Low–Ti Continental Tholeiite Origin of Magmas With Calc‐Alkaline Signature in Transcurrent Settings: The Mississippian Matachel Volcanic Field (SW Iberian Massif)

Abstract

AbstractIn the Mississippian Matachel small volcanic field of the Ossa‐Morena Zone (southern Iberian Massif) outpoured basic‐intermediate lavas exhibit geochemical characteristics of Low‐Ti continental tholeiites and calc‐alkaline lavas. Low‐Ti continental tholeiites integrate two contrasting groups of rocks: basalts (Mg#: 54 to 70; Ti/Zr: 61–79; LaN/LuN: 1.6–2.9; εNdi: +4.0–+6.6; “Group #1”), and basalts and basaltic andesites (Mg#: 43 to 66; Ti/Zr: 36–58; LaN/LuN: 2.5–5.9; εNdi: −0.2–+3.5; “Group #2”). Primitive Group #1 tholeiitic magmas were generated by partial melting of a garnet‐free lherzolite from an enriched lithospheric mantle, near the lithosphere‐asthenosphere thermal boundary layer (with a very limited asthenosphere melting input). Progressive interaction of these magmas with crustal alkali igneous rocks resulted in the formation of the petrological evolutionary trends observed, to a larger extent in the case of Group #2 Low‐Ti tholeiites. Further assimilation of amphibole‐rich calc‐alkaline metaigneous rocks might have originated the basalts and basaltic andesites with calc‐alkaline signature (Mg#: 33 to 56; Ti/Zr: 25–78; LaN/LuN: 2.0–5.6; εNdi: +2.8–+4.8). These exhibit a “Cordilleran‐type” evolutionary trend, though are unrelated to plate convergence. The magmas with calc‐alkaline signature attest to a closed‐system differentiation process controlled by the fractionation of plagioclase, clinopyroxene, magnetite and ilmenite. It is proposed that Mississippian lithospheric‐scale intra‐continental wrenching, unrelated to coeval mantle plume upwelling, reworked complexly docked mantle domains and triggered mantle melting. Enduring mid‐upper crustal processes (magma storage in mid‐crustal chambers and crustal assimilation) likely shaped the latest petrologic and geochemical aspects of the Matachel Low‐Ti tholeiites and related rocks with calc‐alkaline signature.