Andalusite Porphyroblasts Research Articles

Mesoproterozoic age for andalusite in the lower Rocky Cape Group, northwest Tasmania

New U–Th–Pb chemical data from both electron probe microanalysis and U–Pb laser ablation-inductively coupled plasma mass spectrometry on monazite inclusions in giant andalusite porphyroblasts indicate that the lowermost subdivision of the Rocky Cape Group in northwest Tasmania underwent metamorphism at ca 1100 Ma. The Proterozoic Rocky Cape Group is a ∼10 km-thick succession of quartz arenite, siltstone and pelite. These sediments were deposited between ca 1450 and ca 1010 Ma. Metamorphism within the Rocky Cape Group ranges from the prehnite–pumpellyite to upper greenschist facies, locally reaching garnet grade. In the lowermost formation of the Rocky Cape Group, andalusite-bearing mineral assemblages in the Pedder River Siltstone formed prior to or synchronously with monazite development. The andalusite-bearing mineral assemblage indicates relatively low-pressure conditions of <200 MPa, consistent with the stratigraphic thickness of the Rocky Cape Group (∼10 km). However, the presence of the intermediate-temperature biotite–andalusite mineral assemblages (500 °C) in the study area suggests the contribution of additional heat sources. Garnet porphyroblasts were observed ∼2 km away. Similar-age medium-grade metamorphism and magmatism at about 1100 Ma have been observed regionally in the South Tasman Rise and East Tasman Plateau, and in areas that are thought to have been contiguous with northwestern Tasmania such as Dronning Maud Land, central Arizona (Yavapai–Mazatzal) and Belt–Purcell Basin in the southwestern and northwestern Laurentian crustal sequences. The pressure and temperature relationships, mineral associations and regional data suggest additional heat flow was present through the lower part of the Rocky Cape Group. The higher geothermal gradient could be attributed to burial during basin development and deposition of the upper Rocky Cape Group or associated magmatism on the margin of Laurentia during assembly of Rodinia.

Geology and Lithogeochemistry of the Paleoproterozoic Flambeau Cu-Zn-Au Volcanogenic Massive Sulfide Deposit, Wisconsin, USA

Abstract The Paleoproterozoic Flambeau Cu-Zn-Au deposit in northwestern Wisconsin is the only volcanogenic massive sulfide (VMS) deposit to reach production in the Pembine-Wausau terrane of the Penokean orogen, despite the discovery of more than 150 million tonnes of mineralization within 14 deposits since the 1970s. Renewed exploration interest in the region and need for critical minerals has highlighted the importance of the VMS deposits in the region and the need for an improved understanding of their geologic setting. This study examined and sampled 50 drill cores from the Flambeau deposit stored at the Wisconsin state core repository to determine the volcanic, hydrothermal, and tectonic setting of the deposit. The least altered units at the deposit are metadacite and quartz-augen schists that are interpreted to be rapidly emplaced felsic to intermediate crystal-rich tuffs and lapilli tuffs. There are also chlorite ± garnet schists that are interpreted to be mafic sills and/or small flows. The main altered unit is biotite ± andalusite ± white-mica schist that contains porphyroblasts of biotite and/or andalusite that replace the intermediate volcaniclastic units. The ore horizon contains massive sulfide, metachert, and semimassive mineralization within a quartz–white-mica schist. Stratigraphic relationships and chemostratigraphy structurally above and below the ore horizon indicate that the strata are overturned. Despite structural and metamorphic destruction of microtextural evidence, evidence for lateral mineralizing fluid flow in rapidly emplaced, permeable volcanic units, interlayered impermeable to semipermeable units, and macrotextural evidence for lit-par-lit replacement of tuffaceous layers indicate that the Flambeau ores are replacement-style mineralization. Trace element systematics reveal that mafic rock and intermediate rocks are calc-alkalic with low Ti/V values, characteristic of arc environments. Felsic rocks are calc-alkalic, depleted high field strength element (HFSE), FI- to FII-type rhyolites that are also consistent with an arc environment. These lithogeochemical characteristics are similar to those of modern mature oceanic-arc settings. This setting contrasts with other host rocks at nearby deposits (e.g., Lynne, Back Forty) that have petrogenetic evidence for the presence of older Archean basement (i.e., continental crust). Therefore, the geology of the Flambeau deposit highlights the variability of VMS-forming environments within the Penokean orogen and has important local and regional implications for other VMS deposits globally.

