东昆仑布尔汗布达早古生代岩浆弧的形成与演化初探

Research on Formation and Evolution of Early Paleozoic Bulhanbuda Arc in East Kunlun Orogen

东昆仑布尔汗布达弧位于东昆中断裂南侧,不同于北昆仑晚古生代-中生代岩浆弧,是东昆仑早古生代岩浆岩广泛分布的地区,对研究原特提斯洋俯冲-增生过程具有重要意义.报道了布尔汗布达地区早古生代岩浆岩岩石学、地球化学、锆石U-Pb定年、Lu-Hf和Sr-Nd同位素资料,结合以往研究成果,推断布尔汗布达弧与增生造山作用相关的岩浆活动可以划分为寒武纪(515~482 Ma)、中奥陶世(465~463 Ma)和晚奥陶世-早志留世(454~438 Ma)3期.寒武纪岩浆岩是富铌镁铁-超镁铁质岩和偏铝质中高钾钙碱性闪长岩,中奥陶世为弱过铝质高钾钙碱性二长花岗岩,晚奥陶世岩浆岩主要是弱过铝质中高钾钙碱性花岗闪长岩和二长花岗岩,次为中钾钙碱性偏铝质石英闪长岩和拉斑玄武质辉绿岩.3期岩浆活动中酸性侵入岩微量元素组成与弧安山岩(CAA)相似,寒武纪闪长岩Sr/Y比值(22.6~30.0)低,中奥陶世二长花岗岩Sr/Y比值(34.4~37.1)中等,晚奥陶世花岗闪长岩和二长花岗岩Sr/Y比值(49.4)较高,并且均富集全岩放射性成因Sr、Nd同位素;寒武纪闪长岩(^(87)Sr/^(86)Sr)i为0.7151~0.7157,ε_(Nd)(t)值为-7.4~-7.3;中奥陶世二长花岗岩(^(87)Sr/^(86)Sr)i是0.7076~0.7077,ε_(Nd)(t)值是-2.5~-2.4;晚奥陶世花岗闪长岩和二长花岗岩(^(87)Sr/^(86)Sr)i是0.7059~0.7065,ε_(Nd)(t)值是(-3.3~-1.7),闪长岩(^(87)Sr/^(86)Sr)i是0.7069~0.7085,ε_(Nd)(t)值是-6.0~-5.6.锆石Hf同位素组成变化较大,寒武纪闪长岩ε_(Hf)(t)为-6.8~-4.4,中奥陶世二长花岗岩ε_(Hf)(t)是+0.13~+2.90,晚奥陶世花岗闪长岩和二长花岗岩ε_(Hf)(t)为-2.7~9.2,闪长岩ε_(Hf)(t)为-8.6~-2.1.综上所述,推断布尔汗布达弧是原特提斯洋向南俯冲形成的安第斯型陆缘弧,经历了3个重要的演化阶段,早期(515~482 Ma)俯冲带楔状地幔部分熔融形成基性岩浆,同时诱发古元古代下地壳重熔形成中酸性岩浆;中期(465~463 Ma)幔源基性岩浆底侵,下地壳加厚,地壳部分熔融形成小规模酸性岩浆,晚期(454~438 Ma)幔源基性岩浆底侵、加厚下地壳形成,下地壳部分熔融,形成大规模具有埃达克岩地球化学成分的中酸性岩浆岩.

The Bulhanbuda arc is located in the south of the central fault of East Kunlun orogen.Different from the Late Paleozoic-Mesozoic magmatic arc in the North Kunlun,Early Paleozoic magmatic rocks are widely exposed in the Bulhanbuda area and of great significance to the researches of subduction-accretion process of the Proto-Tethyan Ocean.In this study,it reports the petrology,geochemistry,zircon U-Pb ages,Lu-Hf and Sr-Nd isotopic compositions of the Early Paleozoic magmatic rocks from the Bulhanbuda area.This and previous studies indicate that the subduction-related magmatic activities can be divided into three stages:Cambrian(515-482 Ma),Middle Ordovician(465-463 Ma),and Late Ordovician-Early Silurian(454-438 Ma).The Cambrian magmatic rocks contain Nb-enriched mafic-ultramafic rocks and metaluminous high-K calc-alkaline diorite.The Ordovician magmatic rocks are weakly peraluminous high-K calc-alkaline monzogranites.The Late Ordovician magmatic rocks are dominated by weakly peraluminous medium-and high-K calc-alkaline diorite and monzogranite and contain some metaluminous calc-alkaline quartz diorite and tholeiitic dolerite.These three stage intrusive rocks show similar trace element compositions with those of continental arc andesite.The Sr/Y ratios of the Cambrian diorite are low(22.6-30.0),the Sr/Y ratios of the Middle Ordovician monzogranite are medium,and these of Late Ordovician granodiorite and monzogranite are high.All of these intrusive rocks are enriched in radiogenic Sr and Nd isotopes.The Cambrian diorites display high initial ^(87)Sr/^(86)Sr ratios of 0.7151-0.7157 and lowε_(Nd)(t)values of-7.4 to-7.3.The Middle Ordovician monzogranite have initial ^(87)Sr/^(86)Sr ratios of 0.7076-0.7077 andε_(Nd)(t)values of-2.5 to-2.4.The Late Ordovician granodiorite and monzogranite have low initial ^(87)Sr/^(86)Sr ratios(0.7059-0.7065)and negativeε_(Nd)(t)values(-3.3 to-1.7).However,the diorite shows relatively high initial ^(87)Sr/^(86)Sr ratios(0.7069-0.7085)and lowε_(Nd)(t)values(-6.0 to-5.6).The three stage magmatic rocks show wide range of the zircon Hf isotopes.Cambrian diorites have negative ε_(Hf)(t)values(-6.8 to-4.4).Middle Ordovician monzogranites display positiveε_(Hf)(t)values(+0.13 to+2.90).Late Ordovician granidorite and monzogranite show wide range of ε_(Hf)(t)values(-2.7 to+9.2).The diorites mainly yield negativeε_(Hf)(t)values(-8.6 to-2.1).In conclusion,the Bulhanbuda arc is an Andean-type continental margin arc formed during the southward subduction of the Proto-Tethyan Ocean.It has undergone three significant evolutionary stages.In the early stage(515-482 Ma),partial melting of the mantle wedge generated the basic magma,which induced remelting of the Paleoproterozoic lower crust to form intermediate-acid magma.In the middle period(465-463 Ma),the mantle derived basic magma underpenetrated and the thickened lower crust was partially melted to form small-scale acid magma.In the late period(454-438 Ma),the mantle derived basic magma underpenetrated and the thickened lower crust was partially melted to form largescale intermediate-acid magmatic rocks with adakitic geochemical composition.