Abstract Precambrian high-pressure (HP) granulites provide important information for reconstructing ancient continental nuclei. Here we report granulites in Eastern Hebei, North China Craton (NCC). They experienced three metamorphic events related to Neoarchean–early Paleoproterozoic orogenesis. The garnet–clinopyroxene granulite defines the M 1 event at P–T conditions of 11–13 kbar and 780–830°C, while the two-pyroxene granulite was produced during the M 2 event at 7–9 kbar and 850–950°C. Both the garnet–clinopyroxene and two-pyroxene granulites experienced amphibolite retrogression during the M 3 event at 5–7 kbar and 710–730°C. Geochemical compositions of these granulites exhibit affinity to island-arc andesites. Zircon U–Pb dating shows that their magmatic precursors were erupted at c. 2538 Ma, and have experienced two-stage growth of zircon rims at c. 2458 and c. 2285 Ma, respectively. The c. 2458 Ma age may represent orogenic events during the amalgamation of micro-continental blocks of the eastern NCC, and the c. 2285 Ma age may be interpreted as the effect of late Paleoproterozoic magmatism. We suggest that the Neoarchean andesitic protoliths of the granulites were metamorphosed at HP granulite-facies conditions during collision of micro-continental blocks, and then exhumed to shallow levels. These early Paleoproterozoic HP granulites recorded the amalgamation of micro-continental blocks to reach the cratonization of the eastern NCC.

Abstract The Ti-in-zircon thermometer has been widely applied to different high-grade metamorphism rocks owing to its simplicity. Based on the calibration of Ferry & Watson (2007 ; New thermodynamic models and revised calibrations for the Ti-in-zircon and Zr-in-rutile thermometers. Contributions to Mineralogy and Petrology , 154 , 429–437), we used the Ti-in-zircon thermometer to estimate the metamorphic temperatures for low-temperature eclogites of North Qilian and Western Tianshan, China, and compiled HP/UHP eclogites from the literature. The Ti-in-zircon temperatures are generally higher than the estimations by other thermometers, especially when metamorphic temperatures are lower than 650°C. Although temperature exerts the dominant control on Ti content in zircon, other factors (e.g. lattice defect, other trace element substitutions in zircon, disequilibrium zircon growth, and precipitation from metamorphic fluids) may have contributed to the overestimated Ti-in-zircon temperatures.

Abstract Eclogites in the high-pressure (HP) and ultrahigh-pressure (UHP) belts record subduction-zone processes; exhumed eclogites of seafloor protoliths record low-temperature (mostly <600°C), high-pressure and ‘wet’ environments: that is, relatively ‘cold’ subduction with highly hydrous minerals such as lawsonite. In contrast, eclogites formed by the continental subduction record relatively ‘hot’ ( T > 650°C) and ‘dry’ ultrahigh-pressure metamorphic (UHPM) conditions with syncollisional magmatism. Here, we investigate some eclogites from two ophiolite sequences that intercalated in the North Qaidam UHPM belt, which is genetically associated with continental subduction/collision. The observations of lawsonite pseudomorphs in garnets, garnet compositional zoning, mineral and fluid inclusions in zircons, and zircons with distinct trace-element patterns and U–Pb ages all suggest that these eclogites represent two exhumation episodes of subduction-zone metamorphic rocks: the early ‘cold’ and ‘wet’ lawsonite eclogite and the late ‘hot’ and ‘dry’ UHP kyanite eclogite. The early lawsonite-bearing eclogite gives metamorphic ages of 470–445 Ma and the later kysnite-bearing eclogite gives metamorphic ages of 438–420 Ma, with a time gap of c. 7–10 myr. This gap may represent the timescale for transition from oceanic subduction and continental subduction to depths greater than 100 km. We conclude that evolution from oceanic subduction to continental collision and subduction was a continuous process. In addition, we find that titanium contents in zircons have a positive correlation with U contents. Ti-in-zircon thermometry is likely to be invalid or limited for low-temperature eclogites.