Metamorphic Stages Research Articles

An extensive record of orogenesis recorded in a Madagascar granulite

AbstractWe present a comprehensive petrological and geochronological study of a single granulite sample from the lithosphere‐scale Beraketa shear zone in southern Madagascar to constrain the orogenic history of Gondwana assembly in this region. The studied sample provides a panoply of data constraining the prograde, retrograde, and late metasomatic history of the region via the application of Ti‐in‐quartz, Ti‐in‐zircon, Zr‐in‐rutile, and Al‐in‐orthopyroxene thermobarometry; phase‐equilibrium modelling; U–Pb monazite, zircon, and rutile petrochronology; and trace element diffusion chronometry in rutile. Our results reveal five stages of metamorphism along a narrow clockwise P–T path that may have begun as early as 620–600 Ma and certainly by 580–560 Ma, based on the oldest concordant zircon dates. The rock was heated to >725°C at less than 7.5 kbar (Stage 1) before burial to ~8 kbar (Stage 2). By c. 540 Ma, the rock had heated to ~970°C at ~9 kbar, and lost approximately 12% melt (Stage 3), before decompressing and cooling to the solidus at ~860°C and 6.5 kbar within 10 Ma (Stage 4). The vast majority of monazite and zircon dates record Stage 4 cooling and exhumation. Monazite and zircon rim dates as young as c. 510 Ma record subsolidus cooling (Stage 5) and associated symplectite formation around garnet. U–Pb rutile dates record partial resetting at c. 460 Ma; Zr‐ and Nb‐in‐rutile diffusion chronometry link these dates to a metasomatic event that lasted <1 Ma at ~600°C. In addition to chronicling a near‐complete cycle of metamorphism in southern Madagascar, this study constrains the rates of heating and cooling. We estimate that heating (7–14°C/Ma) outpaced reasonable radiogenic heating rates with modest mantle heat conduction. Therefore, we conclude that elevated mantle heat conduction or injection of mantle‐derived magmas likely contributed to regional ultrahigh‐temperature metamorphism (UHTM). Exhumation and cooling from peak metamorphic conditions to the solidus occurred at rates greater than 0.45 km/Ma and 14°C/Ma.

Multi‐stage metamorphic history of the Oki gneisses in Japan: Implications for Paleoproterozoic metamorphism and tectonic correlations in northeastern Asia

AbstractMetamorphicP–Tconditions, monazite chemical ages and zircon U–Pb ages from the gneisses exposed in the Oki belt, Japan, were integrated to unravel the multi‐stage metamorphic history of the belt. Microstructural observations combined with obtainedP–Tconditions and metamorphic ages reveal three distinct stages of metamorphism: M1, M2 and M3. The M1 stage occurredc. 1.85 Ga with high‐Tgranulite‐facies metamorphism (793–803°C and 9.8–12.3 kbar and 738–755°C and 9.1–12.0 kbar in the southwestern and southeastern Oki gneisses, respectively). The age of the M1 stage is well recorded in monazites included in large garnet porphyroblasts and low Th/U metamorphic rims in zircons from the Oki gneisses. The M1 metamorphism was overprinted byc. 230 Ma metamorphism (M2), which occurred at granulite‐facies conditions (817–829°C and 9.0–10.3 kbar) in the southwestern Oki gneisses and at upper amphibolite‐facies conditions (693°C and 5.3 kbar) in the southeastern Oki gneisses. Monazites in small garnets, euhedral zircons and outermost rims of zircons crystallized during this stage. The final metamorphism occurred as retrograde amphibolite‐facies recrystallization (M3) at conditions of 558–638°C and 3.7–4.8 kbar. The inherited cores in zircons yield ages from Paleoarchean to Paleoproterozoic but lack Palaeozoic ages. The detrital zircon distribution and the Paleoproterozoic metamorphic event in the Oki belt support the idea that the Oki gneisses are fragments of a Precambrian terrane rather than Palaeozoic sediments derived from the terrane. Combined with previous studies, we concluded that thec. 1.85 Ga M1 high‐Tgranulite‐facies metamorphism in the Oki belt could be related to that of the Jiao‐Liao‐Ji belt in the eastern North China block via the northern Gyeonggi and Nangrim Massifs on the Korean Peninsula, whereas thec. 250–230 Ma M2 stage could be associated with collision between the North and South China blocks. The Oki belt geologically corresponds to the northern Gyeonggi Massif in South Korea due to their similar Paleoproterozoic and Triassic tectonothermal events.

