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新生代以来广泛发育于青藏高原的各类火山岩(图1a)是印度-亚欧板块碰撞过程中物质与能量调节的最主要表现形式(Wang Qiang et al., 2008)。大规模后碰撞火山岩的形成及演化过程常与青藏高原岩石圈结构调整、地壳加厚、中地壳物质流动及地表隆升等关键科学问题密切相关(Chung Sunlin et al., 1998; Wang Qiang et al., 2016; Guo Zhengfu et al., 2019)。其中羌塘地体北缘中段火山岩出露区因与中-下地壳S波低速带重叠而备受关注(Yang Yingjie et al., 2012; Harker et al., 2014; Wang Qiang et al., 2016; 图1b)。然而,以往对此区域火山岩研究大多数集中于岩相学、显微结构、岩石地球化学方面(Guo Zhengfu et al., 2006, 2019; Wang Qiang et al., 2008; Lai Shaocong et al., 2013),而火山岩中包体的研究相对较少。火山岩中深源捕掳体研究是了解地壳深部物质组成、矿物相态、地球化学模型和热状态的天然岩石探针(Mo Xuanxue, 2009),具有极为重要的研究价值。目前羌塘地体捕掳体研究亦多集中于始新世火山岩内(Li Baohua et al., 2003; Lai Shaocong et al., 2006, 2008),而渐新世以来强烈的构造及火山活动对青藏高原中部地壳结构强烈改造作用使得之前的捕掳体温压计算很难反映目前观察到的地球物理异常,仅有的上新世火山岩内捕掳体为变质程度较深的下地壳麻粒岩(Hacker et al., 2000),而对长英质岩石并无涉及。因此,本文选取青藏高原中部羌塘地体北缘上新世恒梁湖粗安质火山岩内含石榴子石长英质岩石捕掳体为研究对象,在对捕掳体内石榴子石与火山熔体间反应结构以及火山岩斑晶环带进行详细的岩相学观察的基础上,通过多种火山岩矿物化学研究限定其形成的温度和压力条件,进而为约束青藏高原中、下地壳物质组成、矿物相态及热结构提供岩石学依据。
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1 地质背景
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羌塘地块北部以金沙江缝合带与松潘-甘孜地块相邻,南部以班公湖-怒江缝合带与拉萨地块相邻(图1a; Yin An et al., 2000)。地块中部以龙木错-双湖构造带为界又将其分为南羌塘和北羌塘地体(Li Cai et al.,2008)。青藏高原内部发育多阶段的后碰撞火山岩(Chung Sunlin et al., 1998),其中北羌塘多格错仁—枕头崖地区是青藏高原中部出露面积最大的碰撞后火山岩区(Deng Wanming1998; Mo Xuanxue et al., 2009)。研究区火山岩时代可分为始新世(45~33Ma)和中—上新世(5.8~2.5Ma)两个阶段(Dong Yanhui, 2008; Wang Qiang et al., 2008, 2016; Lai Shaocong et al., 2013),其中始新世火山岩主要分布在多格错仁、桌子山、黑虎岭、乱青山、枕头崖及萨宝等区域,岩性以英安岩和安山岩为主,局部发育火山角砾岩,多具有钾玄质或高钾钙碱性性质。中—上新世火山岩在羌塘地块分布面积相对较小,主要分布在太平湖、恒梁湖及东月湖一带,岩性以辉石安山岩、玄武安山岩和粗面岩为主(Mo Xuanxue et al., 2009),部分具有埃达克质特征(Wang Qiang et al., 2016)。恒梁湖火山岩位于此岩区的北缘,出露面积约1350km2,为出露面积最大的上新世火山岩(图2)(Law et al., 2020)。地球物理研究表明北羌塘北缘发育强烈的中-下地壳(20~50km)面波低速与强各向异性异常(Rapine et al., 2003; Duret et al., 2010; Yang Yingjie et al., 2012; Hacker et al., 2014; 图1b),且低速异常中心与区域性火山岩发育范围一致(Lai Saocong et al., 2013; Wang Qiang et al., 2016)。火山岩内发育多种类型的下地壳捕掳体,主要有二辉麻粒岩、紫苏辉石麻粒岩、单斜辉石麻粒岩、长英质麻粒岩等(Jiang Wan et al., 2000; Lai Shaocong et al., 2006)。
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图1 青藏高原大地构造简图和火山岩分布(a,据Chung Sunlin et al., 2005修改)及青藏高原中地壳(30km)横波波速特征(b,据Hacker et al., 2014)
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Fig.1 Sketch tectonic map and the volcanic rocks distribution of the Tibetan Plateau (a, modified from Chung Sunlin et al., 2005), and surface wave characteristics of middle crust (30km) of the Tibetan Plateau (b, after Hacker et al., 2014)
