滇西北红山矽卡岩型铜矿床石榴子石原位成分及其地质意义

红山铜多金属矿床是义敦岛弧南端规模最大的矽卡岩型铜矿床。矽卡岩矿物以石榴子石为主,多以粒状或粒状集合体产出,偶见与透辉石共生。根据穿插关系及光学特征,将该矿床的石榴子石分为早(Grt I)、晚(Grt II)两个世代。本文利用电子探针和LA-ICP-MS原位微区分析技术对两类石榴子石开展了系统研究。石榴子石的SiO_2含量为34. 47%～36. 29%、Ca O含量为32. 48%～34. 59%、FeO含量为20. 58%～28. 17%、Al_2O_3含量为0. 01%～5. 39%,计算获得其属于钙铁榴石-钙铝榴石系列(Gro1-29And69-98)。Grt I较Grt II更富钙铁榴石组分,而Grt II更多发育振荡环带。石榴子石稀土元素总量较低(ΣREE=8. 72×10~(-6)～368×10~(-6))、轻重稀土元素分异明显(LREE/HREE=2. 13～3104)、多正Eu异常(δEu=0. 53～13. 6)。亏损Rb、Ba和Sr等大离子亲石元素(Rb=0. 02×10~(-6)～3. 75×10~(-6),Ba=0. 01×10~(-6)～0. 74×10~(-6),Sr=0. 01×10~(-6)～3. 23×10~(-6)),富集Th、U和Zr等高场强元素(Th=0. 01×10~(-6)～27. 8×10~(-6),U=0. 83×10~(-6)～98. 7×10~(-6),Zr=0. 03×10~(-6)～175×10~(-6))。Grt II的稀土元素总量、LREE/HREE比值、δEu值、Y和U含量总体高于Grt I。石榴子石主量和微量元素含量及变化特征表明,矽卡岩化早期的流体为相对封闭、酸性、氧逸度较高的体系,热液扩散交代作用占主导,多形成无环带的石榴子石;而晚期,流体演化为相对开放、弱酸性-中性、氧逸度相对较低的体系,渗滤交代作用占主导,多形成振荡环带发育石榴子石。此外,石榴子石中较高的Zn含量(Zn=1. 42×10~(-6)～37. 2×10~(-6))说明成矿流体富集Zn,可能暗示了一定的锌成矿前景。     

云南马厂箐铜钼矿床石榴子石LA-ICP-MS原位U-Pb定年及成分研究SCIEI北大核心CSCD

云南马厂箐斑岩-矽卡岩型铜钼矿床位于金沙江-哀牢山成矿带东部,是与喜马拉雅期富碱斑岩侵入有关的大型多金属矿床。本文以其矽卡岩型矿化中的石榴子石为研究对象,利用电子探针和LA-ICP-MS原位U-Pb定年技术开展了成分和年代学测试分析。根据石榴子石的手标本及镜下特征,将其划分为早(Grt I)、中(Grt II)、晚(Grt III)三个世代,其中SiO_(2)、CaO、Al_(2)O_(3)、FeO含量分别为34.15%~36.97%、32.55%~34.40%、0.03%~10.47%、和15.6%~28.3%,属于钙铝榴石-钙铁榴石固溶体系列(Gro_(1-47)And_(51-99)),Grt I较Grt II和Grt III更富含钙铝榴石。三者均富含高场强元素(Th、U、Nb、Ti等)、亏损大离子亲石元素(Rb、Sr、Sc、Y、Sn等)。稀土配分总体呈现轻稀土相对富集、重稀土相对亏损的右倾型。GrtⅠ从核到边Eu均为负异常,GrtⅡ和GrtⅢ核部显示微弱的Eu负异常而边部显示强烈的Eu正异常。石榴子石化学成分及岩相学特征表明早-中期流体处于一个弱氧化、近中性、相对封闭的体系,以扩散交代为主;晚期流体则处于高氧逸度、酸性、相对开放的体系,以渗透交代为主。LA-ICP-MS原位U-Pb同位素测试获得49个石榴子石的U、Th、Pb含量分别为1.95×10^(-6)~56.85×10^(-6)、0.01×10^(-6)~4.74×10^(-6)和0.10×10^(-6)~9.45×10^(-6),T-W图解获得^(206)Pb/^(238)U下交点年龄为34.77±0.38Ma,限定了矽卡岩矿化的时间。综合研究区已有岩浆活动(38~34Ma)、斑岩型矿化(36~34Ma)时限,认为马厂箐斑岩型矿化和矽卡岩型矿化属于同一个岩浆-热液成矿系统的产物。     

