Amphibolite-facies Retrograde Metamorphism Research Articles

Neoproterozoic and middle Paleozoic geological events in the eastern Wolhyeonri complex of the southwestern Gyeonggi Massif, South Korea, and their tectonic correlations in northeastern Asia

The eastern Wolhyeonri complex in the southwestern Gyeonggi Massif, South Korea, which has a close affinity with the northern margin of the South China Craton, primarily consists of Neoproterozoic igneous and sedimentary rocks that were metamorphosed by the middle Paleozoic thermal event. In this study, we present the results from thermobarometry, whole-rock chemistry, and zircon geochronology of rocks in the eastern Wolhyeonri complex near Gwangcheon. The main rock type, such as the migmatitic biotite gneisses, was deposited after ca. 722–680 Ma, and the associated foliated amphibolites exhibit a rift-related Neoproterozoic origin. The ca. 845–820 Ma arc-related metadolerite and mafic layer in the metavolcanic-sedimentary unit are also exposed. Two Neoproterozoic magmatic events support the idea that the southwestern Gyeonggi Massif developed along the northern margin of the South China Craton. These Neoproterozoic rocks experienced upper amphibolite-facies (7–10.5 kbar and 710–735 °C) to low-pressure granulite-facies metamorphism (10.5–12.0 kbar and 810–830 °C) during 424–412 Ma, were associated with ca. 418 Ma leucogranite formation, and then underwent retrograde amphibolite-facies metamorphism. Combined with the results from previous studies, the early-middle Paleozoic orogeny in the southern Gyeonggi Massif is characterized by 470–456 Ma subduction associated with a high P/T metamorphism, and the initiation of intermediate P/T metamorphism from ca. 445 Ma followed by low-pressure granulite-facies metamorphism at 432–403 Ma, and high-pressure granulite-facies metamorphism at 392–381 Ma, indicating that the tectonic setting transitioned from a continental collision to a post-collisional setting between 432 Ma and 392 Ma. These events and the absence of Paleozoic ultrahigh-pressure metamorphism strongly correlated with the Wuyi-Yunkai orogen in the South China Craton, rather than the Qinling belt in the North China Craton. Therefore, the early-middle Paleozoic orogeny in the southern Gyeonggi Massif was caused by the subduction and subsequent collision within the South China Craton.

Cenozoic retrogression and exhumation of the amphibolites in the eastern Gangdese Belt, SW China

Abstract The collision between India and Asia is marked by the Yarlung Zangbo suture zone, which can be traced from Tibet into northwest Yunnan. The suture zone provides a record of the subduction of Neotethyan oceanic lithosphere beneath the Lhasa Block, creating the Gangdese magmatic belt, prior to the India–Asia collision. This paper reports petrological, zircon U–Pb, and whole-rock elemental and isotope data for amphibolite from the extension of the Gangdese belt in northwestern Yunnan. Two metamorphic stages are identified in relict amphibolite-facies rocks preserved in granitic gneisses from the belt. They are early amphibolite-facies metamorphism (M1) and late amphibolite-facies retrograde metamorphism (M2). The assemblage of M1 comprises garnet, amphibole, and plagioclase that formed at 0.89–0.97 GPa and 658–700 °C. Dating of syn-metamorphic zircons assigns this event to the middle Eocene (44–38 Ma). Symplectites (corona) of amphibole, plagioclase, and quartz surrounding garnet formed during retrograde stage M2, at 30–29 Ma, under P–T conditions of 0.41–0.68 GPa and 549–716 °C. The amphibolites display geochemical characteristics similar to those of mid-ocean ridge basalt, with eNd(t) values of +1.8 to +8.9, and are inferred to have formed in a back-arc-basin setting, suggesting that they constitute part of the lithospheric mantle of the Tengchong terrane that lies to the east. The amphibolite-facies metamorphism was related to underplating of the Indian plate during the India–Asia collision, which led to clockwise rotation of eastern Tibet during the middle Eocene, generating amphibolite recrystallization and, subsequently, with ongoing convergence of the two plates, exhumation of the region during the middle Oligocene.

