Amphibolite Facies Metamorphic Conditions Research Articles

Structure of the eastern margin of the East African Orogen in central Madagascar

The island of Madagascar straddles the poorly understood eastern margin of the East African Orogen, which is characterised by a high-grade gneissic basement that was structurally and thermally reworked during the Neoproterozoic collision of the Dharwar craton of India with the Congo/Tanzania/Bangweulu craton of Africa. An east-west traverse across the eastern margin of the Antananarivo block of central Madagascar and the overlying Beforona belt of the Tsaratanana sheet has identified four north-south trending structural domains, which represent the easternmost units of the East African Orogen. Dominant structures in these four domains are ascribed to two main deformation events, D2 and D3, whereas a poorly preserved early foliation and late folding event are classified as D1 and D4, respectively. D2 structures developed during east-west shortening comprise relatively low-strain, upright, north-south trending folds and steeply dipping, planar, north-south trending high-strain zones, both of which are associated with a stretching lineation that plunges gently north-south. D3 structures comprise a >20 km wide mylonitic high-strain zone and smaller scale discrete shear zones that dip gently west and exhibit top-to-the-east movement, again interpreted as a result of east-west shortening. The Antananarivo block and Beforona belt were juxtaposed sometime before D2 folding, which deforms the contact between the two units, but some time after an early granulite-facies metamorphic event that is preserved only in low strain zones of the Antananarivo block. D2 coaxial strain and D3 non-coaxial strain both occurred under amphibolite-facies metamorphic conditions and, although D3 shear zones locally cut D2 structures, these two events need not be temporally distinct. Published geochronological data from rocks exposed along this traverse constrain the likely timing of D2 and D3 to 630–515 Ma. The age, style and magnitude of strain along the eastern Antananarivo block indicates that eastern Madagascar was a focus for deformation during the final stages of collision between the Dharwar and Congo/Tanzania/Bangweulu cratons.

990 and 1100 Ma Grenvillian tectonothermal events in the northern Oaxacan Complex, southern Mexico: roots of an orogen

Inliers of ∼1.0–1.3 Ga rocks occur throughout Mexico and form the basement of the Oaxaquia microcontinent. In the northern part of the largest inlier in southern Mexico, rocks of the Oaxacan Complex consist of the following structural sequence of units (from bottom to top), which protolith ages are: (1) Huitzo unit: a 1012±12 Ma anorthosite–mangerite–charnockite–granite (AMCG) suite; (2) El Catrı́n unit: ≥1350 Ma orthogneiss migmatized at 1106±6 Ma; and (3) El Marquez unit: ≥1140 Ma para- and orthogneisses. These rocks were affected by two major tectonothermal events that are dated using U–Pb isotopic analyses of zircon: (a) the 1106±6 Ma Olmecan event produced a migmatitic or metamorphic differentiation banding folded by isoclinal folds; and (b) the 1004–978±3 Ma Zapotecan event produced at least two sets of structures: (Z1) recumbent, isoclinal, Class 1C/3 folds with gently NW-plunging fold axes that are parallel to mineral and stretched quartz lineations under granulite facies metamorphism; and (Z2) tight, upright, subhorizontal WNW- to NNE-trending folds accompanied by development of brown hornblende at upper amphibolite facies metamorphic conditions. Cooling through 500 °C at 977±12 Ma is documented by 40Ar/ 39Ar analyses of hornblende. Fold mechanisms operating in the northern Oaxacan Complex under Zapotecan granulite facies metamorphism include flexural and tangential–longitudinal strain accompanied by intense flattening and stretching parallel to the fold axes. Subsequent Phanerozoic deformation includes thrusting and upright folding under lower-grade metamorphic conditions. The Zapotecan event is widespread throughout Oaxaquia, and took crustal rocks to a depth of ∼25–30 km by orogenic crustal thickening, and is here designated as Zapotecan Orogeny. Modern analogues for Zapotecan granulite facies metamorphism and deformation occur in middle to lower crustal portion of subduction and collisional orogens. Contemporaneous tectonothermal events took place throughout Oaxaquia, and in various parts of the Genvillian orogen in Laurentia and Amazonia.

