During the early Miocene ignimbrite flare-up, significant parts of the Central Andes (17–20°S) were covered by large-volume ignimbrites. High-precision 206 Pb/ 238 U zircon dates constrain the flare-up in northern Chile at c. 18°S to a 3 myr period, starting with the deposition of the Poconchile ignimbrite at 22.736 ± 0.021 Ma. Of four main pulses, the two largest occurred at 21.924 ± 0.017 and 19.711 ± 0.036 Ma, when the >1000 km 3 in volume Cardones and Oxaya ignimbrites erupted, respectively. The ignimbrites are high-SiO 2 rhyolites and show significant heterogeneities in crystal content, mineral proportions and trace-element compositions. The zoned Oxaya ignimbrite implies incremental extraction of a crystal-poor magma overlying a crystal-rich magma. In contrast, petrological and textural heterogeneities in pumice clasts are spread throughout the Cardones ignimbrite and we propose magma mixing caused by destabilization of multiple magma bodies within a magmatic mush system. Distal and medial deposits of the Cardones ignimbrite, with a maximum welded thickness of at least 1000 m, entirely covered the western flank of the Central Andes, which implies infill of a significant topographic relief. Both compaction and welding resulted in a maximum thickness reduction of around 30% for the Cardones ignimbrite. Supplementary material: U–Pb method and complete data tables, ICP-OES and ICP-MS method and complete data tables, and detailed stratigraphic description of the Cardones ignimbrite are available at http://doi.org/10.6084/m9.figshare.c.2858707 .

15Large volume ignimbrites are excellent spatial and temporal markers for local 16 deformation and structural relief growth, as they completely inundate and bury the 17 underlying palaeo-topography and leave planar surfaces with relatively uniform, low 18 gradient slopes dipping less than 2°. Using one of these planar surfaces as a reference 19 frame, we employ a line-balanced technique to reconstruct the original morphology of 20 an ignimbrite that has undergone post-emplacement deformation. This method allows 21 us to constrain both the amount of post-eruptive deformation and the topography of 22 the pre-eruptive palaeo-landscape. Our test case is the unwelded surface of the 21.

The 21.9 Ma Cardones ignimbrite, a member of the early Miocene Oxaya Formation in northernmost Chile, is a crystal-rich (ca. 40 vol%) rhyolite with an extra-caldera thickness up to 1000 m. Access to core from eight ca. 1 km drill holes enabled sampling and petrological characterisation of pumice throughout the entire thickness of the ignimbrite at an unprecedented range of spatial scales. Mineral chemistry and modes reveal the presence of sanidine-poor and sanidine-rich pumice types, representing two, petrologically distinct magmas with the former being hotter (850-750°C) than the latter (770-670 o C). The sanidine-poor magma is amphibolebearing and has a Ba-rich melt phase (400-1000 ppm), whereas the sanidine-rich magma has a Ba-poor melt phase (<200 ppm) and lacks amphibole. Thermobarometry of antecrystal remnants in the sanidine-poor pumice clasts indicate derivation of rhyodacitic melts from a middle crustal hot zone (950-850°C) where wet, intermediate to mafic magmas stalled and fractionated. Textures and zoning in zircon and plagioclase are consistent with the interpretation that rhyodacitic melts were episodically emplaced into the shallow crust at 6.0 to 8.7 ± 2.0 km depths over a period of ca. 200 ky. Sanidine-rich magma compositions are consistent with the melt phase in the sanidine-poor magma, hence the former could be derived by residual melt segregation from the latter. High-Ba and high-Sr zones in sanidine have compositions up to 28,000 ppm and 400 ppm respectively. Such high concentrations require input of more intermediate magmas and/or partial melting of cumulative magmas. The sanidine-rich magmas likely formed above of the sanidine-poor magmas. However, there is no systematic relationship between the different pumice types and the ignimbrite stratigraphy, suggesting major pre-eruptive destabilisation of the magmatic system led to the amalgamation and mixing of different, melt-dominated lenses and surrounding crystal-mush to form a large heterogeneous eruptible magma body.

Broken crystals have been documented in many large-volume caldera-forming ignimbrites and can help to understand the role of crystal fragmentation in both eruption and compaction processes, the latter generally overlooked in the literature. This study investigates the origin of fragmented crystals in the > 1260 km 3 , crystal-rich Cardones ignimbrites located in the Central Andes. Observations of fragmented crystals in non-welded pumice clasts indicate that primary fragmentation includes extensive crystal breakage and an associated ca. 5 vol% expansion of individual crystals while preserving their original shapes. These observations are consistent with the hypothesis that crystals fragment in a brittle response to rapid decompression associated with the eruption. Additionally, we observe that the extent of crystal fragmentation increases with increasing stratigraphic depth in the ignimbrite, recording secondary crystal fragmentation during welding and compaction. Secondary crystal fragmentation aids welding and compaction in two ways. First, enhanced crystal fragmentation at crystal-crystal contacts accommodates compaction along the principal axis of stress. Second, rotation and displacement of individual crystal fragments enhances lateral flow in the direction(s) of least principal stress. This process increases crystal aspect ratios and forms textures that resemble mantled porphyroclasts in shear zones, indicating lateral flow adds to processes of compaction and welding alongside bubble collapse. In the Cardones ignimbrite, secondary fragmentation commences at depths of 175-250 m (lithostatic pressures 4-6 MPa), and is modulated by both the overlying crystal load and the time spent above the glass transition temperature. Under these conditions, the existence of force-chains can produce stresses at crystal-crystal contacts of a few times the lithostatic pressure. We suggest that documenting crystal textures, in addition to conventional welding parameters, can provide useful information about welding processes in thick crystal-rich ignimbrites.

Generation of silicic magmas leads to emplacement of granite plutons, huge explosive volcanic eruptions and physical and chemical zoning of continental and arc crust [1][2][3][4][5][6][7] . While the time scales for silicic magma generation in the deep and middle crust are prolonged 8 magma transfer into the upper crust followed by eruption is episodic and can be rapid [9][10][11][12] . Ages of inherited zircons and sanidines from four Miocene ignimbrites in the Central Andes indicate a gap of 4.6 Myr between the start of pluton emplacement and onset of super-eruptions, with a 1 Myr cyclicity. Here we show that inherited sanidine crystals were stored at temperatures generation, plutons emplacement, magma chamber formation and how super-eruptions are triggered.We present new 40 Ar/ 39 Ar ages on tens of individual sanidine fragments sampled from early Miocene rhyolitic ignimbrites located in northern Chile. These data are utilised along with 206 Pb/ 238 U single crystal CA-ID-TIMS and laser-ablation ICP-MS zircon geochronology.Terminology for magmatic systems is summarised in methods. Geological backgroundThe Oxaya Formation, located on the western slope of the Central Andes (Figure 1), comprises four large volume (collectively > 2000 km 3 ) regional ignimbrites 25,26 . Their ages are: