2013, Vol. 29
位于华北克拉通西部陆块内的一条重要的古元古代碰撞构造带——孔兹岩带中出露大面积变质表壳岩系(Zhao et al., 1999,2003,2005；Zhai et al., 2000),许多研究者对其内的含尖晶石片麻岩的成因矿物学、矿物相转变和变质演化等方面进行了研究(阎月华等,1991；卢良兆等, 1992,1996；Lu and Jin, 1993；刘福来等,1998；Santosh et al., 2006,2007,2009；Yin,2010；Guo et al., 2012；Liu et al., 2012；蔡佳等,2013)。研究表明,麻粒岩相变泥质岩中尖晶石的化学成分通常接近镁尖晶石(MgAl2O4)-铁尖晶石(FeAl2O4)固溶体的富铁端元组分,并含Zn2+、Fe3+和Ti4+等微量元素。目前,对于孔兹岩带常见的富铝片麻岩中尖晶石的成因,普遍认为是在中-低压麻粒岩相的退变阶段,形成于局部晶域贫Si且高Al/Si比值的条件(阎月华等,1991；Tajčmanová et al., 2007)。部分研究者在土贵乌拉、东坡以及和林格尔等地发现含尖晶石的超高温变质岩(Santosh et al., 2006,2007,2009；Jiao and Guo, 2011；Guo et al., 2012；Liu et al., 2012；Zhang et al., 2012),但有关其成因机制仍存在争议(Hensen,1986；Hensen and Harley, 1990；Harley,1986；Powell and Sandiford, 1988；Das et al., 2001,2003；Kelsey,2008；Tajčmanová et al., 2009；Shimizu et al., 2009)。前人研究表明,尖晶石中微量元素含量的高低可直接影响其稳定的P-T条件,例如尖晶石中Zn含量的升高可将尖晶石+石英矿物组合的稳定域向高压和/或低温方向扩展(Clarke et al., 1989；Nichols et al., 1992；Sack and Ghiorso, 1991；Hand et al., 1994)；另一方面,体系中流体的性质直接影响尖晶石的稳定范围,如随体系fO2的升高,使更多的Fe2+氧化成Fe3+进入尖晶石的晶格中,从而导致含尖晶石矿物组合的稳定域向低温方向移动(Hensen,1986；Powell and Sandiford, 1988；Clarke et al., 1989；Waters,1991；Guiraud et al., 1996；Fitzsimons,1996；White et al., 2002)。
最近我们在大青山-乌拉山变质杂岩带大南沟地区识别出一种含榴尖晶黑云钾长片麻岩,该岩石以贫Si富Al为特征,矿物组合为尖晶石+石榴石+黑云母+钾长石±夕线石±斜长石, 其岩相学特征明显不同于孔兹岩带典型的富铝片麻岩。由于前人的相关报道较少,目前有关该类岩石的变质反应结构、尖晶石的成因机制、变质温压条件、以及变质演化过程中的熔体行为均缺乏深入而系统的研究。有鉴于此,本文在前人研究的基础上,通过详细的野外地质观察,结合室内岩相学、成因矿物学、变质反应结构和相平衡模拟等综合研究,阐明不同晶域尖晶石的分布规律、矿物组合以及相关变质反应结构特征,确定该类岩石形成的P-T条件,为进一步揭示大青山-乌拉山孔兹岩带中特殊岩石类型的成因机制、变质演化特征提供重要基础资料,为进一步建立华北克拉通西部孔兹岩带形成演化的动力学模式提供科学依据。
2 地质背景研究区位于华北克拉通西北缘大青山-乌拉山古元古代孔兹岩带中段,其西侧出露千里山-贺兰山早前寒武纪变质杂岩；东侧则出露集宁-卓资-凉城一带大面积孔兹岩系；北侧紧邻阴山陆块,南侧则与太古代鄂尔多斯陆块相接(图 1a, b)。大青山-乌拉山地区主要出露早前寒武纪变质岩和显生宙沉积岩。此外,在局部地区分布少量中元古代-中生代岩浆岩(李树勋等,1994；Li et al., 2009)。
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图 1 孔兹岩带在华北克拉通的分布和构造位置图(a,b,据Zhao et al., 2005)和大青山-乌拉山变质杂岩带的地质简图及采样位置(c,据徐仲元等,2007) Fig. 1 Distribution of Khondalite Belt in the North China Craton(a,b,after Zhao et al., 2005)and geological map of Daqingshan-Wulashan metamorphic complex belt(c,after Xu et al., 2007) |
