Prograde transformations of medium-pressure amphibolites during their eclogitization

The paper is devoted to mineral equilibria occurring during the transformation of medium-pressure metabasites into eclogites at geothermal gradients of 11–19°C/km; it presents materials on the evolution of the composition and types of prograde zoning in garnet and clinopyroxene, significance and distinctive features of armored inclusions of earlier metamorphic stages in garnets and the possibility of their application in calculating the P-T path of eclogitization. Results of the analysis of mineral assemblages, data on mineral zoning, and evolution of phase equilibria are used to outline depth subfacies of crustal eclogites and describe in detail equilibria in basic and acid rocks of each subfacies. Conditions under which orthopyroxene is stable in the eclogite facies are analyzed within the temperature range of 630–700°C in correlation with the bulk composition of the eclogites. Published materials on high-pressure mineral transformations in eclogite-facies acid gneisses are analyzed.     

Crystallization and very rapid exhumation of the youngest Alpine eclogites (Tauern Window,Eastern Alps) from Rb/Sr mineral assemblage analysis

Multimineral Rb/Sr internal isochrons from eclogite facies rocks of the Eclogite Zone (Tauern Window, Eastern Alps) consistently yield an Early Oligocene age of 31.5±0.7 Ma. This age has been obtained both for late-prograde, dehydration-related eclogitic veins, and for rocks variably deformed and recrystallized under eclogite facies conditions (2.0–2.5 GPa, 600°C). Initial Sr-isotopic equilibria among all phases indicate absence of significant post-eclogitic isotope redistribution processes, therefore the ages date eclogite facies assemblage crystallization. Equilibria also prove that no prolonged pre-eclogite facies history is recorded in the rocks. Instead, subduction, prograde mineral reactions, and eclogitization proceeded rapidly. Fast exhumation immediately after eclogitization, with minimum rates >36 mm/a is inferred from a 31.5±0.5 Ma internal mineral isochron age of a post-eclogitic greenschist facies vein assemblage. Such rates equal typical subduction rates. Late Eocene to Early Oligocene subduction of the European continental margin, with subsequent rapid exhumation of high-pressure nappe complexes has previously been recognized only in the Western Alps. The new data signify synchronous continental collision all along the Alpine belt. Our results demonstrate the unique potential of Rb/Sr assemblage system analysis for precise dating of both eclogite facies and post-eclogitic events, thus for precisely constraining exhumation rates of deep-seated rocks, and for straightforward linkage of petrologic evidence with isotopic ages.     

U-Pb dating of magmatic zircon and metamorphic baddeleyite in the Ligurian eclogites (Voltri Massif,Western Alps)

U-Pb geochronology with ion microprobe (SHRIMP) analysis has been carried out on eclogite-facies rocks of the Beigua Unit, an ophiolitic slice of the Voltri Massif, Western Alps. The investigated samples are eclogites and high-pressure metasomatic rocks (metarodingites and centimetre-sized Ti-clinohumite-bearing dykes). Zircon contained in an eclogitic metagabbro and a metarodingite preserves magmatic zoning patterns and trace element compositions. The zircon ages of 160±1 and 161±3 Ma are interpreted to date the crystallization of the gabbroic protoliths. Ti-clinohumite dykes in the same unit contain baddeleyite crystals in textural equilibrium with Ti-clinohumite, diopside, chlorite and magnetite, which form the eclogite-facies assemblage in these rocks. Baddeleyite also contains inclusions of such minerals, indicating its formation at high pressure. The baddeleyite has cathodoluminescence intensity and chaotic patterns similar to metamorphic zircon. It contains a significant amount of Hf (1.3–1.7 wt%), traces of Ti, Y, Nb, Ta, REE, U and Th. Its chondrite-normalised trace element pattern has strong enrichment in middle REE, positive Ce-anomaly and small negative Eu-anomaly. This represents the first report of baddeleyite formed during regional metamorphism, and suggests that this mineral could (re)crystallize easier than zircon under low-temperature, high-pressure conditions. The age of the baddeleyite is interpreted as likely dating the eclogite-facies metamorphism in the Beigua Unit at 33.6±1.0 Ma. This age is very close to the Early Oligocene age of the overlying Tertiary continental breccias and conglomerates, which contains clasts of high-pressure rocks. This sedimentary record, which is unique for Alpine high-pressure units, is direct evidence of fast exhumation of the Beigua eclogites. The young age for the HP metamorphism of the Beigua ophiolite makes a revision of either the palaeogeography prior to collision, or of the subduction setting in the entire region, necessary.Editorial responsibility: J. Hoefs     

