Major Metamorphic Event Research Articles

Compositional and metamorphic controls on tectonic erosion along a continental subduction-collision zone: Implications from mafic granulites in the northern Sulu orogen

Abstract Subduction erosion has been reported recently in continental subduction-collision zones, and the response of the upper plate remains enigmatic. The Dabie-Sulu orogen is considered to have formed by deep northward subduction of the Yangtze block (lower plate) beneath the North China block (upper plate). However, within the northern Sulu ultrahigh-pressure (UHP) belt, the Haiyangsuo complex has intriguingly been accepted as Neoarchean–Paleoproterozoic metamorphic basement from the North China block, implying subduction-erosion could have occurred. Here, we examined new petrographic, mineral chemical, and published geochronological data from the Haiyangsuo mafic granulites to decipher their multiphase metamorphic evolution. Pressure-temperature (P-T) estimates from pseudosection modeling and geothermobarometry show two major metamorphic events: a medium-pressure granulite-facies event (7.3–8.3 kbar and 830–895 °C) and a later high-pressure granulite-facies event (12.2–16.6 kbar and 800–875 °C). Geochronology and mineral geochemistry link these events to Paleoproterozoic and Triassic metamorphism, respectively. Peak Triassic high-pressure granulite-facies metamorphism was followed by near-isothermal decompression and then near-isobaric cooling. These rocks derive from the upper plate and were metamorphosed at the same time as UHP eclogites, but maximum pressures were substantially lower, and P-T paths did not converge until late-stage exhumation. Evidently, metamorphic basement from the North China block was dragged to lowercrustal to upper-mantle depths (50–60 km) while that from the Yangtze block was subducted deeply, and then they were tectonically juxtaposed during exhumation. These data corroborate thermal-mechanical models that predict tectonic erosion as a major process during continental subduction and collision.

Multiple P–T–d–t paths reveal the evolution of the final Nuna assembly in northeast Australia

AbstractThe final assembly of the Mesoproterozoic supercontinent Nuna was marked by the collision of Laurentia and Australia at 1.60 Ga, which is recorded in the Georgetown Inlier of NE Australia. Here, we decipher the metamorphic evolution of this final Nuna collisional event using petrostructural analysis, major and trace element compositions of key minerals, thermodynamic modelling, and multi‐method geochronology. The Georgetown Inlier is characterised by deformed and metamorphosed 1.70–1.62 Ga sedimentary and mafic rocks, which were intruded by c. 1.56 Ga old S‐type granites. Garnet Lu–Hf and monazite U–Pb isotopic analyses distinguish two major metamorphic events (M1 at c. 1.60 Ga and M2 at c. 1.55 Ga), which allows at least two composite fabrics to be identified at the regional scale—c. 1.60 Ga S1 (consisting in fabrics S1a and S1b) and c. 1.55 Ga S2 (including fabrics S2a and S2b). Also, three tectono‐metamorphic domains are distinguished: (a) the western domain, with S1 defined by low‐P (LP) greenschist facies assemblages; (b) the central domain, where S1 fabric is preserved as medium‐P (MP) amphibolite facies relicts, and locally as inclusion trails in garnet wrapped by the regionally dominant low‐P amphibolite facies S2 fabric; and (c) the eastern domain dominated by upper amphibolite to granulite facies S2 foliation. In the central domain, 1.60 Ga MP–medium‐T (MT) metamorphism (M1) developed within the staurolite–garnet stability field, with conditions ranging from 530–550°C at 6–7 kbar (garnet cores) to 620–650°C at 8–9 kbar (garnet rims), and it is associated with S1 fabric. The onset of 1.55 Ga LP–high‐T (HT) metamorphism (M2) is marked by replacement of staurolite by andalusite (M2a/D2a), which was subsequently pseudomorphed by sillimanite (M2b/D2b) where granite and migmatite are abundant. P–T conditions ranged from 600 to 680°C and 4–6 kbar for the M2b sillimanite stage. 1.60 Ga garnet relicts within the S2 foliation highlight the progressive obliteration of the S1 fabric by regional S2 in the central zone during peak M2 metamorphism. In the eastern migmatitic complex, partial melting of paragneiss and amphibolite occurred syn‐ to post‐ S2, at 730–770°C and 6–8 kbar, and at 750–790°C and 6 kbar, respectively. The pressure–temperature–deformation–time paths reconstructed for the Georgetown Inlier suggest a c. 1.60 Ga M1/D1 event recorded under greenschist facies conditions in the western domain and under medium‐P and medium‐T conditions in the central domain. This event was followed by the regional 1.56–1.54 Ga low‐P and high‐T phase (M2/D2), extensively recorded in the central and eastern domains. Decompression between these two metamorphic events is ascribed to an episode of exhumation. The two‐stage evolution supports the previous hypothesis that the Georgetown Inlier preserves continental collisional and subsequent thermal perturbation associated with granite emplacement.

