Amphibolite Facies Conditions Research Articles

Amphibolites of the metamorphic basement of Wrangel island: age, nature of the protolite and conditions of metamorphism

New data on petrography, geochemistry and geochronology of metamorphic rocks sampled from Neoproterozoic basement of Wrangel Island are presented. Oceanic affinity for mafic rocks underwent greenschist to amphibolite facies conditions (350600C) has been established by geochemical data. Protolith of this rock belongs to products of the evolution of magmatic melts parental for MORB. Protoliths age is 61713 and 69812 Ma was determined by U-Pb isotope dating of Zircons from these rocks (SHRIMP II and La-ICP MS). There are two main stage of metamorphism could be established: Neoproterozoi Early Paleozoi (amphibolite and greenschist facies conditions) and Middle Cretaceous (greenschist facies condition only), Last stage of metamorphism related to Chukcha deformation events accompanied by formation of folded nappes structure and consequent extension. An comparison of studied rocks with samples dredged at Chukcha Borderland evidenced for certain similarity existing between these rock assemblages judging by their petrography and chemistry.

Carbon Isotope Composition and Concentration of Fe-Group Elements in the Sedimentary Organic Matter from Rocks of Different Ages

The Fe/Co ratio (ppm) in the kerogen from different-aged clastic sediments (Early Proterozoic–Mesozoic) varies from 17 to 4100, and δ13C value varies from –14.8 to –32.6‰. The relationship between Fe/Co and δ13C is determined by the composition of microbial biota in paleobasins and superimposed metamorphic processes. The kerogen from the Vendian deposits of the East European Platform, containing a large amount of saprophytes and actinomycetes is characterized by the maximum Co concentration. The kerogen from deposits metamorphosed under amphibolite-facies conditions are characterized by the minimal δ13C and the maximum Fe/Co ratio values.

Mineral assemblages and phase equilibria of metabasites from the prehnite–pumpellyite to amphibolite facies, with the Flin Flon Greenstone Belt (Manitoba) as a type example

AbstractAn exceptionally well‐exposed part of the Flin Flon Greenstone Belt (Manitoba/Saskatchewan) is used to characterize the mineral assemblage evolution associated with prehnite–pumpellyite through amphibolite facies metamorphism of basalts. Data from these rocks are combined with a large literature data set to assess the ability of current thermodynamic models to reproduce natural patterns, evaluate the use of metabasic rocks at these grades to estimate pressure–temperature (P–T) conditions of metamorphism, and to comment on the metamorphic devolatilization that occurs. At Flin Flon, five major isograds (actinolite‐in, prehnite‐ and pumpellyite‐out, hornblende‐in, oligoclase‐in, and actinolite‐out) collectively represent passage from prehnite–pumpellyite to lower amphibolite facies conditions. The evolution in mineral assemblages occurs in two narrow (~1,000 m) zones: the prehnite–pumpellyite to greenschist facies (PP‐GS) transition and greenschist to amphibolite facies (GS‐AM) transition. Across the GS‐AM transition, significant increases in the hornblende and oligoclase proportions occur at the expense of actinolite, albite, chlorite, and titanite, whereas there is little change in the proportions of epidote. The majority of this mineral transformation occurs above the oligoclase‐in isograd within the hornblende–actinolite–oligoclase zone. Comparison with thermodynamic modelling results suggests data set 5 (DS5) of Holland and Powell (1998, Journal of Metamorphic Geology, 16(3):309–343) and associated activity–composition (a–x) models is generally successful in reproducing natural observations, whereas data set 6 (DS6) (Holland & Powell, 2011, Journal of Metamorphic Geology, 29(3):333–383) and associated a–x models fail to reproduce the observed mineral isograds and compositions. When the data from Flin Flon are combined with data from the literature, two main pressure‐sensitive facies series for metabasites are revealed, based on prograde passage below or above a hornblende–albite bathograd at ~3.3 kbar: a low‐pressure ‘actinolite–oligoclase type’ facies series, characterized by the appearance of oligoclase before hornblende, and a moderate‐ to high‐pressure ‘hornblende–albite type’ facies series, characterized by the appearance of hornblende before oligoclase. Concerning the PP‐GS transition, the mineral assemblage evolution in Flin Flon suggests it occurs over a small zone (<1,000 m), in which assemblages containing true transitional assemblages (prehnite and/or pumpellyite coexisting with actinolite) are rare. This contrasts with thermodynamic modelling, using either DS5 or DS6, which predicts a wide PP‐GS transition involving the progressive appearance of epidote and actinolite and disappearance of pumpellyite and prehnite. Patterns of mineral assemblages and thermodynamic modelling suggest a useful bathograd (‘CHEPPAQ bathograd’), separating prehnite–pumpellyite‐bearing assemblages at low pressures and pumpellyite–actinolite‐bearing assemblages at higher pressures, occurs at ~2.3 to 2.6 kbar. Observations from the Flin Flon sequence suggests devolatilization across the GS‐AM transition (average: ~1.8 wt% H2O) occurs over a very narrow interval within the actinolite–hornblende–oligoclase zone, associated with the loss of >75% of the total chlorite. By contrast, modelling of the GS‐AM transition zone predicts more progressive dehydration of ~2 wt% H2O over a >50°C interval. Observations from the field suggest devolatilization across the PP‐GS transition occurs over a very narrow interval given the rarity of transitional assemblages. Modelling suggests fluid release of 1.0–1.4 wt% resulting from prehnite breakdown over a ~10°C interval. This fluid may not be entirely lost from the rock package due to involvement in the hydration of igneous mineralogy across the PP‐GS transition as observed in the Flin Flon sequence.

