The formation time of the Fengshuigouhe (风水沟河) Group in the northwestern Lesser Xing'an Range (小兴安岭),NE China,remains controversial owing to the lake of the precise dating data.This article reports zircon U-Pb ages for the leptynite and gneissic granitoids from the Fengshuigouhe Group in the northwestern Lesser Xing'an Range.The aim is to constrain the formation time and provenance of Fengshuigouhe Group.Field observation indicates that the Fengshuigouhe Group consists of a suit of metamorphic rocks (leptynite) and gneissic granitoids intruding the leptynite,and that both of them are cut by late granitic pegmatite.Zircons from two leptynites are euhedral-subhedral in shape and display oscillatory zoning in CL (cathodeluminescence) images.These detrital zircons give weighted mean ages of 255,291,321,361,469,and 520 Ma.The youngest age of them is interpreted to maximum depositional age of the protoliths of these leptynites.Zircons from gneissic granites are euhedral and subhedral in shape and exhibit typical oscillatory zoning in CL images.The dating results indicate that the gneissic granites were formed in the Early Jurassic (185±2 Ma).Zircons from the late granitic pegmatite are subhedral in shape and exhibit two types in CL images:structureless and oscillatory zoning.The former gives a weighted mean age of 143±1 Ma,considered to represent the timing of crystallization of the pegmatite,the latter yield several groups of ages:178,273,319,482,611,and 788 Ma,representing the crystallization age of inherited or captured zircons entrained by the pegmatite.Taken together,we conclude that the Fengshuigouhe Group in the northwestern Lesser Xing'an Range formed between Late Paleozoic (255 Ma) to Early Mesozoic (185 Ma),rather than Neoproterozoic as previously believed,and that the sediments in the Fengshuigouhe Group were sourced directly from geological bodies in the study area and adjacent regions.
