Role of magma mixing in the petrogenesis of tephra erupted during the 1990-98 explosive activity of Nevado Sabancaya, southern Peru

The Nevado Sabancaya in southern Peru has exhibited a persistent eruptive activity over eight years following a violent eruption in May-June 1990. The explosive activity consisted of alternated vulcanian and phreatomagmatic events, followed by declining phreatic activity since late 1997. The mean production rate of magma has remained low (10(6)-10(7) m(3) per year). The 1990-1998 eruptive episode produced andesitic and dacitic magmas. The juvenile tephra span a narrow range of compositions (60-64 wt% SiO2). While SiO2 contents do vary slightly, they do not show any systematic variation with time. Phenocryst assemblages in the juvenile rocks consist of mainly plagioclase, associated with high-Ca pyroxene, hornblende, biotite, and iron-titanium oxides. Rare fine-grained magmatic enclaves, with angular to subrounded shapes, are contained within some of the juvenile lava blocks, which were expelled since 1992. They have a homogeneous andesitic composition (57 wt% SiO2) and show randomly oriented interlocking columnar or acicular crystals (plagioclase and amphibole), with interstitial glass and a few voids, which define a quench-textured groundmass. Textural, mineralogical and chemical evidence suggests that the 1990-1998 eruptions have mainly erupted hybrid andesites, except for the 1990 dacite. The hybrid andesites contain a mixed population of plagioclase phenocrysts: Ca-rich clear plagioclase (An(40-60)), Na-rich clear plagioclase (An(25-35)), and inversely zoned "dusty-rimmed" plagioclase with a sodic core (An(25-40)) surrounded by a Ca-rich mantle (An(45-65)). Melt-inclusions, wavy dissolution surfaces and stepped zoning within the "dusty-rimmed" plagioclases are compatible with resorption induced by magma recharge events. Chemical and isotopic lines of evidence also show that andesites are hybrids resulting from magma mixing processes. Repeated magma recharge, incomplete homogenisation and different degrees of crustal assimilation may explain the extended range of isotopic signatures. Our study leads to propose an evolution model for the magmatic system at Nevado Sabancaya. The main magma body consisted of dacitic magmas differentiating through extensive open-system crystallization (AFC). Repeated recharge of more mafic magmas induced magma mixing, leading to the formation of hybrid andesites. A partially crystalline boundary layer formed at the interface between the andesites and the recharge magma. The magmatic enclaves were produced by the disruption and dispersion of this andesitic layer as a result of new magma injection and/or sustained tectonic activity. Periodic magma recharge and interactions with groundwater are two processes that have enabled the explosive regime to remain persistent over an 8-year-long period. What precise mechanism triggers the eruptive activity remains speculative, but it may be related either to new magma injection, or to the sustained tectonic activity that occurred at that time in the vicinity of the volcano, or a combination of both.