P,T paths of natural eclogites during metamorphism — a record of subduction

The distribution of iron and magnesium between coexisting garnet and clinopyroxene is expressed by the distribution coefficient K_D^ga-cpx. This coefficient has been experimentally determined as a function of temperature and pressure, and is used to determine the temperature of equilibration of natural eclogites.

The presence of relict zoning in both garnet and clinopyroxene in low to medium temperature eclogites permits evaluation of the P,T path followed by these rocks during prograde metamorphism.

The average P,T path for eclogites of blueschist terranes (type C eclogites) is suggested to be 40°C/Kb during prograde metamorphism. The Tasmanian eclogite (type B) records crystallization along a different P,T path (lower pressure at given temperature) from those of the type C eclogites. Eclogites from the migmatite-gneiss terrances of Poland and Norway do not preserve evidence of prograde metamorphism in mineral zoning but indicate higher temperatures at the metamorphic maximum; pressures were similar to those attained by type C eclogites.     

Variscan vs Cadomian tectonothermal activity in northwestern sectors of the Teplá-Barrandian zone, Czech Republic: constraints from 40Ar/39Ar ages

Five muscovite concentrates from high-grade, pelitic metasedimentary basement rocks exposed in northwestern sectors of the Teplá-Barrandian zone (Czech Republic) record ⁴⁰Ar/³⁹Ar mineral plateau ages which range between ca. 376 and 362?Ma. Hornblende concentrates from metagabbro (Mariánské Lánzě complex) and fine-grained basement amphibolite display plateaux which define ³⁶Ar/⁴⁰Ar vs ³⁹Ar/⁴⁰Ar isotope-correlation ages of ca. 370 and ca. 375 Ma. The mineral ages are interpreted to date relatively rapid cooling through appropriate argon retention temperatures following early phases of Variscan (Early Devonian) regional metamorphism. A slate/phyllite basement sample collected within lower-grade metasedimentary rocks in southeastern portions of the Teplá-Barrandian zone is characterized by an internally discordant ⁴⁰Ar/³⁹Ar whole-rock age spectrum which suggests partial Variscan rejuvenation of intracrystalline argon systems which had cooled through appropriate argon retention temperatures following an initial regional metamorphism at or prior to ca. 500 Ma (Cadomian). Hornblende from undeformed diorite of the Kdyn? complex records a well-defined ⁴⁰Ar/³⁹Ar age plateau which corresponds to an isotope-correlation age of ca. 516?Ma. This is interpreted to date post-magmatic cooling following emplacement.     

Geochemical signatures for eclogite protolith from the Maksyutov Complex, South Urals

We conducted a geochemical study of eclogites (40 samples) from a boudin of the Lower Unit of the Maksyutov Complex in the South Urals in order to determine their protolith nature. The eclogites have major element compositions corresponding to quartz-bearing hypersthene basalts. Trace-element characteristics of the eclogites further suggest that they resemble enriched-type of tholeiites such as E-MORB. The compositional variation of eclogites was likely caused by fractional crystallization of parental melt under hypabyssal conditions, during its intrusion in thinned continental crust shortly before subduction. The high-pressure metamorphism has not affected significantly the major- and trace-element signatures of the protoliths. The compositions of co-existing minerals from the distinguished rock groups do not show significant distinctions. The considerable scatter of P–T estimates of metamorphic conditions does not depend on whole-rock composition. Therefore, the eclogitization was preceded by a chemical differentiation of an initial magmatic source, which is responsible for co-existence of rocks of variable composition in the same boudin. Dikes or sills of tholeiite basalts having geochemical characteristics of E-MORB could be the protoliths for the Maksyutov eclogites.     

