Chapter 5. Kiwitahi Chain Volcanism
The minimum ages of volcanic rocks in Coromandel Volcanic Zone decrease towards the south and west. A trend of generally decreasing ages towards the SSE also occurs in the Kiwitahi chain of volcanic centres, on the western, continental edge of the CVZ. Unlike the complex volcanic stratigraphy in the CVZ, the Kiwitahi centres erupted in spatial isolation and separated by intervals of 1 Myr or more. Nevertheless, the erupted basaltic andesites and andesites are in part comparable to equivalent rocks in the CVZ. This chapter presents new bulk rock geochemical and isotope data for Kiwitahi volcanic rocks in the form of a paper that has been published in Journal of Volcanology and Geothermal Research . The paper documents the development of the Kiwitahi centres, which for the first c. 9 Myr of intermittent eruption had generally comparable compositions to andesites in the CVZ. A notable difference with CVZ andesites, however, is the consistently low enrichment in the highly incompatible elements in Kiwitahi andesites. The youngest Kiwitahi rocks, basaltic andesites erupted between 6.2-5.5 Ma, have markedly different compositions to CVZ andesites and older Kiwitahi andesites, and can be classified as high-Mg andesites. Their trace element distribution is comparable to that of high-Mg andesites in the TVZ. The Kiwitahi chain, and the chain of high-Mg andesite centres in the western TVZ, are interpreted as recording volcanism over the continental edge of the migrating arc magmatic system, the bulk of which at any time was located to the north and east.
Evolving Volcanism at the Tip of a Propagating Arc: the Earliest High-Mg Andesites
Northern New Zealand Mathijs A. Booden, Ian E.M. Smith, Jeffrey L. Mauk, Philippa M. Black Published in Journal of Volcanology and Geothermal Research vol. 195, pp. 83-96.
A NNW-striking string of isolated volcanic centers, the Kiwitahi chain, erupted between 15 and 5.5 Ma in northern New Zealand. Prior to 6.2 Ma, the erupted rocks were plagioclase- and hornblende-dominated andesites, which are geochemically comparable to coeval andesites erupted in the nearby, much larger Coromandel Volcanic Zone (CVZ). Compared to CVZ andesites, however, the Kiwitahi andesites show more subdued incompatible element enrichments, and they generally have relatively unradiogenic Sr isotope compositions. These features, and the small eruption volumes involved, suggest that the Kiwitahi centers formed over the edge of a magmatic system that was centered on the CVZ. The Kiwitahi centers progressively become younger towards the SSE representing the migration over the time of the edge of this magmatic system. Between 6.2 and 5.5 Ma, four centers at the southern end of the chain erupted pyroxene-dominated, high-magnesium andesites that are geochemically unlike coeval andesites in the CVZ, but similar to Quaternary high-Mg andesites erupted along the western edge of the Taupo Volcanic Zone. These are the earliest known high-Mg andesites in northern New Zealand; their appearance may mark the inception of the current configuration where high-Mg andesite eruptions precede regular andesitic volcanism at the leading edge of the arc. Keywords High-Mg andesite; subduction; volcanism; petrogenesis.
The SW Pacific region has had a tumultuous Tertiary tectonic history where the convergence of the Pacific and Australasian plates has given rise to multiple episodes of subduction and back-arc spreading (e.g. Crawford et al., 2003; Schellart et al., 2006; Mortimer et al., 2007; Whattam et al., 2008; Herzer et al., 2009). Tertiary subduction-related volcanism in northern New Zealand has developed in response to dynamic plate configuration readjustments, gradually shifting from Northland to the Taupo Volcanic Zone over the course of 25 million years (Adams et al., 1994; Hayward et al., 2001 and references therein; Briggs et al., 2005). From the Middle Miocene to the Early Pleistocene, the Coromandel Peninsula was the locus of activity (Skinner, 1986), representing what may have been the continental edge of the primarily oceanic Colville Arc (Ballance et al., 1999). A chain of small volcanic centers, the Kiwitahi chain, formed at the edge of this volcanic zone and records 10 million years of intermittent activity (Black et al., 1992) against the backdrop of continuous and evolving andesitic, rhyolitic and basaltic volcanism in the Coromandel Volcanic Zone. The Kiwitahi chain’s eruptive products evolved from plagioclase-dominant andesites to clinopyroxene-dominant high-Mg andesites during this time. This paper presents new geochemical and isotope data that document this evolution and we discuss the relationship of the Kiwitahi volcanic chain to coeval volcanism in the Coromandel Volcanic Zone and to the subsequent eruption of high-Mg andesites in the Taupo Volcanic Zone.
