Island is an exposure above sea-level of part of the crest 38•979), and negatively with Nd ( 143 Nd/ 144 Nd = 0•51310of the Macquarie Ridge. The ridge marks the Australia-Pacific 0•51304). Macquarie Island basaltic glasses are divided into plate boundary south of New Zealand, where the plate boundary two compositional groups according to their mg-number-K 2 O has evolved progressively since Eocene times from an oceanic spreading relationships. Near-primitive basaltic glasses (Group I) have the system into a system of long transform faults linked by short highest mg-number (63-69), and high Al 2 O 3 and CaO contents spreading segments, and currently into a right-lateral strike-slip at a given K 2 O content, and carry microphenocrysts of primitive plate boundary. The rocks of Macquarie Island were formed during olivine (Fo 86-89•5 ). Their bulk compositions are used to calculate spreading at this plate boundary in Miocene times, and include primary melt compositions in equilibrium with the most magnesian intrusive rocks (mantle and cumulate peridotites, gabbros, sheeted Macquarie Island olivines (Fo 90•5 ). Fractionated, Group II, basaltic dolerite dyke complexes), volcanic rocks (N-to E-MORB pillow glasses are saturated with olivine + plagioclase ± clinopyroxene, lavas, picrites, breccias, hyaloclastites), and associated sediments. and have lower mg-number (57-67), and relatively low Al 2 O 3 A set of Macquarie Island basaltic glasses has been analysed by and CaO contents. Group I glasses define a seriate variation within electron microprobe for major elements, S, Cl and F; by Fourier the compositional spectrum of MORB, and extend the compositional transform infrared spectroscopy for H 2 O; by laser ablationrange from N-MORB compositions to enriched compositions that inductively coupled plasma mass spectrometry for trace elements; represent a new primitive enriched MORB end-member. Compared and by secondary ion mass spectrometry for Sr, Nd and Pb isotopes. with N-MORB, this new end-member is characterized by relatively An outstanding compositional feature of the data set (47•4-51•1 low contents of MgO, FeO, SiO 2 and CaO, coupled with high wt % SiO 2 , 5•65-8•75 wt % MgO) is the broad range of K 2 O contents of Al 2 O 3 , TiO 2 , Na 2 O, P 2 O 5 , K 2 O and incompatible trace (0•1-1•8 wt %) and the strong positive covariation of K 2 O with elements, and has the most radiogenic Sr and Pb regional isotope other incompatible minor and trace elements (e.g. TiO 2 0•97-2•1%; composition. These unusual melt compositions could have been Na 2 O 2•4-4•3%; P 2 O 5 0•08-0•7%; H 2 O 0•25-1•5%; La generated by low-degree partial melting of an enriched mantle 4•3-46•6 ppm). The extent of enrichment in incompatible elements in glasses correlates positively with isotopic ratios of Sr ( 87 Sr/ 86 Sr = peridotite source, and were erupted without significant mixing with common N-MORB magmas. The mantle in the Macquarie Island 0•70255-0•70275) and Pb ( 206 Pb/ 204 Pb = 18•951-19•493;

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Macquarie Island is an exposure above sea level of the Macquarie Ridge Complex, on the boundary between the Australian and Pacific plates south of New Zealand. Geodynamic reconstructions show that at ca. 12-9.5 Ma, oceanic crust of the Macquarie Island region was created at this plate boundary within a system of short spreading-ridge segments linked by large-offset transform faults. At this time, the spreading rate was slowing (Ͻ10 mm/yr half-spreading rate) and magmatism was waning. Probably before 5 Ma, and possibly before the extinct spreading ridge had subsided, the plate boundary became obliquely convergent, and crustal blocks were rotated, tilted, and uplifted along the ridge to form the island. Planation by marine erosion has exposed sections through the oceanic crust.The magmatism that built the oceanic crust produced melts similar in composition to the widespread normal to enriched mid-oceanic-ridge basalt (N-to E-MORB) suite found in many spreading ridges, but the melts ranged beyond E-MORB to primitive, highly enriched, and silica-undersaturated compositions. These compositions form one end member of a continuum from MORB but seem not to have been derived from a MORB-source mantle, despite sharing a Pacific MORB isotopic signature. The survival of these primitive melts may be due to their origin in a slow-spreading system that must have been closing down as extension along the plate boundary gave way to transpression, putting a stop to the upwelling of asthenosphere and decompression melting. In a more energetic, faster-spreading system, mixing would have been more efficient, the presence of this end member could not easily have been inferred from its isotopic composition, and the igneous rocks would have resembled a typical N-to E-MORB suite. Macquarie Island may therefore provide a type example of magmatism at a very slow spreading ridge and a clue to the origins of E-MORB.Varne, R., Brown, A.V., and Falloon, T., 2000, Macquarie Island: Its geology, structural history, and the timing and tectonic setting of its N-MORB to E-MORB magmatism, in Dilek, Y.