Comment on “Paleozoic ages and excess Ar in garnets from the Bixiling eclogite in Dabieshan, China: New insights from Ar/Ar dating by stepwise crushing” by Qiu and Wijbrans (2006)

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.     

Constraints on retrograde metamorphism of UHP eclogites in North Qinling,Central China,from 40Ar/39Ar dating of amphibole and phengite

Coesite- and microdiamond- bearing ultra-high pressure (UHP) eclogites in the North Qinling terrane have been widely retrogressed to amphibolites. Previous geochronological studies on these UHP rocks mainly focused on the timing of peak eclogite facies metamorphism. The Kanfenggou UHP metamorphic domain is one of the best-preserved coesite-bearing eclogite occurrences in the North Qinling terrane. In this study, mafic amphibolites and host schists from this domain were collected for ⁴⁰Ar/³⁹Ar dating to constrain their retrograde evolution. Two generations of amphibole are recognized based on their mineral parageneses and ⁴⁰Ar/³⁹Ar ages. A first generation of amphibole from garnet amphibolites yielded irregularly-shaped age spectra with anomalously old apparent ages. Isochron ages of 484–473 Ma and initial ⁴⁰Ar/³⁶Ar ratios of 3695–774 are obtained from this generation of amphibole, indicating incorporation of excess argon. Second generation amphibole occurs in epidote amphibolites yielded flat age spectra with plateau ages of 464–462 Ma without evidence for excess argon. These ages suggest that the amphibolite-facies metamorphism has taken place as early as 484 Ma and lasted until 462 Ma for the North Qinling UHP metamorphic rocks. Phengite from the country-rock schists yielded ⁴⁰Ar/³⁹Ar plateau ages of 426–396 Ma, with higher phengite Si contents associated with the older the plateau ages. Based on our new ⁴⁰Ar/³⁹Ar ages and previous zircon UPb geochronological data, we construct a new detailed pressure-temperature-time (P-T-t) path illustrating the retrograde metamorphism and exhumation rate of the North Qinling eclogites and host schists. The P-T-t path suggests that these UHP metamorphic rocks experienced initial medium-to-high exhumation rates (ca. 8.7 mm/yr) during the Early Ordovician (489–484 Ma), which was mainly derived from buoyancy forces. Subsequently, the exhumation rate decreased gradually from ~0.8 to 0.3 mm/yr from 484 to 426 Ma, which was probably governed by extension and/or erosion.     

Two Contrasting Eclogite Types on the Northern Margin of the Qaidam Basin, Western China

Eclogites have been recently discovered in the Xitieshan area in the middle segment of the northern margin of the Qaidam basin. These eclogites, together with those recognized earlier in the Yuka area of the western segment of the northern margin of the Qaidam basin and in the Dulan area of the eastern segment of the northern margin of the Qaidam basin, form an eclogite belt with a length of 350 km. A comparison of the eclogites from the Yuka and Xitieshan areas suggests that they show different country rocks, microtextures, mineral assemblages, and especially, different peak temperatures, PT paths during decompression and isotopic features. Eclogites from the Yuka area bear evidence of prograde metamorphism, such as prograde mineral relics in garnet and growth zoning of garnet, and hairpin-shaped PT paths with coincidence of the baric and thermal peaks of metamorphism, which reflect rapid burial and uplift. Sm-Nd isotopic determination shows obvious Sm-Nd disequilibrium, and no isochron ages of Early P     

White mica K–Ar geochronology of Sanbagawa eclogites from Southwest Japan: implications for deformation-controlled K–Ar closure temperature

