Metamorphic Temperatures Research Articles

Early Cambrian high pressure/low temperature metamorphism in the southeastern Tarim craton in response to circum-Gondwana cold subduction

The circum-Gondwana subduction initiated by the early Cambrian has been suggested to reflect the establishment of the modern plate tectonics. The metamorphic rocks with low thermobaric (T/P) ratios indicative of cold subduction in the present tectonic regime have not been well investigated. To better understand the circum-Gondwana subduction and to test its possible link with the emergence of the modern plate tectonics, this study focused on blueschist-facies metamorphic rocks in the Altyn Tagh of the southeastern Tarim craton. Mineral assemblage and chemistry, phase equilibrium modelling, and quartz-in-garnet Raman elastic geobarometry reveal that the zoisite blueschist and glaucophane (Gln)-bearing quartz schist in northern Altyn Tagh were metamorphosed to lawsonite to epidote blueschist-facies at 520–545 °C and 16–19 kbar. It reflects high-pressure (HP)/low temperature (LT) metamorphism with low T/P ratios of <300 °C/GPa and thermal gradients of <10 °C/km. These blueschist-facies metamorphic rocks underwent rapid decompression starting at P-T conditions of <495 °C and <9.6 kbar during exhumation. Ar-Ar geochronology records paragonite Ar-Ar plateau ages of 520–506 Ma for the zoisite blueschist samples and phengite Ar-Ar plateau ages of 522–516 Ma for the Gln-bearing quartz schist samples, suggesting that the peak HP/LT metamorphism occurred prior to ca. 522 Ma. Based on new results and available data from the major Gondwana blocks, cold subduction was suggested to profoundly operate along circum-Gondwana in the early Cambrian after the amalgamation of Gondwana. The extensive circum-Gondwana subduction represents the earliest global cold subduction in Earth’s history associated with the establishment of the modern plate tectonics, as directly recorded by the studied early Cambrian blueschist-facies metamorphic rocks and a dramatic drop in the mean T/P of metamorphism since the early Paleozoic.

Mantle Phase Changes Detected From Stochastic Tomography

AbstractStochastic tomography, made possible by dense deployments of seismic sensors, is used to identify phase changes in Earth's mantle that occur over depth intervals. This technique inverts spatial coherences of amplitudes and travel times of body waves to determine the depth and dependence of the spatial spectrum of seismic velocity. This spectrum is interpreted using the predicted thermodynamic stability of mineral composition and phase as a function of temperature and pressure, in which the metamorphic temperature derivative of seismic velocities is used as a proxy for the effects of heterogeneity induced in a region undergoing a phase change. Peaks in the temperature derivative of seismic velocity closely match those found from applying stochastic tomography to elements of Earthscope and Flex arrays. Within ±12 km, peaks in the fluctuation of P velocity at 425, 500, and 600 km depth beneath the western US agree with those predicted by a mechanical mixture of harzburgite and basalt, 180 K cooler than a 1600 K adiabat in the mantle transition zone. A broad peak at 250 km depth may be associated with chemical heterogeneity induced by dehydration of subducted oceanic sediments, and a peak at 775 km depth with a phase change in subducted basalt. Non‐detection of predicted phase changes less than 10 km in width is consistent with the resolution possible with the seismic arrays used in the inversion, including the sharp endothermic phase change near 660 km. These interpretations are consistent with the known history of plate subduction beneath North America.

Metallic lead (Pb) nanospheres discovered in Hadean and Eoarchean zircon crystals at Jack Hills