Alumina Hosts in Fe- and Al-Rich Metapelites from Transangaria (Yenisey Ridge, East Siberia): Distribution, Composition, and Mining Potential

Fe- and Al-rich metapelite from the Transangarian segment of the Yenisey Ridge (East Siberia, Russia) is a potential new source of high-alumina refractories. The rocks have relatively high average contents of Al2O3 (20 wt%) and Fe2O3 (7.91 wt%), moderate K2O (3.44 wt%), and low CaO (0.74 wt%). Their dominant mineral assemblages are andalusite + muscovite + margarite + chlorite + biotite + quartz or staurolite + kyanite or/and andalusite + chlorite + muscovite + biotite + quartz with ±garnet and ±plagioclase. Al2SiO5 polymorphs occur as up to 1.5 cm andalusite porphyroblasts and partial or complete pseudomorphs after andalusite (kyanite and staurolite). Accessories include abundant Fe–Ti oxides and sporadic REE-, Y-, Ca-phosphates; sulfides are negligible. The composition of Al2SiO5 concentrates obtained in laboratory by heavy-media and magnetic separation from ≥0.06 mm fractions meet all requirements for raw material of this type: &gt;56 wt% Al2O3, &lt;42 wt% SiO2, &lt;1 wt% Fe2O3, &lt;1.2 wt% TiO2, and &lt;0.2 wt% (CaO + MgO). The andalusite, kyanite, and mixed ores yield 0.7–4.1 wt%, 0.7–2.2 wt%, and 1.9–6.0 wt% of concentrate, respectively. The best-quality ores rich in Al2SiO5 polymorphs reside in zones of contact and/or dynamic metamorphism superimposed over regional metamorphism of Al-rich rocks.

Polycyclic metamorphic evolution of the Sierra Albarrana Schists (SW Iberian Massif): From low-pressure Ordovician rifting to medium-pressure Variscan overprint

This study presents new information on the polycyclic metamorphic evolution of the Sierra Albarrana Domain (SW Iberian Massif) obtained through a comprehensive assessment of mineral assemblages and petrological modelling (P–T–X pseudosection approach). The pelitic schists in this domain depict the uncommon paragenesis St-Grt-And together with And-Grt-Sil-bearing schists. The first assemblage has been accurately reproduced by pseudosection modelling using the whole rock composition directly analysed by XRF. In contrast, early attempts to model And-Grt-Sil-bearing schists were not as successful. Fe3+ can be a critical element influencing the phase relations in pelites. For this reason, we investigated the influence of the effective amounts of Fe3+ to estimate the oxidation estate during the metamorphism by computing a series of P/T–X pseudosections to explore whether somewhere in P–T–X space the matrix association can be reproduced.The Sierra Albarrana Domain recorded a regional low-pressure metamorphic event accompanied by a complex structural evolution, probably related with the Middle Cambrian-Lower Ordovician rifting developed in the NW Gondwanan margin. This episode has been constrained at ∼4 kbar and 580 °C in the St-Grt-And pelitic schists. Related granitic magma pulses contributed to a local thermal increase of the extending crust (preserved in the And-Grt-Sil-bearing schists), where temperature reaches up to 595 °C. This area shows the syntaxial replacement of coarse-grained andalusite porphyroblasts by sillimanite. This was followed by exhumation, characterised by decompression at ∼2 kbar and 510 °C. After the main pre-Variscan Buchan-type regional metamorphism, a localised medium-pressure metamorphic event, limited to granitic pegmatites and concordant ky-bearing quartz segregation veins was recorded, possibly due to the tectonic thickening corresponding to the Variscan cycle.

Sinistral shear during Middle Jurassic emplacement of the Matancilla Plutonic Complex in northern Chile (25.4° S) as evidence of oblique plate convergence during the early Andean orogeny

Arc magmatism in a continental subduction zone facilitates rheological weakening of the rigid upper plate, and can accommodate the partitioned trench-parallel component of oblique subduction into an intra-arc shear zone. We document a shear zone at latitude 25.4° S near Taltal, Chile that was associated with intrusion of the Matancilla Plutonic Complex at ∼169 Ma to evaluate intra-arc deformation and possible tectonic plate configurations during this time period. Polyphase folding of Paleozoic metasedimentary rocks is overprinted by mylonitic fabrics that are most extensive in a zone up to 1.4 km wide in the thermal aureole of the granodioritic Matancilla pluton, where contact metamorphic andalusite porphyroblasts are synkinematic with fabric development. Mylonite in metasedimentary rocks is overprinted by a ∼130 Ma granodiorite (zircon U–Pb) and by ∼133 Ma postkinematic monazite (U–Pb). Within the Jurassic Matancilla granodiorite, pervasive ductile shear occurs along the intrusive contact while centimeter-scale discrete high-strain zones throughout the pluton are associated with focused hydrothermal alteration and reaction weakening. Mylonitic foliation in the metasedimentary rocks and within the pluton strikes N- to NE and dips steeply, while stretching lineations are subhorizontal on average. Kinematic indicators record dominantly sinistral shear, though some dextral or symmetric indicators and S > L fabrics suggest a component of coaxial strain and flattening. Sinistral strike-slip kinematics in the Matancilla shear zone may indicate that Middle Jurassic convergence had sinistral obliquity that was locally partitioned into the contemporaneous magmatic arc. Sinistral-oblique convergence would require the Phoenix-Farallon spreading center to be north of ∼25° S in the Middle Jurassic, providing a constraint to plate reconstructions during the early Andean orogeny.

