Migmatitic Metapelites Research Articles

Paleoarchean sedimentation and Mesoarchean high-temperature metamorphism in northeastern Brazil: Tracing sources and depositional ages in polymetamorphic rocks

Supracrustal rocks offer a window into tectonic processes of the early Earth, since they are common in the Archean lithosphere. However, these rocks are usually affected by several episodes of metamorphism that can compromise their UPb systematics, leading to equivocal interpretations of depositional ages and sources. In northeast Brazil, supracrustal rocks are frequent within the Archean basement of the São José do Campestre Massif. These metapelitic migmatites show a high-temperature mineral assemblage, with garnet + sillimanite ± spinel and retrograde cordierite, with abundant anatectic melt migration at conditions of upper amphibolite to granulite facies. Zircon UPb dating coupled to trace elements analysis through LA-ICP-MS, as well as zircon internal zoning patterns suggest a maximum depositional age of 3305 ± 16 Ma followed by high-temperature metamorphism in the Mesoarchean, Paleoproterozoic and Neoproterozoic. Mesoarchean high temperature metamorphism occurred between 3084 ± 4 and 3006 ± 6 Ma and generated a wide range of textures that could be grouped in, at least, three stages of zircon growth. The first, during prograde heating, led to dissolution of detrital cores through the process of Ostwald Ripening and reprecipitation in oscillatory zoned rims. The second, probably at peak conditions, occurred above the stability of monazite, as evidenced by high Th/U ratios within zircon grains, rounded shape and sector-zoned cores. The third, during retrograde cooling, is mostly driven by garnet breakdown, and resulted in the crystallization of convoluted rims. Ti-in-zircon temperatures indicate minimum temperatures of 712 ± 21 °C for prograde/retrograde stages and 881 ± 50 °C for peak conditions. The Paleoarchean sedimentation and Mesoarchean metamorphism are coeval with similar events in Kaapvaal and Dharwar Cratons (South Africa and South India, respectively), but show no correlation to any Archean domains in South America to date. Solid-state recrystallization during the Paleoproterozoic (ca. 2.0 Ga) and the Neoproterozoic (ca. 0.6 Ga) correlates with orogenic events in both the Borborema Province and São Francisco Craton, suggesting a common evolution since the Rhyacian.

P–T–t–D records of Early Palaeozoic Andean‐type shortening of a hot active margin: The Dunhuang block in NW China

AbstractHigh‐pressure (HP) granulites form either in the domain of the subducted plate during continental collision or in supra‐subduction systems where the thermally softened upper plate is shortened and thickened. Such a discrepancy in tectonic setting can be evaluated by metamorphic pressure–temperature–time‐deformation (P–T–t–D) paths. In the current study, P–T–t–D paths of Early Palaeozoic HP granulite facies rocks, in the form of metabasic lenses enclosed in migmatitic metapelite, from the Dunhuang block, NW China, are investigated in order to constrain the nature of the HP rocks and shed light on the geodynamic evolution of a modern hot orogenic system in an active margin setting. The rocks show a polyphase evolution characterized by (1) relics of horizontal or gently dipping fabric (S1) preserved in cores of granulite lenses and in garnet porphyroblasts, (2) a N‐S trending sub‐vertical fabric (S2) preserved in low‐strain domains and (3) upright folds (F3) associated with a ubiquitous steep E‐W striking axial planar foliation (S3). Garnet in the granulites preserves relics of a prograde mineral assemblage M1a equilibrated at ~11.5 kbar and ~770–780°C, whereas the matrix granulite assemblage (M1b) from the S1 fabric attained peak pressure at ~13.5 kbar and ~850°C. The granulites were overprinted at ~8–11 kbar and ~850–900°C during crustal melting (M2) followed by partial re‐equilibration (M3) at ~8 kbar and ~625°C. A garnet Lu–Hf age of 421.6 ± 1.2 Ma dates metamorphism M1, while a garnet Sm–Nd age of 385.3 ± 4.0 Ma reflects M3 cooling of the granulites. The mineral assemblage, M1, of the host migmatitic metapelite formed at ~9–12.5 kbar and ~760–810°C, partial melting and migmatization (M2) occurred at ~7 kbar and ~760°C and re‐equilibration (M3) at ~5–6 kbar and ~675°C. A garnet Lu–Hf age of 409.7 ± 2.3 Ma dates thermal climax (M2) and a garnet Sm–Nd age of 356 ± 11 Ma constrains M3 for the migmatitic metapelites. The timing of this late phase is also bracketed by an emplacement age of syntectonic granite dated at c. 360 Ma. Decoupling of M1 and M2 P–T evolutions between the mafic granulites and migmatitic metapelites indicates their different positions in the crustal column, while the shared pressure–temperature (P–T) evolution M3 suggests formation of a mélange‐like association during the late stages of orogeny. The high‐pressure event D1‐M1 is interpreted as a result of Late Silurian–Early Devonian moderate crustal thickening of a thermally softened and thinned pre‐orogenic crust. The high‐temperature (HT) re‐equilibration D2‐M2 is interpreted as a result of Mid‐Devonian shortening of the previously thickened crust, possibly due to ‘Andean‐type’ underthrusting. The D3‐M3 event reflects Late Devonian supra‐subduction shortening and continuous erosion of the sub‐crustal lithosphere. This tectono‐metamorphic sequence of events is explained by polyphased Andean‐type deformation of a ‘Cascadia‐type’ active margin, which corresponds to a supra‐subduction tectonic switching paradigm.