Dating the exhumation of UHP rocks and associated crustal melting in the Norwegian Caledonides

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

Dissolution and replacement creep: a significant deformation mechanism in mid-crustal rocks

Zoning patterns and zoning truncations in metamorphic minerals in a granodioritic orthogneiss indicate that strain and S– C fabrics in these rocks were produced by dissolution, precipitation, and replacement processes, even at epidote–amphibolite facies metamorphic conditions. The metamorphic fabric is defined by alternating layers and folia dominated by quartz, feldspars, and biotite+epidote. Zoning patterns in most metamorphic plagioclase, orthoclase, epidote, and sphene are truncated at boundaries normal to the shortening direction, suggesting dissolution. Interfaces of relict igneous orthoclase phenocrysts that face the shortening direction are embayed and replaced by biotite, epidote, and myrmekitic intergrowths of plagioclase and quartz. Metamorphic plagioclase grains are also replaced by epidote. We interpret these microstructures to reflect strain-enhanced dissolution. The cores of many grains show asymmetric overgrowths with at least two generations of beards, all oriented on the ends of grains that face the extension direction. We interpret these textures to reflect precipitation of components dissolved by deformation-enhanced dissolution. While biotite and quartz probably deformed by dislocation creep, the overall deformation was accommodated by dissolution perpendicular to the shortening direction, and precipitation parallel to it. These chemical processes must have been activated at lower stresses than the dislocation creep predicted from extrapolations of data from experiments in dry rocks. Thus wet crust is likely to be weaker than calculated from these experimental studies.

Geologic constraints on middle-crustal behavior during broadly synorogenic extension in the central East Greenland Caledonides

Structural and U–Pb geochronologic data from the Forsblad Fjord area of East Greenland (72°30′N) indicate close spatial and temporal ties between orogen-parallel shear and extensional deformation during Caledonian orogenesis. This territory is composed of three tectonostratigraphic units separated by two splays of the Fjord Region Detachment System (FRDS), a principal extensional fault system of the East Greenland Caledonides that was active in Silurian time. The oldest Caledonian fabrics in Forsblad Fjord, which developed at upper amphibolite facies metamorphic conditions, indicate that there was north–south, lateral extrusion of ductile middle-crustal material synchronous with approximately east–west shortening. U–Pb dating of synkinematic granitic leucosomes in migmatitic schists and gneisses demonstrates that this process was underway at ca. 425 Ma. Subsequently, east–west extension along the two splays of the FRDS truncated the older fabrics; the structurally highest of these shear zones – the Tindern detachment – was active as early as ca. 424 Ma. This implies either that there was a rapid transition from Caledonian shortening and possible transpressional deformation to post-orogenic collapse, or our preferred interpretation that extension was synorogenic. The FRDS shares many structural characteristics with the South Tibetan Detachment System of the Cenozoic Himalayan orogen but exhibits two important differences. First, while the South Tibetan system developed in the down-going Indian plate during the India–Eurasia collision, the Fjord Region system developed in the overriding Laurentian plate during the collision of Laurentia with Baltica. Second, whereas the exposed extensional structures in the Himalayas developed in the upper crust and are only inferred to have extended to deeper levels, those in the Forsblad Fjord area were demonstrably active at middle-crustal levels. Evidence for broadly coeval extension and contraction at differen t structural levels in both mountain belts emphasizes the general importance of crustal decoupling in the collisional orogenic process, and implies that synorogenic extensional deformation is not strictly an upper crustal phenomenon.

Sm–Nd isotope systematics in garnet from different lithologies (Eastern Alps): age results, and an evaluation of potential problems for garnet Sm–Nd chronometry