Formation of the Nkob talc deposit by contact metamorphism and fluid infiltration into siliceous dolostones (Moroccan Anti-Atlas)

The Nkob talc deposit located in the central Anti-Atlas Pan-African belt is hosted by magnesian and siliceous metacarbonates in the contact aureole of the Ediacaran Amassine granite. It consists of a stratified succession of green serpentine-rich marbles, black dolomitic marbles, talcitites and phlogopitites (variously retrogressed into chloritites) enclosed within andalusite metapelite hornfels and quartzites. Two main metamorphic stages have been defined for the formation of the marbles and talcitites: (1) high-temperature recrystallization occurred under amphibolite facies conditions (>500 °C). Temperature increase during granite emplacement led to the formation of forsterite, tremolite, phlogopite (±diopside) bearing assemblages in siliceous marbles and to transformation of dolomitic (calcite-free) marbles from pure dolostones. This high temperature stage is also characterized by the growth of andalusite porphyroblasts in the metapelites surrounding the deposit. (2) A low temperature retrograde hydrothermal stage characterized by the reactive infiltration of aqueous silica-rich fluid (presumably derived from the granitic body), in greenschist facies conditions (<520 °C). Hydrous silicate minerals, mostly serpentine and talc (+calcite), were formed during this stage within the rocks (after fluid infiltration along grain boundaries) or within veins (marking channeled fluid circulation). Talc in ores formed by two processes at Nkob: acicular talc near the granite formed after the breakdown of tremolite in a temperature range of 350 to 500 °C, while tabular talc in the external aureole formed via reaction between dolomite and siliceous-aqueous fluids in temperatures below 350 °C. The different types of marbles derive from pure to slightly siliceous dolomite but green and green-black marbles were strongly affected by interactions with siliceous-aqueous fluids forming serpentine-phlogopite veins with calcite reaction zones. The talcitite layers were also affected by similar hydrothermal H2O-Si enrichment but their dolomitic precursor probably contained a more important detrital silicate fraction.

Retrieval of P–T–t–d paths in a syn-metamorphic shear zone: Implications for P-T calculations and the Permian–Triassic orogeny on the Korean Peninsula

Crustal-scale shear zones developed on the boundaries of the Precambrian Gyeonggi Massif (GM) can play key role on understanding collisional orogeny during the Late Paleozoic to Early Mesozoic in the Korean Peninsula. Garnet schist formed on the boundary between the GM and Okcheon Metamorphic Belt (OMB) experienced contractional (Dn) and extensional (Dn+1) shearing combined with a clockwise pressure–temperature–deformation (P–T–d) path. Two distinct mineral assemblages were found in the schists, (1) garnet (pre- and/or syn-Dn) + staurolite (syn-Dn and/or inter Dn and Dn+1, and syn-Dn+1) + andalusite (syn-Dn+1)-bearing and (2) garnet (pre- and/or syn-Dn) + epidote-bearing assemblages, depending on bulk-rock compositions. Various compositions (An10–40 in the former assemblage and An45–70 in the latter one) and texture of plagioclase resulted from bulk-rock compositions, heterogeneous strain and dissolution and precipitation during the multiple shearing. The P–T conditions estimated for the early contractional shearing (top-to-the-north reverse shear) are 560 °C–600 °C and 5.5–7.0 kbar based on the intersections between compositional isopleths for pre- and/or syn-Dn garnet and plagioclase (An30–40 & An45–55) calculated in the MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O and MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 systems. Subsequent extensional shearing (top-to-the-south normal shear) was characterized by recrystallization of fine-grained syn-Dn+1 staurolite with growth of syn-Dn+1 andalusite porphyroblasts during cooling and decompression to 560 °C and 3.5 kbar. The contractional and extensional shearing probably occurred at the Permian and the Late Triassic to Early Jurassic, respectively, based on the previous geochronological data. Consequently, inferred P–T–t–d paths on the GM–OMB boundary probably suggest not only crustal thickening followed by thinning, but also multiple amalgamations of several terranes.

