Low-pressure Metamorphism Research Articles

A review of the geology and uranium, gold and iron ore deposits of the Pine Creek Orogen

The Pine Creek Orogen comprises a succession of Paleoproterozoic sedimentary and volcanic rocks, unconformably overlying Neoarchean granitic basement and intruded by Paleoproterozoic mafic rocks and granites. The orogen is subdivided from west to east into the Litchfield Province, Central Domain and Nimbuwah Domain, based on the distinct timing and nature of sedimentation, magmatism and metamorphism. The orogen hosts a wide range of commodities, the most important of which are U and Au. Rifting of Neoarchean basement at 2020 Ma led to deposition of clastic, carbonate, and carbonaceous sedimentary and volcanic rocks in a shallow basin. At 1870 Ma, sedimentation in the Nimbuwah Domain was rapidly followed by burial, I-type granitic magmatism (1867-1860 Ma), compressional tectonism and mid-pressure amphibolite-facies metamorphism (1865-1855 Ma). Major U deposits occur in the Nimbuwah Domain within basal Paleoproterozoic strata, close to tectonised contacts with Neoarchean basement. Metamorphism of the Nimbuwah Domain coincided with sedimentation and volcanism in the Central Domain and Litchfield Province at 1863 Ma. This was followed by extensional high-temperature, low-pressure metamorphism (1855 Ma) and associated felsic and arc-related mafic magmatism (1862-1850 Ma) in the Litchfield Province. At or after this time, greens-chist-facies metamorphism and upright folding and shearing occurred at upper crustal levels in the Central Domain, generating structural traps for subsequent Au- and Fe-bearing fluids. Almost all Au occurrences are associated with late to post orogenic, I-type Cullen Supersuite granites (1835-1820 Ma). Shortly thereafter, platform sediments were deposited in braided rivers across the orogen. The strong spatial heterogeneity in the distribution of U and Au suggests that the pre-existing crustal architecture of the orogen was a significant factor controlling their distribution.

The Cryogenian arc formation and successive high-K calc–alkaline plutons of Socotra Island (Yemen)

The Socotra Island belongs to the southern rifted margin of the Gulf of Aden and occupied in Neoproterozoic times a key position to constrain the age and the nature of the largely hidden Neoproterozoic rocks of the Arabian plate. Our integrated field, petrographic, geochemical and geochronological study in the Neoproterozoic rocks recognises three main successive events: (a) high-temperature ductile deformation and metamorphism forming probably in a compressive or transpressive regime; (b) mafic to intermediate intrusions as vertical sheets, kilometre-scale gabbro laccoliths, mafic dike swarm and lavas which present mainly a depleted arc signature with some evidences of evolution from an enriched-arc signature; (c) felsic intrusions mainly composed of highly potassic calc–alkaline and pinkish granites dated between 840 and 780 Ma. Relationships between the various petrographic types and U–Pb data suggest that these events occurred during a relatively short time span (80 Ma at max). Earlier high-temperature–low-pressure metamorphism stage as well as geochemical signature of mafic rocks show that development of Cryogenian formations of Socotra were controlled successively by an Andean-arc and a back-arc setting. These features cannot be easily reconciled with those of the Arabian–Nubian shield to the west of Socotra and of the Mozambique Belt to the south. We propose that the Socotra basement was developed at an active margin close to the India block in Cryogenian times.

Very low-grade metamorphism of Rheno-Hercynian allochthons (Variscides, Germany): facts and tectonic consequences

The Horre Belt in the SE part of the Rhenish Massif has been regarded as autochthonous because of the apparent lack of metamorphic inversion. We have performed a K-white mica study using powder X-ray diffraction derived Kubler Index (KI) and K-white mica b cell dimension, and the spectrally derived wavelength of the 2,200 nm absorption feature (“2200wvl”) of supposedly allochthonous units and the underlying par-autochthon. KI reveals a slight, but significant inversion of metamorphic grade, which shows that the Horre Belt and the Hessische Schieferserie adjacent to the South were emplaced by thrusting after they had acquired peak metamorphism (M 1) in deeper parts of the orogenic wedge. Only in the westernmost part of the study area, the par-autochthon and the allochthon show the same grade of anchizonal metamorphism. This observation, together with K-white mica b cell dimensions <9.000 A and 2200wvl <2,204 nm, suggests a second, low-pressure metamorphism M 2 subsequent to the emplacement of the allochthon. This is supported by an anomaly of published conodont alteration index data. Our results expand the original width of the Rheno-Hercynian passive margin to at least 580, more probably to 670 or even 750 km. Palinspastic restoration also reveals that the Givetian to Early Carboniferous volcanic belt of the Lahn/Dill area is not only younger than the Emsian MORB-type lavas of the Rheno-Hercynian ocean, but was also separated, from the latter, by a largely non-volcanic belt of at least c. 220 km width. Apparently, the Givetian to Early Carboniferous volcanic episodes were unrelated to the Rheno-Hercynian drift stage.

