U-Pb zircon geochronology, Hf isotope and trace element geochemistry of a unique lower crustal - upper mantle section of a dying slow-spreading mid-ocean ridge (Macquarie Island, Southern Ocean)
Macquarie Island, located in the Southern Ocean, is a section of subaerially exposed oceanic crust formed at the now extinct proto-Macquarie, slow-spreading, mid-ocean ridge. Macquarie Island is unique among oceanic ophiolite sequences in that it is still located in the basin in which it formed. The northernmost part of Macquarie Island is composed primarily of lower crustal gabbro and upper mantle peridotite, and therefore provides a unique window into lower oceanic crust. Here, we report integrated Pb/U zircon ages and Lu-Hf isotopic and trace element data from six samples of the lower crust-upper mantle sequence. Samples consist of two lower crustal gabbros, and three mantle-hosted gabbro dikes/dikelettes and one phlogopite-bearing vein from the upper mantle sequence. ^206 Pb/^238 U SHRIMP-RG zircon error weighted average ages for the lower crustal gabbros are 8.7 ± 0.3 Ma (N = 9) and 9.0 ± 0.2 Ma (N = 13), whereas the mantle-hosted gabbro dikes/dikelettes yield overlapping error-weighted average ages of 8.7 ± 0.2 Ma (N = 9), 8.7 ± 0.3 Ma (N = 8), and 8.9 ± 0.2 Ma (N = 11) (all errors 2σ). The phlogopite vein yielded a slightly younger error weighted average age of 8.5 ± 0.1 Ma (N = 11). Initial epsilon Hf results for zircons from the same samples show a broad distribution ranging from +9.5 to +13.3 for the lower crustal gabbro (N = 28), +7.0 to +16.4 for the gabbro dikes/dikelettes (N = 24), and +8.4 to +12.2 for the phlogopite vein (N = 12). The wide range in values (particularly from the gabbro dikes/dikelettes) is consistent with a heterogeneous source region composed of depleted- and enriched mantle sources. Zircon trace element concentrations also support a heterogeneous source, displaying enrichment in U/Yb relative to N-MORB zircons from the Mid-Atlantic and Southwest Indian Ridge systems. No pattern of enrichment with time is observed within the resolution of the U-Pb zircon dates. We interpret these results to indicate that magmatic construction of Macquarie Island occurred between 8.7 and 9.0 Ma and involved sampling of at least two distinct mantle sources.