The timing of prograde metamorphism in the Pontiac Subprovince, Superior craton; implications for Archean geodynamics and gold mineralization

The Pontiac Subprovince is located in the Superior Province, south of the Abitibi Subprovince. The metasedimentary rocks of the Pontiac Group are characterized by a Barrovian metamorphic gradient increasing from north to south from biotite- through garnet- to staurolite-zone conditions. The Pontiac Subprovince has been interpreted as an accretionary wedge or alternatively as an exotic terrane that was tectonically docked to the Abitibi Subprovince during Archean subduction. However, few studies have attempted to characterize its pressure-temperature-time path to test this hypothesis and little is known about the timing relationship between regional metamorphism and the gold mineralization. An E-W lepidoblastic ductile micaceous foliation defines the regional fabric related to the second episode of deformation. Garnet and staurolite porphyroblasts are interpreted to be late- to post-kinematic to the regional fabric. Locally, cordierite or andalusite porphyroblasts fully pseudomorphed by a polymineralic assemblage of muscovite, feldspar and quartz are wrapped by the main foliation indicating that a potential low pressure - high temperature metamorphism (M1) preceded the main episode of deformation and Barrovian metamorphism (M2). Pressure and temperature (P-T) forward thermodynamic modelling on a water-saturated pseudosection yielded peak conditions of 550–600 °C and 5–6 kbar during a prograde, clockwise P-T path for M2. Lu-Hf dating of garnet from three different locations within the staurolite zone yielded a weighted average age of 2657.5 ± 4.4 Ma (95% confidence level) that is inferred to be representative of these conditions. The age of garnet growth calls into question the previous tectonic interpretation of the Abitibi and Pontiac Subprovinces and is coherent with the new rafted ribbon continent model of Bédard (2018). Because the Canadian Malartic gold mineralization is older or synchronous with the garnet growth episode in the staurolite zone south of the ore body, this garnet age also provides an estimate of the minimum age of the mineralization event, previously dated at 2664 ± 11 Ma with Re-Os on molybdenite.

Pressure-temperature-time evolution of a Variscan garnet-bearing micaschist from the northeastern Fichtelgebirge, NW Bohemian Massif in central Europe

The studied micaschist from the northeastern Fichtelgebirge crystalline complex (FC) contains mm-sized garnet with a foam structure. Staurolite formed at the margin of garnet. Relics of staurolite are enclosed in mm-sized andalusite porphyroblasts. Garnet exhibits prograde concentric zonation with the pyrope component increasing from 1 to 6 mol%. Cores of potassic white-mica grains locally contain Si contents up to 3.15 per formula unit (pfu). The majority of this mica is characterized by Si contents close to 3.00 pfu. Pressure-temperature (P-T) pseudosections, calculated for the micaschist, indicate conditions of 10 kbar and 505 °C for an early metamorphic stage which was followed by a pressure decrease to 5 kbar and 535 °C. Late metamorphic P-T conditions recorded by garnet are around 4.5 kbar and 565 °C, compatible with the presence of staurolite. Andalusite might have metastably formed at this stage or somewhat lower pressures. The U-Th-Pb dating of monazite with the electron microprobe yielded subordinate ages younger than 315 Ma which were assigned to the nearby emplacement of granites in the FC. Some ages older than 395 Ma were related to detrital monazite in the sedimentary protolith of the micaschist. An age cluster at 384.5 ± 1.8 (2σ) Ma is preferentially assigned to the early high-pressure (HP) metamorphism resulting from the collision of Laurussia with a Peri-Gondwanan terrane. Exhumation to 15–20 km depths might have ended at 362.3 ± 1.0 Ma, but possibly the HP metamorphism occurred at this age and the 384.5 Ma age must be then referred to the provenance area of detrital monazite. The age cluster at 325.0 ± 0.7 Ma is related to a second collisional event that caused the overriding of the crystalline nappes of the FC onto the Laurussian crust.

Late Paleoproterozoic clockwise P–T history in the Mahakoshal Belt, Central Indian Tectonic Zone: Implications for Columbia supercontinent assembly

The Paleo/Neoproterozoic Central Indian Tectonic Zone (CITZ) suggested being continuous with the Capricorn Orogen of Western Australia (COWA) and the Trans North China Orogen (TNCO) is the zone of accretion between the North India and the South India Archean Blocks. This study aims to establish the tectonothermal history of the Mahakoshal Belt (MB) in the CITZ and to review its relevance to the suggested correlation of CITZ with COWA and TNCO. In this study, we focus on the metamorphic P–T–t history of phengite–quartz–andalusite–margarite–muscovite–chlorite–biotite schists/phyllites and rare andalusite–corundum–quartz veins in the schists/phyllites. S1 inclusion trails of phengite+biotite+quartz±muscovite±chlorite assemblages in syn/post-S2 andalusite porphyroblasts were stabilized at ∼8kbar, 520°C. Near-isothermal decompression led to andalusite–muscovite stabilization at the expense of phengite-bearing assemblage at <4kbar. Continued isothermal decompression (2–3kbar) led to fluid-aided post-S3 replacement of andalusite porphyroblasts by margarite–muscovite–chlorite pseudomorphs, and andalusite–corundum in the veins were replaced by muscovite–diaspore±chlorite assemblages. U–Th–Pb (total) age determinations suggest that the clockwise P–T path in the MB schists/phyllites occurred at 1.7–1.8Ga, which overlaps with the 1.7–1.8Ga granitoids in the supracrustal belt. This Paleoproterozoic subduction-metamorphism and granitoid emplacement synchronous with exhumation in the MB combined with existing information suggest that the CITZ comprised disparately-evolved crustal domains that accreted over a protracted time between Late Paleoproterozoic and Early Neoproterozoic. The tectonothermal events in the CITZ partly overlap with those in the COWA and TNCO with major omissions, and therefore, these global correlations should await further petrological and robust age determinations in the CITZ.

