【摘要】 对冲绳海槽18个表层样品和11个柱状样品的156个层次中的火山成因斜方辉石进行了电子探针分析,计算了斜方辉石的FS值。结果表明,其FS值具有南高北低的特点,反映出在冲绳海槽从北向南岩浆性质由基性向酸性的变化。各沉积物岩芯中,斜方辉石的FS值自下而上的变化,则反映出冲绳海槽的岩浆性质从早期到晚期,具有由酸性向基性、由基性向酸性、稳定型和波动型四种变化形式。更多还原

【Abstract】 Okinawa Trough is a young back-arc basin. Studying the volcanic rocks in Okinawa Trough is one of the key points to understand the evolution of chinese marginal sea. Recently due to the limit of sampling condition,however,we can not have a integrated and general knowledge of the distribution of volcanic rocks in Okinawa Trough, which distinctly influences our discussion,based on Okinawa Trough, on the inner relationship of Ryukyu trench,Ryukyu island-arc and Okinawa Trough during their evolution, meanwhile we can also not further our understanding about the inner relationship between Okinawa Trough magmatism and submarine hydrothermal activity. In order to resolve these problems,we can supplement our cognition on the basic characteristic of volcanic activity in Okinawa Trough by realizing the information of volcanic recorded in the sediment core,for example by the research of volcanic ashes, volcanic glass and volcanic mineral. Guided by this idea, in this paper we utilize the F_S value of orthopyroxene (ferroaugite mol percent) to discuss the variation characteristics of magma from northern to southern Okinawa Trough. There are large amount of volcanogenic pyroxene in volcaniclastic sediment in Okinawa Trough. They are sandy beige-snuff color and kelly-green-bottle green. They are columnar automorphic crystals. The largest ratio of macroaxis/brachyaxis is 4∶1. Some of the crystals have glazed surface, intact crystal face and crystal edge, and some of have been seriously eroded, which results in the misproportion and rough surfaces of these crystals. Some of crystals are covered by volcanic glass and the grain diameter is 0.5? C0.25mm. In addition most of the pyroxenes have magnetite inclusions which have perfect octahedron and half expose outside of the pyroxene. Shi Jianyou studied the chemical composition of above pyroxenes by the electron microprobe. The research of the chemical composition for 203 pyroxene samples demonstrated that the pyroxene is mainly composed of the orthopyroxene, and less the clinopyroxene. In orthopyroxene, the hypersthene is the major component, the bronzite and ferrohypersthene secondly, and the least is enstatite. While the major component of the clinopyroxene is the augite, diopside secondly, as well as the least is sahlite and enstatitediopside. The volcanic-origin orthopyroxene grains from the 18 surface sediment samples and the 156 subsamples of 11 coring samples in the Okinawa Trough have been analysed by microprobe,and the F_S values of orthopyroxene grains have been calculated.The F_S values of orthopyroxene form the south Okinawa Trough are higher than that from the north Okinawa Trough, that shows, in the Okinawa Trough, from the north to the south, the magma has a variation trend from basic magma to acid magma. In coring samples,according to the variation characteristics of the F_S values of orthopyroxene grains from up to down, it is known that, the magmas in the Okinawa Trough have four kinds of variation types:from acid magma to basic magma, from basic magma to acid magma, stable type and wave tpye. The variation of Okinawa Trough magma character associates with tectonic activity, the inhomogeneity of the magma melting from mantle,the crystal differentiation inside of magma chamber as well as the number of basic material and continental material which collapses in magma channels. Many factors affect the station, which forms the particularity of the variation of magma character at each station. Although we can not receive total bulk of volcanic rock, it is possible to make some reversal development of volcanic rock character in this region by studing the orthopyroxene component in sediment, and this will provide beneficial data on the tectonic setting about this area.更多还原