S.L. de Silva

Gravel-mantled megaripples of the Argentinean Puna: A model for their origin and growth with implications for Mars

Gravel “megaripples” in the Puna of Argentina are the most extreme aeolian megaripples on Earth and are useful analogs for aeolian processes on Mars. Field observations, supplemented by experimental and numerical constraints on wind characteristics and aeolian transport, reveal their conditions of formation and growth to be an aeolian geomorphology “perfect storm.” The bedforms are formed on a substrate of weakly indurated ignimbrite, aeolian deflation of which yields a bimodal lag of lithics and pumice clasts onto an undulating surface. Under normal wind conditions in this region, the lithics are organized into bedforms on local upslopes and “highs” through creep induced by the impact of saltating sand and pumice. The gravel bedforms grow through “shadowing” and trap sand and silt that is gradually kinetically sieved down to “lift” the gravel mantle upwards to form the megaripples. These observations connote that the largest features are not ripples in the sense of migrating bedforms, but rather nucleation sites of wind-transported sediment. Strong control by bedrock topography means that the largest bedform wavelengths are not a result of particle trajectories, and this complicates their comparison with other ripples and may require a new classification. Of relevance to Mars, the Puna megaripples are morphologically and contextually similar to small ripple-like transverse aeolian ridges (TARs). Moreover, the Puna gravels have similar equivalent weight ( mg ) to those composing granule ripples at Meridiani Planum, and their local origin may have implications for the origin of sediment in martian aeolian bedforms. Finally, the stable yet dynamic character of the Puna megaripples could help reconcile current models of TARs with periodic bedrock ridges that may be produced by aeolian erosion.

Catastrophic Caldera-Forming (CCF) Monotonous Silicic Magma Reservoirs: Geochemical and Petrological Constraints on Heterogeneity, Magma Dynamics, and Eruption Dynamics of the 3·49 Ma Tara Supereruption, Guacha II Caldera, SW Bolivia

The 5 65 to 1 80 Ma Cerro Guacha Caldera Complex (CGCC) in the Altiplano–Puna Volcanic Complex of SW Bolivia, with >90% of its >2500 km erupted volume consisting of crystal-rich dacite, has all the characteristics of a ‘monotonous’ magma system. However, it also records minor lithological heterogeneity. Such hand-sample scale heterogeneity is ubiquitous in dominantly ‘monotonous’ magmas, yet remains poorly investigated. Here we explore the heterogeneity in the CGCC, and its implications for the construction and evolution of ‘monotonous’ magma systems. We focus on the Guacha II Caldera (G2C), the younger of two calderas in the complex, because its preto post-climactic eruptive history is fully represented and, although the eruptive products are dominantly dacitic (66–72 wt % SiO2), the juvenile pyroclastic deposits and lavas erupted throughout the history of the G2C define a high-K, calc-alkaline suite of diverse compositions that range from andesite to high-Si rhyolite. The G2C cycle initiated with the effusive eruption of crystal-rich andesite lava. The subsequent explosive phase began with a short-lived plinian eruption of crystalpoor rhyolite pumice. This was immediately followed by the Catastrophic Caldera Forming (CCF) eruption at 3 49 6 0 01 Ma and the deposition of dacite–rhyolite and banded pumice within the >800 km dense rock equivalent (DRE) Tara ignimbrite. A significant volume of magma remained and caused 1 5 km of resurgent uplift. Three crystal-rich dacite–rhyolite post-climactic lava domes (Chajnantor Dome, Rio Guacha Dome, and Chajnantor Lavas) subsequently erupted from separate coexisting melt-rich ‘pods’ within the G2C’s remnant mush. Whole-rock isotope ratios across all lithologies span a significant range in Sr/Sr (0 709380–0 713159) and a relatively narrow range in Nd/Nd (0 512179–0 512297) and dO(qtz) (þ8 38 toþ 8 68%), best reconciled with a twostage assimilation–fractional crystallization (AFC) model. Stage 1 initiated with parental melts from the Altiplano–Puna Magma Body (APMB) fractionating and assimilating crustal lithologies in the upper crust (10–25 km depth) to generate the magma compositions recorded in the andesite lava and the ignimbrite banded pumice. These magmas subsequently accumulated and underwent a second stage of AFC in the uppermost crust ( 800–850 C and 5–9 km depth) to produce the most differentiated magmas recorded in the ignimbrite rhyolite pumices. Although the two-stage AFC model presented here is non-unique, it implies that the basement composition is temporally or spatially variable throughout the 30 km of upper crust beneath the G2C. The origin of some of the VC The Author 2017. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oup.com 227 J O U R N A L O F P E T R O L O G Y Journal of Petrology, 2017, Vol. 58, No. 2, 227–260 doi: 10.1093/petrology/egx012 Advance Access Publication Date: 20 April 2017

Synthesis: PLUTONS: Investigating the relationship between pluton growth and volcanism in the Central Andes

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 ignimbrite-forming eruptions during the past several million years and currently hosts the world’s largest zone of silicic partial melt in the form of the Altiplano-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 large-scale surface uplift that have been found to represent ongoing movement of magmatic fluids in the middle to upper crust. The locations 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 deformation, 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 resistivity, 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.