Extreme effusive eruptions: Palaeoflow data on an extensive felsic lava in the Mesoproterozoic Gawler Range Volcanics

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Journal of Volcanology and Geothermal Research 172 (2008) 148 – 161 www.elsevier.com/locate/jvolgeores

Extreme effusive eruptions: Palaeoflow data on an extensive felsic lava in the Mesoproterozoic Gawler Range Volcanics J. McPhie a,⁎, F. DellaPasqua a , S.R. Allen a , M.A. Lackie b a

School of Earth Sciences and Centre for Ore Deposit Research, University of Tasmania, Private Bag 79, Hobart, Tasmania 7001, Australia b Department of Earth and Planetary Sciences, ARC National Key Centre for Geochemical Evolution and Metallogeny of Continents, Macquarie University, North Ryde, New South Wales 2109, Australia Received 29 May 2005; accepted 10 November 2006 Available online 31 December 2007

Abstract The Gawler Range Volcanics are the eroded remnants of a Mesoproterozoic dominantly felsic large igneous province. The Eucarro Rhyolite is one of three voluminous (N675 km3) units in the upper part of the succession. Eight stratigraphic sections through the Eucarro Rhyolite were sampled for anisotropy of magnetic susceptibility (AMS) and petrofabric analysis in order to determine the source location. The AMS foliations (Kmax/Kint plane) consistently strike northwest and most dip steeply. At sites in the western and central parts of the unit, there is a change from steep southwesterly dips near the base, to vertical or northeasterly dips in the middle and near the top. In the same area, Kmax axes plunge to the south or southwest in the lower parts, and to the north in upper parts, of the unit. At sites in the eastern part of the unit, the pattern is reversed: AMS foliations dip northerly near the base and southerly near the top, and Kmax axes plunge northerly near the base and southerly near the top. Such up-section reversals in dip or plunge directions of AMS parameters resemble the patterns shown by other lavas. A dominance of very steep dips and an up-section reversal in dip direction are also expected for flow bands in felsic lavas. Furthermore, the AMS pattern implies a northeasterly flow direction for the western and central parts of the unit, and a southerly flow direction for the eastern part. The opposing flow directions may indicate that the western and central parts of the unit versus the eastern part belong to different flow lobes, or that they came from separate sources. Lineations defined by preferred alignment of elongate phenocryst intersections on gently dipping surfaces show a less consistent pattern and, alone, could not be used to define palaeoflow directions. This study confirms the existence of felsic lavas that have dimensions comparable with the largest known mafic lavas. © 2007 Published by Elsevier B.V. Keywords: felsic LIP; extensive felsic lava; AMS; phenocryst lineation; Gawler Range Volcanics; Mesoproterozoic

1. Introduction Large igneous provinces (LIPs) occur in continental and oceanic intraplate settings and are typically dominated by voluminous mafic lavas and intrusions (Coffin and Eldhlom, 1993). In addition, some LIPs include significant or even predominant felsic units, particularly those in continental intraplate settings where melting of continental crust contributes to magma production.

⁎ Corresponding author. E-mail address: [email protected] (J. McPhie). 0377-0273/$ - see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.jvolgeores.2006.11.011

The basaltic lavas in mafic LIPs are amongst the most extensive and voluminous eruptive units known; units with volumes in excess of 100 km3 are common (Swanson et al., 1975). There have been important recent advances in understanding their emplacement mechanisms (e.g., Self et al., 1996, 1997) that have major implications for discharge, flow and degassing rates. In bimodal and felsic LIPs, only limited attention has been paid to the physical volcanology of the felsic units. Some felsic units are interpreted to be rheoignimbrites — for example, the quartz latite units in the Etendeka Igneous Province, Namibia (Milner et al., 1992). In other cases, they are lavas, such as the rhyolites of the western Snake River Plain volcanic province (Bonnichsen and Kauffman, 1982). A possible “cauldron” source

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location for the Springbok Quartz Latite in the Etendeka Igneous Province has been inferred on the basis of structural and petrogenetic interpretations (Ewart et al., 2002). Volcanic centres in the western Snake River Plain have been broadly defined on the basis of the distribution and thickness variations of major units (Bonnichsen and Kauffman, 1982). The Mesoproterozoic Gawler Range Volcanics in South Australia qualify as a LIP because a minimum volume of ∼ 70,000 km3 of magma was erupted in less than ∼ 2 million years. The total magma volume is even larger when coeval and co-magmatic plutons of the Hiltaba Suite are included (additional ∼ 30,000 km3). Most of that magma was felsic — basalt and basaltic andesite account for less than ∼ 5% of the preserved volcanic succession. Some of the felsic units are especially voluminous, single emplacement units apparently having minimum volumes of 500 km3 (Allen et al., 2003). The voluminous felsic units in the Gawler Range Volcanics were initially thought to be welded ignimbrites, primarily because of their wide extent and large volume (e.g., Blissett et al., 1993). More recent research has led to the interpretation that they are in fact extensive lavas, or at least that the long-distance transport mechanism was non-particulate (lava-like) (Garner and McPhie, 1999; Morrow and McPhie, 2000; Allen and McPhie, 2002). Rajagopalan et al. (1993) used regional geophysical data to suggest possible source vent locations for the Gawler Range Volcanics, none of which appear to be consistent with the results of the most recent research (e.g., Allen et al., 2003). In this paper, we report results of palaeoflow analysis of one of the voluminous felsic units in the Gawler Range Volcanics. Palaeoflow analysis was undertaken for two reasons: as a further test of the interpreted emplacement as lava flows, and to define outflow paths and hence, source locations. The new results add to current understanding of the facies architecture of felsic LIPs and provide further evidence of the reality of extreme effusive

