新疆阿尔泰康布铁堡组钾质和钠质流纹岩的成因及同位素年代学研究

阿尔泰南缘康布铁堡组酸性火山岩分布在麦兹、克朗、冲呼尔以及阿舍勒等火山-沉积盆地.本文对其中的钾质流纹岩(TM1,铁木尔特地区)和钠质流纹岩(KK7,可可塔勒地区)进行了SHRIMP锆石U-Pb定年以及地球化学的研究.钾质流纹岩的年龄为400.8±8.4Ma(加权平均年龄为394.8±7.9Ma),钠质流纹岩的年龄为402.2±6Ma(加权平均年龄为396.8±5.1Ma).主元素分析结果证明研究区在时间和空间上紧密共生的高钾流纹岩和高钠流纹岩同属于高碱、高硅、低钙、过铝质的钙碱性岩系.相似的稀土元素分配模式和同样亏损高场强元素(Ti、Nb、Ta)的地球化学特征暗示二者的原始岩浆源于上地壳部分熔融.钾质流纹岩富集Rb,Ba、Pb,钠质流纹岩则富集Sr,这种元素分相富集的特征符合在岩浆液态不混溶条件下微量元素在共轭钠质熔体和钾质硅酸盐熔体中的分配原则.由此推论,本区产出的钾质流纹岩和钠质流纹岩可能是上地壳部分熔融形成的高挥发分酸性岩浆液态不混溶的产物.

SHRIMP U-Pb ages and petrology studies on the potassic and podic rhyolites in Altai, North Xinjiang

The acidic volcanic rocks of the Kangbutiebao Formation crop out mainly in Maizi, Kelang, Chonghuer and Ashele volcanic-sedimentary basins on the southern margin of Altai. The SHRIMP zircon U-Pb dating and geochemical characteristics of potassic rhyolites (TM1, Tiemuerte area) and sodic rhyolites (KK7, Keketale area) have been studied. The potassic rhyolites yield a 400.8±8.4Ma(mean age=394.8±7.9Ma)age and the sodic rhyolites 402.2±6Ma(mean age=396.8±5.1Ma)age. Investigations on major element compositions show that the spatially and temporally closely associated high potassic rhyolite and high sodic rhyolite are belonging to a high alkali, high silicon, low calcium and peraluminous calc-alkaline suite. Similar REE distribution pattern and HFSE (Ti, Nb, Ta) depleted geochemical characteristics suggest that their primary magma might be originated from the partial melting of upper crust. However, the potassic rhyolites are enriched in Rb, Ba, Pb, while sodic rhyolites in Sr. The characteristics of phase separation and enrichment accord with the distribution principle of trace elements between conjugated sodic melts and potassic silicate melts under a condition of magmatic liquid immiscibility, therefore, both potassic rhyolites and sodic rhyolites are the products of liquid immiscibility of the acidic magma with high volatile derived from the partial melting of upper crust.