Evidence For Ultrahigh-pressure Metamorphism Research Articles

Chapter 21 Middle thrust sheets in the Caledonide orogen, Sweden: the outer margin of Baltica, the continent–ocean transition zone and late Cambrian–Ordovician subduction–accretion

Abstract Nappes of continental outer and outermost margin affinities (Middle Allochthon) were transported from locations west of the present Norwegian coast and thrust eastwards onto the Baltoscandian foreland basin and platform. They are of higher metamorphic grade than underlying thrust sheets and most are more penetratively deformed. These allochthons are treated here in three groups. The lower thrust sheets comprise Paleoproterozoic crystalline basement (e.g. Tännäs Augen Gneiss Nappe) and greenschist facies, Neoproterozoic, siliciclastic metasedimentary rocks (e.g. Offerdal Nappe). These are overthrust by a Cryogenian−Ediacaran succession intruded by c. 600 Ma dolerites (Baltoscandian Dyke Swarm) with an affinity to mid-ocean ridge basalt containing normal to enriched incompatible element contents (Särv Nappes). The upper sheets are dominated by higher-grade allochthons (Seve Nappe Complex) with similar, mainly siliciclastic sedimentary protoliths, more mafic magmatism and some solitary ultramafic bodies. Within this early Ediacaran continent−ocean transition zone (COT) assemblage, generally metamorphosed in amphibolite facies, some nappes experienced migmatization, and eclogites are present. Evidence of ultrahigh-pressure metamorphism has been obtained from garnet peridotites and eclogites; recently, microdiamonds have been discovered in paragneisses. Subduction of the COT started by the late Cambrian and accretion continued through the Ordovician, prior to the Baltica–Laurentia collision. Thrusting of all these Middle allochthons onto the foreland basin exceeds a distance of 400 km.

UHP metamorphism recorded by phengite eclogite from the Caledonides of northern Sweden: P–T path and tectonic implications

AbstractThe Seve Nappe Complex (SNC) of the Scandinavian Caledonides records a well‐documented history of high pressure (HP) and ultra‐high pressure (UHP) metamorphism. Eclogites of the SNC occur in two areas in Sweden, namely Jämtland and Norrbotten. The Jämtland eclogites and associated rocks are well‐studied and provide evidence for late Ordovician UHP metamorphism, whereas the Norrbotten eclogites, formed during the late Cambrian (Furongian)/Early Ordovician, have not been studied in such detail, especially in terms of the P–T conditions of their formation. Within the studied eclogite, clinopyroxene contains a high‐Na core and two rims: inner, medium‐Na and outer, low‐Na. Garnet consists of a high‐Ca euhedral core, low‐Ca inner rim and medium‐Ca outer rim. A similar pattern occurs within phengite, where high‐Si cores are enveloped by medium and low‐Si rims. The compositions of the mineral cores, inner rims and outer rims reflect three stages in the metamorphic evolution of the eclogite. Applied Quartz‐in‐Garnet geobarometry, coupled with Zr‐in‐rutile geothermometry reveal that garnet nucleation (E0 stage) took place at 1.5–1.6 GPa and 620–660°C. The eclogite peak‐pressure assemblage developed during the E1 stage, it consists of garnet+omphacite+phengite+rutile+coesite? and yields P–T conditions of 2.8–3.1 GPa and 660–780°C as constrained by conventional geothermobarometry and thermodynamic modelling in the NCKFMMnASHT system. Later, lower‐pressure stages E2 and E3 record conditions of 2.2–2.8 GPa, 680–780°C and 2.1 GPa, 735°C, respectively. The prograde metamorphic evolution of the eclogite is inferred from inclusions of epidote, amphibole and clinopyroxene within garnet. The presence of amphibole–quartz–plagioclase symplectites, secondary epidote/zoisite and titanite replacing rutile record the later retrograde changes taking place at <1.5 GPa (referred as E4 stage). The obtained P–T conditions indicate that the Norrbotten eclogites underwent a metamorphic evolution characterized by a clockwise P–T path with peak metamorphism reaching up to coesite stability field within a relatively cold subduction regime (7.8°C/km). The obtained results provide the first evidence for UHP metamorphism in the SNC above the Arctic Circle and document cold subduction regime and multistage exhumation of the deeply subducted Baltican margin at early stage of the Caledonian Orogeny.

