西藏中拉萨地块北部早白垩世晚期控错A型花岗岩的成因及构造环境研究  

Petrogenesis and tectonic setting of the late Early Cretaceous Kong Co A-type granite in the northern margin of Central Lhasa Subterrane, Tibet

作　　者：刘洪[1,2] 李光明 李文昌[1,3] 黄瀚霄[1] 李佑国[2] 欧阳渊[1] 张向飞[1] 周清 LIU Hong;LI GuanMing;LI WenChang;HUANG HanXiao;LI YouGuo;OUYANG Yuan;ZHANG XiangFei;ZHOU Qing(Chengdu Center,China Geological Survey,Chengdu 610081,China;College of Earth Sciences,Chengdu University of Technology,Chengdu 610059,China;College of Land and Resources Engineering,Kunming University of Science and Technology,Kunming 650093,China)

机构地区：[1]中国地质调查局成都地质调查中心,成都610081 [2]成都理工大学地球科学学院,成都610059 [3]昆明理工大学国土资源工程学院,昆明650093

出　　处：《岩石学报》2022年第1期230-252,共23页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(92055314、91955208、42002097);国家重点研发计划(2021YFC2901803、2021YFC2901903);中国地质调查项目(DD20221776、DD20190542)联合资助。

摘　　要：在中拉萨地块北部尼玛控错地区发育着一套碱性长石花岗岩,对该花岗岩体开展成因和形成背景的研究,能为探索班公湖-怒江洋的构造演化提供有价值的信息。用LA-ICP-MS方法测得该花岗岩的锆石^(206)Pb/^(238)U年龄加权平均值为104.9±1.4Ma(MSWD=1.5)和104.6±1.3Ma(MSWD=1.3),表明该岩体形成于早白垩世。花岗岩具有高硅(SiO_(2)=76.75%~77.51%,平均77.27%)、高钾(K_(2)O=4.61%~4.85%,平均4.77%)、高碱(K_(2)O+Na_(2)O=8.24%~8.57%,平均8.44%)、低钙(CaO=0.28%~0.48%,平均0.35%)、低镁(MgO=0.11%~0.16%,平均0.13%)和低铝(Al_(2)O_(3)=11.79%~12.22%,平均12.09%)等特征,里特曼指数(σ)为1.96~2.15(平均2.08),A/NK值为1.06~1.09,A/CNK值为1.01~1.04。这些特征表明控错花岗岩为弱过铝质的高钾钙碱性-钾玄岩系列岩石。控错花岗岩相对富集Zr、Nb、Ce、Y和Hf等微量元素,相对亏损Ti、Ba、Sr和P等微量元素,分异系数(DI)为95.5~96.9(平均:96.3),还具有较高的FeOT/MgO值(5.61~10.22,平均7.26)、10000Ga/Al值(2.78~2.56,平均2.84)、Y/Nb值(2.29~4.97,平均3.57)、Rb/Nb值(11.6~18.2,平均15.2);此外,该岩体还具有较高的全岩Zr饱和温度(875~910℃,平均890℃)和锆石Ti饱和温度(848~919℃,平均890℃),明显的Eu负异常(δEu=0.04~0.09,平均0.06),以及向右缓倾的"V型"稀土元素配分曲线,这些特征表明控错花岗岩为产于碰撞后环境的A2型花岗岩。正的锆石εHf(t)值(4.26~6.38,平均5.16)、相对年轻的锆石Hf地壳模式年龄(tDM2=757~889Ma,平均833Ma)、下地壳与地幔混合特征的(^(87)Sr/^(86)Sr)t(0.7194~0.7407,平均0.7313)、εNd(t)(-3.39~-3.00,平均-3.24)和Pb同位素特征((^(206)Pb/^(204)Pb)_(t)=18.792~18.845,(^(207)Pb/^(204)Pb)_(t)=15.708~15.718,(^(208)Pb/^(204)Pb)_(t)=38.870~38.037),指示控错花岗岩熔融于幔源物质加入的新生地壳。研究结果揭示,控错花岗岩形成于羌塘-拉萨地块碰撞作用下,俯冲板片的断离后,软流圈上涌诱发的地壳部分熔融,并经历了显�A set of alkali feldspar granite develops well in the Kong Co of Nyima County in the northern margin of Central Lhasa Subterrane,Tibet,which can provide invaluable information to explore the geological evolution of the Bangong Co-Nujiang Ocean.The weighted averaged zircon ^(206)Pb/^(238)U ages of the granite measured by LA-ICP-MS are 104.9±1.4 Ma(MSWD=1.5)and 104.6±1.3 Ma(MSWD=1.3),respectively,indicating that it was formed in the late phase of Early Cretaceous magmatism.The rocks in Kong Co granite are grayish-white,with fine-grained equigranular structure and medium coarse-grained porphyritic-like texture.The rock-forming minerals mainly include quartz,potassium feldspar,plagioclase,and a small amount of transparent accessory minerals such as biotite and amphibole,as well as a few melanocratic mineral like magnetite.The granite is characterized by high contents of SiO_(2)(6.75%~77.51%),K_(2)O(4.61%~4.85%),and K_(2)O+Na_(2)O(8.24%~8.57%),with low contents of CaO(0.28%~0.48%),MgO(0.11%~0.16%)and Al_(2)O_(3)(11.79%~12.22%),and a Rittman index(σ43)of 1.96~2.15 and A/CNK ratios of 1.01~1.04.These characteristics reveal that the Kong Co granite is weakly peraluminous high potassium calc-alkaline-potassium basalt series rock.The granite is relatively enriched in Zr,Nb,Ce,Y and Hf,and deficient in Ti,Ba,Sr and P.It shows a highly variation of index(DI:95.50~96.90),and also has high values of FeOT/MgO(5.61~10.22),10000 Ga/Al(2.78~2.56),Y/Nb(2.29~4.97),Y/Nb(2.29~4.97),Rb/Nb(11.6~18.2),which is consistent with A2-type granites produced in post collision environment.In addition,the rock has high whole-rock Zr saturation temperatures(875~910℃)and zircon Ti saturation temperatures(848~919℃),with a significant negative Eu anomaly(δEu=0.04~0.09),and a“V-shaped”rare earth element curve gently inclined to the right.These characteristics also prove the Kong Co granite is A2 type granite.They have relatively positiveεHf(t)values(4.26~6.38),and two-stage Hf model ages(tDM2)ranging from 757 Ma to 889 Ma,with(^(87)Sr/^(8

关 键 词：班公湖-怒江结合带 控错 A型花岗岩 锆石U-PB SR-ND-PB LU-HF 

分 类 号：P588.121[天文地球—岩石学] P597.3[天文地球—地质学]