Peak Pressure Conditions Research Articles

Two‐stage Variscan metamorphism in the Canigou massif: Evidence for crustal thickening in the Pyrenees

AbstractThe Variscan metamorphism in the Pyrenees is dominantly of the low‐pressure–high‐temperature (LP‐HT) type. The relics of an earlier, Barrovian‐type metamorphism that could be related to orogenic crustal thickening are unclear and insufficiently constrained. A microstructural and petrological study of micaschists underlying an Ordovician augen orthogneiss in the core of the Canigou massif (Eastern Pyrenees, France) reveals the presence of two syntectonic metamorphic stages characterized by the crystallization of staurolite (M1) and andalusite (M2), respectively. Garnet is stable during the two metamorphic stages with a period of resorption between M1 and M2. The metamorphic assemblages M1 and M2 record similar peak temperatures of 580°C at different pressure conditions of 5.5 and 3 kbar, respectively. Using chemical zoning of garnet and calculated P–T pseudosections, a prograde P–T path is constrained with a peak pressure at ~6.5 kbar and 550°C. This P–T path, syntectonic with respect to the first foliation S1, corresponds to a cold gradient (of ~9°C/km), suggestive of crustal thickening. Resorption of garnet between M1 and M2 can be interpreted either in terms of a simple clockwise P–T path or a polymetamorphic two‐stage evolution. We argue in favour of the latter, where the medium‐pressure (Barrovian) metamorphism is followed by a period of significant erosion and crustal thinning leading to decompression and cooling. Subsequent advection of heat, probably from the mantle, leads to a new increase in temperature, coeval with the development of the main regional fabric S2. LA‐ICP‐MS U–Th–Pb dating of monazite yields a well‐defined date at c. 300 Ma. Petrological evidence indicates that monazite crystallization took place close to the M1 peak pressure conditions. However, the similarity between this age and that of the extensive magmatic event well documented in the eastern Pyrenees suggests that it probably corresponds to the age of monazite recrystallization during the M2 LP‐HT event.

Monazite as a monitor for melt‐rock interaction during cooling and exhumation

AbstractGranulite facies cordierite–garnet–biotite gneisses from the southeastern Reynolds Range, central Australia, contain both orthopyroxene‐bearing and orthopyroxene‐free quartzofeldspathic leucosomes. Mineral reaction microstructures at the interface of gneiss and leucosome observed in outcrop and petrographically, reflect melt‐rock interaction during crystallization. Accessory monazite, susceptible to fluid alteration, dissolution and recrystallization at high temperature, is tested for its applicability to constrain the chemical and P–T–time evolution of melt‐rock reactions during crystallization upon cooling. Bulk rock geochemistry and phase equilibria modelling constrain peak pressure and temperature conditions to 6.5–7.5 kbar and ~850°C, and U–Pb geochronology constrains the timing of monazite crystallization to 1.55 Ga, coeval with the Chewings Orogeny. Modelling predicts the presence of up to 15 vol.% melt at peak metamorphic conditions. Upon cooling below 800°C, melt extraction and in situ crystallization of melt decrease the melt volume to less than 7%, at which time it becomes entrapped and melt pockets induce replacement reactions in the adjacent host rock. Replacement reactions of garnet, orthopyroxene and K‐feldspar liberate Y, REE, Eu and U in addition to Mg, Fe, Al, Si and K.We demonstrate that distinguishing between monazite varieties solely on the basis of U–Pb ages cannot solve the chronological order of events in this study, nor does it tie monazite to the evolution of melt or stability of rock‐forming minerals. Rather, we argue that analyses of various internal monazite textures, their composition and overprinting relations allow us to identify the chronology of events following the metamorphic peak. We infer that retrograde reactions involving garnet, orthopyroxene and K‐feldspar can be attributed to melt‐rock interaction subsequent to partial melting, which is reflected in the development of compositionally distinct monazite textural domains. Internal monazite textures and their composition are consistent with dissolution and precipitation reactions induced by a high‐T melt. Monazite rims enriched in Y, HREE, Eu and U indicate an increased availability of these elements, consistent with the breakdown of orthopyroxene, garnet and K‐feldspar observed petrographically. Our study indicates that compositional and textural analysis of monazite in relation to major rock‐forming minerals can be used to infer the post‐peak chemical evolution of partial melts during high‐ to ultrahigh‐temperature metamorphism.