Characterization and metamorphic evolution of Mesoproterozoic granulites from Sonapahar (Meghalaya), NE India, using EPMA monazite dating

AbstractThis paper presents three different age domains, obtained by electron microprobe monazite dating, for granulitic gneisses collected from the Shillong-Meghalaya Gneissic Complex in Sonapahar, NE India, which contain radioactive materials, e.g. thorium (3.32–7.20 wt %), uranium (0.133–1.172 wt %) and lead (0.101–0.513 wt %). The microprobe analyses of monazite grains in the rock samples show that the monazites have three different ages ranging from Mesoproterozoic to Neoproterozoic. The oldest age (1571 ± 22 Ma) represents a peak metamorphic event, the youngest dominant age indicates the Pan-African tectonic event (478 ± 7 Ma) and the intermediate age marks the Grenvillian orogeny (1034 ± 91 Ma) or may be a mixing artefact; these ages are located at the cores, rims and intermediate parts of the monazite grains, respectively. The equilibrium mineral phases calculated for the granulitic gneisses from Sonapahar lie in a P–T range from 5.9 kbar/754 °C to 8.3 kbar/829 °C in the NCKFMASH system. Plotting the P–T conditions of the granulitic gneisses reveals a clockwise P–T path. Two major metamorphic events are observed in Sonapahar. The M1 metamorphic stage is represented by peak mineral assemblages of prograde garnet-forming reactions (8.2 kbar/∼713 °C) during Mesoproterozoic time (1571 ± 22 Ma). The M2 metamorphic stage featured decompression (3.9 kbar/∼701 °C) in which garnet–sillimanite broke down to form cordierite along an isothermal decompression path during the Pan-African tectonic event (478 ± 7 Ma).

Ultrahigh Temperature Metamorphic Record of Pelitic Granulites in the Huangtuyao Area of the Huai’an Complex, North China Craton

Pelitic granulite from the Huangtuyao area, occurrs in the Huai’an Complex, is located in the Trans-North China Orogen of the North China Craton. On the basis of petrolography, mineral component, and phase equilibrium modeling studies, the P-T conditions and mineral assemblages of pelitic granulites can be divided into four metamorphic stages: the prograde metamorphic stage M1 defined by the stable mineral assemblage of Grt1 (garnet core)+Pl+Bt+Kfs+Qz+Rt, the peak pressure Pmax stage M2 indicated by Grt2 (garnet mantle)+Kfs±(Ky)+Rt+Qz+Liq (melt), peak temperature Tmax stage M3 characterized by Grt3 (garnet rim)+Sill+Pl+Kfs+Qz+Ilm+Liq, and retrograde stage M4 represented by Grt (in matrix)+Kfs+Sill+Bt+Pl+Qz+Ilm. By using the THERMOCALC V340, the P-T conditions are estimated at ~13.8−14.1 kbar and ~840−850 °C at stage M2, and 7–7.2 kbar and 909–915 °C for the Tmax stage M3, indicating an ultra-high temperature (UHT) metamorphic overprinting during decompression and heating process after high pressure granulite facies metamorphism. The mineral assemblages and their P-T conditions presented a clockwise P-T trajectory for the Huangtuyao pelitic granulites. The major metamorphic events at ~1.95 and ~1.88 Ga obtained by the zircon U-Pb dating suggest that pelitic granulites from the Huangtuyao area has undergone HP granulite metamorphism which probably occurred in the prograde metamorphism and related to the collision between the Ordos and the Yinshan blocks, and afterwards UHT metamorphism is related to crustal extension after continental-continental collision.

Origin of zircon-bearing mantle eclogites entrained in the V. Grib kimberlite (Arkhangelsk region, NW Russia): Evidence from mineral geochemistry and the U-Pb and Lu-Hf isotope compositions of zircon

The concentrations of major and trace elements in minerals, reconstructed whole-rock compositions of zircon-bearing equigranular eclogites from the V. Grib kimberlite pipe located within the Arkhangelsk Diamondiferous Province (North-Western Russia), and results of the U-Pb and Lu-Hf isotope analyses of zircon grains from eclogites and granulite xenoliths are reported. These data suggest that the equigranular eclogites could represent the fragments of mid-ocean-ridge basalt that were metamorphosed during Paleoproterozoic subduction at 1.7–1.9 Ga. The Hf isotope compositions of the eclogitic zircon display uniformity and indicate corresponding Hf-depleted mantle model ages of 2.2–2.3 Ga. The formation of zircon in eclogites could have resulted from interactions with metasomatic/subduction-related fluids just prior to, but associated with, Paleoproterozoic eclogite formation. A link between eclogitic zircon formation and continental lower-crustal rocks can be excluded based on differences in the Hf isotope compositions of eclogitic and granulitic zircon grains. The U-Pb upper intercept age of granulitic zircon of 2716 ± 61 Ma provides a new minimum age constraint for zircon crystallisation and granulite formation. The U-Pb ages obtained from granulitic zircon show two stages of Pb loss at 2.2–2.6 Ga and 1.7–2.0 Ga. The late Paleoproterozoic stage of Pb loss recorded in granulitic zircon is due to the intensive reworking of basement crustal rocks, which was caused by a tectonic process/subduction event associated with equigranular eclogite formation. Our data, along with evidence previously obtained from the V. Grib pipe coarse-granular eclogites, show at least two main subduction events in the lithospheric mantle of the Arkhangelsk region: the Archean (2.8 Ga) and Paleoproterozoic (1.7–1.9 Ga) subductions, which correspond to major magmatic and metamorphic events in the Baltic Shield.

