地质科学2025，Vol.60Issue(4)：1014-1041,28.DOI:10.12017/dzkx.2025.067
中祁连地块北缘寒武纪镁铁质岩成因及其地质意义
Petrogenesis and geological implication of Cambrian mafic rocks at the northern margin of the central Qilian block
摘要
Abstract
Mafic rocks in subduction zones are direct products of mantle magmatic activities and record the properties of the mantle source and the processes of crust-mantle interaction.They are crucial carriers for understanding deep magmatic processes and geodynamic mechanisms.This study investigates two types of mafic rocks exposed in the Datong area of the central Qilian block.Based on field geological investigation and petrographic studies,comprehensive analyses including zircon U-Pb geochronology,in-situ Hf isotopes,and whole-rock geochemistry studies were conducted to reveal the magmatic processes and mantle source characteristics,and to discuss their tectonic setting.The LA-ICP-MS zircon U-Pb ages indicate that the mafic cumulate rocks and OIB-like mafic rocks formed during the Cambrian,with weighted average ages of 515～512 Ma and 509～491 Ma,respectively.Geochemical data reveal that the mafic cumulate rocks are enriched in large-ion lithophile elements(e.g.,K,Rb,Sr,Ba)and depleted in high-field-strength elements(e.g.,Nb,Ta,Th,U),with depleted Nd and zircon Hf isotopic compositions(εNd(t)=-0.1～+2.0,εHf(t)=+9.67～+14.28).These features suggest that they originated from low-degree partial melting of the asthenospheric mantle modified by oceanic slab-derived fluids.In contrast,the Datong OIB-like mafic rocks exhibit enrichment in light rare earth elements(LREE)and no depletion in high-field-strength elements(e.g.,Nb,Ta).Their relatively high La/Sm(4.6～6.9),Nb/Y(1.01～1.87),Nb/U(24.15～67.51),and TiO2/Al2O3(0.09～0.11)ratios suggest that their source was modified by oceanic slab-derived melts outside the rutile stability domain.These rocks have enriched Nd and zircon Hf isotopic compositions(εNd(t)=-2.9～-1.7,εHf(t)=-3.78～+0.38),relatively low 10000×Zn/Fe ratios(10.85～12.47)and higher Dy/Yb(2.24～2.77),indicated that they originated from the lithospheric mantle source dominated by peridotite.Quantitative geochemical simulations suggest that the Datong OIB-like mafic rocks were derived from low-degree partial melting(＜5％)of garnet peridotite.The primary magma underwent approximately 7％～10％ fractional crystallization,with major fractional crystallization mineral phases being olivine(55％)and clinopyroxene(45％).Combined with the regional tectonic setting,this study concludes that both types of mafic rocks formed in a continental arc-back arc setting,and record crust-mantle interaction processes during the southward subduction of the North Qilian oceanic plate beneath the central Qilian block.These findings enhance our understanding of the geodynamic evolution and deep material recycle within marginal arc system of the central Qilian block during the Early Paleozoic.关键词
中祁连地块/寒武纪/镁铁质岩/壳幔相互作用/北祁连洋Key words
The central Qilian block/Cambrian/Mafic rock/Crust-mantle interaction/North Qilian Ocean分类
天文与地球科学引用本文复制引用
段永婷,牛漫兰,王磊,李秀财,李晨,衡哲,朱泰昌,张帅..中祁连地块北缘寒武纪镁铁质岩成因及其地质意义[J].地质科学,2025,60(4):1014-1041,28.基金项目
国家自然科学基金项目(编号:42272246)和安徽省重点实验室自主创新专项(编号:PA2024GDSK0158)资助 （编号:42272246）
