地质力学学报  2021, Vol. 27 Issue (5): 705-718
引用本文
仝来喜, 刘兆, 王彦斌. 东南极茹尔群岛超高温麻粒岩的研究进展[J]. 地质力学学报, 2021, 27(5): 705-718.
TONG Laixi, LIU Zhao, WANG Yanbin. Research progress of the ultrahigh-temperature granulites in the Rauer Group, East Antarctica[J]. Journal of Geomechanics, 2021, 27(5): 705-718.
东南极茹尔群岛超高温麻粒岩的研究进展
仝来喜1, 刘兆1, 王彦斌2    
1. 大陆动力学国家重点实验室, 西北大学地质系, 陕西 西安 710069;
2. 中国地质科学院地质研究所, 北京 100037
摘要:茹尔群岛(又称赖于尔群岛)位于东南极普里兹构造带的东部边缘,是一个由太古宙和中元古代岩石组成的复合高级变质地体。中元古代岩石是含有富Fe-Al的含石榴子石-矽线石的费拉副片麻岩组合,经历了格林维尔和泛非两期变质作用。太古宙正片麻岩是含有富Mg-Al的含假蓝宝石的超高温泥质麻粒岩组合(梅瑟副片麻岩组合),主要由经历超高温变质作用的含假蓝宝石的泥质麻粒岩、富Mg的石榴子石-矽线石泥质片麻岩、斜方辉石-矽线石石英岩、含石榴子石镁铁质麻粒岩和钙硅酸盐麻粒岩等组成。其中,含假蓝宝石泥质麻粒岩中石榴子石变斑晶和矽线石集合体(蓝晶石假象)周围分别发育峰期后由假蓝宝石+斜方辉石和假蓝宝石+堇青石后成合晶组成的典型减压结构。含石榴子石镁铁质麻粒岩中石榴子石变斑晶周围则发育峰期后由斜方辉石+斜长石后成合晶组成的典型白眼圈减压结构。不同研究者得出了具有不同超高温峰期条件、峰期前及峰期后演化历史、不同形式的顺时针变质P-T轨迹。对超高温变质事件发生的时间和构造背景的认识也存在较大分歧,有认为超高温变质事件发生于格林维尔期(~1000 Ma)并与碰撞造山和弧岩浆作用有关,也有研究认为发生于泛非期(~590 Ma或~530 Ma)并与普里兹造山及冈瓦纳大陆聚合有关。因此,为理清该区超高温麻粒岩的变质演化历史和构造背景,需要对其进一步进行详细深入的矿物组合-变质结构分析、P-T轨迹重建及高精度的锆石-独居石U-Pb年代学研究,并进行区域上对比。
关键词超高温麻粒岩    变质P-T轨迹    研究进展    茹尔群岛    东南极    
DOI10.12090/j.issn.1006-6616.2021.27.05.058     文章编号:1006-6616(2021)05-0705-14
Research progress of the ultrahigh-temperature granulites in the Rauer Group, East Antarctica
TONG Laixi1, LIU Zhao1, WANG Yanbin2    
1. State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi'an 710069, Shaanxi, China;
2. Institute of Geology, Chinese Academy of Geological Science, Beijing 100037, China
Abstract: The Rauer Group (Rauer Islands), located in the eastern margin of the Prydz Tectonic Belt in East Antarctica, represents a composite high-grade metamorphic terrane consisting of Archaean and Mesoproterozoic rocks. The Mesoproterozoic rocks contain Fe-Al-rich garnet-sillimanite-bearing Filla Paragneiss associations, and have experienced two phases of metamorphism involving Grenvillian and Pan-African events. The Archaean orthogneisses contain Mg-Al-rich sapphirine-bearing ultrahigh-temperature (UHT) pelitic granulite associations (Mather Paragneiss associations), and they consist mainly of sapphirine-bearing pelitic granulite, Mg-rich garnet-sillimanite-bearing pelitic paragneiss, orthopyroxene-sillimanite quartzite, garnet-bearing mafic granulite and calcsilicate granulite that experienced ultrahigh-temperature metamorphism. In the sapphirine-bearing pelitic granulite, typical post-peak decompression textures around garnet porphyroblasts and sillimanite aggregations (kyanite pseudomorph) developed as symplectite assemblages consisting of sapphirine-orthopyroxene and sapphirine-cordierite respectively. In the garnet-bearing mafic granulite, typical post-peak 'white-eye socket' decompression texture on garnet porphyroblast also developed as symplectite composed of orthopyroxene-plagioclase. Until recently, different researchers derived distinct-type clockwise P-T paths of various peak UHT conditions and pre-peak and post-peak evolution histories, whereas different opinions also exist regarding the timing of UHT metamorphic event and tectonic setting. For example, a UHT metamorphic event was considered to occur either during the Grenvillian period (~1000 Ma) associated with a collisional orogenesis and arc magmatism or during the Pan-African period (~590 Ma or~530 Ma) related to the Prydz orogenesis and the Gondwana continent assembly. Thus, in order to clarify the metamorphic evolution history and tectonic setting of the UHT granulites in the region, further detailed studies on analyses of the mineral assemblages and metamorphic textures and the reconstruction of P-T path as well as high-precesion zircon and monazite U-Pb chronological dating are needed, and regional geological comparison should also be undertaken.
Key words: ultrahigh-temperature granulites    metamorphic P-T paths    research progress    Rauer Group    East Antarctica    
0 引言

