Showing papers on "Metamorphism published in 1995"

Ultrahigh pressure metamorphism

Abstract: 1 Overview of the geology and tectonics of UHPM 2 Experimental and petrogenetic study of UHPM 3 Principal mineralogical indicators of ultrahigh pressure in crustal rocks 4 Structures in ultrahigh pressure metamorphic rocks: a case study from the Alps 5 Creation, preservation, and exhumation of ultrahigh pressure metamorphic rocks 6 The role of serpentinite melanges in the unroofing of ultrahigh pressure metamorphic rocks: An example from the Western Alps of Italy 7 Ultrahigh pressure metamorphic rocks in Western Alps 8 High and ultrahigh pressure eclogites and garnet peridotites in the Scandinavian Caledonides 9 Microcoesites and microdiamonds in Norway: a brief review 10 Ultrahigh pressure metamorphic terrane in Eastern Central China 11 A model for the tectonic history of the high and ultrahigh pressure metamorphic region in East Central China 12 Diamond-bearing metamorphic rocks of the Kokchetav Massif 13 Orogenic ultramafic rocks of ultrahigh pressure (diamond facies) origin

Microdiamond in high-grade metamorphic rocks of the Western Gneiss region, Norway

Abstract: Three grains of microdiamond were recovered from high-grade gneiss exposed in the Western Gneiss region, Norway. Identification and characterization of the diamond grains by Raman and infrared spectroscopy indicate the presence of substitutional impurities of H and N. Primary fluid inclusions in garnet and quartz in the diamond-bearing rock demonstrate the evolution of metamorphic volatile fluids from reduced N 2 -CO 2 compositions during the peak phase of metamorphism, to N 2 -CH 4 ± H 2 O–bearing compositions during retrograde metamorphism. Compatible geologic, petrologic, and fluid composition data imply a metamorphic origin for the microdiamonds; if so, the metamorphic and fluid conditions recorded by the microdiamonds and gneissic host may be applicable to microdiamond investigations in other high-pressure, regionally metamorphosed orogens.

Petrology of ultrahigh‐pressure rocks from the southern Su‐Lu region, eastern China

Abstract: In the Su-Lu ultrahigh-P terrane, eastern China, many coesite-bearing eclogite pods and layers within biotite gneiss occur together with interlayered metasediments now represented by garnet-quartz-jadeite rock and kyanite quartzite. In addition to garnet + omphacite + rutile + coesite, other peak-stage minerals in some eclogites include kyanite, phengite, epidote, zoisite, talc, nyboite and high-Al titanite. The garnet-quartz-jadeite rock and kyanite quartzite contain jadeite + quartz + garnet + rutile ± zoisite ± apatite and quartz + kyanite + garnet + epidote + phengite + rutile ± omphacite assemblages, respectively. Coesite and quartz pseudomorphs after coesite occur as inclusions in garnet, omphacite, jadeite, kyanite and epidote from both eclogites and metasediments. Study of major elements indicates that the protolith of the garnet-quartz jadeite rock and the kyanite quartzite was supracrustal sediments. Most eclogites have basaltic composition; some have experienced variable ‘crustal’contamination or metasomatism, and others may have had a basaltic tuff or pyroclastic rock protolith. The Su-Lu ultrahigh-P rocks have been subjected to multi-stage recrystallization and exhibit a clockwise P-T path. Inclusion assemblages within garnet record a pre-eclogite epidote amphibolite facies metamorphic event. Ultrahigh-P peak metamorphism took place at 700–890° C and P>28 kbar at c. 210–230 Ma. The symplectitic assemblage plagioclase + hornblende ± epidote ± biotite + titanite implies amphibolite facies retrogressive metamorphism during exhumation at c. 180–200 Ma. Metasedimentary and metamafic lithologies have similar P-T paths. Several lines of evidence indicate that the supracrustal rocks were subducted to mantle depths and experienced in-situ ultrahigh-P metamorphism during the Triassic collision between the Sino-Korean and Yangtze cratons.

