Direct evidence of crustal contamination of mantle-derived alkaline magma in the Campos de Calatrava Volcanic Field (SW Spain)

Relatively low degrees of partial melting of the non-convecting subcontinental lithospheric mantle (SCLM) typically produces a low-silica melt enriched in magnesium, iron, and calcium. When in route towards the shallow crust, they may interact with rocks of the whole lithospheric column including the uppermost sections of the mantle and continental crust, inducing substantial modifications to its chemical composition. This interaction, characterized by chemical disequilibrium, usually results in assimilation through partial (or complete) melting and/or mineral reactions between the melt and the country rock. Numerous experimental studies have been conducted to characterize these processes; however, natural examples are also essential for elucidating them. A clear example of this crustal rock assimilation by mantle-derived basalts leading significant variations of chemistry is observed in the Morrón de Villamayor volcano, belonging to the Campos de Calatrava Volcanic Field (Ciudad Real, Spain).  This volcanic edifice originated ca. 7.4 million years ago is mainly composed by ultrapotassic alkali basalt (SiO₂ 39.87-40.89 wt% and K₂O 3.52–4.41 wt%) and consist of dark gray, hipocrystalline, inequigranular and medium-fine-grained volcanic rocks made up of large olivine phenocrysts (Fo=72.08–80.49) with and small clinopyroxene (diopside) microphenocrysts light green (Wo=50.18–53.29; En=44.91–46.38; Fs=1.78–3.42), surrounded of K-Na-rich feldspathoid microliths (leucite and nepheline), clinopyroxenes microliths and small inclusions of ilmenite and titanite. The presence of foids and the enrichment in sodium and potassium indicate that magmas were silica undersaturated basalt. These alkali basalts have abundant white quartzite (cortical) xenoliths, which shown mm to cm reaction rims. The rims are composed of zoned clinopyroxenes, the core of diopside (Wo= 50.13–51.74; En= 44.89–48.67; Fs= 0.30–4.96) with greenish Na-rich rims (aerigine-augite, Q= 71.06–86.31; Ae= 21.99–27.41; Jd= 0.89–1.55), Al-rich saponite (Al₂O₃ 9.38–12.74 wt%), quartz, carbonates, and potassium feldspars (sanidine). The reaction zone produces also olivine alteration by iddingsite (denoting the highly oxidizing character of the environment). In addition to the drastic mineralogical changes, the reaction zone is characterized by depletion in potassium and enrichment (oversaturation) in silica.

Funding
This research was supported by the Autonomous Community of Valencia through the CIAICO/2023/179 project.