Holistic microstructural techniques reveal synchronous and alternating andalusite and staurolite growth during three tectonic events resulted from shifting partitioning of growth vs deformation

Excellent inclusion trails in a sample containing both staurolite and andalusite porphyroblasts are used to demonstrate techniques that allow the intimate relationships between deformation and porphyroblast growth to be recognized, described in detail and understood. This approach reveals three main phases of growth of both mineral phases, some of which was demonstrably synchronous, during three tectonic events. Each main period of growth occurred during the early stages of three deformations that were successively near orthogonal. However, extra periods are distinguishable in andalusite in some of these events because this phase occurs as clusters of large crystals that vary in orientation by 2° to >10°. All foliations defined by all inclusion trails within every porphyroblast inflect/intersect about an axis trending at 025° (called a FIA). This indicates that the direction of the horizontal component of bulk shortening was identical for the first and third of the three deformations recorded by porphyroblast growth. Portions of sigmoidal to slightly spiral-shaped inclusion trails in most porphyroblast clusters locally diverge in opposite directions due to overprinting orthogonal bulk shortening typical of that which forms millipede geometries. These microstructures confirm the role of coaxial bulk shortening in initiating porphyroblast growth in an environment that locally becomes strongly non-coaxial as the deformation intensifies in the same event.In this sample, increasing non-coaxiality as the deformation intensified resulted in the same asymmetry for each of the three events and thus an overall spiral-like shape. Differing stages in the development of these bulk-shortening geometries preserved in adjacent or touching phases negate any role for porphyroblast rotation during ductile deformation. Andalusite and staurolite grew without any inter-reaction in locations where they lie in contact. This multiply repeated growth behaviour initiated within zones of spatially partitioned crenulation deformation of pre-existing foliations. This varied slightly in timing and in the subsequent development of successive surrounding foliations from location to location over distances around 5mm. Growth ceased adjacent to any zone where metamorphic differentiation associated with the development of new foliations initiated. This also varied in timing locally as a result of migrating and/or shifting partitioning of the deformation at similar to smaller scales. A pseudosection for the bulk composition of this sample shows that andalusite and staurolite grow simultaneously only over a very tightly constrained range of P–T conditions. Yet these occurred for at least three tectonic events, two of which produced strong schistosities. This required bulk coaxial orogen-scale deformation with slight crustal thickening during two periods of bulk horizontal shortening balanced by gravitational collapse in the intervening event.

The Sondalo gabbro contact aureole (Campo unit, Eastern Alps): implications for mid-crustal mafic magma emplacement

Contact aureoles rimming plutonic rocks are the locus of metamorphism and deformations witnessing magma emplacement mechanisms in the crust. In this study, structural and petrological observations are combined to mineral equilibria modelling to unravel the polyphase tectono-metamorphic history of the Permian Sondalo gabbro and its host rock, the Campo unit (Eastern Alps). The Campo unit consists of Grt–St–Ms–Bt–Pl–Qtz ± Sil ± And ± Crd mica schists attesting of a Carboniferous prograde P–T path, reaching 6 kbar/600 °C and subsequently 5.6 kbar/650 °C. This metamorphism is coeval with the formation of a sub-vertical NE–SW trending foliation (S1) and its overprint by a sub-vertical NW–SE trending foliation (S2). The heat brought by the Permian intrusives subsequently caused heating of the Campo unit at around 3–4 kbar/540 °C reflected by regional static crystallization of cordierite and andalusite porphyroblasts. During the intrusion of the Sondalo gabbro, thermal peak conditions are recorded by Grt–Sil–Spl–Crd–Ilm granulitic xenoliths at ~5.5 kbar/930 °C, subsequently exhumed at ~4 kbar during the development of a new foliation (S3). This foliation is localized around the pluton and moderately dips away from the centre of the pluton. In the migmatitic contact aureole, Grt–Sil–Bt–Pl–Qtz–Ilm and Grt–Sil–Crd–Spl–Bt–Kfs–Ilm residual rocks bear the new foliation (S3) and document a decompression from 6 kbar/750 °C to 5 kbar/725 °C and from 5.2 kbar/800 °C to reach 4.8 kbar/770 °C, respectively. The P–T–d paths recorded by the host rock and the xenoliths point to a two-step magma emplacement. First the Sondalo gabbro intruded the Campo unit causing heating of the host rock without deformation at 3–4 kbar. Second, the ductile flow along the pluton margins developed a new foliation (S3) during exhumation of the pluton and its immediate contact aureole from 6 to 4 kbar. Altogether, it indicates a progressive increase in mechanical coupling between the pluton and the host rock during magma emplacement.

