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

• Zircons from an amphibolite from northeastern Taiwan are confirmed to be metamorphic.
• Initial εHf(t) values of + 19.2 establish a MORB-like source for the protolith.
• δ18O values of + 1.5 confirm in zircon confirm high-T alteration of the protolith.
• This ~ 87 Ma contact metamorphism is unrelated to the local Arc–Continent collision.

The εHf 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 εHf(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 εHf(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.

The very positive εHf compositions of zircons from amphibolites in NE Taiwan shows that they crystallized in a basalt with MORB–like compositions. The light δ18O values of the same grains show that they crystallized at high but subsolidus metamorphic temperatures in a basalt previously hydrothermally altered by seawater. These results support the argument that the basalts were metamorphosed in the contact aureoles of adjacent granitic intrusions.Figure optionsDownload as PowerPoint slide