Lower Amphibolite Facies Research Articles

Preservation of biosignatures in Neoproterozoic phosphorites metamorphosed at temperatures >450 °C

There is still much controversy over whether apatite and graphite can be used as fingerprint mineral assemblages to preserve biosignatures in ancient sedimentary rocks that experienced medium- to high-grade metamorphism. Herein, we present a comprehensive analysis of graphite and associated phases from meta-phosphorites of the Huangmailing Formation in the South Dabie Orogen to assess possible preservation of biosignatures. Stratigraphic correlations and previous geochronological data suggest that the protoliths of Huangmailing Formation was deposited in the Ediacaran, and was metamorphosed during the Triassic Dabie Orogeny as documented by our apatite UPb dating. Microcrystalline graphite occurs both as inclusions within metamorphic minerals like apatite, and as matrix-hosted disseminations at grain boundaries of other phases. Bulk organic carbon isotopic analyses yield δ13C values between −26.0 ‰ and − 15.2 ‰ and total organic carbon contents from 0.33 to 3.27 wt%. Raman spectra of the two types of graphites yield an average peak metamorphic temperature of 452 °C and 473 °C, consistent with the upper greenschist facies to lower amphibolite facies metamorphic conditions in the area. However, the matrix graphites show a wider range of peak temperature variations, likely containing graphitic carbon with lower crystallinity that was affected by retrograde metamorphic fluids. Notably, two prominent Raman bands at around 325 cm−1 and 395 cm−1 are observed for the graphite hosted in apatite. This, along with the presence of a peak at 2473 eV in the sulfur K-edge XANES spectra of some graphite in apatite, and the co-occurrence of 12C and 14N and local association of 32S with 12C in graphite inclusions determined by TOF-SIMS, suggest the possible presence of C-S-Fe and N-bearing compounds and functional groups. TEM and EDS analysis reveal that graphite inclusions in apatite are closely associated with amorphous silica, and mainly occur as well crystallized, polygonal grains around 100 to 200 nm across and has an expanded (002) lattice spacing between 3.43 Å and 3.64 Å. This lattice expansion is attributable to the presence of heteroatoms such as S, Fe and Si, and N, in the graphene interlayers. In contrast, matrix graphites exhibit a wider range of structural and compositional variations than their counterpart as inclusion in apatite, which probably arise from interactions with metamorphic fluids or incorporation of fluid-deposited graphitic carbon. Our study documents the ubiquitous preservation of biological carbon as graphite inclusions in apatite from Huangmailing meta-phosphorites, likely as a result of metamorphic recrystallization of organic-bearing Ediacaran phosphorites. This suggests that the organic matter trapped in sedimentary apatite during their co-precipitation or early diagenesis was probably less modified during metamorphic recrystallization, thus facilitating the preservation of biosignatures.

Serpentinised Mantle Section of Neoproterozoic Ophiolite at Al‐Barramiya District, North Arabian‐Nubian Shield: Tectono‐Magmatic Evolution and Metamorphism

ABSTRACTThis research focuses on the field observations, petrography, mineral chemistry and geochemistry of the serpentinised peridotite of Al‐Barramiya ophiolitic sequence to place constraints on their magmatic history and their geodynamic evolution. Al‐Barramiya ophiolitic rocks are a dismembered ophiolite which was strongly deformed and metamorphosed under greenschist to lower amphibolite facies. They comprise a mantle section dominated by highly serpentinised peridotite with less metapyroxenite and chromitite, as well as a crustal portion represented by metagabbros. Along shear zones, the ophiolite sequence was affected by several types of alteration. Extensive carbonate alteration is common in the ultramafic section, resulted in talc carbonates, listvenites and magnesite, while rodingitisation is common in the metagabbro resulted in rodingite. Despite the extensive serpentinisation, some fresh relics of primary mantle minerals such as Cr‐spinel, olivine and pyroxenes are preserved sporadically in the serpentinised peridotite. Few Cr‐spinel crystals are sometimes surrounded by subhedral flakes of Cr‐chlorite (kämmererite) that was formed due to replacement of Cr‐spinel during later alteration or regional metamorphism. The serpentinite samples are depleted in the total REE (0.56–1.19 ppm) with slightly negative to slightly positive Eu anomalies (0.89–1.28). The fresh cores of Cr‐spinel have Cr# mostly > 60, and the relics of pyroxenes and olivine are Mg‐rich suggesting that the Al‐Barramiya serpentinites are residual to high degrees of melt extraction. The estimated degrees of partial melting range between 18.2% and 20.7%. All the mineralogical and geochemical characteristics of the ultramafic section of the Al‐Barramiya ophiolites are most consistent with its formation in a fore‐arc setting.

