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1.
Qiu and Wijbrans [Qiu H.-N. and Wijbrans J. R. (2006) Paleozoic ages and excess 40Ar in garnets from the Bixiling eclogite in Dabieshan, China: new insights from 40Ar/39Ar dating by stepwise crushing. Geochim. Cosmochim. Acta70, 2354-2370] present three Ar-Ar age spectra for fluid inclusions in garnet from eclogite at Bixiling in the Dabie orogen, east-central China. These Paleozoic ages of 427 ± 20 to 444 ± 10 Ma are interpreted to represent the first formation of Dabie ultrahigh-pressure (UHP) eclogite and thus require subduction of Yangtze crust to have started much earlier than previously accepted. However, no petrographic evidence, such as mineral inclusions in the garnet relating to the particular metamorphic conditions, is presented to substantiate the proposed UHP metamorphic event. Because garnet growth is not uniquely responsible for UHP eclogite-facies metamorphism, a distinction between UHP and high-pressure (HP) metamorphic events must be made in the interpretation of geochronological results. Available data from mineral Sm-Nd and zircon U-Pb dating of eclogites from the same area have firmly established that the UHP eclogite-facies metamorphism took place at Triassic. Neither the age of UHP metamorphism nor the timing of continental collision is reliably constrained by their presented data; the fluid inclusions in garnet must contain inherited 40Ar from UHP eclogite precursor, without considerable resetting of the Ar-Ar isotopic system during Triassic UHP metamorphism. Therefore, their data are either meaningless, or at best viewed as the age of garnet growth by low-T/HP blueschist/eclogite-facies metamorphism of the UHP eclogite precursor during arc-continent collision in the early Paleozoic. Furthermore, it is critical for metamorphic geochronology to substantiate the timing of UHP metamorphic event by means of zircon U-Pb in situ dating on coesite-bearing domains of metamorphically grown zircon.  相似文献   

2.
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   

3.
Diffusion rates of Sr and O in minerals are often comparable while Nd has a lower diffusion rate during thermal overprint(s); thus, the O isotope systems between metamorphic minerals can serve as an indicator to evaluate whether equilibrium of Rb–Sr and Sm–Nd systems has been preserved in the metamorphic minerals that experienced retrograde metamorphism. This study presents a combination of investigation on Sm–Nd, Rb–Sr, and O isotopic compositions of minerals separated from ultrahigh-pressure eclogite and gneiss that were collected from the main hole of the Chinese Continental Scientific Drilling project located in the Sulu orogen, eastern China. Oxygen isotopic compositions of minerals from gneiss and eclogite yield two temperature groups of 620–740 and 460–590°C, representing diffusion cessation of isotopic exchange during the eclogite-facies recrystallization and later amphibolite-facies retrograde overprint. Rb–Sr mineral regressions of two eclogite samples give consistent Triassic ages of 244 Ma, corresponding to eclogite-facies metamorphism, while the same minerals do not yield meaningful Sm–Nd isochron ages. This phenomenon likely suggests that Rb–Sr isotopic equilibrium was achieved during eclogite-facies metamorphism and preserved during late amphibolite-facies retrogression. In contrast, Sm–Nd isotopic equilibrium between the minerals of eclogite was not achieved under UHP metamorphic conditions. Regressions of epidote and biotite of one gneiss sample give a Triassic Sm–Nd age of 243 ± 34 Ma, corresponding to the time of the eclogite-facies metamorphism, and a Jurassic mineral Rb–Sr age of 187.5 ± 1.8 Ma. These results imply that fluids have played an important role to achievement of the Sm–Nd isotopic equilibrium during eclogite-facies metamorphism and re-equilibration of the Rb–Sr isotopic system during later retrograde overprint.  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.
Lawsonite is an important hydrous mineral that is stable at low‐temperature (LT) and high‐ to ultrahigh‐pressure (HP–UHP) conditions in subducted slabs. The occurrence/absence of lawsonite in eclogite is a significant constraint for the construction of the metamorphic, tectonic and fluid/melt evolution histories of an HP–UHP terrane. However, lawsonite is very rarely preserved in natural eclogites, and accurate judgment of its former existence is a significant challenge for petrologists. At present, whether lawsonite has ever existed in lawsonite‐absent eclogite is mainly judged by (i) pseudomorphs after lawsonite, and (ii) phase equilibria modelling. In this study, major element and trace‐element distributions in multistage minerals were examined in the Ganghe lawsonite‐absent UHP eclogite in the Dabie UHP terrane, eastern China. This work demonstrates that the whole‐rock Sr and light rare earth elements (LREEs) are mainly dominated by epidote; other minerals (garnet, omphacite, quartz, kyanite, barroisite, phengite and accessory minerals) play a very limited role in the Sr and LREEs budgets. Two stages of epidote, which have noticeably different Sr and LREEs contents, were recognized in the eclogite: (i) Epidote porphyroblasts (Ep‐P core), which are suspected to be the pseudomorphic mineral after lawsonite, contain significantly high Sr (7200–10 300 ppm) and LREEs (160–1300 ppm for La). (ii) An earlier stage epidote (Ep‐In core) occurs as inclusions in matrix omphacite, or in omphacite inclusions in the suspected pseudomorphic minerals after lawsonite (SPMAL); this early epidote has significantly lower Sr (990–1890 ppm, average 1495 ppm, n = 17) and LREEs contents (60–110 ppm for La, average 91 ppm, n = 17). All of the existing early‐stage minerals predating the SPMAL have very low contents of Sr and LREEs, and the total amounts of these elements in the early‐stage minerals do not balance those in the SPMAL. This indicates that a missing Ca‐, Al‐, Sr‐ and LREE‐rich mineral, which was previously in equilibrium with the early‐stage minerals, likely existed in the Ganghe eclogite. On the basis of the mineral geochemistry and phase equilibria modelling, we confirm that the missing mineral cannot be anything but lawsonite. This study indicates that examining the mass (im)balance of Sr and LREEs between multistage HP–UHP epidote can be used as a potential method to confirm the previous existence of lawsonite in lawsonite‐absent eclogite.  相似文献   

