Petrogenesis of lawsonite and epidote eclogite and blueschist, Sivrihisar Massif, Turkey

The Sivrihisar Massif, Turkey, is comprised of blueschist and eclogite facies metasedimentary and metabasaltic rocks. Abundant metre‐ to centimetre‐scale eclogite pods occur in blueschist facies metabasalt, marble and quartz‐rich rocks. Sivrihisar eclogite contains omphacite + garnet + phengite + rutile ± glaucophane ± quartz + lawsonite and/or epidote. Blueschists contain sodic amphibole + garnet + phengite + lawsonite and/or epidote ± omphacite ± quartz. Sivrihisar eclogite and blueschist have similar bulk composition, equivalent to NMORB, but record different P–T conditions: ～26 kbar, 500 °C (lawsonite eclogite); 18 kbar, 600 °C (epidote eclogite); 12 kbar, 380 °C (lawsonite blueschist); and 15–16 kbar, 480–500 °C (lawsonite‐epidote blueschist). Pressures for the Sivrihisar lawsonite eclogite are among the highest reported for this rock type, which is rarely exposed at the Earth's surface. The distribution and textures of lawsonite ± epidote define P–T conditions and paths. For example, in some lawsonite‐bearing rocks, epidote inclusions in garnet and partial replacement of matrix epidote by lawsonite suggest an anticlockwise P–T path. Other rocks contain no epidote as inclusions or as a matrix phase, and were metamorphosed entirely within the lawsonite stability field. Results of the P–T study and mapping of the distribution of blueschists and eclogites in the massif suggest that rocks recording different maximum P–T conditions were tectonically juxtaposed as kilometre‐scale slices and associated high‐P pods, although all shared the same exhumation path from ～9–11 kbar, 300–400 °C. Within the tectonic slices, alternating millimetre–centimetre‐scale layers of eclogite and blueschist formed together at the same P–T conditions but represent different extents of prograde reaction controlled by strain partitioning or local variations in fO₂ or other chemical factors.     

Pressure–temperature evolution of lawsonite-bearing eclogites, Pinchi Lake, British Columbia

Abstract Blueschists and retrogressed eclogites are located along the Pinchi Fault Zone (near 54°30'N and 124°W) in central British Columbia. The Pinchi Fault separates rocks of contrasting geological histories, and the blueschists and eclogites occur with ultramafic rocks as fault-bounded blocks. The retrogressed eclogites occur as tectonic blocks, now in glacial debris, a few metres across; blueschists occur in coherent kilometre-sized tracts. Eclogites contain garnet–omphacite–rutile–quartz, with glaucophane, lawsonite and titanite. Some of the lawsonite appears to be stable with omphacite and garnet. Tectonic blocks of eclogite from two different localities have recorded different P–T histories. In one tectonic block, P–T estimates for garnet inclusions in clinopyroxene and lawsonite ( c . 565° C, >13.1 kbar) suggest that garnet and omphacite initially equilibrated outside the stability field of lawsonite. A decrease in temperature, as recorded in garnet rims and matrix clinopyroxene, resulted in crystallization of lawsonite and other retrogressive minerals. Later crystallization of stilpnomelane (locally pseudomorphing garnet), howieite, winchite and actinolite was triggered by an influx of fluid under P–T conditions outside the stability field of garnet. Lawsonite appears to have been stable, suggesting a minimum pressure of about 3 kbar. In the second tectonic block, clinopyroxene inclusions in garnet suggest temperatures near 350° C ( P > 10 kbar) and garnet rims equilibrated with matrix clinopyroxene suggest temperatures near 450° C at pressures above c . 12 kbar.     

Pressure–temperature evolution of lawsonite eclogite in Sivrihisar; Tavşanlı Zone–Turkey

