Shan de Silva

Named for the Andes, andesites (53%–63% SiO2) are the archetypal magma erupted at magmatic arcs. They have been established as the average composition of continental crust and as such are integral to the growth and evolution of the continental crust. However, andesites are quite variable in trace element and isotopic composition reflecting disparate paths of origin. Herein we return to the original site of their identification, the Central Andes, and use a comprehensive dataset of published and unpublished trace elements and isotopes to show that during the past 6 Myr two distinct types of andesite have erupted in the Central Volcanic Zone (CVZ), which correspond with different geodynamic conditions. Consistent with previous work, we confirm that major composite cones and minor centers of the steady state (low magmatic flux) Quaternary CVZ arc have trace element and isotopic characteristics consistent with magma generation/fractionation in the lower crust. Within the Quaternary arc ...

Named for the Andes, andesites (53%-63% SiO 2) are the archetypal magma erupted at magmatic arcs. They have been established as the average composition of continental crust and as such are integral to the growth and evolution of the continental crust. However, andesites are quite variable in trace element and isotopic composition reflecting disparate paths of origin. Herein we return to the original site of their identification, the Central Andes, and use a comprehensive dataset of published and unpublished trace elements and isotopes to show that during the past 6 Myr two distinct types of andesite have erupted in the Central Volcanic Zone (CVZ), which correspond with different geodynamic conditions. Consistent with previous work, we confirm that major composite cones and minor centers of the steady state (low magmatic flux) Quaternary CVZ arc have trace element and isotopic characteristics consistent with magma generation/ fractionation in the lower crust. Within the Quaternary arc centers, there are also significant latitudinal variations that correspond with the age, composition, and P-T conditions of the lower crust. However, in contrast to this prevailing model, in the 21-24°S segment 6-1 Ma andesites from ignimbrites and lava domes associated with the peak of the regional Neogene ignimbrite flare-up have compositions that indicate these andesites are hybrids between mantle-derived basalts and upper crustal lithologies. Since~1 Ma, andesites in young silicic lava domes associated with the regional flare-up are compositionally indistinguishable from proximal Quaternary arc centers, indicating a return to steady-state magmatism and lower crustal production of andesites. We propose that the transition from upper crustal to lower crustal andesite production results from a decrease in mantle heat input and subsequent relaxation of the regional geotherm during the waning of the flare-up event. The two modes of andesite production have significant implications for the production and evolution of the CVZ arc crust. During the flare-up, prodigious amounts of basalt were emplaced into the mid-crust, resulting in the production of large volumes of hybrid intermediate magmas in the mid and upper crust. In contrast, the lower crustal differentiation recorded in the Quaternary steady state arc andesites would result in the formation of a dense crystalline residue in the lower crust and an overall densification of the lower crust. Over time, gravity instabilities associated with this densification may ultimately aid in the delamination of the dense lower crustal root, triggering flare-ups. These differences in andesite production may help explain the cyclicity (flare-up cycles) observed in mature continental arcs and emphasizes that andesite is not a monotonous composition and can vary with depthdependent intra-crustal differentiation related to magmatic flux.

The San Pedro – Linzor volcanic chain located in the Central Andean volcanic zone runs along the western border of the Altiplano-Puna magma body (APMB). The APMB corresponds to a partially molten upper crustal (<25 km depth) MASH-type zone, now thought to be a crystal-mush, related to the eruption of ignimbrites and dacitic domes of the Altiplano-Puna Volcanic Complex. Eruption of the San Pedro – Linzor volcanic chain started 2 Ma ago, generating the NW-SE trending volcanic edifices observed within the chain. This volcanic chain shows a decrease in 87 Sr/ 86 Sr isotope ratios in the orientation of the volcanic chain, from Toconce (>0.7075) to San Pedro (<0.7070) volcanoes. Changes in the isotopic ratios would be associated with different extends of interaction between mantle-derived magmas and the APMB. Thus, erupted lavas in the SE would have assimilated more crustal material than those that evolved in the NW-most part of the volcanic chain.