Divergent metamorphism within the Namche Barwa Complex, the Eastern Himalaya, Southeast Tibet, China: Insights from in situ U–Th–Pb dating of metamorphic monazite

AbstractGeothermobarometry shows that metapelite samples from Namche Barwa Complex (NBC) reached upper amphibolite to near‐granulite facies during the peak metamorphic stage, with similar conditions of ~700–750°C/8–10 kbar, and then experienced retrograde metamorphism at ~630–700°C/4–7 kbar. In situ monazite laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) U–Th–Pb dating suggests divergent metamorphism in the NBC: metapelite on the hanging wall of Namu–La thrust preserved a continuous metamorphic record of >19–3 Ma, whereas metapelite on the footwall yielded age ranges of >18–14 and 8–3 Ma, with a gap in recorded ages between 14 and 8 Ma. Monazite grains in the garnet porphyroblasts, more depleted in the heavy rare earth elements (HREE), yielded the youngest age of ~14 Ma. This is interpreted as the timing of upper amphibolite facies peak metamorphism in the metapelite from the NBC, with the NBC being exhumed coherently thereafter. Furthermore, the discrepancy between reported peak metamorphic ages of high‐pressure granulite (~40–30 Ma, ~25–20 Ma) and mid‐pressure metapelite (~14 Ma, this study) indicate asynchronous subduction–exhumation processes in the NBC. We suggest that crustal flow has played an essential role in exhumation since ~40 Ma, and recent surficial erosion (<8 Ma) intensified the exhumation of the NBC, with young leucogranite (<10 Ma) resulting from decompression melting. From ~3 Ma to the present, the interplay of erosion and tectonic movement caused ubiquitous rapid uplift, resulting in the concomitant exhumation of various types of rocks and the formation of the spectacular high relief between Yarlung Tsangpo Gorge and Namche Barwa Peak.

The reflectance spectra of CV–CK carbonaceous chondrites from the near-infrared to the visible

ABSTRACT Carbonaceous chondrite meteorites are so far the only available samples representing carbon-rich asteroids and in order to allow future comparison with samples returned by missions such as Hayabusa 2 and OSIRIS-Rex, it is important to understand their physical properties. Future characterization of asteroid primitive classes, some of them targeted by sample-return missions, requires a better understanding of their mineralogy, the consequences of the exposure to space weathering, and how both affect the reflectance behaviour of these objects. In this paper, the reflectance spectra of two chemically related carbonaceous chondrites groups, precisely the Vigrano (CVs) and Karoonda (CKs), are measured and compared. The available sample suite includes polished sections exhibiting different petrologic types: from 3 (very low degree of thermal metamorphism) to 5 (high degree of thermal metamorphism). We found that the reflective properties and the comparison with the Cg asteroid reflectance class point towards a common chondritic reservoir from which the CV–CK asteroids collisionally evolved. In that scenario, the CV and CK chondrites could be originated from 221 Eos asteroid family, but because of its collisional disruption, both chondrite groups evolved separately, experiencing different stages of thermal metamorphism, annealing, and space weathering.

Differential exhumation of ultrahigh-pressure metamorphic terranes: A case study from South Altyn Tagh, western China

South Altyn Tagh contains ultrahigh-pressure (UHP) terranes that have been exhumed from ~300 km mantle depth. Previous zircon U–Pb geochronology has yielded an eclogite-facies age of ca. 500 Ma and a high-pressure (HP) granulite-facies retrograde age of ca. 450 Ma in the Jianggalesayi area in the western segment of South Altyn Tagh. However, in the eastern segment (Yinggelisayi and Danshuiquan localities), an age range of 500–480 Ma has been determined, and it remains uncertain as to whether this age range represents the timing of the peak metamorphic stage or the retrograde overprint. Our study of newly discovered retrograde eclogite in the Danshuiquan locality shows that it underwent three stages of metamorphism, under eclogite-facies, HP granulite-facies and amphibolite-facies P–T conditions of 2.5–4.0 GPa and 870–1050 °C, 2.0–1.4 GPa and 830–940 °C, and 0.7–1.3 GPa and 704–880 °C, respectively. The decompression-dominated P–T path evolved mainly after crossing the solidus, indicating marked retrograde modification under melt-bearing conditions. LA–ICP–MS and SIMS zircon U–Pb dating yielded ca. 500 Ma eclogite-facies and ca. 484 Ma granulite-facies retrograde ages and a later retrograde age of ca. 452 Ma. The clockwise P–T–t path indicates rapid exhumation from eclogite-facies to granulite-facies within around 16 Myr, which is faster than that of the UHP rocks in the western segment. Thus, the HP–UHP rocks in South Altyn Tagh suggest a differential exhumation process for the eastern and western segments. The distinct HP–UHT metamorphism and rapid exhumation of (U)HP rocks in the eastern segment likely resulted from local mantle heating. The continuing P–T evolution of the (U)HP rocks under UHT conditions during exhumation led to a pervasive granulite-facies overprint in the eastern segment of the South Altyn Tagh. The rapid exhumation recorded in the eastern segment provides valuable insights into the exhumation mechanism of ultra-deep subducted UHP terranes.

Pressure–temperature–time evolution of subducted crust revealed by complex garnet zoning (Theodul Glacier Unit, Switzerland)

