Ultrahigh-temperature granulite-facies metamorphism and exhumation of deep crust in a migmatite dome during late- to post- orogenic collapse and extension in the central Adirondack Highlands (New York, USA)

This study combines field observations, mineral and whole-rock geochemistry, phase equilibrium modeling, and U-Pb sensitive high-resolution ion microprobe (SHRIMP) zircon geochronology to investigate sillimanite-bearing felsic migmatites exposed on Ledge Mountain in the central Adirondack Highlands (New York, USA), part of an extensive belt of mid-crustal rocks comprising the hinterland of the Mesoproterozoic Grenville orogen. Phase equilibrium modeling suggests minimum peak metamorphic conditions of 960–1025 °C and 11–12.5 kbar during the Ottawan orogeny—significantly higher pressure-temperature conditions than previously determined—followed by a period of near-isothermal decompression, then isobaric cooling. Petrography reveals abundant melt-related microstructures, and pseudosection models show the presence of at least ~15%–30% melt during buoyancy-driven exhumation and decompression. New zircon data document late Ottawan (re)crystallization at ca. 1047 ± 5 to 1035 ± 2 Ma following ultrahigh-temperature (UHT) metamorphism and anatexis on the retrograde cooling path. Inherited zircon cores give a mean date of 1136 ± 5 Ma, which suggests derivation of these felsic granulites by partial melting of older igneous rocks. The ferroan, anhydrous character of the granulites is similar to that of the ca. 1050 Ma Lyon Mountain Granite and consistent with origin in a late- to post-Ottawan extensional environment. We present a model for development of a late Ottawan migmatitic gneiss dome in the central Adirondacks that exhumed deep crustal rocks including the Snowy Mountain and Oregon anorthosite massifs with UHT Ledge Mountain migmatites. Recognition of deep crustal meta-plutonic rocks recording UHT metamorphism in a migmatite gneiss dome has significant implications for crustal behavior in this formerly thickened orogen.

Physicochemical simulation of the conditions for the mafic granulite formation (Bunger Hills, East Antarctica)

The article presents the results of mineral thermobarometry and physico-chemical simulation of the formation of garnet-enstatite gneiss from the Mesoproterozoic metamorphic suite of the Bunger Hills, East Antarctica. As a result, the water activity, temperature and pressure of rock formation were estimated. It is shown that the peak temperature of metamorphism could reach 900 °С or more. Such temperature conditions indicate the prerequisites for the occurrence of UHT-metamorphism during the formation of the East Antarctic Shield.

Geochemical Characterization of Zircon in Fyfe Hills of the Napier Complex, East Antarctica

Ultra-high temperature (UHT) metamorphism plays an essential role in the development and stabilization of continents through accretionary and collisional orogenesis. The Napier Complex, East Antarctica, preserves UHT metamorphism, and the timing is still debated. U–Pb zircon geochronology integrated with rare earth element (REE) and oxygen isotope was applied to a garnet-bearing quartzo-feldspathic gneiss to confirm the timing of UHT metamorphism in Fyfe Hills in the western part of the Napier Complex. The zircons are analyzed using a sensitive high-resolution ion microprobe (SHRIMP). The cathodoluminescence observation and U–Pb ages allowed us to classify the analytical domains into three types: inherited domains (Group I), metamorphic domains (Group II), and U–Pb system disturbed domains (Group III). The REE patterns of Group II are characterized by a weak fractionation between the middle REE and heavy REE, which reinforces the above classification. The 207Pb*/206Pb* ages of Group II have an age peak at 2501 Ma, therefore, the gneiss experienced high temperature metamorphism at 2501 Ma. δ18O of zircons are homogeneous among the three groups (5.53 ± 0.11‰, 5.51 ± 0.14‰, and 5.53 ± 0.23‰), which suggests re-equilibration of oxygen isotopes after metamorphism at ca. 2501Ma under dry UHT conditions.