过去十几年来,超高温(UHT)变质作用研究已成为继超高压(UHP)变质作用研究之后国内外变质地质学领域的另一个重要前沿课题。超高温(UHT)变质作用是指地壳岩石在7~13 GPa压力条件下遭受峰期变质温度超过900 ℃的麻粒岩相变质作用,通常会导致局部或区域性超高温麻粒岩的形成,在FMAS体系发育指示超高温条件的特征变质矿物组合,如假蓝宝石+石英、斜方辉石+矽线石+石英、尖晶石+石英,在KFMASH体系则出现特征的大隅石+石榴子石组合等。斜方辉石以高铝(Al2O3>8%)为特征,金红石富锆,锆石则富钛,在相图中主要位于矽线石的稳定域,其原岩多为富镁和富铝及硅饱和的变泥质岩,变铁岩中则出现倒转的易变辉石,钙硅酸盐麻粒岩中可出现硅灰石+方柱石组合(Harley, 1998a, 2004, 2008)。关于超高温变质作用的特征与成因,国内外已有不少论文对此进行了详细总结(Harley, 1998a, 2004, 2008; Brown, 2007; 魏春景, 2012; Zheng and Chen, 2017; Lei and Xu, 2018)。目前,国际上已有六十余处不同产地含有指示性矿物组合的超高温麻粒岩的报道(Lei and Xu, 2018)。超高温麻粒岩可以形成于不同的地质历史时期,大多位于前寒武纪变质地体中,其形成被认为与超大陆的聚合与裂解过程有关(Brown, 2007)。其中,著名的有南极的纳皮尔杂岩(又称内皮尔杂岩,Napier Complex)(Harley and Motoyoshi, 2000)和茹尔群岛(又称赖于尔群岛,Rauer Group)超高温麻粒岩(Harley, 1998b; Kelsey et al., 2003; Tong and Wilson, 2006)等。近年来,中国也有多处超高温麻粒岩报道,代表性的产地有前寒武纪华北集宁的土贵乌拉和大青山的东坡(Santosh et al., 2007; Guo et al., 2012)及晚古生代的阿尔泰南缘(Li et al., 2014; Tong et al., 2014)等。超高温麻粒岩既可以在伸展环境也可以在挤压环境中形成,关于其构造背景的讨论主要有以下几种观点:由热的弧后盆地的倒转和增厚形成(Brown, 2006, 2007),由大陆碰撞造山期间同岩浆作用加热形成(Moraes et al., 2002; Tong and Wilson, 2006),由岩石圈伸展及镁铁质岩浆底侵或底垫加热形成(Kemp et al., 2007),由于放射性元素长期造山的自加热而形成(Clark et al., 2011),由于洋脊俯冲及板片窗的发育而形成(Guo et al., 2012; Santosh et al., 2012),或由地幔柱活动引起的异常加热而形成(Tong et al., 2014; Liu et al., 2020)等。其中,软流圈地幔上涌及岩石圈地幔减薄有关的活动性大陆张裂造山被认为是最有可能的构造背景(Zheng and Chen, 2017)。

茹尔群岛位于普里兹带(Prydz Belt)的东部边缘,出露著名的含假蓝宝石超高温麻粒岩相变质岩。普里兹带位于东南极大陆,是一条受早古生代泛非期(Pan-African)(~530 Ma)热事件广泛影响的重要高级活动带(Zhao et al., 1992; 赵越等, 1993; Dirks et al., 1993; 刘小汉等, 1995; Dirks and Wilson, 1995; Hensen and Zhou, 1995; Carson et al., 1996; Fitzsimons et al., 1997),主要由东南极普里兹湾(Prydz Bay)沿岸地质露头组成,从东部的茹尔群岛向西南经拉斯曼丘陵(Larsemann Hills)、埃默里冰架(Amery Ice Shelf)延伸到内陆格罗夫山(Grove Mountains)(刘小汉等, 2002; Zhao et al., 2003; Liu et al., 2006, 2007, 2009a; 刘晓春等, 2007, 2013; 刘晓春, 2009, 2018)。普里兹带高级变质岩(包括茹尔群岛超高温麻粒岩)的变质P-T轨迹及其大地构造背景在冈瓦纳(Gondwana)和罗迪尼亚(Rodinia)超大陆聚合过程中所起的作用和意义,目前主要有两种不同的观点:一是该带顺时针近等温减压的P-T轨迹反映了与地壳增厚有关的泛非期(~530 Ma)碰撞造山大地构造背景(Ren et al., 1992; Fitzsimons, 1996; Carson et al., 1997; Liu et al., 2006, 2007, 2009a),因此, 普里兹带是一条代表了早古生代与东冈瓦纳大陆最终聚合有关的碰撞造山带(Hensen and Zhou, 1997; Fitzsimons, 2000; Boger et al., 2001; 刘小汉等, 2002; Harley, 2003; Zhao et al., 2003; 刘晓春等, 2007; Liu et al., 2007胡健民等, 2008; Boger, 2011)。二是由于普里兹带尤其普里兹湾也存在早期格林维尔期(~1000 Ma)构造变质热事件,其变质P-T轨迹由两个无关的高级变质事件组成(Dirks and Hand, 1995; 刘小汉等, 1995; 仝来喜等,1998; Tong and Wilson, 2006; Wang et al., 2007, 2008; 刘晓春, 2009, 2018; Grew et al, 2012; Tong et al., 2014, 2019),因此普里兹带是一条多期变质带(Dirks and Wilson, 1995; 仝来喜等, 1997, 1998; Tong et al., 2002; Tong and Wilson, 2006; Kelsey et al., 2007; 刘晓春, 2009, 2018; Grew et al, 2012),并与罗迪尼亚和冈瓦纳超大陆的聚合有关(刘小汉等, 1995; Wang et al., 2008; Liu et al., 2009b; 刘晓春, 2009; 仝来喜等, 2012; Tong et al., 2014, 2017, 2019)。但由于缺乏代表泛非期碰撞造山的直接标志性岩石组合(如蛇绿岩、高压变质岩及岛弧增生杂岩),普里兹带也被认为仅是东冈瓦纳大陆在早古生代的一条板内(即陆内)活动带, 可能是由于东、西冈瓦纳大陆沿莫桑比克带的东非碰撞造山作用在东冈瓦纳大陆内部的构造调整效应形成的(Phillips et al., 2007; Wilson et al., 2007; 仝来喜等,2012; Tong et al., 2014, 2019; 任留东等,2016)。

