The Pikwitonei Granulite Domain (PGD), located in the northwestern Superior Province, is one of the largest Neoarchean high-grade metamorphic domains in the world, and is a key to understanding the Neoarchean crustal evolution of the Superior Province. Here we report results of a study on ultrahigh temperature (UHT) granulites with a sapphirine + quartz-bearing peak assemblage from the Sipiwesk Lake area in the PGD. Three stages of metamorphic assemblage development are recognized based on petrographic observations: Pre-peak stage is marked by garnet, sillimanite, K-feldspar, and biotite inclusions within sapphirine and orthopyroxene. Peak stage is charactered by the typical UHT associations of sapphirine + quartz and sapphirine + orthopyroxene. The retrograde stage is represented by the retrograde formation of cordierite, biotite and sillimanite in the matrix. Phase equilibrium modelling based on the bulk compositions of sapphirine-bearing granulites suggest that the rocks have experienced extensional UHT metamorphism in excess of 1035 C at pressures of 7.9-9.0 kbar, followed by isobaric cooling process. SHRIMP U-Pb dating of metamorphic zircons in sapphirine-bearing granulite record the timing of the UHT event in the PGD with a weighted mean 207 Pb/ 206 Pb age of 2681 ± 13 Ma. This study in combination with other metamorphic P-T paths and age information reveals that this UHT metamorphism at 2.68 Ga in PGD was generated by upwelling asthenosphere due to slab break-off in collision during the amalgamation of the Superior Province.

We formulate a numerical framework to model the structural patterns emerged from the long-term highly viscous tectonic flow for both two and three spatial dimensions by coupling the discontinuous Galerkin level set method with a finite element Stokes-like flow solver. Our formulation, implemented with adaptive mesh refinement near the material interface, allows for accurate interface capturing and automatic handling of topological splitting and merging. Compared to particle-in-cell family of methods, the level set formulation has the advantage of retaining information on the interface geometry, less memory requirement and the savings of computational expense on the two-way particle-mesh information transfer. Furthermore, our formulation discretizes the level set in the same finite element framework as the flow solver, thus enabling us to fully exploit the advantages of the finite element method such as the flexibility of mesh geometry and the ease of handling anisotropic materials. In order to track the finite deformation in the modelling domain, passive tracer particles are generated at and around locations of interest, whose deformation can be accumulated through arbitrary time interval within the total modelled time span, thus offering a fully dynamical approach for modelling non-steady and inhomogeneous structural patterns. The material distribution and the finite deformation pattern generated from the numerical model can be directly compared with the geological map patterns and the field structural analyses, thus offering the possibility of ground-truthing the modelling results by field evidence.

Located in the southern margin of the Central Asian orogenic belt, the stratigraphy and tectonic setting of the mafic 290−280 Ma Liuyuan Complex have been controversial for decades, with workers arguing for a forearc ophiolite or a continental rift setting. Here, we present the results of a detailed field study, where the Liuyuan Complex was subdivided into troctolite, melatroctolite, layered gabbro, varitextured olivine gabbro, hornblende gabbro, plagiogranite, sheeted dike, and mafic tectonite, in addition to previously identified and studied basalt and chert. All contacts between the igneous facies are intrusive, with gabbroic rocks separated from the overlying basalt by a well-developed and laterally continuous sheeted dike complex. Based on their geochemical affinities, two groups of basalt were identified: group I (low-TiO2) and group II (high-TiO2). A modeled liquid line of descent, assuming perfect mineral fractionation, with a liquidus temperature of 1212 °C, pressure of 1 kbar, fO2 at the quartz-fayalite-magnetite (QFM) buffer, and initial melt H2O of 0.5 wt%, provides an excellent fit to group I lavas, with group II basalts interpreted as having formed from a distinct arc source. The stratigraphy, extended trace-element patterns, and tectonic fingerprinting of the lavas suggest the Liuyuan Complex formed as an ophiolite in a fast-spreading back-arc basin, a setting inconsistent with previously proposed tectonic models for the southern Central Asian orogenic belt. We expand on a tectonic model that proposes the Liuyuan Complex formed as a back-arc to the recently identified Ganquan arc. The back-arc basin was then consumed in a north-dipping subduction zone beneath the active margin of composite Siberia. The magmatic center of this arc migrated southward, likely caused by slab roll-back, with the Liuyuan Complex becoming the basement of an arc. Exhumation of the Liuyuan Complex took place by 267 Ma, as constrained by the age of a subaerial dacite that unconformably overlies the basalts of the Liuyuan Complex.

Orogens are major sites for crustal growth on Earth. However, characterizing orogens with stages is challenging. Here we present Nd isotope mapping results of felsic and intermediate igneous rocks from eight well-studied, typical Phanerozoic orogens. The data illustrate the distribution and areas of isotopic domains ranging from highly primitive to highly evolved, which reflects their final preservation from long-term crustal growth. From the data, we calculated the areal proportion of juvenile crust and divided the orogens into (i) highly primitive (with >70% juvenile crust); (ii) primitive (70-50%; e.g., Altaids with ~58%); (iii) slightly primitive (50–30%; e.g., North American Cordillera (~48%), Newfoundland Northern Appalachian Orogen (~40%), Lachlan Orogen (~31%)); (iv) evolved (30–10%); (v) highly evolved (10–0%; e.g., Tethyan Tibet (~4%), Caledonides (~2%), Variscides (~1%), and Qinling-Dabie (<1%)). Those from (i) to (iii) quantitatively characterize and/or correspond to simple and complex accretionary orogens, and from (iv) to (v) to complex and simple collisional orogens, respectively. Our study presents a new approach for quantitatively characterizing orogens based on compositional architecture through isotope mapping, and for investigating the relationships between orogenesis and continental growth.