Journal of Volcanology and Geothermal Research xxx (2017) xxxxxx

VOLGEO-06071; No of Pages 22

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Journal of Volcanology and Geothermal Research

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Volcanic evolution of Molokai, Hawaii: Implications for the shield topostshield transition in Hawaiian volcanoes

John M. Sinton a,, Deborah E. Eason a, Robert A. Duncan ba Department of Geology and Geophysics, University of Hawaii at Mnoa, Honolulu, HI 96822, United Statesb College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331, United States

Corresponding author.E-mail address: sinton@hawaii.edu (J.M. Sinton).

http://dx.doi.org/10.1016/j.jvolgeores.2017.04.0110377-0273/ 2017 Elsevier B.V. All rights reserved.

Please cite this article as: Sinton, J.M., et al., Vovolcanoes, J. Volcanol. Geotherm. Res. (2017

a r t i c l e i n f o lavas are alkalic hawaiites andmugearites, in contrast to the dominantly

Article history:

Received 28 December 2016Received in revised form 11 March 2017Accepted 6 April 2017Available online xxxx

tholeiitic basalts of the shield stage, an observation further reinforced byMacdonald and Katsura (1964). Macdonald (1968) built on these earlyobservations and recognized a transitional period between the mainshield and later postshield stages, which he referred to as the caldera-filling stage, a term that reflected the observation that by the end ofthe shield stage calderas are no longer a feature of Hawaiian summits.Macdonald (1968) also noted that it was during this transitional, calde-

frequent.

1. Introduction

Hawaiian volcanoes are known to evolve through a series of growthstages (Stearns, 1940; Stearns and Macdonald, 1946; Macdonald, 1968;Macdonald et al., 1983; Clague and Sherrod, 2014), now generallyinterpreted to reflect the passage of the volcano over a region of mantleupwelling or hotspot. Geologic observations of the subaerial portions in-dicate thatmany can be divided into early and late stages on the basis ofthe physical nature of lava successions and their petrographic andchemical character. These stages have come to be known as shield andpostshield stages of volcanism.

Shield successions form by frequent eruptions of very fluid lava(Macdonald, 1968), and consist of a high proportion of basalticphoehoe flows and a very low proportion of pyroclastic deposits(Macdonald et al., 1983). In contrast, the overlying postshield succes-sions typically form a thin capping sequence of thicker-bedded and ligh-ter colored lava flows. Common soil horizons between postshield flowsindicate a marked decrease in eruption frequency, while tephra layersare increasingly common (Macdonald, 1968; Diller, 1982; Macdonaldet al., 1983; Sinton, 1987;Wolfe et al., 1997; Sinton, 2005). The observeddifferences between shield and postshield strata guided geologic map-ping of individual islands and are the basis for the development of astratigraphic lexicon for Hawaiian volcanoes (Macdonald et al., 1983;Langenheim and Clague, 1987; Fig. 1). Notably, the fundamental strati-graphic divisions within most Hawaiian volcanoes preceded the avail-ability of chemical data on them.

Powers (1935) noted petrographic differences among shield andpostshield samples, and Tilley (1950) showed that some postshield

lcanic evolution ofMolokai, H), http://dx.doi.org/10.1016/j.

ra-filling stage that, at least on some volcanoes, eruptions become less

The expanded chemical data constrained by field relations ofMacdonald (1968) demonstrated conclusively that the latter part ofthe highly effusive shield stage is characterized by lava successions inwhich alkalic basalts first appear, commonly interbeddedwith tholeiiticlavas, and some sequences include samples that are chemically grada-tional (compositionally transitional) in character (Macdonald, 1968;Diller, 1982; Feigenson et al., 1983; Sinton, 1987; Zbinden and Sinton,1988; Frey et al., 1990; Chen et al., 1991; Wolfe et al., 1997). In eachcase these sequences all are in the upper parts of stratigraphic unitsthat were designated as shield-stage sequences, e.g., Wailuku Basalt ofWest Maui (Stearns and Macdonald, 1942), Polol Basalt of Kohala(Stearns and Macdonald, 1946), Lower Member of the East MolokaiVolcanics (Stearns and Macdonald, 1947), Honoman Volcanics ofEast Maui (Stearns and Macdonald, 1942), Hmkua Basalt of MaunaKea (Stearns and Macdonald, 1946), Kamaileunu Member of theWaianae Volcanics (Sinton, 1987).

