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[SCG55-23] Distribution of Continental Crust Fragments in the Indian Ocean: IRD/Dropstone or Not—That Is the Question
Keywords:Indian Ocean, Gondwana, Conrad Rise, Continental Crust
The Indian Ocean, formed by the breakup of Gondwana, hosts scattered fragments of continental crust. Evidence for such fragments has been reported from multiple locations, including the Kerguelen Plateau, Mauritius Island, the Afanasiy-Nikitin Rise, and the Conrad Rise.
At the Kerguelen Plateau, a conglomerate layer containing granitoids and metamorphic rocks was recovered during Ocean Drilling Program (ODP) drilling at Elan Bank and interpreted as alluvial deposits from the Gondwana breakup (Ingle et al., 2002). Additionally, zircons dated to 660–1,971 Ma were identified in beach sands from Mauritius Island (Torsvik et al., 2013), while zircons with U-Pb ages of 2.5–3.0 Ga were reported in a 5.7 Ma trachyte (Ashwal et al., 2017). A single zircon grain with a U-Pb age of 2.9 Ga was identified in basalt from the Afanasiy-Nikitin Rise, suggesting assimilation of Gondwana continental crust into hotspot magmas (Sushchevskaya et al., 2022). Recently, Asimus et al. (2025) identified granitic and metamorphic rock clasts of Gondwana origin, including granites and gneisses, in the Central Kerguelen Plateau. The presence of volcanic rinds and melt interaction features suggests their incorporation as xenoliths during magma ascent.
At the Conrad Rise in the southwestern Indian Ocean, metamorphic and granitic rocks up to 15 cm in size, ranging from sub-rounded to angular clasts, have been recovered. The metamorphic rocks exhibit symplectite textures indicative of the isothermal decompression reaction Garnet + Sillimanite → Cordierite + Spinel, indicating a clockwise pressure-temperature evolution, with a metamorphic peak at ca. 1,000 Ma (Ishizuka et al., 2011). Monazite ages of 540–700 Ma and 1,500 Ma have also been reported from these metamorphic rocks (Kobayashi, 2014 MS). The granitic rocks display a wide range of textures and compositions, from medium- to coarse-grained, ranging in color from white to pink. Based on modal compositions, most are classified as alkali granites or granites (Kobayashi et al., 2013). U-Pb dating of zircons from these granitic rocks yielded an age of 1,150 Ma (Kobayashi, 2014 MS). These findings support the hypothesis that continental crust underlies the Conrad Rise, with possible correlations to the Rainer Complex in Antarctica and the Eastern Ghats Belt in India (Ishizuka et al., 2011; Kobayashi et al., 2013).
However, iceberg-rafted debris (IRD) and dropstones transported from Antarctica are well-documented on the Indian Ocean seafloor. Some of these dropstones have been found as far north as the Southwest Indian Ridge at approximately 45°S (Sato et al., 2021). Given this, the hypothesis of continental crust beneath the Conrad Rise requires careful evaluation.
To refine this hypothesis, we analyzed metamorphic and granitic rocks from the region. By comparing these samples with those from the Central Kerguelen Plateau (Asimus et al., 2025) and with known continental dropstones in the Indian Ocean, we aim to clarify the presence and extent of continental crust beneath the Conrad Rise.
At the Kerguelen Plateau, a conglomerate layer containing granitoids and metamorphic rocks was recovered during Ocean Drilling Program (ODP) drilling at Elan Bank and interpreted as alluvial deposits from the Gondwana breakup (Ingle et al., 2002). Additionally, zircons dated to 660–1,971 Ma were identified in beach sands from Mauritius Island (Torsvik et al., 2013), while zircons with U-Pb ages of 2.5–3.0 Ga were reported in a 5.7 Ma trachyte (Ashwal et al., 2017). A single zircon grain with a U-Pb age of 2.9 Ga was identified in basalt from the Afanasiy-Nikitin Rise, suggesting assimilation of Gondwana continental crust into hotspot magmas (Sushchevskaya et al., 2022). Recently, Asimus et al. (2025) identified granitic and metamorphic rock clasts of Gondwana origin, including granites and gneisses, in the Central Kerguelen Plateau. The presence of volcanic rinds and melt interaction features suggests their incorporation as xenoliths during magma ascent.
At the Conrad Rise in the southwestern Indian Ocean, metamorphic and granitic rocks up to 15 cm in size, ranging from sub-rounded to angular clasts, have been recovered. The metamorphic rocks exhibit symplectite textures indicative of the isothermal decompression reaction Garnet + Sillimanite → Cordierite + Spinel, indicating a clockwise pressure-temperature evolution, with a metamorphic peak at ca. 1,000 Ma (Ishizuka et al., 2011). Monazite ages of 540–700 Ma and 1,500 Ma have also been reported from these metamorphic rocks (Kobayashi, 2014 MS). The granitic rocks display a wide range of textures and compositions, from medium- to coarse-grained, ranging in color from white to pink. Based on modal compositions, most are classified as alkali granites or granites (Kobayashi et al., 2013). U-Pb dating of zircons from these granitic rocks yielded an age of 1,150 Ma (Kobayashi, 2014 MS). These findings support the hypothesis that continental crust underlies the Conrad Rise, with possible correlations to the Rainer Complex in Antarctica and the Eastern Ghats Belt in India (Ishizuka et al., 2011; Kobayashi et al., 2013).
However, iceberg-rafted debris (IRD) and dropstones transported from Antarctica are well-documented on the Indian Ocean seafloor. Some of these dropstones have been found as far north as the Southwest Indian Ridge at approximately 45°S (Sato et al., 2021). Given this, the hypothesis of continental crust beneath the Conrad Rise requires careful evaluation.
To refine this hypothesis, we analyzed metamorphic and granitic rocks from the region. By comparing these samples with those from the Central Kerguelen Plateau (Asimus et al., 2025) and with known continental dropstones in the Indian Ocean, we aim to clarify the presence and extent of continental crust beneath the Conrad Rise.