Insights into Akaroa Volcano s Magmatic System through Analysis of Plutonic Lithics

Transcription

1 Insights into Akaroa Volcano s Magmatic System through Analysis of Plutonic Lithics Caroline Lawlor 1,2, Sam Hampton 1, Elisabeth Bertolett 1 1 Department of Geological Sciences, University of Canterbury, Christchurch, NZ 2 Oberlin College Geology Department, Oberlin, OH, USA Abstract Analyzing plutonic material from a volcanic system can provide insights with respect to the magmatic processes occurring within that system. Here, we examine plutonic lithics through a textural, geochemical, and petrographic lens in two locations within Akaroa volcano s magmatic system on Banks Peninsula: (1) Goat Rock dome, and (2) Flea Bay. Goat Rock s plutonic enclaves range from % SiO2 and exhibit conditions of geochemical and textural variation that suggest that they come from a magma body with compositionally distinct layers that experienced mafic recharge or intrusion events. The formation of enclaves on a spectrum from porphyritic to equigranular can be explained by thermal differences between the host magma and the replenishing magma. A comparison of plutonic lithics in other parts of Akaroa volcano support the model of magmatic evolution involving shallow, fractioned magma batches with multiple eruptive sites. Less can be inferred about Flea Bay lithics, but they show varying amounts of alignment and seem to be geochemically similar to plutonic lithics nearby. Geological Setting & Introduction Banks Peninsula is situated on the eastern coast of New Zealand s South Island and formed by multiple eruptive phases of composite shield volcanoes Lyttleton and Akaroa as a result of Miocene intraplate volcanism (Timm et al., 2009). While most igneous material on the peninsula is extrusive, some localities contain plutonic lithics or enclaves. A few prior Banks Peninsula studies have incorporated analysis of plutonic material as well involved textural and geochemical characterization of plutonic enclaves at lava domes (Sewell, 1993; Bertolett, 2014) in attempts to interpret and determine processes and eruptive sequences that have occurred there. Goat Rock one of these domes - is a basaltic trachy-andesitic dome (Tramontano, 2012) produced by Akaroa Volcanic Complex (AVC) and situated in the northeast part of Banks Peninsula. Flea Bay another area where plutonic material was recently found and analyzed for this study - is further south and was formed by lava flows that also originate from AVC (Figure 1). The integration of plutonic rocks into studying volcanic rocks can provide more wholesome understanding of magmatic systems, as plutonic material serves as sort of a composite, larger scale representation of a system and erupted volcanic material is more of an instantaneous portrayal and shows the evolution of magma erupted from the plutonic domain (Bachmann et al., 2007). Although found in a handful of locations in AVC, inclusions of material from the upper mantle and lower crust are rare in the Canterbury region (Sewell, Hobden, and Weaver, 1993) and present valuable insights as to how magmatic systems operate and what processes occur within them. Plutonic material found at Goat Rock takes the form of enclaves

