High-pressure Schists Research Articles

Effect of pressure on the evolution of vitrinite graphitized mesophases: An experimental study on anthracite under high temperature and pressure

In this study, the influence of pressure on the evolution of the intermediate phase of graphitization of vitrinite was determined. Using samples of Taixi anthracite from the Rujigo Coal Mine (Ordos Basin, China), experiments were conducted in a piston cylinder press at a constant temperature of 1073 K and pressures from 500 MPa to 1 GPa. Raman spectroscopy, high-resolution transmission electron microscopy, selective electron diffraction, and electron loss energy spectra were used to characterise the microcrystalline morphology, structural order, and atomic valence states of the samples. Under confined conditions, pressure significantly promoted the graphitization process; however, the effect of different pressures on such process in vitrinite varied. Low pressure (<700 MPa) promoted an ordered growth of aromatic rings; meanwhile, a medium pressure (700–800 MPa) destroyed the microporous structure of vitrinite, leading to a less-ordered carbon layer and a sp3 metastable carbon structure resembling nano-diamond. Notably, high pressure (>900 MPa) promoted the condensation and directionalisation of the carbon layer of the vitrinite by destroying the microporous structure, resulting in a rapid growth of microcrystalline graphite. This suggests a transition to the key stage of anthracite graphitization, which may be an important feature in the formation of intermediate phases. In fact, pressure has been shown to exert an important role in the rupture of microporous structures in vitrinite, whose mechanism may be related to the sp3 metastable carbon formed between the basic structural units in different high-pressure environments. These findings provide insight into the formation mechanism of coal graphite, bringing new understanding of the transition from multi-faceted nanocrystals to graphitization in an orogenic metamorphic zone (e.g., in high-pressure schist and hornblende).

Two Phases of Metamorphism in the High-Pressure Schists in Central Inner Mongolia, China: Implications for the Tectonic Transition From Terminal Subduction of the Paleo-Asian Ocean to Continental Collision

High-pressure (HP) rocks exhumed from subduction zones usually record much warmer geotherms than numerical modelling results, as their peak mineral assemblages are always modified during the exhumation process. The decompressional metamorphic evolution of HP rocks should be considered carefully if using their P–T records to constrain the thermal structure of a subduction zone. The Ondor Sum Group, known as mélanges containing various high P/T metamorphic rocks in central Inner Mongolia, represents a fragment of subducted oceanic crust in the Early Paleozoic, However, the thermal structure of the subduction zone is unavailable due to an absence of exhumed HP rocks with P–T estimates. In this study, the HP schists were newly discovered in the Ondor Sum Group at Airgin Sum in the central Inner Mongolia. The HP schists include a garnet–phengite schist (sample EL01) and a garnet–amphibole schist (sample EL08). The petrography of these rocks and phase equilibrium modelling using THERMOCALC suggest two phases of metamorphism, both of which are characterized by clockwise P–T evolutions involving pre-peak prograde stage, peak-stage, and post-peak decompression stage. The peak P–T conditions of the early-phase metamorphism are constrained by P–T pseudosections to be ∼18 kbar/∼535°C for sample EL01 and 18 kbar/∼500°C for sample EL08. The peak P–T conditions of the late-phase metamorphism are constrained by P–T pseudosections to be ∼8 kbar/532°C in EL01 and ∼7.0 kbar/495°C in EL08. Available U–Pb data of zircons from the HP schists and a granodiorite vein using LA–ICP–MS constrain the timing of early-phase HP metamorphism in the early Paleozoic and the late-phase metamorphism supposed to be in the Devonian based on the previous reported ages. The peak P–T conditions for the early-phase metamorphism were high-P/T conditions with a thermal gradient of ∼8°C/km, pointing to a warm oceanic subduction. The overprinting late-phase metamorphism represents medium-P/T conditions with a geothermal gradient of 22–25°C/km, which we attribute to a collisional thickening process. As a result, we suggest that the HP schists in the Ondor Sum Group represent the terminal stage of subduction of the Paleo-Asian oceanic plate, and that the schists were involved in the continental collision after the closure of the Paleo-Asian Ocean in the Devonian.