6.
Three types of eclogite, together with a serpentinized harzburgite, coexist as blocks within granitic and pelitic gneisses along the Shaliuhe cross section, the eastern part of the North Qaidam continental-type ultrahigh-pressure (UHP) metamorphic belt, NW China. The olivine (Ol1) and orthopyroxene in the harzburgite are compositionally similar to present-day abyssal peridotites. The kyanite–eclogite is derived from a troctolitic protolith, whereas the epidote–eclogite from a gabbroic protolith, both having distinct positive Eu anomalies, low TiO2, and high Al2O3 and MgO. The kyanite–eclogite shows inherited cumulate layering. The phengite–eclogite has high TiO2, low Al2O3 and MgO with incompatible trace elements resembling enriched-type MORB. Sr–Nd isotope data indicate that the protoliths of both kyanite–eclogite and epidote–eclogite ([87Sr/86Sr]i ~ 0.703–0.704; εNd(T) ~ 5.9–8.0) are of mantle origin (e.g., ocean crust signatures). On the other hand, while the lower εNd(T) value (1.4–4.1) of phengite–eclogite is more or less consistent with an enriched MORB protolith, their high [87Sr/86Sr]i ratio (0.705–0.716) points to an additional enrichment in their history, probably in an subduction-zone environment. Field relations and geochemical analyses suggest that the serpentinized harzburgite and the three types of eclogite constitute the oceanic lithological section of an ophiolitic sequence from mantle peridotite, to cumulate, and to upper basaltic rocks. The presence of coesite pseudomorphs and quartz exsolution in omphacite plus thermobarometric calculations suggests that the eclogites have undergone ultrahigh pressure metamorphism (i.e., peak P ≥ 2.7 GPa). The harzburgite may also have experienced the same metamorphism, but the lack of garnet suggests that the pressure conditions of ≤ 3.0 GPa. Zircon U–Pb SHRIMP dating shows that the eclogites have a protolith age of 516 ± 8 Ma and a metamorphic age of 445 ± 7 Ma. These data indicate the presence of a Paleo-Qilian Ocean between Qaidam and Qilian blocks before the early Ordovician. The ophiolitic assemblage may be the relics of subducted oceanic crust prior to the subduction of continental materials during Ordovician–Silurian times and ultimate continent collision. These rocks, altogether, record a complete history of ocean crust subduction, to continental subduction, and to continental collision.  相似文献   

