Excavation of buried Dun Mountain–Maitai terrane ophiolite by volcanoes of the Auckland Volcanic field, New Zealand

Figures & data

Figure 1 Tectonic setting. A, The Auckland Volcanic Field within northern New Zealand. Intraplate volcanic fields labelled in pink. Inset shows relationship to New Zealand tectonic setting. Green line: Dun Mountain–Maitai terrane (M) with the Dun Mountain Belt that generates the Junction Magnetic Anomaly. B, Gravity and magnetic anomalies of the Auckland area, dominated by the effects of the Dun Mountain Belt. Magnetic anomalies after Eccles et al. (2005). Takapuna gravity anomaly after Williams et al. (2006). C, Schematic block diagram of basement structure and terrane subdivision under the Auckland region (not to scale) after Williams et al. (2006). Blue line marks postulated position of in situ Maitai Group sediments. AVF: Auckland Volcanic Field.

Figure 2 The St Heliers Volcanoes. A, Location of the volcanoes (red triangles) within the Auckland Volcanic Field (blue triangles). Yellow rectangles: basement depths after Kenny et al. (2011, 2012) and Edbrooke et al. (1998). MRD: Mount Roskill drill hole, EP: Eden Park drill hole (E Shalev, IESE, University of Auckland, pers. comm. 2013). Green rectangles: basement depths after Davy (2008). t.c.: volcano with xenoliths of terrigenous clastics (greywacke). B, Map of the St Heliers volcanoes, based on figure 6.6 in Searle (1981).

Table 1 Brief summary of lithology and mineralogy of the country rock-derived lithic clasts ejected with phreatomagmatic tuff at Glover Park and Taylors Hill volcanoes. For more detailed descriptions see Spörli & Black (2013).

Type of non-juvenile lithic clast	Mineralogical and lithological characteristics
Foliated (schistose) metabasites
Amphibolites with varying degrees of foliation	Greenish-grey to black clasts dominated by hornblende; less common calcic pyroxene and albite. Rocks range from finely foliated, almost phyllonitic hornblende schist to coarsely foliated types. Multiple sets of epidote veins common. At least one sample has macroscopic garnet. Veined and cut by cataclastic faults.
Severely metasomatised rocks
Rodingite	Yellowish light-coloured aggregates of quartz and fibrous talc with spectacularly zoned quartz veins. Contains hydrogrossular garnet, in some cases concentrated in layers and seams. Protolith was foliated.
Non-foliated metabasites
Gabbro/diorites	Coarse-grained rocks with little or no development of tectonic foliation. Strongly altered, with acicular actinolite replacing clinopyroxene and amphibole. Veined and cut by cataclastic faults.
Pyroxenites	Coarsely crystalline rocks with clinopyroxene crystals up to c. 3 mm; some replaced by metamorphic actinolite. Pervasive cataclasites.
Porphyritic volcanic rocks	Fine-grained mafic rocks, most porphyritic but some grade to aphanitic textures; veined and cut by cataclastic faults.
Metamorphosed sedimentary rocks
Meta-sandstones (greywackes)	Mostly fine-grained finely bedded volcaniclastic meta-sandstones, some veined with prehnite.
Calcareous meta-sandstone	Calcareous meta-sandstone with numerous (20–50% of rock) angular calcite platelets identified as Atemodesma.
Red chert	Massive with quartz veining.
Fault rocks
Cataclasites	Very conspicuous, homogenous to multi-modal rocks, some heavily charged with opaque material, others more translucent in thin section. Present as individual lithic clasts and as components of other clasts.
Non-metamorphosed sedimentary rocks
Sandstone and mudstone (including Waitemata Group)	Weak lithic sandstones and mudstones. No veining observed.

Figure 3 Modes of occurrence of the inclusions; all examples are from the main sampling area below Glover Park volcano (Fig. 2B). A, Fallen block of bedded tuff with dark metabasic rock clasts of two different sizes. B, Fallen block of coarser tuff on the beach. Light-coloured fragments are Waitemata Group sandstones (W); note rounding of some clasts. Dark fragment on lower left is juvenile lava. Hand lens for scale. C, Lava clast with angular fragments of Waitemata Group rocks making a jigsaw pattern. D, Very small lithic clasts associated with bubble trains (arrowed) in a recycled composite lava fragment. Paired yellow arrows show sense of lava flow-shearing along bubble/fracture surfaces. Paper clip is 28 mm long.

Figure 4 Finely foliated metabasite. A, Tracing of a whole thin-section scan. The sequence of structural features is indicated in red. An albite vein cross-cutting cataclasite is shown at a/c. Letter in rectangle locates the micro-photo. B, Micro-photo (plane-polarised light) showing the S₁/S₂ foliation and an F₂ fold. Fsp: feldspar; CC: cataclasite. Green lines and ‘amph’ indicate fanning S₂ amphiboles. C, Photograph of the specimen. ep: epidote; FR?: possible fault repetition of the thick epidote vein. The thin-section face is on the opposite side of the specimen.

Figure 5 Garnet-bearing, coarser metabasite with polygonised feldspar, sample AU58784. The sequence of structural features is indexed in red. A, Tracing of a whole thin-section scan. Location (1) shows reactivation of D₄ top-to-the-left ductile shear by D₇ top-to-the-right cataclastic faulting. B, Micro-photo, plane-polarised light. G: andradite garnet. Note contrast between dusty (saussuritised?) metamorphic feldspars (Fsp) and clear, polygonised (metasomatic?) feldspars (pFsp).

Figure 6 Photomicrographs of various rock types. A, Rodingite: seam of garnets (black) cross-cut by quartz veins. Thin section Searle 5253, crossed polarisers. Note talc/quartz material with schistosity-like fabric (red lines), paralleled by quartz veins, in the upper left-hand corner. The rest of the section is more quartz-rich. B, Pyroxenite: thin section Searle 5233 from Glover Park volcano. Photomicrograph (plane -polarised light) showing transverse section of clinopyroxene crystal (Py) disturbed by cataclastic seams. Am: amphibole-rich interstitial material. The location of this photograph is shown in SF Fig. 3. Note that the photograph is rotated c. 90° counter-clockwise relative to SF Fig. 3. C, Portion of thin section (Searle 5275a) of a fossil-bearing basement sandstone from Glover Park volcano in plane-polarised light, showing shell fragments, clinopyroxene heavy minerals and calcitic veins. See the discussion for the important significance of this specimen.

Figure 7 Schematic cross-section of Glover Park volcano illustrating the lithic-bearing tuff ring and underlying diatreme and country rock. Small angular shapes represent lithic clasts. Not to scale.

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