Tectonic Mode Switches Recorded at the Northern Edge of the Australian Plate During the Pliocene and Pleistocene

AbstractWe report new data from medium‐high grade metamorphic rocks found at the northern margin of the Lengguru Fold Belt in West Papua. The study involved a systematic analysis of cross‐cutting structures to establish the relative timing of deformation, together with isotopic dating to define when these tectono‐thermal events occurred. These data show that the region underwent multiple episodes of deformation within the last six million years. Metamorphic mineral growth was associated with the development of ductile shear zones. This episode occurred during a phase of crustal stretching associated with the formation of a metamorphic core complex. Metamorphic zircon growth at 4.9 to 5.3 Ma was documented in two of the dated samples. These data are interpreted to postdate the peak pressure and temperature conditions of the phase of regional crustal stretching. The shear fabrics associated with the metamorphic core complex were later overprinted by at least two generations of folds. The change in mode from crustal extension to shortening reflects a tectonic mode switch. A subsequent mode switch is documented by numerous brittle extensional faults that cross‐cut the earlier formed ductile fabrics. We interpret ca. 0.75–1.51 Ma (U–Th)/He age data to reflect cooling associated with the later stages of crustal shortening (marked by folds) or the later extensional unroofing of the peninsula. This work demonstrates that an orogen can record multiple tectonic mode switches within several million years. These outcomes should be considered in studies of ancient orogens where analytical uncertainties associated with isotopic dating may mask short‐lived mode switches.

Eocene high-pressure metamorphism and Oligocene retrogression on Naxos, Cyclades, Greece: Significance for Aegean tectonics and 40Ar/39Ar dating in polyphase metamorphic rocks

In this study, we report new microfabrics, pressure–temperature estimates and 40Ar/39Ar ages from high-pressure (HP) rocks of southeastern Naxos in Cyclades, Aegean Sea. Most metapelitic rocks contain two generations of white mica, high-Si phengites crystallized during HP metamorphism and low-Si white mica formed during subsequent retrogressive overprint. 40Ar/39Ar white mica dating of five white mica samples yielded ages of ca. 51–40 Ma, with a mean at 45 Ma for the peak pressure conditions and ca. 35–29 Ma for the retrogressive greenschist-facies overprint, as well as ca. 16 Ma for the strongly sheared rocks. Even though the overprint at the greenschist-facies conditions is at ca. 350 °C and 5 kbar, the phengite ages show that the white mica still records the age of the HP metamorphism. This implies that further factors such as deformed microstructure processes and fluid flow along with temperature-controlled diffusion contributed to Ar loss. The age and conditions of this first stage of greenschist-facies overprint at 35–29 Ma are similar over a large area of the southern Aegean Sea. This event represents an exhumation event, when the continental Adriatic microplate entered the subduction zone. The greenschist-facies marked a thermal pulse during exhumation. Stacking of passive margin cover successions of the subducted continental crust in a caterpillar-like mode would have triggered this, as previously suggested by modelling work.

Metamorphic P-T Path Differences between the Two UHP Terranes of Sulu Orogen, Eastern China: Petrologic Comparison between Eclogites from Donghai and Rongcheng

The Sulu Orogen constitutes the eastern part of the Sulu-Dabie Orogen formed by Triassic collision between the Sino-Korean and Yangtze plates. An HP Slice I and two UHP slices II and III with contrasting subduction and exhumation histories within the Sulu Orogen were postulated. This study presents the metamorphic P-T paths of eclogites from the two UHP belts constructed by petrography, mineral chemistry and Perple_X P-T pseudosection modeling in the MnC(K)NFMASHO system. Eclogites from Slice III mainly consist of omphacite, garnet and quartz, with minor rutile, ilmenite, amphibole and phengite. Eclogites from Slice II show a porphyroblastic texture with epidote porphyroblasts and garnet, omphacite, phengite, quartz and rutile in matrix. Pseudosection modeling reveals that eclogites from Slice II witness a peak metamorphism of eclogite-facies under conditions of 3.1–3.3 GPa and 660–690 oC, and a retrograde cooling decompression process. The eclogites from Slice III record a heating decompressive P-T path with a peak-P stage of 3.2 GPa and 840 oC and a peak-T stage of 2.4 GPa and 950 oC, suggesting an apparent granulite-facies metamorphism overprint during exhumation. Both eclogites recorded clockwise P-T paths with peak P-T conditions suggesting a subduction beneath the Sino-Korean Plate to ~100–105 km depth. Combined with tectonic scenarios from previous studies, it is concluded that the two UHP crustal slices in the Sulu terrane have a similar geodynamic evolution, but the UHP rocks in Slice II exhumed after the eclogitic peak-pressure conditions earlier than that of Slice III. The existence of Slice II diminished the buoyancy force on Slice III, resulting in a granulite-facies overprint on Slice III. The Sulu orogenic belt is made up of different crustal slices that underwent different subduction and exhumation histories, rather than a single unit.