PATTERNS OF DUCTILE DEFORMATION IN ATTICO-CYCLADIC MASSIF

The area of Lavrion constitutes the westernmost part of the Attico-Cycladic massif where the allochthonous Cycladic Greenschist-Blueschist unit overthrusts the para-authochthonous Basal unit. The tectonic contact of these units forms a crustal scale thrust zone which is the continuation of the Evia thrust. Our research was focused on quartz-rich schists of the overlying allochthonous unit. Combination of microstructural, finite strain data and quartz and calcite c-axis fabrics analysis was used to characterize the kinematics of rock flow within the thrust zone. The latter was formed under conditions of progressive exhumation and decompression of the high-pressure schists of the AtticoCycladic massif. A dominant top-to-the-ENE sense of shearing along the thrust zone is inferred by several shear sense criteria. The analysis of several specimens collected from various structural depths manifest that the deformation close to the thrust zone occurred under approximately plane strain conditions and was characterized by an Rxz strain ratio which fluctuates between 3 and 6.5.

A high-pressure folded klippe at Tehuitzingo on the western margin of an extrusion zone, Acatlán Complex, southern México

High-pressure (HP) rocks at Tehuitzingo, on the western margin of the HP belt within the Paleozoic Acatlán Complex (southern México), occur in a klippe that was thrust over low-grade clastic rocks. The youngest detrital zircon cluster in the low-grade rocks yielded U-Pb ages of 481±16Ma, which provide an older limit for deposition. The HP rocks are composed of metabasites, serpentinite, granite (482±3Ma) and mica schist (youngest concordant detrital zircon: 433±3Ma). The schist and granite are inferred to be high-grade equivalents of lower Paleozoic, low-grade rocks exposed elsewhere in the Acatlán Complex, from which they are inferred to have been removed by subduction erosion. Mineral analyses indicate that the subducted rocks underwent HP metamorphism and polyphase deformation at depths of ~50km (~16kbar and 750°C: eclogite facies). Subsequent retrogression passed through epidote-amphibolite to greenschist facies, which was synchronous with W-vergent thrusting over the low-grade clastic rocks. Deposition of the low-grade rocks and thrusting are bracketed between either 481–329Ma (Ordovician-Mississippian), and was followed by F3 synformal folding. Cooling through ca. 385°C is indicated by 329±1 and 316–317±2Ma, 40Ar/39Ar muscovite plateau ages in HP rocks, which are 5–17 my younger than those of the adjacent Piaxtla eclogites suggesting younger exhumation. The petrology, P-T conditions and ages of the Piaxtla Suite is consistent with an extrusion channel within the Acatlán Complex along the active western margin of Pangea during the Carboniferous. Detrital zircon populations in the low-grade psammite (ca. 481, 520–650, 720, 750, 815, 890, 1050 and 2750Ma) and the HP schist (ca. 457–480, 534, 908, 954–1150, 1265, 1845 and 2035Ma) indicate derivation from the Ordovician Acatlán granitoids, Neoproterozoic Brasiliano orogens, 900–750Ma Goiás arc (Amazonia), 1–1.3Ma Oaxaquia, and more ancient sources in Oaxaquia/Amazonia.

