苏鲁地体超高压和退变质时代的厘定:来自片麻岩锆石微区SHRIMP U-Pb定年的证据

锆石微区矿物包体的激光拉曼和阴极发光测试以及相应的SHRIMP U-Pb定年结果表明,苏鲁地体片麻岩锆石微区记录了十分复杂的年代学信息。其中副片麻岩锆石核部记录了345～743 Ma的继承性锆石年龄,标志着原岩碎屑锆石来源的复杂性;含柯石英的锆石微区记录了220～234 Ma的超高压变质年龄;而含石英包体的边部则记录了202～219 Ma的退变质年龄。正片麻岩继承性锆石核部所记录的年龄为574～680 Ma,表明原岩锆石曾经历了部分Pb丢失,原岩的形成年龄应大于680 Ma;含柯石英锆石微区所记录的超高压变质年龄为224～242 Ma;而锆石边部所记录的退变质年龄为209～219 Ma。两类片麻岩锆石微区所隐藏的超高压变质和退变质年龄信息十分相近,平均值分别为229±4Ma和211±4Ma,标志着苏鲁地体超高压变质时代应为印支期,相应的构造抬升速率约5.6 km/Ma。该项成果不仅确定了苏鲁地体超高压变质和退变质时代,而且对于深入探讨苏鲁地体快速折返过程中的动力学机制有着重要的科学意义。     

古生代沉积岩的超高压和退变质时代的确定——以中国大陆科学钻探主孔（CCSD—MH）副片麻岩岩心为例

中国大陆科学钻探工程主孔（CCSD—MH，终孔深度5158．0m）位于南苏鲁东海县的毛北地区。锆石微区矿物包体的激光拉曼测试和阴极发光图象分析结果表明，主孔（CCSD—MH）副片麻岩中的锆石显示明显的光学环带，即具有含低压矿物包体的继承性碎屑锆石的核、含标志性超高压矿物柯石英的变质增生幔以及含石英等（或无任何矿物包体）变质退变边。SHRIMPU—Pb定年结果表明，副片麻岩锆石微区记录了三组具有实际意义的年代学信息。其中锆石核部记录了659～313Ma的继承性碎屑锆石年龄，标志着原岩碎屑锆石来源的复杂性，沉积原岩的形成时代应属于古生代（〈313Ma），     

苏鲁超高压变质带中非超高压花岗质片麻岩的准确识别:来自锆石微区矿物包体及SHRIMP U-Pb定年的证据

通过隐藏在锆石微区矿物包体激光拉曼的系统鉴定和阴极发光图像特征的详细研究,配合相应的锆石微区SHRIMP U-Pb定年测试,发现苏鲁地体超高压变质带中确实存在非超高压变质的花岗质片麻岩。该类岩石中的锆石晶体自核部到边部所保存的矿物包体以不含超高压矿物为特征,相应的阴极发光图像具有典型岩浆结晶锆石的核部和幔部,以及变质的再生边的特点。其中岩浆结晶锆石微区记录的~(238)U-~(206)Pb年龄为404～748Ma,表明原岩中部分锆石可能经历了Pb丢失,也不排除后期热事件因素的影响,原岩的形成年龄应大于748 Ma;而锆石的再生边所记录的~(238)U-~(206)Pb。年龄为204～214 Ma,与研究区经历超高压变质的副片麻岩和花岗质片麻岩锆石微区所记录的苏鲁地体快速折返过程中角闪岩相退变质年龄(~(238)U-~(206)Pb年龄的平均值为211±4 Ma,刘福来等,2003a)十分相似。上述特征表明,苏鲁地体超高压变质带中的部分花岗质片麻岩在超高压变质事件之前就已经形成,但并未“参与”深俯冲—超高压的变质演化过程,而是在苏鲁地体快速折返的角闪岩相退变质过程中与超高压岩片“拼贴”在一起。该项成果不仅为正确识别非超高压变质岩石提供了一个新的研究方法,而且对进一步深入探讨苏鲁地体超高压和非超高压岩片的“拼贴”机制有着重     