Tectonic slicing and mixing processes along the subduction interface: The Sistan example (Eastern Iran)

Suture zones preserve metamorphosed relicts of subducted ocean floor later exhumed along the plate interface that can provide critical insights on subduction zone processes. Mélange-like units are exceptionally well-exposed in the Sistan suture (Eastern Iran), which results from the closure of a branch of the Neotethys between the Lut and Afghan continental blocks. High pressure rocks found in the inner part of the suture zone (i.e., Ratuk complex) around Gazik are herein compared to previously studied outcrops along the belt. Detailed field investigations and mapping allow the distinction of two kinds of subduction-related block-in-matrix units: a siliciclastic-matrix complex and a serpentinite-matrix complex. The siliciclastic-matrix complex includes barely metamorphosed blocks of serpentinized peridotite, radiolarite and basalt of maximum greenschist-facies grade (i.e., maximum temperature of 340 °C). The serpentinite-matrix complex includes blocks of various grades and lithologies: mafic eclogites, amphibolitized blueschists, blue-amphibole-bearing metacherts and aegirine-augite-albite rocks. Eclogites reached peak pressure conditions around 530 °C and 2.3 GPa and isothermal retrogression down to 530 °C and 0.9 GPa. Estimation of peak PT conditions for the other rocks are less-well constrained but suggest equilibration at P < 1 GPa. Strikingly similar Ar–Ar ages of 86 ± 3 Ma, along ~70 km, are obtained for phengite and amphibole from fourteen eclogite and amphibolitized blueschist blocks. Ages in Gazik are usually younger than further south (e.g., Sulabest), but there is little age difference between the various kinds of rocks. These results (radiometric ages, observed structures and rock types) support a tectonic origin of the serpentinite-matrix mélange and shed light on subduction zone dynamics, particularly on coeval detachment and exhumation mechanisms of slab-derived rocks.

Timing and duration of Variscan high-pressure metamorphism in the French Massif Central: A multimethod geochronological study from the Najac Massif

Accurate dating of eclogite-facies metamorphism is of paramount importance in order to understand the tectonic evolution of an orogen. An eclogite sample from the Najac Massif (French Massif Central, Variscan belt) displays a zircon-bearing garnet-omphacite-amphibole-rutile-quartz peak assemblage. Pseudosection modeling suggests peak pressure conditions of 15–20 kbar, 560–630 °C. Eclogite-facies garnet displays Lu-enriched cores and Sm-rich rims and yields a Lu-Hf age of 382.8 ± 1.0 Ma and a Sm-Nd age of 376.7 ± 3.3 Ma. The ages are interpreted as marking the beginning of the prograde garnet growth during the initial stages of the eclogite-facies metamorphism, and the high-pressure (and temperature) peak reached by the rock, respectively. Zircon grains display chondrite-normalized REE spectra with variably negative, positive or no Eu anomalies and are characterized by either enriched or flat HREE patterns. However, they yield a well constrained in situ LA-ICP-MS U-Pb age of 385.5 ± 2.3 Ma, despite this REE pattern variability. Zr zonation in garnet, Y content in zircon and the diversity of zircon HREE spectra may suggest that zircon crystallized prior to and during incipient garnet growth on the prograde P-T path, recording the initial stages of the eclogite-facies conditions. Consequently, the zircon age of 385.5 ± 2.3 Ma, comparable within error with the Lu-Hf age obtained on garnet, is interpreted as dating the beginning of the eclogite-facies metamorphism. Accordingly, the duration of the prograde part of the eclogite-facies event is estimated at 6.1 ± 4.3 Myr. Subsequent exhumation is constrained by an apatite U-Pb age at 369 ± 13 Ma.

Exhumation rates in the Gran Paradiso Massif (Western Alps) constrained by in situ U–Th–Pb dating of accessory phases (monazite, allanite and xenotime)

Exhumation rates for high-pressure metamorphic rocks need to be carefully estimated to decipher tectonic processes in subduction/collision belts. In the Gran Paradiso Massif (Western Alps), the Money Unit crops out as a tectonic window below the Gran Paradiso Unit. According to previous studies, the Gran Paradiso and Money Units reached peak pressure conditions at ~ 18 to 20 kbar, 480–520 °C and ~ 17 to 18 kbar, 500–550 °C, respectively. This yields a maximum difference of ~ 9 to 10 km in the subduction depth reached by these two units during the Alpine history. Thrusting of the Gran Paradiso Unit over the Money Unit led to the simultaneous development of the main foliation under the same metamorphic conditions (~ 12.5 to 14.5 kbar and 530–560 °C) in both units. The thrust contact was subsequently folded and then both units were exhumed together. The relative timing of the growth and dissolution of the accessory phases was assessed by combining thermodynamic modelling with inclusion, textural and chemical (major and trace element) data from both major and accessory phases. The age of monazite constrained the high-pressure metamorphism in both the Gran Paradiso Unit and the Money Unit at 41.5 ± 0.3 and 42.0 ± 0.6 Ma, respectively. Allanite replacing monazite in the matrix has been dated at 32.7 ± 4.2 Ma. The late growth of xenotime associated with the crystallization of biotite pseudomorphs at the expense of garnet (at about 10 kbar) was dated at 32.3 ± 1.0 Ma. Our petrochronological data indicate about 10 m.y. between the peak pressure conditions and the crystallization of xenotime leading to an exhumation rate of the order of 2.2–5 mm/year. The new ages allow to better constrain the timing of the displacement of the thrust defining the lower boundary of the extruding wedge of eclogite-facies rocks.

