The Central Andes is a key global location to study the enigmatic relation between volcanism and plutonism because it has been the site of large ignim briteforming eruptions during the past several million years and currently hosts the world's largest zone of silicic partial melt in the form of the Alti plano Puna Magma (or Mush) Body (APMB) and the Southern Puna Magma Body (SPMB). In this themed issue, results from the recently completed PLUTONS project are synthesized. This project focused an interdisciplinary study on two regions of largescale surface uplift that have been found to represent ongoing movement of magmatic fluids in the middle to upper crust. The loca tions are Uturuncu in Bolivia near the center of the APMB and Lazufre on the Chile Argentina border, on the edge of the SPMB. These studies use a suite of geological, geochemical, geophysical (seismology, gravity, surface defor ma tion, and electromagnetic methods), petrological, and geomorphological techniques with numerical modeling to infer the subsurface distribution, quantity, and movements of magmatic fluids, as well as the past history of eruptions. Both Uturuncu and Lazufre show separate geophysical anomalies in the upper, middle, and lower crust (e.g., low seismic velocity, low resistiv ity, etc.) indicating multiple distinct reservoirs of magma and/or hydrothermal fluids with different physical properties. The characteristics of the geophysical anomalies differ somewhat depending on the technique used-reflecting the different sensitivity of each method to subsurface melt (or fluid) of different compositions, connectivity, and volatile content and highlight the need for integrated, multidisciplinary studies. While the PLUTONS project has led to significant progress, many unresolved issues remain and new questions have been raised.

To better understand the origin of across-strike K 2 O enrichments in silicic volcanic rocks from the Andean Central Volcanic Zone, we compare geochemical data for Quaternary volcanic rocks erupted from three well-characterized composite volcanoes situated along a southeast striking transect between 21° and 22° S latitude (Aucanquilcha, Ollagüe, and Uturuncu). At a given SiO 2 content, lavas erupted with increasing distance from the arc front display systematically higher K 2 O, Rb, Th, Y, REE and HFSE contents; Rb/Sr ratios; and Sr isotopic ratios. In contrast, the lavas display systematically lower Al 2 O 3 , Na 2 O, Sr, and Ba contents; Ba/La, Ba/Zr, K/Rb, and Sr/Y ratios; Nd isotopic ratios; and more negative Eu anomalies toward the east. We suggest that silicic magmas along the arc front reflect melting of relatively young, mafic composition amphibolitic source rocks and that the mid-to deep-crust becomes increasingly older with a more felsic bulk composition in which residual mineralogies are progressively more feldspar-rich toward the east. Collectively, these data suggest the continental crust becomes strongly hybridized beneath frontal arc localities due to protracted intrusion of primary, mantle-derived basaltic OPEN ACCESSGeosciences 2013, 3 634 magmas with a diminishing effect behind the arc front because of smaller degrees of mantle partial melting and primary melt generation.

Cerro Uturuncu, southwest Bolivia, is a high-K, calc-alkaline, composite volcano constructed upon extremely thick continental crust approximately 125 km behind the arc-front of the Andean Central Volcanic Zone (CVZ). Eruptive activity occurred between 890-271 ka in a single phase of volcanism lasting ~620,000 years. The edifice consists of a central cone and several flank vents where dacitic and andesitic lava flows and domes erupted. Volumes of individual eruptive units range from 0.1 to ~10 km 3 ; the composite volume of Uturuncu is ~89 km 3. In this paper, we present new field, petrographic, and geochemical data in an effort to understand the volcanic and magmatic evolution of Uturuncu. Lava flows and domes have a restricted range in whole rock compositions ranging from 61 wt%-67 wt% SiO2; magmatic inclusions contained within these units have a larger range from 53 wt%-64 wt% SiO2. Typical phenocryst assemblages are plagioclase > orthopyroxene > biotite >> quartz and Fe-Ti oxides. Pb isotope ratios are characteristic of the southern CVZ by containing high 207 Pb/ 204 Pb and 206 Pb/ 204 Pb and moderate to high 208 Pb/ 204 Pb. Sr and Nd isotope ratios indicate that Uturuncu magmas were modified by high 87 Sr/ 86 Sr and low 143 Nd/ 144 Nd felsic basement lithology during magma migration and differentiation. In all eruptive units, there is petrographic and geochemical evidence for magma mixing and mingling. In this regard, magma mixing and mingling is considered to be responsible for the small range in lava flow and dome compositions throughout the eruptive history of the center.

The "PLUTONS: Investigating the Relationship between Pluton Growth and Volcanism in the Central Andes" themed issue of Geosphere is dedicated to the memory of Todd Christian Feeley, our friend, colleague, and mentor. He distinguished his career with a long string of insightful papers that integrated the geology and petrology of volcanic rocks. His work was always soundly based in field geology, and his coworkers remember him for his outstanding skills in the field. On Sunday, 21 March 2015, the volcano petrology and the broader geologic community lost a vibrant member. Todd took his own life at the age of 54 after a long battle with depression. Todd was born in September 1961 and grew up in Sunnyvale, California, where he loved to surf. He chose to attend the University of California at Santa Barbara to continue surfing, but during college fell in love with his true calling, geology. He graduated in 1985 and focused his energy in pursuit of his dream to become a professor of geology; Todd earned a Master of Science degree at Oregon State University (Corvallis) in 1989 under Anita Grunder on middle Tertiary volcanism in eastern Nevada and his Ph.D. in geochemistry in 1993 from the University of California-Los Angeles under Jon Davidson on Volcán Ollagüe and the Central Andes. After a postdoctoral position under Michael Dungan in Geneva, Switzerland, Todd started as a research assistant professor of geology in 1996 at Montana State University (Bozeman), where he progressed through tenure to associate professor of geology in 2006, graduating a sizeable number of Master of Science and Ph.D. students. Todd never shied away from sharing his opinion whether in the classroom, at meetings, or behind closed doors, and more often than not spearheaded the conversation on magma, magmatic sources, and how they erupt. Nor did he shy away from using a wide range of sophisticated tools to address problems in volcanic petrology. He worked tirelessly and was always ready to share his geologic and petrologic insight and to see the work to print. His career-long involvement in igneous petrology began in east-central Nevada's Egan Range, mapping and determining the mantle contributions to middle Tertiary calcalkaline volcanic rocks. He soon expanded his work to the Central and Southern Andes, which became his passion, and he continued to return until the summer of 2014, his last field season. He spent much of his early career working on Ollagüe Volcano and the Aucanquilcha complex in the Central Andes and San Pedro-San Pablo Guadal complex in the Southern Andes. He took a short break from work in South America to expand his interests to oxygen and hydrogen isotope studies of Mount St. Helens and Lassen volcanoes, across arc geo

for their analytical assistance. Thanks to the members of PLUTONS research team for insightful discussions and inspiration. I would like to extend my gratitude to my main thesis advisor, Dr. David Mogk, who has acted as my mentor and parent, offering me constructive criticism, unwavering support and thoughtful advice throughout my time at Montana State University. A special thank you to my committee members Drs. Colin Shaw, David Lageson, and Jim Schmitt for offering time, support and resources. I thank Drs. Paul Mueller and Sandy Underwood for assistance and insight. Thank you Gary Michelfelder for your guidance, support, and friendship. I am indebted to Arden Fortenberry and Melanie Baldwin for their assistance. Special thanks to Josh McFarland for his able and entertaining assistance in the field despite altitude, freezing temperatures and rat infestations. Thanks to the company of my friends and fellow students, especially