Here, we report small randomly-distributed crystalline lead (Pb) nanospheres occurring in detrital zircon grains obtained from a weakly metamorphosed Archean conglomerate at Jack Hills, Western Australia, making this the third known global example of this phenomenon. They form in zircon crystals ranging from Hadean (> 4 billion years—Ga) to Eoarchean (> 3.6 Ga) in age, but are absent from Paleoarchean (~ 3.4 Ga) crystals. Unlike previous discoveries of nanospheres in zircon from Precambrian gneisses in Antarctica and India, detrital zircon from Jack Hills shows no evidence of ever undergoing ultra-high temperature (UHT) metamorphism, either before or after deposition, therefore implying that nanospheres can form at temperatures lower than ca. 900 °C. The nanospheres are composed of radiogenic Pb released by the breakdown of uranium (U) and thorium (Th) and are present in zircon irrespective of its U, Th and water contents, its oxygen isotopic composition, and the degree of discordance due to Pb loss or gain. The nanospheres pre-date annealed cracks in the crystals, showing that, once formed, they effectively ‘freeze’ radiogenic Pb in the zircon structure, precluding any further interaction during subsequent geological processes. Both Pb nanoclusters and nanospheres are now reported from Jack Hills, and it appears likely the former is a precursor stage in the formation of the latter. Although the precise mechanism for this transition remains unresolved, a later thermal event is required, but this likely did not reach UHT conditions at Jack Hills.

Northwest Africa 8418: The first CV4 chondrite

AbstractNorthwest Africa (NWA) 8418 is an unusual chondrite whose properties do not exactly match those of any other known chondrite. It has similarities to the CV (Vigarano group), CK (Karoonda group), and CL (Loongana group) chondrites, but its abundance of large calcium‐aluminum‐rich inclusions (CAIs) and the low NiO content (&lt;0.2 wt%) of its matrix olivine ally it most closely with the CV group. The absence of grossular, monticellite, wollastonite, and sodalite from the alteration products of the CAIs; the magnesium‐rich nature of the matrix olivines (Fa38) relative to that of the CV3 chondrites (~Fa50); and the presence of secondary Na‐bearing plagioclase and chlorapatite indicate a metamorphic temperature &gt;600 °C. NWA 8418 contains kamacite, taenite, and troilite, and lacks magnetite and pentlandite. We propose that NWA 8418 be reclassified as a reduced CV4 chondrite, which makes it the first CV chondrite of petrologic type 4.

Protolith age and metamorphic temperature of the Yokokawagawa metamorphic rocks in Nagano Prefecture, central Japan, and comparison with the Sanbagawa metamorphic rocks

We estimated the protolith age and peak metamorphic temperature of the Yokokawagawa metamorphic rocks (YMR) east of the Itoigawa–Shizuoka Tectonic Line using detrital zircon U–Pb dating and Raman carbonaceous material geothermometry, respectively. U–Pb dating of a psammitic rock yielded a youngest age of ∼100 Ma, which corresponds to the protolith age. Raman carbonaceous material geothermometry results for five pelitic rock samples give metamorphic temperatures of ∼350–380 °C. The protolith age is consistent with those of the Sanbagawa metamorphic rocks (SMR), strongly indicating that the YMR are an extension of the SMR. The increase in peak temperature toward intrusive rocks along the eastern margin of the YMR may indicate that the relatively young ages yielded by previous K–Ar dating of the YMR reflect thermal resetting due to contact metamorphism.

Metamorphism and Deformation on Subduction Interfaces: 1. Physical Framework

AbstractA thermal and mechanical framework is presented for analysis of pressure‐temperature (P‐T) data and structural observations from high‐pressure‐low‐temperature (HPLT) terrains. P‐T data from 281 HPLT rocks exhibit two regimes separated at a pressure of ∼1.5 GPa, which corresponds to the modal maximum depth of thrust faulting in subduction zones. At pressures ≲1.5 GPa, interpreted as recording conditions on the plate interface, temperatures increase at about 350°C/GPa and are consistent with conditions calculated for shear stresses of ∼30–100 MPa on the interface. Such shear stresses are high enough to carry several kilometers' thickness of sediment at least to the base of the plate interface. Burial of material on plate interfaces occurs predominantly during large‐to‐great earthquakes; the exhumation phase involves contrasts in ascent rates of adjacent units, because of their differing buoyancies and strengths. In consequence, juxtaposition of unrelated rock types is expected to be ubiquitous, during both descent and ascent. The scarcity of temperatures higher than ∼650°C at pressures ≳1.5 GPa may reflect loss of material from the wedge‐slab interface by buoyant ascent. Exhumation of rocks in the subduction interface requires substantial reduction in shear stress, most plausibly by (near‐)cessation of subduction. During prograde metamorphism temperatures increase smoothly with depth in the plate interface, with almost isothermal compression in the wedge‐slab interface. Following cessation of subduction, rocks rising along the wedge‐slab interface are likely to heat slightly during decompression. Within the plate interface, temperatures drop following the cessation of shear heating, and rocks follow counter‐clockwise hairpin PT paths.