Late Cretaceous eclogite in the Eastern Rhodopes (Bulgaria): evidence for subduction under the Sredna Gora magmatic arc

The Rhodopes in Bulgaria and Greece represent a nappe stack of high-grade units with polymetamorphic history. Constraining the time of metamorphism in individual subunits is essential for unraveling the controversial framework of subduction, exhumation and nappe stacking. Here we present new evidence for Late Cretaceous high-pressure metamorphism in the Eastern Rhodopes. In eclogite from the Byala Reka–Kechros Dome (Kazak eclogite), garnet growth is dated at 81.6 ± 3.5 Ma by Lu–Hf chronometry, indicating that prograde eclogite-facies metamorphism occurred during the Late Cretaceous. Petrological data and modeling suggest peak-pressure conditions of 1.2–1.6 GPa, 570–620 °C. We propose that metamorphism took place in a subduction zone dipping towards north under the Sredna Gora section of the Apuseni–Banat–Timok–Sredna Gora continental magmatic arc. Eclogite-facies metamorphism coincides with the main phase of granitoid intrusions in the Sredna Gora Zone. The site of magmatic activity in this area shifted southward during the Late Cretaceous and arrived in the Eastern Rhodopes at ~ 69 Ma, as shown by granite intrusions of that age only 4 km north of the locality of the dated eclogite sample. This proximity may be explained by south-directed rollback of the subduction zone, although also post-69 Ma tectonic displacement has to be considered. Together with published age data from other parts of the Rhodopes, the new data confirm that multiple subduction events took place between ~ 200 and ~ 40 Ma along this section of the southern European plate boundary.

Reconstruction of multiple P-T-t stages from retrogressed mafic rocks: Subduction versus collision in the Southern Brasília orogen (SE Brazil)

The identification of markers of subduction zones in orogenic belts requires the estimation of paleo-geothermal gradients through pressure-temperature-time (P-T-t) estimates in mafic rocks that potentially derive from former oceanic units once. However, such markers are rare in supracrustal sequences specially in deeply eroded and weathered Precambrian orogens, and reconstructing their metamorphic history is challenging because they are commonly retrogressed and only preserve a few mineral relicts of high-pressure metamorphism. Metamorphosed mafic rocks from Pouso Alegre region of the Neoproterozoic Southern Brasília Orogen outcrop as rare lenses within continental gneisses. They have previously been classified as retrograde eclogites, based on the presence of garnet and the characteristic symplectitic texture replacing omphacite. These rocks were interpreted to mark the suture zone between the Paranapanema and São Francisco cratons. To test the possible record of eclogitic conditions in the Pouso Alegre mafic rocks, samples including the surrounding felsic rocks have been investigated using quantitative compositional mapping, forward thermodynamic modeling and in-situ dating of accessory minerals to refine their P-T-t history. In the metamorphosed mafic rocks, the peak pressure assemblage of garnet and omphacite (Jd20, reconstructed composition) formed at 690±35°C and 13.5±3.0kbar, whereas local retrogression into symplectite or corona occurred at 595±25°C and 4.8±1.5kbar. The two reactions were coupled and thus took place at the same time. A zircon U-Pb age of 603±7Ma was obtained for metamorphic rims and linked to the retrogression stage. Monazite and metamorphic zircon U-Th-Pb ages for the surrounding rocks are at ca. 630Ma and linked to peak pressure conditions similar to the one recorded by the mafic rocks. The low maximal pressure of 14kbar and the high geothermal gradient do not necessarily support subduction process-related metamorphism but, more likely, metamorphism related to continental collision.

Computational Analysis of an Early Direct Injected HCCI Engine Using Bio Ethanol and Diesel Blends as Fuel

Abstract In this paper the work deals with the Experimental analysis of the early direct injected HCCI. The computational analysis of the engine was carried out using CHEMKIN-PRO Software. The computational analysis was carried out using the auto ignition chemistry by means of reduced chemical kinetics. For these investigations, the neat diesel and bio ethanol - diesel blend (E20) fuels were used as fuel and the pressure, combustion and emission characteristics were studied in the equivalence ratio of 0.6 (ɸ = 0.6). The injection timing was advanced to 18° rather than the normal 23° before top dead centre (BTDC) as specified engine conditions. Since pressure and temperature profiles plays a vital role in reaction path at certain operating conditions, an effort has been made here to present a comprehensive reaction path analysis on the formation/destruction of chemical species at peak temperature and pressure conditions.