Geochemistry of Mafic Rocks From the Nagrota–Kathindi Section, Himachal Pradesh, Northwestern Himalaya: A Probable Example of Plume–Lithosphere Interaction

ABSTRACTThe Northwest Himalayan region has a record of several phases of mafic magmatic activity spanning from Precambrian to Cenozoic in a dynamic tectonic setting. Here, we studied detailed petrography and new whole‐rock geochemistry of mafic volcanic and dykes from the Nagrota–Kathindi Section (NKS), Himachal region of the NW Himalaya, to understand the petrogenesis and possible tectonic setting. Both rock types have comparable mineralogical compositions (clinopyroxene + plagioclase + actinolite‐tremolite + chlorite + iron oxides ± hornblende ± epidote ± quartz ± carbonates) overprinted by greenschist to lower amphibolite facies metamorphism. The mafic volcanic and dykes of NKS exhibit subalkaline basalts to basaltic andesites and a typical tholeiite compositional character. The chondrite‐normalized rare earth element pattern exhibits similar LREE‐enrichment and strong HREE‐fractionation, whereas primitive mantle‐normalized multi‐element patterns show pronounced LILE‐enrichment of Rb, Ba, Th, LREE, and HFSE depletion of Nb, K, P, and Ti. The Zr–Y–Nb–Th relationships indicate that both rock types were derived from the plume source, whereas low Nb/La (< 1), similar high large‐ion lithophile element concentrations, and pronounced negative Nb, Zr, P, and Ti anomalies suggest that components other than mantle plume must have been involved in the generation and evolution of both rock types, that is, most likely plume and subcontinental lithosphere mantle (SCLM) interaction. The genesis of parent magma for the NKS volcanic and dykes was derived by 4%–6% and 10%–20% partial melting from the spinel + garnet lherzolite stability field. The majority of the studied samples correspond to spinel + garnet peridotite melting on (Gd/Yb)N versus CaO/Al2O3 diagram, thereby corroborating residual garnet in the mantle restite. All the basalts and dykes from the NK section did erupt/intrude in an intracontinental rift setting based on geochemical discrimination. The key petro‐tectonic processes attributed to the formation of these rocks are as follows: (i) the melting of the ascending plume by adiabatic decompression; (ii) the partial melting of this plume–SCLM source in the melting regime, which produces basaltic magma with a tholeiitic composition; and (iii) the release of heat that provides the thermal condition for melting of SCLM and interaction between upwelling mantle plume and subduction metasomatized SCLM.

Mineral chemistry and geothermobarometry of metasedimentary rocks of Central Menderes Massif, Western Türkiye: Metamorphic evolution and source of metapelitic rocks

The Menderes Massif (MM) is the largest metamorphic massif of Türkiye. This study was carried out to determine the origins, metamorphic conditions and age of garnet-mica schists covering large areas in the Central Menderes Massif (CMM). Detrital zircon dating of a garnet-mica schist derived from a quartzofeldspathic source measured the youngest zircon age to be 450 ± 61 Ma (102 % concordant, Th/U = 0.02) Ma and the oldest zircon age to be 1529 ± 18 Ma (97 % concordant, Th/U = 0.02) Ma. The ƐHf(t) values of the selected zircon grains vary between −27.42 and −3.43, and the 176Lu/177Hf isotope values vary between 0.2820 and 0.2827, and these values are consistent with the values obtained from the orthogneisses and paragneisses of the massif. The presence of five Ordovician zircon grains in garnet-mica schists and the intrusive contact with Triassic granites indicate that the source rock of the schists may have been deposited in the Late Ordovician - Triassic interval. Mineral chemical analyses show that garnets are almandine, biotites are Fe-biotite and white micas are muscovite-phengite in composition, and they are affected by metamorphism (greenschist - lower amphibolite facies) under ∼5 kbar pressure and ∼600 °C temperature conditions. For the first time, rutile UPb dating shows that garnet-mica schists underwent metamorphism 43.1 ± 6.1 Ma ago. PT conditions and metamorphic ages obtained from garnet-mica schists show that the overlying units underwent metamorphism in the greenschist-lower amphibolite facies in the Lutetian (Eocene) period.

The Permian Watershed tungsten deposit (northeast Queensland, Australia): fluid inclusion and stable isotope constraints

The Watershed scheelite deposit is located in an extinct fore-arc basin in the Mossman Orogen of North Queensland. This fore-arc region comprises multiply deformed, Ordovician–Silurian metasedimentary rocks of the Hodgkinson Formation, and it was intruded by Carboniferous–Permian granites of the Kennedy Igneous Association. At Watershed, the Hodgkinson Formation includes strongly deformed skarn-altered conglomerate, psammite and slate units, which record four deformation events that evolved from ductile (D1–3) to brittle–ductile (D4). Early, D1–2 scheelite mineralisation in Carboniferous monzonite and skarn-altered conglomerate formed during regional prograde metamorphism, which reached upper greenschist to lower amphibolite facies conditions. Permian, D4 scheelite mineralisation was deposited in transtensional, shear-related veins, vein haloes and skarn-altered conglomerate during retrograde, lower greenschist facies metamorphism. During D4, four stages of retrograde alteration (retrograde stages 1–4) affected the rocks. Fluid inclusion assemblages in retrograde stage 2, vein scheelite and quartz are characterised by a low-salinity H2O–NaCl ± CH4 fluid (XCH4 <0.01, 1.4–8.0 wt% NaCleq). The fluid inclusions show evidence for fluid mixing between a low (∼0 wt% NaCleq) and a medium (<8 wt% NaCleq) saline fluid. Scheelite mineralisation P–T conditions were determined at ∼300 °C and 1–1.8 kbar (i.e. a depth of 3.7–6.7 km), indicative of a high geothermal gradient (35–75 °C/km), which was likely caused by the heat from the Permian granites. The presence of pyrrhotite and arsenopyrite in D4 veins (retrograde stage 4), plus graphite and methane in the fluid inclusions in scheelite, indicates reduced mineralisation conditions. Oxygen isotope compositions (δ18OVSMOW) of retrograde stage 2 scheelite (+3.8 to +7.3‰), plagioclase (+7.0 to +11.8‰) and quartz (+12.6 to +15.5‰) indicate a fluid temperature of 306 ± 56 °C with δ18OVSMOW values between +4.7 and +8.3‰. Retrograde stage 3 muscovite δDVSMOW (−73.4 to −62.7‰) and δ18OVSMOW (+11.5 to +13.2‰) values were used to calculate the O–H isotopic compositions of the fluids in equilibrium with the minerals at various possible temperatures (250–300 °C). The results are consistent with a metamorphic origin for the mineralising fluid. Sulfur isotope compositions (δ34SCDT between −2.5 and +2.8‰) for vein-hosted, retrograde stage 4 sulfides indicate that sulfur could have come from seawater or seawater sulfate, which is consistent with the local geology, even though this range overlaps with magmatic sulfur isotope compositions. Metamorphic fluids probably originated from devolatilisation reactions in the Hodgkinson Formation during prograde metamorphism. Permian intrusions acted as heat source enhancing metamorphic fluid flow and metal transport.

