U-Pb geochronology of hydrothermal zircon from the Mesoproterozoic Gaoyuzhuang Formation on the northern margin of the North China Block and its geological implications

Abstract

This paper reports LA-ICP-MS zircon U-Pb ages and REE compositions on detrital zircons (Type 1) and hydrothermal zircons (Type 2) from low-metasedimentary rocks in the Mesoproterozoic Gaoyuzhuang Formation, Pingquan area, Hebei Province that provide important constraints on the tectonic evolution along the northern margin of the North China Block. The detrital zircons are characterized by an oscillatory magmatic core, surrounded by a narrow structureless rim in CL images. They yield 207Pb/206Pb ages ranging from 1703 to 2543 Ma with two age peaks at 2473 and 1794 Ma, which is consistent with the Neoarchean and Paleoproterozoic tectono-thermal events recorded in the NCB basement, indicating that the source of the detritus was locally derived. By contrast, the hydrothermal ones are euhedral, sector zoning or internally structureless. They have relatively higher U and Th contents and Th/U ratios (U=139–2918 ppm, Th=35–1327 ppm, Th/U (average) =0.78 vs. U=15–1044 ppm, Th=8–341 ppm, Th/U (average) =0.57 for detrital zircons). Moreover, they are also enriched in REEs relative to detrital zircons and show a lower positive Ce anomaly (ΣREE = 659–2418 ppm vs. 231–611 ppm for detrital zircon; Ce/Ce*=2–13 vs. 33–174 for detrital zircons), similar to known hydrothermal zircons derived from many locations. These characteristics, combined with our field petrographic observations, indicate that the hydrothermal zircons possibly formed from a low temperature aqueous fluid. Twelve concordant or near-concordant analyses on hydrothermal zircons yield weighted mean 206Pb/238U ages of 325–327 Ma, which provide unambiguous evidence that the northern NCB underwent late Paleozoic low temperature hydrothermal modification. This timing of this hydrothermal event is compatible with that of the late Paleozoic magmatic and metamorphic-deformational events occurred on the northern margin of the NCB, it appears likely that the hydrothermal activity resulted from southward subduction of the Paleo-Asian Oceanic plate underneath the NCB during late Paleozoic.