Reworked Precambrian metamorphic basement of the Lhasa terrane, southern Tibet: Zircon/titanite U–Pb geochronology, Hf isotope and geochemistry

Due to the paucity of exposure, the formation and evolution of the Precambrian basement of the Lhasa terrane remain poorly known. Here we report zircon and titanite in situ U–Pb ages, bulk-rock geochemical and zircon Hf isotopic data on the orthogneisses from the Dongjiu area of the southern Lhasa subterrane (SLT), southern Tibet. Geochemical data suggest that the protoliths of the biotite-amphibole gneiss and biotite gneiss are granodiorite and granite, respectively. Inherited magmatic zircon cores from these orthogneisses give protolith crystalline ages of 1520–1506 Ma, whereas the overgrown zircon rims give metamorphic ages of 605–590 Ma. The Mesoproterozoic granitic rocks have bulk-rock εNd(t) values of −3.6 to +0.1 and zircon core εHf(t) values of −4.5 to +2.6, which give similar TDM2 ages of 2.35–2.05 Ga and 2.54–2.10 Ga respectively, suggesting their derivation from partial melting of Paleoproterozoic crustal material. The granitic rocks are also local provenance for the Mesoproterozoic detrital zircons in the Paleozoic strata in the Lhasa terrane. Titanite in situ U–Pb ages further indicate that the Dongjiu orthogneiss experienced more recent metamorphism at ~26 Ma. The mineral assemblage and thermobarometry calculations indicate that the Oligocene metamorphism occurred under medium-pressure (MP) amphibolite-facies conditions (5.4–7.2 kbar, 691–765 °C). We propose that the Dongjiu gneisses represent the Precambrian metamorphic basement of the Lhasa terrane, but have been intensively reworked by metamorphism in the SLT in response to the continued India-Asia convergence since the collision.

Fabrics, geothermometry, and geochronology of the Songshugou ophiolite: Insights into the tectonic evolution of the Shangdan suture, Qinling orogen, China

Abstract The Songshugou ophiolite, located in the northern Qinling belt, consists mainly of metamorphosed mafic and ultramafic rocks recording details of deformation and metamorphism that occurred during subduction, accretion, and collision along the Shangdan suture in the Qinling orogenic belt. Electron backscatter diffraction measurements revealed that the harzburgites are dominated by olivine C-type crystal preferred orientations (CPOs), which were possibly induced by high pressure during slab subduction. Olivine A-type CPOs were also observed in some harzburgites, representing the remnants of the original fabric in oceanic mantle rocks formed in the spreading center of the Shangdan ocean. Coarse-grained dunites are characterized by B-type CPOs, which may have been caused by melt-rock reactions and/or high water contents in a suprasubduction-zone setting during exhumation. Fine-grained dunites are also dominated by B-type CPOs, suggesting that grain-size reduction related to mylonitization did not result in fabric variations. Combined with the mineral assemblages, application of geothermometry suggested that the Songshugou ophiolite has experienced metamorphism and deformation under amphibolite-facies conditions. Zircons from garnet-bearing amphibole schist are characterized by flat heavy rare earth element (HREE) patterns and low Th/U ratios and yielded a mass spectrometry U-Pb age of 500.5 ± 8.8 Ma, representing the peak metamorphic age of the metamafic rocks. Other zircons displayed relative HREE enrichment and a clearly negative Eu anomaly and gave an age of 492.5 ± 3.0 Ma, constraining the time of the exhumation of the ophiolite. Integrated with all the available regional geology, our new fabric, geochemical, and geochronological data suggest that the tectonic evolution of the Songshugou ophiolite can be proximately constrained as subduction at ca. 500 Ma and exhumation at ca. 492 Ma.

