Correlations between silicic volcanic rocks of the St Mary's Islands (southwestern India) and eastern Madagascar: implications for Late Cretaceous India–Madagascar reconstructions

The St Mary's Islands (southwestern India) expose silicic volcanic and sub-volcanic rocks (rhyolites and granophyric dacites)
emplaced contemporaneously with the Cretaceous igneous province of Madagascar, roughly 88–90 Ma ago. The St Mary's Islands
rocks have phenocrysts of plagioclase, clinopyroxene, orthopyroxene and opaque oxide, moderate enrichment in the incompatible
elements (e.g. Zr = 580–720 ppm, Nb = 43–53 ppm, La/Ybn = 6.9–7.2), relatively low initial 87Sr/86Sr (0.7052–0.7055) and near-chondritic initial 143Nd/144Nd (0.51248–0.51249). They have mineral chemical, whole-rock chemical and isotopic compositions very close to those of rhyolites
exposed between Vatomandry–Ilaka and Mananjary in eastern Madagascar, and are distinctly different from rhyolites from other
sectors of the Madagascan province. We therefore postulate that the St Mary's and the Vatomandry–Ilaka–Mananjary silicic rock
outcrops were adjacent before the Late Cretaceous rifting that split Madagascar from India. If so, they provide a valuable
tool to check and aid traditional Cretaceous India–Madagascar reconstructions based on palaeomagnetism, matching Precambrian
geological features, and geometric fitting of continental shelves.

圣玛丽群岛（印度西南部）揭示了与马达加斯加岛白垩纪火成省同期形成的硅质火山岩和亚火山岩（流纹岩和斜长岩质安山岩），距今约8800万至9000万年。圣玛丽群岛的岩石含有斜长石、斜方辉石、正方辉石和暗色氧化物包晶，在不相容元素（如Zr为580-720 ppm，Nb为43-53 ppm，La/Ybn为6.9-7.2）方面有适度的富集，初始87Sr/86Sr比值相对较低（0.7052-0.7055），初始143Nd/144Nd比值接近球粒陨石（0.51248-0.51249）。它们的矿物化学、全岩化学和同位素组成与在马达加斯加东部Vatomandry-Ilaka至Mananjary之间露头的流纹岩非常接近，与马德拉加斯加省其他地区的流纹岩明显不同。因此，我们推测圣玛丽群岛与Vatomandry-Ilaka- Mananjary的硅质岩露头在白垩纪晚期裂谷运动将马达加斯加与印度分开之前是相邻的。如果是这样，它们为检查并辅助基于古地磁学、匹配前寒武纪地质特征和大陆架几何拟合的传统白垩纪印度-马达加斯加重建提供了一个宝贵的工具。