A transitional eclogite- to high pressure granulite-facies overprint on coesite–eclogite at Taohang in the Sulu ultrahigh-pressure terrane, Eastern China

A transitional eclogite- to high-pressure granulite-facies paragenesis (Omp+Pl+Qtz±Grt) after peak coesite–eclogite facies metamorphism and predating the later amphibolite-facies overprint is identified in coesite–eclogite from the Taohang area of the Sulu ultrahigh-pressure (UHP) terrane in eastern China. These minerals were equilibrated at 17 kbar and 820°C. This reveals that fluid infiltration might activate retrograde recrystallisation even at a deep level during the exhumation process of the UHP rocks. The tectono-metamorphic significance of the unusually high pressure overprint is also discussed.     

Mineral evolution of a garnet-pyroxenite nodule within eclogite, eastern Sulu ultrahigh-pressure metamorphic terrane, East China

Detailed microtextural observations and bulk chemical analysis were undertaken on a garnet‐pyroxenite nodule within retrograde eclogites from the NE Sulu ultrahigh‐pressure metamorphic (UHPM) terrane. The results suggest that the protolith was a cumulate from a gabbroic body. The nodule consists primarily of coarse clinopyroxene grains with a very high content of the Ca‐Tschermakite molecule. Microscopic observations and back‐scattered electron images (BSE) demonstrate a complicated intergrowth of clinopyroxene, garnet and ilmenite, which represents the peak metamorphic assemblage. The primary clinopyroxene grains are armoured with a thin garnet corona up to 0.5 mm wide that forms an interconnected network. Within the clinopyroxene grains, four sets of garnet lamellae are distributed along crystallographic planes; locally, a vermicular intergrowth of garnet and diopside is developed. Besides the garnet, parallel arrays of ilmenite blebs are common within the clinopyroxene. Hydrous minerals such as amphibole, zoisite and titanite formed at later stages, and replaced diopside, garnet and ilmenite respectively. The P–T conditions determined for the formation of the garnet lamellae indicate that the garnet pyroxenite experienced UHP metamorphism at the same peak P–T condition as its host eclogite. The very high Ca‐Tschermakite content (31–34 mol.%) of the primary clinopyroxene indicates crystallization at about 9–17 kbar and 1250–1450 °C, and together with the microtextural observations, suggests that the protolith of the garnet pyroxenite was a cumulate from a former gabbroic body, in which case, the host eclogite might represent the gabbroic body.     

青藏高原拉萨地块松多榴辉岩的岩相学特征和变质演化过程

松多榴辉岩出露于拉萨地块的石英片岩中,主要由较为基性的金红石榴辉岩和较为酸性的石英榴辉岩组成。榴辉岩相矿物组合为石榴子石 绿辉石 绿帘石±多硅白云母±石英±金红石。岩石发生了较强烈的退变质作用,退变质矿物有角闪石、绿帘石、石英、钠长石及绿泥石。石榴子石变斑晶具有生长环带结构,变斑晶和基质石榴子石主要落入C类榴辉岩区,少数石榴子石变斑晶边部和基质石榴子石落入B类榴辉岩区;单斜辉石主要为绿辉石,少数Ⅰ世代和Ⅲ世代为普通辉石;角闪石均为钙质角闪石。根据石榴子石-绿辉石-多硅白云母矿物温压计计算,获得的温压范围为630～777℃和2.58～2.70GPa,峰期变质条件接近于石英-柯石英转变线。榴辉岩的原岩经历了从高绿片岩相、角闪岩相、榴辉岩相、角闪岩相到高绿片岩相的变质过程,这反映了与古特提斯洋闭合有关的俯冲进变质作用和随后的折返退变质作用。     