METAMORPHISM AND DEFORMATION OF GOLD-BEARING NEOPROTEROZOIC WONAKA SCHIST BELT, NORTHWEST-NIGERIA.

The role of metamorphism and deformation is indispensable in the occurrences of gold mineralization worldwide. In this work, deformation and metamorphic conditions for gold-bearing Neoproterozoic Wonaka Schist Belt; located around Kutcheri town of Tsafe Local Government of Zamfara State, was investigated. This is achieved using metamorphic litho-minerals obtained from ternary plots via X-Ray fluorescence (XRF) geochemical data, and directly using minerals phases from X-Ray Diffraction (XRD) technique. Index minerals identified from petrographic analysis previously suggest low to medium-grade metamorphism (M1). XRD analysis indicates quartz, albite, oligoclase, microcline, chlorite, and biotite, suggesting greenschist to lower amphibolite facies (M2). Sillimanite, andalusite, kyanite, staurolite, chlorite, biotite, and garnet were identified from the ternary plots using XRF major oxides, indicating upper amphibolite to granulite facies metamorphism (M3). This is typical of prograde metamorphism, granulite facie metamorphic grade is indicated. Na2O/Al2O3 versus K2O/Al2O3 for petrogenetic character suggests shale provenance, while the trace elements spider diagram indicates Wonaka litho-units as co-genetic compositionally, as high concentrations of V and Cr linked the petrogenetic affinity to mafic sources. Three circles of deformations are indicated; ductile deformation (D1) of the paleosome Schist producing foliations and lineation, brittle type (D2) in mid Pan-African and was accompanied by several fractures and felsic intrusions. Late Pan-African (D3) involves the folding of banded orthogneisses, the development of boudinage as well as intense shearing (ductile fault). Geospatial analysis of the fractures suggests that they represent regional Pan-African sutures cross-cutting Nigeria into the Atlantic and up to South American plate. The research therefore concludes that Au-fluid emanating through this regional event, utilizes D2 as channel ways and loci. D3 with M3 engulfed the entire structures repositioning the geometry to its present disposition.

Geochemistry and U-Pb zircon geochronology of the Nkout banded iron formations and associated metasiliciclastic rocks, NW Congo craton, southern Cameroon

ABSTRACT The Nkout banded iron formations (BIFs)-hosted iron deposit is located in the Ntem Complex at the northwestern part of the Congo craton, southern Cameroon. We report geochronological and geochemical data of magnetite BIFs and interlayered mica schists from the Nkout iron ore deposit. The BIFs show typical features of chemical precipitates characterized by seawater-like shale-normalized rare earth elements plus yttrium spectra. Weak positive Eu anomalies indicate the contribution of high-temperature hydrothermal fluids to the BIFs, with low detrital input. The absence of true Ce anomalies indicates that the BIFs were likely to be deposited in anoxic seawater. LA-ICP-MS U-Pb detrital zircon data for the BIFs and associated metasiliciclasltic rocks constrained the maximum depositional age at ca. 2.8 Ga, suggesting that the Nkout BIFs are likely the oldest iron formations in the Ntem Complex. Our zircon U-Pb isotope data indicate that the Nkout succession experienced lower-amphibolite facies metamorphism at ca. 2.0 Ga, which correlates with the regional tectono-thermal event linked to the convergence and assembly of terranes between the Congo and São Francisco cratons during the Eburnean-Transamazonian orogeny.