内蒙古大青山-乌拉山地区出露的早前寒武纪变质岩石类型十分复杂,包括太古代兴和岩群麻粒岩系、古元古代乌拉山岩群和美岱召岩群的变质表壳岩系(孔兹岩系)、太古代深变质岩浆岩、古元古代岩浆岩以及少量古元古代(石榴)基性麻粒岩和斜长角闪岩(卢良兆等, 1992,1996；徐仲元等,2007)。其中,古元古代乌拉山岩群孔兹岩系主要分布于包头以北的哈德门沟、忽鸡沟、大南沟、五当召和鸡灯湾等地,包括两个岩石单元,下部主要是黑云角闪质片麻岩,岩石类型包括含石英辉石斜长角闪岩、含石英钾长/二长角闪岩、斜长角闪岩、角闪斜长片麻岩、黑云角闪斜长片麻岩、黑云角闪二长片麻岩、黑云钾长/二长片麻岩夹辉石磁铁石英岩等(李树勋等,1994)；而上部岩石单元总体与孔兹岩系相当,主要为石榴黑云二长/斜长片麻岩、夕线石榴堇青黑云二长/斜长片麻岩、(石墨)大理岩、长石石英岩、黑云变粒岩等。高精度同位素定年结果表明,乌拉山岩群变质表壳岩的沉积时代为2000~1950Ma,其变质时代在1950~1850Ma(吴昌华等,2006；Xia et al., 2006；Wan et al., 2009；Dong et al., 2013；Zhao et al., 2012)。
3 样品采集与分析方法样品采样点位于内蒙古孔兹岩带大青山-乌拉山地区的大南沟村(图 1c)。该地区出露有典型的孔兹岩系,包括的岩石有：石榴堇青夕线二长片麻岩、石榴夕线黑云斜长/二长片麻岩、石榴长英质粒状岩石和大理岩。局部产出少量含榴尖晶(夕线)黑云钾长片麻岩,其外观呈灰黑色或青灰色,呈互层状或透镜体状产出于石榴长英质粒状岩石或石榴堇青夕线二长片麻岩中,二者呈渐变过渡关系(图 2)。采集的岩石类型包括：石榴长英质粒状岩石(BH28-1)、石榴堇青夕线二长片麻岩(BT28-2)、石榴夕线黑云斜长/二长片麻岩(BT28-3)、含榴尖晶夕线黑云钾长片麻岩(BT28-4)和含榴尖晶(夕线)黑云钾长片麻岩(BH28-5和BH28-6)。
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图 2 大青山-乌拉山变质杂岩带中大南沟地区各类片麻岩野外露头照片 (a)-石榴长英质粒状岩石(BH28-1)；(b)-石榴堇青夕线二长片麻岩(BH28-2)中的石榴石变斑晶；(c)-含榴尖晶黑云钾长片麻岩呈透镜状产出于石榴长英质粒状岩石中；(d)含榴尖晶黑云钾长片麻岩(BH28-5)的矿物组合Grt+Spl+Bt+Kfs Fig. 2 Outcrops of typical Danangou gneisses from the Daqingshan-Wulashan metamorphic complex belt (a)-quartzo-feldspathic rocks(BH28-1);(b)-garnet porphyroblasts in garnet-cordierite-sillimanite-bearing paragneisses(BH28-2);(c)-garnet-spinel-biotite-bearing paragneisses as various lenses distributed within quartzo-feldspathic rocks;(d)-garnet-spinel-biotite-bearing paragneisses(BH28-5)with mineral assemblage of Grt+Spl+Bt+Kfs |
矿物化学成分分析和显微结构特征的观察在中国地质科学院大陆动力学国家重点实验室进行。首先采用JSM-5610LV型扫描电镜(SEM)(日本电子公司JEOL生产)观察样品的显微结构特征,扫描电镜实验条件为：电子束的电压为20kV,焦距20mm,束斑大小为41nm；然后采用英国牛津公司生产的能谱仪(EDS)对样品中矿物化学成分进行半定量测试,同时运用英国牛津公司的INCA软件包进行数据处理(版本4.4)；最后在北京大学造山带与地壳演化教育部重点实验室采用JXA-8100型电子探针仪对上述样品内各矿物的化学成分进行定量分析,仪器测试条件为：加速电压15kV, 束流10nA,电子束斑为1μm,修正方法为PRZ,标样为美国SPI公司的53种标准矿物。主要矿物化学成分测试结果列入表 1-表 4。本文所有矿物代号均采用Whitney and Evans(2010)的资料。
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表 1 含榴尖晶黑云钾长片麻岩中石榴石的化学成分(wt%) Table 1 Chemical composition of garnet in garnet-spinel-biotite-bearing paragneisses(wt%) |
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表 2 含榴尖晶黑云钾长片麻岩中长石的化学成分(wt%) Table 2 Chemical composition of feldspar in garnet-spinel-biotite-bearing paragneisses(wt%) |
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表 3 含榴尖晶黑云钾长片麻岩中黑云母和刚玉的化学成分(wt%) Table 3 Chemical composition of biotite and corundum in garnet-spinel-biotite-bearing paragneisses(wt%) |
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表 4 含榴尖晶黑云钾长片麻岩中尖晶石的化学成分(wt%) Table 4 Chemical composition of spinel in garnet-spinel-biotite-bearing paragneisses(wt%) |
