Mesozoic, not Paleoproterozoic SHRIMP U-Pb zircon ages of two Liaoji Granites, eastern block, North China craton

Abstract

The Paleoproterozoic Jiao-Liao-Ji orogen in the North China craton is composed mainly of the Liaohe Group (and its equivalents) and the Liaoji Granites. The latter have long been considered to be Paleoproterozoic in age, but have not been precisely constrained by reliable isotopic ages, CL and BSE images reveal the existence of both magmatic and xenocrystal/inherited zircons in one rock sample each from the Dandong and Gaoliduntai granites, two major Liaoji Granite intrusions. The magmatic zircons occur as either single grains or rims surrounding or truncating xenocrystal/inherited zircon cores, and are characterized by euhedral, prismatic grains with well-developed oscillatory zoning, whereas the xenocrystal/inherited zircons occur either as cores surrounded by the igneous zircon rims, or as single grains exhibiting weakly zoned bands. The magmatic zircons yielded ages of 157.4 ± 5.7 Ma and 156.3 ± 4.8 Ma for the Dandong and Gaoliduntai granites, respectively, indicating that these two plutons were emplaced in the Mesozoic, not in the Paleoproterozoic as previously considered. The xenocrystal/inherited zircons in these samples yield ages ranging from 2343 to 1971 Ma, much older than that of the magmatic zircons. Although the precise ages of other Liaoji Granite bodies remain unknown, SHRIMP U-Pb zircon data for one sample of each of the Dandong and Gaoliduntai granites require a re-evaluation of previous tectonic models for evolution of the Paleoproterozoic Jiao-Liao-Ji orogen. © 2004 by V. H. Winston and Son, Inc. All rights reserved.