M2 D2 Research Articles

Geochronological constraints on the trans-Hudsonian tectono-metamorphic evolution of the pre-Athabasca basement within the Wollaston-Mudjatik Transition Zone, Saskatchewan

The Hudsonian Pressure-Temperature-Deformation-time-(P-T-D-t) evolution of the pre-Athabasca crystalline basement (>∼1700Ma) of the Wollaston-Mudjatik Transition Zone (WMTZ) highlights two major tectono-metamorphic events M1-D1 and M2-D2. The ages of these two event have been obtained by in-situ LA-ICPMS U-Th-Pb dating of monazite from Grt-Crd bearing pelitic gneiss and U-Pb analyses performed on zircons from pegmatites, using both exposed basement and drill cores from the Wolly–McClean exploration drilling project. The M1-D1 event, interpreted as the burial of the thinned Hearne margin via southward thrusting to pressures varying from 10 to 6kbar, occurred between ca.1840 and 1813Ma. The M2-D2 event, producing the northeast structural trend of the WMTZ, was formed in a sinistral transpressional tectonic regime during the late stage of the Hudsonian oblique collision between ca. 1813 and 1770Ma. Thermobarometric estimates on the M2–D2 assemblages show that the entire studied area was reequilibrated at about 5kbar and 750–825°C. trans-Hudsonian pegmatites are viewed as the main proto-ore of the uranium-rich Athabasca unconformity-type deposits. Formation, transfer and differentiation of these trans-Hudsonian pegmatites are replaced in this P-T-D-t evolution. During the 1840–1813MaM1-D1 partial melting event, the first batches of melt produced in the deep crust are the most likely enriched in uranium. Then, these melts were transferred upwards to the upper crustal levels owing to the development of crustal scale steeply-dipping D2 shear zones, and finally were differentiated to form uranium-enriched pegmatites between 1813 and 1770Ma. Some monazite and zircon grains within retrogressed migmatites recorded a later event at ca. 1720Ma, interpreted as the terminal cooling event down to 300–400°C responsible for partial retrogression of metamorphic assemblages. This age provides new insights into the timing of the onset of the Athabasca sedimentation that should therefore be at least 1710Ma old or younger.

Tectono-metamorphic evolution of the pre-Athabasca basement within the Wollaston–Mudjatik Transition Zone, Saskatchewan

The Paleoproterozoic tectono-metamorphic evolution of the pre-Athabasca basement (∼1.7 Ga) within the Wollaston–Mudjatik Transition Zone (WMTZ) (Saskatchewan, Canada) has been characterized using both exposed basement and drill cores from the Wolly–McClean exploration drilling project. The finite ductile strain pattern of the WMTZ results from the superposition of two tectono-metamorphic events M1–D1 and M2–D2. M1–D1 is associated with the development of a gently dipping foliation striking N90°–N100° and a southward decrease in peak pressures from up to 10 kbar (1 kbar = 100 MPa) in the Cochrane River area down to 6 kbar in the Wolly–McClean exploration drilling project. The M2–D2 event is responsible for the main northeasterly trend of the WMTZ that developed in a sinistral transpressional tectonic regime during the final oblique collision of the Trans-Hudson Orogeny. Thermobarometric estimations on M2–D2 assemblages show that the studied area was reequilibrated at about 4–5 kbar and 750–825 °C. The basement has thus been affected by a differential isothermal decompression event between D1 and D2 that allowed the juxtaposition of the deepest northeastern domains and the Wolly–McClean exploration drilling project, at the same structural level. These results suggest that the basement exposed to the northeast of the Athabasca Basin is not an analog of the basement located beneath the eastern Athabasca Basin where uranium-enriched granitic pegmatites and granites are known. We also suggest that uranium-enriched melts produced during the early M1–D1 stage of partial melting in the deep crust were transferred to the midcrust, owing to D2 shear zones, where they have differentiated to produce uranium-bearing pegmatites.