Orogenesis without collision: Stabilizing the Terra Australis accretionary orogen, eastern Australia

The Neoproterozoic to end-Paleozoic Terra Australis orogen extended along the Gondwana margin of the paleo–Pacific Ocean, and it now provides a detailed record of orogenic activity and continental stabilization within an ongoing convergent, accretionary plate margin. New geochronological data from end-Paleozoic plutonic and volcanic rocks associated with the Gondwanide orogeny in the New England region of eastern Australia, integrated with information on the nature and timing of associated sedimentation, deformation, and metamorphism, allow resolution of a high-fidelity record of orogenesis. At the end of the Carboniferous, around 305 Ma, convergent margin magmatism, which had been active along the western margin of the New England region, terminated and was followed by a short pulse of regional compressional deformation and metamorphism, marking the commencement of the Tablelands phase of Gondwanide orogenesis. Deformation was almost immediately followed by the onset of clastic sedimentation and local calc-alkaline volcanism, dated at 293 Ma, in the extensional Barnard Basin. Emplacement of the two New England S-type granitic suites, the Bundarra and the Hillgrove suites, along with localized high-temperature, low-pressure metamorphism, was essentially contemporaneous, ranging in age from 296 to 288 Ma, and overlapped in time with I-type magmatism and the switch from regional compression to extension and Barnard Basin rifting. The Hunter-Bowen phase of the Gondwanide orogeny commenced with contractional deformation, resulting in termination of sedimentation in the Barnard Basin and regional deformation and metamorphism across New England and into the Sydney and Gunnedah basins to the west at around 265–260 Ma. Contractional loading of the Sydney and Gunnedah basins resulted in their conversion from extensional to foreland basins, which received ongoing pulses of sediment from the New England orogenic welt until 230 Ma. The Hunter-Bowen phase was associated with widespread I-type plutonism and volcanic activity in New England that ceased around 230 Ma, marking the termination of Gondwanide orogenesis. Orogenesis occurred in an evolving convergent plate-margin setting. S- and I-type magmatic activity ranging in age from ca. 300 to 230 Ma represents a stepping out of arc magmatism from the western margin of New England (prior to 305 Ma) into the preexisting arc-trench gap. There is no evidence that deformation was related to the collision of the convergent margin with a major lithospheric mass, and the widespread development of extensional basins in the eastern third of Australia in the Early Permian indicates control by phenomena acting on a continental scale, probably changing plate kinematics associated with the amalgamation of Pangea.

Magnetic overprinting of the Brachina Formation/Ulupa Siltstone, Southern Adelaide Foldbelt, prior to Delamerian deformation

The Proterozoic sediments of the Adelaide Geosyncline were deformed and metamorphosed during the Ordovician Delamerian Orogeny. In much of the Flinders Ranges, the relatively unmetamorphosed sediments, such as those of the Brachina and Bunyeroo formations in the Central Flinders Ranges, are only weakly magnetic. In the Mount Lofty Ranges, where peak metamorphic grades were reached, the Brachina Formation and its equivalent, the Ulupa Siltstone, are extremely magnetic and cause prominent linear magnetic anomalies with amplitudes of 100 to 500 nT that can be traced for more than 50 km. The higher metamorphic grades are accompanied by the growth of metamorphic magnetite within beds that are thought to have originally contained siderite. These magnetic markers have been used to trace the macrostructure of the Southern Adelaide Foldbelt and have been especially useful where outcrop is limited. At the highest grade, where the rocks are andalusite schists, the magnetic anomalies are inconsistent with magnetisations in the direction of the present magnetic field implying that remanence causes a significant proportion of the anomaly. The rocks display a strong thermochemical component, mostly acquired prior to deformation due to the Delamerian Orogeny. Remagnetisation prior to deformation is consistent with the high-temperature low-pressure metamorphism that characterises the eastern Mt Lofty Ranges. This secondary natural remanent magnetisation (NRM) component is similar to that of the nearby post-orogenic Black Hill Norite, which is believed to have intruded during the waning phases of the Delamerian Orogeny and is of the same magnetic polarity. We therefore have magnetic signatures of the same polarity that bracket the Delamerian Orogeny, both pre- and post-folding. The tilt corrected mean direction for the Ulupa Siltstone from 44 samples is D = 231.4°, I = 41.1° (α95 = 5.6°), and a pole position at latitude λp = 46.0°S, longitude φp = 49.6°E (A 95 = 6.4°).

