Tasmanian Geological Survey Record 2001/09 Inclusions of high pressure origin in Tasmanian Cainozoic basalts: A catalogue of localities by J. L. Everard DepartTasmanianment o fGeological Infrast Surveyructu Recreord, E n2001/09ergy and Resources 1 Mineral Resources Tasmania Tasmania 69 1 Inclusion and Megacryst Localities 68 67 66 65 64 SCALE 0 20 40 60 80 100 km 2 3 9 16 15 17 8 4 18,19 13,14 5 6 23 41 7 42 37 45 44 21 43 48,49 26 11 39 46 10 22 38 50 20 30 58 40 57 24 28 47 51 63 25 29 34 55 56 12 35 27 52 53 54 36 31 59,60 61 32 62 33 70 147 71 73 74 81 77 75 72 76 82 78 148 150 149 143 114 144 119 146 145 120 141 121 157 156 159 155 160 162 161 153 163 154 165 180 168 181 184 164 166 169 182 172 183 185,186 178 170 187 189 190 191 Figure 1 Inclusion and megacryst localities. Basalt distribution and 1:50 000 scale quadrangle boundaries are also shown. See figures 2 and 3 and Table 1 for greater detail. Mineral Resources Tasmania PO Box 56 Rosny Park Tasmania 7018 Phone: (03) 6233 8377 l Fax: (03) 6233 8338 Email: [email protected] l Internet: www.mrt.tas.gov.au Contents Abstract ………………………………………………………………………………… 4 Introduction ……………………………………………………………………………… 4 Distribution ……………………………………………………………………………… 4 Host rocks ………………………………………………………………………………… 5 Inclusions ………………………………………………………………………………… 7 Classification …………………………………………………………………………… 7 Cr-diopside suite ………………………………………………………………………… 8 Al-augite suite …………………………………………………………………………… 8 Other inclusions ………………………………………………………………………… 9 Xenocrysts ……………………………………………………………………………… 9 Megacrysts ……………………………………………………………………………… 9 Acknowledgements………………………………………………………………………… 10 References ………………………………………………………………………………… 10 Figures 1. Inclusion and megacryst localities, showing basalt distribution and 1:50 000 scale quadrangle boundaries ………………………………………………………………… 2 2. Inclusion and megacryst localities, southeastern Tasmania ………………………………… 5 3. Inclusion and megacryst localities, southern Midlands …………………………………… 6 4. Localities for various inclusion types …………………………………………………… 12 5. Localities for various inclusion and megacryst types ……………………………………… 13 6. Localities for various megacryst types …………………………………………………… 14 Tables 1. Locations of inclusions of high pressure origin in Tasmanian Cainozoic basalts ……………… 15 2. Xenolith hosts ………………………………………………………………………… 22 While every care has been taken in the preparation of this report, no warranty is given as to the correctness of the information and no liability is accepted for any statement or opinion or for any error or omission. No reader should act or fail to act on the basis of any material contained herein. Readers should consult professional advisers. As a result the Crown in Right of the State of Tasmania and its employees, contractors and agents expressly disclaim all and any liability (including all liability from or attributable to any negligent or wrongful act or omission) to any persons whatsoever in respect of anything done or omitted to be done by any such person in reliance whether in whole or in part upon any of the material in this report. Tasmanian Geological Survey Record 2001/09 3 Abstract Xenoliths, cognate inclusions, xenocrysts and megacrysts of upper mantle or lower crustal origin are recorded in Cainozoic basalts and associated rocks at more than 190 localities throughout Tasmania. Hosts include plugs, lava flows and less commonly pyroclastic rocks or diatreme breccias, and are strongly biased towards undersaturated compositions. By far the most common inclusions are spinel lherzolite xenoliths of the Cr-diopside suite, but at about 30 known sites these are accompanied by Al-augite suite inclusions (mostly websterites and wehrlites). Garnet-bearing ultramafic assemblages are recorded from only two localities. Other xenoliths, of probable lower to mid-crustal origin, include granulite, gabbro, dolerite and anorthosite. Megacryst species reported include clinopyroxene, orthopyroxene, olivine, spinels, kaersutitic amphibole, titanbiotite and titanphlogopite, apatite, alkali feldspar (anorthoclase, sanidine), plagioclase (albite to labradorite) and titanomagnetite. Zircon, corundum (sapphire) and possibly diamond occur as heavy minerals in alluvial deposits derived from Cainozoic basalts, but are very seldom found in situ. Introduction introduced, add 112 m to the eastings and 183 m to the northings. The estimated maximum error of this block The following locality catalogue (Table 1) has been shift is three metres, far less than the imprecision of the compiled using an earlier compilation (Wass and data. Irving, 1976) as a starting point, with the addition of ‘Quadrangle’ (column 5) is for the most part information from subsequent literature (principally synonymous with the 1:50 000 or 1:63 360 scale Geological