Multistage late Neoarchaean crustal evolution of the North China Craton, eastern Hebei
The eastern part of the North China Craton in eastern Hebei Province contains metamorphosed and deformed Neoarchaean to earliest Palaeoproterozoic rocks (~2550-2490 Ma) with some older Archaean rocks. Numerous precise U-Pb zircon ages, structural observations, Nd and Hf isotopic data and whole rock geochemistry (our new data with reassessment and integration of previous work) show that there is not a single protracted event at the end of the Archaean lasting ~60 million years, but that from east to west there are several separate events with their own unique character.The eastern coastal region is dominated by weakly deformed 2530-2510 Ma granites with subordinate granodiorites, diorites and magnesian-gabbros. This suite has inclusions of older 2550-2540 Ma plutonic rocks. ~80 km inland to the west, there are amphibolite facies biotite-rich schists, lenses of BIF, siliceous fuchsite bearing rocks with 3880-3540 Ma zircons, peridotites and orthogneisses, all intruded by younger granites and monzonites. Three amphibolite-granulite facies gneisses and schists with volcano-sedimentary protoliths have 2548-2534 Ma igneous zircon populations, rare inheritance back to >3600 Ma and 2506. +- 6 Ma zircon equated with granulite facies metamorphism. Older tonalitic to granitic polyphase orthogneisses contain 3287. +-. 11 Ma to ~2940 Ma igneous components. Late kinematic high Fe/Mg gabbros, monzonites and granites have ages of 2499. +-. 7 to 2491. +-. 13 Ma. 160-100 km inland, gneissose 2550-2530 Ma quartz-diorite to tonalitic rocks occur as intrusions into, or intercalations with, mafic rocks. These were affected by granulite facies metamorphism but then widely retrogressed. Abundant syn-granulite facies neosome and later shear zones disrupt the early geological relationships. The granulite facies metamorphism is dated at 2503. +-. 5 Ma from several samples, including a syn-granulite facies pegmatite.The Neoarchaean igneous rocks show marked negative Nb, Ti anomalies, LIL enrichment and enrichment of the light REE relative to the heavy REE across the entire ultramafic, basaltic, tonalitic and granitic compositional spectrum: 2550-2530 Ma tonalites and quartz diorites are most important in the west and 2530-2510 Ma granites are most important in the east. Their composition resembles those formed at various stages of magmatic arc evolution at a convergent plate boundary, rather than being plume products. The eastern Hebei rocks thus formed in a complex arc with distinct pulses of plutonism at 2550-2540 and 2530-2510 Ma, with local incorporation of older continental crust. The 2500-2490 Ma thermal event with its associated higher Fe/Mg magmas involved interaction of underplated fractionated gabbroic melt with deep crustal melts, possibly during extension of an already ~50 million years old arc. 2011 Elsevier B.V.
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