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eruptions — those capable of generating felsic lavas comparable in volume and extent to flood basalts. 2. The Gawler Range Volcanics The Gawler Range Volcanics outcrop across an area of ∼25,000 km2 in South Australia (Fig. 1), and throughout most of that area, they are at least 1 km thick. They extend under younger formations to the northeast, giving a total minimum extent of ∼ 90,000 km2. The age of the volcanic succession is ∼1592 Ma, as determined by U/Pb in zircon (Fanning et al., 1988) and the succession is considered to be co-magmatic with a suite of granitoid intrusions that overlap in age (Hiltaba Suite, 1585–1600 Ma; Flint, 1993). The Hiltaba Suite intrusions are exposed in areas bordering the southwestern, western and northwestern margins of the Gawler Range Volcanics, and have been intersected in drill core to the northeast where they are covered by younger formations (Blissett et al., 1993). The combination of intrusions and volcanic units exceeds 100,000 km3 in volume, most of which is felsic in composition. The Gawler Range Volcanics are essentially undeformed and unmetamorphosed. They overlie Archaean and Palaeoproterozoic metamorphic complexes and granitoid intrusions of the Gawler Craton, the setting of which was intracontinental and subaerial at the time the volcanic succession was emplaced (Blissett et al., 1993). Primary textures and compositions of the Gawler Range Volcanics are generally very well preserved. In the lower Gawler Range Volcanics, compositions range from basalt to rhyolite, and a wide variety of lavas, ignimbrites and subordinate sedimentary units is present (Allen et al., 2008this volume). The upper Gawler Range Volcanics comprise three extensive, thick (200–300 m), phenocryst-rich felsic units, the oldest of which is the Eucarro Rhyolite (Allen and McPhie,

Fig. 1. Map showing the distribution of the Eucarro Rhyolite in the southern Gawler Range Volcanics. The inset maps show the location of the Gawler Range Volcanics in relation to the Gawler Craton and Stuart Shelf, in South Australia. The map also shows the AMS and petrofabric (PF) sample locations in the Eucarro Rhyolite. The Eucarro Rhyolite dips very gently to the north, so the sampled sections transect the unit from the base (south) to the top (north). HS, homestead.

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2002; Allen et al., 2003). The two younger units are the Pondanna Dacite and the Moonaree Dacite, each of which has a volume in excess of 500 km3 (Allen et al., 2003). Magma temperatures for the Eucarro Rhyolite and Moonaree Dacite, based on pyroxene and feldspar geothermometry, are relatively high (900 °C to 1100 °C; Creaser and White, 1991; Stewart, 1994). 2.1. The Eucarro Rhyolite The Eucarro Rhyolite was chosen for palaeoflow analysis for three reasons: it is well exposed and well preserved throughout the southern Gawler Ranges (Fig. 1); its internal textural and compositional variations have been established (Allen and McPhie, 2002); it is thick (∼300 m), extensive (∼225 km east–west, b 15 km exposed north–south) and consistently lavalike in texture. Previous work (Allen and McPhie, 2002) has shown that the Eucarro Rhyolite has an evenly porphyritic texture comprising unbroken euhedral phenocrysts in finer groundmass. Variations in the groundmass texture suggest that the Eucarro Rhyolite cooled as a single unit: the base is typically black or brown and has a cryptocrystalline groundmass that was probably originally glassy; the interior is red or pink, columnar-jointed and has a crystalline groundmass (spherulitic or granophyric); the top is amygdaloidal and commonly has a spherulitic or very fine grained groundmass (Fig. 2A). In most of the Eucarro Rhyolite, the dominant phenocrysts are plagioclase, K-feldspar and pyroxene, and there is no flow banding. At the top of some sections, however, the Paney Rhyolite Member, a flow-banded, quartz-phyric rhyolite, occurs above and intermingled with the amygdaloidal, feldspar-pyroxene-phyric rhyolite.

For this study, we sampled the Eucarro Rhyolite along eight sections oriented across strike. Along each section, sites were sampled at the stratigraphically lowest (south) to the stratigraphically highest (north) positions exposed. All the sections had been previously mapped and sampled for textural and geochemical studies (Allen and McPhie, 2002). The sections covered the western and eastern mapped extremities of the Eucarro Rhyolite and were spaced more or less evenly through the central region (Fig. 1). In two central sections (Waganny and Eagle Rock), the Paney Rhyolite Member occurs at the top of the section but was not sampled. Apart from the top where the Paney Rhyolite occurs locally, the Eucarro Rhyolite is texturally uniform and there are no obvious macroscopic indicators of palaeoflow directions or proximity to source. Hence, we sought to determine the original palaeoflow direction using two techniques: anisotropy of magnetic susceptibility (AMS) and a petrofabric technique focusing on the preferred alignments of elongate feldspar phenocrysts. In total, 32 sites were sampled for AMS. At each site, three or four separate samples (A, B, C, D) were taken within an area of ∼100 m2. At 16 of the AMS sites, an additional sample was taken for petrofabric analysis. In situ orientations of all samples were established in the field using both sun and magnetic compasses, and sample site locations were accurately recorded by GPS. 3. AMS methods The magnetic susceptibility of a rock refers to its response to an applied magnetic field. The susceptibility can also be measured in different directions and the results are conventionally expressed in terms of a triaxial ellipsoid defined by the maximum, intermediate