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恒梁湖火山岩主要由陆相中心式溢流、部分为裂隙式溢流火山喷发形成,多以熔岩被或熔岩残丘的形式产出。火山岩不整合覆盖于上三叠统藏夏河群复理石沉积岩,中上侏罗统雁石坪群砂岩、杂砂岩、泥岩及灰岩或第三系唢呐湖组之上(Jin Canhai et al., 2010)。恒梁湖北部粗面岩40Ar/39Ar法坪年龄为5.81±0.06Ma,恒梁湖南部粗面安山岩锆石LA-ICPMS U-Pb年龄为4.8±0.1Ma(Dong Yanhui, 2008),粗面岩Ar-Ar年龄3.2~2.5Ma(Jin Canhai et al., 2010)。
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2 岩相学
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恒梁湖火山岩多呈灰黑色,气孔构造或杏仁构造, 玻基斑状结构或玻晶交织结构。斑晶以斜方辉石、单斜辉石、斜长石、钛铁矿为主。长英质捕掳体与火山岩熔体间界线平直或局部呈港湾状(图3a、b),无明显反应结构。石榴子石仅在捕掳体内呈他形产出,粒径约0.5~4mm,石榴子石内包裹体较少,以石英和锆石为主(图4a、b)。整体观察表明,熔/流体主要沿捕掳体裂隙与石榴子石发生交代反应,背散射图像下石榴子石边缘呈指纹状合晶结构(图4b),分解产物为斜方辉石和斜长石(图4a~c)。斜方辉石周边多发育珍珠状裂纹,可能指示交代反应后熔体快速冷却的过程。斜方辉石发育明显的成分环带或被单斜辉石包裹(图4d、e)。
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长英质矿物在火山岩基质与捕掳体内存在明显的形态及显微结构上的差异,其中火山岩基质内石英捕掳晶与熔体相互作用,发育明显的反应边结构,并形成单斜辉石,周边可见针状磷灰石,反映熔体成分较为富钙(图4d)。火山岩基质内长石颗粒则较为自形,发育熔蚀结构或具有明显的成分环带。捕掳体内长石主要呈他形产出且粒径明显增大(图4c、f)。捕掳体内钛铁矿多产于石榴子石分解结构边缘。
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3 矿物化学
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本文重点对含石榴子石长英质捕掳体内石榴子石反应结构及火山岩内矿物斑晶开展电子探针矿物化学分析,实验在中国地质科学院地质研究所自然资源部深地动力学重点实验室完成,所用仪器为日本电子株式会社JEOL生产的JXA-8100型号探针,实验条件为电压15kV,束斑大小为2~5 μm,每点测试时长为3min,所有元素均采用Kα射线分析结果,所有矿物均采用Ax软件(https://filedn.com/lU1GlyFhv3UuXg5E9dbnWFF/TJBH pages/ax.html)进行离子价位配平和Fe3+含量计算。
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图2 藏北多格错仁—枕头崖—恒梁湖地区地质简图(据1∶25万多格错仁、岗扎日、黑虎岭、玉帽山幅修改, 年龄据Wang Qiang et al., 2008, 2016)
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Fig.2 The simple geological map of Duogecuoren-Zhentouya-Helianghu area, northern Tibetan Plateau (modified after 1∶25000geological maps of Duogecuoren, Gangzhari, Heihuling and Yumaoshan, the volcanic ages are after Wang Qiang et al., 2008, 2016)
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图3 恒梁湖火山岩内含石榴子石长英质岩石捕虏体单偏光照片
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Fig.3 Plane-polarized light images of the garnet-bearing quartzofeldspathic xenolith in the Henglianghu volcanic rock
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(a)—火山熔体沿长英质捕掳体矿物边界与石榴子石反应;(b)—长英质捕掳体平直边界及火山岩基质内斜方辉石; Grt—石榴子石;Pl—斜长石;Qz—石英;Opx—斜方辉石