Introduction: Tyrrhena Patera (~22°S, 104°E), Mars is located within Hesperia Planum, northeast of the Hellas impact structure [1]. The low-lying (<2° slope), heavily dissected central-vent volcano has been interpreted to be composed primarily of pyroclastic deposits [2-4], and probably pyroclastic flows [5, 6]. Hesperia Planum is most commonly interpreted to be composed of fluid lavas [1] although results of recent work [7,8] cannot confidently rule out layered sediments (volcaniclastic or lacustrine). Here, we examine available high-resolution (<19 m/pixel) imagery of Tyrrhena Patera and Hesperia Planum deposits to help constrain their origins. Background: Mandt and others [9] examined Mars Orbiter Camera (MOC) Narrow-Angle images of the Medusae Fosse Formation (MFF) in an attempt to diagnose the lithologies of the MFF. After studying >700 Mars Obiter Camera (MOC) images (resolutions <3.5 m/pixel) of the MFF, a set of deposit-wide characteristics were identified. These...

Abstract Volcanic landforms on the Earth range are shapes and sizes from tiny scoria cones to enormous flood basalt or ignimbrite plateaux. Controls on the final volcanic landform include magma composition and volume, tectonic environment, nature of the crust, and posteruption erosion. Primary volcanic landforms (pre-erosion) can be divided into polygenetic and monogenetic volcanoes. The former include composite, shield, and caldera volcanoes; the latter include mafic minor centers such as scoria and spatter cones, tuff rings and cones, maars and kimberlites, as well as silicic minor centers such as lava domes and coulees. The most obvious volcanic landforms at the planetary scale are spatially, temporally, and magmatically connected volcanic provinces, such as volcanic fields, volcanic arcs, and large igneous provinces. These may contain all of the individual polygenetic and monogenetic volcanic landforms regardless of tectonic environment, providing strong evidence for universal primary controls on the character of Earth's volcanic landforms.

composite volcano Relatively large, long-lived constructional volcanic edifice, comprising lava and volcaniclastic products erupted from one or more vents, and their recycled equivalents. compound volcano Volcanic massif formed from coalesced products of multiple, closely spaced, vents. debris avalanche Catastrophic landsliding of gravitationally unstable volcano flanks resulting in a widely dispersed deposit at the foot of the edifice, typically characterized by a hummocky surface. edifice Constructional volcanic mass. planezes Triangular, flat-faced, facets on volcano flanks formed by the intersection of two master gullies in the upper reaches of a cone. ring plain Region surrounding a volcano beyond lower topographic flanks, over which tephra and mass-wasting products are radially distributed. satellite (or flank, or parasitic) vents Small monogenetic

The long-term thermochemical conditions at which large bodies of silicic magma are stored in the crust is integral to our understanding of the timing, frequency, and intensity of volcanic eruptions, and provides important context for volcano monitoring data. Despite this realization, however, individual magmatic systems also may have unique time-temperature paths, or thermal histories, that are the result of many complex and, sometimes, simultaneous/competing processes, ultimately leading to an incomplete understanding of their long-term thermal evolution. Of recent interest to the volcanology community is the length of time large volumes of eruptible and geophysically detectable magma exist within the crust prior to their eruption. Here we use a combination of diffusion chronometry, trace element, and thermodynamic modeling to quantify the long-term thermal budget of the 2.08 Ma, 630km3 Cerro Galán Ignimbrite (CGI) in NW Argentina, one of the largest explosive volcanic eruptions in...

High spatial resolution U^Pb dates of zircons from two consanguineous ignimbrites of contrasting composition, the high-silica rhyolitic Toconao and the overlying dacitic Atana ignimbrites, erupted from La Pacana caldera, north Chile, are presented in this study. Zircons from Atana and Toconao pumice clasts yield apparent 238U/206Pb ages of 4.11 7 0.20 Ma and 4.65 7 0.13 Ma (2c), respectively. These data combined with previously published geochemical and stratigraphic data, reveal that the two ignimbrites were erupted from a stratified magma chamber. The Atana zircon U^Pb ages closely agree with the eruption age of Atana previously determined by K^Ar dating (V4.0 7 0.1 Ma) and do not support long (s 1 Ma) residence times. Xenocrystic zircons were found only in the Toconao bulk ignimbrite, which were probably entrained during eruption and transport. Apparent 238U/206Pb zircon ages of V13 Ma in these xenocrysts provide the first evidence that the onset of felsic magmatism within the Al...

... John M. Hora 1 ,2 , Brad S. Singer 1 , Brian R. Jicha 1 , Brian L. Beard 1 , Clark M. Johnson 1 , Shan de Silva 3 and Morgan Salisbury 3 ... Noble gasses are normally thought to be very incompatible in minerals and melts (Kelley, 2002). ...