Lopez-Sanchez, Marco A. Aleinikoff, John N. Marcos, Alberto Martínez, Francisco J. Llana-Fúnez, Sergio An example of low-Th/U zircon overgrowths of magmatic origin in a late orogenic Variscan intrusion: the San Ciprián massif (NW Spain) <p>Th/U values in zircon are commonly used to discriminate between metamorphic (Th/U < 0.1) and magmatic (Th/U > 0.1) origin for zircon overgrowths. We test this hypothesis in the San Ciprián massif, a late orogenic granitic intrusion in the hinterland of the Variscan orogeny. Zircon grains from this granite have cores with inherited Ediacaran ages and Th/U > 0.1, whereas zircon mantles yield an age of about 287 Ma, interpreted as the time of crystallization of the granite, and have Th/U < 0.1. Hence, the San Ciprián massif presents an uncommon but unambiguous example of magmatic zircon mantles with Th/U values typical of metamorphic zircon. The most likely causes for the unusually low-Th/U zircon values in the San Ciprián massif are a combination of a U-rich magma composition (owing to a fractionation process) and the absence of other U-enriched accessory minerals. Our work in determining Th/U ratios substantiates the warning previously made by some researchers and precludes the use of Th/U values in zircon as an unequivocal indicator of metamorphic origin in the absence of other chemical, zircon morphology or field-based independent criteria. </p> Variscan orogeny;NW Spain;massif;orogenic granitic intrusion;fractionation process;Th;zircon mantles;Ediacaran ages;zircon morphology;magmatic origin;zircon overgrowths;orogenic Variscan intrusion;San;Cipri;magmatic zircon mantles;Zircon grains;Geology 2016-06-21
    https://geolsoc.figshare.com/articles/dataset/An_example_of_low-Th_U_zircon_overgrowths_of_magmatic_origin_in_a_late_orogenic_Variscan_intrusion_the_San_Cipri_n_massif___NW_Spain_/3454124
10.6084/m9.figshare.3454124.v1