Tissemouminites: A new group of primitive achondrites spanning the transition between acapulcoites and winonaites

AbstractThe Northwest Africa (NWA) 090 meteorite, initially classified as an acapulcoite, presents petrological, chemical, and isotopic characteristics comparable to a group of seven primitive winonaites: Dhofar 1222, NWA 725, NWA 1052, NWA 1054, NWA 1058, NWA 1463, and NWA 8614. Five of these samples were previously classified as acapulcoites or ungrouped achondrites before being reclassified as winonaites based on their oxygen isotopic compositions. These misclassifications are indicative of the particular compositional nature of these primitive achondrites. All contain relict chondrules and a lower closure temperature of metamorphism of 820 ± 20 °C compared to other typical winonaites, as well as mineral elemental compositions similar to those of acapulcoites. The oxygen isotopic signature of these samples, δ17O of 1.18 ± 0.17‰, δ18O of 3.18 ± 0.30‰, and Δ17O of −0.47 ± 0.02, is in fact resolvable from both acapulcoites and winonaites. We investigate the relationship between these eight primitive achondrites, typical winonaites, and acapulcoites, to redefine petrological, mineralogical, and geochemical criteria of primitive achondrite classification. Distinguishing between winonaites, acapulcoites, and this group of eight primitive achondrites can be unambiguously done using a combination of several mineralogical and chemical criteria. A combination of olivine fayalite content and FeO/MnO ratio, as well as plagioclase potassium content allow us to separate these three groups without the absolute necessity of oxygen isotope analyses. NWA 090 as well as the other seven primitive achondrites, although related to winonaites, are most likely derived from a parent body distinct from winonaites and acapulcoites–lodranites, and define a new group of primitive achondrites that can be referred to as tissemouminites.

Skarns at a peridotite contact, Neyriz, Iran: High-temperature melting and metamorphism in a forearc setting

In the Neyriz area of southern Iran unusual skarns are found above serpentinised peridotites at the contact with crystalline limestones. They have been interpreted as the high-temperature product of intrusion by hot peridotite into limestones, as low-temperature rodingites (the product of calcium metasomatism associated with serpentinisation), or a fortuitous juxtaposition of unrelated rocks. Their age is not known. The skarns are wollastonite-pyroxene-calcite rocks in which dark green pyroxenes are fassaites with high Al, Fe and Ti with high Ca-Tschermak's components. The field relations, textures, mineral assemblages and compositions, and melt inclusions in wollastonite and fassaite indicate the skarns formed by melting at the contact between peridotites and limestones with retrograde reactions during cooling forming garnet and anorthite. There are uncertainties in temperature estimates since pressure, XCO2 and other compositional variables are unknown, but melting temperatures were likely to have been close to 1100°C with garnet formation at approximately 900°C. Later alteration of some skarns and formation of rodingites close to the limestone-peridotite contacts occurred during low-temperature Ca metasomatism, probably after emplacement of the ophiolite during Zagros collision. A hot intrusive origin for the skarns appears incompatible with an arc-related supra-subduction origin of the ophiolite inferred from geochemical studies, but recent work in eastern Indonesia shows that during late Neogene subduction rollback, melts formed above hot mantle that intruded highly extended continental crust in a forearc setting. The scale, timing and temperatures of melting and metamorphism are very similar to those of the Late Cretaceous Neyriz ophiolite.

Unravelling the protracted U-Pb zircon geochronological record of high to ultrahigh temperature metamorphic rocks: Implications for provenance investigations

