High-pressure Low-temperature Metamorphism Research Articles

Proterozoic granulite formation driven by mafic magmatism: An example from the Fraser Range Metamorphics, Western Australia

Elevated heat flow and mafic magmatism during lithospheric extension have often been invoked as a mechanism to drive high-temperature low-pressure metamorphism that produces granulite facies mineral assemblages. Typically, however, evidence of the contemporaneous heat source, such as coeval mafic magmatism, is absent. In this study, we present pressure–temperature (P–T) pseudosection analysis combined with U–Pb isotopic data from zircon and monazite that constrain both the conditions and timing of granulite facies metamorphism in the Fraser Range Metamorphics of the Albany-Fraser Orogen in southern Western Australia. These results also elucidate the extremely rapid timing of, sequentially, deposition of sedimentary protoliths, mafic magmatism, partial melting, and metamorphism within the Fraser Zone during the Mesoproterozoic. The youngest detrital zircons, together with the magmatic ages of intrusive rocks, constrain the depositional age of the protoliths to the Fraser Range Metamorphics to between 1334 and 1293Ma. Peak metamorphic conditions at c. 1290Ma were c. 850°C at pressures of 7–9kbar. Peak metamorphism was followed by a period of isobaric cooling at pressures of c. 9kbar. U–Pb zircon ages from leucosomes and metamorphic overgrowths in the metapelitic rocks indicate crystallization of partial melts at 1290Ma, essentially coincident with the emplacement of mafic rocks at 1292Ma. In situ analyses of both matrix hosted monazite and monazite inclusions in garnet yield ages between 1285 and 1268Ma, with no significant age difference between monazite in the two textural positions. Cooling of the Fraser Zone below the Rb-Sr biotite closure temperature (∼400°C) occurred at 1260Ma. Cooling and strengthening of the Fraser Zone rendered it less susceptible to subsequent tectonic events that affected rocks to the north and south of this resistant lozenge. Only rare geochronological evidence of later events can be resolved in recrystallised monazite rims dated at 1234±17Ma.

Tectonic development of the Vardar ocean and its margins: Evidence from the Republic of Macedonia and Greek Macedonia

The tectonic development of the northeast margin of the Korabi-Pelagonian continent and the adjacent Vardar ocean is discussed based on study of nine traverses across the former northeastern margin of the Korabi-Pelagonian continent, represented by the West Vardar subzone in the Republic of Macedonia. A detailed correlation of units is also made across the international border with Greece, coupled with regional comparisons.Precambrian–Palaeozoic units were metamorphosed and intruded by granitic rocks, followed by Mid-Triassic rifting and the construction of a carbonate platform bordering a Late Triassic–Early Jurassic ocean. Neritic deposition was followed by Late Jurassic (syn‐ or pre-Early Tithonian) subsidence with radiolarian and then terrigenous turbidite sedimentation. Overlying ophiolitic rocks are characterised by a relatively thin (<500m), laterally persistent (>200km N–S) sheet of sheared, blocky serpentinite (mainly mantle harzburgite), associated with ophiolite-derived debris flows. The emplaced ophiolitic rocks were subaerially weathered to form local Fe–Ni accumulations. Following marine transgression, a shallow-water carbonate–siliciclastic deposits, including coralgal reefs, accumulated during Late Tithonian–Early Cretaceous. After a hiatus, mixed terrigenous-carbonate gravity flows accumulated in deep water during the Late Cretaceous.The Vardar ocean opened during the Late Triassic–Early Jurassic separating the Korabi-Pelagonian and Serbo-Macedonian continents. Northeastward subduction created a Late Jurassic magmatic arc along the southern margin of the Serbo-Macedonian continent, while future ophiolites formed by supra-subduction zone spreading within the Vardar ocean. These ophiolites were emplaced in response to collision of the subduction trench with the Korabi-Pelagonian continent. The pre-Mesozoic basement detached and subducted undergoing high pressure–low temperature metamorphism during the Late Jurassic (Tithonian), followed by amphibolite/greenschist facies metamorphism and exhumation prior to the Late Cretaceous. A much reduced Vardar ocean survived into the Late Cretaceous until this finally closed during the latest Cretaceous–Early Cenozoic, triggering continental collision and thick-skinned folding/thrusting of the Pelagonian, Vardar and Serbo-Macedonian zones towards the W/SW.

