Diorites and associated intrusive and metamorphic rocks of the Darran Complex, Mount Underwood, Milford, southwest New Zealand

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A Wandres, SD Weaver, D Shelley, JD Bradshaw

The Early Cretaceous Darran Complex of the Median Tectonic Zone near Milford, South Island, New Zealand, consists dominantly of biotite two-pyroxene diorites cut by biotite-hornblende microdiorite and quartz monzodiorite dikes. The host diorite gives a 138 +/2.9 Ma SHRIMP age on zircons, which is interpreted to be the age of igneous crystallisation. The calc-alkaline geochemistry of the host diorites and the microdiorites is attributed to melting of a mantle wedge source fluxed by slab-derived fluids. Ductile deformation (D1) of the host diorite and the microdiorite dikes took place at mid-upper amphibolite facies conditions, with extension lineations indicating a top to the NNE sense of shear. A SHRIMP age of 136 +/1.9 Ma on zircons from a quartz monzodiorite dike injected along D1 shears is statistically indistinguishable from that of the host diorite. This suggests that D1 was synmagmatic and that subduction, during or shortly after magma emplacement, was oblique to the Gondwana margin at c. 138 Ma. The quartz monzodiorite dikes are enriched in Na2O, Al2O3, and Sr, depleted in Y, and have a distinctly adakitic geochemistry. The change in chemistry from calc-alkaline magmas to alkali-calcic adakitic magmas reflects the melting of a mafic, garnet-bearing, essentially plagioclase-free source in the root of a volcanic arc system.

New Zealand Journal of Geology and Geophysics vol. 41, no. 1, p. 1-14
Royal Society of New Zealand