Petrographic and geochemical study of Morrinsville facies, North Island, New Zealand

Abstract

The Late Jurassic to Early Cretaceous Morrinsville facies of greywacke sandstone, North Island, New Zealand, is considered as the younger among the two facies in the Manaia Hill terrane of the Waipapa Composite terrane. These basement rocks consist of fine to coarse-grained greywacke sandstone, interleaved with graded sandstone, siltstone, conglomerate and chipwacke containing mudclasts. They are characterized by petrological monotony, scarcity of bio- and litho-stratigraphic markers, and tectonic complexity due to oblique subduction. Geochemical and petrographic study was undertaken of the Morrinsville greywacke sandstones, including whole rock sediments and conglomerate, which extended from Coromandel to as far south as Te Kuiti. The study has identified at least three different populations of clasts and two groups of sediments. Clast groups include sandstone clasts from Coromandel and South Auckland, and a non-sandstone clasts group mainly from South Auckland, which consists of meta-sedimentary and meta-volcanic rock types. The whole rock population can be broadly divided into two groups, one representing Coromandel and South Auckland area and the other, Central North Island, but within the northern group there are differences noted from Coromandel to South Auckland, but considered not enough to divide them into further groups. The provenance for acidic, medium to high-K Morrinsville rocks from Coromandel to central North Island, was inferred as intermediate (andesitic to dacitic) to felsic (rhyodacitic) igneous types, with continental are to oceanic are tectonic setting. Local influences of extra sources, like quartzose sedimentary type for South Auckland and relatively less evolved volcanic type for central North Island are also proposed based on the relatively more sedimentary lithics in South Auckland and more volcanic lithics in central North Island. The gradual change of intermediate to felsic type from Coromandel to the central North Island may be explained by progressive dissection of a continental arc massif, which appears to be the dominant tectonic setting of the source areas. Comparison of clasts and sediments within and with other groups of sediments from nearby terranes involving Bay of Islands, Murihiku, Maitai, Caples, Torlesse (Axial A, Axial B, Waioeka, and Omaio) terranes, Hunua facies and the Melange Zone present between Bay of Islands terrane and Hunua facies, indicate that sandstone clasts from Coromandel and a group of clasts from South Auckland are similar to their sediment groups and the terrane groups of northern North Island. Therefore, these clasts are considered as either intrabasinal, or recycled from the same or a similar source as the sediments, but the other group of sandstone clasts and the group of non-sedimentary clasts from South Auckland showed no similarities with the terrane and sediment groups involved in comparison. Therefore, the Median Batholith or/and a source from eastern Australia is suggested for them on the basis of a U/Pd zircon study conducted for Morrinsville sediments. Whole rocks from Coromandel and South Auckland show strong similarities with the northern North Island terrane groups, especially with Waioeka subterrane and Melange zone samples, but showed little or no similarities with other terrane groups presently located to the east west and south, this may require a revision of previous postulations about Morrinsville sediments derivation from Murihiku terrane. The sediment group from central North Island shows no affinities with the other sediment groups of Morrinsville facies, but shows similarities with Bay of Islands terrane, which might need remodelling of the present configuration of Morrinsville facies rocks.