2 that exhibit variation in size, texture, and composition. They range in size from <1cm to 17cm in diameter and make up approximately 2% of the dome (DiPadova, 2015). The vast majority of these enclaves display radiating cooling fractures within the surrounding host rock (Figure 2). Plutonic material in Flea Bay was found in what appeared to be a breccia between lava flows that formed the western side of the bay. There was a single sizable chunk of plutonic material found in this location as well as what appeared to be several smaller crystalline cumulates and an ash horizon. Models of AVC s magmatic system and processes have been refined over time (Figure 3). Some of the first studies to explore these processes determined that fractional crystallization and assimilation of wall rock occurred had occurred within AVC (Price & Taylor, 1980; Dorsey, 1988). These are relatively common magmatic processes, but these studies laid the groundwork as well as presented an initial model for further work on understanding the complex s magmatic system through a mineralogical, petrological, and geochemical approach. More recently, there have been assertions of a lithospheric detachment / delamination model for Banks Peninsula, which details the compositional differences in source magma for Lyttleton and Akaroa volcanoes (Timm et al., 2009) as well as an enhanced magmatic model for AVC that suggests the presence of mafic replenishing events and evidence for magma mixing (Hartung, 2011). The most recent model of AVC magmatic evolution further builds upon this and proposes a system that consists of shallow magma batches of varying compositions that may be responsible for cyclic variations in geochemical stratigraphy of lava flows (Johnson, 2012). Though not all of these studies incorporated plutonic material into their analysis, some (Dorsey, 1988; Hartung, 2011) did and used it to enhance a number of assertions. Domes such as Goat Rock make up a significant portion of the locations of plutonic material found in AVC and processes that occurred during their formation can suggest possible processes in the larger magmatic system. Ultramafic xenoliths at Le Bons Bay Peak a dome is close to Goat Rock were grouped into populations based on shared geochemical and textural characteristics. Upon analysis, these plutonic xenoliths suggested various modes of petrogenesis for the different populations at the dome (including a rapid rise to the surface) and proposed the possibility of magmatic underplating on the larger scale of Banks Peninsula to account for geochemical variation (Sewell, Hobden, Weaver, 1993). Recently, plutonic enclaves at Goat Rock have also undergone textural and geochemical analysis to provide enhanced understanding of magmatic processes present at the dome, which have thus far been found to support the most recent model of AVC magmatic evolution involving shallow, chemically diverse magma batches (Tramontano, 2012; Bertolett, 2014). It has been suggested that the existing variation among the enclaves could possibly have stemmed from separate magma bodies or from a single magma chamber with compositionally diverse layers, though more analysis of the various enclave types and textures may be needed for these claims to be further investigated (DiPadova, 2015). Simultaneous petrological, textural, and geochemical analysis of plutonic lithics can provide insights into the nature of magma mixing within a system as well (Plail et al., 2014).

3 The aim of this study is to analyze plutonic lithics at Goat Rock to further explore their textural, geochemical, and petrographic variation and the magmatic processes they may propose, support, and/or dispute for Goat Rock and for AVC in general. These lithics will also be compared with other plutonic material found around AVC to enhance this analysis. The recently found material at Flea Bay will also be characterized as a part of this study. Methods Field methods for this project included taking samples of plutonic lithics from Goat Rock and Flea Bay for geochemical and textural analysis. Several enclave samples have been collected at Goat Rock in recent years, but some of these enclaves have proven exceedingly difficult to obtain using a rock hammer and chisel. Recently, a concrete saw was used to cut around these hard-to-get lithics to facilitate and expedite the collection process (Figure 4). This process yielded 19 enclave samples and a few samples of host material, the majority of which was analyzed as a part of this study. Prior to geochemical and textural analysis, the Goat Rock samples were grouped into populations following the methodology used in Sewell, Hobden, and Weaver, 1993 for their characterization of ultramafic xenoliths based on initial observations of texture, size, shape, grain size, and boundary conditions. These populations shifted as the samples were analyzed and more information came to light. X-ray Fluorescence (XRF) analysis was done at the University of Canterbury to determine major and trace element geochemistry for 11 plutonic lithics, two host rock samples, and an underlying scoria sample from Goat Rock, as well as a host sample and plutonic lithic sample from Flea Bay. Polished thin sections were made for several of these samples as well as for others from the sample set including some smaller crystal cumulates from Flea Bay and three perpendicular thin sections of the larger Flea Bay sample to further explore crystal alignment. These polished thin sections were made to more thoroughly analyze texture and for use in EBSD analysis in further studies. Results Three enclave populations for the Goat Rock lithics were made based on similar geochemistry, texture, and appearance insitu (Figure 5). Due to geochemical spread and significant textural variation, not all of the lithics were grouped into populations because of noteworthy differences in the population-determining factors that, in combination, made them unique within the sample set (Figure 6). No populations were determined at Flea Bay due to limited geochemical data and fewer lithics in general. Texture/Petrography Population 1 (P1) lithics exhibited a generally porphyritic texture with elongate plagioclase phenocrysts (55-65%) that had a slight alignment at most. Lithics in Population 2 (P2) were also porphyritic, but were characterized by

