松潘—甘孜造山带南东段羊房沟石英闪长岩时代、成因及地质意义

Ziron U-Pb chronology and geochemistry of the Yangfanggou quartz diorite in the southeast Songpan-Ganzi orogenic zone

羊房沟石英闪长岩出露于松潘-甘孜造山带南东段,该地区岩浆活动较强烈。LA-ICP-MS锆石U-Pb测年获得石英闪长岩年龄为(205.3±1.3) Ma,形成时代为晚三叠世。石英闪长岩SiO2为58.01%~59.34%,Al2O3为14.90%~15.77%,K2O/Na2O比值为1.69~2.13,显示高钾低钠特点,里特曼指数为1.68~2.12,在K2O-Na2O关系图上落入钾玄岩系列,A/CNK为0.76~0.91,属于准铝质岩石;稀土元素总量(∑REE=183.08×10^-6~283.08×10^-6),为轻稀土元素富集、重稀土相对亏损的右倾型。δEu为0.77~1.00,负铕异常明显。在微量元素原始地壳标准化蛛网图上,岩石富集Rb、Ba、Th等大离子亲石元素、LREE和不相容元素(如U),相对亏损Nb、Ti、P等高场强元素,指示具有岛弧岩浆特征,Rb/Sr、Nb/Ta比值介于壳幔之间,反应具有壳幔岩浆混合成因。结合区域地质背景,认为羊房沟石英闪长岩形成于晚三叠世洋壳俯冲阶段的岛弧环境。

Yangfanggou quartz diorite is located in the Southeast section of Songpan-Ganzi Orogenic Zone. The U-Pb dating of zircons from the quartz diorite using LA-ICP-MS yields an age of (205.3±1.3) Ma,which indicates quartz diorite formed in the late Triassic epoch. Geochemical analysis shows that the SiO2 ranges from 58.01% to 59.34%,Al2O3 ranges from 14.90% to 15.77%,K2O/Na2O ranges from 1.69 to 2.13,and it is rich in K2O but poor in Na2O,rittmann index ranging between 1.68 and 2.12.The rocks is plotted in the potassic basaltic rocks series in the K2O-Na2O diagram,A/CNK ranges from 0.76 to 0.91.This suggests that the Yangfanggou quartz diorite belongs to the aluminum unsaturated granite. In addition,REE of the quartz diorite content varies little with ranges from 183.08×10^-6 to 283.08×10^-6. REE distribution pattern shows that LREE is strong enrichment and HREE is relative depletion,with obviously Eu negative anomaly. The trace elements of primitive mantle standardization on spider diagram reveal that large ion lithophile elements (Rb、Ba、Th) and LREE and incompatible elements (U) are enriched,strength elements Nb、Ti and P are relative depleted. Quartz diorite has magnesium iron inclusions,Rb/Sr、Nb/Ta ratio between crust-mantle. This mirrors the characteristics of crust-mantle mixing origin. In combination with regional geological background,The Yangfanggou quartz diorite has formed in the VAG with Subduction phase of the oceanic crust in the late Triassic epoch.