SHRIMP U-Pb zircon age of gabbro in Aoyougou in the western segment of the North Qilian Mountains, China and its geological implications.

Abstract: SHRIMP U-Pb zircon ages of gabbro at Aoyougou in the western segment of the North Qilian Mountains are reported in this paper. The zircon crystals separated from the gabbro have a euhedral-subhedral shape and a loosely striped inner structure on cathodoluminescence (CL) images. A total of eight spots were analyzed by SHRIMP U-Pb dating and the Th/U ratios at all the analyzed spots are very high, mostly >0.8. The apparent 206Pb/238U ages of all the eight analyses are consistent within errors, with a weighted mean 206Pb/238U age of 503.7 ± 6.4 Ma. The results are significantly different from the SHRIMP U-Pb zircon age data of diabase dike samples collected from the same site reported by previous researchers. Based on the available research results obtained in the area, the authors conclude that: (1) gabbro and diabase in the Aoyougou area were formed in the early Paleozoic rather than the early Mesoproterozoic; and (2) the zircons separated from diabase are usually very complex in origin and much caution is needed in the interpretation of their age data, but, on the contrary, zircons from gabbro are generally of simple and unequivocal origin and consequently their U-Pb ages are more reliable with unambiguous geological significance.