4 abundant plagioclase feldspar phenocrysts (~75%) that were glomeroporphyritic (clustered) with no alignment and had rough edges. Population 3 (P3) contained enclaves that were largely equigranular with noticeable alignment, which is noteworthy since most of the other Goat Rock lithics tend to be generally porphyritic. These lithics also seemed to contain fewer mafic minerals (pyroxenes and possibly olivines; 15%) and contained abundant feldspars. Among the outliers was an enclave (15GR04) that exhibited a schlieren (aligned plagioclase) texture consistent with samples from previous studies of Goat Rock lithics. Sample 15GR11 was generally porphyritic and was the only lithic in the sample set that displayed chilled margins as the enclave had finer grains closer to its contact with the host. In contrast, another outlier (15GR22) had a very sharp, jagged host contact with no change in grain size and also seemed to be texturally in between P1 and P3 enclaves in that it was porphyritic, but its phenocrysts were relatively smaller and less elongate. Another outlier (15GR25) did not have a thin section taken, but appeared to contain a relatively large proportion of darker minerals when viewed in situ. The final outlier (15GR26) is noteworthy because it contains at least two different equigranular textures within the same lithic. The textures appear to have slight compositional differences one of the textures looks similar to that of P3 enclaves whereas the other has finer grained, generally darker crystals - and larger grains are seen at the contact between the two main textures. The two host samples from Goat Rock had occasional phenocrysts (5-7%) with a dark, fine-grained groundmass that exhibited flow banding with some possible shearing structures (Figure 7). Poikolitic textures smaller minerals enclosed by/ growing over larger ones are common on plagioclase crystals in generally porphyritic enclaves. Resorption and sieve textures are also fairly common throughout the enclaves and among occasional phenocrysts in the host rock particularly among plagioclase (perhaps occurring less frequently among P3 enclaves). Albite and Carlsbad twinning is common throughout feldspars on all enclaves. Many of the minerals seem to have brittle fractures as well, which are most common in mafic minerals in nonequigranular enclaves. The Flea Bay samples showed less textural variation, and were most texturally similar to P1 enclaves from Goat Rock in that they are generally porphyritic and have abundant feldspar phenocrysts with slight alignment at times. The three perpendicular thin sections for the larger Flea Bay sample were characterized by a fabric of large, aligned plagioclase crystals interspersed with more mafic minerals similar to other plutonic lithics found in AVC. Thin sections of much smaller lithics from the area contained phenocrysts that ranged from being aligned to randomly oriented and varied in size and shape (Figure 8). Geochemistry Major element geochemistry of the lithics from Goat Rock showed a significant amount of variation in magmatic evolution of the samples, which ranged from 45-55% SiO2 and % alkalis (picrite basalt to mugearite). The two host samples from the dome were consistent and had little difference in geochemistry (see diagram in Figure 9). P1 enclaves had a very consistent geochemistry and were