Petrotectonic framework of granitoids and associated granulites at Nagavalli Shear Zone (NSZ), Eastern Ghats Belt: Evidence of a late transpression orogeny

Megacrystic granitoids associated with migmatitic and metasedimentary gneisses occurring around Nagavalli Shear Zone (NSZ) preserve complex metamorphic and deformation history. Thinly laminated discontinuous banding of quartzofeldspathic layer (S1) in the migmatites is the product of first incipient melting during prograde M1–D1 tectonothermal event. Peak M2–D2 event is manifested by the development of S2 gneissic foliation in all rocks, which is axial planar to rootless folds on S1. Porphyroblastic garnet mantled by leucosomal melt fraction in granitoids, suggest that the rock suffered peak granulite facies metamorphism along with host migmatitic gneisses. The subsequent D3 event deforms differently the massive granitoids and the migmatitic granulite gneisses. The D4 deformation acted as transpression with broad northwest–southeast compression that develops strong discontinuous regional-scale anastomosing shear zones transecting the earlier gneissosity (S2) in the granitoids with prominent sinistral shear sense. It deforms the axial plane of regional folds in migmatites and develops superposed non-plane non-cylindrical folds in outcrop to regional scale. Thus we infer megacrystic granitoids were possibly emplaced in pre- to syn-peak metamorphic event within the host granulites. Granitoids and associated migmatitic gneisses of Late Meso- to Neoproterozoic age suffered subsequent petrotectonic events followed by a sinistral transpression acted along NSZ.

The tectonometamorphic evolution of Southampton Island, Nunavut: Insight from petrologic modeling and in situ SHRIMP geochronology of multiple episodes of monazite growth

Petrologic modeling and in situ SHRIMP monazite geochronology reveal a complex, polycyclic tectonometamorphic history for Southampton Island, Nunavut, that provides further insights into the assembly of the Nuna supercontinent. Although early, cryptic metamorphic events at ca. 2.6–2.5 and 2.3Ga are indicated by monazite ages in an Archean metasedimentary sample, Paleoproterozoic tectonometamorphism is widespread and most profound. Monazite inclusions in garnet from two samples define a lower amphibolite- to upper amphibolite-facies M1–D1 event between 1883±9 and 1879±7Ma. These sampled rocks, as well as pre-1900Ma magmatic rocks, occur on the west side of the island's exposed Precambrian bedrock, outboard of the Hearne craton but on strike with its geophysically defined eastern edge beneath Hudson Bay. This timing and geographic distribution, as well as the orientation of S1 foliations, are consistent with the model of a ca. 1900Ma Rae–Hearne collision, with M1–D1 on Southampton Island reflecting reworking of the Rae upper plate following northeast-directed subduction.Textural variations among monazite dated in two samples tightly bracket the initiation of M2–D2 between 1868±5Ma and 1861±4Ma. M2–D2 monazite crystallized at 1851±5Ma in one sample with a more calcic composition interpreted to have shifted the allanite to monazite reaction to higher temperatures relative to less calcic rocks. Clockwise P–T paths in these recumbently folded high-grade gneisses support a collisional setting, with the timing of M2–D2 consistent with age constraints on Baffin Island for the accretion of Meta Incognita microcontinent to the southeast flank of the Rae craton. The Rae crust of Southampton Island is interpreted as the upper plate during this collision on the basis of the ∼10Myr earlier deformation on Southampton Island compared to the interpreted lower plate (Meta Incognita microcontinent), as well as textural observations in some samples that M2 metamorphism predated D2 deformation. Advective heat transfer would account for the relatively short time inferred between ca. 1870Ma collision and ca. 1868–1861Ma monazite growth during prograde metamorphism.Equant matrix monazite that overgrew the S2 foliation in one sample documents post-D2 metamorphism in the northern part of exposed basement at 1841±4Ma. Four samples from the south record post-D2 monazite growth between 1826±9 and 1815±7Ma. These ages highlight a thermal culmination associated with extensive crustal melting and ca. 1830–1820Ma plutonism attributed to a convergent setting culminating with ca. 1820Ma arrival of the Superior craton. Exhumation and cooling is dated by ca. 1790–1780Ma monazite associated with garnet breakdown.

