Mantle sources of Cenozoic volcanic activities around the South China Sea revealed by geochemical and isotopic data using the principal component analysis (PCA)

Principal component analysis (PCA) was conducted to analyze geochemical and isotopic data and interpret the characteristics and types of mantle sources of Cenozoic volcanic activities around the South China Sea (SCS). Fifteen trace element indicators and five isotopic indicators were surveyed from 623 volcanic rock samples obtained from the SCS, Hainan Island, Fujian–Zhejiang, Taiwan, Vietnam, and Thailand to characterize the geochemical properties of the volcanic rocks, determine the types of mantle sources, and assess the influence degree of each mantle source. Two principal components (PCs) were extracted by PCA based on trace elements and Sr–Nd–Pb isotopic ratios, which are an enriched oceanic island basalt-type mantle plume and a depleted mid-ocean ridge basalt-type spreading ridge. In the Southeast Asian region, the influence of Hainan mantle plume on younger volcanic activities (< 13 Ma) was greater than that on older ones (> 13 Ma) in the same location. PCA was used to verify the mantle plume–ridge interaction model of volcanic activities beneath the expansion center of the SCS and refute the hypothesis that the tension in the SCS is triggered by the Hainan plume. The results of this study demonstrate the efficiency and applicability of PCA to the discussion of mantle sources of volcanic activities and provide a new method with which to analyze geochemical data.

New data on the genesis and evolution of the primitive magmas of Mount Cameroon: contribution of melt inclusions

Mount Cameroon is a Plio-Quaternary volcanic massif, without a central crater, made up ofmore than 140 pyroclastic cones. It is one of the active volcanoes of the Cameroon Line. Mount Cameroon magmatic inclusions are found in microdroplets trapped in the early minerals (olivines) from the pyroclastic products. The analysis of these magmatic inclusions allowed us to find primitive liquids compared to lavas. Major elements study of the magmatic inclusions, trapped in the most magnesian olivines (Mg#84-86) of Mount Cameroon revealed "primitive" liquids of basanite and alkaline basalt type with variable composition compared to the much more uniform basalts of the magmatic series of Mount Cameroon. The study of these trapped liquids shows that: (i)- the original primitive lavas did not undergo the process of evolution by FC, but rather underwent fundamentally (or exclusively) the process of partial melting; (ii) the emitted lavas, evolved essentially by FC; (iii) the variations in the trace element contents of the primitive liquids directly reflect a variation in the rate of partial melting of a homogeneous mantelic source. The very high La/Yb ratios of the Mount Cameroon inclusions (> 20) characterize a garnet lherzolite source. Spectra of the magmatic inclusions show a negative anomaly or depletion in K, Rb and Ba as those of HIMU. The "primitive" liquids and lavas of Mount Cameroon represent a co-genetic sequence formed by varying degrees of partial melting of a source considered as homogeneous.

Tokoro Belt (NE Hokkaido): an exhumed, Jurassic – Early Cretaceous seamount in the Late Cretaceous accretionary prism of northern Japan

The Tokoro Belt exposed in NE Hokkaido (Japan) represents part of a Late Cretaceous accretionary complex, which includes variously metamorphosed volcanic rocks that are interbedded with chert, lenticular limestone and some fore-arc sedimentary rocks. The Tokoro Belt is notably different from other Late Cretaceous accretionary complexes around the Pacific Rim because of widespread occurrence of basalts and volcaniclastic rocks in it. The Nikoro Group, characterized by widespread occurrence of volcanic rocks, is divided into western, eastern and southern sections based on the internal structure, geochemical affinities and metamorphic grades of their volcanic lithologies. OIB (ocean island basalt)-type volcanic rocks with low-grade metamorphic overprint predominate in the western and southern sections, whereas MORB (mid-ocean ridge basalt)- and OIA (ocean island alkaline basalt)-type rocks in the eastern section with partly high-pressure metamorphism make up the northern part of the eastern section. Trace element patterns display transitional trends from MORB to OIA geochemical affinities. OIB-type rocks display trace element characteristics similar to those of shield volcano lavas on Hawaii, rather than small and mainly alkaline, Polynesian hotspot lavas; furthermore, they show significant HREE (heavy rare earth element) enrichment probably caused by plume–ridge interaction. Widespread OIBs in the Tokoro Belt represents tectonic slices of a large (>80 km wide) Hawaiian-style, seamount shield volcano on the Izanagi oceanic plate that was accreted into the continental margin of Far East Asia in the Late Cretaceous.

Isotopic Dating and Geological Significance of Stratiform Orebody in Gejiu Tin Deposit, Yunnan, China

Gejiu tin ore deposit is a famous tin-polymetallic deposit in the world because of its enormous metal reserves. Besides tin, there are copper, lead, zinc, silver, iron, sulphur, tungsten, bismuth, indium and rare earth elements. It was believed that there mainly are skarn-type tin deposit, stratiform tin deposit and basalt-type copper deposit in Gejiu tin orefield. The stratiform tin deposit are distributed in Lutangba, Malage and Huangmaoshan, which are hosted by carbonate rocks of Gejiu formation in Middle Triassic Series. 40Ar-39Ar dating of cassiterite from the sratiform tin deposit in Lutangba yields plateau age of 202.18±2.35Ma and isochron age of 206.81±3.23 Ma respectively. The ages are obviously older than those of the ore of the skarn type deposit of the Yanshanian epoch．The mineralization is the seabed exhalative hydrothermal sedimentary mineralization of the Indosinia epoch.