Petrology of metapelites in the Bugaboo aureole, British Columbia, Canada

AbstractContact metamorphism of greenschist facies Neoproterozoic turbidites by the Cretaceous Bugaboo Batholith in southeastern British Columbia has resulted in a well‐developed contact aureole. The aureole is about 1 km wide and can be divided into three main zones: (i) spotted phyllite zone, extending from the first appearance of spots of cordierite or andalusite to the last occurrence of primary chlorite; (ii) cordierite + andalusite + biotite zone, comprising hornfelses or schists with abundant porphyroblasts of cordierite and andalusite and, at higher grades, fibrolitic sillimanite; and (iii) K‐feldspar zone, characterized by hornfelses and schists that, in the inner part of this zone, are variably migmatitic. Four parts of the aureole were examined, three of which are characterized by schists, and one of which (Cobalt Lake area) is characterized by hornfelses and has exceptional exposure and comparatively unaltered rocks. Petrographic, modal, mineral‐compositional and whole rock‐compositional data were collected from the Cobalt Lake transect, allowing the prograde reaction sequence to be inferred. Notable features of the aureole at Cobalt Lake include: initial development of andalusite and plagioclase at the expense of paragonite‐rich white mica; a narrow interval across which cordierite, andalusite and biotite increase markedly at the expense of chlorite; gradual development of andalusite and biotite at the expense of cordierite and muscovite upgrade of chlorite consumption; and near‐simultaneous development of andalusite + K‐feldspar and sillimanite, the latter indicating a pressure of contact metamorphism of ~3 kbar. In other parts of the aureole, the development of sillimanite downgrade of the initial development of K‐feldspar suggests slightly higher pressures of contact metamorphism. Lack of correspondence between the observed sequence of reactions in the aureole and those predicted thermodynamically suggests that modifications to some of the thermodynamic data or activity–composition models may be required. Textural features in the aureole suggest the influence of kinetic factors on metamorphic recrystallization, including: (i) deformation‐catalysed reaction in the schists compared to the hornfelses, as indicated by different mineral‐growth sequences inferred from microstructures, and (ii) heating rate‐controlled recrystallization, as indicated by the decrease in grain size of hornfelses with increasing metamorphic grade.

High-resolution X-ray computed microtomography: A holistic approach to metamorphic fabric analyses

An intrinsic limitation of studying microstructures in thin section is that their spatial (three-dimensional, 3-D) distribution, shape, and orientation have to be inferred by combining 2-D data from different sections. This procedure always involves some degree of interpretation that in some cases can be ambiguous. Recent advances in high-resolution X-ray computed microtomography have made possible the direct imaging in 3-D of volumes of rock to centimeter scale. This rapidly evolving technology is nondestructive and provides a holistic approach of microstructural analysis that eliminates interpretative procedures associated with 2-D methods. Spatial images can be generated through any part of the rock sample and used as virtual petrographic sections. Our application of this technique to an oriented drill core sample from the classic Orijarvi metamorphic region of southern Finland reveals a number of in situ 3-D aspects, including: (1) the spatial distribution and shape of andalusite porphyroblasts, (2) the geometry of a matrix foliation anastomosing around the porphyroblasts, (3) a millimeter-scale compositional layering that controlled the oscillation of porphyroblasts and sulfide mineralization, and (4) distinct inclusion trail patterns characterizing porphyroblast core versus rim zones. The combined data indicate that the steeply dipping bedding-subparallel foliation that characterizes the Orijarvi area formed by bulk north-south crustal shortening and associated vertical stretching.

Evolution of the Eastern Karakoram Metamorphic Complex, Ladakh, NW India, and its relationship to magmatism and regional tectonics

Abstract The eastern Karakoram terrane, NW India, records crustal evolution in the core of the Himalayan–Tibetan orogen. Recent debate has centred on whether prograde metamorphism, anatexis and leucogranite emplacement were the result of localised shear heating and magma advection within the Karakoram Fault Zone (KFZ) or instead predate the KFZ and were the result of regional tectonometamophic events. Inclusions within andalusite porphyroblasts that grew during 15.7 Ma leucogranite emplacement have fabrics that are random or discordant to the KFZ matrix foliation, indicating that the KFZ initiated after this time. Therefore, earlier anatexis and metamorphism are the result of regional metamorphic events. Amphibole–plagioclase thermobarometry on a c. 17 Ma migmatite melanosome, later exhumed within a transpressional zone of the KFZ, shows that melting occurred at 688 °C and 522 MPa. Amphibolites record an older kyanite-grade metamorphic event that occurred at 677–736 °C and 875–1059 MPa. Metapelites also record a kyanite-grade event which is constrained by Ti-in-biotite thermometry to have occurred at 622 °C and > 650 MPa. The tectonometamorphic history of the eastern Karakoram correlates closely with that of the central Karakoram away from the KFZ. This correlation supports the interpretation that metamorphism and anatexis were regional in extent and also indicates a limited offset of