Petrogenesis and U-Pb zircon geochronology of migmatitzation during Neo-Tethyan Jurassic magmatic arc extension: The Boroujerd example, western Iran

The migmatites in the Boroujerd region, Western Iran are spatially associated with felsic and mafic magmatism. However, the genetic relationship between partial melting and magmatism in the region is unclear. Four metapelite migmatite samples (two melanosome and two leucosome) were selected for U-Pb zircon geochronology to determine the timing of anatexis. Major and trace element compositions of leucosome in the migmatites are consistent with being derived from biotite-breakdown partial melting of metapelites and not from the injection of granites associated with the Boroujerd igneous complex. Cores of zircon from metapelitic migmatite are inferred to be detrital in origin and yield ages of 320–250 Ma. The age of the detrital zircons is compatible with the timing of Paleozoic and Mesozoic magmatism in the Central Iranian micro-plate. The pelitic protolith of the migmatites in the Boroujerd region were probably originated from the Mesozoic continental margin of the Central Iranian microplate. Zircon rims and neoblastic grains yield ages of 220–210 Ma and 170–160 Ma. The geological significance of the older age population is unclear, but the younger population is similar to that of mafic and felsic intrusions in the area and is interpreted to record the general timing of high-temperature–low-pressure contact metamorphism and partial melting. We infer that the Middle Jurassic mafic and felsic magmatism was the principal heat source for the metamorphism and partial melting of the Boroujerd migmatites in response to the subduction of Neotethyan oceanic lithosphere.

Fluid-present and fluid-absent melting of muscovite in migmatites in the Himalayan orogen: Constraints from major and trace element zoning and phase equilibrium relationships

Partial melting of the continental crust may proceed through either influx of free water (hydration, fluid-present melting) or breakdown of hydrous minerals (dehydration, fluid-absent melting). The two types of physicochemical mechanism would be operated for crustal anatexis in nature, and their effects on the composition of anatectic products were closely examined by experimental approaches as well as the study of natural leucogranites from the Himalayan orogen. However, the leucogranites usually experienced several stages of evolution during magmatism, leading to difficulties in retrieving the composition of original melts. This issue is now addressed by studying Himalayan migmatitic metapelites because they generally share similar source rocks and P–T conditions with the leucogranites. In combined with field observations, an integrated study of whole-rock geochemistry, mineral chemistry, zircon geochronology, and phase equilibrium modelling was conducted for migmatitic metapelites from the Nyalam Valley in the Greater Himalayan Sequence. The results show that the migmatitic metapelites share similar clockwise P–T paths. They experienced an early prograde metamorphism through compressional heating to reach peak pressures of 7.3–9.0 kbar at ~700 °C before ca. 31 Ma. Afterwards, the migmatitic metapelites underwent decompressional heating to ~725 °C at 6.0 kbar for upper-amphibolite facies metamorphism at 26–21 Ma. Subsequently, they experienced decompressional cooling to a temperature below the solidus at ca. 18 Ma. The crustal anatexis was dominated by muscovite melting reactions during the decompressional heating from ca. 26.4 to 18 Ma. Despite the similar P–T paths, the migmatitic metapelites experienced different types of melting reactions involving muscovite (i.e., dehydration and hydration). Whereas the hydration melting resulted in a marked increase in An contents (~65 to 75 mol%) and the absence of K-feldspar, the dehydration melting led to relatively low-An (<30 mol%) plagioclase and abundant K-feldspar. The two types of muscovite melting reaction may be common in the Himalayan orogen in the Late Cenozoic. While the fluid-present melting would take place at a shallower depth, the fluid-absent melting would occur at a greater depth. This coupled dehydration-hydration relationship has important implications for the geodynamic mechanism of crustal anatexis in the continental collision zone.

High temperature – low pressure metamorphism during subduction of Neo-Tethys beneath the Iranian plate: evidence for mafic migmatite formation in the Alvand complex (western Iran)

Subduction of Neo-Tethys oceanic lithosphere beneath the Iranian plate during the Mesozoic formed several igneous bodies of ultramafic to intermediate and felsic composition. Intrusion of these magmas into a regional metamorphic sequence (the Sanandaj-Sirjan Zone) caused partial melting and formation of migmatites with meta-pelitic protoliths. The Alvand complex (west Iran) is a unique area comprising migmatites of both mafic and pelitic protoliths. In this area, the gabbroic rocks contain veins of leucosome at their contact with pyroxenite and olivine gabbro. These leucosomes are geochemically and mineralogically different from leucosomes of the meta-pelitic migmatites and clearly show properties of I-type granites. Microscopic observations and whole rock compositions of the mafic migmatite leucosomes show that migmatization occurred through partial melting of biotite, hornblende and plagioclase. Thermobarometric calculations indicate 800 °C and 3.7 kbar for partial melting, although phase diagram modeling demonstrates that the presence of water could decrease the solidus temperature by about 40 °C. Our results suggest an asthenospheric magma upwelling as the source of heat for partial melting of the gabbroic rock during subduction of Neo-Tethys oceanic crust under the western edge of the Iranian plate. The present study also reveals relationships between migmatization and formation of S- and I -type granites in the area.

