Metamorphic-anatectic reworking of continental arc crust: Insights from the North Wulan metamorphic complex in the North Qaidam tectonic belt (northern Tibet)

Tracking the multi-stage metamorphism and exhumation history of felsic gneisses in the South Altyn ultra-high pressure metamorphic belt, Western China

Large granitic batholiths developed in many mountain belts during the late stages of orogenesis. While current melting process research focuses on fluid-absent breakdown of hydrous minerals at amphibolite to granulite facies conditions, petrography, phase equilibrium modelling, and trace element evidence suggests that melt formation due to influx of fluids could be more widespread than previously thought. We investigate Crd bearing diatexites and Kfs bearing migmatitic paragneisses from two locations in the Moldanubian domain of the Bohemian Massif (Czech Republic) for signs of melt formation during water-fluxed or hydrous mineral breakdown melting. We use petrography of thin sections, mineral chemistry by EPMA and trace element compositions from LA-ICP-MS analysis. Samples from Nemojov consist of Crd + Bt + Plg + Qtz + Sill + Ilm ± Kfs and show signs of biotite breakdown to Kfs. It is assumed that pre-existing muscovite was consumed during water-fluxed melting and the subsequent peritectic reaction involves biotite, together with Sill, Plg, and Qtz to form Crd. Diffuse mesosome and leucosome boundaries and the absence of peritectic Kfs suggest water-fluxed melting. The Pohled sample has peritectic Kfs together with Plg + Bt + Ms + Qtz + Ilm ± Grt. This sample has a mm-sized foliation with a clear distinction between melanosome consisting of Plg+ Bt + Ms + Qtz + Ilm and the leucosome comprising Kfs + Qtz ± Grt, reflecting muscovite dehydration melting. Trace element data from both sites show significant differences in Rb, Ba, and Sr, which are dominantly incorporated in micas and feldspars. Variable Rb/Sr ratios can be an indicator for water fluxed melting: since Sr is mainly hosted in Plg and its involvement is stronger during water-fluxed melting, low Rb/Sr values (below 3.5) can be expected in Bt, Plg, and Kfs. This indicates muscovite involvement in Pohled and plagioclase-involvement in the Nemojov samples. Biotite hosts several elements such as LILE and metals like Sc, V, Cr, Co, Ni Nb, Ta, Sn, W. During melting, biotite was consumed via water fluxed melting via the reaction:Bt + Plg + Sill + Qtz + H2O --> Crd + MeltElements like Li and Be are redistributed mainly to Crd, but Rb, Cs, Ba, Sc, V, Cr, Co, Ni, Nb, Ta, Sn, and W were incorporated into the melt (assuming equilibrium between Ilm + Bt). Alkali feldspar and plagioclase have high Ba, Sr, and Pb concentrations, and Zn is enriched in plagioclase. All of this indicates fluid influx, where muscovite released Rb, Cs, and Sn that was taken up by the feldspars. On the other hand, fluid-fluxed plagioclase breakdown resulted in elevated Sr and Ba in biotite and alkali feldspar during melting reactions in Nemojov. Feldspar from Pohled similarly shows high Rb, Sn, and Cs values indicating the influence of muscovite breakdown.