Age (K–Ar phengite)–temperature–structure relations: a case study from the Ishigaki high-pressure schist belt, southern Ryukyu Arc, Japan

AbstractThe Ishigaki high-pressure schist belt in the southern Ryukyu Arc is correlated with the Suo high-pressure schist belt in southwest Japan. The former metamorphic sequence is composed mainly of basic and pelitic schists and is subdivided into three zones, the lower-grade zone A, the medium-grade zone B and the high-grade zone C, based on the mineral assemblages of the basic schists. The K–Ar phengite age gives 188–205 Ma for zone A, 196–206 Ma for zone B and 208–220 Ma for zone C, while the apparent d002 spacing of carbonaceous materials is 3.590–3.437 Å, 3.415–3.390 Å and 3.387–3.364 Å, respectively. The age–d002 relationships suggest that the ages become older with increasing metamorphic temperature. This positive age–temperature relationship in the Ishigaki area contrasts with a negative relationship in the Nishiki area in the Suo belt. The two areas also display a contrasting thermal structure with the former area having an inverted metamorphic gradient and the latter displaying a normal thermal structure. These contrasting age–temperature–structure relationships in the metamorphic belt could be due to different tectonic styles relating to the exhumation of the metamorphic sequences. We suggest that the ages obtained are related directly to the ductile deformation history of the matrix phengite below the closure temperature (500°C) during exhumation of the host rocks. The duration from the beginning of exhumation to the apparent resetting of the phengite K–Ar system was different between the two metamorphic sequences, and significantly longer in the Ishigaki than the Nishiki.

Structural contacts in subduction complexes and their tectonic significance: the Late Palaeozoic coastal accretionary wedge of central Chile

Understanding the contact between the very low-grade metagreywacke of the Eastern Series and high-pressure metamorphosed schist of the Western Series in the Late Palaeozoic accretionary wedge of central Chile is fundamental for the understanding of the evolution of ancient accretionary wedges. We show the progressive development of structures and finite strain from the least deformed rocks in the eastern part of the Eastern Series of the accretionary wedge to high-pressure schist of the Western Series at the Pacific coast. Upright chevron folds of sedimentary layering are associated with an axial-plane foliation, S 1 . As the F 1 folds became slightly overturned to the west, S 1 was folded about west-vergent open F 2 folds and an S 2 axial-plane foliation developed. Near the contact between the Western and Eastern Series S 2 represents a penetrative subhorizontal transposition foliation. Towards the structurally deepest units in the west the transposition foliation becomes progressively flattened. Finite-strain data as obtained by R f /ϕ analysis in metagreywacke and X-ray texture goniometry in phyllosilicate-rich rocks show a smooth and gradual increase in strain magnitude from east to west. Overturned folds and other shear-sense indicators show a uniform top-to-the-west shear sense in moderately deformed rocks, whereas the shear sense is alternating top-to-the-west and top-to-the-east in the strongly flattened high-pressure rocks of the Western Series near the Pacific coast. We interpret the progressive structural and strain evolution across the contact between the two series to reflect a continuous change in the mode of accretion in the subduction wedge. Initially, the rocks of the Eastern Series were frontally accreted to the pre-Andean margin before c . 300 Ma. Frontal accretion caused horizontal shortening, and upright folds and subvertical axial-plane foliations developed. At c . 300 Ma the mode of accretion changed and the rocks of the Western Series were underplated below the Andean margin. This basal accretion caused a major change in the flow field within the wedge and gave rise to vertical shortening and the development of the penetrative subhorizontal transposition foliation. Subsequent differential exhumation was resolved gradually over a wide region, implying that exhumation was not tectonically controlled.

Paleozoic ophiolites and blueschists in Japan and Russian Primorye in the tectonic framework of East Asia: A synthesis