俄罗斯白海活动带Uzkaya Salma地区含绿纤石榴辉岩的岩石学特征及其变质演化

俄罗斯白海活动带Uzkaya Salma地区榴辉岩中发现的绿纤石形成于榴辉岩化早期亚绿片岩相阶段。该绿纤石多以包体形式存在于退变榴辉岩的变斑晶石榴石矿物中,并与榍石、金红石、单斜辉石、绿泥石、绿帘石、石英等矿物伴生,极少量单颗粒绿纤石包裹在基质单斜辉石(透辉石)矿物中,呈浑圆状。绿纤石成分上属于铝绿纤石和铁绿纤石,其中以铝绿纤石为主。在详细的岩相学研究基础上,通过相平衡计算,结合矿物温压计计算结果,发现含绿纤石榴辉岩共经历了4阶段的变质演化:Ⅰ早期进变质阶段,以石榴石中的绿纤石+绿泥石+绿帘石+石英等矿物包裹体为特征,依据实验岩石学研究的矿物组合绿纤石+绿泥石+石英和铁绿纤石+绿帘石稳定域,估算该变质阶段温压条件t=160~320℃,p=0.2~0.8 GPa;Ⅱ峰期榴辉岩相阶段,矿物组合为石榴石+Di-Pl后成合晶推测的绿辉石+金红石±角闪石+石英,石榴石核部镁等值线和绿辉石硬玉分子等值线限定其峰期温压条件为t=725~740℃,p=1.4~1.5 GPa;Ⅲ高压麻粒岩相退变质阶段,矿物组合为石榴石+透辉石+角闪石+斜长石+石英,石榴石-单斜辉石温度计和后成合晶中斜长石钙等值线限定该阶段的温压条件t=725~750℃,p=1.1~1.3 GPa;Ⅳ晚期角闪岩相退变质阶段,矿物组合角闪石+斜长石±黑云母+石英,相平衡计算和角闪石-斜长石温度计限定温压条件为t=670~700℃,p=0.7~0.9 GPa。综上,确定了俄罗斯白海活动带Uzkaya Salma地区含绿纤石榴辉岩具有顺时针的p-T演化轨迹,峰期对应的地温梯度为15℃/km,俯冲进变质阶段经历了绿纤石-绿帘石相变质,由峰期榴辉岩相到退变质高压麻粒岩相具近等温降压的特征。研究表明,板块的"冷"俯冲作用在地球演化早期太古宙时期就可能出现了。     

苏北青龙山超高压变质榴辉岩流体包裹体特征与流体演化

根据青龙山超高压变质榴辉岩中流体包裹体的化学成分、矿物中的分布特征将岩石中的流体包裹体分为五类,即富N2包裹体、高盐度(22.4-略大于23.2wt%NaCl)的NaCl CaCl2 H2O体系流体包裹体、中高盐度(12.6-16.0wt%NaCl)的含Mg2 或Fe2 的NaCl H2O体系流体包裹体、中等盐度(6.4-10.5wt%NaCl)水溶液包裹体和低盐度(3.3-0.2wt%NaCl)的水溶液包裹体。富N2包裹体形成于超高压变质峰期阶段,高盐度的流体包裹体形成于超高压变质岩折返早期固体出溶体出溶阶段,中高盐度的流体包裹体形成于高压变质重结晶作用阶段,中等盐度的流体包裹体形成于角闪岩相变质重结晶作用阶段,低盐度的流体包裹体形成于折返晚期的绿片岩退变质作用阶段。超高压变质峰期阶段和折返早期的高盐度流体和中高盐度的流体主要来自继承原岩中的流体(如含NH4 矿物分解或片麻岩原岩中的有机质分解,名义上无水矿物中羧基水的出溶),晚期角闪岩相退变质阶段的中等盐度的流体除名义上无水矿物中羟基水的出溶外还有外来流体的加入,绿片岩相退变质作用阶段的流体主要为外来流体。     

北秦岭松树沟榴辉岩的确定及其地质意义

松树沟石榴石角闪岩(榴闪岩)呈透镜状产于松树沟超镁铁岩旁侧的斜长角闪岩中,一直以来被认为是形成于接触交代变质或麻粒岩相变质过程。详细岩相学及矿物元素分析,在榴闪岩的基质矿物、石榴石幔部及锆石包体中发现残留的绿辉石,而且石榴石也保存了明显的进变质主、微量元素成分环带,表明松树沟榴闪岩为榴辉岩退变质的产物,至少经历了从角闪岩相到榴辉岩相再到角闪岩相的三阶段顺时针PT演化过程。锆石定年结果得到榴辉岩的变质年龄为500±8Ma,原岩结晶时代为796±16Ma,与秦岭岩群北侧官坡超高压榴辉岩的变质年龄和原岩年龄完全一致,也与北秦岭区域高压-超高压变质时代和原岩的结晶时代一致。表明松树沟榴辉岩与北秦岭造山带已发现的高压-超高压变质岩石一起都应是古生代大陆深俯冲作用的结果,而松树沟超镁铁岩可能是俯冲的大陆板片在折返过程中携带的俯冲隧道中的交代地幔岩。     

