地质力学学报  2021, Vol. 27 Issue (5): 821-834
引用本文
郑光高, 刘晓春, 赵越, 裴军令. 西南极岩浆作用及构造演化[J]. 地质力学学报, 2021, 27(5): 821-834.
ZHENG Guanggao, LIU Xiaochun, ZHAO Yue, PEI Junling. Magmatism and tectonic evolution of West Antarctica[J]. Journal of Geomechanics, 2021, 27(5): 821-834.
西南极岩浆作用及构造演化
郑光高1,2,3, 刘晓春1,2,3, 赵越1,2,3, 裴军令1,2,3    
1. 中国地质科学院地质力学研究所, 北京 100081;
2. 中国地质调查局极地地学研究中心, 北京 100081;
3. 自然资源部古地磁与古构造重建重点实验室, 北京 100081
摘要:西南极主要由哈格冰原岛峰群、南极半岛、瑟斯顿岛、玛丽·伯德地和埃尔斯沃思-惠特莫尔山脉五个各具特色的地壳块体组成。通过综述上述各块体主要的岩浆事件及其构造意义,旨在了解西南极的地质演化过程。西南极最古老的岩石为哈格冰原岛峰群地块的前寒武纪正片麻岩,时代为1238 Ma,记录了中元古代弧岩浆作用,其余四个地块记录了~500 Ma以来的地质演化过程。古生代时期,埃尔斯沃思-惠特莫尔山脉地块处于快速沉降的陆相断陷盆地环境,岩浆活动稀少,与罗斯造山运动形成的弧后伸展有关;玛丽·伯德地地块中—晚古生代发育一套与板块汇聚有关的岩浆作用,形成于活动大陆边缘环境;而南极半岛-瑟斯顿岛地块记录了石炭纪—二叠纪时期弧的发育。各地块的构造背景从侏罗纪开始明显分化,埃尔斯沃思-惠特莫尔山脉地块记录了侏罗纪板内岩浆作用,可能与大火成岩省有关;玛丽·伯德地地块发育的侏罗纪—早白垩世Ⅰ型弧岩浆岩随时间转变为白垩纪中期的A型碱性岩浆岩,经历了由俯冲向裂解机制的转变;南极半岛-瑟斯顿岛地块侏罗纪—白垩纪为弧岩浆活动活跃期,同时也有大火成岩省火山活动的记录,是持续俯冲和裂解相互作用的产物。新生代岩浆作用以南极半岛地块为代表,弧岩浆作用持续到始新世,其时空分布特征与左行错断扩张脊的分段俯冲和碰撞有关。
关键词西南极    岩浆作用    俯冲    陆内伸展    构造演化    
DOI10.12090/j.issn.1006-6616.2021.27.05.067     文章编号:1006-6616(2021)05-0821-14
Magmatism and tectonic evolution of West Antarctica
ZHENG Guanggao1,2,3, LIU Xiaochun1,2,3, ZHAO Yue1,2,3, PEI Junling1,2,3    
1. Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing 100081, China;
2. Research Center of Polar Geosciences, China Geological Survey, Beijing 100081, China;
3. Key Laboratory of Paleomagnetism and Tectonic Reconstruction of Ministry of Natural Resources, Beijing 100081, China
Abstract: West Antarctica is mainly composed of five distinct micro-continental blocks, namely Haag Nunataks, Antarctic Peninsula, Thurston Island, Marie Byrd Land and Ellsworth-Whitmore Mountains. In order to understand the geological evolution of West Antarctica, this paper presents a brief overview of the main magmatic events of the five blocks and their tectonic significance. The oldest rock is the Precambrian orthogneiss from Haag Nunataks with zircon U-Pb age of~1238 Ma, indicating the development of Mesoproterozoic arc magmatism in West Antarctica. The other four blocks preserve the geological records since~500 Ma. During the Paleozoic, the Ellsworth-Whitmore Mountains block was formed in a rapidly subsiding continental rift basin environment which was related to the back-arc extension caused by the Ross Orogeny, and the magmatic activity was rare. A set of convergence-related magmatism occurred in the middle to late Paleozoic in Mary Byrd Land block, which was formed in an active continental margin environment. The Antarctic Peninsula-Thurston Island blocks record the development of the Carboniferous-Permian arc during this time. During the Mesozoic, the tectonic setting of these blocks began to differentiate since the Jurassic. The Ellsworth-Whitmore Mountains block records Jurassic intra-plate magmatism, which may be associated with large igneous province. In Marie Byrd Land, the lithology changed from Ⅰ-type arc magmatic rocks to A-type alkaline magmatic rocks in the Jurassic-Early Cretaceous to the mid-Cretaceous period. This reflects a major change in tectonic setting from subduction to rifting during the mid-Cretaceous. The Jurassic-Cretaceous flare-up in arc magmatism record on the Antarctic Peninsula-Thurston Island blocks with a pulse of Jurassic large igneous provinces. These are the product of the interaction of continuous subduction and rifting. The Cenozoic magmatism was represented by the Antarctic Peninsula block with arc magmatism continuing until the Eocene. The temporal and spatial distribution of the arc magmatism was related to the subduction and collision of spreading ridge which was cut into several segments by sinistral transform faults.
Key words: West Antarctica    magmatism    subduction    intracontinental extension    tectonic evolution    
0 引言