Petrotectonic framework of granulites from northern part of Chilka Lake area, Eastern Ghats Belt, India: Compressional vis-à-vis transpressional tectonics

Granulite-facies rocks occurring north-east of the Chilka Lake anothosite (Balugan Massif) show a complex metamorphic and deformation history. The M1–D1 stage is identified only through microscopic study by the presence of S1 internal foliation shown by the M1 assemblage sillimanite–quartz–plagioclase–biotite within garnet porphyroblasts of the aluminous granulites and this fabric is obliterated in outcrop to map-scale by subsequent deformations. S2 fabric was developed at peak metamorphic condition (M2–D2) and is shown by gneissic banding present in all lithological units. S3 fabric was developed due to D3 deformation and it is tectonically transposed parallel to S2 regionally except at the hinge zone of the F3 folds. The transposed S2/S3 fabric is the regional characteristic structure of the area. The D4 event produced open upright F4 folds, but was weak enough to develop any penetrative foliation in the rocks except few spaced cleavages that developed in the quartzite/garnet–sillimanite gneiss. Petrological data suggest that the M4–D4 stage actually witnessed reactivation of the lower crust by late distinct tectonothermal event. Presence of transposed S2/S3 fabric within the anorthosite arguably suggests that the pluton was emplaced before or during the M3–D3 event. Field-based large-scale structural analyses and microfabric analyses of the granulites reveal that this terrain has been evolved through superposed folding events with two broadly perpendicular compression directions without any conclusive evidence for transpressional tectonics as argued by earlier workers. Tectonothermal history of these granulites spanning in Neoproterozoic time period is dominated by compressional tectonics with associated metamorphism at deep crust.

MICROSTRUCTURAL EFFECTS IN THE STEEL HWS 1.2379 APPLYING THE PROCESS OF PLASMA TRANSFERRED ARC PTA.

ABSTRACTAt present, the industrial sector requires the recovery of tool steel components subjected to severe wear which are built with steel HWS these have the same properties in all tribology directions and have great success in building tooling. In this paper, we recover the previously HWS steel machining with a fillet of 2mm caused by the metallurgical recovery of the component, applying the process of plasma transferred arc PTA which has very good fats dilution properties in comparison to other conventional processes currently applied for the recovery of tooling, such as GTAW, SMAW, SAW. The experiment was conducted with input D2 and M2 with one and two welding seams, making the assessment metallography, hardness and wear tests pin-on-disk. Finding a decrease in the hardness of the interface line, according to the evaluation of wear pin-on-disk, had a higher wear rate of the filler metal in the M2 D2.

Metamorphic and tectonic evolution at 31°36′S across a deep crustal zone from the Sierra Chica of Córdoba, Sierras Pampeanas, Argentina

