183 Summary: Kerguelen Plateau-Broken Ridge—A
Large Igneous Province1
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 35
Ma in the northern Kerguelen Plateau); the submarine igneous basement of
Elan Bank and the northern Kerguelen Plateau had not been previously
- 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, 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.
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