Evidence For Metamorphism Research Articles

The First Samples from Almahata Sitta Showing Contacts Between Ureilitic and Chondritic Lithologies: Implications for the Structure and Composition of Asteroid 2008 TC3.

Almahata Sitta (AhS), an anomalous polymict ureilite, is the first meteorite observed to originate from a spectrally classified asteroid (2008 TC3). However, correlating properties of the meteorite with those of the asteroid is not straightforward because the AhS stones are diverse types. Of those studied prior to this work, 70-80% are ureilites (achondrites) and 20-30% are various types of chondrites. Asteroid 2008 TC3 was a heterogeneous breccia that disintegrated in the atmosphere, with its clasts landing on Earth as individual stones and most of its mass lost. We describe AhS 91A and AhS 671, which are the first AhS stones to show contacts between ureilitic and chondritic materials and provide direct information about the structure and composition of asteroid 2008 TC3. AhS 91A and AhS 671 are friable breccias, consisting of a C1 lithology that encloses rounded to angular clasts (<10 μm to 3 mm) of olivine, pyroxenes, plagioclase, graphite, and metal-sulfide, as well as chondrules (~130-600 μm) and chondrule fragments. The C1 material consists of fine-grained phyllosilicates (serpentine and saponite) and amorphous material, magnetite, breunnerite, dolomite, fayalitic olivine (Fo 28-42), an unidentified Ca-rich silicate phase, Fe,Ni sulfides, and minor Ca-phosphate and ilmenite. It has similarities to CI1 but shows evidence of heterogeneous thermal metamorphism. Its bulk oxygen isotope composition (δ18O = 13.53‰, δ17O = 8.93‰) is unlike that of any known chondrite, but similar to compositions of several CC-like clasts in typical polymict ureilites. Its Cr isotope composition is unlike that of any known meteorite. The enclosed clasts and chondrules do not belong to the C1 lithology. The olivine (Fo 75-88), pyroxenes (pigeonite of Wo ~10 and orthopyroxene of Wo ~4.6), plagioclase, graphite, and some metal-sulfide are ureilitic, based on mineral compositions, textures, and oxygen isotope compositions, and represent at least six distinct ureilitic lithologies. The chondrules are probably derived from type 3 OC and/or CC, based on mineral and oxygen isotope compositions. Some of the metal-sulfide clasts are derived from EC. AhS 91A and AhS 671 are plausible representatives of the bulk of the asteroid that was lost. Reflectance spectra of AhS 91A are dark (reflectance ~0.04-0.05) and relatively featureless in VNIR, and have an ~2.7 μm absorption band due to OH- in phyllosilicates. Spectral modeling, using mixtures of laboratory VNIR reflectance spectra of AhS stones to fit the F-type spectrum of the asteroid, suggests that 2008 TC3 consisted mainly of ureilitic and AhS 91A-like materials, with as much as 40-70% of the latter, and <10% of OC, EC and other meteorite types. The bulk density of AhS 91A (2.35 ± 0.05 g/cm3) is lower than bulk densities of other AhS stones, and closer to estimates for the asteroid (~1.7-2.2 g/cm3). Its porosity (36%) is near the low end of estimates for the asteroid (33-50%), suggesting significant macroporosity. The textures of AhS 91A and AhS 671 (finely comminuted clasts of disparate materials intimately mixed) support formation of 2008 TC3 in a regolith environment. AhS 91A and AhS 671 could represent a volume of regolith formed when a CC-like body impacted into already well-gardened ureilitic + impactor-derived debris. AhS 91A bulk samples do not show a solar wind (SW) component, so they represent sub-surface layers. AhS 91A has a lower cosmic ray exposure (CRE) age (~5-9 Ma) than previously studied AhS stones (11-22 Ma). The spread in CRE ages argues for irradiation in a regolith environment. AhS 91A and AhS 671 show that ureilitic asteroids could have detectable ~2.7 μm absorption bands.

The lawsonite-glaucophane blueschists of Elba Island (Italy)

