Petrology and Petrogenesis of the Monchique Alkaline Complex, Southern Portugal
Monchique is a sizeable subvolcanic ?laccolith, unusual among alkaline complexes in invading sediments and lacking apparent connection with either rifting or orogeny; it may however relate to the opening of the North Atlantic(age = 76 m.y.). The intrusion is predominantly miaskitic syenite, varying...
| Published in: | Journal of Petrology |
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| Main Author: | |
| Format: | Text |
| Language: | English |
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Oxford University Press
1978
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| Online Access: | http://petrology.oxfordjournals.org/cgi/content/short/19/2/171 https://doi.org/10.1093/petrology/19.2.171 |
| Summary: | Monchique is a sizeable subvolcanic ?laccolith, unusual among alkaline complexes in invading sediments and lacking apparent connection with either rifting or orogeny; it may however relate to the opening of the North Atlantic(age = 76 m.y.). The intrusion is predominantly miaskitic syenite, varying irregularly from foyaite to pulaskite with fine-grained nepheline-poor margins but showing no rhythmic or cryptic layering. Minor rock-types include early masses of olivine-free kaersutite—theralites (berondrites) and essexites, bodies of igneous breccias, malignite and agpaite, veins of foyaite—pegmatite and shonkinite, and dykes of lamprophyres and peralkaline tinguaite. Coeval dykes outside the main intrusion include quartz—trachytes, normal (olivine-bearing) basanites and amphibole—picrites. The whole suite may have derived from a basanitic parent under moderately oxidizing conditions. Geochemistry is apparently continuous along the trend Berondrite—Essexite—Foyaite (Malignite)—Pulaskite—Quartz trachyte. As far as Foyaite this parallels the normal oceanic basanite—phonolite trend 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 be explained neither by fractionation nor country-rock assimilation. The enigmatic pulaskites cannot be related directly to the foyaites but might have formed from the same parent under lower pressure conditions; they themselves fractionated to the peralkaline tinguaites. The quartz—trachytes probably originated where foyaite magmas lost alkalis to the siliceous country-rocks, became oversaturated, and then fractionated feldspars. Liquid immiscibility might explain some anomalous monzonitic rocks but otherwise contributed little to the evolution of the complex. |
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