2.62 Ga high‐temperature metamorphism in the central part of the Eastern Dharwar Craton: implications for late Archaean tectonothermal history

Aluminous metasediments occurring in the central part of the Eastern Dharwar Craton (EDC), southern India, develop contrasting mineral assemblages in closely spaced domains. Detailed petrographic, mineral chemical and geothermobarometry showed ‘peak’ metamorphic conditions of 780–820 °C and ~5 kbar that resulted in partial melting of biotite‐bearing protoliths. Initial bulk compositional heterogeneity and different degrees of melt extraction produced diverse restitic mineral assemblages. P–T pseudosections, constructed from effective bulk composition, and consideration of the sequence of mineral reactions, deduced from textural criteria, define a nearly isobaric heating–cooling P–T trajectory. Monazite in the metasediments has a detrital core age of 3161 ± 78 Ma and a metamorphic age of 2625 ± 26 Ma (EPMA chemical age), which is the first record of a pre‐2500 Ma high temperature–low pressure metamorphism event in the EDC. Based on age contemporaneity with the 2.7–2.62 Ga hot juvenile magmas of tonalite–trondhjemite–granodiorite (TTG)‐greenstone accretion and the deduced P–T history, we conclude that heat advecting from rising TTG magmas and mafic magmas (greenstones) as a consequence of mantle melting, resulted in a hot transient mid‐crust during a major late Archaean tectonothermal event in the Eastern Dharwar Craton. All the phenomena are consistent with a recently proposed model involving an arc setting during ridge subduction and consequent opening of a slab window. This study therefore provides new constraints on the thermal evolution of the Eastern Dharwar Craton. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Stratigraphy-related, low-pressure metamorphism in the Hardey Syncline, Hamersley Province, Western Australia

The late Archean Fortescue Group and Paleoproterozoic Hamersley Group are exposed in the Hardey Syncline located at the southwestern edge of the Hamersley Basin, Pilbara Craton, Western Australia. The secondary mineral assemblages and compositions of the basaltic rocks of the Fortescue and Hamersley Groups reveal the metamorphic conditions of the study area. The estimated metamorphic grade ranges from prehnite–actinolite facies (Hamersley Group), through greenschist facies (Fortescue Group), to a transition between greenschist facies and actinolite–calcic plagioclase facies (Fortescue Group), indicating a low-pressure type metamorphic facies series. The metamorphic grade increases northward, which is opposite to the general southward increase of the regional metamorphic grade. Furthermore, the change of metamorphic grade strongly correlates with stratigraphy, and the metamorphic temperature increases with stratigraphic depth. These observations suggest that the metamorphism of the study area was caused by a thermal event before the folding due to the Ophthalmian orogeny that affected most of the Hamersley Province. Considering the presence of 2.2 Ga dolerite sills in the Hardey Syncline and the low-pressure metamorphic condition, it is suggested that the metamorphism of the study area was caused by the 2.2 Ga continental rifting, which is consistent with the reported metamorphic age.

