Melissa Jean Murphy

Abstract: Uplift, exhumation, and denudation of the lower oceanic crust are recorded by sedimentary rocks of Macquarie Island (54°30′S, 158°54′E), which were deposited within the slow-spreading proto–Macquarie spreading ridge between ca. 9 and 12 Ma. Measured stratigraphic sections typically contain basal basaltic breccia lithofacies that are overlain by a thick sequence of enriched mid-ocean-ridge basalt (E-MORB) with thin intercalations of gabbroic sedimentary lithofacies. Basaltic detritus has zeolite to lower-greenschist metamorphic grades typical of the upper oceanic crust, and gabbroic detritus has upper-greenschist to amphibolite metamorphic grades typical of the lower oceanic crust. Breccia clast counts and sedimentary structures indicate that basaltic lithofacies were locally derived from the footwalls of adjacent spreading-related faults. Sedimentary structures, detrital clinopyroxene major- and trace-element geochemistry, and 206Pb/238U zircon geochronology indicate that the gabbroic lithofacies were more distally derived from a Paleogene-aged tholeiitic MORB source. Detrital zircon populations of ca. 27 and ca. 33 Ma correspond to oceanic magnetic anomalies 8o and 13o, respectively, and exclude ca. 8.5 Ma gabbroic rocks of Macquarie Island as a potential source. Geodynamic reconstructions show that anomaly 8o crust from the Southeast Indian Ridge was juxtaposed against the active proto–Macquarie spreading ridge when sedimentary rocks of Macquarie Island were deposited and was a likely source for the gabbroic lithofacies. The proto–Macquarie spreading ridge and Southeast Indian Ridge were connected by the Jurru long-offset transform, which has undergone significant transpression since 27 Ma. This transpression formed a bathymetric transverse ridge that was composed of structurally isolated blocks of heterogeneously aged Paleogene source crust, which provided the source for Macquarie Island's gabbroic sedimentary lithofacies.

Abstract: Oceanic zircon trace element and Hf-isotope geochemistry offers a means to assess the magmatic evolution of a dying spreading ridge and provides an independent evaluation of the reliability of oceanic zircon as an indicator of mantle melting conditions. The Macquarie Island ophiolite in the Southern Ocean provides a unique testing ground for this approach due to its formation within a mid-ocean ridge that gradually changed into a transform plate boundary. Detrital zircon recovered from the island records this change through a progressive enrichment in incompatible trace elements. Oligocene age (33–27 Ma) paleo-detrital zircon in ophiolitic sandstones and breccias interbedded with pillow basalt have trace element compositions akin to a MORB crustal source, whereas Late Miocene age (8.5 Ma) modern-detrital zircon collected from gabbroic colluvium on the island have highly enriched compositions unlike typical oceanic zircon. This compositional disparity between age populations is not complimented by analytically equivalent εHf data that primarily ranges from 14 to 13 for sandstone and modern-detrital populations. A wider compositional range for the sandstone population reflects a multiple pluton source provenance and is augmented by a single cobble clast with εHf equivalent to the maximum observed composition in the sandstone (~ 17). Similar sandstone and colluvium Hf-isotope signatures indicate inheritance from a similar mantle reservoir that was enriched from the depleted MORB mantle average. The continuity in Hf-isotope signature relative to trace element enrichment in Macquarie Island zircon populations, suggests the latter formed by reduced partial melting linked to spreading-segment shortening and transform lengthening along the dying spreading ridge.