Diversity of zircon U-Pb geochronology of meta-sedimentary rocks from the Gaixian Formation, South Liaohe Group, Jiao-Liao-Ji belt: Implications for different provenance and crustal evolution

Abstract

Comprehensive studies of depositional age and source material of the Liaohe Group are significant to the tectonic evolution of the Jiao-Liao-Ji Belt (JLJB). The Gaixian Formation is widely distributed and stratigraphically defined as the uppermost formation of the Liaohe Group. However, our new work reveals that not all meta-sedimentary rocks of the Liaohe Group deposited before the late Paleoproterozoic orogeny. Based on the geochronological results, these meta-sedimentary rocks can be subdivided into two categories. Detrital zircon from the category A meta-sedimentary rocks yield U-Pb age peaks at ca. 2.50 Ga, 2.18 Ga and 2.06 Ga. The youngest age peak of ca. 2.06 Ga, together with the metamorphic age of 1.95–1.84 Ga yielded by pervious studies, indicate their depositional ages of 2.06–1.95 Ga. Compared to the category A, detrital zircon from the category B yield similar 207Pb/206Pb age peaks at ca. 2.54 Ga, ca. 2.17 Ga and ca. 2.05 Ga, but importantly also at ca. 1.87 Ga which was never observed for the category A. The late Paleoproterozoic zircon (~1.87 Ga) include magmatic cores and weakly oscillatory zoned or unzoned rims. These rims yield two age peaks of ca. 1.88 Ga and ca. 1.80 Ga, which are interpreted as inherited partial melting ages. Therefore, the category B meta-sedimentary rocks deposited at some time after ca. 1.80 Ga. These new geochronological data reveal that the category B meta-sedimentary rocks are significantly different with those from the traditional Gaixian Formation. In combination with regional stratigraphy data, it is recommended that the category B meta-sedimentary rocks should be ascribed to the Changhai Group rather than the Liaohe Group.