Geochemistry, U-Pb geochronology and Sm-Nd data from the Paleoproterozoic Western Finland supersuite – A key component in the coupled Bothnian oroclines

The Western Finland (WF) supersuite, characterized by supracrustal rocks and intruding plutonic rocks, forms the core component of the coupled Bothnian oroclines in the central part of the Svecofennian orogen. We present here geochemical-isotope data, detrital zircon age data on sedimentary rocks and age data on granitoid plutons within the WF supersuite in the Pirkanmaa (PB) and Pohjanmaa (PoB) belt representing the southernmost limb of the southern and the hinge zone of the northern Bothnian orocline, respectively. The Tampere belt (TB) in the north and Häme belt (HB) in the south bound the PB.Basaltic to picritic volcanic rocks in the WF supersuite, with a proposed eruption age at ca. 1.91Ga (1.92–1.90Ga), show locally pillow-structures and are typically associated with black shales±cherts±carbonate-bearing rocks. Based on geochemistry and Sm-Nd data a variable MORB-WPB-type origin is evident and an evolution scenario from earlier WPB-type (rifting) to more MORB-affinity (ocean/sea opening) is proposed. Detrital zircon age distributions from sedimentary rocks with ≥1.91Ga maximum depositional age in the TB, PB and HB have been compared and typically the age distributions of <2.2Ga zircons in different areas favor comparable source characteristics. The major metamorphic event at 1885–1875Ma in the WF supersuite is well dated. Although an earlier metamorphic stage at 1.92–1.91Ga is locally probable most of the <1.94Ga overgrowths in the WF supersuite are considered pre-depositional and recycled. Two models are presented to explain the hypothesis of similar pre-1.91Ga evolution in the WF supersuite and lowermost parts of the TB and HB, and contemporaneous arc magmatism at 1.90–1.88Ga in the Skellefte district (SD), Central Finland Granitoid Complex (CFGC), TB and HB: a) all the 1.90–1.88Ga volcanic rocks have formed in one linear arc system from the SD to the HB; b) there have been two separate arcs SD-CFGC-TB and HB of similar age with double-plunging subduction zones. In both models, orogen parallel shortening and buckling is geometrically viable mechanisms for the origin of oroclinal bending, but slab rollback models should also be evaluated in further studies.

Evolution of a Neoproterozoic suture in the Iberian Massif, Central Portugal: New U-Pb ages of igneous and metamorphic events at the contact between the Ossa Morena Zone and Central Iberian Zone

A Neoproterozoic suture is exposed at the contact between the Ossa Morena Zone and the Central Iberian Zone, in the Iberian Massif (Central Portugal), the westernmost segment of the European Variscides. Although, the Cadomian magmatic and tectonometamorphic events have been previously documented, their timing is still poorly constrained, particularly in the inner zones of the suture. We used geochronological (ID-TIMS U-Pb) data to establish the sequence of events, isotopic (Rb-Sr, Sm-Nd) data to characterize the magmatic sources and thermodynamic modelling to determine the maximum P-T conditions attained during the Cadomian metamorphism. The first event, in the future Ossa Morena Zone, is the onset of island arc magmatism represented mainly by tholeiites with a MORB signature. Their igneous crystallization age is unknown, but they are older than ca. 539Ma. This magmatic activity was accompanied by deposition of fine-grained sediments in a Neoproterozoic basin. The second event is the evolution of the Cadomian magmatic arc in different stages. The earliest magmatic stage occurs at ca. 692Ma, which is the oldest igneous age known in the Ossa Morena Zone. It is followed by the generation of subalkaline and peraluminous protoliths at ca. 569Ma, with the isotopic signature of old crustal sources. The final phase of the arc magmatism (ca. 548–544Ma) involved mainly partial melting of continental crust. The range of the main magmatic activity must have been between ca. 569Ma and ca. 544Ma as mentioned for other areas in the Ossa Morena Zone. A major metamorphic event, recorded in metamorphic monazite, zircon and titanite at ca. 540Ma, attained upper amphibolite facies conditions close to the transition to granulite facies (7–8kbar and 640–660°C). It represents the continental arc accretion of the Ossa Morena Zone with the Iberian Autochthon passive margin (future Central Iberian Zone). The Early Ordovician rocks (ca. 483–477Ma) were generated from depleted and juvenile sources. These rocks are strongly deformed and with melting features, display metamorphism at amphibolite facies conditions. They are interpreted as related with the Rheic Ocean.

