmatite in Yakeshi region, Inner Mongolia. The volcanic rocks, which consist of calc-alkaline basalts, andesites and dacites, are characterized by strong enrichment of light rare earth elements (LREE), large ion lithophile elements (LILE) and depletion of high field strength elements (HFSE). SHRIMP zircon U-Pb isotopic dating revealed that the basalt was formed at 373.2±5.3Ma. The zircon εHf(t) values are within a range from +14.67 to +18.67,which are similar to the Hf isotope feature of mid-oceanic ridge basalt (MORB) and modern subduction zone basalt. These characteristics suggest that the basalts were formed in an active continental margin and from the melting of a depleted mantle wedge modified by predominant slab fluids. A new zircon U-Pb chronological date yield an age of 331.2±3.7Ma, interpreted as emplacement age of the granodiorites. They are show adakitic geochemical features, marked with high SiO2 and Sr contents, high Sr/Y ratios (>40), enrichment in LREE and strong depletion in HREE,Y and HFSE, either with no obvious negative or positive Eu anomaly. The positive εHf(t) values (+14.54~+12.78) indicate that the source rocks of these rocks are from a depleted mantle or new underplated basaltic crust. Considering trace element composition, we believe that the granodiorites were probably deprived from an underplated, basaltic-andesitic crust under the arc in Late Paleozoic. The occurrence of the arc volcanic rocks and adakitic granodiorites implies that the oceanic slab subduction has occured from Late Devonian to Early Carboniferous in Yakeshi region, it might be related to the collision and combination between Ergun-Xing'an and Songnen blocks.
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