The architecture of continental arc crust from its surface to base is a crucial aspect of understanding long-term crustal growth and reworking. However, the timing and nature of metamorphism in the middle to lower crust are some of the least-understood components of continental arc systems. In this contribution, we integrate phase equilibrium modeling and zircon geochronology to determine the metamorphic history of garnet amphibolite in the North Wulan metamorphic complex, North Qaidam tectonic belt, northern Tibet, and then provide key insights into the metamorphic nature of the middle to lower crust of an early Paleozoic continental arc, and into orogenic geodynamic processes associated with the closure of the Proto-Tethyan Ocean. Garnet amphibolite contains a peak metamorphic assemblage of garnet + orthopyroxene + hornblende + plagioclase + ilmenite + quartz ± biotite + melt, which defines a peak/near-peak pressure-temperature (P-T) range of 800–860 °C at 5–8.5 kbar, based on phase equilibrium modeling. Rounded inclusions of hornblende, plagioclase, and quartz in garnet porphyroblasts suggest prograde garnet growth at the expense of amphibole. Partial replacement of garnet and hornblende by orthopyroxene suggests high-T decompression after peak pressure. Post-peak partial replacement of garnet by plagioclase–quartz–hornblende or hornblende corona suggests decompression cooling after peak temperature. These results suggest that the garnet amphibolite records a clockwise P-T path with high-T decompression. These currently exposed granulite-facies rocks from the southern part of the North Wulan metamorphic complex represent a middle to lower crustal exposure (root) of an early Paleozoic continental arc–backarc setting (depths of >16–28 km) during the subduction of the Proto-Tethyan Ocean. Zircon petrochronology in garnet amphibolite reveals that protracted metamorphism occurred at ca. 515–449 Ma. These new insights into the early Paleozoic tectonometamorphic evolution of the North Wulan metamorphic complex constrain the northward subduction polarity of the South Qilian Ocean and the tectonic evolution of the North Qaidam tectonic belt. Clockwise P-T paths in continental arc settings are related to the thinning of the thickened arc crust triggered by the steepening of subducted oceanic slab, which is accompanied by slab retreat and rollback and could reflect the migration of continental arc magmatic axes. Our new results tie together high-resolution thermal, magmatic, and metamorphic records to explain the complex evolution of the roots of continental arc systems.

The central part of the Carolina terrane in western South Carolina comprises a 30 to 40 km wide zone of high grade gneisses that are distinct from greenschist facies metavolcanic rocks of the Carolina slate belt (to the SE) and amphibolite facies metavolcanic and metaplutonic rocks of the Charlotte belt (to the NW). This region, termed the Silverstreet domain, is characterized by penetratively deformed felsic gneisses, granitic gneisses, and amphibolites. Mineral assemblages and textures suggest that these rocks formed under high‐pressure metamorphic conditions, ranging from eclogite facies through high‐P granulite to upper amphibolite facies.Mafic rocks occur as amphibolite dykes, as metre‐scale blocks of coarse‐grained garnet‐clinopyroxene amphibolite in felsic gneiss, and as residual boulders in deeply weathered felsic gneiss. Inferred omphacite has been replaced by a vermicular symplectite of sodic plagioclase in diopside, consistent with decompression at moderate to high temperatures and a change from eclogite to granulite facies conditions. All samples have been partially or wholly retrograded to amphibolite assemblages. We infer the following P‐T‐t history: (1) eclogite facies P‐T conditions at ≥ 1.4 GPa, 650–730 °C (2) high‐P granulite facies P‐T conditions at 1.2–1.5 GPa, 700–800 °C (3) retrograde amphibolite facies P‐T conditions at 0.9–1.2 GPa and 720–660 °C. This metamorphic evolution must predate intrusion of the 415 Ma Newberry granite and must postdate formation of the Charlotte belt and Slate belt arcs (620 to 550 Ma).Comparison with other medium temperature eclogites and high pressure granulites suggests that these assemblages are most likely to form during collisional orogenesis. Eclogite and high‐P granulite facies metamorphism in the Silverstreet domain may coincide with a ≈570–535 Ma event documented in the western Charlotte belt or to a late Ordovician‐early Silurian event. The occurrence of these high‐P assemblages within the Carolina terrane implies that, prior to this event, the western Carolina terrane (Charlotte belt) and the eastern Carolina terrane (Carolina Slate belt) formed separate terranes. The collisional event represented by these high‐pressure assemblages implies amalgamation of these formerly separate terranes into a single composite terrane prior to its accretion to Laurentia.