鄂北高压榴辉岩相变质带的变质、变形和流体演化

大别高压超高压变质带从南到北可分成四个带,它们是绿帘蓝片岩带、高压榴辉岩带(南带)、超高压榴辉岩带和高压榴辉岩带(北带).高压榴辉岩相变质带以蓝闪石榴辉岩为代表,并出现多硅白云母、绿帘石、石英、金红石和锆石等变质矿物. 石榴石中含有前榴辉岩相变质形成的矿物包体,并具典型的进变质成分环带.高压榴辉岩中保存了其进、退变质作用全过程中的岩石学和构造信息,即在挤压体制下,表壳岩石经绿帘角闪岩相到榴辉岩相进变质作用和强烈韧性变形;在继续挤压逆冲机制下高压变质岩的大幅度折返,从壳幔边界上升到地壳中、浅层次,并发生绿帘角闪相退变质作用和多期韧性变形;在伸展体制下经滑脱、断块升降、差异抬升高压变质岩块体暴露到地表,并发生绿片岩相退变质作用和韧-脆性变形.高压变质作用过程中存在广泛的流体-岩石相互作用, 气液包裹体和高压含水矿物的稳定产出,是最有力的证据.流体的成分、含量、迁移形式控制着变质反应,是影响高压变质岩形成与保存的热力学和动力学条件.     

柴北缘超高压带中锡铁山榴辉岩的变质时代

锡铁山地体位于柴北缘超高压变质带的中部, 是柴北缘超高压变质带的重要组成部分。该地体由花岗质片麻岩、泥质片麻岩和相对较少的榴辉岩透镜体组成。大部分榴辉岩都经历了不同程度的后生合晶和角闪岩相退化变质改造。虽然近年来进行了大量的锆石U-Pb年代学研究,但榴辉岩相高压-超高压变质的时代一直存在争议,并且以前对锡铁山榴辉岩相变质时代的认识一直与相邻的绿梁山、鱼卡和其东部的都兰等地区的超高压变质年龄有明显的冲突。本文通过锡铁山榴辉岩锆石U-Pb年代学的研究,获得榴辉岩相变质锆石的²⁰⁶Pb/²³⁸U加权平均年龄为433±3Ma,与鱼卡地区榴辉岩的形成年龄一致,代表大陆俯冲时期的高压-超高压变质年龄。该研究对进一步了解锡铁山榴辉岩地体的变质演化和大陆地壳深俯冲有重要意义。     

北秦岭榴辉岩及相关岩石年代学的进一步确定及其对板片俯冲属性的约束

在岩相学观察和锆石CL图像研究的基础上,利用LA ICP MS原位分析方法,对北秦岭官坡超高压榴辉岩和伴生的石榴石角闪岩(榴闪岩)进行了详细的锆石微区微量元素和U Th Pb同位素分析,在榴辉岩样品中得到变质年龄为(502±11)Ma,原岩结晶年龄>(657±18)Ma;在榴闪岩样品中得到原岩结晶年龄为(791±6)Ma,变质年龄为487~503 Ma,角闪岩相退变质年龄为(366±4)Ma。岩石地球化学研究显示,北秦岭官坡地区的榴闪岩具有低Si(SiO2质量分数为4916%~5078%),高Ti(TiO2质量分数为228%~283%)、富集LREE、LILE和大部分的HFSE元素,不显Nb、Ta负异常的板内玄武岩特征,与北秦岭超高压榴辉岩地球化学特征一致。结合两者的野外产状、岩相学特征、锆石形貌和年代学研究结果,表明本文研究的官坡地区的榴闪岩是超高压榴辉岩在抬升过程中在角闪岩相条件下退变质的产物。综合两者的年代学研究结果,得到北秦岭地区超高压榴辉岩的变质年龄为(502±11)Ma,原岩结晶年龄为(791±6)Ma,角闪岩相退变质年龄为(366±4)Ma。研究得到的(502±11)Ma的榴辉岩相变质年龄与前人得到的该榴辉岩围岩超高压泥质片麻岩的变质年龄(507±38)Ma以及北秦岭松树沟地区的超高压长英质片麻岩的变质年龄485~514 Ma一致,表明它们经历了同期超高压变质作用。而且,榴辉岩(502±11)Ma的变质年龄与其原岩的结晶年龄(791±6)Ma存在近300 Ma的时间间隔,表明原岩具有板内玄武岩性质的北秦岭官坡超高压榴辉岩不可能是秦岭古生代大洋板块深俯冲的产物,而可能是已构造就位的古洋壳或裂谷火山岩在古生代随陆壳一起发生大陆深俯冲作用的产物。     

