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Journal Article

The Eucarro Rhyolite, Gawler Range Volcanics, South Australia: A >675 km 3 , compositionally zoned lava of Mesoproterozoic age

S.R. Allen, J. McPhie
Journal: GSA Bulletin
Published: 01 December 2002
GSA Bulletin (2002) 114 (12): 1592–1609.
DOI: https://doi.org/10.1130/0016-7606(2002)114<1592:TERGRV>2.0.CO;2
...S.R. Allen; J. McPhie Abstract The Eucarro Rhyolite is remarkable in being far more voluminous (>675 km 3 ) and extensive than most felsic lavas (<1 km 3 ). Its dimensions are comparable to the largest known volcanic units (felsic ignimbrites and flood basalts). It is one of several...
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View PDF for article title, The <span class="search-highlight">Eucarro</span> <span class="search-highlight">Rhyolite</span>, Gawler Range Volcanics, South Australia: A &amp;gt;675 km 3 , compositionally zoned lava of Mesoproterozoic age
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Figure 11. (A) Schematic cross section of the Eucarro Rhyolite from west (T—Toondulya Bluff) to east (S—Siam) (horizontal axis) and south to north (vertical axis). The Eucarro Rhyolite includes several compositions but is dominated by plagioclase rhyolite that is pyroxene and olivine bearing (western and central zone) or ferromagnesian phase rich (eastern zone). The Eucarro Rhyolite appears significantly thinner at east Paney (PE) where the plagioclase rhyolite pinches out and only quartz rhyolite is present. The Eucarro Rhyolite is also thinner between central Nonning (NC) and east Nonning (NE), coinciding with a concentration of granitic clasts and aphyric and quartz rhyolite pods. Granitic clasts are concentrated in the middle granophyre at Hiltaba (H) and central Nonning and in the amygdaloidal rhyolite at west Nonning (NW). The northern plagioclase rhyolite is commonly vesicular; the largest vesicles occur at west Nonning. Quartz rhyolite occurs within the northernmost margin and has mingled contacts with the plagioclase rhyolite. Quartz rhyolite is thickest at Paney from west Paney (PW) to east Paney (PE). Blank areas represent areas of no outcrop. Other abbreviations are N—Narlaby Well, PC—Paney central. (B) Schematic cartoon of the subsurface magma chamber, showing the compositional variants that contributed to the Eucarro Rhyolite from the western, central, and eastern zones. The basal plagioclase rhyolite is more silica rich in the central and eastern zones, suggesting that the plagioclase rhyolite was erupted from a compositionally zoned magma chamber with a more silica-rich cap. The aphyric and quartz rhyolite magmas may have been present in a single chamber with the plagioclase rhyolite, but it is also plausible that they were in subsidiary magma chambers separated by thin walls that foundered at various stages during the eruption. Granitic-clast accumulations at Hiltaba appear to coincide with a variation in magma type from amphibole bearing (in the west) to pyroxene and olivine bearing. Also, granitic clast concentrations at Nonning coincide with the presence of aphyric and quartz rhyolite, both in the northern margin (northwest) and as pods in the middle granophyre (northeast). These features may be indicating proximity to source vents. Several source vents are proposed to explain the subtle compositional variations between the western and the central zones and the eastern zone as well as the extensive east-trending distribution of the rhyolite

Figure 11. (A) Schematic cross section of the Eucarro Rhyolite from west (T...

Published: 01 December 2002
Figure 11. (A) Schematic cross section of the Eucarro Rhyolite from west (T—Toondulya Bluff) to east (S—Siam) (horizontal axis) and south to north (vertical axis). The Eucarro Rhyolite includes several compositions but is dominated by plagioclase rhyolite that is pyroxene and olivine bearing
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TABLE 1. REPRESENTATIVE XRF ANALYSES OF EUCARRO RHYOLITE

Published: 01 December 2002
TABLE 1. REPRESENTATIVE XRF ANALYSES OF EUCARRO RHYOLITE
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Figure 2. Map of the Eucarro Rhyolite and surrounding units in the western and central zones, showing locations of sections at Toondulya Bluff, Hiltaba, Narlaby Well, and Paney (west, central, east). Dots represent geochemistry sample locations: Bold numbers are analyses given in Table 1. Italic numbers with letters are locations of the photographs in Figures 7–9. Unpatterned areas are covered by Quaternary alluvium. The grid is the 10,000 m Australian Map Grid. Modified from Blissett et al. (1988). Flow-banding orientations for quartz rhyolite from Morrow (1998)

Figure 2. Map of the Eucarro Rhyolite and surrounding units in the western ...

