摘要
西藏安多微陆块作为班公湖-怒江缝合带内的微陆块,记录了新元古代—中生代以来的多期构造热事件,是研究深熔-花岗岩成因的理想对象。为了揭示大洋板块俯冲-折返过程中流体/熔体活动特征,本文结合全岩地球化学、系统的岩石学、锆石内部结构、锆石U-Pb年龄和Lu-Hf同位素进行了综合研究。岩相学观察结果显示混合片麻岩保留了关键野外宏观和微观深熔证据:①浅色体和暗色体相间呈层状分布,伴随微弱的褶皱变形;②石英的边界和钾长石的边界有细小颗粒的集合体,钾长石边部到中间也有不规则的结晶现象;③斜长石和钾长石边界显示出高度尖状、细长或楔形的石英颗粒和长石颗粒,沿着石英和长石颗粒边界有“串珠”结构。阴极发光图像和锆石U-Pb定年结果表明混合岩化片麻岩的锆石具有明显的核-边结构,锆石核部具有明显的振荡环带特征,给出的岩浆结晶年龄约为~510 Ma,边部具有较窄的变质或深熔边。浅色体中锆石具有明显的核-边结构,CL图像显示锆石核部呈高度发光且具有振荡环带,可能是继承岩浆锆石,锆石边部呈现灰暗色到暗色的弱分带或无分带等深熔特征,核部年龄约为510~470 Ma代表原岩结晶年龄,边部年龄为~184 Ma指示熔体结晶年龄。花岗闪长岩的锆石具有典型的岩浆锆石特征,岩浆结晶年龄为~180 Ma,与浅色体的年龄在误差范围内一致。浅色体中深熔成因锆石的ε_(Hf)(184 Ma)值为-5.0~-3.3,花岗闪长岩岩浆锆石的ε_(Hf)(180 Ma)值为-10.97~-5.21。全岩地球化学分析表明TFe_(2)O_(3)、MgO、TiO_(2)、CaO和REEs几乎完全保留在暗色体中,而大量的LILEs(Rb、Sr、K、Ba)则到浅色体中,根据全岩REE特征以及是否携带残留角闪石将浅色体分为Ⅰ型浅色体和Ⅱ型浅色体,其中Ⅰ型浅色体总稀土含量较高,负Eu异常;Ⅱ型浅色体总稀土含量较低,正Eu异常;花岗闪长岩稀土(REE)分布趋势与Ⅰ型浅色体一致,富集大离子亲石元素(Rb、Ba、Th),负Eu异常;综合区域已有资料以及本文获得的野外关系、显微结构、年代学和地球化学结果,表明安多微陆块的黑云斜长片麻岩在俯冲折返阶段发生了黑云母参与的水致部分熔融作用,混合岩中的Ⅰ型浅色体经大规模汇聚、迁移演化和侵位形成了同期花岗闪长岩体。
The Amdo micro-continent in Tibetan Plateau,located within the Bangong Lake-Nujiang River suture zone,has recorded multiple phases of tectonothermal events since the Neoproterozoic-Mesozoic era.Due to its geological significance,it serves as an excellent object for investigating the genesis of the deep-melt granitoids.In this study,we conducted a comprehensive analysis,combining whole-rock geochemistry,systematic petrology,zircon internal structure,zircon U-Pb ages,and Lu-Hf isotopes,to reveal the characteristics of the fluid/melt activities during the subduction-folding process of the oceanic plate.Petrographic observations of the mixed gneiss provide key macroscopic and microscopic evidence of deep melting,including:①interbedded light and dark bodies with laminated distribution and weak fold deformation;②fine-grained assemblages at the boundaries of quartz and potash feldspar,along with irregular crystallization of potash feldspars from edge to middle;③highly acicular,elongate,or wedgeshaped quartz and feldspar grains along the boundaries of plagioclase feldspar and potash feldspar,exhibiting“bead”structures.Cathodoluminescence images and zircon U-Pb dating results show a distinct core-rim structure in the zircons of the mixed-rock gneisses.The zircon cores exhibit a distinctive oscillatory ring,indicating a magma crystallization age of~510 Ma.The zircon rims,on the other hand,display narrow metamorphic or deep-melting features.In the light-colored body,zircons also exhibit an obvious core-rim structure,with a highly luminous core(showed by the CL image)containing an oscillatory ring band,suggesting an inherited magmatic zircon.The zircon rims show deep melting features,such as weakly fractional bands of grayish to dark color or the absence of fractional bands.The core age ranges from 510 to 470 Ma,representing the age of protolithic crystallization,while the rims have an age of~184 Ma,indicating the age of melt crystallization.The zircons in the granodiorite also display typical magmatic zircon characteristics,with a magmatic crystallization age of~180 Ma,consistent with the age of the light-colored body.Theε_(Hf)(184 Ma)values of the deep-melting diagenetic zircons in the light-colored body range from 5.0 to 3.3,whereas those of the granodiorite magmatic zircons range from 10.97 to 5.21.Whole-rock geochemical analyses indicate that TFe_(2)O_(3),MgO,TiO_(2),CaO,and REEs are almost completely retained in the dark-colored body,while a significant amount of LILEs(Rb,Sr,K,and Ba)is present in the light-colored body.The light-colored body is divided into I-type and IItype according to the whole-rock REE characteristics and the presence of residual hornblende.The I-type light-colored body exhibits higher total rare earth content and a negative Eu anomaly,while the II-type light-colored body displays lower total rare earth content and a positive Eu anomaly.The distribution trend of rare earths(REEs)in the granodiorite eclogites aligns with that of the I-type light-colored body,showing enrichment of large ionic proximate elements(Rb,Ba,and Th)and a negative Eu anomaly.By synthesizing the existing regional data and considering the field relationships,microstructure,chronology,and geochemical results obtained in this study,we propose that the biotite plagioclase gneisses of the Amdo microterrane formed during the subduction and folding stages involving hydrous partial melting with the contribution of biotite.Furthermore,we suggest that the I-type light-colored bodies in the mixed rocks contemporaneous formed the granodiorite bodies through large-scale convergence,migratory evolution,and encroachment.
作者
简珍珠
彭银彪
蒋兴洲
高翔宇
纪文涛
李传志
于胜尧
JIAN Zhenzhu;PENG Yinbiao;JIANG Xingzhou;GAO Xiangyu;JI Wentao;LI Chuanzhi;YU Shengyao(Ocean University of China College of Marine Geology,Key Lab of Submarine Geosciences and Prospecting Techniques MOE China,Qingdao,Shandong 266100,China;Laboratory for Marine Mineral Resources,Qingdao National Laboratory for Marine Science and Technology,Qingdao,Shandong 266037,China)
出处
《地质学报》
北大核心
2025年第7期2330-2353,共24页
Acta Geologica Sinica
基金
青岛海洋科学与技术试点国家实验室山东省专项经费(编号2022QNLM05032-3)
国家自然科学基金项目(编号42121005,42372247)
山东省自然科学基金杰出青年基金项目(编号ZR2020JQ16)
泰山学者项目(编号ts20190918,tstp20221112)联合资助的成果。
