Nature and record of igneous activity in the Tonga arc, SW Pacific, deduced from the phase chemistry of derived detrital grains

Abstract

Sedimentary rocks in dredge samples from the Tonga arc, SW Pacific, are simple two-component admixtures of volcanogenic detritus and bioclastic material. The volcanic detritus consists largely of lithic and plagioclase grains, but also includes, in decreasing order of abundance, significant minor amounts of clinopyroxene, orthopyroxene and pigeonite. In addition, amphibole, olivine and opaque oxide minerals are accessory phases and trace amounts of monocrystalline quartz occur in a few samples. Plagioclase grains have high An and low Or contents and fall largely in the bytownite field. Pyroxene grains show progressive Fe enrichment and low Ti contents. Volcanic glasses also show Fe enrichment and have low K20 contents. These features indicate derivation of the volcanic detritus from a low-K tholeiite source. The uniform compositional range and Oligocene to Holocene age range of the detritus is consistent with their derivation from the adjacent Tongan arc, and indicate that Pliocene rifting of the arc and associated back-arc basin formation did not affect the composition and affinities of the magma source.

Evolving heavy mineral assemblages reveal changing exhumation and trench tectonics in the Mesozoic Chugach accretionary complex, south-central Alaska

“…This is unlikely to refl ect erosion of oceanic crust sensu stricto, but probably shows input from intra-oceanic-arc rocks. It is noteworthy that Cawood (1991b) also identifi ed grains with these values in the Neogene Tonga forearc, where their provenance is not ambiguous. A less likely but possible alternative is erosion from the oceanic plateau rocks of the Wrangellia terrane, and there is a Mid-Cretaceous angular unconformity within the Wrangellia stratigraphy, at least in the Chitina area (Fig.…”

“…The tectonic setting of the source rocks from which the pyroxenes were eroded can be constrained through use of the discriminant diagram of Nisbet and Pearce (1977), which was primarily developed to differentiate clinopyroxenes from mafi c magmas, although the volcanic arc basalt fi eld does include clinopyroxene compositions from arc andesites (Cawood, 1991b). In this method, two eigenvectors are calculated based on the chemical composition and crossplotted against one another (Fig.…”

Provenance of the Permian Malužiná Formation sandstones (Malé Karpaty Mountains, Western Carpathians): evidence of garnet and tourmaline mineral chemistry

“…Both modal and bulk rock analyses, however, produce only an overall composition of the source rocks and cannot determine if within-sample provenance mixing took place (c.f. Cawood 1991). On the other hand, the mineral chemistry of detrital heavy minerals documents not only within-sample provenance mixing but also provides more detailed information on source rock characteristics (Asiedu et al 2000).…”

Geoarchaeology of Tonga: Geotectonic and geomorphic controls

“…The frontal arc of the Tongan arc-trench system is termed the Tonga ridge, formed jointly by the active Tofua volcanic arc and the forearc Tonga platform (Figure 2). Backarc spreading has isolated the Tongan islands from the Lau Archipelago capping a remnant arc forming the Lau ridge (Figure 2) where islands expose Miocene volcanic rocks (14-6 Ma) of the ancestral Tongan volcanic arc (Cawood, 1991). The oldest volcanic rocks known from the modern Tofua arc are Pliocene in age.…”