The assessment of detrital zircon age records is a key method in basin analysis, but it is prone to several biases that may compromise accurate sedimentary provenance investigations. High to ultrahigh temperature (HT-UHT) metamorphism (especially if T > 850 °C) is herein presented as a natural cause of bias in provenance studies based on U-Pb detrital zircon ages, since zircon from rocks submitted to these extreme and often prolonged conditions frequently yield protracted, apparently concordant, geochronological records. Such age spreading can result from disturbance of the primary U-Pb zircon system, likewise from (re)crystallization processes during multiple and/or prolonged metamorphic events. In this contribution, available geochronological data on Archean, Neoproterozoic and Palaeozoic HT-UHT metamorphic rocks, acquired by different techniques (SIMS and LA-ICP-MS) and showing distinct compositions, are reassessed to demonstrate HT-UHT metamorphism may result in modes and age distributions of unclear geological meaning. As a consequence, it may induce misinterpretations on U-Pb detrital zircon provenance analyses, particularly in sedimentary rocks metamorphosed under such extreme temperature conditions. To evaluate the presence of HT-UHT metamorphism-related bias in the detrital zircon record, we suggest a workflow for data acquisition and interpretation, combining a multi-proxy approach with: (i) in situ U-Pb dating coupled with Hf analyses to retrieve the isotopic composition of the sources, and (ii) the integration of a petrochronological investigation to typify fingerprints of the HT-UHT metamorphic event. The proposed workflow is validated in the investigation of one theoretical and one natural example allowing a better characterization of the sedimentary sources, maximum depositional ages, and the tectonic setting of the basin. Our workflow allows to the appraisal of biases imposed by HT-UHT metamorphism and resulting disturbances in the U-Pb detrital zircon record, particularly for sedimentary rocks that underwent HT-UHT metamorphism and, finally, suggests ways to overcome these issues.

Geothermometric constraints on the thermal architecture, metamorphism, and exhumation of the Northern Range, Trinidad

AbstractThe Northern Range of Trinidad is composed of Mesozoic passive margin sedimentary rocks that underwent ductile deformation and subgreenschist‐ to greenschist‐facies metamorphism in the early Miocene. Previous studies suggested a two‐stage formation of the Northern Range between the Caribbean and South American plates: an initial collision drove mountain building in the Miocene and subsequent strike‐slip plate motion preferentially exhumed the western segment, producing a westward increase in the metamorphic thermal gradient. However, these studies were not able to resolve whether this gradient was discrete or continuous so the tectonic model awaits testing. In this study we use Raman spectroscopy on carbonaceous material (RSCM), an empirical geothermometer, to constrain peak temperatures across the Northern Range with a greater resolution than was available in previous studies. The RSCM temperatures show an abrupt increase from 337°C ± 10°C in the east to 442°C ± 16°C west of Chupara Point, where a range‐cutting fault (Chupara Fault) had been inferred in previous geologic mapping campaigns. Thus, the discrete thermal discontinuity of ~100°C very likely represents the Chupara Fault. Our RSCM‐derived peak metamorphic temperatures are 50°C to 100°C higher than those from previous estimates, requiring revision of tectonic models to account for deeper burial and greater exhumation. The peak metamorphic conditions determined here, and the deduced timing of faulting from published thermochronological data, are consistent with the two‐stage tectonic model proposed in previous studies.

Subduction and continental collision in the Neoproterozoic: Sanukitoid-like magmatism and paired metamorphism in SE Brazil

Collisional orogens are a periodic manifestation of plate tectonics on Earth. They form by the amalgamation of continental blocks, and they take place along both active and passive continental margins. The Neoproterozoic Southern Brasília Orogen is a collisional orogen related to the formation of West Gondwana, broadly composed of two tectonometamorphic domains, the Upper Tectonostratigraphic Domain (UTD), intruded by subduction-related magmas, and the Lower Tectonostratigraphic Domain (LTD) of passive margin affinity. New metamorphic zircon SHRIMP and LA-ICP-MS U-Pb ages are presented for rocks from both domains, a high-grade mylonitic orthogneiss (∼623 Ma) and a granulitic garnet-sillimanite-K-felspar metapelite (∼621 Ma) from the Upper Tectonostratigraphic Domain, and medium T retro-eclogites (∼616–610 Ma) and a garnet-kyanite-K-feldspar granulitic metapelite (∼615 Ma) from the Lower Tectonostratigraphic Domain. New and compiled U-Pb metamorphic ages, together with published P-T data, indicate the existence of Neoproterozoic coeval metamorphic belts in the Southern Brasília Orogen: an intermediate T/P region (LTD) and a high T/P domain (UTD). This is a characteristic of modern and ancient accretionary and subduction-to-collision orogens. The high T/P domain is dominated by Neoproterozoic sanukitoid-like magmatic rocks emplaced at the pre- to late-collsional stages (∼750–610 Ma) and the most primitive members of these suites (49–53 SiO2 wt.%; #Mg 40–60) are enriched in both compatible and incompatible elements (i.e., Th, Ba, Sr, etc), a characteristic inherited from their subduction-metasomatized garnet-bearing mantle source. Relatively high heat-flux from the mantle to the crust, associated with magmatic activity, combined with heating by radioactive decay in response to crustal thickening, are responsible for ultra-high temperature metamorphism (UHT) in the UTD. Barrovian-style intermediate T/P metamorphism was attained in the Lower Tectonostratigraphic Domain at the same time, in response to crustal thickening.

