Amphibolite Samples Research Articles

In-situ U-Pb titanite age of the Chuncheon amphibolite: Evidence for Triassic regional metamorphism in central Gyeonggi massif, South Korea, and its tectonic implication

We report an in-situ U-Pb age of titanite in the Chuncheon amphibolite, central Gyeonggi massif, using a high-resolution secondary ion mass spectrometer (HR-SIMS). Titanite occurs as common accessory phase in garnet amphibolites, produced by regional metamorphism. Titanite grains separated from two amphibolite samples yield the weighted mean 238U-206Pb ages of 224 ± 14 (2σ) Ma and 239 ± 94 (2σ) Ma, respectively, which are interpreted to suggest a high-grade metamorphism at ca. 220 Ma. This result together with the available ages reported by previous workers suggests that both the Gyeonggi massif and the Imjingang belt experienced the regional Triassic metamorphism, possibly in association with continental collision between the North and South China blocks.

Similar quartz crystallographic textures in rocks of continental earth’s crust (by neutron diffraction data): I. Quartz textures in monomineral rocks

The types of quartz textures found in a large collection of multiphase rocks from different regions of the earth are analyzed. Crystallographic textures of granulite, amphibolite, slate, and gneiss samples are measured, classified, and compared with the similar textures of monomineral rocks.

PROTOLITH COMPOSITION OF CORDIERITE-GEDRITE BASEMENT ROCKS AND GARNET AMPHIBOLITE OF THE BEARPAW LAKE AREA OF THE THOR-ODIN DOME, MONASHEE COMPLEX, BRITISH COLUMBIA, CANADA

The Thor–Odin dome of the Monashee complex, southeastern Canadian Cordillera, British Columbia, contains basement gneiss of North American tectonic affinity, including a distinctive cordierite–gedrite rock that is restricted in occurrence to the basement rocks and a regionally occurring garnet amphibolite. All of these rocks were penetratively deformed and metamorphosed at upper-amphibolite to lower-granulite facies conditions in the Cordilleran orogeny during the Late Cretaceous to Paleocene. The cordierite–orthoamphibole (dominantly gedrite) unit appears to define a discontinuous marker horizon or horizons within the Paleoproterozoic basement paragneiss. The cordierite–gedrite rocks have a unique bulk-rock composition that is characterized by depletions in the alkali elements and Ca, and enrichments in Al, Mg and Fe. They have flat rare-earth element (REE) patterns, depletions in most low field-strength elements (LFSE), and enrichments in high field-strength elements (HFSE). The measured 87Sr/86Sr values for these rocks vary from 0.74923 to 0.85962, and the eNd(today) values vary from −15.3 to −20.6. The cordierite–gedrite rocks are interpreted as Paleoproterozoic mafic volcanic rocks that were hydrothermally altered, likely prior to metamorphism. Hydrothermal alteration explains their distinctive bulk-rock composition. Interlayered and spatially associated lenses of garnet amphibolite are of uncertain age, although they are likely Precambrian, on the basis of their Nd isotopic composition. The garnet amphibolite rocks are characterized by high Mg and Ti values and depletions in the alkali elements. They have flat REE patterns, depletions in most LFSE and enrichments in HFSE. The garnet amphibolite samples have measured 87Sr/86Sr values that vary from 0.70953 to 0.74319, and eNd(today) values that vary from −0.8 to −7.3. The garnet amphibolite rocks are interpreted as Proterozoic metamorphosed mafic dykes or possibly volcanic rocks that postdated the formation of the protolith of the cordierite–gedrite rocks.

Electric signal emissions during repeated abrupt uniaxial compressional stress steps in amphibolite from KTB drilling

Abstract. Laboratory experiments have confirmed that the application of uniaxial stress on rock samples is accompanied by the production of weak electric currents, to which the term Pressure Stimulated Currents – PSC has been attributed. In this work the PSC emissions in amphibolite samples from KTB drilling are presented and commented upon. After having applied sequential loading and unloading cycles on the amphibolite samples, it was ascertained that in every new loading cycle after unloading, the emitted PSC exhibits lower peaks. This attitude of the current peaks is consistent with the acoustic emissions phenomena, and in this work is verified for PSC emissions during loading – unloading procedures. Consequently, the evaluation of such signals can help to correlate the state and the remaining strength of the sample with respect to the history of its mechanical stress.

