XII Congreso Geológico Chileno Santiago, 22-26 Noviembre, 2009 S7_032
Petrologic Constraints on Magma Ascent Rates in the Arc and Back-Arc Near the Magallanes Fault Zone, Austral Chile
Stern, C.R.1, Skewes, M.A.1
(1) Department of Geological Sciences, University of Colorado, Boulder, Colorado 80309-0399, USA:
General
The Magallanes Fault Zone (MFZ) forms the plate boundary between the South American and Scotia plates (Fig. 1). This dominantly strike-slip continental fault generates a complex system of structural responses, including rotation and rifting over a broad area [1-3]. Volcanoes located within the extended area of this fault system include Mt. Burney stratovolcano and Cook Island dome complex, the two southernmost volcanoes in the Austral Volcanic Zone of the Andean arc [4], and the Pali-Aike field of back-arc basalts, the southernmost field of the Patagonian plateau lavas [5,6]. Here we review petrologic constraints on relative differences in magma ascent rates between these two different tectonic environments, both in the area of the MFZ, compared to volcanoes in similar environments further to the north, outside the area of influence of the MFZ.
Pali-Aike back-arc basalts
The alkali olivine basalt of the Pali-Aike volcanic field formed between 3.8 Ma to <10,000 Ka [6] in the northern part of the area of structural complexity generated by the MFZ [2,3]. These basalts erupted from cones and fissures aligned in northwest-southeast, east-west, and southeast-northwest directions, subparallel to the alignment of both the MGZ and normal faults associated with rotation and rifting generated by this fault system. Their chemical compositions are similar to some oceanic island basalts (OIB) consistent with derivation by low degrees of melting of mantle asthenosphere, either due to subduction-induced thermal and mechanical perturbation [5,7], or uprise through a slab-window formed due to subduction of the Chile Rise [6]. A number of lines of evidence indicate that the Pali-Aike basalts ascended from their mantle source, at >80 km, to the surface at rates of 1 to >6 m/sec [8,9], similar to the velocities estimated for the emplacement of kimberlite magmas and more rapidly than
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XII Congreso Geológico Chileno Santiago, 22-26 Noviembre, 2009 olivine basalts in other fields of the Patagonian plateau lavas located north of the area of the MFZ. This evidence includes, first, the presence in these basalts of garnet-peridotite xenolith derived from up to 60-80 km depth [5,10,11]. Garnet-peridotite xenoliths are not found elsewhere in Patagonia plateau lavas. Some of these xenoliths are as large as >40 cm in diameter, which implies rapid turbulent rise of the Pali-Aike magmas, because the density of the peridotite xenoliths (~3.3 gm/cm) is greater than that of basaltic magma (~3.0 gm/cm), and they could not be carried by these magmas to the surface without turbulent motion. Second, petrologic features of the xenoliths, including the lack of evidence for decompression melting along mineral grain boundaries, and the persistence of amphibole, suggest that these peridotites, which equilibrated in the mantle at temperatures between 720 to 1180°C, did not have time to be significantly heated as they were transported to the surface by the basaltic magmas that erupted at temperatures estimated between 1200 to 1290°C [6]. Finally, determination of dehydration profiles in olivine crystals from these xenoliths, when compared with experimentally determined diffusion coefficients for dehydration of olivine, imply ascent rates from 70 km of 6 ± 3 m/sec [9], bringing the xenoliths to the surface in between only 2 to <7 hours.
Mt. Burney and Cook Island adakitic andesite arc volcanoes
The Mt. Burney stratovolcano and the Cook Island dome complex, both active in the Holocene, are the southernmost volcanic centers in the Andean arc (Fig. 1). The Cook Island dome complex occurs on the Scotia plate, along the Beagle Canal south of the Magallanes Fault, in an area below which the slow subduction of the Antarctic plate is highly oblique. Mt. Burney occurs north of the MFZ, above arc-parallel strike-slip faults associated with thias system, in a region of less oblique subduction. Both volcanoes have erupted adakitic andesites formed by melting of subducted oceanic lithosphere, followed by relatively limited interaction with the overlying mantle wedge and continental crust compared to the other four AVZ volcanoes located north, out of the area of the MFZ [4]. The four more northern volcanoes in the AVZ contain strongly corroded, partially oxidized and recrystallized amphibole phenocrysts, suggesting extensive water loss in the upper crust, and groundmass glass with variable composition possibly produced by magma mixing. Both Mt. Burney and Cook Island adakites contain high Mg# and high Cr/(Cr +Al) clinopyroxenes (Fig. 2), typically in the cores of lower Mg# magmatic clinopyroxenes [4]. These high Mg# clinopyroxene cores are interpreted as mantle- derived xenocrysts. Mt. Burney adakites also contain rare high Mg# olivine xenocrysts. The geochemical evidence for the relatively limited interaction of the parental slab- derived adakite melts erupted from Cook Island and Mt. Burney volcanoes, as well the presence of possible mantle-derived clinopyroxene xenocyrsts in their volcanic products, suggest that Cook Island and Mt Burney adakites rose more rapidly to the surface from their subcrustal source regions than andesites and dacites erupted from the other four AVZ volcanoes located to the north of the MFZ.
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XII Congreso Geológico Chileno Santiago, 22-26 Noviembre, 2009 Discussion and Conclusions
Pali-Aike alkali olivine basaltic magma apparently ascended to the surface both from deeper and also more rapidly than other basalts of the Patagonian plateau lavas, as indicated by the larger size of their garnet-peridotite xenoliths, derived from greater depths in the lithosphere than xenoliths that occur in other Patagonian plateau lava fields. Cook Island and Mt. Burney adakitic andesites also apparently rose to the surface more rapidly than adakitic magmas erupted from more northern volcanoes in the AVZ, based on both the geochemical evidence for their more limited interaction with the mantle and crust, and the presence of mantle-derived clinopyroxene xenocrysts in the adakites erupted by these two volcanoes. Thus, it appears that the complex continental fault system generated by the MFZ has provided structural pathways for relatively rapid ascent of magmas in both the arc and back-arc compared to these same environments further to the north outside the influence of this fault system. The data also indicate that although the Mt. Burney and Cook Island adakitic andesites did not ascend as rapidly as the back- arc basalts, these arc magmas did not undergo MASH processes of mixing, assimilation, storage and homogenization as they rose from their source region to the surface.
Figure 1. Map of the six volcanoes in the Andean Austral Volcanic Zone, the southernmost units of the Patagonian plateau lavas including the Pali-Aike field, and the major plate boundaries in southernmost South America.
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XII Congreso Geológico Chileno Santiago, 22-26 Noviembre, 2009 References
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