Contrasting plate-tectonic styles of the Qinling-Dabie-Sulu and Franciscan metamorphic belts

Abstract: The Dabie Mountains are part of the >2000-km-long Qinling-Dabie-Sulu suture zone juxtaposing the Sino-Korean and Yangtze cratons. An eastern extension apparently crosses Korea and lies along the Japan Sea side of Honshu as the Imjingang and Sangun terranes, respectively; a northeastern segment may be present in Sikhote-Alin, Russian Far East. This orogenic belt records late Paleozoic ocean-floor consumption and the Triassic collision of two Precambrian continental massifs in east-central China. Coesite and microdiamond inclusions in strong, refractory minerals of eclogite facies ultrahigh-pressure (UHP) metamorphic rocks in the Dabie-Sulu area attest to profound subduction of a leading salient of the old, cold Yangtze craton, now recovered through tectonic exhumation and erosion. Northward increase in intensity of subsolidus recrystallization of the suture complex is analogous to the internal progression in grade of high-pressure (HP) and UHP metamorphism documented in the Western Alps. In both regions, subduction of narrow prongs of continental material, UHP metamorphism, and return toward midcrustal levels of relatively lower density, buoyant microcontinental blocks resulted from delamination of these rocks from the descending, higher density, oceanic-crust-capped lithospheric plate. Such salients of continental crust represent an integral structural part of the downgoing slab, remain intact, and may be dragged to great depths before disengaging and rising differentially as coherent blocks. UHP parageneses include trace mineralogic relics requiring peak metamorphic conditions of 700–900 ° C and 28–35 kbar or more. In contrast, Pacific-type HP metamorphic belts, as represented by the Franciscan Complex of western California, recrystallized under physical conditions up to 200–500 ° C, 10 ± 3 kbar. In this setting, voluminous quartzo-feldspathic and graywacke debris was carried downward on oceanic-crust-capped lithosphere, choking the subduction zone with incompetent material. Sited between both plates, and strongly adhering to neither, this buoyant, largely sedimentary complex decoupled at 25–30 km depth, and ascended toward the surface. In both Alpine-type intracontinental collision and Pacific-type underflow, light sialic material displaced dense mantle; thus, the return to midcrustal levels was propelled dominantly by body forces.

Oaxaquia, a Proterozoic microcontinent accreted to North America during the late Paleozoic

Abstract: Grenville-age granulite facies rocks in southern, central, and northeastern Mexico have distinctive geologic features that suggest a common tectonic evolution. The similarities include northwest-trending structural grain from Oaxaca to Tamaulipas, massif-type anorthosite-charnockite complexes, protoliths rich in sedimentary rocks of shallow-marine platform or continental rift-related facies devoid of calc-alkaline volcanic rocks, common metamorphism under granulite facies conditions, U-Pb zircon ages of about 1.0 Ga., and an apparently common history of uplift and cooling. Altogether, this evidence strongly suggests a coherent geologic history for this block. Paleontologic data from the overlying sedimentary sequences indicate that Oaxaquia was not part of Laurentia during most of the Paleozoic. This precludes emplacement of Oaxaquia in its present position by simple lateral displacement from the southern United States as well as a Taconic time of emplacement. Oaxaquia was probably emplaced to its present position during late Paleozoic time. The concept of a Mesoproterozoic “Oaxaquia” microcontinent extending for about 1000 km in Mexico needs to be considered in the reconstruction of the Grenville orogen as a whole and for the Paleozoic tectonic interactions between eastern Laurentia and western Gondwana.

Constraints on Pb closure temperature in titanite based on rocks from the Ungava orogen, Canada: Implications for U-Pb geochronology and P-T-t path determinations

Abstract: On the basis of multiequilibrium metamorphic pressure-temperature ( P-T ) estimates, we have determined that 660–700 °;C is a reasonable minimum estimate of the closure T of Pb diffusion in titanite, a temperature significantly higher than previously suggested. Activity-corrected reactions that produce titanite define narrow T stability fields (≈40 °;C) that correspond closely to metamorphic P-T determinations derived for coronitic garnet-clinopyroxene-hornblende samples. Growth of titanite, occurring between 1789 and 1814 Ma in the underthrust Archean basement of the Paleoproterozoic Ungava orogen (northern Quebec), is associated with retrograde metamorphism during underthrusting of the Archean basement (Approx.)1 b.y. after formation of the original granulitic assemblages.