AbstractCollisional orogens commonly include mono‐metamorphic and poly‐metamorphic units, and their different evolution can be difficult to recognize and reconcile. The Theodul Glacier Unit (TGU) in the Western Alps consists of an association of metasedimentary and metamafic rocks embedded within the Zermatt‐Saas tectonic unit. In spite of recent petrological studies, it remains unclear whether these rocks underwent one or multiple metamorphic cycles. In this study, different lithologies from the TGU unit (mafic schist, mafic granofels, and chloritoid schist) were investigated for petrography, quantitative compositional mapping of garnet, thermodynamic modelling, and Lu–Hf garnet dating. The data reveal a coherent mono‐metamorphic history with a β‐shaped pressure–temperature (P–T) path characteristic of oceanic subduction. Garnet Lu–Hf ages yield a restricted garnet crystallization time window between 50.3 and 48.8 Ma (±0.5%, 2SD). A prograde metamorphic stage recorded in garnet cores yields conditions of 490 ± 15℃ and 1.75 ± 0.05 GPa. Maximum pressure conditions of 2.65 ± 0.10 GPa and 580 ± 15℃ were reached at 50.3 ± 0.3 Ma. Initial exhumation was rapid and led to isothermal decompression to 1.50 ± 0.10 GPa within 1 Ma. This decompression was associated with lawsonite breakdown in mafic schist and in mafic granofels, causing intense fluid–rock interaction within and between different lithologies. This process is recorded in garnet textures and trace element patterns, and in the major element composition of K‐white mica. Initial exhumation was followed by re‐heating of ~30℃ at a pressure of 1.50 ± 0.10 GPa. Perturbation of the subduction‐zone thermal structure may be related to upwelling of hot asthenospheric mantle material and transient storage of the unit at the crust–mantle boundary.

Metamorphic stages in mountain belts during a Wilson cycle: A case study in the central Sanandaj–Sirjan zone (Zagros Mountains, Iran)

Polymetamorphic units are important constituents of continent–continent collisional orogens, and rift metamorphic assemblages are often overprinted by subsequent metamorphism during subduction and collision. This study reports the metamorphic conditions and evolution of the Dorud–Azna metamorphic units in the central part of the Sanandaj–Sirjan zone (SSZ), Iran. Here, new geothermobarometry results are integrated with 40Ar/39Ar mineral and Th–U–Pb monazite and thorite ages to provide new insight of polyphase metamorphism in the two different basement units of the SSZ, the lower Galeh-Doz orthogneiss and higher Amphibolite-Metagabbro units. In the Amphibolite-Metagabbro unit, staurolite micaschist underwent a prograde P–T evolution from 640 ± 20 °C/6.2 ± 0.8 kbar in garnet cores (M1) to 680 ± 20 °C/7.2 ± 1.0 kbar in garnet rims (M2). Three Th–U–Pb monazite ages of 306 ± 5 Ma, 322 ± 28 Ma and 336 ± 39 Ma from the garnet-micaschists testify the Carboniferous age of M1 metamorphism. In the same unit, the metagabbro records P–T conditions of 4.0 ± 0.8 kbar and 580 ± 50 °C in the (magmatic) amphibole core (Late Carboniferous intrusion) to 7.5 ± 0.7 kbar and 700 ± 20 °C in the amphibole rim indicating a prograde P–T path during subsequent burial (M1). New 40Ar/39Ar dating of white mica from the staurolite micaschist yielded a staircase pattern ranging from 36 ± 12 Ma to 170 ± 2 Ma. This implies polymetamorphism with a minimum Late Jurassic cooling age through the Ar retention temperature of ca. 425 ± 25 °C after M2 metamorphism and a Paleogene low-grade metamorphic overprint (M3), while 40Ar/39Ar white mica dating of garnet micaschist yielded a plateau age of 137.84 ± 0.65 Ma. We therefore interpret the amphibolite-grade metamorphism M2 to have predated 170 Ma and is likely between 180 and 200 Ma. Furthermore, it is overprinted at about 36 Ma under retrogressive low-grade M3 metamorphism (at temperatures of ~350–240 °C) during final shortening and exhumation. In the underlying Galeh-Doz unit, the Panafrican granitic orthogneiss intruded at P–T conditions of 3.2 ± 4 kbar and 700 ± 20 °C, then it was metamorphosed and deformed at 600 ± 50 °C and 2.0 ± 0.8 kbar (metamorphic stage M1) prior to Late Carboniferous intrusion of mafic dikes. 40Ar/39Ar dating of amphibole from the Galeh-Doz orthogneiss gave plateau-like steps between 260 and 270 Ma, representing the age of cooling through ca. 500 °C after the M1 metamorphic event. Interestingly, the results of this study demonstrate polyphase metamorphic histories in both the Galeh-Doz orthogneiss and Amphibolite-Metagabbro units at different P–T conditions and final thick-skinned Paleogene emplacement of these units over the underlying low-grade metamorphic June Complex. Our findings suggest that both units are affected by high-T/low-P Late Carboniferous orogenic metamorphism along with the bimodal magmatism, as result of rifting. We propose that the Early Jurassic amphibolite-grade M2 metamorphism of the SSZ is correlated with the initial subduction of the Neotethyan Ocean. Eventually, the investigated units reflect various stages of a Wilson cycle, from rifting to initiation of the subduction in final plate collision.

