大兴安岭中段柴河地区玛尼吐组火山岩年代学、地球化学及岩石成因

摘要

The zircon LA ICP MS U Pb dating and geochemistry are reported for the volcanic rocks in Manitu Formation in Chaihe,central Great Xing’an Range in order to discover its formation time,petrogenesis and the regional tectonic setting Petrographically.The volcanic rocks in Manitu Formation are composed of andesite and dacite,but they are trachyte,trachydacite,and dacite based on its chemical composition.Zircon from these volcanic rocks are euhedral-subhedral in shape,with fine-scale oscillatory growing ring.The Th/U ratio of (0.33 2.08)indicates a magmatic origin.The dating results show that the volcanic rocks in Manitu Formation are formed in Late Jurassic from 139 to 148 Ma.The geochemical study shows that the SiO2 of the andesite is between 64.87% 68.65%,Al2 O3 is between 15.53% 1 6.72%,Na2 O+K2 O is between 6.75% 9.22%,K2 O/Na2 O is between 0.63 1.57, falling in the series of high-potassium calc-alkaline.All samples have a similar REE pattern with a significant fractionation of LREE from HREE((La/Yb)N = 1 1.50 14.88)and negative Eu anomalies (δEu=0.45 1.17).The total rare earth element is from 124.95 × 10 -6 to 208.57 × 10 -6 .The trace element geochemistry is characterized evidently by enrichment of Rb,Ba,K,depletion of Nb,P,Ti. Its original magma is derived from the partial melting of the crust,during an extension after the closure of the Mongol-Okhotsk Ocean.%对大兴安岭中段柴河地区玛尼吐组火山岩进行了锆石 LA ICP MS U Pb 年代学和岩石地球化学研究,以了解其形成时代、岩石成因及其所揭示的区域构造背景。玛尼吐组火山岩岩相学鉴定结果为安山岩、英安岩,化学成分显示其为粗面英安岩、粗面岩和英安岩。玛尼吐组火山岩锆石多呈自形半自形晶,振荡环带发育,Th/U=0.33~2.08,指示其岩浆成因。定年结果显示,玛尼吐组火山岩形成于139~148 Ma 的晚侏罗世。岩石地球化学研究表明,玛尼吐组火山岩的 w (SiO2)为64.87%~68.65%, w(Al2 O3)为15.53%~16.72%,全碱 w(Na2 O+K2 O)为6.75%~9.22%,K2 O/Na2 O=0.63~1.57,属于高钾钙碱性系列。所有样品的稀土配分曲线具有相似的特征,稀土丰度总量 w (∑ REE)为(124.95~208.57)×10-6,轻重稀土分馏明显,(La/Yb)N =11.50~14.88,Eu 负异常(0.45~ 1.17),微量元素以富集 Rb、Ba、K,亏损 Nb、P、Ti 为特征。玛尼吐组火山岩原始岩浆来源于地壳岩石的部分熔融,形成于蒙古--鄂霍茨克缝合带闭合后的岩石圈伸展构造环境。