Abstract Ophiolites and high‐pressure (HP) metamorphic rocks are studied to test continuation of Paleozoic and early Mesozoic geological units from Japan to Primorye over the Japan Sea. The early Paleozoic ophiolites are present on both sides, and the late Paleozoic ophiolite of south‐western Japan may also have its counterpart in Primorye. The Shaiginskiy HP schist and the associated Avdakimov gneiss in Primorye, both tectonically underlying the early Paleozoic ophiolitic complex, yield a 250‐Ma phengite and hornblende K–Ar age, which is intermediate between those of the Renge (280–330 Ma) and Suo (170–220 Ma) blueschists in south‐western Japan. This age also coincides with that of the coesite‐bearing eclogites in the Sulu–Dabie suture in China and several medium‐pressure metamorphic rocks in East Asia. On the basis of these results and other geological data, the authors propose the ‘Yaeyama promontory’ model for an eastward extension of the Sulu–Dabie suture. The collision suture warps southward into the Yellow Sea and detours around Korea, turns to the north at Ishigaki Island in the Yaeyama Archipelago of Ryukyu, where it changes into a subduction zone and further continues toward south‐western Japan and Primorye. Most ophiolites from this area represent crust–mantle fragments of an island arc–back‐arc basin system, and the repeated formation of ophiolite–blueschist associations may be due to the repetition of the Mariana‐type non‐accreting subduction and Nankai‐type accreting subduction.

Oscillating modes of orogeny in the Southwest Pacific and the tectonic evolution of New Caledonia

Abstract This paper presents a new interpretation of the tectonic evolution of New Caledonia, based on the extrapolation of detailed structural analysis at several different scales. The coherent high-pressure schist belt of northern New Caledonia contains clear evidence for cyclicity in the orogenic process. Two switches from large-scale crustal shortening to extensional tectonism can be recognized. We propose that orogenesis initially resulted in significant crustal thickening, obduction, and high-pressure (15–20 kbar) metamorphism. There is a close temporal link between ophiolite emplacement and high-pressure metamorphism. The high-pressure rocks were exhumed during the first period of extensional tectonism ( c. 40–36 Ma) as a large coherent terrane (&gt;1500 km 2 ). During the second period of crustal shortening the New Caledonia orogen was folded into upright megafolds. Basin and Range style normal faulting then took place, bringing the folded extensional terrane and the high-pressure rocks to their present crustal level. Metamorphic grade changes in the high-pressure belt are structurally controlled by these late-stage normal faults. However, movement on these faults is responsible for only the very last stages of exhumation.

Tectonic evolution of the Bantimala Complex, South Sulawesi, Indonesia

Abstract The Bantimala Complex of South Sulawesi, Indonesia is an assemblage of northeast-dipping tectonically stacked slices. The slices consist mainly of high pressure metamorphic rocks, radiolarian chert, breccia, sandstone and shale, and melange. In order to understand the tectonic evolution of the Bantimala Complex, we have investigated the lithology, age, stratigraphy, structure and relationships of the components. The K-Ar ages of high P-low T metamorphic rocks suggest that an oceanic plate subducted beneath the Sundaland continent during the Late Jurassic or earliest Cretaceous. The subduction ceased during the Albian, and the high pressure schists were exhumed and eroded at the surface before and during the deposition of middle Cretaceous radiolarian chert. The exhumation of the schists was related to the collision of microcontinents derived from Gondwanaland. The Jurassic shallow marine sedimentary rocks in the Bantimala Complex are possibly remnant fragments of the collided microcontinent. Tectonic stacking of the Bantimala Complex was caused by Neogene subduction and collision of another continental fragment further to the east.

Spessartine garnet+Na-pyroxene-bearing schists from the Horokanai-Kamietanbetsu area in the Kamuikotan zone, Hokkaido, Japan.

Spessartine garnet + Na-pyroxene-bearing schists occur as boudins or lenses in the highpressure schists of the Kamuikotan zone in the Horokanai-Kamietanbetsu area. Their mineral assemblages are spessartine garnet + Na-pyroxene + Na-amphibole + quartz ± chlorite. The Fe-Mg partition coefficient for garnet-pyroxene equilibrium is 91, and the Fe-Mg, Mn-Fe and Mn-Mg partition coefficient for garnet-chlorite equilibrium are 35, 55 and 1913, respectively. Formation temperature obtained is about 300°C, assuming 9.5 kbar pressure. The mineral assemblage of spessartine garnet + Na-pyroxene in lower temperature zone is favored by high-pressure metamorphism.