7.
A combined study of chronometric dating and oxygen isotope analysis for minerals from vein and host eclogite as well as regional country-rock gneiss in the Dabie orogen provides a direct constraint on timing of fluid flow in this orogen formed by continental collision. Oxygen isotope ratios of vein minerals are significantly lower than those of the host eclogite, but comparable with those of the regional gneiss. This suggests the veining fluid came from the regional gneiss (i.e. exhumed slab itself) rather than the host eclogite. While zircon U–Pb and phengite Ar–Ar dating yields ages of 214 to 222 Ma for the eclogite and gneiss, the vein gives a quartz–muscovite Rb–Sr isochron age of 181 Ma and a muscovite K–Ar age of 179 Ma. Thus the veining postdates the Triassic ultrahigh pressure metamorphic event, witnessing postcollisional fluid flow after the orogenic cycle of continental collision.  相似文献   

8.
The Maksyutov metamorphic complex is the first locality where coesite pseudomorphs in garnet were described. The importance of this discovery was not understood until ultrahigh-pressure (UHP) metamorphism was independently recognized in the Dora Maira Massif of the western Alps and the Western Gneiss Region of Norway. The coesite pseudomorphs are significant because they suggest that the lower unit of the Maksyutov complex probably underwent UHP metamorphism at depths greater than 80 km in a paleosubduction zone.

The Maksyutov complex, situated in the southern Ural Mountains of Russia, forms an elongate N-S belt along the boundary between the European and Russian plates. The complex contains two superimposed tectonic unitsa lower eclogite-bearing schist unit that underwent high-pressure (HP) to UHP metamorphism and an upper meta-ophiolite unit subjected to blueschist/greenschist-facies metamorphism. The lower unit lithologies range from quartzofeldspathic, to graphite-rich, to mafic-ultramafic compositions. Mineral assemblages of the metamorphosed mafic rocks include: (1) coesite (as pseudomorphs) + garnet + omphacite + rutile + zoisite; (2) jadeite + quartz (coesite) + garnet + kyanite ± paragonite; (3) garnet + omphacite + barroisite + rutile; and (4) garnet + glaucophane + lawsonite. The upper unit is characterized by sheets of serpentinite that contain lawsonite-bearing metarodingite and rare calcium-rich eclogite. A metamorphosed melange containing blocks of ultramafic, eclogite, and quartz-jadeite rocks is situated between the two units.

The UHP metamorphic event that affected the lower unit is characterized by recumbent folding and shear zones. Subsequent large-scale, left-lateral strike-slip movements deformed both tectonic units. These deep-crustal metamorphic structures are oriented at high angles relative to the younger, N-S-trending Main Uralian thrust and the left-lateral strike-slip movement that displaced the Maksyutov block.  相似文献   