The isotopic composition of zircon and garnet: A record of the metamorphic history of Naxos, Greece

Growth of zircon with respect to that of garnet has been studied using a combination of petrography, U–Pb dating and oxygen isotope analysis. The aim is to document the mechanism and pressure–temperature conditions of zircon growth during metamorphism in order to better constrain the Tertiary metamorphic history of Naxos, Greece. Two metamorphisms are recognised: (1) an Eocene Franciscan metamorphism (M1) and (2) a widespread Miocene Barrovian metamorphism (M2) that increases from greenschist facies up to partial melting. An amphibolite sample contains zircon crystals characterised by a magmatic core and two metamorphic rims, denoted as A and B, dated at 200–270, 42–69, and 14–19 Ma, respectively. The first metamorphic rim A ( δ 18O = 7 ± 1‰) preserves the δ 18O value of the magmatic core (6.2 ± 0.8‰), whereas rim B is characterised by higher δ 18O values (7.8 ± 1.8‰). These observations indicate the formation of A rims by solid-state recrystallisation in a closed system with regard to oxygen and those of B in an open system. Compositional zoning in garnet is interpreted as the result of decompressional heating. Zircon B rims and garnet rims display similar δ 18O values which indicates a contemporaneous growth of garnet and zircon rims during the Miocene Barrovian event (M2). Calcic gneiss and metapelite samples contain zircon crystals with single metamorphic overgrowths aged 41–57 Ma. δ 18O values measured in zircon overgrowths (11.8 ± 1.4‰) from the calcic gneiss are similar to those measured in garnet rims (11.4 ± 1.1‰) from the same rock. This suggests that garnet rims and zircon overgrowths grew during the high pressure–low temperature event in equilibrium with prograde fluids. In the metapelite sample, δ 18O values are similar in garnet cores (14.8 ± 0.2‰) and in zircon metamorphic overgrowths (14.2 ± 0.5‰). As zircon overgrowths have been dated at ca. 50 Ma by U–Pb, garnet cores and zircon overgrowths are interpreted to have grown during the high pressure event. As demonstrated here for the island of Naxos, correlating the crystallisation of zircon with that of metamorphic index minerals such as garnet using stable isotope composition and U–Pb determination is a powerful tool for deciphering the mechanism of zircon growth and pin-pointing zircon crystallisation within the metamorphic history of a terrain. This approach is potentially hampered by an inability to verify the degree of textural equilibrium of zircon with other mineral phases, and the possible preservation (in metamorphic rims) of isotopic signatures from pre-existing zircon when they form by recrystallisation. Nevertheless, this study illustrates the application of this approach in providing key constraints on the timing and mechanism of growth of minerals important to understanding metamorphic petrogenesis.

REDISTRIBUTION OF THE RARE-EARTH ELEMENTS AMONG COEXISTING MINERALS IN METAMAFIC ROCKS ACROSS THE EPIDOTE-OUT ISOGRAD: AN EXAMPLE FROM THE ST. ANTHONY COMPLEX, NORTHERN NEWFOUNDLAND, CANADA