5 the best-fit population overall in terms of chemical and textural similarities. P2 enclaves were geochemically similar to P1 both are classified as hawaiiite though enclaves in this population were slightly less evolved. The sole enclave with geochemical data in P3 was the most evolved notable because it was one of two enclaves more evolved than the host samples. The two host samples from Goat Rock (hawaiite / mugearite) were geochemically similar to past host samples from previous studies. A sample of the underlying scoria through which Goat Rock dome intruded (Tramontano, 2012) was also analyzed and found to be towards the relatively mafic end of the geochemical spectrum (picrite basalt). Among the outliers in the Goat Rock sample set, 15GR04 was, as picrite basalt, towards the more mafic end of the chemical spread and was also geochemically similar to other previously sampled schileren enclaves. XRF analysis was not conducted on sample 15GR11. As mugearite, enclave 15GR22 was more evolved than the host rock along with the P3 lithics. As with texture, this enclave occupies a region between P1 and P3 enclaves with respect to composition. 15GR25 (picrite basalt) was the least evolved member of the Goat Rock sample set by a significant margin. 15GR26 (hawaiite) was compositionally close to P1 and P2 enclaves. Trace element data for Goat Rock samples (Sun/McDon OIB) was fairly consistent and in line what would be expected for comparing AVC rocks to ocean island basalt in that the values fluctuated near 1, since Banks Peninsula is composed largely of ocean island basalts (Figure 10). Nearly all of the samples seem to follow the same trends with the exception of the least evolved enclave (15GR25), which has lower values and noticeably more depletion of Ba, Ce, and Nd among other elements. The sole geochemical data point from Flea Bay was relatively mafic (picrite basalt) and was compositionally similar to 15GR25. Tables from the major and trace element analysis can be found at the back of this report (Tables 1 & 2). Discussion Textural and geochemical variation amongst the Goat Rock enclaves offers some suggestions regarding how these enclaves formed and the nature of the magmatic system from which they came. The different textures present amongst the Goat Rock lithics seem to support Johnson s (2012) AVC model indicating that they came from a single (probably relatively shallow) magma body which is not homogenous, experienced mixing, and may be composed of smaller, geochemically diverse layers. Enclave analysis has suggested the following revisions, insights, and nuances that fit into overall magmatic model and more localized magmatism at Goat Rock: Texture Specific textures aside, enclaves fall along a spectrum from porphyritic to equigranular indicating varying rates of crystallization and cooling. Porphyritic enclaves, such as those in P1 and P2, experienced rapid cooling (quenching) compared to relatively equigranular ones, as in P3. Thus, porphyritic textures occur

6 because the enclaves had less time to evenly crystallize and cool whereas equigranular enclaves cool more slowly and evenly. A depiction of how these processes may occur within a magma chamber comes from Browne, et al. (2005), which explores the formation of porphyritic and equigranular enclaves at Unzen volcano in Japan (see Figure 12 for diagram). Existence of this process is supported by the presence of chilled margins found on a porphyritic sample at the contact with the host. In contrast, equigranular enclaves had no change in grain size approaching the sharp host contact. The range of textures suggests that the enclaves came from the same magma body, albeit one with chemical differences between layers and/or experienced different styles of magma mixing i.e. more equigranular enclaves likely experienced more mixing which may be why they are more evolved- with a source more mafic than the host magma (Browne et al., 2005). Additionally, morphology and strength of the prominent plagioclase framework found in many enclaves may place a control on the enclave size (i.e. an enclave with a more cohesive, touching framework will be more likely to stay intact during transport) as well as extent of magma mixing that occurs (Martin et al., 2006). Petrography Regardless of grain size and texture, some enclaves exhibit varying degrees crystal alignment whereas it is absent in others. Elongate plagioclase phenocrysts are more likely to show alignment in porphyritic enclaves whereas most if not all minerals take part in alignment in more equigranular lithics. The host rock also shows signs of flow banding and shear in places due to the rise and flow of magma. Sieve textures are present on plagioclase crystals and indicate resorption caused by magma mixing and/or changes and fluctuations in temperature and pressure (Browne et al. 2005). Additionally, dissolution occurs in places along the aforementioned chilled margins as individual crystals break off from the enclave and become entrained in the host rock. Geochemistry Porphyritic enclaves tend to be more mafic (less evolved) than equigranular enclaves here, which further supports the notion that the enclaves were formed from a mafic intrusion into a comparatively more silicic (zoned) magma chamber, as equigranular enclaves cooled more slowly and had more opportunity to mix with host magma. However, the presence of equigranular samples that were significantly more evolved than the host further suggests that the magma body is compositionally zoned and these more evolved enclaves experienced a greater degree of mixing with the more felsic areas. Fairly consistent trace element data may indicate that the lithics are related and came from the same magma chamber as opposed to separate ones (a point that was raised by DiPadova (2015)). Geochemical Comparison with other AVC lithics Comparison of plutonic material analyzed here to other previously characterized AVC plutonic lithics raises a number of interesting points and questions (see diagram in Figure 11). The schileren sample collected at Goat Rock is chemically similar to other Goat Rock schileren samples, which further reinforces