Metamorphic evolution of pelitic–semipelitic granulites in the Kon Tum massif (south-central Vietnam)

Pelitic and semipelitic anatectic granulites form one of the major lithological units in Kan Nack complex of the Kon Tum massif (in south-central Vietnam), which comprises HT metamorphic and magmatic rocks including granulites and charnockites is classically regarded as the older part of the Gondwana-derived Indosinia terrain. Metamorphic evolution study of pelitic granulite, the most abundant among granulites exposed in this massif, facilitates to understand that tectonic setting take place during the Indosinian time. The paragenetic assemblages, mineral chemistry, thermobarometry and P–T evolution path of pelitic–semipelitic granulites from Kon Tum massif has been studied in detail. Petrographic feature demonstrates that the pelitic granulite experienced prograde history, from pregranulitic conditions in the amphibolite facies up to the peak granulitic assemblages. Successive prograde reactions led to the temperature-climax giving rise to assemblages with cordierite-hercynite and cordierite-hercynite-K-feldspar. Then, as attested by the mineralogic association occurring in cordieritic coronas, these rocks have been affected by retrograde conditions coeval with a decrease of the pressure. Thermobarometic results show that the highest temperature obtained by ksp/pl thermometry is 850°C and the highest pressure obtained by GASP (Garnet Alumino-Silicate Plagioclase) is 7.8kbar. The obtained clockwise P–T evolution path involving heating decompression, then nearly isothermal decompression and nearly isobar cooling conditions shows that high temperature–low pressure metamorphism of the studied pelitic anatectic granulites of Kan Nack complex occurred possibly in extensional setting during the Indosinian orogeny of 260–240Ma in age.

Paleoproterozoic–earliest Mesoproterozoic basin evolution in the Mount Isa region, northern Australia and implications for reconstructions of the Nuna and Rodinia supercontinents

Paleoproterozoic-earliest Mesoproterozoic sequences in the Mount Isa region of northern Australia preserve a 200 Myr record (1800-1600 Ma) of intracontinental rifting, culminating in crustal thinning, elevated heat flow and establishment of a North American Basin and Range-style crustal architecture in which basin evolution was linked at depth to bimodal magmatism, high temperature-low pressure metamorphism and the formation of extensional shear zones. Rifting initiated in crystalline basement ≥1840 Ma old and produced three stacked sedimentary basins (1800-1750 Ma Leichhardt, 1730-1670 Ma Calvert and 1670-1575 Ma Isa superbasins) separated by major unconformities and in which depositional conditions progressively changed from fluviatilelacustrine to fully marine. By 1685 Ma, a deep marine, turbidite-dominated basin existed in the E and basaltic magmas had evolved in composition from continental to oceanic tholeiites as the crust became increasingly thinned and attenuated. Except for an episode of minor deformation and basin inversion at c. 1640 Ma, sedimentation continued across the region until onset of the Isan Orogeny at 1600 Ma. A near-identical record of crustal thinning and basaltic magmatism accompanied basin formation (lower Willyama Supergroup) in the formerly contiguous Broken Hill region from 1730-1670 Ma. This was followed by further extension and a second phase of basin development that lasted until at least 1640 Ma. Modern-day rifted continental margins preserve a comparable record of crustal thinning and near-continuous basin formation over 100-200 Myr timescales, supporting suggestions that the late Paleoproterozoic-early Mesoproterozoic rift basins of Mount Isa and Broken Hill similarly evolved to continental breakup and formed part of a continental margin sequence no later than 1640 Ma and possibly as early as 1670 Ma. This rifted margin predates assembly and breakup of the Neoproterozoic Rodinia supercontinent to which Australia once belonged and best accords with a pre-Rodinia, SWEAT-like super-continent (Nuna) that matches the E-facing late Paleoproterozoic-early Mesoproterozoic rift sequences of eastern Australia against rocks of comparable age in western Canada. Reconstructions of Rodinia (AUSWUS) based on the distribution of Grenville-age orogenic belts that coincidently position the continental rift sequences of Broken Hill along strike from more juvenile 1700-1650 Ma accreted terranes in the SW United States (Yavapai and Mazatzal provinces) are only possible if the proposed alignment of terranes is not original but an artefact of Neoproterozoic supercontinent assembly. The SWEAT hypothesis avoids this complication but, like AUSWUS, presupposes that eastern Australia and western Laurentia remained juxtaposed throughout the Mesoproterozoic until onset of Rodinia breakup after 830 Ma.