Interpretation of Sm–Nd garnet ages is frequently impaired by one of the following restrictions: (a) high-LREE inclusions, (b) isotopic disequilibrium, and (c) the uncertainty about the closure temperature. These issues are addressed by way of an evaluation of garnet Sm–Nd data from different rock types of the Austroalpine basement units, Eastern Alps, including metabasic eclogites, mica schist and paragneiss, metapegmatite and metagranite. Nd concentration in handpicked garnet varies between 0.021 and 23.1 ppm in metabasites, 0.49 and 17.4 ppm in metapelites and between 0.024 and 4.6 ppm in metapegmatites and metagranites. The overall range of 147Sm 144Nd is 0.15–2.5 in garnet from metabasites, 0.12–3.03 in metapelite garnet and 0.66–7.21 in Mn-rich garnet from metapegmatites and metagranites. A clear negative correlation between Nd concentration and SmNd is observed in garnets from all these lithologies. Therefrom, it is concluded that even optically “clean” garnet separates may contain high-LREE microinclusions, such as epidote-allanite, zoisite, apatite, sphene, monazite or zircon. However, very low Nd concentrations correlated with low SmNd as well as high Nd concentrations (>5 ppm) correlated with fairly high Sm–Nd ratios (0.8) have also been observed. Apart from replicate analyses within as well as between samples with a common PT-history, leaching experiments are a useful technique to elucidate any distorting influence of unequilibrated inclusions on the garnet age, especially if the observed Sm–Nd ratio is low (<0.5). Leaching of garnet separates with HCl (2.5, 5.8 M) produces no obvious element fractionation, but may improve SmNd, and hence age precision, considerably. Isotopic disequilibrium between garnet and other matrix minerals is observed preferentially in basic eclogites, derived from gabbroic precursors. Sm–Nd garnet analysis allows the recognition of several distinct garnet-forming events in the Eastern Alps. (a) A Variscan high-P event is documented in metabasites from the northern–central Ötztal basement around 360–350 Ma, whereas garnet from sillimanite-bearing gneisses dates the Variscan thermal peak in the western part of the same subunit around 345–330 Ma. (b) A long-lived, Permian to Triassic event (285–225 Ma), correlated with crustal extension and low-P metamorphism, is documented by spessartine-rich garnet from metapegmatites as well as almandine-rich garnet cores from mica schist. (c) Age data of garnet from eo-Alpine (Cretaceous) deeply subducted rocks of the southern/eastern Austroalpine units are related to near-peak PT, eclogite- to amphibolite-facies metamorphic conditions (peak: 2 GPa/685 °C), and/or incipient isothermal decompression, due to fast, tectonically driven exhumation (110/100–85 Ma). At cooling rates of 20–30 °C/Ma (exhumation rates: 3–5 km/Ma), the Sm–Nd closure temperature ( T c) for mm-sized garnet in these rocks is estimated at 650–680 °C.

Structural geology, single zircon ages and fluid inclusion studies of the Meatiq metamorphic core complex: Implications for Neoproterozoic tectonics in the Eastern Desert of Egypt

The Meatiq metamorphic core complex (MMCC) formed during the Precambrian as a result of multiple deformation and metamorphism in the Eastern Desert of Egypt. Structural, geochronologic, and fluid inclusion microthermometric analyses reveal a long formation/deformation history for the MMCC. This started with the break-up of Rodinia at ca. 800 Ma and continued until Pan-African collision at ca. 580 Ma. Between 800–780 Ma, rifting continued into sea floor spreading and oceanic crust formation. Synchronously, the Um Ba′anib granite intruded into an approximately 1.14 Ga old crust comprising migmatic amphibolites. Rifting was accompanied by the deposition of quartz- and mica-rich sediments. Between 660 Ma and 620 Ma, convergence between East and West Gondwanaland caused burial of sediments to a crustal depth of approximately 20 km and intrusion of calc-alkaline rocks. Subsequently, the meta-sediments were thrust across the Um Ba′anib granitoid. Deformation of both rock units took place under amphibolite-facies metamorphic conditions. Fluid inclusions with moderate density provide evidence for the retrograde stage of this metamorphic event. Continued oblique convergence between East and West Gondwanaland resulted in a transpressional regime with displacement partition. While nappe stacking continued in foreland domains, the MMCC was exhumed to a depth of 12–15 km in hinterland domains. Extension-related granitoids were emplaced between 620 and 580 Ma. Microthermometric analyses of fluid inclusions suggest a crustal depth of approximately 10–12 km for the transpressional event. Rapid exhumation was accompanied by detachment of the cover nappes and emplacement of syn-tectonic intrusions, which caused local contact metamorphism. Low-density fluid inclusions document high-T, low-P conditions for the contact metamorphism. The final stage of exhumation took place under brittle/ductile conditions at a crustal depth of approximately 3–6 km indicated by water-rich fluid inclusions. The age of this event is constrained by the intrusion of the late- to post-tectonic Arieki adamellite at approximately 580 Ma.