点苍山-哀牢山杂岩带中北段嘎洒地区变沉积岩的成因矿物学与变质演化特征

点苍山-哀牢山变质杂岩带中北段嘎洒地区出露了多种典型的变沉积岩,其中夕线石榴黑云二长片麻岩和二云母片岩保存多期/多阶段矿物相转变特征,本文通过岩相学和矿物化学的综合分析,并结合传统矿物对温压计的估算结果,限定上述典型变沉积岩峰期角闪-麻粒岩相(M1)阶段、近等温减压-高温剪切变形阶段(M2)和晚期退变质(M3)阶段的矿物组合及变质温压条件。峰期角闪-麻粒岩相(M1)阶段的矿物组合为:石榴石(Grt)+板柱状夕线石(Sil1)+黑云母(Bt1)+钾长石(Kfs)+斜长石(Pl)+石英(Qtz)+钛铁矿(Ilm),变质温度压力条件为t=690~750℃,p=690~810 MPa;近等温减压-高温剪切变形阶段(M2)阶段,稳定矿物组合为:Grt+Sil2+Bt2+Kfs+Pl+Qtz+Ilm,黑云母在强烈走滑剪切作用下发生脱水熔融反应:2 Bt→Sil+6(Mg,Fe)O+K_2O+5 Qtz+2 H_2O,石榴石、黑云母和夕线石等受到剪切变形影响而发生强烈定向,形成的温度压力条件为t=650~720℃,p=450~630 MPa;晚期退变质阶段(M_3)的稳定矿物组合为:Qtz+Bt+Ms+Pl,退变的温度压力条件为t=580~640℃,p=400~500MPa。其变质演化p-T轨迹样式具有近等温减压的顺时针型式,表明点苍山-哀牢山变质杂岩带曾经历了一次明显的俯冲-碰撞造山事件,峰期变质可达到角闪-麻粒岩相;在碰撞后的构造折返过程中,上述变质岩石发生强烈的高温剪切变形作用,并伴随着黑云母等含水矿物的脱水熔融。     

夹皮沟片麻杂岩对吉林南部地区太古宙地壳演化的制约——来自岩相学、地球化学、锆石U-Pb年代学方面的证据

吉林南部地区是华北克拉通北缘重要的太古宇出露区。为探讨华北克拉通新太古代构造-岩浆热事件的性质及早前寒武地壳的形成与演化,选择吉林南部夹皮沟TTG片麻杂岩进行研究。研究表明这套岩石高Al(Al2O3=14.97%~16.28%),富Na(Na2O=3.73%~4.54%)、低Mg(MgO=1.96%~2.56%)且平均镁指数为43;富集LREE,亏损HREE,Eu异常并不明显;富集Sr(358×10~(-6)~1114×10~(-6))、Ba(613×10~(-6)~1200×10~(-6))等大离子亲石元素,亏损Nb(6.39×10~(-6)~11.2×10~(-6))、Ta(0.384×10~(-6)~0.679×10~(-6))等高场强元素;Yb(1.17×10~(-6)~1.89×10~(-6))和Y(9.93×10~(-6)~19.3×10~(-6))含量也很低,Sr/Y比值很高(36.0~57.7),以上地球化学特征表明其残留相主要为石榴子石、角闪石和含Ti矿物,没有斜长石,属于高压型TTG岩类。LA-ICPMS锆石U-Pb结果表明其形成于~2509Ma,属于新太古代晚期的产物。结合以往TTG片麻岩类的地球化学研究及Nd模式年龄,这期岩浆事件可能代表古老地壳(~2.8Ga)的重熔。岩相学和锆石形态学研究表明研究区经历了多期变质作用,其中~2509Ma的高级变质事件与岩浆事件几乎同时,可能与地幔柱环境下的岩浆底侵作用有关。     

