Large silicic magma systems

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Journal of Volcanology and Geothermal Research 167 (2007) vii – ix www.elsevier.com/locate/jvolgeores

Preface

Large silicic magma systems Like many fundamental topics in the Earth Sciences, large-scale silicic magmatism has a regular periodicity in the collective consciousness of a broad swath of the geological community that manifests in elevated levels of interest and activity. Collections of modern seminal works can be found in Ash Flow Tuffs (1979, GSA Special Paper 180), Granites and Rhyolites (1981, Journal of Geophysical Research Volume 86, no. B11) and Calderas and Associated Igneous Rocks (1984, Journal of Geophysical Research Volume 89, no. B10) and the regular pulse of the community can be found in the various Hutton symposia going back to 1987. The last few years have been a particularly active time with thematic meetings like the Penrose conference on the Longevity and Dynamics of Rhyolitic Magma Systems (Mammoth, CA, 2001), various State of the Arc (SOTA) meetings (2000, 2003, and 2007), Sierra Nevada/Tuolumne field conference (2005), and numerous sessions at GSA and AGU. It is at two of these meetings in 2005, that this volume was conceived. Olivier Bachmann and Calvin Miller convened a session “The Growth and Evolution of Large Silicic Magma Bodies” at 2005 Joint Assembly (AGU) held in June in pre-Katrina New Orleans, while in July 2005, Shan de Silva, Takeyoshi Yoshida and Kurt Knesel convened the session “Large Scale Silicic Magmatic Systems” at the Asia-Oceanic Geosciences Society (AOGS) meeting in Singapore. These sessions brought together representatives of the volcanic and plutonic communities from across the globe and provided the rare opportunity to coordinate a collection of work that addresses fundamental questions of how large silicic magmatic systems operate. The binding theme to these contributions is the volcano-plutonic relationship and the papers selected for publication herein cover a broad range of approaches that inform this theme. We are particularly pleased that the authorship represents an extensive disciplinary and cultural perspective, and the choice of JVGR as the forum for this volume acknowledges the quality of the 0377-0273/$ - see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.jvolgeores.2007.10.001

publication and its breadth of readership, as well as the desire to continue encouraging the volcanological community to integrate data from both realms. To set the thematic scene, the volume is led off by a review and discussion of the state of the volcano-plutonic relationship by Bachman, Miller, and de Silva. While acknowledging the recalcitrance and longevity of some of the key questions, they try to provide a framework within which the relationship can be understood. Recognizing that the fundamental drive for silicic magmatism is the mantle power input, the connection between the volcanic and plutonic realms is most evident in the upper crust where melt expulsion from silicic crystal mushes results in eruptible volumes and complementary residues that manifest as volcanic and plutonic rocks respectively. Exploring alternatives to previously published mechanisms for rapid extraction of melt in such crystal-rich systems, Davis, Koenders, and Petford present the case for vibroagitation and fluidization at the base of the crystal mush in response to the passage of seismic waves. The conceptual framework is completed with a geophysical perspective on magmatic systems by Lee. Seismic evidence of low velocity layers or zones beneath volcanic systems is often cited as some of the strongest evidence of an active link between the volcanic and plutonic realms (Bachmann et al. and de Silva and Gosnold for instance). However, Lee explains that the relationship between velocity perturbations and active sub-volcanic magma bodies or intrusions is quite opaque. While partially molten zones may indeed be imaged, whether these represent coherent or disseminated bodies is rarely resolvable. More tangible evidence for the link between fossil volcanic and plutonic realms is presented by Sonehara and Haruyama who document the temporal, spatial, and geochemical kinship of the massive Cretaceous Nohi rhyolite province and related granitoids. Three papers in this volume demonstrate that silicic magmas may originate from three major sources: the

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Preface

upper crust, lower crust, and the subducting slab. Silicic systems that originated from each of these are represented here. Kimura and Nagahashi demonstrate that the Takadani Granodiorite and its volcanic equivalent, the Chayano–Ebisutoge pyroclastic deposits, are the product of upper crustal melting. Felsic magmas at the Takamatsu and other stratovolcanoes associated with large caldera systems produced by melting of mafic precursors are documented by Ban and seven co-workers. This is in contrast to the Numazawa volcano, where Yamamoto describes a temporal variation in geochemistry that records an increase in partial melting of the lower crust within the felsic to intermediate sequence of lavas. Completing the trifecta, Tsuchiya, Kimura, and Kagami show that the zoned adakitic to non-adakitic plutons of the Kitakami Mountains in Japan have an origin by slab melting with or without subsequent interaction with mantle wedge or lower crust. These works demonstrate that the primary geochemical variations in these systems can be traced back to the source to pre-eruption/ solidification reservoir flux. The issue of origin of chemical heterogeneity in silicic magmatic systems, which is central to our understanding of the growth and evolution of such systems, is further developed by several papers that are rooted in detailed petrochemical case studies. Collectively these six works address the issue of whether heterogeneity is the result of protracted closed-system differentiation or the result of rapid open-system processes. They all conclude that open-system processes dominate and document the importance of recharge, recycling, and cannibalization. Kaneko and four coworkers reveal rapid cycles of recharge, mixing between resident and recharge magma, and eruption at the classic Aso system — a history also documented by Knesel and Duffield in the Taylor Creek Rhyolite system and by Ban for the aforementioned Takamatsu and similar systems. Tefend and three co-workers reveal the details of silicic systems in the Southwestern Nevada volcanic field that involve distinct magmas that mix and fractionate (see also Ban), while Maeno and Taniguchi reveal that processes in submarine silicic magma systems are very similar. Of particular note are the differing approaches used in these studies; Kaneko et al., Ban et al., Tefend et al., and Maeno and Taniguchi, use traditional bulk geochemical analysis and interpretation but Tefend extend this and demonstrate the utility and resolution of polytopic vector (a.k.a., multivariate or cluster) analysis of bulk geochemical data. Knesel and Duffield base their work on high-precision microanalysis of trace elements in sanidine crystals. Building on the utility of microanalytical techniques, two studies of

