Chris G. Mattinson

Polyphase Tectonothermal History Recorded in Granulitized Gneisses from the North Qaidam HP/UHP Metamorphic Terrane, Western China: Evidence from Zircon U-Pb Geochronology

High-grade gneisses of the north Qaidam high-pressure/ultrahigh-pressure metamorphic terrane enclose minor eclogites and ultramafic rocks. In combination with petrological data, sensitive high-resolution ion microprobe U-Pb geochronology on the granulitized gneisses in the Luliangshan and Xitieshan, western north Qaidam Mountains, reveals a polyphase tectonothermal history including Early Neoproterozoic and Early Paleozoic events. The rocks investigated are two granulite-facies paragneisses and one orthogneiss that both surround garnet peridotite in the Luliangshan, and two paragneisses that enclose eclogite in the Xitieshan. The inherited zircon cores from paragneiss and orthogneiss yield ages between ca. 1000 Ma and ca. 2500 Ma, representing Mesoproterozoic to Archaean source material for these gneisses. A ca. 900 Ma age obtained from one orthogneiss and one paragneiss is interpreted as the age of simultaneous Early Neoproterozoic magmatism and metamorphism. This Early Neoproterozoic tectonothermal event is similar in age to the Jinning orogeny, which is commonly recognized in the metamorphic basement of the south China block, and suggests that the north Qaidam Mountains have an affinity to the south China block. Early Paleozoic metamorphism is recorded in all gneiss samples. In conjunction with cathodoluminescence imagery and mineral inclusions, U-Pb dating of zircons reveals two Early Paleozoic age groups: ca. 450 Ma represents the age of high-pressure granulite metamorphism (Grt + Rt inclusions in zircon; kyanite porphyroblasts), whereas ca. 425 Ma reflects the time of medium-pressure, granulite-facies metamorphism (Pl + Sil inclusions in zircon) and associated anatexis during decompression. The ages obtained suggest that the granulite-facies metamorphism lasted for ~25 m.y. and was related to the Early Paleozoic continental collision between the Qilian and Qaidam blocks, and to subsequent thermal relaxation and exhumation.

High- and Ultrahigh-Pressure Metamorphism in the North Qaidam and South Altyn Terranes, Western China

The North Qaidam and South Altyn terranes extend approximately 1000 km across the northern Tibetan Plateau, and five localities preserve evidence of Early Paleozoic high-pressure (HP) or ultrahigh-pressure (UHP) metamorphism, including the presence of coesite, coesite pseudomorphs, and diamond. A review of the geology, petrology, and geochronology collected over the past 10 years since these localities were discovered supports a correlation of the North Qaidam and South Altyn terranes, offset 350-400 km across the Altyn Tagh fault. Geochronology interpreted to reflect eclogite-facies metamorphism yields ages between 500 and 420 Ma; detailed geochronology from one locality supports a protracted (tens of m.y.) history of HP/UHP metamorphism. Rock associations and geochronology support a passive-margin origin for the protolith of the HP/UHP rocks, which received sediments from a Proterozoic-Late Archean source, and was intruded by Neoproterozoic granites derived from crustal melting.

Epidote-rich talc-kyanite-phengite eclogites, Sulu terrane, eastern China: P-T-fo2 estimates and the significance of the epidote-talc assemblage in eclogite

Abstract Eclogites interlayered with gneiss and minor quartzite in the Qinglongshan near Donghai are characterized by unusually abundant (15.40 vol%) hydrous phases including talc, phengite, and epidote; many also contain kyanite. Garnet hosts both prograde (paragonite, amphibole, epidote) and peak stage (omphacite, epidote, phengite, kyanite) mineral inclusions. Several eclogites contain talc rimmed by barroisite; optically and compositionally similar coarse-grained amphibole in other samples indicates that the reaction Omp + Tlc = Amp has completely consumed talc. Estimated peak conditions of 30.35 kbar, 600.700 °C, are consistent with polycrystalline quartz pseudomorphs after coesite included in garnet, omphacite, epidote, and kyanite, and up to 3.6 Si pfu (11 O atom basis) in phengite. Garnet-epidote oxygen barometry on the peak metamorphic assemblage indicates oxygen fugacities above the Hem-Mag buffer, consistent with the epidote + talc assemblage and 5.20 mol% aegerine component in omphacite. The high oxygen fugacity calculated in this study as well as previously documented negative oxygen isotope values recorded by these rocks may both reflect alteration by oxidizing, meteoric water in a hydrothermal system. Oxidized conditions during peak metamorphism may explain the extreme scarcity of microdiamond in this area. The Ep + Tlc assemblage is stabilized by high oxygen fugacity, and demonstrates that talc-bearing eclogites are not restricted only to unusually Mg-rich bulk compositions.