Evidence for high-P blueschist-facies metamorphism was found in metabasites embedded in calcschists of Eastern Elba Island (Northern Apennines, Italy). Study of immobile trace elements (REEs and HFSEs) in the metabasites indicates an affinity with T- and E-MORBs, and they are interpreted as sill or dykes intruded in the western margin of the Adria continental plate. The minerals are heterogeneously distributed in the rock, constituting mafic and Ca-Al-rich microdomains inherited respectively from the magmatic pyroxene- and plagioclase-rich zones of the original doleritic rock. The peak pressure paragenesis consisted of lawsonite, aegirine-omphacite, glaucophane and chlorite. Former rhombic prisms of lawsonite have been replaced by pseudomorphic clinozoisite (0.1 mm in size) with inclusions of Ms. ± Ab ± Qz. The inclusions are preferentially oriented parallel to one of the diagonals of the basal rhombic sections, indicating that the two diagonals were not crystallographically equivalent. This implies that the protocrystals were orthorhombic prisms with {110} faces, which is the case of lawsonite. The different compositions shown by white mica and epidote in the pseudomorphs (muscovite and Fe3+-poor clinozoisite) and in the matrix (phengite and Fe3+-rich epidote) also suggest the former presence of Mg- and Fe-poor lawsonite in these rocks. Thermodynamic modelling, using the THERMOCALC software and dataset, suggests that metabasites experienced a clockwise P-T path from the lawsonite-blueschist to epidote-blueschist subfacies, down to greenschist facies. The estimated peak pressure conditions are P ≥ 1.6 GPa and 450 < T < 500 °C. Glaucophane grew mainly during the lawsonite-to-epidote transition and is dated at 19.8 ± 1.4 Ma by the 40Ar/39Ar method. This evolution is interpreted in terms: (i) intrusion of basic magmas in carbonatic and pelitic sediments (post-late Cretaceous, pre-Oligocene), (ii) subduction (Oligocene), (iii) collision (early Miocene: 19.8 ± 1.4 Ma), (iv) exhumation and granite intrusion (middle-late Miocene), favored by extensional tectonics. We underline that the eastern part of Elba Island belongs to a blueschist-facies belt that extends from Gorgona Island, in the northwest, to Argentario and Giglio Island in the southeast, so many of the conclusions of this study can be extended to the whole of this belt.

EPMA monazite geochronology of the granulites from Daltonganj, eastern India and its correlation with the Rodinia supercontinent

We report the monazite dates of the granulites from Daltonganj (Palamau), Chhotanagpur granite–gneiss complex (CGGC) which covers the significant part of the granulite blocks in central India by using an electron micro probe analyser dating. The monazite grain varies between 70 and 80 μm and shows the distribution of U, Th and Pb in all monazite grains of both samples. Two different dates were obtained from different monazite grains; the first age suggests that the granulite from CGGC preserves the first remnant of the protolith of the Mesoproterozoic era at ~1424 Ma and second one at ~972 Ma which provides evidence of metamorphism of the protolith. The CGGC rocks preserve four regional metamorphic events, namely M1, M2, M3 and M4. But in this work, two different ages from the Daltonganj granulites were obtained which are similar to the M2 (<1500 Ma, i.e., the age of protolith of the granulitic gneiss) and M3 (1200–930 Ma) metamorphic events as reported in the CGGC. The M3 metamorphism attained its average P–T condition at ~7.35 kbar/792°C, and it represents the prograde metamorphic event. The M3 metamorphic event supported the Grenville-orogeny, and it was responsible for the metamorphism of the magmatic protolith of granulitic gneiss from the CGGC at the time of amalgamation of the Rodinia supercontinent. The Rodinia assembly had occurred through the global Grenville-orogenic events between 1100 and 900 Ma, with continental blocks which exist at that time.

Documentation of shock features in impactites from the Dhala impact structure, India

AbstractThe fundamental approach for the confirmation of any terrestrial meteorite impact structure is the identification of diagnostic shock metamorphic features, together with the physical and chemical characterization of impactites and target lithologies. However, for many of the approximately 200 confirmed impact structures known on Earth to date, multiple scale‐independent tell‐tale impact signatures have not been recorded. Especially some of the pre‐Paleozoic impact structures reported so far have yielded limited shock diagnostic evidence. The rocks of the Dhala structure in India, a deeply eroded Paleoproterozoic impact structure, exhibit a range of diagnostic shock features, and there is even evidence for traces of the impactor. This study provides a detailed look at shocked samples from the Dhala structure, and the shock metamorphic evidence recorded within them. It also includes a first report of shatter cones that form in the shock pressure range from ~2 to 30 GPa, data on feather features (FFs), crystallographic indexing of planar deformation features, first‐ever electron backscatter diffraction data for ballen quartz, and further analysis of shocked zircon. The discovery of FFs in quartz from a sample of the MCB‐10 drill core (497.50 m depth) provides a comparatively lower estimate of shock pressure (~7–10 GPa), whereas melting of a basement granitoid infers at least 50–60 GPa shock pressure. Thus, the Dhala impactites register a strongly heterogeneous shock pressure distribution between &lt;2 and &gt;60 GPa. The present comprehensive review of impact effects should lay to rest the nonimpact genesis of the Dhala structure proposed by some earlier workers from India.