Partial resetting of U–Pb ages during experimental fluid-induced re-equilibration of xenotime

Experimentally metasomatized xenotime was investigated in terms of preservation of U–Pb ages during fluid-induced alterations at 250, 450, 550 and 650 °C. Starting materials for experiments included xenotime (38.4 ± 0.6 Ma), albite, K-feldspar, biotite, muscovite, garnet, SiO2, CaF2, Na2Si2O5 and H2O that roughly replicated metapelitic to granitic composition with high Na, Ca, Si and F activities in the fluid. Despite partial replacement of xenotime by secondary phases including (Y,REE)-rich fluorapatite to Y-rich fluorcalciobritholite in all runs, and an unnamed (Y,HREE)-rich silicate in 250 °C experiment, only two experiments at 550 °C, 800 MPa and 650 °C, 600 MPa resulted in development of faint to moderate patchy zoning in the xenotime. Experimental results indicate higher chemical-stability of xenotime at lower P-T conditions compared to those corresponding to amphibolite facies conditions. The xenotime domains metasomatized via coupled dissolution-reprecipitation reactions are characterized by depletion of U and Pb and younger 206Pb/238U dates of ca. 15–34 Ma, demonstrating preferential removal of Pb and thus the partial resetting of the age record. The U–Pb data for the first time experimentally confirm that xenotime may be affected by re-equilibration induced by fluid or melt with high Na, Ca and F activities via coupled dissolution-reprecipitation reactions that result in removal of radiogenic Pb under temperature conditions of 550–650 °C. These findings have important implications for U–Pb geochronology of igneous (particularly in pegmatites) and metamorphic processes in terms of preservation of U–Pb ages of xenotime that was affected by multiple thermal events in the presence of alkali-rich fluid and/or melt.

On the petrology of brittle precursors of shear zones – An expression of concomitant brittle deformation and fluid–rock interactions in the ‘ductile’ continental crust?

AbstractThe inherited localization model for shear zone development suggests that ductile deformation in the middle and lower continental crust is localized on mechanical anisotropies, like fractures, referred to as shear zone brittle precursors. In the Neves area (Western Tauern Window, Eastern Alps), although the structural control of these brittle precursors on ductile strain localization is well established, the relative timing of the brittle deformation and associated localized fluid flow with respect to ductile deformation remains in most cases a matter of debate. The present petrological study, carried out on a brittle precursor of a shear zone affecting the Neves metagranodiorite, aims to determine whether brittle and ductile deformations are concomitant and therefore relate to the same tectonic event. The brittle precursor consists of a 100–500 µm wide recrystallized zone with a host mineral‐controlled stable mineral assemblage composed of plagioclase–garnet–quartz–biotite–zoisite±white mica±pyrite. Plagioclase and garnet preserve an internal compositional zoning interpreted as the fingerprint of Alpine metamorphism and fluid–rock interactions concomitant with the brittle deformation. Phase equilibrium modelling of this garnet‐bearing brittle precursor shows that metamorphic garnet and plagioclase both nucleated at 0.6 ± 0.05 GPa, 500 ± 20°C and then grew along a prograde path to 0.75 ± 0.05 GPa, 530 ± 20°C. These amphibolite facies conditions are similar to those inferred from ductile shear zones from the same area, suggesting that both brittle and ductile deformation were active in the ductile realm above 500°C for a depth range between 17 and 21 km. We speculate that the Neves area fulfils most of the required conditions to have hosted slow earthquakes during Alpine continental collision, that is, coupled frictional and viscous deformation under high‐fluid pressure conditions ~450°C. Further investigation of this potential geological record is required to demonstrate that slow earthquakes may not be restricted to subduction zones but are also very likely to occur in modern continental collision settings.

Melt-collecting structures and the formation of extraction dykes during syntectonic anatexis of the Camboriú Complex, south Brazil

The Camboriú Complex metamorphic sequence comprises orthogneisses and amphibolites with widespread melting features generated at upper amphibolite facies conditions. Regional to mesoscale structures are shown to control generation, collection and extraction of melts. As partial melting starts it is locally focused as layer-parallel leucosomes in banded gneisses, and isolated pockets of in situ leucosomes in massive amphibolites. The further tightening of cm-to m-scale upright folds and conjugate shear planes oblique to axial-planar position trigger the coalescence and migration of melts, first along the main banding, then along axial planes of folds and finally giving rise to a network where melt is collected. The melt network eventually acquires enough mobility to form m-wide extraction dykes roughly parallel to axial planes of folds, capable of transferring the products of migmatization to upper crustal levels to feed granitic plutons. The orientation and geometry of pure-shear dominated structures in the Camboriú Complex are in accordance with deformation partitioning between this and adjancent shear zones, which is expected for the transpressive character of the Neoproterozoic Southern Brazilian Shear Belt.

Proto-Tethys oceanic slab break-off: Insights from early Paleozoic magmatic diversity in the West Kunlun Orogen, NW Tibetan Plateau

Continental collision and subsequent slab break-off can induce granitoid magmatism of variable compositions and ages. We present new geochronological and geochemical data from granitoids in the eastern part of the West Kunlun Orogen, situated along the northwestern margin of the Tibetan Plateau. Our data reveal a distinct Late Ordovician–middle Silurian (450–428 Ma) magmatic flare-up event after closure of the Proto-Tethys Ocean (Proto-Kunlun Ocean) and following continental collision. Some high-Sr/Y granitoids show variable whole-rock (87Sr/86Sr)i, εNd(t), (206Pb/204Pb)t, and zircon εHf(t) values, indicating that these rocks were derived from heterogeneous sources comprising both juvenile and older material from the lower crust under amphibolite-facies conditions. We identify a linear spatial distribution of small-scale and short-lived magmatism and magmatic diversity (A-/S-/I-type) along the suture zone, and suggest that this Late Ordovician–middle Silurian magmatism in the West Kunlun Orogen was associated with slab break-off of the Proto-Tethys oceanic plate. The results improve our understanding of the closure of the Proto-Tethys Ocean, crustal growth, and the final formation of Gondwana in the NW Tibetan Plateau region.

