Identification of 2.5 Ga Detrital Zircons from Conjoint Area Between Qilian-Qinling Orogenic Belts and Its Geological Implication

Through LA-ICP-MS U-Pb dating of a large number of detrital zircon grains from metamorphic sandstones of Huluhe Group in the Huluhe River of the Qin'an area,the conjoint area between Qilian and Qinling orogenic belts,2 302～2 578 Ma zircon grains were identified.Their 207Pb/206Pb and 206Pb/238U ages coincide well,and CL images exhibit well-developed oscillatory zoning and the Th/U ratios are within 0.11～1.42,implying that they are tectonomagmatic zircons which formed during late Newarchaeozoic.These zircons are the oldest ones discovered in the conjoint area between Qilian and Qinling orogenic belts up to now,which suggests the presence of crust older than high-grade metamorphic Longshan and Qinling groups in the conjoint area between Qilian and Qinling orogenic belts.