Zircon U-Pb and Hf isotope constraints on the petrogenesis of igneous rocks in eastern Taiwan
Date Issued
2015
Date
2015
Author(s)
Shao, Wen-Yu
Abstract
This thesis reports new zircon U-Pb and Lu-Hf isotopic data to better understand the petrogenesis of igneous rocks from eastern Taiwan. These include (1) volcanics from the Coastal Range, Lutao and Lanyu, (2) ophiolitic rocks from the Lichi melange, and (3) glaucophane-bearing rocks from Juisui area, the Yuli belt. Magmatic zircons yielded mean 206Pb/238U ages of 9.2 ~ 4.2 Ma for Coastal Range volcanics, ~2.6 Ma for Lanyu volcanics, ~1.3 Ma for Lutao volcanics, 17.5 ~ 14.1 Ma for east Taiwan ophiolitic rocks, and 16.0 ~ 15.4 Ma for the protolith of Juisui blueschists, respectively. We speculate that the subduction of South China Sea plate beneath the Philippine Sea plate has been initialized since ~20 Ma, and the mean inherited zircon 206Pb/238U ages of ~14.2 Ma from the Chimei igneous complex might be the earliest product of north Luzon arc magmatism. In general, magmatism in the Coastal Range had been terminated soon after the Penglai orogeny (6 ~ 5 Ma) and the final magmatism occurred at ~4.2 Ma. However, the arc magmatism of Lanyu and Lutao still spanned from Pliocene to Pliestocene. Besides, abundant inherited zircons have been obtained from the Chimei igneous complex, Yuemei volcanics, Chimei volcanics, and Lanyu volcanics, providing significant evidence for the existence of Cathaysia-affinity continental fragments rifted by the opening of the South China Sea were underneath the Luzon volcanic arc. The presence of abundant inherited zircons in volcanics suggests the magmas ascent though the old continental crust, feeding the Miocene arc, picked up the ancient zircons through assimilation or partial melting. Based on the increasing zircon uranium concentrations and the decreasing zircon εHf(T) values, we speculate that the continent-ward approach of the Manila trench allowing much more enriched continental material to involve into the mantle wedge. The magmatism of the east Taiwan ophiolite occurred during mid-Miocene, including gabbros of 17.5 ~ 17.4 Ma, diorite of 14.3 Ma and plagiogranites of 14.3 ~ 14.1 Ma. Based on the abundant inherited zircons from the ~17.5 Ma gabbro, we suggest a process of crustal contamination in ETO’s magmas during ~17.5 Ma. Moreover, the zircon Hf isotopic data indicated that the magmas were more depleted through time. In this study, we consider that the ETO originated from the highly depleted forearc magmas due to the subduction initiation of the South China Sea plate and incorporated into the Lichi melange as allochthonous blocks during the late Miocene-early Pliocene Penglai orogeny. The magmatic ages of protolith of Juisui blueschists can be well-constrained at 16.0 ~ 15.4 Ma by the zircon U-Pb ages, instead of exotic/tectonic blocks that were thought to be mid-Cretaceous or older ocean-floor fragments. Thus, we speculate that the Yuli metamorphic belt should have taken place after mid-Miocene. Based on the zircon εHf(T) variations (~32 ε-units) and whole-rock geochemistry (high MnO content), we suggest that the protolith of Juisui blueschists was product which mixed from depleted magmas (mantle-derived), enriched continental material, and Mn-rich deep-sea sediments. The protolith had undergone a high-pressure blueschist facies metamorphism and eventually rapidly exhumed and accreted to the eastern slope of the Central Range during the late Cenozoic. Briefly, the consensus of “paired metamorphic belts” of Taiwan should be thus falsified.
Subjects
zircon U-Pb age
zircon Hf isotope
north Luzon volcanic arc
east Taiwan ophiolite
Juisui blueschist
Type
thesis
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