The petrology of the Separation Point batholith margin, Golden Bay, NW Nelson

Abstract

The northwestern margin of the Separation Point Batholith is compositionally zoned from marginal diorite-tonalite (Ligar Complex) in the west and separated by a ductile shear zone (Wainui Shear Zone) from cogenetic granodiorites and monzogranite in the area of Abel Tasman Point in the east. The Ligar Complex (new name) consists of moderately foliated pyroxene-diorite, quartz diorite and tonalite that has intruded, stoped, and partially assimilated Paleozoic quartzofeldspathic wall rock. Dioritic rocks of the Ligar Complex can be correlated to the Early-Cretaceous Western Fiordland Orthogneiss on the basis of their highly distinctve major, trace and rare earth element geochemistry. Abel Tasman Point consists of a heterogenous, titanite-rich hornblende-biotite granodiorite with minor variations to biotite-hornblende monzogranite (Type-1 granodiorite). The granodiorite contains mafic xenoliths (amphibolite and gabbro) and abundant microgranitoid enclaves in the form of pillow-shaped morphologies with lobate outlines, and as angular xenoliths derived from marginal dioritic rocks. The Type-1 granodiorite is intruded by a finer-grained, moderately foliated granodiorite containing numerous and well dispersed microenclaves (Type-2 granodiorite). Granodiorites are cut by late phase aplites and pegmatites. Field relations, rock textures, mineralogy and geochemistry indicate that the rocks are a product of mixing between contemperaneous dioritic and granitic magmas. Geobarometry indicates that the marginal Ligar Complex dioritic rocks were emplaced at ~6 kbar (~20km), with subsequent granodiorite-monzogranite magmas emplaced at ~2.6 kbar (~8km). Precursor Ligar Complex, and subsequent coeval dioritic magmas are the result of mixing between mantle-derived alkali basalt and high-Al trondhjemitic-type melts derived from partial melting of a metabasaltic protolith at crustal depths in excess of 40km. Separation Point Suite magmas are infered to be the related to the delamination of overthickened continental lithosphere and lower crust following collision between the Median Tectonic Zone and Western Province in the Early Cretaceous.