Integrated results of in situ major-element and rare-earth-element analyses of coexisting Ca amphibole, plagioclase, epidote and titanite have been made from a suite of metamafic rocks from the St. Anthony Complex, Newfoundland, across the epidote-out reaction isograd to examine the redistribution of the REE as a consequence of the breakdown of epidote. Important major-element variations include a decrease in X Cum Amp and an increase in X An Pl across the reaction isograd, and an increase in X Czo Ep within the epidote-amphibolite facies. These major-element changes correlate with variations in ∑REE and the LREE:HREE ratio in several of the phases, and also with systematic changes in D *REE partitioning across the epidote-out isograd. Onuma-type diagrams for the partitioning of the REE between coexisting minerals confirm that the REE substitute at single elastic sites in titanite, epidote and plagioclase, whereas in Ca amphibole they occupy two sites, M 4 and M 4′, with the size of the M 4′ peak being positively correlated with X Cum Amp in Ca amphibole. A REE mass balance across the epidote-out isograd yields very good matches between the measured and reconstructed REE abundances in all cases, indicating that metamorphism was essentially isochemical with respect to the rare-earth elements, and that all the hosts for these elements were analyzed. The following major-element mass-balanced reaction, determined by the method of singular value decomposition from mineral compositions in representative samples of epidote amphibolite and plagioclase amphibolite from this ophiolitic complex, provides a realistic representation of the epidote-out isograd in the study area: 2.94 CaAmp1 + 5.00 Pl1 + 1.00 Ep + 5.21Ttn1 = 3.14 CaAmp2+ 5.65 Pl2 + 5.29 Ttn2. Integration of the calculated stoichiometric coefficients of this reaction with the measured REE contents in each phase provides a good match for the MREE and HREE abundances, but a poorer match for the LREE, which show a deficiency on the product side. However, the results are compatible with the measured increases in LREE in Ca amphibole and plagioclase on the product side of the reaction, as a result of the breakdown of epidote and a reduction in LREE abundance in titanite. We highlight the role of epidote and titanite as the principal carriers of the REE in epidote amphibolite, and that of Ca amphibole as the dominant carrier in plagioclase amphibolite.

The fate of subducted continental margins: Two‐stage exhumation of the high‐pressure to ultrahigh‐pressure Western Gneiss Region, Norway

AbstractThermobarometry suggests that ultrahigh‐pressure (UHP) to high‐pressure (HP) rocks across the Western Gneiss Region ponded at the Moho following as much as 100 km of exhumation through the mantle and before exhumation to the upper crust. Eclogite across the c. 22 000 km2 study area records minimum pressures of c. 8–18 kbar and temperatures of c. 650–780 °C. One orthopyroxene eclogite yields an UHP of c. 28.5 kbar, and evidence of former coesite has been found c. 50 km farther east than previously known. Despite this widespread evidence of UHP to HP, thermobarometry of metapelite and garnet amphibolite samples reveals a surprisingly uniform ‘supra‐Barrovian’ amphibolite‐facies overprint at c. 11 kbar and c. 650–750 °C across the entire area. Chemical zoning analysis suggests that garnet in these samples grew during heating and decompression, presumably during the amphibolite‐facies event. These data indicate that the Norwegian UHP/HP province was exhumed from mantle depths of c. 150 km to lower crustal depths, where it stalled and underwent a profound high‐temperature overprint. The ubiquity of late‐stage supra‐Barrovian metamorphic overprints suggests that large‐scale, collisional UHP terranes routinely stall at the continental Moho where diminishing body forces are exceeded by boundary forces. Significant portions of the middle or lower crust worldwide may be formed from UHP terranes that were arrested at the Moho and never underwent their final stage of exhumation.

Geochemistry and Sm–Nd isotopic characteristics of bimodal volcanic rocks of Juscelândia, Goiás, Brazil: Mesoproterozoic transition from continental rift to ocean basin

The ca. 1.28 Ga Juscelândia Sequence, in Goiás, Brazil, comprises a unit of bimodal metavolcanic rocks, amphibolite and felsic gneiss, intercalated with metapelite and metachert. The bimodal nature of the magmatism is evident as the metavolcanic rocks have basalt and rhyolite compositions. Bimodality is also shown by the trace elements and Nd isotope data. Amphibolites yield positive ε Nd ( T ) values that can be separated into two groups. In the first group, amphibolite samples from lower and middle portion of the sequence are characterized by ε Nd between +2.8 and +5.4. These fairly positive values and enrichment in LREE and LILE suggest they were derived from a depleted mantle source with subsequent assimilation of continental crustal material. The second group is formed by amphibolite of the upper portion of the sequence that have ε Nd around +6.0 and their low LILE and LREE contents resemble normal mid-ocean ridge basalts (N-MORB). Initial 143 Nd / 144 Nd ratios represent the composition of the depleted mantle at 1.28 Ga. Felsic rocks yield negative ε Nd ( T ) values, between ca. −2.8 and −4.5, and Paleoproterozoic model ages ( T DM values in the interval between 1.8 and 2.1 Ga). The original felsic magmas represent the product of melting of older (Paleoproterozoic) continental crust with trace element chemistry indicating that the felsic magmas were the product of continental crustal melts with small mantle contribution. Previous models proposing an oceanic setting for the origin of magmatism of the Juscelândia rocks, within a mid-ocean ridge or back-arc basin are not supported by the data presented here. Transition from a continental rift to an ocean basin represents the most likely tectonic setting for the origin of the Juscelândia rocks.