High-pressure granulites in the Sanggan area, North China craton: metamorphic evolution, P–T paths and geotectonic significance

High‐pressure basic granulites are widely distributed as enclaves and sheet‐like blocks in the Huaian TTG gneiss terrane in the Sanggan area of the Central Zone of the North China craton. Four stages of the metamorphic history have been recognised in mineral assemblages based on inclusion, exsolution and reaction textures integrated with garnet zonation patterns as revealed by compositional maps and compositional profiles. The P–T conditions for each metamorphic stage were obtained using thermodynamically and experimentally calibrated geothermobarometers. The low‐Ca core of growth‐zoned garnet, along with inclusion minerals, defines a prograde assemblage (M1) of garnet + clinopyroxene + plagioclase + quartz, yielding 700 °C and 10 kbar. The peak of metamorphism at about 750–870 °C and 11–14.5 kbar (M2) is defined by high‐Ca domains in garnet interiors and inclusion minerals of clinopyroxene, plagioclase and quartz. Kelyphites or coronas of orthopyroxene + plagioclase ± magnetite around garnet porphyroblasts indicate garnet breakdown reactions (M3) at conditions around 770–830 °C and 8.5–10.5 kbar. Garnet exsolution lamellae in clinopyroxene and kelyphites of amphibole + plagioclase around garnet formed during the cooling process at about 500–650 °C and 5.5–8 kbar (M4). These results help define a sequential P–T path containing prograde, near‐isothermal decompression (ITD) and near‐isobaric cooling (IBC) stages. The clockwise hybrid ITD and IBC P–T paths of the HP granulites in the Sanggan area imply a model of thickening followed by extension in a collisional environment. Furthermore, the relatively high‐pressures (6–14.5 kbar) of the four metamorphic stages and the geometry of the P–T paths suggest that the HP granulites, together with their host Huaian TTG gneisses, represent the lower plate in a crust thickened during collision. The corresponding upper‐plate might be the tectonically overlying Khondalite series, which was subjected to medium‐ to low‐pressure (MP/LP: 7–4 kbar) granulite facies metamorphism with a clockwise P–T path including an ITD segment. Both the HP and the MP/LP granulite facies events occurred contemporaneously at c. 1.90–1.85 Ga in a collisional environment created by the assembly process of the North China craton.     

Tectonic evolution of the North Qinling Orogen from subduction to collision and exhumation: Evidence from zircons in metamorphic rocks of the Qinling Group