plutonic systems focus on U–Pb chronology of zircon crystals to converge with the conclusions from the volcanic systems. Walker and four co-workers document the construction and evolution of the Spirit Mountain Batholith, while Miller and three co-authors focus on the Tuolumne and Mt Stuart batholiths. Both these works document recycling of extant zircons into new magma batches as these batholiths were constructed during several distinct intrusive episodes. Time scales of construction and operation of large silicic magmatic systems are revealed in the temporal record of several of the systems represented but also by the nature of the inquiry. Detailed petrological studies of volcanic systems by Kaneko, Knesel and Duffield, Ban, Maeno and Taniguchi, Yamamoto, reveal rapid time scales on the order of 103 to 104 years for relatively small systems, while much larger systems (both volcanic and plutonic) reported on by de Silva and Gosnold, Sonehara and Haruyama, and Walker et al., are built over timescales of 105 to 106 years. The episodic nature of silicic systems at all scales is more readily revealed in the volcanic records compared with the plutonic systems. This can be traced largely to the nature of the record as discussed by Bachmann et al. – the instantaneous snap shots provided by the volcanic systems versus the terminal annealed records of plutonic systems – as well as to the limits of geochronological resolution of individual plutonic events. Detailed field-based studies continue to provide valuable insights into the operation of large silicic systems. The aforementioned work of Maeno and Taniguchi reveals the details of the physical volcanology of the VEI 7, Holocene eruption Kikai caldera that evacuated the long-lived open magma system. Meanwhile Kawakami, Hoshi and Yamaguchi document how deep erosion reveals details of the volcano-plutonic system of the Kumano Acidic Rocks. Here a major nested caldera system shows variable styles of collapse and resurgence due to laccolith-style intrusion, building on the evidence presented by Walker et al., for the dominance of sheet-like intrusions in the construction of the Spirit Mountain Batholith. The regional tectonic context of the Kumano Acid Rocks and associated volcanism of the Kii Peninsula is examined by Miura and Wada, who emphasize the importance of regional tectonic stresses associated with rotation of Japan in evolution of the magmatic system. They suggest that the massive eruptions coincided with extensional or neutral compressional stresses and evacuated magmas that accumulated during the earlier compressional stress dominated stage. Finally, the persistent themes of periodic operation and piecemeal construction evident throughout this

Preface

volume are integrated at the scale of a magmatic province by de Silva and Gosnold. They reveal that the Altiplano Puna Volcanic Complex, and its plutonic system, was built over ∼10 million years in pulses of distinct magmas that became more intense with time. Rates of magmatism are an order of magnitude higher than typical steady state rates in arcs, suggesting that large silicic magmatic systems are built during “flare-ups” — a theme also developed by Sonehara and Harayama in their study of the equally massive Nohi Rhyolite magmatic system. The potential importance of the feedback between thermal impact of the magmatic systems and the mechanical behaviour of the crust and its role in the development of silicic magmatic systems is discussed by both de Silva and Gosnold, and Miura and Wada. We thank all the attendees of the sessions we convened. We are grateful to those who became contributing authors for their patience and interest in seeing this volume through to the end. No volume like this can be realized without the critical services of the community and we thank the following reviewers for their time and serious contribution to the quality of the volume: Valerio Acocella, George Bergantz, Ilya Bindeman, Cathy Busby, Jim Cole, Drew Coleman, Nelia Dunbar, Derek Elsworth, Lang Farmer, Jim Gill, Wes Hildreth, Mark Jellinek, Glenn Johnson, Katsuya Kaneko, Ben Kennedy, Yan Lavallée, Adam Kent, Hervé Martin, Rodney Metcalf, Mitsuhiro Nakagawa, Michihiko Nakamura, Felix Oberli, Ingrid Ukstins Peate,

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Robert Rapp, Jeremy Richards, Bruno Scaillet, Urs Schaltegger Axel Schmitt, Tom Sisson, Ian Smith, Frank Spera. John Stix, Robert Trumbull, Yutaka Wada, David Wark, Bob Wiebe, Takahiro Yamamoto, and George Zandt and a few others who requested anonymity. We are indebted to Lionel Wilson for his support to take this on and continued support and editorial oversight throughout the process. Finally, this volume could not have been brought to completion without the efforts of Herman Engelen at Elsevier who helped us patiently as we wrestled with the newly installed on-line editorial system.

Shanaka de Silva Oregon State University, USA Olivier Bachmann University of Geneva, Switzerland Corresponding author. Now at University of Washington, USA. Calvin Miller Vanderbilt University, USA Takeyoshi Yoshida Tohuku University, Japan Kurt Knesel University of Queensland, Australia

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