Abstract Based on the nature of ferric sillimanite, an activity model for sillimanite containing Fe 3+ is constructed, tested and adopted to calculate phase equilibria of pelitic compositions under ultrahigh‐temperature (UHT) conditions. The calculated P – T projections and pseudosections suggest that the incorporation of Fe 3+ into sillimanite can fairly solve the current imperfectly topological match between thermodynamic calculations and synthetic experiments, especially at high oxygen fugacity. Fe 3+ in sillimanite remarkably elevates the temperature to switch the parageneses of orthopyroxene + sillimanite (Opx + Sil) and sapphirine + quartz (Spr + Qz) in oxidized metapelites, with an increment around 50–70°C. The calculated compatibility diagrams show that the widely approbatory UHT‐diagnostic mineral assemblages of Opx + Sil and Spr + Qz usually occur in metapelites with high Mg/Fe 2+ ratios, which depend on both bulk‐rock MgO and oxygen fugacity, whereas the metapelites with low Mg/Fe 2+ ratios are characterized by the assemblage of Garnet + sillimanite (Grt + Sil) with or without spinel (Spl) in UHT conditions. Moreover, comprehensive comparisons suggest that the essential petrogenetic framework of natural UHT metapelites is mostly governed by the two metamorphic reactions of Opx + Sil = Spr + Grt and Grt + Sil = Spl + Spr.
Abstract 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.
The South Tianshan high/ultrahigh-pressure (HP–UHP) metamorphic belt, NW China is characterized by extensive pelitic–felsic schists with numerous blocks of eclogite, ultramafic rock and marble. The pelitic–felsic schists preserve two generations of garnet-bearing mineral assemblages: (1) an early generation that involves garnet, phengite, glaucophane, and quartz/coesite with or without paragonite; (2) a late generation that involves albite, hornblendic amphibole, quartz and grossular-rich, pyrope-poor garnet. Early generation garnet has growth zoning involving core to rim increases in pyrope content, coupled with increased or constant grossular content. Pseudosection modeling of these growth zoning textures in three samples (T311, T314, and T316) reflects prograde paths and peak conditions of c. 32 kbar at 550–570°C, c. 22.5 kbar at 550°C, and c. 22–23 kbar at 540–550°C, respectively. These P–T estimates are consistent with those recovered from eclogite blocks hosted by the schists. Phase equilibrium modeling predicts that the early garnet would have mainly grown in mineral assemblages involving lawsonite, jadeite and chloritoid, with or without coesite or carpholite, distinct from the assemblages now in the matrix. The post-peak decompression of the pelitic–felsic schists is inferred to have involved two stages. The early stage decompression is characterized by dehydration reactions involving lawsonite and carpholite at P > 20 kbar, coupled with mode and compositional changes of garnet, glaucophane and phengite. The late-stage decompression after lawsonite disappearance led to the rocks being fluid-absent. Under such fluid-absent conditions, the solid transition of jadeite to albite occurs at P ≈ 14 kbar, and hornblendic amphibole forms at 12–13 kbar. Most garnet grains are somewhat changed in composition, to produce a pyrope-poor and grossular-rich outer rim. In contrast to above-solidus conditions, decompression of the pelitic–felsic schists at subsolidus conditions does not destabilize phengitic muscovite and tends to preserve their peak mineral assemblages if there is no intensive fluid infiltration. In the South Tianshan belt, buoyancy of subducted metasediments with respect to mantle rocks could be one of the major reasons for fast exhumation of the HP–UHP rocks.
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