The Cretaceous blueschist belt, Tavşanlı Zone, representing the subducted and exhumed northern continental margin of the Anatolide–Tauride platform is exposed in Western Anatolia. The Sivrihisar area east of Tavşanlı is made up of tectonic units consisting of i) metaclastics and conformably overlying massive marbles (coherent blueschist unit), ii) blueschist-eclogite unit, iii) marble–calcschist intercalation and iv) metaperidotite slab. The metaclastics are composed of jadeite–lawsonite–glaucophane and jadeite–glaucophane–chloritoid schists, phengite phyllites, and calcschists with glaucophane–lawsonite metabasite layers. The blueschist-eclogite unit representing strongly sheared, deeply buried and imbricated tectonic slices of accreted uppermost levels of the oceanic crust with minor metamorphosed serpentinite bodies consists of lawsonite-bearing eclogitic metabasites (approximately 90% of the field), lawsonite eclogites, metagabbros, serpentinites, pelagic marbles, omphacite–glaucophane–lawsonite metapelites and metacherts. The mineral assemblage of the lawsonite eclogite (garnet + omphacite > 70%) is omphacite, garnet, lawsonite, glaucophane, phengite and rutile. Lawsonite eclogite lenses are enclosed by garnet–lawsonite blueschist envelopes.Textural evidence from lawsonite eclogites and country rocks reveals that they did not leave the stability field of lawsonite during subduction and exhumation. The widespread preservation of lawsonite in eclogitic metabasites and eclogites can be attributed to rapid subduction and subsequent exhumation in a low geothermal gradient of the oceanic crust material without experiencing a thermal relaxation. Peak P–T conditions of lawsonite eclogites are estimated at 24 ± 1 kbar and 460 ± 25 °C. These P–T conditions indicate a remarkably low geotherm of 6.2 °C/km corresponding to a burial depth of 74 km.     

Calculated phase equilibria for MORB compositions: a reappraisal of the metamorphic evolution of lawsonite eclogite

Pseudosections calculated with thermocalc predict that lawsonite‐bearing assemblages, including lawsonite eclogite, will be common for subducted oceanic crust that experiences cool, fluid‐saturated conditions. For glaucophane–lawsonite eclogite facies conditions (500–600 °C and 18–28 kbar), MORB compositions are predicted in the NCKMnFMASHO system to contain glaucophane, garnet, omphacite, lawsonite, phengite and quartz, with chlorite at lower temperature and talc at higher temperature. In these assemblages, the pyrope content in garnet is mostly controlled by variations in temperature, and grossular content is strongly controlled by pressure. The silica content in phengite increases linearly with pressure. As the P–T conditions for these given isopleths are only subtly affected by common variations in bulk‐rock compositions, the P–T pseudosections potentially present a robust geothermobarometric method for natural glaucophane‐bearing eclogites. Thermobarometric results recovered both by isopleth and conventional approaches indicate that most natural glaucophane–lawsonite eclogites (Type‐L) and glaucophane–epidote eclogites (Type‐E) record similar peak P–T conditions within the lawsonite stability field. Decompression from conditions appropriate for lawsonite stability should result in epidote‐bearing assemblages through dehydration reactions controlled by lawsonite + omphacite = glaucophane + epidote + H₂O. Lawsonite and omphacite breakdown will be accompanied by the release of a large amount of bound fluid, such that eclogite assemblages are variably recrystallized to glaucophane‐rich blueschist. Calculated pseudosections indicate that eclogite assemblages form most readily in Ca‐rich rocks and blueschist assemblages most readily in Ca‐poor rocks. This distinction in bulk‐rock composition can account for the co‐existence of low‐T eclogite and blueschist in high‐pressure terranes.     

Formation and preservation of fresh lawsonite: Geothermobarometry of the North Makran Blueschists,southeast Iran

A low‐grade metamorphic “Coloured Mélange” in North Makran (SE Iran) contains lenses and a large klippe of low temperature, lawsonite‐bearing blueschists formed during the Cretaceous closure of the Tethys Ocean. The largest blueschist outcrop is a >1,000 m thick coherent unit with metagabbros overlain by interlayered metabasalts and metavolcanoclastic rocks. Blueschist metamorphism is only incipient in coarse‐grained rocks, whereas finer grained, foliated samples show thorough metamorphic recrystallization. The low‐variance blueschist peak assemblage is glaucophane, lawsonite, titanite, jadeite±phengitic mica. Investigated phase diagram sections of three blueschists with different protoliths yield peak conditions of ~300–380°C at 9–14 kbar. Magnesio‐hornblende and rutile cores indicate early amphibolite facies metamorphism at >460°C and 2–4 kbar. Later conditions at slightly higher pressures of 6–9 kbar at 350–450°C are recorded by barroisite, omphacite and rutile assemblages before entering into the blueschist facies and finally following a retrograde path through the pumpellyite–actinolite facies across the lawsonite stability field. Assuming that metamorphic pressure is lithostatic pressure, the corresponding counterclockwise P–T path is explained by burial along a warm geothermal gradient (~15°C/km) in a young subduction system, followed by exhumation along a cold gradient (~8°C/km); a specific setting that allows preservation of fresh undecomposed lawsonite in glaucophane‐bearing rocks.     