Chafalote Metamorphic Suite (Uruguay): Reflections on the evolution of the Punta del Este Terrane

The Chafalote Metamorphic Suite, located in the Uruguayan sector of the Punta del Este Terrane, southeasternmost Dom Feliciano Belt, comprises semipelitic, migmatitic metapelites, mafic granulites, and calc-silicate/amphibolitic gneisses. These supracrustal rocks occur as roof pendants in a granodiorite belonging to the Cerro Olivo Complex. The main structure of the Chafalote Metamorphic Suite is a composite S0/S1 banding, related to the main deformation event (D1) developed under upper-amphibolite/granulite facies conditions. This work presents an integrated study on the latter rocks, involving different geochronological methods (monazite U-Pb LA-ICP-MS and zircon U-Pb SHRIMP) and whole rock Sm-Nd isotope data in order to better understand the metamorphic evolution of the Chafalote Metamorphic Suite during the amalgamation of the SW Gondwana paleocontinent in the Neoproterozoic. The studied rocks record a peak metamorphic event (M2) occurred at around 660 Ma (monazite U-Pb ages), which was followed by decompression and partial melting (M3 event) at around 640 Ma (zircon U-Pb ages), probably during the late stages of the D1 event. The retrograde methamorphic event (M4) is related to the transition between the D1 and the development of NE-SW striking, low-grade sinistral shear zones (D2 event) and intrusion of syn-to post-orogenic Brasiliano granitoids of the Aiguá Batholith at around 600 Ma. The metamorphic conditions, together with geochronological and isotopic similarities, allow the correlation between the studied high-grade metasedimentary rocks and those from the Paso del Dragón Complex in Uruguay, and from the Várzea do Capivarita, Arroio Grande, and Telho Complex in Brazil. The latter rocks possibly share similar source areas and represent different portions of the Neoproterozoic fore-arc and/or back-arc basins developed in the Dom Feliciano Belt during the Brasiliano/Pan-African orogenic cycle.

Petrology of Spinel-Gedrite-Cordierite Symplectites Replacing Andalusite in Migmatites from the Sarabi Area, Hamedan, Sanandaj–Sirjan Zone, Iran

Metapelitic migmatites in the Hamedan area of Sanandaj–Sirjan Zone, Iran, contain spinel + gedrite + cordierite (± biotite, ± staurolite, ± ilmenite) symplectite, replacing andalusite in metatexite and diatexite. P-T conditions of metamorphism for cordierite-hornfels, andalusite-cordierite-hornfels and migmatites were estimated as 600°C, 2.2 kbar; 645°C, 2.9 kbar and 760°C, 3.7 kbar respectively, using calculated pseudosections for an average composition in the NCKFMASH system. Calculated pseudosection is in accordance with field and petrographic observation, except that symplectite minerals does not appear in the model, suggesting that formation of symplectite was affected by effective bulk compositions (EBC), rather than whole-rock composition. So EBC model was calculated using modal abundance and average composition of symplectite minerals. In pseudosection calculated for EBC, spinel + gedrite + cordierite is stable over wide range of P and T. Peak metamorphic condition for symplectites are estimated as 750°C at 4.1 kbar. Staurolite-bearing symplectites appeared during retrograde isobaric path at 600°C and 4.1 kbar. Based on calculated pseudosections and estimated P and T, an isobaric retrograde anticlockwise P-T path is proposed for the Sarabi migmatites. Metamorphism and migmatization in the area is linked to the major plutonism phases of the Sanandaj–Sirjan Zone in Middle-Jurassic under high T-low P conditions, during the evolution of Neo-Tethys.

Anatexis and fluid regime of the deep continental crust: New clues from melt and fluid inclusions in metapelitic migmatites from Ivrea Zone (NW Italy)

AbstractWe investigate the inclusions hosted in peritectic garnet from metapelitic migmatites of the Kinzigite Formation (Ivrea Zone, NW Italy) to evaluate the starting composition of the anatectic melt and fluid regime during anatexis throughout the upper amphibolite facies, transition, and granulite facies zones. Inclusions have negative crystal shapes, sizes from 2 to 10 μm and are regularly distributed in the core of the garnet. Microstructural and micro‐Raman investigations indicate the presence of two types of inclusions: crystallized silicate melt inclusions (i.e., nanogranitoids, NI), and fluid inclusions (FI). Microstructural evidence suggests that FI and NI coexist in the same cluster and are primary (i.e., were trapped simultaneously during garnet growth). FI have similar compositions in the three zones and comprise variable proportions of CO2, CH4, and N2, commonly with siderite, pyrophyllite, and kaolinite, suggesting a COHN composition of the trapped fluid. The mineral assemblage in the NI contains K‐feldspar, plagioclase, quartz, biotite, muscovite, chlorite, graphite and, rarely, calcite. Polymorphs such as kumdykolite, cristobalite, tridymite, and less commonly kokchetavite, were also found. Rehomogenized NI from the different zones show that all the melts are leucogranitic but have slightly different compositions. In samples from the upper amphibolite facies, melts are less mafic (FeO + MgO = 2.0–3.4 wt%), contain 860–1700 ppm CO2 and reach the highest H2O contents (6.5–10 wt%). In the transition zone melts have intermediate H2O (4.8–8.5 wt%), CO2 (457–1534 ppm) and maficity (FeO + MgO = 2.3–3.9 wt%). In contrast, melts at granulite facies reach highest CaO, FeO + MgO (3.2–4.7 wt%), and CO2 (up to 2,400 ppm), with H2O contents comparable (5.4–8.3 wt%) to the other two zones. Our results represent the first clear evidence for carbonic fluid‐present melting in the Ivrea Zone. Anatexis of metapelites occurred through muscovite and biotite breakdown melting in the presence of a COH fluid, in a situation of fluid–melt immiscibility. The fluid is assumed to have been internally derived, produced initially by devolatilization of hydrous silicates in the graphitic protolith, then as a result of oxidation of carbon by consumption of Fe3+‐bearing biotite during melting. Variations in the compositions of the melts are interpreted to result from higher T of melting. The H2O contents of the melts throughout the three zones are higher than usually assumed for initial H2O contents of anatectic melts. The CO2 contents are highest at granulite facies, and show that carbon‐contents of crustal magmas are not negligible at high T. The activity of H2O of the fluid dissolved in granitic melts decreases with increasing metamorphic grade. Carbonic fluid‐present melting of the deep continental crust represents, together with hydrate‐breakdown melting reactions, an important process in the origin of crustal anatectic granitoids.

