From the Middle Triassic Cima Pape complex (Dolomites; Southern Alps) to the feeding systems beneath active volcanoes: Clues from clinopyroxene textural and compositional zoning

Abstract

Crystal zoning plays a fundamental role in modern volcanology as a key to unravel the geometry and the dynamics of plumbing systems. In this study, a detailed textural and compositional study of clinopyroxene crystals entrained in intrusive, hypabyssal and effusive products from Cima Pape (Dolomites) is coupled with thermobarometric-hygrometricmodels to reconstruct the geometry and evolution of the feeding systembeneath Middle Triassic volcanic edifices. Whole-rock major, trace element distribution and Sr-Nd isotopic signature (87Sr/86Sri = 0.7045–0.7050; 143Nd/144Ndi = 0.51223–0.51228) show that the rocks from Cima Pape are SiO2- saturated and have shoshonitic affinity, and likely belong to the acme of the Mid-Triassic magmatismthat shaped the Southern Alps between 239 and 237.6Ma. Highly porphyritic trachybasaltic to basaltic trachyandesitic volcanic rocks contain a large number of concentric-zoned clinopyroxene crystals. Here, high-Mg# and -Cr2O3, REEdepleted bands (Mg# 80–91; Cr2O3 up to 1.2 wt%) with variable thickness grew between relatively low-Mg# and -Cr2O3 (Mg# 70–77; Cr2O3 < 0.1 wt%) augitic cores and rims. In contrast, the gabbroic to monzodioritic 50- to 300-m-thick sill cropping out belowthe volcanic sequences, though to represent a relic of the shallowest portion of the plumbing system, is mostly made up of unzoned clinopyroxene crystals. Thermobarometric and hygrometric models allowed us to define that a small “mush-type” batchwas located beneath the Cima Pape volcano at depths between 7 and 14 km. Here, augitic clinopyroxene formed in equilibrium with a slightly evolved (basaltic trachyandesitic), H2O-rich melt (Mg# = 43–45; T = 1035–1075 °C; H2O = 2.6–3.8 wt%). Periodic replenishments of the magma batch by primitive (Mg# = 65–70), hotter and relatively H2O-poor (T = 1130–1150 °C; H2O = 2.1–2.8 wt%) basaltic magmas led to the formation of diopsidic bands mantling the already formed augitic cores. Later on, re-equilibration of clinopyroxene with the mixed melt resulted in the formation of low-Mg#, LILE- and LREE-enriched rims. The most Mg-poor micro-phenocrystic clinopyroxene in the volcanic rocks and in the sill records the ultimate and shallowest conditions of crystallization, occurring at T of 975–1010 °C and P comprised between 50 and 150 MPa. Based on the presence of similar zoning in clinopyroxene phenocrysts, a comparison between the Mid-Triassic Cima Pape and active volcanoes was put forward to highlight the potential of studying ancient, entirely exposed volcanic systems for interpreting the feeding systemprocesses acting beneath active volcanoes. At a regional scale, this approach represents a new, powerful tool for investigating the evolution of the Mid-Triassic magmatism in the Southern Alps and shedding light on the interactions between mantle-derived melts and differentiated batches ponding in the crust.