The Calderian orogeny in Wopmay orogen (1.9 Ga), northwestern Canadian Shield

The Wopmay orogen is a Paleoproterozoic orogenic belt formed in part by the accretion of Hottah terrane, an east-facing continental magmatic arc, to the western margin of the Archean Slave craton at ca. 1.88 Ga. The arc-continent collision was responsible for the Calderian orogeny. Just prior to the collision, arc volcanism of the Hottah terrane had migrated trenchward and changed in composition from an aphyric calc-alkaline to a bimodal tholeiitic suite. The change in magmatism, along with subsidence and consequent high-temperature–low-pressure metamorphism of rocks on the upper plate, is attributed to extension and asthenospheric upwelling during rollback of the lower plate. The upwelling led to regional heating of the crust, melting, and generation of the metaluminous to peraluminous Hepburn intrusive suite within the arc just prior to accretion of Hottah terrane onto the Slave craton. During the collision, a 200-km-long, orogen-parallel swarm of mafi c sills was intruded into the lower plate and trench-fill sediments at the passive-margin shelf-slope break. The mafi c magmatism may have been induced by slab breakoff. The Calderian orogeny provides an excellent example of the magmatic history of an arc-continent collision. The Calderian orogeny was short-lived. Within a few million years, an east-dipping subduction zone formed outboard of the accreted terrane, creating volcanic rocks that were erupted atop the eroded HottahSlave collision zone. The short duration of the Calderian orogeny seems typical of arccontinent collisions, perhaps because the attenuated arc crust, coupled with the extended crust of the lower plate, never generates severely overthickened crust. Rise of asthenospheric mantle due to slab breakoff generates rapid uplift, erosion, and collapse of the mountains built during collision.

Proterozoic granites of the Llano Uplift, Texas: A collision-related suite containing rapakivi and topaz granites

Proterozoic granites in the Llano Uplift, central Texas, intruded metamorphic rocks in a southern segment of the Grenville orogen. Three compositional groups of granite are present; two of these have not been previously recognized. The most widespread group occurs in three emplacement styles: (1) irregular plutons (synkinematic) were emplaced at 1119–1116 Ma during low-pressure Grenville metamorphism and deformation; (2) associated with these, there are volumetrically minor rhyolite dikes (1098–1093 Ma) and hypersolvus granite; and (3) large 1093–1072 Ma postkinematic circular to ovoid plutons are the most abundant of this compositional group, and many show rapakivi textures. A second, more felsic group has much higher light (L) to heavy (H) rare earth element (REE) ratios. The third group is highly fractionated, contains topaz locally, is enriched in Rb, Nb, and HREEs, and is depleted in Sr, Zr, and LREEs. No other igneous rocks were emplaced at the same time as the granites at the current level of exposure. Llano granites cannot be considered anorogenic or A-type on the basis of their tectonic setting, timing, or compositions. Sr, Nd, and Pb isotopic data indicate that these granites were derived from Proterozoic mantle and crust. The granites intruded thick continental crust at one margin of Laurentia and completed the consolidation of one segment of the Grenville orogen.

Decoding Mesoproterozoic and Cambrian metamorphic events in Willyama Complex metapelites through the application of Sm–Nd garnet geochronology and P–T pseudosection analysis

The pressure–temperature ( P–T) path for the Palaeoproterozoic Willyama Supergroup rocks in the Olary Domain, South Australia has been reconstructed through detailed petrographic observations, in conjunction with calculation of compositionally specific P–T pseudosections of metapelitic rock units and Sm–Nd garnet geochronology. The P–T path for the Willyama Complex has historically been interpreted to follow a single anticlockwise path, however the results of this study demonstrate that this path can be better described by two metamorphic events (M 1 and M 2) separated by 1100 Ma. The M 1 event occurred at c. 1600 Ma and was associated with high temperature–low pressure metamorphism. Sm–Nd garnet geochronology constrains the timing of garnet growth at c. 1585 Ma. The growth of large andalusite porphroblasts during M 1 exerted a first order control on the bulk composition during the subsequent M 2 event. Mineral chemistries coupled with quantitative phase diagrams constrain peak conditions to be in the order of c. 550°C and 5.5 kbar during M 2. The identification of two metamorphic events calls into question interpretations of metamorphic core complex formation and the single anticlockwise P–T paths being associated with the early stages of the tectonic evolution of the terrain.

