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1.
Rb/Sr internal mineral isochrons in the eclogite facies Marun-Keu metamorphic complex, Polar Urals, Russia, date periods of fluid-rock interaction and record the metamorphic reaction history. The Marun-Keu complex consists of Late Proterozoic to Early Ordovician, mostly igneous rocks that experienced a subduction-related, non-pervasive eclogite facies metamorphism, followed by a local decompression-related amphibolite facies overprint, during the Uralian orogeny. Field observations show that metamorphic reactions as well as ductile deformation are controlled by local availability of a free fluid phase. Isotopic data reveals that availability of fluids similarly exerts control on isotope distribution. From a relic gabbro which has never been infiltrated by free fluids, a premetamorphic Rb/Sr age of 467 ± 39 Ma was obtained. Rb/Sr isochron ages for 14 samples of eclogite and amphibolite facies assemblages, sampled from within or close to metamorphic fluid veins, range from 352 ± 5 Ma to 360 ± 3 Ma. A Sm/Nd isochron for a metagranite yields an age of 354 ± 4 Ma. Taken together, the ages for both prograde and retrograde metamorphic assemblages overlap within analytical uncertainty and yield an average value of 355.5 ± 1.4 Ma, indicating that the metamorphic evolution and incipient exhumation of the Marun-Keu complex proceeded rapidly. The results demonstrate that assemblages preserve their Rb/Sr isotopic signatures as long as they remain devoid of free fluids, and that only fluid-rock interaction may cause Sr isotope redistribution. In addition, the data suggest local fluid-rock equilibrium, low fluid-rock ratios with overall fluid deficiency, and limited fluid mobility at depth. However, some fluids must have been mobile on the km-scale since they can be traced into the suprasubduction zone mantle wedge. Metasomatic veins in the Rai-Iz ophiolite yield a Rb/Sr mineral isochron age of 373.1 ± 5.4 Ma. They are interpreted as evidence for suprasubduction zone metasomatism in an oceanic setting, prior to subduction of the East European margin and associated formation of eclogites in the Marun-Keu complex.We propose that Rb/Sr mineral-isochron ages provide hygrochronological rather than thermochronological constraints. They define the cooling history only in combination with zircon and apatite fission track data. The straightforward interpretation of Rb/Sr mineral ages as cooling ages is obsolete.  相似文献   

2.
Contacts between rocks recording large differences in metamorphic grade are indicative of major tectonic displacements. Low-P upon high-P contacts are commonly interpreted as extensional (i.e. material points on either side of the contact moved apart relative to the palaeo-horizontal), but dating of deformation and metamorphism is essential in testing such models. In the Western Alps, the Piemonte Ophiolite consists of eclogites (T ≈550–600 °C and P≈18–20 kbar) structurally beneath greenschist facies rocks (T ≈400 °C and P≈9 kbar). Mapping shows that the latter form a kilometre-wide shear zone (the Gressoney Shear Zone, GSZ) dominated by top-SE movement related to crustal extension. Rb–Sr data from micas within different GSZ fabrics, which dynamically recrystallized below their blocking temperature, are interpreted as deformation ages. Ages from different samples within the same fabric are reproducible and are consistent with the relative chronology derived from mapping. They show that the GSZ had an extensional deformation history over a period of c. 9 Myr between c. 45–36 Ma. This overlaps in time with the eclogite facies metamorphism. The GSZ operated over the entire period during which the footwall evolved from eclogite to greenschist facies and was therefore responsible for eclogite exhumation. The discrete contact zone between eclogite and greenschist facies rocks is the last active part of the GSZ and truncates greenschist facies folds in the footwall. These final movements were therefore not a major component of eclogite exhumation. Pressure estimates associated with old and young fabrics within the GSZ are comparable, indicating that during extensional deformation there was no significant unroofing of the hangingwall. Since there are no known extensional structures younger than 36 Ma at higher levels in this part of the Alps, exhumation since the final juxtaposition of the two units (at 36 Ma) seems to have been dominated by erosion. Key words: deformation age, eclogite, exhumation, Rb–Sr dating, tectonic.  相似文献   

3.
Seven eclogite facies samples from lithologically different units which structurally underlie the Semail ophiolite were dated by the 40Ar/39Ar and Rb–Sr methods. Despite extensive efforts, phengite dated by the 40Ar/39Ar method yielded saddle, hump or irregularly shaped spectra with uninterpretable isochrons. The total gas ages for the phengite ranged from 136 to 85 Ma. Clinopyroxene–phengite, epidote–phengite and whole‐rock–phengite Rb–Sr isochrons for the same samples yielded ages of 78 ± 2 Ma. We therefore conclude that the eclogite facies rocks cooled through 500 °C at c. 78 ± 2 Ma, and that the 40Ar/39Ar dates can only constrain maximum ages due to the occurrence of excess Ar inhomogeneously distributed in different sites. Our new results lead us to conclude that high‐pressure metamorphism of the Oman margin took place in the Late Cretaceous, contemporaneous with ophiolite emplacement. Previously published structural and petrological data lead us to suggest that this metamorphism resulted from intracontinental subduction and crustal thickening along a NE‐dipping zone. Choking of this subduction zone followed by ductile thinning of a crustal mass wedged between deeply subducted continental material and overthrust shelf and slope units facilitated the exhumation of the eclogite facies rocks from depths of c. 50 km to 10–15 km within c. 10 Ma, and led to their juxtaposition against overlying lower grade rocks. Final exhumation of all high‐pressure rocks was driven primarily by erosion and assisted by normal faulting in the upper plate.  相似文献   