7 the concept of compositionally differentiated layers within the magma chamber that may place a control on the type of textures enclaves from those layers will exhibit. All of the host samples at Goat Rock also group together well, which indicates that the enclaves are significantly more diverse than the host. The two underlying scoria samples from the Goat Rock area (one from this study and one prior) also line up quite well. The increased number of lithics analyzed in this study helps expand upon the known enclave variation at Goat Rock - which could still be expanded on as part of future work to further characterize the variation that exists at the dome. The geochemical spread of plutonic lithics from Haylocks Bay encompass the Flea Bay geochemical data point which raises questions about whether the two are related. Both locations are relatively close to each other (see Figure 1), though this question could perhaps be a topic for future work as only one geochemical sample from Flea Bay was analyzed in contrast to several Haylocks Bay lithics. However, there do not seem to be many more lithics in Flea Bay that are large enough to undergo XRF analysis, though this could be investigated. Overall, Haylock s Bay lithics are generally less evolved than those at Goat Rock. Ultramafic xenoliths from Le Bons Bay Peak (a dome near Goat Rock) are significantly less evolved than the Goat Rock enclaves, which further reinforce the Johnson (2012) magmatic model of shallow, differentiated magma batches by suggesting that the domes have chemically distinct sources. Though Tramontano (2012) already pointed out that the differences in lithics in these locations likely stems from their sources being at different depths, the 2015 Goat Rock data in this study help determine the range of compositions that may be present in Goat Rock enclaves. Magmatic Processes and Evolution Though the proposed processes for the formation for porphyritic and equigranular enclave textures from Browne et al. (2005) work quite well for the scenario at Unzen volcano, the Akaroa magmatic system seems as though it may be more complex. These modes of formation may still apply, but the enclaves at Goat Rock vary along a general spectrum from porphyritic to equigranular as opposed to a stricter binary. Variation of these enclaves may be accounted for by compositional and thermal (or already mentioned in porphyry/equi?) differences within the magma chamber from which the dome is sourced. These differences may take the form of distinct layers within the chamber (Figure 13) and when a mafic intrusion or replenishing event occurs the enclaves could form from various extents of magma mixing depending on thermal differences similar to the processes at Unzen volcano. The lower levels of the magma chamber would have less time to mix with the intruding magma because of a larger temperature difference and would form more porphyritic enclaves. Likewise, as the magma comes into contact with upper levels of the magma reservoir it will have had more time to reach thermal equilibrium and create more equigranular enclave textures. Since porphyritic enclaves tend to be more mafic than equigranular enclaves at Goat Rock it is likely that the layers within this model tend to be more mafic toward the bottom of the chamber and become more felsic moving up the chamber. This sort of chemical spectrum could be a result