Deciphering subduction from exhumation in the segmented Cycladic Blueschist Unit (Central Aegean, Greece)

The Cycladic Blueschist Unit (CBU) represents the northern passive margin of the Adria continental block and ophiolites that are the remnants of the Pindos Ocean, which were affected by high pressure–low temperature metamorphism in the blueschist and eclogite facies during the Eocene. Prior to the Lutetian, the ophiolitic and margin units were thrust towards the SW inside a NE-dipping subduction zone. Two subsequent exhumation stages are characterized by top-to-the-NE senses of shear: i) from mantle depths to lower crustal levels, before 37Ma, which was accommodated by an extensional inversion of the Vardar suture zone, and ii) from deep crustal levels to the near-surface, between 30 and 20Ma, which was accommodated by the North Cycladic Detachment. It is during this late stage of ductile exhumation that the central Cyclades Core Complex was exhumed. Segmentation of the CBU by normal faults started at 12Ma, as recorded by the low temperature thermochronometers, and was coeval with block rotation and folding with NS-trending axes, which were predominantly controlled by the Myrthes–Ikaria strike-slip fault in the centre of the Cyclades. A map restoration of the CBU geometry, prior to segmentation, has been carried out using available structural and paleomagnetic data. The restoration shows that, following the CBU exhumation from mantle depths to crustal levels, a single core complex was exhumed along a single NE dipping detachment.

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.

Tectono-metamorphic evolution of the siliciclastic units in the Middle Tuscan Range (inner Northern Apennines): Mg–carpholite bearing quartz veins related to syn-metamorphic syn-orogenic foliation

This paper accounts for a new finding of High Pressure–Low Temperature (HP-LT) metamorphism documented in continental-derived rocks exposed in the central part of the Middle Tuscan Range, representing the easternmost location, in the Tyrrhenian Region, where blueschist facies metamorphism has been documented. It describes the development mechanism of Mg–carpholite bearing quartz veins in the framework of the deformational events described for the inner Northern Apennines. The HP-LT peak metamorphism can be estimated in P≥1.1GPa and T=370°–420°C as supported by the mineralogical assemblage consisting of quartz–muscovite–chloritoid–pyrophylite–hematite±chlorite±paragonite±Ti-oxides, documented in the aluminous metasediments (pelites and quartzose sandstones) of the Triassic Verrucano succession. This paragenesis defines the S1 schistosity, an axial planar tectonic foliation associated to map- and mesoscale isoclinals folds (F1); S1 schistosity is a composite foliation resulted from a non-coaxial progressive deformation associated to the DE1 collisional event (Late Oligocene–Early Miocene). Mg–carpholite bearing quartz veins developed as tension gashes in en-échelon arrays within brittle/ductile shear zones. The development of quartz veins was guaranteed by the high volumes of Si-rich fluids produced by the dynamic recrystallization process on the siliciclastic rocks.

Tectonic Implications of Early Paleozoic Metamorphism in the Anakie Inlier, Central Queensland, Australia

Well-defined metamorphic zones are developed in pelitic and psammitic rocks of the Late Neoproterozoic to Cambrian Anakie Metamorphic Group of the Anakie Inlier, central Queensland. They are defined by the incoming of biotite, garnet, and andalusite, with or without staurolite. Mineral assemblages indicate that low pressure–high temperature metamorphism is associated with D1, medium pressure–high temperature metamorphism with D2, and retrograde, low pressure–low temperature metamorphism with D3. A mean b cell parameter of 9.035 obtained from K-white micas in the lowest-grade rocks suggests upper intermediate pressure conditions during D2. The timing of the growth of the index minerals indicates that isotherms retreated to the southwest during D2. Phase diagram calculations for both Al-saturated and Al-undersaturated metapelites containing D1 associations indicate pressure-temperature (P-T) conditions of 0.4 GPa and 560°C. These changed to 0.64–0.79 GPa and 580°–640°C as a result of crustal thickening. The samples thus record a history of heating (D1), followed by near-isothermal compression (D2). This P-T pathway shows that contraction rather than extension, as suggested by some authors, occurred during D2. The contractional event is suggested to have taken place during the Delamerian Orogeny as a result of convergence and collisional processes along the former passive margin of Gondwana.