Fig. 2. A. Schematic section through the Eucarro Rhyolite, showing the principal internal textural variations (after Allen and McPhie 2002). B and C. Typical outcrops of the columnar-jointed, middle part of the Eucarro Rhyolite. B, columns in cross-section. C, outcrop ∼15 m high, composed of columnar-jointed Eucarro Rhyolite, Scrubby Peak. D. Equal-area stereonet showing the plunge and trend of columns in columnar-jointed Eucarro Rhyolite. The columns plunge very steeply, implying that the cooling surfaces were near-horizontal.

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and minimum susceptibility directions, Kmax, Kint and Kmin, respectively. The anisotropy of the principal susceptibility axes is generally considered to be inherited from the mechanism of emplacement and can be used to reconstruct flow directions in intrusions (e.g., Knight and Walker, 1988), lavas (e.g., Ellwood 1978; Cañon-Tapia et al., 1996) and ignimbrites (e.g., Ellwood, 1982; Wang et al., 2001). AMS measurements were made on a DIGICO Anisotropy Delineator, interfaced to a dedicated computer. Bulk-rock susceptibility was measured on a Sapphire Instruments S12B magnetic susceptibility and anisotropy meter and on the CSIRO magnetic susceptibility Kappa Balance. The AMS data were subjected to site parametric bootstrap analyses as outlined by Constable and Tauxe (1990) and Tauxe (1998). The output consists of the magnitudes and directions of the maximum (Kmax), minimum (Kmin) and intermediate (Kint) principal susceptibilities. Parameters calculated from the AMS data include: bulk susceptibility [(Kmax + Kint + Kmin) / 3], anisotropy degree (P′ as defined by Jelinek, 1981), magnetic lineation (plunge and trend of Kmax), magnetic foliation (dip and dip azimuth of the Kmax/Kint plane), and the pole to the magnetic foliation (plunge and trend of Kmin). The shape of the susceptibility ellipsoid (prolate, oblate, triaxial, spherical) was determined from analysis of histograms of the bootstrapped eigenvalues associated with the eigenvectors (Tauxe, 1998). The range of magnetic susceptibility (∼90 to 2100 cgs × 10− 6, mostly N 300 × 10− 6) suggests that the AMS characteristics of the Eucarro Rhyolite arise primarily from the preferred orientation of magnetite. For sites with susceptibility less than 300 × 10− 6, pyroxene is probably contributing to the AMS. 4. Petrofabric methods Petrofabric analysis involves measurement of preferred alignments of textural components that can reasonably be expected to have significance with respect to local flow direction. The technique has been used for palaeoflow analysis of both lavas (e.g., Smith and Rhodes, 1972; Smith et al., 1993; Cañon-Tapia and Castro, 2004) and ignimbrites (e.g., Elston and Smith, 1970; MacDonald and Palmer, 1990). The most common textures are elongate vesicles, platy and/or elongate rock or pumice fragments, and elongate crystals. In flow-banded volcanic facies, lineations on flow bands and fold geometry (orientations of axial surfaces and fold axes, fold asymmetry) may also give palaeoflow direction information. With the exception of the Paney Rhyolite Member, the Eucarro Rhyolite is devoid of macroscopic flow bands or other foliations. The only consistent indicator of orientation is the columnar joint pattern. Column axes plunge very steeply and most trend to the south (Fig. 2B) or northwest, implying that cooling surfaces were very gently dipping to the north or southeast, respectively. The samples used for petrofabric analyses were cut into slabs oriented normal to local column axes as no other directional feature is recognizable and consistently present in the field. The surfaces of the cut slabs were stained to accentuate the feldspar phenocrysts and photographed. Images of the slabs were then digitized, and the long axis and lengths of sections through elongate phenocrysts

Fig. 3. Comparison of phenocryst lineation data measured on a single sample (F21) by two operators. Orientations of elongate feldspar phenocrysts were measured on the same five slabs (A, AB, CD, CB, and D) cut from one sample. For four of the slabs, the two operators produced very similar lineations, but for the fifth slab (D), there was a wider spread of lineations and azimuths were different. Nevertheless, the average lineations generated by the two operators are similar.

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Fig. 4. Equal-area stereonets showing AMS data for all Eucarro Rhyolite samples analysed. Kmax, open square; Kint, open triangle; Kmin, open circle. Filled symbols are axis means, shown with associated error ellipses. Sites F14, F73 and F89 do not show error ellipses as the susceptibility ellipsoid is oblate and the associated errors for Kmax and Kint are 90°. The error for Kmin is 25° for sites F14 and F73 and 30° for site F89. Site locations are shown on Fig. 1.

