SHRIMP U–Pb zircon dating of the Valencia del Ventoso plutonic complex, Ossa-Morena Zone, SW Iberia: Early Carboniferous intra-orogenic extension-related ‘calc-alkaline’ magmatism

• The Valencia del Ventoso ‘calc-alkaline’ main pluton is dated at 334 ± 2 Ma, SHRIMP.
• A 15–30 million year younger age decouples magmatism from a subduction context.
• The OMZ plutons and mid-crustal mafic sills are intra-orogenic extension-related.

Mantle- and crustal-derived orogenic magmas provide insights into sources tapped during terrane accretion and supercontinent amalgamation. During Pangea formation volumetrically minor, but common, mafic–intermediate ‘calc-alkaline’ stocks intruded coeval granitoid plutons throughout the European Variscan. The Iberian Peninsula preserves the westernmost segment of this Upper Paleozoic orogeny that resulted from collision of Laurentia and Gondwana. Whether the widespread calc-alkaline mafic–intermediate stocks may be used as petrogenetic tectonomagmatic markers of ancient subduction zones is an important question. Early Carboniferous calc-alkaline plutonic rocks from the Ossa-Morena Zone (OMZ), a northern Gondwana continental block, have traditionally been related to subduction. However, recent studies suggest that the tectonomagmatic regime at that time was continental collision. In this context, a link is made between Early Carboniferous intraplating of ultramafic and mafic sills into the mid-crust, the IBERSEIS Reflective Body (IRB), and the Variscan magmatism in the Olivenza–Monesterio antiform of the OMZ. Here we show that the OMZ Valencia del Ventoso plutonic complex comprising alkaline, calc-alkaline, metaluminous and peraluminous compositions formed during the Visean–Bashkirian, c. 334–320 Ma SHRIMP U–Pb zircon. The calc-alkaline character is, apparently, a result of intra-orogenic extension-related interaction of mantle-derived alkaline mafic and crustal-derived peraluminous felsic magma during terrane accretion. The new age is some c. 15–30 million years younger than previously defined and so coeval with IRB formation. All evidence points towards active subduction having ceased prior to the latest Devonian–earliest Carboniferous. Thus the younger age places the Olivenza–Monesterio antiform calc-alkaline magmatism after collision of the OMZ and the South Portuguese Zone, to the south. During Pangea formation other, comparable, Early Carboniferous extensional events have been described in adjacent terranes in northern Gondwana and Avalonia–Laurentia. Whether orogenic mafic magmatism provokes or results from crustal melting apparently depends on the process controlling, and the timing, of the mantle melting event.

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