石榴长英质粒状岩石(BT28-1)的主要组成矿物为石英(含量约60%~65%)、石榴石(10%)、黑云母(5%)和斜长石(20%~25%),偶见褐红色柱状金红石在基质中分布。石榴石变斑晶核-幔部内可见大量不等粒浑圆状石英颗粒(粒径5~60μm)(图 3a),偶见斜长石和黑云母等包体。
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图 3 黑云石榴长英质粒状岩石(BH28-1)、石榴堇青夕线二长片麻岩(BH28-2)、含榴尖晶夕线黑云钾长片麻岩(BH28-4)和含榴尖晶黑云钾长片麻岩(BT28-5和BT28-6)显微结构照片 (a)-石榴石变斑晶内包裹浑圆状石英和斜长石,石榴石边部围绕黑云母和斜长石(PL图像)；(b)-石榴石变斑晶边部出现堇青石(蚀变)反应边,堇青石中含有残留的夕线石、黑云母和Fe-Ti氧化物(PL图像)；(c、d)-基质中的尖晶石、黑云母和钾长石(PL图像)；(e)-石榴石、尖晶石、钾长石、黑云母和夕线石平衡共生(PL图像)；(f)-黑云母脱水生成细粒钾长石和斜长石,沿黑云母解理缝方向分布(SEM图像) Fig. 3 Typical mineral assemblages and micro-textures in quartzo-feldspathic rocks(BH28-1),garnet-cordierite-sillimanite-bearing paragneiss(BH28-2),garnet-spinel-sillimanite-bearing paragneiss(BH28-4)and garnet-spinel- biotite-bearing paragneisses(BH28-5 and BH28-6) (a)-quartz and plagioclase as inclusions preserved in garnet porphyroblast which is surrounded by fine-grained biotite and plagioclase(PL);(b)-garnet relict rimmed by cordierite corona(pinitized)in which sillimanite,biotite and Fe-Ti oxide are preserved(PL);(c,d)-biotite together with K-feldspar and spinel as matrix minerals in garnet-spinel-biotite-bearing paragneisses(BH28-5 and BH28-6)(PL);(e)-garnet,spinel,K-feldspar,biotite and sillimanite distributed in the matrix of garnet-spinel-sillimanite-bearing paragneiss(BH28-4)(PL);(f)-K-feldspar and plagioclase as newly fine-grained minerals formed by biotite dehydration,and distributed along the cleavages of biotite(SEM) |
石榴堇青夕线二长片麻岩(BT28-2)的主要组成矿物为石榴石(10%~13%)、夕线石(10%~12%)、黑云母(15%~20%)、长石(25%~40%)和石英(20%~25%)。副矿物主要有磁铁矿、金红石和锆石。黑云母颗粒较大(约0.4~1mm),呈片状。石榴石变斑晶以浑圆粒状或反应后的残留状产出,裂隙十分发育。石榴石可包裹粒状石英、黑云母、长石、磁铁矿和金红石等。石榴石边部可出现堇青石(蚀变严重)的退变反应边结构,堇青石内含有残留的细粒石榴石、夕线石、蠕虫状石英、黑云母和不规则状的Fe-Ti氧化物(图 3b),反映峰后减压的退变反应结构,可能的变质反应为：
Grt+Sil+Melt→Crd+Bt+Fe-oxide
含榴尖晶黑云钾长片麻岩(BT28-5和BT28-6)是少见的贫硅富铝岩石,主要组成矿物有尖晶石(10%~13%)、黑云母(25%~27%)、钾长石(25%~28%)、斜长石(13%~16%)和石榴石(10%~13%)(图 3c, d)等,其次为少量斜长石、刚玉和磁铁矿,偶见石英。样品BH28-4(含榴尖晶夕线黑云钾长片麻岩)的矿物组合和变质反应结构与样品BT28-5和BT28-6类似,主要组成矿物有石榴石、黑云母、钾长石、柱状夕线石集合体和尖晶石等(图 3e)。
石榴石变斑晶多为浑圆粒状,粒径约1.5~17mm,可包裹细粒黑云母、尖晶石、钾长石和斜长石等矿物,粒径5~45μm不等。少数石榴石(粒径达~17mm)核-幔部包裹大量黑云母,核部黑云母多为细小鳞片状,向边部粒径变大,且多呈片状。
根据尖晶石的显微结构和形态特征,可进一步分为三种类型：Spl1呈细粒状,和黑云母、长石等共同包裹在石榴石变斑晶中(图 4d)；Spl2呈不规则状,粒径较大(35~85μm),分布在基质中(图 3c-e、图 4a,b,e,f、图 5)；Spl3是由石榴石分解形成,多呈细粒状,与细长鳞片状黑云母、钾长石和斜长石分布在石榴石残晶周围,或呈石榴石假象(图 4b, d)。