Table 1 Anisotropy of magnetic susceptibility and phenocryst lineation data for sample sites along eight stratigraphic sections through the Eucarro Rhyolite. P′ is the corrected anisotropy degree as defined by Jelinek (1981) and relates to the mean tensor. Shape refers to the shape of the anisotropy ellipsoid: P, prolate; O, oblate; T, triaxial; S, spherical as determined from analysis of histograms of eigenvalues of the associated eigenvectors Location

Anisotropy of magnetic susceptibility (AMS)

Phenocryst lineations (EPO)

AMG easting

AMG northing

Facies

AMS site and sample (number of samples)

Kmax Kint Kmin Degree of Shape Foliation EPO Mean No Bulk anisotropy P′ Kmax–Kint sample resultant susceptibility Plunge Trend Plunge Trend Plunge Trend strike/dip cgs 10− 6

Western Eucarro Toondulya Bluff Toondulya Bluff Toondulya Bluff Toondulya Bluff Narlaby Well Narlaby Well Narlaby Well Narlaby Well

496,933 496,764 496,721 504,006 517,802 517,749 518,820 522,322

6,446,732 6,446,475 6,446,707 6,439,691 6,424,954 6,424,691 6,425,319 6,425,313

Red Red Black Red (granophyre) Red Red Red Red

EF01 ABC (15) EF03 ABC (20) EF05 ABCD (20) EF07 ABCD (28) EF13 ABCD (18) EF14 ABCD (22) EF15 ABCD (35) EF16 ABCD (22)

270 322 497 385 345 2078 361 372

Central Eucarro Scrubby Peak Scrubby Peak Scrubby Peak Waganny Waganny Waganny Waganny Waganny Eagle Rock Eagle Rock Eagle Rock Eagle Rock

530,024 530,027 529,696 547,803 540,784 552,198 551,484 551,736 561,451 561,232 562,310 559,825

6,397,691 6,397,471 6,393,919 6,384,473 6,392,762 6,387,328 6,388,494 6,388,051 6,389,328 6,388,060 6,386,823 6,387,669

Red Red Red Black Red Red Red Red Red Red Brown Red

EF20 ABCD (29) EF22 ABCD (29) EF30 ABCD (17) EF27 ABCD (22) EF28 ABCD (23) EF33 ABCD (18) EF38 ABCD (35) EF41 ABCD (29) EF43 ABCD (31) EF44 ABCD (28) EF47 ABCD (36) EF53 ABCD (26)

Eastern Eucarro Coralbignie Coralbignie Coralbignie Coralbignie Nonning Nonning Nonning Nonning Siam Siam Siam Siam

629,000 620,825 623,766 627,411 637,237 653,513 652,860 652,850 666,087 661,764 661,254 659,555

6,386,973 6,395,889 6,394,287 6,390,231 6,399,226 6,397,414 6,396,294 6,392,151 6,389,684 6,391,552 6,390,434 6,396,703

Brown Red Red Red Red (xenocrystic) Red Red Brown Red Red Red (xenocrystic) Red (xenocrystic)

EF81 ABCD (18) EF83 ABCD (33) EF86 ABCD (29) EF89 ABCD (29) EF55 ABCD (29) EF73 ABCD (29) EF75 ABCD (33) EF78 ABCD (32) EF57 ABCD (19) EF60 ABCD (23) EF65 ABCD (30) EF70 ABCD (29)

59 87

114 107

30 3

314 290

9 0

219 200

68 186 355

1.019 1.028 1.087 1.011 1.012 1.025 1.020 1.013

P P S P S O P P

31

169

9

73

58

328

58 76 70

330 330 261

31 8 20

161 95 86

5 12 2

329 258 300 749 119 451 707 625 676 545 234 281

66 40 64 65 65 80 56 50 27 61 71 77

189 189 280 267 57 212 155 187 342 123 172 154

0 50 26 20 6 5 25 25 49 28 8 13

280 20 115 125 313 332 289 311 107 284 284 344

538 388 484 218 418 317 94 284 293 528 402 297

73 16 65 46 69 37 35 27 58 77 73 30

336 108 350 310 122 180 163 22 6 200 25 158

18 3 25 39 5 49 55 63 4 6 5 52

152 199 161 162 19 329 332 190 102 316 279 295

129/81 NE 110/90

24 5 6 14 24 9 21 29 29 8 18 2

10 284 22 29 220 63 29 56 235 19 17 253

1.008 1.012 1.020 1.024 1.027 1.012 1.027 1.029 1.017 1.022 1.033 1.040

P P T T T O T T P O/P T/P T

280/66 193/85 292/84 299/76 130/66 333/81 299/69 325/61 145/61 289/82 287/72 163/88

1 74 3 17 21 16 5 5 32 12 16 21

242 300 253 58 287 78 69 289 194 47 187 55

1.015 1.024 1.023 1.009 1.045 1.014 1.016 1.039 1.042 1.023 1.035 1.017

P T T O P O P O/P T T P T

152/89 NE 210/16 SE 163/87 E 328/73 SW 197/69 SE 347/74 W 325/61 SW 199/85 SE 104/58 N 317/78 SW 097/74 N 299/69 SW

EF4

017

179

ChiSquare value

9.0

238/33 SE EF12 338/85 SW 096/78 N 265/88 S EF16

S SE SW SW NE SW SW SW NE SW SW E

128

56 29.2

(090)

127

0.6

EF21 EF26

018 (030)

302 20.0 42 0.4

EF29

(116)