Correlations on the southern Kaapvaal Craton Margin, 1: Ventersdorp lavas transgressed the Doornberg lineament

Abstract Two small exposures of quartz-porphyritic rocks occur on the farm Zoutpekel 98 in the Marydale Terrane between the Doornberg Fault and Brakbos Shear Zone, apparently overlying Kaapvaal basement granite but lacking clear field relationships due to sand and Dwyka tillite cover. They are lavas and tuffs, metamorphosed in lower amphibolite facies. They contain quartz phenocrysts with a distinctive blue colour, due to metamorphic exsolution of rutile. Microbeam U-Pb zircon dating gives a combined 207Pb/206Pb age of 2 722 ± 3 Ma (seven determinations on four samples), interpreted as the age of extrusion. Three of these samples give the same discordia upper intercept age, but one sample gives discordia intercepts of 2 688 ± 15 and 1 223 ± 120 Ma, thought to reflect metamorphic lead loss related to the ~1 210 Ma Namaqua terrane assembly collisions. The Zoutpekel exposures are coeval with the 2 720 ± 2 Ma Makwassie Formation of the Platberg Group, Ventersdorp Supergroup. They also correspond geochemically to the Makwassie Formation and no other unit of the supergroup. A sample from the T’kuip Formation of the nearest Ventersdorp Supergroup inlier on the Kaapvaal Craton (east of the Doornberg Fault), gives an age of 2 716 ± 8 Ma, also confirming its lithostratigraphic and geochemical correlation with the Makwassie Formation. The Zoutpekel exposures show that not only the Kaapvaal basement granites, but also the supracrustal cover rocks of the Ventersdorp Supergroup, extend southwards across the Doornberg Fault, The Marydale Terrane is thus not an exotic terrane, but probably represents a passive continental margin developed at the beginning of the 1 300 to 1 000 Ma Namaqua-Natal Wilson cycle. The age range of the Ventersdorp Supergroup and the age and stratigraphic correlation of the Marydale Group thrust complex, which straddles the Zoutpekel exposures, will be investigated in two companion papers.

Secular change of tectonic setting in the Archean Takanen greenstone belt, northeastern Karelia Province, Fennoscandian Shield

The target of this study is the Takanen greenstone belt (TGB), a small 11-km-long and east-west oriented supracrustal belt in the northern Lentua complex of the Western Karelia Subprovince (eastern Finland). We present a new geological interpretation of the TGB based on lithology, geochemistry, stratigraphy, and two new zircon U-Pb age determinations and Lu-Hf-in-zircon data. The TGB forms a ���≥800-m-thick synclinal sequence composed of intercalated mafic-���komatiitic and felsic-intermediate volcanic and volcaniclastic rocks (lavas, tuffs, and tuffites) crosscut by granitoid dykes, and metamorphosed in lower amphibolite facies with few primary volcanic textures visible. The felsic-intermediate rocks are calc-alkaline and split into strongly and weakly REE-fractionated groups. The mafic-ultramafic rocks are tholeiitic with flat REE-patterns, but some samples show LREE-enrichment. Massive Fe-sulfide layers (≤��1 m) are associated with volcanoclastic rocks. Komatiites are Al-undepleted and comprise differentiated (peridotite-gabbro) and undifferentiated (dunite/serpentinite) cumulates and thin undifferentiated flows. Channel-facies komatiitic cumulates are found stratigraphically above Fe-sulfide-rich unites and have �potential for komatiite-hosted Ni-deposits. Our new isotope data from two felsic-intermediate volcanic rock samples shows that the TGBs stratigraphically lowermost and uppermost units are 2.96 Ga and 2.71 Ga old, respectively. Based on Hf isotope data, the older 2.96 Ga sample contains reworked crustal material, possibly originating from ~3.2 Ga crust.In contrast, the 2.71 Ga sample is isotopically juvenile (near-chondritic), indicating an episode of juvenile crust formation in the northern Lentua Complex. No unconformity is found between the stratigraphically upper and lower parts of the TGB, but the ~250 Ma age difference (and geochemical indicators) suggests they may have formed in different tectonic settings. The lower part of TGB stratigraphy is correlated with the ~2.94 Ga Luoma Formation in the nearby Suomussalmi greenstone belt. The 2.71 Ga volcanic unit does not have a known equivalent in greenstone belts in Finland, but a corollary is found in Russia, and a tentative correlation to a possible 2.75–2.70 Ga arc-system in Takanen–Khedozero-Bolshozero–Ilomantsi is suggested.

Trace element changes in rutile from quartzite through increasing P–T from lower amphibolite to eclogite facies conditions

Abstract Low concentrations of Na, Ca, K, Fe, Mg and Al in quartzite commonly prevent the crystallization of index metamorphic minerals, inhibiting the obtainability of thermobarometric calculations. Quartzite typically contains quartz, zircon and rutile; therefore, single-element thermometers, such as Zr-in-rutile, may be applied. We investigate changes in trace-element composition of rutile from quartzite through increasing metamorphic conditions. Studied samples derive from a quartzite package (Luminárias Nappe, Minas Gerais, Brazil) where previous thermobarometric constraints on metapelites showed an increasing metamorphic grade southwards, from high-pressure lower amphibolite facies (580°C; 0.9 GPa) to eclogite facies (630°C; 1.4 GPa). Rutile from the lower-grade facies samples show a large spread in Zr concentrations, with the highest values corresponding to temperature estimates higher than metamorphic conditions affecting those units, and thus interpreted as inherited detrital signatures. A narrower spread in Zr concentration is observed in rutile grains from the higher-grade facies, and estimated Zr-in-rutile temperatures agree with previous thermobarometric constraints. Therefore, we show that at 630°C, Zr contents in detrital rutile from quartzites re-equilibrate. The comparison between the quartzite- and metapelite-hosting rutile grains from the same area shows that the resetting of the geothermometer in the latter seems to occur at slightly lower temperatures (∼50°C lower). Supplementary material: A full dataset of analized trace elements in rutile is available at https://doi.org/10.6084/m9.figshare.c.6793887

