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The Miocene Oxaya Formation, exposed along the western Andean slope in northern Chile, represents one of the largest ignimbrite provinces on earth. In this study, magnetic fabric data were acquired from a ~ 1-km-long core drilled vertically through a single cooling unit of the Oxaya Formation the ca.22 Ma Cardones ignimbrite. Samples for magnetic analysis were obtained every 20 m from the fine-grained matrix of the core. Detailed measurements of the variation in bulk magnetic properties, including natural remanent magnetization (NRM), susceptibility, and anisotropy of magnetic susceptibility (AMS), were used to monitor changes in magnetic mineralogy as well as changes in the strength and orientation of the magnetic fabric throughout the flow. AMS ellipsoid orientation and shape reflect rapid deposition from a concentrated granular fluidized flow and constrain both the location of the source caldera for this catastrophic eruption and processes of transport and deposition in this large-volume ignimbrite. After utilizing the magnetic remanence to correct for rotation about the core axis, well-grouped and imbricated petrofabric orientations reveal a well-defined SW (247°) transport direction down a proto-Western Andean slope indicating syn- or post-welding flow and confirming the deformed Lauca caldera as the likely source of the eruption. Systematic variations in fabric shape (T) and intensity (P) with depth reveal a predominately oblate fabric toward the top and base of the flow and predominately prolate fabrics in the center. These vertical changes in fabric reveal that this massive, apparently homogeneous, deposit has a systematic layering. This layering reflects depth-dependent temperature variations, temporal changes in the flow boundary zone during deposition and post-depositional processes. [ABSTRACT FROM AUTHOR]

We report new experimental data on the composition of magmatic amphiboles synthesised from a variety of granite (sensu lato) bulk compositions at near-solidus temperatures and pressures of 0.8–10 kbar. The total aluminium content (Altot) of the synthetic calcic amphiboles varies systematically with pressure (P), although the relationship is nonlinear at low pressures (<2.5 kbar). At higher pressures, the relationship resembles that of other experimental studies, which suggests of a general relationship between Altot and P that is relatively insensitive to bulk composition. We have developed a new Al-in-hornblende geobarometer that is applicable to granitic rocks with the low-variance mineral assemblage: amphibole + plagioclase (An15–80) + biotite + quartz + alkali feldspar + ilmenite/titanite + magnetite + apatite. Amphibole analyses should be taken from the rims of grains, in contact with plagioclase and in apparent textural equilibrium with the rest of the mineral assemblage at temperatures close to the haplogranite solidus (725 ± 75 °C), as determined from amphibole–plagioclase thermometry. Mean amphibole rim compositions that meet these criteria can then be used to calculate P (in kbar) from Altot (in atoms per formula unit, apfu) according to the expression: P kbar = 0.5 + 0.331 8 × Al tot + 0.995 4 × Al tot 2 This expression recovers equilibration pressures of our calibrant dataset, comprising both new and published experimental and natural data, to within ±16 % relative uncertainty. An uncertainty of 10 % relative for a typical Altot value of 1.5 apfu translates to an uncertainty in pressure estimate of 0.5 kbar, or 15 % relative. Thus the accuracy of the barometer expression is comparable to the precision with which near-solidus amphibole rim composition can be characterised. [ABSTRACT FROM AUTHOR]