Petrogenesis and tectonic setting of the Paleoproterozoic Kelle Bidjoka iron formations, Nyong group greenstone belts, southwestern Cameroon. Constraints from petrology, geochemistry, and LA-ICP-MS zircon U-Pb geochronology

ABSTRACT The Kelle Bidjoka Iron Formations (IFs) occur at the northmost margin of the Neoarchean to Paleoproterozoic Nyong group greenstone belts in SW Cameroon. This contribution presents for the first time the petrographical, geochemical, and geochronological data of the Kelle Bidjoka IFs and associated metaigneous (metatonalite) and metasedimentary (pyroxene gneiss) rocks. The studied IFs form a thin unit of 3.21 m thick and comprise magnetite pyroxene gneiss, garnet-bearing magnetite pyroxene gneiss, pyroxene BIFs, and grunerite BIFs. Petrographic studies have revealed mineral assemblages indicating retrograde amphibolite facies metamorphism. Magnetite pyroxene gneiss and pyroxene BIFs show signatures of both seawater and hydrothermal fluids, with magnetite pyroxene gneiss deposited closer to the vent source. In contrast, garnet-bearing magnetite pyroxene gneiss and grunerite BIFs are devoid of hydrothermal characteristics and show significant crustal contamination. The lack of Ce anomaly in most samples indicates anoxic to suboxic conditions during their deposition. The geochemical characteristics of the associated metaigneous and metasedimentary rocks indicate that Kelle Bidjoka IFs were deposited in a submarine volcanic arc environment, similar to the Algoma-type BIFs. Detrital zircon LA-ICP-MS U-Pb dating suggests that the Kelle Bidjoka BIFs were deposited during early Paleoproterozoic (ca. 2423 Ma) and were metamorphosed up to granulite facies during the Eburnean/Transamozian (ca. 2050 Ma) orogeny.

Zircons from a Pegmatite Cutting Eclogite (Gridino, Belomorian Mobile Belt): U-Pb-O and Trace Element Constraints on Eclogite Metamorphism and Fluid Activity

This report presents new data on U-Pb geochronology, oxygen isotopes, and trace element composition of zircon from a pegmatite vein crosscutting an eclogite boudin on Stolbikha Island, Gridino area, Belomorian mobile belt (BMB). The zircon grains occur as two distinct populations. The predominant population is pegmatitic and shows dark cathodoluminescence (CL); about a third of this population contains inherited cores. The second zircon population is typical of granulite and exhibits a well-defined sectorial (mosaic) zoning in CL. Both the inherited cores and sectorial in CL zircons appear to have been captured from metabasites as xenocrysts during the pegmatite vein formation. A U-Pb age of 1890 ± 2 Ma for the main zircon population is interpreted as the age of the pegmatite injection. This value is close to the age threshold for the BMB eclogites (~1.9 Ga) and unambiguously defines the upper age limit for the eclogite metamorphism. The pegmatite formation is thus related to partial melting events that occurred during the retrograde amphibolite-facies metamorphism shortly after the eclogitization. A U-Pb date of 2743 ± 10 Ma obtained for the sectorial in CL zircons is considered as the age of the granulite-facies metamorphism established previously within the BMB. The values of δ18O in the zircon populations overlap in a broad range, i.e., δ18O in the pegmatitic zircons varies from 6.1‰ to 8.3‰, inherited cores show a generally higher δ18O of 6.7–8.8‰, and in the captured granulitic zircons δ18O is 6.2–7.9‰. As a result of fluid attack during the final stage of the pegmatite vein formation, the composition of the pegmatitic zircons in terms of non-formula elements (REE, Y, Ca, Sr, Ti) has become anomalous, with the content of these elements having been increased by more than tenfold in the alteration zones. Our data provide new constraints on the timing of eclogite metamorphism within the BMB and show that the late-stage pegmatite-related fluids exerted a very pronounced influence on trace element abundances in zircon, yet had no significant impact on the isotopic composition of oxygen.

Water Content in Garnet from Eclogites: Implications for Water Cycle in Subduction Channels

Garnet from eclogites often shows very heterogenous and extremely high hydroxyl concentration. Eight eclogite samples were selected from the Sulu ultrahigh-pressure terrane and the Sumdo high-pressure metamorphic belt (Lhasa). The mean hydroxyl concentration in pyrope-rich and almandine-rich garnet varies from 54 to 427 ppm H2O and increases with the retrogression degree of eclogites. TEM observations reveal nanometer-sized anthophyllite exsolutions and clinochlore inclusions in water-rich domains in garnet, where anthophyllite is partly replaced by clinochlore. Because of overlapping of the infrared stretching absorption bands for structural OH in garnet and chlorite, it is impossible to exclude contribution of chlorite inclusions to the estimated hydroxyl concentration in garnet. The broad band near 3400 cm−1 is attributed to molecular water and nanometer-sized chlorite inclusions. Anthophyllite exsolutions may be formed by decomposition of hydrous garnet from ultrahigh-pressure eclogites during exhumation. Significant amounts of water can be stored in garnet from massif eclogites in the forms of hydroxyl in garnet and nanometer-sized inclusions of anthophyllite and clinochlore, as well as fluid inclusions. Amphibolite facies retrograde metamorphism can significantly increase both hydroxyl concentration and water heterogeneity in garnet from massif eclogites. These nano-inclusions in garnet provide a window to trace the water cycle in subduction channels.