Current state of knowledge of the gold mineralization at Imonga-Saramabila, Maniema (DR Congo): a petrographic and mineralogical study of the mineralized vein system

The Great Lakes area in Central Africa forms a large metallogenic province that hosts important deposits of gold mineralization. We present a petrographic, mineralogical and geochemical study of unique borehole samples from the Imonga-Saramabila gold deposit, a historical mine site located in the Maniema Province (DR Congo) in the Karagwe-Ankole belt (KAB) in the Great Lakes area and one of the only places in the Maniema province with accessible boreholes allowing to study the mineralization. The samples are metasedimentary rocks belonging to the Mesoproterozoic Kivu Supergroup, with bedding-parallel meta-igneous rocks. These rocks have undergone upper greenschist to lower amphibolite facies metamorphism, based on the presence of andalusite and chiastolite porphyroblasts, and are affected by hydrothermal alteration. The porphyroblasts formed during peak metamorphism and posterior to a first vein generation. Three additional vein generations were identified at Imonga based on crosscutting relationships, with the second and third events overprinting the porphyroblasts by intense chloritization, and associated with sulfide mineralization. The fourth vein generation is again barren. The first veining event formed pre-folding and the three subsequent generations postdate folding, as concluded based on the relationship of the veins with the cleavage. Only one important folding event is proposed based on the development of only one cleavage. Gold occurs as free gold or is included in pyrite in the second (and maybe third) vein generation. Based on the paragenesis, structural characteristics, and the link between veining and metamorphic minerals, the gold mineralization at Imonga is interpreted to be linked to the early Neoproterozoic (~980 Ma) compressional deformation event, associated with the amalgamation of the Rodinia supercontinent.

Origin and evolution of Neoproterozoic metaophiolitic mantle rocks from the eastern Desert of Egypt: Implications for tectonic and metamorphic events in the Arabian-Nubian Shield

The mantle rocks from Kadaboura and Madara areas represent sections of dismembered ophiolitic complexes developed during the Neoproterozoic in the Eastern Desert of Egypt, which is located in the northwestern corner of the Arabian–Nubian Shield. The Kadaboura mantle rocks comprise serpentinites and serpentinized dunites, whereas those of the Madara consist of serpentinites and serpentinized pyroxenites.Despite the serpentinization of the studied mantle rocks, few relicts of primary chromite, olivine and pyroxene are preserved. Chromite is partly altered having unaltered Al-rich chromite cores surrounded by Fe-rich chromite and Cr-rich magnetite rims. The unaltered Al-rich chromite cores show compositions equilibrated at temperatures mostly below ~500-600°C, which is a temperature comparable to that estimated for primary chromite in greenschist up to lower amphibolite facies rocks. The high Cr# [100×Cr/(Cr+Al)= 47-76] of the unaltered chromite cores and the Mg-rich nature of the olivine relicts (Fo91–94) indicate that the studied mantle rocks were produced from a highly depleted mantle that experienced high degrees of melt extraction (mostly ~30-40%). This range of melt extraction resembles that estimated for supra-subduction zone peridotites, but higher than that in abyssal and passive margin peridotites. Furthermore, the clinopyroxene relicts show compositions comparable to those from the Mariana forearc peridotites. Bulk-rock geochemistry also reflects derivation from an extremely depleted and a highly refractory mantle source. Modelling of rare-earth elements suggests that the studied mantle rocks were possibly formed by the interaction of their highly depleted harzburgitic mantle precursors with subduction-related melts/fluids during their evolution in a fore-arc basin of the supra-subduction zone.The proposed geodynamic model suggests that the oceanic lithosphere generated during the seafloor spreading of the Mozambique Ocean was emplaced in the upper plate of the intra-oceanic subduction zone, in which the formely depleted Neoproterozoic mantle of the Arabian-Nubian Shield experienced mature phases of hydrous melting, extreme depletion and enrichment.

Timescales and mechanisms of felsic lower continental crust formation: Insights from U-Pb geochronology of detrital zircon (Malenco Unit, eastern Central Alps)

