【摘要】 本文为首次报道东准噶尔北塔山蛇绿混杂岩的锆石SHRIMP U-Pb年龄及氧同位素。北塔山蛇绿混杂岩出露于扎河坝-阿尔曼泰蛇绿混杂岩东南端,中蒙边境附近。岩石地球化学表明,其主要为岛弧玄武岩,稀土元素球粒陨石标准化模式表现为轻稀土相对富集,微量元素的N-MORB标准化蛛网图显示富集大离子亲石元素(LILE),存在Nb和Ta负异常,显示了受消减带影响的信息,其可能是准噶尔地区洋壳俯冲-消减的产物。东准噶尔北塔山蛇绿混杂岩中辉长岩的锆石获得了3组SHRIMP U-Pb年龄,第一组锆石的年龄为494±3Ma(n=14,χ2=1.16),对应δ18O加权平均值为5.2±0.3‰(1σ),与幔源锆石δ18O值(5.3±0.3‰)相一致,代表了辉长岩的形成年龄,亦代表了该蛇绿岩形成时代。辉长岩中第二组锆石的SHRIMP UPb年龄范围为412±13Ma和259~264Ma,对应锆石δ18O值为6.5±0.1‰~11.1±0.1‰,表明它们受到后期高δ18O壳源流体/熔体不同程度的改造。研究区与玄武岩呈侵入接触关系的花岗斑岩锆石SHRIMP U-Pb年龄为407±2Ma,代表了该花岗斑岩的形成年龄,并约束了阿尔曼泰洋盆的闭合时限可能为晚志留纪-早泥盆纪。而其对应锆石δ18O值为6.1±0.2‰(1σ),反映了岩浆物质可能主要来源于下伏玄武质洋壳。本文的研究结果不仅表明北塔山蛇绿岩形成于晚寒武世,而且进一步证明它是扎河坝-阿尔曼泰蛇绿岩带的东南延伸。更为重要的是,本文的研究还证明锆石O同位素研究与锆石SHRIMP定年研究相结合是合理解释蛇绿岩中锆石年龄复杂性的有效途径之一。更多还原

【Abstract】 This paper firstly reports SHRIMP zircon U-Pb dating and O isotope composition of the Beitashan ophiolitic mélange in the East Junggar. Beitashan ophiolitic mélange is located in the southeastern end of the Zhaheba-Aermantai ophiolite belt,which southeasterly extends via the China-Mongolia border to Mongolia. Geochemical data show that most of basalts hold geochemical signatures of island arc magmatic rocks,such as enriched LREE relative to HREE in the chondrite-normalized REE pattern,and NMORB normalized spider diagram shows enrichment in LILE and Nb,Ta negative anomalies,suggesting that they likely resulted from island arc magmatism. Zircons from a gabbro sample which from the ophiolite yielded three groups of SHRIMP U-Pb ages. The first group give a weighted mean age of 494 ± 3Ma( n = 14,χ2= 1. 16) and weighted mean δ18O value of 5. 2 ± 0. 3‰( 1σ),consistent with the mantle value of 5. 3 ± 0. 3‰,and therefore we interpret the age of 494 ± 3Ma as the formation age of the gabbro,as well as the ophiolite. The second and third groups,which yielded ages of 412 ± 13Ma( n = 6,χ2= 0. 21) and 259 ~ 264 Ma with the δ18O values ranging from 6. 5 ± 0. 1‰ to 11. 1 ± 0. 1‰,significantly higher than the mantle value. We interpret the age of second group as results of varied degrees of modification by fluids or melts,which have the high δ18O values and the third group age as results of the hydrothermal fluids with high δ18O value. A granite porphyry,which intrudes ophiolite complex,yielded a weighted mean age of 407 ±2Ma and a weighted mean δ18O value of 6. 1 ± 0. 2‰( 1σ). We interpret the age as the formation time of the porphyry and the δ18O value as an indicator for the granitic melt originated from partial melting of basaltic oceanic crustal materials. The formation time of the porphyry constrained the closure of Aermantai ocean basin in the Late Silurian-Early Devonian. These results not only demonstrate that the Beitashan ophiolite formed during Late Cambrian but also further confirm that the Beitashan is the southeastern extension of the Zhaheba-Aermantai ophiolite belt. Importantly,our current research illustrates that combination of zircon O isotope analyses with zircon U-Pb dating is one of the most efficient technique to reasonably interpret the complexity of the age spectrum that has usually been detected in ophiolites.更多还原