10

EF40

069

96 85.9

EF48 EF54

007 089

187 27.0 36 17.9

EF82 EF84 EF87

055 154 099

148 61.3 147 63.7 84 19.9

EF74 EF76

031 121

41 7.5 29 17.0

EF61 EF69

(034) 066

3.3

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Stratigraphic section

11 3.1 129 54.9

153

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were measured and recorded. Only phenocryst intersections with length-to-width ratios greater than two were measured. For each sample, the apparent lineation azimuth was calculated using Georient 9.2 (Holcombe, 1996) and the degree of phenocryst alignment was evaluated statistically by the ChiSquare method (Rusnak, 1957; Masuda et al., 1999). Chi-Square values greater than 4.6 indicate a confidence level greater than 90% in the degree of phenocryst alignment in the plane of the section. In this study, samples with Chi-Square values less than 4.6 were considered unsuccessful and discarded. In an attempt to evaluate the reproducibility of the results, one set of slabs (sample EF21) was measured by two operators (Fig. 3). For four of the five slabs, the results for the two operators are very similar. There is a substantial discrepancy in distribution for the fifth slab (D). However, when results from all five slabs are pooled, the average apparent lineations generated by the two operators differ by ∼20°. We conclude that the results are affected by operator-related variations but that different operators are likely to produce broadly similar results (that is, within the same sector). 5. AMS and petrofabric results Of the samples from the 32 AMS sites, those from 30 sites have yielded distinctly anisotropic susceptibilities (Fig. 4; Table 1), and the degree of anisotropy ranges from 1% to 4%, although it is generally around 2%. The remaining sites have susceptibilities

which define a sphere (rather than an ellipsoid). Site F05 has the largest anisotropy but is not lithologically consistent and hence gives a spherical distribution of the susceptibility. Collectively, the AMS data are characterised by northwest– southeast striking, steeply dipping Kmax/Kint foliations (AMS foliations) and steeply plunging Kmax axes (Fig. 5). The AMS foliations have more or less the same orientation in all sections across the ∼ 225 km extent of the Eucarro Rhyolite (Fig. 6). The western and central parts of the unit (Fig. 6A,B) show reasonably systematic stratigraphic variations in the dip direction of the AMS foliations and in the plunge direction of the Kmax axes. In the lower and middle parts of the unit, the AMS foliations dip to the southwest and the Kmax axes mainly plunge to the south or southwest, whereas in the upper part, the AMS foliations dip to the northeast and Kmax axes plunge northerly. In the eastern part of the unit (Fig. 6C), the AMS foliations and Kmax axes also show up-section reversals in dip direction or trend, respectively, but in an opposite sense to that in the rest of the Eucarro Rhyolite: near the base, the AMS foliations dip to the northeast and the Kmax axes plunge to the north, whereas near the top, the AMS foliations dip to the southeast or southwest and the Kmax axes plunge to the southwest. Twelve of the 16 petrofabric samples show preferred alignments of elongate sections through feldspar phenocrysts, having Chi-Square values varying from 7.5 to 63.7 (Fig. 7; Table 1). In three samples (F26, F29, F61), elongate phenocryst

Fig. 5. Equal-area stereonets for: (A) all the poles to the Kmax/Kint foliation (filled circle; equivalent to Kmin poles) measured for samples from the Eucarro Rhyolite; (B) poles to the Kmax/Kint foliation measured for samples from western, central and eastern sections; (C) poles to the Kmax/Kint foliation measured for samples from the base, middle and top of stratigraphic sections through the Eucarro Rhyolite in the central area (area B on Fig. 1); (D) poles to the Kmax/Kint foliation measured for samples from the base, middle and top of the easternmost stratigraphic section through the Eucarro Rhyolite at Siam (Fig. 1).

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Fig. 6. Map of sections through the Eucarro Rhyolite, showing the sample locations, strikes and dips of the magnetic foliation (Kmax/Kint plane), trend of the AMS lineation (Kmax), and phenocryst lineations for (A) western, (B) central and (C) eastern areas. Large arrows give the local flow directions inferred from the AMS foliation and lineation data.

sections are not sufficiently abundant to provide enough measurements (b 100 measurements per sample). In F16, 127 elongate sections through phenocrysts were measured but they are not preferentially aligned (Chi-Square value b 1). The petrofabric data show considerable variations, both in single traverses across the Eucarro Rhyolite and laterally along strike (Fig. 7; Table 1). However, in nine of the twelve sites, elongate sections through phenocrysts are aligned preferentially in northerly, northeasterly or easterly directions. At the remaining three sites, elongate sections through phenocrysts define lineations that trend NW–SE, between ∼120° and 160° (or 300° and

340°). The sites with NW–SE lineations occur in the western area (F12, Narlaby Well section), and in two eastern sections (F84, Coralbignie section, and F76, Nonning section) (Fig. 6A,C). 6. Discussion 6.1. Implications for emplacement mechanisms Recent AMS research concentrating on basaltic lavas (e.g., Cañon-Tapia et al., 1995, 1996, 1997; Walker et al., 1999; Cañon-Tapia and Pinkerton, 2000; Cañon-Tapia, 2004) has

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Fig. 7. Rose diagrams of preferred orientations of elongate feldspar phenocrysts measured on slabs taken from the twelve Eucarro Rhyolite samples that yielded statistically valid results. Sample locations are shown on Fig. 1. Lineation directions are given in Table 1. n is the number of elongate feldspar phenocrysts measured.