The Cretaceous Yuhuashan igneous complex contains abundant xenoliths of high‐grade metamorphic rocks, with the assemblage garnet ± hypersthene + biotite + plagioclase + K‐feldspar + quartz. The biotite in these samples has high TiO2 (>3.5%), indicating high‐T metamorphism (623–778 °C). P–T calculations for two felsic granulites indicate that the peak metamorphism took place at 880–887 °C and 0.64–0.70 GPa, in the low pressure/high temperature (LP‐HT) granulite facies. Phase equilibrium modelling gives equilibrium conditions for the peak assemblage of a felsic granulite of >0.6 GPa and >840 °C, consistent with the P–T calculations, and identifies an anticlockwise P–T–t path. LA‐ICPMS U–Pb dating of metamorphic and detrital zircon from one xenolith reveals that the granulite facies metamorphism took place at 273.6 ± 2.2 Ma, and the protolith was a sedimentary rock deposited later than 683 Ma. This represents the first Late Palaeozoic (Variscan) granulite facies event identified in the South China Block (SCB). Coupled with other geological observations, the LP‐HT metamorphic conditions and anticlockwise P–T–t path suggest that Variscan metamorphism probably occurred in a post‐orogenic or intraplate extensional tectonic setting associated with the input of external heat, related to the underplating of mantle‐derived magma. Based on P–T estimates and the comparison of the protolith composition with mid‐ to low‐grade metamorphic rocks in the area, it is suggested that the mid‐lower crust under the Xiangshan–Yuhuashan area consists mainly of these felsic granulites and gneisses, whose protoliths were probably subducted to these depths during the Early Palaeozoic orogeny in the SCB, and underwent two episodes of metamorphism during Early Palaeozoic and Late Palaeozoic time.

The Lesser Antilles volcanic arc is known for its magmatic diversity and unusually abundant plutonic xenoliths. Xenoliths from Petit St. Vincent (Grenadines’ archipelago) are particularly interesting because of their textural and petrogenetic range. Here we combine petrographic observations, Electron Backscatter Diffraction (EBSD) analysis, major and trace element chemistry of xenoliths and lavas and geochemical and thermal modelling to explore the construction of arc crust beneath Petit St. Vincent. Petit St. Vincent xenoliths are dominated by calcic plagioclase, clinopyroxene and amphibole, and can be divided into two main categories, igneous and meta-igneous. Igneous xenoliths typically have cumulate textures; meta-igneous xenoliths range texturally from those that preserve vestiges of primary magmatic fabrics to intensely deformed varieties characterised by grain-size reduction and foliation development. Meta-igneous xenoliths also contain the most calcic plagioclase (An98–100). The presence of both meta-igneous and igneous xenoliths provides evidence for reworking of older arc crust and antecedent igneous intrusions. The latter have a protolith composition similar to high-MgO, low-Sr picrites and high-Ca, high-Sr ankaramites from the neighbouring islands of Petite Martinique and Grenada. The meta-igneous xenoliths derive from older, mafic arc crust present at the onset of subduction. Trace element chemistry and EBSD analyses of meta-igneous xenoliths are consistent with a complex history of re-melting and deformation mediated by chlorine-bearing H2O rich fluids (including melts). Thermal modelling supports crustal reworking through repeated magma intrusions and indicates that the observed thermal structure and thickness of crust beneath Petit St. Vincent could have developed on a timescale of approximately 4 million years at rates compatible with the regional arc magma flux. Based on evidence from thermodynamic models and exhumed ancient arc crust sections, Collins et al. (Nature Geoscience, 13, 331–338, 2020) have proposed that water-fluxed melting may be an important aspect of deep arc crust sections world-wide. Textures and mineralogy of xenoliths from Petit St. Vincent, including their characteristic high-An plagioclase, testify to such a process beneath an active, intra-oceanic arc.

