Abstract:

Basic dikes occurring within the Dongfangzhongsha farmland in SW Hainan Island are mainly composed of plagioclase, monoclinic pyroxene and a small amount of hornblende. These basic dikes are charactered by such geochemical features as SiO₂=52.39%-53.44%, K₂O+Na₂O=5.02%-5.71%, K₂O=2.45%-3.29% and high K₂O/Na₂O value of 0.91-1.63. These rocks are also featured by enrichment in LREEs and LILEs, slight Eu negative anomaly (δEu=0.76-0.84), insignificant Sr and P negative anomalies as well as significant Nb-Ta and Ti negative anomalies. These signatures show these basic dikes may come from fluid metasomatic mantle resource. The representative basic dike (11HN31A) yielded the LA-ICP-MS zircon U-Pb ages of (242.2±2.6) Ma with MSWD=0.074 (n=14), interpreted as the crystallization age, which is consistent with those in east Hainan Island. Our new data, together with the available data, indicate the Indosinian basic dikes formed under the post-orogenic extensional tectonic setting in Hainan Island,in Early Triassic (-240 Ma).

Key words: basic dikes, elemental geochemistry, LA-ICP-MS zircon U-Pb dating, SW Hainan Island