海南岛晚古生代裂谷作用

THE LATE PALAEOZOIC RIFTING IN HAINAN ISLAND, CHINA

海南岛于晚古生代发生了裂谷作用。在石炭纪发育了由玄武岩与流纹岩组成的双峰火山岩。在晚古生代地层中广泛地发育了多层受裂谷断裂控制的河流相山间型砾岩。以碎屑岩为主的晚古生代沉积中,主体部分为海相,下部及上部为陆相。碎屑岩的岩石学及岩石化学研究表明,其形成的构造背景为大陆裂谷。裂谷演化到由大陆裂谷向陆间裂谷过渡的阶段后,即告夭折,裂谷盆地为一个向西开口的海湾。

There are a lot of facts indicating the presence of Late Palaeozoic rifting in Hainan Island, China. One of the facts is the occurrence of the Carboniferous bimodal volcanic rock belt with basalt and rhyolite association, which extends in a nearly E-W direction and for about 80 km in the western part of the island. The volcanic rocks are interbedded, with metamorphosed fossiliferous sandstones, siltstones and shales, and even with conglomerates in some places. The basalts are mainly present in the west part of the belt, and the rhyolites are in the east. The maximam thickness of an interbedded rhyolite formation is about 20m, and one of the interbeded basalt formations is more than 140 m. No intermediate volcanic rocks have been found. There are gaps between 52% and 72% in the SiO_2 conents of the volcanic rocks and between 51.05 and 69.2 for their differentiation index (D. I.). Based on their characteristics in trace elements and REE the basalts are referred to as A-MORB, and are similar with those basalts located in the axis part of the Afar Rift. Conglomerates are widespread in several Late Palaeozoic formations. The thickest one is up to 300m. They are mainly of fluvial origin and their distribution is controlled by the rift faults. They may be identified as the intermountainous conglomerates, which are typical for continental rift regions. The Late Palaeozoic sandstones are mainly of marine deposition, whereas the lower part of early Carboniferous sandstone and the upper part of late Permian sandstone are alluvial and diluvial deposits as judged by grain, analysis. Petrological and petrochemical studies on the Late Palaeozoic detrital rocks indicate that they were deposited under a continental rift setting. Three sedimentary assemblages have been recognized. They are the lower detrital one of the early Early Carboniferous; the carbonate-detrital one of the middle-late Early Carboniferous to Late Permian; and the upper detrital one of the Early Trias. Such sgries are conformable to the tectonic evolutionary rule of rift. The lower detrital assemblage lies unconformably on the Palaeozoic formations and contains sandstones which are quite rich in detritus of phyllite and slates, the formation of which signifies the start of the rift event. The carbonate-detrital assemblage is composed of interbedded carbonates and detrital rocks. According to the distribution of fossils, the facies change of the carbonates and the depositional characteristics, it is suggested that the rift basin was a bay opening westward to the sea. The upper detrital assemblage is composed of conglomerates, sandstones and shales. Abundant detritus of phyllite, schist and migmatite is present in the conglomerates. There is a remarkable unconformity contact between the Triassic conglomerates and the Palaeozoic metamorphosed rocks. The formation of the third assemblage indicates the closing of the rift, accompanied by the deformation, metamorphism and migmatization of the sediments in the rift. The facts above mentioned show that the rifting was developed on the continental crust, ending at the stage of its transition from an intra-continental rift to inter-continental one.