Zircon U-Pb chronology of Longshan complex by LA-ICP-MS and its geological significance
陇山杂岩的LA-ICP-MS锆石U-Pb年龄及其地质意义

Longshan complex located in the southwestern margin of North China Craton ( NCC) , is a set of metamorphosed volcanic-sedimentary assemblage, including granitic gneisses, amphibolite, aluminum gneisses and marbles. Its tectonic position is comparable with the Lesser Qinling zone to the East. The granitic gneisses from Longxian -Guanshan cross section are geochemically high in SiO2 and Al2O3 contents, and Na2 O/K2 O (>2.0) , Sr/Y (>20) and La/Yb (?) (>10) ratios, resembling the geochemical characteristics of Archaean TTG ( trondhjemite-tonalite-granodiorite) suite. The in situ U-Pb dating on zircons by LA-ICP-MS, yields three age groups at 1. 90Ca ( 1. 90 - 1. 95Ga) , 2. 35Ga ( 2. 35 - 2. 40Ga) and 2. 50Ga (2.45 - 2. 50Ga). The analytical points for the first age group have a Th/U ratio ranging from 0 to 0.08, suggesting their metamorphic origin, whereas the analytical points for the last two age groups have Th/U ratios higher than 0. 1, implying their igneous origin. And the 2. 5Ga. represents the formation age of tonalitic gneisses. The geochemical and geochronologic results are closely related to the temporal evolution of the basement of the North China Craton, indicating that Longshan complex, together with Taihua complex, Sushui complex, Dengfeng complex, should make up the basement outcrops along southern margin of the North China Craton ( NCC). The extensive and intensive 1. 9Ga metamorphic events indicate the south margin of Ordos block are also affected by the Pre-Rodinia supercontinent event.