南苏鲁超高压变质地体中罗迪尼亚超大陆裂解事件的记录

通过苏鲁超高压变质地体南部不同类型超高压变质岩石的原岩重塑．揭示超高压变质岩的原岩形成于由大陆玄武质岩石、辉长岩、表壳岩和花岗岩组成的被动陆缘拉伸构造环境。中国大陆科学钻探主孔中不同类型超高压变质岩石的锆石SHRIMP U-Pb定年表明。花岗质片麻岩原岩年龄为780～680Ma；榴辉岩、石榴角闪岩的原岩年龄为765～730Ma，副片麻岩中包含了730Ma、680Ma、621Ma和较年轻的继承性碎屑锆石和结晶锆石年龄。结合前人的研究成果表明，苏鲁超高压变质地体南部正片麻岩类和榴辉岩的原岩所代表的花岗岩浆和基性岩浆活动为罗迪尼亚超大陆形成后的新元古代裂解事件的产物．而副片麻岩的原岩为新元古代．古生代时期形成的扬子被动陆缘的沉积-火山表壳盖层，它们与结晶基底一起在240～220Ma期间经历了超高压变质作用。     

大别超高压变质带的进变质、超高压和退变质时代的准确限定:以双河大理岩中榴辉岩锆石SHRIMP U-Pb定年为例

在大别超高压变质带的双河地区存在一种特殊类型的榴辉岩,该类榴辉岩主要以似层状、条带状以及不规则透镜体赋存于大理岩中。矿物组成主要为石榴石、绿辉石以及少量的金红石、白云石、菱镁矿等。沿石榴石和绿辉石边缘常退变为角闪石+斜长石等,有的岩石完全退变为斜长角闪岩。激光拉曼和阴极发光综合分析表明,该类榴辉岩中的锆石可划分为两种类型:继承性碎屑锆石和变质锆石。继承性碎屑锆石十分少见,阴极发光图象具有明显的双层结构,即强发光的核和弱发光的边,核部和边部的包体矿物分别为Pl+Ap和Qtz+Pl。SHRIMP U-Pb定年结果表明。继承性碎屑锆石核部记录的207Pb／206Pb年龄为2701±15Ma,Th/U比值明显偏高为1．05,稀土元素配分模式显示重稀土明显富集,具有典型岩浆结晶锆石的特点;边部记录的207Pb／206Pb年龄为1801±12Ma-1753±22Ma,Th／U比值则明显偏低,为0．19-0．22之间,稀土元素配分模式显示重稀土相对平坦,具有典型变质锆石的特点。上述特征表明该类继承性碎屑锆石可能来源于太古代的基底,并经历了早元古代变质热事件的改造。新生的变质锆石无论是矿物包体还是阴极发光图象均与继承性碎屑锆石存在明显差异。有的变质增生锆石具有弱发光的核(阴极发光图象呈灰色)和强发光的边(阴极发光图象呈白色)。核部包体矿物组合为Qtz +Grt+Omp+Phe+Dol+Ap,具有典型石英榴辉岩相矿物组合特征,而边部则保存含柯石英的超高压包体矿物组合Coe+Grt +Omp+Mgs+Arg+Ap,表明该类锆石的核部和边部分别形成于俯冲进变质阶段和超高压变质阶段。另一部分变质增生锆石具有强发光的核(阴极发光图象呈白色)和弱发光的边(阴极发光图象呈黑色)。核部保存的标志性超高压包体矿物组合为Coe+Grt+Omp+Mgs+Arg+Ap,边部则保存Qtz+Cal等退变矿物组合,有的则缺乏矿物包体,表明该类锆石自超高压变质阶段开始生长,并经历了后期退变质作用的改造。从不同微区矿物包体组合的性质及其转变特征可以明显看出,自石英榴辉岩相进变质阶段到超高压峰期变质阶段存在下列转变反应:Qtz→Coe和Dol→Mgs+Arg;而自超高压峰期变质阶段到后期退变质阶段则存在下列退变反应:Coe→Qtz和Arg→Cal。SHRIMP U-Pb定年结果表明,含石英榴辉岩相矿物包体的锆石微区记录的206Pb／238U年龄为249-241Ma,加权平均值为244±4Ma,代表了深俯冲石英榴辉岩相进变质阶段的变质年龄;含柯石英等超高压矿物包体的锆石微区记录的206Pb／238U年龄为239-231Ma,加权平均值为234±3Ma,代表超高压阶段的峰期变质年龄;而含石英和方解石的退变边记录的206Pb／238U年龄为219-211Ma,加权平均值为216±6Ma,应代表后期折返阶段的角闪岩相退变质年龄。上述两类变质增生锆石微区的Th／U比值和稀土元素配分模式十分相似,Th／U比值变化于0．02- 0．18之间,稀土元素配分模式显示重稀土相对平坦,稀土元素总量明显低于继承性碎屑锆石,具有典型变质锆石的特点。根据锆石微区矿物包体的化学成分,采用Grt-Omp和Grt-Omp-Phe温压计,结合前人的变质反应实验资料的综合分析,确定榴辉岩的原岩在深俯冲过程的石英榴辉岩相进变质阶段的变质温压条件为T=588-668℃,P=1．7-1．8GPa;超高压峰期阶段的温压条件为T=784-849℃,P>5．5GPa;而构造折返过程中角闪岩相退变质阶段的温压条件为T=550-720℃,P=0．8～1．4GPa。由此可见,大别超高压变质岩的原岩——元古代(部分可能为太古代)的陆壳物质在早三叠纪发生俯冲至55- 60km深处,并经历了石英榴辉岩相变质作用。随后这些变质岩石继续深俯冲至165～175km的地幔深处,于中三叠纪发生了超高压变质作用,石英榴辉岩相矿物组合转变为超高压榴辉岩相矿物组合。最后这些超高压变质岩石发生构造折返,至晚三叠纪抬升到约30km的中下地壳深度,并经历了角闪岩相退变质作用的改造,超高压榴辉岩相矿物组合退变为角闪岩相矿物组合。由此推断,大别超高压变质带俯冲和折返速率分别为11-12km Myr-1和7．5-8．1km Myr-1。该项成果不仅确定了大别超高压变质地体的石英榴辉岩相进变质-超高压榴辉岩相峰期变质-角闪岩相退变质的年代谱系,而且对于重塑大别超高压变质地体的快速俯冲-折返的动力学模式有着重要的科学意义。     