8 of fractional crystallization, which may also be the cause of alignment in schileren enclaves due to compaction as liquid melt is extracted (Bertolett, 2014). Goat Rock is probably not supplied by separate magma bodies due to the significant and varying degree of mixing that seems to have affected most plutonic lithics at the dome (Bégué et al., 2014). It is likely that Goat Rock fits into the AVC magmatic system as the product of one of the shallow, fractionated magma batches described by Johnson (2012) (Figure 14). The dome may have a single, separate, compositionally layered magma source that stems from lower magma reservoirs that feed the AVC system. Goat Rock seems to share similarities with other lava domes containing plutonic inclusions suggested to originate from layered magma bodies. For example, Mt. Helen is a lava dome at Lassen Volcanic Center in California, USA that contains 3-19% plutonic lithics that range from 56 61% SiO2 and chemically consistent host rock. Though this is higher than Goat Rock s estimated 2% lithic abundance and occupies a higher silica range, similar formation processes involving thermal differences as a key reason for textural variation are suggested (Feeley, Wilson, & Underwood, 2008). Geochemical variation is attributed largely to multiple layers within the magma chamber, though the circumstances are more specific and defined than at Goat Rock. Nonetheless, the similar conclusions drawn between the two comparable domes help reinforce this model for Goat Rock enclave petrogenesis. Flea Bay Less can be inferred about Flea Bay lithics compared to the ones at Goat Rock due to the limited size of the Flea Bay sample set and its position in a brecciated lava flow. However, since the geochemical point from Flea Bay falls within the range of lithics from nearby Haylock s Bay, the two may share a source, though it is difficult to do more than speculate at this point. The larger lithic was the only one that showed moderate plagioclase alignment and that alignment could be looked at in more detail as three perpendicular EBSD polished thin sections were made for that sample as a part of this project. Future Work Future work that could be done in relation to this topic includes doing electron backscatter diffraction (EBSD) analysis on the polished thin sections used for this project to gain more detailed information about the mineralogy and composition of different types of lithics. In a more mathematically inclined direction, polytopic vector analysis has shown to be a useful approach to determining the extent of magma mixing versus fractional crystallization within magma using major and trace element data. This type of analysis was later applied to Browne et al. (2006) s data from Unzen volcano to suggest the possibility of multiple different types of mafic magma involved in recharge events (Vogel et al., 2008) and could be used at Goat Rock or elsewhere in AVC to more quantitatively characterize magmatic processes. Due to the abundance of plutonic enclaves at Goat Rock, the variation discussed here likely is not representative of all the variation in enclaves throughout the dome so that could also be examined in more detail.

9 Additionally, doing similar geochemical and textural analysis of plutonic lithics at domes or other sites including a more in-depth look at the Flea Bay lithics - within AVC could yield information about localized magmatism and has the potential provide further insights about the larger magmatic system. Conclusion The following can summarize the primary conclusions presented here: Goat Rock contains enclaves that range from at least wt % SiO2 and can be split into multiple groupings due to variations in texture, geochemistry, and other descriptive factors, which suggests that they came from a magma chamber with distinct compositional layers that experienced replenishment / intrusion from more mafic magma(s). A layered model is also supported by the existence of enclaves that are more significantly more felsic than the host rock of the dome. This layered model could be compared or tested at other parts within the larger magmatic system to see how often it may occur in shallow AVC magma batches. The chamber exclusively supplies Goat Rock and seems to reinforces the model for Akaroa s magmatic evolution presented by Johnson (2012). Grain size relationships indicate that enclaves cooled at different rates depending on proximity to the replenishing magma / intrusion and chemical variation results from various amounts of magma mixing between the intrusion and zoned chamber. Thus, multiple types of enclave textures on the spectrum from porphyritic to equigranular could be formed from a single intrusion event. Flea Bay plutonic lithics exhibit varying degrees of alignment, but the lack of samples in that location makes it difficult to infer much about their origin, though they fall within the geochemical range of Haylock s Bay plutonic lithics. Acknowledgements Frontiers Abroad and the University of Canterbury made this project possible. Many thanks are due to Rob Spiers and Stephen Brown for the polished thin sections and XRF analysis respectively, as this project could not have been completed without their assistance. Additional thanks are due to Darren Gravely for help with determining some possible magmatic processes within this system and to other Frontiers Abroad geology students for sample collection assistance.

10 Figures Figure 1: Banks Peninsula and the locations of the two areas (Goat Rock and Flea Bay) where plutonic material was collected for analysis. Other locations where plutonic material exists, has been analyzed, and could be compared to the data in this study are included (red stars). There have also been other studies involving analysis of enclaves at Goat Rock. Figure 2: Plutonic lithics in situ in the form of enclaves at Goat Rock (L) and as fragments within a brecciated lava flow at Flea Bay (R). The lithic pictured from Flea Bay was the largest plutonic lithic found in that location by several centimeters whereas enclaves make up ~2% of Goat Rock and range in size from <1 to 17cm in diameter. Most of the Goat Rock enclaves exhibit radiating cooling fractures highlighted by arrows - in the immediately surrounding host material that are more prominent for larger lithics.

