High-pressure Granulite Facies Research Articles

Constraining the age of high-pressure metamorphism of paragneisses from the Eastern Himalayan Syntaxis using zircon petrochronology and phase equilibria

Abstract The metamorphic evolution of the Eastern Himalayan Syntaxis (EHS) is of primary importance in understanding the tectonic framework of the India–Asia collision. However, the age of formation and exhumation of the high-pressure granulites from the EHS remains controversial. In this paper, we report the age of metamorphism of high-pressure (HP) granulite-facies paragneisses from the Namche Barwa Complex (NBC). These paragneisses consist of variable proportions of garnet, biotite, plagioclase, K-feldspar, quartz, rutile and ilmenite. Using phase equilibrium modelling of garnet growth combined with partitioning of rare earth elements (REEs) between zircon and garnet, we link multistage zircon growth to garnet growth to date the prograde stage to the interval 32.6–25.2 Ma, the pressure peak to c. 25–24 Ma and the retrograde stage to the interval 23.5–13.1 Ma. Taking into account published ages from the literature, for the NBC we propose that the prograde to peak metamorphism persisted until c. 25–24 Ma. This was followed by exhumation and melt crystallization during the interval 24–8 Ma, and cooling through the amphibolite facies after c. 8 Ma.

The relationship between systematic metamorphic patterns and collisional processes along the Qinling–Sulu–Odesan collisional belt between the North and South China Cratons

Abstract Along the Qinling–Sulu–Odesan collisional belt, the peak metamorphic conditions reflect systematically changing metamorphic regimes from east to west: ultra-high temperature (UHT) granulite-facies metamorphism in the Odesan area; high pressure (HP) eclogite-facies metamorphism in the Hongseong area; ultra-high pressure (UHP) eclogite-facies metamorphism in the Sulu, Dabie and Hong'an areas; HP eclogite-facies metamorphism in the Tonbai area; epidote–amphibolite- and blueschist facies metamorphism in the Wudang area; and HP granulite-facies metamorphism in the western Qinling area. Additionally, the Triassic post-collisional igneous rocks regionally intruded at both ends of the collision belt, in the west Qinling and Hongseong–Odesan areas. These observations indicate that the following collisional processes occurred along the Qinling–Dabie–Odesan collisional belt. The North China Craton and South China Craton first collided at their eastern margins (the Odesan area) with an angle of approximately 60° between their long axes. Consequently, the amount of oceanic slab subducted before the collision increased from the Odesan area to the Sulu area, resulting in an increase in the depth of slab break-off. After slab break-off occurred in the Sulu area, the lateral tearing force strengthened, decreasing the depth of slab break-off from the Sulu area to the western Qinling area as collision propagated westward.

Shared metamorphic histories of various Palaeoproterozoic granulites from Datong–Huai’an area, North China Craton (NCC): constraints from zircon U–Pb ages and petrology

ABSTRACTDifferent tectonic interpretations have been proposed for the various spatially associated Palaeoproterozoic granulite-facies lithologies (metasedimentary rocks, metabasites, and felsic granulites) from north-central part of the North China Craton, which hinges primarily on controversies about metamorphic histories of these granulites, especially on the timing of peak metamorphism. Published data exhibit two controversial peak metamorphic ages of 1950–1900 Ma and 1850–1800 Ma. We report here LA-ICPMS U–Pb zircon ages of seven representative granulite-facies samples of different lithologies to constrain the timing of metamorphism, and then discuss their geological significance. Most zircon grains from these rocks display weak core-and-rim structures and yield two comparable group metamorphic ages of 1970–1900 Ma and 1880–1790 Ma, although their formation ages vary from Neoarchaean to Palaeoproterozoic. The older population metamorphic ages are interpreted to approximate timing of high-pressure granulite-facies metamorphism, and the younger population ages as the approximate timing of intermediate- to low-pressure granulite-facies metamorphism. Combined with recent petrological studies, we propose these granulites have shared metamorphic histories at least since ~1970–1900 Ma, and they are probably formed in one single metamorphic cycle in response to crustal-scale subduction–collision–exhumation processes involved in Palaeoproterozoic mobile belt.

Petrologic and zircon U–Pb geochronological characteristics of the pelitic granulites from the Badu Complex of the Cathaysia Block, South China