Time–space relationships among structural and metamorphic aureoles related to granite emplacement: a case study from the Serre Massif (southern Italy)

AbstractTectonic and thermal perturbations, related to emplacement of granodiorite in the upper continental crust, have been investigated in the late-Hercynian basement exposed in southern Calabria (Italy). Here, the structural aureole is marked by the presence of a major rim fold adjacent to the intrusive contact for a length of at least 20 km. Geometrical analysis of the structural aureole and related foliations, lineations and crenulations reveals that the perturbed zone is at least 3000 m wide and characterized by an open synform trending nearly parallel to the intrusive contact. This pattern is compatible with a laccolith-like mode of magma emplacement, related to the accretion of the pluton that shouldered weak phyllitic and slaty wall rocks. The metamorphic aureole, about 1800 m wide, is characterized by biotite, cordierite and andalusite that appear sequentially in spotted schists and hornfelses approaching the intrusive contact. The peak assemblage equilibrated between 535 and 590°C at pressures between 175 and 200 MPa, confirmed by Al-in-hornblende barometry on granodiorite. Microstructural analysis allowed the inference of a time lag between the thermal and tectonic perturbations. With the aid of thermal modelling it was possible to quantify the time required to reach the peak temperature at a distance from the intrusive contact where cordierite spots and andalusite porphyroblasts clearly overprint crenulations. This estimate represents the time limit to accomplish deformation in the inner portion of the aureole and thus indicates a minimum strain rate of 4 × 10−14s−1within the country rocks during granodiorite intrusion.

Permo-Triassic changes in bulk crustal shortening direction during deformation and metamorphism of the Taebaeksan Basin, South Korea using foliation intersection/inflection axes: Implications for tectonic movement at the eastern margin of Eurasia during the Songrim (Indosinian) orogeny

The Permo-Triassic Songrim (Indosinian) orogeny in South Korea was a major tectonic event involving complicated continental collisions at the eastern margin of Eurasia. Previous studies have examined the structural and metamorphic features of the Songrim orogeny in each of the Paleozoic terranes of the orogenic belt (i.e., the Taebaeksan Basin, the Okcheon Basin, and the Imjingang Belt), but correlations of these features among the terranes remain uncertain. The aim of this paper is to reveal deformation history including bulk crustal shortening directions in the Taebaeksan Basin, and to correlate the tectono-metamorphic evolution of the Taebaeksan Basin with other Phanerozoic mobile belts in eastern Asia based on a combined analysis of foliation intersection/inflection axes (FIA) trends and metamorphic P–T and T–t (time) paths. The orientations and relative timing of FIA preserved as inclusion trails within porphyroblasts of andalusite, chloritoid, garnet, and staurolite reveal two age groups of inclusion trails in the Pyeongan Supergroup at the northeastern margin of the Taebaeksan Basin. These microstructures indicate the development of early NNW–NNE-trending structures and fabrics, followed by later E–W-trending ones. These observations suggest a change in the orientation of bulk crustal shortening from E–W to N–S during the Songrim orogeny. Based on the similar microstructures and temperature–time paths of the three Paleozoic terranes, we interpret that the E–W bulk crustal shortening influenced the eastern part of the Korean Peninsula during the early stages of the Songrim orogeny, presumably related to amalgamation between the proto-Japan terrane and the eastern margin of Eurasia, whereas the N–S bulk crustal shortening was stronger in the western part of the peninsula during the later stages of the orogeny, related to collision between the South and North China blocks.

Polymetamorphism accompanied switching in horizontal shortening during Isan Orogeny: Example from the Eastern Fold Belt, Mount Isa Inlier, Australia

Mesoproterozoic low-P/high-T volcano-sedimentary rocks of the Soldiers Cap Group, the southeastern corner of Mount Isa Inlier, record a complex polymetamorphic history that accompanied four periods of bulk horizontal shortening directed NE–SW, N–S, W–E and NW–SE during the long-lived Isan Orogeny (~1650–1500Ma). Low-P/high-T metamorphism (M1) prevailed during a period of NE–SW bulk horizontal shortening as indicated by the early growth of cordierite porphyroblasts, which entrap monazite grains that gave an average age of 1649±12Ma. This was followed by medium-P/high-T (M2) and high-P/high-T (M3) metamorphisms that accompanied periods of N–S and W–E bulk horizontal shortening, respectively. Growth of first generation garnet, andalusite and staurolite porphyroblasts having an average age of 1645±7Ma identify the former, whereas growth of the 1591±10Ma second generation garnet, staurolite and andalusite porphyroblasts together with fewer kyanite porphyroblasts characterizes the latter.The sediments of the Soldiers Cap Group were deposited in a tectonic setting that has the characteristics of an intra-continental rift basin. The upper limit of the age of sedimentation is constrained by detrital zircons at 1654±4Ma suggesting that tectonism and metamorphism were either active during the final stage of the basin filling or immediate after deposition.Introduction of mafic dykes and sills at different stratigraphic levels over a wide time span was the major source of heat.

