Hf and O isotopic evidence for metamorphic crystallization of zircon during contact metamorphism of Fenniaolin metabasalts, Tananao complex, Taiwan

Abstract

Abstract The eHf and δ18O compositions of zircons separated from an amphibolite in the Tananao complex of northeast Taiwan have been analyzed in order to test the hypothesis that these zircons are metamorphic in origin. A previous study found the U–Pb age of 18 zircons to be 86 ± 1 Ma. The 176Hf/177Hf ratio of 13 of these zircons is rather constant with an average value of 0.283263 ± 0.000037 (2σ SD) and a weighted mean of initial eHf(t) value of + 19.2. The δ18O compositions of these same 13 grains and of 7 other grains vary from + 1.2 to + 2.0‰ with a weighted mean of + 1.5 ± 0.43‰ (2σ SD). The very positive eHf(t) values show that the protolith of this amphibolite had a primitive composition, consistent with N-MORB. The low δ18O values are not consistent with typical mantle values of ~ 5.3; rather we interpreted them to reflect metamorphic growth of zircon at greenschist facies conditions in a basalt already hydrothermally altered by seawater. In keeping with other studies, we conclude [1] that the host amphibolite was basalt of MORB composition extensively altered by seawater shortly after extrusion, [2] that the crystallization of the metamorphic zircons was caused by contact metamorphism of the host amphibolite by the 87 Ma intrusion of the adjacent granodiorite, and [3] that this metamorphism was not caused by the Neogene collision of the Philippine Sea plate with the south China margin.