三江上叠裂谷盆地人支雪山组火山岩是赋存火山成因块状硫化物矿床(VHMS)的重要层位,但对其形成时代一直存在着争议.本文对几家顶一带人支雪山组火山岩进行了LA-ICPMS锆石U-Pb年代学研究,结果显示,两件流纹岩样品中锆石23个分析点的<'206>Pb/<'238>U年龄分别为247.4±2.1 Ma和249.1±1.6 Ma,因此人支雪山组火山岩形成于早三叠世(249～247 Ma).地球化学资料表明,人支雪山组火山岩形成于一个伸展的地球动力学背景,由此本文认为金沙江结合带在早三叠世已进入弧-陆碰撞后的伸展时期.%It is generally accepted that the volcanics-hosted massive sulfide deposits were widely derived from the marine settings, such as island arc, back-arc basin, mid-ocean ridges, and intraplate spreading centers.Different tectonic regimes are typically marked by changes in the composition of the associated marine volcanic rocks and different ore mineral assemblages in volcanic massive sulfide deposits.Volcanic rocks from Renzhixueshan Formation in Shangdie rift basin of Sanjiang is an important layer hosting volcanic massive sulfide deposits.However, the time of their formation has been controversial.The major rock types of Renzhixueshan Formation are basalt, rhyolites and bimodal volcanic rocks.LA-ICPMS U-Pb zircon dating of two rhyolite samples from Renzhixueshan Formation in Jijiading area yields concordant 206Pb/238U ages of 247.4 ±2.1 Ma and 249.1 ± 1.6 Ma, respectively, implying that these rocks were erupted in early Triassic (249～247 Ma).The geochemical and isotopic characteristics indicate that the basaltic volcanic rocks were probably produced by partial melting of an enrichment mantle.The similar Sr-Nd isotopic compositions of rhyolite to the basalt suggest that they were probably triggered by the same tectonic event.These characteristics of volcanic rocks in Renzhixueshan Formation suggest that they were probably formed in an extending dynamic settings.Therefore, the authors hold that Jinshajiang suture had already been an extending setting after the arc-continental collision during the early Triassic in the Jijiading area of east Tibet.At the same time, the extensional settings probably played an important role in the formation of volcanics-hosted massive sulfide deposits in the Jinsha orogenic belt.
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