Eclogite-, amphibolite- and blueschist-facies rocks from Diego de Almagro Island (Patagonia): Episodic accretion and thermal evolution of the Chilean subduction interface during the Cretaceous

Few localities in the Patagonian Andes expose remnants of the Mesozoic Chilean paleo-accretionary complex. We focus on the Diego de Almagro Island high-pressure/low-temperature (HP/LT) Complex, a pluri-kilometer thick sequence comprising metavolcanic rocks with oceanic affinities and metasedimentary rocks. In this study, the deepest segments of the Chilean subduction interface in Patagonia are characterized for the first time. Despite its apparent homogeneity, the complex is actually composed of two tectonic units with distinct ages of metamorphism and thermal evolution: the garnet amphibolite (GA) and the underlying blueschist (BS) units.The GA unit mafic rocks exhibit epidote, phengite, titanite, rutile, chloritoid and paragonite inclusions in prograde garnet I, diopside + albite intergrows replacing omphacite inclusions in garnet II, and relict omphacite (XJd45) included in edenitic-pargasitic amphiboles. Thermobarometric results show that these rocks were buried along a relatively cold prograde path (c. 11°C/km) and reached eclogite-facies near peak pressure conditions (c. 550–600°C, 1.6 GPa). The GA unit underwent a pervasive stage of amphibolitization during decompression at c. 1.3 GPa. Field and petrological observations, together with multi-mineral Rb-Sr dating, indicate that amphibolitization of the GA unit took place along the subduction interface at c. 120Ma in a slightly warmer subduction regime (c. 13–14°C/km), in agreeement with the formation of coetanoeus amphibolites at c. 35km. The underlying BS unit (i) yields four consistent Rb-Sr deformation ages of c. 80Ma, i.e. 40Ma younger than the overlying rocks from the GA unit; (ii) exhibits slightly cooler peak metamorphic conditions (c. 520–550°C, 1.6 GPa) indicating burial along a prograde path of c. 10°C/km (iii) does not show amphibolite-facies overprint as seen in the GA unit. After a long residence time under amphibolite-facies conditions, the amphibolitized rocks of the GA unit partly recrystallized during blueschist-facies deformation at around 80Ma, as shown by silica-richer rims around phengite and glaucophane overgrowths around hornblende.These new results: (i) shed light on the dynamics of the deep Chilean paleo-accretionary wedge, (ii) reveal that the GA unit underwent two stages of exhumation, likely juxtaposed with the BS unit near 40km depth, and (iii) suggest that the subduction thermal gradient on Diego de Almagro Island fluctuated between c. 10–11°C/km and 13–14°C/km during Cretaceous times.

Maruyamaite, K(MgAl2)(Al5Mg)Si6O18(BO3)3(OH)3O, a potassium-dominant tourmaline from the ultrahigh-pressure Kokchetav massif, northern Kazakhstan: Description and crystal structure

Maruyamaite, ideally K(MgAl 2 )(Al 5 Mg)Si 6 O 18 (BO 3 ) 3 (OH) 3 O, was recently approved as the first K-dominant mineral-species of the tourmaline supergroup. It occurs in ultrahigh-pressure quartzofeldspathic gneisses of the Kumdy-Kol area of the Kokchetav Massif, northern Kazakhstan. Maruyamaite contains inclusions of microdiamonds, and probably crystallized near the peak pressure conditions of UHP metamorphism in the stability field of diamond. Crystals occur as anhedral to euhedral grains up to 2 mm across, embedded in a matrix of anhedral quartz and K-feldspar. Maruyamaite is pale brown to brown with a white to very pale-brown streak and has a vitreous luster. It is brittle and has a Mohs hardness of ~7; it is non-fluorescent, has no observable cleavage or parting, and has a calculated density of 3.081 g/cm 3 . In plane-polarized transmitted light, it is pleochroic, O = darkish brown, E = pale brown. Maruyamaite is uniaxial negative, ω = 1.634, ɛ = 1.652, both ±0.002. It is rhombohedral, space group R 3 m , a = 15.955(1), c = 7.227(1) A, V = 1593(3) A 3 , Z = 3. The strongest 10 X-ray diffraction lines in the powder pattern are [ d in A( I )( hkl )]: 2.581(100)(051), 2.974(85)(132), 3.995 (69)(240), 4.237(59)(231), 2.046(54)(162), 3.498(42)(012), 1.923(36)(372), 6.415(23)(111), 1.595(22)(5.10.0), 5.002(21)(021), and 4.610(20)(030). The crystal structure of maruyamaite was refined to an R 1 index of 1.58% using 1149 unique reflections measured with Mo K α X-radiation. Analysis by a combination of electron microprobe and crystal-structure refinement gave SiO 2 36.37, Al 2 O 3 31.50, TiO 2 1.09, Cr 2 O 3 0.04, Fe 2 O 3 0.33, FeO 4.01, MgO 9.00, CaO 1.47, Na 2 O 0.60, K 2 O 2.54, F 0.30, B 2 O 3 (calc) 10.58, H 2 O(calc) 2.96, sum 100.67 wt%. The formula unit, calculated on the basis of 31 anions pfu with B = 3, OH = 3.24 apfu (derived from the crystal structure) and the site populations assigned to reflect the mean interatomic distances, is (K 0.53 Na 0.19 Ca 0.26 □ 0.02 ) ∑X=1.00 (Mg 1.19 Fe 2+ 0.55 Fe 3+ 0.05 Ti 0.14 Al 1.07 ) ∑Y=3.00 (Al 5.00 Mg 1.00 )(Si 5.97 Al 0.03 O 18 )(BO 3 ) 3 (OH) 3 (O 2− 0.60 F 0.16 OH 0.24 ). Maruyamaite, ideally K(MgAl 2 )(Al 5 Mg)(BO 3 ) 3 (Si 6 O 18 )(OH) 3 O, is related to oxy-dravite: ideally Na(MgAl 2 )(Al 5 Mg)(BO 3 ) 3 (Si 6 O 18 )(OH) 3 O, by the substitution X K → X Na.

