Genesis of rhyolitic ignimbrites and lavas from distinct sources at a deep crustal level: field, petrographie, chemical and isotopic (Sr, Nd) constraints in the Tazekka volcanic complex (Eastern Morocco)

Abstract

This study reports field, petrographic, geochemical and isotopic (Nd, Sr) data on the rhyolitic ignimbrites and lavas which erupted concurrently with mafic lavas onto the continental basement of Eastern Morocco during late Variscan times. Throughout the volcanic cycle up to 50 km 3 of crystal-rich ignimbrites erupted from individual or network-forming feeder-dykes over an area of 75 km 2. Crystal-poor lavas, extruding episodically in the central Tazekka complex, produced a small pile of less than 0.1 km 3 in volume. Both types of silicic eruptions interacted with their mafic counterparts, but only during the first part of the cycle. The rhyolites have high-K and subalkaline compositions. ORG (ocean ridge granite)-normalized trace element patterns show a significant LFS vs. HFS element fractionation and a weak negative Ta anomaly, similar to the M-type granites. On account of the compositional gap between the silicic and mafic magmas the genesis of the former by anatexis of an intermediate crust is proposed. This is confirmed by the different initial Nd isotopic ratios of the rhyolites (ϵ Nd 325 Ma = −3.1 to −5.2) and the mafic lavas (ϵ Nd 325 Ma = 2.9 to 0.6). The geochemical data on the two silicic magmas types, together with the field and petrographic observations, cannot be explained by invoking the origin of one magma from the other because, (1) the trace element contents of the ignimbrites and the lavas are negatively correlated to the REE fractionation ( La Yb ratio); (2) the ignimbrites have slightly higher Nd isotopic ratios than the lavas; (3) Eu ∗ correlates negatively with the crystal abundance of each rhyolitic facies; (4) the erupted volumes are disproportionate to each other and there is an absence of any compositional trend during the eruptive cycle. The best explanation for the data is the simultaneous formation of rhyolitic ignimbrites and lavas by melting of spatially distinct sources within a heterogeneous crust. This accords with the isotopic data, which also display a binary mixing relationship between the crustal sources. REE fractionation implies a marginally lower degree of melting for the lava-forming magma. Based on positive correlations from the lavas to the ignimbrites of Th, Hf and Zr, and also Ti and Fe + Mg (with a gap between the two rock types), the ignimbrite-forming melt would require higher melting temperatures than the lava-forming melt, with both melts being produced from biotite-rich crustal sources. The variability in the textures of numerous cognate lithic fragments, in the ignimbrites, associated with all the previously described features, suggests, following the model of Huppert and Sparks (1988), the genesis of the pumiceous, voluminous silicic magma at depth in the source region. The fragments reflect various states of stress within a convective viscous melt in which crystallization progressed from the core to the periphery of the chamber (melting roof theory) and in which vesiculation was favoured by the positive correlation between water solubility and temperature at depth. The final pyroclastic state of the magma would have been reached at depth as soon as the walls of the reservoir were disrupted, leading to fissural eruptions of great volumes of pulverized rhyolitic material.