Metamorphic P-T conditions and variation of REE between two garnet generations from granulites in the Sør-Rondane mountains, East Antarctica

In this paper, we describe the metamorphic conditions of Fe-rich granulite and variations in rare earth elements (REE) between peak garnet porphyroblasts and secondary garnet coronae. The Fe-rich granulites were collected from Vesthaugen, Sor-Rondane Mountains, East Antarctica, and consist mainly of cordierite, garnet, spinel, perthite, K-feldspar, plagioclase, and orthopyroxene or sillimanite. Temperatures estimated from perthitc–mesoperthitic feldspar compositions, experimentally calibrated geothermobarometers and the modeling of P-T pseudosections suggest that the rocks experienced peak ultrahigh-temperature (UHT) metamorphic conditions of 900–950 °C and 5.0 ± 0.5 kbar. Spinel contains quartz inclusions that also provide evidence for UHT metamorphism. Evidence of partial melting is characterized by the presence of leucocratic bands. The second generation of garnet occurs as coronae around spinel, formed during isobaric cooling following the peak conditions of UHT metamorphism. Garnet coronae and garnet porphyroblasts have distinct trace element patterns. Textural evidence and REE geochemistry suggest that the development of garnet coronae was controlled by (1) the REE composition of reactant phases and melt and/or (2) the crystallization of HREE-rich accessory phases (e.g., zircon and monazite) during secondary garnet growth.

(U‐Th)/He zircon dating of Chesapeake Bay distal impact ejecta from ODP site 1073

AbstractSingle crystal (U‐Th)/He dating has been undertaken on 21 detrital zircon grains extracted from a core sample from Ocean Drilling Project (ODP) site 1073, which is located ~390 km northeast of the center of the Chesapeake Bay impact structure. Optical and electron imaging in combination with energy dispersive X‐ray microanalysis (EDS) of zircon grains from this late Eocene sediment shows clear evidence of shock metamorphism in some zircon grains, which suggests that these shocked zircon crystals are distal ejecta from the formation of the ~40 km diameter Chesapeake Bay impact structure. (U‐Th/He) dates for zircon crystals from this sediment range from 33.49 ± 0.94 to 305.1 ± 8.6 Ma (2σ), implying crystal‐to‐crystal variability in the degree of impact‐related resetting of (U‐Th)/He systematics and a range of different possible sources. The two youngest zircon grains yield an inverse‐variance weighted mean (U‐Th)/He age of 33.99 ± 0.71 Ma (2σ uncertainties n = 2; mean square weighted deviation = 2.6; probability [P] = 11%), which is interpreted to be the (U‐Th)/He age of formation of the Chesapeake Bay impact structure. This age is in agreement with K/Ar, 40Ar/39Ar, and fission track dates for tektites from the North American strewn field, which have been interpreted as associated with the Chesapeake Bay impact event.

Lewis Cliff 86211 and 86498: Metal-sulfide liquid segregates from a carbonaceous chondrite impact melt

Abstract The petrogenesis of the ungrouped iron meteorite Lewis Cliffs (LEW) 86211 and its proposed pair LEW 86498 has remained elusive in the decades since their discovery in Antarctica. Wasson (1990) and Kracher et al. (1998) noted the enrichment in the siderophile refractory elements, fine-grained texture, and high abundances of sulfides in LEW 86211 as features that are both difficult to explain and that set it apart from other iron meteorites. In this work, we investigate the pairing and formation of these two ungrouped iron meteorites using a combination of petrography, electron microprobe analyses, and secondary ion probe analyses of oxygen-three isotope of olivine. Similarities in petrographic features and phase compositions further support the initial pairing of LEW 86211 and 86498. The bulk composition of LEW 86211 (Wasson, 1990) closely resembles those of separated chondritic metallic components (e.g., Kong and Ebihara, 1997), which indicates this pairing group formed directly from this portion of a chondrite. The metal-sulfide cellular textures and mineral compositional trends are consistent with LEW 86211 and 86498 forming by rapid cooling of the FeNiS immiscible liquid of a larger chondritic impact melt unit. Previous bulk oxygen-three isotope analyses (Clayton and Mayeda, 1996) combined with the in situ oxygen-three isotope analyses from this work are consistent with LEW 86211 having a carbonaceous chondrite provenance. LEW 86211 is most similar to CR chondrites in its oxygen-three isotope signatures, but may not be from an established carbonaceous chondrite group. The silicate inclusions in LEW 86211 and 86498 record evidence of pre-impact metamorphism and later reduction related to contact with the metal-sulfide impact melt liquid. The silicate inclusions appear to have been engulfed by metal-sulfide liquids rather than part of the impact melted unit. Additionally, the size of this sulfide-dominated pairing group compared to the volume of sulfides and metal in unmelted CR chondrite suggests that these meteorites originated from a much larger carbonaceous chondrite impact melt body than has been previously recognized (e.g., Lunning et al., 2016).