Metamorphism and geochronology of high-pressure mafic granulites (retrograded eclogites?) in East Cathaysia terrane of South China: Implications for Mesozoic tectonic evolution

AbstractHigh-pressure mafic granulites (retrograded eclogites?) were discovered as minor lenses enclosed in garnet-kyanite-cordierite gneiss from the Badu Complex of the East Cathaysia terrane in South China. These rocks consist mainly of garnet, clinopyroxene, hornblende, quartz, and rutile/ilmenite with or without omphacite pseudomorphs that are indicated by clinopyroxene + sodic plagioclase symplectic intergrowths. Mineral textures and reaction relationships suggest three metamorphic stages: (1) an eclogite-facies stage (M1) characterized by the mineral assemblage of garnet + clinopyroxene (omphacite) + hornblende + rutile + quartz; (2) a high-pressure granulite-facies (M2) stage mainly represented by garnet + clinopyroxene + plagioclase + hornblende + rutile + quartz in the matrix; and (3) an amphibolite retrograde stage (M3) defined by hornblende + plagioclase + ilmenite + quartz symplectites surrounding garnet porphyroblasts. Conventional geothermometers and geobarometers in combination with phase equilibria modeling constrain metamorphic P–T conditions of 15.8–18.2 kbar/625–690 °C (M1), 11.8–14.5 kbar/788–806 °C (M2), and 5.4–6.4 kbar/613–668 °C (M3), respectively. Two-staged decompression processes are defined after the peak pressure, which suggests a two-staged exhumation of these deeply buried rocks. Secondary ion mass spectrometry (SIMS) zircon U-Pb dating and trace element analysis show that the high-pressure metamorphism occurred at 240–244 Ma. Complete early Mesozoic orogenic processes characterized by initial subduction and/or crustal thickening and subsequent exhumation followed by rapid uplift are reconstructed for this part of the East Cathaysia terrane, South China.

Vein topology, structures, and distribution during the prograde formation of an Archean gold stockwork

Abstract The Archean Cheechoo stockwork gold deposit is hosted by a felsic intrusion of tonalitic-granodioritic composition and crosscutting pegmatite dikes in the Eeyou Istchee James Bay area of Quebec, Canada (Archean Superior craton). The evolution of the stockwork is characterized herein using field relationships, vein density, and connectivity measurements on drill core and outcrop zones. The statistical distribution of gold is used to highlight mechanisms of stockwork emplacement and gold mineralization and remobilization. Two statistical populations of gold concentration are present. Population A is represented by gold grades below 1 g/t with a lognormal cumulative frequency. It is widespread in the hydrothermally altered (albite and quartz) and mineralized facies of the pluton. It is controlled by the development of quartz-feldspar-diopside veins as shown by the similar lognormal distribution of grades and vein density and by the correspondence of grades with network connectivity. Diopside and actinolite porphyroblasts in deformed veins within sodic and calcsilicate alteration zones are evidence for auriferous vein emplacement prior to the amphibolite facies peak of metamorphism. Population B (>1 g/t) is erratic and exhibits a strong nugget effect. It is present throughout the mineralized portion of the pluton and in pegmatites. This population is interpreted as the result of gold remobilization during prograde metamorphism and pegmatite emplacement following the metamorphic peak. The pegmatites are interpreted to have scavenged gold emplaced prior to peak metamorphism. These results show the isotropic behavior of the investigated stockwork during regional deformation and its development during the early stages of regional prograde metamorphism.

Petrogenesis and tectonic significance of Neoarchean (~2.6 Ga) alkaline ultrapotassic granitic gneisses from the southeastern margin of the North China Craton: Constraints from U-Pb dating, Hf isotope and petrogeochemistry

The Wuhe complex (WC), exposed at the southeastern margin of the North China Craton (NCC), is an important constituent of the Eastern Block of the NCC. In order to better understand the Precambrian crustal composition and evolution of this region, a comprehensive investigation on zircon geochronology and Hf isotopes, as well as whole-rock geochemistry and Sr-Nd-Pb isotopes was conducted on alkaline ultrapotassic granitic gneisses from the WC at Mashan. These gneisses are considered as meta-igneous rocks based on geochemical and mineralogical criteria (with special reference to zircon cathodoluminescence images). They are characterized by modally abundant alkali feldspar (more than 60%) and quartz (more than 30%) while plagioclase is rare. Mafic minerals are sodic amphiboles such as arfvedsonite and eckermannite, aegirine and rare biotite. Minor constituents are rutile, muscovite, apatite and barite. Major elements geochemistry shows high SiO2 (69.85%–74.51%) and K2O + Na2O (9.67%–12.17%) contents, high K2O/Na2O (7.41–22.53) ratios, and relatively low MgO (0.37%–0.87%) and CaO (0.10%–0.23%) contents. Trace elements geochemistry shows significant depletions of Nb, Y, Ce, Ga, and REE (rare earth elements) relative to anorogenic granites. These features suggest that the magmatic protoliths of the studied gneisses belong to ultrapotassic silica-saturated alkaline series from an extensional background, perhaps in a subduction-related rifting environment. As concerning isotope geochemistry, their 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios are 16.6297–17.1877, 15.4454–15.5066, and 36.6036–38.0304, respectively, and εNd(t) values vary from −1.7 to +9.1. Two samples yielded zircon 207Pb/206Pb ages of 2615 ± 4 Ma and 2617 ± 5 Ma, respectively, representing their precursor ages. The zircon igneous core domains exhibit oscillatory growth zoning with positive εHf (t) values (+2.5 − +6.6). These data, coupled with chondrite-normalized and primitive-mantle-normalized element patterns, suggest that the precursors of the studied granitic gneisses were mainly derived from a subduction-modified ultrapotassic syenitic parental magma, and may be considered as a particular group of A-type granites, involved in an important crustal growth and reworking event at ~2.6 Ga. These rocks experienced a granulite-facies metamorphic stage accompanied by partial melting, as testified by clinopyroxene + rutile + K-feldspar + spinel + quartz + apatite inclusions in zircon metamorphic domains which were dated at ~2.5 Ga. The occurrence of this metamorphic stage is also supported by the lower ΣREE contents, negative Eu anomalies, and high Ti-in-zircon temperatures (>800 °C) of metamorphic zircon mantles dated at ~2.5 Ga. Eventually, the studied rocks suffered a later ~1.85 Ga metamorphic overprinting, possibly related to the Paleoproterozoic collisional orogeny recorded in the region.