Thermal gradient and timing of high‐T–low‐P metamorphism in the Wongwibinda Metamorphic Complex, southern New England Orogen, Australia

AbstractThe Wongwibinda Metamorphic Complex (WMC) is a high‐temperature, low‐pressure (HTLP) belt in the southern New England Orogen. It is characterized by a high metamorphic field gradient exposed in variably metamorphosed siliceous turbidites. The Abroi Granodiorite and the Rockvale and Tobermory adamellites, S‐type granitoids of the Hillgrove Plutonic Suite, intrude the metaturbidites. Six samples of metaturbidite were studied from an ∼3 km long field traverse. Integrated petrography, mineral chemistry, and mineral equilibria modelling indicate a peak metamorphic temperature of 350–450 °C in the lowest grade rocks and ∼660 °C in the highest‐grade rocks. Maximum pressure does not exceed 3.5 kbar anywhere, implying a maximum depth of 12 km and indicating an average vertical gradient of at least 55 °C km−1, though our calculations suggest this is not linear. Metamorphic isograds show no apparent relationship with distance to the exposed margins of the Hillgrove Suite granitoids. Electron microprobe U–Th–Pb monazite data indicate a date of 296.8 ± 1.5 Ma for the thermal peak of the HTLP metamorphism. Laser ablation inductively coupled plasma mass spectrometry indicates a zircon U–Pb crystallization age of 290.5 ± 1.6 Ma for the Abroi Granodiorite, confirming that the pluton post‐dates the peak HTLP metamorphism. Consequently, magmatic advective heat transfer from depth via emplacement of a large volume of granitoid is unlikely to be the key local driver of the high‐grade metamorphism. It is concluded that published evidence of an extensional geodynamic setting around the Carboniferous–Permian boundary supports conductive heat transfer as the key driver of HTLP metamorphism for the WMC. It is not possible to exclude magmatic advective heat transfer via emplacement of mantle derived basaltic magmas in the deeper crust.

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.

New age data from the Dzirula massif, Georgia: Implications for the evolution of the Caucasian Variscides

The Caucasus mountain belt has a complex tectono-thermal history that is as yet poorly understood due to the scarcity of reliable geochronological information. Here, we report new monazite and zircon data from the Dzirula massif in the southern Caucasus, which permit development of a model for Variscan LP-HT (low pressure–high temperature) metamorphism in this region. Data are presented for two key lithologies of the Dzirula massif: 1) a group of variably deformed granitoid (mainly granodioritic-tonalitic) rocks and 2) metapelitic LP-HT cordierite-biotite-sillimanite migmatites and paragneisses. Electron-microprobe Th-U-total Pb monazite dating demonstrates that the LP-HT metamorphism in the Dzirula massif is Variscan, and occurred around 330 Ma. LA-ICP-MS zircon dating reveals that the granitoids include two different magmatic series. An older, mainly tonalitic, series is of Lower Cambrian age. Younger intrusions, including gabbros and diorites, but mainly tonalites and granodiorites, are Variscan in age. The older series show ductile deformation features and, along with the metapelites, experienced Variscan high-grade metamorphism that resulted in penetrative blastic recrystallization and anatexis. The younger series is generally undeformed, but often shows a magmatic foliation, and variable alteration under greenschist facies conditions. It is suggested that the Variscan intrusions facilitated the regional LP-HT metamorphic event at 330 to 340 Ma. Age and petrographic data from the Greater Caucasus imply a similar evolution as observed in the Dzirula massif. The Dzirula massif can thus be used as a proxy to model the evolution of the Caucasian Variscides.

Carboniferous high-potassium I-type granitoid magmatism in the Eastern Pontides: The Gümüşhane pluton (NE Turkey)

The Gümüşhane pluton, a high-K calc-alkaline I-type granodiorite/granite complex, forms an important component of the pre-Liassic basement of the Eastern Pontides (NE Turkey). In its eastern part, the pluton shows a compositional zonation ranging from biotite–hornblende granodiorite in the NW through biotite–hornblende granite to leucogranite/granophyre in the SE. Numerous mafic microgranular enclaves (up to ∼ 40 cm in diameter) suggest the former presence of globules of mafic melt during crystallization. Emplacement of the pluton occurred during the latest Early Carboniferous, as shown by the 320 ± 4 Ma 40Ar– 39Ar biotite/hornblende and 324 ± 6 Ma LA-ICP-MS U–Pb zircon ages. In Harker diagrams, samples of the different rock types exhibit well-defined data trends. With increasing SiO 2, the abundances of TiO 2, Al 2O 3, Fe 2O 3 tot, MnO, MgO, CaO, P 2O 5 and Sc decrease, but those of K 2O and Rb increase. However, the variations of Sr, Ba, (La/Yb) cn, Sr/Y and ∑ REEs vs. SiO 2 form distinctive groupings, which cannot be explained by a simple fractional crystallization. Chondrite-normalized (cn) REE patterns of granodiorite/granite samples show concave-upward shapes with (La/Yb) cn ranging from 5.2 to 12.4 and Eu/Eu* from 0.84 to 0.47, while there is almost no fractionation of the middle REE relative to the heavy REE. In primitive mantle-normalized element concentration diagrams, all rocks display marked negative anomalies in Ba, Nb/Ta, Sr, P and Ti, but positive anomalies in K and Pb. These geochemical features imply a fractionating mineral assemblage of clinopyroxene, amphibole and plagioclase without significant involvement of garnet. The granophyres are, on the other hand, characterized by higher K 2O/Na 2O and Rb/Sr ratios, lower (La/Yb) cn ratios (1.3 to 4.8) and more pronounced negative anomalies in Ba, Nb/Ta, Sr, Eu, P and Ti. Initial ε Nd values range from − 3.78 to − 5.30 and Nd model ages from 1.38 to 163 Ga. The magmas of the granite/granodiorite portion were probably generated by partial melting of high-potassic amphibolitic rocks, and those of the granophyres by a relatively felsic micaceous crustal source. The Gümüşhane pluton was emplaced at the wake of the low-pressure–high-temperature metamorphism, and is regarded as a late phase of Hercynian orogeny in the Eastern Pontides.