Evidence for a 2 Ga subduction zone: Eclogites in the Usagaran belt of Tanzania

Abstract: U-Pb geochronology on metamorphic minerals from a 35-km-long belt of eclogite-facies rocks in central Tanzania yields a Paleoproterozoic age of 2 Ga for the time of metamorphism. Peak metamorphic conditions found in eclogites (± kyanite) and metapelites reached about 750 °C and 18 kbar. A clockwise pressure-temperature path is deduced from mineral zonations, inclusion relations, and retrograde reaction textures. Near-isothermal decompression can be explained by erosion or tectonically controlled exhumation that followed tectonic thickening of the crust during subduction. Trace and rare earth element geochemistry indicates a mid-ocean ridge basaltlike mantle source for the precursors of the mafic members of the eclogite-facies rock suite. All the observations combined indicate that these high-pressure rocks are the oldest-known large-scale outcrops of eclogites formed during subduction of oceanic lithosphere. Linking eclogite formation to a Paleoproterozoic subduction event adds credibility to models of crust dynamics that advocate the operation of plate-tectonic processes early in Earth9s history. The paucity of Precambrian eclogites may then be addressed as a problem of preservation rather than lack of formation.

Late Archaean crust-mantle interactions: geochemistry of LREE-enriched mantle derived magmas. Example of the Closepet batholith, southern India

Abstract: The Closepet batholith in South India is generally considered as a typical crustal granite emplaced 2.5 Ga ago and derived through partial melting of the surrounding Peninsular Gneisses (3.3 to 3.0 Ga). In the field, it appears as a composite batholith made up of at least two groups of intrusions. (a) An early SiO2-poor group (clinopyroxene quartz-monzonite and porphyritic phyritic monzogranite) is located in the central part of the batholith. These rocks display a narrow range in both initial 87Sr/86Sr (0.7017–0.7035) and ɛNd(−0.9to −4.1). (b) A later SiO2-rich group (equigranular grey and pink granites) is located along the interface between the SiO2-poor group and the Peninsular Gneisses. They progressively grade into migmatised Peninsular Gneisses, thus indicating their anatectic derivation. Their isotopic characteristics vary over a wide range (87Sr/86Sr ratios=0.7028–0.7336 and ɛNd values from-2.7 to-8.3, at 2.52 Ga). Field and geochronological evidence shows that the two groups are broadly contemporaneous (2.518–2.513 Ga) and mechanically mixed. This observation is supported by the chemical data that display well defined mixing trends in the ɛSr vs ɛNd and elemental variation diagrams. The continuous chemical variation of the two magmatic bodies is interpreted in terms of interaction and mixing of two unrelated end-members derived from different source regions (enriched peridotitic mantle and Peninsular Gneisses). It is proposed that the intrusion of mantle-derived magmas into mid-crustal levels occurred along a transcurrent shear zone; these magmas supplied additional heat and fluids that initiated anatexis of the surrounding crust. During this event, large-scale mixing occurred between mantle and crustal melts, thus generating the composite Closepet batholith. The mantle-derived magmatism is clearly associated with granulite facies metamorphism 2.51±0.01 Ga ago. Both are interpreted as resulting from a major crustal accretion event, possibly related to mantle plume activity.

Ar/Ar geochronology of ultrahigh‐pressure metamorphism in central China

Abstract: The Dabie Shan of China contain the largest areal exposure of ultrahigh-pressure regional metamorphic rock known on Earth. The thermal history of these unusual rocks is central to understanding the tectonic processes responsible for their creation, preservation, and exhumation. Published ages of ultrahigh-pressure metamorphism and subsequent cooling range from Archean to Jurassic. By analyzing 21 Dabie hornblende, phengite, and biotite samples by the 40Ar/39Ar method, we find that (1) cooling from peak metamorphic temperatures to ∼300°C occurred between about 206 and 178 Ma; (2) widespread Cretaceous ages reflect reheating by a postultrahigh-pressure magmatic/extensional episode; (3) 40Ar/39Ar and K/Ar ages older than 230 Ma, and perhaps 210 Ma, are geologically meaningless due to the incorporation of excess 40Ar; and (4) Dabie ultrahigh-pressure metamorphic rocks are temporally related to blueschist-facies rocks farther west in the suture zone.