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(a)—Volcanic melt reaction with garnet along mineral boundaries of quartzofeldspathic xenolith; (b)—straight boundary ofquartzofeldspathic xenolith and orthopyroxene in volcanic matrix; Grt—garnet; Pl—plagioclase; Qz—quartz; Opx—orthopyroxene
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3.1 石榴子石
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长英质捕掳体内石榴子石主要以铁铝榴石和镁铝榴石组分为主,两者摩尔百分含量均大于40%,其中石榴子石核部成分为Alm46.3~47.6Prp42.3~43.5Grs6.4~6.9Sps3.7~3.8,边部成分为Alm41.8~48.3Prp41.3~46.2Grs6.2~8.3Sps3.6~3.9,且铁铝榴石含量略高于镁铝榴石,电子探针结果显示,石榴子石整体上边部与核部成分差异不大,仅边部成分变化范围略大(附表1,http://www.geojournals.cn/dzxb/ch/reader/view_abstract.aspx?file_no=202200798&flag=1,图5a)。
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3.2 斜长石
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恒梁湖火山岩内斜长石分为三类(附表2,http://www.geojournals.cn/dzxb/ch/reader/view_abstract.aspx?file_no=202200798&flag=1,图5b),即石榴子石反应边周围与斜方辉石共生的斜长石(Plg),长英质捕掳体内斜长石(Pli)以及火山岩基质内斜长石(Plm)。同时,对部分基质内斜长石斑晶背散射下亮色核部(Plmc)成分亦进行了电子探针分析。其中基质内斜长石核部明显具有更高的An值(67~70),属于拉长石或倍长石。基质内斜长石边部An值为31~39,属于中长石。长英质捕掳体内斜长石成分较为集中,石榴子石反应边存在透长石和更长石两种长石,其中更长石An值为24~27,透长石Or值为61~67。
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3.3 辉石
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火山岩内辉石具有多样的产出位置和成分特征,其中斜方辉石和单斜辉石电子探针成分分别见附表3和附表4(http://www.geojournals.cn/dzxb/ch/reader/view_abstract.aspx?file_no=202200798&flag=1),辉石分类图解见图5c。
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3.3.1 斜方辉石
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根据火山岩内斜方辉石产出位置将其分为:捕掳体内石榴子石周边细粒斜方辉石(Opxg),基质内较自形斜方辉石(Opxm)及被单斜辉石包裹的斜方辉石(Opxc),部分斜方辉石发育成分环带,可分为核部(Opxmc)和边部(Opxmr)。火山岩基质内被单斜辉石包裹的斜方辉石MgO含量最高,FeO含量最低,分别为26.21%~26.58%和13.95%~14.24%,对应的Mg#值(Mg2+×100/( Mg2++ Fe2+))为76.6~77.2,其Al2O3含量为2.23%~2.57%,属于古铜辉石;而捕掳体内石榴子石周边斜方辉石FeO和Al2O3含量最高,MgO含量最低,分别为26.48%~28.84%、3.02%~5.02%和14.00%~15.85%,对应Mg#值为46.4~51.6;属铁紫苏辉石;基质内较自形的斜方辉石FeO和MgO含量介于上述两者之间,分别为19.25%~23.38%和19.79~22.22%,Mg#值为60.1~67.2,具有最低的Al2O3含量(0.89%~2.10%),属紫苏辉石。且大多数斜方辉石显示正环带特征,即背散射图像下斜方辉石亮度较高的边部与核部相比具有相对较高的FeO和较低的MgO含量,指示熔体温度降低过程(附表3)。
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图4 羌塘北缘恒梁湖上新世火山岩内石榴子石长英质岩石捕虏体及捕虏晶背散射图像
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Fig.4 Back-scattering images of garnet-bearing quartzofeldspathic xenolith and phenocrysts in the Henglianghu Pilocene volcanic rock, north margin of Qiangtang terrane