shown that the most reliable results for palaeoflow analysis require measurements of AMS at a series of sites through single emplacement units. These studies suggest that both AMS foliations and the principal susceptibility axes can provide information on flow direction in lavas. The basaltic lavas measured show a shift in plunge direction of the principal susceptibility axes from up-flow near the base to down-flow near the top; similarly, AMS foliations dip up-flow near the base, and down-flow near the top (Cañon-Tapia et al., 1997; Cañon-Tapia, 2004). The up-section reversals in AMS directions reflect vertical variation in shear. In many cases, the reversal is recorded by the Kmax axes and Kmax/Kint foliations. Cañon-Tapia et al. (1996, 1997) have established that the reversal in dip/plunge direction may also be shown by other AMS parameters. The use of AMS on felsic lavas is in its infancy. The only detailed work to date (Cañon-Tapia and Castro, 2004) concentrated on a small-volume (b 2 km3) rhyolite dome. Furthermore, eight of the ten sample sites were distributed around the dome margin and at the dome surface. Nevertheless, the results showed that Kmax/Kint foliations are parallel to flow bands and that Kmax is approximately parallel to a mineral preferred fabric defined by microlites. Studies of AMS in ignimbrites have shown that the Kmax axis is parallel to the outflow direction, and can be horizontal or else plunge gently up-flow (imbrication). In general, these studies do not consider the Kmax/Kint foliation, and rely

instead on the Kmax axis to determine flow direction. In addition, stratigraphic variations in AMS patterns in ignimbrites differ significantly from those in lavas. Several studies have reported results from sites spaced stratigraphically upward through single sections, varying from just a few metres thick (e.g., Bandelier Tuff, MacDonald and Palmer, 1990), to 120 m thick (Bishop Tuff, Palmer et al., 1996) and 250 m thick (Fish Canyon Tuff, Ellwood, 1982). The conclusion from the two more recent studies was that stratigraphic variations in Kmax declination can be quite substantial on the scale of metres, whereas the third, earlier study found that the AMS directions do not vary significantly. None of the stratigraphic sections shows any systematic reversal in the plunge directions of the AMS major axes. The AMS results from stratigraphic sections through the Eucarro Rhyolite are dominated by Kmax/Kint foliations that are steeply dipping with more or less consistent NW–SE strikes. No near-horizontal or very gently dipping foliations were measured, possibly because all sample sites lie at some distance above basal contacts. Nevertheless, up-section reversals in the dip directions of the Kmax/Kint foliations are evident throughout the unit (Fig. 6). All but four out of thirty (87%) Kmax plunges are moderate to steep (N30°), and trends are either unidirectional or else show an up-section reversal in concert with the up-section reversals shown in the AMS foliations. Using the patterns shown by basaltic lavas as a guide, we conclude that the AMS characteristics of the Eucarro

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Fig. 8. Cartoon showing a reconstruction of the pattern of AMS foliations (dashed lines) in the Eucarro Rhyolite lava. The pattern is similar to that expected for flow bands in felsic lava. Also shown are the mapped internal variations within the Eucarro Rhyolite, including the vesicular facies which occurs toward the top, the facies with black or dark brown, formerly glassy groundmass which typically occurs near the base, and columnar joints which are perpendicular to the base of the unit base. A. Cross-section view along a reconstructed SW–NE transect. B. Map view of the internal variations as seen on a horizontal plane that contains XX′. Such a plane is a reasonable approximation of the present-day eroded surface of the Eucarro Rhyolite, and mainly exposes the middle, columnar-jointed part of the unit where the measured AMS foliations are very steeply dipping or vertical.

Rhyolite are consistent with emplacement by non-particulate (lava-like) flow. In small felsic lavas and domes, flow bands are commonly near-vertical, especially away from the basal contacts (e.g.,

Cole, 1970; Merle, 1998), and at least in one case (Cañon-Tapia and Castro, 2004), are parallel to Kmax/Kint foliations. Hence, the pattern of Kmax/Kint foliations in the Eucarro Rhyolite (Fig. 8) mimics what one would expect for the orientation of

Fig. 9. (A) Simplified map showing the extent of the Eucarro Rhyolite, arrows for palaeoflow directions determined by interpretation of AMS data, and the main Hiltaba Suite granitoid intrusions southwest of the Gawler Range Volcanics. Based on Flint (1993, Fig. 5.2). (B) Schematic southwest–northeast cross-section showing the possible southwesterly magma source for the western and central Eucarro Rhyolite lava flow in relation to the present level of erosion. The unpatterned areas on both diagrams are occupied by pre-Mesoproterozoic basement formations.