Computational Analysis of an Early Direct Injected HCCI Engine with Turbocharger Using Bio Ethanol and Diesel Blends

In this paper the work deals with the computational analysis of early direct injected HCCI engine with turbocharger using the CHEMKIN-PRO software. The computational analysis was carried out in the base of auto ignition chemistry by means of reduced chemical kinetics. For this study the neat diesel and Bio ethanol diesel blend (E20) were used as fuel. The inlet pressure was increased to 1.2 bar to simulate the turbocharged engine operation. The injection time was advanced to 18° before top dead centre (BTDC) i.e., 5° BTDC than normal injection time of 23° BTDC. The equivalence ratio was kept at 0.6 (ɸ=0.6) and the combustion, emission characteristics and chemical kinetics of the combustion reaction were studied. Since pressure and temperature profiles plays a very important role in reaction path at certain operating conditions, an attempt had been made here to present a complete reaction path investigation on the formation/destruction of chemical species at peak temperature and pressure conditions. The result showed that main draw backs of HCCI combustion like higher levels of unburned hydrocarbon emissions and carbon monoxide emissions are reduced in the turbocharged operation of the HCCI engine when compared to normal HCCI engine operation without turbocharger.

Transition from dehydration to hydration during exhumation of the Sanbagawa metamorphic belt, Japan, revealed by the continuous P–T path recorded in garnet and amphibole zoning

To investigate the water budget in deep accretionary prisms, we undertook a detailed petrologic analysis of garnet-bearing amphibolites in the subduction-origin high-pressure Sanbagawa metamorphic belt, Japan. We obtained a continuous P–T path of exhumation by the application of differential thermodynamics to compositional zoning in both garnet and amphibole. The P–T path is characterized by three stages of decompression: (1) heating decompression from 1.5 GPa and 550 °C to 1.1 GPa and 600 °C; (2) isothermal decompression toward 0.8 GPa and 580 °C; and (3) cooling decompression to 0.3 GPa and 400 °C. Comparison with the results of numerical modeling of the thermal structures of subduction zones suggests that the peak-pressure condition is warmer than the slab surface temperature in a typical subduction zone, but is consistent with subduction of a young slab during the Sanbagawa metamorphism. The heating decompression just after the peak-pressure condition is explained by heating from the mantle wedge during ascent of the metamorphic body in the accretionary prism; heating stops when the metamorphic body reaches crustal depths. Mass balance calculations for the mineral reactions in an amphibolite reveal that the dehydration associated with the growth of Ca-poor garnet takes place during heating decompression, whereas hydration takes place during cooling decompression by the breakdown of garnet and pargasite to form chlorite and actinolite. Such a transition from dehydration to hydration in the exhuming metabasite body in an accretionary prism generates fluids between the continental Moho and the slab surface and may explain the seismic activity above the subduction plate boundary, such as supra-slab earthquakes.

