Plutonic Xenoliths Reveal the Timing of Magma Evolution at Hualalai and Mauna Kea, Hawaii
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Plutonic xenoliths reveal the timing of magma evolution at Hualalai and Mauna Kea, Hawaii J.A. Vazquez Department of Geological Sciences, California State University–Northridge, Los Angeles, California 91330-8266, USA P.J. Shamberger J.E. Hammer Department of Geology and Geophysics, University of Hawaii, 1680 East-West Road, Honolulu, Hawaii 96822, USA ABSTRACT Hawaiian volcanoes evolve through stages that have been delimited by the compositions of their erupted lavas. New in situ 238U-230Th and U-Pb dating of single zircons from leucocratic plutonic xenoliths erupted at Mauna Kea and Hualalai volcanoes, Hawaii, reveals that impor- tant episodes of magmatic evolution are not necessarily refl ected in the stratigraphy or com- positions of erupted lavas. Zircons from Mauna Kea diorites form a heterogeneous population with apparently bimodal ages of ca. 125 ka and ca. 65 ka, suggesting fractionation, intrusion, and crystal recycling about the time of transition of postshield volcanism from basaltic to hawaiitic compositions. Hualalai syenogabbros and diorites record extreme fractionation and generation of alkalic magma at 41 ± 9 ka and 261 ± 28 ka, indicating that alkalic magma was generated ~130,000 yr before the shield to postshield transition inferred from lava stra- tigraphy, and that coeval evolution of chemically distinct magma reservoirs at shallow and deep levels may have characterized the shield stage. These episodes at Hualalai did not cause eruption of evolved lavas, indicating that extreme differentiation in Hawaiian volcanoes is not necessarily followed by eruption of highly evolved magma. Keywords: uranium-series method, magma chambers, magmatic differentiation, cumulates, volcanism. INTRODUCTION Sinton, 1987). In the paradigm of Hawaiian vol- While Mauna Kea follows the archetypal pro- Highly evolved magmas, such as trachytes cano evolution, stages and the timing of their gression of Hawaiian volcanism, Hualalai departs and rhyolites, are typically volumetrically transitions are delimited by the composition of from the paradigm in several respects. Based on minor relative to basalts at oceanic volcanoes erupted lavas. However, we present new results the exposed tholeiite and alkalic lava stratigra- but are important because they mark decreases from 238U-230Th and U-Pb dating of zircons phy, the timing of Hualalai’s shield to postshield in the supply of magma from the mantle (e.g., from leucocratic plutonic xenoliths indicating transition has been inferred to be between ca. Clague, 1987) or melting of subvolcanic crust that lava stratigraphy is an incomplete monitor 130 ka and 100 ka (Moore and Clague, 1991; (e.g., Bohrson and Reid, 1998). At Hawaiian vol- of magmatic evolution within subsurface res- Cousens et al., 2003). Postshield alkalic basalts canoes, eruptions of highly evolved magmas are ervoirs. Our results indicate that diorites from have erupted at Hualalai since at least 25 ka, especially rare and signal the fi nal stages of edi- Mauna Kea record postshield evolution over with the most recent erupted at A.D. 1800–1801 fi ce construction when waning magma supply tens of thousands of years when the depth of (Moore et al., 1987; Moore and Clague, 1991). leads to cooling and extreme fractional crystalli- magma storage increased and highly evolved Hualalai is distinctive because trachytes erupted zation of magma reservoirs (Clague, 1987; Frey lavas began erupting. In contrast, diorites and at ca. 100 ka from its three rift zones during the et al., 1990; Fodor, 2001; Shamberger and Ham- syenogabbros from Hualalai record generation apparent beginning of the postshield stage rather mer, 2006). Much of the mass of Hawaiian vol- of evolved alkalic magma during the shield than near the end (Clague and Bohrson, 1991; canoes is generated during the tholeiitic shield stage, and growth of a deep composite pluton Cousens et al., 2003), as at other Hawaiian vol- stage, when a volcano is centered on the hot- over an ~210,000 yr interval. canoes. These trachytes have Pb isotope compo- spot and the degree of mantle melting, magma sitions that differ from the shield-stage tholeiites supply, and eruption rate are greatest (Clague, MAGMATISM AT MAUNA KEA AND and oceanic crust that presumably compose 1987). During the shield stage, tholeiitic magma HUALALAI VOLCANOES most of the subvolcanic basement, which rules is typically stored in a shallow (~3–7 km depth) Mauna Kea and Hualalai are both in their out an origin by anatexis (Cousens et al., 2003). reservoir beneath the summit and rifts (Clague, postshield stages and have erupted highly Instead, their compositions suggest derivation 1987; Klein et al., 1987). Transitional and alkalic evolved magmas. At Mauna Kea, a gradual by >90% fractional crystallization of alkalic or magmas, such as hawaiite and mugearite, begin transition from shield to postshield volcanism transitional basalt (Cousens et al., 2003; Sham- erupting as a volcano moves away from the started at ca. 370 ka, and the