MK-1 Orthopyroxene—A New Potential Reference Material for In-Situ Microanalysis

Orthopyroxene, an important phase in mantle-derived rocks, has become a powerful tool to unravel mantle nature and magma processes. However, the applications have been hindered by the lag in the development of analytical techniques, such as shortage of reference materials. Orthopyroxene grains derived from an ultramafic intrusion at the Mogok metamorphic belt (Myanmar) were evaluated for the potential use of orthopyroxene as a reference material for in-situ microanalysis. Approximately 20 g of 0.5–3 mm pure orthopyroxene grains were separated under binocular microscope and analyzed using EPMA, LA-ICPMS, and bulk analytical methods (XRD, XRF, and solution-ICPMS) for major and trace elements at four institutions. Eleven core-to-rim profiles carried out using EPMA and twelve core-to-rim profiles determined using LA-ICPMS suggest that MK-1 orthopyroxene grains are sufficiently homogeneous, with RSD < ±2% (1σ) for major elements (Mg, Si, and Fe) and RSD < ±10% (1σ) for trace elements (Na, Al, Ca, Ti, Cr, Co, Zn, Ni, Mn, Sc, and V). In addition, the composition of MK-1 orthopyroxene was also measured by XRF and solution-ICPMS measurements in two different laboratories, to compare with the results measured using EPMA and LA-ICPMS. The results indicated a good agreement with RSE < ±2% (1σ) for major elements and RSE < ±5% (1σ) for most trace elements, except for Na (±9.73%) and Ti (±6.80%). In an overall assessment of these data, MK-1 orthopyroxene can be considered as a reference material for in-situ microanalysis, which would provide solid trace elements data for a better understanding of mantle source and magmatic evolution.

Short-lived intra-oceanic arc-trench system in the North Qaidam belt (NW China) reveals complex evolution of the Proto-Tethyan Ocean

Recognition of any intra-oceanic arc-trench system (IOAS) could provide invaluable information on the tectonic framework and geodynamic evolution of the vanished ocean basin. The Tanjianshan Complex and mafic-ultramafic rocks along the North Qaidam ultra-high pressure metamorphic belt in NW China record the subduction process of the Proto-Tethyan Ocean. Four lithotectonic units, including island arc, ophiolite, forearc basin, and accretionary complex, are recognized based on detailed field investigation. They rest on the northern margin of the Qaidam block and occur as allochthons in fault contact with underlying high-grade metamorphic rocks. The ophiolite unit mainly consists of ultramafic rocks, 527−506 Ma gabbro, 515−506 Ma plagiogranite, dolerite, and massive lava. High-Cr spinels in serpentinite, dolerite with forearc basalt affinity, and boninitic lava collectively indicate a forearc setting. The accretionary complex, exposed to the south of the ophiolite complex and island arc, is highly disrupted and contains repeated slices of basalt, 495−486 Ma tuff, chert, limestone, and mélange. Tuffs with positive zircon εHf(t) values indicate derivation from a nearby juvenile island arc. These lithotectonic units, as well as the back-arc basin, are interpreted to constitute a Cambrian IOAS that formed during the northward subduction of the Proto-Tethyan Ocean. Combined with regional geology, we propose a new geodynamic model involving short-lived Mariana-type subduction and prolonged Andean-type subduction to account for the complex evolution of the Proto-Tethyan Ocean. The reconstruction of a relatively complete IOAS from the North Qaidam belt not only reveals a systematic evolution of intra-oceanic subduction but also advances our understanding of the subduction and accretion history of the Proto-Tethyan Ocean.