南极大陆是地球上最古老的大陆块体之一,经历并保存了地球演化中一些重大的地质事件。南极大陆可划分为东南极地盾、西南极活动带以及夹于其间的横贯南极山脉(Transantarctic Mountains)三个构造单元(Elliot, 1975; 陈廷愚等, 2008)。其中,西南极活动带东靠横贯南极山脉,西邻太平洋,由哈格冰原岛峰群(Haag Nunataks)、南极半岛(Antarctic Peninsula)、瑟斯顿岛(Thurston Island)、玛丽·伯德地(Marie Byrd Land)和埃尔斯沃思-惠特莫尔山脉(Ellsworth-Whitmore Mountains)五个各具特色的地壳块体组成(Dalziel and Elliot, 1982; Storey et al., 1988a)。与东南极地盾相比,西南极活动带相对年轻、构造不稳定,除哈格冰原岛峰群地块有前寒武纪基岩出露外,其他四个地块是自~500 Ma以来,沿着与冈瓦纳古太平洋边缘的洋-陆汇聚带形成的。其中,南极半岛、瑟斯顿岛和玛丽·伯德地三个地块构成了冈瓦纳大陆中生代太平洋边缘的一部分,而埃尔斯沃思-惠特莫尔山脉地块包含了古生代晚期—中生代早期冈瓦纳褶皱带的一部分(Storey et al., 1988a)。因此,在冈瓦纳古陆聚合、裂解和离散过程中,西南极的主体地块不同程度上记录了冈瓦纳古陆演化的重要信息。文章简要总结和评述了西南极主要的岩浆作用事件及其所反映的构造环境,并探讨了西南极的地质演化过程。

1 前寒武纪岩浆作用

西南极最古老的岩石出露在埃尔斯沃思-惠特莫尔山脉地块和南极半岛地块之间的哈格冰原岛峰群地块(图 1图 2Storey et al., 1994; Wareham et al., 1998),面积约2 km2,岩性为强变形和片理化的花岗闪长质-闪长质片麻岩,被细晶岩-伟晶岩和微粒花岗岩(microgranite)岩席侵入(Riley et al., 2020)。花岗闪长质片麻岩中的主要构造组构呈南北走向,在剪切带和褶皱重叠部位有明显的二次变形,致使区域平面组构发生变形(Storey and Dalziel, 1987; Riley et al., 2020)。由Rb-Sr全岩等时线方法获得花岗闪长质片麻岩的原岩结晶年龄为1176±76 Ma,细晶岩-伟晶岩和微粒花岗岩的年龄稍微年轻,分别为1003±18 Ma和1058±53 Ma(Millar and Pankhurst, 1987)。

AP—南极半岛地块;EWM—埃尔斯沃思-惠特莫尔山脉地块;HN—哈格冰原岛峰群地块;MBL—玛丽·伯德地地块;TI—瑟斯顿岛地块 AP-Antarctic Peninsula; EWM-Ellsworth-Whitmore Mountains; HN-Haag Nunataks; MBL-Marie Byrd Land; TI-Thurston Island 图 1 南极洲大地构造单元划分(据Elliot, 1975修改) Fig. 1 Division of the geotectonic units of Antarctica (modified after Elliot, 1975)

图 2 哈格冰原岛峰群及埃尔斯沃思-惠特莫尔山脉地块位置图(据Flowerdew et al., 2007a修改) Fig. 2 Location map for Haag Nunataks and the Ellsworth-Whitmore Mountains block (modified after Flowerdew et al., 2007a)

后期经U-Pb锆石年代学方法验证,该花岗闪长质片麻岩、细晶岩和微粒花岗岩年龄分别为1238±4 Ma、1064±4 Ma和1056±8 Ma(Riley et al., 2020),进一步确证了在西南极存在元古代岩体,重新修订了哈格冰原岛峰群地块片麻岩被两期岩浆侵入的先后次序。同位素组分分析显示,这些岩石具有极低的初始87Sr/86Sr比值(0.70293~0.70360;Millar and Pankhurst, 1987),以及正的εNd(t)值(+3.0~+5.2;Storey et al., 1994)和εHf(t)值(+1.6~+9.2;Flowerdew et al., 2007a),表明这些岩石起源于新生地壳的部分熔融,未受到明显的地壳物质混染,可能形成于一个中元古代的岩浆弧环境(Millar and Pankhurst, 1987; Jordan et al., 2020)。哈格冰原岛峰群地块的年代学和岩石特征与邻近年轻的南极半岛地块不同,但与南非的纳塔尔湾(Natal Embayment)和东南极基底的年龄和岩石学特征相似(Jordan et al., 2020)。因此,哈格冰原岛峰群地块可能是在约1200 Ma的原卡拉哈里克拉通(proto-Kalahari Craton)增生边缘发育形成的(Jacobs et al., 2008; Jordan et al., 2020)。

2 古生代岩浆作用 2.1 埃尔斯沃思-惠特莫尔山脉地块古生代沉积作用

与哈格冰原岛峰群地块毗邻的是埃尔斯沃思-惠特莫尔山脉地块(图 1图 2),出露有南极洲的最高峰文森山(Mount Vinson,海拔4892 m)。埃尔斯沃思-惠特莫尔山脉地块是在弧后背景下,形成于与伸展作用有关的快速沉降的陆相断陷盆地环境(Curtis, 2001),岩浆活动较少,基底未暴露地表。然而,碎屑锆石Hf同位素特征表明(Flowerdew et al., 2007a),埃尔斯沃思-惠特莫尔山脉地块存在元古代基底,且与哈格冰原岛峰群地块片麻岩时代相似(Jordan et al., 2020)。

