西秦岭中—晚三叠世板块碰撞事件的记录:来自合作地区火山岩的锆石U-Pb年龄和地球化学证据

Records of Middle-Late Triassic Plate Collision Events on the West Qinling: Evidence from Zircon U-Pb Age and Geochemistry of the Volcanic Rocks in the Hezuo Area

西秦岭造山带发育了大量印支期钙碱性花岗岩,其中在甘肃合作地区以早印支期形成的为主,并伴有少量中酸性火山岩浆活动。合作地区火山岩主要分布在德乌鲁、扎油沟口一带,整体以安山质晶屑熔岩、安山质熔岩为主,并含英安斑岩、英安岩、火山角砾岩、晶屑岩屑凝灰岩、杏仁状安山岩的中酸性火山岩组合。德乌鲁安山质晶屑熔岩锆石U-Pb测年结果为(237.7±1.9) Ma,扎油沟口安山质熔岩锆石U-Pb测年结果为(234.8±2.1) Ma,限定了其喷发时代为晚三叠世早期。岩石地球化学分析结果显示该区火山岩属于富钾、高铝、高镁钙碱性系列;Rb、Ba、Th、U、La、Ce、Sr元素强烈富集,Ti、Y、Yb等元素略显富集;稀土总量偏低,LREE/HREE值为6.79~17.15;δEu为0.76~0.86,略显负铕异常。以上结果指示火山岩物质源于富集地幔,成岩过程或成岩后有壳源物质的强烈混染。扬子板块与华北板块汇聚时期,其间小洋盆与微板块发生强烈的挤压、俯冲,在后碰撞松弛阶段,出现了强烈的壳幔物质交换和岩浆上升的现象。多次的碰撞挤压-后碰撞松弛运动,在研究区域形成了一系列晚三叠世不同时间段的火山岩。

Strong magmatic activities dominated by the Indosinian high potassic calc-alkaline granitoids in Western Qinling orogenic belt. A small number of volcanic rocks were developed. The volcanic rocks from the Hezuo area of West Qinling, as reposed in this paper, are intermediate-acid rocks which contain mainly tuffaceous lava with volcanic breccia and dacite.The andesite was limited by the zircon U-Pb age of(237.7±1.9) Ma in the Dewulu area, the andesite was limited by the zircon U-Pb age of(234.8±2.1) Ma in the Zhayougoukou area, indicating that these volcanic rocks erupted in the early Late Triassic. The results show that andesite belongs to the calc-alkaline series and shows the characteristics of potassium-rich volcanic rocks. They are characterized by enrichment in Rb, Ba, Th, U,La, Ce, Sr, low REE(LREE/HREE=6.79-17.15) and relative depletion in δEu(δEu=0.76-0.86).Hezuo volcanic rocks were generated by the mantle, some of them might also be related to the contamination of upper crustal materials. Strong extrusion and subduction proceed between some small ocean basins and some small plates, when the initial collision between the North China block and Yangtze block. The mantle materials were contaminated with upper crustal materials and the magma rose after the initial collision. A series of volcanic rocks of different time periods were formed in the late Triassic after repetitive plate collisional crushing-relaxation.