北苏鲁威海地区超基性岩的原岩形成时代和超高压变质时代

超基性岩是苏鲁超高压变质地体中一类特殊且十分重要的岩石类型,它们通常呈规模不一的块状、条带状或不规则透镜状 (体) 赋存于区域大面积出露的花岗质片麻岩中。锆石中矿物包体激光拉曼测试、阴极发光图像分析和不同性质锆石LA-ICP-MS U-Pb定年等综合研究结果表明,北苏鲁威海地区含橄榄石辉石岩 (样品W1和W2) 中锆石的成因十分复杂,可进一步划分3种不同类型锆石。其中第一类锆石呈自形-半自形晶,阴极发光图像显示清晰的岩浆结晶环带,矿物包体主要为Ol+Cpx+Ap, 记录的²⁰⁷Pb/²⁰⁶Pb年龄为1835~1845Ma,应代表含橄榄石辉石岩的原岩形成时代;第二类为变质重结晶锆石,呈半自形-他形晶,阴极发光图像显示模糊的岩浆结晶环带,矿物包体与第一类完全一致,记录的²⁰⁶Pb/²³⁸U年龄变化范围大,为250~784Ma之间,表明部分继承性岩浆结晶锆石明显受到后期岩浆-变质热事件的影响而发生不完全重结晶和Pb丢失,进而使其记录的年龄相对偏新;第三类锆石呈他形晶,为典型的变质锆石,阴极发光图像十分均匀,矿物包体相对少见,主要为Grt+Cpx,记录的²⁰⁶Pb/²³⁸U年龄为230~234Ma, 且与苏鲁地体榴辉岩及其围岩中含柯石英锆石微区记录的超高压变质年龄 (225~235Ma) 十分一致,应代表含橄榄石辉石岩的峰期超高压变质时代。超基性岩中超高压变质锆石的准确识别表明苏鲁地体在峰期超高压变质阶段的确存在流体,流体的存在对超高压变质锆石的形成起着至关重要的作用。该项研究不仅准确厘定北苏鲁威海地区超基性岩的原岩形成时代和超高压变质时代,而且对于深入探讨苏鲁-大别超高压地体流体行为、演化规律及其水-岩相互作用机理具有重要的科学意义。     