茹尔群岛作为普里兹带的重要组成部分,自从含假蓝宝石超高温麻粒岩首次从该区报道以来(Harley and Fitzsimons, 1991),不同研究者对其变形-变质演化历史、超高温变质事件的时间及构造背景进行了研究,并得出了不同形式的进变质和峰期后演化的P-T轨迹,以及超高温变质事件发生于不同的时间和构造背景的结论(Dirks and Wilson, 1995; Sims and Wilson, 1997; Harley, 1998b; Tong and Wilson, 2006; Kelsey et al., 2007; Wang et al., 2007; Wilson et al., 2007; Harley et al., 2009; Hokada et al., 2016; Clark et al., 2019)。因此,对茹尔群岛超高温麻粒岩研究进展进行了概括性梳理和总结,并找出该区超高温麻粒岩研究中仍然存在的一些科学问题,希望可以作为下一步研究的方向。

1 区域地质背景

茹尔群岛位于东南极普里兹湾地区的东部,东北面距新太古代西福尔丘陵(Vestfold Hills)麻粒岩地体仅10 km(Harley, 1998b),西南面距普里兹带的布拉特滨海陡崖(Brattstrand Bluffs)约30 km(图 1a)。茹尔群岛是一个以正片麻岩为主的多期麻粒岩相变质地体,其岩石单元主要由活化的太古宙长英质正片麻岩、中元古代复合片麻岩及其中的层状副片麻岩组成,其中太古宙长英质正片麻岩单元中发育变形变质的多期元古代镁铁质岩墙群(Harley, 1987; Dirks et al., 1994; Sims et al., 1994)。活化的太古宙正片麻岩主要分布于茹尔群岛的东北部和西南部,而中元古代复合片麻岩则分布在其余的岛屿和半岛上(图 1a)。中元古代复合片麻岩主要由花岗质正片麻岩及其中的费拉副片麻岩单元(Filla Paragneiss)组成,花岗质正片麻岩具有1050~1000 Ma的SHRIMP锆石U-Pb年龄(Kinny et al., 1993)。活化的太古宙长英质正片麻岩被认为代表了茹尔群岛的古老基底,其上为中元古代的沉积盖层序列(费拉表壳岩),其正副片麻岩具有中元古代1520~1160 Ma的Sm-Nd模式年龄(Sheraton et al., 1984)。费拉副片麻岩单元主要由富铁的变泥质-变砂质副片麻岩、浅色片麻岩、含石榴子石的钙硅酸盐麻粒岩及少量镁铁质麻粒岩组成,较早期被认为在晚元古代的格林维尔事件(1100~1000 Ma)期间经历了一次主要的中压麻粒岩相构造变质事件,具有峰期变质条件为0.7~0.9 GPa、820~860 ℃,峰期后具有0.2~4 GPa减压的顺时针P-T演化轨迹(Harley, 1988; Harley and Fitzsimons, 1991)。

a—茹尔群岛岩石单元分布图(显示茹尔群岛(Rauer Group, RG)、西福尔丘陵(Vestfold Hills, VH)、拉斯曼丘陵(Larsemann Hills, LH)、布拉特滨海陡崖(Brattstrand Bluffs, BB)、姊妹岛(Søstrene Island, SI)和伯林根群岛(Bolingen Islands, BI)在普里兹湾的位置,断续线指示一条主要由500 m宽的面状高应变带定义的梅瑟剪切带);b—梅瑟半岛的岩石单元和超高温麻粒岩样品位置(断续线代表识别出的厘米级至米级高级剪切带);c—主要的片麻理的投影;d—主要的矿物拉伸线理的投影 (a) Lithological unit map of the Rauer Group. Insert shows the locations of the Rauer Group (RG), the Vestfold Hills (VH), the Larsemann Hills (LH), the Brattstrand Bluffs (BB), Søstrene Island (SI) and the Bolingen Islands (BI), the dashed line shows the major Mather Shear Zone defined by a ~500 m wide planar high-strain zone; (b) Lithological distribution and sample locations on Mather Peninsula, the dashed lines show representative recognized cm- to m-wide high-grade shear zones; (c) Stereographic projections for the major foliation orientations; (d) Mineral elongation lineations on Mather Peninsula 图 1 茹尔群岛与梅瑟半岛地质图(Tong and Wilson, 2006) Fig. 1 Geological maps of the Rauer Group and Mather Peninsula (Tong and Wilson, 2006)

