Reconstruction of Hualca Hualca volcano (southern Peru) based on geomorphological and geological evidence and 40Ar/39Ar dating

Abstract

The volcanic history of many of the western Central Andes volcanoes, including Hualca Hualca in the northern Ampato Volcanic Complex, remains poorly constrained. Based on an updated 1:50,000-scale geological map, new cross-sections of the Ampato Complex, a compilation of published dates, new geochemical data and 40Ar/39Ar ages, we present new insights into its volcanic evolution. Our study identifies 7 principal geological units (4 were not reported on the previous geologic map) and 8, mostly constructive, volcanic phases. The 40Ar/39Ar dating supports that Hualca Hualca began its formation during the Early Pleistocene (>1.6 Ma) with the establishment of an ancient stratovolcano. This volcano underwent a massive sector collapse to the north that created a U-shaped large amphitheater. This collapse likely resulted in the temporary damming of the Colca River, though no dates or associated deposits have been identified. Subsequent eruptions led to the formation of a smaller volcano along the amphitheater's scar that emplaced andesitic and dacitic lavas. This volcano dubbed modern Hualca Hualca holds the main summit. One of the longest lavas of this edifice reach the Colca valley and was previously dated at 610 ka. Sometime around 550 ka, volcanic activity migrated to the interior of the amphitheater forming the Nevado de Puye, Cruz del Condor, and Ahuashune domes that produced lavas of andesitic and dacitic composition. Around 164 ka a dacitic lava flow was emitted between Nevado del Puye and the base of the modern Hualca Hualca inside the horseshoe crater. The youngest known collapse of the Hualca Hualca complex involved parts of Nevado de Puye dome and the modern Hualca Hualca volcano, emplacing a debris avalanche that again dammed the Colca River and formed an upstream temporary lake that emplaced lacustrine and volcaniclastic deposits. After this collapse, no other volcanic deposit of the Hualca Hualca complex has been identified. Subsequently, glacial erosion during the local Last Glacial Maximum (17–16 ka) modified the volcanic landscape. Geochemical characteristics of Hualca Hualca suggest that their magmas were produced in a metasomatized mantle wedge and likely underwent crustal contamination during their ascent through the thick continental crust.