White mica (phengite and paragonite) K–Ar ages of eclogite-facies Sanbagawa metamorphic rocks (15 eclogitic rocks and eight associated pelitic schists) from four different localities yielded ages of 84–89 Ma (Seba, central Shikoku), 78–80 Ma (Nishi-Iratsu, central Shikoku), 123 and 136 Ma (Gongen, central Shikoku), and 82–88 Ma (Kotsu/Bizan, eastern Shikoku). With the exception of a quartz-rich kyanite-bearing eclogite from Gongen, white mica ages overlap with the previously known range of phengite K–Ar ages of pelitic schists of the Sanbagawa metamorphic belt and can be distinguished from those of the Shimanto metamorphic belt. The similarity of K–Ar ages between the eclogites and surrounding pelitic schists supports a geological setting wherein the eclogites experienced intense ductile deformation with pelitic schists during exhumation. In contrast, phengite extracted from the Gongen eclogite, which is less overprinted by a ductile shear deformation during exhumation, yielded significantly older ages. Given that the Gongen eclogite is enclosed by the Higashi-Akaishi meta-peridotite body, these K–Ar ages are attributed to excess ⁴⁰Ar gained during an interaction between the eclogite and host meta-peridotite with mantle-derived noble gas (very high ⁴⁰Ar/³⁶Ar ratio) at eclogite-facies depth. Fluid exchange between deep-subducted sediments and mantle material might have enhanced the gain of mantle-derived extreme ⁴⁰Ar in the meta-sediment. Although dynamic recrystallization of white mica can reset the Ar isotope system, limited-argon-depletion due to lesser degrees of ductile shear deformation of the Gongen eclogite might have prevented complete release of the trapped excess argon from phengites. This observation supports a model of deformation-controlled K–Ar closure temperature.     

Paleozoic ages and excess Ar in garnets from the Bixiling eclogite in Dabieshan, China: New insights from Ar/Ar dating by stepwise crushing

The ⁴⁰Ar/³⁹Ar stepwise crushing technique is applied for the first time to date garnet from ultra-high-pressure metamorphic (UHPM) eclogites. Three garnet samples from the Bixiling eclogites analyzed by ⁴⁰Ar/³⁹Ar stepwise crushing yield regular, predictable age spectra, and a clear separation between excess ⁴⁰Ar and concordant plateau and isochron ages. All three age spectra begin with high apparent ages followed by step by step decreasing ages, and finally age plateaux with apparent ages in the range from 427 ± 20 to 444 ± 10 Ma. The data points constituting the age plateaux yield excellent isochrons with radiogenic intercept ages ranging from 448 ± 34 to 459 ± 58 Ma, corresponding to initial ⁴⁰Ar/³⁶Ar ratios from 292.1 ± 4.5 to 294.5 ± 6.7, statistically indistinguishable from the modern air. The high initial ages are interpreted to derive from secondary fluid inclusions containing excess ⁴⁰Ar, whereas the plateau ages are attributed to gas from small primary fluid inclusions without significant excess ⁴⁰Ar. The plateau ages are interpreted to approximate the time of garnet growth during initial UHPM metamorphism. Phengite analyzed by laser stepwise heating yielded a complicated two-saddle age spectrum with a scattered isochron corresponding to age of 463 ± 116 Ma and initial ⁴⁰Ar/³⁶Ar ratio of 1843 ± 1740 indicative of the presence of extraneous ⁴⁰Ar within phengite. These concordant isochron ages measured on minerals diagnostic of eclogite grade metamorphism strongly suggest that Dabie UHPM eclogites were first formed in the early Paleozoic, during the same event that caused the Qinling-Northern Qaidam Basin-Altyn Tagh eclogites.     

The metamorphic complex of Beloretzk, SW Urals, Russia — a terrane with a polyphase Meso- to Neoproterozoic thermo-dynamic evolution