点苍山-哀牢山杂岩带多期变质作用:嘎洒地区变沉积岩锆石微量元素与U-Pb年代学制约

点苍山-哀牢山杂岩带位于青藏高原东南缘,为云南三江地区一条重要的造山带,由扬子板块和印支板块于晚二叠世-中晚三叠世碰撞拼合而成。杂岩带主要由各类副片麻岩、片岩、石英岩、大理岩和斜长角闪岩构成,岩石发育强烈糜棱岩化和深熔作用。本文选取哀牢山北段新平嘎洒地区变沉积岩为研究对象,通过对变沉积岩锆石的阴极发光图像、微量元素、矿物包裹体组合、表面形态和U-Pb年龄的综合研究,揭示出嘎洒地区哀牢山杂岩经历了两期变质事件:其中,含石榴子石斜长二云母片岩中30颗变质锆石获得了较为一致的206Pb/238U年龄215±6Ma~227±5Ma,加权平均年龄为222.3±1.2Ma(n=30,MSWD=0.27),这些锆石具有浑圆状或椭圆状形态、较为均匀的阴极发光图像、平坦的HREE配分模式((Lu/Gd)N=0.73~4.08)和弱的负Eu异常,这些特征与典型的高级变质岩中变质锆石相似,而锆石的Th/U比值较为分散为0.06~0.84,平均值为0.45,可能与变质过程中富Th矿物独居石分解有关。变质年龄与杂岩带中点苍山和元阳地区变质岩中、晚三叠世变质年龄极为吻合,指示这期变质事件与中-晚三叠世古特提斯洋闭合-造山有关,标志着点苍山-哀牢山杂岩带为三江地区一条重要的古缝合线。此外,嘎洒地区夕线石榴黑云二长片麻岩的岩相学特征显示,岩石经历了石榴子石的转熔作用,除两颗锆石年龄为35.4Ma外,28颗锆石(增生边)给出了误差范围内较为一致的206Pb/238U年龄(27.3±0.5Ma~31.9±0.5Ma),加权平均年龄为29.4±0.53Ma(n=28,MSWD=2.0)。这些锆石的增生边中的矿物包裹体组合为夕线石+钾长石+黑云母+石英+独居石,且具有较低的Th/U比值(0.01~0.1),平坦的重稀土(HREE)配分模式((Lu/Gd)N=0.45~7.59)和中等程度的负Eu异常,这些特征指示该类锆石为典型的变质锆石。变质年龄与新生代红河-哀牢山剪切带内大量发现的同剪切岩浆岩、变质岩的年龄较为一致,指示这期年轻的变质事件与岩石圈尺度大规模剪切运动有关。此外,两类变沉积岩中6颗继承性碎屑锆石的年龄分布范围为528~783Ma,这些锆石具有锥形的锆石形态,清晰的振荡环带,表面发育蚀痕和凹坑,较高的Th/U比(0.1),陡倾的HREE配分曲线,表明这些锆石为经过剥蚀-搬运-沉积的岩浆锆石,具有继承性碎屑锆石的特征,说明哀牢山杂岩变沉积岩中至少应包含新元古代和早古生代的沉积物源,指示研究区哀牢山杂岩带部分岩石并不属于真正意义上的扬子结晶基底。     

北秦岭超高压榴辉岩中长英质脉体的锆石U-Pb年龄及其地质意义

北秦岭地区的早古生代超高压变质作用是整个秦岭-桐柏-红安-大别-苏鲁造山带内最古老的超高压变质记录, 代表华南与华北板块之间最早的一期增生碰撞事件, 然而目前对于该地区早古生代的构造演化过程仍存在较大争议.对北秦岭官坡超高压榴辉岩中的一个长英质脉体开展了详细的锆石形态学、微量元素和U-Pb年代学研究.结果表明, 脉体中的锆石呈自形的棱柱状晶形, 发育弱的振荡环带、面状分带或无明显分带特征, 具有高的HREE、Y、U含量, 低的Th含量和Th/U比值, 说明锆石生长自含水熔体.另外, 这些变质锆石具有LREE亏损、HREE相对富集的配分模式以及明显的Eu负异常, 表明含水熔体形成于角闪岩相退变质过程.锆石的206Pb/238U加权平均年龄为494±10 Ma (MSWD=2.2), 与北秦岭超高压变质作用的峰期年龄(490.4±5.8 Ma)在误差范围内基本一致.北秦岭地体经历深俯冲作用之后发生快速折返, 并在折返过程中发生角闪岩相退变质作用诱发俯冲板片部分熔融产生含水熔体.      