Magmatic and metamorphic history of Paleoarchean tonalite–trondhjemite–granodiorite (TTG) suite from the Singhbhum craton, eastern India

Texturally controlled dating of zircon from Paleoarchean tonalite–trondhjemite–granodiorites of the Older Metamorphic Tonalitic Gneisses and the Singhbhum Granite batholith (Phases I, II, and III) from the Singhbhum craton in eastern India reveals a polycyclic evolution of the Archean crust. The granitoid suites were emplaced in two pulses at 3.45–3.44Ga and 3.35–3.32Ga. Tonalites and trondhjemites of the Older Metamorphic Tonalitic Gneisses were emplaced at ca. 3.45–3.44Ga together with Phase III of the Singhbhum Granite pluton while granites belonging to the Older Metamorphic Tonalitic Gneisses were emplaced at ca. 3.35–3.32 together with Phase I and Phase II of the Singhbhum Granite pluton. Both crustal units underwent an early phase of relatively high-grade metamorphism at 3.30–3.28Ga followed by extensive fluid-induced alteration during low-grade metamorphism at 3.19–3.12Ga, and 3.02–2.96Ga. The two units have also been marginally affected at ca. 2.52Ga and 1.06Ga by major metamorphic events in the North Singhbhum Mobile Belt and the Singhbhum shear zone at the northern margin of the craton. The zircon grains in granites have inherited cores with ages of ca. 3.61Ga and 3.46–3.41Ga and with well-developed oscillatory growth zonation which suggests the granitic magmas were derived by partial melting of an igneous precursor or sedimentary rocks derived from an igneous source. The emplacement of the expansive granitoids belonging to the Older Metamorphic Tonalitic Gneisses and the Singhbhum Granite was synchronous with the amphibolite-facies metamorphism (ca. 3.32Ga) of older meta-igneous and metasedimentary rocks belonging to the Older Metamorphic Group. Major felsic crust formation in the craton occurred in a narrow time interval between 3.46 and 3.32Ma with minor contributions of material as old as 3.6Ga. The complex polycyclic evolution of the Paleoarchean crust in the Singhbhum craton can account for the wide range of often disparate ages obtained using whole rock isochron dating techniques with some of the isochron dates being geologically meaningful while others representing mixing lines or disturbance of the isotopic systems during metamorphism.

Ages of protolith and Neoproterozoic metamorphism of Al-P-bearing quartzites of the Veredas formation (Northern Espinhaço, Brazil): LA-ICP-MS age determinations on relict and recrystallized zircon and geodynamic consequences

The N-S running Espinhaço fold belt, situated in the São Francisco craton in central-eastern Brazil, separates this craton in a western and an eastern part along the Paramirim Corridor, a Brasiliano-Panafrican (∼650–500Ma) deformation zone. The (meta)sediments of the Espinhaço basin consist of several synthems with dominantly siliciclastic deposits. Among them, Al- and P-rich, lazulite-bearing blue quartzite, the amphibolite-facies equivalent of a phosphatic sandstone, of the Veredas Formation in the Northern Espinhaço form a conspicuous horizon. Zircon crystals from these quartzites have been dated using laser ablation inductively coupled plasma mass spectrometry. The zircon crystals are both detrital and metamorphic in origin, the latter exhibiting highly unusual textures produced by dissolution–reprecipitation reaction and concomitant replacement of metamict areas: (REE, Y, Th, U) zircon+fluid=zircon+xenotime. Detrital zircons reveal relict age groups of 3.1–3.3Ga, 2.6–2.7Ga, 2.0–2.3Ga, and 1.5Ga. The end of sedimentation and erosion of the magmatic and metamorphic rocks of the Sao Francisco craton can be estimated therefore as ≤1.5Ga. The major metamorphic event recorded by reprecipitated zircon is Neoproterozoic and documented by a lower-intercept age of 634±19Ma, which was calculated from analyses of individual spots on19 grains with a common relict Orosirian-Rhyacian age of 2032±31Ma, temporal equivalent to Transamazonian. The Neoproterozoic age of the metamorphosed zircons of the Vereda formation corresponds to the Brasiliano-Panafrican remobilization of the São Francisco-West Congo craton along the eastern side of the Espinhaço fold belt in the Paramirim Corridor.