ZIRCONS FROM ROCKS OF THE MURZINKA-ADUI METAMORPHIC COMPLEX: GEOCHEMISTRY, THERMOMETRY, POLYCHRONISM, AND GENETIC CONSEQUENCES

Transformation of the oceanic crust into the continental one in orogenic belts is an important problem in petrological studies. In the paleocontinental sector of the Urals, a key object for tracing the stages of metamorphism and investigating the origin of anatectic granites is the Murzinka-Adui metamorphic complex. We have analyzed trace elements in zircons and established their genesis, sources, crystallization conditions, and stages of metamorphic events and granite generation in this complex. Zircons compositions were determined by the LA-ICP-MS method. Temperatures were calculated from Ti contents in the zircons. We distinguish three geochemical types of zircons, which differ in the ratios of light and heavy REE, U, Th, Ti, Y and show different values of Ce- and Eu-anomalies and Zr/Hf ratios, which are indicative of different crystallization conditions, as follows. Type I: minimal total LREE content; clear negative Eu- and Ce- anomalies; features of magmatic genesis; crystallization temperatures from 629 to 782 °C. Type II: higher contents of Ti, La, and LREE; low Ce-anomaly; assumed crystallization from highly fluidized melts or solutions. Type III: low positive Eu-anomaly; high REE content; low Th/U-ratio; zircons are assumed to originate from a specific fluidized melt with a high Eu-concentration. Ancient relict zircons (2300–330 Ma) in gneisses and granites show features of magma genesis and belong to types I and II. Such grains were possibly inherited from granitoid sources with different SiO2 contents and different degrees of metamorphism. Based on the geological and petrogeochemical features and zircon geochemistry of the Murzinka-Adui complex, there are grounds to conclude that the material composing this complex was generated from the sialic crust. The main stages of metamorphism and/or granite generation, which are traceable from the changes in types and compositions of the zircons, are dated at 1639, 380–370, 330, and 276–246 Ma. Thus, transformation of the oceanic crust into the continental one was a long-term and complicated process, and, as a result, the thickness of the sialic crust is increased in the study area.

Late Variscan (315 Ma) subduction or deceptive zircon REE patterns and U–Pb dates from migmatite‐hosted eclogites? (Montagne Noire, France)

AbstractEclogites hosted in sillimanite‐bearing migmatites in the Montagne Noire dome (French Massif Central) have an oceanic protolith and recorded a prograde P–T path from ~19.5 kbar, 700°C to the pressure peak at ~21 kbar, 750°C (pseudosection modelling), suggesting metamorphism in a subduction setting. Subsequent exhumation to low‐P high‐T (LP–HT) conditions (~6 kbar, 730°C) is constrained by the equilibration conditions of the embedding migmatite. In samples of a fresh and a retrogressed eclogite, all zircon crystals but one display similar REE patterns (no Eu anomaly, flat HREE), usually ascribed to crystallization under eclogite‐facies conditions. Yet, the U–Pb apparent ages of zircon crystals from both eclogites spread from c. 360 Ma to a dominant data cluster at c. 315 Ma. The c. 315–310 Ma zircon U–Pb dates obtained from the embedding migmatite are interpreted as the age of crystallization of the partial melt during the LP–HT metamorphic stage. First‐order geological evidence, in particular the sedimentary record, excludes the existence of a subduction zone in the region at this period. Unless calling upon a major reappraisal of the tectonics of the European Variscan orogen, this suggests an ambiguous relation between REE patterns and U–Pb dates in the zircon population. Various scenarios that could account for the observations are discussed. Combining our data with the results of a previously published Sm–Nd dating of garnet, and regional considerations, we consider that 360 Ma is the best approximation of the minimum age of the eclogite‐facies event. We hypothesize that the eclogites formed farther north and were transferred to their present location by lower‐crustal flow. It is inferred that during the subsequent exhumation, eclogite‐facies zircon grains recrystallized and underwent partial to total resetting of their U–Pb system, whereas the REE system remained mostly unmodified. These results caution against the use of REE patterns as the only criterion to associate a specific zircon age with HP metamorphism in eclogites occurring in migmatitic domes.

