大兴安岭中段索伦地区玛尼吐组火山岩年代学、地球化学及其构造背景

索伦地区玛尼吐组火山岩岩相学鉴定为安山岩、粗安岩和英安岩,化学成分显示以玄武粗面安山岩、粗安岩为主,少量流纹岩。LA-ICPMS锆石U-Pb定年结果显示其形成于130~132 Ma之间的早白垩世。地球化学特点上主要属高钾钙碱性系列。稀土总量较高(95.19~198.38)×10~(-6),轻重稀土分馏较明显(LREE/HREE=6.38~10.59),弱的负铕异常(δEu=0.68~1.01),富集大离子亲石元素(K、Rb、Th)和轻稀土元素,亏损高场强元素(Nb、Ta、P、Ti)。微量元素具高Sr(多499×10~(-6))和低Yb(最大值为2.83×10~(-6)3.6×10~(-6))、Y(多29.9×10~(-6))和高Sr/Y和低Rb/Sr比值,显示了弧火山岩的特征。正的ε_(Hf)(t)值(7.3~13.7),锆石~(176)Hf/~(177)Hf平均值为0.282964,显示玛尼吐组安山质岩浆源区主要为岩石圈地幔和新增生下地壳的部分熔融,且岩浆形成的过程中受到了流体的改造作用。玛尼吐组火山岩形成于岛弧构造环境,该岛弧构造背景的产生可能与受古太平洋板块俯冲作用有关。

Zircon U-Pb ages,geochemical characteristics and tectonic implications of volcanic rocks from Manitu Formation of Suolun area in central Great Xing'an Range

The volcanic rocks in Manitu Formation of Suolun area are composed of andesite, trachy-andesite and dacite petrographically,but they are mainly basaltic andesite or trachy-andesite with minor rhyolite based on chemical composition. Zircon U-Pb dating results show that they formed in Early Cretaceous from 130 to 132 Ma. These volcanic rocks belong to high-K calc-alkaline series. They are high in total rare earth elements (∑REE=95.19×10-6～198.38×10-6), with high LREE/HREE(6.38～10.59) and negative Eu anomalies (δEu=0.68～1.01). The volcanic rocks are enriched in LILE(K, Rb, Th)and LREE, but depleted in HFSE(Nb, Ta, P, Ti). Trace elements are characterized by high Sr (>499×10-6), low Yb (the maximum is 2.83×10-6, <3.6×10-6) and Y (majority <29.9×10-6), and high ratio of Sr/Y and low ratio of Rb/Sr, indicating the characteristic of arc volcanic rocks. The volcanic rocks have positive values of εHf(t) (7.3～13.7), and the initial 176Hf/177Hf ratios for magmatic zircons is 0.282 964, suggesting that the andesitic magma of Manitu Formation was derived from the lithosphere mantle and partial melting of newly accreted lower crust, and is affected by the subduction fluid. Volcanic rocks of Manitu Formation formed in the island arc tectonic environment caused by the subductions of the Paleo-Pacific Plate beneath the Eurasian continent.