Cold subduction zone in northern Calabria (Italy) revealed by lawsonite–clinopyroxene blueschists

Lawsonite blueschists are important markers of cold subduction zones, subjected to intense fluid circulation. This is because lawsonite preservation in exhumed blueschists and eclogites is typically linked to cold exhumation paths, accompanied by hydration. In the Catena Costiera (Calabria, southern Italy), lawsonite–clinopyroxene blueschists of the Diamante–Terranova Unit, affected by ductile shearing and retrogression, are exposed. Blueschists contain zoned clinopyroxene crystals, showing core–rim compositional variation from diopside to omphacite and hosting primary inclusions of lawsonite and titanite. Thermodynamic modelling of phase equilibria in the NCKFMASHTO system revealed peak metamorphic conditions of 2.0–2.1 GPa and 475–490°C for the Alpine subduction in Calabria. The subsequent post-peak metamorphic evolution mainly proceeded along a decompression and cooling path up to ~1.1 GPa and ~380°C. The final exhumation stages are recorded in the sheared blueschists where a mylonitic to ultramylonitic foliation developed at ~0.7 GPa and 290–315°C. Therefore, the P–T evolution of the Diamante–Terranova blueschists mostly occurred in the stability field of lawsonite, sustained by H₂O-saturated conditions during the exhumation path. The results of this study indicate that the blueschists underwent peak metamorphic conditions higher than previously thought, reaching a maximum depth of ~70 km under a very cold geothermal gradient (~6.6°C/km), during the Eocene subduction of the Ligurian Tethys oceanic crust in Calabria.     

P–T path of high-pressure/low-temperature rocks and tectonic implications in the western Tianshan Mountains, NW China

The Chinese western Tianshan high-pressure/low-temperature (HP–LT) metamorphic belt, which extends for about 200 km along the South Central Tianshan suture zone, is composed of mainly metabasic blueschists, eclogites and greenschist facies rocks. The metabasic blueschists occur as small discrete blocks, lenses, bands, laminae or thick beds in meta-sedimentary greenschist facies country rocks. Eclogites are intercalated within blueschist layers as lenses, laminae, thick beds or large massive blocks (up to 2 km² in plan view). Metabasic blueschists consist of mainly garnet, sodic amphibole, phengite, paragonite, clinozoisite, epidote, chlorite, albite, accessory titanite and ilmenite. Eclogites are predominantly composed of garnet, omphacite, sodic–calcic amphibole, clinozoisite, phengite, paragonite, quartz with accessory minerals such as rutile, titanite, ilmenite, calcite and apatite. Garnet in eclogite has a composition of 53–79 mol% almandine, 8.5–30 mol% grossular, 5–24 mol% pyrope and 0.6–13 mol% spessartine. Garnet in blueschists shows similar composition. Sodic amphiboles include glaucophane, ferro-glaucophane and crossite, whereas the sodic–calcic amphiboles mainly comprise barroisite and winchite. The jadeite content of omphacite varies from 35–54 mol%. Peak eclogite facies temperatures are estimated as 480–580 °C for a pressure range of 14–21 kbar. The conditions of pre-peak, epidote–blueschist facies metamorphism are estimated to be 350–450 °C and 8–12 kbar. All rock types have experienced a clockwise P–T path through pre-peak lawsonite/epidote-blueschist to eclogite facies conditions. The retrograde part of the P–T path is represented by the transition of epidote-blueschist to greenschist facies conditions. The P–T path indicates that the high-pressure rocks formed in a B-type subduction zone along the northern margin of the Palaeozoic South Tianshan ocean between the Tarim and Yili-central Tianshan plates.     

Preservation of high‐P rocks coupled to rock composition and the absence of metamorphic fluids

Eclogites, blueschists and greenschists are found in close proximity to one another along a 1‐km coastal section where the Cyclades Blueschist Unit (CBU) is exposed on SE Syros, Greece. Here, we show that the eclogites and blueschists experienced the same metamorphic history: prograde lawsonite blueschist facies metamorphism at 1.2–1.9 GPa and 410–530°C followed, at 43–38 Ma, by peak blueschist/eclogite facies metamorphism at 1.5–2.1 GPa and 520–580°C. We explain co‐existence of eclogites and blueschists by compositional variation probably reflecting original compositional layering. It is also shown that the greenschists record retrogression at 0.34 ± 0.21 GPa and T = 456 ± 68°C. This was spatially associated with a shear zone on a scales of 10–100‐m and veins on a scale of 1–10‐cm. Greenschist facies metamorphism ended at (or shortly after) 27 Ma. We thus infer a period of metamorphic quiescence after eclogite/blueschist facies metamorphism and before greenschist facies retrogression which lasted up to 11–16 million years. We suggest that this reflects an absence of metamorphic fluid flow at that time and conclude that greenschist facies retrogression only occurred when and where metamorphic fluids were present. From a tectonic perspective, our findings are consistent with studies showing that the CBU is (a) a high‐P nappe stack consisting of belts in which high‐P metamorphism and exhumation occurred at different times and (b) affected by greenschist facies metamorphism during the Oligocene, prior to the onset of regional tectonic extension.     