大青山-乌拉山变质杂岩带石拐地区富铝片麻岩成因矿物学与变质演化

大青山-乌拉山变质杂岩中石拐地区富铝片麻岩出露于华北克拉通孔兹岩带中段,包括夕线石榴堇青二长片麻岩、紫苏石榴黑云二长片麻岩和夕线石榴黑云二长片麻岩,与基性麻粒岩彼此呈互层或夹层产出.根据岩相学观察、成因矿物学和变质反应结构的系统研究,结合地质温压计估算以及相平衡模拟的综合分析,揭示石拐地区富铝片麻岩的变质演化可划分为四个变质阶段.其中,早期进变质阶段(M1)矿物组合以石榴石核部及其包裹的细粒矿物黑云母+石英+斜长石±夕线石±钾长石±尖晶石为特征;峰期变质阶段(M2)的稳定的矿物组合为石榴石+基质中粗粒夕线石+黑云母+石英+斜长石+钾长石±磁铁矿±钛铁矿,形成的温压条件为T=840 ～ 860℃,P=10.0～10.5kbar;峰后近等温减压阶段(M3)以石榴石边部发育含堇青石的后成合晶为特征,并发生一系列典型的减压反应:Grt+ Sil+ Qz→Crd、Grt+ Melt→Crd+ Bt+ PI和Grt+ Melt→Crd+ Qz±P1,形成新的矿物组合为石榴石+堇青石+黑云母+斜长石+石英±夕线石±紫苏辉石,相应的温压条件为T=720～ 800℃和P=5.6 ～6.1kbar;晚期角闪岩相降温阶段(M4)的矿物组合是石榴石+石榴边部细粒黑云母+斜长石+石英+磁铁矿±钾长石±钛铁矿,记录的温压条件为T=616 ～661℃和P=3.4 ～5.2kbar.石拐地区富铝片麻岩及相关岩石具有典型的近等温减压的顺时针P-T轨迹,峰后经历了近等温减压和近等压降温的变质演化阶段.上述研究结果表明,石拐地区富铝片麻岩曾卷入到华北克拉通西部的阴山陆块和鄂尔多斯陆块间的俯冲-碰撞造山及随后的快速隆升的演化过程.     

Discovery of scandium enrichment and its geological significance in the Dawanshan Neoproterozoic metamorphic basalts from the Mouding area in Central Yunnan Province,ChinaSCIEI北大核心CSCD

钪(Sc)是世界各国竞相争夺的关键金属矿产资源之一。滇中牟定大弯山变质玄武岩厚度>36.5m,出露面积0.5km^(2),形成时代为新元古代南华纪(781.3±1.9Ma)。本文对该变质玄武岩开展了全岩地球化学分析、全自动矿物分析(TESCAN TIMA)观测、NPPM薄片区域面扫和单矿物原位LA-ICPMS分析等研究,结果显示变质玄武岩全岩Sc含量为47.0×10^(-6)~97.9×10^(-6),平均含量为69.1×10^(-6),钪氧化物(Sc_(2)O_(3))平均含量为106×10^(-6),变质玄武岩空间Sc矿化特征稳定,具有形成钪矿资源的潜力。同时,变质玄武岩共伴生有钛和铁矿化,全岩TiO_(2)含量为2.57×10^(-2)~6.13×10^(-2),平均为4.25×10^(-2);TFe含量为13.3×10^(-2)~23.7×10^(-2),平均为17.7×10^(-2)。Sc可能存在类质同象和离子相两种赋存形式,类质同象形式Sc主要赋存于钛铁矿和金红石矿物中,钛铁矿中Sc含量为70.0×10^(-6)~168×10^(-6),平均值为108×10^(-6);金红石中Sc含量高达297×10^(-6);而磁铁矿、黑云母等矿物中Sc含量较低,均低于全岩Sc含量,对全岩Sc矿化贡献较小。牟定大弯山Sc矿化与以往报道侵入岩及其风化壳中Sc矿化在富集特征、赋存岩性和载体矿物等方面不同,是变质火山岩中新发现的Sc矿化信息,显示了较好的找矿潜力,对Sc资源勘查和研究具有重要指示意义。     