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图 4 含榴尖晶黑云钾长片麻岩(BH28-5和BH28-6)显微结构照片 (a)-石榴石转变形成细粒斜长石和鳞片状黑云母(PL图像)；(b)-石榴石分解形成细粒尖晶石+钾长石±斜长石(PL图像)；(c)-石榴石转变形成细粒斜长石和鳞片状黑云母(SEM图像)；(d)-石榴石变斑晶内包裹细粒夕线石、黑云母和尖晶石,石榴石边部分布斜长石、尖晶石、石英和钾长石(SEM图像)；(e)-正条纹长石中的钾长石主晶和斜长石客晶(SEM图像)；(f)-正条纹长石中的斜长石包体(SEM图像) Fig. 4 Typical mineral assemblages and micro-textures in garnet-spinel-biotite-bearing paragneisses(BH28-5 and BH28-6) (a)-fine-grained plagioclase and biotite surrounding garnet relict(PL);(b)-newly symplectite of Spl+Kfs±Pl surrounding garnet relict(PL);(c)-fine-grained plagioclase and biotite surrounding garnet relict(SEM);(d)-sillimanite,biotite and spinel as fine-grained inclusions in garnet porphyroblast which is rimmed by newly mineral assemblage of plagioclase,spinel,quartz and K-feldspar(SEM);(e)-exsolution lamellae of plagioclase identified within matrix perthite(SEM);(f)-plagioclase as inclusions preserved within matrix perthite(SEM) |
黑云母可分为三种类型：Bt1呈细小鳞片状包裹在石榴石核部或呈片状包裹在石榴石幔-边部；Bt2位于基质中,与尖晶石和钾长石等矿物共同产出,粒径相对较大,略具定向排列；Bt3是在晚期降温冷却阶段由石榴石分解形成,形态明显不同于Bt2,呈细长鳞片状分布在石榴石的边部(图 4c)。
浅色矿物主要是条纹长石,少量斜长石(图 4e, f),偶见石英。条纹长石有两种类型：分布在石榴石变斑晶的边部；在基质中与黑云母、尖晶石等共同产出。斜长石可分为四种类型：以细粒状包裹在石榴石变斑晶中；由石榴石降温分解形成而呈不规则状围绕在石榴石边部；呈浑圆状包裹在基质正条纹长石内部(图 4f)或位于条纹长石粒间(图 4e)；正条纹长石内出溶的斜长石呈细长叶片状定向排列。此外,偶见基质尖晶石包裹细粒钾长石。
样品BH28-4(含榴尖晶夕线黑云钾长片麻岩)中的夕线石主要包括两种类型：呈细粒,沿黑云母解理缝分布；呈粗粒针-柱状集合体,与黑云母平衡共生(图 3e)。
此外,含榴尖晶黑云钾长片麻岩保存多种后成合晶结构。部分石榴石边部可分解形成Kfs+Spl3±Pl的反应边结构(图 4b,d),可能发生如下变质反应：
Grt+Sil+Melt→Kfs+Spl+Pl(Álvarez-Valero et al., 2007)
在样品BH28-4(含榴尖晶夕线黑云钾长片麻岩)中还可观察到峰期和峰后阶段黑云母发生脱水熔融现象,形成钾长石等(图 3f),可能发生的变质反应：
Bt+Sil+Qz±Pl→Grt±Kfs±Melt(Patiño Douce and Johnston, 1991)
针柱状夕线石集合体和鳞片-片状黑云母分布在石榴石变斑晶周围,是上述变质反应的逆反应的产物。
部分石榴石边部被细小鳞片状黑云母和细小粒状斜长石、石英所环绕(图 4a, c,d),可能的退变质反应为：
Grt+Melt→Bt+Qz±Pl(Vielzeuf and Montel, 1994；Dharma Rao et al., 2012)
在局部贫硅微域可观察到尖晶石固溶体(solid solution,缩写为Splss)中常含有刚玉+磁铁矿的新生矿物相(图 5),即尖晶石内部或边部出现粒状刚玉和零星分布的磁铁矿,表明晚期降温或是氧化作用促使尖晶石内的Al和Fe析出,发生如下转变反应：