Preservation of subduction‐related prograde deformation in lawsonite pseudomorph‐bearing rocks

Lawsonite pseudomorphs are used to identify and distinguish the kinematic records of subduction and exhumation in blueschist‐facies rocks from Syros (Cyclades; Greece). Lawsonite is a hydrous mineral that crystallizes at high‐pressure and low‐temperature conditions. During decompression, lawsonite is typically pseudomorphed by an aggregate dominated by epidote and paragonite. Such aggregates are easily deformable and if deformation occurs after the lawsonite breakdown, the pseudomorphs are difficult to distinguish from the matrix. The preservation of the lawsonite crystal shape, despite complete retrogression, indicates therefore that the host blueschist rock has not been affected by penetrative deformation during exhumation, thus providing indication of strain‐free conditions. Therefore, tracking the lawsonite growth and destabilization along the P–T path followed by the rocks during a subduction/exhumation cycle provides information about the subduction/exhumation‐related deformation. Using microstructural observations and P–T pseudosections calculated with thermocalc , it is inferred that top‐to‐the‐south sense of shear preserved in lawsonite pseudomorph‐bearing blueschists on Syros occurred during the prograde metamorphic path within the lawsonite stability field, and is therefore associated with subduction. On the contrary, the deformation with a top‐to‐the‐north sense of shear is observed in surrounding rocks, where lawsonite pseudomorphs are deformed or apparently lacking. This deformation occurred after the lawsonite breakdown during exhumation. At the regional scale, exhumation‐related deformation is heterogeneous, allowing the preservation of lawsonite pseudomorphs in significant volumes of blueschists of the central and southern Cyclades. It is argued that such successive shearing deformation events with opposite senses more likely correspond to an exhumation process driven by slab rollback, in which subduction and exhumation are not synchronous.     

Subduction and accumulation of lawsonite eclogite and garnet blueschist in eastern Australia

Lawsonite eclogite and garnet blueschist occur as metre-scale blocks within serpentinite mélange in the southern New England Orogen (SNEO) in eastern Australia. These high-P fragments are the products of early Palaeozoic subduction of the palaeo-Pacific plate beneath East Gondwana. Lu–Hf, Sm–Nd, and U–Pb geochronological data from Port Macquarie show that eclogite mineral assemblages formed between c. 500 and 470 Ma ago and became mixed together within a serpentinite-filled subduction channel. Age data and P–T modelling indicate lawsonite eclogite formed at ~2.7 GPa and 590°C at c. 490 Ma, whereas peak garnet in blueschist formed at ~2.0 GPa and 550°C at c. 470 Ma. The post-peak evolution of lawsonite eclogite was associated with the preservation of pristine lawsonite-bearing assemblages and the formation of glaucophane. By contrast, the garnet blueschist was derived from a precursor garnet–omphacite assemblage. The geochronological data from these different aged high-P assemblages indicate the high-P rocks were formed during subduction on the margin of cratonic Australia during the Cambro-Ordovician. The rocks however now reside in the Devonian–Carboniferous southern SNEO, which forms the youngest and most outboard of the eastern Gondwanan Australian orogenic belts. Geodynamic modelling suggests that over the time-scales that subduction products accumulated, the high-P rocks migrated large distances (~>1,000 km) during slab retreat. Consequently, high-P rocks that are trapped in subduction channels may also migrate large distances prior to exhumation, potentially becoming incorporated into younger orogenic belts whose evolution is not directly related to the formation of the exhumed high-P rocks.     