Complexity of characterizing granitoids in high-grade terranes: An example from the Neoarchean Verbaard granitoid, Limpopo Complex, Southern Africa

Identifying the spatial distribution or extent of individual granitoids in Archean high-grade terranes can be complicated, with the effect of overprint events resulting in localized varieties. The case study presented here highlights this issue from the Limpopo Complex high-grade terrane in southern Africa. The Beit Bridge Complex segment preserves a variety of temporally and spatially associated Neoarchean granitoids. Of the different granitoids, the ~2.65–2.62 Ga biotite-bearing Verbaard granitoid is characterized by a conspicuous network of garnet-bearing leucocratic veins. Unlike these leucocratic veins with more or less straight line outline, garnet-bearing leucosomes with irregular outline are associated with surrounding migmatitic metapelites and have biotite-bearing melanocratic margins. Garnet from the Verbaard granitoid and migmatitic metapelite have comparable mineral chemical characteristics. The most common variety is characterized by low XCaGrt = 0.03–0.04 and the less common one has higher XCaGrt (0.05–0.06: granitoid; 0.08: migmatitic metapelite). No rim-core-rim zoning is observed in garnets from the host Verbaard granitoid and metapelite. However, garnets in the leucocratic veins and leucosomes exhibit distinct zoning with a decrease of XCa from 0.08 to 0.03. Phase equilibria modeling using PERPLE_X software reveals that such variation in grossular content of garnets reflects decompression from ~9.5 to ~6.5 kbar at 760–800 °C for both rocks. During decompression, melts were segregated trapping garnet including the high XCa variety. Both the Verbaard granitoid and migmatitic metapelite were affected by the segregated melts. Difference in composition between the modeled melts and leucosome/leucocratic vein is accounted in terms of different degree of a melt loss. Whole-rock geochemical characteristics of garnet- and leucocratic vein-free domains of the Verbaard granitoid indicate that they are similar to medium-pressure TTGs derived from the melting of metabasalts and are comparable to those of the Alldays granitoid. Thus, the Verbaard granitoid represents a local manifestation of the voluminous (widespread) Alldays granitoid magmatism in the specific (Verbaard) area.

Zircon and monazite petrochronologic record of prolonged amphibolite to granulite facies metamorphism in the Ivrea-Verbano and Strona-Ceneri Zones, NW Italy

In order to improve the understanding of thermal-tectonic evolution of high-grade terranes, we conducted a systematic study of textures, REE content and U-Pb ages of zircon and monazite grains extracted from migmatitic metapelites across the amphibolite to granulite facies metamorphic gradient exposed in the Ivrea-Verbano and Strona-Ceneri Zones (Italy). This study documents the behaviour of these accessory minerals in the presence of melt. The absence of relict monazite grains in the metasediments and the gradual decrease in the size of inherited zircon grains from amphibolite to granulite facies cores indicate partial to total dissolution of accessory minerals during the prograde path and partial melting. The retrograde path is marked by (i) growth of new zircon rims (R1 and R2) around inherited cores in the mesosome, (ii) crystallisation of stubby zircon grains in the leucosome, especially at granulite facies, and (iii) crystallisation of new monazite in the mesosome. Stubby zircon grains have a distinctive fir-tree zoning and a constant Th/U ratio of 0.20. Together, these features reflect growth in the melt; conversely, the new zircon grains with R1 rims have dark prismatic habits and Th/U ratios < 0.1, pointing to growth in solid residues. U-Pb ages obtained on both types are similar, indicating contemporaneous growth of stubby zircon and rims around unresorbed zircon grains, reflecting the heterogeneous distribution of the melt at the grain scale.In the Ivrea-Verbano Zone the interquartile range (IQR) of U-Pb ages on zircon and monazite are interpreted to represent the length of zircon and monazite crystallisation in the presence of melt. Accordingly, they provide an indication on the minimum duration for high-temperature metamorphism and partial melting of the lower crust: 20 Ma and 30 Ma in amphibolite and granulite facies, respectively. In amphibolite facies, zircon crystallisation between 310 and 294 Ma (IQR) is interpreted to reflect metamorphic peak condition and earlier retrograde history; conversely, monazite crystallisation between 297 and 271 Ma (IQR) reflects cooling under ~750 °C to a temperature close to the solidus. In granulite facies, zircon crystallisation between 295 and 265 Ma (IQR) is interpreted to reflect high-temperature conditions, which were attained after peak of metamorphism during isothermal decompression and subsequent cooling under ~850–950 °C. The observed decrease of U-Pb ages in metamorphic zircon and monazite from amphibolite to granulite facies (i.e. from the middle to the lower crust) is interpreted to record slow cooling and crystallisation of the Variscan orogenic root at the transition from orogenic collapse to opening of the Tethys Ocean.