An integrated geological study of the tectono-metamorphic evolution of the metamorphic complex of Beloretzk (MCB) which is part of the eastern Bashkirian mega-anticlinorium (BMA), SW Urals, Russia shows that the main lithological units are Neoproterozoic (Riphean and Vendian age) siliciclastic to carbonate successions. Granitic, syenitic and mafic intrusions together with subaerial equivalents comprise the Neo- and Mesoproterozoic magmatic rocks. The metamorphic grade ranges from diagenetic and very low grade in the western BMA to high-grade in the MCB. The N–S trending Zuratkul fault marks the change in metamorphic grade and structural evolution between the central and eastern BMA. Structural data, Pb/Pb-single zircon ages, ⁴⁰Ar/³⁹Ar cooling ages and the provenance signature of Riphean and Vendian siliciclastic rocks in the western BMA give evidence of Mesoproterozoic (Grenvillian) rifting, deformation and eclogite-facies metamorphism in the MCB and a Neoproterozoic (Cadomian) orogenic event in the SW Urals. Three pre-Ordovician deformation phases can be identified in the MCB. The first SSE-vergent, isoclinal folding phase (D₁) is younger than the intrusion of mafic dykes (Pb/Pb-single zircon: 1350 Ma) and older than the eclogite-facies metamorphism. High P/low T eclogite-facies metamorphism is bracketed by D₁ and the intrusion of the Achmerovo granite (Pb/Pb-single zircon: ≤970 Ma). An extensional, sinistral, top-down-to-NW directed shearing (D₂) is correlated with the first exhumation of the MCB. E-vergent folding and thrusting (D₃) occurred at retrograde greenschist-facies metamorphic conditions. The tremolite ⁴⁰Ar/³⁹Ar cooling age (718±5 Ma) of amphibolitic eclogite and muscovite ⁴⁰Ar/³⁹Ar cooling ages (about 550 Ma) of mica schists indicate that a maximum temperature of 500±50 °C was not reached during the Neoproterozoic orogeny. The style and timing of the Neoproterozoic orogeny show similarities to the Cadomian-aged Timan Range NW of the Polar Urals. Geochronological and thermochronological data together with the abrupt change in structural style and metamorphism east of the Zuratkul fault, suggest that the MCB is exotic with respect to the SE-margin of the East European Platform. Thus, the MCB is named the ‘Beloretzk Terrane’. Recognition of the ‘Beloretzk Terrane’ and the Neoproterozoic orogeny at the eastern margin of Baltica has important implications for Neoproterozoic plate reconstruction and suggests that the eastern margin of Baltica might have lain close to the Avalonian–Cadomian belt.     

Formation and exhumation of blueschists and eclogites from NE Oman: new perspectives from Rb–Sr and 40Ar/39Ar dating

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.     

Comparison of 40Ar-39Ar and Rb-Sr Data on Phengites from the UHP Brossasco-Isasca Unit (Dora Maira Massif, Italy): Implications for Dating White Mica

Different lithologies (impure marble, eclogite and graniticorthogneiss) sampled from a restricted area of the coesite-bearingBrossasco–Isasca Unit (Dora Maira Massif) have been investigatedto examine the behaviour of ⁴⁰Ar–³⁹Ar and Rb–Srsystems in phengites developed under ultrahigh-pressure (UHP)metamorphism. Mineralogical and petrological data indicate thatzoned phengites record distinct segments of the P–T path:prograde, peak to early retrograde in the marble, peak to earlyretrograde in the eclogite, and late retrograde in the orthogneiss.Besides major element zoning, ion microprobe analysis of phengitein the marble also reveals a pronounced zoning of trace elements(including Rb and Sr). ⁴⁰Ar–³⁹Ar apparent ages (35–62Ma, marble; 89–170 Ma, eclogite; 35–52 Ma, orthogneiss),determined through Ar laserprobe data on phengites (step-heatingand in situ techniques), show wide intra-sample and inter-samplevariations closely linked to within-sample microchemical variations:apparent ages decrease with decreasing celadonite contents.These data confirm previous reports on excess Ar and, more significantly,highlight that phengite acted as a closed system in the differentlithologies and that chemical exchange, not volume diffusion,was the main factor controlling the rate of Ar transport. Conversely,a Rb–Sr internal isochron from the same eclogite yieldsan age of 36 Ma, overlapping with the time of the UHP metamorphicpeak determined through U–Pb data and thereby corroboratingthe previous conclusion that UHP metamorphism and early retrogressionoccurred in close succession. Different phengite fractions ofthe marble yield calcite–phengite isochron ages of 36to 60 Ma. Although this time interval matches Ar ages from thesame sample, Rb–Sr data from phengite are not entirelyconsistent with the whole dataset. According to trace elementvariations in phengite, only Rb–Sr data from two wet-groundphengite separates, yielding ages of 36 and 41 Ma, are internallyconsistent. The oldest age obtained from a millimetre-sizedgrain fraction enriched in prograde–peak phengites mayrepresent a minimum age estimate for the prograde phengite relics.Results highlight the potential of the in situ ⁴⁰Ar–³⁹Arlaser technique in resolving discrete P–T stages experiencedby eclogite-facies rocks (provided that excess Ar is demonstrablya negligible factor), and confirm the potential of Rb–Srinternal mineral isochrons in providing precise crystallizationages for eclogite-facies mineral assemblages. KEY WORDS: ⁴⁰Ar–³⁹Ar dating; Rb–Sr dating; phengite; SIMS; UHP metamorphism     