Paleoproterozoic (ca. 1.87–1.69 Ga) arc-related tectonothermal events on northcentral Yeongnam Massif, South Korea and its tectonic implications: Insights from metamorphism, geochemistry and geochronology

The Paleoproterozoic basement of the Korean Peninsula consists of three massifs, the Nangrim, Gyeonggi and Yeongnam Massifs, from north to south. In the Muju area of the northcentral Yeongnam Massif, amphibolites intruded the Paleoproterozoic metasedimentary rocks and orthogneisses. The amphibolites can be divided into Type-I and Type-II amphibolites. The whole-rock geochemical data and zircon Lu–Hf composition of the amphibolites suggest that the Type-I and Type-II amphibolites formed in an island and continental arc tectonic setting, respectively, and both amphibolites were derived from enriched mantle sources. Zircon U–Pb dating reveals that the Type-I and Type-II amphibolites were emplaced at ca. 1.87 Ga and 1.69 Ga, respectively. The surrounding metasedimentary rock (migmatite) underwent upper amphibolite facies (720–730 °C and 7.3–7.4 kbar) metamorphism and then underwent amphibolite facies retrograde metamorphism (520–622 °C and 1.7–3.5 kbar). The zircon U–Pb dating reveals that the peak metamorphism occurred at ca. 1.87–1.86 Ga. The geothermal gradient (approximately 32 °C/km) during peak metamorphism correlates with that of the arc area, indicating that metamorphism occurred in an arc tectonic setting. These data suggest that the northcentral Yeongnam Massif underwent arc-related magmatic and metamorphic events during ca. 1.87–1.69 Ga. This finding indicates that the northcentral Yeongnam Massif does not correlate well with the Nangrim and northern Gyeonggi Massifs, which are correlated with the eastern North China Craton, because the Nangrim and Gyeonggi Massifs underwent postcollisional igneous activity during ca. 1.88–1.84 Ga.

Petrology of Garnet Amphibolites from the Hualong Group: Implications for Metamorphic Evolution of the Qilian Orogen, NW China

The Qilian Orogen marks the junction of the North China, South China and Tarim cratons. The mechanism of continental growth during the formation of the orogen remains unclear. Based on detailed fieldwork, we present a systematic study of petrography, mineral chemistry and phase equilibria of garnet amphibolites from the Hualong Group, which represents the Precambrian basement in the southern accretionary belt of the Qilian Orogen. The garnet amphibolites mainly consist of amphibole, plagioclase, garnet and quartz, with minor pyroxene, biotite and ilmenite. A peak stage of upper amphibolite facies to low-temperature granulite facies metamorphism and retrograde metamorphism in the amphibolite facies affected the samples. Garnet has a homogeneous composition of Alm66–71Grs14–17Prp9–12Sps3–5, amphibole is ferro-hornblende, biotite belongs to the ferro-biotite species and pyroxene is dominated by orthopyroxene with few clinopyroxene. Pseudosection modeling of the garnet amphibolite samples indicates clockwise P-T paths. The samples witness peak metamorphism at conditions of ~4.9–6.3 kbar and ~755–820 oC in the upper amphibolite facies to lowtemperature granulite facies, and retrograde cooling and decompression at conditions of ~2.5–3.1 kbar and ~525–545 oC. It is inferred that peak metamorphism with high temperature and low pressure occurred at ca. 450 Ma during northward subduction of the South Qilian oceanic crust beneath the central Qilian Block. When continental collision occurred between the central Qilian and the Qaidam blocks, the Hualong Block was accreted onto the South Qilian accretionary complex and experienced amphibolite facies retrograde metamorphism at ca. 440 Ma.

