Continental arc dynamics: a plutonic-volcanic marriage in New Zealand

The importance of hornblende-bearing intermediate intrusive rocks to the petrogenetic evolution of subduction-related silicic magmas has long been recognized and obvious evidence of cumulates with up to 40 modal % hornblende exists in exposed sections of arc crust exhumed along orogenic belts. Although these ‘mushes’ of intermediate composition (andesite-dacite) can be erupted together with crystal-poor rhyolites in active volcanic systems (e.g. monotonous intermediates), a detailed comparison with dioritic to grandioritic plutons has yet to be attempted. This paucity of direct volcano-plutonic comparison may be rooted in the assumption that these plutonic rocks often only record the waning stages of magmatism, whereas the volcanic rocks record the high flux stages and, therefore, they cannot be effectively compared. In an attempt to bridge that gap, we have compared the textural, petrological, and geochemical variations of rocks from the present-day active Okataina Volcanic Complex in the Taupo Volcanic Zone (TVZ) and Cretaceous Bungaree Intrusives (BI), both found in New Zealand and related to the same subduction system (although 140 Ma apart). Unique crystal-rich, hornblende-bearing basalt-dacite cognate lapilli and blocks were erupted coincident with, and following, the caldera-forming silicic Matahina eruption from the TVZ (330 ka). The andesite cumulate rocks (up to 50% vol. crystals) are the first of this type reported from the TVZ and are interpreted to represent a fragment of the intermediate progenitor to the large volume silicic magmas erupted at the surface. These volcanic juvenile clasts are very similar to rocks of intermediate composition in the lower section of the BI. Both can be defined as cumulates by: 1) textural evidence such as crystal alignment indicating settling or adcumulate glomerocrysts, 2) geochemical evidence such as a positive Eu anomaly indicative of feldspar accumulation, and 3) high Zr content, indicative of zircon accumulation. In addition, as commonly observed in mid- to upper-crustal plutons, mafic sheets crosscut the BI diorite cumulate within the lower section of the BI. The margins of these sheets are composed of fine-grained hornblende + plagioclase + oxide rock with a coarse interior. This texture along the sheet margin is similar to that seen in fine-grained hornblende-bearing basalt pumice of the Matahina - the only difference being that the BI mafic sheet cooled enough to stabilize plagioclase. Our textural and geochemical comparison of these two calc-alkaline magmas with intermediate composition provides a link between the plutonic and volcanic realms in continental arc settings, and suggests that plutonic sequences in the mid- to upper-crust contain valuable information on the volcanogenic period of the system, not merely the waning stages of the magmatic cycle.