New Lu–Hf and Sm–Nd geochronology constrains the subduction of oceanic crust during the Carboniferous–Permian in the Dabie orogen

Eclogites from the Huwan shear zone in the western Dabie were investigated in terms of their P–T evolution, geochemistry, and combined Lu–Hf and Sm–Nd geochronology. Trace element and isotope data suggest a normal mid-ocean ridge rather than an intraplate or ocean island setting for the protoliths of the eclogites. Electron microprobe analyses of representative garnets show typical prograde zoning profiles. Estimated peak metamorphic temperatures of 540–590 °C most likely did not exceed the closure temperature of the Lu–Hf and Sm–Nd systems. The consistent Lu–Hf and Sm–Nd ages, therefore, most likely reflect garnet growth and are interpreted to reflect high-pressure eclogite-facies metamorphism due to the occurrence of omphacite inclusions from core to rim in garnets and the spherical geometry effect despite the well-preserved prograde zoning in the garnets. The high-pressure mineral assemblage of the eclogite yielded a statistically robust Lu–Hf age of 260.0 ± 1.0 Ma (2σ, 10 points, MSWD = 1.0) and a Sm–Nd age of 260.4 ± 2.0 Ma (2σ, 9 points, MSWD = 1.4), which are younger than the Carboniferous zircon U–Pb ages of ca. 310 Ma. The new Lu–Hf and Sm–Nd data, in combination with published geochronological data, define two distinct Carboniferous and Permian population ages for the oceanic-type eclogites from the Huwan shear zone, which may require that these rocks experienced two episodes of high-pressure metamorphism within less than 50 Myr.     

Isotopic Geochronological Evidence for the Caledonian Xiongdian Eclogite in the Northwestern Dabie Mountains,China

A typical HP/MT (high pressure/medium temperature) eclogite from Xiongdian, northwestern Dabie Mountains, has been geochronologically studied using the single-zircon U-Pb, 40Ar-39Ar and Sm-Nd methods. Prismatic zircons occurring as inclusions within garnets define a minimum crystallization age of 399.5±1.6 Ma. 40Ar-39Ar dating on amphibole gives a plateau age.of 399.2 ± 4 Ma, which is interpreted as a retrogression age of amphibolite facies. This integrated study enables us to conclude that the age of high-pressure metamorphism is older than 399.5 ± 1.6 Ma, suggesting Caledonian collision between the North China and Yangtze plates. Round zircon within the aggregate of quartz and muscovite gives a concordant age of 301± 2 Ma, reflecting a later retrogression event. An age profile of post-eclogite metamorphism is documented, including amphibolite facies metamorphism at 399.2 Ma shortly after eclogitization and later retrogressive metamorphism at 301 Ma. Sm-Nd mineral isochron of garnet+omphacite gives     

Eocene age of eclogite metamorphism in Pakistan Himalaya: implications for India-Eurasia collision