茹尔群岛活化的太古宙正片麻岩具有3470 Ma、3270 Ma及2800 Ma的岩浆结晶年龄(Kinny et al., 1993; Harley et al., 1998),而一个变形的层状镁铁质侵入杂岩则具有2840 Ma的侵入年龄(Harley et al., 1998)。这些活化的太古宙正片麻岩被认为在2500 Ma前经历了一次高级变质作用(Harley and Kelly, 2007)。在太古宙的英云闪长质正片麻岩中分布着一些镁橄榄石大理岩、钙铁榴石矽卡岩、富镁铝的变泥质麻粒岩捕虏体或布丁,由于其经历了超高温变质作用且主要分布于梅瑟(又称马瑟)半岛(图 1b),被称为梅瑟副片麻岩(Mather Paragneiss)(Harley, 1998b)。梅瑟副片麻岩是一个分布不很广泛但却很重要的岩石单元,其岩石类型主要包括含石榴子石-斜方辉石-矽线石-假蓝宝石的泥质混合岩、相关富镁的石榴矽线变泥质麻粒岩、紫苏矽线石英岩、含斜方辉石浅色片麻岩、含石榴子石镁铁质麻粒岩、含石榴子石钙硅酸盐麻粒岩及石榴子石英岩等(图 2; Harley, 1998b; Tong and Wilson, 2006),大多以5 m×15 m大小的捕虏体或无根的褶皱或布丁产出于太古宙正片麻岩围岩之中(Harley, 1998b)。近年来,在托克勒岛又发现了含假蓝宝石-石英的超高温变质的泥质麻粒岩(Harley, 2008)。梅瑟副片麻岩以继承锆石具有太古宙的源区年龄而区别于中元古代的费拉副片麻岩(Harley, 2014)。

a—富镁铝的超高温含假蓝宝石变泥质麻粒岩和互层的紫苏矽线石英岩(参照物铅笔长约15 cm);b—富镁的石榴矽线泥质片麻岩及其中的粗粒含石榴子石浅色体(参照物地质锤长约30 cm);c—富铁铝的石榴矽线泥质片麻岩透镜体,发育明显东南向倾的矿物拉伸线理(L6)和片麻理(S6)(参照物记号笔长约14 cm);d—含石榴子石镁铁质麻粒岩布丁(参照物地质锤长约30 cm) (a) Mg-Al-rich UHT Spr-bearing metapelitic granulite and interlayered Opx-Sil-bearing quartzite, the pencil is 15 cm long as a scale; (b)Mg-rich Grt-Sil-bearing metapelitic gneiss and coarse-grained Grt-bearing leucosome, the hammer is 30 cm long as a scale; (c) Fe-Al-rich Grt-Sil-bearing metapelitic gneiss lens develop obvious SE-dipping mineral elongation lineation (L6) and foliation (S6), the marker is 14 cm long as a scale; (d) Grt-bearing mafic granulite boudin, the hammer is 30 cm long as a scale. 图 2 代表性超高温麻粒岩的野外照片 Fig. 2 Representative field photographs of the UHT granulites on Mather Peninsula

茹尔群岛具有典型的多期变质演化历史,最初被认为经历了至少四期高级构造变形过程,其中主要的变质变形事件(D1、D2、D3)被认为发生于镁铁质岩墙的侵入之前,而在麻粒岩相条件下镁铁质岩墙发生变形变质的事件被认为发生于D4期高级构造变形事件期间。D4期高级构造变形事件导致了茹尔群岛南部南倾的开阔到紧闭直立褶皱的形成,以及该区南北向和东西向直立的韧性剪切带的发育。D5期变形则与面状伟晶岩的侵入有关(Harley, 1987)。在基底片麻岩中的平卧等斜褶皱和陡倾的矿物拉伸线理被认为发生于岩墙侵入前的一期主要变形事件(D3)期间,并形成于晚元古代1030~1000 Ma(Harley and Fitzsimons, 1991)。之后进一步的构造研究显示,茹尔群岛的大部分构造与广泛的高级韧性剪切变形事件有关,发生于D6期构造变形事件期间,并导致了该区面状高应变带的广泛发育(Sims et al., 1994; Sims and Wilson, 1997)。在整个茹尔群岛,区域一致的向东南倾向的矿物拉伸线理普遍平行于褶皱轴和布丁颈(Dirks and Wilson, 1995)。早期的构造一般作为低应变带中重复褶皱的镁铁质麻粒岩和长英质片麻岩层被局部保留,而几乎所有早期的面理在面状高应变带中或多或少都被强烈置换,其置换程度从厘米级到千米级变化(Dirks et al., 1994; Sims et al., 1994)。