Local rapid exhumation and fast cooling in a long-lived Paleoproterozoic orogeny

The exhumation and cooling rates of high-grade metamorphic rocks are crucial for inferring orogenic processes and understanding the regimes of heat transport in Earth's crust. Quantification of these rates remains challenging for Precambrian terranes, because the temporal resolution of geochronology becomes coarser in deeper geologic time. This limitation is partly reflected by a striking lack of Proterozoic or older short-duration events (<10 Myr), most documented cases of fast metamorphism are confined to the Phanerozoic. In this study, we use garnet geospeedometry to explore the metamorphic rates of Paleoproterozoic high-grade rocks from two representative areas within the long-lived (1.95–1.80 Ga) Jiao-Liao-Ji orogenic belt, North China Craton. The pelitic granulites in the Taipingzhuang area record high-pressure granulite-facies (HPG) metamorphism of ∼12 kbar and ∼800 °C, followed by a fast decompression-cooling to ∼5 kbar and ∼600 °C within ∼5 Myr, at ca. 1.87 Ga. The pelitic granulites in the Rizhuang area document a brief (<1 Myr) thermal excursion to ultra-high-temperature (UHT) metamorphism of ∼8 kbar and ∼940 °C at ca. 1.85 Ga, followed by a fast cooling to ∼600 °C within 1–5 Myr. In light of available geological data, the fast decompression-cooling of HPG granulites is interpreted as the syn-collisional exhumation of thickened lower crustal segments at ca. 1.87 Ga, most likely through tectonic extrusion. The thermal excursion transiently reaching UHT conditions is inferred to be triggered by localized syn-metamorphic mafic intrusions in association with magmatic underplating during post-collisional extension at ca. 1.85 Ga. These metamorphic pulses were interspersed within the protracted Paleoproterozoic orogenesis and require geodynamic processes resembling modern plate tectonics. Notably, these ancient rapid events are beyond the temporal resolution of commonly-used in-situ geochronology that tends to yield apparent longer durations given errors and uncertainties. We therefore note that most ancient metamorphic rates might be underestimated using geochronological data, and recommend garnet geospeedometry as a promising alternative approach. The largely similar rates recorded by Paleoproterozoic and Phanerozoic orogens, as well as high-pressure metamorphism at 1.9–1.8 Ga, support the operation of modern plate tectonics in Paleoproterozoic time.

UHT Metamorphism Peaking Above 1100 °C with Slow Cooling: Insights from Pelitic Granulites in the Jining Complex, North China Craton

The peak temperature and duration of ultrahigh-temperature (UHT) metamorphism are critical to identify and understand its tectonic environment. The UHT metamorphism of the Jining complex in the Khondalite Belt, North China Craton is controversial on the peak temperature, time and tectonic setting. A representative sapphirine-bearing granulite sample is selected from the classic Tianpishan outcrop for addressing the metamorphic evolution and timing. The rock is markedly heterogeneous on centimetre scale and can be divided into melanocratic domains rich in sillimanite (MD-s) or rich in orthopyroxene (MD-o), and leucocratic domains (LD). On the basis of detailed petrographic analyses and phase equilibria modelling using THERMOCALC, all three types of domains record peak temperatures of 1120–1140 °C and a series of post-peak cooling stages at 0·8–0·9 GPa to the fluid-absent solidus (∼890 °C), followed by sub-solidus decompression. The peak temperature for MD-s is constrained by the coexistence of sillimanite-I + sapphirine + spinel + quartz, where sillimanite-I contains densely exsolved aciculae of hematite, yielding reintegrated Fe2O3 contents up to 2·1–2·3 wt %. The post-peak cooling evolution involves the sequential appearance of K-feldspar, sillimanite-II + garnet, orthopyroxene and biotite, where sillimanite-II is exsolution-free and contains variable Fe2O3 contents of 1·3–1·8 wt %. The peak temperature for MD-o is constrained by the sapphirine + orthopyroxene assemblage, where orthopyroxene has a maximum AlIV of 0·22 (Al2O3 = 9·5 wt %) in the core. The cooling evolution involves the sequential appearance of K-feldspar, garnet and biotite, and the decreasing AlIV (0·22→0·17) from core to rim in orthopyroxene. The peak temperature for LD is constrained by the inferred K-feldspar-absent assemblage and the maximum anorthite content of 0·11 in K-feldspar. The cooling evolution involves the crystallization of segregated melts, exsolution of supra-solvus ternary feldspars and growth of biotite. The Al in orthopyroxene, Fe2O3 in sillimanite and anorthite in K-feldspar are good indicators for constraining extreme UHT conditions although they depend differently on bulk-rock compositions. In-situ SHRIMP U–Pb dating of metamorphic zircon indicates that the UHT metamorphism may have occurred at >1·94 Ga and the cooling under UHT conditions lasted over 40 Ma. The extreme UHT metamorphism in the Jining complex is interpreted to be triggered by the advective heating of intraplate hyperthermal mafic magmas together with a plume-related hot mantle upwelling, following an orogenic crustal thickening event.