Zircon U–Pb ages and O–Nd isotopic composition of basement rocks in the North Qinling Terrain, central China: evidence for provenance and evolution

The Qinling Group was previously interpreted as the oldest Precambrian basement unit of the North Qinling Terrain, recording its formation and early crustal evolution. The Qinling Group consists predominantly of gneisses, amphibolites, and marbles, which underwent multi-phase deformation and metamorphism. In order to better constrain the provenance and tectonic setting of this group and the evolution of the North Qinling orogenic belt, in situ U–Pb dating and oxygen isotopic analysis of zircons in combination with whole-rock geochemistry and Sr–Nd isotope analysis was performed on the two dominant rock types, amphibolite and felsic gneiss. Felsic gneisses exhibit enrichment of LREEs and LILEs (Rb, Ba, Th, K, Pb), negative Eu anomalies and depletion of HFSEs (Nb, Ta, P, Ti). The rocks have slightly elevated δ18O values (6.5–9.3 ‰) and initial e Nd values of −4.6 corresponding to two-stage Nd model age of 1.99 Ga. Amphibolites are also enriched in LILEs and LREEs and depleted in Nb and Ta and have homogeneous δ18O values (5.0–6.0 ‰), but higher initial e Nd values (2.8–3.3) and younger two-stage Nd model ages (1.29–1.24 Ga) compared to the gneisses. The zircon age record indicates that the gneisses and amphibolites were formed in a ~960 Ma volcanic arc environment rather than in a rift setting as previously suggested. A major metamorphic event took place during the Early Paleozoic. Based on the age spectrum of detrital zircons, the Qinling Group is interpreted as an autonomous geological unit, which was mainly derived mostly from 1,000 to 900 Ma old granitoid rocks. The North Qinling Terrain can be regarded as a remnant of the Grenville orogenic belt with an early Neoproterozoic evolution different from that of the North and South China blocks.

Electron-microprobe Th–U–Pb monazite dating in Early-Palaeozoic high-grade gneisses as a completion of U–Pb isotopic ages (Wilson Terrane, Antarctica)

The electron microprobe (EMP) Th–U–Pb monazite bulk chemical dating method was applied to granulite-facies rocks of the Wilson Terrane in Antarctica. A combination of this method to isotopic U–Pb-SHRIMP ages for the evaluation of metamorphic processes required the analysis of reference monazites. These can be subdivided into three groups: a) Monazite with variable total Pb at constant Th (e.g. VK-1) is unsuitable for EMP data evaluation; b) Monazite with highly variable total Pb and Th, but with at least some Th/Pb approximating an apparent isochrone (e.g. MPN) is partly useful; and c) Monazite with constant Th/Pb at high Th (e.g. Madmon monazite) is best suitable for the combined approach and can be additionally used to improve the Th calibration for EMP. Study of monazite in grain mounts and in thin sections led to partly different but complementary results: Older monazites with EMP ages up to 680Ma occur mainly in a grain mount from diatexite and metatexite and are interpreted as detrital relics. Some of these monazites show structures and mineral–chemical zonation trends resembling metasomatism by alkali-bearing fluids. A marked mobility of Th, P, Ce, Si and U is observed. The age of the metasomatic event can be bracketed between 510 and 450Ma. Furthermore, in the grain mount and in numerous petrographic thin sections of migmatites and gneisses, the EMP Th–U–Pb and SHRIMP U–Pb monazite data uniformly signal a major metamorphic event with a medium-pressure granulite facies peak between 512 and 496Ma. Subsequent isothermal uplift and then amphibolite-facies conditions between 488 and 466Ma led to crystallisation of pristine monazite. The high-grade metamorphic event, related to the Ross Orogeny, can be uniformly traced more than 600km along strike in the Wilson Terrane.

Fission track thermochronology of the Beni Bousera peridotite massif (Internal Rif, Morocco) and the exhumation of ultramafic rocks in the Gibraltar Arc

The Beni Bousera peridotite massif and its metamorphic surrounding rocks have been analyzed by the fission track (FT) method. The aim was to determine the cooling and uplift history of these mantle and associated crustal rocks after the last major metamorphic event that dates back to the Lower Miocene–Upper Oligocene time (∼22–24 Ma). The zircon FT analyses give an average cooling—i.e., below 320 °C—age of ∼19.5 Ma. In addition, the apatite FT data give an average cooling—i.e., below 110 °C—age of ∼15.5 Ma. Taking into account the thermal properties of the different thermochronological systems used in this work, we have estimated a rate of cooling close to 50 °C/Ma. This cooling rate constrains a denudation rate of about ∼2 mm year−1 from 20 to 15 Ma. These results are similar to those determined in the Ronda peridotite massif of the Betic Cordilleras documenting that some ultrabasic massifs of the internal zones of the two segments of the Gibraltar Arc have a similar evolution. However, Burdigalian sediments occur along the Betic segment (Alozaina area, western Betic segment) unconformably overlying peridotite. At this site, ultramafic rock was exposed to weathering at ages ranging from 20.43 to 15.97 Ma. Since the Beni Bousera peridotite was still at depth until 15.5 Ma, we infer that no simple age projection from massif to massif is possible along the Gibraltar Arc. Moreover, the confined fission track lengths data reveal that a light warming (∼100 °C) has reheated the massif during the Late Miocene before the Pliocene–Quaternary tectonic uplift.