Creep of mafic dykes infiltrated by melt in the lower continental crust (Seiland Igneous Province, Norway)

A dry mafic dyke from a continental lower-crustal shear zone in the Seiland Igneous Province (northern Norway) experienced syn-kinematic melt-rock interaction. Viscous shearing occurred at T≈800°C, P≈0.75–0.95GPa and was coeval with infiltration of felsic melt from adjacent migmatitic metapelites. The dyke has a mylonitic microstructure where porphyroclasts of orthopyroxene, clinopyroxene and plagioclase are wrapped by a fine-grained (4–7μm) polyphase mixture of clinopyroxene+orthopyroxene+plagioclase+quartz+ilmenite±K-feldspar±apatite. Microstructural observations and electron backscatter diffraction analysis indicate that the porphyroclasts deformed by a combination of dislocation glide and fracturing, with only a limited record of dislocation creep, recovery and dynamic recrystallization. We identified diffusion creep as the dominant deformation mechanism in the mixture based on the small grain size, phase mixing and weak crystallographic preferred orientation of all phases (interpreted as the result of oriented grain growth during viscous flow). The polyphase mixture did not form by dynamic recrystallization or by mechanical fragmentation of the porphyroclasts, but rather by melt-rock interaction. Thermodynamic models indicate that the syn-kinematic mineral assemblage results from the chemical interaction between a pristine mafic dyke and ca. 10vol.% of felsic melt infiltrating from the adjacent partially molten metapelites. Extrapolation of laboratory-derived flow laws to natural conditions indicates that the formation of interconnected layers of fine-grained reaction products deforming by diffusion creep induces a dramatic weakening in the mafic granulites, with strain rates increasing up to 2–3 orders of magnitude. The reaction weakening effect is more efficient than the weakening associated with melt-assisted diffusion creep in the presence of up to 10vol.% of infiltrated melt without formation of fine-grained reaction products. Thus, we conclude that weakening by dramatic grain-size reduction through nucleation of fine-grained material that subsequently deforms by diffusion creep is a major effect of melt infiltration during lower crustal shearing.

Timescales and mechanisms of batholith construction: Constraints from zircon oxygen isotopes and geochronology of the late Variscan Serre Batholith (Calabria, southern Italy)

The late Variscan Serre Batholith in central Calabria represents the middle portion, c. 13-km-thick, of a tilted crustal section continuously exposed from lower-crustal granulites to upper-crustal phyllites. The batholith is zoned, consisting of several granitoid types that were emplaced at depths ranging from c. 23 to c. 6km. Deep, strongly foliated quartz diorites and tonalites were emplaced into migmatitic metapelites, the intermediate level granitoids are weakly foliated to unfoliated porphyritic granodiorites and monzogranites, and the shallowest bodies are two-mica granodiorites and granites, grading upward to biotite ± amphibole granodiorites, emplaced into paragneisses and phyllites. Five samples, representative of the main granitoid types in terms of both composition and emplacement depth, have been dated by SHRIMP. Zircon from a lower-crustal quartz diorite gave an emplacement age of 297.3±3.1Ma; two middle-crustal strongly peraluminous K-feldspar megacrystic granites were emplaced at 296.1±1.9Ma and 294.9±2.7Ma; a middle- to upper-crustal two-mica monzogranite was emplaced at 294.2±2.6Ma and finally, an upper-crustal weakly peraluminous granodiorite from the batholith roof was emplaced at 292.2±2.6Ma. These results are consistent with the development of the batholith by incremental multipulse overaccretion, each granitoid body being in a dominantly rigid state before intrusion of the next, with little or no possibility of magma mixing. The difference in age between the oldest and youngest granitoids, from the batholith floor and roof, respectively, is 5.1±4.0Ma, providing an upper limit of about 9Ma on the time taken for batholith construction. The presence in the c. 296–294Ma granitoids of c. 305–302Ma anatectic zircon with varied Th/U and oxygen isotope compositions indicates a time interval of 8–9Ma between incipient partial melting in the lower crust and magma emplacement in the middle crust. The emplacement of the first granitoid bodies into the top of the lower crust was controlled tectonically by the activation of a deep-seated shear zone. The shallowest granodiorites were emplaced during the waning stages of the shear zone activity, producing late- to post-tectonic contact metamorphism in the upper-crustal phyllites and mylonitic paragneisses.

Rapid extensional unroofing of a granite–migmatite dome with relics of high-pressure rocks, the Podolsko complex, Bohemian Massif

AbstractThe Podolsko complex, Bohemian Massif, is a high-grade dome that is exposed along the suprastructure–infrastructure boundary of the Variscan orogen and records snapshots of its protracted evolution. The dome is cored by leucocratic migmatites and anatectic granites that enclose relics of high- to ultrahigh-pressure rocks and is mantled by biotite migmatites and paragneisses whose degree of anatexis decreases outwards. Our new U–Pb zircon ages indicate that the leucocratic migmatites were derived from Early Ordovician (c. 480 Ma) felsic igneous crust; the same age is inferred for melting the proto-source of the metapelitic migmatites. The relics of high- to ultrahigh-pressure rocks suggest that at least some portions of the complex witnessed an early Variscan subduction to mantle depths, followed by high-temperature anatexis and syntectonic growth of the Podolsko dome in the middle crust at c. 340–339 Ma. Subsequently, the dome exhumation was accommodated by crustal-scale extensional detachments. Similar c. 340 Ma ages have also been reported from other segments of the Variscan belt, yet the significance of this tectonothermal event remains uncertain. Here we conclude that the 340 Ma age post-dates the high-pressure metamorphism; the high temperatures required to cause the observed isotopic resetting and new growth of zircon were probably caused by heat input from the underlying mantle and, finally, the extensional unroofing of the complex requires a minimum throw of about 8–10 km. We use this as an argument for significant early Carboniferous palaeotopography in the interior of the Variscan orogen.