The Dabie-Sulu orogenic peridotites: Progress and key issues

大别-苏鲁造山带橄榄岩原岩属性可分为壳源和幔源橄榄岩, 它们在矿物结构、变质演化以及全岩和单矿物化学成分上存在一系列差别. 幔源橄榄岩来自于俯冲板块之上的古老华北克拉通岩石圈地幔, 记录了俯冲板块与地幔楔之间相互作用的信息. 石榴橄榄岩峰期条件下都显示石榴石和尖晶石共存, 石榴石和尖晶石的稳定性和成分都受全岩富化程度控制, 尖晶石成分不能用来判别部分熔融程度. 难熔幔源纯橄岩中普遍发育低Mg高Ca橄榄石取代早期斜方辉石结构, 指示硅不饱和熔体-橄榄岩反应过程, 它可能是导致纯橄岩Re-Os同位素年龄偏年轻的原因之一. 幔源橄榄岩/辉石岩中的金红石、钛斜粒硅镁石和锆石是俯冲地壳熔/流体迁移高场强元素进入地幔楔的直接矿物学证据, 只有通过精细的原位元素和同位素工作, 才能准确限定交代介质来源、时间和交代过程. 大陆俯冲带地幔楔氧逸度变化范围宽泛(FMQ=5.50~1.75), 并随深度增加而减小, 但其氧逸度计算结果受矿物组合、温压条件以及折返过程中的脱氢-氧化作用等因素影响. 本文对橄榄岩-辉石岩复杂体系相关系、大陆俯冲之前的地幔过程、地壳交代对地幔楔元素-同位素-氧逸度以及物理性质的改造等重要研究方向提出了建议.

High-pressure whiteschists from the Ti-N-Eggoleh area (Central Hoggar, Algeria): A record of Pan-African oceanic subduction

The Ti-N-Eggoleh area (Sérouènout Terrane, Central Hoggar, Algeria) comprises mainly a high-pressure Neoproterozoic metamorphic formation consisting of talc–kyanite–quartz whiteschists, chlorite schists, marbles, sulphide ores, partially serpentinized peridotites and partially amphibolitized eclogites, and reminiscent of an ophiolitic mélange that was metamorphosed and strongly deformed under eclogite-facies conditions. Major and trace elements indicate that the whiteschists underwent intense hydrothermal alteration, with Mg enrichment and leaching of alkalis and Ca, prior to high-pressure metamorphism. The main talc–kyanite–quartz paragenesis is stable within a large multivariant P–T field extending from high-P amphibolite to eclogite facies; the Tschermak substitution in talc marginally constrains peak pressure conditions to P>11kbar and 600<T<~800°C. The subsequent development of cordierite±sapphirine±corundum symplectites and coronae at the contact between talc and kyanite was due to isochemical and almost univariant reactions (Tlc+Ky+Qtz→Crd; Tlc+Ky→Crd+Crn; Tlc+Ky→Crd+Spr) which combined together to produce hybrid microstructures. These metamorphic reactions indicate a thermal overprint under granulite-facies conditions (T=650–860°C; P<12kbar). The peraluminous Mg-rich sapphirine observed in the symplectites reaches one of the highest degrees of Tschermak substitution ever reported for this mineral (with n=3.6), intermediate between the 13:19:5 (n=3) and 3:5:1 (n=4) theoretical compositions. The neighbouring eclogites record a similar P–T evolution: after the eclogite-facies metamorphic peak, they underwent partial amphibolitization and a subsequent high-temperature overprint under granulite-facies conditions that led to partial dehydration. The Ti-N-Eggoleh series is interpreted as the product of the thermal alteration of oceanic rocks that were subducted prior to the continental collision that formed the West Gondwana orogenic belt during the Neoproterozoic Pan-African orogeny. The decompression associated with the early phase of exhumation was followed by an important increase in temperature towards granulite-facies conditions, possibly determined by the intrusion of abundant mafic rocks in this region due to delamination of the lithospheric mantle. The Ti-N-Eggoleh area and its high-pressure meta-ophiolitic series apparently belong to the Sérouènout Terrane, which stretches along the eastern margin of the Western Gondwana orogenic belt and consists mainly of oceanic metasediments; they are possibly markers of an ancient, yet unidentified, subduction and suture zone.

Structural, petrological and chemical analysis of syn‐kinematic migmatites: insights from the Western Gneiss Region, Norway