Discovery of Disulfane (H2S2) in Fluid Inclusions in Rubies from Yuanjiang, China, and Its Implications

Raman spectra of fluid inclusions in gem rubies from Yuanjiang deposit (China) within the Ailao Shan-Red River (ASRR) metamorphic belt showed the presence of compounds such as CO2, COS, CH4, H2S, and elemental sulfur (S8), accompanied by two bands at approximately 2499 and 2570 cm−1. These two frequencies could be assigned to the vibrations of disulfane (H2S2). This is the second case of the sulfane-bearing fluid inclusions in geological samples reported, followed by the first in quartzite from Bastar Craton of India. The H2S2 was likely in situ enclosed by the host rubies rather than a reaction product that formed during the cooling of H2S and S8, suggesting sulfanes are stable at elevated temperatures (e.g., >600 °C). By comparing the lithologies and metamorphic conditions of these two sulfane-bearing cases (Bastar and Yuanjiang), it is suggested that amphibolite facies metamorphism of sedimentary sequence that deposited in a continental platform setting might favor the generation of sulfanes. Sulfanes may play an important role in the mobilization of Cr that is essential for ruby crystallization.

Bulk Crustal Properties and Layered Velocity Structure in Fujian, SE China: Constraints From P and S Receiver Functions

Multiple tectonic events since the Neoproterozoic era have framed the present-day lithosphere in the Fujian province affiliated with the eastern part of the South China Block. Comprehensive information of the crustal structure and bulk properties can aid to understand the geological features and tectonic processes of still much debate in this region. An attempt is made in this study to explore crustal thickness and internal velocities across Fujian using the teleseismic receiver functions (RFs). The H-V stacking of joint P and S RFs improves to simultaneously estimate crustal thickness, average Vp and Vs, and derived Vp/Vs ratio and bulk sound speed in three backazimuth sectors for each of 17 stations. Furthermore, a Neighborhood Algorithm nonlinear inversion of P RFs is employed to determine the layered structures of Vs and Vp/Vs beneath all the stations. Results indicate the crustal thickness varies from at most ∼35 km in northwest Fujian to 30–35 km in the inland mountains and 27–30 km in the southeastern coasts. The inferred Moho geometry is nonplanar or inclined across the Zhenghe-Dapu (ZD) and Changle-Zhaoan (CZ) fault zones, especially in the southern ZD fault area. The average Vp/Vs suggests that the crust is predominantly felsic in the Wuyi-Yunkai orogen and intermediate-to-mafic in the Cretaceous magmatic and metamorphic zones. A high-velocity upper crust along the coastline is revealed, which attributes to the Pingtan-Dongshan metamorphic belt. At the sites near the ZD fault zone, the intracrustal negative discontinuity occurs at a shallower depth of ∼15 km marking an abrupt Vs decrease into the low-velocity mid-to-lower crustal layer, probably linked to the closed paleo-rift basin remnants. The lower crust across the Fujian is generally characterized by relatively lower Vs and higher Vp/Vs (1.80–1.84) consistent with those of the mafic-ultramafic rocks, which do not support the proposed extensive magmatic underplating in the Late Mesozoic.

Hydrogeochemical Characteristics of Uranium and Radon in Groundwater from the Goesan Area of the Ogcheon Metamorphic Belt (OMB), Korea

Uranium and radon concentrations in groundwater from the Goesan area of the Ogcheon Metamorphic Belt (OMB), central Korea, whose bedrock is known to contain the highest uranium levels in Korea, were analyzed from 200 wells. We also measured the uranium concentrations in the bedrock near the investigated wells to infer a relationship between the bedrock geology and the groundwater. The five geologic bedrock units in the Goesan area consist of Cretaceous granite (Kgr), Jurassic granite (Jgr) and three types of metasedimentary rocks (og1, og2, and og3). The percentages of the groundwater samples over 30 μg/L (maximum contaminant level, MCL of US EPA) were 2.0% of the 200 groundwater samples; 12% of Kgr and 1.8% of Jgr exceeded the MCL, respectively. Overall, 16.5% of the 200 groundwater samples exceeded 148 Bq/L (alternative maximum contaminant level, AMCL of US EPA); 60.0% of Kgr and 25.0% of Jgr exceeded the AMCL, but only 0% of og1, 7.9% of og2, and 2.6% of og3 exceeded the value, respectively. No direct correlation was found between uranium concentration and radon concentration in water samples. Radon has a slightly linear correlation with Na (0.31), Mg (−0.30), and F (0.36). However, uranium behavior in groundwater was independent of other components. Based on thermodynamic calculation, uranium chemical speciation was dominated by carbonate complexes, namely the Ca2UO2(CO3)3(aq) and CaUO2(CO3)32− species. Although uraniferous mineral phases designated as saturation indices were greatly undersaturated, uranium hydroxides such as schoepite, UO2(OH)2 and U(OH)3 became possible phases. Uranium-containing bedrock in OMB did not significantly affect radioactive levels in the groundwater, possibly due to adsorption effects related to organic matter and geochemical reduction. Nevertheless, oxidation prevention of uranium-containing bedrock needs to be systematically managed for monitoring the possible migration of uranium into groundwater.

