Deception Island (South Shetland Islands), discovered in 1820, is one of the most active volcanoes in Antarctica with more than 20 eruptions (including the historic eruptions of 1967, 1969 and 1970) and three documented volcanic unrest events (1992, 1999 and 2014-15) over the past two centuries. Today, Deception Island hosts two scientific bases, which operate every year during the Austral summer and is also one of the most popular tourist destinations in Antarctica. The island consists of a composite volcano with a 8.5 x 10 km centrally located caldera dated as 3,980 ± 125 yr. BP. During the caldera-forming event, between 30 and 60 km3 (Dense Rock Equivalent-DRE) of magma erupted in the form of dense basaltic-andesitic pyroclastic density current deposits. During the last decades, Deception Island has been intensively investigated but some aspects regarding the magmatic processes associated with the formation of its collapse caldera are still under debate. However, characterizing the magmatic conditions and processes that triggered the huge explosive event is crucial to understand the past (and future) magmatic and volcanic evolution of the island. The objective of the present study is to perform an exhaustive petrological and geochemical study (mineral and juvenile glass geochemistry) of the Outer Coast Tuff Formation (OCTF), the main syn-caldera depositional unit. Our primary results confirm the existence of two different magmas coexisting, and interacting, prior to (and during) the caldera-forming event . Mineral analyses also allow shedding further light on the magmatic processes occurring in the magma system before the eruption (e.g. fraction crystallization, magma mixing). The presence of alteration minerals such as palagonite and zeolites suggest syn- and post-eruptive magma-water interaction. This research is part of POLARCSIC and PTIVolcan research initiatives. This research was partially funded by the MINECO grants POSVOLDEC(CTM2016-79617-P)(AEI/FEDER-UE) and VOLGASDEC (PGC2018-095693-B-I00)(AEI/FEDER, UE). This research is also supported by the PREDOCS-UB grant.