Precambrian geology of China

Precambrian geology of China

Síntesis geocronológica del magmatismo, metamorfismo y metalogenia de Costa Rica, América Central

Una recopilación completa de 651 determinaciones de edades radiométricas desde 1968 (415 dataciones 40Ar/39Ar, 211 K/Ar, 5 U/Th, 4 Rb/Sr, 2 U/Pb y de fisión termocronología con zircón), tanto aquellas publicadas desde 1968 como muchas nuevas, proporcionan un marco completo de la estratigrafía ígnea de Costa Rica y las inferencias acerca de la edad del metamorfismo y los eventos metalogenéticos. Las rocas ígneas del Jurásico Superior temprano al Eoceno Medio (~ 160 a ~ 41 Ma), corresponden principalmente con acreciones ofiolíticas. Durante el Campaniano (~ 71 Ma) comienza a establecerse la actual zona de subducción, conformada por rocas volcano-sedimentarias, con una composición desde básica a félsica. Sin embargo, las rocas volcánicas primarias, subalcalinas, in situ y abundantes, aparecen hasta después de los ~ 29 Ma. Los intrusivos e hipoabisales de granitoides hasta gabroides (plutones, stocks, diques y sills), están presentes de SE a NW en: la cordillera de Talamanca (~ 12,4 a 7,8 Ma), cerros de Escazú (~ 6,0 a 5,9 Ma), fila Costeña (~ 18,3 a 16,8 y ~ 14,8 a 11,1 Ma), Tapantí-montes del Aguacate-Carpintera (~ 4,2 a 2,2 Ma) y Guacimal (~ 6,4 a 5,2 Ma). Las rocas asociadas al arco con edades entre los 29 y 11 Ma (denominado Frente Proto- Volcánico), están presentes en la llanura de San Carlos y en el sur de Costa Rica. La ubicación y la edad de las rocas ígneas indican que el arco dio un giro de 20° al NW entre 15 y 8 Ma, con un polo de rotación que se centró en el sur de Costa Rica. Esta rotación se atribuye a la deformación en la placa superior (acortamiento en el sur y extensión en el noroeste), acompañado por el retroceso de la trinchera hacia el sur. A los ~ 3,45 Ma, el arco volcánico en el sur cesó su actividad, mientras que el vulcanismo adakítico localizado persistió en la cordillera de Talamanca (4,2 - 0,95 Ma) debido a la subducción de las Placa Cocos. El Frente Paleo-Volcánico está representada por rocas del arco (8 - 3,5 Ma) que se extienden a lo largo de Costa Rica, de forma paralela al arco volcánico moderno y fue seguido por el evento efusivo Monteverde (2,1 - 1,1 Ma), que progresivamente se retrocedió hacia el NE. El Frente Neovolcánico se estableció entre el 2,1 y el presente, con el crecimiento de los actuales volcanes, mayoritariamente hace unos 0,6 Ma y, en general, están representados por tres episodios, no isocrónicos entre volcanes: volcanes ancestrales entre 1,61 y 0,85 Ma (Proto-Cordillera), en parte coetánea con Monteverde; un evento importante cerca de ~ 0,74 - ~ 0,2 Ma (Paleo-Cordillera), y una actividad volcánica relativamente pequeña, pero activa aún en 0,25 - 0 Ma (Neo-Cordillera). El vulcanismo submarino en aguas del Pacífico costarricense, está representado por la serranía Fisher (30,0 Ma y 19,2 Ma), la cordillera del Coco (14,5 - 0,6 Ma) y el vulcanismo subaéreo de la isla del Coco (2,2 - 1,5 Ma). Los principales eventos magmáticos, metamórficos y metalogéneticos están relacionados con los principales eventos geotectónicos, incluyendo discordancias regionales en Cretácico Superior (~ 82 Ma), Eoceno Medio (~ 45 Ma) y el Mioceno Superior (~ 8 Ma).

The crust beneath the Polish Sudetes: evidence from a gneiss xenolith in Tertiary basanite from Paszowice

Xenoliths found in basaltic lavas provide useful information about the materials of the Earth crust and mantle sampled by the lava on its way to the surface, and they help to extend our interpretations based on geophysical data. A garnet-biotite-hornblende-gneiss xenolith (min. ca. 40 cm across) was drilled at a depth of ca. 18 m in a Tertiary basanite flow at Paszowice village near Jawor, in the easternmost part of the Kaczawa Mountains, central Sudetes (SW Poland). Based on petrography, geochemistry and thermobarometry, the xenolith is interpreted to have been formed at deep crustal levels from a metasedimentary rock. The maximum PT conditions recorded in the gneiss (T 700–740oC, P ca. 10 kbar) indicate that the main metamorphic event took place at a depth of 25–30 km, thus corresponding to the present-day lower crustal level defined on geophysical constraints. The gneiss xenolith contains four zircon populations, as revealed by U-Pb SHRIMP analyses: (1) 1.7 – 2.1 Ga, (2) 547 – 623 Ma, (3) 473 – 494 Ma, and (4) 354 – 437 Ma. The zircons of the youngest group (4) are interpreted to have originated during a metamorphic episode, at ca. 374±3 Ma, corresponding well with the HT-MP major metamorphic event in the neighboring Góry Sowie gneisses and migmatites. A range of features (e.g. the good preservation of the xenolith, lack of significant chemical and thermal interaction with the lava) suggest that the gneiss was sampled by the rising lava on the final path of the magma to the surface, probably from a depth of less than ca. 10 km. That means that the upper crust below this region (underlying the low-grade metamorphic Kaczawa Complex) contains gneisses originally derived from deep-crustal levels, and similar to those locally exposed in the neighborhood (e.g. in the Góry Sowie Massif to the south-east). This interpretation extends our knowledge about the structure and composition of the crust in the eastern part of the European Variscides, which otherwise is inferred only from geophysical data.