Petrographical study of paragneisses in Northeast of Golpayegan: migmatization and evidences of retrograde metamorphism

Petrographical study of paragneisses in Northeast of Golpayegan: migmatization and evidences of retrograde metamorphism

Shock‐induced microtextures in lunar apatite and merrillite

AbstractApatite and merrillite are the most common phosphate minerals in a wide range of planetary materials and are key accessory phases for in situ age dating, as well as for determination of the volatile abundances and their isotopic composition. Although most lunar and meteoritic samples show at least some evidence of impact metamorphism, relatively little is known about how these two phosphates respond to shock‐loading. In this work, we analyzed a set of well‐studied lunar highlands samples (Apollo 17 Mg‐suite rocks 76535, 76335, 72255, 78235, and 78236), in order of displaying increasing shock deformation stages from S1 to S6. We determined the stage of shock deformation of the rock based on existing plagioclase shock‐pressure barometry using optical microscopy, Raman spectroscopy, and SEM‐based panchromatic cathodoluminescence (CL) imaging of plagioclase. We then inspected the microtexture of apatite and merrillite through an integrated study of Raman spectroscopy, SEM‐CL imaging, and electron backscatter diffraction (EBSD). EBSD analyses revealed that microtextures in apatite and merrillite become progressively more complex and deformed with increasing levels of shock‐loading. An early shock‐stage fragmentation at S1 and S2 is followed by subgrain formation from S2 onward, showing consistent decrease in subgrain size with increasing level of deformation (up to S5) and finally granularization of grains caused by recrystallization (S6). Starting with 2°–3° of intragrain crystal‐plastic deformation in both phosphates at the lowest shock stage, apatite undergoes up to 25° and merrillite up to 30° of crystal‐plastic deformation at the highest stage of shock deformation (S5). Merrillite displays lower shock impedance than apatite; hence, it is more deformed at the same level of shock‐loading. We suggest that the microtexture of apatite and merrillite visualized by EBSD can be used to evaluate stages of shock deformation and should be taken into account when interpreting in situ geochemically relevant analyses of the phosphates, e.g., age or volatile content, as it has been shown in other accessory minerals that differently shocked domains can yield significantly different ages.

Petrological and geochronological evidence for Paleoproterozoic granulite-facies metamorphism of the South Liaohe Group in the Jiao-Liao-Ji Belt, North China Craton

To better understand the formation of high-temperature metamorphic rocks, we present a detailed petrological and geochronological study of recently discovered cordierite-bearing granulites of the South Liaohe Group in the Jiao-Liao-Ji Belt, North China Craton. Petrographic observations indicate that four distinct mineral assemblages are present in the granulites: pre-peak amphibolite facies (M1), peak granulite facies (M2), post-peak decompression (M3), and late cooling retrogression (M4). M1 is preserved as fine-grained inclusions in the cores of garnet grains, represented by quartz + plagioclase + biotite + ilmenite. M2 is interpreted to have comprised garnet + sillimanite + plagioclase + quartz + biotite + ilmenite + melt. M3 was characterized by the formation of cordierite + sillimanite symplectites and cordierite + quartz coronas replacing garnet. M4 is indicated by the formation of staurolite, accompanied by the crystallization of melt. A combination of multi-equilibria geothermobarometers and pseudosection modeling constrains the P–T conditions of the M1, M2, M3, and M4 stages to P = 0.66–0.71 GPa and T = 620–650 °C, P = 0.96–1.10 GPa and T = 790–840 °C, P = 0.62–0.65 GPa and T = 725–785 °C, and P = 0.43–0.55 GPa and T = 595–625 °C, respectively. Zircon and monazite U–Pb dating yields three distinct and meaningful ages for the granulites: (1) 2200–2100 Ma for the granulite protoliths, (2) ca. 1945 Ma for the near-peak metamorphism, and (3) 1851–1839 Ma for the post-peak and late retrogressive metamorphism. Thus, a clockwise P–T–t path is determined for the cordierite-bearing granulites of the South Liaohe Group. Petrological and geochronological evidence from regional metamorphic rocks in the Jiao-Liao-Ji Belt suggests that the belt experienced a continuous orogenesis from 1950 to 1800 Ma.

