Mineral and whole rock geochemistry of the spinel peridotites from the East Taiwan Ophiolite, Southeast Taiwan

The East Taiwan Ophiolite (ETO) is hosted within Miocene and Pliocene turbidites and agglomerates of southeast Taiwan. The ophiolite suite of rocks is dismembered and appears as coherent ';blocks' within the Lichi mélange distributed around the southern Coastal Range. The original stratigraphy of the ETO, as exposed in the larger blocks, consists of a sequence of incompletely metamorphosed gabbro, diabase and peridotite capped by pelagic red shale and an overlying sequence of originally glassy basaltic pillow lavas and volcanic breccias with intercalated red shale. The spinel peridotites consist mostly of serpentinized harzburgite with minor amounts of lherzolite. The peridotites have bulk rock Al2O3 content between 5.5 wt% and 8.9 wt% and Mg-numbers between 90.1 and 91.3 whereas the Cr-numbers of the spinel range between 40 and 55. The initial results indicate the peridotites were formed at an ocean-ridge setting as they are similar to other peridotites dredged at ocean-ridge settings and collected from ophiolites interpreted to be ocean-ridge settings. A previous interpretation suggests that the ETO represents the western terminus of the Philippine Sea plate and was analogous to mature, marginal oceanic crust however recent work based on geochemistry of the basaltic rocks suggests that the ETO formed in a mid-ocean ridge environment related to the opening of the South China Sea during the middle Miocene. The new geochemical data from this study are consistent with a mid-ocean ridge setting and that the ETO likely represents new oceanic crust formed during the opening of the South China Sea.