The Tamrau Block of NW New Guinea records late Miocene–Pliocene collision at the northern tip of the Australian Plate

The Tamrau Block is a geological terrane located in the Bird’s Head Peninsula of NW New Guinea. Relatively little is known about this terrane and how it relates to other parts of western New Guinea. The geological history and provenance of this terrane is difficult to resolve because it is found at the boundary between the Australian, Philippine Sea, Caroline, and Pacific plates. This segment of the plate boundary records episodes of deformation, magmatism, and exhumation associated with the interaction of these three tectonic plates. For instance, the southern edge of the Tamrau Block is bounded by a major strike-slip fault zone (the Sorong Fault Zone), previous studies imply that the terrane may have been translated some distance along this fault zone during the Cenozoic. We seek to understand the geological history and provenance of the Tamrau Block. To do this, we report new field observations made during a three-month field campaign in a poorly exposed region, together with new petrographic and geochronology data. U–Pb detrital zircon age spectra were obtained from five sedimentary and metamorphic sequences and the results were combined with existing biostratigraphic age data to reassess the stratigraphy and tectonic history of the region. The oldest rocks in the Tamrau Block (the Tamrau Formation) represent Jurassic–Cretaceous passive margin sediments. These were deformed and metamorphosed in at least three distinct events – the first phase of which involved amphibolite facies conditions (and produced the first reported instance of metamorphic kyanite from the Bird’s Head), possibly in the Oligo–Miocene. These Jurassic–Cretaceous rocks are unconformably overlain by the Ajai Limestone. Both units are cross-cut and overlain by intrusives and eruptives of the middle Miocene Moon Volcanics. The heat associated with these intrusives baked the Tamrau Formation and the overlying Ajai Limestone, resulting in andalusite growth in the Tamrau Formation and hornfelsing of the limestones. Deposition of the overlying Koor Formation occurred from the middle–late Miocene, and was partially contemporaneous with volcanism. An episode of crustal shortening occurred after the deposition of the Koor Formation with the development of asymmetric folds and steeply-inclined reverse faults. This is marked by a late Miocene–Pliocene break in deposition between the Koor Formation and undeformed Opmorai Formation (∼10.5–4.5 Ma) and records the collision of part of an oceanic island arc (Tosem Block) to the Tamrau Block, an event also recognised in other parts of New Guinea. A comparison of the U–Pb detrital zircon age spectra from sedimentary rocks within the Tamrau Block and those reported in other studies indicate that there is no relationship between the rocks of the Tamrau and Kemum blocks (which are in faulted contact). Instead, the U–Pb detrital age data share similarities with rocks from the Lengguru Fold and Thrust Belt and Weyland Overthrust in the south-east. We therefore propose that the Tamrau Block was transported westwards ∼300 km to its current position along the Sorong Fault Zone after the late Miocene–Pliocene collision between the Tosem Block and the northern margin of the Australian Plate.

Geometric and tectonic analysis of Ad-Damm mega-scale fold: implication of Neoproterozoic Transpressive Regime in the west-central Arabian Shield

A mega-scale fold is exposed near Jeddah city in the western-central part of the Arabian Shield. This fold extends NE for about 35-km long and 12-km width and occupies the northwestern block of a distinct linear feature known as Ad-Damm Shear Zone. It is composed essentially of foliated, sheared, and mylonitized metamorphic and plutonic rocks. Para and ortho-gneisses were an amphibolite facies condition whereas greenschist facies units were represented by metasedimentary and metavolcanics. Image processing and field-based structural analysis, in addition to micro structural investigations of the mega-scale fold, indicating that the ductile dextral-sense of shear was overprinted by a brittle regime associated with the evolution of Ad-Damm shear zone. The Ad Dam mega-scale fold is a shear-related structure, north plunging overturned anticline. Mylonitization and dextral shear-related structures were developed during a transpressional regime. The Ad-Damm fold is a structural system that illustrates the evolution history of the Arabian Shield associated with the development of the NE-wrench fault. This fault started as an early plastic followed later by a brittle tectonic regime. The present study indicates that Ad-Damm shear zone and several sub-parallel dextral shears of NE-trends which located in the western side of the Arabian Shield represent a conjugate antithetic shear to the NW- trending sinistral shear of the Najd fault system.