fi rst eruptions of berger and Hammer, 2006). The alkalic affi nity hotspot and enters a postshield stage character- alkalic basalt occurred at ca. 250 ka (Huang and and unusual timing of evolved magmagenesis at ized by decreased mantle melting and magma Frey, 2003). Postshield alkalic lavas and cinder Hualalai suggest atypical processes at depth. supply, freezing of shallow reservoirs, and frac- cones cap both rift and off-rift areas of Mauna tionation of magma in deep (~20 km depth) res- Kea (Wolfe et al., 1997), with some vents as INSIGHT FROM XENOLITHS ervoirs (Clague, 1987; Frey et al., 1990; Wolfe young as ca. 4.5 ka (Porter, 1979). The youngest A unique perspective of magmatic evolu- et al., 1997). Trachytes are typically erupted at postshield lavas are the most evolved and least tion and extreme differentiation at Mauna Kea the end of the postshield stage (Clague, 1987; abundant (Wolfe et al., 1997). and Hualalai is provided by leucocratic plutonic © 2007 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, August August 2007; 2007 v. 35; no. 8; p. 695–698; doi: 10.1130/G23495A.1; 2 fi gures; Data Repository item 2007179. 695 xenoliths hosted in postshield lavas and tephra RESULTS free trachyte to rhyolite glass (Table DR3), cones. At Mauna Kea, leucocratic xenoliths are Mauna Kea zircons (n = 10) yield an apparent as well as plagioclase, alkali feldspar, and clino- 238 230 +84 σ diorite and rare quartz tonalite (Fodor, 2001). U- Th isochron age of 76−48 ka (2 ), with a pyroxene. REE concentrations in Hualalai zircons The diorites are frozen hawaiite and mugearite mean square of weighted deviates (MSWD) of (Table DR4) are typical of zircon from evolved magmas formed by >60% fractional crystalliza- 3.7 that is outside the 95% confi dence interval alkalic magmas (Fig. DR2). tion of alkalic basalt, whereas the quartz tonalite expected for a single age population (cf. Mahon, fractionated from tholeiitic magma (Fodor, 2001). 1996), and show an apparent range of ages within DISCUSSION Leucocratic xenoliths from Hualalai are alkalic each xenolith (Fig. 1). Based on the mixture Crystallization of Zircon in Mauna Kea and and include syenogabbros, diorites, monzodio- model of Sambridge and Compston (1994), and Hualalai Magmas rites, and anorthosites, which represent liquid or assuming an initial Th-isotope activity ratio such The high REE concentrations and signifi - cumulate compositions (Shamberger and Ham- as that of young Hawaiian lavas (1.03 ± 0.06; cant Eu anomalies (Table DR4; Fig. DR2) sug- mer, 2006) previously identifi ed as “syenites” Sims et al., 1999), the Mauna Kea zircons can gest that zircon saturation in diorite magma at (e.g., Moore et al., 1987; Clague, 1987; Cousens represent a bimodal population with ages of Hualalai was achieved partly by fractionation of +88 +24 σ et al., 2003). Bulk compositions, phase equilibria, 122−48 ka and 64−20 ka (2 ) in proportions of plagioclase. The large variability (up to several and mineral chemistry indicate that Hualalai’s 35% and 65%, respectively (Fig. 1). orders of magnitude) in REE, U, and Th con- leucocratic xenoliths are frozen magmas that Hualalai zircons form two groups defi ned centrations of the zircons (Tables DR1–DR4), crystallized at pressures of ~300–700 MPa near by their 238U-230Th composition (Table DR1) as well as a range of euhedral to anhedral crys- the bottom of the oceanic crust and are the prod- and apparent isochron ages (Fig. 1). One group tal shapes, suggests an assortment of growth ucts of ≥65% fractional crystallization of a tran- (n = 25) yields a weighted isochron age of 41 ± conditions in melt-rich to melt-poor magma. sitional or alkalic basalt parent (Shamberger and 9 ka (2σ) with an MSWD of 1.5, and another The trachyte inclusions in Hualalai zircons are Hammer, 2006). Based on petrologic evidence group (n = 24) yields a weighted isochron age of frozen melts captured late in the crystalliza- +60 σ indicating that the ca. 100 ka trachytes fraction- 257−46 ka (2 ; MSWD = 0.69). U-Pb analyses of tion sequence of their parent diorite magma, as ated from magmas such as those represented by zircons (n = 24; Table DR2) from the older group evidenced by the anhedral shape of their host the leucocratic xenoliths, Shamberger and Ham- yield a common Pb and 230Th disequilibrium- zircons (Fig. DR1). Nevertheless, early growth mer (2006) concluded that either Hualalai’s active corrected (Schärer, 1984) 238U-206Pb age of 261 ± and accumulation produced cumulus zircons in magma reservoir deepened and fractionated over 28 ka (2σ; MSWD = 1.8), which is concordant some xenoliths (Clague and Bohrson, 1991). The only an ~20,000 yr interval at the shield to post- with their 238U-230Th isochron age (Fig. 1). Zir- leuco cratic xenoliths, ca. 100 ka trachytes, and shield transition, or a shallow shield reservoir cons from single Hualalai xenoliths fall in one melt inclusions fall within the general fraction- existed simultaneously above a deep, spasmodi- age group or the other, except for two zircons ation trend for suites of mafi c-to-felsic alkalic cally active and fractionating reservoir during a from two ca.