The first evidence of UHP metamorphism in the Polar Urals (Russia)

A new ultrahigh-pressure (UHP) metamorphic terrane, the Marun-Keu eclogite–peridotite–gneiss complex was established in the Polar Urals (Russia). Owing to the P–T parameters of formation of garnet peridotites obtained for the first time, it was established that they experienced UHP metamorphism with a peak in the diamond stability field (39 kbar, 830oC) due to subduction of rocks of the Marun-Keu complex to the mantle (a depth of more than 100 km). Relics of plagioclase peridotites preserved in central parts of large peridotite bodies and petrochemical data are evidence that Marun-Key garnet peridotites cannot be of mantle origin. A basic-ultrabasic intrusion, which formed in the upper crust, can be considered as their protolith. Later, rocks of the massif plunged to the mantle along the subduction zone and metamorphosed under high-P to ultrahigh-P conditions.

Discovery of coesite in eclogite from Keqike Jianggalesayi: new evidence for ultrahigh-pressure metamorphism in South Altyn Tagh, northwestern China

Discovery of coesite in eclogite from Keqike Jianggalesayi: new evidence for ultrahigh-pressure metamorphism in South Altyn Tagh, northwestern China

Understanding continental subduction: A work in progress

Understanding continental subduction: A work in progress

Discovery of diamond in the Tromsø Nappe, Scandinavian Caledonides (N. Norway)

AbstractWe report the first finding of diamond in crustal rocks from the Tromsø Nappe of the North Norwegian Caledonides. Diamond occurs in situ as inclusions in garnet from gneiss at Tønsvika near Tromsø. The rock is composed essentially of garnet, biotite, white mica, quartz and plagioclase, minor constituents include kyanite, zoisite, rutile, tourmaline, amphibole, zircon, apatite and carbonates (magnesite, dolomite, calcite). The microdiamond, identified by micro‐Raman spectroscopy, is cuboidal to octahedral in shape and ranges from 5 to 50 μm in diameter. The diamond occurs as single grains and as composite diamond + carbonate inclusions. Diamond vibration bands show a downshift from 1 332 to 1 325 cm−1, the majority of Raman peaks are centred between 1 332 and 1 330 cm−1 and all peaks exhibit a full width at half maximum between 3 and 5 cm−1. Several spectra show Raman bands typical for disordered and ordered graphite (sp2‐bonded carbon) indicating partial transformation of diamond to graphite. The calculated peak P–T conditions for the diamond‐bearing sample are 3.5 ± 0.5 GPa and 770 ± 50 °C. Metamorphic diamond found in situ in crustal rocks of the Tromsø Nappe thus provides unequivocal evidence for ultrahigh pressure metamorphism in this allochthonous unit of the Scandinavian Caledonides. Deep continental subduction, most probably in the Late Ordovician and shortly before or during the initial collision between Baltica and Laurentia, was required to stabilize the diamond at UHP conditions.

U–Pb dates and trace-element geochemistry of zircon from migmatite, Western Gneiss Region, Norway: Significance for history of partial melting in continental subduction

The Western Gneiss Region (WGR), Norway, is dominated by migmatitic gneiss that contains inclusions of eclogite, some of which contain evidence for ultrahigh-pressure metamorphism. To evaluate geochemical and age relationships between host migmatite and eclogite, we obtained LA–ICP–MS U–Pb dates and trace-element analyses for zircon from a variety of textural types of leucosome, from layer-parallel to crosscutting. Zircon textures (euhedral, oscillatory- and sector-zone grains) indicate a likely magmatic origin of the leucosomes. Caledonian U–Pb zircon dates from zircon rim and near-rim regions are as old as 410–406Ma, coeval with previously determined ages of high- and ultrahigh-pressure metamorphism of WGR eclogite. Trace-element analyses obtained simultaneously with U–Pb dates indicate crystallization of zircon under garnet-present conditions in the majority of leucosomes. Other zircons, including those from crosscutting pegmatite, yield younger ages (as young as 385Ma), coinciding with dates determined for amphibolite-facies retrogression of eclogite; trace-element analyses suggest that these zircons grew under plagioclase-present (garnet-absent) conditions. Combined age and trace-element data for leucosome zircons record the transition from high-pressure (garnet-present, plagioclase-absent) crystallization to lower-pressure (plagioclase-present) crystallization. If the euhedral zircons that yield ages coeval with peak or near-peak UHP metamorphism represent crystallization from anatectic leucosomes, these results, combined with field and petrographic observations of eclogite–migmatite relationships, are consistent with the presence of partially molten crust in at least part of the WGR during continental subduction. The decreased viscosity and increased buoyancy and strain weakening associated with partial melting may have assisted the rapid ascent of rocks from mantle to crustal depths.

