Metamorphic Stages Research Articles

Palaeoproterozoic turbidite deposition in the Liaodong Penisula, northeastern North China craton – Constraints from the Gaojiayu formation of the Liaohe Group

The Palaeoproterozoic sedimentary environment is difficult to reconstruct because rocks often experience multiple stages of deformation and metamorphism; however, it is critical to understand the Precambrian evolution of sedimentary basin. In this study, we report two un-metamorphosed sections in the Gaojiayu Formation of the North Liaohe Group, central part of the Jiao-Liao-Ji orogenic belt, eastern North China Craton. Rocks consist of shales, carbonaceous slates, calcareous mudstones and sandstones, marlstones, calcarenites, and brecciated limestones in these sections. Detailed field observations indicate that all kinds of original bedding and syn-depositional structure developed in these rocks. Microscope observations suggest that detritus in rocks predominantly contain feldspars, quartz, and calcites, occasionally with volcanic fragments and pyrites. Palaeocurrent study indicates that the provenance of detritus in rock of these two sections was from NW to SE. Detrital heavy minerals mainly consist of pyrite and hematite, occasionally with barite, which are all from the Li'eryu Formation that contains these minerals in sedimentary layers. The increase of percentages of pyrite in a cross-section indicate that the sedimentary environments transformed from an oxidizing to a reducing depositional environment because the depositional setting became deeper. According to the mafic intrusion dated in this study and regional geochronological data such as the age of underlying volcanic rocks, the depositional age of the Gaojiayu Formation is constrained between 2.17 and 2.16 Ga and 2.11 Ga. Archaean basement (2.5 Ga) is recognized as the major provenance providing clastic materials for the Gaojiayu Formation; volcaniclastic rocks in the Li'eryu Formation were also a significant source. Accordingly, in this paper we interpret the Gaojiayu Formation as turbiditic sedimentary rocks that were deposited in an arc-related abyssal and semi-abyssal environment, probably in an extensional back-arc basin setting. Palaeoproterozoic turbidite deposition combined with a 2.17 Ga volcanic arc developed in the study area, we conclude that the modern trench-arc-basin system may had already existed in the early Palaeoproterozoic and deeply affected the tectonic evolution of the Jiao-Liao-Ji orogenic belt, northeastern North China Craton.

Newly discovered MORB-Type HP garnet amphibolites from the Indus-Yarlung Tsangpo suture zone: Implications for the Cenozoic India–Asia collision

High pressure (HP) and ultrahigh pressure (UHP) oceanic rocks remain very scarce along the Indus-Yarlung Tsangpo suture zone in the Tibetan Plateau. Such rock types, however, could provide new and important insight into the understanding of the India-Asia collision tectonics and regional metamorphic processes. This contribution focuses on newly discovered HP garnet amphibolites that occur in the North Ayilari range along the Indus suture zone in the Western Himalaya. Petrographic and mineralogical studies as well as pressure–temperature (P–T) calculations show that the garnet amphibolites underwent at least three metamorphic stages. A prograde amphibolite-facies stage at 0.9–1.2 GPa and 605–680°C (M1) was followed by HP granulite-facies metamorphism at 680–750°C and 1.1–1.5 GPa (M2) and cooling to amphibolite-facies metamorphism at 628–650°C, 0.6–0.8 GPa (M3). Zircon and apatite U–Pb dating shows that the garnet amphibolites experienced granulite-facies conditions at 33–32 Ma and a cooling age of 12.5 ± 2.6 Ma at temperatures of 375–550 °C. The garnet amphibolites have light rare earth element (LREE)-depleted distribution patterns, low (87Sr/86Sr)i = 0.70727–0.70833, but high εNd(t) (+8.9 – +9.8) in whole-rock, and high εHf(t) (+13.9 – +16.0) values of zircon, which are similar to those of normal mid-ocean ridge basalt. This confirms that the protolith represents an oceanic slice that dominantly sourced from depleted mantle material. Geochemical, geochronological and geothermobarometrical data demonstrate that the garnet amphibolites represented a part of the Neo-Tethyan ocean, that experienced a profound amphibolite facies metamorphism before reaching HP granulite-facies conditions at ca. 32 Ma as a consequence of crustal thickening during the India–Asian collision. Thus, the studied HP oceanic-type slice represents an important puzzle piece that, together with other continental portions that experienced different P-T conditions such as HP and UHP eclogite and different metapelites and quartzo-feldspathic gneisses, helps to decipher the tectonic evolution of the Western Himalaya orogen, from oceanic subduction to continental collision and subsequent exhumation.