Low pressure migmatites from the Sanandaj-Sirjan Metamorphic Belt in the Hamedan region (Iran)

Low pressure migmatites from the Sanandaj-Sirjan Metamorphic Belt in the Hamedan region (Iran)Migmatites with evidence for low pressure metamorphism and partial melting occur adjacent to the Alvand Plutonic Complex in the Hamedan region of Iran. They show stromatic, schollen, diktyonitic and massive structure. Sillimanite/andalusite/(kyanite)-garnet- and cordierite-K-feldspar-andalusite-spinel-bearing migmatites are the most common rock types. Some of the granitic intrusions contain xenocrysts which resemble the porphyroblasts of nearby migmatites (e.g. sillimanite, andalusite, cordierite and garnet). Although migmatitic rocks of the region are located near the granitic intrusions, the degree of partial melting is not related to intrusions and is irregular. It appears that partial melting and migmatization pre-date the intrusion of major granitic bodies in the region. Leucosomes in stromatic migmatites are commonly parallel to bedding planes and are mostly formed by metamorphic segregation and/orin situpartial melting (showing mafic selvedges, pinch and swell structures). The melt fraction and migmatite type depend on the chemical composition of parent rocks and the distribution of high strain zones. The formation of thin leucosomes in the stromatic migmatites was controlled by short-range melt movement along the grain boundaries. Melt-rich layers are constrained by pre-existing compositional layering and foliation. Peak metamorphic conditions of ~650 °C and ~300 MPa are consistent with the observed mineral assemblages and the presence of melt in the investigated migmatites.

Variscan intra-orogenic extensional tectonics in the Ossa-Morena Zone (Évora-Aracena-Lora del Rı́o metamorphic belt, SW Iberian Massif): SHRIMP zircon U-Th-Pb geochronology

Abstract Following a Middle–Late Devonian ( c . 390–360 Ma) phase of crustal shortening and mountain building, continental extension and onset of high-medium-grade metamorphic terrains occurred in the SW Iberian Massif during the Visean ( c . 345–326 Ma). The Évora–Aracena–Lora del Rı́o metamorphic belt extends along the Ossa–Morena Zone southern margin from south Portugal through the south of Spain, a distance of 250 km. This major structural domain is characterized by local development of high-temperature–low-pressure metamorphism ( c . 345–335 Ma) that reached high amphibolite to granulite facies. These high-medium-grade metamorphic terrains consist of strongly sheared Ediacaran and Cambrian–early Ordovician ( c . 600–480 Ma) protoliths. The dominant structure is a widespread steeply-dipping foliation with a gently-plunging stretching lineation generally oriented parallel to the fold axes. Despite of the wrench nature of this collisional orogen, kinematic indicators of left-lateral shearing are locally compatible with an oblique component of extension. These extensional transcurrent movements associated with pervasive mylonitic foliation ( c . 345–335 Ma) explain the exhumation of scarce occurrences of eclogites ( c . 370 Ma). Mafic-intermediate plutonic and hypabyssal rocks ( c . 355–320 Ma), mainly I-type high-K calc-alkaline diorites, tonalites, granodiorites, gabbros and peraluminous biotite granites, are associated with these metamorphic terrains. Volcanic rocks of the same chemical composition and age are preserved in Tournaisian–Visean ( c . 350–335 Ma) marine basins dominated by detrital sequences with local development of syn-sedimentary gravitational collapse structures. This study, supported by new U–Pb zircon dating, demonstrates the importance of intra-orogenic transtension in the Gondwana margin during the Early Carboniferous when the Rheic ocean between Laurussia and Gondwana closed, forming the Appalachian and Variscan mountains.

Modelling the thermal evolution of a collisional Precambrian orogen: High heat production migmatitic granites of southern Finland