Ammonium partitioning and nitrogen-isotope fractionation among coexisting micas during high-temperature fluid-rock interactions: examples from the New England Appalachians

Despite recent advances in the field of N-isotope geochemistry, our understanding of the behavior of this element in the solid earth remains limited by a lack of fundamental information regarding the partitioning of ammonium and isotopic fractionation of N among coexisting mineral and fluid phases. Study of N behavior in regionally metamorphosed rocks provides the opportunity to assess intermineral NH 4 + partitioning and N-isotope fractionation among coexisting micas during metamorphism and affords an application of the N system as a tracer of high-T fluid-rock interactions. Analyzed mica samples range in δ 15N air from +3.3 to +11.9‰, and contain 9 to 1820 ppm N. The outcrop at Townshend Dam, Vermont, allows examination of N behavior across-strike on a relatively small scale, and samples from western Maine demonstrate the effect of varying metamorphic conditions on N behavior in metapelites. Δ 15N bt-w.mica(δ 15N biotite-δ 15N white-mica) ranges from −0.9 to +2.7‰ (for all samples from both suites, mean = +0.36‰, with 1σ = 0.79‰), with samples containing a separate paragonite white-mica phase showing the greatest range (−0.12 to +1.02‰; mean = 0.58‰, 1σ = 1.03‰). Thirteen samples containing only Na-poor muscovite (six from Townshend Dam, seven from Western Maine) have mean Δ 15N bt-w.mica of 0.07‰ (1σ = 0.41‰). In both suites, biotite nearly always contains more N than coexisting white mica, but N w.mica/N bt also shows some significant scatter (mean N w.mica/N bt = 0.46, with 1σ = 0.34). The thirteen samples, containing only a Na-poor, muscovitic white-mica phase, have mean N w.mica/N bt = 0.39 with 1σ = 0.26, similar to that reported by others for other metamorphic suites containing only muscovite as the white-mica phase. There is no obvious suggestion of equilibrium N-isotopic fractionation among coexisting micas at epidote-amphibolite to amphibolite-facies metamorphic conditions, although NH 4 + appears to partition systematically among coexisting biotite and white mica. Significant scatter in both Δ 15N bt-w.mica and NH 4 + partitioning (conceivably the result of differential closure to exchange during cooling or of retrograde replacements) could, however, obscure observation of small equilibrium intermica fractionations related to the characteristics of the interlayer sites in which NH 4 + resides. Samples most unlike the mean in both Δ 15N bt-w.mica and N w.mica/N bt contain abundant chlorite, some of which is likely retrograde, based on petrographic observations [chl/(chl+bt) >0.3]. Thus, retrograde replacement of biotite by chlorite may have been accompanied by fluid-mineral N-isotope exchange, perhaps involving the production of fine-grained, retrograde N-bearing white mica observed petrographically for some samples. It is also possible that sampling at scales greater than those of N-isotope equilibrium domains (e.g., across fine interlayers) results in some scatter because of varying relative modal proportions of the two micas in adjacent fine interlayers. Although further investigation of the extent of retrograde reequilibration of mica N systematics is warranted, the observed lack of systematic N-isotope fractionation among coexisting micas, and the reasonably systematic NH 4 + partitioning data for these phases provide important preliminary constraints for attempts to model N-isotope behavior in fluid-rock systems. These results, and other attempts to calibrate N-isotope fractionation through field studies, point to the conspicuous lack of experimentally determined mica-fluid N-isotope fractionation factors.

The influence of country rock structural architecture during pluton emplacement: the Loch Loyal syenites, Scotland

The late Caledonian Loch Loyal syenites comprise three related, but geographically separate, composite intrusions: the Ben Loyal, Ben Stumanadh and Cnoc nan Cuilean bodies. They were emplaced (c. 426 Ma) into Moine and Lewisian rocks of the Moine and Naver thrust sheets previously deformed by Caledonian NW-directed folding and thrusting at amphibolite facies metamorphic conditions. The intrusions lie mainly within a zone of large-scale cross-folds in which the regional foliation trends NW-SE at high angles to regional orogenic strike (NNE-SSW). The cross folds are underlain by a broad, ESE-dipping zone of high strain known as the Ben Blandy shear zone. The NW basal contact of the Ben Loyal body dips generally SE and strikes sub-parallel to the foliation in the underlying Ben Blandy shear zone. The higher parts of this syenite, together with those of the Ben Stumanadh and Cnoc nan Cuilean intrusions, form a series of variably coalesced dykes trending NW-SE sub-parallel to the foliation in the pre-existing zone of cross-folds. There is little evidence for significant country-rock deformation during emplacement of the syenites. Minor components of top-to-the-SE extension occurred along the NW boundary of the Ben Loyal body, whilst minor dextral shearing is associated with the emplacement of the NW-SE, vertical sheet-like units that are thought to form the SE part of this intrusion and the other Loch Loyal syenites. Generally contact-parallel magma tic-state deformation fabrics are dominant in most of the intrusions and are thought to reflect limited magma pressure-driven, internal deformation of the igneous bodies during and immediately after emplacement. The geometry of these fabrics and the Loch Loyal syenites as a whole has been directly controlled by the pre-existing structural architecture of the surrounding country rocks. This appears to be a previously overlooked feature of many plutons emplaced at mid- to upper crustal depths below the region influenced by the free surface. The siting and emplacement of the Loch Loyal syenites may have been initially facilitated by late Caledonian gravity-driven extension of the Moine Thrust sheets. Following the initial phase of intrusion, it is also possible that the presence of magma actually promoted further nappe collapse in the adjacent country rocks. There is no compelling reason to link the upper crustal extension observed during emplacement to the genesis of the alkaline magmas in the underlying mantle. The siting and ascent of the syenites at depth may be linked to the presence of steeply-dipping structures in the Lewisian autochthon buried beneath the Moine Nappe.