埃尔斯沃思-惠特莫尔山脉地块出露最老的岩石单元为寒武纪的赫里蒂奇群(Heritage Group),沉积物组合厚约7.5 km,主要岩性为碎屑沉积岩、玄武质火山碎屑岩和含化石的碳酸盐岩,形成于532~505 Ma期间(Curtis, 2001; Craddock et al., 2017a; Jordan et al., 2020)。对赫里蒂奇群锆石年龄数据分析表明,这些沉积物来自于东南极或澳大利亚地区(Castillo et al., 2017),但也有学者提出是劳伦(Laurentian)和南非沉积物的混合物源(Craddock et al., 2017a)。而Hf同位素组分等特征表明,该沉积物源来源于非洲的南部或当地的基底(Flowerdew et al., 2007a)。尽管沉积物来源存在争议,但人们普遍认为,在寒武纪时期,埃尔斯沃思-惠特莫尔山脉地块位于其当前位置的北面,并接受来自冈瓦纳超大陆内部的沉积物(Jordan et al., 2020)。晚寒武世—泥盆纪期间,埃尔斯沃思-惠特莫尔山脉地块沉积了约3 km的浅海相砂岩,形成了克拉什锡特群(Crashsite Group),标志着一段相对的构造平静期(Curtis, 2001)。晚石炭世—二叠纪冈瓦纳冰川(Gondwanan glaciation)时期,埃尔斯沃思-惠特莫尔山脉地块沉积形成的一套怀特奥特砾岩(Whiteout Conglomerate),覆盖在克拉什锡特群之上(Craddock et al., 2017a)。随后,一套厚约1 km、以浅海相砂岩为主的二叠纪极星组(Polarstar Formation)形成,并覆盖在怀特奥特砾岩冰碛层之上,该组地层上层粉砂岩和煤含量增加,标志着由海相三角洲向陆上滨海平原过渡(Craddock et al., 2017a; Jordan et al., 2020)。此外,与极星组互层的火山凝灰岩发育有中—晚二叠世的棱柱状锆石,表明凝灰岩与当时古太平洋边缘的一次大型岩浆喷发事件有关(Elliot et al., 2015; Craddock et al., 2017a; Jordan et al., 2020)。

2.2 玛丽·伯德地地块中—晚古生代发育与汇聚有关的岩浆作用

根据地理位置和分离地块的演化历史,玛丽·伯德地地块可分为西部的罗斯省(Ross Province)和东部的阿蒙森省(Amundsen Province)(图 1图 3Pankhurst et al., 1998; Yakymchuk et al., 2015)。玛丽·伯德地地块出露相对较老的侵入岩统称为福特花岗闪长岩套(Ford Granodiorite suite),广泛出露在玛丽·伯德地西部的福特群岭(Ford Ranges)地区,Rb-Sr全岩等时线年龄为380~353 Ma(Adams, 1987),后经U-Pb锆石验证,年龄主要分布在375~345 Ma之间(Pankhurst et al., 1998; Mukasa and Dalziel, 2000; Yakymchuk et al., 2015; Nelson and Cottle, 2018)。此外,福特花岗闪长岩套侵入到玛丽·伯德地地块最古老的斯旺森组(Swanson Formation)岩石单元中,该组地层是沿冈瓦纳边缘沉积形成的、以浊积岩和复理石为主的新元古代—寒武纪的沉积序列(Adams, 1986; Pankhurst et al., 1998; Mukasa and Dalziel, 2000; Jordan et al., 2020)。同时,在福特群岭的福斯迪克山(Fosdick Mountains)发育有一套混合岩-花岗岩杂岩,其出露的正、副片麻岩分别是福特花岗闪长岩套和斯旺森组岩石高级变质的产物(Yakymchuk et al., 2015)。福斯迪克混合岩-花岗岩杂岩中的花岗岩的U-Pb锆石年龄为372~353 Ma(Siddoway and Fanning, 2009; Yakymchuk et al., 2015),二云母花岗岩的U-Pb独居石年龄约为359 Ma和351 Ma(Tulloch et al., 2009)。

图 3 玛丽·伯德地地块位置图(据Mukasa and Dalziel, 2000修改) Fig. 3 Location map for the Marie Byrd Land block (modified after Mukasa and Dalziel, 2000)

福特花岗闪长岩套的主要岩性为钙碱性、准铝质-弱过铝质的Ⅰ型花岗闪长岩-二长花岗岩,其初始87Sr/86Sr值为0.704~0.706,εNd(t)值为-3.4~-1.0(Pankhurst et al., 1998),εHf(t)值在-5.3~+1.9之间,δ18O同位素值为7‰~10.5‰(Yakymchuk et al., 2015);福斯迪克山混合岩的εHf(t)值在-9.8~-1.3之间,δ18O同位素值为8.5‰~10.6‰(Yakymchuk et al., 2015)。同位素组分表明,该岩套在成岩过程中有一定比例的陆壳物质参与。该岩套微量元素显示出典型的大陆弧或碰撞后环境的特征(Mukasa and Dalziel, 2000),表明这些晚泥盆世—早二叠世岩石由新生地壳和陆壳物质混染而成,形成于一个活动大陆边缘环境(Yakymchuk et al., 2015)。与此同时,这些岩石与维多利亚地(Victoria Land)北部的阿德默勒尔蒂(Admiralty)侵入杂岩(390~350 Ma)、新西兰西部省的卡拉梅亚(Karamea)岩套(371~305 Ma)和澳大利亚东部塔斯马尼亚(Tasmanides)地区的墨尔本(Melbourne)地体(约360 Ma)同时期岩浆活动形成的岩石具有相关性(Mukasa and Dalziel, 2000; Yakymchuk et al., 2015),表明冈瓦纳太平洋边缘演化早期是以汇聚有关的岩浆作用为主,处于汇聚板块边缘环境。