柴北缘鱼卡河榴辉岩的变质演化——石榴石成分环带及矿物反应结构的证据

柴北缘鱼卡河榴辉岩的典型矿物组合为石榴石-绿辉石-多硅白云母-金红石。其中粗粒石榴石变斑晶普遍保存进变质生长环带,从核部到边部石榴石的化学成分、包体矿物的种类和粒度皆呈现出规律的分带性。岩相学和矿物化学研究进一步表明,该榴辉岩经历了前榴辉岩相、榴辉岩相及后榴辉岩相三个主要变质演化阶段。前榴辉岩相以石榴石核部成分及核部包体矿物组合石榴石(GrtⅠ) 角闪石(AmpⅠ) 斜长石(PⅡ) 石英(Qtz)为特征,P-T 估算结果为450～500℃和0.6～0.7GPa。榴辉岩相变质阶段又可细分为早期、峰期榴辉岩和退变角闪榴辉岩三个亚相。早期榴辉岩亚相以石榴石幔部成分和幔部包体矿物组合石榴石(GrtⅡA) 绿辉石(OmpⅡA) 多硅白云母(PheⅡA)±黝帘石(Zoi) 金红石(Ru)为代表,估算的温压条件为580～640℃和2.4～2.5GPa;峰期榴辉岩相以石榴石的边部(GrtⅡB)及基质中绿辉石(OmpⅡB)和多硅白云母(PheⅡB)的核部为代表,矿物组合为 GrtⅡB OmpⅡB PheⅡB Ru,估算的 P-T 条件为620～680℃和3.0～3.4GPa;退变角闪榴辉岩相以共生的石榴石的最边部(GrtⅡC)、基质绿辉石(OmpⅡC)和多硅白云母(PheⅡC)的边部及镁红闪石(AmpⅡ)组合为代表,矿物组合为 GrtⅡC OmpⅡC AmpⅡ PheⅡC,估算的 P-T 条件为700～720℃和2.3～2.4GPa。后榴辉岩阶段主要为麻粒岩-高角闪岩相,以绿辉石分解形成透辉石 钠长石冠状体以及进一步分解形成韭闪石 斜长石,铁红闪石分解形成浅闪石 斜长石为代表,P-T 估算结果为550～600℃和0.6～1GPa。温压估算结果表明,鱼卡河榴辉岩经历了升温升压—升温降压—降温降压的一个顺时针 P-T 演化轨迹,它记录了从俯冲-超高压变质-抬升的连续的演化过程。峰期变质条件为630～680℃和3.0～3.4GPa,已达超高压变质范畴。榴辉岩中进变质矿物组合和生长环带的保存说明榴辉岩的形成经历了相对快速的俯冲和折返的动力学过程。     

内蒙古集宁三岔口夕线堇青石榴二长片麻岩变质作用及年代学研究

三岔口夕线堇青石榴二长片麻岩出露于华北克拉通孔兹岩带东部的集宁变质杂岩中,该岩石保留了典型的减压反应结构。系统的岩相学观察和矿物化学研究结果表明,三岔口地区夕线堇青石榴二长片麻岩保存了两个阶段变质作用的矿物组合及相应的变质反应结构,其中峰期变质阶段M1的矿物组合为石榴石+夕线石+黑云母+石英+斜长石+钾长石+钛铁矿±磁铁矿;峰后减压的M2-1变质阶段矿物组合为石榴石+堇青石+夕线石+石英+斜长石+钾长石+钛铁矿±尖晶石,M2-2阶段的矿物组合为石榴石+堇青石+黑云母+石英+斜长石+钾长石+钛铁矿+磁铁矿±尖晶石。自峰期变质阶段到峰后减压阶段,典型的转变(减压)反应包括:Grt+Sil+Melt→Crd+Bt+Fe-oxide和Grt+Sil→Crd+Spl。相平衡模拟结合传统地质温压计限定峰期变质阶段的温压条件为T=852~862℃,P=9.3~10.2kbar;峰期后近等温减压的M2-1和M2-2阶段的温压条件分别为854~880℃和7.0~7.4kbar,820~848℃和5.3~6.4kbar。三岔口夕线堇青石榴二长片麻岩记录了典型的近等温减压型的顺时针P-T轨迹。阴极发光图像特征显示夕线堇青石榴二长片麻岩存在大量变质锆石,LA-ICP-MS U-Pb定年结果表明,所有变质锆石记录了十分一致的207Pb/206Pb年龄,其变质时代为1912±11Ma。变质作用历史说明内蒙古孔兹岩带东段的集宁地体卷入了华北克拉通西部的阴山陆块和鄂尔多斯陆块之间的俯冲-碰撞,并经历了古元古代(~1912Ma)的麻粒岩相变质作用后快速折返至地表的过程。     

