Compositional Evolution of the Variscan Intra-Orogenic Extensional Magmatism in the Valencia del Ventoso Plutonic Complex, Ossa-Morena Zone (SW Iberia): A View from Amphibole Compositional Relationships

Abstract

The Ossa-Morena Zone (OMZ), SW Iberia, has numerous Lower Carboniferous compositionally zoned plutons that formed in a Variscan intra-orogenic extensional setting. This magmatism shows a wide compositional variation comprising alkaline, transitional, and calc-alkaline suites. The calc-alkaline suite was produced by hybridization of alkaline magmas with felsic melts generated by crustal anatexis related to the intrusion of mafic magmas in the middle crust. In this work, we present a textural and mineralogical study of the Variscan Valencia del Ventoso main pluton from the OMZ to track the compositional evolution of magmas during hybridization using constraints from amphibole compositions and to determine the P-T conditions of emplacement using amphibole-based thermobarometry. This pluton exhibits reverse zoning with an inner facies containing alkaline dolerites, gabbros, and quartz diorites, an intermediate facies with transitional diorites, and an outer facies with calc-alkaline quartz diorites to monzogranites. Magmas from the intermediate and border facies crystallized under oxidizing conditions at relatively low temperatures (range: 640–760 °C) and ca. 280–300 MPa, implying near H2O-saturated conditions. These rock facies show mineralogical evidence of hybridization between alkaline to mildly alkalic and calc-alkaline magmas. The former is inferred from the occurrence of antecrysts of labradorite-andesine, high-Ti pargasite-hastingsite, and biotite with deficiency in tetrahedral-site occupancy, a distinctive feature of biotite from the inner facies alkaline dolerites. This contrasts with later crystallization from the calc-alkaline magma of andesine-oligoclase, low-Ti magnesiohornblende-edenite, and biotite with full tetrahedral-site occupancy. Constraints from amphibole-melt compositional relationships in antecrystic high-Ti amphibole suggest that the alkaline magmatic component could have a high- to ultra-K affinity.

Episodic Precipitation of Wolframite during An Orogen: The Echassières District, Variscan Belt of France

“…The geological setting for wolframite b mineralization in the Echassières district is comparable to the one inferred for the Erzgebirge province in central Europe, which experienced widespread post-collisional peraluminous magmatism, with only relatively limited contamination by mantle-derived magma [64]. In contrast, an important mantle contribution is evidenced by numerous calc-alkaline plutons in the Ossa-Morena Zone of the Variscan Iberic belt [81]. As for the Echassières district, the main event of W-Sn mineralization in the Erzgebirge (325-315 Ma; refs.…”

Editorial for Special Issue “Distribution of Major- and Trace-Elements in Igneous Minerals”

“…Cambeses et al [7] provide textural and phase relationship constraints on magma hybridization in the Valencia del Ventoso Plutonic Complex that was generated during the Variscan intra-orogenic extensional magmatism of the Ossa-Morena Zone (SW Iberia). The authors determine the P-T conditions of magma crystallization using Al-in-hornblende barometry and amphibole-plagioclase thermobarometry.…”