The Rocks In The Coastal Range, Eastern Taiwan: Petrology, Geochemistry And Tectonic Implications

Taiwan is situated in the active subduction-collision region between the southeasternEurasian Plate and the Philippine Sea Plate. The island of Taiwan is the product of the collision ofthe Luzon Arc with the Eurasian Continent (arc-continent collision). The Coastal Range of easternTaiwan with Lutao and Lanhsu islets represents the northern segment of the Luzon Arc, which wascreated as a result of eastward subduction of the South China Sea oceanic lithosphere beneath thePhilippine Sea plate. The northern Luzon Arc first accreted to the Eurasian Continent margin andformed the Coastal Range. In the graduation thesis I study the petrography, geochemistry andchronology for the igneous rocks, sedimentary rocks and mélanges from the Coastal Range, andsummarize the temporal and spatial variation of petrographic and geochemical characteristicscombined with the tectonic background, finally conclude the evolution of the Coastal Rangeduring the arc-continent collision.The Tuluanshan Formation in the Coastal Range (Chimei and Chengkuangao volcanoes),Lutao and Lanhsu is mainly composed of andesitic rocks. But there are some differences inpetrography. Pyroxene phenocrysts are common in the andesites from the Chimei volcano besidesthe dominated plagioclase phenocrysts. But hornblende phenocrysts extensively turn up in theandesites from the Chengkuangao volcano. There is no biotite phenocryst in the andesites from theCoastal Range, while it widely turns up in the andesites from Lutao and Hsiaolanhsu. It is shownthe magma evolution could be advanced with the northern Luzon Arc getting younger southward.The temporal and spatial variation also could be shown in geochemistry. An alkali-rich trend andstronger fractionation of REE could been shown southward in younger rocks, and the values of εNdbecome more negative in younger andesites. In a word it is shown that the rocks are contaminatedby continental materials more intensively southward in younger rocks. These continental materialscould be sediments from the Eurasian continent but not the lower continent crusts by petrographic,geochemical and tectonic analyses. The magma has evolved from low-K tholeiite series tomedium-K calc-alkaline series and the arc has already reached to semi-mature stage southwardfrom an immature intra-oceanic arc. All the igneous rocks of the Tuluanshan Formation have thesame magma source forming by partial melting of lower crust with a little mantle and they aredifferent in lithology because of partial melting in varying degree.The Takangkou Formation in the Coastal Range was turbidite facies in the forearc basin,including the lower Fanshuliao Formation and upper Paliwan Formation. The sandstones in theFanshuliao Formation contain more andesitic clastics and their zircons are all magmatic zircon,forming in middle Miocene from10to8Ma mainly. It is indicated that these sediments weremainly from the Luzon Arc because of the basins were far away from the Eurasian Continent atthat time. Their sedimentation centers are in the eastern basins. The sandstones in the PaliwanFormation contain more slate and metamorphic sandstone clastics. The zircons in the PaliwanFormation are mainly magmatic zircon and some metamorphic zircons. Their U-Pb datingresults yield dominant middle Proterozoic (1300-1200Ma)、early Proterozoic (2500-1800Ma) and late Permian to Triassic (260-216Ma). These zircon are from the Eurasian Continent. Theseforearc basins were closer to the Eurasian Continent margin and the sedimentation centers weremoving westward when the Paliwan Formation formed.The Lichi mélange, contained in a fine-grained deformed matrix, consists of a jumble ofblocks of sandstone, siltstone, gabbro, diabase, granite, basalt, andesite, limestone, ophiolite, slateand metamorphic sandstone. The Lichi mélange shows varying degrees of fragmentation of strata,mixing and shearing. The petrographic and chronological evidences show that the matrix could beorigin of the forearc basin. The Huatung Ridge, a Popup formed by back-thrust in the forearcbasin now, could be the proto-mélange. The Ridge was accreted to the accretionary prismwestward with slate and matermorphic sandstone blocks from accretionary prism. At the sametime the Taiyuan forearc basin and its volcanic basement were thrust westward over the PopupRidge taking andesite, gabbro, limestone and oceanic crust blocks into the mélange.The Luzon Arc was created in middle Miocene time and its volcanism started at about16Ma.The period of initial volcanism ranges from16Ma to14Ma, and the active period ranges from13Ma to9Ma. The sedimentation in the forearc basin has began with the arc derived FanshuliaoFormation before late Miocene, which contains andesitic breccias and tuffaceous sandstones. TheChimei volcano in the north of the Luzon Arc firstly was thrusted westward onto the Eurasiancrust in late Miocene (~6.5Ma). Its volcanic activity was terminated because of moving away fromthe magma chamber in the subduction zone. The initial arc-continent collision was started. TheLuzon Arc was getting close to the Eurasian Continent and the accretionary prism was upliftedwith continuous subduction. The turbidites from the Eurasian Continent and the accretionaryprism deposited in the forearc basin and the Paliwan Formation formed. During the collision,strikep-slip faults would have been developed in the arc volcano to induce transtensionmovements and the intra-arc basins formed. The forearc basin began to thrust wesward in latePliocene. The proto-mélange Ridge formed because of the Paliwan Formation back-thrust at about3.5Ma. Then two westward thrustings of the forearc basin talked above induced the Lichi mélange.The volcanic island, forearc basin and mélange all collided north-westward with the EurasianContinent margin and they were accreted to the east of the Central Range so that the CoastalRange and the Longitudinal Valley formed at about2Ma.