Facies Stage Research Articles

Metamorphism and P-T Evolution of High-Pressure Granulites from the Fuping Complex, North China Craton

Granulite facies rocks provide important keys to evaluating collisional metamorphism in orogenic belts. The mafic granulites of Baoding in the Fuping Complex of the North China Craton occur within the Trans-North China Orogen (TNCO), a major Paleoproterozoic collisional orogen. Here, we present results from detailed investigations on newly discovered garnet pyroxenite, garnet two-pyroxene granulite, and garnet-bearing-plagioclase amphibolite using petrographic, mineralogical, geochemical, and zircon U-Pb dating methods. Our results show that the Fuping Complex metamorphic evolution in this study evolved in four stages: prograde (M1), high-pressure granulite facies (M2), granulite facies (M3), and retrograde (M4) stages. The mineral assemblage of the prograde stage (M1) consists of Amp + Pl + Q within garnet cores. The mineral assemblage of high-pressure granulite facies at the peak stage (M2) consists of Gt + Cpx + Pl + Q ± Amp, forming the garnet pyroxenite. The granulite facies stage M3 is characterized by the occurrence of orthopyroxene, with a mineral assemblage of Gt + Cpx + Opx + Amp+ Pl + Q. The early retrograde stage M4-1 includes clinopyroxenes scattered inside amphiboles, following the breakdown of garnet and clinopyroxene. The mineral assemblage of this stage comprises Amp + Pl + Q + Ilm ± Cpx. Later, in the late retrograde stage M4-2, the composition of amphiboles changed to actinolite, and epidote and chlorite started to appear in the matrix. Traditional geothermobarometry yielded P-T conditions of 700~706 °C and 6.0~6.2 kbar for prograde stage M1, 854~920 °C and 13.0~13.8 kbar for high-pressure granulite facies stage M2, 912~939 °C and 8.1~9.9 kbar for M3, 661~784 °C and 3.1~4.4 kbar for M4-1, and 637~638 °C, 1.1~1.3 kbar for M4-2, along a clockwise P-T path with a nearly isothermal decompression (ITD) and slight heating. Zircon LA-ICP-MS U-Pb dating constrains the timing of the high-pressure granulite facies metamorphic event to be between 1.83 and 1.86 Ga. Geochemical features suggest that the protoliths of the high-pressure granulites may have formed in an island arc environment within a convergent margin setting. Together with results from previous studies, our data suggest that the ~1.85 Ga metamorphic age recorded in the Fuping Complex represents a regional metamorphism in the TNCO, associated with the subduction–collision and assembly of the Eastern and Western Blocks of the NCC.

Orogen-parallel discontinuity of the Apennines subduction zone in Southern Italy as seen from mantle wedge seismic structure

We investigate the seismic structure of the mantle wedge of the Apennines subduction zone (Central Mediterranean) using teleseismic receiver function (RF). We inverted RF for both isotropic and anisotropic properties of the mantle wedge, from below the overriding Moho to the “plate boundary”, i.e. the interface that separate the slab from the mantle wedge. Given the distribution of the seismic network, we are able to map out the change in the elastic properties at the transition between southern apennines and the Calabrian arc, given by the change in the subduction style (i.e from the subduction of continental materials to oceanic plate). We found that the anisotropy in the mantle wedge is similar between all seismic stations, generally highly anisotropic (> 10%), with a direction of the symmetry axis that rotates clockwise from North to South, following the Calabrian arc geometry and likely indicating the mantle flow driven by the slab retreat. The elastic properties of the subducted crust are more heterogeneous. To the North, the subducted crust shows a highly anisotropic (> 10%) behavior, and it occurs at larger depth (around 70 km depth), where to the South anisotropy is less intense (around 7%) and the subducted crust is shallower (around 60 km depth). These results point out a change in the subduction style that can be given by either a change in the metamorphic phase (more evolved blueschist facies stage to the North, initial greenschist facies stage to the South) or a different origin for the subducted materials (continental to the North and oceanic to the South). The differences in the anisotropic behavior of the subducted crust are reflected in the topography of the plate boundary, which becomes shallower from North to South, suggesting the existence of either a step in the slab topography or a more gentle ramp.

Multiphase-solid fluid inclusions in HP-LT eclogite facies rock (Zavkhan Terrane, Western Mongolia): evidence for the evolution from saline to hypersaline fluids during metamorphism in subduction zone

