PGS: Crustal-Scale Mass and Heat Transfer During the Run-up to a Super Eruption

The May, 2014, meeting of the Potomac Geophysical Society will be held May 15th at the Fort Myer Officers’ Club in Arlington, VA (http://www.jbmhhmwr.com/index/Maps_and_Directions.html) in the Glassed-in room in the Fife and Drum (main dining room).

Crustal-Scale Mass and Heat Transfer During the Run-up to a Super Eruption, James E. Quick1, Silvano Sinigoi2, Gabriella Demarchi2, Ian Richards1, Rita Economos3 (1Huffington Department of Earth Sciences, Southern Methodist University, Box 750395, Dallas, TX 75205. 2Dipartimento di Scienze della Terra, Università di Trieste, via Weiss 8, 34127 Trieste, Italy. 3Department of Earth and Space Sciences, University of California, Los Angeles, CA 90095-1567

Abstract: A virtually complete crustal section in the Sesia Valley of northwest Italy provides a unique opportunity to directly constrain crustal-scale transfer of mass and heat beneath a continental volcanic system. Capping the section, a bimodal volcanic complex containing a >15-km-diameter rhyolitic caldera is intruded by a 7- to 8-km-thick granitic pluton that is rooted in migmatitic paragneiss at mid-crustal levels. An 8-km-thick gabbroic body, “magmatically underplated” at >15 km depth, intrudes the paragneiss. Consistent with a cause-and-effect relationship between magmatic underplating in the deep crust and silicic plutonism and volcanism at high crustal levels, SHRIMP U/Pb zircon ages for volcanic, granitic and underplated gabbroic rocks cluster within a relatively narrow time window of ~290 to ~280 Ma. A Concordia age of 282 + 0.75 Ma on zircons from the caldera ignimbrite indicates that caldera formation occurred late in the evolution of this magmatic system. Field relations and geochemistry constrain the thermal history of the Sesia section and the processes of magmatic underplating, crustal anatexis and assimilation, and hybridization during its magmatic evolution. Magmatic underplating was accommodated by crustal extension, which is recorded by structures produced by the flow of gabbroic cumulates downward and away from a small magma chamber perched near the top of the intrusion. Heat from the underplated gabbro induced anatexis in country-rock paragneiss, producing granitic melts that migrated higher in the crust. Eu and Ba enrichments, εNd < -2.5, 87Sr/86Sr >0.7075, and δ18O > 8 indicate that the parental melt of the underplated gabbro had ingested ~24% to 40% assimilant consisting of paragneiss previously stripped of a granitic component. Peraluminous compositions of the granitic and volcanic rocks indicate that anatectic melting of metapelitic paragneiss was a contributing source, but 87Sr/86Sr ~ 0.710 and whole-rock δ18O ranging from 10 to 11.5 are intermediate between the compositions of the paragneiss (87Sr/86Sr ~ 0.715 and δ18O from 11 to >15) and the underplated gabbro, indicating that late-stage, residual melts produced by fractional crystallization of the underplated gabbro also contributed to the formation of granite and silicic volcanic rocks. A 1-D, finite-difference thermal model utilizing these constraints and incorporating advection and energy-constrained assimilation reproduces the thicknesses of lithologies observed in the field and indicates that: (1) focused delivery of mantle melt was efficient in driving anatexis in overlying crustal rocks, (2) migration of anatectic melts to the upper crust was efficient in removing heat from the underplated gabbro and restricting its thermal impact on the overlying crust, and (3) growth of lower- and upper-crustal plutons involved incremental assembly under conditions favoring creation of large volumes of crystal mush rather than large classic magma chambers.

Bio:  James E. Quick is the Associate Vice President for Research, Dean of Graduate Studies, and Professor of Earth Sciences at Southern Methodist University (SMU). Prior to his appointment at SMU, he was employed by the US Geological Survey for 27 years in various capacities including Team Chief Scientist for the Eastern Regional Geologic Mapping and Earth Surface Processes Teams, and Program Coordinator for the Volcano Hazards Program. He earned a BSc in Geology at UCLA, an MSc in Mineralogy and Petrology at the University of Minnesota, and a PhD in Geology at Caltech. For the last 25 years, Professor Quick’s research has focused on the Sesia Valley in northern Italy as a natural laboratory to understand the processes that influence the composition and the formation of igneous rocks. In this area, he and his Italian colleagues have demonstrated that tilting and uplift during the Alpine orogeny exposed the magmatic system beneath a Permian supervolcano to an unprecedented depth of >25 km. These results, which were recognized by the Geological Society of Italy with the Capellini Medal in 2010, led to the creation of a UNESCO Geopark in 2013, and resulted in Professor Quick’s induction in January as an honorary citizen of Borgosesia, the principal city of the Sesia Valley. In his presentation, Professor Quick will use boundary conditions developed in the Sesia Valley to address crustal-scale mass transfer and thermal evolution within an evolving magmatic system during the lead up to a super eruption.

Gate, open 24 hours a day (http://www.jbmhhmwr.com/index/Maps_and_Directions.html). Reservations are not necessary, however, we need a head count, so, if you wish to attend dinner ($25), please inform Bob Fraser at 540-888-3001540-888-3001 or via E-mail at fraser.robert@comcast.net.If you wish, please feel free to attend the talk without dinner. Non-members and guests are welcome. Visit the PGS web site at http://www.potomacgeophysical.com for new meeting announcements, etc. Please send changes of address or email tofraser.robert@comcast.net.

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