Continental accretionary tectonics of western Pangea and the formation of Sn, W and Li deposits

The Variscides of Europe and northwestern Africa formed during the collision of the vast Peri-Gondwana shelf of Africa / Arabia with Laurussia eventually leading to the assembly of western Pangea. Late- to post-orogenic granites with Li and W mineralization were emplaced throughout the entire belt, but only complexes of the former Armorican spur (Kroner et al., 2013), i.e., Cornwall, the Erzgebirge and N-Iberia, host primary Sn mineralization of economic value. These deposits are related to late-orogenic high temperature metamorphism (N-Iberia, Erzgebirge) and to post-orogenic advective heat transfer from the mantle (Cornwall, Erzgebirge), respectively. Thus, it is not the tectonic setting of magmatism that controls the distribution of Sn deposits, but the protoliths (which formed long before supercontinent assembly) and the accretionary development before collision (Romer and Kroner, 2022). Common to all Gondwana-derived crustal fragments in the Variscan belt is the presence of voluminous, early Ordovician siliciclastic sedimentary units deposited on the Peri-Gondwana shelf during the breakup of Pannotia. These sediments represent deeply weathered continental sediments that have been redeposited from the interior of Gondwana to its margins (Romer and Kroner, 2016) and are slightly metal enriched (residual enrichment). The weathering is important as it affects the later melting behavior of these sedimentary rocks. The change from a passive to an active continental margin setting of the hyperextended Gondwana shelf results in the accretion and stacking of shelf sediments during the prolonged formation of Pangea. This tectonic accumulation locally resulted in metal redistribution within the former shelf sediments, leading to domains that are depleted or enriched. Such redistribution may lead to extreme Sn enrichments as in local metasedimentary sequences of the Erzgebirge (Bohemian Massif) that contain Sn-rich and cassiterite bearing peak metamorphic minerals, clearly showing prograde Sn enrichment. The distinctive feature of the Armorican spur of the Gondwana plate is its prolonged subduction accretion tectonics lasting from the early Devonian to the early Carboniferous. As the Armorican spur hosts the most important Variscan Sn deposits, we argue that metal enrichment in continental accretionary complexes is an essential step in the formation of tin specialized granitic melts.

Kroner, U. and R. L. Romer: Two plates - Many subduction zones: The Variscan orogeny reconsidered, Gondwana Research, 24/1, 298-329, https://doi.org/10.1016/j.gr.2013.03.001, 2013.

Romer, R. L. and U. Kroner: Phanerozoic tin and tungsten mineralization—Tectonic controls on the distribution of enriched protoliths and heat sources for crustal melting, Gondwana Research, 31, 60-95, https://doi.org/10.1016/j.gr.2015.11.002, 2016.

Romer, R. L. and U. Kroner: Provenance control on the distribution of endogenic Sn-W, Au, and U mineralization within the Gondwana-Laurussia plate boundary zone, New Developments in the Appalachian-Caledonian-Variscan Orogen, 25-46, https://doi.org/10.1130/2021.2554(02), 2022.