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图 5 含榴尖晶黑云钾长片麻岩(BH28-5和BH28-6)矿物相转变反应结构显微结构照片 (a)-含榴尖晶黑云钾长片麻岩(BH28-6)主要组成矿物为石榴石+黑云母+尖晶石+钾长石(PL图像)；(b)-图(a)局部尖晶石分解形成刚玉+磁铁矿的放大照片(PL图像)；(c)-尖晶石转变为刚玉+磁铁矿的退变反应结构(SEM图像)；(d)-图(c)中尖晶石转变为细小粒状刚玉+磁铁矿的放大照片(SEM图像) Fig. 5 Micro-textures of garnet-spinel-biotite-bearing paragneisses(BH28-5 and BH28-6) (a)-garnet,biotite,spinel and K-feldspar as matrix minerals in garnet-spinel-biotite-bearing paragneiss(BH28-6)(PL);(b)-magnified image of transformation reaction Spl→Crn+Mag as shown in Fig. 5(a)(PL);(c)-retrogressive texture showing reaction of Splss+O2→Spl+Mag+Crn(SEM)in garnet-spinel-biotite-bearing paragneiss(BH28-5);(d)-magnified image of transformation reaction Splss+O2→Spl+Mag+Crn as shown in Fig. 5(c)(SEM) |
Splss+O2→Spl+Mag+Crn(Viertel and Seifert, 1981；Waters,1991)。
5 矿物化学 5.1 石榴石本文对变斑晶和残晶石榴石均进行了化学成分测试,其结果列入表 1中,所有石榴石均以富含FeO为特征,其中铁铝榴石端元组分变化于67.4~77.6之间,镁铝榴石端元组分为17.9~28.5,钙铝榴石端元组分为3.7~6.2,而锰铝榴石端元组分则含量极低,为0.1~0.5。石榴石的XMg(Mg2+/(Fe+Mg2+))值变化于0.190~0.297之间。石榴石的核-边成分略有变化,核部相对富镁(0.295＜XMg＜0.297),而边部相对富铁(0.190＜XMg＜0.244)(图 6)。此外,残晶石榴石的化学成分与变斑晶石榴石边部成分大体相当(表 1)。
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图 6 含榴尖晶黑云钾长片麻岩中石榴石的Prp-(Alm+Sps)-Grs图解 Fig. 6 Prp-(Alm+Sps)-Grs diagram of garnet from garnet-spinel-biotite-bearing paragneisses |
含榴尖晶黑云钾长片麻岩的浅色矿物主要为正条纹长石,含少量斜长石。An-Ab-Or图解(图 7)显示样品中的斜长石属于钠长石-奥长石组分(An端元含量为10~31；表 2),其中包裹在石榴石核部的斜长石比石榴石边部分解而成的斜长石更接近Ab端元(图 7)。在同一样品内,基质斜长石和石榴石边部的斜长石成分相近,只是石榴石边部斜长石的An端元含量略高。基质条纹长石中的钾长石主晶接近纯Or端元组分(Or为90；表 2)。
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图 7 含榴尖晶黑云钾长片麻岩中斜长石的An-Ab-Or图解 Fig. 7 Ternary plot of An-Ab-Or for plagioclase from garnet-spinel-biotite-bearing paragneisses |
黑云母XMg值变化于0.502~0.604之间,而TiO2含量在2.02%~3.16%(表 3)。位于石榴石核部、基质和石榴边部的三类黑云母(分别为Bt1,Bt2和Bt3)的化学成分对比显示,Bt2和Bt3的TiO2含量和XMg值明显低于Bt1。相反,Bt1中Al2O3和FeOT的含量则明显低于Bt2和Bt3。Bt2和Bt3的成分相近,只是Bt2中的Al2O3含量略高于Bt3。样品中黑云母的Ti含量明显低于内蒙典型的孔兹岩(富铝片麻岩)(蔡佳等,2013),由于未见黑云母出溶形成的钛铁矿或金红石,故这可能与全岩成分低Ti有关。
5.4 尖晶石含榴尖晶黑云钾长片麻岩中尖晶石的化学成分总体变化不大(表 4)。尖晶石主要是铁尖晶石和镁尖晶石的固溶体(0.155＜XMg＜0.208)。ZnO的含量均很低(＜0.3%),明显低于内蒙孔兹岩带典型富铝片麻岩的尖晶石,并含少量Cr2O3(＜0.30%),而TiO2和MnO的含量均很低。Fe3+/(Fe3++Fe2+)比值小于0.071。Cr-Al-Fe3+图(图 8a)表明基质中的尖晶石整体较石榴石边部的Fe3+高。Al-Fe2+-Fe3+图(图 8b)显示尖晶石均落在Al-Fe2+线上,其中包裹在石榴石内的尖晶石的Fe2+最低,表明尖晶石的化学成分与直接接触的镁铁矿物相及其共生组合有关(刘平华等,2011)。在XMg-Zn图解(图 9)中,研究区样品中的尖晶石Zn含量和XMg值均明显偏低,与华北克拉通孔兹岩带(Santosh et al., 2007；Tsunogae et al., 2011；Liu et al., 2012)、南印度Madurai地体(Sajeev et al., 2006)、新疆南阿尔金(张建新和孟繁聪,2005)、东南极Sør Rondane山脉(Asami et al., 2007)和斯里兰卡Highland杂岩(Kriegsman and Schumacher, 1999)等超高温变质岩中尖晶石Zn含量十分一致,但后者XMg值变化范围较大,且普遍高于研究区样品中尖晶石的XMg值(图 9)。阿尔及利亚(Ouzegane et al., 2003)超高温变质岩中尖晶石的Zn含量和XMg值均明显高于本研究区样品。相比之下,马达加斯加(Markl et al., 2000)、东南极Jetty半岛(Hand et al., 1994)、南韩Odesan地区(Oh et al., 2006)中-高温变质岩中尖晶石的Zn含量和XMg值也均高于本研究区样品,因此,尖晶石中Zn含量的高低不仅受控于变质温度条件,而且可能与寄主岩石的化学成分存在一定的成因关系。