Middle-Late Triassic metamorphism of the Guajira Arch-basement: Insights from zircon U–Pb and Lu–Hf systematics

The Guajira Arch is located in the northernmost portion of South America and holds the record of several stages of magmatism and metamorphism. We performed LA-ICP-MS U–Pb zircon geochronology coupled with Lu–Hf isotope geochemistry on 11 samples from a high-grade gneissic basement hosting interleaved complexes of younger meta-granitoids and meta-sediments to unravel its tectono-metamorphic evolution. The oldest rocks correspond to a Rodinia-type basement at ca. 1100-880 Ma, affected by mafic magmatism during the Permian at ca. 272 Ma, anatexis during the Carnian at ca. 230 Ma, updoming and the formation of an extensional detachment fault during the Norian at ca. 224 Ma concluding with Jurassic magmatism since ca. 190 Ma. Trace element geochemistry shows that Triassic zircons are depleted in HREE and yield lower Th/U ratios than Jurassic zircons enriched both in HREE, Yb, and Y. Hf systematics indicates a Triassic to Jurassic evolution towards more radiogenic compositions. The lithological associations, structural setting, U–Pb ages, and Hf isotope geochemistry of the Triassic-Jurassic rocks at the Guajira Arch characterize an active margin setting of a hot orogenic belt in NW-Gondwana, comprising the Central Cordillera of Colombia, the Santa Marta Massif, the Merida Andes, the Perijá Range, the Santander Massif and the para-autochthonous terranes south of the Ouachita-Marathon Suture (Oaxaquia, Acatlán, and Maya), and thus vindicate the existence of a conjugate margin with Laurentia until the Late Triassic, preceding the breakup of western Pangaea. The Triassic anatectites and Jurassic granitoids from the Guajira Arch yield Proterozoic Hf TDMi ages which are indistinguishable from the autochthonous Proterozoic basement and thus render an allochthonous origin by terrane accretion highly improbable. Instead, these data point to melting of the continental crust by the incorporation of primitive material reflecting a two-stage process at the boundary of a large underlying mantle convection cell (i) Triassic slab steepening, subduction arrest, updoming anatexis at ca. 230-223 Ma and (ii) Early Jurassic arc-magmatism at ca. 190-165 Ma due renewed convergence. Furthermore, we propose that the structural setting, lithology, and isotope geochemistry of the Guajira Arch units, fit into the style of a cordilleran core-complex emplaced in a roll-back extensional setting preceding Pangaea break-up.

Effects of thiourea on the skull of Triturus newts during ontogeny.

BackgroundIn amphibians, thyroid hormone (TH) has a profound role in cranial development, especially in ossification of the late-appearing bones and remodeling of the skull. In the present study, we explored the influence of TH deficiency on bone ossification and resulting skull shape during the ontogeny of Triturus newt hybrid larvae obtained from interspecific crosses between T. ivanbureschi and T. macedonicus.MethodsLarvae were treated with two concentrations of thiourea (an endocrine disruptor that chemically inhibits synthesis of TH) during the midlarval and late larval periods. Morphological differences of the cranium were assessed at the end of the midlarval period (ontogenetic stage 62) and the metamorphic stage after treatment during the late larval period.ResultsThere was no difference in the ossification level and shape of the skull between the experimental groups (control and two treatment concentrations) at stage 62. During the late larval period and metamorphosis, TH deficit had a significant impact on the level of bone ossification and skull shape with no differences between the two treatment concentrations of thiourea. The most pronounced differences in bone development were: the palatopterygoid failed to disintegrate into the palatal and pterygoid portions, retardation was observed in development of the maxilla, nasal and prefrontal bones and larval organization of the vomer was retained in thiourea-treated larvae.ConclusionsThis implies that deficiency of TH caused retardation in development and arrested metamorphic cranium skeletal reorganization, which resulted in divergent cranial shape compared to the control group. Our results confirmed that skull remodeling and ossification of late-appearing bones is TH–dependent, as in other studied Urodela species. Also, our results indicate that TH plays an important role in the establishment of skull shape during the ontogeny of Triturus newts, especially during the late larval period and metamorphosis, when TH concentrations reach their maximum.

Nature and timing of anatectic event of the Hida Belt (Japan): Constraints from titanite geochemistry and U-Pb age of clinopyroxene-bearing leucogranite

The Hida Belt, central Japan, is a continental fragment, which was once a part of the crustal basement of the East Asian continental margin. It consists mainly of Permo-Triassic granite-gneiss complexes with both syn-to-late-metamorphic migmatite or granite bodies. Clinopyroxene-bearing leucogranite, locally called as ‘Inishi’-type migmatite, occurs characteristically in the migmatite zone associated with amphibolite and marble. The leucogranite is characterized by the presence of coarse-grained diopside–hedenbergite series clinopyroxene and titanite in plagioclase-dominated matrix. Clinopyroxene contains abundant calcite inclusions. Euhedral titanite with high Th/U ratios (~2.8–7.8) and REE contents (~4514–14,069 μg/g) contains polycrystalline ‘granitic’ inclusions. Those mineralogical features indicate the involvement of carbonate during anatexis. Considering a nominal pressure of 0.4–0.7 GPa of the Hida gneiss, Zr-in-titanite thermometry yields a temperature of 730–810 °C and 770–850 °C at aTiO2 = 0.5 and 1, respectively. The titanites show highly variable U/Pb (238U/206Pb = 15.0–24.0) and Pb (207Pb/206Pb = 0.172–0.419) isotope ratios, and the scattered trend follows a discordia line with a lower intercept at 225.4 ± 1.9 Ma. This titanite age would represent the timing of regional anatexis that have occurred in a later stage of the regional metamorphism of the Hida Belt. A high apparent thermal gradient at middle crustal levels might have been caused by regional extension.

