Font Size: a A A

Geochemistry Of Cenozoic Alkali Basalts From The South China Sea And Its Geodynamical Significance

Posted on:2009-03-12Degree:DoctorType:Dissertation
Country:ChinaCandidate:Q S YanFull Text:PDF
GTID:1100360245958611Subject:Marine geology
Abstract/Summary:PDF Full Text Request
The South China Sea is one of the largest marginal basins in the west Pacific, which lies at triple junction among three large plates, i.e., Eurasian plate, Indo- Australia plate and Pacific plate (or Philippine plate ). A large scale intraplate volcanism after the cessation of seafloor spreading, affects a broad areas including the South China Sea itself, southeast China, Indochina peninsula and several rifted micro-continental segments in the South China Sea. A comprehensive study on Cenozoic alkali basalts is not only helpful to understand the deep mantle process of the intraplate volcanism occurred in the South China Sea, but also significant for understanding of the deep geodynamical regime for the formation and evolution of the South China Sea and a series of oil-bearing basins.The whole rock K-Ar/Ar-Ar ages for basaltic rock from the South China Sea range from 3.8 Ma to 7.9 Ma, which suggestS that they are products of magmatism since late Miocene, as is consistent with those of alkali basalt from around the South China Sea. Petrographic studies suggest that the mineral assemblage is composed of olivine, clinopyroxene and plagioclase, which is consistent with common mineral assemblage of a characteristic alkali basalt. The average value for mantle potential temperatures (Tp) beneath the South China Sea, calculated from olivine-melt equilibrium, is 1661℃, which imply that there may exist thermally anomaly in the mantle beneath the South China Sea, and provide a important evidence for the Hainan Plume. During the ascent form the magma origin to the surface, the temperatures and pressures of evolved magmas in different high level magma chambers calculating from clinopyroxene - melt equilibrium are as follows, (1) equilibrium temperatures (°K) for tephrite, trachybasalt and basalt are 1535~1498, 1429 and 1369, respectively; (2) equilibrium pressure for tephrite is 1.46~1.48 GPa , which corresponds to magma chamber at depth of ~ 49km (the middle-lower part of oceanic lithosphere); (3) equilibrium pressure for trachybasalt is 0.76 GPa , which corresponds to magma chamber at depth of ~ 25 km (the middle-upper part of oceanic lithosphere); (4) equilibrium pressure for basalt is 0.44 GPa , which corresponds to magma chamber at depth of ~ 15 km (the upper part of oceanic lithosphere). The quenching temperature of magma erupted on the surface, calculated from plagioclase– melt equilibrium, is 989℃.Major element compositions suggest that rock types are mainly tephrite, trachybasalt and basalt in subordination, and all rocks belong to alkali series. Large ion lithophile elements (LILE) and high field strength elements (HFSE), e.g., Nb, Ta, Ti and Y, are enriched, and Yb, Sc, Sr and heat-producing elements, e.g. K, Th, U, are relatively depleted. In a whole, the distributional patterns for trace elements and rare earth elements are similar to those of global intraplate OIB average value. Sr-Nd-Pb isotopic data of these basaltic rocks strongly suggest there is a binary mixing model in origin with Dupal Pb anomaly, one is a depleted mantle end-member (DMM), and the other is EM2 component which may be not result from subcontinental lithospheric mantle, but the Hainan plume originated from core-mantle boundary. Dupal anomaly is not only limited to South Hemisphere, and there have no relationship in Dupal anomaly between the South China Sea and South Hemisphere. The petrogenesis for basaltic rock described below, the Hainan plume contributes to partial melting of source rock thermally and materially. During the ascent of magma from the origin to the surface, they experienced fractional crystallization and/or cumulation to different extent in different level magma chamber, and there have no obvious mixing inprint with continental lithospheric mantle and crust.Hainan plume are introduced into a preliminary model about the formation and evolution of the South China Sea which this study newly builds. The model is as follows, (1), 50-32Ma. Integrated effects of the collision between Indian ocean plate and Euro-Asian plate and its resulting in the retrogression of Pacific plate, created a extensional tectonic setting, which provided a channel for the ascent of mantle plume. (2), 32-21Ma. When the head of mantle plume arrived at asthenosphere, it immediately interacted with the spreading center of the South China Sea by lateral material flow, which enhanced spreading spead. During 26-24Ma, there took place ridge jump, which adjusted the spreading center from nearby 18°N(i.e., present-day center of NW sub-basin) to nearby 15.5°N(i.e., present-day center of East sub-basin). (3), 21-15.5Ma. With the mantle plume effect gradually enhancing, the hotspot-spreading center interaction is more and more intensive, and about 21Ma there took place ridge jump again, and induced the open of SW sub-basin. (4), 15.5-0 Ma. Due to the collision between the predestined affinity of Indo-Australian plate and Sunda continent, the spreading stopped. And subsequently earlier formed oceanic crust subducted along Nansha trench and Manila trench. However, the mantle plume still existed up to now. Since Pliocene A large amount of alkali basalt erupted in the South China Sea and its adjacent areas are a actual evidence.
Keywords/Search Tags:K-Ar geochronology, Mineral chemistry, Olivine, Geochemistry, Sr-Nd-Pb isotopes, Dupal anomaly, Hainan plume, the formation and evolution of the South China Sea, the South China Sea
PDF Full Text Request
Related items