Metamorphic P-T-t Path of UHT Granulites from the North Tongbai Orogen, Central China

Although zircon U-Pb geochronometer has been widely used for dating metamorphism in moderate- to high-grade metamorphic rocks, it is still difficult to link the zircon U-Pb age to pressure and temperature (P-T) conditions. In this study, zircon trace elements and Hf isotopes and REE partitioning between zircon and garnet are adopted to track the formation condition of zircon in the granulites from North Tongbai Orogen, Central China. Combined with previous metamorphic P-T path results, a quantitative integrated anticlockwise P-T-t path was established for Tongbai granulites. These granulites recorded an early low-P heating followed by a dramatic pressure increase. Evidence for the prograde history (M1) is provided by cordierite, orthopyroxene and biotite inclusions in garnet. The prograde metamorphism occurred at around 443±3 Ma, with P-T conditions of ca. 730–820 °C and 920 °C and 8.5–10 kbar and the peak metamorphism age is ca. 432±4 Ma. At around 419 Ma, the granulites suffered an amphibolite-facies retrograde metamorphism (M3), represented by the replacement of garnet by biotite and plagioclase, and clinopyroxene by amphibole, with metamorphic condition of ca. 700 °C and ca. 7 kbar. The last retrograde metamorphism (M4) is a greenschist-facies overprint with an age of ca. 404 Ma. It is concluded that the metamorphism of Tongbai granulite lasted for more than 40 Ma, including a stage of more than 20 Ma granulite-facies metamorphism. The prolonged granulite-facies metamorphism resulted from the continuous northward subduction of the Shangdan oceanic crust beneath the North Qinling terrane.

Regional Middle Paleozoic metamorphism in the southwestern Gyeonggi Massif, South Korea: Its implications for tectonics in Northeast Asia

The Hongseong area in the southwestern Gyeonggi Massif in South Korea is considered to represent the eastward extension of the Qinling–Dabie–Sulu collision belt in China. We have carried out zircon U–Pb SHRIMP dating and P–T estimations of the gneisses and amphibolites in the eastern Wolhyeonri complex within the Hongseong area in order to constrain their metamorphic and tectonic evolutions. The protoliths of the migmatitic biotite gneisses formed during the Neoproterozoic and underwent granulite-facies metamorphism (750–880°C, 12–15kbar) at 442–413Ma. These rocks subsequently experienced amphibolite-facies retrograde metamorphism at 585–660°C and 7.5–10.3kbar. Mylonitic biotite gneiss, hornblende gneiss, and folded amphibolite in the study area yield metamorphic ages that range from 429 to 420Ma. The protoliths of some garnet amphibolites that formed at 470–456Ma are arc magmatic rocks; they experienced metamorphism at the boundary between amphibolite- and eclogite-facies (ca. 625–700°C and 13–15.5kbar) before 418Ma and underwent retrograde amphibolite-facies metamorphism (ca. 625–700°C and 8–9kbar) at 418–405Ma. These data suggest that a regional intermediate-P/T metamorphic event occurred during the Middle Paleozoic. In contrast, Paleoproterozoic augen gneiss blocks enclosed in the Deokjeongri gneiss complex preserve evidence of high-pressure (HP) metamorphism (840–960°C, 17–21.8kbar) at 234–230Ma, which are similar to the previously reported results from eclogite blocks in this area. The occurrence of Middle Paleozoic regional metamorphism before the Permo-Triassic HP metamorphism in the Hongseong area may be correlated with the Middle Paleozoic metamorphism in the Qinling belt in China; such regional metamorphic events were caused by the collision of microcontinents with the North or South China Cratons prior to the collision between the North and South China Cratons in the Permo-Triassic.

Eocene granulite-facies metamorphism prior to deformation of the Mianhuadi mafic complex in the Ailao Shan-Red River shear zone, Yunnan Province, SW China

The Mianhuadi mafic complex in the Ailao Shan-Red River (ASRR) shear zone, SW China, consists of granulite and mylonite, formed by granulite-facies peak metamorphism and retrograde amphibolite-facies metamorphism, with later local mylonitic deformation. Textures and mineral relics indicate that the protolith, represented by the mineral assemblage (M0) of olivine, clinopyroxene, plagioclase and Fe-Ti oxides, was emplaced as a mafic intrusion in the Late Permian as part of the Emeishan large igneous province. Two kinds of metamorphic mineral assemblages were identified: the peak assemblage (M1) of garnet+orthopyroxene+clinopyroxene+plagioclase+magnetite+ilmenite, and the retrograde assemblage (M2) composed of hornblende surrounding orthopyroxene and garnet, and hornblende+spinel symplectite around garnet. Pressure-temperature estimates yield 657–722°C and 7–9kbar, and 649–676°C and 5–6kbar for the M1 and M2 stages, respectively, roughly defining a clockwise P-T path with near-isothermal decompression following the peak metamorphism. These metamorphic conditions are quite unlike those (pressure up to 14kbar) of the Late Permian-Middle Triassic metamorphic event in the region. In addition, foliated biotite from mylonites has an 40Ar-39Ar plateau age of 34.3±0.4Ma, indicating that the ASRR shear zone was active at or before the Late Eocene. Combined with previous data, our new results indicate that the granulite-facies metamorphism of the Mianhuadi mafic complex occurred most likely during the Eocene, followed by rapid exhumation and ductile sinistral shearing in the Late Eocene. The formation of the granulite was likely related to the upwelling of the asthenosphere due to the delamination of overthickened lithospheric mantle after the India-Asia collision.