The recognition of the Indosinian Orogeny in the South China block has been controversial and difficult because of strong weathering and thick cover. High temperature (HT) and high pressure (HP) metamorphic rocks related to this orogeny were considered to be absent from this orogenic belt until the recent discovery of eclogite and granulite facies meta–igneous rocks, occurring as lenses within the meta–sedimentary rocks of the Badu Complex. However, metamorphic state of these meta–sedimentary rocks is still not clear. Besides, there have been no geochronological data of HT pelitic granulites previously reported from the Badu Complex. This paper presents petrographic characteristics and zircon geochronological results on the newly discovered kyanite garnet gneiss, pyroxene garnet gneiss and the HT pelitic granulites (sillimanite garnet gneiss). Mineral assemblages are garnet + sillimanite + ternary feldspar + plagioclase + quartz + biotite for the HT pelitic granulite, kyanite + ternary feldspar + garnet + sillimanite + plagioclase + quartz + biotite for the kyanite garnet gneiss, and garnet + biotite + pyroxene + plagioclase + ternary feldspar + quartz for the pyroxene garnet gneiss, respectively. Decompressional coronas around garnet grains can be observed in all these pelitic rocks. Typical granulite facies mineral assemblages and reaction textures suggest that these rocks experienced HP granulite facies metamorphism and overprinted decompression along a clockwise P–T loop. Results from integrated U–Pb dating and REE analysis indicate the growth of metamorphic zircons from depleted heavy REE sources (100–50 chondrite) compared with detrital zircons derived from granitic sources (typically > 1000 chondrite). Metamorphic zircons in HP granulite exhibit no or subdued negative Eu anomalies, which perhaps indicate zircon overgrowth under eclogite facies conditions. The zircon overgrowth ages range from 250 to 235 Ma, suggesting that HP granulite (eclogite) to granulite facies metamorphism of these supracrustal rocks occurred in the Early–Middle Triassic. Based on the presence of HP granulite facies pelitic rocks, it is inferred that significant underthrusting was involved during the Indosinian Orogeny which introduced these supracrustal rocks to lower crustal levels.

Petrological Investigations and Zircon U‐Pb Dating of High Pressure Felsic Granulites from the Yushugou Complex, South Tianshan, China

AbstractAs a window of insight into the lower crust, high pressure granulite has received much attention since last decade. Yushugou high pressure granulite‐peridotite Complex was located in the northeast margin of Southern Tianshan, NW China. Previous ideas agreed that the peridotite unit in Yushugou, combined with the ultramafic rocks in Tonghuashan and Liuhuangshan, represent an ophiolite belt. However, the metamorphic evolution and tectonic mechanism of the Yushugou high pressure (HP) granulite remain controversial. Petrological investigations and phase equilibrium modelling for two representative felsic granulite samples suggest two stages metamorphism of the rocks in Yushugou Complex. Granulite facies metamorphism (Stage I) with P‐T conditions of 9.8–10.4 kbar at 895–920°C was recorded by the porphyroblastic garnet core; HP granulite facies metamorphism (Stage II) shows P‐T conditions of 13.2–13.5 kbar at 845–860°C, based on the increasing grossular and decreasing pyrope contents of garnet rims. The Yushugou HP felsic granulites have recorded an anticlockwise P‐T path, characterized by the temperature decreasing and pressure increasing simultaneously. The LA‐ICP‐MS isotopic investigations on zircons from the felsic granulite show that the protolith ages of the granlulites are ∼430 Ma, with two age groups of ∼390 Ma and 340–350 Ma from the metamorphic rims of zircon, indicating the Stage I and II metamorphic events, respectively. A tectonic model was proposed to interpret the processes. The investigated felsic granulite was derived from deep rooted hanging wall, with Stage I granulite facies metamorphism of ∼390 Ma, which may be related to the Devonian arc magmatic intrusion; Stage II HP granulite facies metamorphism (340–350 Ma) may due to the involvement of being captured into the subducting slab and experienced the high pressure metamorphism.

The metamorphic evolution of Salma-type eclogite in Russia: Constraints from zircon/titanite dating and phase equilibria modeling

Combined geochronology with phase equilibria modeling, the P-T-t evolution of metamorphic rocks can be deciphered. Four different representative samples, including retrogressed eclogite, garnetite, paragneiss and tonalite–trondhjemite–granodiorite (TTG) gneiss have been selected for detailed petrography and U/Pb dating from the Belomorian Eclogite Province in Russia. Zircon cores yield Archean ages for the protoliths of these samples. Zircon rims of the TTG gneiss have recorded Neoarchean anataxis at ca 2.73 Ga, whereas 1.91–1.92 Ga zircon rims of the garnetite and the paragneiss have dated high-pressure granulite facies metamorphism. Titanite in the retrogressed eclogite has recorded amphibolite facies retrogression at ca 1.86 Ga. Phase equilibria modelings for retrogressed eclogite, garnetite, and paragneiss define P-T conditions of 17–18 kbar/below 790 °C for the peak eclogite facies metamorphism, 11–12 kbar/870–900 °C for the high-pressure granulite facies metamorphism, and 8–10 kbar/650–710 °C for titanite’s stability which corresponds to the amphibolite facies metamorphism. A clockwise P-T path is inferred for the Salma-type eclogite, which is characterized by thermal relaxation from the peak eclogite facies to the high-pressure granulite facies, and nearly isobaric cooling from the high-pressure granulite facies to the amphibolite facies.

