PAPERS Department of Geology • The University of Queensland VOLUME 12 number 3 Editor: S.H. HALL Zeolites in the Main Range Volcanics, Queensland D.J. DRYSDALE P.264-268 Bracewellite and the origin of "Merumite" D.J. DRYSDALE P.269-277 Perlitic texture and other fracture patterns produced by hydration of glassy rocks D.J. DRYSDALE P.278-285 Lithium aluminium silicate minerals and pollucite from Meldon, Devon and San Piero in Campo, Elba D.J. DRYSDALE P.286-293 Polytype 2H molybdenites with high rhenium contents D.J. DRYSDALE P.294-303 Date of publication: December 1991 ZEOLITES IN THE MAIN RANGE VOLCANICS, QUEENSLAND by D.J. Drysdale ABSTRACT. Amygdaloidal zeolites from the Main Range Volcanics, Queensland, in the collections of the Queensland Museum and the Geology Museum, Queensland University, are dominated by chabazite and natrolite. Stilbite and analcite are the only other species represented. Zeolites are present in a number of amygdaloidal horizons at several levels in the pile and there is no evidence for vertical zonation of species. INTRODUCTION Zeolites present in amygdales in basalt lavas that build up thick piles of more or less constant composition are characteristically distributed in depth related zones that are regional in scale and defined by the incoming and outgoing of marker species. This zonation is interpreted as reflecting heat flow in the lava pile during zeolitization and is supposed to be a measure of regional geothermal gradient. High heat flow, as at mid-ocean ridges and spreading centres in continental environments, is the main factor that controls the nature and extent of regional zonation. Local zonal distributions of zeolite species have also been observed, related to local heat flow domes about individual volcanic centres, active geothermal centres, dyke swarms, highly fractured zones, shallow intrusions, and the roots of deeply eroded major volcanoes. No study has been made of the zeolites present in the pile of the Main Range Volcanics, Queensland. THE MAIN RANGE VOLCANICS, QUEENSLAND, AND THEIR ZEOLITES Stratigraphy of the Main Range Volcanics is described by Stevens (1969), Grenfell (1984) and Ewart and Grenfell (1985). The volcanics are late Oligocène to early Miocene lava flows, products of anorogenic volcanism, that cover an area of 4300 square kms. and attain a maximum thickness of about 900m. Dips are shallow, generally less than 3 degrees and most commonly westward. The southern Main Range (south of a line from Laidley to Allora) comprises two formations of subequal thickness: the Governors Chair Volcanics (lower), a mildly alkaline basalt - comendite association with several thick trachyte horizons (including the Steamers Trachyte Member); and the Superbus Basalt (upper), exposed in most of the southern Main Range, with a maximum recorded thickness of 440 m of predominently hawaiite lavas with alkaline olivine basalt and mugearite. The Toowoomba Basalt of the northern Main Range passes laterally into the Suberbus Basalt. Pap. Dep. Geol. Univ. Qd., 12(3): 264-268, Dec., 1991. 265 Some zeolites listed in Whitehouse (1937) are from the Main Range Volcanics: chabazite associated with stilbite in basalt, Toowoomba; chabazite in decomposed basalt. Freestone Creek; chabazite associated with calcite and natrolite in basic volcanic tuff. Spring Bluff - Harlaxton railway cutting; and chabazite associated with calcite, magnesite and natrolite in decomposed basalt. Mount Davidson. Stevens (1965, 1969, 1984) notes species at a number of localities: at about 975m on Mount Mitchell and Mount Cordeaux an amygdaloidal zone rich in chabazite indicates an upper margin of a basalt flow, for 75m above this the lavas are highly amygdaloidal and generally altered, and at higher elevations amygdales occur but are less abundant and confined to flow tops; zeolites occur in basalt 1km past the Boonah border gate on White Swamp Road; porphyritic basalt with chabazite occurs below the Steamers Trachyte Member on the road to Queen Mary's Falls and just below Dagg's Falls; at Harlaxton there is natrolite-bearing basalt; and at Teviot Falls there are amygdaloidal basalts with natrolite and analcite. Stevens also notes markedly amygdaloidal basalts containing chabazite are found at just over 915m between Mount Cordeaux and Acacia Plateau and at lower levels to the west of this line but it is not known whether these occurrences are at the same stratigraphic horizon. The collections of the Queensland Museum and the Geology Museum of the University of Queensland house 45 specimens of zeolitic amygdaloidal basalts identifiable as coming from the Main Range Volcanics in the Ipswich and Warwick 1: 250,000 sheet areas. Chabazite dominates (present in 34 specimens), followed by natrolite (9 specimens). Locality