SHRIMP U–Pb and Sm–Nd data for the Araxá Group and associated magmatic rocks: constraints for the age of sedimentation and geodynamic context of the southern Brası́lia Belt, central Brazil

Abstract SHRIMP U–Pb and Sm–Nd isotopic data for Araxa Group micaschist, associated amphibolite and intrusive tonalite were used to estimate the age and tectonic setting of deposition of the original detrital sediments, in the southern part of the Neoproterozoic Brasilia Fold Belt. One amphibolite sample from a layer interleaved tectonically within the metasedimentary rocks has the U–Pb zircon age of 838±20 Ma. Its tholeiitic composition and depleted mantle isotopic signature ( e Nd =+5.1) suggests that it represents a tectonic slice of the former oceanic lithosphere separating the Sao Francisco and Amazon continents. U–Pb ages of detrital zircon grains and T DM model ages of the Araxa Group micaschists show a bimodal pattern, interpreted as the result of provenance from two contrasting sources: (i) young juvenile sources, probably represented by igneous rocks of the Goias Magmatic Arc in the west and (ii) Paleoproterozoic sialic sources, most probably belonging to the Sao Francisco Craton, in the east. The detrital population is largely dominated by ca. 666–682 Ma old zircon grains derived from both mafic and felsic igneous source rocks. Detrital metamorphic grains and rims dated at ca. 715 and 643 Ma indicate that the original basin received sediments from Neoproterozoic sources with a previous metamorphic overprint, implying that young metamorphic rocks have been uplifted, exposed and eroded, providing sediment for at least part of what is presently mapped as the Araxa Group. The youngest detrital zircon age of ca. 643 Ma represents the upper limit for deposition of the original detrital sediments. Their minimum age is constrained by the crystallisation of an intrusive tonalite at 638±11 Ma. The data suggest, therefore, that the Araxa Group metasediments were deposited and metamorphosed within a very short time interval, most probably during the tectonic events related with the accretion of the Goias Magmatic Arc to the eastern part of the Brasilia Belt and final closure of the Goias Ocean.

Geologia e geocronologia da suíte metamórfica colorado e suas encaixantes, SE de Rondônia: implicações para a evolução mesoproterozóica do SW do cráton Amazônico

Combined geochronological (U-Pb, 40Ar/39Ar, and Rb-Sr) geological data help define an important tectonomagmatic event in the Colorado do Oeste and Cabixi regions, southeastern Rondônia, Brazil, and characterize the Colorado Metamorphic Suite (CMS), well represented by rock assemblages which experienced deformation and metamorphic recrystallization (upper-amphibolite facies). The suite is composed of: 1) porphyritic monzogranite associated with amphibolite (bimodal magmatism), 2) interlayered clastic and chemical metasedimentary rocks (sillimanite schists and iron formation), 3) muscovite-garnet leucogranite, and 4) mafic-ultramafic intrusive rocks. The CMS mafic rocks occur as undeformed isolated bodies of layered coarse-grained metagabbro, still preserving typical cumulate igneous texture. U-Pb zircon isotopic data for three fractions of sample RO-10 define a discordia with an upper intercept (crystallization) age of 1352 + 4/-3 Ma (MSWD = 0.18). The porphyritic ortogneisses (RO-15; monzogranite and amphibolite) yielded a Rb-Sr whole rock isochron age of 1360 + 45 Ma, and 87Sr/86Sr i = 0.7040 + 0.0012 (MSWD = 9.2). One sample of aplite that is subparallel to the main regional foliation has furnished an age of 1360 + 13 Ma. The 40Ar/39Ar data for muscovite from a anatectic leucogranite (RO-14) yielded plateau ages of 1312 + 3 Ma (grain 1), and 1303 + 2 Ma/1305 + 2 Ma (grain 2). Grain 3 presented an heterogeneous isotopic spectrum, the integrated age being 1289 + 2 Ma. Hornblende from two amphibolite samples yielded 40Ar/39Ar plateau ages of 1313 + 4 Ma; 1313 + 6 Ma and 1312 + 3 Ma (RO-18), and 1325 + 3 Ma; 1326 + 2 Ma; 1330 + 3 Ma (RO-19). The weighted-mean age is 1319 + 10 Ma, and is interpreted as the best estimated age for regional metamorphic cooling. These data suggest that southeastern Rondônia was affected by a tectonomagmatic event at ca. 1.36 - 1.32 Ga, predating the evolution of the Nova Brasilândia Terrane (1215 - 1110 Ma). However, the unequivocal characterization of an orogeny refered to the time interval 1.36 - 1.32 Ga is still uncertain due to scarcity of geochronological data in the region. Similar ages are reported for correlatable intrusive rocks which cut the polydeformed basement rocks of central Rondônia. Thus, these ages are compatible with available Mesoproterozoic tectonic models for the SW Amazonian Craton.

