柴北缘西端盐场北山二长花岗岩年代学、地球化学及其Hf同位素特征

Geochronology,Geochemistry,and Hf Isotope of Yanchangbeishan Adamellite of Lenghu Area in Qinghai

盐场北山二长花岗岩位于柴北缘西端青海冷湖地区.岩体高硅（74.98%~76.92%）,富钾（4.44%~5.93%）,贫镁（0.04%~0.07%）和钙（0.43%~0.69%）.里特曼指数介于2.43~2.79属于高钾钙碱性岩石系列,铝饱和指数平均为1.01,属弱过铝质花岗岩.岩体富集大离子亲石元素（K、Rb、Ba、Pb）,亏损高场强元素（Nb、Ta）,具岛弧花岗岩地球化学特征.LA-ICP-MS锆石 U-Pb 测年结果表明：岩体形成于252＆#177;3 Ma；锆石 Hf 同位素初始比值176 Hf/177 Hf 分布于0.282958~0.283032之间,并具有极高的正εHf （t ）值（8.75~14.50）,其平均两阶段模式年龄 T DM2（Hf）为385 Ma,反映岩体源区可能为玄武质下地壳.另外,εHf （t ）基本位于亏损地幔演化线附近,指示该玄武质下地壳来源于亏损地幔.结合区域地质背景和岩石地球化学特征,认为盐场北山二长花岗岩与泥盆纪镁铁-超镁铁岩有关,由于宗务隆裂谷小洋盆的向南俯冲,引发其上覆玄武质新生地壳发生熔融形成,进而揭示晚二叠世末柴北缘处于火山弧或活动陆缘的构造环境.

The adamellite develops in Yanchangbeishan of Lenghu area in Qinghai,which is located in the west segment of the north margin of Qaidam.The samples have higher SiO2 （74.98% - 76.92%）and K2 O （4.44% - 5.93%）contents,but lower MgO （0.04%-0.07%）and CaO （0.43%-0.69%）contents.The Rittmann indices of the rock range in 2.43-2.79,which shows that it belongs to high potassium calc-alkaline series.The alumina saturation average index is 1.01,which indicates that the adamellite belongs to peraluminous granites.Moreover,the samples are intensively depleted in HFSEs （Nb,Ta）,with enriched LILEs （K,Rb,Ba,Pb）, reflecting the characteristics of island-arc magma.LA-ICP-MS zircon U-Pb dating shows that the formation age of the rock is 252 ＆#177; 3 Ma.The zircon Hf isotope analysis reveals that 1 76 Hf/1 77 Hf ratios,varying in 0.282 958 to 0.283 032,have high positiveεHf （t ） （8.75-14.50）and two-stage Hf model ages （averaged at 385 Ma）.In the ^1 76 Hf/^1 77 Hf-εHf （t ）discrimation diagrams,these zircons are plotted nearly on the depleted mantle evolution line.It can be concluded that the rock chiefly derived from the new basaltic lower crust which is originated from depleted mantle.Based on the regional geological background and the geochemical characteristics of the mas-ses,we consider that Yanchangbeishan adamellite bears similar characteristics of its primary magma with mafic-ultramafic rocks of De-vonian,which were supposed to be originated from partial melting of the young basaltic lower crust related to subduction of Zongwulong small oceanic basin from south.Thereby,it reveals that the north margin of Qaidam was in volcanic arc or active continen-tal margin tectonic environment during Permian epoch.