苏鲁高压—超高压变质地体的陆—陆碰撞深俯冲剥蚀模式

中国苏鲁高压—超高压变质地体由2个不同时代的变质基底组成.南苏鲁(临沭—连云港地区)中不同类型高压—超高压变质岩石的原岩形成于由大陆玄武质岩石、辉长岩、表壳岩和花岗岩组成的被动大陆边缘拉伸构造环境.研究表明南苏鲁高压—超高压变质岩石的原岩所代表的花岗岩浆和基性岩浆作用为罗迪尼亚超大陆形成后的新元古代(780-700Ma)裂解事件的响应.北苏鲁(青岛—威海)超高压变质地区的花岗质片麻岩锆石SHRIMPU-Pb定年表明,变质基底的年龄是2400Ma(或 & 2400 Ma),并经历了1800-1700Ma和-200Ma的变质事件,研究表明苏鲁高压—超高压变质地体由2个不同时代变质基底组成,北苏鲁的变质基底属于北中国板块胶辽朝地块的一部分,形成时代比南苏鲁基底老得多,其与南苏鲁地块之间的界限位于五莲以北到海阳所以南一线.由于在北苏鲁含柯石英的透辉石石英岩锆石SHRIMPU-Pb定年获得精确超高压峰期变质年龄为(234.1±4.2)Ma,退变质年龄为(218.2±1.5)Ma,表明南、北苏鲁2个不同时代基底地块同时经历了超高压变质作用.根据上述事实,提出苏鲁高压—超高压变质地体的陆—陆碰撞俯冲剥蚀新模式,即扬子板片在240-220Ma的深俯冲作用中拽动上部胶辽朝板片的—部分老变质基底岩石向下俯冲至大于100km的深度,并形成楔形俯冲剥蚀体,之后又与南苏鲁俯冲板片一起快速折返上来。      

北苏鲁超高压变质带中斜长角闪岩的成因

在北苏鲁超高压变质带的威海地区的正片麻岩中,存在大量规模不一的含石榴石斜长角闪岩的透镜体或不规则团块。锆石中矿物包体的激光拉曼测试、阴极发光图像分析、稀土元素和微量元素以及Lu-Hf同位素的LA-(MC)-ICP-MS测试等综合研究结果表明,含石榴石斜长角闪岩 (WH17) 只存在一种成因类型的锆石,即变质锆石。该类锆石自核部到边部均保存了典型的超高压包体矿物组合：柯石英 (Coe)+石榴石 (Grt)+绿辉石 (Omp)+多硅白云母 (Phe)+金红石 (Rt)+磷灰石 (Ap),相应的阴极发光图像自核部到边部十分均匀,具有典型变质锆石的特点。锆石核部和边部的稀土元素含量特征和配分模式也十分相似,主要表现为轻稀土元素相对亏损,而重稀土元素相对平坦,无Eu异常 (Eu/Eu^*=0.94~1.04), 具较明显的正Ce异常 (Ce/Ce^*=83.4~111.0),低的Th/U (<0.02) 和 Lu/Hf (<0.00010) 比值以及极低的 Th (1×10^-6~4×10^-6) 含量。锆石核部到边缘的重稀土元素亏损与石榴石稳定有关,而无Eu异常则与斜长石分解关系密切。上述特征表明,含石榴石斜长角闪岩中的锆石均是在超高压变质阶段形成的变质锆石,而研究样品中继承性岩浆结晶锆石的缺乏,是由于含石榴石斜长角闪岩在原岩形成时Zr的成分明显匮乏所致。SHRIMP U-Pb定年结果表明,含柯石英锆石自核部到边部记录了十分一致的²⁰⁶Pb/²³⁸U 年龄,变化于 240.2±3.3Ma 到220.2±2.8Ma 之间,加权平均年龄为229.8±2.0Ma, 这组年龄与前人对苏鲁超高压变质带中榴辉岩、正片麻岩、副片麻岩和大理岩等含柯石英锆石微区记录的年龄完全吻合,应代表苏鲁地体超高压变质时代。Lu-Hf同位素研究结果表明,含石榴石斜长角闪岩中变质锆石的Hf同位素特征与苏鲁超高压变质带中榴辉岩的继承性岩浆锆石和变质锆石均存在本质差别,其中¹⁷⁶Hf/¹⁷⁷Hf=0.00001~0.00003, ¹⁷⁶Hf/¹⁷⁷Hf (_t)=0.282052~0.282107, 相应的ε_Hf(t)=-21.1~-18.8, 模式年龄t_DM2=2.14~2.24Ga,这些Lu-Hf同位素特征与围岩正片麻岩含柯石英锆石微区的Hf同位素性质十分相似, 充分表明了北苏鲁含石榴石斜长角闪岩在超高压变质阶段形成的变质锆石所必须的Zr和Hf成分不是来自体系本身,而是来源于围岩正片麻岩。     