Physical and mechanical properties of selected amphibolite core samples from the Kola Superdeep Borehole KSDB‐3

ABSTRACTLaboratory tests of physical and mechanical properties were performed on amphibolites representing two units from the Kola Superdeep Borehole KSDB‐3 (Russia) − the Proterozoic Karelian Complex (depths of 3043 m, 3530 m and 4389 m) and the Archean Kola–Belomorian Complex (depths of 7951 m, 8942 m and 9904 m). Obtained grain density and bulk density values are nearly identical for all tested samples, but porosities increased slowly with depth. Marked differences in strength properties were found between amphibolite samples from the Karelian Complex and amphibolite samples from the Kola–Belomorian Complex. The uniaxial compressive strength of the samples from the lower part of the borehole is significantly smaller than in samples from the upper part; a similar trend was found for triaxial tests. The values of the deformation modulus, Young's modulus and Poisson's ratio obtained from samples of the Karelian Complex are considerably higher than those from the Kola–Belomorian Complex. Observed mechanical properties are explained by the variable grain‐size distribution and by the spatial arrangement of main rock‐forming minerals. These properties are also influenced by recovery of rocks from great depths.

The spectrum of thermally stimulated currents in rock samples from KTB drilling: preliminary results

The technique of thermally stimulated depolarization currents (TSDC) was applied to fine grained amphibolite rock samples extracted from KTB drilling. Each sample was cooled down to liquid nitrogen temperature and consequently heated at a constant rate b = 4 K/min while at the same time it was shorted by a sensitive electrometer. Thermocurrent glow curves were detected in the range from 180 to 360 K. By applying the TSDC method, an effort was made to carry out a preliminary analysis of the TSDC spectrum.

Polyphase evolution and reaction sequence of compositional domains in metabasalt: a model based on local chemical equilibrium and metamorphic differentiation

AbstractEclogitic garnet amphibolite samples from the Southern Steep Belt of the Central Alps show evidence of several stages of metamorphic evolution and exhumation. A method for unravelling this evolution is presented and applied to these samples. It involves a combination of detailed petrographic analysis and microchemical characterization with quantitative models of the thermodynamically stable phase relations for specific compositional domains of each sample. Preserved mineral relics and textural evidence are compared to model predictions to identify the important irreversible reactions. The interpretation of the exhumation history is thus based on the consistency of a wide spectrum of observations with predicted phase diagrams, leading to robust reconstruction of a pressure–temperature (P–T) path even where the mineralogical relics in samples are insufficient, due to retrogression, to warrant application of multi‐equilibrium thermobarometric techniques.The formation of compositionally different domains in the metabasalt samples studied is attributed to prograde growth of porphyroblasts (e.g. garnet, plagioclase, zoisite) in the matrix, implying substantial metamorphic differentiation at the scale of a few millimetres. Chemical interaction among different domains during the subsequent P–T evolution is shown to have been very limited. This led to different reaction sequences during exhumation, in which relics preserved in different domains reflect a range of continually changing P–T conditions. For samples from a single outcrop, we deduce a Barrovian prograde path to eclogite facies (23 ± 3 kbar, 750 ± 50°C), followed by (rapid) decompression to 8 ± 1 kbar and 675 ± 25°C, and a final heating phase at similar pressures reaching 750 ± 40°C. This evolution is attributed to the Alpine cycle involving subduction→collision and slab breakoff→extrusion of tectonic fragments that make up the Southern Steep Belt of the Central Alps. Copyright © 2000 John Wiley & Sons, Ltd.