Tectonometamorphic evolution of the Permo-Triassic Songrim (Indosinian) orogeny: evidence from the late Paleozoic Pyeongan Supergroup in the northeastern Taebaeksan Basin, South Korea

The deformed and metamorphosed middle Pennsylvanian to Early Triassic (?) rocks of the Pyeongan Supergroup, occurring in northeastern flank of the Paleozoic Taebaeksan Basin, east central South Korea, record the impact of the Late Permian–Triassic Songrim (Indosinian) collisional orogeny in the eastern Eurasia margin. Analysis of meso- and microstructures and their relationship with porphyroblast growth has revealed that these metasedimentary rocks were affected by four deformation events (D1–D4). During the polyphase deformations, successive foliations with alternately steep (S1 and S3) and flat-lying (S2 and S4) orientations were produced with growth of chloritoid, andalusite and kyanite porphyroblasts during D1 and D2, and growth of garnet, staurolite and andalusite porphyroblasts during D3 and D4. SHRIMP U–Pb detrital zircon ages (330–310 Ma) from metapelites in the oldest unit of the Pyeongan Supergroup in the study area, combined with the previous data on sandstone compositions, indicate magmatic activity that was contemporaneous with the initial deposition of the Pyeongan Supergroup. This also suggests that the Pyeongan Supergroup was deposited in an arc-related foreland basin at an active continental margin during the Late Carboniferous and Permian time. Two contractional deformation events (D1 and D3) and two events related to gravitational flattening (D2 and D4), along with Barrovian-type medium-pressure regional metamorphism, may have resulted from the Late Permian–Triassic Songrim orogeny, related to collision between the North and South China blocks and/or collision between the Eastern Eurasia margin and the proto-Japan terrane.

Radical changes in bulk shortening directions during orogenesis: Significance for progressive development of regional folds and thrusts

Polyphase folding and metamorphism of rocks in the eastern Mount Isa Inlier resulted from four distinct orogenic cycles. These consist of NE–SW shortening (O 00) from 1672 to ∼1648 Ma, involving a succession of five phases of deformation, which was responsible for the development of the first generation NW–SE striking macroscopic folds, plus cleavages that are mainly only preserved in porphyroblasts. N–S shortening (O 1) followed to 1600 Ma, involving at least three phases of deformation. This period of N–S directed orogenesis rotated large portions of the NW–SE trending regional folds into W–E trends and the dominance of locally well-preserved W–E trending sub-vertical matrix foliations as well as the intensification of foliation parallel to bedding. Subsequent W–E shortening (O 2) from 1600 to ∼1515 Ma was accompanied by the development of N–S trending folds. These dominate the Mount Isa Inlier and where best developed involved the formation of at least three near orthogonal foliations. Where these large-scale folds regionally are weakly developed or not visible, only one sub-vertical foliation has developed. Finally NW–SE shortening (O 3) produced locally developed SW–NE trending folds and weak crenulations until ∼1500 Ma. The asymmetry of steep to gentle pitch changes in inclusion trails preserved in garnet porphyroblasts that developed during O 00 indicates thrusting to the SW. Similar inclusion trail asymmetries in garnet, staurolite and andalusite porphyroblasts that developed in O 1, indicate thrusting to the N confirming a change in thrust direction across the Wonga Duchess belt to the W. Formation of the Meerenurker roof thrust near Mount Isa by thrusting to the SW during O 00 with subsequent development of the Kokkalukkanurker duplex below by thrusting to the S during O 1 reduces the displacement required for the roof thrust from 234 to ∼65 kms. Foliations preserved in O 00 garnets are truncated by foliations associated with the development of O 1 in the matrix. O 1 garnet, staurolite and andalusite porphyroblasts preserve inclusion trails that are commonly truncated against a differentiated crenulation cleavage in the matrix that formed during O 2. The inclusion trails entrapped within O 00 and O 1 generated porphyroblasts reveal that the foliations that progressively developed in the matrix during these orogenies were composite fabrics resulting from the effects of multiple discrete orthogonal increments in each case. Numerous phases of sub-vertical foliation development due to horizontal bulk shortening were followed by phases of gently dipping foliation formation due to gravitational collapse that accompanied thrusting during O 00 and O 1.