Survey Bulletins and Explanatory Reports), Geological Atlas sheets published by the Tasmania unpublished data supplied by F. L. Sutherland, and Department of Mines from 1956 to 1994. ‘1:25000 map’ observations of the author and colleagues. (column 6) refers to both topographic sheets published The petrology of mantle-derived and other high at that scale and digital geological maps produced by pressure inclusions from eastern Australia, including Mineral Resources Tasmania since 1995. Sheets that Tasmania, has been reviewed by several authors are not available as geological maps at these scales are including Wass and Irving (1976), Sutherland (1979), listed in italics. It will be noted that for most localities Sutherland and Hollis (1982), O‘Reilly et al. (1989) and relevant 1:50 000 or 1:63 360 scale, but at present not Beyer et al. (2002). No new petrological data is 1:25 000 scale geological maps, are available. For a presented here, and only a brief review of the small number of localities (e.g. Flinders Island and petrology of the inclusions and megacrysts is parts of the Central Plateau), the only systematic attempted. References to more detailed information on geological maps available are at 1:250 000 scale. individual localities are given; the principal published sources are Varne (1977), Sutherland (1974), Brown and McClenaghan (1982), Sutherland et al. (1984, 1996) Distribution and Beyer et al. (in prep.). The general map (fig. 1) shows that high pressure The number in the first column of Table 1 links a brief inclusions have been found in most districts in which description of the geographic locality (column 2) to Cainozoic basalts occur. The density of known positions shown on the attached maps (Figures 1 to 6). localities is partly a function of terrain (accessibility Some of the localities listed are relatively insignificant, and exposure) and intensity of geological mapping, and contain only very sparse and/or small inclusions but is largely real. or xenocrysts. As noted, several localities refer to There are relatively few sites in the extensive, mainly multiple flow remnants from the same eruptive mildly alkalic lava flows of the northwest coast, source, and may not represent discrete, separate although much of this country has been cleared for eruptions. agriculture or forestry, and is geologically relatively Australian map grid co-ordinates are given (columns 3 well known. No localities are known from the and 4) to a nominal precision of either 10 or 100 metres. dominantly tholeiitic lava pile east of Waratah, which Locations which are considered substantially less is locally more than 350 m thick. There may be precise are indicated (c.; circa), usually because the additional undiscovered localities in remote areas of inclusions are inferred to be present over a substantial the far northwest (e.g. the Savage River area), a region area (e.g. in a flow). In a few cases, the location is that has not been mapped in detail. There are poorly known (indicated by ?), often because of vague numerous localities in northeast Tasmania, especially or ambiguous descriptions in the literature. Most from the plugs and extensive flows (partly olivine locations have been checked and refined if necessary nephelinite) of the Ringarooma River area. The against available geological maps, and many have strongest concentration (about 75 sites) is in the been field checked by the author. The datum is Zone 55 southern Midlands region (fig. 3), a region of of the Australian Geodetic Datum 1966 (AGD66), numerous small plugs and flow remnants of alkalic which is used on all recent topographic and geological basalts (and some highly detailed mapping). This also maps of Tasmania. To convert to geocentric applies to a lesser degree to southeastern Tasmania co-ordinates (GDA94) which are now being (fig. 2), apart from the extreme south where Cainozoic Tasmanian Geological Survey Record 2001/09 4 121 139 140 BRIGHTON 157 156 BUCKLAND 159 155 162 151 158 160 153 161 163 154 152 180 HOBART 167 SORELL 181 165 166 184 168 179 164 182 169 186 172 170 183 185 171 173 176 175 174 187 178 177 189 190 188 191 KINGBOROUGH TASMAN Figure 2 Inclusion and megacryst localities, southeastern Tasmania. Basalt distribution, major roads and 1:50 000 quadrangle boundaries are also shown. basalt is absent. There are few known localities in the North Queensland (<5 Ma) (Duncan and McDougall, northern Midlands, western Central Plateau or upper 1989), where large xenoliths occur in cinder cones, Derwent Valley, which are regions of dominantly maars and diatremes (e.g. O’Reilly et al., 1989). Such tholeiitic basalt, although the latter area is relatively features are relatively easily eroded and are not poorly known. common in Tasmania, probably