A geological section at 31°36′S cut the main structures of a representative area of the deep crust in the Sierra Chica (Sierras Pampeanas of Córdoba, Argentina), where associations of high-grade gneisses, migmatites, marbles, amphibolites and ultramafic rocks are recognized. A group of ultramafic bodies composed of serpentinized spinel harzburgites are associated with a basement affected by a M2–D2 regional metamorphism that developed the main S2 metamorphic foliation. Applying different thermobarometers to the varieties of garnet sillimanite gneisses and leucocratic garnet gneisses, a range of P–T conditions of 768–840 °C and 7.3–8.6 kb were obtained for M2. A strong D3 ductile deformation structured the ultramafic bodies, more viscous than the associated metamorphic rocks, generating large west-vergent sheath folds with tubular geometry. This constrictional deformation was developed in a simple shear regime, under high-temperature conditions, with a tectonic transport direction toward the northwest (∼N 300°). Later, decompression and cooling down to amphibolite facies conditions occurred in all the rocks of the area during a retrograde M3 event. A range of P–T conditions of 588–710 °C and 5.2–7.3 kb were calculated for M3 in the gneisses of the area. The Sierra Chica ultramafic rocks outcrop as folded bodies, isolated between them and locally parallel to the S2 metamorphic foliation, therefore they cannot be correlated as a regional belt that could be interpreted as an ancient suture. Our hypothesis is that the ultramafic bodies are upper mantle relics of earlier emplacements into an accretionary prism that was entirely reworked during the Pampean orogeny. This same outcrop-pattern can also be recognized in the other ultramafic bodies in the Sierras de Córdoba. Hence, we suggest to avoid the use of the terms Eastern and Western Belts for the treatment of the ultramafic rocks in the Sierras de Córdoba. Una sección geológica a los 31°36’ de latitud sur atraviesa perpendicularmente las principales estructuras de un sector representativo de la corteza profunda en la Sierra Chica (Sierras Pampeanas de Córdoba, Argentina), donde se reconocen asociaciones de gneises, migmatitas, mármoles, anfibolitas y rocas ultramáficas. Un grupo de cuerpos ultramáficos compuestos por harzburgitas espinélicas serpentinizadas se encuentra asociado a un basamento afectado por un metamorfismo regional M2–D2 de alto grado, que desarrolló la foliación metamórfica principal S2. Aplicando diferentes termobarómetros a las variedades de gneises biotíticos sillimaníticos y gneises granatíferos leucocráticos, se obtuvieron rangos de temperaturas de 768–840 °C y de presiones de 7.3–8.6 kb para el evento M2. Se determinó que una fuerte deformación dúctil D3 estructuró los cuerpos ultramáficos, de mayor viscosidad que las rocas metamórficas asociadas, generando grandes estructuras plegadas en vaina con geometría tubular, con vergencia al oeste. Esta deformación constriccional se habría desarrollado en un régimen de cizalla simple, en condiciones de alta temperatura, con una dirección de transporte tectónico hacia el noroeste (∼N 300°). Posteriormente, durante un evento retrógrado M3, se habrían producido la descompresión y enfriamiento de todas las rocas del área hasta condiciones de la facies de anfibolitas. Se calcularon rangos de temperaturas de 588–710 °C y de presiones de 5.2–7.3 kb para el evento M3 en los gneises del área. Las rocas ultramáficas de la Sierra Chica se encuentran como cuerpos plegados aislados entre sí y localmente concordantes con la foliación metamórfica, por lo que no se pueden correlacionar como una única faja regional que pueda interpretarse como una antigua sutura. Nuestra hipótesis es que los cuerpos ultramáficos serían escamas del manto superior emplazadas tectónicamente en un prisma de acreción. Este mismo patrón de afloramiento puede reconocerse en los otros cuerpos ultramáficos de las Sierras de Córdoba. De acuerdo con esto, en este trabajo se sugiere evitar el uso de los términos Fajas Oriental y Occidental para el tratamiento de las rocas ultramáficas en las Sierras de Córdoba.

Permian to Cretaceous polymetamorphic evolution of the Stewart River region, Yukon‐Tanana terrane, Yukon, Canada: P–T evolution linked with in situ SHRIMP monazite geochronology

AbstractTextural relationships and the trace element chemistry of accessory minerals and garnet can provide the linkage between in situ SHRIMP ages and quantitative pressure–temperature data that is required to decipher complex polymetamorphic and polydeformational histories. Application of these methods to lower amphibolite facies rocks of the Stewart River area, Yukon (Canada) yields robust new constraints on the tectonic evolution of central Yukon Tanana Terrane (YTT).A TIMS U/Pb titanite age of 365–350 Ma is interpreted to date low‐P metamorphism (M1) and D1 deformation associated with arc plutonism above an east‐dipping subduction zone. Monazite inclusions in garnet porphyroblasts record a transition from low to high pressure (∼9 kbar and 600 °C) at c. 239 Ma. These data help to establish a c. 260–240 Ma tectonometamorphic event (M2–D2) reflecting intra‐arc thickening during west‐dipping subduction of Slide Mountain Ocean. Another transition from low‐ to high‐P (M3–D3; 7.8 kbar and 595 °C), dated by c. 195–187 Ma monazite, is interpreted to reflect the change from regional contact metamorphism during arc plutonism to internal duplication of YTT during initial collision of YTT with the North American craton.The Mt Burnham (north‐eastern) region records a different history because of its proximity to later plutons and its late exhumation via extensional faulting. Monazite growth at 146 Ma dates ∼9 kbar metamorphism (M4), interpreted to reflect a previously unrecognized period of plutonism associated with auriferous quartz veins in the Klondike region. Monazite growth at 114–107 Ma reflects low‐P (<4.6 kbar) contact metamorphism (M5) accompanying regional plutonism and extension.