4.
任云飞  陈丹玲  宫相宽  刘良 《地球科学》2019,44(12):4009-4016
硬柱石是大洋冷俯冲带的代表性矿物之一,富含水和Sr、Pb及稀土等微量元素,其形成和分解对于俯冲带流体活动、壳幔水和微量元素循环、地幔楔交代和熔融及岛弧岩浆作用等具有重要影响.但由于硬柱石对温度和压力的改变非常敏感,在板片折返过程中很容易分解,因此目前全球出露的硬柱石榴辉岩极为稀少.总结了榴辉岩中早期硬柱石存在的识别标志,并据此确定柴北缘超高压带西段鱼卡地区的含蓝晶石榴辉岩和斜黝帘石榴辉岩是峰期硬柱石榴辉岩退变质改造的结果.该发现说明柴北缘成为继大别造山带之后全球第二例出露硬柱石榴辉岩的大陆俯冲型造山带.利用相平衡计算方法恢复了这两种榴辉岩的变质演化过程,其中含蓝晶石榴辉岩的P-T轨迹和峰期变质条件均与区内大陆俯冲型含柯石英多硅白云母榴辉岩相似,而斜黝帘石榴辉岩峰期变质温压则略低.锆石定年获得含蓝晶石榴辉岩和斜黝帘石榴辉岩的变质时代分别为437 Ma和436 Ma,与带内已有超高压榴辉岩相变质时代相同,同时获得含蓝晶石榴辉岩的原岩结晶时代为1 273 Ma.相似的变质P-T轨迹和变质时代表明含蓝晶石榴辉岩与同剖面含柯石英多硅白云母榴辉岩共同经历了大陆深俯冲作用.这一结果表明,硬柱石榴辉岩并非大洋冷俯冲带特有,决定榴辉岩中是否出现硬柱石的主要因素是原岩成分和变质条件.在鱼卡地区,榴辉岩的矿物组合中能否出现硬柱石的最主要控制因素是原岩中的Mg含量,由高Mg#的基性岩变质形成的榴辉岩峰期矿物组合中易出现硬柱石.   相似文献   

5.
Rb–Sr multimineral isochron data for metamorphic veins allow to date separate increments of the mineral reaction history of polymetamorphic terranes. Granulite facies rocks of the Lindås nappe, Bergen Arcs, Norway, were subducted and exhumed during the Caledonian orogeny. The rocks show petrographic evidence for two distinct events of local fluid infiltration and vein formation, along fractures and shear zones. The first occurred at eclogite facies (15–21 kbar, 650–750°C) and a later one at amphibolite facies conditions (8–10 kbar, 600°C). The presence of fluids enabled local metamorphic equilibration only near fluid pathways. In fluid-absent domains, preexisting assemblages were metastably preserved. This resulted in a heterogeneity of metamorphic signatures on meter to μm-scales. Well-preserved granulite facies rocks preserve their Proterozoic Rb–Sr mineral ages, as does the U–Pb system of zircon in most lithologies. Six Rb/Sr multimineral isochron ages for eclogite facies veins and their immediate wallrocks date the fluid-induced eclogitization at 429.9 ± 3.5 Ma (2σ, weighted average, MSWD = 0.39). An eclogite facies vein has yielded metamorphic zircon with concordant U–Pb ages of 429 ± 3 Ma, identical to the U–Pb age of 427.4 ± 0.9 Ma for zircon xenocrysts in an amphibolite facies vein. Seven Rb/Sr mineral isochron ages date amphibolite-facies fluid infiltration at 414.2 ± 2.8 Ma (MSWD = 1.5), an age value testifying to residence of the rocks in the deep orogenic crust at temperatures >600°C for nearly 15 Ma. The new data show that Rb–Sr mineral isochron ages effectively date fluid-induced (re)crystallization events rather than stages of cooling. The direct link between isotopic ages and distinct petrographic equilibrium assemblages aids to constrain the evolution of rocks in the P–T-reaction-time space, which is essential for understanding exhumation histories and the internal dynamics of orogens in general.  相似文献   