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(a)—石榴子石长英质岩石捕虏体内石榴子石与熔体反应形成斜方辉石;(b)—石榴子石边部指纹状合晶结构,斜方辉石周边发育珍珠状裂纹,(a)图中红色方框范围;(c)—长英质岩石中长石与火山岩熔体接触;(d)—石英捕掳晶与火山熔体反应形成单斜辉石,细粒斜方辉石发育暗色核部和明亮边部;(e)—单斜辉石内包裹斜方辉石,单斜辉石发育成分环带;(f)—火山岩基质内单斜辉石与斜长石斑晶平直边界,斜长石发育明显成分环带(彩色图像为对应背散射离散色图);Grt—石榴子石;Pl—斜长石;Qz—石英;Opx—斜方辉石;Cpx—单斜辉石;Ilm—钛铁矿;Zrn—锆石;Ap—磷灰石
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(a)—Garnet and volcanic melt interaction forms orthopyroxene; (b)—symplektite textures surround garnet rim and pearl cracks surround fine grained orthopyroxene (red box in Fig.4a); (c)—plagiocalse in quartzofeldspathic xenolith and interaction with melt; (d)—quartz xenocryst interaction with volcanic melt forms clinopyroxene; (e)—orthopyroxene included by clinopyroxne and the later show distinct compositional zoning; (f)—straight boundary between clinopyroxene and zoned plagioclase in volcanic matrix (the small colored image in each figure is the discrete color back-scattering image); Grt—garnet; Pl—plagioclase; Qz—quartz; Opx—orthopyroxene; Cpx—clinopyroxene; Ilm—ilmenite; Zrn—zircon; Ap—apatite
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3.3.2 单斜辉石
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火山岩内单斜辉石属于普通辉石和透辉石(图5c),根据单斜辉石的产出位置分为:石英捕掳晶周边的他形单斜辉石(Cpxq),基质内与长石共生的单斜辉石(Cpxp)及包裹斜方辉石的单斜辉石。单斜辉石幔部(Cpxom)和边部(Cpxor)成分具有明显的差异。其中Cpxom具有最高的MgO含量,为15.15%~15.78%,Mg#值为78.8~80.0,其Al2O3含量仅次于与斜长石伴生的单斜辉石,为2.98%~3.50%;Cpxq则具有最高的Al2O3含量,两个位置测试分析值均为4.93%;Cpxor和Cpxq具有较低的Mg#值(61.3~70.1),MgO含量分别为12.79%~14.07%和12.76%~13.19%,其中Cpxq在所有类型中具有最低的Al2O3含量,为0.58%~0.80%。
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3.4 钛铁矿
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本文仅对捕掳体内3颗钛铁矿进行了电子探针分析(附表4;图5d)。钛铁矿含少量Mg和Mn,其中MgTiO3组分含量为8.0%~19.7%,MnTiO3组分含量为4.2%~4.8%,较高的MgTiO3组分含量可能指示其形成温度较高(Ghiorso et al., 1991)。
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4 温压计算
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应用PTQuick软件中石榴子石-斜方辉石压力计(Nikitina et al., 2010)、石榴子石-斜方辉石-斜长石-石英压力计(Perkins et al., 1985; Eckert et al., 1991)与二辉石温度计(Wood et al., 1973; Well, 1977; Bertrand et al., 1985; Putorka, 2008)对含石榴子石周边的相变结构进行综合计算,获得温压范围为950~1050℃,1.30~1.66GPa (图6)。另外,依据火山岩950~1050℃的温度,对基质内相接触的单斜辉石和斜长石较为富钠的边部(图4f)应用单斜辉石-斜长石压力计(McCarthy et al., 1998)计算,获得1.26~1.33GPa的压力范围。应用石榴子石-斜长石-矽线石-石英压力计(Newton et al., 1981; Holdaway, 2001),对捕掳体内石榴子石核部成分与大颗粒长石进行压力估算,获得1.18~1.53GPa压力范围。