Petrology and phase equilibrium modeling of granulites from Obudu in the Benin-Nigerian Shield, Southeastern Nigeria: implications for clockwise P-T evolution in a collisional orogen

The Eastern Nigerian terrane forms the eastern flank of the Trans-Saharan Orogenic Belt within the Benin-Nigerian Shield. We report new petrological and P–T data of metamorphic rocks (migmatitic gneisses with subordinate granite gneiss, meta-quartz diorite, and meta-ultramafic rock) from Obudu in the southeastern flank of the Benin-Nigerian Shield for unraveling the tectono-metamorphic signatures of the eastern margin of the Trans-Saharan Orogenic Belt. The migmatized pelitic gneiss contains a peak mineral assemblage of plagioclase + rutile + garnet + biotite + K-feldspar + quartz ± liquid, based on which the peak metamorphic conditions are constrained as 850–890 °C and 9–10 kbar using phase equilibria modeling, optimal thermobarometry, and garnet-biotite geothermometry. Retrograde conditions were also estimated for a plagioclase + quartz + garnet + biotite + cordierite + rutile + sillimanite assemblage as 3.8 kbar/590 °C to 6.8 kbar/780 °C. The peak condition under granulite facies confirmed a marked increase of metamorphic grade from the greenschist facies in the western Benin-Nigerian Shield through the middle-upper amphibolite facies in the north-central Nigerian terranes to the granulite facies in the eastern part of the Benin-Nigerian Block. The results presented in this study are the first evidence of granulite-facies metamorphism from the easternmost parts of the Benin-Nigerian Shield. The clockwise path obtained in this study might suggest a continent-continent collisional setting for the evolution of this region related to the closure of the Goias-Pharusian Ocean and the formation of the Trans-Saharan Orogenic Belt.

Minor and trace element variations in chromite from the Songshugou dunites, North Qinling Orogen: Evidence for amphibolite-facies metamorphism

The Songshugou dunites are part of the ultramafic sequence of the Songshugou ophiolite within the Qinling Group metamorphic rocks, which are distributed in the northern part of the Shang–Dan Suture Zone (SDSZ) in central China. The dunites include lenses of coarse- and medium-grained dunite set in a fine-grained dunite host. Disseminated chromite within these dunites can be classified into three microstructural types: (1) partly altered chromite with primary cores surrounded by porous chromite rims; (2) porous chromite with abundant pores that are filled with chlorite; and (3) homogeneous chromite. The primary magmatic cores of the partly altered chromite (Chr) is depleted in Cr and enriched in Al and Mg. Porous chromite includes Cr-rich Chr and ferritchromite (Fe-Chr) that is enriched in Fe2+ but depleted in Al. Homogeneous chromite is mainly Cr-magnetite (Cr-Mag) that is enriched in Fe3+ and extremely depleted in Al. The primary Cr-poor chromite was gradually transformed to porous Cr-rich Chr and/or by Fe-Chr during metamorphism. Reaction of Cr-poor Chr with olivine (Ol) produces chlorite (Chl) and secondary Chr enriched in Cr and Fe2+. Mg and Al diffuse away from primary Chr, leaving a residual Al- and Mg-depleted, Cr-rich Chr and Fe-Chr. Porous Chr is enriched in Zn, Co, Mn, but depleted in Ga, Ni, and Sc, due to the fluid-assisted formation of Chl within the pores of porous chromite. Zn, Co, and Mn from the metamorphic fluids diffuse toward the cores of chromite. The elements Sc, Ti, V, and Ni diffuse from Cr-poor Chr core to Cr-mag rim with increasing Fe3+/(Fe3+ + Fe2+). Late precipitation of Cr-mag occurred under oxidizing and hydrous conditions. The infiltration of fluid produced non-porous Fe3+-rich Chr or Cr-mag along grain boundaries or fractures within chromite. These fluids further promoted the reordering of octahedral sites, enhancing higher mobilization and enrichment of Ga, Ni, Sc, V, and Ti. The infiltration of high oxygen fugacity fluids promoted diffusion of Ga, Ni, Sc, and Ti toward the rims, and diffusion of Zn, Co, and Mn toward the cores. All chromite grains from the Songshugou dunites have low Mg# [molar 100 × Mg/(Mg + Fe2+)] values, and high Cr# [molar 100 × Cr/(Cr + Al)] and Fe3+/(Cr + Al + Fe3+), similar to metamorphic chromite formed under amphibolite-facies conditions. The observed positive Zn–Co–Mn anomalies and negative Ga–Ni–Sc anomalies in chromite, and the crystallization of Chl, are fingerprints of amphibolite-facies metamorphism. The compositions of chromite, the inferred mineral assemblage in which it occurs, and mineral equilibria modeling of the assemblage all suggest amphibolite-facies metamorphic temperatures ranging from 600 °C to 700 °C.