The North Qinling Block (NQB) is an important segment of the Qinling Orogen in Central China. Here we report the results from SIMS geochronology and oxygen isotopes, as well as LA-MC-ICPMS Hf isotopic analyses on zircon grains from a suite of metamorphic rocks (felsic gneisses, garnet plagioclase amphibolites, and retrograde eclogite dikes) in the Qinling Group of the NQB. The age data show that these rocks underwent at least two episodes of metamorphism with the peak at 483–501 Ma, followed by 454–470 Ma retrograde metamorphism. These results are generally coeval with the periods of 500–480 Ma for peak metamorphism and 460–420 Ma for retrograde metamorphism previously obtained from the HP/UHP metamorphic rocks of the NQB. During the prograde and retrograde metamorphism, widespread fluid and melt circulation within the block has been identified from the geochemical features of the metamorphic zircons. The fluids that circulated in the felsic gneisses and retrograde eclogite dikes originated from the dehydration of altered oceanic basalts as inferred from the exceedingly low Th/U ratios, positive ε_Hf(t) (> 5) and extremely δ¹⁸O (10.01–13.91‰) values in metamorphic zircons. In contrast, the melt involved in the formation of garnet plagioclase amphibolites appears to have been derived from continental sediments interlayered with the oceanic basalts since zircons crystallized during the peak and retrograde metamorphism show typical magmatic features with high U and Th contents and Th/U ratios and enriched Hf (ε_Hf(t) = − 5.42 to − 0.18) and oxygen isotope composition (δ¹⁸O around 8‰). Geochronological and geochemical features of the magmatic cores of the clear core-rim textured zircons demonstrate that the protoliths of the gneisses were intermediate-acid volcanic rocks erupted before Neoproterozoic (800 Ma), which is further supported by the intrusion of basaltic magma of asthenospheric origin as represented by protoliths of retrograde eclogite dikes, with the oldest magmatic zircon formed at 789 Ma. The protoliths of garnet plagioclase amphibolites appear to be altered oceanic basalts but had been significantly affected by the melt during the metamorphism. Combined with the previous studies, the Qinling Group experienced overall subduction in the Early Paleozoic. The NQB as represented by the Qinling Group was most likely a discrete micro-block in the Neoproterozoic, and underwent deep subduction in the Cambrian (483–501 Ma) and exhumation in Ordovician (454–470 Ma). We propose that the NQB preserves a complete cycle of tectonic evolution of an orogen from an oceanic basin spreading, and micro-continent formation to deep subduction and exhumation.     

华北克拉通中部造山带早前寒武纪变质演化历史评述

根据变质作用程度不同,华北中部造山带早前寒武纪基底可以进一步分为高级区和花岗-绿岩带。前者变质程度可达高角闪岩相-麻粒岩相,包括太华、吕梁、阜平、恒山、怀安、宣化等杂岩,花岗-绿岩带的变质程度较低,多为绿片岩相-角闪岩相,包括登封、赞皇、五台等杂岩。已有变质演化研究表明,高级区恒山、怀安和宣化杂岩中的麻粒岩和(或)退变榴辉岩记录的峰期变质压力最高,恒山杂岩、阜平杂岩和宣化杂岩中的麻粒岩记录的峰期变质温度最高;花岗-绿岩带中的赞皇杂岩和五台杂岩出露高压斜长角闪岩和高压变泥质岩。中部带各变质杂岩中可识别出早期进变质、峰期、峰后快速降压和晚期冷却等变质阶段,拥有顺时针近等温降压型的变质作用P-T轨迹,与华北克拉通中部的俯冲碰撞有关。大量变质年代学数据显示,中部带各变质杂岩中至少记录了~1.85Ga、~1.95Ga和~2.5Ga三组变质年龄,其中,~1.85Ga的变质年龄占据了主导地位,大致与区域片麻理形成的时间一致,代表变质高峰期时代;~1.95Ga的变质年龄代表了峰期前的某个进变质片段;~2.5Ga的变质年龄则指示了更早一期的变质事件,推测与古老陆块~2.5Ga所遭受的大量幔源岩浆的侵入和底垫作用有关。然而,变质年龄与变质阶段的对应关系尚不明确。     

Metamorphic evolution of eclogite and associated garnet-mica schist in the high-pressure metamorphic Maksyutov complex, Ural, Russia

