Petrogenesis of the Greenhills Complex, Southland, New Zealand: magmatic differentiation and cumulate formation at the roots of a Permian island-arc volcano

Abstract

A Permian ($265 Ma) intrusive complex which formed as a magmatic feeder reservoir to an immature island-arc volcano is fortuitously exposed in southern New Zealand. Known as the Greenhills Complex, this intrusion was emplaced at shallow crustal levels and consists of two layered bodies which were later intruded by a variety of dykes. Cumulates, which include dunite, olivine clinopyroxenite, olivine gabbro, and hornblende gabbro-norite, are related products of parent-magma fractionation. Both primary (magmatic) and secondary platinum-group minerals occur within dunite at one locality. Using the composition of cumulus minerals, mafic dykes and melt inclusions, we have determined that the parent magmas of the complex were hydrous, low-K island-arc tholeiites of ankaramitic affinities. Progressive magmatic differentiation of this parent magma generated fractionated melt of high-alumina basalt composition which is now preserved only as dykes which cut the Complex. Field evidence and cumulus mineral profiles reveal that the magma chambers experienced turbulent magmatic conditions during cumulate-rock formation. Recharge of the chambers by primitive magma is likely to have coincided with eruption of residual melt at the surface. Similar processes are inferred to account for volcanic-rock compositions in other parts of this arc terrane and in modern island-arc systems.

Mafic and ultramafic rocks, and platinum mineralisation potential, in the Longwood Range, Southland, New Zealand

“…Plagioclase-rich rocks (leucogabbro, tending anorthositic, many troctolites) are rich in Al 2 O 3 , CaO, and Sr. MgO variations are similar to those from the nearby Greenhills Complex ( Fig. 1; Spandler et al 2003) except that in mafic rocks, CaO increases with decreasing MgO owing to calcic plagioclase enrichment. Feenrichment is modest, with total FeO/MgO values ranging from 0.39 to 2.57 (lowest in the most olivine-rich rocks, highest in some gabbronorites and gabbros).…”

“…10), indicating a lack of association with chromite (cf. Greenhills Complex; Spandler et al 2000Spandler et al , 2003; Fig. 1).…”

SHRIMP ion probe zircon geochronology and Sr and Nd isotope geochemistry for southern Longwood Range and Bluff Peninsula intrusive rocks of Southland, New Zealand

“…2B), are dominated by noritic gabbros showing layering and micro-textures that are also consistent with a cumulate origin. Spandler et al (2003) argued that the Green Hills intrusion is largely cumulate and formed as a magmatic feeder and storage system below an intra-oceanic, subductionrelated volcanic arc, and although the other intrusions (Bluff and Flat Hill) lack significant ultramafic cumulates, they are likely to have formed in the same way as part of a sub-arc, volcanic plumbing system.…”

“…The nature of the Median Batholith is important to our understanding of the assembly of New Zealand in the context of the Pacific margin of Gondwana. Precise delineation of the boundaries of the various Median interpretations with the Western and Eastern Provinces is problematic because in a number of key areas, intrusive rocks of broadly similar character are major components of all three terranes; most of these intrusives have petrological characteristics expected of rocks formed in subduction-related magmatic settings (e.g., Coombs et al 1976;Houghton & Landis 1989;Muir et al 1998;Spandler et al 2003). The combination of geochemistry and geochronology offers one of the best ways of distinguishing and grouping these intrusives and thereby more precisely defining the boundaries between the various terranes.…”

The origin of the Popiel peridotite (Western Sudetes, SW Poland): Metamorphism of the island arc tholeiitic cumulate

“…The com posi tion of spinel, which is char ac ter ized by very low Cr# (<0.2) and high Mg# (0.5-0.8), does not fit those typ i cal of arc-root ultrabasic rocks (cf. DeBari and Coleman, 1986;Spandler et al, 2003), which are richer in Cr. The spinel is com monly in timately over grown with mag ne tite (see Fig.…”