Geochemistry of banded iron formations and their host rocks from the Central Eastern Desert of Egypt: A working genetic model and tectonic implications

Banded iron formations (BIF) occur intercalated with Neoproterozoic volcanosedimentary units in 13 localities in the Egyptian Central Eastern Desert (CED). These units are characterized by magnetite and hematite-rich layers alternating with quartz-rich layers containing andradite–rich garnet, epidote, ± calcite in the southern parts of CED, and chlorite ± stilpnomelane, and calcite in the north. Localized hydrothermal alteration affected most deposits and is manifested by secondary Ca–bearing minerals. All BIFs and their host rocks were strongly deformed and metamorphosed under greenschist to epidote amphibolite facies conditions during the collisional stage of the Pan African orogeny.Geochemically, CED BIFs have higher Fe/Si compared to Algoma, Superior, or Rapitan BIF types. All BIFs have rare earth element – Y patterns similar to those of modern day oceanic water, with a few samples displaying a weak positive Eu anomaly. All BIFs have high SiO2/Al2O3 and Fe/Ti, and low Al/(Al + Fe + Mn), which suggest a hydrogenous origin with hydrothermal contributions and low detrital component. Geochemical trends, Y/Ho, and Pr/Yb values suggest deposition of Wadi El Dabbah BIF closest to land and Um Nar and Wadi Kareim closest to hydrothermal vents. Host metavolcanic and metavolcaniclastic rocks show chemical signatures indicative of an immature oceanic arc setting with MORB affinities for the southern areas and back- or fore-arc basin affinities for the northern localities.These results lead to the conclusion that CED BIFs and their host rocks formed in small intra-arc basins and sloped or terraced silled basins in the back- and fore-arc areas surrounding an immature island arc. Restricted circulation of hydrothermal fluids in these basins concomitant with arc volcanism increased Fe+2 and Si in solution. During periods of arc quiescence, oxidation of Fe2+ led to deposition of Fe-oxyhydroxide. Diagenesis formed fine-grained magnetite, whereas subsequent hydrothermal alteration and metamorphism formed porphyroblastic magnetite and specularite.

Minor and trace element variations in chromite from the Songshugou dunites, North Qinling Orogen: Evidence for amphibolite-facies metamorphism

The Songshugou dunites are part of the ultramafic sequence of the Songshugou ophiolite within the Qinling Group metamorphic rocks, which are distributed in the northern part of the Shang–Dan Suture Zone (SDSZ) in central China. The dunites include lenses of coarse- and medium-grained dunite set in a fine-grained dunite host. Disseminated chromite within these dunites can be classified into three microstructural types: (1) partly altered chromite with primary cores surrounded by porous chromite rims; (2) porous chromite with abundant pores that are filled with chlorite; and (3) homogeneous chromite. The primary magmatic cores of the partly altered chromite (Chr) is depleted in Cr and enriched in Al and Mg. Porous chromite includes Cr-rich Chr and ferritchromite (Fe-Chr) that is enriched in Fe2+ but depleted in Al. Homogeneous chromite is mainly Cr-magnetite (Cr-Mag) that is enriched in Fe3+ and extremely depleted in Al. The primary Cr-poor chromite was gradually transformed to porous Cr-rich Chr and/or by Fe-Chr during metamorphism. Reaction of Cr-poor Chr with olivine (Ol) produces chlorite (Chl) and secondary Chr enriched in Cr and Fe2+. Mg and Al diffuse away from primary Chr, leaving a residual Al- and Mg-depleted, Cr-rich Chr and Fe-Chr. Porous Chr is enriched in Zn, Co, Mn, but depleted in Ga, Ni, and Sc, due to the fluid-assisted formation of Chl within the pores of porous chromite. Zn, Co, and Mn from the metamorphic fluids diffuse toward the cores of chromite. The elements Sc, Ti, V, and Ni diffuse from Cr-poor Chr core to Cr-mag rim with increasing Fe3+/(Fe3+ + Fe2+). Late precipitation of Cr-mag occurred under oxidizing and hydrous conditions. The infiltration of fluid produced non-porous Fe3+-rich Chr or Cr-mag along grain boundaries or fractures within chromite. These fluids further promoted the reordering of octahedral sites, enhancing higher mobilization and enrichment of Ga, Ni, Sc, V, and Ti. The infiltration of high oxygen fugacity fluids promoted diffusion of Ga, Ni, Sc, and Ti toward the rims, and diffusion of Zn, Co, and Mn toward the cores. All chromite grains from the Songshugou dunites have low Mg# [molar 100 × Mg/(Mg + Fe2+)] values, and high Cr# [molar 100 × Cr/(Cr + Al)] and Fe3+/(Cr + Al + Fe3+), similar to metamorphic chromite formed under amphibolite-facies conditions. The observed positive Zn–Co–Mn anomalies and negative Ga–Ni–Sc anomalies in chromite, and the crystallization of Chl, are fingerprints of amphibolite-facies metamorphism. The compositions of chromite, the inferred mineral assemblage in which it occurs, and mineral equilibria modeling of the assemblage all suggest amphibolite-facies metamorphic temperatures ranging from 600 °C to 700 °C.

