蒙古-鄂霍茨克洋俯冲的记录:额尔古纳地区八大关变质杂岩的证据

Subduction record of Mongol-Okhotsk Ocean: Constrains from Badaguan metamorphic complexes in the Erguna massif,NE China

本文对额尔古纳地块西缘八大关杂岩进行了锆石LA-ICP-MS U-Pb定年和岩石地球化学分析,以确定该套杂岩的形成时代及其构造属性。原定义为＂佳疙瘩组＂的八大关杂岩主要由黑云角闪斜长片麻岩和花岗质糜棱岩组成。LA-ICP MS锆石U-Pb研究表明,3个样品的锆石发育典型的岩浆振荡环带,高Th/U（0.13~1.42）,轻稀土元素亏损,重稀土元素富集,具有强烈的正Ce异常和强烈的负Eu异常等特征,表明锆石均属于岩浆成因。测年结果表明2个黑云角闪斜长片麻岩形成时代分别为210±2Ma、214±2Ma,花岗质糜棱岩的原岩年龄为203±3Ma;样品中同时存在~501Ma和~795Ma的捕获/继承锆石。上述结果显示八大关杂岩的形成时代应为晚三叠世而不是前人认为的新元古代,而捕获锆石则显示与东北其它地块具有相同的构造演化历史。地球化学研究显示,八大关杂岩具有高钠、铝等特点,A/CNK=0.86~1.05,A/NK=1.53~1.97,为准铝质到弱过铝质钙碱性系列;轻稀土元素富集、重稀土元素亏损（La/Yb）N=6~31,Eu弱亏损（Eu/Eu＊=0.50~1.01）,具有较高的Sr含量（在378×10-6~598×10-6之间）及低的Yb含量（在0.71×10-6~3.50×10-6之间）;微量元素原始地幔标准化蛛网模式图显示,富集Rb、Ba、K及Sr等大离子亲石元素、强烈亏损Nb、Ta、P、Ce及Ti等高场强元素。这些地球化学特征表明八大关杂岩形成于活动大陆边缘的岛弧环境。因此,八大关杂岩应形成于蒙古-鄂霍茨克洋向额尔古纳地块俯冲的大地构造背景,为蒙古-鄂霍茨克洋在三叠纪晚期南向俯冲提供了关键证据。

LA-ICP-MS zircon U-Pb dating and geochemical data are reported from the Badaguan complex at the western margin of the Erguna massif in NE China, with the aim of constraining formation time and the regional tectonic setting. The Badaguan complex, which was originally defined as the Jiageda Formation, is mainly composed of biotite-bearing amphibolitic gneiss and granitic mylonite. The zircon LA-ICP-MS U-Pb analysis shows that the zircons are of typical magmatic origin with oscillatory zoning, higher concentrations of Th, U, and REE. Th/U ratios of zircons are between 0. 13 and 1.42. Chondrite-normalized REE were loss of light rare earth elements and significant enrichment of heavy rare earth elements. Zircons have a strong positive Ce anomaly and negative Eu anomalies strongly, which show that these zircons are magmatic in origin. The dating results indicate that the protolith ages of the two biotite- bearing amphibolitic gneisses are 210 ± 2Ma and 214 ± 2Ma, respectively. The protolith age of the granitic mylonite yielded weight mean age of 203 - 3 Ma, existing -501Ma and - 795Ma capture zircon ages. Thus the formation age of the Badaguan complex is at Late Triassic rather than Late Proterozoic as previously considered. Furthermore, the capture zircon ages display that the same tectonic evolution history between the Erguna massif and other NE massifs. Geochemical studies have shown that the Badaguan complex, characterized by high Na2O, Al2O3, A/CNK =0. 86 - 1.05, A/NK = 1.53 - 1.97, suggesting aluminum to the weakly peraluminous calc-alkaline series, enrichment in light REE and depletion in heavy REE with （La/Yb）N ratios of 6 -31. They have weakly negative Eu abnormality （Eu/Eu＊ = 0. 50 -1.01 ）, high Sr content （378 × 10^-6- 598 × 10^-6） and low Yb content （0. 71 × 10^-6 - 3.50 × 10^-6）. Their primitive mantle normalized trace element spider diagrams show they are enrich in large ion lithophile element Rb, Ba, K and Sr, and are strongly depleted in high field-strength element Nb, Ta, P, Ce and Ti. This geochemical data indicates that the Badaguan complex formed in an active continental margin or island arc setting. We suggest that the tectonic setting of the Badaguan complex was Mongol-Okhotsk Ocean subduction beneath the Erguna massif, which provides key evidence for the subduction of Mongol- Okhotsk Ocean at Triassic.