2.3 南极半岛-瑟斯顿岛地块古生代岩浆作用及弧的发育

南极半岛是一个弧形地块(图 1图 4),已发现最古老的基底为东部伊登冰川(Eden Glacier)的晚寒武世—早奥陶世(487~485 Ma)闪长质片麻岩,代表了早期的弧岩浆活动,经历了晚二叠世的变质和混合岩化作用,并可能为南极半岛地块晚古生代增生杂岩提供有效的碎屑物源(Riley et al., 2012a; Jordan et al., 2020)。南部帕默地(Palmer Land)西部也发现志留纪(~430 Ma)弧岩浆活动的存在(Millar et al., 2002; Jordan et al., 2020)。半岛东部的塔吉特山丘(Target Hill)花岗质片麻岩和条带状片麻岩的时代为泥盆纪—石炭纪(397±8 Ma、393±1 Ma和327±9 Ma),记录了泥盆纪岩浆作用的存在,且经历了石炭纪的变质作用改造(Millar et al., 2002)。二叠纪时期,南极半岛地块的阿迪湾(Adie Inlet)、巴斯琴峰(Bastion Peak)和伊登冰川(Eden Glacier)地区广泛的记录了280~255 Ma之间的花岗质岩浆作用和变质作用(Riley et al., 2012a)。与此同时,南极半岛地块出露的特里尼蒂半岛群(Trinity Peninsula Group)变质沉积岩,标志着沉积作用的启动,且沉积作用始于晚石炭世时期,延续至二叠纪时期,是在一个活动大陆边缘弧前环境的上覆板块沉积形成的,代表了弧俯冲发育的早期阶段(Bradshaw et al., 2012; Jordan et al., 2020)。格雷厄姆地(Graham Land)北部和东部的特里尼蒂半岛群碎屑锆石数据分析表明,该地区以二叠纪时代为主,峰期年龄在285~260 Ma之间。帕默地南部的晚二叠世埃瑞红层(Erewhon Beds)来源于硅质火山岩,同样包括263 Ma和275 Ma的峰期年龄,表明二叠纪弧火山作用的存在,并从半岛的北部延伸到半岛的南部(Jordan et al., 2020)。

图 4 南极半岛地块地质简图(据Vaughan and Storey, 2000郑光高等,2015修改) Fig. 4 Geological sketch map of the Antarctic Peninsula block (modified after Vaughan and Storey, 2000; Zheng et al., 2015)

瑟斯顿岛地块位于南极半岛地块南端(图 1图 5),其摩根湾(Morgan Inlet)地区的花岗闪长质片麻岩U-Pb锆石年龄为349±2 Ma(εHf(t)=+2~+10)、门泽尔角(Cape Menzel)正片麻岩时代为347±1 Ma(εHf(t)=+11)、金陡崖(King Cliffs)正片麻岩时代为338±2 Ma(εHf(t)=+2.1)。这些岩石富集大离子亲石元素,亏损高场强元素,具有弧岩浆岩的亲属性(Leat et al., 1993; Riley et al., 2017b; Nelson and Cottle, 2018),表明瑟斯顿岛地块石炭纪弧岩浆作用的存在,且岩浆源区为新生陆壳物质,形成于俯冲环境。

图 5 瑟斯顿岛地块位置图(据Riley et al., 2017b修改) Fig. 5 Location map for the Thurston Island block (modified after Riley et al., 2017b)
3 中生代岩浆作用 3.1 埃尔斯沃思-惠特莫尔山脉地块侏罗纪板内岩浆作用

埃尔斯沃思-惠特莫尔山脉地块的侏罗纪花岗岩侵入到寒武纪—二叠纪变形地层中,形成了区内中部现有的主体岩体(Storey et al., 1988b)。埃尔斯沃思-惠特莫尔山脉地块花岗岩的U-Pb锆石年龄范围主要在200~168 Ma,且绝大多数集中在早侏罗世晚期(177~174 Ma),是在较短时间内侵位的(Craddock et al., 2017b)。这些花岗岩具有高放射性成因的87Sr/86Sr,初始87Sr/86Sr比值为0.7068~0.7232,εNd(t)值在约-12~-1.5之间,指示在成岩过程中,古老陆壳物质的参与且占主导地位(Wareham et al., 1998; Craddock et al., 2017b)。地球化学特征显示,中侏罗世花岗岩具有较高的Nb、Rb、Sm、Y、Th和Yb,缺失与俯冲相关花岗岩的地球化学特征,属于典型的板内花岗岩(Storey et al., 1988b; Craddock et al., 2017b; Jordan et al., 2020)。因此,埃尔斯沃思-惠特莫尔山脉地块侏罗纪花岗岩的发育与裂谷作用有关,形成于板内拉张环境。

南非和东南极的干旱台地-弗拉尔(Karoo-Ferrar)侏罗纪基性大火成岩省的岩浆作用时代主要为~183 Ma,很好的限定了冈瓦纳裂解期间的一个非常短暂的同步侵位事件(Encarnación et al., 1996; Burgess et al., 2015)。其中,弗拉尔大火成岩省很可能形成于由古太平洋边缘俯冲变化而形成的伸展和减压熔融环境(Jordan et al., 2020)。虽然西南极地区未找到直接归属于弗拉尔或干旱台地大火山岩省的岩浆岩石,但埃尔斯沃思-惠特莫尔山脉地块出露有早—中侏罗世的板内花岗岩(Burgess et al., 2015; Jordan et al., 2020)。同时,埃尔斯沃思-惠特莫尔山脉地块花岗岩套与弗拉尔超群(Ferrar Supergroup)具有相似的Sr和Nd同位素特征,且该地块哈特丘陵(Hart Hills)辉长岩具有弗拉尔岩套(Ferrar suite)相似的地球化学属性,反映出埃尔斯沃思-惠特莫尔山脉地块侏罗纪花岗岩套和弗拉尔辉绿岩两者之间可能具有成因联系(Storey et al., 1988b; Craddock et al., 2017b)。综合表明,西南极可能存在与冈瓦纳裂解有关的中侏罗世双峰式岩浆岩套,形成了南极-南大西洋地区广泛岩浆活动的一部分。同时,埃尔斯沃思-惠特莫尔山脉地块中侏罗世花岗岩,可能是由热地幔上涌引起的地壳熔融和地幔来源的弗拉尔岩浆直接分馏共同作用的结果(Craddock et al., 2017b; Jordan et al., 2020)。

此外,惠特莫尔山西利格山(Mount Seeling)花岗岩的U-Pb锆石年龄为207.96±0.06 Ma,该花岗岩比埃尔斯沃思-惠特莫尔山脉地块的主体侏罗纪花岗岩要早~30 Ma,可解释为一个单独的岩浆事件,证明埃尔斯沃思-惠特莫尔山脉地块三叠纪末期岩浆作用的存在,尽管目前尚不清楚这种岩浆作用的时空分布范围(Craddock et al., 2017b)。有学者认为西南极三叠纪花岗岩与沿南美南部和西南极太平洋边缘的安第斯型俯冲有关,属于弧岩浆岩(Millar et al., 2001; Craddock et al., 2017b)。因此,该花岗岩的发现,为西南极三叠纪岩浆弧的持续性发育提供了证据,且具有负εNd(t)值(-2.4),表明其在成岩过程中有古老陆壳物质的贡献(Craddock et al., 2017b; Jordan et al., 2020)。