大别—苏鲁超高压变质带内部的浅变质岩

大别苏鲁超高压变质带内部零星出露有若干呈构造残片状产出的浅变质岩，主要以变质碎屑岩－千枚岩－大理岩组合为代表，遭受过低绿片岩相变质和脆－韧性变形作用的改造，与围岩均为构造接触（断层或韧性剪切带）。微古生物化石研究表明，其原岩为震旦纪前后扬子板块北缘的浅海相沉积。同位素年代学研究指示，它们经历过加里东期和印支期构造热事件的影响，与区域超高压岩石经受的构造热事件时间一致；氧同位素研究得到，部分浅变质岩原岩曾遭受过高温大气降水热液蚀变，与区域超高压岩石经受的构造热事件时间一致；氧同位素研究得到，部分浅变质岩原岩曾遭受过高温大气降水热液蚀变，与区域超高压岩石的同位素特征一致。这些浅变质岩的原岩为扬子板块北缘震旦系沉积岩及其中的火山碎屑岩，构造上具有板块俯冲过程中的刮削岩片－构造加积楔的产状和形成机制，因此可以是大陆板块俯冲加积楔的一部分。     

左权变质杂岩区早前寒武纪变质演化及其构造指示

左权变质杂岩构造上位于华北中部带中南段,向东紧邻赞皇变质杂岩。研究区广泛发育长英质黑云斜长片麻岩和斜长角闪岩,无典型变泥质岩石出露,斜长角闪岩多以似层状或透镜状方式产于似层状的片麻岩中,二者在局部地区侵入接触关系明显。该地区可识别出三期变形作用和三期变质作用,区域片麻理所代表的第二期变形作用(D2)与峰期变质作用(M2)事件相对应。杂岩区含榴黑云斜长片麻岩和含榴斜长角闪岩中较好地保留了多个阶段的变质作用信息,本文重点研究其变质演化过程。含榴黑云斜长片麻岩中仅保留峰期阶段矿物组合,变质条件为730℃/8.5kbar。含榴斜长角闪岩记录了3个阶段的变质矿物组合,第一阶段矿物组合(M1)为进变质矿物组合,以石榴石变斑晶内部的早期包裹体及其临近的石榴石核部为代表,即Grt1+Pl1+Amp1+Qtz±Bt1±Chl1±Ilm±Ap,该阶段的温度和压力范围分别为:608~643℃/5.2~5.5kbar;第二阶段矿物组合(M2)为变质峰期矿物组合,主要由石榴石XMn最低的"边部"和基质矿物(Grt2+Amp2+Pl2+Qtz±Cpx2±Bt2±Ep2±Ilm±Ap)组成。最高变质温度大于670℃,最高变质压力大于9.4kbar。第三阶段矿物组合(M3)为退变质减压矿物组合,其典型代表是石榴石边部发育的Pl3+Hbl3+Cum3+Qtz±Bt3后成合晶矿物组合,呈细粒交生状结构特征,该阶段温压估算范围为:611~627℃/5.1~5.9kbar。左权变质杂岩区岩石变质程度虽明显低于赞皇变质杂岩区(Tmax812℃,Pmax12.5kbar),但两杂岩区岩石拥有类似的变质演化特征,均记录了包含近等温降压型(ITD)退变质片段的顺时针P-T轨迹,指示碰撞造山环境。结合中部带其它杂岩区的变质演化特征,推测左权变质杂岩卷入了晚太古代-早元古代末期华北克拉通东、西部陆块之间的碰撞造山过程。     