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flow bands in the interior of a viscous felsic lava, and like flow bands, the pattern probably developed in response to laminar shear during outflow. 6.2. Source location for the Eucarro Rhyolite Allen and McPhie (2002) speculated that the overall east–west elongate distribution of the Eucarro Rhyolite could indicate eruption from multiple sources located on an extensive fissure. There are no textural or structural features in the exposed Eucarro Rhyolite that imply close proximity to a vent. Hence, the sources were probably located beyond the present extent of the unit. Allen and McPhie (2002) also recognized subtle textural and compositional differences between the western and central versus the

eastern parts of the unit, and the eastern part is separated from the rest by a gap in outcrop (Fig. 1). They concluded that the eastern and other parts of the Eucarro Rhyolite were erupted from different vents tapping compositionally distinct parts of the source magma. The new AMS data on the Eucarro Rhyolite provide further support for the conclusion that the central and western versus the eastern parts of the unit had different source locations. For the western and central parts, up-section reversals in AMS foliation dip directions and Kmax axes trends imply outflow from sources located to the southwest (Fig. 9). The data indicate a southwesterly source for some 110 km of the total 225 km lateral extent of the Eucarro Rhyolite. A southwesterly source for the western and central parts of the Eucarro Rhyolite is consistent with two other aspects of its

Fig. 10. Relationships between AMS lineation (Kmax) and phenocryst lineations in selected samples of the Eucarro Rhyolite. (A) Sample F48 shows a strong phenocryst lineation parallel to the steeply plunging Kmax in the Kmax/Kint plane. The phenocryst intersection lineation measured on the palaeohorizontal plane is almost perpendicular to the trend of Kmax. (B) In sample F84, the Kmax/Kint plane dips very gently and is therefore close to the plane in which the phenocryst intersection lineations were measured. However, the trend of Kmax differs from the lineation defined by the elongate feldspar phenocrysts by ∼ 50°.

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distribution and internal variations. The texturally and compositionally distinct member of the Eucarro Rhyolite, the Paney Rhyolite Member, occurs along part of its northern margin in the western and central parts of the unit (Allen and McPhie, 2002). The quartz-phyric Paney Rhyolite is intermingled with the plagioclase-phyric rhyolite that dominates the Eucarro Rhyolite. It is not found intermingled with more southerly, stratigraphically lower parts of the Eucarro Rhyolite. The confinement of the Paney Rhyolite to the northern margin is consistent with it being a compositionally distinct batch at the leading edge of the lava flow, advancing from a southerly or southwesterly source, and now found at the distal termination. In addition, flow bands in the Paney Rhyolite are steeply dipping or vertical and have northwesterly, east–west or northeasterly strikes, suggesting that outflow was broadly northerly (Allen and McPhie, 2002). In the eastern part of the Eucarro Rhyolite, Kmax/Kint foliations are parallel to those in the rest of the unit but in general, their dip directions are opposite. The plunges of Kmax axes are consistently to the north in lower parts and to the south in upper parts, of the unit. Both patterns indicate flow from the north, more or less opposite to that for the other sections (Fig. 6C). The Eucarro Rhyolite extends for ∼ 40 km farther east from Siam but is poorly exposed, and hence was not sampled. The different flow direction found for the eastern part could be indicating that it belongs to a separate flow lobe. For example, adjacent flow lobes in the Badlands Rhyolite, Idaho, have followed orthogonal flow paths (Manley, 1996). The eastern part may have been erupted from the southwesterly source, like the rest of the unit, or else, an independent emplacement unit erupted from a northerly source vent. Older volcanic units in the lower Gawler Range Volcanics are exposed to the southwest of the Eucarro Rhyolite and include a variety of rhyolitic lavas and ignimbrites (Allen et al., 2008-this volume). A volcanic centre at Menninnie Dam (Fig. 9) has been recognized as the source of a thick, lithic-rich welded ignimbrite and a small rhyolitic lava dome (Roache et al., 2000). However, neither the location nor the size of this centre appear compatible with the southwesterly source region for the western and central parts of the Eucarro Rhyolite indicated by the AMS data. Beyond the present southwestern limit of outcrop of the Gawler Range Volcanics, the principal lithostratigraphic units are the deformed Palaeoproterozoic and Archaean basement complexes and Mesoproterozoic Hiltaba Suite granitoid intrusions (Flint, 1993). Both of these major units are extensively covered by Quaternary sand dunes but have been delineated on regional aeromagnetic maps. The granitoid intrusions have a broadly northwest–southeast distribution, and some separate plutons appear to be elongate in a northwest–southeast direction, parallel to the inferred trend of the southwesterly source region for the Eucarro Rhyolite (Fig. 9). A link between the Eucarro Rhyolite source and one or more of the Hiltaba Suite granitoid plutons is plausible but cannot be evaluated further. A substantial thickness (perhaps more than 2°km?) of the Mesoproterozoic section has been removed by erosion, and what remains is only locally exposed.