Thus, two different evolutionary events characterize the decliningeruptive phases of Hawaiian volcanoes - the period of compositionaltransition to alkalic volcanism (within the upper parts of sequencesmapped as shield successions), and the change in lava flow color andthickness and the nature of eruptions that are the signature of mappedpostshield stratigraphic units. The earlier transition to alkalicmagmatismhas not been mapped in any volcano, primarily because thin-beddedalkalic basalts and hawaiites are rarely distinguishable from tholeiitic ba-salts in the field, coupled to the problem that tholeiites and alkalic lavasare commonly interbedded. In contrast, eight Hawaiian volcanoes (Fig. 1,including West Molokai based on data presented in this paper), havemapped postshield sequences, wherein a fundamental change in erup-tive character has been identified, and whose areal distribution hasbeen determined. Some of these volcanoes record a significant composi-tional change across the boundary, such as frommixed tholeiites, alkalicbasalts and hawaiites in the upper part of the Wailuku Basalt of West

awaii: Implications for the shield to postshield transition inHawaiianjvolgeores.2017.04.011

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Fig. 1. Stratigraphic lexicon of Hawaiian volcanoes showing oldest available ages and age of demise in Ma, based mainly on compilations of Macdonald et al. (1983) and Sherrod et al.(2007b), with additional information from Clague and Calvert (2009), Garcia et al. (2012), Sinton et al. (2014), and Sherrod et al. (2015). Age of zero indicates eruptions in the 20th or21st centuries. There likely are scattered postshield lava flows at the top of the Waimea Canyon Basalt on Kauai (Clague and Dalrymple, 1988), that have not been separatedstratigraphically. This compilation follows Macdonald (1968) in designating a late shield stage characterized by declining eruption rates and first appearance of alkalic basalts.

2 J.M. Sinton et al. / Journal of Volcanology and Geothermal Research xxx (2017) xxxxxx

Maui to the benmoreites and trachytes of the Honolua Volcanics (e.g.,Sherrod et al., 2007a). In other volcanoes the compositional change, ifany, is more subtle with alkalic basalts and hawaiites occurring on bothsides of the mapped stratigraphic contacts at East Maui and Waianaevolcanoes.

The presence of two different manifestations of declining magmasupply inHawaiian volcanic evolution gives rise potentially to two alter-natives for assigning stage boundaries. The initial concept was an out-growth of mapping of individual volcanoes (Stearns, 1940, 1946) andmany subsequent contributions retained the relationship betweenmapped stratigraphic units and the volcanic stages represented bythem (e.g., Macdonald, 1968; Macdonald et al., 1983; Sinton, 1987;Zbinden and Sinton, 1988; Presley et al., 1997). An alternative is the as-signment of volcanic stages on the basis of rock type or chemical criteria(e.g. Clague and Dalrymple, 1987; Frey et al., 1990; Wolfe et al., 1997),although whether the alkalic postshield should begin with eruption ofthe first alkalic lava or the last erupted tholeiite is unclear (Clague andSherrod, 2014). Neither of these lithologic milestones corresponds typ-ically to the first-order stratigraphical relations revealed by geologicmapping. The tholeiitic-to-alkalic chemical criterion for volcanic stagealso leads to ambiguities, such as the presence of tholeiitic basalts inthe traditionally designated postshields of Huallai (Huallai Volcanics)and the Kolekole Basalt of Waianae volcano.

1.1. The shield to postshield transition in Hawaiian volcanoes

The appearance of magmas with alkalic affinity late in the shieldstage can confidently be attributed to a decline inmean extent of partialmelting of theunderlyingmantle as the volcanomigrates away from thecenter of upwelling, as predicted from experiments on mantle analogsystems (e.g., Yoder and Tilley, 1962; Green and Ringwood, 1967), andsupported by geochemical modeling (Feigenson et al., 1983; Presley etal., 1997). Furthermore, the change in chemistry tends to coincide

Please cite this article as: Sinton, J.M., et al., Volcanic evolution ofMolokai, Hvolcanoes, J. Volcanol. Geotherm. Res. (2017), http://dx.doi.org/10.1016/j.

with a decline in eruption frequency, confirming a general correspon-dence between decreasing mean extent of mantle melting and de-creased magma supply to the volcano.

Many petrological studies have concluded that postshield magmasevolved at pressures higher than the 12 kbar typical of shield-stagemagmas, and the postshield stage has become associated with the dis-appearance of shallow chambers and eruption from magma reservoirsresiding near the base of the crust or within the upper mantle (e.g.,Clague, 1987a; Frey et al., 1990; Sinton, 2005; Shamberger andHammer, 2006; Clague and Sherrod, 2014). These moderately differen-tiated alkalicmagmas tend to be degassed and aphyric, producing short,thick, a lava flows from vents composed primarily of cinder, makingt