地质学报2025，Vol.99Issue(7)：2330-2353,24.DOI:10.19762/j.cnki.dizhixuebao.2023183
藏北安多地区侏罗纪地壳深熔作用与花岗岩成因
Jurassic anatexis and granite genesis in Amdo area,northern Tibetan Plateau
摘要
Abstract
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:① interbedded light and dark bodies with laminated distribution and weak fold deformation;② fine-grained assemblages at the boundaries of quartz and potash feldspar,along with irregular crystallization of potash feldspars from edge to middle;③ highly acicular,elongate,or wedge-shaped 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.The εHf(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 TFe2O3,MgO,TiO2,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 Ⅱ-type 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.关键词
混合片麻岩/浅色体/深熔作用/花岗岩成因/班公湖-怒江缝合带Key words
migmatites/leucosome/anatexis/granite genesis/Bangong-Nujiang suture zone引用本文复制引用
简珍珠,彭银彪,蒋兴洲,高翔宇,纪文涛,李传志,于胜尧..藏北安多地区侏罗纪地壳深熔作用与花岗岩成因[J].地质学报,2025,99(7):2330-2353,24.基金项目
本文为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)和泰山学者项目(编号ts20190918,tstp20221112)联合资助的成果. （编号2022QNLM05032-3）