甘肃敦煌红柳峡地区石榴石斜长角闪岩的变质特征、锆石U-Pb年龄及地质意义

甘肃敦煌红柳峡石榴石斜长角闪岩原岩为岛弧拉斑玄武岩,形成于不成熟岛弧.其典型矿物组合为:石榴石(Grt)-角闪石(Amp)-透辉石(Di)-斜长石(Pl),其中较粗大的变斑晶石榴石中通常保存进变质生长环带.根据岩相学和矿物化学研究得出,该区石榴石斜长角闪岩经历了四个变质演化阶段:早期进变质阶段(M1)、变质高峰期阶段(M2)、近等温减压阶段(M3)和晚期降温退变质阶段(M4).M1阶段以石榴石核部成分及其核部包体矿物组合石榴石(Grt Ⅰ)+角闪石(AmpⅠ)+斜长石(Pl Ⅰ)+石英(Qtz)为特征,P-T估算结果为n=550 ～ 600℃,P=0.3 ～0.5GPa;M2阶段以变斑晶石榴石幔部成分(GrtⅡ)及基质中的矿物组合角闪石(AmpⅡ)+透辉石(Di)+斜长石(+PlⅡ)+石英(Qtz)为特征,P-T估算结果为T=650 ～ 780℃,P=0.8 ～0.9GPa;M3阶段的代表性结构为石榴石边缘(GrtⅢ)形成由绿色角闪石(AmpⅢ)和斜长石(PlⅢ)组成的“白眼圈”,该阶段温度估算结果为630 ～ 700℃,压力大幅降低;M4阶段透辉石(Di)开始向角闪石(AmpⅣ)和斜长石(PlⅣ)转化,该阶段压力变化不大,主要是温度的降低.温压估算结果表明,红柳峡石榴石斜长角闪岩完整记录了从早期升温升压的进变质到后来的近等温减压再到最后的降温退变质的一个顺时针的PTt演化轨迹,反映的构造演化历史为:板块俯冲到地壳深部遭受高温变质作用,然后该区出现拉张的构造体制,随之地壳减薄,但是原来处于地壳深部的岩石并未出现大幅度折返和抬升,而是仍然处于原来的深度,慢慢的冷却直至正常的地温梯度.锆石U-Pb定年结果表明,红柳峡石榴石斜长角闪岩原岩的结晶年龄为1611±6Ma,该期年龄代表的构造事件与哥伦比亚超大陆的裂解事件一致,结合前人的研究成果我们推断,塔里木板块可能与哥伦比亚超大陆的裂解有联系,且当时位于塔里木东北缘的敦煌地块还是岛弧的构造环境,后来在中元古代塔里木运动中有一次板块俯冲碰撞事件,引起了一次高级变质作用(高角闪岩相),使当时的基性火山岩变质形成如今含有石榴石的斜长角闪岩.     

大兴安岭北部漠河地区早奥陶世A型花岗岩锆石U- Pb年代学、地球化学及Hf同位素研究

大兴安岭北部漠河地区黑云母二长花岗岩中锆石自形程度较好,主要为长柱状,具有较为清晰的韵律环带结构,表明锆石岩浆成因。锆石LA-ICP-MS U-Pb年代学研究显示岩石形成时代为早奥陶世(481±5Ma)。岩石地球化学研究显示,岩石属于钙碱性、弱过铝质系列花岗岩,高硅(SiO_2=71.42%～72.57%)、富碱(ALK=8.11%～8.76%)、贫镁(MgO=0.52%～0.54%)。10000Ga/Al值为2.73～2.85,Zr+Nb+Ce+Y含量374×10~(-6)～495×10~(-6),岩浆形成温度801～897℃,判断岩石为A型花岗岩。岩石有中等的负Eu异常(δEu=0.24～0.35),Rb/Sr值为1.18～1.38、Ba/La值为8.7～12.4、Mg~#值为23.3～30.3,及低Ni(1.51×10~(-6)～5.59×10~(-6))、Cr(6.60×10~(-6)～13.4×10~(-6))、V(20.0×10~(-6)～21.6×10~(-6))含量。岩石的ε_(Hf)(t)值为-2.22～5.57,二阶段模式年龄(t_(DM2))变化于1021～1453Ma之间,主要源于中新元古代增生地壳。结合区域构造演化及构造判别,认为岩石形成于额尔古纳地块与西伯利亚板块拼合造山后的伸展背景。     

Genetic Mechanism of Mineral Inclusions in Zircons from the Khondalite Series, Southeastern Inner Mongolia

The early Precambrian khondalite series is widely distributed in the Jining-Zhuozi-Fengzhen-Liangcheng area, southeastern Inner Mongolia. The khondalite series mainly consists of sillimanite garnet potash feldspar (or two-feldspar) gneiss and garnet biotite plagioclase gneiss. These gneissic rocks have commonly experienced granulite-facies metamorphism. In zircons separated from sillimanite garnet potash feldspar gneisses, many mineral inclusions, including Sil, Grt, Ky, Kfs, Qtz and Ap, have been identified by the Laser Raman spectroscopy. Generally, prograde metamorphic mineral inclusion assemblages such as Ky + Kfs + Qtz + Ap and Ky + Grt + Kfs + Qtz are preserved in the core of zircon, while peak granulite-facies metamorphic minerals including Sil + Grt + Kfs + Qtz and Sil + Grt + Kfs + Qtz + Ap are identified in the mantle and rim of the same zircon. However, in some zircons are only preserved the peak metamorphic minerals such as Sil + Grt + Kfs + Qtz and Sil + Grt + Kfs + Qtz + Ap from core to ri     