Latitudinal transition of mesopelagic larval fish assemblages in the eastern central Atlantic

This study presents a broad-scale view of larval fish species distributions in the eastern Central and North Atlantic and places it in a hydrographic context. Pelagic fish larvae, including metamorphic stages, were sampled to 1000 m depth across a 46° latitudinal transect from the equator to the Bay of Biscay. By analysing species composition and relating it to hydrographic parameters, we were able to identify 5 assemblages: Equatorial, Tropical, Subtropical, Temperate-bloom and Temperate-nobloom. We also categorized 4 groups of species by association with specific hydrographic parameters. Tropical species, including Vinciguerria nimbaria, Ceratoscopelus warmingii, and Hygophum macrochir, are associated with high sea surface temperature. Tropical-subtropical species, such as Diogenichthys atlanticus, Benthosema suborbitale and Electrona risso, are associated with high temperature in the upper 200 m, i.e. the epipelagic zone. The Temperate species group, dominated by Benthosema glaciale and Maurolicus muelleri, is associated with high chlorophyll-a in the epipelagic zone, and these species were abundant where the spring bloom occurred. Species with the broadest latitudinal distributions, such as Cyclothone braueri and Argyropelecus hemigymnus, here termed Cosmopolitan species, are associated with high salinity in the epipelagic zone. The Cape Verde Frontal Zone seems to act as a one-way distributional barrier, confining many Tropical larval species to the South Atlantic Central Water mass, while allowing more northerly-distributed species to cross southward. This may be related to a dependence of Tropical species on high near-surface water temperatures, which decrease sharply at the Cape Verde Frontal Zone. An oxygen minimum zone had no significant effect on larval abundance or species richness, although there were some taxonomic differences. For example, phosichthyids, including Vinciguerria nimbaria, were not found within the oxygen minimum zone, while the family Melamphaidae thrived. Our results suggest that the dominant hydrographic factors that affect the distribution of fish larvae in the eastern Central and North Atlantic vary by latitude and by species. The Cape Verde Frontal Zone and oxygen minimum zone play complex roles, affecting different taxonomic groups in various ways.

Compositional variations, thermometry, and probable parental magmas of Archean chromite from the Sargur greenstone belt, Western Dharwar Craton (India)

The chromite and magnesite deposits from Talur, Sindhuvalli, and Doddakanya in the layered ultramafic-mafic plutonic sequence are a part of the Archean Sargur greenstone belt within the Western Dharwar Craton of the Indian shield. The ultramafic bodies in these three areas are highly sheared and comprise of serpentinized dunite with chromitite bodies along with harzburgite and pyroxenite. Detailed electron microprobe studies reveal intense compositional variability of the chromite grains in silicate-rich chromitite (~ 50 vol% of chromite) and serpentinite (~ 2 vol% of chromite) throughout the entire ultramafic sequence. In Talur, the chromites in massive chromitites (~ 60–70 vol% of chromite) are characterized by relatively lower Mg# (0.28–0.39) compared to chromites from massive chromitites in Sindhuvalli (Mg# = 0.63–0.69). The accessory chromite in dunite from Talur shows higher Cr#, Fe2O3, TiO2, and MnO content than other localities. The range of Fo content of cumulus olivine in dunites from Talur (Fo91-92) is relatively lower than the dunite from Doddakanya (Fo92-93) and net-textured chromitite from Sindhuvalli (Fo95-96). The range of Mg# in the intercumulus clinopyroxene in Talur net-textured chromitite varies between 94 and 99, whereas, in the Sindhuvalli net-textured chromitite, the intercumulus clinopyroxene shows a lower range of Mg# (= 96–97). Chromites in massive and silicate-rich chromitites, and from dunite contain several rounded to subhedral inclusions of silicate minerals of various sizes (~ 4–20 μm). Spherical inclusions of olivine (Fo94) are present in accessory chromites of dunite (Fo87). The Mg-rich clinopyroxene inclusions have Cr2O3 content from 1.5 to 2.3 wt%. The clinopyroxene inclusions in the Talur schlieren-banded chromitite are more magnesian (Mg# = 95–98) compared to the inclusions in chromites of the Sindhuvalli net-textured chromitite (Mg# = 96–97). The euhedral phlogopite inclusions in chromites from the Sindhuvalli net-textured chromitite are Mg-rich (~ 31.09 wt% MgO) with a significant amount of K2O (~ 9.28 wt%). The chlorite inclusion in chromites from the Talur schlieren-banded chromitite is also Mg-rich (~ 39.32 wt% MgO) with Al2O3 ~ 21.01 wt% and a significant amount of Cr2O3 (~ 3.51 wt%). Clinopyroxene (along with phlogopite) inclusions always occur as independent grain testifying their primary nature and might be genetically linked with the chromite crystallization from a primary high-Mg silicate magma. Chlorite and dolomite inclusions may be related to secondary hydrothermal alteration events. The average re-equilibration temperature of the olivine-chromite pairs in the Sindhuvalli net-textured chromitite and Doddaknaya dunite of around 545 °C represents the first stage of metamorphism under reducing conditions. Subsequently, the second hydrothermal episode involved fluids evolving to an average temperature of about 327 °C calculated from the accessory chromite and olivine pairs from Talur dunite, consistent with the abundance of magnesite veins throughout the Talur mines. The computed values of Al2O3, TiO2, and FeO/MgO ratio of the melt show that the parental melt associated with the chromitites in the Sargur greenstone belt has a close resemblance with the parental melt of the chromitites in the Nuggihalli schist belt, which is low-Al komatiitic. In the tectonic discrimination diagrams, most of the primary chromite compositions from Sargur plot within the field of chromites from komatiites from the Archean greenstone belts. Parental komatiitic magma fractionated in the subvolcanic conduits within the thin lithospheric crust and formed the sill-like layered ultramafic-mafic bodies with chromite deposits. These conduits fed the overlying extrusive system that developed the ultramafic-mafic volcanic sequence of the Sargur greenstone belt in the Western Dharwar Craton.