6.
The youngest known ultrahigh‐pressure (UHP) rocks in the world occur in the Woodlark Rift of southeastern Papua New Guinea. Since their crystallization in the Late Miocene to Early Pliocene, these eclogite facies rocks have been rapidly exhumed from mantle depths to the surface and today they remain in the still‐active geodynamic setting that caused this exhumation. For this reason, the rocks provide an excellent opportunity to study rates and processes of (U)HP exhumation. We present New Rb–Sr results from 12 rock samples from eclogite‐bearing gneiss domes in the D'Entrecasteaux Islands, and use those results to examine the time lag between (U)HP metamorphism and later ductile thinning, penetrative fabric development and accompanying metamorphic retrogression at amphibolite facies conditions during their exhumation. A Rb–Sr age for a sample of mafic eclogite (with no preserved coesite) from the core zone of the Mailolo gneiss dome (Fergusson Island) provides a new estimate of the timing of HP metamorphism (5.6 ± 1.6 Ma). The strongly deformed quartzofeldspathic and granitic gneisses (90–95% by volume) that enclose variably retrogressed relict blocks of mafic eclogite (5–10% by volume) yield Rb–Sr isochron ages from 4.4 to 2.4 Ma. For the UHP‐bearing gneisses of Mailolo dome, previously published U–Pb ages on zircon and our Rb–Sr isochron ages are consistent with a mean time lag of 2.2 ± 1.5 Ma (~95% c.i.) for passage of the rock between eclogite and amphibolite facies conditions. New thermobarometric data indicate that the main syn‐exhumational foliation developed at amphibolite facies conditions of 630–665 °C and 12.1–14.4 kbar. These pressure estimates indicate that the lower crust of the Woodlark Rift was unusually thick (>40 km) at the time of the amphibolite facies overprint, possibly as a result of accumulation and underplating of UHP‐derived material from below. Our data imply a minimum unroofing rate of 10 ± 7 mm year?1 (~95% c.i.) for the (U)HP body from minimum HP depths (73 ± 7 km) to lower crustal depths. This minimum unroofing rate reinforces previous inferences that the exhumation from the mantle to the surface of the gneiss domes in the D'Entrecasteaux Islands took place at plate tectonic rates. On the basis of previous structural studies and the new thermobarometry, we attribute the high (cm year?1) exhumation to diapiric ascent of the partially molten terrane from mantle depths, with a secondary contribution from pure shear thinning of the terrane after its arrival in the crust.  相似文献   

7.
董杰  魏春景  张建新 《地球科学》2019,44(12):4004-4008
南阿尔金造山带是目前报道的具有最深俯冲记录的大陆超高压变质带,其内出露有高压-超高温麻粒岩,它们对深入理解大陆地壳岩石超深俯冲与折返过程具有重要意义.介绍了对南阿尔金巴什瓦克地区长英质麻粒岩和基性麻粒岩的岩相学、矿物化学、相平衡模拟及锆石U-Pb年代学研究成果.其中基性麻粒岩主要记录了深俯冲大陆地壳折返过程的变质演化:包括高压榴辉岩相、高压-超高温麻粒岩相、低压-超高温麻粒岩相及随后的近等压降温演化阶段;长英质麻粒岩除了记录与基性麻粒岩相似的折返过程外,还记录了从角闪岩相到超高压榴辉岩相的进变质演化过程.结合已有研究资料,确定超高压榴辉岩阶段峰期条件> 7~9 GPa和>1 000℃,可达到斯石英稳定域.锆石年代学显示两种岩石类型的原岩和变质年龄均分别在900 Ma和500 Ma左右.变质作用与年代学研究表明,南阿尔金大陆地壳岩石在早古生代发生超深俯冲至200~300 km后,折返至加厚地壳底部发生高压-超高温变质作用,随后被快速抬升至地壳浅部发生低压-超高温变质作用并经历迅速冷却.   相似文献   

8.
Rocks within the Zermatt-Saas ophiolite of the western Alps have undergone eclogite facies metamorphism during subduction prior to the Alpine collision. The metamorphic history of these rocks is well defined, with eclogitic assemblages being followed by the limited growth of blueschist assemblages of glaucophane and paragonite. Subsequent greenschist alteration occurs adjacent to faults, veins and metasediments. Away from such sources of water, retrogression is very limited. Sm-Nd isotopic analyses of an essentially unretrogressed eclogitic metabasalt suggest that eclogite facies metamorphism occurred at 52 ± 18 Ma. The large uncertainty is due to the presence of very small amounts of Nd-rich epidote present as inclusions within garnet. As the closure temperature of garnet to Sm & Nd is thought to be >600C, resetting due to post-high-pressure diffusion is thought to be insignificant. Given the fine-grained protolith to the sample analysed, and its extensive deformation under eclogite facies conditions, incomplete homogenization of pre-metamorphic isotopic variations is also considered unlikely to be responsible for the young age. A Tertiary age of eclogitization means that models of early Alpine evolution based on the cessation of high-pressure metamorphism in the Cretaceous need to be revised.  相似文献   

