Detrital modes of new zealand graywackes

Abstract

Graywacke, used here for any dark indurated sandstone, and argillite are the dominant pre-Tertiary rocks underlying much of New Zealand. The strata, mainly Permian to Jurassic, can be divided into two facies: (1) the western marginal facies of orderly, fossiliferous strata exposed in the New Zealand marginal syncline; and (2) the eastern axial facies of strongly folded and faulted, and partly schistose, rocks that occupy the keel of the New Zealand geosyncline. The two facies are in fault contact along the length of New Zealand. Each facies is 15–20 km thick, and records deposition, probably on oceanic crust, at a rate ≈125 ± 25 m/million years (m.y.) for ≈150 m.y. The western marginal facies apparently was deposited on a rapidly subsiding shelf that was positioned as a major topographic bench between an oceanic trench, site of sedimentation of the turbidites of the eastern axial facies, and a volcano-plutonic arc, probable provenance of the detritus in the two facies. The two facies were metamorphosed at low grade under relatively high pressures and low temperatures, probably in the sub-trench belt of paired metamorphic belts, with the provenance region being the sub-arc belt of relatively low pressure and high temperature. The graywackes of the western marginal facies apparently record a progressive erosional modification of the volcano-plutonic provenance, from a volcanic terrane that shed detritus into lower horizons of the sequence to a plutonic terrane that shed detritus into higher horizons of the sequence as dissection bit into the igneous source rocks of the arc. The most common graywackes of the eastern axial facies are more quartzose than those of the western marginal facies, and may have received plutonic detritus that bypassed the shelf between the arc and the trench. Feldspatholithic “volcanic” graywackes are poor in quartz, rich in pyriboles, bear mainly plagioclase feldspar, and contain mostly volcanic rock fragments. Lithofeldspathic “plutonic” graywackes are relatively high in quartz and mica, bear K-feldspar as well as plagioclase, and contain abundant metamorphic as well as volcanic rock fragments. Analogous paired suites of graywackes may be common in the circum-Pacific region.