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“向地球深部进军”是解决地质重大战略科技问题的突破口。为探索地球内部的结构和物质组成,20世纪50年代末60年代初,科学家就提出了“莫霍钻” 的宏伟计划,但是由于难度大,钻探成本高,迄今为止“莫霍钻”梦想尚未实现。进入21世纪后,科学家关于深部地壳和“莫霍钻”的梦想重新被点燃。2016年实施的国际大洋发现计划360航次在西南印度洋中脊亚特兰斯滩完成了莫霍面钻探的第一阶段任务,钻探至海底以下789.7m的辉长岩(Li Jiantao et al.,2020)。通过对综合大洋钻探计划(Integrated Ocean Drilling Program, IODP)大洋钻探建议书情况进行统计分析,发现近年来向IODP提交的深部地壳和莫霍面钻探建议书明显增加,建议位置包括超慢速扩张背景下的西南印度洋,慢速扩张背景下的大西洋洋中脊大洋核杂岩,快速扩张背景下的东太平洋夏威夷海域、南加利福尼亚海域和科科斯板块洋壳,弧后扩张背景下的哥斯拉裂谷大洋核杂岩,以及洋内俯冲背景下的西北太平洋博宁海沟陆坡洋壳,位置分布见图1。然而,其他构造背景下的深部地壳和莫霍面钻探建议仍为空白。
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图1 全球海域现代洋壳上发现的大洋斜长花岗岩空间分布(统计文献来源见表1)
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Fig.1 Spatial distribution of oceanic plagiogranite sites discovered in modern oceanic crust by Remote Operated Vehicle (ROV), dredge and drilling (data are from Table1)
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南海是西太平洋最大的边缘海,IODP349、367/368/368X大洋钻探表明,其岩石圈破裂机制不同于开阔大洋,为“板缘张裂”盆地(Wang Pinxian et al.,2019;林间等, 2020)。据最新研究,南海海底扩张形成的洋壳年龄约为33~15Ma(Li Chunfeng et al., 2014)。作为西太平洋边缘研究程度最高的边缘海,在南海开展深部地壳和莫霍面钻探不仅可揭示边缘海扩张这一构造背景下的地壳结构和莫霍面的地质属性,还可揭示地球深部的物质与能量循环、探索地球深部的生命迹象,实现人类“莫霍钻”梦想。Zhong Lifeng et al.(2018)报道了南海东部次海盆管事平顶海山拖网获得的大洋斜长花岗岩,通过同位素年代学、地球化学研究,提出了大洋斜长花岗岩成因模式。本文拟综合分析全球海域现代洋壳上发现的大洋斜长花岗岩案例,总结其分布规律、岩石组合特点和成因。通过对比分析,探讨南海管事平顶海山大洋斜长花岗岩成为南海深部地壳和莫霍面钻探潜在优选区的可能性,并提出管事平顶海山的关键地质科学问题和下一步调查研究建议。
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1 全球洋壳大洋斜长花岗岩研究进展
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大洋斜长花岗岩最早用于描述蛇绿岩中的浅色岩(Coleman et al.,1975),我国目前已知的122处蛇绿岩(包括疑似蛇绿岩)中有32处蛇绿岩中发现有花岗质岩石,包括闪长岩、斜长花岗岩和钠(奥)长花岗岩(张旗等, 2001)。广义上,它是镁铁质-超镁铁质岩中出现的长英质岩石的统称,包括闪长岩、石英闪长岩、英云闪长岩以及奥长花岗岩(Coleman et al.,1975)。这类岩石主要由石英、斜长石(钠-更长石)和少量暗色矿物(含量＜10%)组成,基本不含或含少量(＜10%)碱性长石,以高SiO2、Na2O、CaO和低K2O(≤0.8%)为特征,富含副矿物,如锆石、磷灰石、榍石等,主要产于蛇绿岩套中(Koepke et al.,2007; Grimes et al.,2013; Furnes et al.,2017)。随着深海拖网、有缆遥控水下机器人(Remote Operated Vehicle, ROV)和大洋钻探的实施,在洋壳中也逐渐发现有类似的浅色岩, 证实了大洋中存在大洋斜长花岗岩(Aumento,1969; Coleman et al.,1975; Aldiss, 1981; Dick et al.,2002)。
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1.1 大洋斜长花岗岩分布
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大洋斜长花岗岩主要分布在洋壳和蛇绿岩套中(Coleman et al.,1975; Aldiss, 1981; Koepke et al.,2007; Grimes et al.,2013; Furnes et al.,2017; Li Wei et al.,2021)。洋壳上的大洋斜长花岗岩稀少且罕见,本文对全球现代洋壳上发现的大洋斜长花岗岩共16个案例进行了统计分析,空间分布见图1,统计情况见表2,形成分布规律认识如下:
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从地理分布上看,大洋斜长花岗岩分布广泛,在北大西洋、南大西洋、西南印洋度、中印度洋、西北太平洋、东太平洋等海域均有发现,但主要分布在北大西洋和西太平洋。从构造背景上看,大洋斜长花岗岩在多种不同构造背景下形成的洋壳上均有发现,如西南印度洋超慢速扩张构造环境,大西洋、西北太平洋、西印度洋和中印度洋慢速扩张构造环境,东太平洋快速扩张构造环境,南海等边缘海扩张构造环境,伊豆-小笠原-马里亚纳(IBM)岛弧、九州-帕劳海脊、Amami海底高原等洋内俯冲构造环境,但主要分布在大西洋慢速扩张洋中脊、印度洋超慢速和慢速扩张洋中脊和洋内俯冲构造形成的洋内弧上。