Multi-stage metamorphism of the South Altyn ultrahigh-pressure metamorphic belt, West China: insights into tectonic evolution from continental subduction to arc–backarc extension

The South Altyn ultrahigh-pressure (UHP) metamorphic belt is claimed to host the deepest subducted continental crust based on the discovery of former stishovite, and thus can provide unique insights into the tectonic evolution from deep continental subduction and exhumation to arc–backarc extension. In this paper, we present detailed studies of petrography, mineral chemistry, phase equilibria modelling and zircon U-Pb dating for three representative samples involving garnet amphibolite (A1531 & A1533) and associated garnet-biotite gneiss (A1534) from the UHP belt. Three phases of metamorphism are inferred for the rocks. The first phase high pressure (HP)–UHP-type eclogite facies is represented by the mineral assemblages of garnet and phengite inclusions in zircon and garnet cores with the high grossular (XGrs = 0.33–0.34). The Si contents of 3.40–3.53 and 3.24–3.25 p.f.u. in phengite inclusions yield pressure conditions of &gt;1.7–2.3 GPa for A1533 and 2.5–2.55 GPa for A1534 at a fixed temperature of 770 °C. The second phase medium-pressure (MP)-type overprinting of garnet amphibolite facies shows P–T conditions of 0.8–1.2 GPa/750–785 °C based on the stability fields of corresponding mineral assemblages, the measured isopleths of Ti contents in biotite and amphibole cores, and XGrs in garnet. The third phase low-pressure (LP) type overprinting includes early-stage heating to peak granulite facies followed by cooling towards a late-stage amphibolite facies. The peak granulite facies is represented by the high Ti amphibole mantle, high Zr titanite and the intergrowths of clinopyroxene + ilmenite in A1533 & A1531, with P–T conditions of 800–875 °C/0.80–0.95 GPa. The late-stage is defined by the solidus assemblages, giving P–T conditions of 0.5–0.7 GPa/720–805 °C. U-Pb geochronology on metamorphic zircons from A1533 and A1534 gives three ages of c. 500 Ma, c. 482 Ma and c. 460 Ma. They are interpreted to represent the HP–UHP, MP and LP types of metamorphism respectively, based on cathodoluminescence images, mineral inclusions and trace element patterns. Combining the regional geology and metamorphic evolution from the Altyn Orogen, a tectonic model is inferred, including the following tectonic scenarios. The small Altyn Microcontinent was subducted to great mantle depths with dragging of the surrounding vast oceanic lithosphere to undergo the HP–UHP eclogite facies metamorphism during the early subduction stage (c. 500 Ma) of the Proto-Tethys Ocean. Then, the subducted slabs were exhumed to a thickened crust region to be overprinted by the MP-type assemblages at c. 482 Ma. Finally, an arc–backarc extension was operated within the thickened crust region due to the retreat of subduction zones. It caused evident heating and the LP-type metamorphic overprinting at c. 460 Ma, with a fairly long interval of 30–40 Myr after the HP–UHP metamorphism, distinct from the short interval of &lt;5–10 Myr in the Bohemian Massif.