We present results of thermal and geochemical modelling of Paleoproterozoic southern Finland, which is characterized by formation of high heat production migmatitic granites, post-orogenic granitoids, and high temperature–low pressure metamorphism at 1.85–1.78 Ga during late Svecofennian time. Metamorphic grade is mostly amphibolite facies (4–5 kbar, 650–700 °C), but also granulite facies (6 kbar, up to 800 °C) occurs. Our results suggest that the thermal evolution of the area can be modelled as a process of rapid crustal thickening (up to 70 km) in a plate collision at ca. 1870–1860 Ma ago, followed by conductive heating of the crust, simultaneous exhumation and partial melting of middle crust, and emplacement of the melts in the upper crust as late orogenic high heat production granites at about 1825 Ma ago. The upper crust collapsed gravitationally at about 1820 Ma but the crust was still thick (57 km) when the main orogenic era terminated about 1800 Ma ago. In modelling the thermal evolution we applied crustal heat production values constrained by geochemical data, and a conservative estimate, i.e., relatively low value for mantle heat flow density. The modeled temperatures at 30–50 km depths in the thickened crustal stack exceed the dehydration melting temperatures of micas (at 40 km 750 °C for muscovite and 800 °C for biotite, respectively) 25–35 Ma after the collision and thickening of crust. The results explain the high temperature–low pressure metamorphism in southern Finland as an inevitable consequence of crustal thicknening leading to an increase of the total heat production of the crust. Crustal thicknening is the main heat source for melting and metamorphism of the orogeny. Advective and latent heat brought by the melts further increased the temperatures at the emplacement depth and contributed to the granulite facies metamorphism that is spatially related to the migmatitic granites. Geochemical models suggest that the high concentrations of U and Th in the migmatitic granites can be attributed to about 20 vol% partial melting of deeply buried sediments and volcanic rocks. About 36–71% of the original U, 18–54% of Th and 34–46% of K, respectively, in the source rock types entered the melt phase. No anomalous concentrations of these elements in the source rock are required. In addition to explaining the migmatitic late orogenic granite and post-orogenic granitoid genesis, the collision model implies that the middle-lower crust and the upper mantle continued warming until the Mesoproterozoic times, which suggests a causal link between the Svecofennian thermal evolution and the extensive bimodal rapakivi granite and anorthosite magmatism in the Fennoscandian Shield about 200–250 Ma later.

The Nagssugtoqidian orogen in South-East Greenland: Evidence for Paleoproterozoic collision and plate assembly

The 200 km wide, east-west trending Paleoproterozoic mobile belt of the Ammassalik region of South-East Greenland contains a diverse assemblage of Paleoproterozoic and Archean rocks, variably affected by Paleoproterozoic deformations and high-grade low or high pressure metamorphism. By using previous field and geochemical data combined with new zircon dating and zircon trace element geochemistry, this mobile belt is confirmed as a 1870 to 1840 Ma collisional orogen, which contains one or more 1900 to 1880 Ma magmatic suites and northern and southern Archean basement terranes. The most studied 1900 to 1880 Ma magmatic suite is the Ammassalik Intrusive Complex, which is dominated by diorites (with arc-like geochemical signatures and with Paleoproterozoic Nd depleted mantle model ages), which was intruded into sedimentary rocks, (with predominantly Paleoproterozoic detrital zircons). Both these associated rock types show 1900 to 1880 Ma moderate pressure granulite facies metamorphism (7 kbar, ≥ 850°C). Paleoproterozoic mylonites separate the Ammassalik Intrusive Complex from Archean orthogneisses containing 1870 Ma eclogite (≥ 11 kbar, 650 –700°C) and high-pressure granulite facies assemblages in Palaeoproterozoic diabase dike remnants. Associated with these Archean gneisses are pelitic metasediments, marbles and orthoquartzites (with Archean detrital zircon with complex 1870 to 1740 Ma metamorphic rims) that contain kyanite, thereby also showing high-pressure metamorphism. In the Ammassalik area we propose that one or more Paleoproterozoic magmatic arcs with syn-magmatic moderate pressure, high temperature metamorphism were emplaced over the edge of southern Archean continental crust. This resulted in at least doubling of crustal thickness, causing the transient 1870 Ma eclogite to high-pressure granulite facies metamorphic conditions in the buried southern Archean terrane. Archean orthogneisses in the north of the mobile belt preserve low-pressure Archean granulite facies assemblages, and are interpreted as a different terrane at a higher structural level, which was juxtaposed with the southern eclogite-bearing Archean terrane and the lower pressure metamorphism Paleoproterozoic arc rocks during the collisional event(s). This new information from the Ammassalik region is used in a synthesis of Paleoproterozoic crustal accretion and collisional orogeny in Greenland.

