Search

Magma recharge into a differentiated reservoir is one of the main triggering mechanisms for explosive eruptions. Here we describe the petrology of the eruptive products of the last explosive eruption of Tutupaca volcano (southern Peru) in order to constrain the pre-eruptive physical conditions (P-T-XH2O) of the Tutupaca dacitic reservoir. We demonstrate that prior to the paroxysm, magma in the Tutupaca dacitic reservoir was at low temperature and high viscosity (735 ± 23 °C), with a mineral assemblage of plagioclase, low-Al amphibole, biotite, titanite, and Fe-Ti oxides, located at 8.8 ± 1.6 km depth (233 ± 43 MPa). The phenocrysts of the Tutupaca dacites show frequent disequilibrium textures such as reverse zonation, resorption zones, and overgrowth rims. These disequilibrium textures suggest a heating process induced by the recharge of a hotter magma into the dacitic reservoir. As a result, high-Al amphibole and relatively high-Ca plagioclase phenocryst rims and microlites were formed and record high temperatures from just before the eruption (840 ± 45 °C). Based on these data, we propose that the recent eruption of Tutupaca was triggered by the recharge of a hotter magma into a highly crystallized dacitic magma reservoir. As a result, the resident dacitic magma was reheated and remobilized by a self-mixing process. These magmatic processes induced an enhanced phase of dome growth that provoked destabilization of the NE flank, producing a debris avalanche and its accompanying pyroclastic density currents. [ABSTRACT FROM AUTHOR]

We describe sedimentological variations of the block-rich debris avalanche deposits and associated pyroclastic density current deposits emplaced around 1802 CE from Tutupaca volcano in southern Peru. We use these exceptionally well-preserved features to document the collisional shearing contact between the avalanche and coeval pyroclastic density currents. Furthermore, we show how the first stages of the edifice collapse and syn -cataclastic emplacement process affect the block-size distributions. With field observations, we describe imbricated block clusters, jigsaw cracks and striations related to elongated ridge structures on the deposit surface. Sedimentological and statistical methods (Fourier Shape analysis and Shape Preferred Orientation measured on 208 blocks and 566 mesoscale structures) help us to characterize the cataclastic gradient and establish the collisional relationships between different units. We determine that the proximal impacted deposits and block lithofacies from ridges may be related to distal block units around ∼10 km run-out distance. Different block clusters indicate a kinematic transition between avalanche units to pyroclastic density currents. Block shape parameters help to differentiate rounded blocks resulting from matrix abrasion with and striated blocks from ridges related to proximal imbricated block clusters. From the statistical dataset, a few equations have been developed indicating a common cataclastic origin with a co-genetic evolution of block lithofacies during sequential syn -cataclastic emplacement. The dome collapse is associated with a specific granular flow regime between avalanche and pyroclastic density currents with secondary reworking. Cyclic impact waves contribute to block cluster growth. Clusters are disaggregated during shock propagation. The inherited shapes of the block lithofacies with a/b = 1.2–2 and ellipse = 0.2–2.5 indicate the reworking by impact waves. A multidirectional switch to mass spreading in the median zone between 2 and 6 km may be considered with secondary flow and segregation waves. A basal frictional regime with striations is differentiated from collisional cataclastic flow, generating polymodal grooves during peak velocity at the flow front. Impact forces around ∼15.7 × 1010 N are implied by suggested clast velocities around 8.86 m.s−1 and the transitional regime between avalanche units and pyroclastic density currents between 15.5 and 39.6 m.s−1. An extensional disaggregation with the fractal dimensions (D) of the surrounded matrix between 0.6 and 2.8 characterizes the granular transport. A collisional shearing contact probably operated between avalanche units and pyroclastic density currents, which contribute to co-genetic evolution of block clusters from median to frontal distal zones. In the distal zone, abraded block clusters and tilted blocks are related to frontal reworking by impact wave. The cataclastic gradient of avalanche units is correlated with the pyroclastic flow regime. Semi-quantitative analysis of block clusters provides information about syn -emplacement processes during sequential impact waves related to volcanic debris-avalanche units and pyroclastic density currents. • A combined approach helps to correlate the block clusters of avalanche deposits. • Striations are associated with the subsequent pyroclastic density currents. • Sequential events of syn -emplacement processes have been established. [ABSTRACT FROM AUTHOR]