Fluid inclusions in high- and ultrahigh-pressure metamorphic rocks provide direct information on the composition of the fluids that evolved during metamorphism and fluid-rock interactions in deep subduction zones. We investigate the fluid inclusions in the Khungui eclogite of the Zavkhan Terrane, Central Asian Orogenic Belt. Fluid inclusions are observed in garnet and quartz in the eclogite samples that underwent metamorphism during subduction. The primary fluid inclusions in quartz are composed of liquid and vapor with high salinities (15.7–16.4 wt.% NaCl eq.), whereas the secondary fluid inclusions in quartz are classified as: relatively high salinity (Type I:12.5–16.3 wt.% NaCl eq.) and low salinity (Type II:6.7–10.6 wt.% NaCl eq.). The garnet shows compositional zoning from Ca-poor cores to Ca-rich rims, and the rims that grew during the eclogite-stage metamorphism (2.1–2.2 GPa at 580–610 °C) preferentially contain numerous primary fluid inclusions. The primary fluid inclusions in garnet are commonly bi-phases (liquid and vapor); however, some are multiphase-solid fluid inclusions composed of fluids (liquid and vapor) and combinations of several minerals (halite, quartz, apatite, calcite, biotite, chlorite, and actinolite). Bi-phase fluid inclusions preferentially occur in the inner parts of the Ca-rich garnet rim, whereas multiphase-solid fluid inclusions occur along the margins of the Ca-rich rim. We hypothesize that the multiphase-solid fluid inclusions are formed via interactions between trapped fluids, trapped minerals, and the host garnet during exhumation. By combination of FIB–SEM and synchrotron X-ray CT analyses, the detailed occurrences, volumes, and compositions of the solid phases in the fluid inclusion was analyzed. We then conduct mass balance analysis to reconstruct accurate fluid compositions using data from the FIB–SEM and synchrotron X-ray CT images of the multiphase-solid fluid inclusion. The results of these analyses reveal that (1) fluid changed from an H2O-dominated saline fluid (13–16 wt. % NaCl eq.) at the prograde to the earlier eclogite stage to H2O–CO2-dominated hypersaline fluid at later eclogite stage (~ 32 wt. % NaCl eq., 7.3 wt. % CO2 and ~ 19 molal dissolved cations); (2) a variety of mineral assemblages in multiphase-solid fluid inclusions are produced by post-entrapment reactions between the trapped hypersaline fluid, trapped minerals and the fluid host mineral. The evolution of fluids from saline to hypersaline during the eclogite facies stage is probably caused by the formation of hydrous minerals (i.e., barroisite) under a near-closed system.

Tectonic erosion and deep subduction in Central Tibet: Evidence from the discovery of retrograde eclogites in the Amdo microcontinent

AbstractThe Amdo microcontinent which separates the Qiangtang terrane to the north and the Lhasa terrane to the south is a key terrane for reconstructing the tectonic evolution of Central Tibet. We report the new finding of retrograde eclogites within the Amdo microcontinent in this study. The eclogites are characterized by peak metamorphic mineral assemblages of garnet, omphacite, rutile and quartz and underwent a four‐stage metamorphic evolution, including a peak eclogite facies stage (M1) at ~20–24 kbar and 580–620°C, followed by an HP granulite facies decompression stage (M2) at ~13–15 kbar and 750–780°C, a subsequent MP‐UHT granulite facies heating stage (M3) at 8–10 kbar and >840°C and a final amphibolite facies retrogression (M4) at 5.3–6.0 kbar and 560–580°C. The eclogites exhibit rare earth element distribution patterns and trace element abundances similar to those of N‐MORB and arc‐related volcanics, with depleted whole‐rock εNd(t) values of 3.4 to 4.2, and are inferred to have formed in a back‐arc basin tectonic setting. Zircon and rutile U–Pb dating yields a protolith age of 226 ± 5 Ma, a peak eclogite facies metamorphic age of 190 ± 1 Ma, an HP granulite facies metamorphic age of 179 ± 1 Ma and an amphibolite facies retrograde age of 172 ± 1 Ma. The clockwise P–T–t paths and the oceanic protolith signature of retrograde eclogites suggest that part of the back arc basin was subducted to depths of ~80 km. Tectonic erosion associated with the subduction of the Amdo microcontinent beneath the Tethys Ocean accounts for the deep subduction of the back‐arc basin and the absence of arc magmatic rocks in the northern Amdo microcontinent.

Newly recognized retrograde eclogites overprinted by high‐temperature metamorphism in the Amdo microcontinent, Central Tibet: Implications for subduction erosion during continental subduction

The Amdo microcontinent located within the Bangong–Nujiang Suture Zone (BNSZ) holds one of the important keys to understand the tectonic evolution of Central Tibet before the Qiangtang‐Lhasa terrane collision during the Mesozoic. Newly discovered eclogites were reported within the Amdo microcontinent in this study, and their metamorphic evolution is deciphered by petrographic observations, pseudosection modelling, geochemistry, and zircon U–Pb dating. The eclogites are characterized by peak metamorphic mineral assemblages of garnet, omphacite, rutile, and quartz, and underwent a four‐stage metamorphic evolution, including eclogite facies stage (M1) at ~20–24 kbar and 580–620°C, followed by HP granulite facies decompression stage (M2) at ~13–15 kbar and 750–780°C, subsequent MP–HT granulite facies heating stage (M3) at 8–10 kbar and >840°C, and final amphibolite facies retrogression (M4) at 5.3–6.0 kbar and 560–580°C. The eclogites exhibit rare earth element (REE) distribution patterns and trace element abundance similar to those of N‐MORB and arc‐related volcanics, which are inferred to have formed in a back‐arc Basin tectonic setting. Zircon U–Pb dating yields eclogite facies metamorphic age of 190 ± 1 Ma. The clockwise P–T–t paths and the oceanic protolith signature of retrograde eclogites suggest that part of the back‐arc Basin was subducted to the depths of ~80 km. The subsequent ultra‐high temperature metamorphic overprinting may be related to the upwelling of the asthenosphere. Tectonic erosion associated with the subduction of the Amdo microcontinent beneath the Tethys Ocean accounts for the deep subduction of the back‐arc Basin and the absence of arc magmatic rocks in the northern Amdo microcontinent.