Metamorphic and magmatic evolution of the Paleoproterozoic gneisses in the Sancheong area, Yeongnam Massif, South Korea, and their implications to the tectonics in the Northeast Asia

The Sancheong area in the southwestern Yeongnam Massif mainly consists of Precambrian paragneisses (quartz-feldspathic and garnet gneiss), orthogneiss (porphyritic gneiss) and anorthosite and is important for an understanding the tectonic evolution of the southern part of the Korean Peninsula and Northeast Asia. The Precambrian paragneisses in the Sancheong area underwent intermediate-P/T metamorphism (quartz-feldspathic gneiss; 730°C and 12.2kbar). The intermediate-P/T metamorphism was overprinted by regional low-P/T metamorphism (750–820°C and 7.0–9.0kbar) which was followed by amphibolite-facies retrograde metamorphism. In contrast, the orthogneiss only experienced the low-P/T (750–800°C and 8.0–8.7kbar) metamorphism followed by the amphibolite-facies retrograde metamorphism. Zircon U–Pb dating indicates that the intermediate-P/T metamorphism likely occurred at ca. 1880–1920Ma and that the low-P/T metamorphism occurred at ca. 1855–1859Ma. The whole-rock geochemical data and zircon U–Pb ages indicate that the protolith of the porphyritic gneiss and the anorthosite were emplaced in a post-collisional tectonic setting during ca. 1872–1880Ma and ca. 1860Ma, respectively. These data suggest that the Sancheong area underwent continental collision associated with the intermediate-P/T metamorphism (ca. 1920–1880Ma) and post-collisional igneous activity associated with the low-P/T metamorphism during the Paleoproterozoic (ca. 1856–1880Ma). Similar Paleoproterozoic tectonic events were recently reported from the eastern Cathaysia Block, suggesting that the Yeongnam Massif may have a close affinity to the eastern Cathaysia Block.

Using detrital zircons from late Permian to Triassic sedimentary rocks in the south-eastern Central Asian Orogenic Belt (NE China) to constrain the timing of the final closure of the Paleo-Asian Ocean

We present new detrital zircon U–Pb–Hf isotope data for seven samples from late Permian to Triassic sedimentary units in Jilin Province, Northeast (NE) China, and use these data to constrain the timing of final closure of the Paleo-Asian Ocean and the late Permian to Triassic tectonic evolution of the Changchun–Yanji suture belt. The late Permian to Early Triassic sedimentary rocks contain material derived from both the northern margin of the North China Craton and the eastern segment of the Central Asian Orogenic Belt, suggesting that final closure of the Paleo-Asian Ocean occurred during the late Permian at latest. Metamorphic zircons within two metasedimentary units record amphibolite-facies retrograde metamorphism and metamorphic recrystallization events at ∼233Ma, postdating the peak of the collisional event in this area and providing an upper limit for the termination of orogenesis. In comparison, metamorphic zircons with younger ages of ∼223 and ∼210Ma from Middle–Late Triassic sandstones in eastern Jilin Province provide evidence of later hydrothermal events. These results, combined with the presence of voluminous middle Late Triassic terrestrial sedimentary rocks with bidirectional provenances in this area and coeval A-type granites and rhyolites in adjacent areas, suggest that this region underwent post-orogenic extension after the final closure of the Paleo-Asian Ocean. In brief, we suggest that the final closure of the Paleo-Asian Ocean along the Changchun–Yanji belt occurred during the late Permian at latest and that the associated collisional orogeny terminated at ∼233Ma (i.e. early Late Triassic). This belt was subsequently dominated by post-collisional extensional setting during the middle Late Triassic.

The metamorphic evolution of Paleoproterozoic eclogites in Kuru-Vaara, northern Belomorian Province, Russia: Constraints from P-T pseudosections and zircon dating

