Tuesday, January 05, 2016

Contrary Evidence From North China Craton That Plate Tectonics Didn't Start Until MesoProterozoic

Detrital zircon U-Pb, Lu-Hf, and O isotopes of the Wufoshan Group: Implications for episodic crustal growth and reworking of the southern North China craton


Zhang et al


The Paleoproterozoic orogens of the North China Craton (NCC) provide important windows to evaluate the crustal evolution history during early Precambrian. Here we report results from SIMS zircon geochronology and oxygen isotopes, as well as LA-ICPMS zircon Hf isotopic analyses on detrital zircons from Mesoproterozoic-Neoproterozoic sandstones of the Wufoshan Group from the southern NCC to probe the Paleoarchean-Paleoproterozoic crustal evolution. Our data show episodic magmatism during the Paleoarchean-Neoarchean and Paleoproterozoic as inferred from the 207Pb/206Pb age range of 3514-2751 Ma and the well-defined 207Pb/206Pb weighted mean ages of 2669 ± 15 Ma, 2510 ± 8 Ma, 2402 ± 39 Ma, 2298 ± 36 Ma, 2179 ± 10 Ma, 2058 ± 16 Ma, 1940 ± 6 Ma, 1874 ± 7 Ma and 1819 ± 5 Ma. The similarity in U-Pb geochronology and Hf isotopes of the Archean detrital zircons from the Wufoshan Group to those from the Paleoproterozoic Songshan Group reveal that the 2669 Ma zircons record a major phase of Neoarchean crustal growth, and that 2510 Ma zircons correspond to the latest Neoarchean crustal reworking. The 18O-depleted isotopic features displayed by some detrital zircons are absent in those from Songshan Group, and together with the rarity of core-rim textures which were widespread in Archean zircons from the Songshan Group, we suggest that the source materials for the Mesoproterozoic-Neoproterozoic Wufoshan Group are different from the Archean cratonic source for the Paleoproterozoic Songshan Group. We propose that the zircons were probably sourced from the newly-formed Trans-North China Orogen (TNCO). Our Paleoproterozoic zircon data provide robust evidence for continuous and complex reworking of the Mesoarchean to early Neoarchean crust in the southern NCC during Early Paleoproterozoic at ca. 2402 Ma, 2298 Ma, and 2179 Ma. The Middle Paleoproterozoic ages (2058 Ma, 1940 Ma, and 1874 Ma) mark a partial cycle of oceanic crust subduction to continental collision during the formation of the TNCO. The extreme oxygen isotopic variation with the fairly restricted range in Hf isotopes demonstrates that the components from both high-temperature and the low-temperature rock-water interaction induced by the release of hydrothermal fluids from the subducted altered oceanic crust must have played a significant role during the growth of the zircons. The Paleoproterozoic thermal event at 1819 Ma records post-collisional uplifting, as evidenced by an apparent negative correlation in the ɛHf(t) vs. δ18O plot, possibly inherited from the mixing of the ancient crust and the juvenile input during the oceanic subduction and continental collision. Integrated with the information from granulite facies metamorphic rocks in the orogenic root of the TNCO, our data suggest that the episodic Paleoproterozoic magmatism was mainly induced by the tectonics of plate subduction and subsequent continental collision, leading to the final cratonization of the NCC. Following the Mesoproterozoic-Neoproterozoic sedimentation of the Paleoproterozoic zircons, except for the Pb loss during the subsequent thermal events, there was no major alteration in the Hf and oxygen isotopic compositions. Thus, the remarkably large variation of oxygen isotopes from the lower than normal mantle to the supracrustal value (δ18O = 2.28 to 10.09 ‰) in the Paleoproterozoic detrital zircons further demonstrates that these zircons came from the uplifted and exposed upper part of the TNCO. Our detrital zircon data provide convincing evidence for the repeated crustal reworking in Early Paleoproterozoic and the onset of modern-style plate tectonics in the NCC in Middle Paleoproterozoic.

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