摘要
华南陆块东部在中生代时期主要受古特提斯和古太平洋两大构造域的影响,发育多期岩浆活动和大规模成矿作用,但两大构造域的转换时间及过程存在较大争议.基性岩是研究软流圈、岩石圈地幔和俯冲板片相互作用的理想对象,本文系统厘定了华南陆块东部中生代基性岩(228～69 Ma)的时空格架及元素-同位素时空演变规律,取得了以下主要认识:(1)侏罗纪和白垩纪基性岩按照岩相学和元素-同位素地球化学特征均可分为OIB、类OIB和"岛弧''/岛弧型三类,它们来源于不同的地幔源区;(2)侏罗纪"岛弧"型基性岩的时空分布和元素-同位素地球化学性质明显有别于晚三叠世基性岩和白垩纪岛弧型基性岩,说明它们并非是与古太平洋板块俯冲相关的岛弧岩浆,其"岛弧"性质很可能是继承了源区的特征;(3)两大构造域转换的起始时间为早侏罗世早期(约200Ma),完成转换的最终时间为中-晚侏罗世之交(约170Ma);(4)古太平洋构造域自晚侏罗世以来对华南陆块东部的影响越来越明显,白垩纪时期的板片交代影响范围仅局限于赣江断裂以东地区;(5)白垩纪基性岩主要形成于两个峰期(约125和90Ma),与古太平洋板块的两期俯冲-回撤作用有关.
Abstract
The eastern South China Block(SCB)was mainly influenced by Paleo-Tethyan and Paleo-Pacific dynamic domains during the Mesozoic,accompanied by intensive magmatism and mineralization.However,the tectonic transition and evolution of these two dynamic domains remain debated.The tectonic transition has significant effects on the tectonic,sedimentary and magmatism activities in the eastern SCB.The mafic igneous rocks are excellent indicators for exploring the interaction among asthenosphere,lithosphere mantle and subducted slab.The spatio-temporal framework of Mesozoic mafic rocks is critical for unravelling the tectonic transition and evolution.Thus,this paper complied the geochronology and geochemistry data of Mesozoic mafic igneous rocks in the eastern SCB,aiming to constrain the tectonic transition and the influence of Paleo-Pacific Plate subduction.The formation ages of the Mesozoic mafic igneous rocks(228-69 Ma)mainly fall into five age-clusters at ca.228-223 Ma,ca.197-169 Ma,ca.168-146 Ma,ca.143-115 Ma,and ca.113-69 Ma.The Triassic mafic igneous rocks in the eastern SCB are enriched in large-ion lithophile elements(LILE)and high-field strength elements(HFSE)with significant Nb-Ta positive anomalies.They have high(87Sr/86Sr)i ratios and low εNd(t)values,deriving from partial melting of an enriched lithosphere mantle and forming in an post-collision setting associated with the Paleo-Tethyan dynamic domain.The Jurassic and Cretaceous mafic igneous rock can be geochemically classified into OIB-,OIB-like-and"arc"/arc-types,originating from different mantle sources.The OIB and OIB-like mafic rocks were derived from asthenosphere and asthenosphere-lithosphere interaction,respectively.The Cretaceous arc mafic rocks were derived from a mantle wedge which was modified by Paleo-Pacific subducted slab-derived melts/fluids.The Cretaceous mafic rocks show geochemical variations from the interior(OIB and OIB-like-types)to coastal provinces(arc-type).From the interior to coastal provinces,the Cretaceous mafic igneous rocks exhibit a decreasing trend of Nb/Yb,Nb/La,Ce/Pb ratios and εNd(t)values across the Ganjiang Fault,suggesting the increasing contribution of Paleo-Pacific subducted slab-derived components.Such spatio-temporal framework indicates that the slab metasomatism associated with Paleo-Pacific Plate subduction during the Cretaceous was probably limited to the east of the Ganjiang Fault.The Jurassic"arc"-type mafic igneous rocks are marked by enrichment in LILE and depletion in HFSE with significant Nb-Ta positive anomalies.However,they have Nb/La,whole-rock εNd(t)and(206Pb/204Pb)i,and zircon εHf(t)values higher than the Cretaceous arc mafic rocks.Furthermore,the Jurassic"arc"-type mafic rocks mainly occurred at the west of the Ganjiang Fault,distinct from the Cretaceous arc mafic rocks.Such spatial-temporal and geochemical signatures suggest that the Jurassic"arc"-type mafic rocks were not genuine arc rocks which were associated with Paleo-Pacific Plate subduction.Instead,their"arc"signatures appear to have been inherited from their mantle source.The Early-Middle Jurassic mafic rocks occurred in the EW-trending Nanling Range,and the contemporaneous sedimentary rocks share similar detrital provenance to the Late Triassic sedimentary rocks.The eastern SCB was not influenced by Paleo-Pacific Plate subduction during the Early-Middle Jurassic,and inherited the tectonic regime from Paleo-Tethyan dynamic domain.The Late Jurassic mafic rocks occurred as a NE-trending,and the basin type,detrital provenance and tectonic regime changed,suggesting a significant effect from Paleo-Pacific Plate subduction on the eastern SCB.Thus,the tectonic transition from Paleo-Tethyan to Paleo-Pacific dynamic domains likely occurred at ca.200 Ma initially,and completed at ca.170 Ma finally.The Paleo-Pacific dynamic domain has significant influences on the eastern SCB since Late Jurassic.
基金项目
国家自然科学基金(42330302)
国家重点研发计划(2023YFF0803701)
南方海洋科学与工程广东省实验室(珠海)项目(SML2023SP206)
NETL
NSTL
万方数据