9.
In the Chinese southwestern Tianshan (U)HP belt, former lawsonite presence has been predicted for many (U)HP metamorphic eclogites, but only a very few lawsonite grains have been found so far. We discovered armoured lawsonite relicts included in quartz, which, on its part, is enclosed in porphyroblastic garnet in an epidote eclogite H711‐14 and a paragonite eclogite H711‐29. H711‐14 is mainly composed of garnet, omphacite, epidote and titanite, with minor quartz, paragonite and secondary barroisite and glaucophane. Coarse‐grained titanite occasionally occurs in millimetre‐wide veins in equilibrium with epidote and omphacite, and relict rutile is only preserved as inclusions in matrix titanite and garnet. H711‐29 shows the mineral assemblage of garnet, omphacite, glaucophane, paragonite, quartz, dolomite, rutile and minor epidote. Dolomite and rutile are commonly rimed by secondary calcite and titanite respectively. Porphyroblastic garnet in both eclogites is compositionally zoned and exhibits an inclusion‐rich core overgrown by an inclusion‐poor rim. Phase equilibria modelling predicts that garnet cores formed at the P‐peak (490–505 °C and 23–25.5 kbar) and coexisted with the lawsonite eclogite facies assemblage of omphacite + glaucophane + lawsonite + quartz. Garnet rims (550–570 °C and ~20 kbar) grew subsequently during a post‐peak epidote eclogite facies metamorphism and coexisted with omphacite + quartz ± glaucophane ± epidote ± paragonite. The results confirm the former presence of a cold subduction zone environment in the Chinese southwestern Tianshan. The P–T evolution of the eclogites is characterized by a clockwise P–T path with a heating stage during early exhumation (thermal relaxation). The preservation of lawsonite in these eclogites is attributed to isolation from the matrix by quartz and rigid garnet, which should be considered as a new type of lawsonite preservation in eclogites. The complete rutile–titanite transition in H711‐14 took place in the epidote eclogite facies stage in the presence of an extremely CO2‐poor fluid with X(CO2) [CO2/(CO2 + H2O) in the fluid] <<0.008. In contrast, the incomplete rutile–titanite transition in H711‐29 may have occurred after the epidote eclogite facies stage and the presence of dolomite reflects a higher X(CO2) (>0.01) in the coexisting fluid at the epidote eclogite facies stage.  相似文献   

10.
A variety of eclogites from an east-west transect across the North-East Greenland eclogite province have been studied to establish the timing of high pressure (HP) and ultrahigh-pressure (UHP) metamorphism in this northern segment of the Laurentian margin. Garnet + omphacite ± amphibole + whole rock Sm-Nd isochrons from a quartz eclogite, a garnet + omphacite + rutile eclogite and a partially melted zoisite eclogite in the western HP belt are 401±2, 402±9 and 414±18 Ma, respectively. Corresponding sensitive high-resolution ion microprobe (SHRIMP) 206Pb/238U ages of metamorphic zircon in the same samples are 401±7, 414±13, and 393 ±10 Ma. Metamorphic zircon domains were identified using morphology, cathodoluminescence (CL) imaging, U, Th, Th/U and trace element contents. Zircon from the quartz eclogite and the garnet + omphacite + rutile eclogite are typical of eclogite facies zircon with rounded to subhedral shapes, patchy to homogenous CL domains, low U, and very low Th and Th/U. The partially melted eclogite contains euhedral zircons with dark, sector-zoned, higher U, Th and Th/U inherited cores. Three cores give a Paleoproterozoic 207Pb/206Pb age of 1,962±27 Ma, interpreted as the age of the leucogabbroic protolith. CL images of the bright overgrowths show faint oscillatory zoning next to homogenous areas that indicate zircon growth in the presence of a HP melt and later recrystallization. Additional evidence that zircon grew during eclogite facies conditions is the lack of a Eu anomaly in the trace element data for all the samples. These results, combined with additional less precise Sm-Nd ages and our earlier work, point to a Devonian age of HP metamorphism in the western and central portions of the eclogite province. An UHP kyanite eclogite from the eastern part of the transect contains equant metamorphic zircon with homogeneous to patchy zoning in CL and HP inclusions of garnet, omphacite and kyanite. These zircons have slightly higher U, Th and Th/U values than the HP ones, no Eu anomaly, and are thus comparable to UHP zircons in the literature. The 206Pb/238U age of these zircons is 360±5 Ma, much younger than the HP eclogites. The same sample gives a Sm-Nd age of 342±6 Ma. Unlike the HP eclogites, the Sm-Nd age of the UHP rock is ca. 20 Ma younger than the U-Pb zircon age and most likely records slow cooling through the closure temperature, since peak temperatures were in excess of 900°C. Widespread HP metamorphism of both the Laurentian and Baltica continental margins marks the culmination of this continent–continent collision in the Devonian. Carboniferous UHP conditions, though localized in the east, suggest a prolonged collisional history rather than a short-lived Scandian orogeny. The traditional Silurian Scandian orogeny should thus be extended through the Devonian.  相似文献   