Deep subduction and exhumation of micro-continents in the Proto-Tethys realm: Evidence from metamorphism of HP-UHT rocks in the North Qinling Orogen, central China

The North Qinling Orogen in central China contains a typical continental high- and ultrahigh-pressure (HP-UHP) metamorphic belt, but its metamorphic evolution remains controversial. We report a combined investigation on petrology, geochronology and phase modelling for felsic granulite (samples SN1504 & SN1517) and garnet clinopyroxenite (sample SN1505) from the Songshugou area. Four stages of metamorphic evolution are constrained: (i) A possible prograde from HP amphibolite facies to peak UHP eclogite facies stage (~3–8 GPa/700–1080 °C) recorded by the chemical zoning of garnet core and mantle in sample SN1504 and also the presence of supersilicic titanite in sample SN1505. (ii) A post-peak decompression evolution is recorded by the formation of coronary garnet and plagioclase around kyanite in sample SN1504 and high-Ti amphibole and plagioclase in sample SN1505, including the evolution from eclogite facies to HP–ultrahigh temperature (UHT) conditions of 1.3–2.2 GPa/900–980 °C for sample SN1504 and 1.0–2.1 GPa/960–1010 °C for SN1505, respectively. (iii) Further decompression is revealed from the growth of corundum + spinel in local domains in sample SN1504, suggesting a low-pressure (LP) condition of ~0.8 GPa/950 °C. (iv) A subsequent cooling evolution is recorded by thin films of K-feldspar + quartz + biotite in sample SN1504 and low-Ti amphibole in sample SN1505, indicative of melt crystallization towards the solidus with conditions of 830–770 °C/0.55–0.8 GPa and 880–820 °C/0.8–0.9 GPa, respectively. Zircon and monazite U-Pb dating yields three metamorphic ages of c. 521 Ma, c. 503 Ma and c. 500–480 Ma, which are interpreted to represent the prograde HP amphibolite facies stage, the peak eclogite facies stage and the late cooling stage, respectively. The metamorphic P–T–t path suggests a complete history of deep continental subduction to depths of >90–250 km, followed by rapid exhumation to crustal depths for the Qinling micro-continent during the early Proto-Tethys evolution.

The Middle Permian (Capitanian) carbonate platform evolution and stratigraphic architecture of a Neo‐Tethys rim basin, Central Pakistan

Here we present the stratigraphic architecture and carbonate platform evolution of the Middle Permian (Capitanian) succession (Wargal Formation) of Trans‐Indus Ranges, central Pakistan. The exposed carbonate sequence consists of thin‐ to thick‐bedded bioclastic, nodular, fine‐grained, and sandy limestone. Based on detailed petrographic studies, seven microfacies assemblages have been recognized, suggesting diverse depositional environments including mudflats, lagoon, sand shoal, and middle shelf. The identified age‐diagnostic larger benthic foraminiferal species enabled to assign a Middle Permian (Capitanian) age to the studied section of the Wargal Formation. Based on the vertical arrangements of depositional environments in time, three Transgressive Systems Tracts (TSTs), two Regressive Systems Tracts (RSTs), and 11 high‐resolution fourth/fifth‐order parasequences were delineated. The depositional and sequence stratigraphic analyses of the studied section inferred evolution of the Middle Permian carbonate platform in three stages: (i) marginal marine facies development in the lower part followed by (ii) transitional stage in the middle part, and (iii) finally, aggrading shallow marine facies stage in the upper part. The evolution of carbonate platform can be linked with the cessation of major rifting followed by thermal cooling and establishment of marine Tethyan settings at the margin of the rift flank basin. The diverse facies variation in the second stage of platform evolution argues strongly for the local intra‐plate stresses. Based on petrographic, SEM, and EDS studies, it is concluded that the studied section of Wargal Formation carries porosity values from a range of 2–8% with an average of 5% and can be called a low to moderate hydrocarbon reservoir in the study area.