Metamorphic evolution and SIMS U–Pb geochronology of orthopyroxene‐bearing high‐P semipelitic granulite in the Fuping area, middle Trans‐North China Orogen

AbstractThe Fuping area lies in the middle part of the Trans‐North China Orogen, which is a critical region for understanding the metamorphic–tectonic evolution of this orogenic belt in the Precambrian. The newly discovered orthopyroxene‐bearing semipelitic granulite in this area is a coherent stratigraphic unit, among which three generations of metamorphic mineral assemblages (peak, post‐peak and cooling retrograde) have been identified based on petrographic observation and mineral chemical analysis. The peak metamorphic mineral assemblage mainly consists of porphyroblastic garnet plus matrix minerals including biotite, plagioclase, orthopyroxene, quartz as well as accessory minerals including pyrite, monazite, zircon and apatite. Geothermobarometry calculations and phase equilibria modelling confirm that the representative samples record different P–T conditions and metamorphic stages due to retrogressive metamorphism, with the highest P–T conditions reaching high‐P granulite facies. The retrieved clockwise metamorphic P–T paths pass from 12.5 to 13.5 kbar/855–880℃ (peak stage) through 8.5–11 kbar/850–886℃ (post‐peak stage) and to a speculative cooling phase (retrograde stage), reflecting near isothermal decompression (ITD) followed by near isobaric cooling (IBC). This P–T path is interpreted to reflect a subduction and/or collision event followed by a rapid exhumation. Secondary ion mass spectrometry (SIMS) U–Pb dating on metamorphic monazite and zircon yielded metamorphic ages ranging from 1,825 to 1,815 Ma, possibly constraining a retrograde metamorphic age. Therefore, the semipelitic granulite in the Fuping area records the subduction/collision event between the Eastern and Western Blocks of the North China Craton during the Late Palaeoproterozoic.

Metamorphosis in Insect Muscle: Insights for Engineering Muscle-Based Actuators.

One of the major limitations to advancing the development of soft robots is the absence of lightweight, effective soft actuators. While synthetic systems, such as pneumatics and shape memory alloys, have created important breakthroughs in soft actuation, they typically rely on large external power sources and some rigid components. Muscles provide an ideal actuator for soft constructs, as they are lightweight, deformable, biodegradable, silent, and powered by energy-dense hydrocarbons such as glucose. Vertebrate cell lines and embryonic cultures have allowed critical foundational work to this end, but progress there is limited by the difficulty of identifying individual pathways in embryonic development, and the divergence of immortal cell lines from these normal developmental programs. An alternative to culturing muscles from embryonic cells is to exploit the advantages of species with metamorphic stages. In these animals, muscles develop from a predefined pool of myoblasts with well-characterized contacts to other tissues. In addition, the endocrine triggers for development into adult muscles are often known and tractable for experimental manipulation. This is particularly true for metamorphic muscle development in holometabolous insects, which provide exciting new avenues for tissue engineering. Using insect tissues for actuator development confers additional benefits; insect muscles are more robust to varying pH, temperature, and oxygenation than are vertebrate cells. Given that biohybrid robots are likely to be used in ambient conditions and changing environments, this sort of hardiness is likely to be required for practical use. In this study, we summarize key processes and signals in metamorphic muscle development, drawing attention to those pathways that offer entry points for manipulation. By focusing on lessons learned from in vivo insect development, we propose that future culture designs will be able to use more systematic, hypothesis-driven approaches to optimizing engineered muscle. Impact statement This review summarizes our current understanding of metamorphic muscle development in insects. It provides a framework for engineering muscle-based actuators that can be used in robotic applications in a wide range of ambient conditions. The focus is on identifying key processes that might be manipulated to solve current challenges in controlling tissue development such as myoblast proliferation, myotube formation and fusion, cytoskeletal alignment, myotendinous attachment and full differentiation. An important goal is to gather findings that cross disciplinary boundaries and to promote the development of better bioactuators for nonclinical applications.

Petrological Characteristics of Suevite Varieties in the Kara Astrobleme, Pay-Khoy, Russia

The paper presents the first detailed mineralogical and petrographic information on the clastic components of three suevite varieties from the Kara astrobleme in the Pay-Khoy Ridge, Russian Arctic, including features of their morphology, inner structure, chemical and mineral phase composition of the suevite impact glasses. Data on the suevites are consistent with the earlier classification of the impactites. All of the suevite varieties contain indicator minerals of the impact process and show evidence of all principal stages of impact metamorphism and postimpact hydrothermal mineral-forming processes. Data acquired in the course of this study can be utilized to identify new and transitional types of suevites to more comprehensively characterize the Kara impactites and to build up a model for the origin of this unique geologic object and other astroblemes.