A geochronological investigation of two rocks with an eclogitic assemblage (omphacite-garnet-quartz-rutile) from the High Himalaya using the Sm/Nd, Rb/Sr, U/Pb and Ar/Ar methods is presented here. The first three methods outline a cooling history from the time of peak metamorphism at 49±6 Ma recorded by Sm/Nd in garnet-clinopyroxene to the closure of Rb/Sr in phengite at 43±1 Ma and U/Pb in rutile at 39–40 Ma. The Sm/Nd isotopic system was fully equilibrated during eclogitization and has not been disturbed since; its mineral ages may date the peak metamorphic conditions (650±50°C at 13–18 kbar: Pognante and Spencer, 1991). The Ar/Ar data reveal the presence of substantial amounts of excess ⁴⁰Ar in hornblende, and yield a statistically acceptable but geologically meaningless phengite plateau age of 81.4±0.2 Ma, inconsistent with Sm/Nd, Rb/Sr and U/Pb. This questions the use of such a chronometer for the dating of high-pressure assemblages. The results imply a Late Palaeocene or Early Eocene subduction of the northern Indian plate margin in NW Himalaya. The fact that eclogites are restricted to NW Himalaya may be the result of a peculiar p-T-t path associated with a high convergence rate during the first indentation, in contrast to the later and slow subduction in Central and Eastern Himalaya.     

Rb/Sr record of fluid-rock interaction in eclogites: The Marun-Keu complex, Polar Urals, Russia

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.     

Dating the geologic history of Oman’s Semail ophiolite: insights from U-Pb geochronology

Eclogites from the deepest structural levels beneath the Semail ophiolite, Oman, record the subduction and later exhumation of the Arabian continental margin. Published ages for this high pressure event reveal large discrepancies between the crystallisation ages of certain eclogite-facies minerals and apparent cooling ages of micas. We present precise U-Pb zircon (78.95 ± 0.13 Ma) and rutile (79.6 ± 1.1 Ma) ages for the eclogites, as well as new U-Pb zircon ages for trondhjemites from the Semail ophiolite (95.3 ± 0.2 Ma) and amphibolites from the metamorphic sole (94.48 ± 0.23 Ma). The new eclogite ages reinforce published U-Pb zircon and Rb-Sr mineral-whole rock isochron ages, yet are inconsistent with published interpretations of older ⁴⁰Ar/³⁹Ar phengite and Sm-Nd garnet dates. We show that the available U-Pb and Rb-Sr ages, which are in tight agreement, fit better with the available geological evidence, and suggest that peak metamorphism of the continental margin occurred during the later stages of ophiolite emplacement.     

Closed system behaviour of argon in osumilite records protracted high‐T metamorphism within the Rogaland–Vest Agder Sector,Norway

Here, we present results of the first ⁴⁰Ar/³⁹Ar dating of osumilite, a high‐T mineral that occurs in some volcanic and high‐grade metamorphic rocks. The metamorphic osumilite studied here is from a metapelitic rock within the Rogaland–Vest Agder Sector, Norway, an area that experienced regional granulite facies metamorphism and subsequent contact metamorphism between 1,100 Ma and 850 Ma. The large grain size (~1 cm) of osumilite in the studied rock, which preserves a nominally anhydrous assemblage, increases the potential for large portions of individual grains to have remained essentially unaffected by the effects of diffusive argon loss, potentially preserving prograde ages. Step‐heating diffusion experiments yielded a maximum activation energy of ~461 kJ/mol and a pre‐exponential factor of ~8.34 × 10⁸ cm²/s for Ar diffusion in osumilite. These parameters correspond to a relatively high closure temperature of ~620°C for a cooling rate of 10°C/Ma in an osumilite crystal with a 175 µm radius. Fragments of osumilite separated from the sample preserve a range of ages between c. 1,070 and 860 Ma. The oldest ages are inferred to date the growth of coarse‐grained osumilite during prograde granulite facies regional metamorphism, which pre‐date contact metamorphism that has historically been ascribed to the growth of osumilite in the region. The majority of fragments record ages between c. 920 and 860 Ma, inferred to reflect the growth of osumilite and/or diffusive argon loss during contact metamorphism. The retention of old ⁴⁰Ar/³⁹Ar dates was facilitated by the low diffusivity of Ar in osumilite (i.e. a closed system), large grain sizes, and anhydrous metamorphic conditions. The ability to date osumilite with the ⁴⁰Ar/³⁹Ar method provides a valuable new thermochronometer that may constrain the timing and duration of high‐T magmatic and metamorphic events.