Published: 01 December 2002
Figure 2. Map of the Eucarro Rhyolite and surrounding units in the western and central zones, showing locations of sections at Toondulya Bluff, Hiltaba, Narlaby Well, and Paney (west, central, east). Dots represent geochemistry sample locations: Bold numbers are analyses given in Table 1 . Italic
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Figure 3. Map of the Eucarro Rhyolite and surrounding units in the eastern zone, showing locations of sections at Nonning (west, central, east) and Siam. The grid is the 10,000 m Australian Map Grid. Key to symbols in Figure 2

Figure 3. Map of the Eucarro Rhyolite and surrounding units in the eastern ...

Published: 01 December 2002
Figure 3. Map of the Eucarro Rhyolite and surrounding units in the eastern zone, showing locations of sections at Nonning (west, central, east) and Siam. The grid is the 10,000 m Australian Map Grid. Key to symbols in Figure 2
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Figure 4. Variations in phenocryst populations within the Eucarro Rhyolite, determined by image analysis of slabs and thin sections. (A) Abundance of K-feldspar vs. plagioclase vs. quartz. The plagioclase rhyolite is predominantly quartz poor, apart from the base and amygdaloidal northern margin. The quartz rhyolite is relatively quartz rich and more variable in the proportions of these three phases. (B) Feldspar size sorting (σϕ) vs. feldspar median diameter (Md). The parameters for sorting (ϕ84–ϕ16)/2 and median diameter (ϕ50) are after Inman (1952). Plagioclase rhyolite samples plot in a relatively well defined field (solid circle), whereas quartz rhyolite samples show more scatter (dashed oval). Feldspars within the quartz rhyolite have a wider size range, although most are smaller than those within the plagioclase rhyolite. Samples of mingled plagioclase rhyolite and quartz rhyolite (oval of fine dashes) have sorting similar to the quartz rhyolite and median diameter similar to plagioclase rhyolite. (C) Total volume of feldspar phenocrysts at various stratigraphic positions (schematic). Plagioclase rhyolite samples from the basal and middle parts show only small variations in feldspar abundance compared to the northernmost quartz rhyolite. Symbols as in B. (D) Volume and size of ferromagnesian phases coarser than 0.25 mm in plagioclase rhyolite. At Nonning, ferromagnesian phases are larger and more abundant than elsewhere. Symbols as in B

Figure 4. Variations in phenocryst populations within the Eucarro Rhyolite,...

Published: 01 December 2002
Figure 4. Variations in phenocryst populations within the Eucarro Rhyolite, determined by image analysis of slabs and thin sections. (A) Abundance of K-feldspar vs. plagioclase vs. quartz. The plagioclase rhyolite is predominantly quartz poor, apart from the base and amygdaloidal northern margin
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Figure 5. Major and trace element compositions of Eucarro Rhyolite samples (recalculated 100% anhydrous). The sample set comprises 48 plagioclase rhyolite, 13 quartz rhyolite, 2 aphyric rhyolite, and 2 granitic clasts. (A) SiO2 vs. Al2O3. Most plagioclase rhyolite samples plot between 71 and 72 wt% SiO2 (“most plagioclase rhyolite” field). (B) SiO2 vs. Rb. The clear trend indicates that most samples are unaltered. (C) SiO2 vs. Fe2O3. The decrease in iron with increasing SiO2 can be attributed to fractionation of pyroxene and titanomagnetite. (D) SiO2 vs. Ba. Ba decreases with SiO2 owing to fractionation of plagioclase. (E) SiO2 vs. La. As relatively immobile elements, REEs increase with fractionation within the plagioclase rhyolite. La decreases within the quartz rhyolite. La within the aphyric rhyolite and granitic clasts is highly variable. (F) SiO2 vs. Zr. Depletion of Zr at high SiO2 values is probably a result of late-stage fractionation of accessory minerals including zircon. The size of the symbols is larger than the analytical error

Figure 5. Major and trace element compositions of Eucarro Rhyolite samples ...