Geochemical and Sm–Nd isotopic study of amphibolites in the Cathaysia Block, southeastern China: evidence for an extremely depleted mantle in the Paleoproterozoic

Geochemical and Sm–Nd isotopic results are reported for late Paleoproterozoic mafic amphibolites from SW Zhejiang and NW Fujian, parts of the Cathaysia Block of SE China. Two suites of contemporaneous amphibolites are distinct in their geochemical characteristics. Group 1 samples, from NW Fujian, have chemical compositions of transitional and alkali basalts, show LREE-enriched patterns and plot mainly in the field of within-plate basalt on a number of trace element discrimination diagrams. Group 2 rocks, from SW Zhejiang, have tholeiitic compositions and are characterized by flat to LREE-depleted patterns and fall into the MORB field. All the amphibolite samples have high εNd(T) values of +5.6 to +8.5 (T=1766±19 Ma). A positive correlation between εNd(T) and Nb/Th suggests possible mixing of a mantle-derived magma and a crustal component, with the least-contaminated samples having very high εNd(T) values (+8∼+8.5) and Nb/Th ratios of ∼13. The geochemical and isotopic characters and close temporal relationship of these two suites of amphibolites suggest that their magmatic precursors were likely formed in an environment similar to an ensialic rift developing into a proto-oceanic basin (e.g. the Gulf of Tadjoura). The exceptionally high εNd(T) values of up to +8.5 for some of the amphibolites suggest the presence of a time-integrated extremely depleted mantle source beneath Cathaysia during the Paleoproterozic.

西南日本内帯大江山地域の普甲峠変成沈積岩に産する４〜４．４億年含藍晶石緑れん石角閃岩

The Fuko Pass metacumulate is a tectonic block (4.5×1.5 km in size) within the Oeyama peridotite body in the Inner Zone of southwestern Japan. It was metamorphosed under the high-pressure and moderate-temperature condition to form a mineral assemblage hornblende+clinozoisite+kyanite+paragonite+albite+rutile. K-Ar ages were determined on hornblende separates from four epidote amphibolite samples, yielding 443-403 Ma. This suggests a Siluro-Ordovician subduction-related metamorphic event for the Fuko Pass metacumulate, which is distinct from the Late Paleozoic high-P/T type metamorphic events of the Renge metamorphic belt in the Inner Zone of southwestern Japan. The Fuko Pass metacumulate is correlated, in age, with the Early Paleozoic high-P/T type schists (445-402 Ma) of the Kurosegawa klippe in the Outer Zone.

Variscan P–T deformation paths from the southwestern Aiguilles Rouges massif (External massif, western Alps) and their implication for its tectonic evolution

Petrological and structural investigations in metapelites and amphibolites were undertaken to unravel the Variscan tectonic and metamorphic evolution of the southwestern Aiguilles Rouges massif. The geometry of planar and linear structures indicates a transpressive deformation regime with a change from predominantly subhorizontal to subvertical movements during its evolution. In metapelite samples from gneiss areas, garnet zonation trends have been correlated with chemical variations in plagioclase and biotite by microstructural observations. Applied conventional cation-exchange and net-transfer geothermobarometers yielded anti-clockwise P–T-deformation paths. Clockwise P–T-deformation paths resulted from (Na, Ca)-amphibole zonations of amphibolite samples from a different lithotectonic unit. Notable differences in P–T conditions and the general shape of P–T paths for the gneisses and amphibolites indicate that the units do not represent a continuous metamorphic zonation and underwent no common tectono-metamorphic evolution throughout the entire deformation. The combination of P–T-deformation paths and structural investigations revealed first-stage thrust tectonics followed by a transpressive deformation which was responsible for the uplift of the southwestern Aiguilles Rouges massif.