Ultrahigh temperature metamorphism recorded in the Lüliang Complex, Trans-North China Orogen: P–T–t evolution and heating mechanism

Identifying ultrahigh temperature (UHT) metamorphism from hot orogens and establishing its pressure–temperature–time (P–T–t) path will provide notable insights into tectonic evolution and heat source (mantle/mafic magma vs crustal radioactive elements) responsible for the extreme conditions. We report here a combined investigation of petrography, mineral chemistry, phase equilibria modelling, geothermobarometer calculation, geochronology and heat source for mafic and pelitic granulites from the Lüliang Complex, Trans-North China Orogen (TNCO), a representative Paleoproterozoic hot collisional orogen. The rocks record clockwise P–T paths, including granulite-facies peak and post-peak retrograde stages. Garnet + pyroxene + high-Ti hornblende-bearing assemblage in the mafic granulite and garnet + feldspar + sillimanite-bearing assemblage in the pelitic granulite constrain the thermal peaks at 8.2–10 kbar and 880–959 °C, suggesting UHT conditions. Zircon and monazite U–Pb dating, hornblende 40Ar/39Ar dating and Ti-in-zircon thermometer calculation indicate that the rocks may have experienced sluggish retrograde metamorphism from 1.92 to 1.80 Ga, with a cooling rate of 2.0–2.5 °C/Ma. The newly discovered UHT metamorphism, combined with previous geological data, validates a long-lived tectono-thermal evolution from 1.97 to 1.80 Ga in the TNCO. The calculated heat flow produced by the decay of radioactive elements in the thickened crust is 37–77 mW/m2 (with an average of 56 mW/m2), which is high enough to achieve UHT conditions. Therefore, we suggest that radioactive heating can be an important heating mechanism for UHT metamorphism in a long-lived hot orogen like the TNCO.

Permian-Triassic flood basalts in the pre-Jurassic basement of the Arctic zone of the West Siberian platform

Research subject. Permian-Triassic flood basalts in the basement of the Arctic zone of the West Siberian Platform locate mainly in graben-rift structures. Flood basalts in this region remain to be understudied in comparison with other areas of its distribution, mainly due to the significant depth of their occurrence (4–6 km).Materials and methods. 36 core samples from 11 superdeep and deep boreholes were studied. Isotopic ratios were measured on mass spectrometers NEPTUNE PLUS (Nd, Sm) and TRITON PLUS (Rb, Sr). Bitumen were studied using a Raman spectrometer LabRAM HR800 Evolution. The Raman spectra were deconvoluted (“Peak fitting” procedure), and the bitumen conversion temperature was estimated.Results. About half of the samples of volcanic rocks underwent metamorphism of the prehnite-pumpellite and locally greenschist facies or intense low-temperature hydrothermal alteration. The studied basalts are close to typical flood basalts and are somewhat similar to island-arc volcanic rocks in terms of their geochemical characteristics. For the first time, thin inclusions of bitumen were found in the amygdalae of Permian-Triassic basalts in the superdeep borehole Tyumenskaya SG-6 at a depth of 7310.6 m. A high similarity of the studied volcanics by geochemical characteristics and the isotopic composition of Sr and Nd with the flood basalts of the Siberian platform is shown.Conclusions. The presence of a negative Ta, Nb, Ti anomaly, as well as a negative Ce anomaly, in some of the analyzed samples indicates a possible contamination of the basalts by island arc volcanics and volcanogenic-sedimentary rocks. The temperature of transformation of bitumen in inclusions in basalts from the well Tyumenskaya SG-6 according to Raman spectroscopy is estimated at 150–300°C and generally corresponds to the temperature of metamorphism of the host basalts. The presence of bitumen in the amygdalae may indicate the migration of hydrocarbons through the basalts.