9.
Fluid availability during high‐grade metamorphism is a critical factor in dictating petrological, geochemical and isotopic reequilibration between metamorphic minerals, with fluid‐absent metamorphism commonly resulting in neither zircon growth/recrystallization for U‐Pb dating nor Sm‐Nd isotopic resetting for isochron dating. While peak ultra‐high pressure (UHP) metamorphism is characterized by fluid immobility, high‐pressure (HP) eclogite‐facies recrystallization during exhumation is expected to take place in the presence of fluid. A multichronological study of UHP eclogite from the Sulu orogen of China indicates zircon growth at 216 ± 3 Ma as well as mineral Sm‐Nd and Rb‐Sr reequilibration at 216 ± 5 Ma, which are uniformly younger than UHP metamorphic ages of 231 ± 4 to 227 ± 2 Ma as dated by the SHRIMP U‐Pb method for coesite‐bearing domains of zircon. O isotope reequilibration was achieved between the Sm‐Nd and Rb‐Sr isochron minerals, but Hf isotopes were not homogenized between different grains of zircon. The HP eclogite‐facies recrystallization is also evident from petrography. Thus this process occurred during exhumation with fluid availability from decompression dehydration of hydrous minerals and the exsolution of hydroxyl from nominally anhydrous minerals. This provides significant amounts of internally derived fluid for extensive retrogression within the UHP metamorphosed slabs. Based on available experimental diffusion data, the consistent reequilibration of U‐Pb, Sm‐Nd, Rb‐Sr and O isotope systems in the eclogite minerals demonstrates that time‐scale for the HP eclogite‐facies recrystallization is c. 1.9–9.3 Myr or less. This provides a maximum estimate for duration of the fluid‐facilitated process in the HP eclogite‐facies regime during the exhumation of deeply subducted continental crust.  相似文献   

10.
Composite granite–quartz veins occur in retrogressed ultrahigh pressure (UHP) eclogite enclosed in gneiss at General's Hill in the central Sulu belt, eastern China. The granite in the veins has a high‐pressure (HP) mineral assemblage of dominantly quartz+phengite+allanite/epidote+garnet that yields pressures of 2.5–2.1 GPa (Si‐in‐phengite barometry) and temperatures of 850–780°C (Ti‐in‐zircon thermometry) at 2.5 GPa (~20°C lower at 2.1 GPa). Zircon overgrowths on inherited cores and new grains of zircon from both components of the composite veins crystallized at c. 221 Ma. This age overlaps the timing of HP retrograde recrystallization dated at 225–215 Ma from multiple localities in the Sulu belt, consistent with the HP conditions retrieved from the granite. The εHf(t) values of new zircon from both components of the composite veins and the Sr–Nd isotope compositions of the granite consistently lie between values for gneiss and eclogite, whereas δ18O values of new zircon are similar in the veins and the crustal rocks. These data are consistent with zircon growth from a blended fluid generated internally within the gneiss and the eclogite, without any ingress of fluid from an external source. However, at the peak metamorphic pressure, which could have reached 7 GPa, the rocks were likely fluid absent. During initial exhumation under UHP conditions, exsolution of H2O from nominally anhydrous minerals generated a grain boundary supercritical fluid in both gneiss and eclogite. As exhumation progressed, the volume of fluid increased allowing it to migrate by diffusing porous flow from grain boundaries into channels and drain from the dominant gneiss through the subordinate eclogite. This produced a blended fluid intermediate in its isotope composition between the two end‐members, as recorded by the composite veins. During exhumation from UHP (coesite) eclogite to HP (quartz) eclogite facies conditions, the supercritical fluid evolved by dissolution of the silicate mineral matrix, becoming increasingly solute‐rich, more ‘granitic’ and more viscous until it became trapped. As crystallization began by diffusive loss of H2O to the host eclogite concomitant with ongoing exhumation of the crust, the trapped supercritical fluid intersected the solvus for the granite–H2O system, allowing phase separation and formation of the composite granite–quartz veins. Subsequently, during the transition from HP eclogite to amphibolite facies conditions, minor phengite breakdown melting is recorded in both the granite and the gneiss by K‐feldspar+plagioclase+biotite aggregates located around phengite and by K‐feldspar veinlets along grain boundaries. Phase equilibria modelling of the granite indicates that this late‐stage melting records P–T conditions towards the end of the exhumation, with the subsolidus assemblage yielding 0.7–1.1 GPa at <670°C. Thus, the composite granite–quartz veins represent a rare example of a natural system recording how the fluid phase evolved during exhumation of continental crust. The successive availability of different fluid phases attending retrograde metamorphism from UHP eclogite to amphibolite facies conditions will affect the transport of trace elements through the continental crust and the role of these fluids as metasomatic agents interacting with the mantle wedge in the subduction channel.  相似文献   

