Analysis and correlation of igneous clast geochemistry and petrography from four Mesozoic conglomerates
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
The Torlesse terrane encompasses three-quarters of the emergent New Zealand micro-continent and much speculation has been made as to the likely source for the detritus of which it is composed. This research concentrates on an examination of the igneous (plutonic and volcanic) clasts from three Torlesse conglomerates, one from the Rakaia terrane of Triassic age, and two from the Pahau terrane, of Early Cretaceous age, to obtain a better understanding of the characteristics of this source terrane. The Early Cretaceous Ethelton conglomerate comprises predominantly metaluminous I-type and weakly peraluminous evolved I-type granitoids that relate mineralogically and geochemically to the rhyolitic and dacitic volcanic clasts present and are thought to represent one cogenetic suite derived by partial melting of a common igneous source. The Mount Saul conglomerate contains a predominance of volcanic clasts and a variety of granitoid types. Some volcanic clasts relate to the I-type, and evolved I-type granitoids present. A-type peralkaline to weakly peraluminous volcanic clasts geochemically display a subduction related signature, that reflects source characteristics. A-type magmatism indicates an event unassociated with represented intrusives, either related to anorogenic magmatism or to a change to extensional tectonism. The predominant granodiorite character and low K content of Lake Hill conglomerate S type granitoid clasts contrasts with that of the other conglomerates. The dacitic and rhyolitic volcanic clasts examined from Lake Hill exhibit distinctive minerals indicative of I-types and relate to the evolved I-type granitoid clasts also present. The middle Cretaceous conglomerate located on Pitt Island, Chatham Islands is also examined to provide a comparison to the Torlesse conglomerates and an insight into source characteristics prior to continental separation between New Zealand and West Antarctica. The Chatham Islands conglomerate contains a variety of granitoid and volcanic types that are petrographically and geochemically comparable with the Torlesse conglomerates. The granitoid clasts are predominantly leucosyenogranites and leucomonzogranites. The volcanic clasts are rhyolites and dacites. Geochemically I-type, S-type and highly evolved/A-types are identified. The clasts from all four conglomerates examined exhibit trace element characteristics indicative of subduction-related magmatism. The granite type and highly felsic nature of most clasts indicates intrusion and extrusion occurred along an active continental margin that had attained full crustal thickness. The mineralogy and geochemistry of conglomerate clasts compares favourably with numerous volcano-plutonic continental provinces around the Pacific margin that are believed to represent the former Gondwana margin.