Internal or external magma oceans in the earliest protoplanets – Perspectives from nitrogen and carbon fractionation

Protoplanets growing within ∼1 Ma of the Solar System's formation underwent large-scale melting due to heat released by the decay of 26Al. When the extent of protoplanetary melting approached magma ocean (MO)-like conditions, alloy melts efficiently segregated from the silicates to form metallic cores. The nature of the MO of a differentiating protoplanet, i.e., internal or external MO (IMO or EMO), not only determines the abundances of life-essential volatiles like nitrogen (N) and carbon (C) in its core and mantle reservoirs but also the timing and mechanism of volatile loss. Whether the earliest formed protoplanets had IMOs or EMOs is, however, poorly understood. Here we model equilibrium N and C partitioning between alloy and silicate melts in the absence (IMO) or presence (EMO) of vapor degassed atmospheres. Bulk N and C inventories of the protoplanets during core formation are constrained for IMOs and EMOs by comparing the predicted N and C abundances in the alloy melts from both scenarios with N and C concentrations in the parent cores of magmatic iron meteorites. Our results show that in comparison to EMOs, protoplanets having IMOs satisfy N and C contents of the parent cores with substantially lower amounts of bulk N and C present in the parent body during core formation. As the required bulk N and C contents for IMOs and EMOs are in the sub-chondritic and chondritic range, respectively, N and C fractionation models alone cannot be used to distinguish the prevalence of these two end-member differentiation regimes. A comparison of N and C abundances in chondrites with their peak metamorphic temperatures suggests that protoplanetary interiors could lose a substantial portion of their N and C inventories with increasing degrees of thermal metamorphism. Provided the thermal metamorphism induced-loss of N and C from the protoplanetary interiors prior to the onset of core formation was efficient, the earliest formed protoplanets, as predicted by previous thermo-chemical models, are more likely to have undergone IMO differentiation resulting in the formation of N- and C-poor cores and mantles overlain by N- and C-rich undifferentiated crusts.

Origin of Nitrogen Isotopic Variations in the Rocky Bodies of the Solar System

Noncarbonaceous (NC; inner solar system) meteorites have lower 15N/14N ratios than carbonaceous (CC; outer solar system) meteorites. Whether this is evidence of a primordial heterogeneity of N reservoirs in the protosolar disk remains unclear. In this study, I consider the N isotopic compositions of meteorite (chondrite, achondrite, and iron meteorite) parent bodies as a function of their growth zones. Despite the 15N/14N ratios of CC meteorites being generally higher than NC meteorites, there is a substantial overlap between them. Late-stage mixing of isotopically distinct reservoirs cannot explain this overlap. 15N/14N ratios of meteorites, independent of the growth zones, are correlated with the accretion ages of their parent bodies. A common correlation of the 15N/14N ratios of NC and CC chondrites with their peak metamorphic temperatures suggests that N isotopic compositions of meteorites were likely set by a universal time-dependent process—thermal evolution of their parent bodies by radiogenic heating. Therefore, heterogeneous N isotopic compositions of meteorites do not allude to isotopically heterogeneous primitive N reservoirs in the protosolar disk. Rather, it is likely that the N isotopic compositions of meteorites are a direct reflection of a differential response of labile 15N-rich and refractory 15N-poor components in common organic precursors to variable degrees of parent body processing. Consequently, the isotopic ratios of N, and other highly volatile elements like C and H, in meteorites do not reflect the isotopic compositions of primitive volatile reservoirs in the protosolar disk and thus cannot be used independently to cosmolocate volatile reservoirs in the disk.