The NNW-striking Kiwitahi Volcanic chain comprises nine volcanic centers that erupted from 15 to 5 Ma (Black et al., 1992). The centers generally become younger towards the SSE. The Kiwitahi chain flanks the western boundary of the NNW-striking, 300 by 20-40 km Hauraki Rift. This is a double half-graben that comprises a western half-graben, which is downthrown 2-3.5 km along the median, normal, west-dipping Kerepehi Fault; a buried median horst; and an eastern fault angle depression along the normal, west-dipping Hauraki Fault. The Hauraki Rift boundary is locally offset along NE-striking faults (Hochstein and Nixon, 1979; Hochstein et al., 1986; de Lange and Lowe, 1990). The Hauraki Rift is probably no older than 7 Ma (Hochstein and Ballance, 1993) and therefore post-dates all but the youngest Kiwitahi centers. Voluminous subduction-related volcanic rocks occur to the east of the Hauraki Rift as the Coromandel Volcanic Zone (CVZ) and include basalts, andesites and rhyolites (Fig. 5.1; Skinner, 1986). The CVZ was initially (18-12 Ma) dominated by andesitic volcanism. From 12 Ma, silicic eruptions occurred (Carter et al., 2003; Nicholson et al., 2004) that were joined from 9 Ma by sporadic basaltic eruptions (Adams et al., 1994). Locally in the CVZ, the inception of andesitic volcanism preceded the inception of silicic volcanism, and andesitic volcanism generally continued until after the cessation of significant silicic volcanism (Skinner, 1986). An extinct NNE-striking volcanic arc, the Colville Ridge, impinges on New Zealand at the CVZ. The Colville arc was active at least as early as 16.7 Ma (Mortimer et al., 2010) and it is likely that at least part of the CVZ represented the continental tip of this largely oceanic arc, in a similar tectonic configuration to the current Taupo Volcanic Zone-Kermadec Arc system
The Kiwitahi Volcanic Chain
The Kiwitahi centers erupted through and onto Jurassic greywacke basement rocks of the Waipapa Terrane (Skinner, 1972; Edbrooke, 2001). Lavas, breccias, agglomerates and dykes occur and likely represent the remains of small cone volcanoes or monoeruptive centers. The Kiwitahi centers generally become younger towards the SSE (Black et al., 1992). We view this as analogous to the general SSE migration of the volcanic front in the CVZ (Skinner, 1986). Only the youngest, southernmost centers show preserved volcanic landforms. Because the Kiwitahi eruptions were widely separated in time, it appears likely that the magmas from the different centers were not directly genetically related to each other. We distinguish the following petrologic groups. (1) The four oldest centers (NE Waiheke Island, Ness Valley, Miranda and Pukekamaka (Waiheke-Pukekamaka)) formed between 15 and 10 Ma (following K-Ar ages of Black et al., 1992) characteristically have hornblende. They comprise porphyritic, plagioclase- or hornblende-dominant (+ pyroxene) basaltic andesite, andesite and minor dacite with 15-50 vol% crystal content (Fig. 5.2A-B). Hornblende crystals are typically subhedral rounded and opaque-rimmed, suggesting resorption by the melt, whereas plagioclase and pyroxene typically form euhedral to subhedral, commonly angular crystals. The least-porphyritic rocks (15-20 vol% crystals, Pukekamaka center) have hornblende phenocrysts with only subordinate plagioclase and pyroxene. (2) The 6.7 Ma Tahuna center erupted plagioclase + pyroxene andesites. Tahuna rocks are petrographically comparable to the andesites of the older centers, but lack hornblende (Fig. 5.2C). (3) The four youngest centers (Ruru, Te Tapui, Maungakawa and Maungatapu (Ruru-Maungatapu)) erupted between 6.2 and 5.5 Ma at the southern end of the Kiwitahi chain. Ruru, Te Tapui and Maungakawa are small eroded cones aligned along a 305º trend; Maungatapu is another cone located ~10 km further north. The Ruru-Maungatapu centers comprise porphyritic basaltic andesites with typically less than 30% crystals (Fig. 5.2D-F). Plagioclase occurs as a minor phenocryst phase at Ruru, the oldest center, but not in the subsequently formed centers. Angular to sub-rounded, typically eu- to subhedral clinopyroxene is the dominant phenocryst in rocks at each center. Petrographically similar clinopyroxene crystals also occur with Fe-Ti oxides in glomeroporphyritic aggregates, usually without plagioclase. Subordinate olivine occurs in some samples. Groundmasses comprise fine-grained plagioclase + pyroxene + Fe-Ti oxides. The dominance of pyroxene and the near-absence of phenocrystic plagioclase sets these rocks apart from the plagioclase-dominant andesites that erupted in the same timeframe to the NNE in the CVZ, and from the Waiheke-Pukekamaka and Tahuna centers. In addition to the centers described above, basaltic andesites of the Stony Batter center erupted between 6.9-8.3 Ma on Waiheke Island in close spatial association with the much older NE Waiheke center (Kuschel, 1988; Black et al., 1992). The Stony Batter rocks are porphyritic olivine ± clinopyroxene basaltic andesites with plagioclase in the groundmass (Smith et al., 1993). Their eruption at the northern end of the Kiwitahi chain does not conform to the south-directed younging trend defined by the other centers. Petrographically and chemically they are similar to 8.4-10 Ma basaltic andesites of the Ti Point center in Northland, and with those have been interpreted as the product of backarc magmatism associated with the CVZ (Smith et al., 1993; Smith et al., 1995).
Chapter 1 Introduction
1.2. Motivation and Aims
1.3 Approach and Methodology
1.4 Thesis Outline
Chapter 2 The North Island Volcanic Succession
2.1 Geologic Setting of Northern New Zealand
2.2 The North Island Volcanic Succession
2.3Hydrothermal Alteration of the Volcanic Succession
Chapter 3 Northland Arc Volcanism
3.2.Geochemistry of the Early Miocene Volcanic Succession of Northland, New Zealand, and
Implications for the Evolution of Subduction in the Southwest Pacific
Chapter 4 Coromandel Volcanic Zone Volcanism
4.2.The Transition from Intermediate to Silicic Volcanism in the North Island of New Zealand
4.3 .Volcanic development of the Coromandel Volcanic Zone
Chapter 5 Kiwitahi Chain Volcanism
5.2..Evolving Volcanism at the Tip of a Propagating Arc: the Earliest High-Mg Andesites in Northern New Zealand
Chapter 6 Application to Geochemical Exploration
6.2.Quantifying Metasomatism in Epithermal Au-Ag Deposits: a Case Study from the Waitekauri Area, New Zealand
6.3..Determination of Protolith Composition in Whitianga Group Rhyolites
Chapter 7 Synthesis
7.1Evolving Volcanic Style
7.2. Volcanism and Mineralisation
7.3..Avenues for Future Research
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