Metasedimentary garnet-mica schists are interlayered with metabasic garnet–omphacite schists and enclose eclogite boudins in the high-pressure metamorphic Maksyutov complex in the Southern Urals, Russia. These three rock types were investigated in one outcrop and compared chemographically and thermobarometrically. The Fe/Mg distributions between garnet rim–omphacite and garnet rim–phengite pairs indicate different equilibration temperatures for the three samples, with the lowest temperature (500°C, >1.5 GPa) for the eclogite boudin, an intermediate temperature (630°C, >1.7 GPa) for the foliated eclogite and the highest temperature (650°C, >1.7 GPa) for the garnet-mica schist. The garnets in garnet-mica schist enclose abundant chloritoid relics and the Fe/Mg distribution between chloritoid and garnet records an earlier high-temperature stage (650°C, >2.0 GPa) before the garnet rim–phengite temperatures were reached. Together with some minimum- and maximum-pressure estimates three different prograde pressure–temperature paths and a common retrograde metamorphic evolution are interpreted from the chemographic and thermobarometric data. The different early metamorphic evolutions and conditions confirm the variability of protoliths, which are also indicated by different U/Pb zircon and rutile ages.     

SHRIMP U–Pb zircon dating of quartz-bearing eclogite from the Sanbagawa Belt, south-west Japan: implications for metamorphic evolution of subducted protolith

In order to decipher the origin of eclogite in the high‐P/T Sanbagawa metamorphic belt, SHRIMP U–Pb ages of zircons from quartz‐bearing eclogite and associated quartz‐rich rock (metasandstone) were determined. One zircon core of the quartz‐rich rock yields an extremely old provenance age of 1899 ± 79 Ma, suggesting that the core is of detrital origin. Eight other core ages are in the 148–134 Ma range, and are older than the estimated age for trench sedimentation as indicated by the youngest radiolarian fossil age of 139–135 Ma from the Sanbagawa schists. Ages of metamorphic zircon rims (132–112 Ma) from the quartz‐rich rock are consistent with metamorphic zircon ages from the quartz‐bearing eclogite, indicating that eclogite facies metamorphism peaked at 120–110 Ma. These new data are consistent with both the Iratsu eclogite body and surrounding highest‐grade Sanbagawa schists undergoing coeval subduction‐zone metamorphism, and subsequent re‐equilibration under epidote amphibolite facies conditions during exhumation.     

The Kabul Block (Afghanistan), a segment of the Columbia Supercontinent,with a Neoproterozoic metamorphic overprint

We report field relationships, petrography and isotopic ages from two superposed basement units of the Kabul Block, the so called Lower Sherdarwaza and Upper Welayati formations. The Sherdarwaza Formation is represented mostly by migmatites and gneisses that are derived from pelitic and psammitic lithologies with lenses and layers of mafic and carbonate rocks. Several bodies of orthogneisses are also exposed in the Sherdarwaza Formation. The Upper Welayati Formation is characterized by micaschist, quartzite and amphibolites. SHRIMP U–Pb data on zircon from the orthogneiss in the Sherdarwaza Formation indicates a Neoarchean age of ca 2.5–2.8 Ga for their magmatic crystallization. The rocks exhibit granulite facies conditions of 5–7 kbar and 800 °C that are documented by the presence of orthopyroxene and Ti-rich biotite in the orthogneiss and by olivine and phlogopite in some calc-silicate rocks at contact with marble. A Paleoproterozoic age of ca. 1.85–1.80 Ga for this metamorphism was obtained using U-Pb SHRIMP dating on zircon and U-Th dating on monazite. Mineral textural relations also show a younger amphibolite facies metamorphism that is documented in both the Sherdarwaza and Welayati formations. This metamorphism occurred at relatively higher pressure conditions of up to 9 kbar at ca. 650 °C, compared to the granulite facies event. A Neoproterozoic age of ca 0.85–0.9 Ga, for this metamorphism is confirmed by Ar-Ar data on biotite and white mica as well as by U-Th data on monazite. By combining the presented results on the metamorphic petrology, geochronology and geochemistry, we conclude that: (1) The Kabul basement is a fragment of an Archean block (craton); (2) the ca. 1.85–1.8 and 0.9–0.85 Ga metamorphism marks an important orogenic events for the basement rocks of the Kabul Block which was stabilized during the early Precambrian; (3) the two metamorphic ages correlate well with global-scale orogenies related to the assembly of the Paleoproterozoic Columbia and Neoproterozoic Rodinia supercontinents; (4) based on metamorphic characteristics and ages, the Kabul basement rocks show an affinity to the Neoarchean rocks of the Tarim and/or South China cratons.     

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。