Friday, November 27, 2015

Formation of the Cratons During the Orosirian PaleoProterozoic

Zircon U-Pb ages of Paleoproterozoic mafic granulites from the Huai’an terrane, North China Craton (NCC): Implications for timing of cratonization and crustal evolution history


Zhang et al


The North China Craton (NCC) is considered to have been incorporated into the Columbia supercontinent during late Paleoproterozoic. The Huai’an terrane located in north-central part of the NCC is an importan tectonic window to investigate the Paleoproterozoic crustal evolution history of this craton, particularly due to the wide occurrence of high-grade metamorphic rocks including retrograded eclogites and high-pressure granulites. Available data show two controversial peak metamorphic ages of these rocks as 1.95–1.90 Ga or ∼1.85–1.80 Ga. Thus, determining the precise timing of metamorphism of the high-pressure granulites is important in reconstructing the collisional history. In this study, we present zircon U-Pb ages, bulk chemistry, and mineral inclusions in zircons and matrix mineral assemblages from the high-pressure granulites to evaluate the timing of metamorphism. Our results reveal ca. 2.05–2.0 Ga ages from magmatic zircons in these rocks representing the timing of emplacement of mafic magmas. The metamorphic zircons yield two groups of ages at 1.95–1.90 Ga and ∼1.85∼1.80 Ga. Clinopyroxene, plagioclase, titanite, and apatite occur as common mineral inclusions within the ∼1.85–1.80 Ga metamorphic zircons. The chemical compositions of clinopyroxene and plagioclase occurring as inclusions are very similar to those of the rim compositions of those minerals in the matrix, suggesting their formation during retrograde granulite metamorphism, and are distinctly different from those of the core domains of the minerals in the matrix which represent high-pressure granulite metamorphism. These results indicate that the metamorphic zircons formed during retrograde granulite metamorphism dated as ∼1.85–1.80 Ga and the age range of ∼1.95–1.90 Ga corresponds to high-pressure granulite metamorphism. The dual nature of the geological and geochronological features suggest that the Archean gray gneisses in the Huai’an terrane represent the basement rocks detached from the Khondalite Belt of the NCC after collision. Integrating with published data, a double-side collision with scissor-like suturing model is proposed for the Paleoproterozoic cratonization of the NCC.

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