Multi-Stage Fluid System Responsible for Ore Deposition in the Ossa-Morena Zone (Portugal): Constraints in Cu-Ore Deposits Formation

The Mociços Cu-deposit is part of a cluster of ancient copper mines in the Sousel-Barrancos metallogenic belt in the Ossa-Morena Zone at the SW Iberia. The orebodies develop along NNW-SSE quartz-carbonate-sulfides veins with pyrite and chalcopyrite as the main sulfide phases, and ore emplacement has been attributed to copper remobilization from the metasedimentary host-rocks, though no detailed studies were conducted. A novel multi-stage fluid circulation model is hereby proposed, supported by petrography and fluid inclusion data evidencing the P–T–V-x evolution of the deposit. Stage (i) is an early metamorphic stage with a predominance of carbonic fluids, identified in highly deformed milky quartz (QzI), with estimated pressures between 338 and 486 MPa compatible to the regional metamorphic events (greenschist facies). Stage (ii) corresponds to a late-metamorphic manifestation of H2O–NaCl–CO2 fluids, with low-salinity (eq. w(NaCl) from 0.4 to 5.0%) and CO2 dominated. Stage (iii) in which ore emplacement took place and is characterized by dominant multisolid H2O–NaCl hypersaline, halite-bearing fluid inclusions (eq. w(NaCl) from 29.3 to 44.3%) with an H2O–NaCl–CO2 endmember and features indicative of magmatic-hydrothermal brines. Many of these inclusions homogenize by halite dissolution, with pressures as high as 320 MPa, and the coexistence of both fluids in the same fluid inclusion assemblages (FIA) could indicate phase separation caused by fluid pressure variations. Although there is no direct evidence of the magmatism responsible for these fluids, the geodynamic settings could favor deep-seated magmatism. Stage (iv) is characterized by low-salinity (eq. w(NaCl) from 0.18 to 15.57%) and low-temperature (68 to 160°C) primary two-phase fluid inclusions hosted by late-stage quartz (QzIV), suggesting a late-meteoric fluid circulation phase responsible for the leaching, oxidation and supergene enrichment observed at surficial levels. Throughout the P–T evolution of the system a decrease in pressure and temperature was registered, especially in fluid inclusions hosted in quartz from the sulfide bearing veins, suggesting that the transition from ductile to brittle regimes might have favored ore deposition.

The first report on circulating microRNAs at Pre- and Post-metamorphic stages of axolotls.

MicroRNAs (miRNAs) are endogenously coded small RNAs, implicated in post-transcriptional gene regulation by targeting messenger RNAs (mRNAs). Circulating miRNAs are cell-free molecules, found in body fluids, such as blood and saliva, and emerged recently as potential diagnostic biomarkers. Functions of circulating miRNAs and their roles in target tissues have been extensively investigated in mammals, and the reports on circulating miRNAs in non-mammalian clades are largely missing. Salamanders display remarkable regenerative potential, and the Mexican axolotl (Ambystoma mexicanum), a critically endangered aquatic salamander, has emerged as a powerful model organism in regeneration and developmental studies. This study aimed to explore the circulating miRNA signature in axolotl blood plasma. Small RNA sequencing on plasma samples revealed 16 differentially expressed (DE) circulating miRNAs between neotenic and metamorphic stages out of identified 164 conserved miRNAs. Bioinformatics predictions provided functional annotation of detected miRNAs for both stages and enrichment of DE miRNAs in cancer-related and developmental pathways was notable. Comparison with previous reports on axolotl miRNAs unraveled common and unique members of the axolotl circulating miRNome. Overall, this work provides novel insights into non-mammalian aspects of circulating miRNA biology and expands the multi-omics toolkit for this versatile model organism.

Cenozoic retrogression and exhumation of the amphibolites in the eastern Gangdese Belt, SW China

Abstract The collision between India and Asia is marked by the Yarlung Zangbo suture zone, which can be traced from Tibet into northwest Yunnan. The suture zone provides a record of the subduction of Neotethyan oceanic lithosphere beneath the Lhasa Block, creating the Gangdese magmatic belt, prior to the India–Asia collision. This paper reports petrological, zircon U–Pb, and whole-rock elemental and isotope data for amphibolite from the extension of the Gangdese belt in northwestern Yunnan. Two metamorphic stages are identified in relict amphibolite-facies rocks preserved in granitic gneisses from the belt. They are early amphibolite-facies metamorphism (M1) and late amphibolite-facies retrograde metamorphism (M2). The assemblage of M1 comprises garnet, amphibole, and plagioclase that formed at 0.89–0.97 GPa and 658–700 °C. Dating of syn-metamorphic zircons assigns this event to the middle Eocene (44–38 Ma). Symplectites (corona) of amphibole, plagioclase, and quartz surrounding garnet formed during retrograde stage M2, at 30–29 Ma, under P–T conditions of 0.41–0.68 GPa and 549–716 °C. The amphibolites display geochemical characteristics similar to those of mid-ocean ridge basalt, with eNd(t) values of +1.8 to +8.9, and are inferred to have formed in a back-arc-basin setting, suggesting that they constitute part of the lithospheric mantle of the Tengchong terrane that lies to the east. The amphibolite-facies metamorphism was related to underplating of the Indian plate during the India–Asia collision, which led to clockwise rotation of eastern Tibet during the middle Eocene, generating amphibolite recrystallization and, subsequently, with ongoing convergence of the two plates, exhumation of the region during the middle Oligocene.