Metamorphic P–T paths and zircon u–pb ages of Paleoproterozoic metabasic dykes in eastern Hebei and northern Liaoning: Implications for the tectonic evolution of the North China Craton

Metabasic dykes are described from Qianxi (JD1462) and Qinglong (JD15158) in the eastern Hebei and Qingyuan (15Q18) in the northern Liaoning that clearly crosscut the foliation of Neoarchean tonalitic–trondhjemitic–granodioritic (TTG) gneisses and supracrustal rocks in the Longgang Block. Petrographic observations and phase modeling using THERMOCALC suggest that these metabasic dykes have experienced metamorphism of high-pressure (HP) granulite facies and amphibolite facies characteristic of clockwise P–T paths. Their peak P–T conditions were constrained to be 11.5 ± 0.6 kbar/815 ± 62 °C for sample JD1462, 8–9 kbar/570–585 °C for JD15158 and 10.7 ± 0.6 kbar/780 ± 70 °C for 15Q18, corresponding to apparent geothermal gradients of 18–19 °C/km. Zircon U–Pb dating yields metamorphic ages of ∼1.82 Ga for samples JD15158 and JD1462, interpreted to represent the peak stage of amphibolite facies and the post-peak decompression stage of HP granulite assemblage. Zircons in sample 15Q18 yield four age groups of ∼1.84 Ga, 1.94 Ga, 2.12 Ga and 2.49 Ga, interpreted to represent the ages, respectively, of the post-peak decompression stage of HP granulite assemblage, a mixed result from zircons of ∼1.84 Ga and ∼2.12 Ga, the emplacement of basic dykes, and the captured zircon grain from country TTGs. Thus, the metamorphic evolution of metabasic dykes may correlate to a crustal thickening event occurred close to 1.84–1.82 Ga, which is separated from the collision event at ∼1.95 Ga prevailed in the North China Craton. The metabasic dykes show weakly right-declined REE patterns and are depleted in Nb, Ta and Ti, inferred to represent a within-plate extensional setting at ∼2.12 Ga, consistent with the magmatic events occurred in the Trans-North China Orogen and Jiao-Liao-Ji Belt. As a consequence, the Archean Longgang Block was largely influenced by the Paleoproterozoic magmatic and metamorphic events.

Dumortierite- and corundum-bearing quartz–feldspar–mica rocks of the Belomorian eclogite province: An example of melting of phengite + quartz

In the Salma eclogite of the Belomorian eclogite province, a dumortierite–phengite–corundum–bearing quartz–feldspar rock has been studied: its primary HP mineral paragenesis included garnet, phengite, and quartz. The phengite–quartz rocks were formed during dehydration and/or melting of boroncontaining rocks when they were dipped in the Meso- Neoarchaean subduction zone to a depth of not less than 70 km. As a result of the subsequent superimposed high-temperature metamorphic events under PT conditions of high-pressure granulite facies, the phengite in quartz underwent incongruent dehydration melting with formation of complex polymineral pseudomorphs, consisting of feldspars, biotite, newly formed muscovite, kyanite, corundum, and dumortierite. New estimates of the metamorphic temperature (850–900°C according to the melting reactions of phengite and the dumortierite field of stability; about 1000°C by the reintegrated composition of feldspar–mesoperthite) that affected the HP parageneses of Salma eclogitized rocks are at least 50–100°C (or even more) higher than them estimated earlier.

Paleoproterozoic high-pressure-high-temperature pelitic granulites from Datong in the North China Craton and their geological implications: Constraints from petrology and phase equilibrium modeling

Paleoproterozoic granulite-facies metapelites are widespread in northern part of the North China Craton (NCC), and their metamorphic conditions and P-T paths provide important constraints for tectonic evolution of the NCC. However, it is still controversial on the metamorphic evolution of these rocks due to variations of retrograde modification. In combination with detailed petrological studies, geological mapping, and geochronology, we here present new evidence from Gushan, Datong, conjunction zone of the Ji’ning Complex and Huai’an Complex to constrain P-T evolution of the metapelites. Field investigations reveal that the pelitic granulites are spatially associated with high-pressure (HP) mafic granulites, showing concordant characteristics of deformation and metamorphism. Fine mineralogical and microstructural studies reveal that kyanite-bearing HP granulite-facies indicators preserved in garnet porphyroblasts of the pelitic granulites. It suggests the previously assumed moderate- to low-pressure (MP-LP) and high- to ultrahigh-temperature (HT-UHT) pelitic granulites have experienced HP granulite-facies metamorphism.Four metamorphic generation assemblages (M1–M4) are recognized in the studied samples. The pre-Pmax assemblages (M1) are characterized by mineral inclusions within garnet core, including Bt-Qz-Rt±Ms±Liq, representing the late stage of prograde metamorphism. The Pmax assemblages (M2, HP granulite-facies) are represented by garnet mantle and its inclusion-type minerals, i.e., Grt-Ky-Kfs-Bt-Rt-Liq-Qz±Ms±Zn Spl. It is followed by heating until the maximum of temperature stage (M3, Tmax), featured by disappearance of biotite and transition from kyanite into sillimanite, with mineral assemblages of Grt-Sil-Kfs-Liq-Qz-Ilm±Zn Spl. The cooling stage (M4) is featured by breakdown of garnet to Bt+Qz±Pl±Sil±Ilm. Phase equilibrium modeling and titanium-in-biotite thermometer are combined to constrain the P-T evolution of the studied samples. The P-T conditions for the M1–M4 are respectively estimated around ∼11–11.5kbar/∼770–790°C, ∼10.3–13.8kbar/∼820–835°C, ∼6.5–9.5kbar/∼850–925°C, ∼6kbar/∼645–761°C, which defined a clockwise P-T path. Zircon U-Pb ages of two representative samples demonstrated comparable age clusters of ∼1950Ma and ∼1850Ma, which are interpreted to represent the timing of HP and MP-LP granulite-facies stages, respectively. Combining with other studies of the NCC, we propose that the pelitic granulites and associated HP mafic granulites have experienced the common metamorphic processes from peak stage (HP-HT granulite-facies) to retrograde metamorphism. Due to intensive and pervasive overprint of sillimanite-type granulite-facies metamorphism, the HP granulite-facies mineral assemblages of the pelitic granulites are partially or completely reworked. The metamorphic peak conditions and relatively high thermal gradients of these granulite-facies rocks suggest that their formation is probably in response to a crustal-scale subduction-collision process during the late Paleoproterozoic.

