早三叠世-晚白垩世古太平洋俯冲:沙巴的弧前岩浆记录

Early Triassic-Late Cretaceous Paleo-Pacific subduction zone along the East Asia continental margin:Fore-arc igneous records in Sabah of NE Borneo

厘定东亚大陆边缘中生代弧前岩浆记录以限定其俯冲边界及其时序,是解码古太平洋俯冲体系的关键.基于婆罗洲所处特殊大地构造位置且其具有弧前指示意义的基性-超基性岩石报道较少,本文选择菲律宾巴拉望构造带南延之婆罗洲沙巴地区开展了系统研究.对昔加麦火成岩、古达、特鲁比和达弗尔湾蛇绿岩基性岩及相关岩石的全岩Ar-Ar和锆石U-Pb年代学、全岩元素和Sr-Nd-Pb同位素地球化学的研究表明:沙巴基底发育有前三叠系岩石,构造上亲缘于华夏陆块或地处巽他古陆东缘.原定义为“沙巴非蛇绿岩的昔加麦基底岩石”的安山岩和英云闪长岩等形成于251~179 Ma,具埃达克质岩石地球化学属性,87Sr/86Sr(i)=0.70285~0.70307,εNd(t)=+6.5~+7.7,具太平洋洋中脊玄武岩(Mid-Ocean Ridge Basalt,MORB)型Pb同位素组成,源自俯冲板片派生组分交代的地幔源区,是古太平洋俯冲体系的重要组成部分.限定出沙巴地区蛇绿岩的形成时代介于早-中侏罗世至晚白垩世(约185~85 Ma),其中特鲁比蛇绿岩可能较早发育(约185~140 Ma)、古达蛇绿岩主体形成于早白垩世(约135~112 Ma)、而达弗尔湾蛇绿岩形成持续至晚白垩世(约85 Ma).沙巴蛇绿岩中镁质岩石发育具相似Sr-Nd-Pb同位素组成的MORB型、高铌和富铌基性岩,其87Sr/86Sr(i)=0.70291~0.70567,εNd(t)=+6.6~+10.2,(206Pb/204Pb)i=17.91~19.00、(207Pb/204Pb)i=15.43~15.58和(208Pb/204Pb)i=37.61~38.47,其源区受板片派生熔体的交代改造,形成于古太平洋俯冲弧前背景.该研究限定出东亚大陆边缘的古太平洋西向俯冲至少自早三叠世(约251 Ma)即已启动、并持续至晚白垩世(约85 Ma),具长寿命(>150 Ma)“多阶段幕式进撤”的安第斯型俯冲特征,其白垩纪俯冲前锋位于现今中国台湾东部、菲律宾西部,经沙巴、延入古晋带、甚至帕朗卡拉亚一线.该地区构造格局直至晚白垩世末期-新生代早期才转换为现今西太平洋俯冲体系.

East Asia is composed of discrete continental fragments derived from the northern margin of East Gondwana since the Paleozoic.Along its eastern margin,a giant Mesozoic arc igneous zone develops,which extends from Coastal South China into SE Vietnam and Peninsular Malaysia through the northern South China Sea,and then into West Borneo.Identification of Mesozoic fore-arc magmatism along the East Asian continental margin is a key to decoding the Paleo-Pacific supra-subduction system.However,the mafic-ultramafic rocks with the indicative of a fore-arc tectonic setting are poorly documented so far.This paper focuses on the Sabah tectonic zone of NE Borneo that is tectonically proposed as either the southern extension of the Palawan mélange zone or the eastern extension of the Sarawak Kuching tectonic zone.We present zircon U-Pb geochronology and Lu-Hf isotopes,as well as whole-rock Ar-Ar ages,and elemental and Sr-Nd-Pb isotopic data for the igneous rocks in Sabah.The igneous rocks are collected from(1)the previously-defined Segama non-ophiolite basement and(2)the Kudat,Telupid and Darvel Bay ophiolite sequences of the Sabah ophiolite basement,in order to determine the“where,when and what”of the Paleo-Pacific arc-trench gap in the East Asia continental margin.Our data show that the Sabah basement might be formed at pre-Triassic in age and has a tectonic affinity to the Cathaysia of the South China Block.The Sabah ophiolite basement might form between the Early-Middle Jurassic and Late Cretaceous period(~185–85 Ma),the Telupid,Kudat and Darvel Bay ophiolite sequences of which mainly generate at~185–140,~135–112,and~135–85 Ma,respectively.The mafic rocks from the Telupid,Kudat and Darvel Bay ophiolite sequences coexisted with the radiolarian cherts and deep-sea turbidities,and are geochemically classified as the MORB-like,high-Nb and Nb-enriched mafic rocks with similar Sr-Nd-Pb isotopic compositions.Their 87Sr/86Sr(i)andεNd(t)range from 0.70323 to 0.70460 and+7.3 to+10.2,respectively.(206Pb/204Pb)i=17.91–18.43,(207Pb/204Pb)i=15.43–15.55,and(208Pb/204Pb)i=37.61–38.44.Their generation might be related to input of slab-derived melt in the plagioclase-rich MORB mantle wedge source.The Segama andesite and tonalite,which have previously been mapped as the Sabah non-ophiolite basement or“Segama Valley Felsic Intrusive Rocks”,were dated at 251–179 Ma via zircon U-Pb dating.They are geochemically classified as adakites with depletion in Nb-Ta,87Sr/86Sr(i)of 0.70287–0.70297,εNd(t)of+7.2–+9.8,and the Pacific MORB-like Pb isotopic compositions,indicating their origin of the mantle wedge source modified by slab-derived fluids in response to the Triassic Paleo-Pacific subduction.Such rock-associations and geochemical signatures suggest the generation of the mafic rocks in the fore-arc setting of the Paleo-Pacific trench-arc system.Our work firstly identified the Earliest Triassic-Early Jurassic adakite,Jurassic-Cretaceous MORB-like,arc-like,high-Nb and Nb-enriched mafic rocks,and revealed the magmatic flare-up of~251–203,~185–153,~135–112 and~95–85 Ma in Sabah being compatible with those in Coastal South China and West Borneo.It is proposed that Sabah in NE Borneo was tectonically located at the southeastern most margin of the East Cathaysia of South China at the Mesozoic period.The Segama igneous rocks and Sabah ophiolite basement collectively evidenced the development of the Sabah long-lived supra-subduction zone,and formed in the fore-arc setting of the East Asian Andeantype active continental margin.The Paleo-Pacific subduction was initiated no later than the Earliest Triassic(~251 Ma)and continued till the Late Cretaceous(~90 Ma)period along the East Asian active continental margin.Such long-lived(>150 Ma)Andean-type Paleo-Pacific subduction is characterized by“multi-staged pulsing slab subduction and retraction”.The arc-trench gap might extend from East Taiwan(China)into Sarawak Kuching zone,even the Palangkaraya area of West Borneo,across Sabah(Telupid,Kudat and Lahad Datu)and Palawan.Such an accretionary orogenesis ended in the Latest Cretaceous,and then evolved into the Cenozoic western Pacific marginal sea.