The Metamorphism and Its Tectonic Implications of Indosinian High Pressure Granulites from the Badu Complex of the Cathaysia Block, Southwestern Zhejiang Province, South China

AbstractThe Badu Complex is the oldest metamorphic rock in Cathaysia Block which experienced several episodes of metamorphism Especially indosinian metamorphic reworking in the southwestern Zhejiang Province, South China. The degree of indosinian metamorphism reaches granulite facies. However, there is still insufficient understanding of the characteristics of the Indosinian granulite metamorphism in the Cathaysia and many interpretations of its tectonic significance. Therefore, we present detailed petrology, mineral chemistry and LA‐ICP‐MS zircon U‐Pb age in this paper from pelitic granulites of the Badu Complex, which is composed of “sillimanite + garnet + cordierite + spinel + biotite + k‐feldspar” assemblage and garnet pyroxenite with garnet amphibolite which is consists of “garnet + clinopyroxene + orthopyroxene + amphibole + plagioclase”. By comprehensive study we get following new findings: Pelitic granulites record four stages of metamorphic mineral assemblages, including prograde (M1), pressure peak (M2), Peak (M3) and post‐peak decompressional and then cooling (M4) stages. The prograde M1 assemblage consists of garnet1 (core) + staurolite + kyanite + biotite + quartz ± rutile ± chlorite; The pressure peak M2 assemblage consists of garnet1 (mantle) + sudoite + rutile + kyanite + corundum + biotite + quartz; The peak M3 have garnet2 (rim‐mantle) + biotite + sillimanite + quartz ± K‐feldspar ± plagioclase ± ilmenite assemblag; the M4 stage is consist of garnet + cordierite + biotite + sillimanite + quartz + ilmenite ± spine ± K‐feldspar. The garnet pyroxenite and garnet amphibolites have experienced three stages of metamorphic evolution. Peak high‐pressure granulite facies stage M2 consists of garnet + sahlite ± ilmenite ± quartz; Post‐peak near isothermal decompression medium granulite facies stage M3 is characterized by typical decompression reaction textures and assemblage of orthopyroxene + plagioclase (An=90–92); amphibolites facies retrograde metamorphic stage M4 is characterized by amphibole + plagioclase (An=33–35) + ilmenite ± sahlite ± quartz mineral assemblage.By means of phase equilibrium simulation and traditional thermobarometer, P‐T conditions of 785–820 °C and 8.9–9.9 kbar for M3 stage, 780–860 °C and 5.7–6.2 kbar for decompressional M4 stage, 705–720 °C and 4.5–4.7 kbar for cooling M4 stage in pelitic granulites were obtained. And also 11.6–12.5 kbar and 780–840 °C for M2 stage, 7.4–8.2 kbar and 800–880 °C for M3 stage, 6.6–7.5 kbar and 500–560 °C for M4 stage were obtained in garnet pyroxenite and garnet amphibolite. A clockwise P‐T path is confirmed in the two type rocks of the Badu Complex which reflected a near‐isothermal decompressional metamorphic process. The peak metamorphism can reach high‐pressure granulite facies. In addition, the mineral assemblage of garnet + rutile + kyanite + corundum in the peak metamorphic stage of pelitic granulite indicates that it may underwent ultra‐high‐pressure metamorphism, and the acidic plagioclase exsolution of clinopyroxene in garnet pyroxenite also suggests that it may be retrograded eclogites, which indicates that the deeper Cathaysian block may have eclogite metamorphism.Analyses of LA‐ICP‐MS zircon U‐Pb dating indicate that the metamorphic age of pelitic granulite is 233.5 Ma–subduction/collision followed by rapid exhumation and cooling events. The events may relate with the amalgamation of the Indochina Block‐South China Block North China Block in the paleo‐Tethyan domain.

