Chemical mineralogy of the Monchique alkaline complex, southern Portugal

Abstract

Some 500 microprobe analyses from 43 rock samples, covering the entire range of major and minor intrusive rock-types, define the following ranges of mineral composition: plagioclases (An_70-30), alkali feldspars (Or_98-20 and Ab_90–99), olivines (Fo_85-82), clinopyroxenes (aluminous titanaugite through salite and aegirine-augite to acmite), amphiboles (subsilicic kaersutite through pargasite to hastingsite, edenite or katophorite), biotites (titanbiotite to titaniferous manganiferous lepidomelane). Varied discontinuous reaction relationships are evident petrographically between these minerals (e.g. amphibole overgrowths on pyroxenes or biotites), but most appear to reflect reequilibration during slow magmatic cooling, or perhaps local disequilibrium effects, and there is little evidence for significant subsolidus, hydrothermal or deuteric modification of the primary mineralogy.

Although these mineral ranges are reasonably typical of differentiated alkaline gabbroic-syenitic intrusions, Monchique also shows many unusual features: e.g. a restricted stability range for olivine, the absence of amphiboles from all rocks with 54<%SiO₂<58, a lack of alkali amphiboles corresponding to the acmite-rich pyroxenes, the presence of acmite-poor pyroxenes and aluminous biotites in peralkaline rocks, and irregular Ti variation in biotites. Mineral/ host-rock relationships also show peculiarities: e.g. Mg/ (Mg+Fe) ratios of mafic minerals and An contents of plagioclases increase as host-rock fractionation index (FI) increases from gabbroic (FI 30–40%) to alkali feldspar-bearing essexitic (FI c. 50) rocks. Thereafter, Mg ratios decrease only slightly, such that many malignites and miaskitic syenites (FI 70–90) carry pyroxenes and biotites which are no more, and sometimes less evolved than those in the gabbroic rocks (FI 30–40).

Such features confirm earlier suggestions from whole-rock geochemistry, that a major evolutionary process in the complex was a relatively discontinuous ‘segregation’ of already somewhat evolved essexitic magma into more basic and more felsic magmas, rather than an incremental gabbro→syenite crystal fractionation. They also confirm that the complex represents neither a single in situ differentiating magma pulse, nor a series of simply related pulses, but an irregular and largely irresolvable juxtaposition of magma-batches of widely varying compositions and evolutionary histories.