Shocked quartz grains in the early Cambrian Vakkejokk Breccia, Sweden—Evidence of a marine impact

AbstractHere we present a study of the abundance and orientation of planar deformation features (PDFs) in the Vakkejokk Breccia, a proposed lower Cambrian impact ejecta layer in the North‐Swedish Caledonides. The presence of PDFs is widely accepted as evidence for shock metamorphism associated with cosmic impact events and their presence confirms that the Vakkejokk Breccia is indeed the result of an impact. The breccia has previously been divided into four lithological subunits (from bottom to top), viz. lower polymict breccia (LPB), graded polymict breccia (GPB), top sandstone (TS), and top conglomerate (TC). Here we show that the LPB contains no shock metamorphic features, indicating that the material derives from just outside of the crater and represents low‐shock semi‐autochthonous bombarded strata. In the overlying, more fine‐grained GPB and TS, quartz grains with PDFs are relatively abundant (2–5% of the grain population), and with higher shock levels in the upper parts, suggesting that they have formed by reworking of more distal ejecta by resurge of water toward the crater in a marine setting. The absence of shocked quartz grains in the TC indicates that this unit represents later slumps associated with weathering and erosion of the protruding crater rim. Sparse shocked quartz grains (&lt;0.2%) were also found in sandstone beds occurring at the same stratigraphic level as the Vakkejokk Breccia 15–20 km from the inferred crater site. It is currently unresolved whether the sandstone at these distal sites is related to the impact or just contains rare reworked quartz grains with PDFs.

Eclogites in peridotite massifs in the Western Gneiss Region, Scandinavian Caledonides: Petrogenesis and comparison with those in the Variscan Moldanubian Zone

Eclogite lenses and boudins are volumetrically minor, but petrologically important, features of peridotite massifs worldwide. In the Western Gneiss Region of the Scandinavian Caledonides, eclogites in the Almklovdalen and Raubergvik peridotites originated as basaltic to picrobasaltic dikes, comprising both olivine–normative and nepheline–normative types, with a wide variation in Mg–number from 34 to 65. Positive anomalies for Pb and Sr and negative anomalies for Zr and Hf reflect a subduction signature in the basic melts, and rare–earth element modelling requires 20% to 70% fractional crystallization, combined with 20% to 70% assimilation of peridotite. Clinopyroxenes in eclogites have a wide variation in εNd(0) from +68 to −26, which is comparable to that for associated garnet peridotites and pyroxenites, +55 to −38, and a range in 87Sr/86Sr from 0.7021 to 0.7099, which is much larger than that in peridotites and pyroxenites, 0.7014 to 0.7033.Plagioclase and amphibole inclusions in eclogite garnet provide evidence for prograde metamorphism, which attained a maximum temperature of ~775 °C and pressure of ~25 kb. Such conditions are allofacial with those of associated garnet peridotites and pyroxenites, which equilibrated at ~825 °C and ~37 kb. Eclogites yield mixed Sm-Nd isochron ages, as do the peridotites and pyroxenites, but ages in eclogites are <1000 Ma, and those in peridotites and pyroxenites are >1000 Ma. Three eclogites yield Ordovician U-Pb ages for rutile at 440 ± 12, 445 ± 51, and 480 ± 29 Ma, which are coeval with the Taconic Orogeny and are consistent with a Laurentian provenance for the host peridotites.Eclogites in both Norwegian and Czech peridotites originated from melts passing through a mantle wedge above a subduction zone, and both suites exhibit subduction geochemical signatures, although they differ dramatically in petrogenesis. Eclogites in Norwegian peridotites initially crystallized as relatively low–pressure, plagioclase–bearing basaltic or gabbroic dikes and subsequently recrystallized to high–pressure eclogite, whereas most eclogites in Variscan Moldanubian peridotites crystallized directly from magmas at high pressure to produce eclogite facies assemblages.