11.
This study presents in situ strontium (Sr) isotope and Sr content data on multi-stage epidote crystals from ultrahigh-pressure (UHP) eclogites and omphacite–epidote veins therein at Ganghe (Dabie terrane, China), determined using LA-MC-ICP-MS. The Ganghe eclogites occur as lenses in mainly leucocratic UHP gneisses, and therefore, our data provide insights into the origin, composition, and transport scale of the discrete multi-stage fluids in UHP eclogites during the subduction and exhumation of a continental crust. Four textural types of epidote that record compositional and isotopic signatures of fluid at various metamorphic P–T conditions have been distinguished based on petrographic observations and compositional analyses. They are (1) fine-grained high-pressure (HP) epidote inclusions (Ep-In) in omphacite that define the earliest stage of epidote formation in the eclogite; (2) coarse-grained UHP epidote porphyroblasts (Ep-P) that contain omphacite with Ep-In inclusions in the eclogite; (3) fine-grained HP epidote in omphacite–epidote veins (Ep-V) as well as (4) the latest-stage epidote in disseminated amphibolite-facies veinlets (Ep-A), which crosscut the Ep-P or matrix minerals in the eclogite and HP vein. Both Ep-P and Ep-V crystals exhibit significant and complex chemical zonations with respect to the XFe (= Fe/(Fe + Al)) ratio and Sr content. In contrast to the varying Sr contents, Ep-In, Ep-P, and Ep-V have similar and narrow ranges of initial 87Sr/86Sr ratios (from 0.70692 to 0.70720 for Ep-In, from 0.70698 to 0.70721 for Ep-P, and from 0.70668 to 0.70723 for Ep-V), which are significantly different from those in Ep-A (from 0.70894 to 0.71172). The initial 87Sr/86Sr ratio of Ep-A is closer in value to the initial Sr isotopic composition of the gneisses (from 0.710790 to 0.712069) which enclose the UHP eclogite. These data indicate different sources of the eclogite-facies fluids and retrograde amphibolite-facies fluid in the Ganghe eclogites. The HP–UHP fluids responsible for the large amounts of hydrous minerals in the eclogites were internally derived and buffered. The omphacite–epidote veins were precipitated from the channelized solute-rich HP–UHP fluids released from the host eclogite. However, hydrated amphibolite-facies metamorphism during exhumation was mainly initiated by the low-Sr and high-87Sr/86Sr external fluid, which infiltrated into the eclogite from the surrounding gneisses. The eclogite-facies fluids in the Ganghe eclogites were locally derived, whereas the infiltration of the retrograde amphibolite-facies fluid from the gneisses required a long transport, most likely longer than 80 m. This study highlights that the in situ Sr isotopic analysis of multi-stage epidote can be employed as a powerful geochemical tracer to provide key information regarding the origin and behavior of various-stage subduction-zone metamorphic fluids.  相似文献   

12.
The north Qilian high‐pressure (HP)/low‐temperature (LT) metamorphic belt is composed mainly of blueschists, eclogites and greenschist facies rocks. It formed within an Early Palaeozoic accretionary wedge associated with the subduction of the oceanic crust and is considered to be one of the best preserved HP/LT metamorphic belts in China. Here we report new lawsonite‐bearing eclogites and eclogitic rocks enclosed within epidote blueschists in the North Qilian Mountains. Five samples contain unaltered lawsonite coexisting with omphacite and phengite as inclusions in garnet, indicating eclogite facies garnet growth and lawsonite pseudomorphs were observed in garnet from an additional 11 eclogites and eclogitic rocks. Peak pressure conditions estimated from lawsonite omphacite‐phengite‐garnet assemblages were 2.1–2.4 GPa at temperatures of 420–510 °C, in or near the stability field of lawsonite eclogite, and implying formation under an apparent geothermal gradient of 6–8 °C km?1, consistent with metamorphism in a cold subduction zone. SHRIMP U‐Pb dating of zircon from two lawsonite‐bearing eclogitic metabasites yields ages of 489 ± 7 Ma and 477 ± 16 Ma, respectively. CL images and mineral inclusions in zircon grains indicate that these ages reflect an eclogite facies metamorphism. An age of 502 ± 16 Ma is recorded in igneous cores of zircon grains from one lawsonite pseudomorph‐bearing eclogite, which is in agreement with the formation age of Early Ordovician for some ophiolite sequences in the North Qilian Mountains, and may be associated with a period of oceanic crust formation. The petrological and chronological data demonstrate the existence of a cold Early Palaeozoic subduction zone in the North Qilian Mountains.  相似文献   