Partial melting during exhumation of Paleozoic retrograde eclogite in North Qaidam, western China

Ultrahigh pressure (UHP) rocks are often overprinted by high pressure (HP) granulite-facies metamorphism during exhumation. Here we investigate retrograde eclogite from the Lvliangshan area in North Qaidam which is associated with abundant felsic veins. The eclogite adjacent to the vein shows conversion to garnet amphibolite. Petrographic, mineralogical, geochemical, geochronological data suggest that the eclogite has been experienced peak eclogite facies metamorphism at c. 440Ma, followed by granulite facies retrogression at c. 420Ma, and amphibolite facies overprinting at <420Ma. The retrograde eclogite has microstructural records for partial melting with elongated plagioclase grains occurring as veinlets along the garnet-ilmenite grain boundaries and interspersed in clinopyroxene. Moreover, highly cuspate, wedge–shaped and triangular plagioclase grains are distributed along the garnet boundaries, and banded felsic veins (Pl+Qz) occur along the boundaries of amphibole in garnet amphibolite. Whole-rock geochemistry suggests that the felsic veins have 438.1–1106ppmSr and 2.87–12.18ppm Y with Sr/Y ratios from 81.98–166.2, exhibit pronounced positive Eu anomaly, and depletion of HREE, resembling those of adakitic rocks. Felsic veins contain zircon grains with an U-Pb age of ∼422Ma, similar to the age of HP granulite–facies metamorphism (420Ma) of retrograde eclogite. Combined with the identical Hf isotope compositions between the granulite–facies zircon grains from the host retrograde eclogite and those from the felsic vein, we infer closed system partial melting during HP granulite facies retrogression of the retrograde eclogite. The tonalitic (high Sr/Y ratio and enrichment of LREE) chemical feature, low Rb/Sr and high Ba/Rb ratios of the felsic veins suggest that zoisite and sodic amphibolite dehydration-melting together account for the formation of felsic veins during exhumation from the eclogite–facies to granulite–facies stages.

Spatial distribution, P–T–t paths, and tectonic significance of high-pressure mafic granulites from the Daqingshan–Wulashan Complex in the Khondalite Belt, North China Craton

High-pressure (HP) mafic granulites in the Daqingshan–Wulashan Complex from the central segment of the Khondalite Belt of the North China Craton occur as irregular lenses or deformed dike swarms within dioritic–granitic gneisses and meta-sedimentary rocks. The HP mafic granulites contain three distinct mineral assemblages: a peak HP granulite-facies assemblage (M1) that consists of garnet+clinopyroxene+plagioclase+amphibole±quartz±Fe–Ti oxides; a post-peak decompression assemblage (M2) that is characterized by symplectites of orthopyroxene+clinopyroxene+plagioclase±amphibole±Fe–Ti oxides; a late cooling assemblage (M3) represented by symplectites of amphibole+plagioclase+Fe–Ti oxides. Pseudosection modeling using THERMOCALC in the NCFMASHTO system and conventional thermobarometers constrain the P–T conditions of the M1, M2, and M3 assemblages to P=1.25–1.40GPa and T=760–815°C, P=0.72–0.80GPa and T=825–860°C, and P=0.69–0.81GPa and T=645–730°C, respectively. In-situ U–Pb dating of zircons show that the protolith ages of the HP mafic granulites are 2100–2000Ma and the timing of peak HP granulite-facies metamorphism is 1960–1940Ma. The medium- to low-pressure amphibolite- to granulite-facies retrogression occurred mainly from 1920 to 1820Ma. Petrographic investigations, mineral compositions, metamorphic reaction histories, P–T calculations, and geochronological data define a clockwise P–T–t path involving near-isothermal decompression and near-isobaric cooling, which suggests that the Daqingshan–Wulashan Complex underwent initial crustal thickening from 1960 to 1940Ma, followed by relatively rapid exhumation, cooling, and retrogression from 1920 to 1820Ma. This P–T–t path was likely the result of the Paleoproterozoic continental–continental (or arc) collision during the amalgamation of the North China Craton.

