METAMORPHIC CHARACTERISTICS OF THE CHENXING METABASITE MASSIF IN TUNCHANG AREA, HAINAN ISLAND, SOUTH CHINA AND ITS TECTONIC IMPLICATION

Abstract

The petrology, mineral paragenese and microstructure suggest that the protoliths for the Chenxing metabasite massif in Tunchang area, Hainan Island probably are composed of gabbroic rocks (coarse-grained amphibolites), gabbroic to diabasic rocks (medium to fine-grained and medium to coarse-grained amphibolites), and pillowed lava (fine-grained and/or massive amphibolites). These rocks are characterized not only by main mineral assemblages of amphiboles, plagioclase and chlorite which show at least two or more than two generations, but also by well-developed ductile-brittle shear deformations, being related to regional/thermal metamorphism and/or sea-floor metamorphism. The mineral chemistry of zoned amphiboles in the metabasites indicates a progressive evolution at early stage, being consistent with increasing Ti, Al and (K + Na) from core to rim. By contrary, the reverse trend for Ti, Al and K + Na contents reflects another retrogressive evolution. The whole rock and amphibole separate yield a Sm-Nd isochronic age of 128 ± 12 Ma, whereas the Rb-Sr whole-rock ages are 131.8 ± 6.2 Ma and 344 ± 11 Ma, respectively. Three successive and contrasting tectono-thermal-metamorphic events are clearly identified. The first stage of metamorphism ~527 ~450 Ma) records (sub) greenschist-amphibolite facies conditions, due probably to sea-floor metamorphism, being shown by the first generation of actinolite, hornblende and orthoamphibole cores. "Peak" metamorphic pressure and temperature conditions clustering around values of ~9 GPa and ~700°C indicate a transitional amphibolite-granulite facies metamorphism, being marked by the pervasive ductile-shear deformation and the pargasitic horblende-pargasite rims related to the subduction and collision event of ~330 ~240 Ma age. The third stage is associated with the regional/ther Mal retrograde metamorphism at P/T = ~0.1 ~0.3 GPa/~ 300 ~ 400°C conditions, due to emplacement of the Yanshanian granites (~130 ~90 Ma). All the study point to pressure increasing with increasing temperature on the prograde metamorphic path, followed by retrograde metamorphism (i. e. an anticlockwise P-T-t path), which characterized the Chenxing metabasite massif most likely as the relics of an Ordovician oceanic crust due to rifting and oceanization from subduction of the Paleo-Pacific ocean, and to closure of the young ocean and subsequent collision of the South China (including Hainan Island) with the Indochina.