Masters Thesis

Construction of continental crust by deep crustal fractional crystallization and garnet pyroxenite root development: geochemical evidence from Fiordland, New Zealand

A key problem in understanding the growth of continental crust in magmatic arcs centers on the mechanisms that control geochemical diversification, particularly in deep crustal MASH (melting, assimilation, storage, and homogenization) zones where high heat flow facilitates differentiation processes. The Malaspina Pluton in Fiordland, New Zealand is a particularly well exposed suite of gabbro to diorite that was emplaced into the root of the Gondwana arc at 12–14 kbars during a brief 5 myr interval from ca. 120 to 115 Ma. Previous studies on related rocks in the Western Fiordland Orthogneiss (WFO) have hypothesized that partial melting of mafic source rocks controlled the geochemical characteristics of the WFO, particularly heavy rare earth element depletions and high Sr/Y values (>>40). Similarly, partial melting of lower crust has been implicated as a mechanism for producing the characteristic tonalite-trondhjemite-granodiorite (TTG) suites in the adjacent, mid crustal Separation Point Suite; however, the production of large volumes of mafic to intermediate magmas by partial melting alone is difficult to reconcile with thermal and mass balance considerations. The issue is compounded by the brief interval over which these large volumes of melts were emplaced into the crust (e.g., 5 Myr in the case of the Malaspina Pluton). To better understand magma sources and processes of geochemical diversification in the Malaspina Pluton, we integrate literature data with 36 new whole rock x-ray fluorsecence analyses and 17 new ICP-MS solution whole-rock analyses. Our results reveal that primitive gabbros (49.2–52.6 wt.% SiO2) in the Malaspina Pluton are magnesian, metaluminous and calc-alkalic to alkali-calcic in composition. They are characterized by high Al2O3 (>18.3–20.6 wt.%) and Sr/Y values (>46–223), but low CaO (6.7–8.6 wt.%), Mg# (40–50), Y (7–24 ppm) and heavy rare element concentrations. Zircon δ18O values for these rocks range from 5.67–5.75‰, and overlap entirely with the mantle oxygen isotope field (5.3 ± 0.6 ‰; 2SD: Valley, 2003). These results are not consistent with a direct mantle-derived melt, and therefore there is still some missing differentiation process in the history of the Malaspina Pluton. Post emplacement, geochemical trends are consistent with garnet + clinopyroxene (or omphacite) + plagioclase + amphibole controlling the liquid line of descent. Using average and evolved Malaspina diorite as daughter compositions and primitive Malaspina gabbros as starting compositions, major element mass balance models also predict a cumulate assemblage of garnet + clinopyroxene (or omphacite) + plagioclase ± amphibole, and rutile with an approximately 1:1 ratio of melt-to-cumulate by volume. These results are consistent with seismic velocity data for the Doubtful Sound region which demonstrate the existence of a high velocity body (Vp > 7.5 km/s) centered on the Malaspina Pluton that extends to 40 km depth and are likely the remnants of the Cretaceous mafic root.

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