Coesite in eclogite from the North Qaidam Mountains and its implications

Coesite provides direct evidence for ultrahigh pressure metamorphism. Although coesite has been found as inclusions in zircon in paragneiss of the north Qaidam Mountains, it has never been identified in eclogite. In this contribution, based on petrographic observations and in situ Raman microprobe spectroscopy, coesite was identified as inclusions in garnet of eclogite from the Aercituoshan, Dulan UHP metamorphic unit, north Qaidam Mountains. Coesite is partly replaced by quartz, showing a palisade texture. This is the first report on coesite in eclogite from the north Qaidam Mountains, and is also supported by garnet-omphacite-phengite geothermobarometry (2.7–3.25 GPa, 670–730°C). Coesite and its pseudomorphs have not been found in eclogites and associated rocks of other units of the north Qaidam Mountains. Further studies are required to confirm if all metamorphic units in the north Qaidam Mountains underwent the ultrahigh-pressure metamorphism.

Quartz exsolution in clinopyroxene is not proof of ultrahigh pressures: Evidence from eclogites from the Eastern Blue Ridge, Southern Appalachians, U.S.A.

Oriented quartz needles in clinopyroxene have become one of the diagnostic indicators of ultrahighpressure (UHP) metamorphism. The presence of apparently exsolved quartz is taken as evidence of decompression of a non-stochiometric Ca.Eskola component (Ca 0.5 ⃞ 0.5 AlSi 2 O 6 , CaEs) that is presumed to be stable only at UHP conditions. Eclogite from the Eastern Blue Ridge, North Carolina, contains clinopyroxene (Jd 20 CaTs 5 Ac 5 CaEs 0 Di 65 Hd 5 ) with oriented needles of quartz and calcic amphibole that appear to have exsolved together. The quartz + amphibole intergrowths are surrounded by 1.5 µm haloes of neoformed pyroxene (Jd 10 CaTs 10 Ac 5 CaEs 0 Di 70 Hd 5 ). The modes of quartz, amphibole, and clinopyroxene haloes were determined using BSE images, and reintegrated with the host clinopyroxene. Viewing the quartz and amphibole needles down the c-axis of the pyroxene host provides a better estimate of their proportions than in prismatic sections. Reintegrated pyroxene compositions were nearly identical to the analyzed host pyroxene with no CaEs component. Clinopyroxene with CaEs solid solution has been repeatedly synthesized at UHP conditions. However, examination of the phase equilibria usually cited as evidence for CaEs stability at conditions of ≥25 kbar shows that clinopyroxene with 10 mol% CaEs is stable well within the quartz field, and provides a pressure minimum similar to the albite = jadeite + quartz barometer. Exsolution of quartz and associated amphibole is commonplace in clinopyroxene from the Blue Ridge eclogite that lacks coesite or other evidence for UHP metamorphism. The presence of a diluted (5.10%) CaEs component in clinopyroxene does not require UHP conditions.

ULTRAHIGH-PRESSURE METAMORPHISM OF GARNET PERIDOTITES FROM POHORJE MTS. (EASTERN ALPS, SLOVENIA)