Burial of supracrustal sedimentary material is one of the processes that contribute to the formation of the lower continental crust. We investigate the rate of this process in Permian lower continental crust from the Malenco Unit (eastern Central Alps) by constraining the age of protolith deposition and subsequent high-grade metamorphism of felsic, garnet-rich granulites. Detrital zircon cores, many of which are small (<50 μm), were dated by SIMS and yield concordant dates ranging from 387 ± 21 Ma (2σ) to 2380 ± 16 Ma, with the vast majority of the dates falling in the range 450–1000 Ma. For the seven cores in each sample that gave the youngest concordant dates, replicate analyses were carried out. None of these cores had all measured U-Pb dates agree within error, indicating significant disturbance by the Permian granulite facies metamorphism. A 206Pb/238U age of 424 ± 6 Ma for a core with two overlapping replicate analyses is the best available constraint on the maximum depositional age of the sedimentary protolith. This maximum depositional age also coincides with the first significant population in the probability density plot of detrital core dates, which is a double peak at ∼425 Ma and ∼450 Ma.Zircon detrital cores are separated from metamorphic overgrowths by a small seam of zircon that is riddled with tiny inclusions of quartz, biotite and muscovite. The first generation of metamorphic zircon rims has a 206Pb/238U age of 272.9 ± 2.8 Ma, displays steep heavy rare earth element (HREE) patterns and a moderate negative Eu-anomaly, and returns Ti-in-zircon temperatures of 740–780 °C. The second generation of metamorphic rims has a 206Pb/238U age of 263.8 ± 2.6 Ma, a flat HREE pattern and a pronounced negative Eu-anomaly, and gave Ti-in-zircon temperatures of 780–810 °C. Collectively these observations indicate that the first zircon rim formed on the prograde path at the onset of partial melting where muscovite was present but the rocks contained little garnet and no K-feldspar. Therefore, the metapelites resided at lower amphibolite facies, subsolidus conditions up until the intrusion of a Permian gabbro, which was emplaced shortly after the formation of the first generation of metamorphic zircon and caused heating to granulite facies conditions and widespread partial melting. There is no record of high-pressure metamorphism preceding granulite facies conditions, indicating that felsic lower crust in the Malenco unit was not formed by relamination.The Malenco unit was situated at the north-eastern active margin of Gondwana 420 Ma ago. Subduction-related accretion of sediments at lower crustal levels shortly after their deposition could explain the formation of this felsic lower crust that resided at subsolidus conditions for up to 100 My prior to Permian extension, gabbro intrusion and granulite facies metamorphism.

An oceanic core complex preserved in the Squanga Lake ophiolite, northern Atlin terrane, Yukon

The Squanga Lake ophiolite of southern Yukon exhibits a non-Penrose pseudostratigraphy with mantle tectonites separated from upper crustal rocks by an extensional detachment zone (Squanga Lake detachment zone, SLDZ), compatible with formation in an oceanic core complex. Spinel, orthopyroxene and clinopyroxene mineral compositions indicate dynamic re-crystallisation during exhumation of ophiolitic mantle rocks. The rocks and structures of the SLDZ also preserve evidence of exhumation: 1) clasts of primitive olivine-cumulates in the SLDZ mélange represent excised lower oceanic crust; 2) structures in the SLDZ indicate ductile to brittle conditions; 3) temperatures along the SLDZ span from upper- to lower-amphibolite facies conditions. We obtained 248.77 ± 0.22, 248.53 ± 0.41, and 248.84 ± 2.09 Ma U-Pb zircon ages from upper crustal gabbro, a gabbro fragment within the SLDZ mélange and a dyke within the SLDZ implying syn-kinematic magmatism. Calc-silicate alteration associated with cross-cutting diabase dykes yielded a UPb titanite age of 247 ± 10.5 Ma. All igneous rocks show similar geochemical signatures with ubiquitous subduction input. Our data indicate that Squanga Lake ophiolite preserves an oceanic core complex, formed in a back-arc basin environment.

Metamorphic P-T paths and zircon U-Pb ages of Paleoproterozoic metabasic dykes in the Jiapigou area, South Jilin Province: Implications for the tectonic evolution of the North China Craton

The distribution and tectonic evolution of the Paleoproterozoic orogenic belts in the North China Craton (NCC) remain controversial. Especially, the extent of the Paleoproterozoic magmatic and metamorphic influences on the Archean Longgang Block needs further demonstration. This paper presents detailed petrographic and mineralogical analyses, phase equilibria modeling and zircon dating for three metabasic dyke samples in the Jiapigou area, South Jilin Province, in order to discuss their metamorphic evolution and tectonic implications. Petrographic observation and phase modeling using THERMOCALC suggest that these metabasic dykes have experienced the metamorphism with clockwise P-T paths under low-amphibolite facies, high-amphibolite facies and high-pressure granulite facies. Their peak P-T conditions were constrained to be 0.6–0.9 GPa/580–650 °C (JN1929), ∼0.95 GPa/757 °C (JN1918) and ∼1.15 GPa / 820 °C (JN1932), according to the stability fields of mineral assemblage and isopleths of Ti content in amphibole, anorthite component (XAn) in plagioclase, pyrope content (XPy) and grossular content (XGrs) in garnet. The peak conditions correspond to apparent geothermal gradients of 19–22℃/km, being medium-pressure type. Zircon U-Pb dating shows that the metabasic dykes have a protolith crystallization age of 2199 ± 5 Ma (MSWD = 0.43), and a metamorphic age of 1855 ± 41 Ma (MSWD = 0.21). The protolith ages, metamorphic characteristics and ages of the metabasic dykes in South Jilin can be well compared with those for metabasic dykes in East Hebei and North Liaoning, indicating that the northern part of the Longgang Block was extensively affected by the mid Paleoproterozoic magmatism, and was involved in an orogen to be metamorphically reworked during the late Paleoproterozoic (1.82–1.85 Ga). In addition, this orogeny mainly occurred along the northern margin of the craton, which is different from the ∼1.95 Ga orogeny in the NCC.

