Progressive Build-Up of a Transcrustal System beneath an Adakite-Like Volcanic Complex (Chachimbiro, Ecuador): An Example of an Embryonic Porphyry Cu System

Abstract Arc magmas display global trends of increasing adakite-like indices (e.g. Sr/Y, La/Yb) with increasing crustal thickness, which are interpreted as the result of an increasingly deeper evolution of the magmas in a thick crust. Several volcanic edifices in continental arcs display a transition from normal to adakite-like magmas during their geologically short lifetimes and are precious examples to understand in detail how adakite-like signatures are acquired by magmas in thick continental arcs. Understanding the temporal transition from normal to adakite-like magmas has important implications on fundamental geological processes that are associated with adakite-like magmas, like the genesis of porphyry Cu deposits. The Quaternary Ecuadorian arc hosts numerous volcanic edifices featuring this transition during the last ~1 Ma, among which the Chachimbiro Volcanic Complex (CVC). The CVC records a history of effusive and explosive eruptions during the last ~400 ka that is characterized by progressively increasing adakite-like indices (e.g. Sr/Y, La/Yb), similar to that observed in magmatic systems associated with supergiant porphyry Cu deposits. It is, therefore, a suitable example to investigate the magmatic processes associated with these changes and their potential implications for the formation of porphyry Cu deposits. Here, we provide an extensive dataset on major and trace element geochemical compositions of the three main phenocryst minerals (pyroxene, amphibole, plagioclase) of the CVC. We retrieve thermobarometric data of amphiboles and pyroxenes and discuss the occurrence of different compositional clusters of the three phenocryst minerals in the frame of the ~400 ka temporal evolution of the CVC. Our data show that the oldest products of CVC, andesitic lava flows of the CH1 unit, were the result of staging of mantle-derived magmas in the lower crust and subsequent establishment of an upper crustal magma reservoir where plagioclase- and pyroxene-dominated fractionation occurred. After a magmatic lull of ~180 ka, volcanic activity resumed with effusive and explosive products of the CH2 and CH3 units characterized by more felsic compositions (high-SiO2 andesite to dacite). Thermobarometric data and contrasting REE patterns of amphiboles suggest sampling by magma coming from depth of an extensive mid- to upper crustal system at this time. The CH4 unit (~6 ka) consists of pyroclastic products which have the most evolved (rhyodacitic) composition of the whole CVC. Thermobarometric data and REE patterns of amphiboles suggest that also at this stage magmas ascending from depth sampled an extensive transcrustal system from mid- to upper crustal levels. For all evolutionary stages of the CVC, bulk rocks convey a signature that corresponds to a deeper-seated magmatic differentiation compared to magmas in equilibrium with phenocrystic minerals, which crystallized in mid- to upper crustal portions of the transcrustal system and were mechanically incorporated by magmas ascending from depth. Our study documents the progressive build-up of a transcrustal system over 400 ka during the transition to adakite-like magmatism favourable to porphyry Cu deposit mineralization, which could represent an embryonic porphyry-related magmatic system.