Chronology of Paleozoic metamorphism and deformation in the Blue Ridge thrust complex, North Carolina and Tennessee

The Blue Ridge province in northwestern North Carolina and northeastern Tennessee records a multiphase collisional and accretionary history from the Mesoproterozoic through the Paleozoic. To constrain the tectonothermal evolution in this region, radiometric ages have been determined for 23 regionally metamorphosed amphibolites, granitic gneisses, and pelitic schists and from mylonites along shear zones that bound thrust sheets and within an internal shear zone. Samples were collected along a northwest-southeast traverse across three crystalline thrust sheets located west of the Grandfather Mountain window. Ordovician metamorphic mineral ages (Llanvirnian to Caradocian) are recorded in the highest thrust sheets within the Blue Ridge thrust complex. Six samples of metapelite and amphibolite from the Spruce Pine thrust sheet and five samples of basement gneiss and metagabbro from the Pumpkin Patch thrust sheet yield Sm-Nd and Rb-Sr garnet ages of 460 to 450 Ma and hornblende ages of 472 to 451 Ma. In addition, multiple mineral isochron ages of 457+ or -2 and 458+ or -17 Ma were obtained from a Pumpkin Patch metagabbro. The garnet ages from the Pumpkin Patch thrust sheet (458, 455, and 451 Ma) are similar to those from the structurally overlying Spruce Pine thrust sheet (460, 456, 455, 455, and 450 Ma). Both thrust sheets exhibit similar upper amphibolite-facies conditions. Because of the high closure temperature for garnet, the garnet ages are interpreted to date growth at or near the peak of Taconic metamorphism. Devonian metamorphic ages are recognized in the Spruce Pine thrust sheet, where Sm-Nd and Rb-Sr garnet ages of 386 and 393 Ma and mineral isochron ages of 397+ or -14 and 375+ or -27 Ma are preserved. Hornblendes record similar 40 Ar/ 39 Ar, Sm-Nd, and Rb-Sr ages of 398 to 379 Ma. Devonian 40 Ar/ 39 Ar hornblende ages are also recorded in the structurally lower Pumpkin Patch thrust sheet. The Devonian mineral ages are interpreted to date a discrete tectonothermal event, as opposed to uplift and slow cooling from an Ordovician metamorphic event. This interpretation is based in part on the fact that Devonian and Ordovician mineral ages are intermixed spatially within the Spruce Pine and Pumpkin Patch thrust sheets. 40 Ar/ 39 Ar muscovite ages from regionally metamorphosed rocks indicate that much of the Blue Ridge cooled slowly to temperatures of about 375 degrees C by the Late Mississippian. White micas from mylonite zones separating thrust sheets record 40 Ar/ 39 Ar plateau ages of 323, 327, 328, and 336 Ma and Rb-Sr ages of 324, 324, 326, and 326 Ma. These reflect the age of Late Mississippian mylonitization (mica growth below or near respective Sr and Ar closure temperatures). The Mississippian mylonitization is interpreted to represent thrusting and initial assembly of crystalline sheets associated with the Alleghanian orogeny. The composite thrust stack of the Blue Ridge complex was subsequently thrust northwestward along the Linville Falls fault during middle Alleghanian orogeny (about 300 Ma).

Geochemistry and Sr-Nd-Pb isotopic systematics of the Ogcheon amphibolites from the central Ogcheon Belt, Korea: Implication for the source heterogeneity.

A detailed geochemical and isotopic study was performed on the Ogcheon amphibolites from the central Ogcheon Belt, Korea, to investigate the source characteristics and petrogenesis. Major and trace element data indicate that the parental rocks of the amphibolites are mostly transitional basalts, except for a few tholeiitic basalts. The tholeiitic basalts are less enriched in incompatible elements than transitional basalts and are geochemically close to primary magmas. Some transitional basalts with low SiO2 ( 16 wt%) are clearly distinguished from others and are believed to be derived from a ferro-basaltic magma. On the basis of several contamination parameters (e.g., La/Nb ratio, Nb/Th ratio, Nb and Ta anomaly), we suggest that crustal contamination has not significantly influenced the chemistry of the amphibolites. All amphibolite samples are moderately enriched in incompatible elements and plot in within-plate fields on a number of tectonic discriminant diagrams, and are consistent with previous works. The parental magmas of the amphibolites were derived from a heterogenous mantle source. Sm-Nd isotope whole rock data define a poor linear array. Considering variations in incompatible elements and Nd isotopes, the linear array may be not a true isochron but rather a mixing pseudoisochron due to source heterogeneity.

