Article Geochemistry 3 Volume 4, Number 1 Geophysics XX Month 2003 XXXX, doi:10.1029/2003GC000560 GeosystemsG G ISSN: 1525-2027 AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society 1 Chronology, chemistry, and origin of trachytes from Hualalai 2 Volcano, Hawaii 3 Brian L. Cousens 4 Ottawa-Carleton Geoscience Centre, Department of Earth Sciences, Carleton University, 1125 Colonel By Drive, 5 Ottawa, Ontario, Canada, K1S 5B6 ([email protected]) 6 David A. Clague 7 Monterey Bay Aquarium Research Institute, 7700 Sandholt Road, Moss Landing, California, USA, 95039 8 Warren D. Sharp 9 Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California, USA, 94709 10 [1] Hualalai Volcano is unique among Hawaiian volcanoes in that it possesses a relatively high 11 proportion of evolved, trachytic lavas that were erupted at the beginning of the alkalic, postshield phase 12 of volcanism. These evolved lavas yield insights into magma sources, magma supply rates, and the 13 evolution of the subvolcanic magmatic plumbing system at this time. Trachyte lavas are exposed at the 14 Puu Waawaa pumice dome and Puu Anahulu flow, as blocks in maars on the south flank of the volcano, 40 39 15 and as flows in water wells drilled on the west flank of Hualalai. New Ar/ Ar dates show that the Puu 16 Waawaa and Puu Anahulu complex is 114 ka, a block from the Waha Pele maar is 103 ka, and water 17 well trachytes range from 107 to 92 ka in age, indicating a range for trachyte volcanism of 20 ka. Nd and 18 Pb isotopic compositions overlap with younger alkalic basalts from Hualalai but are distinct from 19 Hualalai tholeiitic basalts and Pacific mid-ocean ridge basalts, linking the trachytes to alkalic parental 20 magmas that underwent extensive crystallization to yield trachytic residual magmas. Both Sr and O 21 isotopic ratios are higher in the trachytes than in Hualalai alkalic lavas, which is best explained by 22 reaction with, or assimilation of, altered Hualalai shield basalts at shallow depth. Major, trace element, 23 and isotopic variations between trachytes are consistent with their evolution by fractional crystallization 24 from a Puu Anahulu parent. The short time gap between the end of tholeiitic volcanism (<133 ka) and 25 the onset of trachytic, alkalic volcanism and the lack of deep-origin xenoliths place the magma reservoir 26 within which the trachytes evolved rapidly at shallow (<7 km) depth. Whereas Mauna Kea and Kohala 27 volcanoes produced small volumes of highly evolved lavas as magma supply rates dwindled through the 28 postshield stage, postshield magma intrusion rates at Hualalai were lowest during trachyte formation and 29 increased through a more recent period of alkalic basalt eruptions. Subtle rare earth element and 30 radiogenic isotopic distinctions between trachytes from the three localities indicate that the roof of the 31 shallow magma reservoir may have been irregular, allowing some trachytes to evolve independently from 32 others. 33 Components: 14,506 words, 9 figures, 5 tables, 1 dataset. 34 Keywords: Hawaii; magma chamber; magma evolution; postshield stage; geochronology. 35 Index Terms: 3640 Mineralogy and Petrology: Igneous petrology; 3655 Mineralogy and Petrology: Major element 36 composition; 3670 Mineralogy and Petrology: Minor and trace element composition. 37 Received 27 March 2003; Revised 25 July 2003; Accepted 28 July 2003; Published XX Month 2003. Copyright 2003 by the American Geophysical Union 1 of 27 Geochemistry 3 cousens et al.: hualalai volcano trachytes Geophysics 10.1029/2003GC000560 Geosystems G 38 Cousens, B. L., D. A. Clague, and W. D. Sharp, Chronology, chemistry, and origin of trachytes from Hualalai Volcano, 39 Hawaii, Geochem. Geophys. Geosyst., 4(1), XXXX, doi:10.1029/2003GC000560, 2003. 41 1. Introduction itic lavas from the submarine northwest rift zone 81 drape a reef at 430 m below sea level that is dated 82 42 [2] Hawaiian volcanoes are composed dominantly at 133 ka, indicating that terminal tholeiitic erup- 83 43 of basaltic lavas, and include only small volumes tions are younger than 133 ka [Moore and Clague, 84 44 of more differentiated rocks [Macdonald et al., 1992]. 