Reply to Comment on “Ultra-high pressure and ultra-reduced minerals in ophiolites may form by lightning strikes” by Griffin et al., 2018: No evidence for transition zone metamorphism in the Luobusa ophiolite

Reply to Comment on “Ultra-high pressure and ultra-reduced minerals in ophiolites may form by lightning strikes” by Griffin et al., 2018: No evidence for transition zone metamorphism in the Luobusa ophiolite

Ultramafic–Mafic Assemblage of Plutonic Rocks and Hornblende Schists of Shirshov Rise, Bering Sea, and Stalemate Ridge, Northwest Pacific: Geodynamic Interpretations of Geochemical Data

The paper presents data on plutonic and metamorphic rocks dredged during Cruise 249 of the German R/V Sonne to the Stalemate Ridge, Northwest Pacific Ocean and the Shirshov Rise, western Bering Sea. Dredges in the northwestern sector of the Stalemate Ridge and central portion of the Shirshov Rise show that the plutonic and metamorphic rocks obtained here are amazingly similar. Our petrologic and geochemical data led us to view the rocks as members of a mafic–ultramafic assemblage typical of cumulate portions of ophiolite complexes and backarc spreading centers. The plutonic complexes of the Shirshov Rise and Stalemate Ridge show similarities not only in the petrography and mineralogy of their protoliths but also in the character of their metamorphic transformations. Plutonic rocks from both areas display mineralogical evidence of metamorphism within a broad temperature range: from the high-temperature amphibolite facies to the greenschist facies. Relations between the index mineral assemblages indicate that the metamorphic history of plutonic complexes in the Stalemate Ridge and Shirshov Rise proceeded along a retrograde path. Hornblende schists accompanying the plutonic rocks of the Stalemate Ridge and Shirshov Rise are petrographically close to foliated amphibolites in subophiolitic metamorphic aureoles. Within the framework of geodynamic interpretations of our results, it is realistic to suggest that the examined plutonic complexes were exhumed from subduction zones of various age.

Eclogite facies metabasites from the Paleoproterozoic Nyong Group, SW Cameroon: Mineralogical evidence and implications for a high-pressure metamorphism related to a subduction zone at the NW margin of the Archean Congo craton

High- to ultrahigh-pressure metamorphic assemblages consisting of garnet-omphacitic clinopyroxene bearing mafic rocks have been identified within the Paleoproterozoic Nyong Group in SW Cameroon, at the northwestern margin of the Archean Congo craton. These rocks were investigated in detail and for the first time evidence for eclogite facies metamorphism at ca 25 kbar and 850 °C is provided. A clockwise P-T path with nearly isothermal decompression (ITD) is deduced from mineral zoning and textural relationships characterized by mineral recrystallization and multi-layered coronitic overgrowths of plagioclase and clinopyroxene surrounding garnet porphyroblasts. These P-T conditions imply a burial depth greater than 90 km, at lower geothermal gradient of ca 10 °C/km. The geochemical signature of ten representative rock samples show that two groups of eclogite facies rocks genetically originate from mostly basaltic and basaltic andesite compositions, with a characteristic upper mantle-derived tholeiitic trend. Moreover, their chondrite and MORB normalized REE and trace element concentrations are characterized by nearly flat REE patterns with very little to no Eu anomaly, (La/Sm)N ≥ 1 and Zr/Nb ≤ 10, as well as a gradual depletion from LREE to HREE with also very little to no Eu anomaly, but (La/Sm)N < 1, Zr/Nb > 10 and negative anomalies in Th, K, Nb, Ta, Sr, Zr and Ti consistent with mid-ocean ridge basalt (MORB) contaminated by a subduction component or by a crustal component.Previous available geochronological data coupled with our new petrological, mineralogical and geochemical findings clearly indicate that the eclogite facies metabasites from the Eburnean Nyong Group between 2100 and 2000 Ma represent one of the oldest subducted oceanic slab or trace of a suture zone so far recorded within the West Central African Fold Belt (WCAFB). The geodynamic implications of these eclogites suggest a subduction-related process followed by a rapid exhumation of their protoliths, therefore, providing critical information corroborating that plate tectonic processes operated during the Paleoproterozoic.

Petrology and Rare Earth Elements Mineral Chemistry of Chadegan Metabasites (Sanandaj-Sirjan Zone, Iran): Evidence for Eclogite-Facies Metamorphism during Neotethyan Subduction

The metabasites of Chadegan, including eclogite, garnet amphibolite and amphibolite, are forming a part of Sanandaj–Sirjan Zone. These rocks have formed during the subduction of the Neo–Tethys ocean crust under Iranian plate. This subduction resulted in a subduction metamorphism under high pressuremedium temperature of eclogite and amphibolites facies condition. Then the metamorphic rocks were exhumed during the continental collision between the Afro–Arabian continent and the Iranian microcontinent. In the metabasite rocks, with typical MORB composition, garnet preserved a compositional zoning occurred during metamorphism. The magnesium (XMg) gradually increases from core to rim of garnets, while the manganese (XMn) decreases towards the rim. Chondrite–normalized Rare Earth Element patterns for these garnets exhibit core–to–rim increases in Light Rare Earth Elements. The chondrite–normalized REE patterns of garnets, amphiboles and pyroxenes display positive trend from LREEs to Heavy Rare Earth Elements (especially in garnet), which suggests the role of these minerals as the major controller of HREE distribution. The geochemical features show that the studied eclogite and associated rocks have a MORB origin, and probably formed in a deep–seated subduction channel environment. The geothermometry estimation yields average pressure of ~22 kbar and temperature of 470–520°C for eclogite fomation. The thermobarometry results gave T = 650–700°C and P ≈ 10–11 kbar for amphibolite facies.