Análise microestrutural e de eixos-c de quartzo na Seção Aimorés (MG)-Colatina (ES)

This work presents microstructural and crystallographic c-axis pattern data in high grade metamorphic rocks in the Aimores (MG) - Colatina (ES) section. Undullatory extinction in several minerals, bent twins in plagioclase and recrystallization of quartz as ribbons and chessboard patterns, which are features consistent with a high temperature deformation path (550 - 700 o C). The c-axis pattern suggests basal - prism[c] switch and form type-II crossed girdles related to non-coaxial deformation. The kinematic indicators at the microscopic scale suggest compressive top-to-SW-SSW and extensive top-to-NE-NNE movements, suggesting kinematic superposition. The presence of cordierite implies a low pressure metamorphism path, possibly related to the exhumation history

Constraining the metamorphic evolution of a cryptic hot Mesoproterozoic orogen in the Central Indian Tectonic Zone, using P– T pseudosection modelling of mafic intrusions and host reworked granulites

In this study, we reconstruct the metamorphic pathways of reworking of a deep crustal granulite terrane in the Central Indian Tectonic Zone, using Archaean/Palaeoproterozoic (?) polycyclic felsic granulites and two groups of intrusive Mesoproterozoic mafic granulites, a coarse-grained noritic gabbro and a fine-grained gabbroic norite. The granulite terrane, locally referred to as the Bhandara–Balaghat granulite domain, is bounded by the South Indian Block and a Grenville-aged, younger tectonic domain of the Central Indian Tectonic Zone. The granulites and the mafic intrusions were multiply deformed and metamorphosed in the early Mesoproterozoic. Using P– T pseudosection modelling of host felsic granulites and the mafic intrusions, two distinct metamorphic events (BM 2 and BM 3) with contrasting P– T paths have been established. The P– T path of BM 2 metamorphism has a clockwise sense, having an important prograde segment of heating of more than 250 °C with pressure fall, followed by cooling. The peak BM 2 metamorphism has been constrained at ∼6 kbar, ∼725 °C. Emplacement of the mafic intrusions was broadly coincident with the low-pressure metamorphism, and they underwent a phase of subsolidus cooling. During subsequent metamorphism, BM 3, the mafic intrusions and the host felsic granulites were re-metamorphosed along a counterclockwise P– T path. This led to tectonic burial of the mid-crust to ∼9.4 kbar, ∼760 °C, which was followed by cooling accompanying pressure decline. Although, the clockwise P– T path is generally interpreted in terms of thermal relaxation of crust following thrusting, the scale of heating (∼250 °C) during initial decompression, documented here, is much larger than modelled for tectonically thickened crust, but consistent with magmatically active extensional zones. Based on this evidence and also the syn-metamorphic mafic intrusions, we present an alternate interpretation in terms of mid-crustal extension for the BM 2 metamorphism. In contrast, the counter clockwise P– T path during BM 3, recording cooling following prograde burial is explained by tectonic thickening of a hot mid-crust. Collating available geochronological data, the two metamorphic events appear to indicate tectonic switching from lithospheric extension to contraction in the early Mesoproterozoic. The findings provide the first quantitative constraints on an early Mesoproterozoic hot orogen at the craton–mobile belt interface in the Central Indian Tectonic Zone.

Major and trace element chemistry and Sm–Nd age correlation of magmatic pegmatite garnet overprinted by eclogite-facies metamorphism

Meta-pegmatites are widespread in the polymetamorphic Koralpe–Saualpe basement, an Austroalpine unit with eo-Alpine high-pressure overprint. Major and trace element data of coarse-grained garnet and equilibrium relationships with other pegmatite minerals indicate that garnet crystallized during pegmatite emplacement. Whole rock and garnet separates from the meta-pegmatites yield Sm–Nd isochrons with Late Permian–Early Triassic ages of 254 ± 2–239 ± 2 Ma. Surprisingly, the Cretaceous eclogite facies metamorphic overprint did not produce pervasive material exchange between garnet and matrix. Only the outermost 10–50 μm thick rims of cm-sized garnet crystals show diffusive major element re-equilibration. Detailed Sm–Nd investigations, using different separation techniques demonstrate isotopic equilibration of garnet, inclusions and the whole rock. High-precision age information is provided by garnet with extremely high 147Sm/ 144Nd ratios of 2−10, best-fit isochrons of multiple garnet magnetic fractions and reproducible results. However, age variations between different samples and between different crystals separated from single samples range outside analytical uncertainties. The Sm–Nd age results, covering a time-span of 18 Ma, are interpreted as reflecting long-term, though continuous garnet crystallization during pegmatite-emplacement. This was supposedly related to low pressure metamorphism with widespread formation of metamorphic andalusite in the metapelitic pegmatite host-rock. Protracted magmatic and metamorphic processes are in line with a tectonic scenario of crustal scale extension and initial rifting causing highly perturbed geothermal gradients, partial melting of metapelitic rocks and enhanced magmatic activity as postulated for the Austroalpine basement in Permian–Triassic time.