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从岩石组合上看,多数大洋斜长花岗岩站位同时发现辉长岩、辉绿岩、橄榄岩等深层洋壳岩石,且大洋钻探发现的大洋斜长花岗岩常呈脉状分布在辉长岩中(Alt et al.,2007; Silantyev et al.,2008; Chen Yanhong et al.,2019)。仅菲律宾海九州-帕劳海脊Komahashi-Daini海山、Amami高原、菲律宾海IBM岛弧北段Daisan-West Sumisu海丘以及南海管事平顶海山拖网只发现大洋斜长花岗岩,未发现其他深层火成岩组合。
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从深度上看,大洋斜长花岗岩可发育于不同深度。例如东太平洋Cocos板块大洋钻探1256站位深度1405~1471m、大西洋中脊断裂带大洋钻探1275站位深度23.5~185.5m、西南印度洋中脊Atlantis浅滩735站位深度506~940m和1473站位深度373~754m均零星发育大洋斜长花岗岩(Alt et al.,2007; Silantyev et al.,2008; Chen Yanhong et al.,2019)。
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1.2 大洋斜长花岗岩成因
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关于大洋斜长花岗岩的岩石学成因尚存在争议,目前主要有四种模式。
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(1)玄武质岩浆结晶分异成因模式。该种模式认为大洋斜长花岗岩是大洋玄武质岩浆结晶分异的后期产物(Aldiss, 1981; Beccaluva et al.,1999; Niu et al.,2002)。Dixon et al.(1983)开展了玄武质岩浆在含水条件下的实验,证实了含水玄武质岩浆在结晶分异后期能够形成大洋斜长花岗岩。
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(2)岩浆不混溶成因模式。该种模式认为大洋斜长花岗岩是岩浆不混溶作用形成的(Philpotts, 1982),Dixon et al.(1979)通过实验证实了这一模式。有学者认为西南印度洋中脊发现的部分长英质岩脉与富氧化物含铁辉长岩存在密切关系,并认为可能是硅酸盐岩浆不混溶形成的(Dick et al.,2000)。
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(3)辉长岩部分熔融成因模式。Koepke et al.(2004)利用大洋钻探获取的辉长岩开展实验,提出了含水辉长岩部分熔融模式。在该模式下,有学者还进一步提出了剪切熔融模式和俯冲洋壳脱水熔融模式。在剪切熔融模式中,剪切带内变质镁铁质洋壳发生部分熔融,形成长英质脉(Pedersen et al.,1984; Shen Xiaoming et al.,2018);在俯冲洋壳脱水熔融模式下,俯冲洋壳在高压环境下通过脱水熔融也能形成大洋斜长花岗岩(Yoshikawa et al.,2007)。
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(4)快速扩张背景下的席状岩墙熔融成因模式。该种模式认为经受过热液蚀变作用的席状岩墙可能会发生部分熔融,形成大洋斜长花岗岩(Koepke et al.,2007)。特鲁多斯蛇绿岩研究支持这一模式(Gillis et al.,2002)。
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TiO2含量是区分大洋斜长花岗岩成因的重要指标。一系列实验表明辉长岩部分熔融成因形成的大洋斜长花岗岩TiO2含量较低,而玄武质岩浆结晶分异和岩浆不混溶成因形成的大洋斜长花岗岩TiO2含量较高。Koepke et al.(2007)通过综合分析大洋斜长花岗岩的TiO2含量,认为辉长岩部分熔融是大洋斜长花岗岩的主要成因模式。辉长岩部分熔融可发生在超慢速扩张、慢速扩张、快速扩张、洋壳俯冲等构造环境(Koepke et al.,2007; Yoshikawa et al.,2007)。慢速扩张洋中脊上的海水容易通过高温剪切带渗入地壳深部,导致辉长岩发生部分熔融成为新岩浆,形成大洋斜长花岗岩(Koepke et al.,2007),模式见图2。
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前三种成因模式下的大洋斜长花岗岩常形成于洋壳辉长岩中,形成深度较大;第四种成因模式下的大洋斜长花岗岩形成于席状岩墙中,形成深度较小(Koepke et al.,2007)。因此,若前三种成因模式形成的大洋斜长花岗岩被拖网、深潜或浅部钻探发现,则指示大洋斜长花岗岩可能早先形成于洋壳深层,后来在断裂等地质作用下被抬升至海底浅部或被剥蚀至海底而被发现。例如,根据Escartín et al.(2011)提出的适用于慢速扩张洋中脊地壳结构的Chapman模型(图3),发育大洋斜长花岗岩的大西洋慢速扩张洋中脊Markov深渊大洋核杂岩(Savel'eva et al.,2006)其原来应为洋壳深层物质,经拆离断层作用被剥离至海底表面,伴生岩石有橄榄岩、辉长岩等深部岩石。
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2 南海管事平顶海山大洋斜长花岗岩研究进展