85 45 1983]. This contrasts strongly with oceanic islands 46 elsewhere, particularly with those in the Atlantic [4] The north slope of the volcano is punctuated 86 47 Ocean, where highly differentiated rocks are com- by the 1.6 km-diameter Puu Waawaa trachyte 87 48 mon [Baker, 1975; Baker et al., 1974; Le Roex, pumice cone and a >275 meter-thick trachyte flow 88 49 1985; Ridley et al., 1974; Schmincke, 1976]. If from that cone, termed Puu Anahulu, which to- 89 50 present, differentiated rocks on Hawaiian volca- gether form the Waawaa Trachyte Member of the 90 51 noes are usually confined to late in the alkalic Hualalai Volcanics (Figure 1). The horseshoe- 91 52 postshield stage that caps tholeiitic, shield-building shaped cone is breached to southeast, and although 92 53 basaltic lavas. These differentiated lavas are most no exposures exist to show the relationship of the 93 54 commonly hawaiites and mugearites, rarely ben- cone to the flow, the cone is interpreted to have 94 55 moreites, but more evolved lavas such as trachyte been built early in the eruption followed by the 95 56 are extremely rare [e.g., MacDonald and Katsura, extrusion of the trachyte flow [Stearns and Mac- 96 57 1964; Spengler and Garcia, 1988; Wolfe et al., donald, 1946]. This eruptive event first tapped the 97 58 1997]. Hualalai Volcano on the island of Hawaii is gas-rich, aphyric upper layer of the magma reser- 98 59 distinctive in that trachytes are relatively common voir, producing the pumice cone, and then tapped 99 60 and were erupted at the beginning rather than near volatile-poor, more crystal-rich magma at depth to 100 61 the end of the postshield phase of volcanism on produce the trachyte flow. Situated within the 101 62 that volcano [Moore et al., 1987]. Given the northwest rift zone of Hualalai, these are the oldest 102 63 scarcity of these highly evolved lavas at Hawaii dated lavas on the surface of the volcano (ca. 103 64 and the abruptness of their appearance at the onset 105 ka) and are surrounded by younger alkalic 104 65 of alkalic postshield volcanism at Hualalai, the basalt flows [Clague, 1987; Stearns and Macdon- 105 66 conditions under which they formed are factors ald, 1946]. Puu Waawaa is composed of crudely 106 67 critical in understanding how magma supply rates bedded layers of fine pumice, banded pumice, and 107 68 and the subvolcanic magma conduit/reservoir sys- obsidian blocks. The cone is covered by the late 108 69 tem vary with time. Pleistocene Pahala Ash and a 30-cm thick soil 109 layer [Clague and Bohrson, 1991]. Puu Anahulu, 110 70 2. Geology of Hualalai Volcano the massive trachyte flow emanating from Puu 111 Waawaa, is composed of at least two flows of 73 112 71 [3] Most of the surface of Hualalai Volcano, the and 76 m thickness, separated by a 40 m-thick 113 72 third youngest on the island of Hawaii, consists of layer of pumice. In addition to the trachyte cone 114 73 alkalic basalts of the Hualalai Volcanics [Clague et and flow, trachyte (underlain by tholeiitic basalt) 115 74 al., 1980; Moore et al., 1987]. Tholeiitic basalts are has also been recovered in several water wells 116 75 found beneath the alkalic lavas in some drill holes, drilled on the northwest and west flanks of Hua- 117 76 as blocks in a tuff at Waha Pele, and have been lalai Volcano. Blocks and chips of trachyte are 118 77 dredged from the northwest rift zone of Hualalai, found in basaltic maar deposits at Waha Pele, as 119 78 suggesting that the alkalic basalts at the surface well as in cinder deposits at two different vents 120 79 form a thin veneer over tholeiitic shield basalts near Waha Pele [Clague and Bohrson, 1991]. 121 80 [Clague, 1982; Moore and Clague, 1992]. Tholei- Syenite xenoliths are found as loose blocks on 122 2of27 Geochemistry 3 cousens et al.: hualalai volcano trachytes Geophysics 10.1029/2003GC000560 Geosystems G Figure 1. Map of Hualalai volcano showing trachyte localities (filled stars) and water sampling location (open star). Surface trachyte flows shown in orange, 1800–1901 alkalic basalt flows shown in green. Huehue Ranch location shown as yellow box. Numbers in circles indicate highway numbers. Photo (upper right inset) of Puu Waawaa cone (center) and Puu Anahulu flow (left foreground), looking south, courtesy of Richard Moore. 123 the surface near Malekule, just west of Hainoa type on Hualalai that is rare on other Hawaiian 132 124 Crater, that are interpreted to be cumulates from volcanoes (e.g., Mauna Kuwale rhyodacite flow, 133 125 the magma reservoir within which the trachytes Oahu, trachyte domes, West Maui, Hawi flows 134 126 evolved [Moore et al., 1987]. Pronounced aero- and domes, Kohala [Macdonald et al., 1983]). 135 127 magnetic and gravity lows over the summit and 128 rift zones of Hualalai indicate that these areas are [5] Petrographically, all trachytes from Hualalai 136 129 underlain by low-density, non-magnetic rocks, are very similar [Clague and Bohrson, 1991]. 137 130 presumably trachyte, thinly mantled by alkalic Obsidian and pumice from Puu Waawaa are 138 131 basalt flows. Thus trachyte is an important rock glassy and aphyric. Holocrystalline trachyte con- 139 3of27 Geochemistry 3 cousens et al.: hualalai volcano trachytes Geophysics 10.1029/2003GC000560 Geosystems G A. B. Figure 2. Photomicrographs of: A. syenite fragment from summit of Hualalai and B. Waha Pele trachyte block. 140 sists largely of laths of anorthoclase feldspar with clase) with potassium feldspar exsolution streaks 151 141 minor amounts of magnetite, edenitic amphibole, (Figure 2b).