由于茹尔群岛太古宙基底岩石比相邻的新太古代西福尔丘陵具有更老的结晶形成年龄(Kinny et al., 1993; Harley et al., 1995, 1998),二者具有不同的太古宙地质演化历史,因此认为茹尔群岛太古宙基底岩石是不同于西福尔丘陵的地体单元,称为茹尔地体(Rauer Terrane)(Harley and Kelly, 2007)。茹尔群岛中元古代费拉副片麻岩经历的一期主要的晚元古代(~1000 Ma)麻粒岩相高级构造变质事件(Kinny et al., 1993; Harley et al., 1998),又被称为茹尔构造事件(Rauer Tectonic Event),而叠加的早古生代泛非期(~530 Ma)构造变质事件则被称为普里兹构造事件(Prydz Tectonic Event;Harley and Kelly, 2007)。

2 超高温麻粒岩矿物组合与结构特征

茹尔群岛典型的超高温麻粒岩主要包括富镁铝的含假蓝宝石变泥质麻粒岩、不含假蓝宝石的石榴紫苏堇青变泥质麻粒岩、石榴矽线变泥质片麻岩、紫苏矽线石英岩和含石榴子石镁铁质麻粒岩等,并发育一系列变质反应结构,不同研究者已对其进行了详细的研究和报道(包括会议论文;Harley and Fitzsimons, 1991; Harley, 1998b; Kelsey et al., 2003Tong and Wilson, 2006; Kelsey et al., 2007; Harley et al., 2009; Clark et al., 2019),这里列出一些代表性的变质反应结构(图 3),对其矿物组合和变质反应结构分别进行概括描述。

a—石榴子石变斑晶中的斜方辉石、矽线石、黑云母及斜长石等包裹体;b—矽线石集合体(蓝晶石假象)周围发育的峰期后假蓝宝石和堇青石后成合晶组合;c—浅色体中粗粒的假蓝宝石、斜方辉石及钾长石组合(视域宽2.0 mm);d—石榴子石变斑晶周围发育的典型的峰期后斜方辉石和假蓝宝石后成合晶组合;e—石榴子石变斑晶中的蓝晶石包裹体(视域宽1.0 mm);f—具有蓝晶石假象的矽线石变斑晶显示扭折变形特征;g—紫苏矽线石英岩中,富铝紫苏辉石变斑晶周围发育的峰期后堇青石和石英后成合晶组合;h—含石榴子石镁铁质麻粒岩中,单斜辉石周围的斜方辉石和斜长石反应边组合,石榴子石变斑晶周围的斜方辉石、斜长石及磁铁矿后成合晶组合 (a) Opx-Sil-Bt-Pl inclusions in garnet porphyroblast; (b)Post-peak Spr-Crd symplectite assemblage around Sil aggregates (Ky pseudomorph); (c)Coarse-grained Spr-Opx-Ksp assemblage in leucosome (sight width 2.0 cm); (d)Typical post-peak Spr-Opx symplectite assemblage on garnet porphyroblast; (e) Ky inclusions in garnet porphyroblast (sight width 1.0 cm); (f) Sil porphyroblast with Ky pseudomorph shows features of kink deformation; (g)Post-peak Crd-Qtz symplectite assemblage on Al-rich Opx prophyroblast in Opx-Sil-bearing quartzite; (h)Opx-Pl corona assemblage on Cpx grain and Opx-Pl-Mt symplectite assemblage on Grt prophyroblast in Grt-bearing mafic granulite 图 3 超高温麻粒岩中代表性的镜下显微照片 Fig. 3 Representative microphotographs of the UHT granulites on Mather Peninsula

含假蓝宝石变泥质麻粒岩:在梅瑟半岛,该岩石主要由石榴子石(Grt)、斜方辉石(Opx)、矽线石(Sil)、假蓝宝石(Spr)、堇青石(Crd)、钾长石(Ksp)、斜长石(Pl)、黑云母(Bt)及少量尖晶石(Spl)等组成,矽线石集合体显示蓝晶石(Ky)假象特征,不含石英(Qtz),因此为硅不饱和麻粒岩。石榴子石变斑晶中含有小的斜方辉石、定向的矽线石与黑云母等包裹体(图 3a),指示其代表峰期前的矿物组合。样品tlx61中的矽线石集合体显示蓝晶石假象(图 3b),表明峰期前到峰期的进变质阶段发生了蓝晶石向矽线石的转变反应:Ky→Sil。样品tlx20的浅色体中出现粗粒的假蓝宝石(图 3c),说明其为峰期矿物组合(Tong and Wilson, 2006),而不是之前认为的是峰期后的变质组合(Harley and Fitzsimons, 1991; Harley, 1998b)。因此,其峰期超高温变质矿物组合为粗粒的Grt-Opx-Sil-Spr-Ksp。矽线石集合体周围发育假蓝宝石和堇青石后成合晶(图 3b),石榴子石变斑晶周围发育斜方辉石和假蓝宝石后成合晶(图 3d),这两个结构指示峰期后经历了典型的减压过程,其变质反应分别为:Sil→Spr+Crd, Grt→Opx+Spr。另外,石榴子石变斑晶周围还发育斜方辉石、假蓝宝石和矽线石,斜方辉石和堇青石后成合晶(Harley, 1998b),同样指示了峰期后的减压过程,其变质反应分别为:Grt→Opx+Spr+Sil, Grt→Opx+Crd。此外,一些黑云母周围还发育晚期斜方辉石反应边和粒间钾长石,可能反映了退变质的冷却或减压熔融过程。在托克勒岛的超高温麻粒岩产地,Harley et al. (2009)发现在含假蓝宝石变泥质麻粒岩中,石榴子石变斑晶里蓝晶石假象周围发育假蓝宝石和石英组成的后成合晶结构,指示了进变质阶段蓝晶石向矽线石的转变:Ky→Sil,以及峰期典型超高温变质组合的形成:Grt+Sil→Spr+Qtz,由于该岩石含石英,因此属于硅饱和的麻粒岩。