11 A B Figure 3: Development of a magmatic model for Akaroa Volcano, which includes a preliminary model supporting assimilation and fractional crystallization (A; Dorsey, 1988), a figure asserting the presence of mafic magma replenishment, magma mixing, and convection in two stages (B; Hartung, 2011), and the most recent model which builds on the others and proposes shallow, chemically diverse magma batches (C; Johnson, 2012). Analysis of plutonic material has played a role in determining and reinforcing some of the concepts suggested in these models. C

12 Figure 4: (photo: Elisabeth Bertolett) Some of the authors using a concrete saw and other equipment from the University of Canterbury to cut around plutonic enclaves at Goat Rock, which were then chiseled out of the host rock. Many of the enclaves have been difficult to extract using just a rock hammer and chisel since they have relatively flat surfaces and tend not to protrude from the host rock, so this method allows a wider variety of lithics to be sampled. Population Characteristics Geochem: Hawaiite; % silica, % alkalis Larger lithics (at least 10x12cm in situ), sharp smooth contact, strong radiating fractures in host material, some crystal alignment within larger plag. crystals Geochem: Hawaiite; 47-48% silica, ~5.7% alkalis Sharp contact with host, similar tabular, raggedty, glomeroporphyritic plagioclase fabric Geochem: Mugearite, ~55% silica, 8.5% alkalis; more evolved than host samples sharp contact, mid size lithic, similar textures in thin section (relatively equigranular, floworiented fabric), plagioclase-rich Samples 3 total; 2 have thin sections, 2 total; 2 have geochem data, 1 2 total; 1 geochem/thin section geochem thin section Representative thin section 15GR20 15GR09 15GR14 Figure 5: Population table for Goat Rock lithics based on texture, geochemistry, petrography, and other characteristics. The thin section in plane polarized and crosspolarized light for representative samples from each population are shown.

13 Figure 6: Table for outliers that don t quite fit into the populations shown in Figure 5. Figure 7: Photomicrographs of the two host samples (15GR24H and 15GR22H; top and bottom respectively) from Goat Rock in cross-polarized light. Flow banding is present and the top image contains possible evidence of shearing.

14 Figure 8: Photomicrographs in XPL for assorted thin sections from Flea Bay. The upper left which shows clear phenocrysts alignment - and right images are two of the three perpendicular thin sections from the largest plutonic lithic (FBBP17A) found in the area during this investigation. The bottom images show two of the smaller crystalline cumulates (FB17B at left; FB17C at right). The smaller lithics were quite weathered and not much (if any) alignment is visible. These thin sections also contain groupings of more mafic minerals.

15 Figure 9: Rock type diagram (Cox-Bell-Pankhurst 1979) for all Goat Rock samples that were collected as a part of this project. Goat Rock populations are highlighted by ovals (red = population 1; blue = 2; orange = 3). The outliers that don t fit into these populations are labeled and can be compared with the population and outlier tables in the preceding figures.

16 15GR25 Figure 10: Spider diagram (Sun/McDon OIB) for displaying trace/ REE data for Goat Rock samples. Most of the samples seem to follow the same trends with the exception of the most mafic enclave in the sample set (15GR25).

17 Figure 11: Rock type diagrams for comparison of other AVC plutonic lithics to Goat Rock and Flea Bay lithics from this study. Plutonic lithics from Haylock s Bay (top) are generally less evolved than Goat Rock lithics, but encompass the geochemical region where the Flea Bay lithic resides. Le Bons Bay Peak ultramafic xenoliths (middle) are less significantly less evolved than nearby Goat Rock lithics*. Previously collected Goat Rock samples (bottom) fall within the range of the lithics collected in this study and show geochemical similarities among host composition (green circle) schileren samples (purple circle) and scoria samples (blue circle). Legend: Red circles = Goat Rock 2015 Green triangles = Flea Bay 2015 (2 samples, one enclave that is picrite basalt and a host that is picrite basalt / hawaiiite) Blue Stars: Top: Dorsey (1988) samples Middle: Sewell, Hobden, and Weaver (1993) samples Bottom: Bertolett (2014) samples White stars: Top: Hartung (2011) samples Bottom: Tramontano (2012) samples * Le Bons Bay Peak data points may be approximate due to slight difficulty in adjusting the geochemical data for loss on ignition.