11.
Major and trace‐element zoning in garnet, in combination with Rb–Sr, Sm–Nd and Lu–Hf geochronology, provide evidence for a protracted garnet growth history for the Zermatt‐Saas Fee (ZSF) ophiolite, western Alps. Four new Lu–Hf ages from Pfulwe (c. 52–46 Ma) and one from Chamois (c. 52 Ma) are very similar to a previously published Lu–Hf age from Lago di Cignana. Overall, the similarity of geochronological and garnet zoning patterns suggests that these three localities had a similar prograde tectonic history, commensurate with their similar structural position near the top of the ZSF. Samples from the lower part of the ZSF at Saas Fee and St. Jacques, however, produced much younger Lu–Hf ages (c. 41–38 Ma). Neither differences in whole‐rock geochemistry, which might produce distinct garnet growth histories, nor rare‐earth‐element zoning in garnet, can account for the age differences in the two suites. This suggests a much later prograde history for the lower part of the ZSF, supporting the idea that it was subducted diachronously. Such a model is consistent with changes in subduction vectors based on plate tectonic reconstructions, where early oblique subduction, which produced long prograde garnet growth, changed to more orthogonal subduction, which corresponds to shorter prograde garnet growth. Six new Rb–Sr phengite ages range from c. 42 to 39 Ma and, in combination with previously published Rb–Sr ages, constrain the timing of the transition from eclogite to upper greenschist facies P–T conditions. The proximity of the ZSF in the Saas Fee region to the underlying continental Monte Rosa unit and the similarity of peak‐metamorphic ages suggest these two units were linked for part of their tectonic history. This in turn indicates that the Monte Rosa may have been partly responsible for rapid exhumation of the ZSF unit.  相似文献   

12.
 This integrated study of the sedimentology, magnetostratigraphic chronology and petrography of the mostly continental clastics of the Oligocene to Miocene Swiss Molasse Basin underpins a reconstruction of facies architecture and delineates relationships between the depositional evolution of a foreland-basin margin and exhumation phases and orogenic events in the adjacent orogen. A biostratigraphically based high-resolution magnetostratigraphy provides a detailed temporal framework and covers nearly the whole stratigraphic record of the Molasse Basin (31.5–13 Ma). Three transverse alluvial fan systems evolved at the southern basin margin. They are characterized by distinct petrographic compositions and document the exhumation and denudation history of the growing eastern Swiss Alps. Enhanced northward propagation of the orogenic wedge is interpreted to have occurred between 31.5 and 26 Ma. During the period 24–19 Ma, intense in-sequence and out-of-sequence thrusting took place as Molasse strata were accreted to the orogenic wedge. A third active tectonic phase, possibly caused by backthrusting of the Plateau Molasse, probably occurred between ca. 15 and 13 Ma. Fan head migration between 31.5 and 13 Ma is probably controlled by the structural evolution of the thrust front due to Molasse accretion and backthrusting. Received: 11 March 1998 / Accepted: 12 March 1999  相似文献   

13.
Laser Raman spectroscopy and cathodoluminescence (CL) images show that zircon from Sulu‐Dabie dolomitic marbles is characterized by distinctive domains of inherited (detrital), prograde, ultrahigh‐pressure (UHP) and retrograde metamorphic growths. The inherited zircon domains are dark‐luminescent in CL images and contain mineral inclusions of Qtz + Cal + Ap. The prograde metamorphic domains are white‐luminescent in CL images and preserve a quartz eclogite facies assemblage of Qtz + Dol + Grt + Omp + Phe + Ap, formed at 542–693 °C and 1.8–2.1 GPa. In contrast, the UHP metamorphic domains are grey‐luminescent in CL images, retain the UHP assemblage of Coe + Grt + Omp + Arg + Mgs + Ap, and record UHP conditions of 739–866 °C and >5.5 GPa. The outermost retrograde rims have dark‐luminescent CL images, and contain low‐P minerals such as calcite, related to the regional amphibolite facies retrogression. Laser ablation ICP‐MS trace‐element data show striking difference between the inherited cores of mostly magmatic origin and zircon domains grown in response to prograde, UHP and retrograde metamorphism. SHRIMP U‐Pb dating on these zoned zircon identified four discrete 206Pb/238U age groups: 1823–503 Ma is recorded in the inherited (detrital) zircon derived from various Proterozoic protoliths, the prograde domains record the quartz eclogite facies metamorphism at 254–239 Ma, the UHP growth domains occurred at 238–230 Ma, and the late amphibolite facies retrogressive overprint in the outermost rims was restricted to 218–206 Ma. Thus, Proterozoic continental materials of the Yangtze craton were subducted to 55–60 km depth during the Early Triassic and recrystallized at quartz eclogite facies conditions. Then these metamorphic rocks were further subducted to depths of 165–175 km in the Middle Triassic and experienced UHP metamorphism, and finally these UHP metamorphic rocks were exhumed to mid‐crustal levels (about 30 km) in the Late Triassic and overprinted by regional amphibolite facies metamorphism. The subduction and exhumation rates deduced from the SHRIMP data and metamorphic P–T conditions are 9–10 km Myr?1 and 6.4 km Myr?1, respectively, and these rapid subduction–exhumation rates may explain the obtained P–T–t path. Such a fast exhumation suggests that Sulu‐Dabie UHP rocks that returned towards crustal depths were driven by buoyant forces, caused as a consequence of slab breakoff at mantle depth.  相似文献   

