Contemporaneously emplaced submarine volcaniclastic deposits and pillow lavas from multiple sources in the island arc Brook Street Terrane, Southland, New Zealand

Figures & data

Figure 1. Location map with Riverton section field area boxed. Also shown are intrusive rocks of the Median Batholith, and the largely volcanic Brook Street Terrane (the western contact of the terrane is not exposed on this coast). Basement geology based on Turnbull and Allibone (2003).

Figure 2. Lithofacies map and generalised vertical successions (GVS). Outcrop along the coast is almost continuous; inland it is sparse. Numbered circles mark field localities. Bold line on inset map shows the location of the cross-section (Figure 3A) relative to mapped geology.

Figure 3. A, Cross-section along the coast, showing lateral relationships among lithofacies. This section is vertically exaggerated (×2) so relationships can be better seen. The strips of lapilli tuff in section E–F are a schematic representation of lapilli tuff filling channels in tuff (see text). Argillite and tuff are in part interbedded (not illustrated). B, Logged stratigraphic section of the western side of the sequence (that represented in the Western GVS of Figure 2).

Figure 4. Features of these rocks include: A, Reverse-graded beds in lapilli tuff, with black arrows showing the direction of grading. B, Lapilli tuff filling channels within tuff; C, Peperitic contact between pillow lava below and tuff above. D, Argillite showing Bouma sequences (dotted lines denote boundaries between successive sequences; notebook for scale has 20 cm spine). E, Clasts in lapilli tuff at site 29, showing the distinction between aphyric clasts and those with abundant amygdales, with quartz cement in the matrix. F, Examples of clasts within the tuff, including those bearing chlorite-filled amygdales, and those with albite laths. G, Amygdale in pillow lava showing pumpellyite rim and mixed centre: regions with different crystal sizes are also visible. H, Dike, showing fresh clinopyroxene and altered plagioclase phenocrysts and chlorite pseudomorphing crystal shapes. Thin section images in plane polarised light.

Table 1. Summary of lithofacies features.

Lithofacies	Contacts	Composition	Features	Alteration
Pillow lava	All or most pillows are intrusive and locally have peperitic contacts with lapilli tuff both beneath and above, and tuff above.	Finely crystalline, groundmass of albite and clinopyroxene with secondary chlorite and pumpellyite; aphanitic to few phenocrysts of albite and clinopyroxene.	10–70 cm distinct pillow shapes, some with dark once-glassy rinds. Abundant 2–6 mm amygdales of quartz, chlorite, prehnite, pumpellyite; often forming concentric rings, some with centre of solid quartz; quantity increases towards pillow rims.	Some with networks of chlorite veins.
	Inter-pillow matrix is present in most pillow lava outcrops.	Some with 2–10 mm dark angular basalt clasts; in other places pale-coloured and sandy-textured with clasts of basalt, albite, and clinopyroxene.	Filling gaps between pillows in most places; some weak laminations.	All highly altered; some entirely recrystallised to coarse quartz, prehnite, epidote, chlorite, and calcite.
Lapilli tuff	Peperitic contacts with pillow lava.	2–50 cm subangular clasts of pillow lava, dike fragments in some places; medium sand matrix including small basalt clasts, isolated amygdales, and crystals of feldspar, quartz, and clinopyroxene.	0.2–2 m bedding, varying in thickness and nature over the area; normal and reverse grading, sequences of coarse beds alternating with 2–4 finer graded beds, isolated fine beds with few larger clasts.	Quartz veins, quartz cement in matrix in places; alteration to quartz and chlorite; some pyrite and chalcopyrite.
Tuff	Conformably overlies lapilli tuff; peperitic contacts with pillow lava; channel zone where lapilli tuff appears as coarse channel fill in tuff.	Very fine to coarse sand (varying over area); grains of basalt, devitrified glass, clinopyroxene, feldspar, and quartz crystals.	0.1–1 m beds, commonly with laminations, cross-laminations in places; some weak lensing.	Quartz veins, bioturbation, cubic pyrite crystals; pseudomatrix; clay minerals.
Argillite	Sharp incised contact with underlying tuff, interbedded toward contact in places; sharp contact with overlying tuff.	Silt to fine sand grains of quartz, albite, calcite, and clinopyroxene; calcite cement.	0.1–1 m beds, with laminations and cross-bedding forming Bouma Tb-e sequences.	Quartz veins, fine pyrite crystals, bioturbation, iron staining, calcite concretions.
Dikes	Cut through all other lithologies with chilled, often undulating margins; apophyses are common into tuff and lapilli tuff, with some engulfed domains.	Most porphyritic with a fine groundmass of feldspar, clinopyroxene, and titanite, and phenocrysts of clinopyroxene and plagioclase; some with coarser groundmass, some aphyric.	0.2–1.5 m thick dikes; most with flow banding (denoted by bands of varying vesicularity and phenocryst content). Few small (0.2–0.4 mm) amygdales of chlorite, prehnite, pumpellyite, and quartz. Some zoned phenocrysts.	Some at a similar level of hydrothermal alteration to the surrounding rock, others much fresher; some with iron staining.

Figure 5. Clinopyroxenes are mostly augite and diopside.

Figure 6. Measured feldspar compositions. End-member albite and orthoclase probably formed during alteration.

Figure 7. A, Chondrite-normalised rare earth element (REE) and B, Primitive mantle-normalised trace element profiles for the averages of the cores of augite crystals for each sample. Normalising values from Sun and McDonough (1989).

Figure 8. Calculated model whole-rock A, Chondrite-normalised rare earth element and B, Primitive mantle-normalised trace element profiles, compared to whole-rock data from Spandler et al. (2005), showing similar patterns with a few notable differences (see text, Supplement). Normalising values from Sun and McDonough (1989).

Figure 9. Plots of A, Y/Yb vs. Nb/Yb after Maurice et al. (2012) and B, Ba/Yb vs. Nb/Yb after Pearce and Stern (2006). Key in bottom right applies to both plots.

Figure 10. Map showing inferred volcanic depositional systems and positions of two small volcanic centres. Positions of depositional fan lobes and branching channel indicated by features in outcrop. Argillite is derived from the main volcano’s central vent(s); tuff and lapilli tuff from satellite vents form the marked lobes. Trace element sample numbers are marked on the map; positive Pb anomaly = oval symbols, negative Pb anomaly = rectangles.

Figure 11. depositional system scenarios for the Riverton section (not to scale); A, Single lobe of a submarine fan, with lapilli tuff-filled channels. B, Large-scale lapilli tuff-filled channel with tuff turbidites forming levees.

Figure 12. interpretative cross-section of the Riverton section along the Colac Bay coast, showing a proposed structure and sequence of events for deposits of the western vent (not to scale): A, Initial generation of pillow lava is accompanied by generation of hyaloclastite which is redeposited in a submarine fan lobe by a series of channel-forming mass flows. B, A second (observed) generation of pillow lava intrudes these deposits to form peperite.

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Data availability statement

The data that support the findings of this study are openly available in Pangaea https://doi.pangaea.de/10.1594/PANGAEA.917577, reference number PDI-23688.