西昆仑大红柳滩地区花岗岩类侵位时代与成因

Age and petrogenesis of granitoids in Dahongliutan area,West Kunlun

西昆仑大红柳滩地区花岗岩类分布广泛,与伟晶岩型锂(铍)矿化时空关系密切。为了约束其地质特征、岩石成因以及对伟晶岩型锂(铍)成矿的作用,文章对白龙山闪长岩、俘虏沟闪长岩和大红柳滩二云母花岗岩进行了岩石学、矿物学、地球化学和年代学研究工作。地球化学数据揭示白龙山闪长岩和俘虏沟闪长岩明显富MgO、CaO和TiO_(2),属于准铝质-弱过铝质、高钾钙碱性岩浆系列,结合已有全岩Sr-Nd、锆石Lu-Hf等同位素数据,白龙山闪长岩和俘虏沟闪长岩涉及新生地壳的贡献,其源区贫黏土,可能以砂屑岩为主。全岩锆饱和温度计约束岩浆温度分别为762~795℃(平均781℃)和769~773℃(平均771℃)。角闪石温压计约束白龙山闪长岩中角闪石结晶温度为718~760℃(平均734℃),压力介于125~208 MPa之间(平均151 MPa);俘虏沟闪长岩中的角闪石结晶温度为729~776℃(平均741℃),压力介于133~231 MPa之间(平均176 MPa)。白龙山闪长岩和俘虏沟闪长岩锆石LA-ICP-MS U-Pb年龄分别为(213.67±0.61)Ma(MSWD=1.4,n=39)和(211.73±1.02)Ma(MSWD=1.9,n=14)。大红柳滩二云母花岗岩中发育电气石、白云母和石榴子石等富铝矿物,明显富SiO_(2)和K_(2)O,贫MgO、CaO和TiO_(2),属于过铝质、高钾钙碱性岩浆系列,由壳源物质重熔而成,源区可能是泥质岩。全岩锆饱和温度计约束其温度为755~773℃(平均764℃)。所含锆石往往发育继承核,其年龄介于207~2490 Ma之间。最年轻的岩浆锆石约束岩浆侵位可能发生在(211.20±1.1)Ma(MSWD=0.34,n=10),这些岩浆锆石随后被热液增生边包裹,U-Pb年龄约束岩浆-热液事件发生在(185.10±0.89)Ma(MSWD=4,n=5),可能记录了后期伟晶岩侵位所诱发的热液活动。大红柳滩地区同时发育I型和S型花岗岩,形成于中温(734~781℃)、中压(151~176 MPa)的条件,岩浆-热液活动持续时间长达~33 Ma,为形成超大型规模矿床提供有利条件。

In the West Kunlun orogenic belt,there are well developed Triassic granitoids that may be associated with pegmatite Li(Be)mineralization.In order to constrain its geological characteristics,petrogenesis,and their impact on pegmatitic lithium(beryllium)mineralization.In this paper,petrology,mineralogy,geochemistry and geochronology studies were carried out for the Bailongshan diorite,Fulugou diorite and Dahongliutan two-mica granite from the Dahongliutan district.Geochemical analyses reveal that the Bailongshan diorite and Fulugou diorite are significantly rich in MgO,CaO and TiO_(2),and belong to metaluminous-peraluminous,high-K calc-alkaline series.Based on a comprehensive review of available whole-rock Sr-Nd and zircon Lu-Hf data,we propose that the Bailongshan diorite and Fulugou diorite have a source region characterized by low clay content(possibly dominated by sandstone)and potential contributions from juvenile crust.The whole rock Zr saturation temperatures are 762~795℃(averaged at 781℃)for the Bailongshan and 769~773℃(averaged at 771℃)for the Fulugou diorites.Amphibole thermobarometry yields temperatures of 718~760℃(averaged at 734℃)and pressure of 125~208 MPa(averaged at 151MPa)for the Bailongshan diorite,and 729~776℃(averaged at 741℃)and 133~231MPa(averaged at 176 MPa)for the Fulugou diorite.Zircon LA-ICPMS U-Pb dating reveals an emplacement age of(213.67±0.61)Ma(MSWD=1.4,n=39)for the Bailongshan diorite and(211.73±1.02)Ma(MSWD=1.9,n=14)for the Fulugou diorite.In case of the Dahongliutan two-mica granite,there are abundant aluminum-rich minerals such as tourmaline,muscovite and garnet.The two-mica granite has higher SiO_(2) and K_(2)O,but lower MgO,CaO and TiO_(2) in comparison to the Bailongshan and Fulugou diorites,and belongs to peraluminous,high K calc-alkaline series.By contrast,the Dahongliutan two-mica granite is mainly sourced from partial melting of pelitic rocks.Its Zr saturation temperature falls into the range of 755~773℃(averaged at 764℃).The zircons hosted by two-mica granite commonly have inherited nuclei,and their U-Pb ages are extremely variable,ranging from 207 Ma to 2490 Ma.The youngest magmatic zircon constrains the emplacement at(211.20±1.1)Ma(MSWD=0.34,n=10).Subsequently,these magmatic zircons were enveloped by hydrothermal overgrowth rims,and the U-Pb age constrains the magmatic-hydrothermal event to have occurred at(185.10±0.89)Ma(MSWD=4,n=5),possibly recording a later hydrothermal event induced by the emplacement of barren pegmatite.The Dahongliutan area concurrently developed I-type and S-type granites,formed under conditions of medium temperature(734~781℃)and pressure(151~176 MPa),with magma-hydrothermal activity lasting~33 Ma,providing favorable conditions for the formation of super large pegmatite deposits.