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图5 恒梁湖火山岩内矿物化学成分图解
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Fig.5 Mineral chemical features in the Henglianghu volcanic rocks
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(a)—石榴子石;(b)—斜长石;(c)—辉石;(d)—钛铁矿;图中各矿物分类与附表1~4相同
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(a)—Garnet; (b)—plagioclase; (c)—pyroxene; (d)—ilmenite; minerals classification are same with Appendix 1~4
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5 讨论
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青藏高原加厚陆壳中、下部的物质组成、热状态以及局部熔融程度多通过地震波结果进行反演和推测(Yang Yingjie et al., 2012; Hacker et al., 2014)。而对于青藏高原中部的北羌塘地块,多种地球物理观测手段皆揭示其中下地壳存在或曾经发生过部分熔融(Hacker et al., 2014)。然而,由于深源样品难以获得,迄今为止青藏高原还未建立基于岩相学和微观结构基础上的中、下地壳岩石学结构剖面(Lai Shaocong et al., 2006)。以往捕虏体岩石学结构与成分研究多集中在始新世火山岩内(Li Baohua et al., 2003; Lai Shaocong et al., 2006, 2008),但岩石圈结构的调整可以在＜30Ma的时间尺度内发生,如造山带从形成榴辉岩下地壳到最终拆沉结束,其时间跨度＜15Ma(Gao Shan et al., 2003),而陆缘弧下地壳发生石榴辉石岩堆晶到拆沉进入地幔的周期一般在10~30Ma时间跨度内发生(Lee et al., 2015)。因而始新世火山岩内捕虏体不能代表目前地球物理观测结果。目前对于青藏高原上新世以来火山岩捕掳体研究极其匮乏,仅Hacker et al.(2000)对太平湖火山岩(3.4~2.5Ma)内多种壳源捕掳体进行了论述,其计算的30~50km深度的温度可达800~1000℃,然而其主要研究火山岩内基性和富铝变沉积岩捕掳体,并未涉及含石榴子石的长英质岩石捕掳体。
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图6 羌塘北缘恒梁湖上新世火山岩含石榴子石长英质岩石捕掳体及斑晶地质温压计PTQuick计算结果
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Fig.6 PTQuick geothermobarometers calculation results of the garnet-bearing quartzofeldspathic xenolith and phenocryst in the Pliocene Henglianghu volcanic, north margin of Qiangtang terrane
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石榴子石成分往往具有一定的指示意义,本文石榴子石具有极高的镁铝榴石组分(41%~46%,摩尔百分比),与下地壳岩石经历高温—超高温麻粒岩相变质后所形成的石榴子石成分类似(Jiao Shujuan et al., 2020)。本文获得的火山熔体与石榴子石相互作用的温度亦与华北克拉通集宁地区超高温变质麻粒岩变质温度940~1050℃相近(Santosh et al., 2007; Jiao Shujuan et al., 2011; Liu Shoujie et al., 2012; Li Xianwei et al., 2018)。另外,与超高温泥质麻粒岩内石榴紫苏花岗岩(Wang Luojuan et al., 2018)中石榴子石成分相比具有更高的Mg#值和钙铝榴石端元组分,指示其可能形成于更高的压力条件。Hacker et al.(2014)认为羌塘地壳普遍经历过始新世部分熔融,但长英质熔体因其黏度较高而导致其不能有效地长距离迁移。另外,结合研究区内广泛发育始新世下地壳熔融形成的埃达克质火山岩(Wang Qiang et al., 2005, 2008; Lai Shaocong et al., 2013),本文推测上新世恒梁湖火山岩熔体上升过程中可能捕获了深部地壳熔融的残留熔体,该含石榴子石长英质熔体与火山熔体具有相近的温度,捕获后快速上升,两者接触边界间无明显的反应关系,而当下地壳火山熔体上升至中上地壳捕获长英质沉积岩时,由于石英颗粒和火山岩间存在强烈的成分差异,随即发生物质成分的交代而形成细粒单斜辉石(图4d),同时相对富钙熔体成分亦导致斜方辉石被单斜辉石包裹(图4e),随后伴随压力和温度降低,基质内单斜辉石的Mg#值和铝含量逐渐降低。
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6 结论