A petrographic history of martian meteorite ALH84001: Two shocks and an ancient age

Abstract: ALH84001 is an igneous meteorite, an orthopyroxenite of martian origin. It contains petrographic evidence of two shock metamorphic events, separated by thermal and chemical events. The evidence for two shock events suggests that ALH84001 is ancient and perhaps a sample of the martian highlands. From petrography and mineral chemistry, the history of ALH84001 must include: crystallization from magma, a first shock (impact) metamorphism, thermal metamorphism, low-temperature chemical alteration, and a second shock (impact) metamorphism. Originally, ALH84001 was igneous, an orthopyroxene-chromite cumulate. In the first shock event, the igneous rock was cut by melt-breccia or cataclastic veinlets, now bands of equigranular fine-grained pyroxene and other minerals (crush zones). Intact fragments of the cumulate were fractured and strained (now converted to polygonized zones). The subsequent thermal metamorphism (possibly related to the first shock) annealed the melt-breccia or cataclastic veinlets to their present granoblastic texture and permitted chemical homogenization of all mineral species present. The temperature of metamorphism was at least 875 C, based on mineral thermometers. Next, Mg-Fe-Ca carbonates and pyrite replaced plagioclase in both clasts and granular bands, producing ellipsoidal carbonate globules with sub-micron scale compositional stratigraphy, repeated identically in all globules, The second shock event produced microfault offsets of carbonate stratigraphy and other mineral contacts, radial fractures around chromite and maskelynite, and strain birefringence in pyroxene. Maskelynite could not have been preserved from the first shock event, because it would have crystallized back to plagioclase. The martian source area for ALH84001 must permit this complex, multiple impact history. Very few craters on young igneous surfaces are on or near earlier impact features. It is more likely that ALH84001 was ejected from an old igneous unit (Hesperian or Noachian age), pocked by numerous impact craters over its long exposure at the martian surface.

Mineral parageneses and metamorphic P-T paths of ultrahigh-pressure eclogites from Kyrghyzstan Tien-Shan

Abstract: Eclogites occur in three districts of the northern and southern parts of Tien-Shan Three eclogites collected from the Aktyuz, Makbal and Atbashy districts were analyzed; the P-T paths of three eclogites were estimated by analyzing compositional growth zoning and retrograde reaction of garnet and omphacite Aktyuz and Makbal eclogites have not preserved the prograde path An Aktyuz eclogite that underwent a quartz eclogite facies metamorphism (about T = 600°C, P = 12 kbar) has recorded three stages of retrograde metamorphism Four stages of retrograde metamorphism were recognized in a Makbal eclogite; the garnet-omphacite geothermometer gave about T = 560°C at 20 kbar as the highest metamorphic condition Garnet from a garnetchloritoid-talc schist of the Makbal district includes quartz pseudomorphs after coesite; some units evidently underwent a low-temperature part of coesite eclogite fades metamorphism Prograde and retrograde paths were recognized in an Atbashy eclogite; five stages of metamorphic reaction were observed in the Atbashy sample The prograde path from stage I to stage III has been recorded in garnet and omphacite in which quartz pseudomorphs after coesite are included The peak metamorphism of stage III took place at about 660°C at 25 kbar The stages IV and V are retrograde UHP eclogite facies metamorphism took place twice in Kyrghyzstan The Aktyuz and Atbashy eclogites gave Rb-Sr mineral-isochron ages of about 750 Ma and 270 Ma, respectively The K-Ar age of paragonite from the Makbal eclogite is about 480 Ma

Geotectonic evolution of diamondiferous paragneisses, Kokchetav Complex, northern Kazakhstan: The geologic enigma of ultrahigh‐pressure crustal rocks within a Paleozoic foldbelt

Abstract: The Kokchetav Complex is a tectonic mega-melange consisting of seven pre-Ordovician units (units I-VII) of contrasting lithologies and P–T conditions of metamorphism, overlain and/or intruded by four post-recrystallization entities. Most of the constituent rock types display affinities with continental crust; paraschists and paragneisses, which carry biogenically produced carbon, clearly were laid down near the surface of the Earth. Microdiamond (and rare coesite) inclusions are contained in strong, refractory garnet, zircon, clinopyroxene, and kyanite, some of the constituent neoblastic phases of this metasedimentary unit. Systematic mineral parageneses and textural relationships support the hypothesis that the metamorphic assemblages represent a close approach to chemical equilibrium at the time of formation. Metamorphism of diamond-bearing paragneisses and schists transpired at 535 ± 5 Ma; physical conditions included minimum pressures of 40 kbar and temperatures exceeding 900 °C. Other associated units contain mineralogic evidence of somewhat lower to considerably lower pressures and temperatures: observed magnesite + diopside pairs, coesite, grossular-pyropic garnet, potassic clinopyroxene, Si-rich phengite, barroisite-crossite(?), aluminous titanite and/or Al-rutile, and the assemblage talc + kyanite + garnet all testify to relatively elevated pressures of formation. The metamorphosed lithotectonic units represent individual, discrete stages in what initially may have been a continuous P-T series, but intense post-metamorphic dislocation has resulted in the preservation of a chaotically mixed sequence rather than an unbroken gradation in preserved conditions of metamorphism. Only units I-III, and probably VIb may represent portions of a dismembered subduction zone lithologie assemblage. The uplift to mid-crustal levels and cooling of the mega-melange took place by about 515–517 Ma, at which time the complex was stabilized as a part of the Kazakhstan microcontinental collage. An hypothesized Late Vendian-Early Cambrian subduction of the Kazakhstan-North Tianshan(?) microcontinental salient to depths exceeding 125 km, followed by decoupling from the descending oceanic crust-capped lithospheric plate is held responsible for the ultrahigh-pressure metamorphism of the Kokchetav Complex. Inasmuch as vestiges of a calc-alkaline volcanic/plutonic arc of approximately Early Cambrian age are preserved as only scattered relics in the general region, the plate-tectonic setting may have involved an intra-oceanic, Marianas-type, incipient arc which was subsequently removed through transform faulting or erosion.

