Monday, July 28, 2014

Evidence of a Very Slow Motion Continental Collision Between the Rhycian to Orosirian PaleoProterozoic

Geochronology and geochemistry of the Paleoproterozoic meta-basalts from the Jiao-Liao-Ji Belt, North China Craton: Implications for petrogenesis and tectonic setting


Li et al


Geochronology, geochemical and isotopic studies were carried out on the Paleoproterozoic meta-basalts and related rocks from the Jiao-Liao-Ji Belt, North China Craton, to understand the mantle source characteristics and the geodynamic setting. U-Pb isotopic dating using the LA-ICPMS method on zircons from the metamorphosed volcanic rocks reveals that they formed at ca. 2204 - 2158 Ma and were metamorphosed at ca. 1895 - 1919 Ma, respectively. The meta-basalts are of calc-alkali affinity and have arc-like geochemical compositions. They are enriched in LREEs ((La/Yb)N = 3.5 - 5.1) and LILEs (Sr, Ba, Rb), depleted in HFSEs (Nb, Ta, Zr, Hf, Ti), and show variable high ɛNd(t) values (+2.4 to +4.5). The basaltic rocks experienced significant fractional crystallization and minor crustal contamination during magma evolution. Modeling suggests that the parent magma of the meta-basalts was derived from 20% partial melting of spinel-garnet lherzolites that were previously metasomatized by subduction zone fluids/melts in the Late Archean. Petrological, geochronological and geochemical data suggest that the Paleoproterozoic volcanic rocks formed in an active continental margin setting and were subsequently deformed and metamorphosed to amphibolite facies due to the arc-continental collision at ca. 1.9 Ga, rather than in a continental rifting setting as previously thought by most workers.

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