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南海管事平顶海山位于南海东北海域,靠近马尼拉海沟,目前尚未有正式发布的海底地理实体名称,仅在相关文献中出现(张伙带等, 2017),也有学者称为蓬莱海山(Zhong Lifeng et al.,2018)。根据磁条带研究,Briais et al.(1993)提出南海在海底扩张过程中曾发生洋脊跃迁事件,Li Chunfeng et al.(2014)认为东部次海盆洋脊跃迁时间约为23.6Ma左右。管事平顶海山位于南海古扩张脊附近,构造位置独特(图4)。
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图2 慢速扩张洋中脊辉长岩部分熔融成因模式 (据Koepke et al.,2007修改)
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Fig.2 Model for hydrous partial melting and the genesis of oceanic plagiogranites at slow spreading ridges (modified after Koepke et al.,2007)
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(a)—洋壳发育断裂; (b)—海水沿断裂渗入辉长岩; (c)—辉长岩发生部分熔融
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(a)—Faults develop in oceanic crust; (b)—sea water penetrates into gabbros along faults; (c)—partial melting happens in gabbro
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广州海洋地质调查局与国内学者合作,对2012年利用“海洋四号”科考船在管事平顶海山获取的拖网岩石样品开展了岩石学、地球化学、40Ar/39Ar年代学分析等研究,证实了其为MORB型大洋斜长花岗岩(Zhong Lifeng et al.,2018)。因此管事平顶海山拖网样品成为南海现代洋壳上目前为止发现的唯一大洋斜长花岗岩样品。
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图3 形成大洋核杂岩的Chapman模型 (据Escartín et al.,2011)
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Fig.3 Chapman model for oceanic core complex formation (after Escartín et al.,2011)
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管事平顶海山拖网大洋斜长花岗岩样品的辉石单矿物和全岩40Ar/39Ar年龄分别为32.3±0.5Ma和28.9±1.9Ma(Zhong Lifeng et al.,2018),表明管事平顶海山是南海现代洋壳上利用40Ar/39Ar测年方法发现的年龄最老的海山之一,且比该位置推测的磁条带年龄(Li Chunfeng et al.,2014)要老。
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关于管事平顶海山大洋斜长花岗岩的岩石学成因,Zhong Lifeng et al.(2018)推测其由含水辉长岩部分熔融形成,证据如下:① 大洋斜长花岗岩的稀土元素配分模式和微量元素组成与含水辉长岩部分熔融形成的大洋斜长花岗岩相似;② 大洋斜长花岗岩显示残留辉长结构,部分辉石尚未完成蚀变为角闪石,与辉长岩部分熔融成因的大洋斜长花岗岩相似;③ 大洋斜长花岗岩疑似经历了剪切作用。综合以上因素,Zhong Lifeng et al.(2018)提出了管事平顶海山大洋斜长花岗岩形成演化模式(图5):南海初始扩张时管事平顶海山形成典型的Penrose层状洋壳结构,层3辉长岩在岩浆房边缘具有较强的韧性,在中后期扩张过程中管事平顶海山层2和层3之间发生韧性剪切变形,导致海水进入层3辉长岩,使辉长岩发生部分熔融,形成大洋斜长花岗岩。
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3 南海管事平顶海山调查研究及莫霍面钻探展望
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前人认为管事平顶海山大洋斜长花岗岩为含水辉长岩部分熔融成因模式(Zhong Lifeng et al.,2018)。基于对全球大洋斜长花岗岩分布规律和成因模式的研究,本文推测部分熔融成因模式的管事平顶海山大洋斜长花岗岩意味着其可能形成于洋壳深层,在断裂等后期地质作用下被剥蚀至海底而被拖网发现,成为了解南海洋壳深部岩浆过程和洋壳结构的重要窗口, 可能是南海深部地壳和莫霍面钻探的优选区。
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图4 管事平顶海山位置(据Li Chunfeng et al.,2014修改)
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Fig.4 Location of Guanshi Guyot (modified after Li Chunfeng et al.,2014)
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此外,从全球范围看,海山和海底高原位置发现的大洋斜长花岗岩与其他构造背景发现的大洋斜长花岗岩伴生岩石存在明显的差异 (仅菲律宾海九州-帕劳海脊Komahashi-Daini海山、Amami海底高原、菲律宾海IBM岛弧北段Daisan-West Sumisu海丘以及南海管事平顶海山拖网只发现大洋斜长花岗岩,未发现其他深层火成岩组合)。在管事平顶海山开展工作可以进一步揭示发育斜长花岗岩海山区深部岩石类型、岩浆过程和地球动力学机制的特殊性。