Late Paleozoic geodynamic evolution of the western North Patagonian Massif and its tectonic context along the southwestern Gondwana margin

In this study, we propose a geodynamic model covering sedimentation, metamorphism, magmatism, and exhumation processes for the western North Patagonian Massif basement. The youngest detrital zircon population ages (ca. 369 ± 8 Ma) obtained in a schist sample constrains the sedimentation stage to the Carboniferous Period. A first prograde metamorphic stage (M1-D1) produced the main foliation (S1) under greenschist conditions (~ 500 °C; < 4.5 Kbars). This stage was possibly linked to the regional Carboniferous magmatism event (330–300 Ma). The Permian magmatism (ca. 290 Ma) likely induced partial melting and migmatization of the deepest metasedimentary suite. This event corresponds to the second prograde metamorphic stage (M2) that reached amphibolite conditions (690 °C and 6.5 Kbars). The beginning of the basement uplift corresponds to the first retrograde metamorphic stage possibly developed during Permian – Triassic times (265–235 Ma). This event was triggered by NE-SW compression (σ1) and developed folds (D2-F2), second foliation (S2), micro-textural quartz deformation, and a retrograde evolution path for the garnet-bearing lithofacies. The final stage of the basement exhumation corresponds to the second retrograde metamorphic stage (D3) developed by NNW-SSE compression and linked to open folds (F3) in the Cushamen Formation. The characteristics of the western North Patagonian Massif geodynamic evolution and the adjacent basement regions suggest a paleotectonic subduction setting for the southwestern Gondwana margin during the late Paleozoic times.

P–T path, zircon U–Pb ages and tectonic implications of Devonian pelitic granulites in the southern Chinese Altay orogen

Abstract The Chinese Altay orogen is an important part of the giant Central Asian Orogenic Belt. However, its tectono–metamorphic evolution is still a matter of debate. Here, we report Devonian pelitic granulites at the Wuqiagou area in the southern Chinese Altay orogen. They are characterized by an assemblage of garnet + orthopyroxene + cordierite ± spinel + biotite + plagioclase ± K–feldspar + quartz + Fe–Ti oxides. Based on petrographic features and P–T estimates, together with phase equilibrium modeling in the NCKFMASHTO system, three metamorphic stages can be distinguished as follows: (1) pre–peak prograde (M1) stage, with small biotite–plagioclase– or spinel–bearing assemblages preserved as inclusions in garnets and P–T conditions of

Petrogenetic re-examination of spinel + quartz assemblage in the Larsemann Hills, East Antarctica

We present textural evidence of spinel (low-Zn) and quartz from two samples of quartzofeldspathic rocks from the Larsemann Hills, East Antarctica. The present occurrence is unusual for the fact that the direct grain contact is preserved unlike most other occurrences where these two phases are separated by reaction products during retrograde and/or prograde metamorphic stages. Textural features further indicate that Ti-biotite was part of the prograde assemblage that underwent partial melting to produce spinel. The maximum temperature estimated from the rock is 870 °C. Monazite grains are abundant in the rock and these are mostly homogeneous in terms of U, Th and Pb distribution. In situ U–Th-total Pb EPMA data of these monazite yield a group age of 527 ± 8 Ma. Textural and geochronological data match with the second (M2) stage of metamorphism of the Larsemann Hills region as evident from the existing geological record. Although spinel and quartz was considered to be a part of peak M2 assemblage in earlier studies, preservation of their grain contact is enigmatic. We consider the grain contact metastable and its preservation possibly resulted from significant overstepping of cooling reaction due to fast reaction kinetics.

Chromitites from an Archean layered intrusion in the Western Dharwar Craton, southern India

Chromite deposits hosted in a layered ultramafic–mafic intrusion in the Archean Chithradurga greenstone belt are part of a suture that divides the Western and Central Dharwar craton blocks in southern India. Serpentinised ultramafic rocks including dunite, peridotite, and phlogopite-rich clinopyroxenite occur with massive chromitite. The olivine compositions show two distinct groups, one with Fo ~91–92 olivines in the massive chromitite, and another group with olivine Fo values of ~95–96, in the serpentinised dunite. The chromite occurs as massive chromitite and as an accessory phase in dunite. The former is characterized by high Cr# (Cr/(Cr + Al) × 100 = 67–70) and moderate to high Mg# (Mg/(Mg + Fe) = 52–66); The latter shows a larger variation in Cr which is likely related to sub-solidus re-equilibration of the chromite during the serpentinization event. In the massive chromitites, sub-solidus re-equilibration of chromite is less significant or absent. Bulk-rock major and trace element analyses of the peridotites reveal a composition less magnesian than depleted mantle rocks, with low TiO2, Na2O, CaO and high Cr, Ni content, a general depletion in Sr and enrichment in Th, U. The chromitites contain variable PGE abundances (ΣPGE = 8-244 ppb) with enrichments of Os, Ir and Ru, relative to Pt and Pd . These features suggest that the chromitites are cumulates formed from a highly magnesian parental magma with komatiitic affinity, derived by large degrees of partial melting, , which underwent crustal contamination. The ReOs isotope data demonstrate a minimum Re-depletion model age (TRD) of 2895 ± 24 Ma for the serpentinized dunite (MY11/3), and a more robust age of 3120 ± 12 Ma for two chromitite samples, which we interpret as the best estimate of the magmatic age of this ultramafic intrusion. Abundant zircon grains also occur within these ultramafic rocks with UPb ages spanning from ca. 2.9 Ga to ca. 2.4 Ga, with a dominant age population in the range 2.8–2.6 Ga. We interpret the upper intercept age at ca. 2.9 Ga to represent the initial crystallization of zircon in the ultramafic rocks, potentially as a result of an early stage of metamorphism and hydrothermal alteration. The 2.5–2.4 Ga zircon ages reflect a late-stage high-grade metamorphic overprint.