Karakoram fault zone rocks cool in two phases

The Karakoram fault zone of Ladakh, India, is a major strike-slip boundary along which the tectonic evolution of Tibet has been accommodated. In this paper 40 Ar/ 39 Ar isotopic age data are integrated with structural and metamorphic data to infer an exhumation history for granites and low- to intermediate-grade metamorphic rocks exposed adjacent to two strands of the fault zone. Near Tangtse, leucogranites which crystallized c . 17 Ma ago are cut by the Karakoram fault zone, indicating that fault movement was initiated subsequent to 17 Ma. The 40 Ar/ 39 Ar data give temperature-time histories which indicate that the &lt;17 Ma metamorphic rocks within the fault zone were exhumed differentially relative to the adjacent (and already cooled to &lt;150°C) Ladakh Batholith, while cooling progressively through amphibolite to below greenschist facies temperatures. Two phases of rapid cooling of Karakoram fault zone rocks are evident, one at c . 17 Ma to c . 13 Ma, and another, following an intervening period of relatively slow cooling, starting at c . 8 Ma and continuing until at least 7 Ma. Uplift since c . 17 Ma via dextral oblique thrusting has resulted in exhumation from amphibolite facies metamorphic conditions, possibly with a larger thrust component from 17 to 13 Ma, followed by a change to dominant strike-slip motion at about 13 Ma. Rapid cooling starting at c . 8 Ma at greenschist facies temperatures is probably the result of renewed oblique thrusting. It is notable that the two episodes of rapid cooling recorded in Karakoram fault zone rocks coincide with episodes of exhumation in the Pakistani Karakoram and also in southern Tibet.

Origin of pseudotachylite in the lower Witwatersrand Supergroup, Vredefort Dome (South Africa): constraints from metamorphic studies

Small, sub-millimetre to centimetre thick, pseudotachylite veins in metapelitic rocks of the lower Witwatersrand Supergroup in the collar of the Vredefort Dome are recrystallized to a biotite±cordierite paragenesis, indicating the attainment of lower amphibolite facies metamorphic conditions ( T ≥ 500°C) following their formation. The veins occur along ubiquitous anastomosing fracture networks displaying millimetre- to centimetre-scale displacements and truncate, and incorporate fragments of, an older, ca. 2.05 Ga, mid-amphibolite facies, peak metamorphic assemblage. The post-pseudotachylite metamorphism is attributed to the same event that caused highly variable annealing of ca. 2.02 Ga shock microdeformation features in rocks across the dome. Together with the disproportionately large volume of pseudotachylite in the rocks relative to that found in tectonic provinces, and the absence of evidence of genetically related large-scale shear or fault zones, this metamorphic evidence suggests an origin for the pseudotachylite related to the 2.02 Ga Vredefort bolide impact event that produced the dome. Possible origins for the pseudotachylite include shock brecciation and melting, post-shock decompression melting, and friction melting associated with the post-impact high-velocity excavation and modification phases of crater formation. The post-pseudotachylite metamorphism is attributed to the combined effects of an elevated pre-impact crustal geotherm (∼25°C/km) related to the waning stages of the 2.05 Ga peak metamorphism, release of elastic strain energy from the impact shock event and contact effects beneath the impact melt body.