Multistage growth and reworking of the Palaeoproterozoic crust in the Bergslagen area, southern Sweden: evidence from U–Pb geochronology

The Svecofennian Domain of the Fennoscandian Shield constitutes a considerable volume of Palaeoproterozoic crustal growth, 2.1–1.86 Ga ago, in between the Archaean craton in the NE and the 1.85–1.65 Ga Transscandinavian Igneous Belt (TIB) in the south and west. The Bergslagen area is a classical ore province located in the southwestern part of the Svecofennian Domain of south-central Sweden. Its northern part is dominated by volcanic and plutonic rocks of a magmatic arc with continental affinity, while the SE part is made up by a sedimentary basin. The Bergslagen area shows a metamorphic zonation from lower to middle amphibolite facies in the north to upper amphibolite facies and locally granulite facies in the south; a small greenschist area exists in the west. Identifying the age spectra of inherited components, magmatic crystallization, as well as metamorphic episodes, provide important constraints on the geodynamic evolution of this centrally located piece of the Shield.U–Pb zircon SIMS data presented in this paper complement the previous, regionally scattered TIMS data from this area. Magmatic zircons from two felsic metavolcanic rocks and two amphibolites (metagabbros) yield 1888±12, 1892±7 and 1887±5, 1895±5 Ma, respectively; i.e. within the 1.91–1.86 Ga range previously obtained for Early Svecofennian magmatism in Bergslagen. An augen gneiss from southern Bergslagen, assigned to the earliest TIB generation, yield an intrusive age of 1855±6 Ma. Metamorphic monazites from the same rock indicate that deformation and elevated thermal activity prevailed 1.83–1.82 Ga ago (TIMS). Metamorphic zircons in high-grade metasedimentary rocks from the south and west yield ages of 1793±5 and 1804±10 Ma, in accordance with ages for regional peak metamorphism and migmatite formation found elsewhere in the southern Svecofennian province of Sweden. More importantly, a few zircon crystals and overgrowths in rocks from the north indicate an early metamorphic episode at c. 1.87 Ga, indicating that Bergslagen has experienced two major metamorphic events. Detrital and inherited zircons span the range 2.78–1.90 Ga, with an apparent gap at 2.45–2.1 Ga, which further emphasize previous observations of a major juvenile (&lt;2.1 Ga) and a minor Archaean provenance. This, and in particular the 1.94–1.91 Ga crystals present in the c. 1.89 Ga amphibolites, support the suggestion of a former Palaeoproterozoic pre-1.91 Ga crust in the Bergslagen area.

Th–U–Pb monazite geochronology of the Lüliang and Wutai Complexes: Constraints on the tectonothermal evolution of the Trans-North China Orogen

Recent lithological, metamorphic and geochronological studies show that the basement of the North China Craton (NCC) formed by collision of two discrete Archean to Paleoproterozoic blocks (Eastern and Western Blocks) along a Paleoproterozoic orogen, named the Trans-North China Orogen. However, the timing of the collision is controversial and post-collisional exhumation has not been precisely dated. This study applies the electron microprobe Th–U–Pb monazite dating technique to determine the ages of the peak metamorphic event and subsequent exhumation of the low- to medium-grade Lüliang and Wutai Complexes in the Trans-North China Orogen. Electron microprobe Th–U–Pb monazite data for the Lüliang Complex reveal five ThO 2 */PbO age ranges: (1) 1940–1938 Ma, (2) 1880–1847 Ma, (3) 1795–1755 Ma, (4) 1720–1703 Ma, and (5) ∼1648 Ma. Of these age ranges, the first four have also been recorded in the Wutai Complex, which yields ThO 2 */PbO ages of (1) ∼1930 Ma, (2) 1838–1822 Ma, (3) ∼1793 Ma, and (4) ∼1719 Ma. The oldest age range of 1940–1930 Ma is interpreted to record the widespread emplacement of mafic dikes in rocks of the Lüliang and Wutai Complexes. The age range of 1882–1822 Ma is in accord with the SHRIMP U–Pb ages of metamorphic zircons and mineral Sm–Nd and 40Ar/ 39Ar ages obtained for other complexes in the orogen, and is interpreted as the age of the major metamorphic event caused by amalgamation between the Eastern and the Western Blocks. The age range of 1795–1755 Ma is consistent with emplacement of large scale unmetamorphosed mafic swarms at 1800–1765 Ma, interpreted as the time of post-orogenic extension. The 1720–1703 Ma and ∼1648 Ma ages are considered to date later multiple stages of hydrothermal alteration, since monazites with such young ages only occur along fractures in older monazite grains and at the rims. These new monazite ThO 2 */PbO ages support the tectonic model for the evolution of the North China Craton that envisages discrete Eastern and Western Blocks colliding along the Trans-North China Orogen at ∼1.85 Ga and then undergoing post-collisional extension in the period 1795–1755 Ma.

