博士 / 國立臺灣大學 / 地質學研究所 / 88 / Being part of an active mountain belt formed by oblique collision of the Luzon arc with Asia, northern Taiwan and offshore islets mark with a series of late Pliocene-Quaternary volcanoes, named Northern Taiwan Volcanic Zone (NTVZ), whose eruptions have been conventionally ascribed to westward propagation of the Ryukyu volcanic arc. Based on comprehensive sample collection and high-quality chemical analysis, this study presents overall geochemical characteristics of the NTVZ volcanic rocks for better understanding their petrogenesis and relation with the tectonic evolution of the Taiwan mountain belt and nearby Ryukyu subduction system. Consequently, it is proposed that this young volcanic zone resulted from post-collisional lithospheric extension associated with collapse of the northern Taiwan mountain belt.
The NTVZ began around late Pliocene (~2.8~2.6 Ma) in the Mienhuayu, Chiweiyu and Tatun volcano group and was active in many localities in the Quaternary. The youngest eruption has been dated as ~0.11 Ma in the Tatun Volcano Group. The NTVZ is composed dominantly of calc-alkaline andesites and basalts, with subordinate low-K tholeiitic, high-K shoshonitic, and ultrapotassic rocks in several outcrops. From the Chiweiyu in the northeast to the Tsaolingshan in the southwest, the NTVZ mafic rocks display a spatial geochemical variation characterized by a southwestward increase in the potash content coupled with silica undersaturation. Additionally, a southwestward increase in enrichment of the large ion lithophile elements (LILE) and light rare earth elements (LREE) is also observed. Except for the Mienhuayu magmas, all NTVZ volcanic rocks show "arc-like" geochemical features represented by significant enrichments of LILE and Pb enrichment and depletion of the high field strength elements (HFSE). This indicates that their mantle source has been affected by subduction-related processes. In addition to such a 烞ubduction component, trace element and Nd-Sr isotope systematics of the NTVZ magmas suggest involvement of two mantle source components, represented by the Mienhuayu and Tsaolingshan lavas, respectively, in magma generation. The Mienhuayu rocks are high-Mg basaltic andesites (SiO252.4~54.1wt.%; MgO5.9~8.1wt.%) which have a rather flat spidergram pattern and high Nd isotope ratios (Nd+5.1~+7.2). Thus, they are most likely derived from decompression melting of an ascended asthenospheric mantle, which is similar to the mantle source of E-MORB. On the other hand, the Tsaolingshan lavas are high-Mg potassic absarokites (MgO14.2~16.3wt.%; K2O2.9~5.6wt.%), marking with extremely enriched LILE and Pb, moderately enriched LREE and relatively depleted HFSE and HREE. These geochemical features are similar to certain post-collisional potassic magmas from the Tethyan orogenic belt. Showing the highest Sr and lowest Nd isotope ratios (Nd-1~+0.8; 87Sr/86Sr0.705402~0.705509) among the NTVZ volcanics, the Tsaolingshan magmas are therefore interpreted as products of small degree melting from a phlogopite-bearing harzburgite lithospheric mantle source metasomatized recently by hydrous fluids from the Ryukyu subduction zone. In the Nd-Sr isotope correlation diagram, the Mienhuayu and Tsaolingshan magmas define the "end-members" so that other NTVZ volcanics plot between a binary mixing trend. This suggests that the NTVZ magmas may be explained as derived from interaction between the depleted asthenospheric and enriched lithospheric mantle sources. Accordingly, a two component partial melting calculation is performed for modeling the incompatible trace element variation patterns of the NTVZ volcanic rocks in the hope to explain the observed spatial geochemical variation.
Whereas the collisional tectonism is still undergoing in central and southern Taiwan, structural and seismological data demonstrate that the northern part of the Taiwan mountain belt has been subjected to an extensional deformation in the Quaternary. Thus, the northern Taiwan orogen has virtually gone through mountain''s build-up and is in the process of collapsing. Geological and seismic data from offshore regions, furthermore, show that the northern Taiwan mountain belt has stepped into extensional collapse since the Plio-Pleistocene boundary accompanying widespread magmatic activities. Therefore, it is proposed that the NTVZ resulted from post-collisional lithospheric extension owing to collapse of the northern Taiwan mountain belt, but does not represent part of the western Ryukyu volcanic arc. Derived from the decompression melting of an ascended astheosphere, emplacement of the Mienhuayu magmas suggests that extensional collapse of the northern Taiwan mountain belt may have been related to removal or delamination of the basal lithospheric mantle. As a result, the asthenospheric upwelling caused not only decompression melting but also perturbation of the geotherm that hence induced partial melting of the metasomatized lithospheric mantle to produce the NTVZ magmas. The spatial geochemical variation observed in NTVZ, which might be explained by southwestward decrease in degrees of partial melting in the mantle source region, reflects southwestward weakening of extension due to ongoing collision in the central-southern Taiwan mountain belt. The post-collisional extension in northern Taiwan, furthermore, might have played a role in reactivation of opening in the middle Okinawa Trough, and gave way to its rapid southwestward propagation with associated development of the Ryukyu subduction zone west of ~124?. This nascent subduction, in turn, resulted in abundant submarine volcanoes which delineate an embryo volcanic front along the southern margin of the southernmost Okinawa Trough. In this case, the southernmost part of Okinawa Trough is not a "fore-arc" basin as previously alleged, but represents an "atypical" back-arc basin which developed broadly synchronously or prior to its arc-trench system in this particular collision/extension/subduction tectonic environment.
| Identifer | oai:union.ndltd.org:TW/088NTU00138001 |
| Date | January 2000 |
| Creators | Kuo-Lung Wang, 王國龍 |
| Contributors | Sun-Lin Chung, Chang-Hwa Chen, 鍾孫霖, 陳中華 |
| Source Sets | National Digital Library of Theses and Dissertations in Taiwan |
| Language | zh-TW |
| Detected Language | English |
| Type | 學位論文 ; thesis |
| Format | 169 |
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