Discovery and geological significance of high-pressure mafic granulites in the Pingdu–Anqiu area of the Jiaobei Terrane, the Jiao–Liao–Ji Belt, the North China Craton

High-pressure (HP) mafic granulites in the Pingdu–Anqiu area of the Jiaobei Terrane, in the southwestern segment of the Jiao–Liao–Ji Belt of the North China Craton, occur as irregular lenses or deformed dike swarms within the Paleoproterozoic graphite-bearing paragneisses and marbles. Petrographic examination has revealed three distinct metamorphic mineral assemblages: a peak HP granulite-facies assemblage (M1) that consists of garnet+clinopyroxene+plagioclase+quartz±amphibole±Fe–Ti oxides, a post-peak decompression assemblage (M2) that is characterized by symplectites of orthopyroxene+clinopyroxene+plagioclase±amphibole±Fe–Ti oxides, and a late cooling assemblage (M3) represented by symplectites of amphibole+plagioclase+Fe–Ti oxides. Pseudosection modeling using THERMOCALC in the NCFMASHTO system and conventional thermobarometers constrained the P–T conditions of the M1, M2 and M3 assemblages to P=1.28–1.44GPa and T=757–805°C, P=0.50–0.80GPa and T=780–840°C, and P=0.55–0.73GPa and T=665–730°C, respectively. An integrated study involving laser Raman spectroscopy and scanning electron microscope analysis of mineral inclusions, cathodoluminescence imaging, and in-situ U–Pb dating of zircons showed that the protolith ages of the HP mafic granulites are mainly 2200–2000Ma and that the timing of the peak HP granulite-facies metamorphism ranges from 1950 to 1900Ma, as recorded by the cores of metamorphic zircons. The medium- to low-pressure amphibolite- to granulite-facies retrogression occurred mainly at 1900–1800Ma, as recorded by the rims of some zircon grains as well as zircon grains that contain inclusions of clinopyroxene+orthopyroxene+plagioclase+amphibole+sphene. Comprehensive petrographic, mineralogical, and geochronological investigations of the HP mafic granulites defined a clockwise P–T–t path involving near-isothermal decompression and near-isobaric cooling, which further suggests that the Jiaobei Terrane had underwent initial crustal thickening during 1950–1900Ma, followed by relatively rapid exhumation, cooling, and retrogression in the period 1900–1800Ma. This P–T–t path was probably generated by the Paleoproterozoic collisional orogenesis of the North China Craton.

Adakitic Rocks Resulting from Partial Melting of Metabasite at High-Pressure Granulite-Facies Condition during Continental Collision

Adakitic Rocks Resulting from Partial Melting of Metabasite at High-Pressure Granulite-Facies Condition during Continental Collision

Neoproterozoic eclogite- to high-pressure granulite-facies metamorphism in the Mozambique belt of east-central Tanzania: A petrological, geochemical and geochronological approach

This study investigated Neoproterozoic (Pan-African) eclogite- and high-pressure-granulite (E-HPG) facies rocks from the Mozambique belt of east-central Tanzania, collected close to the town of Ifakara and the adjacent Furua area from different tectonic settings, the Palaeoproterozoic Usagaran and the Neoproterozoic Mozambique belt. The studied rocks are E-HPG facies granite- and diorite-gneisses and a meta-gabbroic rock, which are retrogressed to amphibolite- and greenschist-facies conditions. Four different clockwise P-T paths were constructed. The first P-T path for a granodioritic gneiss displays peak metamorphic conditions at ~830°C and ~13.0kbar. The second P-T path for a quartz dioritic gneiss shows peak metamorphic conditions of ~920°C and ~14.9kbar. The third P-T path for a mafic granulite shows peak metamorphic conditions of ~820°C and ~13.2kbar. A fourth P-T path for a monzodioritic gneiss also displays peak metamorphic conditions of up to ~810°C and ~14.9kbar. Evidence for all four P-T paths is provided by mineral chemical and modal abundance calculations in combination with textural observations in thin sections. Zircon ages indicate that the east-central part of the Mozambique belt in Tanzania consists of granite-, granodiorite- and monzodiorite gneisses with Mesoarchaean (~2915Ma), Neoarchaean (~2637–2676Ma) and Palaeoproterozoic (~1873–1926Ma) protolith ages. Early Neoproterozoic (Tonian) igneous zircons were found in the mafic granulite with an age of ~989Ma. Late Neoproterozoic (Cyrogenian) igneous zircons were found in a dioritic and monzodiorite gneiss with ages of ~748Ma and ~718Ma, respectively. Metamorphic zircons extracted from Qtz-monzodiorite and granodiorite gneisses yielded ages of ~640Ma and are considered to approximate the peak of regional E-HPG metamorphism. We suggest that this high-grade metamorphic event was caused by the collision of fragments of East and West Gondwana during the Pan-African orogeny, associated with ocean closure and the formation of Andean-type continental arc domains, which ended in the formation of a collisional belt.