U\u2013Pb zircon geochronology and phase equilibria modelling of HP-LT rocks in the Ossa-Morena Zone, Portugal

The Ossa-Morena Zone (OMZ) has a complex geological history including both Cadomian and Variscan orogenic events. Therefore, the OMZ plays an important role in understanding the geodynamic evolution of Iberia. However, the P–T–t evolution of the OMZ is poorly documented. Here, we combine structural and metamorphic analyses with new geochronological data and geochemical analyses of mafic bodies in Ediacaran metasediments (in Iberia known as Série Negra) to constrain the geodynamic evolution of the OMZ. In the studied mafic rocks, two metamorphic stages were obtained by phase equilibria modelling: (1) a high-pressure/low-temperature event of 1.0 ± 0.1 GPa and 470–510 °C, and (2) a medium-pressure/higher-temperature event of 0.6 ± 0.2 GPa and 550–600 °C. The increase in metamorphic temperature is attributed to the intrusion of the Beja Igneous Complex (around 350 Ma) and/or the Évora Massif (around 318 Ma). New U–Pb dating on zircons from the mafic rocks with tholeiitic affinity yields an age between 815 and 790 Ma. If the zircons crystallised from the tholeiitic magma, their age would set a minimum age for the pre-Cadomian basement. The ca. 800 Ma protolith age of HP-LT tholeiitic dykes with a different metamorphic history than the host Série Negra lead us to conclude that: (1) the HP-LT mafic rocks and HP-LT marbles with dykes were included in the Ediacaran metasediments as olistoliths; (2) the blueschist metamorphism is older than 550 Ma (between ca. 790 Ma and ca. 550 Ma, e.g., Cadomian).

Metamorphic evolution and U-Pb geochronology of metapelite, northeastern Wutai Complex: Implications for Paleoproterozoic tectonic evolution of the Trans-North China Orogen

The Wutai Complex lies in the middle part of the Trans-North China Orogen, mainly consisting of metamorphosed volcanic-sedimentary rocks (named as “Wutai Group”) and Late Archean TTG gneisses. Garnet biotite schist from the northeastern part of the Wutai Group (lower Shizui Subgroup) mainly consists of garnet porphyroblast plus biotite, staurolite, kyanite, sillimanite/fibrolite, plagioclase, quartz as well as accessory minerals including ilmenite, magnetite, hematite, apatite, monazite and zircon. Petrographic observation and mineral chemistry indicate that these metapelites underwent the sequential prograde (M1), peak (M2) and retrograde (M3) metamorphic stages. Geothermobarometry computation and phase equilibrium modelling show that the representative samples record clockwise P-T paths passing from 3–7 kbar/570–630 °C (M1) through 9–10 kbar/615–660 °C (M2P) to 6–9 kbar/655–680 °C (M2T) and finally to a speculated exhumation stage (M3), indicative of orogenic metamorphism. SRHIMP and LA-ICP-MS U-Pb dating on inherited detrital zircons yielded three age peaks of ~2.7 Ga, ~2.5 Ga and ~2.1 Ga. The youngest 207Pb/206Pb age of 2064 ± 21 Ma of detrital zircon possibly constrains the maximum depositional age of the Shizui Subgroup. SIMS and SHRIMP U-Pb dating of metamorphic monazite and zircon constrain the metamorphic age of the garnet-bearing mica schist to be 1853–1823 Ma. Combined with previous data, the Wutai Complex was demonstrated to have been involved in a prolonged subduction-collision process (1.97–1.82 Ga) between the Western and Eastern Blocks in the Paleoproterozoic.

Peak Metamorphic Conditions of Garnet Amphibolite from Luk Ulo Complex, Central Java, Indonesia: Implications for Medium-Pressure/High-Temperature Metamorphism in the Central Indonesian Accretionary Collision Complex