Qaidam block situated in the interior of Rodinia and Gondwana: New magmatic and metamorphic constraints

Some Precambrian microcontinental blocks, generally embedded in the Phanerozoic orogenic belts, are keys to constrain supercontinent assembly and dispersal as well as play an important role in the ocean-continent transition. The Qaidam block is considered as a Precambrian continental fragment surrounded by Paleozoic orogens. The previous reconstruction models commonly placed the Qaidam block in the peripheries rather than the interiors of the Rodinia and Gondwana supercontinents due to the lack of two global events at 1.2–1.0 Ga and 0.65–0.5 Ga. In this study, we present new zircon U–Pb–Hf isotope data and whole-rock geochemical data for the high-grade orthogneisses and low-grade metavolcanic rocks in the western Qaidam block. Zircon U–Pb dating displays three age groups at approximately 0.55 Ga, 0.75 Ga, and 1.1 Ga. The Ediacaran to early Cambrian age in zircon rims from orthogneisses is interpreted as a metamorphic origin, consistent with the timing of the latest Ediacaran–early Cambrian ultrahigh temperature metamorphism recorded in Mg-Al and mafic granulites. The late Mesoproterozoic age in magmatic zircons of orthogneisses and the middle Neoproterozoic age of metavolcanic rocks reflect two magmatic events. Combining with wide magmatic and metamorphic records of 1.0–0.9 Ga in the Qaidam block, we suggest that the Qaidam block experienced a prolonged subduction–collision process during the late Mesoproterozoic to early Neoproterozoic, followed by a 0.8–0.75 Ga anorogenic event related to the continental rifting environment within an already assembled Rodinia. These data show that the Qaidam block likely maintained a long-term connection with East Gondwana from the late Mesoproterozoic to the latest Cambrian and occupied interior positions in Rodinia and Gondwana rather than in their periphery. It probably did not separate from East Gondwana until the latest Ediacaran–early Cambrian.

Effects of contact metamorphism on δ13C, δ15N, and XPS nitrogen functional forms of inertinite-rich Witbank coals (South Africa): Indications for possible alteration by hydrothermal fluids

In this study, δ13C, δ15N, and nitrogen functional forms were used to evaluate the effects of contact metamorphism on inertinite-rich Main Karoo Basin coals (South Africa). Eighteen samples were collected at 0.25 m intervals from the western and eastern sides of a ∼ 2.6 m thick dolerite dyke emplaced into the No. 2 Seam; an unaltered coal from the same seam was included as a control (Witbank Coalfield). Inertinite (semifusinite and inertodetrinite) dominated the organic composition of the samples. δ13C and δ15N for the unaltered coal were − 21.1‰ VPDB and 1.5‰ Air, respectively. For the intruded coals from the western side, δ13C ranged between −22.4 and −21.4‰ VPDB, and δ15N between 1.2 and 2.6‰ Air. The δ13C for samples from the eastern side ranged between −21.8 and −20.7‰ VPDB, and between 1.8 and 3.0‰ Air for δ15N. Differences in δ15N between the unaltered and the intruded coals were generally more significant than in δ13C, especially for samples from the eastern side. The positive shifts in δ15N may reflect reactions between organic nitrogen in coal and mobile nitrogen transported by hydrothermal fluids. The absence of a relationship between δ15N and distance from the dyke, may reflect secondary, localized contact metamorphism due to hydrothermal fluid migration within the intruded seam. The circulation of a fluid transporting 15N-rich moieties, apparently more active on the eastern side, was supported by the higher ash yields, δ15N, and Ntotal/Corganic ratios. The correlation between Ntotal/Corganic and ash yields suggests deposition of NH4+ − bearing inorganic components preferentially into the eastern side. Similar to the unaltered coal (N-5 = 47.24 at.%), N-pyrrolic was the dominant nitrogen functional form of the intruded coals, reflecting the developmental pathways for the inertinite macerals as opposed to the contact metamorphism. The low proportion of graphitic (quaternary) nitrogen, irrespective of sample distance from the dyke, suggests metamorphic temperatures lower than 900 °C.