Published: 01 December 2002
Figure 5. Major and trace element compositions of Eucarro Rhyolite samples (recalculated 100% anhydrous). The sample set comprises 48 plagioclase rhyolite, 13 quartz rhyolite, 2 aphyric rhyolite, and 2 granitic clasts. (A) SiO 2 vs. Al 2 O 3 . Most plagioclase rhyolite samples plot between 71
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Figure 6. Generalized stratigraphic section through the Eucarro Rhyolite from south to north. Small inset figure shows how the stratigraphic section relates to south-to-north traverses through the Eucarro Rhyolite. The section represents a single cooling profile; the coarsest groundmass textures are in the middle pink granophyre. Some parts (lower red, pink, northern mottled brown-green) have two varieties of groundmass textures. The plagioclase rhyolite and quartz rhyolite have a gradational, mingled contact. Contacts between black, brown, red, pink, or mottled parts are gradational. The base is more silicic (higher SiO2, greater abundance of quartz and K-feldspar) than the middle granophyre and amygdaloidal part. The northernmost quartz rhyolite contains the highest abundance of quartz, K-feldspar, and SiO2. The pink granophyre (middle) and amygdaloidal mottled brown-green (northern margin) varieties include granitic and mafic clasts, along with abundant megacrysts. Phenocryst components are given in the pie diagrams

Figure 6. Generalized stratigraphic section through the Eucarro Rhyolite fr...

Published: 01 December 2002
Figure 6. Generalized stratigraphic section through the Eucarro Rhyolite from south to north. Small inset figure shows how the stratigraphic section relates to south-to-north traverses through the Eucarro Rhyolite. The section represents a single cooling profile; the coarsest groundmass textures
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Figure 8. Eucarro Rhyolite from the northern margin. (A) Amygdaloidal plagioclase rhyolite at Siam. (B) Mingled plagioclase rhyolite and quartz rhyolite at Paney. (C) Amygdaloidal plagioclase rhyolite from Paney in plane-polarized light. The groundmass is intensely spherulitic. Spherulites have been overprinted by randomly oriented feldspar laths. Feldspar phenocrysts (f) are turbid. Scattered embayed quartz phenocrysts are present (q). Scale bar represents 1 mm. (D) Polished slab of flow-banded quartz rhyolite from Paney. Smaller, dark phenocrysts are quartz. Scale bar represents 1 cm. (E) Intricately flow-banded quartz rhyolite from Paney under plane-polarized light. Relatively abundant quartz phenocrysts and scattered turbid feldspar phenocrysts occur within a cryptocrystalline groundmass. Scale bar represents 1 mm. (F) Two spherulites from the quartz rhyolite at Paney under plane-polarized light. Spherulites are composed of very fine, tightly clustered, needles. Scale bar represents 0.25 mm

Figure 8. Eucarro Rhyolite from the northern margin. (A) Amygdaloidal plagi...

Published: 01 December 2002
Figure 8. Eucarro Rhyolite from the northern margin. (A) Amygdaloidal plagioclase rhyolite at Siam. (B) Mingled plagioclase rhyolite and quartz rhyolite at Paney. (C) Amygdaloidal plagioclase rhyolite from Paney in plane-polarized light. The groundmass is intensely spherulitic. Spherulites have
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Figure 1. Simplified geologic map of the Gawler Range Volcanics (GRV) in South Australia. The Eucarro Rhyolite is one of the most extensive units of the southern Gawler Range Volcanics, extending from Toondulya Bluff (west) to Siam (east). Older units (south) and the younger Yardea Dacite (north) conformably enclose the Eucarro Rhyolite. Both the Eucarro Rhyolite and Yardea Dacite dip very gently northward. Outlines of the western and central (Fig. 2) and eastern zones (Fig. 3) are marked. Geologic boundaries are modified from Blissett et al. (1988). Inset: The location and distribution of the Gawler Range Volcanics in the Gawler craton, South Australia

Figure 1. Simplified geologic map of the Gawler Range Volcanics (GRV) in So...

Published: 01 December 2002
Figure 1. Simplified geologic map of the Gawler Range Volcanics (GRV) in South Australia. The Eucarro Rhyolite is one of the most extensive units of the southern Gawler Range Volcanics, extending from Toondulya Bluff (west) to Siam (east). Older units (south) and the younger Yardea Dacite (north
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Figure 10. Representative compositional data (SiO2 vs. Al2O3) for the Eucarro Rhyolite from (A) the western and central zone to (B) the eastern zone. Basal plagioclase rhyolite from Narlaby Well (Nb), east Paney (Pb), and Nonning (Nob) are all more silica rich and more highly fractionated than their main middle parts (“most plagioclase rhyolite” field), whereas at Toondulya Bluff, the base (Tb) plots in the most plagioclase rhyolite field and is less silicic than the two samples of the middle granophyre (T). Amygdaloidal plagioclase rhyolite (Y) and two samples of the plagioclase rhyolite from Hiltaba (H) have lower SiO2, probably owing to contamination by mafic clasts. Quartz rhyolite is the most fractionated with the highest silica contents. Two samples from the eastern zone plot close to the quartz rhyolite field and appear to be mixtures of plagioclase rhyolite and quartz rhyolite (mixed plagioclase rhyolite and quartz rhyolite)

Figure 10. Representative compositional data (SiO 2 vs. Al 2 O 3 ) for the...

