High-grade metamorphism during Archean–Paleoproterozoic transition associated with microblock amalgamation in the North China Craton: Mineral phase equilibria and zircon geochronology

• Granulite facies metamorphism along the margin of a microblock at 800–920 °C and up to 14 kbar in the NCC
• Magmatic protoliths emplaced at ca. 2.56–2.54 Ga, followed by metamorphism at ca. 2.48 Ga
• Metamorphism associated with subduction–collision tectonics in late Archean

Metamorphic regimes in Archean terranes provide important keys to the plate tectonic processes in early Earth. The North China Craton (NCC) is one of the ancient continental nuclei in Asia and recent models propose that the cratonic architecture was built through the assembly of several Archean microcontinental blocks into larger crustal blocks. Here we investigate garnet- and pyroxene-bearing granulite facies rocks along the periphery of the Jiaoliao microcontinental block in the NCC. The garnet-bearing granulites contain peak mineral assemblage of garnet + clinopyroxene + orthopyroxene + magnetite + plagioclase + quartz ± biotite ± ilmenite. Mineral phase equilibria computations using pseudosection and geothermobarometry suggest peak P–T condition of 800–830 °C and 7–8 kbar for metamorphism. Isopleths using XMg of orthopyroxene and XCa of garnet in another sample containing the peak mineral assemblage of garnet + orthopyroxene + quartz + magnetite ± fluid yield peak P–T conditions of 860–920 °C and 11–14 kbar. Geochemical data show tonalitic to granodioritic composition and arc-related tectonic setting for the magmatic protoliths of these rocks. Zircon LA-ICP-MS analyses yield well-defined discordia with upper intercept ages of 2562 ± 20 Ma (MSWD = 0.94) and 2539 ± 21 Ma (MSWD = 0.59) which is correlated with the timing of emplacement of the magmatic protolith. A younger group of zircons with upper intercept ages of 2449 ± 41 Ma (MSWD = 0.83); N = 6 as 2449 ± 41 Ma (MSWD = 0.83; N = 6) and 2480 ± 44 Ma (MSWD = 1.2; N = 9) constrains the timing of metamorphism. Zircon Lu–Hf data show dominantly positive εHf(t) values (up to 8.5), and yield crustal residence ages (TDMC) in the range of 2529 to 2884 Ma, suggesting magma sources from Meso-Neoarchean juvenile components. The high temperature and medium to high pressure metamorphism is considered to have resulted from the subduction–collision tectonics associated with microblock amalgamation in the NCC at the end of Archean. Together with the evidence for high pressure metamorphism from an adjacent locality, our results correlate with models that predict paired metamorphism at the Archean–Proterozoic transition with the onset of modern style plate tectonics.

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