14.
俯冲隧道是俯冲板片与上覆板块之间的剪切带,也是高压—超高压变质岩折返和深部流/熔体活动的通道。大别山超高压变质岩分布广泛,变形程度差异很大,是研究大陆俯冲隧道中岩石变质- 变形过程的理想地区。本文系统总结了前人对中大别双河地区超高压变质岩的岩石学和年代学研究成果,在双河地区开展了地质填图、应变分析和三维构造重建。通过将超高压变质岩的变形特征与P- T- t轨迹结合,识别出超高压变质岩折返过程中的三期韧性变形。在双河北部发现了一个上盘向NW剪切的千米尺度的榴辉岩相鞘褶皱,枢纽向SE倾伏,倾伏角约20°,与榴辉岩、片岩和长英质片麻岩的拉伸线理平行,表明超高压变质岩初始折返阶段的流体活动使榴辉岩的强度显著降低,榴辉岩与围岩一起发生韧性变形。该期变形被角闪岩相退变质阶段上盘向NW的剪切叠加,此时应变集中于片麻岩、片岩、大理岩等非能干层,强度较高的榴辉岩成为构造透镜体。而绿片岩相变质阶段上盘向SE方向的剪切与早白垩世北大别花岗片麻岩穹隆的形成有关。对双河南部弱变形花岗片麻岩的锆石U- Pb定年揭示了757±14 Ma的原岩年龄和 240~216 Ma的变质年龄,与双河北部含柯石英强变形花岗片麻岩类似,暗示其也经历了三叠纪超高压变质作用及随后的角闪岩相退变质作用。通过计算长英质片麻岩的有效黏度,发现无水碱长花岗片麻岩的有效黏度高于黑云斜长片麻岩,折返阶段的流体活动使超高压变质岩的强度显著降低,当局部的流体活动不足以弱化碱长花岗岩体时,应变集中于黑云斜长片麻岩。因此,大陆俯冲隧道中的应变分布受矿物组成、流体活动和岩体规模的共同影响。  相似文献   

15.
Geothermobarometric and geochronological work indicates a complete Eocene/early Oligocene blueschist/greenschist facies metamorphic cycle of the Cycladic Blueschist Unit on Naxos Island in the Aegean Sea region. Using the average pressure–temperature (P–T) method of thermocalc coupled with detailed textural work, we separate an early blueschist facies event at 576 ± 16 to 619 ± 32°C and 15.5 ± 0.5 to 16.3 ± 0.9 kbar from a subsequent greenschist facies overprint at 384 ± 30°C and 3.8 ± 1.1 kbar. Multi‐mineral Rb–Sr isochron dating yields crystallization ages for near peak‐pressure blueschist facies assemblages between 40.5 ± 1.0 and 38.3 ± 0.5 Ma. The greenschist facies overprint commonly did not result in complete resetting of age signatures. Maximum ages for the end of greenschist facies reworking, obtained from disequilibrium patterns, cluster near c. 32 Ma, with one sample showing rejuvenation at c. 27 Ma. We conclude that the high‐P rocks from south Naxos were exhumed to upper mid‐crustal levels in the late Eocene and early Oligocene at rates of 7.4 ± 4.6 km/Ma, completing a full blueschist‐/greenschist facies metamorphic cycle soon after subduction within c. 8 Ma. The greenschist facies overprint of the blueschist facies rocks from south Naxos resulted from rapid exhumation and associated deformation/fluid‐controlled metamorphic re‐equilibration, and is unrelated to the strong high‐T metamorphism associated with the Miocene formation of the Naxos migmatite dome. It follows that the Miocene thermal overprint had no impact on rock textures or Sr isotopic signatures, and that the rocks of south Naxos underwent three metamorphic events, one more than hitherto envisaged.  相似文献   