A potential method to confirm the previous existence of lawsonite in eclogite: the mass imbalance of Sr and LREEs in multistage epidote (Ganghe,Dabie UHP terrane)

Lawsonite is an important hydrous mineral that is stable at low‐temperature (LT) and high‐ to ultrahigh‐pressure (HP–UHP) conditions in subducted slabs. The occurrence/absence of lawsonite in eclogite is a significant constraint for the construction of the metamorphic, tectonic and fluid/melt evolution histories of an HP–UHP terrane. However, lawsonite is very rarely preserved in natural eclogites, and accurate judgment of its former existence is a significant challenge for petrologists. At present, whether lawsonite has ever existed in lawsonite‐absent eclogite is mainly judged by (i) pseudomorphs after lawsonite, and (ii) phase equilibria modelling. In this study, major element and trace‐element distributions in multistage minerals were examined in the Ganghe lawsonite‐absent UHP eclogite in the Dabie UHP terrane, eastern China. This work demonstrates that the whole‐rock Sr and light rare earth elements (LREEs) are mainly dominated by epidote; other minerals (garnet, omphacite, quartz, kyanite, barroisite, phengite and accessory minerals) play a very limited role in the Sr and LREEs budgets. Two stages of epidote, which have noticeably different Sr and LREEs contents, were recognized in the eclogite: (i) Epidote porphyroblasts (Ep‐P core), which are suspected to be the pseudomorphic mineral after lawsonite, contain significantly high Sr (7200–10 300 ppm) and LREEs (160–1300 ppm for La). (ii) An earlier stage epidote (Ep‐In core) occurs as inclusions in matrix omphacite, or in omphacite inclusions in the suspected pseudomorphic minerals after lawsonite (SPMAL); this early epidote has significantly lower Sr (990–1890 ppm, average 1495 ppm, n = 17) and LREEs contents (60–110 ppm for La, average 91 ppm, n = 17). All of the existing early‐stage minerals predating the SPMAL have very low contents of Sr and LREEs, and the total amounts of these elements in the early‐stage minerals do not balance those in the SPMAL. This indicates that a missing Ca‐, Al‐, Sr‐ and LREE‐rich mineral, which was previously in equilibrium with the early‐stage minerals, likely existed in the Ganghe eclogite. On the basis of the mineral geochemistry and phase equilibria modelling, we confirm that the missing mineral cannot be anything but lawsonite. This study indicates that examining the mass (im)balance of Sr and LREEs between multistage HP–UHP epidote can be used as a potential method to confirm the previous existence of lawsonite in lawsonite‐absent eclogite.     

Coexistence of garnet blueschist and eclogite in South Tianshan,NW China: dependence of P–T evolution and bulk‐rock composition

Coexisting garnet blueschist and eclogite from the Chinese South Tianshan high‐pressure (HP)–ultrahigh‐pressure (UHP) belt consist of similar mineral assemblages involving garnet, omphacite, glaucophane, epidote, phengite, rutile/sphene, quartz and hornblendic amphibole with or without paragonite. Eclogite assemblages generally contain omphacite >50 vol.% and a small amount of glaucophane (<5 vol.%), whereas blueschist assemblages have glaucophane over 30 vol.% with a small amount of omphacite which is even absent in the matrix. The coexisting blueschist and eclogite show dramatic differences in the bulk‐rock compositions with higher X(CaO) [=CaO/(CaO + MgO + FeO_total + MnO + Na₂O)] (0.33–0.48) and lower A/CNK [=Al₂O₃/(CaO + Na₂O + K₂O)] (0.35–0.56) in eclogite, but with lower X(CaO) (0.09–0.30) and higher A/CNK (0.65–1.28) in garnet blueschist. Garnet in both types of rocks has similar compositions and exhibits core–rim zoning with increasing grossular and pyrope contents. Petrographic observations and phase equilibria modelling with pseudosections calculated using thermocalc in the NCKMnFMASHO system for the coexisting garnet blueschist and eclogite samples suggest that the two rock types share similar P–T evolutional histories involving a decompression with heating from the P_max to the T_max stage and a post‐T_max decompression with slightly cooling stage, and similar P–T conditions at the T_max stage. The post‐T_max decompression is responsible for lawsonite decomposition, which results in epidote growth, glaucophane increase and omphacite decrease in the blueschist, or in an overprinting of the eclogitic assemblage by a blueschist assemblage. Calculated P–X(CaO), P–A/CNK and P–X(CO₂) pseudosections indicate that blueschist assemblages are favoured in rocks with lower X(CaO) (<0.28) and higher A/CNK (>0.75) or fluid composition with higher X(CO₂) (>0.15), but eclogite assemblages preferentially occur in rocks with higher X(CaO) and lower A/CNK or fluid composition with lower X(CO₂). Moreover, phase modelling suggests that the coexistence of blueschist and eclogite depends substantially on P–T conditions, which would commonly occur in medium temperatures of 500–590 °C under pressures of ~17–22 kbar. The modelling results are in good accordance with the measured bulk‐rock compositions and modelled temperature results of the coexisting garnet blueschist and eclogite from the South Tianshan HP–UHP belt.     