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目前管事平顶海山只有一块拖网大洋斜长花岗岩样品,仍缺乏精确的原位岩石样品和地球物理资料联合约束海山区的洋壳结构、岩浆过程和地球动力学机制。因此,还有以下重要科学问题有待进一步解决:① 管事平顶海山的岩石类型有多少种?② 管事平顶海山地壳和上地幔结构如何?地壳结构是更倾向于Penrose模型还是Chapman模型?③ 管事平顶海山莫霍面埋深如何?④ 管事平顶海山大洋斜长花岗岩何时、如何剥露海底?⑤ 管事平顶海山大洋斜长花岗岩年龄和磁条带年龄不一致的原因?⑥ 管事平顶海山大洋斜长花岗岩与南海洋脊跃迁、马尼拉俯冲等南海形成演化关键过程关系如何?为解决以上科学问题,需要加大对管事平顶海山的调查研究力度,开展电视抓斗取样、ROV观测、浅钻等可精确定位的地质取样,获取海山准确位置的新鲜大洋斜长花岗岩样品,寻找辉长岩等大洋斜长花岗岩容易出现的岩石组合,进一步揭示管事平顶海山的物质组成和大洋斜长花岗岩成因模式。开展重磁、深反射地震、海底地震仪(Ocean Bottom Seismometer, OBS)、大地电磁等地球物理综合调查,建立管事平顶海山地壳和上地幔结构模型,查明断裂分布与性质,揭示莫霍面深度,进一步判断管事平顶海山是否南海深部地壳和莫霍面钻探的优选区。
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图5 管事平顶海山三维地形图(据张伙带等, 2017)和拖网大洋斜长花岗岩样品(据Zhong Lifeng et al.,2018)
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Fig.5 3D topographic map of Guanshi Guyot (after Zhang Huodai et al.,2017) and oceanic plagiogranite sample (after Zhong Lifeng et al.,2018)
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图6 管事平顶海山拖网大洋斜长花岗岩成因模式(据Zhong Lifeng et al.,2018修改)
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Fig.6 Idealized conceptual model of oceanic plagiogranite formation in oceanic lithosphere (modified after Zhong Lifeng et al.,2018)
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莫霍面钻探是一项钻探时间长、钻探技术要求高、钻探难度大、钻探成本高的复杂性、战略性工程计划,选址要求综合考虑科学意义重大、工程技术可行、钻探成本较低等综合因素。因此,在南海开展莫霍面钻探选址,不仅要考虑地壳厚度小,同时还要考虑:莫霍面钻探的具体科学目标是什么?莫霍面钻探的工程技术难度如何?莫霍面钻探的费用如何?综上所述,建议分阶段有序地推进南海莫霍面钻探计划,第一阶段可以先钻进基岩数百米,在不断试验与升级海底硬岩钻探技术装备、积累工程技术试验应用经验的同时,聚焦洋壳增生过程与机制、洋壳长英质岩浆作用过程与地球动力学机制、洋壳生物圈特征及其控制因素、洋壳和海水相互作用、洋壳在全球碳循环中的作用、南海洋脊跃迁等关键科学问题,产出阶段性科学研究成果,后面阶段再逐渐向更深洋壳钻进,最终实现莫霍面钻探目标。
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4 结论
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本文聚焦“钻穿地壳,进入上地幔”这一大洋钻探主题,通过系统总结全球海域现代洋壳上发现的大洋斜长花岗岩的分布规律、岩石组合特征和成因模式,对比分析南海管事平顶海山大洋斜长花岗岩特征,形成以下认识:
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(1)通过统计分析全球海域现代洋壳上通过拖网、ROV、大洋钻探等手段发现的大洋斜长花岗岩案例共16个,总结了大洋斜长花岗岩的形成分布规律。大洋斜长花岗岩在多种不同构造背景形成的洋壳上均有发现,但主要分布在超慢速扩张、慢速扩张和洋内俯冲构造环境;多数大洋斜长花岗岩站位发现辉长岩等深部地壳岩石组合,呈大洋斜长花岗岩主要呈脉状零散分布于辉长岩中;海山和海底高原位置发现的大洋斜长花岗岩与其他构造背景发现的大洋斜长花岗岩伴生岩石存在明显的差异。
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(2)大洋斜长花岗岩的岩石学成因尚存在争议,目前共有四种成因模式,包括玄武质岩浆结晶分异成因模式、岩浆不混溶成因模式、辉长岩部分熔融成因模式和席状岩墙熔融成因模式。前三种成因模式均指示大洋斜长花岗岩形成深度可能较大,而第四种成因模式指示大洋斜长花岗岩形成深度可能较小。
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(3)南海管事平顶海山拖网发现MORB型大洋斜长花岗岩,前人根据地球化学研究推测为辉长岩部分熔融成因模式。基于对全球大洋斜长花岗岩分布规律和成因模式的研究,本文推测管事平顶海山大洋斜长花岗岩很可能为深部地壳物质剥露海底,有望成为南海深部地壳和莫霍面钻探的潜在优选区,但需要开展进一步的调查研究验证推测。
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致谢:衷心感谢编辑和审稿专家的细心审阅和宝贵性的意见与建议。
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参考文献