16.
T. M. Boundy  K. Mezger  E. J. Essene   《Lithos》1997,39(3-4):159-178
The U-Pb and Sm-Nd dating of deep crustal rocks from the Bergen Arcs system helps resolve enigmatic aspects of the tectonic evolution of the Caledonian Orogen in western Norway and yields insights into the arrested stages of eclogite development within the granulites of the area. The U-Pb dating of zircon from one of the eclogite facies shear zones yields an upper intercept age of 945 ± 5 Ma [all errors two standard deviations (2σ)], which is similar to other zircon ages from the granulite facies protolith. The age is interpreted to represent the time of late Proterozoic (Sveconorwegian) granulite metamorphism. The U-Pb ages of sphene and epidote show that the eclogites formed early in the evolution of the Caledonian Orogen (pre-Scandian phase) at about 460 Ma. An eclogite facies quartz vein yields a Sm-Nd whole rock-garnet isochron of 440 ± 12 Ma that may reflect the onset of cooling immediately after peak eclogite facies conditions, although the Sm-Nd systematics reveal some isotopic disequilibrium within the sample. In tandem with previous 40Ar/39Ar age determinations from, an adjacent eclogite of 450 Ma for hornblende and 430 Ma for muscovite, these data indicate that < 30 Ma elapsed between formation of the eclogites and the initial stages of cooling and exhumation to at least mid-crustal levels. This corresponds to minimum cooling rates of 14 °C/m.y. The timing relations suggest that the formation and exhumation of these eclogites from the overlying Caledonian Nappe wedge in western Norway are related to an early phase of crustal subduction during or somewhat before the major phase of continent-continent collision.

The short period of time between the formation of the eclogites and the initial stages of exhumation and rapid cooling is consistent with the only partial and localized transformation of the granulite to eclogite. Isolated occurrences of eclogite within the granulite, the formation of eclogite along metasomatic fronts and the formation of hydrous eclogite facies minerals within the “dry” granulite all point to the importance of fluids in the transformation and re-equilibration of the granulite to eclogite. Together, field and isotopic data demonstrate that both the localized and limited access of fluids and the rapid cycling of continental crust through the deepest portions of the orogen to upper crustal levels resulted in the preservation of the arrested stages of eclogite formation and survival of the granulites metastably through eclogite facies conditions.  相似文献   


17.
Diffusion modeling of zoning profiles in eclogite garnets from three different tectonic units of Mt. Dabie, UHPM unit, HPM unit and northern Dabie, was used to estimate the relative time span and cooling rates of these rocks. Modeling result for the Huangzhen eclogite garnet shows that the maximal time span for the diffusion-adjustment process is about 22 Ma since the peak-temperature metamorphism, which is the maximum time span from amphibolite facies metamorphism to greenschist facies metamorphism. The Bixiling eclogites had subjected to a cooling process at a rate of - 10℃/Ma from 750℃ to 560℃ during 20 Ma. The second cooling stage of the Raobazhai eclogite following granulite-facies metamorphism is an initial fast cooling process at a rate of about 25℃/Ma and then slowed down gradually. All these belong to a coherent Dabie collision orogen with differences in subduction depth and exhumation/uplifting path.  相似文献   

18.
Eclogite lenses in marbles from the Dabie-Sulu ultrahigh-pressure (UHP) terrane are deeply subducted meta-sedimentary rocks. Zircons in these rocks have been used to constrain the ages of prograde and UHP metamorphism during subduction, and later retrograde metamorphism during exhumation. Inherited (detrital) and metamorphic zircons were distinguished on the basis of transmitted light microscopy, cathodoluminescence (CL) imaging, trace element contents and mineral inclusions. The distribution of mineral inclusions combined with CL imaging of the metamorphic zircon make it possible to relate zircon zones (domains) to different metamorphic stages. Domain 1 consists of rounded, oblong and spindly cores with dark-luminescent images, and contains quartz eclogite facies mineral inclusion assemblages, indicating formation under high-pressure (HP) metamorphic conditions of T = 571-668℃and P = 1.7-2.02 GPa. Domain 2 always surrounds domain 1 or occurs as rounded and spindly cores with white-luminescent images. It contains coesite edogite facies mineral inclusion assemblages, indicating formation under UHP metamorphic conditions of T = 782-849℃and P > 5.5 GPa. Domain 3, with gray-luminescent images, always surrounds domain 2 and occurs as the outermost zircon rim. It is characterized by low-pressure mineral inclusion assemblages, which are related to regional amphibolite facies retrograde metamorphism of T = 600-710℃and P = 0.7-1.2 GPa. The three metamorphic zircon domains have distinct ages; sample H1 from the Dabie terrane yielded SHRIMP ages of 245±4 Ma for domain 1, 235±3 Ma for domain 2 and 215±6 Ma for domain 3, whereas sample H2 from the Sulu terrane yielded similar ages of 244±4 Ma, 233±4 Ma and 214±5 Ma for Domains 1, 2 and 3, respectively. The mean ages of these zones suggest that subduction to UHP depths took place over 10-11 Ma and exhumation of the rocks occurred over a period of 19-20 Ma. Thus, subduction from～55 km to > 160 km deep mantle depth took place at rates of approximately 9.5-10.5 km/Ma and exhumation from depths >160 km to the base of the crust at～30 km occurred at approximately 6.5 km/Ma. We propose a model for these rocks involving deep subduction of continental margin lithosphere followed by ultrafast exhumation driven by buoyancy forces after break-off of the UHP slab deep within the mantle.  相似文献   