DOI:10.17014/ijog.7.3.225-239Garnet amphibolites, which suggest medium-pressure/high-temperature (MP/HT), are widely recognized in Luk Ulo Complex, Central Java. Their occurrences associated with high-pressure/low-temperature (HP/LT; eclogite, blueschist) metamorphic rocks in the Luk Ulo Complex will provide important constraint on the geodynamic model of Central Indonesian Accretionary Collision Complex (CIACC). This study aimed to estimates P-T metamorphic condition of garnet amphibolite from Luk Ulo Complex by using mineral parageneses, thermodynamic data, and NCKFMASHO pseudosection. Prograde stage assemblages represented by inclusions in the garnet, which are garnet core, zoisite, titanite, apatite, and quartz. Mineral coexistences at the peak P-T condition are garnet rim, magnesio-hornblende, zoisite, titanite, quartz, albite, and phengite. The retrograde stage represented by secondary minerals fill the crack in the garnet and other minerals, which are chlorite and quartz. P-T metamorphic condition of garnet amphibolite can only be interpreted from the peak metamorphic stage. The temperature of the garnet amphibolite is estimated using the garnet-amphibole and garnet-phengite geothermometers. Meanwhile, the pressure condition is estimated from phengite geobarometer. The results were compared to the stability and compositions of the phases in NCKFMASHO pseudosection in order to constrain the peak P-T metamorphic conditions. It is concluded that the peak P-T metamorphic condition for garnet amphibolite is 0.9 - 1.4 GPa and 558 - 606 ºC. The estimated peak P-T metamorphic temperature is higher compared to the previously published gradient geothermal of eclogite and tourmaline-eclogite in the Luk Ulo Complex. The MP/HT (amphibolite) and HP/LT (blueschist and eclogite) metamorphic rocks could have similar metamorphic ages if both footwall and hanging wall had initially very high thermal gradients and the rate of subduction was very slow (10 km/Ma or less).

Development and distribution rules of the main Neoproterozoic source and reservoir strata in the Yangtze Block, Southern China

In recent years, significant breakthroughs in gas exploration have been made in Neoproterozoic strata in the Yangtze Block, in southern China, as the Anyue super gas field was discovered in the central Sichuan region. The potential of Neoproterozoic exploration in other parts of the Yangtze Block remains a question. Defining the development and distribution rules of Neoproterozoic source and reservoir strata is an important reference for the next step of deep-zone oil and gas explorations in the Yangtze Block. Based on field surveys and sample collections, the analyses of many predecessor field sections and drilling logging data of the Neoproterozoic in the Yangtze Block, source-rock series tests and analyses, as well as analyses of reservoir thin sections, and the distribution rules of the source and reservoir strata were determined by industrial mapping methods. Three sets of source rocks are found in the Neoproterozoic in Yangtze Block, namely, the Datangpo Formation, Doushantuo Formation, and the third member of the Dengying Formation. From top to bottom, the thermal evolutions in the three sets of source rocks show an increasing trend, with the Dengying Formation reaching the stage of over-maturation, while that of Doushantuo and Datangpo formations enter thermal metamorphism stages. The source rocks of the Datangpo Formation are relatively limited in distribution, displaying a northeastward rift style, whereas those of the Doushantuo Formation are distributed widely with two thickness centers. The Neoproterozoic reservoirs in the Yangtze Block developed in the Dengying Formation, with the carbonate reservoir spaces mainly comprising caves, dissolved bores, or pores and cracks. The thickness center of the dolomite reservoirs in Dengying Formation are mainly located in the Middle and Upper Yangtze carbonate platform marginal zones, which are distributed in the eastern Sichuan Basin, and in the Mianyang–Changning rifting platform margin belts in the central Sichuan Basin and the northern Hubei areas. Based on the typical reservoir anatomy, the hydrocarbon accumulation rules of the Neoproterozoic in the Yangtze Block are summarized as “near the rift trough or the passive continental margin hydrocarbon generation centers, adjacent to Karst bioherm-beach reservoir, and stable structural zones afterwards”.

The Zirconometry and Thermochronology of Migmatized Gneisses of the Gondaray Metamorphic Complex (Greater Caucasus)

The results of the isotope dating of the crystalline basement of the Greater Caucasus (the Elbrus subzone of the Main Caucasus Range) within of the high-temperature gneiss–migmatite area of the Gondaray metamorphic complex is discussed. The relationships between regional metamorphism, migmatization, and heat flows in the lithosphere are considered. The measurements of the zircon isotope compositions were performed on a SHRIMP-II ion microprobe at the CIR VSEGEI (St. Petersburg). All zircon crystals from gneiss sample 526 have a zonal structure with ancient detrital cores. Almost all U–Pb isotope dating points lie on the concordant line and show a wide age range from 320–1000 Ма, partially inherited from detrital zircon grains from primary pelitic sediments. The youngest ages (320 Ма) were measured in the regeneration zones of zircon grains recrystallized during stage of the anatexis and migmatization. The other points of detrital zircons form an age cluster of 540–1000 Ма, which indicates their origin from different magmatic sources that existed during the formation of the protometamorphic protolith. Several detrital zircon grains yielded a Cambrian age; this is evidence for an Early Paleozoic age of the metamorphic protolith, which was traditionally suggested to be Precambrian or even Archean. The ages of the rims of recrystallized zircons (320 Ма) have a direct correlation with the postmetamorphic granitoid ages of the Greater Caucasus. The thermochronological modeling of the transformation of the Gondaray metamorphic complex during the retrogressive stage of metamorphism demonstrates that its cooling from the temperature of migmatite crystallization (650°С) to the closure of the temperature of the K–Ar biotite isotope system (350°С) was relatively rapid (the rate of cooling was 8–10°С/Ma) under subisobaric conditions and during a time range approximately 30–40 Ма.