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图 8 含榴尖晶黑云钾长片麻岩中尖晶石的Cr-Al-Fe3+(a)和Al-Fe2+-Fe3+(b)成分图解 Fig. 8 Compositional diagrams of spinel in garnet-spinel-biotite-bearing paragneisses |
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图 9 含榴尖晶黑云钾长片麻岩和其它地区高级变质岩中尖晶石的XMg-Zn图解 数据来源：超高温变质岩：内蒙土贵乌拉(Santosh et al., 2007)；内蒙东坡(Tsunogae et al., 2011)；内蒙和林格尔(Liu et al., 2012)；南阿尔金(张建新和孟繁聪,2005)；南印度Madurai地体(Sajeev et al., 2006)；斯里兰卡Highland杂岩(Kriegsman and Schumacher, 1999)；东南极Sør Rondane山脉(Asami et al., 2007)；阿尔及利亚In Ouzzal地体(Ouzegane et al., 2003)；中-高温变质岩：马达加斯加(Markl et al., 2000)；东南极Jetty半岛(Hand et al., 1994)；南韩Odesan地区(Oh et al., 2006) Fig. 9 Diagram of XMg versus Zn(a.p.f.u)for spinel in Danangou garnet-spinel-biotite-bearing paragneisses and high-garde rocks from worldwide terranes Data source: ultrahigh temperature metamorphic rocks: Tuguiwula,Inner Mongolia(Santosh et al., 2007); Dongpo,Inner Mongolia(Tsunogae et al., 2011); Heling’er,Inner Mongolia(Liu et al., 2012); South Altyn Tagh(Zhang and Meng, 2005); Madurai Block,southern India(Sajeev et al., 2006); Highland Complex,Sri Lanka(Kriegsman and Schumacher, 1999); Sør Rondane Mountains,East Antarctica(Asami et al., 2007); In Ouzzal terrane,Algeria(Ouzegane et al., 2003); Medium-high temperature metamorphic rocks: Madagascar(Markl et al., 2000); Jetty Peninsula,East Antarctica(Hand et al., 1994); Odesan area,South Korea(Oh et al., 2006) |
由于研究区含榴尖晶黑云钾长片麻岩几乎不含石英,很多常规变泥质岩石的传统地质温压计并不适用于该类岩石各阶段的P-T条件估算。然而,P-T视剖面图可以展示特定全岩成分下的变质矿物相平衡、模拟矿物成分和摩尔分数等值线(魏春景和周喜文,2003),因此,可以采用该方法进行矿物相转变与变质反应关系、P-T条件的系统研究。
本文研究的含榴尖晶黑云钾长片麻岩样品(BH28-5)明显不同于孔兹岩带典型的富铝片麻岩特征,前者贫Si富Al,仅偶见石英,尖晶石含量很高(约10%~12%),在基质中与黑云母、钾长石和石榴石平衡共生,或由石榴石分解形成。由于研究样品内尖晶石的Zn含量很低(＜0.3wt%),讨论尖晶石稳定域时可不予考虑Zn的影响。在岩相学、矿物相转变及变质反应性质综合研究的基础上,利用程序THERMOCALC 3.33(Powell and Holland, 1988, 2009年更新)及内部一致热力学数据库(tcds55,Holland and Powell, 1998)对样品BH28-5进行P-T视剖面图计算。