The microarea composition of garnet and biotite from the Ji’an Group in Tonghua district and its significance as an indicator of metamorphicP-T-t path

The garnet (Grt) and biotite (Bt) from gneisses of the Ji’an Group are characterized by diffusion zoning at the rim, but equilibrium composition of metamorphic peak is usually remained in extensive interior area. Garnet with growth zoning is also found in the kyanite zone. In the light of microarea compositional variation of Grt and Bt, the temperature and pressure at the progressive, peak and post-peak metamorphic stages are determined by correctly using Grt-Bt thermometer and GASP barometer. On this basis, a counterclockwiseP-T-t path can be constructed, which reflects the closing process of an ensialic rift belt in this region during the Early Proterozoic. The project was financially supported by the State Educational Commission Ph. D. Station Foundation (No. 96018702).     

Zonal REE + Y profiles in garnet and their genetic implications: A case study of metapelites from the northern Ladoga region

The objective of this study is to provide insights into the REE and Y behavior during garnet porphyroblast formation in staurolite-bearing schists as a constituent of Late Paleoproterozoic metapelites of the Ladoga Complex. The MnNCKFMASH P–T pseudosection for a single sample and Grt–Bt thermometry indicate that the garnet core grew at 520°C and under 7.0–7.2 kbar in the Grt–Bt–Pl–Chl–Ms–Zo field, whereas the garnet rim was equilibrated at 590–600°C and under 3.5–4.0 kbar. The measured zoning profiles are strongly depleted in REE + Y in the garnet core containing high Mn and Ca concentrations. The intermediate zone of garnet is enriched in La, Ce, Pr, and Nd (inner LREE + Nd annulus), as well as in Dy, Er, Yb, Lu, and Y (outer HREE + Y + Dy annulus). According to pseudosection analysis, these peaks were probably produced owing to breakdown of epidote-group minerals (allanite, REE-rich epidote) at T < 535°C and P > 6.5 kbar. Towards the rim, the HREE + Y contents gradually decrease, whereas MREE (Sm, Eu, Gd) display an inverse trend. The rim also exhibits a negative Eu anomaly. The former tendency reflects an increase in temperature during garnet crystallization and partitioning of elements between garnet and monazite. It is thought that the latter is linked to oppositely directed change in garnet-monazite partition coefficients for HREE and MREE with increasing temperature.     

Petrology and zircon U–Pb dating of well‐preserved eclogites from the Thongmön area in central Himalaya and their tectonic implications

The discovery of eclogites is reported within the Great Himalayan Crystalline Complex in the Thongmön area, central Himalaya, and their metamorphic evolution is deciphered by petrographic studies, pseudosection modelling, and zircon dating. For the first time, omphacite has been found in the matrix of eclogites taken from a metamorphic mafic lens. Two groups of garnet have been identified in the Thongmön eclogites on the basis of major and rare earth elements and mineral inclusions. Core and intermediate sections of garnet represent Grt I, in which the major elements (Ca, Mg, and Fe) show a nearly homogenous distribution with little or weak zonation. This Grt I displays an almost flat chondrite‐normalized HREE pattern, and the main inclusions are amphibole, apatite, quartz, and abundant omphacite. Grt II, forms thin rims on large garnet grains, and is characterized by rim‐ward Ca decrease and Mg increase and MREE enrichment relative to HREE and LREE. No amphibole inclusions are found in Grt II, indicating the decomposition of amphibole contributed to its MREE enrichment. Two metamorphic stages, recorded by matrix minerals and inclusions in garnet and zircon, outline the burial of the Thongmön eclogites and progressive metamorphic processes to the pressure peak: (a) the assemblage of amphibole–garnet–omphacite–phengite–rutile–quartz, with the phengite interpreted as having been replaced by Bt+Pl symplectites, represents the prograde amphibole eclogite facies stage M1₍₁₎, (b) in the peak eclogite facies [stage M1₍₂₎], amphibole was lost and melting started. Based on the compositions of garnet and omphacite inclusions, M1₍₁₎ is constrained to 19–20 kbar and 640–660°C and M1₍₂₎ occurred at >21 kbar, >750°C, with appearance of melt and its entrapment in metamorphic zircon. SHRIMP U–Pb dating of zircon from two eclogite samples yielded consistent metamorphic ages of 16.7 ± 0.6 Ma and 17.1 ± 0.4 Ma respectively. The metamorphic zircon grew concurrently with Grt II in the peak eclogite facies. Thongmön eclogites characterized by the prograde metamorphism from amphibolite facies to eclogite facies were formed by the continuing continental subduction of Indian plate beneath the Euro‐Asian continent in the Miocene.     

