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
- 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 ¾35 Ma in the
northern Kerguelen Plateau); the submarine igneous basement of Elan Bank and the northen
Kerguelen Plateau had not been previously sampled.
- 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.
- 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.
- 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, ~26 m of a braided river conglomerate was intercalated with basaltic
flows; the clasts in this conglomerate show the wide range of rock types that were subaerially
exposed on Elan Bank. Most notable are clasts of garnet-biotite gneiss, a rock type that is
characteristic of continental crust, thereby showing that a continental fragment is present in
this oceanic environment.
Leg 183 Introduction
Leg 183 Table of Contents