Sedimentary and volcaniclastic record of a mid-ocean spreading ridge: Macquarie Island, Southern Ocean
thesisposted on 2022-03-28, 09:48 authored by Ryan Adam Portner
Sedimentary and volcaniclastic rocks formed within mid-ocean ridge spreading centers have received relatively little documentation compared to their counterparts found in continental margin and volcanic arc systems. Results presented in this thesis characterize a diverse sedimentary and volcaniclastic lithofacies assemblage exposed on the Macquarie Island ophiolite, which formed within a waning slow-spreading mid-ocean ridge. Petrography, geochemistry, and geochronology of detritus indicate that the sedimentary rocks were derived from submarine exposures of faulted Oligocene to Miocene age oceanic crust. The interaction of fault-derived gravity flows with a rugged mid-ocean ridge sea floor and strong bottom current of the Southern Ocean produced a unique depositional environment. Low grade zeolite metamorphism and in-situ uplift of Macquarie Island sedimentary rocks aided the preservation of delicate sedimentary structures otherwise deformed during continental obduction or drilling of modern day midoceanridge sediment. Isotope and trace element geochemistry of sandstone, gabbroic colluvium, and basalt indicates that Miocene Macquarie Island crust was derived from a much more enriched mantle source compared to the Oligocene age detritus. The recorded covariance in enrichment with time may be attributed to changes in spreading direction and shortening of spreading segments. Alternatively, long-offset transforms associated with the shortened segments likely separated a heterogeneous mantle source. This is supported by paleogeographic reconstructions that show >300km of offset between sources for the Oligocene detritus and Macquarie Island crust. Long-offset transforms along the waning spreading center also played a role in limiting magma mixing, which is preserved by geochemically variable sideromelanein volcaniclastic rocks. Distinct geochemistry within individual volcaniclastic rock units is attributed to discrete eruption episodes, which were primarily associated with non-explosive quench fragmentation processes along the steep slopes of growing pillow cones and active fault scarps. Rare explosive eruptions also occurred in relatively enriched magma compositions containing both fractionated and volatile components.