Thick‐skinned versus thin‐skinned thrusting: Rheology controlled thrust propagation in the Variscan collisional belt (The southeastern Bohemian Massif, Czech Republic ‐ Austria)

The Variscan nappe assembly within the southeastern Bohemian Massif includes (1) crystalline nappes which were transported from initial granulite to amphibolite facies conditions to uppermost crustal levels, (2) nappe assembly of footwall units under very low to low‐grade metamorphic conditions, and (3) frontal, thin‐skinned imbrication of flysch sequences which were deposited within a deep and narrow foreland basin during loading of the lower plate. Thick‐skinned thrusting of hinterland tectonic units resulted in an inverted metamorphic zonation and contrasts with the thin‐skinned tectonic character of foreland units. 40Ar/39Ar hornblende and muscovite plateau ages from hanging wall units reflect complete rejuvenation of intracrystalline argon isotopic systems during Late Variscan thrusting (between ∼ 340 Ma and 325 Ma). The 40Ar/39Ar mineral ages together with previously published 40Ar/39Ar and Sm/Nd data record systematically decreasing Variscan mineral ages structurally downward into footwall units. This is explained by rapid exhumation and cooling synchronously with thrusting along crustal‐scale ramps during foreland‐directed nappe propagation. The location of decollement zones within specific structural levels was apparently controlled by crustal‐scale rheological con‐trasts. Late Variscan cooling ages contrast with Cadomian 40Ar/39Ar hornblende and muscovite ages recorded within the structural basement immediately below the major Variscan thrust plane where intracrystalline isotopic systems were only partially rejuvenated during Variscan thrusting. These record a sequence of Cadomian magmatic and metamorphic events in foreland units including (1) pre intrusive metamorphism and deformation under amphibolite facies metamorphic conditions (&gt;610 Ma), (2) greisen formation during a Cadomian collisional event (∼ 600 Ma), and (3) intrusion of calcalkaline granitoid bodies with I‐type affinities (∼ 590 Ma).

Evolution of deep-crustal normal faults: constraints from thermobarometry in the Grenville Orogen, Ontario, Canada

The Robertson Lake shear zone (RLSZ) is a low-angle (∼30° dip), plastic to brittle extensional shear zone located in the southeastern Grenville Orogen, Ontario, Canada. Thermobarometric data have been collected to evaluate the effects of extension on regional metamorphic field gradients. Upper greenschist facies metamorphic conditions are preserved in the footwall and upper amphibolite facies metamorphic conditions in the hanging wall. However, steep gradients in metamorphic pressure are present adjacent to the Robertson Lake shear zone. In the footwall pressures increase from 550 MPa to 800 MPa within 8 km of the RLSZ and in the hanging wall pressures decrease from 900 MPa to 500 MPa within 12 km of the RLSZ. The pressure gradients are interpreted to be a product of isostatic flexural rotations in response to extension along the zone. The degree of rotation indicated by the baric gradients in the hanging wall and footwall are compatible and require an initial shear zone orientation of 60–90°, which suggests reactivation of an initially high-angle terrane boundary. Previous studies provide constraints of a depth extent of 40 km for the RLSZ and crustal thickness of 60 km during extension. Thus, extensional rotations were accommodated by decoupling and regional-scale flow in the lower third of the crust.

Isotopic constraints on Pan-African metamorphism in Dronning Maud Land, Antarctica

Garnet SmNd isotopic data are presented for selected samples from the metamorphic terrane of the H.U. Sverdrupfjella, western Dronning Maud Land, and yield Pan-African ages ranging from 680 to 415 Ma. Reproducibility of the garnet data is outside analytical error, but this does not affect the consistency of ages obtained. The garnet ages are considerably younger than the inferred initial crystallization age, based on petrographical and fabric analysis, structural, and whole-rock and zircon isotopic data which support initial growth during the Grenvillian orogeny (∼ 1200-1100 Ma). The garnet ages, however, are indistinguishable from a large number of RbSr mineral ages. The possibility that the garnets remained above the blocking temperature until Pan-African exhumation is discounted. Polymetamorphism of the metamorphic terrane associated with both the Grenbillian and Pan-African orogenes is favoured. The garnet ages are interpreted as reset ages, but the possibility that the garnets crystallized during the Pan-African event cannot be discounted. In this case, the data imply that the Pan-African orogeny attained lower granulite- to amphibolite-facies metamorphic conditions, as indicated by geothermometry and geobarometry calculations involving garnet. In either case, the H.U. Sverdupfjella area was subjected to significant Pan-African heating to temperatures of the order of 600–650°C.