滇西鲁甸金沙江结合带新发现退变榴辉岩

在滇西鲁甸地区金沙江结合带新发现退变榴辉岩,其在野外呈透镜体状产于石榴子石白云母石英片岩中.利用电子探针及激光拉曼分析发现石榴子石和锆石中残留绿辉石包体.石榴子石及基质中的白云母为多硅白云母(Si(p.f.u)=3.27~3.53),指示岩石经历了高压变质作用过程.石榴子石发育进变质生长成分环带.岩相学及矿物化学特征显示,退变榴辉岩大致经历了进变质角闪岩相、峰期榴辉岩相、早期退变质以及晚期强退变这4个世代矿物组合,各阶段典型的矿物组合依次为Grt+AmpI+Qtz、Grt+Omp+Rt+Qtz+Phe、Pl+Di+AmpⅡ+Ilm+Spn+Qtz、AmpⅢ+Pl+Czo+Ilm+Qtz.该新发现对金沙江结合带复杂的变质演化P-T-t轨迹样式及年代格架、以金沙江洋为代表的整个西南三江地区古特提斯洋-陆俯冲-碰撞-造山的复杂构造演化历史以及微陆块的拼贴机制等关键科学问题的解决提供了极为重要的素材和更多的约束,具有重要的科学意义.      

Geochemistry and geochronology of multi-generation garnet: New insights on the genesis and fluid evolution of prograde skarn formation

Origin of garnet in skarn (magmatic vs. hydrothermal) and the prograde skarn fluid evolution are still controversial. Two generations of garnet (Grt1, Grt2) were identified at the Tongshankou deposit: Grt1 is anisotropic with oscillatory zoning and resorbed boundary, whilst Grt2 grew around Grt1 and formed oscillatory rims. In-situ LA-ICP-MS U-Pb dating of Grt1 and Grt2 yielded a lower intercept ²⁰⁶Pb/²³⁸U age of 142.4 ± 2.8 Ma (n = 57; MSWD = 1.16) and 142.3 ± 9.6 Ma (n = 60; MSWD = 1.06), respectively, coeval with the ore formation and ore-related granodiorite emplacement. Positive Eu anomaly, non-CHARAC Y/Ho value and low TiO₂ content, together with the mineral assemblages indicate that both Grt1 and Grt2 have a hydrothermal origin. The existence of melt and melt-fluid inclusions in Grt1, together with similar LREE-enriched patterns to the granodiorite, further indicate that Grt1 may have formed in the magmatic-hydrothermal transition. Higher U contents and LREE-enriched patterns of Grt1 indicate that fluid I is mildly acidic pH and low fO₂. The inner gray Grt2 rims (Grt2A) is HREE-enriched with low U contents, indicating that fluid II has nearly neutral pH and high fO₂. The wider Y/Ho range and LREE-enriched patterns of the outer light-gray Grt2 rims (Grt2B) show that the evolved magmatic fluid II had mixed with an external fluid, characterized by being mildly acidic pH and with high fO₂. Our results suggest that the prograde skarn-forming fluids can be multistage at Tongshankou, and the mixing of meteoric water may have been prominent in the prograde skarn stage.     

Phase Equilibria of Hornblende-Bearing Eclogite in the Western Dabie Mountain,Central China

The high-pressure (HP) eclogite in the western Dabie Mountain encloses numerous hornblendes,mostly barroisite.Opinions on the peak metamorphic P-T condition,PT path and mineral paragenesis of it are still in dispute.Generally,HP eclogite involves garnet,omphacite, hornblendes and quartz,with or without glaucophane,zoisite and phengite.The garnet has compositional zoning with X_(Mg) increase,X_(Ca) and X_(Mn) decrease from core to rim,which indicates a progressive metamorphism.The phase equilibria of the ...     