The Majorqaq Belt: A record of Neoarchaean orogenesis during final assembly of the North Atlantic Craton, southern West Greenland

Over the past 4.5 billion years (Ga) the Earth has undergone a secular change in tectonic regimes. Archaean tectonics was a markedly different process than observed today, involving a hotter planet with a thinner lithosphere, thicker oceanic crust, and smaller proto-continents. The transition to a more modern type of plate tectonics required the initiation of subduction of oceanic lithosphere. The onset of subduction in the Meso- to Neoarchaean (ca. 3.2 to 2.5Ga) is shown by geochemical signatures indicative of crustal recycling and petrologic evidence of tectonically driven thickening of the continental crust. This transition is recorded in the rock record of the North Atlantic Craton, which preserves a history of crust formation through to terrane accretion and the collision of crustal blocks. Here we present petrologic, chronologic, chemical and geophysical evidence from the Majorqaq Belt, a ~100km wide east–west striking zone, defined by a high gravity anomaly, voluminous metasedimentary units, olivine-rich ultramafic enclaves, and anomalously high K2O, Ni, and MgO stream-sediment chemistry. We use a 207Pb/206Pb zircon LA-ICP-MS age of 2.557±0.005Ga (2σ) from an upper-amphibolite facies metapelitic migmatite to propose Neoarchaean suturing of the Maniitsoq crustal block to the North Atlantic Craton. Ti-in-zircon temperatures and zircon–garnet REE distribution coefficients constrain zircon crystallisation to 815–729°C, which is at, or near the solidus during leucosome crystallisation. Our data reveal a clockwise P–T path with a high-pressure assemblage of garnet–kyanite–plagioclase–rutile–biotite–quartz which grew at an apparent thermal gradient of ~660°C/GPa during subduction-driven compression of surface-derived sediments to a minimum-peak-pressure of ~1.15GPa at ~760°C followed by the growth of gedrite–cordierite–sillimanite–ilmenite at minimum-peak-temperature conditions of ~810°C at ~0.94GPa. We propose the evolution of the Majorqaq belt formed from the collision of the Maniitsoq block with the proto-North Atlantic Craton following closure of an oceanic basin with south-dipping subduction, providing a potential source of volatiles for the ca. 2.55 Ga Qôrqut granite complex situated ~ 150 km further southwards. These conditions necessitate orogenesis and indicate that plate tectonics was active by ca. 2.56Ga during the final assembly of the North Atlantic Craton.

Granulite-facies metamorphic events in the northwestern Ubendian Belt of Tanzania: Implications for the Neoarchean to Paleoproterozoic crustal evolution

We present a geological evolution model for the Paleoproterozoic Ubendian Belt. This model is deduced from the metamorphic histories of metasediments and metamafites combined with previously obtained crust formation and metamorphic ages obtained from different rock types of the Katuma Block in the NW Ubendian Belt.Geothermobarometry and pseudosection modelling of metabasites indicate that the granulite-facies coronas containing garnet–clinopyroxene–quartz–hornblende formed at about 8.9–6.6kbar and 790–700°C. The formation of the corona textures is attributed to the post magmatic cooling history in the deep crust following their intrusion at about 2.65Ga. This period correlates with the age of deposition of sediments in the Katuma Block, as deduced from the age of detrital zircon grains. The metamorphic P–T path of these sediments contrasts with that of the Archean mafic rocks. The common occurrence of sillimanite pseudomorphs after cm-sized kyanite crystals in migmatitic metapelites provides evidence that an early stage of metamorphism took place in the kyanite stability field whereas the subsequent peak metamorphism was characterised by the stability of the mineral assemblage sillimanite-garnet/cordierite-K-feldspar. Modelling of the XMg ratios of compositionally homogenous cores of garnet porphyroblasts together with GASP barometry suggest peak P-T conditions of about 7kbar and 770°C. The formation of plagioclase coronas around garnet in metapelites, the decrease in XMg and an increase of the spessartine fraction in rims of garnet porphyroblasts point to a near isothermal uplift after peak metamorphism.Texturally controlled in situ U–Th–total Pb microprobe dating of monazite in metapelites resulted in two ages for metamorphic events. The monazite of the two dated samples is mostly complex and patchy zoned. The cores record ages of 1957±10Ma and 1967±16Ma, whereas the rims give ages of 1837±6Ma and 1848±16Ma. As the two ages of monazite growth zones (core and rims) are found in monazite of the rock matrix and in monazite inclusions of garnet porphyroblasts, we conclude that garnet growth occurred during or after the second metamorphic event at 1840Ma. This interpretation is in agreement with the depletion of HREE and Y in the monazite rims. We correlate the second, high-grade event with the collisional stage between the Tanzania Craton and the Bangweulu Block. The first event that preceded the collision for about 120Ma is attributed to the kyanite grade metamorphism during accretionary processes and associated calc-alkaline magmatism along the continental margin of the Tanzania Craton.Combining our new data with those of previous studies on the geochemistry and zircon geochronology we develop a new evolutionary model for the Paleoproterozoic orogenic cycle. The geologic history in the Ubendian Belt began in the Neoarchean (2.7–2.6Ga) with a magmatic crust formation phase in an active continental margin setting. In the following Neoarchean–Paleoproterozoic (2.65–2.05Ga) stage of a tectonically inactive, passive continental margin the protoliths of metabasites cooled under near-isobaric conditions and sediments were deposited on the Neoarchean crust. Subsequently, there was a protracted period of subduction (2.05–1.84Ga) at an active continental margin, which was associated with magmatic additions and metamorphic events during tectonic accretions, that led to kyanite-grade metamorphism in wedge sediments. The final collision at 1.84Ga leading to garnet–sillimanite–cordierite grade metamorphism in metapelites most likely was responsible for the exhumation of the 1880–1860Ma MORB-type eclogites in the Ubendian Belt.

