大兴安岭北部新林地区早侏罗世火山岩的发现及其地质意义

    DISCOVERY OF THE EARLY JURASSIC VOLCANIC ROCKS IN XINLIN AREA OF NORTHERN DAXINGANLING MOUNTAINS: Geological Implication

    • 摘要: 大兴安岭北部新林地区战备村一带新发现早侏罗世酸性火山岩.共测试2个锆石LA-ICP-MS U-Pb年龄.第一个测年样品为绢英岩化流纹岩,谐和图上测点偏离谐和线右侧为典型的不谐和年龄,与谐和线相交年龄为192.0±1 Ma,MSWD=1.2,为流纹岩喷出后的冷却结晶年龄;第二个测试样品为弱硅化流纹岩,谐和年龄为178.0±1 Ma,系微量放射性Pb丢失的结果,其成岩时代为应与绢英岩化流纹岩时代相近.岩石化学及地球化学具有高Ti流纹岩和Ⅰ型流纹岩特征.流纹岩的Mg#值(0.43~0.73)及CIWP标准矿物中紫苏辉石含量(0.25%~0.96%)佐证了流纹岩是由基性岩浆的结晶-分异演化而来.

       

      Abstract: The Early Jurassic acid volcanic rocks are newly discovered in Zhanbeicun Formation of Xinlin area in northern Daxinganling Mountains, from which two samples of zircon LA-ICP-MS U-Pb ages are determined. The first is sericitized rhyolite, with the testing point deviating to the right side of concordant curve on the concordia diagram-a typical discordant age, yielding the intersecting age with concordant curve of 192.0±1 Ma(MSWD=1.2), which is the cooling crystallization age of rhyolites after extrusion. The second one is weakly silicified rhyolite with a concordant age of 178.0±1 Ma, which is the result of the loss of radioactive trace element Pb, and the rock-forming age should be close to that of the sericitized rhyolite. The volcanic rocks are petrochemically and geochemically characterized by high Ti rhyolite and I-type rhyolite. The Mg# value of the rhyolite(0.43-0.73) and content of hypersthene in CIWP norm minerals (0.25%-0.96%) support that the rhyolite is derived from crystallization-differentiation of basic magma.

       

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