Eoarchean to Neoproterozoic crustal evolution of the Mantiqueira and the Juiz de Fora Complexes, SE Brazil: Petrology, geochemistry, zircon U-Pb geochronology and Lu-Hf isotopes

The Mantiqueira and the Juiz de Fora Complexes (MQC and JFC) in the Araçuaí-Ribeira orogenic system, southeastern Brazil, have been regarded as parts of the Neoproterozoic to Cambrian Brasiliano Orogeny formed by a series of collisions of the São Francisco and the Congo Cratons. Here we report new petrological, geochemical, and zircon U-Pb and Lu-Hf isotopic data on felsic to mafic orthogneisses from the complexes with a view to evaluate the timing and P-T conditions of the collisional event as well as protolith formation. Metamorphic P-T estimates for orthogneisses indicate amphibolite-facies conditions of 650–730 °C and 5.8–9.1 kbar from the MQC, whereas slightly higher (upper amphibolite- to lower granulite-facies) conditions of 680–770 °C and 6.0–7.7 kbar were obtained from the JFC. Geochemical, zircon U-Pb geochronological, and Lu-Hf isotopic data of felsic orthogneisses indicate that the MQC was formed by a series of Neoarchean (ca. 2.67 and 2.61 Ga) volcanic arc magmatisms with magma sourced from Eoarchean to Paleoarchean (ca. 3.82–3.42 Ga) crustal components. In contrast, slightly younger U-Pb ages of ca. 2380 Ma and ca. 2050–1960 Ma were obtained from the JFC marking the timing of volcanic arc magmatism as Early to Middle Paleoproterozoic, with magma sourced from Paleoarchean (∼3.45 Ga) crust with addition of juvenile crustal components. The recycled ancient crustal components were also supplied for late Neoproterozoic (ca. 600 Ma) continental arc magmatism of the JFC. The high-grade metamorphism is possibly related to the final collision of the Congo and São Francisco Cratons at 590–560 Ma, which broadly compares with the formation of the Kuunga Orogen.

Origin and thermometry of graphites from Itapecerica supracrustal succession of the southern Sao Francisco Craton by C isotopes, X-ray diffraction, and Raman spectroscopy

ABSTRACTIn the mines of the Nacional de Grafite Company around Itapecerica (Minas Gerais), located in the southern Sao Francisco Craton, occurs a supracrustal succession of high-grade metamorphic rocks including quartzite, garnet-biotite gneiss, and graphite schist formed in the Palaeoproterozoic (2.0 Ga). During metamorphic processes, organic matter was progressively transformed into graphite. From four graphite samples of three different mines (two samples from high-grade metamorphic graphite schist and two generated by hydrothermal recrystallization of the graphite schist), the origin and formation temperature of this mineral was obtained by C isotopes, X-ray diffraction (XRD), and Raman spectroscopy. The values of δ13C range between −21.23 and −27.89 ‰, indicating that the source of the graphite was a primitive biogenic carbon material. High-grade metamorphic graphites show average temperatures around 729°C, while hydrothermal recrystallizated graphites (vein-graphites) show temperatures around 611°C by XRD, which correspond to granulite- to amphibolite facies conditions. The hydrothermal process with percolation of C-O-H fluids leads to a decrease in the crystal size along stacking direction (Lc(002)) when compared with the previously formed high-grade metamorphism graphites. An update of the current tectonic model about the collisional process during Rhyacian-Orosirian orogeny in the Sao Francisco Craton is proposed to insert the formation of the Itapecerica graphite-rich metasedimentary sequence.

Arc-related high-K magmatism in the Ceuta Peninsula (Internal Rif, Spain): discovery and consequences

AbstractWe document the occurrence of micro-diorite magmatic sills, with magmatic enclaves, in the Ceuta Peninsula within metapelites from the Lower Sebtides units (Internal Rif). All magmatic rocks show a primary magmatic mineralogy and geochemical signature diagnostic for high-K calc-alkaline to shoshonitic island arc magmatism. Moreover, these rocks are significantly affected by secondary metamorphic transformations under greenschist- to amphibolite-facies conditions, regionally dated atc. 21 Ma. Geometric relationships between the sills and the main regional foliation, developed under intermediate-pressure granulite-facies conditions atc. 28 Ma, demonstrate that the sills emplaced during the late stage of this main tectonic event. New U–Pbin situanalyses of monazite performed on the micro-diorite sills provide an age of 20.64 ± 0.19 Ma, coherent with this chronological framework and interpreted as the age of greenschist-facies re-equilibration. The discovery of pre-Miocene high-K calc-alkaline to shoshonitic arc-related magmatism is clearly consistent with the subduction context proposed for the Alboran Basin evolution, according to geophysical investigations. In this framework, the Lower Sebtides units could be considered as part of the upper plate of the subduction system, while the Upper Sebtides must be regarded as the lower subducted plate.