The rare occurrence of Precambrian eclogite has been used to argue when plate tectonics initiated. Well-preserved kyanite-bearing eclogites were collected in the Kuru-Vaara quarry, northern Belomorian Province, Fennoscandian Shield. Textural observations and phase equilibria modeling were applied to constrain their P-T evolution. The eclogite is characterized by the peak pressure assemblage garnet+omphacite+kyanite+amphibole+quartz. The garnet has inclusions of zoisite, omphacite and kyanite. This omphacite with high jadeite (up to 33mol%) and 5% Ca-Eskola content bears oriented rods of quartz, amphibole, and rutile. Omphacite was replaced by diopside+plagioclase±orthopyroxene symplectites and garnet surrounded by plagioclase coronas. Kyanite was mantled by the assemblage sapphirine+spinel+plagioclase±scapolite. The symplectites of diopside+plagioclase±orthopyroxene are further replaced by amphibole. P-T estimates of peak eclogite facies conditions are c. 18–20kbar and 720–820°C. The granulite facies assemblages formed at c. 11–13kbar and 870–915°C. Zircon U-Pb dating reveals the protolith ages of 2802–2929Ma for the eclogites. The c. 1896Ma zircon ages correspond to the time of prograde metamorphism in eclogite facies. The age of 1885±7Ma for a corundum-bearing mica pegmatite vein that intruded into the eclogites reflects the infiltration of fluid at amphibolite facies conditions. Thus, the Kuru-Vaara eclogite was metamorphosed during the Paleoproterozoic, not the Archean. The P-T path of Kuru-Vaara eclogites was characterized by eclogite facies metamorphism overprinted by granulite facies and retrograde amphibolite facies metamorphism within c. 10Ma. This process reflects the fast exhumation of the eclogites. The increase in temperature from eclogite to granulite facies in the HT-UHT domain suggests a hot Precambrian tectonic environment.

The metamorphic evolution from ultrahigh-temperature to amphibolite facies metamorphism in the Odaesan area after the collision between the North and South China Cratons in the Korean Peninsula

The Odaesan Gneiss Complex (Odesan Gneiss Complex) is the eastern end of the Hongseong–Odaesan collision belt in the Korean Peninsula, which is an extension of the Dabie–Sulu collision belt between the North and South China cratons. The Odaesan Gneiss Complex mainly consists of banded and migmatitic gneisses with porphyritic granitoids and amphibolites. The garnet-bearing banded gneisses can be subdivided into garnet–biotite and garnet–orthopyroxene banded gneisses. At the beginning of the post-collision stage, the banded gneisses underwent regional ultrahigh-temperature metamorphism (902–950°C/8.8–9.4kbar) at ca. 247–245Ma due to the heat supplied from underplated basic magma, which was generated by the partial melting of the lithospheric mantle caused by the heat supplied from the asthenospheric mantle. As a result of the continuous extensional force, the study area (lower crust) uplifted onto the middle crust depths, and then the study area underwent prograde granulite facies metamorphism from 660°C and 8.7kbar to 750–760°C and 6.3–6.5kbar at ca. 227Ma, causing migmatization, which erased the ultrahigh-temperature metamorphism in most of the study area. The ultrahigh-temperature metamorphism was preserved only in the garnet–orthopyroxene banded gneisses due to their very low water contents. During migmatization, the garnet–biotite banded gneisses were retrograded into upper granulite facies due to the relatively abundant water compared with the garnet–orthopyroxene gneisses. Finally, the study area was uplifted to a shallow depth and locally underwent amphibolite facies retrograde metamorphism (575–680°C and 3.1–4.5kbar). In addition, Paleoproterozoic metamorphic (ca. 1930–1886Ma) and post-collisional magmatic events (ca. 1847Ma) are identified based on SHRIMP age dating. These ages agree well with the regional Paleoproterozoic metamorphic and post-collisional magmatic activities reported from other areas of the Gyeonggi Massif.

Multistage Svecofennian metamorphism: Evidence from the composition and U-Pb age of titanite from eclogites of the Belomorian Mobile Belt

The first geochemical study of titanite from eclogites and associated rocks of the Belomorian Mobile Belt (BMB) by secondary ion mass spectrometry made it possible to establish the compositional features of this mineral in the garnet-bearing and garnet-free assemblages. Titanite from garnet-bearing assemblages is characterized by upward convex REE pattern and lowered HREE content relative to LREE, as well as the average GdN/YbN ratio around 16.5. Titanite from metaultrabasic rock inherits the specific features of the host rock, which should be taken into account when comparing with titanite from metagabbro. Results of U-Pb (TIMS) dating of titanite confirms the identification of the early and late stages of the Svecofennian metamorphism in the studied areas of BMB: early metamorphism with the peak eclogite facies conditions at around 1900 Ma, retrograde amphibolite facies metamorphism at 1870–1880 Ma, and late allochemical metamorphism accompanied by the pegmatite formation with an age of 1840 Ma.