The metamorphic PT history of Precambrian Belomorian eclogites (Shirokaya Salma), Russia

AbstractThis petrological study details representative well‐preserved and partly retrogressed eclogite samples from the Shirokaya Salma locality in the Belomorian Province (Russia) to resolve ambiguity in the PT evolution of one of the oldest eclogite occurrences on Earth. Based on microstructures, eclogite and granulite facies mineral assemblages comprise omphacite, garnet (interior), quartz, rutile±phengite and diopside, garnet (modified rim), plagioclase, orthopyroxene, amphibole, quartz, rutile/ilmenite±melt respectively. Omphacite and Na‐bearing diopside feature breakdown textures with multiple oriented quartz–amphibole and orthopyroxene lamellae, while diopside forms symplectite with plagioclase. Garnet often exhibits a particular compositional annulus in both major and trace elements (Y+REE). We interpret the high‐Ca and inclusion‐rich garnet (core) with outwards Grs decrease and Prp increase as relict of prograde growth. The mantle‐rim domain of garnet, containing omphacite, diopside, plagioclase and rutile formed during eclogitization. Compositionally homogeneous garnet porphyroblasts are most likely outcomes of high‐T modifications. A progressive zonation of Y+REE from core to rim in several garnets is related to resorption at high temperatures. Both geothermometric and pseudosection calculations reveal a typical orogenic clockwise trajectory for the studied eclogites, and their metamorphic PT path includes a possible pre‐eclogitic stage (M0) at <630°C, ~16–19 kbar, a peak eclogite facies stage (M1) at 19–21 kbar, 670–750°C, a granulite facies overprint (stage M2) from 14–16 kbar, 830–870°C (M2a) to 10–11 kbar, 880–910°C (M2b), and retrogression to <9–11 kbar, <650–680°C (stage M3). It agrees well with our previous estimates for other Salma eclogites with Palaeoproterozoic ages.

Persistence of partial melting in the southern North China Craton: Evidence from Paleoproterozoic migmatites of the Taihua Complex

The Taihua Complex is exposed in the southern North China Craton (NCC), it has experienced a long and complicated history of multiple crustal growth, magmatism, metamorphism and deformation during the early Precambrian. Abundant leucosomes are widely distributed within the migmatites of the Taihua Complex, which occurred as irregular lenses, blocks, layers or dikes with several thickness of millimeters to tens of centimeters wide. Meanwhile, former presence of melt and typical microstructures related to partial melting also can be recognized in the thin sections under the microscope, those provide compelling evidences that the Taihua Complex has extensively suffered partial melting, and can be regarded as the best target to decipher multistage anatexis during metamorphism as well as assembly of the whole NCC. However, the timing and nature of the partial melting in the Taihua Complex have not been well studied up to now. In this contribution, we present an integrated in situ zircon U-Pb ages and Hf isotope compositions for the Paleoproterozoic migmatites from the Taihua Complex in the southern NCC. Both the metasedimentary and metamorphized plutonic rocks of the Taihua Complex have extensively suffered partial melting. The ca. 2.50 Ga magmatic zircons or inherited magmatic cores from the migmatites and leucosomes present similar Hf isotope composition to those of the contemporaneous depleted mantle at ca. 2.50 Ga, suggesting that ca. 2.50 Ga is a major period of crustal growth in the southern NCC as well as the whole NCC. The ages of metamorphic zircons obtained from leucosomes, melanosomes and migmatites show a large time interval of 1909–1798 Ma, indicating that the Taihua Complex has experienced a persistence of partial melting event during Late Paleoproterozoic that lasted more than 100 Ma. The crustal anatexis in the southern NCC probably occurred during the post-peak upper amphibolite- to granulite facies stage with melting temperature of at least 652–753 °C. The decompression melting and widespread migmatization of the Taihua Compelx are probably due to extension related to exhumation of the previously thickened crust during Paleoproterozoic collision between the Eastern and Western block, or between the Southern Archean Block and other micro-continents. The main collision-related assembly and crustal thickening events in the southern NCC likely occurred at ca. 1.95 Ga, rather than ca. 1.85 Ga as previously believed.

Multiple Metamorphic Events Recorded within Eclogites of the Chandman District, SW Mongolia

The eclogite-bearing Alag Khadny metamorphic complex in the Lake Zone, SW Mongolia occupies the central region of the Central Asian Orogenic Belt, the largest Phanerozoic orogenic belt in the world. The complex consists mainly of orthogneisses intercalated with eclogites and micaschists in a mélange zone. Most of eclogites are strongly amphibolitized. In this study, we examined petrography and mineral chemistry of eclogites and amphibolitized eclogites, respectively. The result of our research shows that Chandman eclogites experienced multiple events of metamorphism in throughout their subduction and subsequent collision history. We revealed that eclogites were subjected to blueschist facies metamorphism before the peak eclogite facies stage. In addition, we have studied amphibolitized eclogite, and revealed that another distinct progressive medium pressure (MP) epidote-amphibolite facies metamorphic event took place in the eclogite, consistent with collision process. The multiple events of metamorphism in eclogites have been revealed by zonation textures of HP amphiboles zoned with glaucophane→barroisite→Mg-hornblende and MP amphiboles zoned with actinolite/winchite→barroisite→Mg-hornblende/tschermakite/Fe-pargasite. These amphiboles with different zonation textures reflect their metamorphic history of subduction to collision events.