Polyphase and anticlockwise P-T evolution for Franciscan eclogites and blueschists from Jenner, California, USA

High-grade exotic blocks in the Franciscan Complex at Jenner, California, show evidence for polydeformation/metamorphism, with eight distinct stages. Two parallel sets of mineral assemblages [(E) eclogite, and (BS) laminated blueschist] representing different bulk chemistry were identified. Stage 1, recorded by parallel aligned inclusions (S1) of crossite + omphacite + epidote + ilmenite + titanite + quartz (E), and glaucophane + actinolite + epidote + titanite (BS) in the central parts of zoned garnets, represents the epidote blueschist facies. The onset of a second stage (stage 2) is represented by a weak crenulation of S1 and growth of garnet. This stage develops a well-defined S2 foliation of orientated barroisite + epidote + titanite (E), or subcalcic actinolite + epidote + titanite (BS) at c. 90d? to S1, with syntectonic growth of garnet, defining the (albite-)epidote-amphibolite facies. A third stage, with aligned inclusions of glaucophane + (subcalcic) actinolite + phengite parallel to S2 in the outermost rims of large garnet grains, is assigned to the transitional (albite-)epidote-amphibolite/(garnet-bearing) epidote blueschist facies. The fourth stage represents the peak metamorphism, and was identified by unorientated matrix minerals in the least retrograded samples. In this stage the mineral assemblages garnet + omphacite + glaucophane + phengite (E) and garnet + winchite + phengite + epidote (BS) both represent the eclogite facies. Stage 5 is represented by the retrogression of eclogite facies assemblages to the epidote blueschist facies assemblages crossite/glaucophane + garnet + omphacite + epidote + phengite (E), and glaucophane + actinolite + epidote + phengite (BS), with the development of an S5 foliation subparallel to S2. Stage 6 represents a crenulation of S5, with the development of a well-defined S6 crenulation cleavage wrapping around relics of the eclogite facies assemblages. This crenulation cleavage is further weakly crenulated during a D7 event. Post-D7 (stage 8) is recorded by the growth of lawsonite + chlorite ± actinolite replacing garnet, and by veins of lawsonite + pumpellyite + aragonite and phengite + apatite. The different, yet coeval, mineral parageneses observed in rock types (E) and (BS) are probably due to differences in bulk chemistry. The metamorphic evolution from stage 1 to stage 8 seems to have been broadly continuous, following an anticlockwise P-Tpath: (1) epidote blueschist (garnet-free) to (2) (albite-)epidote-amphibolite to (3) transitional epidote blueschist (garnet-bearing)/(albite-)epidote-amphibolite to (4) eclogite to (5) epidote blueschist (garnet-bearing) to (6-7) epidote blueschist (garnet-free) facies to (8) lawsonite + pumpellyite + aragonite-bearing assemblages. This anticlockwise P-T path may have resulted from a decreasing geothermal gradient with time in the Mesozoic subduction zone of California at early or pre-Franciscan metamorphism.     

Blueschists of the Amassia-Stepanavan Suture Zone (Armenia): linking Tethys subduction history from E-Turkey to W-Iran

The Amassia–Stepanavan blueschist-ophiolite complex of the Lesser Caucasus in NW Armenia is part of an Upper Cretaceous-Cenozoic belt, which presents similar metamorphic features as other suture zones from Turkey to Iran. The blueschists include calcschists, metaconglomerates, quartzites, gneisses and metabasites, suggesting a tectonic mélange within an accretionary prism. This blueschist mélange is tectonically overlain by a low-metamorphic grade ophiolite sequence composed of serpentinites, gabbro-norite pods, plagiogranites, basalts and radiolarites. The metabasites include high-P assemblages (glaucophane–aegirine–clinozoisite–phengite), which indicate maximal burial pressure of ∼1.2 GPa at ∼550°C. Most blueschists show evidence of greenschist retrogression (chlorite—epidote, actinolite), but locally epidote-amphibolite conditions were attained (garnet—epidote, Ca/Na amphibole) at a pressure of ∼0.6 GPa and a temperature of ∼500°C. This LP–MT retrogression is coeval with exhumation and nappe-stacking of lower grade units over higher grade ones. ⁴⁰Ar/³⁹Ar phengite ages obtained on the high-P assemblages range between 95 and 90 Ma, while ages obtained for epidote-amphibolite retrogression assemblages range within 73.5–71 Ma. These two metamorphic phases are significant of (1) HP metamorphism during a phase of subduction in the Cenomanian–Turonian times followed by (2) exhumation in the greenschist to epidote-amphibolite facies conditions during the Upper Campanian/Maastrichtian due to the onset of continental subduction of the South Armenian block below Eurasia.     