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6.3. Comparison of AMS and petrofabric data The consistent pattern shown by the Eucarro Rhyolite AMS data contrasts with the wider variation in lineations obtained by petrofabric analysis based on elongate sections through feldspar phenocrysts. Phenocryst lineations appear to fall into two classes — one is more or less parallel to the flow direction inferred from the AMS foliation data, and the other is more or less perpendicular. The flow-parallel results are roughly twice as common as the flow-perpendicular results. However, in the absence of the AMS foliation data, confident interpretation of the petrofabric lineations would have been impossible. Other studies have shown that, although there may be local complications, Kmax and preferred mineral orientations are likely to coincide (e.g., Cañon-Tapia and Castro, 2004). We have not found such a consistent relationship. For some samples, we cut surfaces parallel to the Kmax/Kint foliation orientation and found cases where there is a strong mineral alignment parallel to Kmax (e.g., Fig. 10A), and other cases where no such strong preferred orientation could be identified (e.g., Fig. 10B). We have not attempted to further investigate the relationship between the AMS and phenocryst apparent lineation data but speculate that there are at least two sources of complication. One relates to our method for obtaining the petrofabric data. In particular, we measured apparent elongate mineral lineations on gently dipping planes approximating the orientation of the Eucarro Rhyolite base, whereas the AMS foliation planes are generally very steeply dipping. The other relates to the fact that the magnetic fabric is defined by magnetite, whereas the petrofabric data are based exclusively on the apparent preferred alignment of elongate feldspar phenocrysts. In particular, these two mineral phases are likely to experience different durations and intensities of shear, reflecting differences in their physical properties (e.g., size, shape, density) and timing of crystallization. 7. Conclusions In sections through the Eucarro Rhyolite, AMS foliations (Kmax/Kint planes) typically strike northwest–southeast and are very steeply dipping. For sections in the western and central parts of the unit, the dip direction is southwest near the base and in the middle, and northeast near the top. Kmax axes mainly plunge to the south in lower and middle levels of the unit and, at a few sites, show a reversal to northerly plunges near the top. For the eastern part of the Eucarro Rhyolite, Kmax axes mainly plunge to the north in lower and middle levels of the unit and reverse to southerly plunges near the top. The pattern shown by the AMS parameters is consistent with northwesterly palaeoflow of the western and central parts of the Eucarro Rhyolite, and southerly palaeoflow of the eastern part. The pattern could be indicating that the western and central versus the eastern parts belong to different flow lobes, or that they came from separate source vents. The second case implies that the Eucarro Rhyolite in fact comprises two separate but coeval emplacement units. Co-magmatic Hiltaba Suite granitoid intrusions that occur to the southwest of the Eucarro Rhyolite could be the deeply eroded roots of the southwesterly eruptive centre(s).

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The up-section reversal in directions of AMS parameters is consistent with the plagioclase-phyric part of the Eucarro Rhyolite being a lava, or else having been emplaced throughout its present extent in a non-particulate lava-like mechanism. The pattern is also consistent with most of the Eucarro Rhyolite (plagioclase-phyric rhyolite accounts for N99% of the total unit volume) having been emplaced and having cooled as a single unit, as already recognized from internal textural variations. Hence, it appears that felsic lavas in bimodal and felsic LIPs can be almost as gigantic in volume and extent as basaltic lavas. Lineations defined by alignment of elongate intersections of feldspar phenocrysts are also present in the Eucarro Rhyolite. In most samples (about two-thirds), the feldspar phenocrysts appear to be aligned parallel to general flow directions inferred from the Kmax/Kint foliation pattern, and in the remainder, the apparent alignment is roughly perpendicular. Acknowledgements Financial support for this research came from an Australian Research Council Discovery Grant awarded to J McPhie. We thank Sue Daly, Michael Schwarz and Gary Ferris of Primary Industries and Energy South Australia for many valuable discussions and generous logistic support in the field, and Kath McMahon for undertaking the AMS measurements. Edgardo Cañon-Tapia, Curtis Manley and James White provided critical reviews and helpful comments, all of which are much appreciated. References Allen, S.R., McPhie, J., 2002. The Eucarro Rhyolite, Gawler Range Volcanics, South Australia: a N675 km3, compositionally zoned lava of Mesoproterozoic age. Geological Society of America Bulletin 114, 1592–1609. Allen, S.R., Simpson, C., McPhie, J., Daly, S.J., 2003. Stratigraphy, distribution and geochemistry of widespread felsic volcanic units in the Mesoproterozoic Gawler Range Volcanics, South Australia. Australian Journal of Earth Sciences 50, 97–112. Allen, S.R., McPhie, J., Ferris, G., Simpson, C., 2008. Evolution and architecture of a large felsic igneous province in western Laurentia: the 1.6 Ga Gawler Range Volcanics, South Australia. J. Volcanol. Geotherm. Res. 172, 132–147 (this volume). Blissett, A.H., Creaser, R.A., Daly, S.J., Flint, R.B., Parker, A.J., 1993. Gawler Range Volcanics. In: Drexel, J.F., Preiss, W.V., Parker, A.J., (Eds.), The Geology of South Australia. Vol.1, The Precambrian. Geological Survey of South Australia, Bulletin 54, 107–131. Bonnichsen, B., Kauffman, D.F., 1982. Physical features of rhyolite lava flows in the Snake River Plain volcanic province, southwestern Idaho. Special Paper — Geological Society America 212, 119–145. Cañon-Tapia, E., 2004. Flow direction and magnetic mineralogy of lava flows from the central parts of the Peninsula of Baja California, Mexico. Bulletin of Volcanology 66, 431–442. Cañon-Tapia, E., Castro, J., 2004. AMS measurements on obsidian from the Inyo Domes, CA: a comparison of magnetic and mineral preferred orientation fabrics. Journal of Volcanology and Geothermal Research 134, 169–182. Cañon-Tapia, E., Pinkerton, H., 2000. The anisotropy and magnetic susceptibility of lava flows: an experimental approach. Journal of Volcanology and Geothermal Research 98, 219–233. Cañon-Tapia, E., Walker, G.P.L., Herrero-Bervera, E., 1995. Magnetic fabric and flow direction in basaltic Pahoehoe lava of Xitle Volcano, Mexico. Journal of Volcanology and Geothermal Research 65, 249–263.

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