Microtectonics and 39Ar- 40Ar dating of high pressure metamorphic rocks of the south Ryukyu Arc and their bearings on the pre-Eocene geodynamic evolution of Eastern Asia

The pre-Eocene metamorphic rocks found in the Yaeyama Islands (the southern part of the Ryukyu Arc) have disputed relationships with the metamorphic formations of Japan and Taiwan. They are divided into the Tomuru and Fusaki formations. 1. (1) The Tomuru Formation is composed of high pressure (HP) metamorphic rocks derived from oceanic sediments and the uppermost oceanic crust. The first deformation phase is characterized by a synmetamorphic NW-SE trending stretching lineation ( L 1). Kinematic analysis shows that the deformation corresponds to a subhorizontal ductile shear directed along L 1 from the northwest to the southeast. 2. (2) The Fusaki Formation or chaotic formation is an olistostrome of Triassic or Lower Jurassic radiolarite olistoliths. The age of the matrix is assumed to be late Jurassic. It is slightly schistose and only a S-vergent deformation phase is observed. It is assumed that the HP schists overthrust the Mesozoic olistostrome. 39Ar- 40Ar step-heating experiments have been carried out on phengite and Na amphibole from the Tomuru Formation. Phengite and zoned barroisite-crossite give well defined plateaus at 225 ± 4.8 Ma and 237 ± 6.3 Ma respectively. They are interpreted as the age of the ductile deformation. Crossite yields an integrated data of 104.8 ± 8.3 Ma, interpreted as the age of the thrusting of the HP schists above the chaotic formation. The lithostratigraphy, petrology, microtectonics and 39Ar- 40Ar ages of the HP schists and the chaotic formation are similar to the Permian Sangun HP schists and the Jurassic Tanba olistostrome of the Inner Belt of southwestern Japan respectively. There, the Sangun schists also overthrust the Tanba olistostrome during the Mesozoic orogeny. The same Tanba-like Mesozoic olistostrome is also found in Taiwan, but was intensely deformed and metamorphosed in Miocene times. The geodynamic collision model proposed to account for the late Permian to early Triassic orogen of southwestern Japan is extended to the South Ryukyu and Taiwan. Conversely, the Mesozoic Sanbagawa HP schists and the upper Jurassic to lower Miocene turbidites and olistostrome (Sanbosan-Shimanto zones) of southwestern Japan are neither exposed in the south Ryukyu Arc, nor in Taiwan. Hypotheses are discussed that call upon tectonic erosion or non-deposition to account for this absence.

Sulfides, oxides and sphene in high-pressure schists from New Caledonia

In the New Caledonia high-pressure schists pyrite, pyrrhotite, chalcopyrite, rutile and sphene are common phases while hematite and ilmenite are rare and magnetite is absent. The parageneses of these minerals were clarified from their occurrence as inclusions in garnet, from phase relations in the Cu-Fe-S and Fe-Ti-O-S systems, and from phase rule considerations for the multi-component system. The sulfur fugacity estimated for pelites and basites containing pyrrhotite, pyrite and rutile increased with increasing metamorphic grade; the oxygen fugacity in these schists was less than 10−27.6 bars at 400° C, 10 kb and 10−22.3 bars at 500° C, 11 kb. Among the other components of the metamorphic fluid in pelites, H2O was major, CH4, CO2 and H2S minor, and H2, CO, COS and SO2 rare. The fluid composition altered with advancing metamorphic grade, such that H2O decreased while CO2, CH4 and H2S increased, and this change was linked to concurrent massive decarbonization in the rock matrices.