High Throughput Petrochronology and Sedimentary Provenance Analysis by Automated Phase Mapping and LAICPMS

The first step in most geochronological studies is to extract dateable minerals from the host rock, which is time consuming, removes textural context, and increases the chance for sample cross-contamination. We here present a new method to rapidly perform in-situ analyses by coupling a fast Scanning Electron Microscope (SEM) with Energy Dispersive X-ray Spectrometer (EDS) to a Laser Ablation Inductively Coupled Plasma Mass Spectrometer (LAICPMS) instrument. Given a polished hand specimen, a petrographic thin section, or a grain mount, Automated Phase Mapping (APM) by SEM/EDS produces chemical and mineralogical maps from which the X-Y coordinates of the datable minerals are extracted. These coordinates are subsequently passed on to the laser ablation system for isotopic analysis. We apply the APM+LAICPMS method to three igneous, metamorphic and sedimentary case studies. In the first case study, a polished slab of granite from Guernsey was scanned for zircon, producing a 609 ± 8 Ma weighted mean age. The second case study investigates a paragneiss from an ultra-high pressure terrane in the North Qaidam terrane (Qinghai, China). 107 small (25 μm) metamorphic zircons were analysed by LAICPMS to confirm a 419 ± 4 Ma age of peak metamorphism. The third and final case study uses APM+LAICPMS to generate a large provenance dataset and trace the provenance of 25 modern sediments from Angola, documenting longshore drift of Orange River sediments over a distance of 1500 km. These examples demonstrate that APM+LAICPMS is an efficient and cost effective way to improve the quantity and quality of geochronological data.

Tectonic erosion related to continental subduction: An example from the eastern North Qaidam Mountains, NW China

Handling editor: Jaroslaw Majka Abstract Tectonic erosion has been commonly recognized in oceanic subduction zones, but evidence of tectonic erosion related to continental subduction is rarely reported. In this contribution, we suggest a tectonic erosion process along the North Qaidam Mountains of NW China during the Early Palaeozoic continental subduction. To the north of the North Qaidam ultra-high pressure (UHP) metamorphic belt, the microcontinental Oulongbuluke Block (OB) is characterized by Precambrian metamorphic basement. New and existing zircon U–Pb data show that the OB underwent Early Palaeoproterozoic (2.4–2.3 Ga) magmatism and Late Palaeoproterozoic (c. 1.9 Ga) high-grade metamorphic and anatectic events. The North Qaidam UHP metamorphic belt has been considered to result from the northward subduction of the Qaidam continental block consisting mainly of Meso-Neoproterozoic orthogneiss and paragneiss. However, our new data show some gneisses enclosing eclogites in the eastern North Qaidam UHP metamorphic belt (Dulan UHP metamorphic unit) experienced 2.4–2.3 Ga magmatism and 1.9–1.8 Ga metamorphism, similar to those of the OB, prior to Early Palaeozoic UHP metamorphism. These data suggest that tectonic erosion occurred during north-dipping subduction: the Precambrian basement from the upper plate (OB) was transported in the subduction zone to more than 80 km depth, resulting in UHP metamorphism related to Early Palaeozoic continental subduction prior to their exhumation.

Phase equilibrium modelling and implications for P-T determinations of medium-temperature UHP eclogites, North Qaidam terrane, China

Handling Editor: Richard White ABSTRACT In contrast to low‐T eclogites (garnet growth zoning preserved) or high‐T eclogites (garnet diffusionally homogenized at peak conditions), medium‐temperature eclogites pose additional challenges to P–T determinations due to the partial preservation of garnet zoning. The Dulan area, in the southeastern part of North Qaidam ultrahigh‐pressure terrane, exposes minor eclogites hosted by ortho‐ and paragneisses. Four fresh, medium‐temperature eclogites contain the paragenesis Grt+Omp+Rt+Qz/Coe+Ph±Ky±Zo. In all samples, garnet XMg shows little zoning, suggesting diffusional modification, and precludes the use of pyrope+almandine+grossular isopleth intersections to determine a P–T path. However, in one sample, sharp zoning in grossular content suggests that grossular growth compositions are preserved. Since garnet pyrope+almandine compositions appear to be modified, we instead use the intersections of grossular and garnet volume isopleths to define a prograde P–T path. This approach yields a path from ~17 kbar and ~410°C to ~35 kbar and ~625°C with a gradient of ~5–9°C/km through the lawsonite stability field. Peak P–T conditions determined from the intersection between Si pfu in phengite and Zr‐in‐rutile isopleths are ~26– 33 kbar and ~625–700 °C for the four eclogites. These conditions are close to the limit of the lawsonite stability field, suggesting that fluid released from lawsonite breakdown may have promoted re‐equilibration at these conditions. These peak conditions are also in good agreement (within 3 kbar and 50°C) with garnet–omphacite–phengite (±kyanite) thermobarometry in three of the four samples. We regard the phengite–rutile constraints as more reliable, because they are less sensitive to uncertainties associated with ferric iron compared to conventional thermobarometry. Phase equilibrium modelling predicts that the retrograde assemblage of amphibole+zoisite formed at ~60 km. Infiltration of external fluids was likely required for the growth of these hydrous minerals. Based on the comparison of P–T estimation methods applied in this study, we propose that the garnet grossular+volume isopleths can recover the prograde P–T path of medium‐temperature eclogites, and that the combination of phengite+rutile isopleths represents a more robust approach to constrain peak P–T conditions.