information is detailed but difficulties with the lateral conelation of lavas make it impossible to work out a stratigraphy for the museum specimens. Mount Mitchell (5 specimens). Mount Cordeaux (5 specimens), the Toowoomba district (15 specimens). Mount Superbus (3 specimens) and the Killamey district (9 specimens) dominate the collections. Main Range localities represented in the Australian Museum collections have only chabazite and natrolite (pers. comm. Dr Lin Sutherland). The work of Stevens and Grenfell suggests it may be that zeolitic amygdaloidal basalts are restricted to a few highly amygdaloidal horizons in the lava pile. The Dagg's Falls and Queen Mary's Falls road occurrences are in basalts not far below the Steamers Trachyte Member, the Boonah border gate and Teviot Falls localities are probably also below the Steamers Trachyte Member, the zeolite occunences on Mount Cordeaux are well up in the Superbus Basalt, the occurrence near Acadia Plateau is in the Governors Chair Volcanics below the Steamers Trachyte, those on Mount Mitchell are also in the Superbus Basalt, and the Harlaxton locality is in the Toowoomba Basalt. DISCUSSION The Main Range assemblages differ from those observed in most other thick piles of basalt lavas in two respects: 266 1. The range of species is limited relative to assemblages from the classic localities of Ireland, Iceland, and the Faeroes (Cunie 1910, Walker 1962 and references therein, Betz 1981, Mehegan et al. 1982 and references therein). Table Mountain, Colorado (Kile and Modreski 1988), and Paterson, New Jersey (Peters et al. \91Sy, and assemblages from the Parana Basin, Brazil (Murata et al. 1987), the Deccan Basalt, India (Jeffery et al. 1988), Nova Scotia, Canada (Bujak & Donohoe 1980), and the Siletz River Volcanics, Oregon (Keith & Staples 1985). 2. There is no zonation of species as described from Ireland, Iceland, Nova Scotia, and Brazil (references as above). For other localities zonation is suspected but not proven: Betz (1981) notes that it appears that zeolite zoning is present in the Faeroes but does not describe a scheme; at Table Mountain, Colorado, locality dependant variations in individual species descriptions and occurrence that are analogous to regional variations in Ireland, Iceland, the Faeroes, India and Brazil are noted by Kile and Modreski (1988) but lack of complete locality information for many specimens makes a comprehensive evaluation of regional variation uncertain; and Jeffery et al. (1988) found the zones distinguished by earlier workers in the Deccan basalts to be absent from an area of 15,000 sq. kms. and the zeolites to be unzoned or zoned according to a pattern not yet recognised. Acccounts of lava piles lacking zonation of zeolites are rare. The Siletz River Volcanics, Oregon, (Keith & Staples 1985) are a thick pile of zeolitized flows devoid of any regional zonation, and in the volcanics of north-eastern Azerbaijan, Iran, (Comin - Chiaramonti et al. 1979), the distribution of zeolite species does not show any relationship to the volcanic stratigraphy. The Siletz River Volcanics is the only well documented example of a thick pile of lavas in which the zeolite species lack a vertical zonal distribution. Keith and Staples (1985) suggest zeolitization here occuned during a low temperature (60 - 70°) hydrothermal event, or by reaction of cold (circa - 10°) meteoric water with basalt over a long time, and that the concept of localised heat flow from sources such as individual eruptive centres seems to be a reasonable explanation of the inegular distribution of the zeolites. Murata et al. (1987) suggest a fundamental cause of the lack of zonation here may be low heat flow in the region, a result of the subduction of oceanic plate under this part of the North American plate, and that the cavities contain different assemblages of zeolites distributed at random because sealing off of cavities from one another through precipitation of minerals (in this 267 case montmorillonite) at different times led to their minerals crystallizing in closed systems. There may be other examples of lack of vertical zonation of zeolite species in basalt sequences. For example, Birch (1988) notes there has been no evaluation of zonation, either vertically or horizontally, in the distribution of zeolite species in the Older Volcanics, Flinders, Victoria, but Hall (in Birch, 1989) observes the zeolites are restricted to exposures along the coast and have not been detected in the flows further inland, the majority are found in relatively thin and markedly vesicular flows which are amongst the youngest in the sequence, and montmorillonite as a vesicle lining is the most common secondary mineral and the first mineral to have formed in the cavities. Coulsell (1980)