Evolution of a tourmaline-bearing lawsonite eclogite from the Elekdağ area (Central Pontides,N Turkey): evidence for infiltration of slab-derived B-rich fluids during exhumation

An undated high-pressure low-temperature tectonic mélange in the Elekda area (central Pontides, N Turkey) comprises blocks of MORB-derived lawsonite eclogite within a sheared serpentinite matrix. In their outer shells, some of the eclogite blocks contain large (up to 6 cm) tourmaline crystals. Prograde inclusions in poikiloblastic garnet from a well-preserved eclogite block are lawsonite, epidote/clinozoisite, omphacite, rutile, glaucophane, chlorite, Ba-bearing phengite, minor actinolite, winchite and quartz. In addition, glaucophane, lawsonite and rutile occur as inclusions in omphacite. These inclusion assemblages document the transition from a garnet-lawsonite-epidote-bearing blueschist to a lawsonite eclogite with the peak assemblage garnet + omphacite I + lawsonite + rutile. Peak metamorphic conditions are not well-constrained but are estimated approximately 400–430°C and >1.35 GPa, based on Fe–Mg exchange between garnet and omphacite and the coexistence of lawsonite + omphacite + rutile. During exhumation of the eclogite–serpentinite mélange in the hanging wall of a subduction system, infiltration of B-rich aqueous fluids into the rims of eclogite blocks caused retrogressive formation of abundant chlorite, titanite and albite, followed by growth of tourmaline at the expense of chlorite. At the same time, omphacite I (X_Jd=0.24–0.44) became unstable and partially replaced by omphacite II characterized by higher X_Jd (0.35–0.48), suggesting a relatively low silica activity in the infiltrating fluid. Apart from Fe-rich rims developed at the contact to chlorite, tourmaline crystals are nearly homogeneous. Their compositions correspond to Na-rich dravite, perhaps with a small amount of excess (tetrahedral) boron (~5.90 Si and 3.10 B cations per 31 anions). ¹¹ B values range from –2.2 to +1.7. The infiltrating fluids were most probably derived from subducting altered oceanic crust and sediments.     

A counter-clockwise P–T path for the Voltri Massif eclogites (Ligurian Alps, Italy)

Integrated petrological and structural investigations of eclogites from the eclogite zone of the Voltri Massif (Ligurian Alps) have been used to reconstruct a complete Alpine P–T deformation path from burial by subduction to subsequent exhumation. The early metamorphic evolution of the eclogites has been unravelled by correlating garnet zonation trends with the chemical variations in inclusions found in the different garnet domains. Garnet in massive eclogites displays typical growth zoning, whereas garnet in foliated eclogites shows rim‐ward resorption, likely related to re‐equilibration during retrogressive evolution. Garnet inclusions are distinctly different from core to rim, consisting primarily of Ca‐, Na/Ca‐amphibole, epidote, paragonite and talc in garnet cores and of clinopyroxene ± talc in the outer garnet domains. Quantitative thermobarometry on the inclusion assemblages in the garnet cores defines an initial greenschist‐to‐amphibolite facies metamorphic stage (M1 stage) at c. 450–500 °C and 5–8 kbar. Coexistence of omphacite + talc + katophorite inclusion assemblage in the outer garnet domains indicate c. 550 °C and 20 kbar, conditions which were considered as minimum P–T estimates for the M2 eclogitic stage. The early phase of retrograde reactions is polyphase and equilibrated under epidote–blueschist facies (M3 stage), characterized by the development of composite reaction textures (garnet necklaces and fluid‐assisted Na‐amphibole‐bearing symplectites) produced at the expense of the primary M2 garnet‐clinopyroxene assemblage. The blueschist retrogression is contemporaneous with the development of a penetrative deformation (D3) that resulted in a non‐coaxial fabric, with dominant top‐to‐the‐N sense of shear during rock exhumation. All of that is overprinted by a texturally late amphibolite/greenschist facies assemblages (M4 & M5 stages), which are not associated with a penetrative structural fabric. The combined P–T deformation data are consistent with an overall counter‐clockwise path, from the greenschist/amphibolite, through the eclogite, the blueschist to the greenschist facies. These new results provide insights into the dynamic evolution of the Tertiary oceanic subduction processes leading to the building up of the Alpine orogen and the mechanisms involved in the exhumation of its high‐pressure roots.     