Rare peak and ubiquitous post-peak zircon in eclogite: Constraints for the timing of UHP and HP metamorphism in Erzgebirge, Germany

Erzgebirge eclogite occurs in three high-pressure (HP) units, including one UHP unit (Unit 1). Each sample investigated contains either magmatic or metamorphic zircon but never both. This is in contrast to many literature examples of eclogite with more than one generation of zircon. We infer that metamorphic zircon formed at the expense of a magmatic precursor, and that the process either went to completion (consuming all magmatic zircon) or did not occur at all. Metamorphic zircon is only present in UHP eclogite and one HP eclogite from Unit 2. Another sample from Unit 2 and one from Unit 3, which experienced the lowest metamorphic PT conditions of all samples, contain only magmatic zircon. The U-Pb SIMS age of c. 540 Ma of igneous zircon, being somewhat younger than average Hf model ages (i.e., 567 and 591 Ma), is attributed to crystallization in the basaltic or gabbroic precursor that formed from a juvenile mantle source.The time of peak metamorphism is recorded by only two zircons from UHP eclogite giving ages of 360.5 ± 5.4 Ma and 359.7 ± 5.3 Ma that well agree with a previous three-point Sm-Nd isochron of 360 ± 7 Ma. All other grains gave younger ages, mainly between 340 and 330 Ma but covering a range of c. 350–315 Ma, which is attributed to retrogression at amphibolite-facies conditions. We infer (i) that this post-eclogitic zircon replaced peak-metamorphic grains (only in UHP eclogite) as well as magmatic zircon and (ii) that peak-metamorphic zircon did not form in non-UHP eclogite. Growth of peak and post-peak zircon was governed by fluid and/or melt. The latter is related to decompressional melting in felsic host rocks (and possibly also to late-orogenic granitic magmatism). Fluid is either of external origin (melt-derived or from dehydration reactions in subducting footwall units) or due to decompressional release of internal fluid (molecular water from fluid inclusions or structural water in omphacite and garnet). The new and former age data can best be reconciled with a two-stage exhumation process. Initial, buoyancy-driven exhumation was fast and terminated as the HP/UHP slices reached lower crustal levels where the rocks presumably rested for 10–20 million years, followed by slow exhumation associated with gravitational collapse of the Variscan orogen. The majority of metamorphic zircon (with ages of 340–330 Ma) grew at lower crustal levels, where the rocks presumably rested for 10–20 million years, and other grains (younger than 330 Ma) during the final, slow exhumation that partly overlapped with the time of granitic magmatism.

Structures, strain analyses, and 40Ar/39Ar ages of blueschist‐bearing Heilongjiang Complex (NE China): Implications for the Mesozoic tectonic evolution of NE China

The high‐pressure Heilongjiang Complex belongs to a Jurassic accretionary complex in Yilan area, NE China. The complex is located in the junction of three orogenic systems in the NE Asia: the E‐W‐striking Central Asian Orogenic Belt (CAOB), the N‐S‐striking Paleo‐Pacific system, and the ENE‐WSW‐striking Mongol‐Okhotsk system. Thus, a detailed tectono‐metamorphic analysis coupled with accurate radiometric dating of the Heilongjiang Complex provided in this study represent important evidence for understanding the complex interactions between the three orogenic systems in the Late‐Palaeozoic to Mesozoic. New structural and microstructural data combined with the strain analysis highlight the presence of three distinct deformation events affecting the high‐pressure Heilongjiang Complex. The first deformation event D1 is related to development of blueschist facies prolate to plane strain fabrics that were subsequently reworked by the blueschist‐amphibolite facies event D2 that transposed all previous structures by the main west‐dipping schistosity. This planar fabric was folded by upright F3 folds and heterogeneously reworked by greenschist facies E‐W‐striking vertical cleavage during large scale N‐S shortening. Each deformation event was related to specific strain fabrics and metamorphic conditions highlighting dynamic evolution of the subduction complex during burial, exhumation, and final shortening. The first D1 event is related to westward directed prograde constrictional flow and progressive dismembering of competent mafic, ultramafic layers, and quartzites in highly stretched meta‐sedimentary matrix. This mélange was folded by recumbent detachment folds resulting in emplacement of the exhumed unit progressively reaching amphibolite facies conditions atop of W‐WSW‐dipping channel. This event is associated with dominant flattening implying increasing mechanical coupling between the colliding Songliao and Jiamusi blocks. Subsequently, the accreted subduction complex was reworked during N‐S‐directed horizontal shortening. This event was associated with an important post‐buckle flattening of fold limbs and rotation of competent blocks with inherited D1 and D2 strain ellipsoids along F3 fold axes. Our new 40Ar‐39Ar plateau ages on muscovite and phengite (173–177 Ma), together with previous radiometric data, are interpreted as reflecting exhumation of subduction channel during the blueschist‐amphibolite facies event while the N‐S shortening (D3) probably operated in Late Jurassic as indicated by some previously published cooling ages (~140–160 Ma). The westward‐oriented subduction and exhumation fabrics of the high‐pressure Heilongjiang accretionary complex followed by N‐S‐directed upright folding are interpreted to reflect transition from E‐W Pacific subduction regime to N‐S‐directed shortening related to the closure of the Mongol‐Okhotsk Ocean.