13.
A combined study of internal structure, U-Pb age, and Hf and O isotopes was carried out for metamorphic zircons from ultrahigh-pressure eclogite boudins enclosed in marbles from the Dabie orogen in China. CL imaging identifies two types of zircon that are metamorphically new growth and recrystallized domain, respectively. The metamorphic zircons have low Th and U contents with low Th/U ratios, yielding two groups of 206Pb/238U age at 245 ± 3 to 240 ± 2 Ma and 226 ± 4 to 223 ± 2 Ma, respectively. Anomalously high δ18O values were obtained for refractory minerals, with 9.9 to 21.4‰ for garnet and 16.9‰ for zircon. This indicates that eclogite protolith is sedimentary rocks capable of liberating aqueous fluid for zircon growth during continental subduction-zone metamorphism. Most of the zircons are characterized by very low 176Lu/177Hf ratios of 0.000001-0.000028, indicating their growth in association with garnet recrystallization. A few of them falling within the older age group have comparatively high 176Lu/177Hf ratios of 0.000192-0.000383, suggesting their growth prior to the formation of garnet in the late stage of subduction. The variations in the Lu/Hf ratios for zircons can thus be used to correlate with garnet growth during eclogite-facies metamorphism. In either case, the zircons have variable εHf (t) values for individual samples, suggesting that their protolith is heterogeneous in Hf isotope composition with localized fluid availability in the bulk processes of orogenic cycle. Nevertheless, a positive correlation exists between 206Pb/238U ages and Lu-Hf isotope ratios for the metamorphically recrystallized zircons, suggesting that eclogite-facies metamorphism in the presence of fluid has the identical effect on zircon Lu-Hf and U-Th-Pb isotopic systems. We conclude that the zircons of the older group grew in the presence of fluid during the subduction prior to the onset of peak ultrahigh-pressure metamorphism, whereas the younger zircons grew in the presence of fluid released during the initial exhumation toward high-pressure eclogite-facies regime.  相似文献   

14.
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   

15.
《International Geology Review》2012,54(10):1270-1293
ABSTRACT

The Chinese southwestern Tianshan HP–UHP/LT metamorphic complex possesses well-preserved mafic layers, tectonic slices/blocks, boudins/lens of different sizes, and lithology embedded within dominant metavolcanoclastics. A recent study on the ultra-high pressure (UHP) eclogite revealed a short timescale of exhumation (≤10 Ma, ~315 ± 5 Ma). However, controversies still exist on some key questions: (1) the reasonable interpretation of spatially close-outcropped high pressure (HP) and UHP slices with respect to regional geodynamics, and (2) if the previous regional scatter Ar–Ar ages proved the existence of internally coherent sub-belts or troubled by dating on samples with notable 40Ar retention. This study focusses on detailed P–T–time (phengite Ar closure) recovery of samples from a HP eclogite lens and its host rock, the UHP thick-layered eclogite. Based on data from bulk–rock, microprobe analysis, and muscovite Ar–Ar chronological dating, we link phengite growth to potential garnet growth stages via thermodynamic modelling. Facilitated by the P–T–Ar retention% graph, we collect all the regional muscovite Ar–Ar data together with results in this study for evaluating the significance of regional muscovite Ar–Ar ages and set back to geodynamics. According to modelling results, the HP lens eclogite reached peak metamorphism at ~550°C, 2.50 GPa with an Ar–Ar muscovite plateau age of 316.9 ± 1.0 Ma that could date the mass phengite growth event during prograde metamorphism. In contrast, the UHP layered eclogite experienced UHP peak burial at ~510°C, 2.95 GPa, and then to HP peak metamorphism at ~560°C, 2.60 GPa with ~311.6 ± 0.7 Ma plateau age that may constrain the cooling age during early exhumation. Noteworthy, both of them share a quite similar early exhumation path despite bearing contrasting prograde metamorphic experiences. With considering updated regional exhumation pattern, this might imply the existence of a potential deep juxtaposing (capture) process between HP slices and exhumating UHP complex, at about 45–60 km depth along subduction plate interface.  相似文献   