3.2 玛丽·伯德地地块白垩纪与裂解有关的岩浆作用启动及陆内伸展作用

玛丽·伯德地地块沿原太平洋边缘部位的侏罗纪—早白垩世早期岩浆作用非常有限(Riley et al., 2017b)。福斯迪克山北部的白垩纪花岗岩中,仅有一颗继承锆石年龄(181±11 Ma)记录了早侏罗世岩浆活动的存在(Korhonen et al., 2010)。此外,在福特群岭东缘地区几个花岗质岩体获得的K-Ar或Rb-Sr年龄介于165 Ma与135 Ma之间(Adams, 1987);在玛丽·伯德地东部派恩艾兰湾(Pine Island Bay)也有花岗质岩石产出,其Ar-Ar年龄约147 Ma,代表晚侏罗世—早白垩世早期的岩浆记录(Kipf et al., 2012)。

玛丽·伯德地地块出露有两套白垩纪中期的花岗质岩石,统称为伯德海岸花岗岩(Byrd Coast Granite)。相对较早的岩浆作用沿整个玛丽·伯德地地块边缘广泛出露,除了在玛丽·伯德地东部厄尔利陡崖(Early Bluffs)和麦肯齐群岛(McKenzie Islands)两地区分别获得103.4±0.3 Ma和96±1 Ma的年轻年龄外,其他地区U-Pb锆石年龄主要集中在129~110 Ma之间(Mukasa and Dalziel, 2000)。相对较晚的岩浆作用主要分布于玛丽·伯德地西部的鲁伯特-霍布斯特海岸(Ruppert-Hobbs Coast)地区,而玛丽·伯德地东部地区缺乏,这期岩浆作用的时代在102~95 Ma之间,其中,兰德冰川(Land Glacier)地区U-Pb锆石年龄为102~98 Ma(Mukasa and Dalziel, 2000),爱德华兹七世半岛(Edward Ⅶ Peninsula)的Rb-Sr年龄为101~95 Ma(Weaver et al., 1992)。早期侵入体为钙碱性、准铝质的Ⅰ型闪长岩、花岗闪长岩和二长花岗岩,87Sr/86Sr比值为0.7056~0.7079(Adams, 1987; Mukasa and Dalziel, 2000),微量元素组分具有富集大离子亲石元素、亏损高场强元素和明显Nb负异常特征,形成环境与弧背景有关,可能是大洋沿冈瓦纳太平洋边缘早白垩世俯冲的结果(Mukasa and Dalziel, 2000)。晚期侵入体为高分异、非造山的A型正长岩和石英正长岩等,高度富集不相容元素、K2O>Na2O,且部分侵入岩的SiO2含量可达80wt%,具有高而宽泛的初始87Sr/86Sr值(0.7080~0.7212)和负εNd(t)值(-7.7~-3.6)(Weaver et al., 1992; Korhonen et al., 2010),表明该类岩石可能由非均质大陆地壳物质熔融而成,形成于陆内伸展环境。

此外,普林斯山(Mount Prince)花岗岩年龄为110±1 Ma,是在玛丽·伯德地西部鲁伯特-霍布斯特海岸发现的最年轻的弧岩浆岩,其内被大量的基性和中性岩脉侵入,标志着与裂解有关岩浆作用的启动。其中一个岩脉的年龄为101±1 Ma(Mukasa and Dalziel, 2000),表明白垩纪中期110~101 Ma之间的~10 Ma期间,玛丽·伯德地地块沿岸地区地球动力学机制由俯冲向陆内伸展转变。然而,玛丽·伯德地东部麦肯齐群岛~96 Ma的Ⅰ型花岗闪长岩的产出表明,弧岩浆作用在玛丽·伯德地西部停止后,在玛丽·伯德地东部仍持续活动了一段时间(~14 Ma)。

有关学者基于南大洋板块的重建,认为在侏罗纪末期,在西南极原太平洋部位形成了一个由太平洋、法拉隆(Farallon)和菲尼克斯(Phoenix)板块组成的三联点构造体系,并持续到白垩纪时期(Hilde et al., 1977; Barker, 1982)。在~100 Ma,菲尼克斯-太平洋脊与冈瓦纳原太平洋陆缘由西向东斜向俯冲和碰撞,从而造成玛丽·伯德地西部俯冲作用停止,转为陆内伸展过程;而玛丽·伯德地东部地区俯冲作用持续了一段时间后,随洋脊与陆缘接触及碰撞后停止俯冲(Mukasa and Dalziel, 2000)。因此,玛丽·伯德地地块岩性的穿时性特征,可能与菲尼克斯-太平洋脊由西向东依次俯冲到冈瓦纳西缘之下而终止俯冲有关。

3.3 南极半岛-瑟斯顿岛地块中生代弧岩浆活动

三叠纪岩浆作用在南极半岛格雷厄姆地北部不发育,主要出露在格雷厄姆地南部和帕默地地区,并延伸到瑟斯顿岛地块,岩浆活动时代在239~202 Ma。其中,格雷厄姆地南部地区,以斯塔布斯山口(Stubbs Pass)和凯西角(Cape Casey)等花岗闪长岩为代表,其时代介于239~209 Ma之间(Riley et al., 2012a);帕默地地区,以乔治六世海峡的片麻岩和花岗闪长岩为代表,年龄为233~202 Ma(Millar et al., 2002);瑟斯顿岛地块,以布拉姆霍尔山(Mount Bramhall)花岗质岩石为代表,年龄为239±4 Ma(Riley et al., 2017b)。这些三叠纪岩浆岩是在汇聚大陆边缘弧背景下侵位形成的,并在南极半岛地块南部西缘发育有伴生的亚历山大岛(Alexander Island)的勒梅群(LeMay Group)俯冲增生杂岩(Willan, 2003; Jordan et al., 2020)。