Metamorphic Evolution and Tectonic Implications of the Granulitized Eclogites from the Luliangshan Terrane in the North Qaidam Ultrahigh Pressure Metamorphic Belt, NW China: New Constraints from Phase Equilibrium Modeling

The granulitized eclogites from the Luliangshan terrane of the North Qaidam UHP metamorphic belt occur as lenses within pelitic gneisses and orthogneisses. Combined petrologic data and phase equilibrium modeling indicate a multi-stage metamorphic history of the granulitized eclogites: (1) an earlier eclogite facies metamorphism (P>18.5 kbar, T> 830 °C) is deduced from omphacite relics in the matrix and rare omphacite inclusions within garnet. The possible assemblage is garnet+omphacite+rutile+ quartz; (2) the early stage of high pressure granulite facies assemblages (garnet+clinopyroxene+ plagioclase+rutile+quartz+liquid) developed in the early decompression process has a P-T regime of 17.5 kbar and 852–858 °C, constrained by plagioclase and clinopyroxene inclusions in garnet. The late stage of high pressure granulite assemblages (garnet+clinopyroxene+amphibole+plagioclase+rutile+quartz+liquid) records an isothermal decompression process with the pressure successively declining from 17.5 to 14.7 kbar and to 11.3 kbar at 858 °C; (3) the later medium pressure granulite facies assemblage (garnet+ orthopyroxene+ clinopyroxene+amphibole+plagioclase+ilmenite+liquid+quartz) indicates a drop in pressure and rise in temperature at P-T conditions of 7.6–7.7 kbar and 878–883 °C; (4) retrogressive amphibolite facies stage, which is represented by amphibole+plagioclase kelyphitic rims around garnet, formed under conditions of <5 kbar and <650 °C. The preservation of medium pressure granulite facies assemblage and the garnet composition feature constrain a following isobaric cooling path during late exhumation. This process suggests a clockwise P-T path and indicates that the granulitized eclogites record a high grade “Barrovian” metamorphic overprint at the middle-lower crust during exhumation. The present data show that the Luliangshan terrane is a “hot” HP-UHP terrane.

Timing and nature of the partial melting processes during the exhumation of the garnet–bearing biotite gneiss in the southern Altyn Tagh HP/UHP belt, Western China

Partial melting of the garnet (Grt)–bearing biotite gneiss from the Jianggalesayi area in the southern Altyn Tagh high pressure (HP) – ultra high pressure (UHP) metamorphic belt has been identified and reported here. Felsic veins were distributed along the foliation plane and deformed together with the Grt–bearing biotite gneiss, which dominated the main outcrop and preserved diagnostic features of anataxis. Highly cuspate and irregular quartz (Qz)-melt and plagioclase (Pl)-melt surrounded or stretched into biotite and/or quartz grains with obvious eroded boundaries further provided the microstructural evidences for partial melting of the gneiss. CL images showed that zircons from the Grt–bearing biotite gneiss were characterized by multizone structures of core – Rim I – Rim II – Rim III. The different zircon zones from the gneiss had integrated recordings of the protolith ages 836–1613 Ma; the peak metamorphic age of 503 ± 5 Ma; the retrograded age of 456 ± 4 Ma; and the anatectic age of 418 ± 3 Ma. Zircons from felsic veins also exhibited core – rim structures on CL images. The felsic veins yielded similar protolith ages (1099–1072 Ma) and anatectic age (417 ± 2 Ma) as the host gneiss. The zircons from the host gneiss and felsic veins had plotted identical crustal evolution curves in the εHf(t) vs. U-Pb age diagram with identical TDM ages. The U-Pb ages and Lu-Hf isotope both suggested that the felsic veins were internally derived by the partial melting of the host gneiss. Moreover, the high content of Al2O3 and Rb in felsic veins indicated that the decomposition of biotite accounted for the partial melting of the gneiss. The temperatures calculated by Ti–in–zircon thermometer for the anatectic zircons from the felsic veins (582–694 °C) were identical to the conditions of the amphibolite facies stage of the Grt–bearing biotite gneiss (T = 619–646 °C), which indicated that the crystallization of the melt had taken place during the amphibolite facies stage of the exhumation. The anatectic ages (418 ± 3 Ma and 417 ± 2 Ma) of the gneiss and felsic veins were determined to be within the range of the fourth episode of the early Paleozoic magmatism (420–385 Ma), which had formed in a post-collisional extensional setting. Therefore, the combination of the magmatic heat events and stress release may have played an important role in the partial melting of the gneiss.