Paleozoic HP granulite-facies metamorphism and anatexis in the Dulan area of the North Qaidam UHP terrane, western China: Constraints from petrology, zircon U–Pb and amphibole Ar–Ar geochronology

HP mafic and felsic adakitic granulite bodies in the Dulan area of the North Qaidam ultrahigh-pressure (UHP) terrane record high-pressure (HP) granulite-facies metamorphism and anatexis and provide temporal and tectonic constraints on deep subduction of continental crust and its subsequent exhumation. Mafic HP granulite components dominate the main outcrop and preserve features diagnostic of anatexis and locally may be described as migmatite. The HP mafic granulites comprise garnet, clinopyroxene, plagioclase and quartz. The felsic granulite (leucosome) is mainly composed of K-feldspar+plagioclase+quartz+kyanite+garnet. Detailed zircon U–Pb and amphibole Ar–Ar geochronology, combined with trace element geochemistry, indicate peak metamorphism for the mafic HP granulite at 434±3 to 435±3Ma and peak metamorphism and partial melting for the felsic HP granulite at 433±5 to 438±4Ma, which overlaps the ages of UHP metamorphism for adjacent eclogite (430–446Ma). 40Ar/39Ar amphibole ages of 423 to 432Ma represent amphibolite-facies retrograde metamorphism and indicate rapid cooling during exhumation of the HP granulite bodies. Our geochronological data, combined with field relationships, petrology and geochemistry suggest that HP granulite-facies metamorphism and the partial melting that produced adakitic melts represent the same tectonic event. In this case, the felsic HP granulites (leucosome) formed from an adakitic melt derived from partial melting of mafic HP granulite in the overriding plate in a relatively higher geothermal gradient (15–18°C/km), leaving garnet-cumulate and/or meta-ultramafic (mainly garnet pyroxenite) as the residual component. In contrast, the nearby UHP eclogite is thought to have formed in the subducted plate in a relatively lower geothermal gradient (6–10°C/km). Penecontemporaneous metamorphic ages but different geothermal gradients between HP granulites and related UHP eclogite define a possible paired metamorphic belt generated in a subduction–collision setting associated with the North Qaidam continental collisional orogeny during the Late Ordovician–Early Silurian.

Metamorphic P–T–t evolution of mafic HP granulites in the northeastern segment of the Tarim Craton (Dunhuang block): Evidence for early Paleozoic continental subduction

The Dunhuang block, which constitutes the northeastern segment of the Tarim Craton, is located between the Central Asian Orogenic Belt (CAOB) to the north and the Altyn–Qilian Orogenic Belt to the south. The present study reveals that the early Paleozoic HP mafic granulites from the Dunhuang block underwent four stages of metamorphism: prograde amphibolite-facies (M1), peak high-pressure (HP) granulite-facies (M2) and two late stages of amphibolite-facies retrograde metamorphism. Based on phase equilibrium modeling, P–T conditions of the four stages of metamorphism are estimated at 720–750°C and 11–13kbar, 760–800°C and 14–16kbar, 690–720°C and 8–8.5kbar, and at <6kbar and <640°C respectively. Peak granulite-facies metamorphism is characterized by a low geothermal gradient of ca. 16°C/km. Zircon U–Pb dating shows that the HP granulite-facies metamorphism occurred at ca. 431Ma and the early retrograde amphibolite-facies overprint at ca. 403Ma. Thus the investigated mafic rocks here reveal a clockwise P–T–t path involving burial heating before peak granulite-facies metamorphism and subsequent decompression–cooling with an uplift rate of ca. 0.8km/Ma. This, together with a continental affinity of the HP metamorphic rocks, indicates that the Dunhuang block experienced a collisional orogenesis during the early Paleozoic.

Polymetamorphism in the mainland Lewisian complex, NW Scotland – phase equilibria and geochronological constraints from the Cnoc an t’Sidhean suite

