Petrology and Petrogenesis of the Monchique Alkaline Complex, Southern Portugal

Monchique is a sizeable subvolcanic ?laccolith, unusual amongalkaline complexes in invading sediments and lacking apparentconnection with either rifting or orogeny; it may however relateto the opening of the North Atlantic(age = 76 m.y.). The intrusionis predominantly miaskitic syenite, varying irregularly fromfoyaite to pulaskite with fine-grained nepheline-poor marginsbut showing no rhythmic or cryptic layering. Minor rock-typesinclude early masses of olivine-free kaersutite—theralites(berondrites) and essexites, bodies of igneous breccias, maligniteand agpaite, veins of foyaite—pegmatite and shonkinite,and dykes of lamprophyres and peralkaline tinguaite. Coevaldykes outside the main intrusion include quartz—trachytes,normal (olivine-bearing) basanites and amphibole—picrites. The whole suite may have derived from a basanitic parent undermoderately oxidizing conditions. Geochemistry is apparentlycontinuous along the trend Berondrite—Essexite—Foyaite(Malignite)—Pulaskite—Quartz trachyte. As far asFoyaite this parallels the normal oceanic basanite—phonolitetrend with major and trace elements and minerals (except olivine)behaving as in normal fractionation; the absence of nepheline—monzonites,creating a Daly Gap, may merely reflect high fractionation efficiency.The apparent evolution across the thermal barrier in Ne—Ks—Qtz,with a reversal in some major element trends, however, can beexplained neither by fractionation nor country-rock assimilation.The enigmatic pulaskites cannot be related directly to the foyaitesbut might have formed from the same parent under lower pressureconditions; they themselves fractionated to the peralkalinetinguaites. The quartz—trachytes probably originated wherefoyaite magmas lost alkalis to the siliceous country-rocks,became oversaturated, and then fractionated feldspars. Liquidimmiscibility might explain some anomalous monzonitic rocksbut otherwise contributed little to the evolution of the complex.