南极半岛地块侏罗纪岩浆侵入作用主要发育在东缘,记录的时代为199~156 Ma(Pankhurst et al., 2000; Flowerdew et al., 2006; Leat et al., 2009; Riley et al., 2012a);火山作用在南部帕默地地区时代为184~178 Ma(Pankhurst et al., 2000; Hunter et al., 2006),而北部格雷厄姆地地区时代集中在173~150 Ma之间(Pankhurst et al., 2000; Hunter et al., 2005; Riley et al., 2012b; Jordan et al., 2014),这些火山活动较好的对应于南美巴塔哥尼亚Chon Aike省识别出的三期火山活动事件(V1: ~183 Ma,V2: ~170 Ma和V3: ~155 Ma)(Pankhurst et al., 2000)。尤其是在帕默地的波斯特山组(Mount Poster Formation)和布伦尼克组(Brennecke Formation)获得的年龄分别为183.4±1.4 Ma和184.2±2.5 Ma(Pankhurst et al., 2000),与第一期火山活动时间基本吻合。沿南极半岛地块冈瓦纳边缘向南的瑟斯顿岛地块,道林山(Mount Dowling)硅质凝灰岩年龄确定为182~181 Ma(Riley et al., 2017b),也与Chon Aike Province冈瓦纳裂解岩浆作用事件的时代相一致,表明南极半岛地块南部(帕默地)的波斯特山组和布伦尼克组与瑟斯顿岛的道林山火山活动的年代重叠。从岩性上看,瑟斯顿岛地块硅质火山活动类似于帕默地主要的硅质凝灰岩和熔结凝灰岩。在Sr-Nd同位素组成上,道林山硅质火山岩的成分与布伦尼克组的流纹质凝灰岩成分接近,说明两者具有内在的成因联系(Riley et al., 2017b)。在此基础上,有学者在古大陆重建时,认为~180 Ma时期瑟斯顿岛地块位于块体旋转部位,此时与南极半岛地块南部是并列的,但对于瑟斯顿岛地块旋转的角度,存在旋转180°和旋转90°等不同的认识(Veevers, 2012; Elliot et al., 2016; Riley et al., 2017b)。南极半岛地块的晚侏罗世岩浆作用较少,时代为162~150 Ma,主要零星出露在南极半岛西缘的阿弗斯岛(Avers Island)和阿德莱德角(Adelaide Island)等地区(Pankhurst et al., 2000; Riley et al., 2012b; Jordan et al., 2014)。此外,瑟斯顿岛地块晚侏罗世花岗质岩浆活动广泛分布于其西部和南部的兰福尔峰(Landfall Peak)、亨德森圆丘(Henderson Knob)、黑尔冰川(Hale Glacier)、诺克森山(Mount Noxon)、辛普山(Mount Simpson)、博格森山(Mount Borgeson)、郎冰川(Long Glacier)和谢尔顿岬(Shelton Head)等地,Rb-Sr年龄在152~142 Ma之间,可能代表了复合岩基的一部分(Pankhurst et al., 1993; Riley et al., 2017b)。

白垩纪是横跨南极半岛和瑟斯顿岛两地块的大陆边缘弧岩浆作用的主要阶段。在全球板块重组时期,白垩纪中期岩浆作用沿冈瓦纳整个原太平洋广泛分布。南极半岛地块广泛记录的岩浆侵入时代为118~102 Ma(Tangeman et al., 1996; Vaughan et al., 2002; Flowerdew et al., 2005; Riley et al., 2018; Zheng et al., 2018),火山喷发年龄有114 Ma、107 Ma和103 Ma(Leat et al., 2009; Riley et al., 2012b; 郑光高等, 2017)。南极半岛的南设得兰群岛(South Shetland Islands)地区,白垩纪中期岩浆作用主要出露在利文斯顿岛(Livingston Island)西南拜尔斯半岛(Byers Peninsula)的内格罗丘陵(Negro Hill,102.4±1.3 Ma)和希勒夫角(Cape Shirreff,109.19±0.46 Ma; Haase et al., 2012)。此外,南极半岛格雷厄姆地西缘还出露有几处时代为~85 Ma的晚白垩世侵入体,如蒂克森角(Cape Tuxen)的石英闪长岩、德马里亚山(Mount Demaria)的花岗质闪长岩和屈韦维尔岛(Cuverville Island)的花岗斑岩脉(Tangeman et al., 1996; 郑光高等, 2017; Zheng et al., 2018);南设得兰群岛的格林尼治岛(Greenwich Island)斯帕克角(Spark Point)和罗伯特岛(Robert Island)科珀曼半岛(Coppermine Peninsula)存在~82 Ma火山作用事件(Haase et al., 2012)。在此阶段,影响南极半岛帕默地主要构造事件为白垩纪中期~107 Ma左旋走滑挤压和~103 Ma右旋走滑挤压的两期帕默地事件(Vaughan et al., 2012b)。瑟斯顿岛地块的白垩纪岩浆作用同样发生在110~95 Ma左右,但不像南极半岛地块出露的那样广泛(Jordan et al., 2020)。

