Blueschists in the South Sepik region of Papua New Guinea are one of several poorly known occurrences along the northern margin of New Guinea's Central Orogenic Belt. It is believed that they formed in a northward-dipping subduction zone, beneath a Palaeogene arc system now accreted along the north coast of New Guinea, and that they now lie within a Tertiary continent-arc collision zone. The South Sepik blueschists, or Tau Blueschist, are situated near the centre of New Guinea, on the northern fall of the island's axial mountains, and are mostly contained in an allochthonous east-west lens (55 x 8 km) known as the Tau body. There are also some much smaller allochthons to the east of the Tau body. The blueschists occur within the late-Mesozoic to Eocene Salumei Formation, which is believed to be a tectonic mixture of mostly politic sediments, derived from the continent to the south, and ophiolite fragments and other volcanogenic rocks, related to the arc to the north. Near the Tau body, the Salumei Formation is metamorphosed from prehnite-pumpellyite to low-greenschist grade. Stratigraphic constraints and isotopic dating indicate an Oligocene to early Miocene metamorphism of the Tau Blueschist, Salumei Formation, and other metamorphic rocks in the region. The blueschists are mostly massive to well-foliated mafic schists, containing abundant blue amphibole, but there are also some intimately associated pelitic-calcareous-graphitic metasediments. Within the Tau body the metamorphic grade increases towards the north, from lawsonite rocks to high-grade epidote blueschists. The lowest grade blueschists are pumpellyitic lawsonitic metabasites, known only from river float. In addition, there are zones and isolated occurrences of coarsely crystalline mafic tectonic blocks, within and just north of the Tau body. These include high-grade blueschist, eclogite and amphibolite. There is a complete spectrum from blueschist to eclogite. Whole-rock chemistry indicates, that all basic rocks are probably metamorphosed mafic ophiolite. The mineralogy and mineral chemistry is similar to other circum-Pacific blueschists, particularly those in New Caledonia. The blue amphibole is glaucophane or crossite, but ferroglaucophane occurs in the lawsonite zone. It is sometimes accompanied by Ca-amphibole, ranging from actinolite, in the lawsonite zone, to barroisite, in the epidote zone. Some metabasites and metasediments contain grossularitic almandine garnet, spessartine rich at low grade to pyropic at high grade. Sodic pyroxene is not particularly common in the blueschists, and is generally an acmitic variety, although omphacite occurs in eclogites. Paragonite is common in the epidote zone, often associated with phengite, while chlorite and albite are present at all grades, generally in small amounts. Calcite, dolomite and chloritoid occur in metasediments of the epidote zone. The unique assemblage lawsonite-glaucophane-chloritoid was found. Apart from zoning in minerals, and minor retrograde alteration, most rocks appear to be equilibrium assemblages, and obey the phase rule. The conditions of metamorphism are estimated from available experimental studies, and calculations based on mineral thermodynamic data. Equilibria involving lawsonite, epidote, grossular, albite, paragonite and quartz suggest 395°C and 7.5 kbar at the disappearance of lawsonite. Metasediments in the middle of the epidote zone yield a minimum of 420°C from the calcite-dolomite geothermometer, and 444° ± 20°C from coexistence of chloritoid, garnet, albite paragonite and quartz. A pressure of 10-11 kbar at 500°C is calculated from garnet, epidote, albite, paragonite and quartz in the highest grade blueschists. Corrections to the muscovite-paragonite solvus for pressure and non-binary components give plausible temperatures and pressures for coexisting white micas. Mineral stabilities, and equilibria between paragonite, jadeite, albite, grossular and zoisite, indicate a maximum of 550°C and 15 kbar for eclogites, and that current versions of the garnet-clinopyroxene thermometer are not giving accurate results in eclogites with high jadeite or acmite in the pyroxene. From buffer equilibria between calcite and Ca-Al silicates in metasediments, the non-ideal mole fraction of CO2 is estimated to be about 0.02 in the epidote zone, and 0.002 in the lawsonite zone. Graphite, pyrite and pyrrhotite in metasediments allow calculation of fluid composition in the system C-H-O-S, and show that very minor CH and H2S were present. Water activities may have been high in some eclogites. The metamorphic conditions require that both blueschists and eclogites were formed in a subduction system, and support the continent-arc collision hypothesis. However, some form of active and rapid transport is needed to bring these rocks back to shallow levels, and the term "retrojection" is coined.
| Identifer | oai:union.ndltd.org:ADTP/276132 |
| Date | January 1980 |
| Creators | Ryburn, Roderick James |
| Publisher | ResearchSpace@Auckland |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | Items in ResearchSpace are protected by copyright, with all rights reserved, unless otherwise indicated., http://researchspace.auckland.ac.nz/docs/uoa-docs/rights.htm, Copyright: The author |
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