由于孔兹岩带经历了多期麻粒岩相变质(卢良兆等,1992)并伴随部分熔融,应考虑多种主量元素对平衡矿物组合的影响,故选择最接近实际岩石化学组分及矿物组合的体系Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe2O3(NCKFMASHTO),其中有关矿物固溶体的活度-成分关系引用的参考文献包括：石榴石(White et al., 2007),钾长石和斜长石(Holland and Powell, 2003),黑云母和熔体(White et al., 2007),尖晶石-磁铁矿(White et al., 2002),石英、铝硅酸盐和刚玉为纯端元组分。岩石中MnO含量很低(＜0.1%),故在相平衡计算中未予考虑。岩石中各氧化物的摩尔百分含量是通过下列步骤获得的： 首先选取典型薄片BH28-5中均匀且未蚀变的部分,然后估算石榴石(13%)、黑云母(27%)、尖晶石(12.5%)、钾长石(28%)、斜长石(16%)、刚玉(1%)和磁铁矿(1%)的矿物体积百分含量,最后将这些矿物的电子探针数据加权平均而得到其摩尔百分含量,如图 10所示。
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图 10 含榴尖晶黑云钾长片麻岩(BH28-5)的P-T视剖面图 图中黄色域代表峰期矿物组合的稳定域,实线代表蓝晶石-夕线石相变线 Fig. 10 Calculated P-T pseudosection for garnet-spinel-biotite-bearing paragneiss(BH28-5) The yellow area highlights the peak mineral assemblage. The black bar corresponds to kyanite-sillimanite equilibria line |
P-T视剖面图(图 10)中出现的贫Si矿物为尖晶石和刚玉。尖晶石的消失线(图 10中spl线)随着温度的升高向高压方向移动,石榴石的消失线(图 10中grt线)在920℃以下整体近平行于T(温度)轴,表明石榴石的分解受压力影响较大,而在920℃以上,石榴石消失线的变化趋势受温度的影响更大。相反,磁铁矿的消失线(图 10中mag线)近平行于P(压力)轴,说明磁铁矿主要受温度控制。固相线出现在~850℃,而通常变泥质岩的固相线出现在~650℃,这种差异的原因可能是贫Si残留体的原岩在~870℃形成了峰期矿物组合liq+bt+grt+spl+mag+sil+kfs+pl,并随着温度的降低至~850℃发生大量早期熔体的迁移,使得峰期矿物组合被保留,而不出现低熔组分(White et al., 2001)。通过石榴石幔-边部z(g)值(Ca/(Fe2++Mg+Ca))和基质尖晶石核部的x(sp)值(Fe2+/(Fe2++Mg))等值线的计算(图 10),确定峰期矿物组合bt+grt+spl+mag+sil+kfs+pl+liq稳定的温压条件在830~870℃和8.3~8.6kb,位于夕线石的稳定域。
6.2 尖晶石成因尖晶石可产出于多种高级变质岩中(Tajčmanová et al., 2009),其形成受到全岩化学成分、变质反应和P-T条件等诸多因素的影响。孔兹岩带富铝片麻岩中的尖晶石均赋存于贫硅微域,例如以包裹体形式存在于石榴石或夕线石晶体中,或以细粒包体存在于石榴石边部的堇青石中,相应的变质反应可能为Grt+Sil→Crd+Spl(Harris,1981),这种尖晶石的形成不仅受控于P-T条件,而且与局域体系的化学成分存在密切的成因关系。通过系统岩相学观察,将含榴尖晶黑云钾长片麻岩中的尖晶石划分出三种类型,其中,Spl2是基质中的尖晶石,与黑云母没有明显转变反应结构,二者接触关系平直且粒径相近,这种尖晶石可能是在岩石发生部分熔融后,熔体迁出后的残留相(贫Si富Al)重结晶形成,主要受控于全岩化学成分；Spl3多呈细粒状,与黑云母、钾长石和斜长石分布在残留的细粒石榴石周围,是由石榴石分解形成,因此该种尖晶石形成明显受寄主岩石化学成分和P-T条件的共同影响。
值得注意的是,孔兹岩带的富铝片麻岩中处于不同微构造域的尖晶石Zn含量普遍较高(一般>3%,未发表数据)。部分研究者(Atkin,1978；Stoddard,1979；Moore and Reid, 1989；Heimann et al., 2005)提出十字石为尖晶石提供Zn,由于Zn2+优先进入含四次配位Fe2+的十字石中,在早期进变质过程中,含Zn十字石脱水分解并释放Zn。另一部分研究者认为黑云母为尖晶石提供Zn(Morimoto et al., 2004；Tajčmanová et al., 2007,2009)。然而,不同于富铝片麻岩中的尖晶石,研究区含榴尖晶黑云钾长片麻岩中的尖晶石Zn含量普遍很低。Cesare(2000)提出无石英参与反应的黑云母脱水熔融将形成低Zn尖晶石,相应的反应为：Bt+Sil+Grt+vapour→Spl+Melt(Montel et al., 1986)。然而系统的岩相学观察显示,本文研究的含榴尖晶黑云钾长片麻岩中黑云母脱水熔融形成细粒钾长石+斜长石(图 3f),这些新生矿物相沿黑云母解理缝方向分布,并未见到尖晶石相。由此推断,基质中大量低Zn尖晶石的形成与寄主岩石贫Si富Al有关,体系中的Zn、Si是在熔体迁出过程中被带出,剩余的贫Si、Zn且富Al的残留相通过麻粒岩相变质形成低Zn的尖晶石。