AbstractThe metamorphic evolution of rocks cropping out near Stoer, within the Assynt terrane of the central region of the mainland Lewisian complex of NW Scotland, is investigated using phase equilibria modelling in the NCKFMASHTO and MnNCKFMASHTO model systems. The focus is on the Cnoc an t’Sidhean suite, garnet‐bearing biotite‐rich rocks (brown gneiss) with rare layers of white mica gneiss, which have been interpreted as sedimentary in origin. The results show that these rocks are polymetamorphic and experienced granulite facies peak metamorphism (Badcallian) followed by retrograde fluid‐driven metamorphism (Inverian) under amphibolite facies conditions. The brown gneisses are inferred to have contained an essentially anhydrous granulite facies peak metamorphic assemblage of garnet, quartz, plagioclase and ilmenite (±rutile, K‐feldspar and pyroxene) with biotite, hornblende, muscovite, chlorite and/or epidote as hydrous retrograde minerals. P–T constraints imposed by phase equilibria modelling imply conditions of 13–16 kbar at &gt;900 °C for the Badcallian granulite facies metamorphic peak, consistent with the field evidence for partial melting in most lithologies. The white mica gneiss comprises a muscovite‐dominated matrix containing porphyroblasts of staurolite, corundum, kyanite and rare garnet. Previous studies have suggested that staurolite, corundum, kyanite and muscovite all grew at the granulite facies peak, with partial melting and melt loss producing a highly aluminous residue. However, at the inferred peak P–T conditions, staurolite and muscovite are not predicted to be stable, suggesting they are retrograde phases that grew during amphibolite facies retrograde metamorphism. The large proportion of mica suggests extensive H2O‐rich fluid‐influx, consistent with the retrograde growth of hornblende, biotite, epidote and chlorite in the brown gneisses. P–T conditions of 5.0–6.5 kbar at 520–550 °C are derived for the Inverian event. In situ dating of zircon from samples of the white mica gneiss yield apparent ages that are difficult to interpret. However, the data are permissive of granulite facies (Badcallian) metamorphism having occurred at c. 2.7–2.8 Ga with subsequent fluid driven (Inverian) retrogression at c. 2.5–2.6 Ga, consistent with previous interpretations.

Magnetic fabrics in garnet peridotites–pyroxenites and host felsic granulites in the South Bohemian Granulites (Czech Republic): Implications for distinguishing between primary and metamorphism induced fabrics

In the high-grade Moldanubian Zone of the European Variscides, numerous bodies of ultramafic rocks occur embedded in granulite. The anisotropy of magnetic susceptibility and its low-field variation as well as the anisotropy of magnetic remanence were used to investigate magnetic fabrics of some ultramafic bodies and host granulite. In granulite, the magnetic foliation is roughly parallel to the metamorphic foliation and the magnetic lineation is near the mineral alignment lineation. In ultramafite, the magnetic foliation is relatively scattered spatially, but mostly oriented in a different way than that in granulite. The magnetic lineation is also scattered, but still relatively well defined spatially. Again, its orientation is mostly different than that of granulite. The magnetic fabric in ultramafic rocks is therefore different from that in the host granulite even though both rock types underwent at least partially common structural history. The componental movements forming the granulite fabric, mostly during amphibolite facies retrograde metamorphism, were evidently not intensive enough to strongly overprint the magnetic fabric of ultramafite. The ultramafite is therefore strong enough to maintain its pre-metamorphism fabric even at such high temperatures and pressures that are characteristic of high amphibolite facies retrograde metamorphism.

EVIDENCE OF EVAPORITES IN THE GENESIS OF THE VANADIAN GROSSULAR 'TSAVORITE' DEPOSIT IN NAMALULU, TANZANIA

The Namalulu tsavorite (vanadian grossular up to 0.80 wt.% V 2 O 3 ) deposit in northeastern Tanzania occurs in metasomatized graphitic gneiss overlain by a dolomitic marble unit. These rocks belong to the Neoproterozoic metamorphic Mozambique Belt. The graphitic gneiss is composed of quartz, V-bearing kyanite (up to 0.4 wt.% V 2 O 3 ), V-bearing muscovite (up to 1.0 wt.% V 2 O 3 ), V-bearing rutile (up to 1.9 wt.% V 2 O 3 ), and graphite. This rock is affected by calcic metasomatism precipitating calcite and mobilizing the elements for tsavorite crystallization in quartz-calcite veins. The dolomitic marble unit hosts an anhydrite-gypsum-dolomite lens (lens I) and a calcite-scapolite-diopside-sulfides-graphite lens (lens II). This last unit is characterized by the presence of F-bearing minerals (tremolite, phlogopite, tainiolite, titanite), with up to 9.4 wt.% F for tainiolite, and Ba-bearing minerals (feldspar, phlogopite), with up to 7.5 wt.% BaO for feldspar. Lithium (up to 2.0 wt.% Li 2 O) and boron (up to 110 ppm) are also present in tainiolite, as well as in F-bearing tremolite. This is the first description of metamorphic tainiolite. The protoliths of the Namalulu rocks correspond to organic-rich black shales, pure and magnesian carbonates, and evaporites. They were presumably deposited in a marine coastal sabkha located at the eastern margin of the Congo-Kalahari cratons bounded by the Mozambique Ocean during early Neoproterozoic. Tsavorite formed in the Ca-metasomatized gneiss during amphibolite facies retrograde metamorphism at 5.6–6.7 kbar and 630 ± 30 °C.