Energetics of metamorphic crystallization

Metamorphic crystallization necessitates nucleation of new grains. Associated with this process is an energy barrier which requires an input of energy sufficient to make the net change of free energy with nucleus growth decrease so that the process of grain crystallization will proceed. Temperature increase and elastic strain are widely accepted as capable of including metamorphic crystallization. Evaluation of these suggests that an energy input on the order of 0.x cal gm^?1 is commonly enough to overcome the energy barrier and induce metamorphic crystallization. Both processes are necessarily timebound to the time of energy input. Conservative quantitative evaluations of the increase in interfacial free energy by grain size reduction, and of the energy increase resulting from increased dislocation density of grains, show that energetically, these may be equally capable of inducing metamorphic crystallization. These processes can store energy in the system; later release of that energy by metamorphic crystallization may occur under stress and temperature conditions much different from those that accompanied the input of the energy. Furthermore, the formation of a new set of grains will necessarily eliminate the evidence of the precursor state, whether fine granulation or a condition of high dislocation density in the grains of the system.Experiments have demonstrated the existence and properties of tiny short-lived hot spots on the surfaces of sliding solids. From this we infer the likelihood of such high spot temperatures being realized at grain boundaries during penetrative deformation. The energy concentrated at these spots may help to overcome the energy barrier to nucleation and grain growth and may stimulate formation of stable grains and the progressive elimination of metastable grains during deformation. This is a syntectonic process, but recognizing that syntectonic metamorphic crystallization is most characteristic of regionally dynamothermally metamorphosed terranes, the importance of grain boundary hot spots in providing energy for metamorphic crystallization may be very great.     

Uranium in metamorphic rocks

The distribution of U has been studied in two metamorphic rock-series with a gradient of regional metamorphism. One series ranges from the lowest greenschist to amphibolite facies and the other one shows increasing metamorphic grade from amphibolite to granulite facies. Several medium and high pressure granulitic inclusions from alkali basalts were also analyzed. The abundances of U in the rocks do not appear to be affected by metamorphism below the granulite facies grade. Granulites are depleted in U in comparison with equivalent rocks of amphibolite facies grade. There are also differences in their U distribution, as the bulk of U in amphibolite facies rocks is located along the fractures and cleavage planes of ferro-magnesian minerals and in U-rich accessories, while in granulites, most of the U resides in accessory minerals. It seems that the depletion of U in granulites is due to a loss of U which is not located in accessory minerals or in the crystal structure of rock-forming minerals and may also be related to a migration of hydrous fluids, perhaps during dehydration.     

Fluids in metamorphic rocks

Basic principles for the study of fluid inclusions in metamorphic rocks are reviewed and illustrated. A major problem relates to the number of inclusions, possibly formed on a wide range of P–T conditions, having also suffered, in most cases, extensive changes after initial trapping. The interpretation of fluid inclusion data can only be done by comparison with independent P–T estimates derived from coexisting minerals, but this requires a precise knowledge of the chronology of inclusion formation in respect to their mineral host.

The three essential steps in any fluid inclusion investigation are described: observation, measurements, and interpretation. Observation, with a conventional petrographic microscope, leads to the identification and relative chronology of a limited number of fluid types (same overall composition, eventually changes in fluid density). For the chronology, the notion of GIS (Group of synchronous inclusions) is introduced. It should serve as a systematic basis for the rest of the study. Microthermometry measurements, completed by nondestructive analyses (mostly micro-Raman), specify the composition and density of the different fluid types. The major problem of density variability can be significantly reduced by simple considerations of the shape of density histograms, allowing elimination of a great number of inclusions having suffered late perturbations. Finally, the interpretation is based on the comparison between few isochores, representative of the whole inclusion population, and P–T mineral data. Essential is a clear perception of the relative chronology between the different isochores. When this is possible, as illustrated by the complicated case of the granulites from Central Kola Peninsula, a good interpretation of the fluid inclusion data can be done. If not, fluid inclusions will not tell much about the metamorphic evolution of the rocks in which they occur.     

Uranium and boron distributions related to metamorphic microstructure-evidence for metamorphic fluid activity

The distribution of uranium and boron in polymetamorphosed, granulite facies schists and gneisses has been studied using particle track methods. The concentration and distribution of these elements when examined in relation to mineralogy and microstructure provide an insight into: (1) the behaviour of U and B in metamorphism, (2) the activity of a fluid phase in the metamorphic processes and (3) the nature of chemical processes during schistosity development. A low concentration of primary U occurs in micro-inclusions of apatite and zircon (many are metamict) in the granulite facies M₁ assemblage. This assemblage which lacks B, except for zoned sillimanite, has undergone a localized retrograde metamorphism RM₁ characterised by hydrous alteration products containing abundant U and B. The RM₁ metamorphism is attributed to fluids generated during granulite facies dehydration reactions. A schistosity S₂ defined by M₂ fibrolite aggregates overprints the M₁ events. It is associated with (1) intragranular U concentrated in M₂ apatite and titanium bearing minerals and (2) abundant intergranular U within the fibrolite aggregates. High B contents also occur with the fibrolite. The S₂ schistosity appears to develop in a metamorphic environment containing a fluid enriched in U and B.     