6.3 岩石成因含榴尖晶黑云钾长片麻岩中的尖晶石含量很高且岩石贫硅,不同于典型的富铝片麻岩特征,前人对其成因的研究相对较少。含榴尖晶黑云钾长片麻岩呈互层状或透镜状产出于石榴长英质粒状岩石或石榴堇青夕线二长片麻岩中。在石榴堇青夕线二长片麻岩中可以观察到石榴石边部具有典型的堇青石退变反应边结构,含榴尖晶黑云钾长片麻岩也保留多种后成合晶结构(图 3~图 5),反映峰后退变质阶段矿物的演化特点,然而,有关该类岩石的成因尚不十分清楚。它的形成可用多种成因模式来解释,其中第一种模式为,含榴尖晶黑云钾长片麻岩与石榴长英质粒状岩石是同一套岩石的不同成分层,共同经历了麻粒岩相变质作用；另一种成因模式,如Riesco et al.(2004)结合岩石成因格子和P-T视剖面图,讨论了不饱和硅熔融机制和不含石英的矿物组合间的转变,认为在接触变质条件下,早期产生的熔体发生迁移后,残留的硅不饱和组分通过转变形成一系列新的贫硅矿物,如尖晶石和刚玉。而这种硅不饱和岩石自身的部分熔融仅能产生极少量的熔体以致不会迁出。石榴长英质粒状岩石在相对低温条件下(650℃),黑云母可以发生脱水熔融反应：Bt+Sil+Qz±Pl→Grt±Kfs±Melt(Patiño Douce and Johnston, 1991),产生早期熔体、钾长石和石榴石,同时长英质矿物也可以发生部分熔融,形成富Si熔体。熔体的不断产生导致岩石更易产生塑性变形(McKenzie,1987；Sawyer,2001；陈意,2008),从而引起富Si熔体脱离母岩随之迁出(即熔体丢失),难熔的残留相更加贫Si,而富集Fe、Mg等,经麻粒岩相变质作用和重结晶作用形成以含尖晶石为特征的新的矿物组合。
如前所述,含榴尖晶黑云钾长片麻岩经历了峰后退变质作用,并伴随着典型的退变质反应,其一是沿石榴石边部分解成细小鳞片状黑云母和细粒斜长石；其二是石榴石分解形成钾长石+尖晶石±斜长石。在温度持续降低且体系fO2不断增加的条件下,尖晶石内的Fe和Al析出形成磁铁矿和刚玉。根据前人的研究,当岩石贫Si且富Mg和Al组分时,经历超高温条件(>900℃)时会形成一系列硅不饱和矿物,如尖晶石、假蓝宝石、刚玉等(Riesco et al., 2004; Kelsey et al., 2005),但是本文研究的含榴尖晶黑云钾长片麻岩并没有出现超高温标志矿物组合,表明其峰期变质温度并未达到超高温条件,这与相平衡模拟结果一致。图 10中模拟的峰期组合为Bt+Grt+Spl+Mag+Sil+Kfs+Pl+Liq,与研究区含榴尖晶黑云钾长片麻岩峰期矿物组合特征完全一致,稳定的温压条件为830~870℃和8.3~8.6kb,应代表研究区该类岩石峰期变质的P-T条件。此外,由于岩石明显贫Si,石榴石的分解不可能形成相对富Si、Mg的堇青石,这正是研究区含榴尖晶黑云钾长片麻岩不含堇青石的原因所在。
7 结论通过对大青山-乌拉山变质杂岩带大南沟地区含榴尖晶黑云钾长片麻岩的岩相学、成因矿物学、变质反应结构及相平衡模拟等方面的综合分析研究,得出以下结论：
(1) 大青山-乌拉山变质杂岩带大南沟地区出露的含榴尖晶黑云钾长片麻岩是一种典型的贫硅富铝岩石,主要以形态各异的透镜体或以夹层、互层的形式赋存于石榴长英质粒状岩石和石榴堇青夕线二长片麻岩中。
(2) 大南沟地区含榴尖晶黑云钾长片麻岩中的尖晶石,以极低的Zn含量(XZn=Zn/(Fe2++Mg+Zn)＜0.006)为主要特征。矿物相转变特征表明,尖晶石是在峰期及峰后连续的退变质过程中形成的,首先在峰期变质阶段形成尖晶石,与石榴石等矿物共生形成麻粒岩相矿物组合；然后在退变质过程中,由石榴石发生转变反应Grt+Sil+Melt→Kfs+Spl+Pl,形成钾长石+尖晶石±斜长石的后成合晶；接着,在晚期降温和氧化作用条件下,进一步发生退变质反应Splss+O2→Spl+Mag+Crn,形成尖晶石-刚玉-磁铁矿新的退变矿物组合。
(3) P-T 视剖面图研究结果显示,大南沟地区含榴尖晶黑云钾长片麻岩峰期矿物组合为Bt+Grt+Spl+Mag+Sil+Kfs+Pl+Liq,形成的温压条件为830~870℃和8.3~8.6kb。
(4) 大南沟含榴尖晶黑云钾长片麻岩,可能是早期富Si的长英质粒状岩石经含水矿物脱水熔融和长英质矿物部分熔融形成的熔体析出迁移后,残留的贫Si组分经麻粒岩相变质作用而成。
致谢 北京大学造山带与地壳演化教育部重点实验室电子探针室舒桂明老师在矿物成分测试过程中给予了指导和帮助；大陆动力学国家重点实验室扫描电镜和能谱实验室陈方远老师在实验中给予建议和指导；中国地质科学院地质研究所刘超辉博士和肖玲玲博士在野外提供了很大的帮助；北京大学魏春景教授,中国科学院地质与地球物理研究所陈意副研究员和焦淑娟博士对本文提出了十分宝贵的修改意见和建议；在此一并表示衷心谢意。| [] | Álvarez-ValeroAM, CesareB, KriegsmanLM. 2007. Formation of spinel-cordierite-feldspar-glass coronas after garnet in metapelitic xenoliths: Reaction modelling and geodynamic implications. Journal of Metamorphic Geology, 25(3): 305–320. |
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