16.
In our paper we supposed that the Paleozoic ages of the Bixiling eclogitic garnets by 40Ar/39Ar crushing in vacuo were related to the UHP metamorphism. Zhao and Gao [Zhao Z.-F. and Gao T.-S. (2007) Comment on “Paleozoic ages and excess 40Ar in garnets from the Bixiling eclogite in Dabieshan, China:Newinsights from 40Ar/39Ar dating by stepwise crushing” by Qiu and Wijbrans (2006). Geochim. Cosmochim. Acta71(24), 6046-6050] dispute our conclusions and re-interpret them as “the age of garnet growth by low-T/HP blueschist/eclogite-facies metamorphism of the UHP eclogite precursor during arc-continent collision in the early Paleozoic” without presenting any new petrological evidence. Based on our present understanding of 40Ar/39Ar crushing technique and petrological observations, we do not agree with their re-interpretation and still prefer our original interpretation.  相似文献   

17.
We have identified by laser micro-Raman spectroscopy that inclusions of coesite occur together with other eclogite-facies mineral phases within metamorphic zircons separated from the large eclogite body at Ulsteinvik–Dimnøy on Hareidland. This is the first identification of coesite from this portion of the northwestern Western Gneiss Region (WGR) and supports continuity of ultrahigh-pressure (UHP) metamorphism between the documented coesite occurrences on Stadlandet to the south and the microdiamond and coesite pseudomorph localities on Fjørtoft in the Nordøyene to the north. The zircons, first analysed by U–Pb TIMS in 1973, have been re-analysed and have yielded a much more precise age of 401.6±1.6 Ma, that overlaps with the previously determined age. Our discovery of coesite and the indication of a close to 402 Ma formation age add to a growing number of mid–late Early Devonian ages that signal that the UHP metamorphism in this part of west Norway occurred relatively late in the Caledonian orogenic cycle. These observations should be incorporated in geodynamic models for the exhumation of these rocks and for the metastable preservation of eclogite-facies mineralogies.  相似文献   

18.
The Zhujiachong eclogite in the south‐eastern Dabieshan ultra‐high‐P terrane has been overprinted during retrograde metamorphism, with the development of garnet‐amphibolite mineral assemblages in most rocks in the outcrop. This study is focused on providing age constraints for the retrograde amphibolite facies and greenschist facies mineralogy by 40Ar/39Ar dating. By applying a novel approach of combining three different techniques for extracting argon: laser stepwise heating of single grains and small separates, a spot fusion technique by UV‐laser ablation microprobe on polished sections and an in vacuo crushing technique for liberating radiogenic argon from fluid inclusions, it is demonstrated that an internally consistent thermal history can be derived. The 40Ar/39Ar ages indicate that phengite formed before 265 Ma, probably during the ultra‐high‐P event. Ages associated with amphibolite facies retrograde metamorphism range from 242 to 217 Ma by the analyses of amphibole. Ages of c. 230 Ma were found for the symplectite matrix that formed during retrogression from eclogite pyroxene. Late stage hydrothermal activity leading to the formation of coarse‐grained paragonite and fluid inclusions in vein amphibole was dated at c. 200 Ma. These age results agree well with the mineral crystallization sequence observed from thin‐sections of the retrograded eclogite: phengite → paragonite and amphibole in matrix → amphibole in the corona.  相似文献   

19.
Koen de Jong   《Lithos》2003,70(3-4):91-110
In order to attempt to further constrain the age of the early Alpine tectonic evolution of the Mulhacén Complex and to explore the influence of inherited isotopes, micas from a small number of well-characterised rocks from the Sierra de los Filábres, with a penetrative tectonic fabric related to the exhumation of eclogite-facies metamorphic rocks, were selected for 40Ar/39Ar and Rb–Sr dating.