Upper plate exhumation during continental collision: An example from the paranapanema paleocontinental active margin, SE Brazil

The Southern Brasília Orogen is the result of Ediacaran continental collision between the active margin of the Paranapanema paleocontinent and the passive margin of the São Francisco paleocontinent. The active margin was deformed into the Socorro-Guaxupé Nappe and the associated Embu Domain. This paper presents new U–Pb zircon data and geothermobarometric estimates that allow to reconstruct the metamorphic evolution in these upper plate units. A pre-collisional metamorphism M1 with an age peak in metamorphic rims of zircon grains in the range 680–630 Ma is related to the development of the Socorro-Guaxupé magmatic arc with batholiths dated in a similar time span, 690 - 630 Ma. This metamorphism reached medium pressure granulite facies, now exposed by the tilted uplift and erosion due to the collision. Another peak is detected in metamorphic rims of zircon grains, in the range 630–590 Ma, labeled M2, related to the collision and corresponds to the main metamorphism in the lower plate and accretionary wedge-like nappe stack, underlying the Socorro-Guaxupé Nappe. This nappe stack was affected by greenschist facies metamorphism in the autochthonous underlying craton, grading to high pressure granulite facies in the upper nappe, with local occurrences of retroeclogite lenses. In the Socorro-Guaxupé Nappe and Embu Domain this main metamorphism (M2) is characterized by three metamorphic zones decreasing in grade with distance from the suture: a granulite facies orthopyroxene bearing zone; a high temperature amphibolite facies zone and a lower amphibolite facies zone. Geothermobarometric results in mineral associations related to M2 show temperatures decreasing from about 770 °C to ∼620 °C and pressures decreasing from about 9 kbar close to the suture to about 5 kbar in the southern and western parts of the studied region. On the upper Paranapanema plate, relative to the Socorro-Guaxupé Nappe and Embu Domain units, at the base of the plate, an older metamorphism (M1) is verified with temperatures between 960 and 890 °C and pressures between 11 and 9 kbar, reequilibrated at approximately 700 °C with a pressure of about 9 kbar (M2, continental collision). For this reason, it can be assumed that the rim of the Paranapanema plate was exhumed during the continental collision, since the temperature and pressure of the metamorphic peak determined during the continental collision are lower than those determined for the pre-collisional magmatic arc metamorphism.

Cambrian magmatism in the Vlahina Mt. (SW Bulgaria) – correlation with the Vertiskos Unit (Serbo-Macedonian Massif)

The Rhodope massif and the Kraishte Zone are parts of the Internal Balkanides separated by Gabrovdol detachment. In the most southwestern part of Bulgaria one of the units of the Serbo-Macedonian Massif (the western part of the Rhodope Massif) the Vertiskos Unit crops out. Part of this unit represented by metagabbro-metadiorites is exposed in an area of about 60 km2 in the central part of the Vlahina Mountain. These rocks are metamorphosed at lower amphibolite facies conditions (T = 630–670 °C and P = 0.85–1.05 GPa). LA-ICP-MS U-Pb dating of zircons display concordant age of these rocks of 541±2.6 Ma. The petrological and isotopic investigations show significant differences with the rocks of the Vertiskos Unit and allow us to assign these rocks to the basement of the Kraishte zone (Struma Unit).

Metamorphic patterns and zircon U–Pb dating of the Xilingol complex in Inner Mongolia, China: Implications for rifting metamorphism and tectonic evolution in eastern Central Asian Orogenic Belt

The Xilingol complex is one of the maximal outcropped metamorphic terranes in the eastern segment of the Central Asian Orogenic Belt. Detailed petrological and geochronological study of the Xilingol complex will provide valuable insights into the orogenic process and closure of the Paleo-Asian Ocean. We present a comprehensive study of petrology, pseudosection modelling, and zircon U-Pb geochronology of representative rocks collected in Daqing Pasture, including two-mica quartz schist, epidote amphibolite, biotite plagioclase gneiss and quartzite. The Xilingol complex in Daqing Pasture can be preliminarily divided into the north and south segments based on field and petrological observations. The north segment comprises two-mica quartz schist, biotite quartz schist, amphibolite and minor epidote amphibolite, whereas the south segment consists mainly of biotite gneiss, amphibolite and quartzite, with varying degrees of migmatization developed. Combined pseudosection modelling and composition isopleths of garnet and biotite constrain a peak P–T condition of ~660 °C, ~5.5 kbar for the garnet-bearing two-mica quartz schist in the north segment, corresponding to low amphibolite facies metamorphism. The garnet-bearing biotite plagioclase gneisses in the south segment are interpreted to experience high amphibolite facies metamorphism at a peak condition of ~750–790 °C, ~5.5–8.0 kbar, followed by nearly isobaric cooling. The variations of peak P–T estimates within Daqing Pasture, coupled with the spatial heterogeneities of lithological associations between different outcropped terranes, reveal an overall northward decreasing metamorphic grade for the Xilingol complex. Metamorphic zircons from three biotite plagioclase gneisses in the south segment yield apparent 206 Pb/ 238 U age spreads of 397–348 Ma, 363–294 Ma and 341–301 Ma, respectively. These partially overlapping zircon ages are an indication of discrete metamorphic events involving localized thermal metamorphism at depth in the Middle Devonian and long-lasting Buchan-type metamorphism in the Carboniferous, with intermediate data probably related to a mixture of multiple growth domains. Five metamorphic zircons with low Th/U ratios from the epidote amphibolite in the north segment yield a concordia age of 284.8 ± 1.9 Ma, reflecting the diachronism of the Late Paleozoic Buchan-type metamorphism. Summarizing all U-Pb data of inherited magmatic zircons and metamorphic zircons, a distinct U-Pb age continuum is retrieved to elucidate a successive process from sedimentary diagenesis of forearc sedimentary formations to deep burial in the Early Devonian, and then to subsequent multi-stage metamorphic overprinting from the Middle Devonian to the Permian. The isograd patterns, P–T–t paths (clockwise paths and long-term duration) and spatial distribution of metamorphic terranes of the Xilingol complex are more compatible with an active continent rift setting rather than paired metamorphic belts in the arc-basin system, revealing post-collisional extension of the Early-Middle Paleozoic orogenic lithosphere of eastern CAOB in the Late Paleozoic. • The Xilingol complex underwent long-lasting Buchan-type metamorphism in the Carboniferous to Permian. • Metamorphic patterns of the Xilingol complex are compatible with continent rift setting. • Zircon U-Pb age continuum reveals a successive process from diagenesis to deep burial to multiple metamorphic episodes.