Ultradeep metamorphic rocks: The retrospective viewpoint

Abstract: Ultradeep, or ultra-high-pressure (UHP), metamorphic rocks, formed from crustal protoliths within the stability field of coesite at pressures >2.5–3.0 GPa corresponding to depths >80–120 km, occur locally though regionally distributed in at least five continental areas. Their recognition is solely based on characteristic minerals and mineral assemblages calibrated by experimental high-pressure studies. Detailed petrographic and microprobe work, especially on mineral inclusions, in favorable cases allows the derivation of prograde PT paths during subduction and of retrograde ones during exhumation. Commonly, the gneisses adjacent to the UHP rocks do not exhibit signs of ultradeep metamorphism, apparently because the kinetics of their mineral reactions are sufficiently fast to allow complete reequilibration to shallower PT conditions during the retrograde path. It is also possible, however, that UHP equilibria were not attained throughout the rock volumes subducted, but only along zones of shearing and fluid introduction. If it is true that not all UHP metamorphic rocks return to the crustal orogenic belts, but some continue to be subducted to greater mantle depths, the classical geochemical pattern of a one-way mass transfer from mantle to crust throughout the Earth's history is at stake. The assumed gradual growth of continents may have had a counterpart of continent destruction during collision events. Most recent experimental studies at high pressures and relatively low temperatures show that at least three new hydrous (Mg)A1-silicates exist that were not found in nature thus far, but may be characteristic minerals in the cold portions of old subduction zones, thus extending the water retentivity of subducting slabs to greater, and hitherto unexpected, depths.

The hydrodynamics of contact metamorphism

Abstract: During contact metamorphism, fluid production from magma and wall rocks, thermal expansion of fluid, thermal and chemical buoyancy, topography, and deformation of rocks all interact to drive fluid flow. An understanding of how these processes interact is necessary to predict the evolution of fluid flow, fluid pressures, and permeability; to evaluate mechanisms of ore deposition, metasomatism, and heat transfer; and to apply petrologic, field, and geochemical data to infer hydrologic histories. Results from finite-difference models that account for many of these processes reveal the hydrodynamics of contact metamorphism. A systematic evolution in flow system from lithostatic fluid pressures and expulsion of internally generated fluids, to hydrostatic fluid pressures and circulation of externally derived fluids is predicted to occur with time, increasing permeability, and decreasing depth. Fluid production from magmas and wall rocks is the dominant process elevating fluid pressures toward lithostatic values during prograde metamorphism; other processes, such as strain or thermal expansion of the fluid, are less significant. For typical water contents of magmas and wall rocks, permeabilities of ≤1 μd (10 −18 m 2 ) are required to produce lithostatic fluid pressures in the aureole. Consideration of processes controlling permeability suggests that fluid flow is likely intergranular during prograde metamorphism and channelized during retrograde metamorphism. Balance of rates of thermal fracturing and mineral deposition implies that permeability should increase above 1 μd during cooling, and thus fluid pressures should drop abruptly toward hydrostatic values once cooling and fracturing ensues. Rocks near the contact cool first, and complex mixing of magmatic, metamorphic, and meteoric fluids is likely in this region. If permeability increases during cooling, meteoric fluids may dominate the total fluid budget even though rocks interact with only magmatic and metamorphic fluids during prograde metamorphism. Many broad features of the models are similar to patterns observed in porphyry copper deposits. Models with ad hoc assumptions of permeability near the contact are required to produce significant up-temperature fluid flow during prograde metamorphism, a pattern predicted from some reaction-transport models.