Coexisting different types of zoned garnet in kyanite‐quartz eclogites from the Sanbagawa metamorphic belt: Evidence of deformation‐induced lithological mixing during prograde metamorphism

AbstractGarnet grains in Sanbagawa quartz eclogites from the Besshi region, central Shikoku commonly show a zoning pattern consisting of core and mantle/rim that formed during two prograde stages of eclogite and subsequent epidote–amphibolite facies metamorphism, respectively. Garnet grains in the quartz eclogites are grouped into four types (I, II, III, and IV) according to the compositional trends of their cores. Type I garnet is most common and sometimes coexists with other types of garnet in a thin section. Type I core formed with epidote and kyanite during the prograde eclogite facies stage. The inner cores of types II and III crystallized within different whole‐rock compositions of epidote‐free and kyanite‐bearing eclogite and epidote‐ and kyanite‐free eclogite at the earlier prograde stage, respectively. The inner core of type IV probably formed during the pre‐eclogite facies stage. The inner cores of types II, III, and IV, which formed under different P–T conditions of prograde metamorphism and/or whole‐rock compositions, were juxtaposed with the core of type I, probably due to tectonic mixing of rocks at various points during the prograde eclogite facies stage. After these processes, they have shared the following same growth history: (i) successive crystal growth during the later stage of prograde eclogite facies metamorphism that formed the margin of the type I core and the outer cores of types II, III, and IV; (ii) partial resorption of the core during exhumation and hydration stage; and (iii) subsequent formation of mantle zones during prograde metamorphism of the epidote–amphibolite facies. The prograde metamorphic reactions may not have progressed under an isochemical condition in some Sanbagawa metamorphic rocks, at least at the hand specimen scale. This interpretation suggests that, in some cases, material interaction promoted by mechanical mixing and fluid‐assisted diffusive mass transfer probably influences mineral reactions and paragenesis of high‐pressure metamorphic rocks.

High‐pressure metamorphic evolution of eclogite and associated metapelite from the Chuacús complex (Guatemala Suture Zone): Constraints from phase equilibria modelling coupled with Lu‐Hf and U‐Pb geochronology

AbstractAs is common in suture zones, widespread high‐pressure rocks in the Caribbean region reached eclogite facies conditions close to ultrahigh‐pressure metamorphism. Besides eclogite lenses, abundant metapelitic rocks in the Chuacús complex (Guatemala Suture Zone) also preserve evidence for high‐pressure metamorphism. A comprehensive petrological and geochronological study was undertaken to constrain the tectonometamorphic evolution of eclogite and associated metapelite from this area in central Guatemala. The integration of field and petrological data allows the reconstruction of a previously unknown segment of the prograde P–T path and shows that these contrasting rock types share a common high‐pressure evolution. An early stage of high‐pressure/low‐temperature metamorphism at 18–20 kbar and 530–580°C is indicated by garnet core compositions as well as the nature and composition of mineral inclusions in garnet, including kyanite–jadeite–paragonite in an eclogite, and chloritoid–paragonite–rutile in a pelitic schist. Peak high‐pressure conditions are constrained at 23–25 kbar and 620–690°C by combining mineral assemblages, isopleth thermobarometry and Zr‐in‐rutile thermometry. A garnet/whole‐rock Lu‐Hf date of 101.8 ± 3.1 Ma in the kyanite‐bearing eclogite indicates the timing of final garnet growth at eclogite facies conditions, while a Lu‐Hf date of 95.5 ± 2.1 Ma in the pelitic schist reflects the average age of garnet growth spanning from an early eclogite facies evolution to a final amphibolite facies stage. Concordant U‐Pb LA‐ICP‐MS zircon data from the pelitic schist, in contrast, yield a mean age of 74.0 ± 0.5 Ma, which is equivalent to a U‐Pb monazite lower‐intercept age of 73.6 ± 2.0 Ma in the same sample, and comparable within errors with a less precise U‐Pb lower‐intercept age of 80 ± 13 Ma obtained in post‐eclogitic titanite from the kyanite‐bearing eclogite. These U‐Pb metamorphic ages are interpreted as dating an amphibolite facies overprint. Protolith U‐Pb zircon ages of 167.1 ± 4.2 Ma and 424.6 ± 5.0 Ma from two eclogite samples reveal that mafic precursors in the Chuacús complex originated in multiple tectonotemporal settings from the Silurian to Jurassic. The integration of petrological and geochronological data suggests that subduction of the continental margin of the North American plate (Chuacús complex) beneath the Greater Antilles arc occurred during an Albian‐Cenomanian pre‐collisional stage, and that a subsequent Campanian collisional stage is probably responsible of the amphibolite facies overprint and late syncollisional exhumation.