A relic slice of archean–early Paleoproterozoic basement of Jiaobei Terrane identified within the Sulu UHP belt: Evidence from protolith and metamorphic ages from meta-mafic rocks, TTG–granitic gneisses, and metasedimentary rocks in the Haiyangsuo region

A great quantity of metamorphic rocks (Haiyangsuo Complex), including high-pressure (HP) mafic granulites with ‘red-eye-circle’ textures, retrogressive products of Grt-, Qtz-, Ep-, and Cpx-bearing amphibolites, TTG–granitic gneisses, and metasedimentary rocks, is distributed extensively within the Haiyangsuo region, northeastern Sulu ultrahigh-pressure (UHP) belt. The Haiyangsuo Complex crops out over an area of at least 70km2, rather than the 15-km2 area mentioned in previous studies. Based on integrated studies of petrography, mineral phase transformations, mineral inclusions, cathodoluminescence (CL) images of zircons, and zircon U–Pb LA–ICP–MS and amphibole Ar–Ar age dates, together with previous P–T condition estimates, abundant Neoarchean to Early Paleoproterozoic igneous protoliths, including ca. 2.6–2.5Ga meta-mafic rocks and ca. 2.7Ga, ca. 2.55–2.5Ga, and ca. 2.4Ga TTG–granitic gneisses, were identified in the Haiyangsuo region, whereas the Late Paleoproterozoic (∼1.7Ga) and Neoproterozoic (ca. 800–650 Ma) protolith ages common in the Sulu HP–UHP rocks have not been observed. The metasedimentary rocks in the region studied have a deposition age of ca. 2.1–1.9Ga, showing protolith ages similar to those of the voluminous Khondalite series of the Jingshan and Fenzishan groups from the Jiaobei Terrane, southwestern Paleoproterozoic Jiao–Liao–Ji orogenic belt.Two discrete episodic HP granulite–facies metamorphic events were identified for the first time in the meta-mafic rocks of the region and their country rocks. The first episodic HP granulite-facies metamorphism (M1) is characterized by the relict coarse-grained mineral assemblage of garnet (Grt)+clinopyroxene (Cpx)+plagioclase (Pl)±quartz (Qtz), which formed under P–T conditions of 740–850°C and 1.0–1.26GPa. In contrast, the second HP granulite-facies metamorphism (M21) is characterized by ‘red-eye-circle’ textures, with a typical fine-grained mineral assemblage of Grt+Cpx+Pl±Qtz surrounding the relict coarse-grained HP granulite-facies minerals of M1. It formed under P–T conditions of 765–845°C and 1.14–1.75GPa. Both of the HP granulite-facies metamorphic events were overprinted and replaced by a late amphibolite-facies retrogression (M22), which is typified by a fine-grained mineral assemblage of Amp+Pl+Grt and formed under P–T conditions of 580–660°C and 0.6–0.8GPa. Abundant metamorphic zircon U–Pb dating revealed that the two discrete episodic HP granulite-facies metamorphic events in the Haiyangsuo Complex occurred and recorded in metamorphic zircon domains with HP granulite-facies mineral inclusions at Late Paleoproterozoic (ca. 1.87–1.86Ga) and Middle Triassic (ca. 245–225Ma), respectively, and that each was overprinted by a subsequent amphibolite-facies retrogression at∼1.84Ga and 220–214Ma, respectively. Various metamorphic rocks (Haiyangsuo Complex) in the Haiyangsuo region have rock association, petrographical, geochronological, and P–T evolution affinities to the Paleoproterozoic Jiaobei Terrane, SW Jiao–Liao–Ji Belt, but are distinguished from the Sulu UHP rocks. Hence, the Haiyangsuo Complex is exotic to the Sulu UHP belt and can be regarded as an inseparable part of the Jiaobei Terrane during the Paleoproterozoic. The unique Middle Triassic HP granulite-facies metamorphic event first identified within the Haiyangsuo Complex coincides with that (245–225Ma) of the adjacent Jiaodong Terrane, NE Sulu UHP belt, indicating that the Haiyangsuo Complex was also involved in Triassic collision–subduction and experienced HP granulite-facies metamorphism at a lower crustal level. The ultimate formation of the Haiyangsuo Complex along the southeastern margin of the Jiaobei Terrane, SW Jiao−Liao−Ji orogenic belt, was genetically related to the collision between the North China and South China cratons. Thus, the juxtaposition of the Haiyangsuo Complex to the Sulu UHP metamorphic belt took place in the Middle–Late Triassic rather than in the Jurassic.