-
Aldiss D T. 1981. Plagiogranites from the ocean crust and ophiolites. Nature, 289: 577~578.
-
Alt J C, Teagle D A, Umino S, Miyashita S, Banerjee N R, Wilson D S. 2007. IODP Expeditions 309 and 312 drill an intact section of upper oceanic basement into gabbros. Scientific Drilling, 4: 4~10.
-
Aumento F. 1969. Diorites from the mid-Atlantic ridge at 45°N. Science, 165(3898): 1112~1113.
-
Beccaluva L, Chinchilla-Chaves A L, Coltorti M, Giunta G, Sienna F, Vaccaro C. 1999. Petrological and structural significance of the Santa Elena-Nicoya ophiolitic complex in Costa Rica and geodynamic implications. European Journal of Mineralogy, 11(6): 1091~1107.
-
Briais A, Patriat P, Tapponnier P. 1993. Updated interpretation of magnetic anomalies and seafloor spreading stages in the South China Sea: implications for the Tertiary tectonics of Southeast Asia. Journal of Geophysical Research: Solid Earth, 98(B4): 6299~6328.
-
Brunelli D, Sanfilippo A, Bonatti E, Skolotnev S, Escartin J, Ligi M, Ballabio G, Cipriani A. 2020. Origin of oceanic ferrodiorites by injection of nelsonitic melts in gabbros at the Vema Lithospheric Section, Mid Atlantic Ridge. Lithos, 368: 105589.
-
Chen Yanhong, Niu Yaoling, Wang Xiaohong, Gong Hongmei, Guo Pengyuan, Gao Yajie, Shen Fangyu. 2019. Petrogenesis of ODP hole 735B (Leg 176) oceanic plagiogranite: partial melting of gabbros or advanced extent of fractional crystallization? Geochemistry, Geophysics, Geosystems, 20: 2717~2732.
-
Coleman R, Peterman Z. 1975. Oceanic plagiogranite. Journal of Geophysical Research, 80(8): 1099~1108.
-
Dick H J B, Natland J H, Alt J C, Bach W, Bideau D, Gee J S, Haggas S, Hertogen J G, Hirth G, Holm P M. 2000. A long in situ section of the lower ocean crust: results of ODP Leg 176 drilling at the Southwest Indian Ridge. Earth and Planetary Science Letters, 179(1): 31~51.
-
Dick H J B, Ozawa K, Meyer P S, Niu Y, Robinson P T, Constantin M, Hebert R, Maeda J, Natland J H, Hirth G, Mackie S. 2002. Primary silicate mineral chemistry of a 1. 5 km section of very slow spreading lower ocean crust: ODP hole 735B, Southwest Indian Ridge. In: Natland J H, Dick H J B, Miler D J, Von Herzen R, eds. Proceedings of the Ocean Drilling Program, Scientific Results, 1~60.
-
Dick H J B, Kvassnes A J S, Robinson P T, Macleod C J, Kinoshita H. 2019. The Atlantis Bank Gabbro Massif, southwest Indian Ridge. Progress in Earth and Planetary Science, 6(1): 1~70.
-
Dixon S, Rutherford M J. 1979. Plagiogranites as late-stage immiscible liquids in ophiolite and mid-ocean ridge suites: an experimental study. Earth and Planetary Science Letters, 45(1): 45~60.