澜沧江杂岩带小黑江-上允地区蓝片岩的成因及变质演化

通过对澜沧江杂岩带小黑江-上允地区蓝片岩的岩相学、地球化学、成因矿物学以及相平衡模拟的综合研究,阐述蓝片岩的原岩以及变质演化过程。地球化学分析结果显示,蓝片岩具有一致的稀土元素配分模式,具弱Eu正或负异常,稀土元素和微量元素特征与OIB相似,其原岩可能为OIB型玄武岩。详细矿物学研究表明,本区蓝片岩记录了俯冲峰期蓝片岩相变质和峰期后绿片岩相变质两个变质阶段,其矿物组合分别为蓝闪石+钠长石+多硅白云母+绿泥石+绿帘石和蓝闪石+钠长石±阳起石+绿泥石+绿帘石。通过Na_2O-Ca O-Fe O-MgO-Al_2O_3-SiO_2-H_2O-O体系相平衡计算,得到两个阶段的压力范围分别约为0.95 GPa和0.40 GPa。     

大别山超高压变质带内浅变质岩层再认识

本文依据浅变质岩层所保存的韵律层理、沉积构造、火山角砾一凝灰结构及残存的微古植物组合等,以及硅酸盐、稀土、微量元素分析成果的判别,对浅变质岩层进行了再认识,认定该浅变质岩是一套经受浅变质作用改造的火山碎屑沉积岩层;并就建立其地层序列,区域地层对比进行了论述;对其存在意义和时代归属进行了讨论.     

Development of inverted metamorphic isograds in the western metamorphic belt, Juneau, Alaska

An inverted metamorphic gradient is preserved in the western metamorphic belt near Juneau, Alaska. The western metamorphic belt is part of the Coast plutonic–metamorphic complex of western Canada and southeastern Alaska that developed as a result of tectonic overlap and/or compressional thickening of crustal rocks during collision of the Alexander and Stikine terranes. Detailed mapping of pelitic single-mineral isograds, systematic changes in mineral assemblages, and silicate geothermometry indicate that thermal peak metamorphic conditions increase structurally upward over a distance of about 8 km. Peak temperatures of metamorphism increase progressively from about 530 °C for the garnet zone to about 705 °C for the upper kyanite–biotite zone. Silicate geobarometry suggests that the thermal peak metamorphism occurred under pressures of 9–11 kbar. The metamorphic isograds are in general parallel to the tonalite sill that is regionally continuous along the east side of the western metamorphic belt, although truncation of the isograds north of Juneau indicates that the sill intrusion continued after the isograds were established. Our preferred interpretation of the cause of the inverted gradient is that it formed during compression of a thickened wedge of relatively wet and cool rocks in response to heat flow associated with the formation and emplacement of the tonalite sill magma. Garnet rim compositions and widespread growth of chlorite suggest partial re-equilibration of the schists under pressures of 5–6 kbar during uplift in response to final emplacement and crystallization of the tonalite sill. The combined results of this study with previous studies elsewhere in the western metamorphic belt indicate that high-T/high-P metamorphism associated with the collision of the Alexander and Stikine terranes was a long-lived event, extending from about 98 Ma to about 67 Ma.     

包含变质岩分类三要素的主要变质岩分类表

变质岩分类的三要素是:变质岩的物质成分(化学成分、矿物成分)、变质岩的组构(结构、构造)和变质岩的成因(变质作用类型和形成变质岩的物理化学条件).由于变质岩的化学成分、矿物成分、组构特征和形成变质岩的地质环境十分复杂,致使至今尚无以变质岩分类三要素为基础的、内容比较完善的分类方案.本文中主要变质岩的分类是以其分类三要素为基础编制的,首次将不同成因的变质岩类并列于同一表中、将鉴定变质岩的主要标志性矿物成分和组构特征列入同一分类表中.该分类对鉴定变质岩石具有可操作性和实用性,分类表中涵盖了自然界主要的变质岩石.