对南极半岛地块侏罗纪—白垩纪岩浆作用的时代进行频率直方图分析,发现主要集中在190~170 Ma和120~80 Ma两个阶段(Zheng et al., 2018)。早阶段的岩石呈现出较高的初始87Sr/86Sr比值和负εNd(t)值,具有冈瓦纳古陆的亲属性(Millar et al., 2001; Vaughan and Livemore, 2005; Riley et al., 2017a);然而,南奥克尼岛(South Orkney Islands)斯科舍俯冲增生杂岩时代为200~180 Ma(Trouw et al., 1998; Vaughan and Livemore, 2005; Flowerdew et al., 2007b; Wendt et al., 2008),表明在此阶段,南极半岛地块东缘处于伸展环境,而南极半岛地块西缘则处于俯冲环境。晚阶段的岩浆作用呈现两种显著不同的同位素特征,其中南极半岛地块南部白垩纪中期岩浆作用主要受控于转换挤压变形作用,主要表现为高的初始87Sr/86Sr值和负εNd(t)值(Scarrow et al., 1996; Vaughan et al., 2012a; Riley et al., 2018);其他部位出露的白垩纪火成岩,其εHf(t)和εNd(t)值均为较大的正值,属于典型的岛弧岩浆岩(Zheng et al., 2018)。此时,象岛(Elephant Island)地区发育一套同期斯科舍俯冲增生杂岩(120~80 Ma;Trouw et al., 1998; Vaughan and Livemore, 2005; Wendt et al., 2008)。瑟斯顿岛地块晚侏罗世—早白垩世岩石大部分初始87Sr/86Sr比值为0.705~0.706,εNd(t)值在-4~+2之间,表明其岩浆来源于轻微富集的地幔或新生下地壳物质,形成于俯冲环境(Pankhurst et al., 1993)。同时,瑟斯顿岛地块弧岩浆作用在~90 Ma左右停止,开始发育东西走向的晚白垩世基性到酸性岩脉,表明该时期可能是太平洋板块和菲尼克斯板块的扩张中心与大陆边缘俯冲带开始碰撞,构造体制从俯冲挤压向伸展拉张过渡(Leat et al., 1993)。

4 新生代岩浆作用

西南极新生代岩浆活动以南极半岛-南设得兰群岛地区为代表,记录冈瓦纳古陆的最后裂解和离散的过程。南极半岛地块西缘发育古近纪早期的侵入体,U-Pb锆石年龄在62~54 Ma之间,典型的有布思岛(Booth Island)花岗闪长岩-闪长岩(61.6±0.5 Ma、59.3±1.1 Ma)、茹格拉角(Jougla Point)闪长岩和辉绿玢岩(56.3±0.9 Ma、55.1±0.9 Ma)及杜梅岛(Doumer Island)闪长岩(54.4±1.0 Ma)等(Leat et al., 1995; Veevers and Saeed, 2013; Zheng et al., 2018)。该期侵入体呈现出大离子亲石元素富集、高场强元素亏损,具有正的εNd(t)值(+3.6~+7.2)和εHf(t)值(+6.7~+12.2),属于岛弧岩浆岩(Zheng et al., 2018; Cui et al., 2019)。南设得兰群岛是发育在一个片岩和变沉积岩硅质基底上的(?)侏罗纪—第四纪的岛弧(Smellie et al., 1984),新生代岛弧岩浆作用主要出露在纳尔逊岛(Nelson Island)和乔治王岛(King George Island)地区,岩性主要为玄武-安山质火山岩,并发育有少量的基性和中性岩脉,Ar-Ar年龄集中在61~35 Ma之间(Kim et al., 2000; Wang et al., 2009; Kraus et al., 2010; Nawrocki et al., 2010; Pańczyk and Nawrocki, 2011; Haase et al., 2012; Gao et al., 2018)。南设得兰群岛史密斯岛(Smith Island)地区发育的斯科舍俯冲增生杂岩的时代在65~47 Ma之间(Trouw et al., 1998; Vaughan and Livemore, 2005; Wendt et al., 2008),与新生代弧岩浆岩的时代重叠,表明两者是俯冲体系不同部位出露的岩性组合。此外,新近纪末期—第四纪火山作用记录在南极半岛地块北部的詹姆斯·罗斯岛(James Ross Island)和邓迪岛(Dundee Island)等地,岩性为碱性玄武质火山岩,时代在6~0.132 Ma之间(Kristjánsson et al., 2005; Smellie et al., 2006),表明岩浆活动在南极半岛地块持续至今。

年龄数据资料表明,在南极半岛地块西缘地区的新生代弧岩浆作用主要发生在54 Ma以前,且中新生代弧岩浆作用在半岛北部有自东向西迁移的趋势(Zheng et al., 2018);而在南设得兰群岛地区,新生代岛弧岩浆作用则主要发生在35 Ma以前,且中新生代弧岩浆作用有自西南向东北迁移的趋势(Pankhurst and Smellie, 1983)。如上文所述,持续到白垩纪时期,冈瓦纳原太平洋存在太平洋-法拉隆-菲尼克斯板块三联点体系,白垩纪中期从玛丽·伯德地西部开始斜向由西朝东向海沟处俯冲和碰撞。南极半岛地块目前位于南极板块、斯科舍板块和德雷克板块的交汇部位(Barker, 1982),其中,南极板块是太平洋板块的一部分,纳兹卡(Nazca)板块是法拉隆板块向南美板块之下俯冲的残余,而德雷克板块是菲尼克斯板块现在所能见到的部分(刘小汉等, 1991)。在~50 Ma时期,该太平洋-菲尼克斯扩张脊左旋错断成若干扩张脊分支,西部的扩张脊分支首先到达南极半岛的亚历山大岛西缘,随后各扩张脊分支由西向东依次到达海沟处发生碰撞及停止俯冲,弧岩浆作用随即相应的依次停止活动。在4 Ma左右,随着板块俯冲和洋脊扩张作用的全部停止,已俯冲在南极半岛地块之下的板块继续下沉,由于缺失扩张洋脊的推动力,使得下沉板块产生回卷,最终导致布兰斯菲尔德海峡(Bransfield Strait)弧后盆地的打开(Grunow et al., 1992; Johnson, 1996)。南极半岛-南设得兰群岛板内碱性玄武岩的爆发,正是由于太平洋-菲尼克斯扩张脊分段俯冲碰撞后的结果。此过程中,南美与南极半岛之间的伸展作用,以及斯科舍俯冲带的弧后伸展作用,导致了德雷克海峡通道在~34 Ma时期打开,从而使南极绕极流得以发展,形成现今南极洲的位置格局(Eagles et al., 2006; Jordan et al., 2020)。