Two-stage granulite formation in a Proterozoic magmatic arc (Ongole domain of the Eastern Ghats Belt, India): Part 1. Petrology and pressure–temperature evolution

The Ongole domain of the Proterozoic Eastern Ghats Belt, India, is dominated by charnockites and enderbites with enclaves of migmatitic metapelites. Reaction textures indicate two generations of mineral growth. Spinel–hercynite solid solution+quartz-bearing Fe–Al granulites provide critical evidence for an ultrahigh-temperature (UHT) metamorphism. The association of spinel and quartz, now separated by garnet and sillimanite coronas, suggests that the rocks underwent T>950°C peak metamorphism at P=6.5–7kbar, as constrained from P–T pseudosection. Coarse-grained orthopyroxene in garnet+cordierite-bearing metapelites shows the highest Al2O3 content up to 8.2wt.%, suggesting T=950–1000°C. Temperatures estimated from mesoperthite and plagioclase pairs and other geothermometers (T=900–1000°C) further support ultrahigh-temperature metamorphism in the Ongole domain. The absence of cordierite in retrograde mineral assemblages points to isobaric cooling, giving rise to a near isobaric heating–cooling trajectory. The second generation of garnet, occurring as thick overgrowths over coarse-grained garnet porphyroblasts, in metapelites and especially in charnoenderbites have higher grossular contents than the porphyroblasts. Thermobarometry and P–T pseudosections indicate that they formed at a higher pressure but lower temperature (ca. 780°C, 9.5kbar) during a second metamorphic event. Orthopyroxene+sillimanite±kyanite±spinel symplectites replacing cordierite is most likely formed during this second metamorphic event. The presence of sillimanite, kyanite as well as andalusite along with coarse-grained retrograde biotite in the same rock marks the last stage of the second metamorphic event characterized by isobaric cooling just below the aluminosilicate triple point. This further suggests near-isothermal decompression from higher pressures. UHT metamorphism at low pressures during the first metamorphic event is most likely caused by magma emplacement. The higher pressure but lower temperature conditions achieved during the second metamorphic event is due to crustal thickening during collision of continental blocks. Near-isothermal decompression to ca. 4kbar points to subsequent rapid exhumation of the over thickened crust. The two-stage evolutionary history of the Ongole domain fits well to the expected evolution of a magmatic arc, in which the first can be attributed to magmatic heat advection during arc growth and the second to crustal thickening during collision.

The Feiran–Solaf metamorphic complex, Sinai, Egypt: Geochronological and geochemical constraints on its evolution

Ion-microprobe U–Th–Pb geochronological data are presented for fourteen samples from the Feiran–Solaf metamorphic complex in the Sinai Peninsula, Egypt. These data reveal a succession of magmatic and metamorphic events from ca. 1000Ma to <600Ma. The oldest age of ∼1.0Ga is obtained from zircon in Solaf zone metapsammites, representing the age of a locally restricted acidic igneous source which eroded and was deposited as arkose sometime after ∼975±10Ma. A similar ∼1.0Ga age from granodioritic biotite gneisses is interpreted as dating its crystallization. A biotite gneiss sample bordering the Solaf granodioritic gneiss yields an age of 792±7Ma, suggesting at least two phases of arc development over ca. 200Ma. While detrital zircon from migmatitic metapelites indicates input of material as old as ∼1.0Ga, the probable maximum sedimentation age for their immature pelitic protolith, which was likely deposited in an active continental marginal basin within a continental arc, is 656±6Ma. These metapelites were metamorphosed at 603±6Ma.Leucosome and mesosome samples from migmatitic biotite-hornblende gneisses of the Feiran zone yield polyphase zircon with ca. 700Ma cores interpreted as dating igneous crystallization, while overgrowths of 627±7Ma and 592±10Ma define the time of high-grade metamorphism. A biotite-hornblende gneiss sample from the Feiran-zone contains zircon that defines a protolith crystallization age at 623±6Ma, with rim overgrowths defining a metamorphic age of 587±7Ma. Zircon from the diorite body separating the Feiran and Solaf zones yields crystallization ages of 617±4Ma and 602±4Ma for samples collected from the periphery and core, respectively, while zircon from a late-tectonic granodiorite sample intruding the metapsammitic gneisses yields a crystallization age of 589±6Ma.High spatial resolution age dating from the Feiran–Solaf metamorphic complex and intruding magmatic rocks thus reveals four episodes of arc magmatism at 799–785Ma, 713–692Ma, 629–613Ma, and 606–583Ma. The timing of regional high-grade metamorphism is constrained by the zircon rims in the migmatitic metapelites and migmatized biotite-hornblende gneisses which record two distinct events at 627±7Ma and 592±10Ma, interpreted to represent peak and isothermal decompressional metamorphic events, synchronous with heat flow from concurrent young magmatism.

Mineral chemistry, petrology and geochemistry of the Sebago granite-pegmatite system, southern Maine, USA