An integrated EPMA-EBSD study of metamorphic histories recorded in garnet

Growth histories recorded in garnet grains in metasedimentary rocks from the Sanbagawa belt in Japan and the Mogok belt in Myanmar were analyzed using an effective combination of electron backscatter diffraction (EBSD) and electron probe microanalysis (EPMA) data. Garnet in the Sanbagawa metapelite has inner and outer zones that formed in the eclogite and epidote-amphibolite facies stages, respectively. Based on EPMA element mapping, this garnet appears to have grown as a single crystal with a temporal break in growth between the inner and outer zones that occurred during exhumation. The EBSD data, however, document that the garnet grain is composed of four domains. The misorientation angles of crystallographic orientations between the domains are as large as 59°, and domain boundaries crosscut the growth zoning and the compositional boundary between the inner and outer zones. Sets of quartz grains included in the garnets on either side of the domain boundaries sometimes share the same crystallographic orientation with misorientation angles less than 4°. The garnet grains formed via a three-step process of prograde crystallization of polycrystalline garnet during the eclogite facies stage (inner zone) → resorption around garnet rims and along domain boundaries during exhumation → crystallization of the outer zone and in the domain boundaries during the prograde epidote-amphibolite facies stage. The garnet porphyroblasts in the Mogok pelitic gneisses, which formed during prograde metamorphism to the upper amphibolite-granulite facies (0.6–1.0 GPa/780–850 °C), are now separated into segments of various sizes by mosaic or symplectite aggregates of biotite, plagioclase, and quartz or monomineralic biotite veins. The segment texture formed at about 0.3–0.4 GPa/610–650 °C or lower-grade conditions. The EBSD analysis shows that most of the segments share the same crystallographic orientation with misorientation angles less than 4° and show no evidence of deformation and/or rotation processes after segmentation. These data suggest that the Mogok sample did not experience dynamic deformation of the garnet grains after the resorption and segmentation stage and may have been exhumed under static conditions from depths of 9–12 km.

Early Mesozoic retrograded eclogite and mafic granulite from the Badu Complex of the Cathaysia Block, South China: Petrology and tectonic implications

The high-grade metamorphic terrane in the Badu region along the northeastern Cathaysia Block in South China preserves retrograded eclogites and mafic granulites. Here we present the petrology, mineral phase equilibria and P-T conditions based on pseudosection computations, as well as zircon U-Pb ages of these rocks. Mineral textures and reaction relationships suggest four metamorphic stages for the retrograded eclogite as follows: (1) eclogite facies stage (M1), (2) clinopyroxene retrograde stage (M2), (3) amphibole retrograde stage (M3), and (4) chlorite retrograde stage (M4). For the mafic granulite, three stages are identified as: (1) plagioclase-absent stage (M1), (2) granulite facies stage (M2) and (3) amphibolite facies stage (M3). Metamorphic evolution of both of the rock types follows clockwise P-T path. Conventional geothermometers and geobarometers in combination with phase equilibria modelling yield metamorphic P-T conditions for each metamorphic stage for the eclogite as 500–560°C, 23–24kbar (M1), 640–660°C, 14–16kbar (M2), 730–750°C, and 11–13kbar (M3). The chlorite retrograde stage (M4) is inferred to have occurred at lower amphibolite to greenschist facies conditions. Phase equilibria modelling of the mafic granulite shows P-T conditions for each metamorphic stage as 600–720°C, >13kbar (M1) and 860–890°C, 5–6kbar (M2) and M3 at amphibolite facies conditions. LA-ICPMS zircon U-Pb dating and trace element analysis show that the high pressure metamorphism occurred at 245–251Ma. Protolith age of the mafic granulite is 997Ma, similar to that of the mafic to ultramafic rocks widely distributed in the Cathaysia Block and also along the Jiangnan belt. Subduction of ancient oceanic lithospheric materials (or crustal thickening) during Mesozoic and formation of eclogites suggest that the Cathaysia Block was perhaps in the Tethyan oceanic domain at this time. The granulite formation might have been aided by Mesozoic mafic magma underplating associated with lithospheric delamination, heating and retrogression of the eclogite accompanied by rapid uplift.

Prograde and retrograde evolution of eclogite from Adrar Izzilatène (Egéré-Aleksod terrane, Hoggar, Algeria) determined from chemical zoning and pseudosections, with geodynamic implications

Adrar Izzilatène in the Egéré-Aleksod terrane of the LATEA metacraton (Hoggar, Algeria) exposes one of the best preserved examples of eclogite facies metamorphism in Hoggar. Three distinct stages of metamorphic development are recognised, namely, the pre-peak stage (M1), characterised by garnet, amphibole, epidote, quartz and rutile, the peak eclogite facies stage (M2), consisting of omphacite, garnet, edenite, epidote, quartz and rutile, and the retrograde stage (M3), where initial decompression resulted in the appearance of plagioclase, the development of pargasite+plagioclase kelyphites and finally the formation of anhydrous plagioclase+diopside coronas. Porphyroblastic omphacite has a jadeite content of up to XJd=0.36, which is the highest yet observed for eclogite facies rocks from the Tuareg Shield. Garnet growth zoning patterns are characterised by flat profiles in the cores (XAlm=0.55–0.60; XPrp=0.12–0.16; XGrs=0.26–0.30) before showing a decrease in almandine to XAlm=0.45, coupled to an increase in pyrope to XPrp=0.29 and decrease in grossular to XGrs=0.26 at the rims. Calculated P–T–MH2O pseudosections show that the prograde M1 assemblage equilibrated at 13–14kbar and 580°C, before pressure and temperature increased to 19kbar and 650–700°C at fluid-saturated conditions during peak metamorphism. Retrogression involved near-isothermal decompression to 8–9kbar and 700–750°C at fluid-undersaturated conditions. Prograde-to-peak metamorphism of the Izzilatène eclogite could have involved either oceanic or continental subduction, followed by exhumation as the area was obducted towards the LATEA metacraton during the Pan-African orogeny and the assembly of Western Gondwana.