A single phengite grain from an amphibolite yielded an 40Ar/39Ar laser step heating plateau age of 86.9±1.2 Ma (2σ; 70% 39Ar released) and an inverse isochron age of 86.2±2.4 Ma with an 36Ar/40Ar intercept within error of the atmospheric value. Induction furnace step heating of a biotite separate from a gabbro relic in an eclogite yielded a weighted mean age of 173.2±6.3 Ma (2σ; 95% 39Ar released). These ages are diagnostic of excess argon (40ArXS) incorporation, as they are older than independent age estimates for the timing of eclogite-facies metamorphism and intrusion of the gabbros. 40ArXS incorporation probably resulted from restricted fluid mobility in the magmatic rocks during their metamorphic recrystallisation.

Rb–Sr whole-rock–phengite ages of graphite-bearing mica schists from Paleozoic rocks (Secano unit) show a dramatic variation (66.1±3.2, 40.6±2.6 and 14.1±2.2 Ma). An albite chlorite mica schist from the Mesozoic series of the Nevado–Lubrín unit has a whole-rock–mica–albite age of 17.2±1.9 Ma, which is within error of an 40Ar/39Ar plateau age published previously and of the youngest Rb–Sr age of the Paleozoic series obtained in this study. The significant spread in Rb–Sr ages implies that progressive partial resetting of an older isotopic system has occurred. The microstructure of the samples with pre-Miocene Rb–Sr ages reveals incomplete recrystallisation of white mica and inhibited grain growth due to the presence of graphite particles. This interpretation agrees with previously published, disturbed and slightly dome-shaped 40Ar/39Ar age spectra that may point similarly to the presence of an older isotope component. The progressively reset Rb–Sr system is a relic of Variscan metamorphism of the Paleozoic series of the Mulhacén Complex. In contrast, the origin of the ca. 17.2 Ma old sample from the Mesozoic series precludes any isotopic inheritance, in agreement with its pervasive tectono-metamorphic recrystallisation during the Miocene.

Exhumation of the eclogite-facies Mulhacén Complex occurred in two stages with contrasting rates of about 22.5 mm/year during the early phase and 9–10 mm/year during the late phase; the latter with a cooling rate in the order of 330 °C/Ma.  相似文献   


20.
The Malpica–Tui complex (NW Iberian Massif) consists of a Lower Continental Unit of variably deformed and recrystallized granitoids, metasediments and sparse metabasites, overridden by an upper unit with rocks of oceanic affinities. Metamorphic minerals dated by the 40Ar/39Ar method record a coherent temporal history of progressive deformation during Variscan metamorphism and exhumation. The earliest stages of deformation (D1) under high-pressure conditions are recorded in phengitic white micas from eclogite-facies rocks at 365–370 Ma. Following this eclogite-facies peak-metamorphism, the continental slab became attached to the overriding plate at deep-crustal levels at ca. 340–350 Ma (D2). Exhumation was accompanied by pervasive deformation (D3) within the continental slab at ca. 330 Ma and major deformation (D4) in the underlying para-autochthon at 315–325 Ma. Final tectonothermal evolution included late folding, localized shearing and granitic intrusions at 280–310 Ma.

Dating of high-pressure rocks by the 40Ar/39Ar method yields ages that are synchronous with published Rb–Sr and Sm–Nd ages obtained for both the Malpica–Tui complex and its correlative, the Champtoceaux complex in the French Armorican Massif. The results indicate that phengitic white mica retains its radiogenic argon despite been subjected to relatively high temperatures (500–600 °C) for a period of 20–30 My corresponding to the time-span from the static, eclogite-facies M1 peak-metamorphism through D1-M2 eclogite-facies deformation to amphibolite-facies D2-M3. Our study provides additional evidence that under certain geological conditions (i.e., strain partitioning, fluid deficiency) argon isotope mobility is limited at high temperatures, and that 40Ar/39Ar geochronology can be a reliable method for dating high pressure metamorphism.  相似文献   


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