SHRIMP U-Pb geochronology and metamorphic history of the Smallefjord sequence, NE Greenland Caledonides

Abstract: SHRIMP U-Pb zircon ages have been obtained for a sample of metasedimentary gneiss from the Smallefjord sequence in the NE Greenland Caledonides. Pre-1000 Ma zircons are interpreted as being detrital in origin; the source region contained late Archaean rocks (or younger sediments or inheritance-rich granites derived from them) and several groups of Proterozoic rocks as young as c. 1035 Ma. Zircons with 206 Pb/ 238 U ages between c. 980 Ma and 890 Ma are interpreted to have grown during a discrete high grade metamorphic event. Six analyses provided a mean 206 Pb/ 238 U age of 955 ± 13 Ma, which is interpreted as recording 9Grenvillian9 metamorphism of the Smallefjord sequence. This supports regional correlation of the Smallefjord sequence with the Krummedal sequence of central E Greenland which has previously yielded evidence for tectonothermal activity at c. 1000 Ma, and provides a further indication of a northern arm of the Grenville-Sveconorwegian orogen in the North Atlantic region. Further growth of metamorphic zircon within the Smallefjord sequence occurred during the Caledonian orogeny at 445 ± 10 Ma (Upper Ordovician). This is consistent with estimates of a Middle-Upper Ordovician age for Caledonian orogenesis in central E Greenland. It contrasts with previously published SHRIMP U-Pb zircon dating of Lower Devonian (404 ±6 Ma) metamorphism within the basement gneisses of the Dove Bugt area north of the Smallefjord sequence outcrop. SHRIMP dating of zircons therefore indicates polyphase Palaeozoic metamorphism within the NE Greenland Caledonides.

Retention of isotopic memory in garnets partially broken down during an overprinting granulite-facies metamorphism: Implications for the Sm-Nd closure temperature

Abstract: Garnets from an early granulite event that have partially broken down to orthopyroxene-bearing assemblages during a second regional granulite-facies metamorphism were used to test the closure temperature of the Sm-Nd system in garnet. The garnets from Sostrene Island, Prydz Bay, Antarctica, retain a memory of both the physical conditions of their formation and their Sm-Nd age, notwithstanding an effective diameter for diffusion of only 1 mm. Because only minor isotopic resetting of these garnets has taken place during the overprinting granulite event, the closure temperature for the Sm-Nd system in garnet in the present case must be >700–750 ° C. The results show that chronological constraints are essential for the unambiguous interpretation of reaction textures in terms of thermotectonic evolution. The garnet Sm-Nd dating method is particularly well suited for this purpose.

Lower crustal rejuvenation and growth during post-thickening collapse: Insights from a crustal cross section through a Variscan metamorphic core complex

Abstract: The Variscan lower crust of Europe acquired its main features (seismic layering, intrusion of mantle-derived magmas, granulite-grade metamorphism) when the upper crust underwent post-thickening extension through gravitational collapse. We suggest that post-thickening collapse is a geodynamic process that contributes to the growth and differentiation of continental lithosphere in collapsed mountain belts.

Kisseynew metasedimentary gneiss belt, Trans-Hudson orogen (Canada): Back-arc origin and collisional inversion

Abstract: The Kisseynew metasedimentary gneiss belt initially developed in a back-arc setting. The ages of detrital zircons and crosscutting plutons constrain turbidite sedimentation to 1.855-1.841 Ga, postdating early deformation and metamorphism associated with arc-arc and arc-continent collisions affecting much of the Trans-Hudson orogen. These turbidites are interpreted to have been deposited in a back-arc basin behind a retreating subduction boundary. The Kisseynew domain and its southern flank are marked by 1.84-1.83 Ga magmatic rocks that may be related to subduction of back-arc basin oceanic crust. Collapse of the Kisseynew "basin' by ductile fold-and-thrust deformation began during 1.84-1.83 Ga magmatism and continued through peak metamorphism at ca. 1.820-1.805 Ga. -from Authors

