Evolution of the Taupo Volcanic Center New Zealan

Authors: Sarah E Gelman Chad D Deering Francisco J Gutierrez Olivier Bachmann

Publish Date: 2013/08/04

Volume: 166, Issue: 5, Pages: 1355-1374

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Abstract

The 20 ka ~01 km3 Omega dacite which erupted shortly after the 265 ka Oruanui supereruption compositionally stands out among Taupo Volcanic Zone TVZ magmas which are overwhelmingly characterized by rhyolites 90  by volume The previously reported presence of inherited zircons in this zirconundersaturated magma has provided unequivocal evidence for the involvement of uppercrustal material in a 1–10 year timescale prior to the Omega eruption However whether this crustal involvement is characterized by wholesale melting of preexisting crust or subordinate bulk assimilation into an already differentiated magma body remains unclear To disentangle these processes we describe the mineral chemistry of the major phases present in the Omega dacite and determine intensive parameters describing magma chamber conditions Dominantly unimodal populations of plagioclase An50–60 orthopyroxene Mg from 58 to 68 and clinopyroxene Mg from 65 to 73 along with coexisting equilibrium pairs of Fe–Ti oxides constrain preeruptive temperatures to 850–950 °C a pressure between ~3 and 7 kbars and an oxygen fugacity of ~NNO MELTS thermodynamic modeling suggests that this phase assemblage is in equilibrium with the bulk rock and glass compositions of the Omega dacite at these estimated P–T–fO2 preeruptive conditions Combining these petrological observations with insights into conductive thermal models of magma–crust interactions we argue that the Omega dacite more likely formed in the midtolower crust via protracted processing through fractional crystallization coupled with some assimilation AFC Incorporation of crustal material is likely to have occurred at various stages with the inherited zircons and potentially parts of glomerocrysts representing late and subordinate uppercrustal assimilants This petrogenetic model is consistent with the presence of a differentiating crustal column consisting of a polybaric fractional crystallization and assimilation history On the basis of petrological thermal and geophysical considerations uppercrustal reservoirs which feed largescale rhyolitic volcanism in the TVZ most likely take the form of large longlived crystal mush zones Following large eruptions such as the Oruanui event this mush is expected to crystallize significantly up to 70–80 vol crystals due to syneruptive decompression Hence the Omega dacite immediately postdating the Oruanui event potentially represents incoming deeper recharge of lessevolved magma that was able to penetrate the nearly solidified uppercrustal mush Over the past 20000 years similar intermediate recharge magmas have incrementally reheated reconstructed and reactivated the uppercrustal mush zone allowing a gradual return to rhyolitic volcanism at the Taupo Volcanic CenterSilicic magmas are thought to form through the combination of two endmember processes crystal fractionation from mantlederived mafic progenitors and partial melting of preexisting crust The degree to which each of these processes has dominated throughout the Earth’s history has major implications for the physical processes involved in and the rate of continental crust formation On a more local scale understanding the sources of silicic magmas in a given magmatic province can provide insight into the thermal petrological and tectonic structure of a region Despite more than a century of research attempting to disentangle the relative contributions of mantle versus preexisting crust in the petrogenesis of evolved magmas eg Bunsen 1851 Daly 1914 Bowen 1928 the complex geochemical signatures and frequent mixing of those two sources still fuel vigorous debate For example through a wide variety of experimental observational and theoretical techniques some studies suggest a limited addition of preexisting crust to a more mafic crystallizing progenitor that eventually evolves to produce silicic magma eg Halliday et al 1991 Thompson and Connolly 1995 Geist et al 1998 Sisson et al 2005 Simon et al 2007 Jagoutz et al 2009 while others suggest a dominant contribution from recycled crust eg Conrad et al 1988 Huppert and Sparks 1988 Petford et al 2000 Riley et al 2001 Price et al 2005 A striking example of this controversy comes from oceanic arc lavas where in the same year two different studies reported opposite interpretations for similar rocks in the same tectonic region Haase et al 2006 Smith et al 2006With the advent of highprecision mass spectrometry and the broad use of isotopic tracers the geochemical tools available to improve our understanding of magmatic processes occurring within the crust have grown considerably in the past few decades Notably it has been shown that opensystem behavior that is contributions of mass from both mafic progenitor magmas as well as from country rocks is the rule rather than the exception in magmatic systems regardless of tectonic setting eg Francis et al 1980 Taylor 1980 Halliday et al 1991 Dungan and Davidson 2004 Charlier et al 2007 Davidson et al 2007 McCurry and Rodgers 2009 Likewise an everexpanding library of experimentally and theoretically derived phase equilibria eg MELTS Ghiorso and Sack 1995 and the library of studies upon which it is built have broadened our available petrological tools while the advancement of computational fluid dynamics and numerical heat transfer has most recently elucidated the possible physics at work in magmatic systems and their plausible relationships to the data we can collect at the Earth’s surface eg Bergantz 1989 Petford and Gallagher 2001 Babeyko et al 2002 Spera and Bohrson 2004 Annen et al 2006 Dufek and Bachmann 2010 Gutierrez and Parada 2010 Huber et al 2010 The goal of this study is to integrate the use of a few of these tools into a more unified approach toward examining the source of silicic magmatism within a spatially and temporally wellconstrained systemThe Taupo Volcanic Zone TVZ in the northern island of New Zealand is ideally suited for this integrated study due to its relatively wellconstrained crustal structure and welldocumented eruptive history Wilson et al 1995 Smith et al 2005 In particular the Taupo Volcanic Center location of the 265 ka 530 km3 Oruanui eruption Wilson et al 2006 affords us a unique opportunity to study the mechanisms involved in formation of silicic magma following the Oruanui caldera collapse as the uppercrustal silicic magmatism was reestablished Here we focus on one of the first postOruanui eruptions the 20 ka Omega dacite which constitutes only 01 km3 of magma Although small in volume the appearance of this composition of magma is important since dacite has been proposed as the intermediate progenitor to the voluminous erupted rhyolites Deering et al 2008 Deering et al 2011a although dacites themselves rarely reach the surface in the TVZ ≪1  by volume of eruptive productsWe present petrological observations and mineralogical data constraining the P–T–fO2 and water content under which the Omega dacite likely formed Using these constraints we also introduce thermal models to explore the influence that crustal melting has on the final composition volume and melt fraction of the Omega magma reservoir Finally we integrate our petrological observations with the thermal constraints to unravel the source contributions in the formation of the Omega dacite and discuss the potential insight that has been gained for understanding the formation of largerscale silicic magmatism in the TVZ in generalLocation map of the Taupo Volcanic Zone New Zealand A star marks the location of the primary outcrop of the Omega dacite located near the northern shore of Lake Taupo The young TVZ boundary is drawn after Wilson et al 1995 The location of the Mesozoic metasedimentary terranes is drawn in green after New Zealand Geological Survey 1972

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