Retrograde fluid evolution at the Rampura Agucha Pb-Zn-(Ag) deposit, Rajasthan, India

Fluid inclusions in mineralized graphite-sillimanite-mica schist from the Rampura-Agucha Pb-Zn-(Ag) deposit, Rajasthan, northwest India, have been investigated by microthermometry and Raman microspectrometry. Three different main types of fluid inclusions in quartz can be distinguished: (1) gaseous (CO2, partially mixed with CH4-N2), (2) low salinity aqueous inclusions (0–8 eq. wt% NaCl) and (3) high salinity aqueous inclusions (NaCl ± MgCl2-CaCl2). Low density CO2-rich and low salinity H2O inclusions are contemporaneous and occur, together with CH4-N2 inclusions, in close association with sulfide mineral inclusions. This indicates immiscibility between the gaseous and aqueous phase and participation of these fluids during the deposition or remobilization of the ore, which occurred over a wide P (1220 to 200 bar) and T (450 to 250 °C). Raman spectra of graphite indicate upper greenschist-facies metamorphic conditions, although host rocks have been metamorphosed at upper amphibolite-facies metamorphic conditions. This indicates that graphite re-equilibrated with the CO2-rich phase during retrograde metamorphism.

Anatomy of the Mondoñedo Nappe basal shear zone (NW Spain)

This paper analyses a major shear zone from the Iberian Hercynian belt which forms the basal thrust of the Mondoñedo Nappe. The shear zone developed by ductile deformation under amphibolite facies metamorphic conditions and later by brittle-ductile deformation in greenschists facies. Folds in the shear zone are asymmetric, very tight, 1C or similar class and frequently developing sheath geometries. The sheath folds originated by non-coaxial flow superimposed on earlier irregularities. The fabric of quartzitic rocks in the shear zone changes from bottom to top from ultramylonites through blastomylonitic rocks to non-mylonitic tectonites. c-axis fabrics vary across the shear zone, but show a dominant monoclinic symmetry. The blastomylonitic rocks include the fabrics representing the highest temperatures. The main foliation of the schists results from flattening of an earlier foliation, recording occasional microfolds. The use of different kinematic criteria has allowed an analysis of their validity as well as an assessment of movement direction towards the foreland of the orogen.

The Scotia–Quaal metamorphic belt: a distinct assemblage with pre- early Late Cretaceous deformational and metamorphic history, Coast Plutonic Complex, British Columbia

The Scotia–Quaal metamorphic belt extends from Hawkesbury Island to Work Channel between the early Late Cretaceous Ecstall and the Paleogene Quottoon plutons. The belt consists of a Proterozoic?–Paleozoic metasedimentary and metavolcanic sequence, the Middle Devonian Big Falls orthogneiss, Early Jurassic orthogneiss, and Jurassic or Cretaceous mafic and ultramafic intrusive rocks. The assemblage may be correlative with Nisling terrane lithologies and may have shared a common history, at least from Early Jurassic time on, with rocks of Stikine terrane.Strong planar and linear fabrics, abundant folds, and scarcity of kinematic indicators characterize the belt's deformational style. Development of a strong foliation followed by three episodes of folding occurred between emplacement of the Middle Devonian Big Falls orthogneiss and early Late Cretaceous Ecstall intrusion. Paleogene fabrics occur in the Quottoon pluton and in the easternmost 1.5 km of the Scotia-Quaal belt. If a major Paleogene shear zone is postulated to explain the disparity in cooling dates, metamorphic histories, and structural styles between the western and central Coast Plutonic Complex, then it does not traverse the Scotia–Quaal metamorphic belt, but must be located within or at the western edge of the Quottoon pluton.Medium-pressure, epidote–amphibolite to upper amphibolite facies metamorphic conditions are preserved in the central region. Metamorphic grade increases gradually across the belt from west to east and from south to north. Regional metamorphism outlasted regional deformation. Contact metamorphism associated with Quottoon and Ecstall intrusion is apparent only in rocks of the southern region where regional metamorphic grades are lowest.