New evidence for ultrahigh-pressure metamorphism (UHPM) in the Eastern Alps is reported from garnet peridotites of Pohorje Mts. in Slovenia. In this area, an eo- Alpine UHPM has been recently documented in the eclogites (Janak et al. 2004). These eclogites are closely associated with metaultrabasites - predominantly serpentinised dunite and harzburgite with garnet peridotite remnants. The country rocks of eclogites and metaultrabasites are amphibolites, orthogneisses, paragneisses and micaschists. All these rocks belong to the Lower Central Austroalpine basement unit of the Eastern Alps, exposed in the proximity of the Periadriatic fault. Ultramafic rocks have experienced a complex metamorphic history. At least four stages of recrystallization have been identified in the garnet peridotite based on an analysis of reaction textures and mineral compositions. Stage I is a high-temperature protolith assemblage of olivine + orthopyroxene + clinopyroxene + Cr-spinel. Aluminous pyroxenes occur as inclusions in garnet, chromian spinel is preserved in the matrix. Stage II – an ultrahigh-pressure stage is defined by matrix assemblage garnet + olivine + orthopyroxene + clinopyroxene + Cr-spinel. Garnet contains up to 67 mol% of pyrope, olivine has 90 mol% of forsterite, orthopyroxene is low in Al2O3 (~0.8 wt%) and spinel has a Cr* ~ 50. Stage III – a decompression stage is manifested by formation of kelyphitic rims of high-Al orthopyroxene, aluminous spinel and pargasitic hornblende replacing garnet. Due to retrogression, garnet shows a decrease in MgO. Stage IV – is represented by formation of tremolitic amphibole, chlorite, serpentine and talc. P-T estimates based on geothermobarometric calculatios a) Fe-Mg exchange between garnet, olivine and orthopyroxene thermometers, b) the Al-in-orthopyroxene barometer indicate that the peak of metamorphism (stage II) occurred at ~820-900oC and 3-3.5 GPa. This is consistent with previous estimation of very high P-T conditions in metaultrabasites by Hinterlechner-Ravnik et al. (1991) and the associated eclogites (Janak et al. 2004). These results suggest that the mantle fragment (garnet peridotite) and the crustal fragment (eclogite) in the Pohorje Mts. both experienced a common UHPM during the Cretaceous orogeny. We propose that UHPM resulted from deep subduction of a continental slab which incorporated peridotites from an overlying mantle wedge.

Carbonate-Bearing UHPM Rocks from the Tso-Morari Region, Ladakh, India: Petrological Implications

Evidence of ultrahigh-pressure metamorphism (UHPM) of subducted Indian continental crust in the form of carbonate-bearing coesite eclogite is preserved in the Tso-Morari Crystalline Complex (TMC) in eastern Ladakh, India. These eclogites, which occur as boudins in kyanite/sillimanite—grade rocks of the Puga Formation, contain essential mineral assemblages (garnet, clinopyroxeneomphacite, phengite, rutile, epidote-zoisite/clinozoisite and quartz), as well as coesite, talc, kyanite, magnesite, aragonite, dolomite, and Mg-calcite. Coesite, magnesite, and dolomite occur as inclusions in zoned garnet. The carbonate-bearing coesite eclogite underwent three stages of metamorphism—prograde, peak, and retrograde. The prograde assemblage is characterized by the presence of magnesite and a SiO2 polymorph, which is stable throughout the metamorphic process from the prograde to retrograde stage. At ultrahigh-pressure (27 kbar) and a temperature of 650°C, quartz transforms to coesite. Peak metamorphism was characterized by the development of coesite in garnet coexisting with high-Si phengite, clinopyroxene, magnesite, aragonite, dolomite, zoisite/clinozoisite, kyanite, and talc at a pressure of >39 kbar and temperature of >750°C. This is in good agreement with the estimated peak pressure and temperature judging from the composition of phengite, jadeite barometry, and garnet-clinopyroxene, garnet-phengite thermometry. Enstatite formed with talc and kyanite at a pressure of >31 kbar and temperature of 750°C. With a subsequent decrease in pressure, retrogression is constrained by the development of chlorite and chloritoid, which surround the garnet at a minimum pressure of 4-5 kbar and temperature of <500°C. Mineral assemblages in the carbonate-bearing coesite eclogite reveal that prograde metamorphism started with greenschist-facies conditions and reached the ultrahigh-pressure eclogite facies, passing through the intermediate blueschist facies. During UHP metamorphism, pressure abruptly doubled with a slight change of temperature, defining a geothermal gradient of 6–7°C/km. The UHP material was brought back to the surface along a path by rapid and almost isothermal exhumation.