Timing, geochemistry and tectonic setting of Ni-Cu sulfide-associated intrusions of the Halls Creek Orogen, Western Australia

The Lamboo Province of the East Kimberley region of Western Australia contains a record of crustal and tectonic evolution from ca. 1912 to 1808 Ma. In this interval, current models indicate an ocean basin was systematically closed from ca. 1880 Ma until collision of the Kimberley (Western Zone) and North Australia Cratons (Eastern Zone) at the ca. 1830 Ma Halls Creek Orogeny. Within this orogen, the Central Zone has two proposed origins: (1) an intra-oceanic island-arc; and (2) an exotic continental fragment. In this study, we investigated the origin of Ni-Cu mineralized mafic-ultramafic intrusions found in the Central Zone. We performed U-Pb SHRIMP geochronology on 21 samples, together with O-isotopes in zircon and whole-rock geochemistry, to understand the formation of the intrusions and their host mineralization within the tectonic evolution of the Lamboo Province. Geochronology demonstrates that the majority of intrusions are synchronous at ca. 1848–1838 Ma (ca. 1844 ± 2 Ma), and were intruded into the <1869 Ma Tickalara metasedimentary package (with 1895–2810 Ma detrital zircons). The Ni-Cu mineralized Copernicus pyroxenite was dated at 1823 Ma. Whole-rock geochemistry indicates a subduction origin for the mafic-ultramafic magmatism, with trace element data trending with modern continental-arc magmas, and lacking distinct assimilation trends with the Tickalara metasedimentary host-rock. Oxygen isotope data (δ18O ca. 5.6–12.8‰), supports this interpretation. The occurrence of more mantle-derived compositions, in early metamafic dykes, suggests a setting for the Central Zone that included both mantle- and arc-derived magmas. This, together with evidence for syn-magmatic high-temperature metamorphism, suggests a continental back-arc setting in the Central Zone at 1845 Ma. This interpretation, as well as the Archean continental zircon detritus identified in the Tickalara metasediments, indicates the Central Zone is a continental fragment rather than an island arc. Subsequently, this study demonstrates that Paleoproterozoic continental arcs are a prospective setting for orthomagmatic Ni-Cu-PGE systems.

Evidence for regional metamorphism in a continental rift during the Rodinia breakup

Continental rifts are tectonic zones where substantial material and heat can be transferred from the asthenospheric mantle into the crust. Because of their high thermal gradients, they were hypothesized as a setting for regional high-temperature (HT)/low-pressure (LP) metamorphism. However, the cited example is located in an intracontinental setting, where not only the compressional regime was prominent during collisional orogeny but also the envisaged continental rift failed to run into rupture. This led to ambiguous identification of the continental rift in intraplate regions and thus rejection of this hypothesis. In the present study, we identify for the first time typical HT/LP metamorphic rocks that show the occurrence of both andalusite and sillimanite in a Neoproterozoic continental rift zone. Andalusite O isotopes confirm the continental rift setting, and comparison with zircon O isotopes supports the metamorphic origin of aluminosilicates. Pseudosection calculations indicate that the HT/LP metamorphism occurred at 1.0–3.5 kbar and 560–660 °C. U-Pb dating of metamorphic titanite yields ages of ca. 750 Ma for the HT/LP metamorphism, consistent with the peak age of Rodinia breakup. Premetamorphic protoliths show bimodal lithochemistry and arc-like geochemical signatures, suggesting that they are rift magmatic rocks. These rocks would be produced during the development of continental rifting on a Grenvillian accretionary orogen that formed due to the Rodinia assembly. The continental rifting is coupled with stretching of the thinned orogenic lithosphere, accounting for the high heat flow from the asthenospheric mantle into the upper crust for the HT/LP metamorphism at shallow depths. Heat flow estimates from crustal Th, U and K contents are significantly lower than that from peak mineral assemblages, indicating that the anomalously high heat flow from the asthenospheric mantle did occur in the continental rift. Therefore, continental rifts are an important site for high thermal gradients and thus for regional HT/LP metamorphism.