Clinopyroxene geochemistry and zoning as a petrogenetic tool to assess the crystallization conditions of the alkaline Talzast Dome (Middle Atlas, Morocco)

Abstract

Located in the SW border of the Middle Atlas Causse, the Talzast Dome comprises highly SiO₂-undersaturated nephelinites with singular features. This Miocene volcanic dome displays a zoned structure whose rock types change from the periphery towards the centre regarding texture and mineralogical composition. The external zone of the dome is made of ankaratrite (a melanocratic variety of olivine nephelinite displaying a porphyritic microlitic texture), the intermediate zone is composed of fasinite (feldspar-free medium-grained variety of melteigite), and the central zone consists of coarse-grained to pegmatitic ijolite named talzastites in honour of the Talzast area. Such coarse-grained and pegmatitic ijolite were previously unknown in the entire Middle Atlas Miocene-Quaternary magmatic province.

The Talzast Dome nephelinites contain abundant clinopyroxene phenocrysts and microcrysts, with complex zoning patterns. Three distinct types of clinopyroxene were identified based on their zoning patterns: a) type 1 exhibits reverse zonation with low-Mg# rounded resorbed cores (Mg# = 66–79), surrounded by high-Mg clear rims (Mg# = 72–84); b) type 2 phenocrysts generally display normal zonation, with high-Mg cores (Mg# = 75–85) and low Mg rims (Mg# = 53–75); c) type 3 phenocrysts are euhedral and show no zonation pattern. The textural and compositional features of the studied clinopyroxenes suggest the presence of antecrysts and autocrysts, reflecting complex magmatic processes.

Thermobarometric Clinopyroxene-liquid and Clinopyroxene-only calculations for the studied rocks revealed a wide P-T range, varying from 0.04 to 14.73 kbar (±1.5 kbar) and 1008 to 1233 °C (±52–60 °C), respectively. Furthermore, the data indicates the existence of three main magma storage zones at depth: a) a deep magma reservoir (≃ 27 km); b) an intermediate magma reservoir (≃10 km); and c) a shallow magma reservoir (≃ 2 km). Whole-rock geochemistry suggests that the Talzast Dome rocks resulted from magmatic differentiation processes that occurred in a shallow magmatic reservoir, as indicated by the detailed mineral-scale analysis of the clinopyroxene crystals. Ankaratrite represent the least differentiated rock, followed by fasinites. Talzastites are the most differentiated products of this dome. The genesis of pegmatitic rock-types imply the influence of volatile agents, concentrated after the bulk of fractional crystallization had occurred.