Stretching of the deep crust at the slow-spreading Southwest Indian Ridge

The 500 metres of gabbro drilled in the Southwest Indian Ocean during ODP Leg 118 allow direct observation of early, high-temperature extensional deformation in the deep crust of a slow-spreading rift valley. About 30% of the recovered gabbros have been plastically stretched at progressively decreasing temperatures (granulite- to lower amphibolite-facies metamorphic conditions), increasing water/rock ratio, and probably increasing deviatoric stresses. The early solid-state strain in granulite facies metamorphic conditions produced low-dipping foliations and low strain gradients. At this stage, mylonitic shear zones only formed in oxide-rich gabbros. Strain localization in the deepest and hottest part of the oceanic crust thus appears to have been influenced by the magmatic stratigraphy. In upper to lower amphibolite-facies metamorphic conditions, the ductile strength of the gabbro was probably higher, and deformation concentrated in mylonitic shear bands, which may have provided access to seawater-derived fluids. Strain localization at this stage was accompanied by brittle failure and boudinage, in the mylonitic foliation, of individual porphyroclasts, or of thick gabbro intervals. The influence of such brittle failure on hydrothermal circulations was critical: it allowed for extensive crust/seawater interactions at high temperatures (upper amphibolite facies conditions). In a spreading context where continuous magma supply provides heat and new crustal material to the axial region, the mechanical behaviour of the ridge axis lithosphere should be controlled by continuous ductile deformation in granulite-facies conditions. This may lead to the steady-state “necking” configuration described by Tapponnier and Francheteau (1978). Alternatively, a discontinuity of the magma supply, allowing the lower crust of the axial region to cool below granulite-facies temperatures, should cause mylonitic shear zones characteristic of amphibolite-facies deformation to extend deeper into the axial domain, eventually leading to the detachment fault configuration proposed by Karson (in press). As magma supply to slow-spreading oceanic ridges is likely to be discontinuous, necking and faulting periods should alternate through time.

Strain transfer at continental scale from a transcurrent shear zone to a transpressional fold belt: The Patos-Seridó system, northeastern Brazil

During the Brasiliano-pan-African orogeny, a complex continental-scale pattern of east-west transcurrent shear zones and northeast-trending fold belts formed in the northern and central Borborema province of northeastern Brazil. The east-west shear zones have been usually regarded as slightly younger features, but the study of the most spectacular case of intersection between these two structures, the Patos shear zone and the Serido transpressional belt, leads to a different tectonic model. Satellite imagery and structural, petro-logical, and geophysical data support the interpretation that these structures (1) are in structural continuity and (2) formed simultaneously under amphibolite facies metamorphic conditions that led to partial melting. This suggests a model of strain transfer at the scale of the orogen: at the eastern end of the Patos east-west dextral shear zone, the strain that accommodated the relative motion of the northern block was transferred to the northeast-trending Serido belt, where it resulted in folding, strike-slip faulting, and stretching parallel to the strike of the belt.

Age of amphibolitic metamorphism in the ophiolitic unit of the Morais allochthon (Portugal): implications for early Hercynian orogenesis in the Iberian Massif

Structurally lower portions of the allochthonous Morais complex are comprised of an ophiolitic unit exposed in two contrasting structural units separated by a ductile thrust fault. The lower unit is dominated by variably foliated amphibolite whereas the upper unit is largely comprised of metaperidotite and metagabbro. Most of the ophiolitic unit has recrystallized synkinematically under amphibolite facies metamorphic conditions ( c. 500°C and 5 kbar). Amphiboles formed during this metamorphism are typically magnesio-hornblende. An amphibole concentrate from the lower structural unit records a 39 Ar/ 40 Ar plateau age of 397.3 ± 4 Ma. An amphibole concentrate from the upper structural unit displays an internally discordant 39 Ar/ 40 Ar age spectrum corresponding to a total gas age of 431.1 ± 27.3 Ma. Gas fractions evolved from each amphibole concentrate at intermediate and high experimental temperatures are characterized by similar apparent K/Ca ratios and yield well-defined 36 Ar/ 40 Ar v. 39 Ar/ 40 Ar isotope correlations which result in similar ages of 384.2 ± 5.2 and 392.4 ± 7.3 Ma. These are considered to be more reliable than the plateau ages and are interpreted as dating the last cooling through those temperatures required for intracrystalline retention of argon within constituent amphibole grains. The associated tectonothermal event represented a first stage of Hercynian orogenesis in the Iberian massif. It was probably related to oceanic closure and associated obduction of granulitic/eclogitic massifs which at present structurally overlie the ophiolitic unit.