Cryptic crustal growth identified through Variscan post-collisional lamprophyre-granite composite dykes, French Massif Central

Abstract

The processes that control crustal growth by addition of new mantle-derived material to the crust and the recycling of crustal components that were introduced into the mantle during subduction remain poorly understood in collisional orogens. This is largely due to the fact that the sub-orogenic mantle has, in many instances, been fertilized by the introduction of crustal components during previous subduction events, and lacks the typical depleted mantle signature. Here, we present a study of lamprophyric–granitic composite dykes from the northern margin of the post-collisional Variscan Aigoual granite, French Massif Central (FMC). At Aigoual, lamprophyres and granites have gradational contacts and similar geochemical characteristics which indicate that the two were co-magmatic. UPb dating of zircon yields crystallization ages between 311 ± 3 Ma and 313 ± 3 Ma for dykes, identical within uncertainty with the ages of 312 ± 3 Ma and 313 ± 3 Ma obtained for the emplacement of the Aigoual granite. The lamprophyres are metaluminous to slightly peraluminous and display enrichment in both compatible (Fe, Mg, Ni, Cr) and incompatible elements (K, LILE, LREE) and have crustal isotopic signatures in both radiogenic (Sr, Nd and Hf) and stable (O) isotope systems. This dual crust/mantle signature of the lamprophyres can be interpreted to reflect partial melting, within the spinel stability field, of an enriched mantle that was metasomatised by the addition of subducted sediments. The granites are peraluminous, high-K and have shoshonitic affinities. Their primitive mantle-normalized trace-element patterns are similar to the lamprophyres and the isotopic compositions of the two rock types overlap. The high Cr, Ni, Fe, and Mg contents of the granites are consistent with a mantle component in the magmas. Our results suggest that between 65 and 80 wt% of material in the lamprophyres and granites was derived from the depleted mantle, and the rest from recycled crustal material. Although blurred from an (conventional) isotopic point of view, this magmatism contributed to the crustal growth via the post-collisional magmatism of the FMC. Coupled with their high preservation potential, post-collisional sites might thus represent significant sites of crustal growth.