利用碎屑鋯石探討菲律賓呂宋島之地體構造演化及恆春半島中新統之沉積物來源

Tectonic development and sediment provenances of the Luzon Island and Hengchun Peninsula from detrital zircon analysis

蔡佳欣(Chia-Hsin Tsai)

指導教授 ： 徐澔德

國立臺灣大學/理學院/地質科學研究所/碩士(2017年)

https://doi.org/10.6342/NTU201700466

摘要

呂宋島位在菲律賓群島最北端，其西側的南中國海海洋地殼與東側的菲律賓海板塊皆隱沒至呂宋島之下，造成此島有著相當複雜的地體構造。然而，目前呂宋島缺乏較完整的岩漿特性和年代分佈資料。另一方面，整個東南亞被視為新生代大陸地殼增生的重點地區，因此本研究為了瞭解此地的岩石地球化學特徵與岩漿演化過程，針對呂宋島河砂中的碎屑鋯石進行鈾鉛定年及鉿同位素分析。此外，根據馬尼拉海溝目前的位置，位處呂宋島北側的恆春半島理應同樣隸屬於菲律賓海板塊。這個現象讓我們想一併探討是否呂宋島亦為恆春半島的沉積物源區之一，並也想瞭解恆春半島在碰撞前的沉積機制。因此，本研究亦將恆春半島與呂宋島的碎屑鋯石進行年代及鉿同位素特徵的比對，並建立出恆春半島的古地理演化模式。
本研究在呂宋島的主要河流中採集了14個河砂樣品，它們的碎屑鋯石鈾鉛年齡顯示呂宋島最老的岩漿年代可追溯到中生代，但大部分集中於始新世至中新世和第四紀。而呂宋島碎屑鋯石的鉿同位素初始值 [εHf(T)] 全為正值（+10 到 +20），顯示此地區的岩漿主要來自虧損地函，且未受到明顯大陸地殼的混染。這個結果也顯示呂宋島確實為一大陸地殼初生的範例。此外，我們在恆春半島的中新統採集了10個砂岩樣品。相較於呂宋島，絕大部份的恆春半島碎屑鋯石鈾鉛年齡老於新生代且集中在白堊紀至侏羅紀、早二疊紀和元古宙。再者，恆春碎屑鋯石的鉿同位素初始值大部份是負值。因此，從年代與地球化學特徵的差異，本研究可以否定呂宋島為恆春半島的沉積物源之可能性。由於恆春鋯石年齡峰值多半能和中國華南主要岩漿事件年代互相對應，因此華南地區仍應該是主要的恆春半島沉積物源。經過與前人資料的對比，本研究推測恆春半島中新統的沉積物源主要應來自福建的閩江、九龍江、早期被抬升的古台灣島以及一個從華南被分離出來的大陸陸塊。然而，對於恆春半島中30-15 Ma的鋯石，本研究認為其主要來源應該是當時恆春東側之增積岩體上的混同層物質。因此，恆春半島沉積物應不僅由華南一帶提供，而是有多個來源。而在後來隱沒碰撞的過程中，這些沉積物都從海床上被刮起，併入增積岩體當中且移動至海溝的東側成了現今的模樣。

關鍵字

鈾鉛定年 ； 鉿同位素 ； 碎屑鋯石 ； 沉積物源 ； 呂宋島 ； 恆春半島

並列摘要

The Philippines and Taiwan are both in the western Philippine Sea Plate that has complicated tectonics. Located in the northern Philippine Archipelago that is an amalgamation of tectonic terranes, the Luzon Island is in the middle of two subduction zones. As the hanging wall of both sides of trench systems, the Luzon Island has an elusive tectonic structure beneath. The whole southeastern Asia is believed to be a critical area for accretionary orogeny and juvenile crustal formation in Cenozoic, but the comprehensive ages of the magmatic events and the evolution of the Luzon Island are not well constrained. In order to examine the magma properties in this area, this study analyzed U-Pb ages and hafnium isotopic ratios in detrital zircons from the Luzon Island. Moreover, north of the Luzon Island, the Hengchun Peninsula is also located on the eastern side of the Manila Trench. This implies the possibility that sediments in the Hengchun Peninsula may be originated from the Luzon Island since they both sit on the Philippine Sea Plate. Hence, this study also investigated the sedimentary relationship between the two places by the comparison of detrital zircon ages and geochemical properties. Furthermore, this study also constructed a paleogeography model for the Hengchun Peninsula.
Fourteen riversand samples were collected from the major rivers in the Luzon Island. The U-Pb ages of the detrital zircons show that the magmatic events mostly occurred in Eocene to Miocene and Quaternary in this area. However, some of the events in the central Luzon can be traced back to Mesozoic. Albeit the presence of old zircons, all the samples in Luzon Island show positive εHf(T) values that range from +10 to +20. These results indicate the Luzon Island is dominated by juvenile magma, and few continental material has been involved in the construction of this island. Thus, the Luzon Island could be an important locus for Asian accretionary orogenesis and Cenozoic crustal growth. In the Hengchun Peninsula, ten sandstone samples were collected in the Miocene Series. Their data demonstrate the detrital zircon ages are mostly older than Cenozoic and concentrate in Cretaceous to Jurassic, early Permian and even Precambrian. Moreover, the εHf(T) values for the samples in the peninsula are mostly negative. Since these data are significantly different from those of the Luzon Island, the results imply the Luzon Island is unlikely to be the sedimentary provenance of Hengchun. The zircon U-Pb ages of the Miocene Series in Hengchun could well correspond to the major magmatic events in southeastern China. Through the comparison with previous studies, the Miocene Series in the Hengchun Peninsula might come from Minjiang, Jiulongjiang, proto-Taiwan and a rifted continental fragment which is separated from SE China. Nevertheless, some of the younger zircons (30-15 Ma) should be originated from the mélange in the accretionary prism in the east. These sediments were scratched up from the sea floor and moved to the eastern side of the trench during the subduction and collision in southern Taiwan.

並列關鍵字

U-Pb dating ； Hafnium isotope ； Detrital zircon ； Sedimentary provenance ； Luzon Island ； Hengchun Peninsula

參考文獻

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