石榴紫苏堇青变泥质麻粒岩:在兰尼岛,该岩石主要由石榴子石、斜方辉石、堇青石、黑云母、斜长石、石英等组成,含有少量金红石、钛铁矿等,虽然不含矽线石和假蓝宝石,但石榴子石-斜方辉石矿物对温压计算、金红石中Zr温度计算结果及相图模拟显示其经历了峰期超高温(>900 ℃)变质作用过程(Clark et al., 2019)。

石榴矽线变泥质片麻岩:分为富镁的和富铁的两类,主要由石榴子石、蓝晶石及其假象、矽线石、斜长石、钾长石、石英等组成,含有少量的黑云母、堇青石、尖晶石、金红石、钛铁矿和石墨等,前者含有比后者更多的金红石,后者比前者更粗粒并以团块或透镜体位于前者之中(图 3c)。在样品tlx1a中,石榴子石变斑晶里存在蓝晶石包裹体(图 3e),指示峰期前的进变质阶段位于蓝晶石稳定域。在样品tlx67中,具有蓝晶石假象的粗粒的矽线石变斑晶显示特征的扭折变形特征(图 3f),其周围含有尖晶石和堇青石的反应边,指示峰期后的减压过程:Sil→Spl+Crd。

紫苏矽线石英岩:主要由斜方辉石、矽线石、石英等组成,含有少量的斜长石、钾长石、堇青石和黑云母,具有典型的峰期超高温变质矿物组合Opx-Sil-Qtz。在斜方辉石周围发育细粒的堇青石和石英后成合晶组合(图 3g),指示峰期后典型的减压过程,其减压变质反应为:Opx+Sil→Crd+Qtz。

含石榴子石镁铁质麻粒岩:主要由石榴子石、单斜辉石、斜方辉石、角闪石(Amp)、斜长石等组成,含有少量的石英、黑云母、钾长石、不透明矿物磁铁矿(Mt)等,矿物结构关系指示峰期矿物组合为Grt-Cpx-Opx-Amp-Pl-Qtz。在样品tlx6a中,粗粒的单斜辉石周围发育相对粗粒的斜方辉石和斜长石反应边结构,而石榴子石变斑晶周围发育相对细粒的斜方辉石、斜长石及磁铁矿后成合晶结构(图 3h),前者指示了一个前进的加热或减压加热相关的变质反应过程:Grt+Cpx+Amp→Opx1+Pl+熔体,而后者指示了一个典型的峰期后减压变质反应:Grt+Qtz→Opx2+Pl+Mt。

3 超高温变质条件与变质P-T轨迹

关于茹尔群岛超高温麻粒岩的变质P-T轨迹,迄今为止,不同研究者给出了具有不同超高温峰期条件、峰期前及峰期后演化历史、不同形式的顺时针变质P-T轨迹(图 4)。例如,Harley and Fitzsimons(1991)最初报道了梅瑟半岛具有非常高温变质条件的含假蓝宝石变泥质麻粒岩残留体,其峰期变质条件为1.0~1.2 GPa和1000~1050 ℃,并具有峰期后高温和中温后两次减压的P-T演化过程(图 4; Harley and Fitzsimons, 1991)。之后,Harley(1998b)在石榴子石变斑晶中斜方辉石包裹体的保留的成分环带得出超高温峰期前具有一个进变质阶段的升温升压的过程,整体具有一个峰期后等温减压的顺时针P-T演化轨迹(图 4)。但是,Kelsey et al.(2003)的研究则认为峰期超高温变质条件没有那么高,只达到了950~970 ℃和1.0~1.1 GPa,并具有一个简单的峰期后减压冷却的P-T演化轨迹(图 4)。然而,Tong and Wilson(2006)通过对含假蓝宝石变泥质麻粒岩、石榴矽线变泥质片麻岩及含石榴子石镁铁质麻粒岩的详细的野外和室内岩相学研究,发现超高温变质峰期前存在早期进变质阶段的蓝晶石假象、包裹体以及峰期形成的粗粒的假蓝宝石矿物,认为峰期前进变质阶段的升温升压发生在蓝晶石稳定域,具有峰期温压条件1.2 GPa和960~970 ℃及峰期后温压条件1.0 GPa和1000 ℃,经历了一个先减压加热再减压冷却的顺时针P-T演化轨迹(图 4)。而Kelsey et al.(2007)的进一步研究仍然认为峰期条件为950 ℃和1.0 GPa,但是具有一个由进变质阶段升温升压、峰期后减压冷却组成的紧闭的发卡型的顺时针P-T演化轨迹(图 4)。另外,Harley et al.(2009)对托克勒岛新发现的含假蓝宝石和石英的超高温泥质麻粒岩的初步研究结果显示,峰期超高温变质条件为1.3~1.4 GPa和~1000 ℃,其中石榴子石里的蓝晶石假象指示峰期前进变质阶段位于蓝晶石稳定域,因此,Harley (2016)修改了其之前的P-T轨迹,认为进变质阶段的升温升压最初从矽线石稳定域进入蓝晶石稳定域,之后在超高温阶段又进入矽线石稳定域(图 4)。而最近,Clark et al.(2019)对兰尼岛不含假蓝宝石超高温变泥质麻粒岩的研究结果显示,其峰期变质条件为0.9 GPa和910~950 ℃,并具有一个先等压加热后等温减压再等压冷却的顺时针P-T演化轨迹(图 4)。