-
Dixon S, Rutherford M J. 1983. The origin of rhyolite andplagiogranite in oceanic crust: an experimental study. Journal of Petrology, 24(1): 1~25.
-
Engel C G, Fisher R L. 1975. Granitic to ultramafic rock complexes of the Indian Ocean ridge system, western Indian Ocean. Geological Society of America Bulletin, 86(11): 1553~1578.
-
Escartín J, Canales J. 2011. Detachments inoceanic lithosphere: deformation, magmatism, fluid flow, and ecosystems. Eos, Transactions American Geophysical Union, 92(4): 31.
-
Furnes H, Dilek Y. 2017. Geochemical characterization and petrogenesis of intermediate to silicic rocks in ophiolites: a global synthesis. Earth-Science Reviews, 166: 1~37.
-
Gillis K M, Coogan L A. 2002. Anatectic migmatites from the roof of an ocean ridge magma chamber. Journal of Petrology, 43(11): 2075~2095.
-
Grimes C B, Ushikubo T, Kozdon R, Valley J W. 2013. Perspectives on the origin of plagiogranite in ophiolites from oxygen isotopes in zircon. Lithos, 179: 48~66.
-
Haraguchi S, Ishii T, Kimura J I, Ohara Y. 2003. Formation of tonalite from basaltic magma at the Komahashi-Daini Seamount, northern Kyushu-Palau Ridge in the Philippine Sea, and growth of Izu-Ogasawara (Bonin)-Mariana arc crust. Contributions to Mineralogy and Petrology, 145(2): 151~168.
-
Hickey-Vargas R. 2005. Basalt and tonalite from the Amami Plateau, northern West Philippine basin: new Early Cretaceous ages and geochemical results, and their petrologic and tectonic implications. Island Arc, 14(4): 653~665.
-
Johnson J A, Hickey-Vargas R, Fryer P, Salters V, Reagan M K. 2014. Geochemical and isotopic study of a plutonic suite and related early volcanic sequences in the southern Mariana forearc. Geochemistry, Geophysics, Geosystems, 15(3): 589~604.
-
Koepke J R, Feig S T, Snow J, Freise M. 2004. Petrogenesis of oceanic plagiogranitesby partial melting of gabbros: an experimental study. Contributions to Mineralogy and Petrology, 146(4): 414~432.
-
Koepke J R, Berndt J, Feig S T, Holtz F. 2007. The formation of SiO2-rich melts within the deep oceanic crust by hydrous partial melting of gabbros. Contributions to Mineralogy and Petrology, 153(1): 67~84.
-
Li Chunfeng, Xu Xing, Lin Jian, Sun Zhen, Zhu Jian, Yao Yongjian, Zhao Xixi, Liu Qingsong, Kulhanek D K, Wang Jian, Song Taoran, Zhao Junfeng, Qiu Ning, Guan Yongxian, Zhou Zhiyuan, Williams T, Bao Rui, Briais A, Brown E A, Chen Yifeng, Clift P D, Colwell F S, Dadd K A, Ding Weiwei, Almeida I H, Huang Xiaolong, Hyun S, Jiang Tao, Koppers A A P, Li Qianyu, Liu Chuanlian, Liu Zhifei, Nagai R H, Peleo-Alampay A, Su Xin, Tejada M L G, Trinh H S, Yeh Y C, Zhang Chuanlun, Zhang Fan, Zhang Guoliang. 2014. Ages and magnetic structures of the South China Sea constrained by deep tow magnetic surveys and IODP Expedition 349. Geochemistry, Geophysics, Geosystems, 15(12): 4958~4983.
-
Li Jiantao, Mara P, Schubotz F, Sylvan J B, Burgaud G, Klein F, Beaudoin D, Wee S Y, Dick H J, Lott S. 2020. Recycling and metabolic flexibility dictate life in the lower oceanic crust. Nature, 579: 250~255.
-
Li Wei, Liu Nina, Nayak R, Ma Yaoliang, Wang Jinjun, Hu Xichong, Pang Jiehui, Huang Weile, Zhong Yun, Liu Weiliang. 2021. An island arc origin of Jurassic plagiogranite in the Shiquanhe ophiolite, western Bangong Suture, Tibet: zicon U-Pb chronology, geochemistry, and tectonic implications of Bangong Meso-Tethys. Geological Survey, 56(8): 3941~3958.
-
Lin Jian, Sun Zhen, Li Jiabiao, Zhou Zhiyuan, Zhang Fan, Luo Yiming. 2020. South China Sea basin opening: lithospheric rifting and interaction with surrounding subduction zones. Science & Technology Reviews, 38(18): 35~39 (in Chinese with English abstract).
-
Nakamura K, Morishita T, Chang Q, Neo N, Kumagai H. 2007. Discovery of lanthanide tetrad effect in an oceanic plagiogranite from anocean core complex at the Central Indian Ridge 25°S. Geochemical Journal, 41(2): 135~140.