Role of aqueous fluids during low pressure partial melting of pelites in the Adamello pluton contact aureole (Italy)

Phase equilibrium modelling of the amphibolite facies metamorphism in the Yelapa-Chimo Metamorphic Complex, Mexico

Age and geochemistry of western Hoh-Xil–Songpan-Ganzi granitoids, northern Tibet: Implications for the Mesozoic closure of the Paleo-Tethys ocean

The latest Neoarchean–Paleoproterozoic evolution of the Dunhuang block, eastern Tarim craton, northwestern China: Evidence from zircon U–Pb dating and Hf isotopic analyses

Petrogenesis of Late Devonian–Early Carboniferous volcanic rocks in northern Tibet: New constraints on the Paleozoic tectonic evolution of the Tethyan Ocean

The Twelve Mile Bay assemblage (TMBa) forms the high-strain interior of the Twelve Mile Bay shear zone (TMBsz), a major ductile decollement zone within the western Canadian Grenville orogen. Metasupracrustal gneiss within the TMBa preserves evidence for an early granulite facies (˜10–11 kbar and ˜840°C) metamorphism overprinted by amphibolite facies (˜5–7 kbar and ˜650°C) assemblages that define the high-strain shear zone fabric. U–Pb zircon ages for TMBa samples were determined by LA-ICP-MS. A low-strain amphibolite pod with partially preserved granulite facies assemblage and textures yielded an anchored discordia intercept of 1157 ± 11 Ma and 207Pb/206Pb weighted average of 1146 ± 10 Ma. Three higher strain samples with recrystallized amphibolite facies assemblages all yield younger ages, with 207Pb/206Pb weighted averages of 1125 ± 16, 1110 ± 8, and 1095 ± 17 Ma. Phase equilibrium modelling shows that up to 40 vol.% anatectic melt could have been produced in TMBa pelitic rocks during peak metamorphic conditions, and thus, much of the package likely would have been substantially weakened during the early stages of TMBsz development. Strain apparently continued to accumulate within the TMBa until ca. 1100 Ma, concurrent with pegmatite dike emplacement and hydration along the base of the overlying interior Parry Sound domain (iPSD), perpetuating TMBsz activity during cooling and exhumation to shallower crustal levels. Similarities between the TMBa and the upper parts of the basal PSD (bPSD), in terms of timing and conditions of metamorphism and shearing, as well as structural position relative to the overlying iPSD allochthon, indicate that these units are likely correlative. The composite bPSD–TMBa system appears to have contemporaneously localized strain within the middle orogenic crust during early to middle stages of Grenvillian collision, providing a petrologically constrained mechanism for the long distance transport of mid-crustal nappes predicted in thermal-mechanical models of continental collision for this area.

The Cerro Uyarani Metamorphic Complex on the Bolivian Altiplano: New constraints on the tectonic evolution of the Central Andean basement between ∼1.8 and 1.0 Ga

Eclogites overprinted in the granulite facies from the Ďumbier Crystalline Complex (Low Tatra Mountains, Western Carpathians)Metabasites with evidence for breakdown of former eclogites and recrystallization under granulite facies conditions occur in the Ďumbier Crystalline Complex of the Low Tatra Mountains, Central Western Carpathains. Textural relationships, phase equilibrium modelling and thermobarometry have been used to determine theP-Tevolution of these rocks. Omphacite diagnostic for the eclogites facies stage is absent but its former presence is inferred from the symplectitic intergrowths of clinopyroxene + plagioclase. The re-equilibration in high-pressure granulite facies conditions is demonstrated by the assemblage garnet + clinopyroxene (< 10 % Jd) + plagioclase + quartz. The phase equilibrium modelling using THERIAK-DOMINO program and conventional geothermobarometry suggest theP-Tconditions of 750-760 °C and 1.1-1.5 GPa for the high-pressure granulite stage. Orthopyroxene formed in the clinopyroxene + plagioclase symplectites and kelyphites and coronas around garnet atP-Tconditions of ca. 0.7-1.0 GPa and 650-700 °C.P-Tevolution of granulitized eclogites is interpreted as the result of two metamorphic events; early Variscan eclogite facies metamorphism was followed by granulite facies thermal overprint in the Carboniferous time. The second metamorphic event was crucial for breakdown of eclogites, these are only seldom preserved in the pre-Alpine basement of the Western Carpathians.

Evolution of fluids and melts in deeply subducted continental crust: Insights from an UHP eclogite–vein system in the Dabie terrane, China