FERROHÖGBOMITE-2S2N IN THE DIATEXITES OF WESTERN SANGILEN, SOUTH-EASTERN TUVA, RUSSIA

In the Western Sangilen (Tuva–Mongolia microcontinent, Central Asian Orogenic Belt, Russia) rare mineral ferrohögbomite-2S2N, for the first time, was found in a deep-seated contact aureole of the early Paleozoic Bayan-Kol gabbro-monzodiorite intrusion. The rocks are diatexites of roof pendant, where metamorphics contact gabbronorite. Diatexites formed as a result of strong melting and desilication of quartz-bearing kyanite-staurolite schists (M1) during progressive thermal metamorphism (M2) near the Bayan-Kol intrusion with peak at 950 °C, 7–8 kbars. Ferrohögbomite-2S2N grains up to 50 μm in size found in ilmenite-corundum-magnetite-hercynite pseudomorphs. The later formed during progressive stage of thermal metamorphism M2 after regional staurolite M1 and are in cordierite-gedrite-plagioclase restite blocks of diatexites. Ferrohögbomite also appeared as thin (1–5 μm) rims around individual grains of hercynite, corundum and magnetite in a rock matrix. According to microtextural features ferrohögbomite grains formed during post-peak metamorphic stage after minerals of pseudomorphs. A univariant mineral reaction of ferrohögbomite-2S2N formation calculated quantitatively in Fe-Al-Ti-Zn-H2O system with program Mathematic 9.0 using electron microprobe chemical composition of minerals of pseudomorphs. The reaction is 0.96Spl+0.194Ilm+0.116Mag+0.036Crn+0.163H2O=0.163Hgb. A local equilibrium value during formation of ferrohögbomite was probably correlated with pseudomorphs size and thus do not transcend 1 mm3. Thermobarometry yields metamorphic temperature and pressure of ferrohögbomite formation as T=665 °C, P=5.8 kbar, which correspond to late retrograde stage of thermal metamorphism of diatexites. To present day in Russia, högbomite group mineral was known only on South Urals and Aldan Shield in Eastern Siberia.

Localized equilibrium and mineralogic effects on trace element distribution and mobility in highly metamorphosed Eucrite Elephant Moraine (EET) 90020

Eucrites exhibit a range of igneous and metamorphic textures and geochemistries that can be used to study the evolution of early planetary differentiation and crust formation in the solar system. We integrated petrologic/textural observations, in-situ geochemical analyses, and thermodynamic modeling to explore the petrogenesis of Elephant Moraine (EET) 90020, an unbrecciated meteorite. We identified microdomains that record relatively high metamorphic temperatures and concluded that diffusion processes likely modified EET 90020 during and/or after peak thermal conditions. There is little evidence that partial melting caused the distribution of minor and trace elements within or among the microdomains. Trace element linear transect measurements within the microdomains imply that phosphate minerals strongly controlled trace element distributions throughout the sample. The discrepancy between the observed metamorphic textures, major element chemistry, and the trace element distributions is a consequence of differing chemical mobility. Multiple processes are influencing geochemistry within a single sample which has implications for the development of petrogenetic models that seek to reconcile the differences observed between eucrite geochemical groups.

Constraint on the temperature of A-type magma from contact metamorphic aureole, Biesituobie batholith, West Junggar in NW China, Central Asian Orogenic Belt

The Biesituobie A-type batholith in West Junggar in NW China of the Central Asian Orogenic Belt contains metapelite xenoliths derived from the contact metamorphic aureole. These xenoliths could be divided into two types: biotite-muscovite-andalusite hornfels and cordierite-alkaline feldspar-sillimanite-corundum gneiss, indicating prograde metamorphism. The phase equilibrium modeling on the cordierite-alkaline feldspar-sillimanite-corundum gneiss sample yielded a peak condition of P = 3.0−4.0 kbar, T = 760−800 °C. Similarly, the Na-in-cordierite geothermometer yielded a peak condition of T = 771−780 °C. The temperature of the peak metamorphism could be considered as a lower limit of the temperature of the Biesituobie A-type batholith. On the contrary, the Ti-in-zircon geothermometer applied to the Biesituobie A-type batholith zircons yields a mean temperature of 672 ± 22 °C. It suggested that the application of Ti-in-zircon geothermometer on A-type magma may involve a &amp;gt;100 °C underestimation. Zircon U-Pb dating indicates that the age of the contact metamorphism is between 263 and 286 Ma, consistent with the age of the pluton at 274 Ma. The result of 760−800 °C calculated from the contact metamorphic aureole of the Biesituobie batholith put a lower limit on considering the temperature condition of the A-type granite series from a metamorphic constraint.