Tectonothermal Records in Migmatite-Like Rocks of the Guandi Complex in Zhoukoudian, Beijing: Implications for Late Neoarchean to Proterozoic Tectonics of the North China Craton

Migmatite-like rocks transformed from strongly metamorphosed and deformed enclave-bearing felsic plutons usually make people confuse with the true migmatites and mistake in interpretation of their petrogenesis and tectonic implications. Here we report a suite of rocks that have long been called as migmatites from the Guandi complex in Zhoukoudian region, southwest of Beijing. The rocks are dominated by felsic gneisses with garnet-free amphibolites. Field occurrence, petrography and geochemistry indicate that the felsic gneisses and amphibolites were metamorphosed from protoliths of intermediate-acid and basic igneous rocks, respectively. New LA-ICP-MS zircon U-Pb dating and geothermobarometry study further reveal that precursor magmas of the two types of rocks were emplaced at 2.54-2.56 Ga and the rocks subsequently underwent medium P / T -type metamorphism with upper amphibolite facies conditions of 0.55-0.90 GPa and 670-730℃ at~2.48-2.50 Ga. Geochemically, precursor magmas of the amphibolites were suggested to be derived from an enriched lithospheric mantle source in continental arc setting, and those of the felsic gneisses are characterized by tonalitic to trondhjemitic magmas that are usually considered to be generated by partial melting of hydrated, thickened metamorphosed mafic crust with garnet as residues, suggesting that the rock associations are not of true migmatites but migmatite-like rocks. Our study reveal that protoliths of the migmatite-like rocks from the Guandi complex, were likely formed via magmatism in a continental arc setting, followed by accretion and collision of the continental arc as well as the intro-oceanic arc terranes to the Eastern Block of the North China Craton in the transition from the Late Neoarchean to Early Paleoproterozoic.

Metavolcanic rocks and orthogneisses from Porongos and Várzea do Capivarita complexes: A case for identification of tectonic interleaving at different crustal levels from structural and geochemical data in southernmost Brazil

The Dom Feliciano Belt (DFB), part of the Mantiqueira Province in southern Brazil and Uruguay, results from multiple Neoproterozoic collisions leading to the construction of the Gondwana supercontinent. Therefore, complex structural relations are expected, as it is the case for the Porongos Metamorphic Complex (PMC) and Várzea do Capivarita Complex (VCC). Several papers have described structural features from both complexes, and some evolution models were proposed for them. However, very few papers have tried to correlate them, and recent geochronological and provenance studies carried out in both complexes pointed out some inconsistencies in these models. Such studies demonstrate that part of the protoliths of metasedimentary and metaigneous rocks of PMC and VCC, of different metamorphic grades, could have been deposited in the same sedimentary environment. The geological meaning of such data is still poorly explained and requires a better understanding of the original geometrical relations between PMC and VCC. This paper focuses on the metavolcanic rocks from two subareas near the northeastern tip of the Porongos Metamorphic Complex area, and on the previously published data for the VCC. In order to investigate the structural and kinematic history of this part of PMC, detailed structural mapping, petrography, whole-rock and mineral chemistry studies were carried out in selected subareas. Metavolcanic rocks from Subarea 1 and Subarea 2 are metadacites and metarhyolites bearing a geochemical imprint of arc magmatism at ca. 790 Ma. Structural investigation of the area reveals that the main deformation phase D1 is mostly compressive and reached the metamorphic peak at amphibolite facies conditions. D2 and D3 are late deformation phases which develop open folds and axial plane cleavages. The most conspicuous D1 structure is an alternation of non-mylonitic S1 and mylonitic-S1, developed over an originally horizontal foliation, which suggests strain concentration in sub-horizontal shear zones interpreted as related to a fold and thrust belt evolution. The scattering of stretching lineation (L1) measurements along a great circle in stereoplots is interpreted to result from shearing along S1 planes, similar to what is described for the progressive deformation of VCC nearby. The present dataset supports the interpretation that PMC and VCC have shared a single volcano-sedimentary basin which subsequently achieved different crustal levels. A W-directed collisional event at 650 Ma metamorphosed the rocks and generated thrust-folds in both complexes. As indicated by recent studies in the local literature, deformed metavolcanics rocks of ca. 578 Ma magmatic age form concordant sheets in the PMC. Such data suggest a metamorphic-deformational event younger than ca. 578 Ma, which, in our view, is probably related to the reactivation of the contractional structures generated at ca. 650 Ma. This younger event must have folded the eastern part of the PMC generating shallow-level folds, and thrusted the rocks onto the eastern side to place them on top of the PMC western-side rocks. Thus, it is possible that PMC rocks register two thrusting episodes, one during peak metamorphic conditions (ca. 650 Ma) and one under retrograde metamorphic conditions (ca. 578 Ma).