AbstractEvidence of melting is presented from the Western Gneiss Region (WGR) in the core of the Caledonian orogen, Western Norway and the dynamic significance of melting for the evolution of orogens is evaluated. Multiphase inclusions in garnet that comprise plagioclase, potassic feldspar and biotite are interpreted to be formed from melt trapped during garnet growth in the eclogite facies. The multiphase inclusions are associated with rocks that preserve macroscopic evidence of melting, such as segregations in mafic rocks, leucosomes and pegmatites hosted in mafic rocks and in gneisses. Based on field studies, these lithologies are found in three structural positions: (i) as zoned segregations found in high‐P(ultra)mafic bodies; (ii) as leucosomes along amphibolite facies foliation and in a variety of discordant structures in gneiss; and (iii) as undeformed pegmatites cutting the main Caledonian structures. Segregations post‐date the eclogite facies foliation and pre‐date the amphibolite facies deformation, whereas leucosomes are contemporaneous with the amphibolite facies deformation, and undeformed pegmatites are post‐kinematic and were formed at the end of the deformation history. The geochemistry of the segregations, leucosomes and pegmatites in theWGRdefines two trends, which correlate with the mafic or felsic nature of the host rocks. The first trend with Ca‐poor compositions represents leucosome and pegmatite hosted in felsic gneiss, whereas the second group with K‐poor compositions corresponds to segregation hosted in (ultra)mafic rocks. These trends suggest partial melting of two separate sources: the felsic gneisses and also the included mafic eclogites. TheREEpatterns of the samples allow distinction between melt compositions, fractionated liquids and cumulates. Melting began at high pressure and affected most lithologies in theWGRbefore or during their retrogression in the amphibolite facies. During this stage, the presence of melt may have acted as a weakening mechanism that enabled decoupling of the exhuming crust around the peak pressure conditions triggering exhumation of the upward‐buoyant crust. Partial melting of both felsic and mafic sources at temperatures below 800 °C implies the presence of an H2O‐rich fluid phase at great depth to facilitate H2O‐present partial melting.

Early Ordovician metabasites from the Spanish Central System: A remnant of intraplate HP rocks in the Central Iberian Zone

The rocks at Tenzuela represent the only known outcrop of HP metabasites in the internal sectors of the Iberian Belt, in the vast Central Iberian Zone (CIZ). These rocks appear as lenses of meta-tholeiites of continental affinity intruded within the Neoproterozoic metasedimentary sequences. The mafic types have high Th/Yb and Ta/Yb ratios that suggest, combined with initial εNd values ranging from +4.4 to +6.0, derivation from a slightly enriched mantle source. SHRIMP zircon data indicate an Early Ordovician age (206Pb/238U mean age of 473±2Ma) for this magmatic event. However, the U–Pb data do not yield a constrained age for the high-pressure metamorphic event undergone by the metabasites, which nevertheless can be estimated at just before 335Ma, during the Variscan collision. The HP metabasites rarely overpass peak pressure conditions of 15kbar, and thus are remarkably lower than those of eclogites related to oceanic subduction in ophiolite- or allochthonous complexes from NW Iberia. On the other hand, these pressures are similar to those of the metabasites intercalated in orthogneisses along the Coimbra–Córdoba Shear Zone, in the boundary with the Ossa-Morena Zone (OMZ). In this regard, the new geochronological data set together with already published ages shows that the subduction-related Iberian eclogites (allochthonous complexes) are older (Lower to Middle Devonian) than the HP metabasites from more intracontinental settings, mostly recrystallized at lower HP conditions during the Visean. Thus, the studied metabasites should form part of an inner, diachronous belt of HP rocks within the European Variscan orogen, far away from the main subduction front. Nevertheless, the suggested geodynamic scenario needs future confirmation by accurate geochronological data for the HP metamorphism.

Coesite and diamond inclusions, exsolution microstructures and chemical patterns in ultrahigh pressure garnet from Ceuta (Northern Rif, Spain)

Garnet from diamondiferous granulites of Ceuta (Betic–Rif cordillera, Spain and Morocco) contains a variety of inclusion types. To better understand the evolution of these rocks during the ultrahigh pressure event, two samples (1 and 2) were selected for the detailed study of garnet. Primary inclusions of apatite, quartz, coesite, rutile and retrograded pyroxene, and exsolution microstructures of rutile characterize garnet from sample 1, whereas exsolution microstructures of quartz, coesite, apatite and rutile, and inclusions formed from a melt characterize garnet from sample 2, indicating that peak metamorphic conditions were recorded by sample 2. In contrast, the chemical patterns of garnet suggest an inverse situation. Garnet from sample 1 has high Ca- and low Mn contents and high XMg, characteristic of growth at high pressure and temperature whereas garnet from sample 2 shows high Mn and low Ca contents and low XMg, characteristic of garnet formed at lower temperature and pressure. The contrasting compositions are interpreted as reflecting differences in the position of the metamorphic path followed by both samples relative to the solidus: Garnets from sample 1 are interpreted as formed below the solidus whereas garnets from sample 2 are interpreted as formed in the presence of a melt, which caused notable enrichment of garnet in Mn and depletion in Ca relative to garnet from sample 1. Due to extensive low-pressure Hercynian melting that caused generalized migmatization and melt mobilization, whole-rock composition of the samples notably changed, thus preventing the accurate estimation of the physical conditions characterizing the older ultrahigh pressure event. Estimations based on experimental determinations of the phosphorous solubility in garnet suggest that peak pressure conditions were on the order of 6–7GPa, which put the origin of the studied crustal rocks at depths greater than 200km.