-
Niu Y, Regelous M, Wendt I J, Batiza R, O'hara M J. 2002. Geochemistry of near-EPR seamounts: importance of source vs. process and the origin of enriched mantle component. Earth and Planetary Science Letters, 199(3-4): 327~345.
-
Pedersen R B, Malpas J. 1984. The origin of oceanic plagiogranites from the Karmoy ophiolite, western Norway. Contributions to Mineralogy and Petrology, 88(1): 36~52. Philpotts A. 1982. Compositions of immiscible liquids in volcanic rocks. Contributions to Mineralogy and Petrology, 80(3): 201~218.
-
Savel'eva G N, Bortnikov N S, Peyve A A, Skolotnev S G. 2006. Ultramafic rocks from the Markov Deep in the rift valley of the Mid-Atlantic Ridge. Geochemistry International, 44(11): 1105~1120.
-
Shen Xiaoming, Zhang Haixiang, Wang Qiang, Saha A, Ma Lin. 2018. Zircon U-Pb geochronology and geochemistry of devonian plagiogranites in the Kuerti area of southern Chinese Altay, northwest China: petrogenesis and tectonic evolution of Late Paleozoic ophiolites. Geological Journal, 53: 1886~1905.
-
Silantyev S A, Kostitsyn Y A, Cherkashin D V, Dick H J B, Kelemen P B, Kononkova N N, Kornienko E M. 2008. Magmatic and metamorphic evolution of the oceanic crust in the western flank of the MAR crest zone at 15°44′N: investigation of cores from sites 1275B and 1275D, JOIDES Resolution Leg 209. Petrology, 16(4): 353~375.
-
Silantyev S A, Koepke J, Ariskin A A, Anosova M O, Krasnova E A, Dubinina E O, Suhr G. 2014a. Geochemical nature and age of the plagiogranite-gabbronorite association of the oceanic core complex of the Mid-Atlantic ridge at 5°10′S. Petrology, 22(2): 109~127.
-
Silantyev S A, Portnyagin M, Krasnova E G, Hauff F, Werner R F, Kuzmin D V. 2014b. Petrology and geochemistry of plutonic rocks in the Northwest Pacific Ocean and their geodynamic interpretation. Geochemistry International, 52: 179~196.
-
Tani K, Dunkley D J, Chang Q, Nichols A R, Shukuno H, Hirahara Y, Ishizuka O, Arima M, Tatsumi Y. 2015. Pliocene granodioritic knoll with continental crust affinities discovered in the intra-oceanic Izu-Bonin-Mariana Arc: syntectonic granitic crust formation during back-arc rifting. Earth and Planetary Science Letters, 424: 84~94.
-
Wang Pinxian, Huang Chiyue, Lin Jian, Jian Zhimin, Sun Zhen, Zhao Minghui. 2019. The South China Sea is not a mini-Atlantic: plate-edge rifting vs. intra-plate rifting. National Science Review, 6(5): 902~913.
-
Yoshikawa M, Ozawa K. 2007. Rb-Sr and Sm-Nd isotopic systematics of the Hayachine-Miyamori ophiolitic complex: melt generation process in the mantle wedge beneath an Ordovician island arc. Gondwana Research, 11(1-2): 234~246.
-
Zhang Huodai, Zhu Benduo, Guang Yongxian, Yang Shengxiong. 2017. Topographic features of the seamounts in the central basin of the South China Sea: based on multi-beam bathymetric data. Marine Geology & Quaternary Geology, 37(6): 149~157 (in Chinese with English abstract).
-
Zhang Qi, Zhou Guoqing. 2001. Ophiolites of China. Beijing: Science Press (in Chinese).
-
Zhong Lifeng, Cai Guanqiang, Koppers A A P, Xu Yigang, Xu Hehua, Gao Hongfang, Xia Bin. 2018. 40Ar/39Ar dating of oceanic plagiogranite: constraints on the initiation of seafloor spreading in the South China Sea. Lithos, 302-303: 421~426.
-
林间, 孙珍, 李家彪, 周志远, 张帆, 罗怡鸣. 2020. 南海成因: 岩石圈破裂与俯冲带相互作用新认识. 科技导报, 38(18): 35~39.
-
张伙带, 朱本铎, 关永贤, 杨胜雄. 2017. 基于多波束数据的南海海盆洋壳区海山地形特征. 海洋地质与第四纪地质, 37(6): 149~157.
-
张旗, 周国庆. 2001. 中国蛇绿岩. 北京: 科学出版社.