Deformation history inferred from magnetic fabric in the southwestern Okcheon metamorphic belt, Korea

Magnetic fabric and rock-magnetic studies have been carried out for the non-fossiliferous, low- to medium-grade metasedimentary rocks in the southwestern part of the Okcheon metamorphic belt (OMB). Two major metamorphic events in the study area were previously recognized: regional metamorphism ( M 1) in the late Carboniferous to early Permian and contact metamorphism ( M 2) due to the intrusion of granite in the middle Jurassic. The metamorphic grade of the study area increases from the biotite zone in southeast through the garnet zone to the staurolite zone towards the northwest. Magnetic fabrics of the study area are generally well defined and can be characterized according to the metamorphic zones. Magnetic foliation is the dominant magnetic fabric in the biotite zone, while magnetic lineation prevails in both garnet and staurolite zones. We interpret the metamorphism-related deformation history of the study area based on magnetic fabrics, magnetic mineralogy and previously reported metamorphic evolution as follows. Penetrative NW-dipping cleavage, represented by magnetic foliation, was formed in the study area by prevailing NW–SE shortening event during the M 1 regional metamorphism in the late Carboniferous–early Permian. This shortening event is interpreted to be associated with the collisional event between the North and South China blocks. Cleavages dipping steeply to the southeast in the staurolite zone are attributed to the pressure exerted from the intrusion of Jurassic granite in the northwestern area.

Ultrahigh-temperature metamorphism followed by two-stage decompression of garnet–orthopyroxene–sillimanite granulites from Ganguvarpatti, Madurai block, southern India

The Mg–Al granulites from Ganguvarpatti consist of orthopyroxene–sillimanite–garnet ± quartz as peak assemblage, with a few porphyroblasts of cordierite and sapphirine. These assemblages were strongly overprinted by late symplectites and coronas. Orthopyroxene inclusions in garnet and porphyroblast cores have the highest X Mg (0.80) and Al2O3 content (10.7 wt%). The estimated near-peak metamorphic conditions (1,000±50°C and 11 kbar) using garnet–orthopyroxene geothermobarometry are consistent with those determined using a petrogenetic grid. The proposed multi-stage evolution process implies an initial decompression, deduced from multi-phase symplectites, followed by cooling during biotite formation. Further late decompression is explained from the orthopyroxene rims on biotite. This proposed P–T path thus suggests a unique and complex evolution history for the UHT granulites of southern India. Present results are comparable with similar adjacent terranes in the Gondwana supercontinent, but the lack of structural and geochronological data makes a link with any major metamorphic event uncertain.

Cretaceous40Ar/39Ar detrital mica ages in Tertiary sediments shed a new light on the Eo-Alpine evolution

Clastic sediments deposited in the syn-orogenic Tertiary Piedmont Basin in northwest Italy represent the depositional counterpart of the cooling/exhumation and erosion of Western Alpine rocks over the last 30- 35 Myr. 40 Ar/ 39 Ar analyses of detrital white micas from Oligocene-Miocene sediments and present-day river sands show, in addition to younger Tertiary age groups, a wide range of Cretaceous ages. Pronounced well defined Late Cretaceous (~70-90 Ma) and Early Cretaceous (~105-120 Ma) age clusters are recorded in Lower to Upper Miocene sediments, forming discrete age groups with a contribution to the total detrital population of as high as 58%. This age pattern of discrete age peaks is remarkably constant and can be followed up-sequence through different formations spanning a time interval for sedimentation of >20 Myr. Our new detrital mica ages may be the result of either excess Ar in the source rock, as commonly assumed for HP mica Ar ages from the internal western Alps, or of inherited Ar, and consequently be representative of real geological events. The first scenario would imply that incorporation of excess Ar in minerals can lead to non-random detrital age populations which could then mistakenly be interpreted as representative of real geological events. The second scenario would imply that during the last Eocene thermal event, pre-existing micas were only partially overprinted and the presence of older ages are the result of real Cretaceous metamorphic events of the Western Alpine orogen. We argue that our new data derived from the sedimentary record, in particular from the time interval from Serravallian to Present, cannot easily be explained as simply being due to incorporation of excess argon. We therefore interpret these ages to be representative of cooling following major metamorphic events in the Alpine orogen. The new argon data from the sediments in combination with the data from the rocks exposed in the orogen today point to a complex Mesozoic history of the internal Alpine orogen.