CALIBRATION COEFFICIENTS OF CONNECTION BETWEEN THE LANGMUIR ADSORPTION EQUATION AND V.N. OPARINA’S KINEMATIC EXPRESSION FOR PENDULUM-TYPE WAVES ACCORDING TO LOCAL MEASUREMENTS AT THE COAL DEPOSITS OF KUZBASS

The article shows that at present there are experimental and theoretical prerequisites for establishing formalized relationships between deformation-wave and physicochemical processes occurring in real multiphase rock massifs. The calibration coefficients of the relationship in the correspondence operator between the adsorption equation and the kinematic expression for pendulum waves were established, verified in comparison with the available measurements in laboratory and field experiments for coals of the entire metamorphic series: quantitative values of the physicochemical invariant were determined for coals of different stages of metamorphism and structural parameter of the geomedium, as well as the velocity of propagation of a longitudinal wave in geoblocks-carriers of pendulum waves and translational movement of "colliding" geoblocks for different stages of metamorphism. It is shown that the values of the obtained dispersion allow one to take into account all the variety of the main stages of metamorphism of Kuzbass coal deposits in the theory and forecast of outburst hazard, i.e. take into account the real qualities of coal seams.

A Small Proportion of Breeders Drive American Bullfrog Invasion of the Yellowstone River Floodplain, Montana

The American bullfrog (Lithobates catesbeianus) is a non-native invader of aquatic habitats across the northwestern United States. It recently invaded the Yellowstone River, Montana, and has spread to over 140 km of floodplain habitat. We analyzed seven microsatellites in 528 tadpoles sampled across nearly the entire Yellowstone River invasion (about 140 river km) to characterize invasion genetics, compare our results with those of a recent mtDNA study (Kamath et al. 2016), and to inform control efforts. Microsatellite variation supports the mtDNA-based hypothesis of at least two independent introductions to the floodplain from genetically divergent populations in the midwestern United States, followed by massive range expansion. One introduction is associated with the upstream extent of the invasion near Park City, Montana and the other more broadly with downstream populations. All sites were characterized by small effective numbers of breeders (Nb; harmonic mean = 9.97), and therefore, a small proportion of highly successful adults may drive the invasion by producing large families. Microsatellites and mtDNA produced discordant estimates of genetic admixture between the upstream and downstream invasions, which may reflect small effective population size. Although we observed isolation by distance using both types of markers, microsatellites appear to reflect population structure resulting from secondary contact between the two introductions, as opposed to structure resulting from equilibrium between gene flow and genetic drift. Most sites showed evidence for genetic bottlenecks, which supports the recent history of invasion. Small Nb paired with known high localized extinction rates following colonization suggests focused removal of post metamorphic life stages at sites less likely to go extinct on their own could help limit invasion by bullfrogs.

Visualization of Sox10-positive chromatoblasts by GFP fluorescence in flounder larvae and juveniles using electroporation.

Japanese flounder are left-right asymmetrical, with features, such as dark, ocular-side specific pigmentation. This pigmentation arises during metamorphic stages, along with the asymmetric differentiation of adult-type chromatophores. Additionally, among juveniles, tank-reared specimens commonly show ectopic pigmentation on their blind sides. In both cases, neural crest-derived Sox10-positive progenitor cells at the dorsal fin base are hypothesized to contribute to chromatophore development. Here, we developed a method to visualize Sox10-positive cells via green fluorescent protein (GFP) fluorescence to directly monitor their migration and differentiation into chromatophores in vivo. Electroporation was applied to introduce GFP reporter vectors into the dorsal fin base of larvae and juveniles. Cre-loxP system vectors were also tested to enable cell labeling even after a decrease in sox10 expression levels. In larvae, undifferentiated Sox10-positive progenitor cells were labeled in the dorsal fin base, whereas newly differentiated adult-type chromatophores were seen dispersed on the ocular side. In juveniles, Sox10-positive cells were identified in the connective tissue of the dorsal fin base and observed prominently in areas of ectopic pigmentation, including several labeled melanophores. Thus, it was suggested that during metamorphic stages, Sox10-positive cells at the dorsal fin base contribute to adult-type chromatophore development, whereas in juveniles, they persist as precursors in the connective tissue, which in response to stimuli migrate to generate ectopic pigmentation. These findings contribute to elucidating pigmentation mechanisms, as well as abnormalities seen in hatchery-reared flounders. The electroporation method may be adapted to diverse animals as an accessible gene transfer method in various research fields, including developmental and biomedical studies.