Comparison of the high-pressure schist belts of New Caledonia and Sanbagawa, Japan

The high-pressure schist terranes of New Caledonia and Sanbagawa were developed along the oceanic sides of sialic forelands by tectonic burial metamorphism. The parent rocks were chemically similar, as volcanic-sedimentary trough or trench sequences, and metamorphic temperatures in both belts were 250° to 600° C. From phase equilibria curves, total pressures were higher for New Caledonia (6–15 kb) than for Sanbagawa (5–11 kb) and the estimated thermal gradients were 7–10° C/km and 15° C/km respectively.PT paths identify the higher pressure in New Caledonia (P differences 2 kb at 300° C and 4 kb at 550° C) with consequent contrast in progressive regional metamorphic zonation for pelites in the two areas: lawsonite-epidote-omphacite (New Caledonia) and chlorite-garnet-biotite (Sanbagawa). In New Caledonia the Na-amphibole is dominantly glaucophane and Na-pyroxenes associated with quartz are Jadeite (Jd95–100) and omphacite; in Sanbagawa the amphibole is crossite or riebeckite and the pyroxene is omphacite (Jd50). For both areas, garnet rims show increase in pyrope content with advancing grade, but Sanbagawa garnets are richer in almandine. Progressive assemblages within the two belts can be equated by such reactions as:New Caledonia Sanbagawa glaucophane+paragonite+H2O⇌albite+chlorite+quartz glaucophane+epidote+H2O⇌albite+chlorite+actinolite and the lower pressure Japanese associations appear as retrogressive phases in the New Caledonia epidote and omphacite zones. The contrasts inPT gradient, regional zonation and mineralogy are believed due to differences in the tectonic control of metamorphic burial: for New Caledonia, rapid obduction of an upper sialic plate over an inert oceanic plate and sedimentary trough; and for Sanbagawa, slower subduction of trench sediments beneath a relatively immobile upper plate.

Coalification and graphitization in high-pressure schists in New Caledonia

The northern portion of the Tertiary high-pressure schist belt in New Caledonia contains, from west to east, a metamorphic progression from lawsonite-albite facies through glaucophanitic greenschists to eclogitic albite-epidote amphibolites. This belt is flanked to the west by Upper Cretaceous-Eocene metasediments, of prehnite-pumpellyite grade. Paraschists throughout this whole sequence contain abundant carbonaceous material which shows a progressive metamorphism from coal to graphite. Structural analysis of lithostatic load and oxygen isotope data have provided a PT profile for the carbon metamorphism. In the prehnite-pumpellyite metasediments, phytoclasts were progressively coalified to anthracite rank under PT conditions which extended up to 3 kb/255 ° C at the lawsonite isograd where graphite first appears. On the high grade side of the lawsonite isograd a transitional mixed zone of continued coalification and graphitization occurred within the PT range 3 kb/255 ° C to 5.5 kb/335 ° C which included the ferroglaucophane isograd. Immediately beyond this zone all phytoclasts were completely graphitized before the epidote isograd was reached at 6.3 kb/ 390 ° C. The prevailing metamorphic environment retarded coalification, but accelerated graphitization, under conditions of high pressure and a low temperature gradient (7 ° C/km) that had been generated within the sedimentary pile by rapid tectonic thickening and consequent deep burial.

Blueschist ophiolites in the melange zone, northern New Caledonia

A regional melange zone, 150 km long and 30 km wide, forms the southern boundary and structural capping to a high-pressure blueschist belt in northern New Caledonia. The disrupted country rocks in the melange zone are Mesozoic metagrey-wackes and Eocene chert-limestone sequences which have been penetrated from below by tectonically-injected ophiolite slivers containing metamorphosed serpentinite, gabbro, dolerite, basalt, tuff, chert and shale. An ocean crust origin for these rocks is indicated by chemical, mineralogical and radiometric data from coastal outcrops at Anse Ponandou on the northeast coast. The age (41 m.y.), metamorphic environment (350 ° C at 7 kb), and mineral association (acmitic jadeite-riebeckite-pyropic spessartine-pistacitic epidote-lawsonite-high Si phengite) are significantly different from those of the adjacent regional high-pressure schist belt, indicating a separate structural site for blueschist metamorphism of buried ophiolitic ocean crust during early Tertiary orogenesis.

High-pressure schists in Northern New Caledonia

High-pressure schists in Northern New Caledonia