Geochemical and Nd isotopic compositions of the Palaeoproterozoic metasedimentary rocks in the Kongling complex, nucleus of Yangtze craton, South China block: implications for provenance and tectonic evolution

AbstractPalaeoproterozic metasedimentary rocks, also referred to as khondalites, characterized by Al-rich minerals, are extensively exposed in the nucleus of the Yangtze craton, South China block. Samples of garnet–sillimanite gneiss in the khondalite suite were collected from the Kongling complex for Nd isotopic and elemental geochemical study. These rocks are characterized by variable SiO2 contents ranging from 35.71 to 58.07 wt%, and have low CaO (0.45–0.84 wt%) but high Al2O3 (18.56–29.04 wt%), Cr (174–334 ppm) and Ni (42.5–153 ppm) contents. They have high CIW (Chemical Index of Weathering) values (90.4–94.7), indicating intense chemical weathering of the source material. The samples display light rare earth elements (LREE) enrichment with negative Eu anomalies (Eu/Eu*=0.40–0.68), and have flat heavy rare earth elements (HREE) patterns. The high contents of transition elements (e.g. Cr, Ni, Sc, V) and moderately radiogenic Nd isotopic compositions suggest that the paragneisses might be those of first-cycle erosion products of predominantly mafic rocks mixing with small amounts of felsic moderately evolved Archaean crustal source. Geochemical and Nd isotopic compositions reveal that at least some of the protoliths of Kongling khondalite were sourced from local pre-existing mafic igneous rocks in a continental arc tectonic setting. Combined with documented zircon U–Pb geochronological data, we propose that the Palaeoproterozoic high-pressure granulite-facies metamorphism, rapid weathering, erosion and deposition of the khondalites in the interior of the Yangtze craton might be related to a Palaeoproterozoic collisional orogenic event during 2.1–1.9 Ga, consistent with the worldwide contemporary orogeny, implying that the Yangtze craton may have been an important component of the Palaeoprotorozoic Columbia supercontinent.

Meso-Neoproterozoic Grenville-Sveconorwegian intracontinental orogen: history, tectonics, geodynamics

The objective of this paper is to represent the main features inherent to Grenville-Sveconorwegian Orogen (GSNO) and to propose a model of tectonic and geodynamic evolution of this orogen based on the results of research concerning similar Precambrian tectonic units in the East European Craton. The studies of the conditions and settings related to origin and evolution of GSNO are of special interest, because it is located geographicaly and in a certain sense ideologically in the center of Rodinia, a supposed Neoproterozoic supercontinent. GSNO originated in the MezoNeoproterozoic in the inner region of the Lauroscandia continent. At present, the synformal tectonic structure of GSNO is divided into two portions: Grenville sector along the southeastern margin of the Canadian Shield, and Sveconorwegian sector in the southwestern Scandinavia. The integrity of Lauroscandia was twice disturbed in the MezoNeoproterozoic when oceanic structures resembling the Atlantic Ocean were formed. Later on, the continuity of the continent was restored with the involvement of oceanic lithosphere subduction and accretion and obduction of the island-arc and oceanic terranes. We distinguish two stages in the GSNO history: (1) ‘preparatory’ stage (from ~1.90 to ~1.16 Ga), and (2) formation of GSNO proper (from ~1.19 to ~0.90 Ga). The manifestations of granulite-facies metamorphism were repeatedly recorded before the Grenville Orogeny at 1.67–1.66, 1.47–1.45, 1.37–1.35, and 1.20–1.18 Ga. The Ottawan stage of the Grenville metamorphism proper is dated between 1.16 and 1.05–1.03 Ga. Metamorphism at the base of Allochthonous Belt corresponds to high-pressure granulite facies and, in a number of places, to hightemperature eclogite facies (800–900 °C at pressure in the range between 14 and 20 kbar). The age of metamorphism of rocks within Paraautochthonous Belt is 1.05–0.95 Ga; metamorphic grade increases from the greenschist facies near the Grenville front to the high-pressure amphibolite facies near Allochthon Boundary Thrust showing an inverted metamorphic zoning. High-pressure granulite-facies metamorphism is characteristic of Sveconorwegian sector; and high-temperature eclogites are observed locally at the base of the allochthonous complexes and within the paraautochthonous complexes. A distinctive feature of GNSO is the abundant occurrence of specific intrusive magmatism. Massifs of anorthosite-mangerite-charnockite-granite (AMCG) and anorthosite-rapakivi granite (ARG) complexes formed 1.8–1.5 Ga ago frame the orogen as a wide arc. In the internal region of GSNO, these complexes were formed successively at 1.16–1.13, 1.09–1.05, 0.99–0.96, and 0.93–0.92 Ga. Later on, after the intrusion, the massifs unevenly underwent granulite-facies metamorphism. The high-temperature magmatism and metamorphism, numerous repeated thermal pulses and enormous crustal body that underwent high-temperature transformation point to a mantle plume as the most adequate source of thermal energy. The model of intracontinental development of GSNO comes into conflict with popular ideas, which assume origination of this orogen as a result of the collision and welding of the formerly distant continents (Laurentia, Baltica and Amazonia), which, as suggested, completed the assembly of Rodinia supercontinent. A conclusion is drawn that the concept of tectonic position and geodynamic evolution of GSNO, which is not a counterpart of the Tibet-Himalayan Orogen, should be revised.