Published: 01 December 2002
Figure 10. Representative compositional data (SiO 2 vs. Al 2 O 3 ) for the Eucarro Rhyolite from (A) the western and central zone to (B) the eastern zone. Basal plagioclase rhyolite from Narlaby Well (N b ), east Paney (P b ), and Nonning (No b ) are all more silica rich and more highly
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Figure 7. (A and B) Photomicrographs of black and brown plagioclase rhyolite from the base of the Eucarro Rhyolite. (A) Black plagioclase rhyolite from Toondulya Bluff. Phenocrysts are mainly feldspar, some of which are crystal fragments (x); sparse embayed quartz (q) is also present. Ferromagnesian phases (clinopyroxene and olivine) are relatively fresh. The groundmass shows intricate, streaky flow bands in plane-polarized light. Scale bar represents 1 mm. (B) Brown plagioclase rhyolite from the base at Narlaby Well, showing a slightly streaky groundmass in plane-polarized light. Scattered embayed quartz phenocrysts (q) are present. Scale bar represents 1 mm. (C–F) Red and pink middle granophyre. (C) Polished slab of the lower red granophyre from Narlaby Well, showing an evenly porphyritic texture with scattered feldspar phenocrysts and small dark patches that are either small, round, quartz phenocrysts or larger altered ferromagnesian phases. Scale bar represents 1 cm. (D) Lower red granophyre from Nonning under plane-polarized light. The groundmass comprises scattered spherulites (0.5 mm) and interconnected microgranular domains of quartz and feldspar. Phenocrysts include turbid feldspar (up to 4 mm) and embayed quartz (q). Scale bar represents 1 mm. (E) Upper red granophyre from Narlaby Well under cross-polarized light. Open spherulites comprising 13-μm-wide feldspar laths are separated by granophyric domains. Scale bar represents 0.25 mm. (F) Pink interior granophyre from Hiltaba under cross-polarized light. Coarse vermicular granophyre in the groundmass surrounds turbid feldspar phenocrysts (f). Scale bar represents 1 mm

Figure 7. (A and B) Photomicrographs of black and brown plagioclase rhyolit...

Published: 01 December 2002
Figure 7. (A and B) Photomicrographs of black and brown plagioclase rhyolite from the base of the Eucarro Rhyolite. (A) Black plagioclase rhyolite from Toondulya Bluff. Phenocrysts are mainly feldspar, some of which are crystal fragments (x); sparse embayed quartz (q) is also present
Journal Article

The fluorine link between a supergiant ore deposit and a silicic large igneous province

Jocelyn McPhie, Vadim Kamenetsky, Sharon Allen, Kathy Ehrig, Andrea Agangi, Adam Bath
Journal: Geology
Published: 01 November 2011
Geology (2011) 39 (11): 1003–1006.
DOI: https://doi.org/10.1130/G32205.1
... : Chemical Geology , v. 263 , p. 1 – 18 , doi: 10.1016/j.chemgeo.2008.10.005 . Allen S.R. McPhie J. , 2002 , The Eucarro Rhyolite, Gawler Range Volcanics, South Australia: A >675 km 3 , compositionally zoned lava of Mesoproterozoic age : Geological Society of America Bulletin , v. 114...
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INTERESTING PAPERS IN OTHER JOURNALS

Journal: Economic Geology
Published: 01 May 2003
Economic Geology (2003) 98 (3): 671–682.
DOI: https://doi.org/10.2113/gsecongeo.98.3.671
... of neodymium(III) complexation in chloride solutions —Art A. Migdisov and A.E. Williams-Jones 4311 Standard states for the activities of mineral surface sites and species —Dimitri A. Sverjensky 17 The Eucarro Rhyolite, Gawler Range Volcanics, South Australia: A >675 km 3 , compositionally zoned...
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Tectonic Framework and Evolution of the Gawler Craton, Southern Australia

Martin Hand, Anthony Reid, Liz Jagodzinski
Journal: Economic Geology
Published: 01 December 2007
Economic Geology (2007) 102 (8): 1377–1395.
DOI: https://doi.org/10.2113/gsecongeo.102.8.1377
... the auspices of a National Geoscience Agreement between PIRSA and Geoscience Australia. The Australian Research Council and PIRSA are acknowledged for support of this project via funding to Linkage Grant no. LP0454301. REFERENCES Allen , S.R. , and McPhie , J. , 2002 , The Eucarro Rhyolite...
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