||The formation of ophiolite recorded the processes of growth, movement, subduction, and obduction of oceanic crusts. The occurrence of ophiolite is thus an important evidence of the movement of plates. Geochemical characteristics of ophiolite recorded different tectonic environments, such as the ocean ridges, volcanic arcs and plumes for the growth of ophiolite. Study of alteration and secondary mineral assemblages of rocks in an ophiolite suite can help us to understand the evolution of ophiolite. Similar evidence of the movement of plates, the Eastern Taiwan ophiolite (ETO) was discovered in the Guanshan area of Eastern Taiwan. Previous researches suggested that the ETO went through oceanic-ridge axis metamorphism under greenschist facies to amphibolite facies conditions, and later was affected by off-axis metamorphism under zeolite facies condition. The ETO was emplaced into Lichi mélang during the expansion of the South China Sea. In this research, we studied petrography, minerals composition, whole rock composition, and isotope geochemistry of the ETO rocks in order to understand their geochemical and mineralogical characteristics, and how the alterations of ETO affected the element mobilization and redistribution.|
The major rock types of the ETO samples in this research are serpentinites, gabbros and basalts. The protoliths of the serpentinites include harzburgite and wehrlite. The gabbros are classified as bytownite gabbro, diopside gabbro, hornblende gabbro, norite and gabbro. The basalts are classified into dolerite, basalt and glassy basalt. The serpentinized wehrlite was also overlapped by rodingitization. The gabbro metamorphism under amphibolite facies and then affected by uralitization and rodigitization. The basalts were mainly affected by metamorphism under zeolite facies to green schist condition, hornblende and tschermakite could also be found partially. The veins of K-feldspar and analcime are common in the seriously altered basalt samples from the south Jiawu stream. The glassy basalts could be divided into fresh glass, palagonite and microcrystalline basalt, and part of the glassy basalts contain amygdule of gyrolite, thomsonite and analcime.
The serpentinization released Mg and Fe, and enriched Cs, Ba, Sr, Pb, and Eu. The rodingitization reduced SiO2, increased CaO, meanwhile depleted Rb, Th, U, Sr, Nb, and Ta, and enriched Ba and Eu. The palagonitization depleted SiO2, Al2O3, CaO, Na2O, and MgO; increased H2O, TiO2 and FeO, and enriched Cs, Rb, Ba, Th, U, Nb, Ta, K and Pb. The serpentine replaced by talc was required the participation of SiO2 and its low concentration of rare earth element represented that the protolith was more depleted ultramafic rock. The relationship between the degree of alteration and elements mobility showed that K-feldspar and zeolite were the major minerals of hosting Cs, Rb, Ba and Sr. Besides, Eu and Sr were related with alteration of plagioclase. The plots of spinel composition, trace elements, and Pb isotopes showed that the ETO might have the same origin as Philippine ophiolite. And the tectonic environment of ETO may be affected by suprasubduction zone or volcanic arc environments.