| Summary: | 碩士 === 國立成功大學 === 地球科學系 === 103 === Geochemical studies on the lavas from the northern end of the North Luzon arc (NLA) at eastern Taiwan are scarce and have rarely been included in the NLA evolution history. Lai and Song (2013) combined lithofacies analysis, bathymetric and isotopic data to propose four volcanic centers in the Coastal Range. However, the geochemical features and significance of these volcanic
rocks have not been addressed in detail. In addition to those in the Coastal Range, volcanic breccias also occur ~8 km east of the middle-south section of the Coastal Range on the coastline at Sansiantai (SST). These volcanic breccias are fresher but have been inferred as exotic blocks with unidentified source. In this study, the petrography, mineral chemistry, major and trace
elements, and Sr-Nd-Hf isotopic compositions of the volcanic rocks from SST were determined and compared to those of the lavas from the aforementioned volcanic centers to trace the sources. The SST volcanic rocks are characterized by breccias of centimeter to meter size imbedded in greenish or brownish tuff. Based on their colors, these breccias were tentatively classified into “monolithologic” and “polylithologic” types. The monolithologic breccias (MB) are relatively fresh and mainly blackish, while the polylithologic breccias (PB) are characterized by varying colors, possibly representing products from different eruptive episodes. The tuff samples were also analyzed to provide constraints on the genesis of the breccias. The SST volcanic rocks contain olivine, plagioclase and two pyroxene phenocrysts without hornblende, contrasting to the occurrence of hornblende phenocrysts in the samples from the Coastal Range. The tuff, however, contains higher proportions of clay minerals, reflecting higher degrees of alteration. The outcrop occurrences and the petrography in the section where tuff and breccias intersected suggested a pyroclastic flow origin for the SST volcanic rocks. Sansiantai is adjacent to the Chengkuangao volcano. Nevertheless, the differences in major element and trace element compositions as well as isotopic signatures between Sansiantai and Chengkuangao samples (Lai, 2012) imply distinct magmatic sources. The compositional variations and fractional crystallization modeling also suggest that the volcanic rocks from the four volcanoes and the SST area cannot be related to each other by fractional crystallization processes. Source contamination modeling suggests that the isotopic signatures of Sansiantai samples can be explained by addition of authigenic sediment components into the depleted mantle. Nevertheless, the role of altered oceanic crust (AOC) is also an essential component in the sources of the NLA lavas.
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