-
摘要
本文紧密围绕IODP“面向2050年大洋钻探科学框架”中的“地球深部探测”旗舰计划和“莫霍面”梦想,研究并总结全球现代洋壳发现的大洋斜长花岗岩的分布规律、岩石组合特征和成因模式,探讨发育大洋斜长花岗岩的南海管事平顶海山是否为深部地壳和莫霍面钻探潜在优选区。统计结果表明大洋斜长花岗岩在多种不同构造背景形成的洋壳上均有发现,包括西南印度洋超慢速扩张构造环境,大西洋、西北太平洋、西印度洋和中印度洋慢速扩张构造环境,东太平洋快速扩张构造环境,南海等边缘海构造环境,伊豆-小笠原-马里亚纳(IBM)岛弧、九州-帕劳海脊、Amami海底高原等洋内俯冲构造环境。多数大洋斜长花岗岩呈脉状零散分布于辉长岩中,意味大洋斜长花岗岩可能形成于洋壳深部,在后期断裂等地质作用下被剥蚀而更容易被发现,成为了解洋壳深部岩浆过程和洋壳结构的一个窗口。管事平顶海山位于南海东部次海盆古扩张脊附近,拖网获得MORB型大洋斜长花岗岩,前人基于地球化学数据认为其可能为辉长岩部分熔融形成。本文推测管事平顶海山大洋斜长花岗岩很可能为洋壳深部物质剥露海底,是南海的一个重要构造窗口,有望成为南海深部地壳和莫霍面钻探的潜在优选区,但需要开展进一步调查研究以验证推测:① 海山大洋斜长花岗岩为拖网所得,位置误差较大,需开展可精确定位的电视抓斗、浅钻或有缆遥控水下机器人(ROV)调查;② 海山目前只发现大洋斜长花岗岩,需调查海山是否发育辉长岩等深部地壳岩石组合;③ 需开展重磁、深反射地震、海底地震仪(OBS)、大地电磁等调查研究,建立管事平顶海山洋壳和上地幔结构模型,查明断裂带分布,揭示莫霍面深度。
Abstract
This paper closely focuses on the flagship project “deep earth exploration and the dream of Moho discontinuity in the 2050 Science Framework: Exploring Earth by Scientific Ocean Drilling” released by IODP. The discussion focuses on the possibility that the Guanshi Guyot of the South China Sea where oceanic plagiogranite was found, is a potential preferred area for deep crust and Moho discontinuity drilling, by studying the distribution pattern, associated intrusive rocks and genetic model of oceanic plagiogranites obtained around the global modern oceanic crust. The results show that oceanic plagiogranites were discovered under different tectonic backgrounds, including the ultraslow spreading background of southwest Indian Ocean, the slow spreading background of the Atlantic Ocean, the northwest Pacific Ocean, the middle Indian Ocean and the Indian Ocean, as well as the rapid-spreading background of the eastern Pacific Ocean, the marginal sea seafloor-spreading background such as the South China Sea, the intra-oceanic subduction background forming intra-oceanic arcs such as the Izu-Bonin-Mariana arc (IBM), the Kyushu-Palau ridge, and the Amami submarine plateau. Most oceanic plagiogranites are scattered in gabbro rocks as veins, which means that the oceanic plagiogranites were probably initially formed in the deep oceanic crust, but later were exhumed to the seabed surface or sub-surface due to faults and other geological process, making them easily discoverable and providing a window for studying the magma process and structure of the oceanic crust. The Guanshi Guyot is located near the fossil spreading ridge of the eastern sub-basin in the South China Sea, and a rock sample of MORB-type oceanic plagiogranite was obtained by dredging. Geochemical data of the rock samples indicates that it may have been formed by partial melting of gabbro. Therefore, the oceanic plagiogranite of the Guanshi Guyot probably came from the exhumation of deep oceanic crustal rock and is expected to be an important tectonic window and a potential preferred area for deep crustal and Moho drilling in the South China Sea. In order to verify this speculation, further work is suggested as follows: (1) As the previous oceanic plagiogranite was obtained by dredging, it has a large positioning error. Therefore, it is necessary to use an accurate positioning survey means, such as TV grab, shallow drilling or Remote Operation Vehicle (ROV) to obtain new rocks on the Guanshi Guyot. (2) At present, only oceanic plagiogranites were found on the Guanshi Guyot. It is necessary to investigate whether other deep oceanic crustal rocks such as gabbros are developed on the Guanshi Guyot. (3) It is necessary to conduct supplementary survey on gravity, magnetic, deep reflection seismic, Ocean Bottom Seismometer (OBS), magnetotelluric to reveal the crust and upper mantle structure characteristics, the faults distribution and the depth of the Moho discontinuity from a geophysical perspective.
Keywords
South China Sea ； Guanshi Guyot ； oceanic plagiogranite ； deep crust ； Moho discontinuity