Multiple Metamorphic Events at Broken Hill, Australia. Evidence from Chloritoid-bearing Parageneses in the Nine-Mile Mine Region

Pressure (P) and temperature (T) conditions for two distinct metamorphic events are quantified in the Nine-Mile Mine region of the central Broken Hill Block, New South Wales, Australia. Parageneses of the earlier event formed contemporaneously with D2 which produced a pervasive foliation. The event occurred at conditions around 650°C and 4–5 kbar, but may have been preceded by higher peak temperatures, possibly during an early M1 event. Thus the event described here is called M2. This is supported by geochronological studies that report at least two high-grade partial melting events in the region. A later paragenesis, here called M3, overprinted statically the M2–D2 and D3 fabrics. PT conditions during M3 reached 480°C at 5 kbar. Possible PT paths between M2 and M3 are inferred from the interpretation of chloritoid-bearing parageneses using (1) a new calculated petrogenetic grid, (2) calculated compatibility diagrams and (3) thermodynamic pseudosections. In particular, two possible PT paths are discussed: (a) a direct path involving negligible pressure change during cooling and (b) cooling with minor decompression from M2 and much later compression and heating to M3. PT path (b) is interpreted from pseudosections. It is also considered more likely because: (1) the orientation and position of the regional isograds for M2 and M3 do not coincide across the Broken Hill Block and (2) D3 is interpreted as a substantial shortening event. Thus, a much larger P difference would be expected between M2 and M3 if the two events occurred on the same PT path. The interpretation of independent PT paths for M2 and M3 allows the possibility that the M3 event is much younger than the mid-Proterozoic high-grade events.

Successive overprinting granulite facies metamorphic events in the Anmatjira Range, central Australia

The Anmatjira Range and adjacent Reynolds Range, central Australia, comprise early Proterozoic metasediments and othogneisses that were affected by three, and possibly four, temporally distinct metamorphic events, M1–4, and deformation events, D1–4, in the period 1820–1590 Ma. The north‐western portion of the range, around Mt Stafford, preserves the effects of ±1820 Ma M1‐D1, and shows a spectacular lateral transition from muscovite + quartz‐bearing schists to interlayered andalusite‐bearing migmatites and two‐pyroxene granofelses that reflect extremely low‐pressure granulite facies conditions, over a distance of less than 10 km. Orthopyroxene + cordierite + garnet + K‐feldspar + quartz‐bearing gneisses occur at the highest grade, implying peak conditions of ±750°C and 2.5 ± 0.6 kbar. An anticlockwise P–T path for M1 is inferred from syn‐ to late‐D1 sillimanite overprinting andalusite, petrogenetic grid considerations and quantitative estimates of metamorphic conditions for inferred overprinting assemblages. The effects of M1 have been variably overprinted to the south‐east by a c. 1760 Ma M2–D2 event. Much of the central Anmatjira Range, around Ingellina Gap, comprises orthogneiss, deformed during D2, and metapelites that have M1 andalusite and K‐feldspar overprinted by M2 sillimanite and muscovite. The south‐eastern portion of the range, around Mt Weldon, comprises metasediments and orthogneisses that were completely recrystallized during M2–D2, with metapelitic gneisses characterized by spinel + sillimanite + K‐feldspar + quartz‐bearing assemblages that suggest peak M2 conditions of >750°C and 5.5 ± 1 kbar. Overprinting parageneses in metapelitic gneisses imply that D2 occurred during essentially isobaric cooling. A third granulite facies event, M3, affected rocks in the Reynolds Range, immediately to the south of the Anmatjira Range, at c. 1730 Ma. A possible fourth event, M4, with a minimum age of c. 1590 My affected both Ranges, but resulted in only minor overprinting of M1–3 assemblages. The superimposed effects of M1–4, mapped for the entire Anmatjira–Reynolds Range area, indicate that only minor or no dislocation of the regional geology occurred during any of the metamorphic and accompanying folding, events. Although the immediate cause of each of the metamorphic events involved advection, the ultimate causes were external to the metasediments and most probably external to the crust.