Pre-Hercynian magmatism in the Eastern Pyrenees (Cap de Creus and Albera Massifs) and its geodynamical setting

Abstract

In the Eastern Pyrenees Hercynian massifs, basic and acid magmatic associations occur in metasedimentary series of probable Cambrian Late Precambrian age. Basic magmatic rocks occur in the lower part of the metasedimentary series as metagabbros and metabasalts derived from low-Ktholeiites and quartz-tholeiites. They are characterized by low niobium and thorium contents (< 5 ppm) and their multi-element and REE patterns are flat. They are interpreted as fractionated E-MORB mantle partial melts. Acid magmatic rocks are: 1) Metarhyolites and dacitic metatuffs intercalated at different levels in the series. They are characterised by low to moderate niobium and thorium contents (7 -12 ppm). Their multi-element patterns are analogous to those of crustal melts, and the REE patterns are characteristic of progressively fractionated calcalkaline magmas. These rocks are derived from aluminous calcalkaline crustal magmas. 2) Rhyolitic metaporphyries mainly in the upper part of the series, although thin sills are also present in the lower part. These rocks are highly impoverished in incompatible and REE elements and display low titanium, phosphorus and strontium contents. Their REE patterns are flat with a significant europium anomaly. The rocks represent highly differentiated aluminous calcalkaline crustal magmas. 3) A quartz-monzonitic metaporphyry sill in the lowest part of the series. It represents a meta-aluminous magma with alkaline affinity and, although its incompatible element patterns are analogous to those of the metavolcanics, its REE patterns are more fractionated and exhibit a stronger impoverishment in HREE. The assemblage of acid rocks can be ascribed to a magmatic episode developed by crustal partial melting and likely also to subsequent differentiation. The interbedded acid and basic volcanics in the lower part of the series suggest that both magmatic events overlapped in time, although no genetic connection exists since the acid and basic rocks are of crustal and mantle origin respectively. The magmatism probably took place in a continental back-arc basin.