Neoproterozoic (~828 Ma) Expansion of Back-Arc Basin: Implications fromGeochronology and Geochemistry of the Diabase and Flyschoids in Kaihua

As a record of the process for expansion of the back-arc basin, the Xieyuanling diabase dyke swarms and the Yangling tuffs found interlayer in the flyschoids of Luojiamen Formation along the two sides of the Suzhuang-Shizhu fault in Kaihua area, western Zhejiang, and which is the eastern segment of Jiangnan orogeny. The U-Pb dating of zircon yields a lower limit age of 828.2±8.7 Ma for the Xieyuanling diabase,and another age of 830.9±4.9 Ma for the Yangling tuffs. Geochemical analysis indicates that the diabases have high Al₂O₃ (15.82%–17.09%), low TiO₂ (0.64%–1.37%) and poor K₂O (0.01%–0.04%). In the chondrite- and primitive mantle-normalized trace element diagram, all diabase samples show flat patterns,with (La/Yb)_N=1.02–1.78 and (Ce/Yb)_N=0.93–1.72, shows the similar characteristics of MORB or BABB. And they also enriched in Sr, Ba, Th and Pb, with Nb/U=11.67–28.17, La/Nb=1.21–2.02, Th/Ta=1.79–2.86, that indicate the formation process of diabase must have been contaminated by weak crustal materials. Therefore, a spreading of back-arc basin occurred with the paleo-South-China plate northwestwardly subducted in Neoproterozoic, it suggested the Shuangxiwu island arc rifted from the Yangtze block simultaneously, and the Yangtze block had not collided with the Cathaysia block at least at ca. 828 Ma or later.