Decoding the complex internal chemical structure of garnet porphyroblasts from the Zermatt area,Western Alps

Garnet is a prototypical mineral in metamorphic rocks because it commonly preserves chemical and textural features that can be used for untangling its metamorphic development. Large garnet porphyroblasts may show extremely complex internal structures as a result of a polycyclic growth history, deformation, and modification of growth structures by intra‐ and intercrystalline diffusion. The complex internal structure of garnet porphyroblasts from garnet–phengite schists (GPS) of the Zermatt area (Western Alps) has been successfully decoded. The centimetre‐sized garnet porphyroblasts are composed of granulite facies garnet fragments overgrown by a younger generation of grossular‐rich eclogite facies garnet. The early granulite facies garnet (G‐Grt) formed from low‐P, high‐T metamorphism during a pre‐Alpine orogenic event. The late garnet (E‐Grt) is typical of high‐pressure, low‐temperature (HPLT) metamorphism and can be related to Alpine subduction of the schists. Thus, the garnet of the GPS are polycyclic (polymetamorphic). G‐Grt formation occurred at ~670 MPa and 780°C, E‐Grt formed at ~1.7 GPa and 530°C. The G‐Grt is relatively rich in Prp and poor in Grs, while E‐Grt is rich in Grs and poor in Prp. The Alm content (mol.%) of G‐Grt is 68 of E‐Grt 55. After formation of E‐Grt between and around fragmented G‐Grt at 530°C, the GPS have been further subducted and reached a maximum temperature of 580°C before exhumation started. Garnet composition profiles indicate that the initially very sharp contacts between the granulite facies fragments of G‐Grt and fracture seals of HPLT garnet (E‐Grt) have been modified by cation diffusion. The profiles suggest that Ca did not exchange at the scale of 1 µm, whereas Fe and Mg did efficiently diffuse at the derived maximum temperature of 580°C for the GPS at the scale of 7–8 µm. The Grt–Grt diffusion profiles resulted from spending c. 10 Ma at 530–580°C along the P–T–t path. The measured Grt composition profiles are consistent with diffusivities of log D_MgFe = ?25.8 m²/s from modelled diffusion profiles. Mg loss by diffusion from G‐Grt is compensated by Fe gain by diffusion from E‐Grt to maintain charge balance. This leads to a distinctive Fe concentration profile typical of uphill diffusion.     

Anatomy of garnets in a Jurassic granite from the south-eastern margin of the North China Craton: Magma sources and tectonic implications

The Late Jurassic Jingshan granite located at the south-eastern margin of the North China Craton contains abundant garnets which can be subdivided into three types based on texture and composition: (i) euhedral garnet in mafic biotite and garnet rich enclave (Grt I), (ii) coarse-grained garnet (Grt II) in the host granite, and (iii) small euhedral garnet in aplite (Grt III). In general, Grt I has higher FeO, CaO and lower MnO contents than Grt II. Grt III has higher Mn, but lower Ca contents than others. Grt I has lower MREE and HREE contents than Grt II. Grt III has prominent and distinctly negative Eu anomaly as well as higher MREE composition compared to the others. Systematic variations in oxygen isotope compositions are observed among the three garnet types, with δ¹⁸O values of <3.8‰ in most of Grt I, 3.8–4.7‰ in most Grt II (for inclusion-free garnets), and typically >4.7‰ in Grt III. Some of the Grt II and Grt III display two distinct zonings with cores having similar major and trace element compositions to Grt I.Cathodoluminescence (CL) images revealed that the zircons from different garnet-bearing samples possess fine-scale oscillatory zoned magmatic rims with inherited cores. In situ zircon U–Pb dating and trace element analyses show that the dark-luminescent magmatic rims all have Jurassic concordia ages (∼160 Ma) and similar trace element patterns. Most of the inherited cores also display similar Triassic ages of 210–236 Ma, which is similar to the ages of ultrahigh pressure (UHP) metamorphic rocks of the Dabie–Sulu orogen (230 Ma). In addition, Jurassic concordia ages were also found in a zircon inclusion in Grt I, implying that the Grt I was formed shortly before the main magmatic event. The age data suggest that the three different garnet types may be genetically related and modified by cogenetic magmatic events.Based on the zircon U–Pb ages from different garnet-bearing samples, the major element, trace element, oxygen isotope, and zoning textures of the three kinds of garnet we suggest that Grt I may be peritectic garnet, whereas Grt II and III are probably the results of magmatic dissolution–precipitation processes and re-equilibration of garnets with changing magmatic conditions during melting, differentiation, crystallization, and cooling within the granite. We conclude from the oxygen isotopic character of the garnets and ages of the zircons that the source rocks for the Jingshan granites are from Dabie–Sulu orogen representing the South China Craton.