High-temperature metamorphism of the Yushugou ophiolitic slice: Late Devonian subduction of seamount and mid-oceanic ridge in the South Tianshan orogen

The South Tianshan Orogenic Belt (STOB), representing the southern segment of the Central Asian Orogenic Belt (CAOB), underwent a long-lived and subduction-related accretionary orogenic process. Revealing the petrogenesis of high-pressure (HP) metamorphic ophiolitic slices within this orogen is of crucial importance to understanding the geodynamic evolution of the STOB. In this study, we carry out a petrological, geochemical and geochronological study of HP mafic granulites from the Yushugou ophiolitic slice within the South Tianshan Accretionary Complex. Our results combined with previously published data suggest that the Yushugou mafic granulites, including garnet-clinopyroxene granulite, garnet two-pyroxene granulite and garnet-orthopyroxene granulite, are generally subalkaline to alkaline basalts, and show geochemical characteristics of MORB and OIB. The nominally anhydrous minerals of the mafic granulites contain certain but trace amounts of water in the manner of structural OH and sub-microscopic fluid inclusions. The granulites have a possible protolith age of ca. 400Ma and metamorphic age of 390–360Ma, and underwent HP and high-temperature (HT) granulite-facies metamorphism under conditions of 12–14kbar and 840–950°C and low H2O activity. Our study indicates that the Yushugou ophiolitic slice was probably derived from seamount that formed at mid-oceanic ridge closing to the oceanic trench and subduction zone during the Early Devonian, and then underwent metamorphism and deformation as a result of the subduction of the seamount and associated spreading ridge during the Middle to Late Devonian. Therefore, the Yushugou HP ophiolitic slice provides an important information of the Paleozoic tectonic evolutionary of the STOB.

Mid-Neoproterozoic (ca. 830-800 Ma) metamorphicP-Tpaths link Tarim to the circum-Rodinia subduction-accretion system

Long-lived exterior accretionary orogeny shapes tectonothermal evolution of the peripheral building blocks of supercontinents and leads to considerable crustal growth. However, such accretionary orogeny has only been locally recognized for the Rodinia supercontinent. Here a suite of newly discovered mid-Neoproterozoic high-grade metamorphic rocks in the northern Tarim Craton, NW China, are used to test the exterior accretion hypothesis for Rodinia. These rocks occur as dark-colored mafic and calc-silicate boudins in impure marbles and mica schists. Geochemical data suggest a protolith of arc-related basalts metasomatized by Ca-rich fluids. Mineral assemblages, phase diagram modeling, and mineral compositions for a garnet pyroxenite and a garnet clinopyroxene gneiss reveal upper amphibolite to high-pressure granulite facies peak metamorphism (660–700°C, 11–12 kbar) following a counterclockwise P-T path, which is characterized by prograde burial and heating, followed by near-isothermal burial and retrograde exhumation and cooling. This P-T path is interpreted to have recorded crustal thickening of an earlier magmatic arc transformed to a fore arc by subduction erosion and subsequent burial along bent isotherms near the subduction channel. All studied samples record ca. 830–800 Ma metamorphic zircon U-Pb ages, which probably date the early exhumation and cooling according to Ti-in-zircon temperatures, zircon rare earth element patterns, and Hf isotopes. This is the first mid-Neoproterozoic P-T-t path in Tarim, and it provides metamorphic evidence for a mid-Neoproterozoic advancing-type accretionary orogeny, which is coeval with the initial breakup events of Rodinia and thus links Tarim to the circum-Rodinia accretion system, supporting the peripheral subduction model.

Granulite-facies metamorphism of the Palaeoproterozoic – early Palaeozoic gneiss domains of NE Mozambique, East African Orogen

AbstractGranulite-facies metamorphism recorded in NE Mozambique is attributed to three main tectonothermal events, covering more than 1400 Ma from Palaeoproterozoic – early Palaeozoic time. (1) Usagaran–Ubendian high-grade metamorphism of Palaeoproterozoic age is documented in the Ponta Messuli Complex by Grt-Sil-Crd-bearing metapelites, estimated to pressure (P) 0.75 ± 0.08 GPa and temperature (T) 765 ± 96°C. The post-peak P-T path is characterized by decompression followed by near-isobaric cooling. (2) Irumidian medium- to high-pressure granulite-facies metamorphism is evident in the Unango and Marrupa complexes of late Mesoproterozoic – early Neoproterozoic age. High-pressure granulite-facies is documented by Grt-Cpx-Pl-Rt-bearing mafic granulites in the northwestern part of the Unango Complex, with peak conditions up to P = 1.5 GPa and T = 850°C. Medium-pressure granulite-facies conditions recording P of c. 1.15 GPa and T of 875°C are documented by Grt-Opx-Cpx-Pl assemblage in mafic granulites and charnockitic gneisses of the central part of the Unango Complex. (3) Tectonothermal activity during the Ediacaran–Cambrian Kuunga Orogeny is recorded in the Mesoproterozoic gneiss complexes as amphibolite facies to medium-pressure granulite-facies metamorphism. Granulite facies are documented by Grt-Opx-Cpx-Pl-bearing mafic granulites and charnockitic gneisses, reporting P = 0.99 ± 13 GPa at T = 738 ± 84°C in the Unango Complex and P = 0.92 ± 18 GPa at T = 841 ± 135°C in the Marrupa Complex. This metamorphism is attributed to crustal thickening related to overriding of the Cabo Delgado Nappe Complex, and shorthening along the Lurio Belt during the early Palaeozoic Kuunga Orogeny.