18 Figure 12: Magmatic model for formation of porphyritic (P-type) and equigranular (E-type) enclaves in Unzen Volcano, Japan (from Browne et al., 2005) which portrays processes largely similar to formation of enclaves along the spectrum from porphyritic to equigranular over a geochemical spread at Goat Rock. First, a mafic intrusion enters the more silicic host magma chamber (which in the case of Goat Rock is compositionally zoned) (a). Due to the temperature difference between the intrusion and the host magma, the intruding magma begins to cool. The interface between this magma and the various zones experiences the most rapid cooling and crystallization which results in formation of P-type enclaves, which break off and rise up from the intrusion (b). P- types are likely more mafic due to the lack of time and opportunity to mingle extensively with the host magmas. While this is happening, the interior of the intrusion cools slowly by more thorough mixing and conduction with the host magma zones and crystallizes to form equigranular textures (c). E-type enclaves then form as bits with these textures break off of the intrusion due to movement in the magma chamber (d) (perhaps by convection, another replenishing event, etc.).

19 (b) (a) Figure 13: Possible formation* processes for Goat Rock enclaves. As a mafic intrusion or replenishing event is emplaced in the chemically stratified magma chamber, thermal differences between it and the host magma cause magma at the interface to cool quickly, forming porphyritic textured mafic enclaves that don t have the opportunity to undergo much mixing (a). Magma further from he interface may end up with less porphyritic textures as it doesn t experience rapid cooling. As the intruding magma reaches upper levels of the chamber, it has had more time to approach thermal equilibrium and so magma at the interface cools at a slower rate, forming comparatively equigranular textured enclaves that have more time to undergo mixing (b). Silica content in the host layers increases further upward in the chamber. The intruding magma may contain crystals that are then incorporated into the enclaves upon mixing. *The number of layers and nature of the intrusion depicted may not be accurate.

20 Figure 14: How Goat Rock might fit into the overall AVC magmatic system (based on Johnson, 2012). Goat Rock (and perhaps other domes in the region) may be sourced from a shallow, chemically stratified magma chamber that is in turn sourced from deeper reservoirs.

21 Table 1: Major Element (XRF) Data for all samples Table 2: Trace Element Data for Goat Rock samples (ppm)

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23 (2014). Characterisation of mafic enclaves in the erupted products of Soufrière Hills Volcano, Montserrat Geological Society London Memoirs, Price, R., & Taylor, S. (1980). Petrology and Geochemistry of the Banks Peninsula Volcanoes, South Island, New Zealand. Contributions to Mineralogy and Petrology, 72, Sewell, R. J., B. J. Hobden, and S. D. Weaver. "Mafic and Ultramafic Mantle and Deep Crustal Xenoliths from Banks Peninsula, South Island, New Zealand." New Zealand Journal of Geology and Geophysics 36.2 (1993): Web. Timm, C., K. Hoernle, P. Van Den Bogaard, I. Bindeman, and S. Weaver. "Geochemical Evolution of Intraplate Volcanism at Banks Peninsula, New Zealand: Interaction Between Asthenospheric and Lithospheric Melts." Journal of Petrology 50.6 (2009): Web Tramontano, S. Reconstructing the Magmatic History of Goat Rock Intrusive, Banks Peninsula, New Zealand. (2012): Unpublished manuscript. Vogel, T. A., P. J. Hidalgo, L. Patino, K. S. Tefend, and R. Ehrlich (2008), Evaluation of magma mixing and fractional crystallization using whole-rock chemical analyses: Polytopic vector analyses, Geochem. Geophys. Geosyst., 9, Q04020, doi: /2007gc