Chicheng high-pressure granulites record the paleoproterozoic tectonic evolution in the northern North China Craton

High-pressure (HP) mafic granulites are widely distributed along the Trans-North China Orogen (TNCO), and they provide important insights into the timing and tectonic processes involved in the formation of the TNCO. We discovered three types of HP granulite, including mafic, intermediate, and felsic granulites, exposed in Chicheng area, in the northern segment of the TNCO. Their mineral assemblages of prograde, peak, and post-peak metamorphic stages were identified based on petrographic investigations. SIMS U-Pb zircon analyses yielded an upper intercept age of 2547 ± 38 Ma and a lower intercept age of 1916 ± 100 Ma for one HP mafic granulite and a discordant 207Pb/206Pb age of 2479 Ma with a weighted mean 207Pb/206Pb age of 1905 ± 9 Ma for the other HP mafic granulite. For the HP intermediate granulite, an upper intercept age of 2544 ± 20 Ma was obtained. The HP felsic granulite yielded an upper intercept age of 2524 ± 23 Ma and a concordant 207Pb/206Pb age of 1906–1907 Ma. These results reveal that the protoliths of HP granulites were formed at ~ 2.5–2.55 Ga and underwent high pressure granulite facies metamorphism at ~ 1.9 Ga. The elevated Hf isotopic compositions of the metamorphic zircons from two granulite samples indicate that metamorphic fluids that have high 176Hf/ 177Hf ratios but low 176Lu/177Hf ratios were involved in their metamorphism. The low 18O isotopic features of the zircons from the intermediate granulite suggest alteration by δ18O-depleted seawater or meteoric fluids at high temperatures. By integrating geochronological data from previous studies, we infer that the formation of the TNCO may have begun as early as ~ 1.9 Ga and lasted until ~ 1.8 Ga, supporting the hypothesis that it was formed by a long-lived tectono-metamorphic event

Tectono-metamorphic evolution of the proto-Andean margin of Gondwana: Evidence of internal high-grade metamorphism along the northern portion of the Famatinian orogen, Sierra de Aconquija, Sierras Pampeanas Orientales, Argentina

The present work gives a detailed analysis of the metamorphic and structural evolution of the back-arc portion of the Famatinian Orogen exposed in the southern Sierra de Aconquija (Cuesta de La Chilca segment) in the Sierras Pampeanas Orientales (Eastern Pampean Sierras). The Pampeanas Orientales include from north to south the Aconquija, Ambato and Ancasti mountains. They are mainly composed of middle to high grade metasedimentary units and magmatic rocks.At the south end of the Sierra de Aconquija, along an east to west segment extending over nearly 10 km (Cuesta de La Chilca), large volumes of metasedimentary rocks crop out. The eastern metasediments were defined as members of the El Portezuelo Metamorphic–Igneous Complex (EPMIC) or Eastern block and the western ones relate to the Quebrada del Molle Metamorphic Complex (QMMC) or Western block. The two blocks are divided by the La Chilca Shear Zone, which is reactivated as the Río Chañarito fault.The EPMIC, forming the hanging wall, is composed of schists, gneisses and rare amphibolites, calc– silicate schists, marbles and migmatites. The rocks underwent multiple episodes of deformation and a late high strain-rate episode with gradually increasing mylonitization to the west. Metamorphism progrades from a M1 phase to the peak M3, characterized by the reactions: Qtz + Pl + Bt ± Ms → Grt + Bt2 + Pl2 ± Sil ± Kfs, Qtz + Bt + Sil → Crd + Kfs and Qtz + Grt + Sil→ Crd. The M3 assemblage is coeval with the dominant foliation related to a third deformational phase (D3).The QMMC, forming the foot wall, is made up of fine-grained banded quartz – biotite schists with quartz veins and quartz-feldspar-rich pegmatites. To the east, schists are also overprinted by mylonitization. The M3 peak assemblage is quartz + biotite + plagioclase ± garnet ± sillimanite ± muscovite ± ilmenite ± magnetite ± apatite.The studied segment suffered multiphase deformation and metamorphism. Some of these phases can be correlated between both blocks. D1 is locally preserved in scarce outcrops in the EPMIC but is the dominant in the QMMC, where S1 is nearly parallel to S0. In the EPMIC, D2 is represented by the S2 foliation, related to the F2 folding that overprints S1, with dominant strike NNW – SSE and high angles dip to the E. D3 in the EPMIC have F3 folds with axis oblique to S2; the S3 foliation has striking NW – SE dipping steeply to the E or W and develops interference patterns. In the QMMC, S2 (D2) is a discontinuous cleavage oblique to S1 and transposed by S3 (D3), subparallel to S1. Such structures in the QMMC developed at subsolidus conditions and could be correlated to those of the EPMIC, which formed under higher P-T conditions. The penetrative deformation D2 in the EPMIC occurred during a prograde path with syntectonic growth of garnet reaching P-T conditions of 640°C and 0.54 GPa in the EPMIC. This stage was followed by a penetrative deformation D3 with syn-kinematic growth of garnet, cordierite and plagioclase. Peak P-T conditions calculated for M3 are 710°C and 0.60 GPa, preserved in the western part of the EPMIC, west of the unnamed fault.The schists from the QMMC suffered the early low grade M1 metamorphism with minimum PT conditions of ca 400°C and 0.35 GPa, comparable to the fine schists (M1) outcropping to the east. The D2 deformation is associated with the prograde M2 metamorphism. The penetrative D3 stage is related to a medium grade metamorphism M3, with peak conditions at ca 590°C and 0.55 GPa.The superimposed stages of deformation and metamorphism reaching high P-T conditions followed by isothermal decompression, defining a clockwise orogenic P-T path. During the Lower Paleozoic, folds were superimposed and recrystallization as well as partial melting at peak conditions occurred. Similar characteristics were described from the basement from other Famatinian-dominated locations of the Sierra de Aconquija and other ranges of the Sierras Pampeanas Orientales.