U-Pb sphene dating of metamorphism: the importance of sphene growth in the contact aureole of the Red Mountain pluton, Laramie Mountains, Wyoming

The relative importance of thermal diffusion versus new growth or recrystallization on U-Pb isotopic data from sphene is assessed through a study of amphibolites and granite gneisses within the contact aureole of the Red Mountain pluton, Laramie anorthosite complex, Wyoming. Samples were collected along a traverse approximately perpendicular to the margin of the intrusion over a distance of 0.13 to 2.65 km from the contact. The 207Pb/206Pb ages of sphene from amphibolite samples range between 1.43 Ga, the intrusive age of the Red Mountain pluton, to 1.78 Ga, the age of regional metamorphism. The 207Pb/206Pb ages of sphene in rocks metamorphosed above 700° C are within error of the intrusive age of the pluton, and appear to have resulted from diffusional resetting of preexisting sphene and the metamorphic growth of additional sphene at 1.43 Ga. At greater distance from the contact the 207Pb/206Pb ages range from 1.45 to 1.72 Ga. This 300 million year spread in ages is interpreted to result from two periods of sphene growth, one produced during regional metamorphism at 1.78 Ga and another generation of newly grown or recrystallized sphene that formed during contact metamorphism at 1.43 Ga. These two age populations may be identified on the basis of petrographic textures, the morphologies and color differences of grain separates as well as by the U-Pb systematics. In rocks metamorphosed to temperatures less than 700° C, sphene growth was the dominant process controlling the response of the U-Pb isotope system to contact metamorphism. Sphene grew well outside the zone of obvious contact metamorphism. The U-Pb sphene ages were reset by diffusion only at high temperatures, supporting the experimentally determined closure temperature estimates for the U-Pb system in sphene of around 650° C (Cherniak 1993). This study demonstrates that U-Pb ages of sphene can be used to date metamorphism not only in areas with a simple geologic history, such as igneous intrusion or single metamorphic or deformational events, but also to date multiple events so long as different generations of sphene can be identified and separated.

Geochronology and geochemistry of eclogites from the Mariánské Lázně Complex, Czech Republic: Implications for Variscan orogenesis

The Marianske Lazně complex (MLC) is located in the Bohemian Massif along the north-western margin of the Tepla-Barrandian microplate and consists of metagabbro, amphibolite and eclogite, with subordinate amounts of serpentinite, felsic gneiss and calcsilicate rocks. The MLC is interpreted as a metaophiolite complex that marks the suture zone between the Saxothuringian rocks to the north-west and the Tepla-Barrandian microplate to the south-east. Sm-Nd geochronology of garnet-omphacite pairs from two eclogite samples yields ages of 377±7, and 367±4 Ma. Samples of eclogite and amphibolite do not define a whole rock Sm-Nd isochron, even though there is a large range in Sm/Nd ratio, implying that the suite of samples may not be cogenetic. Eclogites do not have correlated ɛNd values and initial 87Sr/86Sr ratios. Five of the eight eclogite samples have high ɛNd values (+10.2 to +7.1) consistent with derivation from a MORB-like source, but variable 87Sr/86Sr ratios (0.7033 to 0.7059) which probably reflect hydrothermal seawater alteration. Three other eclogite samples have lower ɛNd values (+ 5.4 to −0.8) and widely variable 87Sr/86Sr ratios (0.7033 to 0.7096). Such low ɛNd values are inconsistent with derivation from a MORB, source and may reflect a subduction or oceanic island basalt component in their source. The MLC is an important petrotectonic element in the Bohemian Massif, providing evidence for Cambro-Ordovician formation of oceanic crust and interaction with seawater, Late Devonian (Frasnian-Famennian) high- and medium-pressure metamorphism related to closure of a Saxothuringian ocean basin, Early Carboniferous (Visean) thrusting of the Tepla terrane over Saxothuringian rocks and Late Visean extension.