1998/03/01 by Anekant M Wandres, S. D. Weaver, David Shelley +1 · 2 citations
Earth and Planetary Sciences · #Geological and Geochemical Analysis #High-pressure geophysics and materials #earthquake and tectonic studies #Geology #Geochemistry #Diorite #Mafic #Dike #Zircon #Pluton #Porphyritic #Petrology #Quartz #Tectonics #Paleontology
paper · pdf · doi:10.1080/00288306.1998.9514786
openalex publication_date 1998/03/01 · openalex created_date 2025/10/10 · openalex updated_date 2026/06/26
Abstract 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 (D 1 ) 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 D 1 shears is statistically indistinguishable from that of the host diorite. This suggests that D 1 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 Na 2 O, Al 2 O 3 , 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.