5 西南极构造演化

西南极出露的最古老的岩石为哈格冰原岛峰群片麻岩,时代为~1238 Ma,记录了中元古代的弧岩浆作用。而西南极主体所记录的~500 Ma以来的地质演化过程,在很大程度上与罗斯增生造山带(Ross Orogeny;~510 Ma)的发育有关。该造山带发生在西冈瓦纳和澳大利亚-南极板块(Kuunga造山带)缝合之后,使俯冲的位置由冈瓦纳碰撞前杂岩区向原太平洋边缘迁移(Cawood, 2005; Boger, 2011; Pereira et al., 2018)。这一造山事件导致被动沉积边缘的变形,形成了弧后盆地沉积环境,沉积形成了埃尔斯沃思-惠特莫尔山脉地块古生代的地层单元,并导致了随后的地壳增生到冈瓦纳边缘。沿冈瓦纳原太平洋边缘的汇聚,导致了西南极地区寒武纪—奥陶纪浊积层序的沉积及随后的增生,以及泥盆纪—石炭纪(375~345 Ma)花岗质岩石的侵入,这些早古生代的岩石记录在玛丽·伯德地罗斯海省、瑟斯顿岛和南极半岛东部地区均有发现(Pereira et al., 2018)。

晚石炭世时期,冈瓦纳与劳伦古陆的碰撞使罗斯造山运动终止,随后是冈瓦纳造山运动(Gondwanide Orogeny)。这一造山运动的转变,在玛丽·伯德地、南极半岛和瑟斯顿岛各地块中触发了晚古生代—中生代一系列、大规模钙碱性花岗质为主的弧岩浆作用(Pereira et al., 2018)。同时,埃尔斯沃思-惠特莫尔山脉地块西利格山发育晚三叠世(~208 Ma)与弧有关的岩浆作用,表明冈瓦纳西缘俯冲作用的持续性及可能影响其内侧部位。

侏罗纪时期,各地块的构造背景开始明显分化。冈瓦纳的侏罗纪裂解涉及到一个区域性的双峰式岩浆事件,埃尔斯沃思-惠特莫尔山脉地块的侏罗纪板内花岗岩套与弗拉尔岩套是早—中侏罗世大火成岩省的一部分,代表了大陆地壳幔源岩浆作用驱动的深部热事件,为地壳伸展和冈瓦纳大陆裂解起到了促进作用(Craddock et al., 2017b)。玛丽·伯德地地块在白垩纪中期爆发了一次大规模的岩浆活动,岩性由侏罗纪早期—早白垩世的Ⅰ型花岗质岩石及相关的火山岩,转变为白垩纪中期的A型花岗质岩石,反映了白垩纪中期构造体制从俯冲向陆内裂解的转变。同时,玛丽·伯德地地块西部在~100 Ma俯冲作用的停止,很可能是由于侏罗纪末期形成的太平洋-法拉隆-菲尼克斯板块三联点体系,其中的太平洋-菲尼克斯分支脊在白垩纪中期由西向东朝冈瓦纳古太平洋边缘俯冲和碰撞的结果,从而极大地改变了西南极这一地区的构造格架(Jordan et al., 2020)。南极半岛-瑟斯顿岛地块侏罗纪—白垩纪活跃的弧岩浆活动是持续俯冲和西南极裂解相互作用的结果。新生代以来,左行错断成若干分支的太平洋-菲尼克斯扩张脊自~50 Ma起,由西向东分段俯冲及碰撞到南极半岛的亚历山大岛西缘至南设得兰群岛的海沟部位,形成了南极半岛和南设得兰群岛新生代岩浆作用的时空格局。在此期间,~34 Ma时期德雷克海峡通道打开,从而使南极绕极流得以发展,形成现有西南极的构造格局。

6 结论与建议

西南极哈格冰原岛峰群地块记录了中元古代弧岩浆作用,而南极半岛、瑟斯顿岛、玛丽·伯德地和埃尔斯沃思-惠特莫尔山脉四个地块记录的是显生宙以来的地质演化过程。古生代时期,埃尔斯沃思-惠特莫尔山脉地块处于快速沉降的陆相断陷盆地环境,与罗斯造山运动形成的弧后伸展有关;玛丽·伯德地地块中—晚古生代发育一套与板块汇聚有关的岩浆作用,形成于活动大陆边缘环境;南极半岛-瑟斯顿岛地块记录了石炭纪—二叠纪时期弧的发育。自侏罗纪开始,各地块的构造背景明显分化,埃尔斯沃思-惠特莫尔山脉地块记录了侏罗纪板内岩浆作用,可能与大火成岩省有关;玛丽·伯德地地块发育的侏罗纪—早白垩世Ⅰ型弧岩浆岩随时间转变为白垩纪中期的A型碱性岩浆岩,经历了由俯冲向裂解机制的转变;南极半岛-瑟斯顿岛地块侏罗纪—白垩纪为弧岩浆活动活跃期,同时也有大火成岩省火山活动的记录,是持续俯冲和裂解相互作用的产物。新生代岩浆作用以南极半岛地块为代表,弧岩浆作用持续到始新世,其时空分布特征与左行错断扩张脊的分段俯冲和碰撞有关。

由于地理位置和南极科学考察条件保障的制约,自长城站建立以来,中国在西南极地质考察工作主要集中在南设得兰群岛地区,近十年开始涉足南极半岛格雷厄姆地北部和玛丽·伯德地罗斯省的部分地区,但中国在西南极大部分领域地质考察仍属空白。文章通过综合相关学者们的研究成果,对西南极主要的岩浆活动和构造演化进行简要综述,深入的研究工作急需后续实地考察系统展开。同时,希望有更多学者加入到南极地质考察队伍,一起探索神秘的南极大陆。

致谢: 野外工作得到中国第33、34和36次南极科学考察队相关单位和个人、智利南极研究所等合作单位的大力支持,审稿人提出了建设性修改意见,在此一并表示衷心的感谢。

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