First evidence for ultrahigh-pressure metamorphism in the North- East Greenland Caledonides

The first evidence for ultrahigh-pressure ( P ) metamorphism in the Greenland Caledonides is reported from kyanite eclogites and associated host gneisses on an island in Jokelbugt. Polycrystalline quartz inclusions in garnet and omphacite exhibit palisade quartz rims that are a diagnostic feature of quartz pseudomorphs after coesite, thus providing textural evidence for ultrahigh- P conditions. Geothermobarometry on the mineral assemblage garnet + omphacite + kyanite + quartz and/or coesite ± phengite confirms the microstructural interpretation of ultrahigh- P metamorphism. Peak pressure and temperature conditions (∼972 °C at 3.6 GPa) are well within the coesite stability field. The host gneisses are more retrograded than the kyanite eclogites and only record high- P conditions of 2.5 GPa at 826 °C; however, garnet contains polycrystalline quartz inclusions with radial fractures, suggesting that the gneisses also were subject to ultrahigh- P conditions. The presence of high- and ultrahigh- P metamorphism along the Laurentian and Baltica margins, the high temperatures recorded by the ultrahigh- P terranes in Greenland and Norway, and the absence of mantle peridotites in Greenland suggest that crustal thickening may have played an important role in the formation of an extensive orogenic root in the Greenland and Scandinavian Caledonides.

Characteristics of UHP Pelites, Gneisses, and Other Unusual Rocks

The main felsic lithologies experiencing ultrahigh-pressure metamorphism (UHPM) are described, with a focus on rock association, nature, and composition of the peak minerals, as well as nature of the protolith. Felsic rocks usually preserve poor evidence of UHPM, being pervasively deformed and retrogressed during exhumation. Locally, however, unstrained lithologies are preserved that show UHPM minerals and even microstructures inherited from the protolith. Among the number of localities where UHPM has been reported, the following have been considered in detail— the southern Dora-Maira massif and Lago di Cignana in the Western Alps, the Dabie Shan-Sulu region in northeastern China, the Kokchetav massif in northern Kazakhstan, and the Western Gneiss Region of the Scandinavian Caledonides. At Lago di Cignana, metasediments of oceanic affinity are carbonate-bearing garnet-phengite-quartz schist and oxidized manganiferous quartz schists, all preserving rare coesite relics. In the Dora-Maira massif, the Brossasco-Isasca Unit includes coesite-bearing phengite-jadeite-almandine-quartz micaschist, pyrope whiteschist, jadeite-kyanite quartzite, sodic whiteschist, and jadeite-almandine orthogneiss. In the Dabie Shan-Sulu region, felsic rocks with clear evidence of UHPM include jadeite quartzite, talc-kyanite quartzite, and other kyanite-bearing quartzites. Moreover, other lithotypes like aegirine orthogneiss, paragneiss, and micaschist also may have been subjected to UHPM. In the Kokchetav massif, microdiamond-bearing rocks include garnet-biotite gneisses and schists, garnet-pyroxene and pyroxene-carbonate-garnet rocks, and possibly also whiteschists. Other localities where UHPM felsic rocks have been reported include the northern Tien Shan, northern Mali, the Maksyutov Complex in the southern Urals, and the Western Gneiss Region of the southwestern Caledonides. In felsic rocks, the most “common” UHP minerals are coesite, clinopyroxenes, garnets, epidotes, kyanite, phengite, talc, aragonite, dolomite, and accessory rutile, titanite, apatite, tourmaline, and zircon. Exceptionally, rare or unique phases also may be found as inclusions armored in more competent minerals. In the highest-P areas, microdiamond occurs, its growth being strongly controlled by the composition of the fluid phase. Mineral chemistry is sensitive to both bulk-rock chemical composition and P-T conditions: garnets are solid solutions of the pyralspite series, the Mn-rich members being high in calderite; pyroxenes are jadeite-diopside solid solutions; epidotes include allanite, zoisite, clinozoisite-epidote solid solutions, and piemontite; and micas are highly celadonitic phengites. In rocks with microdiamond, extreme UHPM conditions are further suggested by the unusual composition of some minerals, such as garnet high in Si, Y, and Yb, or Na-pyroxenes rich in K. Clear evidence of partial melting during UHMP is missing, although peak conditions are close to or above the minimum granite wet solidus. This may be due to either the aH2O < 1, or to melt extraction.