Metamorphic Characteristics Of The Paleozoic Metabasites In The Tunchang Area, East-central Hainan Island, And Its Tectonic Implication

This paper examines the metamorphic characteristics of the Paleozoic metabasites in the Tunchang area, East-central Hainan Island, China with respect to the tectonic processes in connection with the evolution of Paleo-Tethys. The petrology, the mineral paragenese and the microstructure suggested that the protoliths for these metabasites are probably composed of gabbroic rocks (coarse-grained amphibolites), gabbroic-diabasic rocks (medium to fine-grained and coarse-grained amphibolites), and pillowed lava (fine-grained and/or massive amphibolites). The overall mineral assemblages for these rocks are amphibole+ plagioclase(An=:0∽91) + chlorite + epidote, accessory phases: ilmenite + titanite ± rutile ± zircon ± tourmaline. Minor muscovite, prehnite and K-feldspar, and abundant Fe (Cu) -Ni sulphide commonly occur as secondary minerals. Most minerals, e.g., amphiboles, are characterized by at least two generations, and the well-developed ductile-brittle shear deformations are shown by the development of folds, shear foliations or cleavages, S-C fabrics, NNE to NNW-striking stretching lineations and mylonitic microstructures. The mineral chemistry of zoned amphiboles in the metabasites indicates a progressive evolution from greenschist, amphibolite facies to amphibolite-granulite facies, being consistent with increasing Ti, Al and (K + Na) from core to rim. However, Ti, Al and (K+Na) decreasing likely indicate another retrogressive evolution from amphibolite-granulite or amphibolite to low greenschist facies. The whole-rock and amphibole separate yield a Sm-Nd isochronic age of 128 ± 12 Ma, whereas the Rb-Sr whole-rock ages is 131.8 ± 6.2 Ma and 311.1 ± 7.9 Ma, repsectively. Three successive and contrasting tectono-thermal-metamorphic events are clearly identified, based on the zoned amphiboles, the mineral paragenese, the microstructure and the Sr-Nd isochronic age, as well as on the regional geology, the granitic magmatism and the tectonic development of South China. The first stage of metamorphism(~330 Ma) records (sub) greenschist-amphibolite facies conditions, probably due to sea-floor metamorphism shown by the first generation of actinolite,hornblende and orthoamphibole cores. The second stage(~310-260 Ma) of "peak" metamorphic pressure and temperature conditions cluster around values of ∽ 9 kbar and ∽ 700 ℃, which show a transitional amphibolite-granulite facies metamorphism, and are marked by the pervasive ductile-shear deformation, the nappe stacking and the pargasitic horblende-pargasite rims. The third stage(~130 Ma) relates to the regional/thermal retrograde metamorphism at P/T= ∽ 1-3 kbar/ ∽ 300-400 ℃ conditions. 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 these metabasites as an origin of intraoceanic arc/suprasubduction zone-like tectonic setting. Thus the Tunchang area metabasites most likely represent the upper layers of an incomplete, dismembered ophiolite body, owing to the northerly subduction of the eastern Paleo-Tethys.A possible model to delineate the petrogenesises of these metabasites and the Paleozoic tectonic evolution in the Hainan Island has been proposed in this paper. The model further supports that the NE-SW-trending Baisha fault of the Hainan Island is a Late Paleozoic-Earliest Mesozoic suture. The Hainan Island Block had been composed of the northwest Island (separated from South China) and the northeast Island (separeated from Indochina), which have been sutured along the Baisha fault during Late Paleozoic-Earliest Mesozoic.