Metamorphism of the Yilan amphibolites from the Heilongjiang Complex and deformation of the granodioritic mylonites from the Jiamusi Massif, Northeastern China

The Heilongjiang Complex records the evolution history of the Mudanjiang Ocean, which is of great significance to constrain the amalgamation process of the Jiamusi and Songnen‐Zhangguangcai Range massifs. We carried out petrological, chronological, geochemical, mineral chemical and electron backscatter diffraction (EBSD) fabric analyses on the Heilongjiang Complex in the Yilan area. The latest zircon U–Pb dating results show that the protolith ages of the amphibolite are 261.3 ± 3.0 Ma and 261.8 ± 3.3 Ma, while that of the granodioritic mylonite is 207.8 ± 2.2 Ma. The amphibolites and granodioritic mylonite are enriched in large‐ion lithophile elements (e.g., Rb, Ba, and Sr) and light rare earth elements, with depletion in high‐field‐ strength elements (e.g., Nb, Ta, Zr, and Hf), indicating that the formation of the protoliths of the amphibolites and granodioritic mylonite is related to the subduction of the Mudanjiang oceanic plate under the Songnen‐Zhangguangcai Range Massif. The amphibolites and granodioritic mylonite both experienced two periods of metamorphic and deformation events. The metamorphic degree of the early period of metamorphism is low amphibolite facies, which records a clockwise P–T path from early increased temperature and pressure to a late isothermal depressurization. The P–T path reveals that this period of metamorphism is associated with the collision between two massifs. The later period of metamorphism reaches low greenschist facies, accompanied by deformation, which may be related to the rapid exhumation of the Heilongjiang Complex. This study provides a new perspective for exploring the collision and collage process of the Jiamusi and Songnen‐Zhangguangcai Range massifs.

Comparison between Raman spectra of carbonaceous material and carbon isotope thermometries in low-medium grade meta-carbonates: Implications for estimation of metamorphic temperature condition

The changes in the physicochemical conditions of the carbonaceous material during progressive metamorphism in metapelitic rocks is widely used as a geothermometer with the aid of Raman spectroscopy. However, the application of this technique in carbonate rocks has not been established yet. Here, we compare Raman spectroscopy of carbonaceous material thermometry and carbon isotope thermometry in low- to medium-grade metacarbonate rocks from the Archean Chitradurga Schist Belt in the Dharwar Craton, India. The carbonates in the lowermost Bababudan Group have metamorphosed under lower amphibolite facies metamorphic conditions giving consistent estimates for both Raman spectra of carbonaceous material thermometry (460–592 °C) and carbon isotope thermometry (450–560 °C). Contrastingly, in the Vanivilas Formation, the carbonaceous material with very fine flaky morphology in the grain boundary has slightly lower crystallinity, when compared to the coarse-grained ones near the vein boundary. Nevertheless, the carbon isotope thermometry estimated a lower temperature around 400 °C. The inconsistencies between the temperature estimates are ascribed to the pervasive post-metamorphic aqueous hydrothermal fluid infiltration indicated by the presence of numerous criss crossing quartz veins. This is also corroborated by the lowering of the oxygen isotope ratio of the carbonates near the vein boundary by 2.3‰, but without much difference in the carbon isotope ratios. The coarsening of the carbonaceous material near the vein boundary signifies fluid assisted recrystallisation, that enhanced the crystallinity as evidenced in the Raman spectra. Moreover, the negative δ13C shift in the carbonaceous material (c. –8.5 to –13‰) was possibly due to recrystallization following partial CO2 degassing. In the Chitradurga Schist Belt, the Bababudan Group and the Vanivilas Formation have regionally metamorphosed under lower amphibolite facies condition, possibly related to the widespread granitic intrusion at c. 2.61 Ga and later affected by hydrothermal event at c. 2.5 Ga.