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.

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.

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.

Complex chemical zoning in eclogite facies garnet reaction rims: the role of grain boundary diffusion

In metapelites of the Saualpe complex (Eastern Alps) continuous 10 µm to 20 µm wide garnet reaction rims formed along biotite-plagioclase and biotite-perthite interfaces. The pre-existing mineral assemblages are remnants of low pressure high temperature metamorphism of Permian age. The garnet reaction rims grew during the Cretaceous eclogite facies overprint. Reaction rim growth involved transfer of Fe and Mg components from the garnet-biotite interface to the garnet-feldspar interface and transfer of the Ca component in the opposite direction. The garnets show complex, asymmetrical chemical zoning, which reflects the relative contributions of short circuit diffusion along grain boundaries within the polycrystalline garnet reaction rims and volume diffusion through the grain interiors on bulk mass transfer. It is demonstrated by numerical modelling that the spacing of the grain boundaries, i.e. the grain size of the garnet in the reaction rim is a first order control on its internal chemical zoning.

Permian metamorphic event in the Alps

In addition to the well-known Variscan and Alpine orogenic cycles, a Permian thermal event occurred in the European Alps. The geodynamic background of this event is not well understood. Here we suggest that the event is a reflection of lithospheric thinning accompanied by magmatic underplating causing partial melting of the lower crust and low-pressure–high-temperature metamorphism. The event was terminated by Early Triassic opening of the Meliata ocean and was followed by sag-stage subsidence during slow lithospheric cooling. This last stage of the evolution allowed the onset of the Mesozoic marine evolution with the orogen-wide formation of Triassic carbonate platforms well known from the Dolomites and the Northern Calcareous Alps. The combined evidence for magmatism, metamorphism, and late subsidence identifies the Permian event of the Alps as an excellent example of underplating events suggested as a cause for low-pressure–high-temperature terrains worldwide.

High-pressure–low-temperature metamorphism of metasedimentary rocks, southern Menderes Massif, western Turkey

Kilometer-scale lenses of quartz-rich metasedimentary rocks crop out in a discontinuous belt along the southern margin of the Menderes Massif, Turkey, and preserve evidence for high-pressure–low-temperature (H P–L T) metamorphism related to subduction of a continental margin during Alpine orogeny. Kyanite schist, quartzite, and quartz veins contain kyanite + phengite + Mg-chlorite, and the veins also contain magnesiocarpholite. A deformed carbonate metaconglomerate juxtaposed with the quartzite-dominated unit does not contain H P index minerals, and likely represents the tectonized boundary of the siliceous rocks with adjacent marble. The H P–L T rocks (10–12 kbar, 470–570 °C) record different pressure conditions than the adjacent, apparently lower pressure Menderes metasedimentary sequence. Despite this difference there is disagreement as to whether these H P–L T rocks are part of the Menderes sequence or are related to the tectonically overlying Cycladic blueschist unit. If the former, the entire southern Menderes Massif experienced H P–L T metamorphism but the evidence has been obliterated from most rocks; if the latter, rocks recording different metamorphic-kinematic conditions experienced different tectonic histories and were tectonically juxtaposed during thrusting. Based on observations and data in this study, the second model better accounts for the differences in P– T-deformation histories of the southern Menderes Massif rocks, and suggests that the H P–L T rocks are not part of the Menderes cover sequence.