Petrogenesis and chemogenesis of oceanic and continental orogens in Asia: Current topics, Part I

Petrogenesis and chemogenesis of oceanic and continental orogens in <scp>A</scp>sia: Current topics, <scp>Part I</scp>

Composite metamorphic history recorded in garnet porphyroblasts of Sambagawa metasediments in the Besshi region, central Shikoku, Southwest Japan

AbstractThis report describes an investigation of the composite metamorphic history recorded in garnet porphyroblasts of Sambagawa metasediments, and presents discussions of the possible distribution of eclogite facies lithologies in the Besshi region, central Shikoku, southwest Japan. Garnet grains usually show chemically composite zoning with resolved inner and overgrown outer segments, respectively. The inner segment usually contains paragonite as a sodic phase inclusion and rarely contains omphacite or glaucophane, whereas the outer segment rarely includes albite but no paragonite. The inner segment often includes quartz grains that preserve high residual pressures corresponding to the eclogite facies conditions, whereas the outer segment includes quartz grains that preserve lower residual pressures corresponding to the epidote–amphibolite facies conditions. For this composite‐zoned garnet, the assemblage of sodic phase inclusions and the values of the residual pressure of quartz inclusions imply the following successive metamorphic path: prograde eclogite facies stage → decompression and hydration reaction stage → prograde epidote–amphibolite facies stage. Metasediments containing this composite‐zoned garnet with such evidence for eclogite facies conditions are more widely distributed than the eclogitic lithologies that contain the assemblage of omphacite + garnet. These results imply that the combination of (i) the chemical composite zoning of garnet, (ii) the nature of sodic phase inclusions, and (iii) the residual pressure of quartz inclusions is useful as criteria to ascertain the true distribution of the lithologies that have experienced eclogite facies metamorphism for the Sambagawa metasediments, which was difficult to recognize in earlier studies.

Metamorphic evolution of relict lawsonite‐bearing eclogites from the (U) HP metamorphic belt in the Chinese southwestern Tianshan

AbstractIn the Chinese southwestern Tianshan (U)HP belt, former lawsonite presence has been predicted for many (U)HP metamorphic eclogites, but only a very few lawsonite grains have been found so far. We discovered armoured lawsonite relicts included in quartz, which, on its part, is enclosed in porphyroblastic garnet in an epidote eclogite H711‐14 and a paragonite eclogite H711‐29. H711‐14 is mainly composed of garnet, omphacite, epidote and titanite, with minor quartz, paragonite and secondary barroisite and glaucophane. Coarse‐grained titanite occasionally occurs in millimetre‐wide veins in equilibrium with epidote and omphacite, and relict rutile is only preserved as inclusions in matrix titanite and garnet. H711‐29 shows the mineral assemblage of garnet, omphacite, glaucophane, paragonite, quartz, dolomite, rutile and minor epidote. Dolomite and rutile are commonly rimed by secondary calcite and titanite respectively. Porphyroblastic garnet in both eclogites is compositionally zoned and exhibits an inclusion‐rich core overgrown by an inclusion‐poor rim. Phase equilibria modelling predicts that garnet cores formed at the P‐peak (490–505 °C and 23–25.5 kbar) and coexisted with the lawsonite eclogite facies assemblage of omphacite + glaucophane + lawsonite + quartz. Garnet rims (550–570 °C and ~20 kbar) grew subsequently during a post‐peak epidote eclogite facies metamorphism and coexisted with omphacite + quartz ± glaucophane ± epidote ± paragonite. The results confirm the former presence of a cold subduction zone environment in the Chinese southwestern Tianshan. The P–T evolution of the eclogites is characterized by a clockwise P–T path with a heating stage during early exhumation (thermal relaxation). The preservation of lawsonite in these eclogites is attributed to isolation from the matrix by quartz and rigid garnet, which should be considered as a new type of lawsonite preservation in eclogites. The complete rutile–titanite transition in H711‐14 took place in the epidote eclogite facies stage in the presence of an extremely CO2‐poor fluid with X(CO2) [CO2/(CO2 + H2O) in the fluid] &lt;&lt;0.008. In contrast, the incomplete rutile–titanite transition in H711‐29 may have occurred after the epidote eclogite facies stage and the presence of dolomite reflects a higher X(CO2) (&gt;0.01) in the coexisting fluid at the epidote eclogite facies stage.