Carbon isotope thermometry in marbles of the Adirondack Mountains, New York

Abstract: Carbon isotope thermometry has been applied to coexisting calcite and graphite in marbles from throughout the Adirondack Mountains, New York. Eighty-nine calcite-graphite pairs from the amphibolite grade NW Adirondacks change systematically in temperature north-westwards from 680 to 640 to 670° C over a 30-km distance, reflecting transitions from amphibolite facies towards granulite facies to the north-west and to the south-east. Temperature contours based on calcite-graphite thermometry in the NW Adirondacks parallel mineral isograds, with the orthopyroxene isograd falling above 675° C, and indicate that regional metamorphic temperatures were up to 75° C higher than temperatures inferred from isotherms based on cation and solvus thermometry (Bohlen et al. 1985). Fifty-five calcite-graphite pairs from granulite grade marbles of the Central Adirondacks give regional metamorphic temperatures of 670–780° C, in general agreement with cation and solvus thermometry. Data for amphibolite and granulite grade marbles show a 12%oo range in δ13Ccal and δ13Cgr. A strong correlation between carbon isotopic composition and the abundance of graphite (Cgr/Crock) indicates that the large spread in isotopic compositions results largely from exchange between calcite and graphite during closed system metamorphism. The trends seen in δ13C vs. Cgr/Crock and δ13Ccal vs. δ13Cgr could not have been preserved if significant amounts of CO2-rich fluid had pervasively infiltrated the Adirondacks at any time. The close fit between natural data and calculated trends of δ13C vs. Cgr/Crock indicates a biogenic origin for Adirondack graphites, even though low δ13C values are not preserved in marble. Delamination of 17 graphite flakes perpendicular to the c-axis reveals isotopic zonation, with higher δ13C cores. These isotopic gradients are consistent with new graphite growth or recrystallization during a period of decreasing temperature, and could not have been produced by exchange with calcite on cooling due to the sluggish rate of diffusion in graphite. Samples located >2km from anorthosite show a decrease of 0.5-0.8%oo in the outer 100 μ of the grains, while samples at distances over 8 km show smaller core-to-rim decreases of c.0.2%oo. Correlation between the degree of zonation and distance to anorthosite suggests that the isotopic profiles reflect partial overprinting of higher temperature contact metamorphism by later granulite facies metamorphism. Core graphite compositions indicate contact metamorphic temperatures were 860–890° C within 1 km of the Marcy anorthosite massif. If samples with a significant contact metamorphic effect (Δ(cal-gr) <3.2%oo) are not included, then the remaining 38 granulite facies samples define the relation Δ13C(cal-gr) = 3.56 ± 106T-2 (K).

Variscan Sm-Nd and Ar-Ar ages of eclogite facies rocks from the Erzgebirge, Bohemian Massif

Abstract: The Erzgebirge Crystalline Complex (ECC) is a rare example where both 'crustal' eclogites and mantle-derived garnet-bearing ultramafic rocks (GBUs) occur in the same tectonic unit. Thus, the ECC represents a key complex for studying tectonic processes such as crustal thickening or incorporation of mantle-derived material into the continental crust. This study provides the first evidence that high- pressure metamorphism in the ECC is of Variscan age. Sm-Nd isochrons define ages of 333+6 (Grt-WR), 337 f 5 (Grt-WR), 360 f 7 (Grt-Cpx-WR) (eclogites) and 353 f 7 Ma (Grt-WR) (garnet- pyroxenite). @Arf9Ar spectra of phengite from two eclogite samples give plateau ages of 348 f 2 and 355 f 2 Ma. The overlap of ages from isotopic systems with blocking temperatures that differ by about 300" C indicates extremely fast tectonic uplift rates. Minimum cooling rates were about 50" C Myr-'. As a consequence, the closure temperature of the specific isotopic system is of minor importance, and the ages correspond to the time of high-pressure metamorphism. Despite textural equilibrium and metamorphic temperatures in excess of W"C, clinopyroxene, garnet and whole rock do not define a three-point isochron in three of four samples. The metamorphic clinopyroxenes seem to have inherited their isotopic signature from magmatic precursors. Rapid tectonic burial and uplift within only a few million years might be the reason for the observed Sm-Nd disequilibrium. The cNd values of the eclogites (+4.4 to +6.9) suggest the protoliths were derived from a long-term depleted mantle, probably a MORB source, whereas the isotopically enriched garnet-pyroxenite (E~,, - 2.9) might represent subcontinental mantle material, emplaced into the crust prior to or during collision. The similarity of ages of the two different rock types suggests a shared metamorphic history.