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

In 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] <<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) (>0.01) in the coexisting fluid at the epidote eclogite facies stage.     

Eclogite Occurrences in the Southern Tianshan High-Pressure Belt,Xinjiang, Western China

Eclogite pods and layers of eclogites in mafic blueschists are the most prominent witnesses of eclogite facies metamorphism in the southern Tianshan high-pressure belt of Western China. The P-T evolution was deciphered in order to understand the geotectonic framework of these rocks. Geothermobarometric evaluation for matrix assemblages reveals peak metamorphic condition between 14 and 21 kbar at 500 to 600°C. Prograde growth zoning and mineral inclusions in garnet are evidence for a clockwise P-T path. The presence of intimate eclogite/metabasic blueschist interlayering and the high P-T estimates suggest that the eclogite-facies rocks from the southern Tianshan high-pressure belt formed in a tectonic regime similar to the Franciscan type. Therefore the southern Tianshan high-pressure belt is most important in order to resolve the Paleozoic tectonic evolution of central Asia.     

Prograde and retrograde history of the Junction School eclogite,California, and an evaluation of garnet–phengite–clinopyroxene thermobarometry

Quantitative thermobarometry of inclusions in zoned garnet from a Franciscan eclogite block record a counter-clockwise P–T path from blueschist to eclogite and back. Garnet retains prograde zoning from inclusion-rich Alm₅₂Grs₃₀Pyp₆Sps₁₂ cores to inclusion-poor Alm₆₂Grs₂₅Pyp₁₂Sps₁ mantles, with overgrowths of highly variable composition. Barometry using the Waters–Martin version of the garnet–phengite–omphacite thermobarometer yields conditions of 7–15 kbar, 400–500°C (garnet cores), 18–22 kbar, ∼550°C (mantles), and 10–14 kbar, 350–450°C (overgrowths), in agreement with clinozoisite–sphene–rutile–garnet–quartz barometry. These pressures are ∼10–15 kbar less than those obtained using more recent, fully thermodynamic calibrations of the phengite–omphacite–garnet thermobarometer. Low early temperatures suggest that the block was subducted in a thermally mature subduction zone and not at the inception of subduction when prograde temperature is expected to be higher. Franciscan high-grade blocks likely represent crust subducted throughout the history of this convergent margin, rather than only at the inception of the subduction zone.     

Metamorphic evolution of lawsonite eclogites from the southern Motagua fault zone,Guatemala: insights from phase equilibria and Raman spectroscopy

Lawsonite eclogite (metabasalt and metadolerite) and associated metasedimentary rocks in a serpentinite mélange from an area just south of the Motagua fault zone (SMFZ), Guatemala, represent excellent natural records of the forearc slab–mantle interface. Pseudosection modelling of pristine lawsonite eclogite reproduces the observed predominant mineral assemblages, and garnet compositional isopleths intersect within the phase fields, yielding a prograde P–T path that evolves from 20 kbar, 470 °C (M1) to 25 kbar, 520 °C (M2). The dominant penetrative foliation within the eclogite blocks is defined by minerals developed during the prograde evolution, and the associated deformation, therefore, took place during subduction. Thermometry using Raman spectra of carbonaceous material in metasedimentary rocks associated with the SMFZ eclogites gives estimates of peak‐T of ～520 °C. Barometry using Raman spectroscopy shows unfractured quartz inclusions in garnet rims retain overpressures of up to ～10 kbar, implying these inclusions were trapped at conditions just below the quartz/coesite transition, in agreement with the results of phase equilibrium analysis. Additional growth of Ca‐rich garnet indicates initial isothermal decompression to 20 kbar (M3) followed by hydration and substantial cooling to the lawsonite–blueschist facies (M4). Further decompression of the hydrated eclogite blocks to the pumpellyite–actinolite facies (3–5 kbar, 230–250 °C) is associated with dehydration and veining (M5). The presence of eclogite as m‐ to 10 m‐sized blocks in a serpentinite matrix, lack of widespread deformation developed during exhumation and derived prograde P–T path associated with substantial dehydration of metabasites within the antigorite stability field suggest that the SMFZ eclogites represent the uppermost part of the forearc slab crust sampled by an ascending serpentinite diapir in an active, moderate‐T subduction zone.