[B-CG05] Decoding the history of Earth: From Hadean to the present

[BCG05-P06] Geochemistry of the Upper Triassic basalts of the Sambosan Belt, Kyushu and Shikoku, and their relationship with Wrangellia LIP magmatism

*Tomonari Kandabashi¹, Katsuyuki Yamashita³, Kohei Kawakami¹, Tetsuji Onoue² (1.Department of Earth and Planetary Sciences, Graduate School of Science, Kyushu University, 2.Department of Earth and Planetary Sciences, Faculty of Science, Kyushu University, 3.Graduate School of Natural Science and Technology, Okayama University)

Keywords:basalt, Sambosan Belt, Carnian

The Carnian Pluvial Episode (CPE) was a phase of global environmental change and biotic turnover that occurred during the Carnian in the Late Triassic. The CPE is considered to have been caused by a large igneous province (LIP) magmatism in a mid-oceanic realm of the Panthalassa Ocean, which resulted in the eruptions of the Wrangellia flood basalts (FB) in northwestern Canada. In addition to the Wrangellia FB, Carnian oceanic basalts of an intraplate origin have been recognized in the Sambosan Belt, Southwest Japan. Although the contemporaneous emplacement of these oceanic and flood basalts may suggest a single LIP origin, the relationship is hypothetical, due to a lack of geochemical data from basaltic rocks of the Sambosan Belt. Here, we present the major and trace elements and Sr isotopic data of the Sambosan basalts in Kyushu and Shikoku, Southwest Japan, to constrain the origin of the basaltic rocks of the Sambosan Belt and their relationship with Wrangellia FB volcanism.
The basaltic rocks of the Sambosan Belt in the study area crop out as exotic blocks in a mudstone matrix or displaced blocks in basaltic volcaniclastic rocks. XRF whole rock analyses indicate these basalts are classified as alkaline basalts with high Na₂O concentrations. N-MORB normalized spider diagrams for the Sambosan basalts show the enrichment of large-ion lithophile elements (LILE) and high field strength elements (HFSE), suggesting an affinity with ocean island basalts (OIB). Discrimination diagrams based on least-mobile elements (e.g., Nb, Zr, and Y), suggest that these basalts have higher Zr/Y ratios than MORB with high Nb/Zr ratios (Nb/Zr>0.1), which shows the similarity with the within-plate basalts (WPB). There is no negative correlation between Sr/Ca and Sr isotopic (⁸⁷Sr/⁸⁶Sr) ratios, which suggest an early diagenetic overprint by precipitation of carbonate minerals may not occur. Initial Sr isotopic ratios of the Sambosan basalts, fall in the range of 0.703 to 0.707. These ratios are not as depleted as MORB sources and indicate the Sambosan basalts were formed from an isotopically enriched mantle source. In addition, most basalts belong to the range of 0.703 to 0.705, which is similar to the ratios of Wrangellia FB.
Our data suggest that the Sambosan basalts preclude significant involvement of a subduction zone and MORB components, and were derived from a mantle plume source in a mid-oceanic setting.