Orogenic systems record locations where subduction or continent-continenta collision occurs, and reflect the primary means by which continents grow in mass and are assembled. Our research in orogenic systems includes work on high-pressure and ultra-high-pressure (HP/UHP) eclogite terranes, as well as modern volcanic arcs.
Metamorphic history of the Western Alps
The Alps remain a classic laboratory for studying the tectonics of continent-continent collision. The Zermatt-Saas ophiolite complex of the Western Alps is a remnant of the Tethys Ocean that was subjected to high (HP) and ultra-high pressure (UHP) metamorphism, followed by rapid exhumation during the Alpine Orogeny. Despite over a century of structural studies and decades of geochronological research in this region, the age and duration of HP to UHP metamorphism, the maximum pressures and temperatures the Zermatt-Saas ophiolite and structurally underlying basement nappes experienced, and the rate at which these units were buried and exhumed remain unresolved problems. These questions continue to be hotly debated, and are critical to understanding of the geodynamic processes in orogenic belts. We have focused on the Alps because this orogenic belt is young enough so that temporal relations may be worked out in detail, the exposures are superb, the maximum temperatures were relatively cool, below the blocking temperatures of several geochronometers, and the prior geologic work provides a excellent framework upon which to build new studies. In addition to work on the Zermatt-Saas complex, we have extended our work to the Monte Rosa nappe (a fragment of European basement) and the Sesia nappe (a fragment of African crust).
The strong core-to-rim zonation in Lu/Hf ratios of garnet indicate that Lu-Hf ages should be skewed to the early periods of garnet growth as compared to the Sm-Nd ages, and indeed this is the case. These results suggest that prograde garnet growth occurred over perhaps 40 m.y. or more, which is on the order of plate velocities that have been estimated for subduction of Tethys crust during the Alpine Orogeny. The very slow rates of subduction are consistent with the highly oblique nature of subduction of Tethys crust from the Late Cretaceous to the middle Tertiary. For comparison, U-Pb zircon geochronology, a very common geochronological system, places no constraint on the age of metamorphism at specific P-T conditions. Sm-Nd and Rb-Sr date the peak of metamorphism, and all HP/UHP units, regardless of different prograde paths, were uplifted as a single unit ~38-40 m.y. ago. Such multi-geochronometer approaches, tied to metamorphic petrology, are essential in complex orogenic belts, where no single age can reflect the geologic history of such terranes.
Our work in the Alps has involved UW-Madison graduate students Tom Lapen, Nancy Mahlen, and Eva Szilvagyi, as well as collaborators at the University of Lausanne.
The Cascade Arc – Crater Lake and Mt. Shasta
The lab has a long history of working in the Cascades, including previous work on Mt. Lassen and Mt. Adams. New work has focussed on Crater Lake and Mt. Shasta. Crater Lake is located in southern Oregon in the central part of the Cascade arc. The caldera was formed by the climactic eruption of Mt. Mazama at 7.7 ka, which emplaced ~50 km3 of primarily rhyodacitic magma on slopes and drainages surrounding the collapsed edifice. Overlapping in age is Mt. Shasta, the largest volcano in the Cascades, which lies south of Crater Lake, in northern California. Four cone-building stages occurred at Shasta, one of which, Shastina, is similar in age, composition, and volume as the 7.7 ka eruption of Mt. Mazama, although the Shastina eruption did not produce a pyroclastic caldera.
At Crater Lake and Shasta, U-Th isotopes indicate a much greater role for the lower crust than has been previously recognized in magmas at this and other Cascade volcanoes. Both volcanoes have strong 230Th excesses, which our models suggest is due to assimilation of mafic lower crust in the presence of garnet produced through the breakdown of amphibole and plagioclase during dehydration melting, given the strong affinity of garnet for U over Th. This process is likely common in arc settings, but the Th-excess signature is obscured in most arcs in the world due to a 238U-excess subduction fluid overprint. At Shasta, changes in (230Th/232Th)0 ratios correlate with eruptive volume, where the most voluminous stage (Misery Hill) is inferred to have the largest proportion of crustal melt and highest (230Th/232Th)0 ratios. The volume-(230Th/232Th)0 relations at Shasta are accompanied by correlations with 87Sr/86Sr ratios, where the most radiogenic Sr is associated with the largest eruptive volumes, indicating that the largest magmatic episodes produced the largest amount of lower crustal interaction.
Building on earlier work in the Cascades by UW-Madison graduate student Garret Hart and post-doc Brian Jicha, current work on Crater Lake and Shasta has been done by UW-Madison graduate students Meagan Ankney and Allison Wende.
Dynamics of a caldera-scale rhyolitic magma reservoir: Laguna del Maule
The Laguna del Maule (LdM) volcanic field, located in central Chile, is a superlative example of late Pleistocene to Holocene rhyolitic volcanism. At least two catastrophic caldera forming eruptions have occurred at LdM in the last 1.5 million years and the spatial distribution and major and trace element compositions of the most recent rhyolitic eruptions suggests they could be the product of a large, shallow magma reservoir. An interdisciplinary research team spearheaded by UW-Madison professor Brad Singer is in the midst of a multi-year petrologic and geophysical investigation of the volcanic field initiated in response to unprecedented volcanic unrest, including ongoing uplift at rates in excess of 20 cm/yr since 2007.
Measurements of Sr, Pb, and Th isotopic tracers and major and trace element contents of whole rocks and mineral phases will be interpreted within a geochronologic framework determined by 230Th-238U crystallization ages and 40Ar/39Ar eruption ages. This work is being done by graduate student Nathan Andersen. Thus, we will test if the recent silicic eruptions were issued from a common, integrated magma reservoir and more generally, characterize the thermo-chemical evolution of the LdM magma reservoir in time and space. This record of Pleistocene to Holocene magmatic processes will be combined with tomographic and time-series geophysical studies of the modern magma system in order to create new dynamic models of the assembly and eventual eruption of large silicic magma systems.
The plumbing systems of large volcanic centers: Shallow pluton emplacement and evolution
Working with UW-Madison professor Brad Singer in the Ar/Ar geochronology group in the Department of Geoscience, graduate student Allen Schaen is currently investigating the construction of the ~6 Ma compositionally zoned Risco Bayo and Huemul granitoid plutons in the Chilean Andes (~36°S). Their close proximity (18 km) and compositional similarity to the caldera-scale rhyolitic Laguna del Maule volcanic field (currently uplifting at 25 cm/yr due to magma intrusion; see project above) offers unique possibilities in understanding silicic magma reservoir formation from the plutonic perspective. A detailed petrochronologic campaign using both U-Pb LASS-ICP-MS and chemical abrasion-TIMS of zircon and titanite, along with 40Ar/39Ar thermochronology of biotite and hornblende will document the crystallization/cooling histories of individual magmatic domains, revealing rates and mechanisms of pluton assembly in the upper crust. Petrochronologic findings will be linked to phase equilibria predictions of MELTS modeling and detailed fabric analysis to constrain T-X-t and flow relationships of different magmatic domains. Observations from Laguna del Maule (extrusion/intrusion rates, spatio-temporal pattern of mafic/rhyolitic volcanism, etc.) will help to temper interpretations with an overarching goal of determining whether this plutonic system ever produced large volumes of eruptible rhyolite. Coeval silicic ignimbrites in the area provide additional exterior tests to geochemically and temporally fingerprint if they represent erupted products from the Riso Bayo-Huemul plutonic system. In addition, whole rock 87Sr/86Sr TIMS analyses of individual magmatic domains will help to constrain the role of assimilation and mixing within the upper crust as this epizonal plutonic system formed.