Geochemical evolution in crustal magma reservoirs: Trace-element and SrNdO isotopic variations in two continental intraplate series at Monts Dore, Massif Central, France

Abstract

Both silica-saturated and -undersaturated series lavas occur at Mont-Dore (French Massif Central), a large stratovolcano whose activity began at 3 Ma with a caldera collapse which produced rhyolitic pyroclastics. Rhyolitic lavas and heterogeneous trachyandesites then were erupted until 1.4 Ma, with a short tephritic and phonolitic episode at 2 Ma. After erosion and basaltic eruptions, major volcanic activity resumed with trachytic pyroclastics and abundant heterogeneous trachyandesites from 1.0 to 0.25 Ma. Major- and trace-element data for both series are roughly indistinguishable below 55 wt.% SiO 2, but well separated above 58 wt.% SiO 2. Compatible- and incompatible-element variations reflect the fractionation of olivine, clinopyroxene, plagioclase, amphibole and feldspars as well as accessory phases such as sphene, zircon and apatite in the more differentiated lavas (trachytes, rhyolites and phonolites). For the silica-saturated series, Sr- and Nd-isotope variations are well correlated with SiO 2 contents and δ 18O-values. These isotopic variations are interpreted to be a product of AFC processes within the upper crust and emphasize the role of continental crust in the saturated-series magma evolution. For the undersaturated series, tephrites and phonolites display slightly higher 87 Sr 86 Sr ratios than basanites probably due in part to secondary hydrothermal or alteration processes as indicated by δ 18O-values. However, their 143 Nd 144 Nd ratios are identical to those of basanites, suggesting that either no crustal contamination occurred in the undersaturated series, or that the contaminant was crust of less distinct radiogenic isotope composition. The genesis of rhyolites via an AFC process is related to the development of a shallow magmatic reservoir. This began as a dense network of anastomosed dykes having variable width in relation with the tectonic state of the granitic basement in the first 5 km. Once formed, the reservoirs expanded through the crust with successive pulses of basic magma and were periodically refilled providing heterogeneous and less contaminated trachyandesite magmas. For the silica-undersaturated series, fractional crystallization may have occurred either at great depth or in ancient inactive reservoirs of the saturated series after exhaustion of the potential upper-crustal contaminants by the saturated magmas, or may have been accompanied by an AFC process that involved young igneous crust of similar radiogenic isotope compositions.