Pre- to syn-extension melt-assisted nucleation and growth of extensional gneiss domes: The western French Massif Central (Variscan belt)

Evidence for melt-assisted development of extensional gneiss domes is provided using the combined analysis of geological, geophysical, and geochronological data on the Millevaches dome and the adjacent Argentat fault (AF), in the western Massif Central, France. The Millevaches dome consists of mica-schists associated with monzogranites (344–342 Ma), migmatites (337–334 Ma) and leucogranites (329–318 Ma). These melts were emplaced before, at the onset of, and during the slip of the AF (∼335–315 Ma), respectively. The AF is a 5-km-wide, crustal-scale, westward dipping shear zone along which S 3 foliation strikes NNW and L 3 lineation trends WNW. Kinematic indicators show uniform top-to-the WNW sense of shear coeval with retrograde low-pressure metamorphism, indicative of a normal-sense movement along the AF. The Millevaches dome is interpreted to be an extensional gneiss dome cored by migmatites and monzogranite derived from a mid-crustal molten layer inherited from collision and modified by addition of syn-extension leucogranites. The dome formed by crustal-scale normal shearing along the AF and coeval exhumation of deep-seated rocks in the footwall. Pre-extension melts have allowed dome nucleation at the onset of extension by localizing normal shearing within the molten layer. Syn-extension melts, given their quantity and emplacement mode within the dome, have enhanced the persistence of dome growth and its ultimate geometry.

Diffusion-controlled development of silica-undersaturated domains in felsic granulites of the Bohemian Massif (Variscan belt of Central Europe)

Plagioclase rims around metastable kyanite crystals appear during decompression of high-pressure felsic granulites from the high-grade internal zone of the Bohemian Massif (Variscan belt of Central Europe). The development of the plagioclase corona is a manifestation of diffusion-driven transfer of CaO and Na2O from the surrounding matrix and results in isolation of kyanite grains from the quartz- and K-feldspar-bearing matrix. This process establishes Si-undersaturated conditions along the plagioclase- kyanite interface, which allow crystallization of spinel during low-pressure metamorphism. The process of the plagioclase rim development is modeled thermody- namically assuming local equilibrium. The results combined with textural observations enable estimation of equilibration volume and diffusion length for Na and Ca that extends ~400-450 and ~450-550 lm, respectively, around each kyanite crystal. Low esti- mated bulk diffusion coefficients suggest that the diffusion rate of Ca and Na is controlled by low diffusivity of Al across the plagioclase rim.

Evidence of contrasting low-grade metamorphic conditions from clay mineral assemblages in Triassic Alpujárride-Maláguide transitional units in the Betic Cordilleras, Spain

Triassic sequences from ‘intermediate units’ between the Alpujarride and the Malaguide complexes (Betic Cordilleras, Spain) of the westernmost part of the Cordilleras (Casares area) occur as four superimposed tectonic units; the uppermost unit shows lithological characteristics similar to those of the Malaguide complex, changing progressively at increasing depth, towards lithologies typical of the Alpujarride complex. The units studied, with a maximum thickness of ~400 m, record important variations in metamorphic pressures, according to the b parameter of white micas: from low-pressure metamorphism (in the upper unit) to high-pressure facies series (in the deepest one). The mean b values range from 8.988 A in the uppermost unit (Crestellina) to 9.042 A in the lowermost one (Jubrique). The lowest metamorphic grade is represented by mineral assemblages consisting of phengite + intermediate Na-K white mica ± Fe-chlorite ± sudoite ± pyrophyllite, which record temperatures of ~300°C and pressures of 1.5–3 kbar. At increasing tectonic depth, intermediate Na-K mica and pyrophyllite disappear and the metamorphic assemblages consist of phengite ± paragonite ± margarite + Mg-chlorite ± sudoite, which record minimum pressures of ~7 kbar and temperatures in the order of 400–450°C. These mineral assemblages provide evidence of the passage from collisional to extensional geotectonic settings. The units showing different metamorphic patterns were juxtaposed tectonically, after the development of metamorphic mineral assemblages.