ĐẶC ĐIỂM ĐỊA HÓA THẠCH HỌC ĐÁ MAGMA BAZAN VÀ ĐẶC ĐIỂM NGUỒN MANTI KHU VỰC BIỂN ĐÔNG VÀ LÂN CẬN TRONG KAINOZOI

The East Vietnam Sea is one of the largest marginal basins in western Pacific Ocenan, formed by breaking of continental margin in the Late Mesozoic. Geochemical data of the Miocene - Pleistocene bazanic samples collected in the East Sea and neighboring areas show two major eruption trends that reflect the formation and development of the region. The early eruption event is characterized by low alkaline, TiO2, Na2O, K2O and P2O5, and high SiO2 group, comprising olivine and tholeiitic bazans. The later eruption demonstrates high alkaline, TiO2, Na2O, K2O and P2O5, and low SiO2 group, mainly generated by central-type volcanic eruptions, consisting of alkaline olivine and olivine bazans. Distinctive geochemistry of the volcanic rocks within the East Vietnam Sea and adjacent areas is illustrated by wide range of Magnesium index (Mg#= 35-75). At the values of Mg#>65, the relation between Mg# and major oxides is unclear. In contrast, Mg#65 (Olivine differentiation) the isotope ratios start changing. The primitive components are computed based on the principle of olivine compensation. The computed results show that the critical pressure for Tholeiite melting was estimated from ~11.97-20.33 Kb (ca. 30 - 60 km deep) and the Alkaline melting pressure varies from ~16.87-34.93 Kb (corresponding to the depths of ~60 km to 100 km). The continuous range of melting pressures suggests two trends of tholeiitic and alkaline eruptions occurr at various depths in the same magmatic source. Hight temperature and melting pressure of the primitive magma are dependent on partial melting pressure. Possibly, this process was triggered by the asthenosphere intrusion resulted from the closure of the Neo-Tethys following the India - Eurasia collision. This event has not only made the mantle hotter and easily melted but also triggered the opening of the marginal seas, including the East Vietnam Sea.Biển Đông là một trong những biển rìa lớn nhất thuộc rìa tây Thái Bình Dương hình thành do phá vỡ một bộ phận của rìa lục địa vào cuối Mesozoi. Kết quả thành phần địa hóa các đá bazan tuổi Miocen - Pleitocen khu vực Biển Đông và lân cận cho thấy hai xu thế phun trào chính phản ánh quá trình hình thành và phát triển khu vực. Xu thế phun trào sớm có đặc điểm thấp Kiềm, TiO2 và P2O5 và cao SiO2 chủ yếu là các Tholeit, Olivin bazan. Xu thế phun trào muộn thường cao Kiềm, TiO2 và P2O5 và cao SiO2, chủ yếu tập trung tại các khu vực phun trào kiểu trung tâm, thành phần là các bazan Olivin và Nephenin. Đặc trưng địa hóa của đá núi lửa khu vực Biển Đông và lân cận là sự phân bố rộng của chỉ số Magie (Mg#=35-75). Tại giá trị Mg#>65, quan hệ giữa Mg# và các hợp phần oxit chính là không rõ ràng. Ngược lại, tại giá trị Mg

Establishing calculation method for chemical composition of primitive magma in the Cenozoic in South Central coast region and the adjacent continental shelf of Vietnam

The calculations which determine the chemical composition of the primitive magma are simple but they show changes in the temperature and pressure states of the magma source. The method is based on the addition of the chemical composition of the Olivine to the major element composition of the eruptive rocks which follows the formula: Ci = Ci-1+ 0.1 * Ci-1Ol. In accordance with the characteristics of the study area, we have made new additions to the calculation method. The calculation results are highly accurate when tested and compared with the chemical composition of the eruptive rocks. The chemical composition of the primitive magma solution is used to calculate the temperature and pressure states in the magma source. The results show that there is a difference in temperature and pressure in the source at different tectonic positions in the study area. Accordingly, the South Central coast region and the adjacent continental shelf are divided into two main types of eruptions. The first type of volcanic eruptions occurs at locations where major faults intersect and they are located north of the study area. The second type of volcanic eruptions in the form of a single volcano is located to the south of the study area and the southeastern continental shelf, and occurs in intracontinental extension structure

Composition of volcanic rocks from the Alchan Basin, Northwestern Primorie

Geological, petrochemical, and geochemical data are reported for volcanic rocks of a Cretaceous pull-apart basin in the Tan Lu strike-slip system, Asian continental margin. A comparison of these volcanic rocks with magmatic rocks from typical Cenozoic transform margins in the western North America and rift zones of Korea made it possible to distinguish some indicator features of transform-margin volcanic rocks. Magmatic rocks from strike-slip extension zones bear island-arc, intraplate, and occasionally depleted MORB geochemical signatures. In addition to calc-alkaline rocks there are bimodal volcanic series. The rocks are characterized by high K2O, MgO, and TiO2 contents. They show variable enrichment in LILE relative to HFSE, which is typical of island-arc magmas. At the same time they are rich in compatible transition elements, which is a characteristic of intraplate magmas. Trace element distribution patterns normalized to MORB or primitive mantle usually show a negative Ta-Nb anomaly typical of suprasubduction settings. Their Ta/Nb ratio is lower, whereas Ba/Nb, Ba/La, and La/Yb ratios are higher than those of some MORB and OIB. In terms of trace element systematics, for example, Ta-Th-Hf, Ba/La-(Ba/La)_n, (La/Sm)_n-La/Hf, and others, they fall within the area of mixing of magmas from several sources (island arc, intraplate, and depleted reservoirs). Magmatic rocks of transform settings show a sigmoidal chondrite-normalized REE distribution pattern with a negative slope of LREE, depletion in MREE, and an enriched or flat HREE pattern. Magmas with mixed geochemical characteristics presumably originated in a transform margin setting in local extension zones under influence of mantle diapirs, which caused metasomatism and melting of the lithosphere at different levels, and mixing of melts from different sources in variable proportions