19.
Although ultrahigh‐pressure (UHP) metamorphic rocks are present in many collisional orogenic belts, almost all exposed UHP metamorphic rocks are subducted upper or felsic lower continental crust with minor mafic boudins. Eclogites formed by subduction of mafic lower continental crust have not been identified yet. Here an eclogite occurrence that formed during subduction of the mafic lower continental crust in the Dabie orogen, east‐central China is reported. At least four generations of metamorphic mineral assemblages can be discerned: (i) hypersthene + plagioclase ± garnet; (ii) omphacite + garnet + rutile + quartz; (iii) symplectite stage of garnet + diopside + hypersthene + ilmenite + plagioclase; (iv) amphibole + plagioclase + magnetite, which correspond to four metamorphic stages: (a) an early granulite facies, (b) eclogite facies, (c) retrograde metamorphism of high‐pressure granulite facies and (d) retrograde metamorphism of amphibolite facies. Mineral inclusion assemblages and cathodoluminescence images show that zircon is characterized by distinctive domains of core and a thin overgrowth rim. The zircon core domains are classified into two types: the first is igneous with clear oscillatory zonation ± apatite and quartz inclusions; and the second is metamorphic containing a granulite facies mineral assemblage of garnet, hypersthene and plagioclase (andesine). The zircon rims contain garnet, omphacite and rutile inclusions, indicating a metamorphic overgrowth at eclogite facies. The almost identical ages of the two types of core domains (magmatic = 791 ± 9 Ma and granulite facies metamorphic zircon = 794 ± 10 Ma), and the Triassic age (212 ± 10 Ma) of eclogitic facies metamorphic overgrowth zircon rim are interpreted as indicating that the protolith of the eclogite is mafic granulite that originated from underplating of mantle‐derived magma onto the base of continental crust during the Neoproterozoic (c. 800 Ma) and then subducted during the Triassic, experiencing UHP eclogite facies metamorphism at mantle depths. The new finding has two‐fold significance: (i) voluminous mafic lower continental crust can increase the average density of subducted continental lithosphere, thus promoting its deep subduction; (ii) because of the current absence of mafic lower continental crust in the Dabie orogen, delamination or recycling of subducted mafic lower continental crust can be inferred as the geochemical cause for the mantle heterogeneity and the unusually evolved crustal composition.  相似文献   

20.
U–Pb and Rb–Sr dating was undertaken in combination with P–T estimates to (1) constrain the time of ultrahigh-pressure (UHP) eclogite formation in the Stadlandet UHP province of Norway, (2) date later crustal melting–migmatization of the eclogite country gneisses, and (3) temporally trace post-migmatite cooling and retrogression under amphibolite facies metamorphic conditions. In contrast to earlier U–Pb studies which used accessory minerals from the gneisses, we focused on the direct dating of minerals defining the HP assemblage. For the eclogite, rutile and omphacite fractions were analyzed for U–Pb, and from an adjacent migmatite leucosome titanites and K-feldspar. For Rb–Sr dating, phengite was measured for the eclogite, and biotite for two leucosome layers of the migmatite–eclogite complex. A U–Pb age of 389±7 (2σ) Ma is obtained if the full set of 12 rutile and five omphacite analyses is regressed (MSWD: 16), and 389±2 Ma for those nine data which strictly satisfy isochron conditions (MSWD: 0.78). The 389-Ma age is interpreted to date equilibration and freezing of the eclogite paragenesis at maximum temperatures of 770 °C, reached during decompression to 1.8 GPa. Decompression from 2.8 to 1.8 GPa occurred in the partial melting domain of granitic crust, with the migmatites being dated at 375±6 Ma by titanite and K-feldspar from an eclogite-adjacent granitic leucosome. This titanite age also shows that the U–Pb chronometer in rutile is very robust to high temperatures—it remained a closed system for at least 14 million years, at temperatures in excess to 650 °C. After decompression and migmatization, exhumation is accompanied by rapid cooling to reach the 300 °C isograde by 357± 9 Ma, determined by a biotite isochron for a leucosome in a slightly shallower structural level. In considering that the time of maximum pressure is bracketed by early zircon crystallization during subduction and later omphacite–rutile equilibration in the eclogites, an exhumation rate of 5 mm/year is deduced for initial exhumation, occurring between 394 and 389 Ma. For subsequent cooling from 770 to 600 °C, we obtain a rate of 2.3±1.3 mm/year. First stages of exhumation most likely occurred under an overall compressional regime, whereas Devonian basin formation is associated to detachment movements during 389–375 Ma exhumation. This period of extension is followed by a much younger, decoupled thermal phase at 327±5 Ma, occurring under static conditions within very restricted zones, most likely in association with the circulation of fluid phases along old discontinuities. Initial isotopic signatures of Sr and Pb substantiate Paleo- to Meso-Proterozoic crust formation times of the Stadlandet UHP province precursor lithologies.  相似文献   

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