图 4 茹尔群岛超高温麻粒岩不同研究者得出的不同形式的变质P-T演化轨迹 Fig. 4 Various metamorphic P-T paths derived by different researchers for the UHT granulites in the Rauer Group
4 超高温变质事件发生的时间和构造背景

对茹尔群岛的超高温麻粒岩变质作用发生的时间及其大地构造背景的认识一直存在较大分歧。例如,茹尔群岛和普里兹湾沿岸其他露头最早一起被认为是属于东南极几个太古宙陆核(如纳皮尔杂岩、西福尔丘陵及南查尔斯王子山(southern Prince Charles Mountains))之间广泛的晚元古代格林维尔期高级活动带的一部分(Sheraton et al., 1984; Black et al., 1987; Fitzsimons and Harley, 1991; Tingey, 1991)。茹尔群岛麻粒岩峰期后减压的变质P-T轨迹反映了晚元古代(~1000 Ma)增厚地壳伸展垮塌的构造过程(Harley, 1988)。同一时期,Harley and Fitzsimons(1991)报道了梅瑟半岛含假蓝宝石变泥质麻粒岩残留体,并推其经历的非常高温条件的构造变质事件可能发生于中元古代之前更早的新太古代时期。

之后,广泛的构造-变质和年代学研究显示了拉斯曼丘陵和普里兹湾沿岸其他露头的大部分高级构造作用和变质作用是发生于早古生代的泛非期(~530 Ma)(Ren et al., 1992; 赵越等, 1993; Dirks et al., 1993; Zhao et al., 1995; 刘小汉等, 1995; Dirks and Wilson, 1995; Hensen and Zhou, 1995; Carson et al., 1996Carson et al., 1997; Fitzsimons et al., 1997),而这导致了对茹尔群岛超高温麻粒岩的重新研究。例如,Hensen and Zhou(1997)认为太古宙岩石缺乏~1000 Ma事件的记录,他们和Sims and Wilson(1997)均认为该区超高温变质事件可能发生于早古生代泛非事件期间,并与冈瓦纳大陆聚合有关。而Harley(1998b)对该区超高温麻粒岩的进一步研究认为,超高温变质事件既可能发生于新太古代,也可能是晚元古代(~1000 Ma)或早古生代(~530 Ma),其变质P-T历史是不清楚的。Kelsey et al.(2003)则根据超高温麻粒岩变质P-T轨迹的研究,认为之前的增厚地壳伸展垮塌的构造模型需要修改,并推测该区所有麻粒岩相变质作用可能发生于早古生代泛非期。

然而,Tong and Wilson(2006)通过研究发现,该区超高温峰期前存在早期进变质阶段高压蓝晶石矿物残留体,通过详细区域对比认为早期进变质阶段及峰期超高温变质事件应发生于晚元古代的格林维尔事件(~1000 Ma)期间,并可能与晚元古代的碰撞造山和高温弧岩浆作用有关,而叠加的中温中压变质事件应发生于早古生代泛非期(~530 Ma)。之后,Wang et al. (2007)从超高温含假蓝宝石麻粒岩里锆石的幔部获得的格林维尔期锆石U-Pb年龄为~995 Ma,与锆石边部获得的泛非期年龄(~530 Ma)进一步支持了该看法。与此同时,Kelsey et al.(2007)对梅瑟超高温变质副片麻岩和费拉副片麻岩进行了独居石Th-Pb定年,从前者没有获得格林维尔期年龄数据,于是认为超高温变质事件仍然发生于早古生代(540~520 Ma);从后者获得了两期变质年龄数据,则认为后者经历了两期构造变质事件。但与此同时,Wilson et al.(2007)通过单矿物Ar-Ar年龄研究,认为该区及普里兹带早古生代高级构造变质事件代表了一期广泛的板内造山和陆内剪切带的发育。另外,Harley et al.(2009)对托克勒岛(Torckler Island)发现的超高温含假蓝宝石麻粒岩中的独居石进行了EPMA定年,认为茹尔群岛超高温变质事件可能发生于~590 Ma期间,早于普里兹带早古生代高级变质事件的时间(~530 Ma)。Hokada et al.(2016)对梅瑟超高温变质副片麻岩又进行了锆石和独居石的U-Th-Pb年龄测定,尽管获得了几个900~700 Ma的锆石年龄,但也认为由于太古宙围岩没有晚元古代~1000 Ma事件记录,该区超高温变质事件可能发生于590~580 Ma期间。最近,Clark et al.(2019)对不含假蓝宝石的超高温麻粒岩中的锆石和独居石也进行了U-Pb年龄测定,获得了514 Ma的年龄结果,认为其偏年轻可能是受到了超高温事件影响,且超高温变质事件可能发生于早古生代普里兹构造事件(~530 Ma)期间。

