1. Leg 183 Summary: Kerguelen Plateau-Broken Ridge—A Large Igneous Province¹

Shipboard Scientific Party²

ABSTRACT

Most of the Kerguelen Plateau and Broken Ridge formed as a single giant oceanic plateau in Cretaceous time. During Ocean Drilling Program Leg 183, igneous basement rock and sediment cores were obtained from five sites on the Kerguelen Plateau and two on Broken Ridge. Based on the recovery of basalt, other igneous rocks, and interbedded and overlying sediment, we found that

1. From south to north, the age of the uppermost crust forming this very large igneous province (LIP) decreases, possibly in steps (i.e., ~110 Ma in the southern Kerguelen Plateau, ~85 to 95 Ma in the central Kerguelen Plateau, Broken Ridge, and Elan Bank, and 35 Ma in the northern Kerguelen Plateau); the submarine igneous basement of Elan Bank and the northern Kerguelen Plateau had not been previously sampled.
2. The growth rate of the LIP at five of seven basement sites was sufficient to form a subaerial landmass. This was most spectacularly revealed at central Kerguelen Plateau Site 1138 by wood fragments in a dark brown sediment overlying the subaerially erupted lava flows, a result consistent with the charcoal and wood fragments in sediments overlying igneous rocks at Site 750 in the southern Kerguelen Plateau.
3. The terminal stage of volcanism forming the LIP included explosive eruptions of volatile-rich felsic magmas formed from cooling basaltic magmas that were trapped within the crust when the flux of basaltic magma from the mantle decreased.
4. Previous geochemical studies of basalt from the southern Kerguelen Plateau and eastern Broken Ridge had identified a component derived from continental crust (e.g., Mahoney et al., 1995), but the mechanism for incorporation of a continental component into the oceanic plateau was unconstrained. Possible processes range from recycling of continental material into a deep mantle plume to contamination of mantle-derived basaltic magma by fragments of continental crust isolated in the embryonic Indian Ocean crust during rifting of Gondwana. At Site 1137 on Elan Bank, a 26-m sequence of fluvial conglomerate was intercalated between basaltic flows; the clasts in this conglomerate show that a wide range of rock types were subaerially exposed on Elan Bank. Most notable are clasts of garnet-biotite gneiss, a rock type that is commonly found only in continental crust, thereby indicating that a continental fragment is present in this oceanic environment.