内蒙古正镶白旗碎斑熔岩地球化学特征

GEOCHEMICAL FEATURES OF THE PORPHYROCLASTIC LAVA FROM BAIQI, INNER MONGLIA, CHINA

白旗碎斑熔岩是一种特殊成因的火山岩,属太平洋岩系钙碱质系列火山岩。以富亲石微量元素为特征。稀土配分属铕亏损且轻稀土富集型。形成白旗碎斑熔岩的原始岩浆来自于地壳深部,乃是上地幔与地壳共同混熔分异的产物。

As a special volcanic rock, the porphyroclastic lava is a major rock type outcropping in the Mesozoic volcanic basin in the Baiqi area, Inner Monglia. Studies show that the rock belongs to rhyolites of the Pacific calc-alkalic rock series, with chemical characteristics of volcanics in orogenic belts. The trace elements in the lava can be divided into two groups. Li, Rb, Cs, U, Th and F of the first group are relatively high and enriched during the late period of magmatism while Ti, V, Cr, Mn, Co and Ni of the second group are relatively low and tend to decrease with increasing differentiation degree of magma. The REE distribution patterns show a LREE enrichment but a HREE depletion, with remark able Eu deficiency in the lava. The average δ18O% value is estimated at 9.975%, very close to that of syntectic granite. In the diagram of Sr isotope evolution for the crust and upper mantle the data points of porphyroclastic lava from the Baiqi area are found falling between the continental line and the basalt line, implying that the original magma was derived from syntexis and differentiation of the upper mantle and deeper crust. It is postulated that the Yenshanian orogenic movement and underthrusting of the Pacific plate toward the continent of China and of the Kura plate toward the Eurasian plate may have provided a heat source for the melting of deep-seated material.