5 总结和认识

综上所述,位于东南极普里兹带东部边缘的茹尔群岛是一个不同于西福尔丘陵的、典型的、多期变质的复合高级地体(茹尔地体),经历了新太古代、晚元古代格林维尔(~1000 Ma)(茹尔构造事件)和早古生代泛非(~530 Ma)(普里兹构造事件)多期变质作用(Harley and Kelly, 2007)。茹尔群岛超高温麻粒岩(梅瑟副片麻岩)保留了一系列进变质、峰期超高温变质、峰期后减压和冷却等多阶段矿物组合与反应结构(Harley, 1998b; Tong and Wilson, 2006; Kelsey et al., 2007)。至目前,不同研究者得出了不同的超高温峰期变质P-T条件、不同的峰期前和峰期后演化过程的变质P-T轨迹。对超高温变质事件发生的时间及构造背景的认识也存在较大分歧,一种观点认为茹尔群岛超高温麻粒岩变质事件发生于泛非期(~530 Ma或~590 Ma; Kelsey et al., 2007; Harley et al., 2009; Hokada et al., 2016; Clark et al., 2019),并且只经历了一个单一的泛非期与碰撞构造有关的P-T轨迹。另一种观点则认为,茹尔群岛超高温含假蓝宝石变泥质麻粒岩和镁铁质麻粒岩的变质P-T演化轨迹由两个高级构造变质事件组成,进变质到超高温阶段可能与早期格林维尔期(~1000 Ma)碰撞造山及弧岩浆作用有关,而叠加的中温中压退变质阶段应反映了早古生代泛非期(~530 Ma)高级构造变质事件(Tong and Wilson, 2006; Wang et al., 2007)。

另外,茹尔群岛具有中元古代原岩年龄的富铁铝的费拉副片麻岩和具有太古宙原岩年龄的富镁铝的梅瑟副片麻岩,二者被认为经历了相似的超高温变质P-T轨迹(Kelsey et al., 2003; Tong and Wilson, 2006)。独居石年龄指示前者经历了晚元古代格林维尔期(~1000 Ma)和早古生代泛非期(~530 Ma)两期构造变质事件(Harley, 2003; Kelsey et al., 2007)。而近年来的研究显示,在茹尔群岛东面相邻的新太古代西福尔丘陵,中元古代侵入的镁铁质岩墙在早新元古代(960~940 Ma)经历了麻粒岩相高级构造变质事件,证明部分新太古代西福尔丘陵也卷入了格林维尔期高级构造变质事件(Liu et al., 2014)。这说明西福尔丘陵和茹尔地体都经历了格林维尔期高级构造变质事件。然而到目前,太古宙岩石中似乎没有明确的格林维尔期的年龄记录(Kinny et al., 1993; Harley et al., 1998; Hokada et al., 2016),只有含假蓝宝石超高温麻粒岩中锆石幔部有该期的年龄记录(Wang et al., 2007)。

因此,尽管目前取得不少进展,茹尔群岛超高温变质事件发生的时间、超高温麻粒岩的变质P-T演化轨迹及其大地构造背景仍一直存在很大争议。茹尔群岛位于中国中山站所处的拉斯曼丘陵以东约100 km处,是国际上研究超高温麻粒岩及其大地构造背景的理想天然实验室。所以,为理清该区超高温麻粒岩的变质演化历史和构造背景,需要对其进一步进行详细深入的野外地质构造关系、室内矿物组合-变质结构分析、P-T轨迹重建及高精度锆石-独居石U-Pb年代学研究,并进行区域上对比。例如,该区含假蓝宝石等超高温麻粒岩和太古代围岩片麻岩是否经历了格林维尔期(~1000 Ma)的构造变质事件?超高温麻粒岩中独居石记录的年龄(590~580 Ma)是反映一个更早的不同于普里兹构造事件(~530 Ma)的独立事件,还是属于普里兹造山事件的一部分?茹尔群岛含石榴石子镁铁质麻粒岩的超高温变质作用是否可以和姊妹岛(Søstrene Island)和拉斯曼丘陵含石榴石子镁铁质麻粒岩记录的峰期超高温变质作用进行对比(Thost et al., 1991; Tong et al., 2019)?这些科学问题对认识和理解普里兹带东部边缘的构造性质、该区超高温麻粒岩完整的变质P-T演化历史、罗迪尼亚和冈瓦纳超大陆的形成以及聚合过程具有极其重要的科学意义。

致谢: 感谢国家海洋局极地考察办公室和中国极地研究中心在第27次和第33次南极考察期间给与的后勤支持和保障。感谢特邀主编刘晓春研究员盛情邀约撰写本文。感谢任留东研究员和王伟研究员的仔细审阅和建设性的修改意见。

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