U-Pb Isotope Geochronology and Granitic Gneisses Nanoparticles from the Xiaomei Shear Zone: Implication for Caledonian Tectonic in the Hainan Island, China

Abstract

U-Pb ages of zircons from the granitic gneisses in the Xiaomei shear zone, Hainan Island, provide constraints on the age of granitic gneisses and tectonic evolution of Caledonian orogeny in Hainan Island. Zircons extracted from granitic gneisses are rounded and subrounded and characterized by oscillatory overgrowths enclosing inherited cores. The Early Devonian (414–411 Ma) ages of inherited zircons from the Xiaomei granitic gneisses are consistent with tectonic event that uplifted the Hainan island, resulting in the absence of Devonian strata. Meanwhile, nanoparticles are found in the granitic gneisses, including agglomerated nanoparticles and spherical nanoparticles in the ductile shear zone. The spherical nanoparticles in the shear zone are believed to have experienced a two-stage formation, from the linear spherical nanoparticles to planar spherical nanoparticles. With rise in temperature due to frictional heating during the shearing, spherical nanoparticles are deformed to agglomerated nanoparticles and the inherited zircons probably are recrystallized at 411 Ma.