The origin of microcontinental terranes

Slivers of ancient crust make up part of the collages of accreted terranes found in many ancient orogens.  How they form is not well-known.   Clues might lie in modern microcontinents that still remain surrounded by oceanic lithosphere, such as Jan Mayen, the Seychelles and the East Tasman Plateau.  Geologists from the Universities of Sydney, and Aarhus and the Geological Survey of Canada believe that such fragments of continental crust form early in the evolution of passive margins, as a result of plume activity followed by asymmetric sea-floor spreading (Müller, D.M. 2001.  A recipe for microcontinent formation.  Geology, v.  29, p. 203-206).

One suspected microcontinent in the southern Indian Ocean is the Kerguelen Plateau – its shape is odd.  In the few places where it breaches surface in the Kerguelen Archipelago, there are rare occurrences of silicic plutonic rocks.  However, evidence from dredged samples seems to show that most of the Plateau formed by plume-related basaltic volcanism that began at the same time as the formation of the Rajmahal Traps in Bangladesh (about 117 Ma ago).  ODP drilling now reveals fluviatile sediment layers that contain high-grade gneisses, whose ages range back to the Proterozoic (Nicolaysen, K. and many others 2001.  Provenance of Proterozoic garnet-biotite gneiss recovered from Elan Bank, Kerguelen Plateau, southern Indian Ocean.  Geology, v.  29, p. 235-238).  The authors do not see this as directly supporting a Kerguelen microcontinent, but the formation of the plateau close to eastern India around 110 Ma ago, from where abundant Precambrian crustal debris would have been shed.  However, the presence of continental geochemical signatures in Kerguelen Plateau basalts, otherwise having plume affinities, might indicate a fragment of former Gondwanan lithosphere at the core of the Plateau, akin to the now exposed Danakil block in the nascent Red-Sea – Afar rift in NE Africa, that spalled off during the break-up of the Mesozoic supercontinent.


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