Corresponding author The Permian (293 ± 2 Ma) Sebago Pluton is a homogeneous, two-mica granite situated in the Oxford pegmatite field, southwestern Maine. Surrounding the pluton is an area designated as the Sebago Migmatite Domain (SMD) dominated by metapelitic migmatites and diatexites with subordinate intrusions of heterogeneous, two-mica and biotite granites, pegmatitic leucogranites and granitic pegmatites. The Sebago Pluton plus the SMD formerly defined the extent of the Sebago Batholith. Most of the granitic pegmatites and bodies of pegmatitic leucogranites occur either within, or barely outside, the margins of the SMD. The pegmatitic leucogranite facies displays units typical of fertile granites (granites that produce granitic pegmatites) and include: megacrystic graphic K-feldspar, sodic aplite and potassic pegmatite pods hosting rare tourmaline, beryl and columbite–tantalite. Over 100 granitic pegmatite bodies (Sebago Pegmatite Group – SPG) intrude the outer portions of the SMD and neighboring granitoids and metasedimentary rocks. The pegmatite population includes mineralogically simple to complexly zoned pegmatites that are characterized by a LCT-type (Li, Cs and Ta) geochemical signature, extensive replacement of primary zones and gem-bearing miarolitic cavities. Sebago granites are strongly peraluminous and show rare-element enrichment typical of evolved fertile granites (K/Rb = 87–257), Rb/Tl = 10.6–71.3, Ba/Rb = 0.18–5.04, Al/Ga = 1419–1749, Zr/Sn = 1.53–43.9). The SPG shows high levels of Be, Nb > Ta, P, Li and B with subordinate enrichment in Rb and Cs. Moderate to high levels of rare-element fractionation are encountered in pegmatitic K-feldspar (K/Rb ≈ 17, K/Cs ≈ 90, Rb/Tl ≈ 75), muscovite (K/Rb ≈ 6.6, K/Cs ≈ 14.8, Rb/Tl ≈ 127), beryl (Na/Li ≈ 1.77, Cs 2 O ≈ 3.15 wt. %), garnet [Mn/(Mn + Fe) ≈ 0.60], manganotantalite and ixiolite/wodginite [Mn/Mn + Fe) ≈ 0.98, Ta/(Ta + Nb) ≈ 0.80–0.93]. Evidence that supports the pegmatitic leucogranites as the likely parent to the SPG includes the close spatial distribution of the pegmatites to the leucogranite bodies, texturally and mineralogically similar units observed within the leucogranite and the neighboring pegmatites plus gradual, yet overlapping, rare-element fractionation from the leucogranites to the associated pegmatites. A few pegmatites (e.g., the Lord Hill pegmatite and amethyst-bearing pegmatites) show NYF tendencies unlike any other pegmatites of the SPG. Differences in fractionation degree, evolution and/or relation to another fertile granite–pegmatite system may be responsible for this apparently anomalous group of pegmatite dikes.

Time and duration of Variscan high-temperature metamorphic processes in the south European Variscides: constraints from U-Pb chronology and trace element chemistry of zircon

The lower crust of the Serre massif (Calabria, southern Italy) provides a window into the mid- to lower crust of the south European Variscan orogenic belt. Previously, zircon U-Pb ages were employed to date high-temperature processes affecting this portion of the Variscan crust. The present paper reports new LA-ICP-MS U-Pb data on the zircon of a deformed quartz-monzodiorite dike and of three mafic granulites sampled at the base of the lower crust section. Determination of trace elements on zircon, including rare earth elements (REE), has been also performed. The end of the Variscan exhumation, dated by anatectic zircon from migmatitic metapelites, and the growth-modification of zircon with respect to the growth of Variscan metamorphic garnet have been assumed as “time markers”. The concordant zircon ages of the metamorphic basic rocks cover a range from 744 ± 20 Ma to 231 ± 10 Ma with high age density from 357 ± 11 Ma to 279 ± 10 Ma, a few ages comprised between 418 ± 14 Ma and 483 ± 12 Ma and between 505 ± 11 Ma and 593 ± 14 Ma. Zircon from the deformed quartz-monzodiorite dike evidences a minimum age of emplacement of 323 ± 5 Ma. Most of the analysed zircon domains dated between 357 ± 11 Ma to 279 ± 10 Ma from garnet-bearing metabasic rocks show flat patterns of heavy rare earth elements (HREE), as expected in the case of their simultaneous growth with garnet. This allows to consider (1) zircon domains giving Variscan ages as “metamorphic” with specific geological significance, and (2) zircon domains with ages ranging from 564 ± 17 Ma to 593 ± 14 Ma as dating the emplacement of the magmatic protoliths as shown by internal microtextures, fractionated patterns of HREE and Th/U ratios (0.16–0.19). The Variscan zircon ages (357–279 Ma) reflect effects of crustal thickening, peak metamorphism and subsequent multistage Variscan decompression documented by the statistically significant clusters of ages around 347–340 Ma, 323–318 Ma, 300–294 Ma and 279 Ma. The U-Pb zircon ages of the metabasic rocks suggest a period of about 60–70 Ma for granulite facies metamorphism and anatectic conditions. Literature data indicate that the migmatitic metapelites of the upper part of the Serre crust section also underwent a long period, about 40 Ma, of granulite facies metamorphism and anatectic conditions. A diachronism emerges through the time comparison of the Variscan evolution between the upper and the lower portions of the Serre deep crust. The duration of the Variscan processes defined in Calabria is comparable to that of other south European Variscan blocks.

Carbon isotope anatomy of a single graphite crystal in a metapelitic migmatite revealed by high-spatial resolution SIMS analysis

High-spatial resolution carbon isotope analyses of natural graphite using secondary ion mass spectrometry (SIMS), together with conventional mass spectrometry techniques, demonstrate isotopic heterogeneity within single graphite crystals precipitated from a partially melted metamorphic rock. SIMS 13C/12C measurements were calibrated using an internal graphite standard previously analyzed by conventional isotope ratio mass spectrometry, which gave a reproducibility of 0.3‰ (1σ) at a spatial resolution of 2–3 μm. This resolution helped to identify an unusual carbon isotope distribution in a single graphite crystal from a metapelitic leucosome, showing remarkable core to rim variations with sharp δ13C steps up to 10‰. The results suggest that the graphite crystal grew from one edge to other forming layers perpendicular to the c-axis. The sharp isotopic steps indicate the presence of disequilibrium carbon isotope zoning in graphite and points to the possible existence of carbon isotope sector zoning. Intra-crystalline carbon isotope disequilibrium in graphite is believed to have resulted from the difference in diffusivity between 12C and 13C in the growth medium to the interface of graphite precipitation in different growth sectors.