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(1)本文首次报道了青藏高原中部上新世火山岩捕掳体内代表深部地壳高温熔融的熔体残余含石榴子石长英质岩石,其中石榴子石的Mg#值为41~46,与下地壳超高温麻粒岩相变质过程中石榴子石成分相近。
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(2)地质温压计揭示含石榴子石长英质捕掳体形成压力为1.18~1.53GPa,深度约39~50km。火山岩熔体与捕掳体相互作用的温压范围为950~1050℃,1.30~1.66GPa,深度约43~55km,长英质熔体与火山岩形成深度相近,此深度与地球物理所观察的地壳面波低速区及强各向异性区深度范围一致,指示深部岩石部分熔融是形成地球物理异常的重要原因。
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致谢:电子探针测试中获得中国地质科学院地质研究所毛小红助理研究员及李鸣嘀实验员的帮助,两位审稿人建设性的意见为本文的进一步的提升助益颇多,在此一并表示感谢!
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摘要
本文以羌塘地体北缘出露面积最大的上新世恒梁湖火山岩为研究对象,对火山岩内含石榴子石长英质捕掳体的岩相学、显微结构及矿物化学综合研究,揭示出此类捕掳体系深部地壳超高温部分熔融残余,其中石榴子石以高Mg#值(41~46)为特征。传统地质压力计估算其发生部分熔融的压力为1.18~1.53 GPa,对应深度约39~50 km,火山岩与捕掳体相互作用的温压条件为950~1050 ℃,1.30~1.66 GPa,对应深度约43~55 km。火山岩熔体快速上升冷却过程中,熔体沿捕掳体裂隙或边缘与石榴子石发生反应形成斜方辉石和斜长石,而火山岩基质内辉石及长石表现出强烈的成分环带。此研究结果首次揭示了北羌塘北缘中-深地壳岩石部分熔融所产生的熔体及其岩相学和矿物化学特征,亦为此区域地壳深部面波低速异常及强各向异性提供了深部岩石学方面的限定。
Abstract
This study is focussed on the largest Pliocene (3.2~2.5 Ma) volcanic rocks from the Henglianghu area in the north margin of the Qiangtang terrane. Detailed petrographical and mineral chemical work on the garnet-bearing quartzofeldspathic xenolith and phenocrysts revealed that melt in the deep crust (40~50 km) was trapped when the volcano erupted. The garnets in xenolith are characterized by extremely high Mg# (Mg2+×100/(Mg2++Fe2+)) values (41~46) which indicated they were formed under ultrahigh temperature metamorphism. Garnet-plagioclase geobarometer calculation for the xenolith yields 1.18~1.53 GPa corresponding to 39~50 km depth. Clinopyroxene-orthopyroxene geothermometers applied to the volcanic melt give temperature ranges from 950℃ to 1050℃. During ascending stage,volcanic melt interacted with the garnet along the xenolith fractures or mineral boundaries and transformed to orthopyroxene and plagioclase. Pressure estimated by garnet-orthopyroxne-plagiocalse geobarometers is 1.30~1.66 GPa (43~55 km). Pyroxene and plagioclase in the matrix exhibit distinct compositional zoning asthe volcanic rock composition evolved. Our results not only first exhibit the texture and mineral chemical features of melt in the middle-deep crust of the northern Qiangtang terrane but also provide petrological constraints on the low S-wave and high anisotropy anomaly in this region.
