Antecrysts and their effect on rock compositions: The Cretaceous lamprophyre suite in the Catalonian Coastal Ranges (NE Spain)

Abstract

Cretaceous lamprophyres in the Catalonian Coastal Ranges are isolated, cm- to m-thick intrusions of camptonite, exceptionally rich in crystal populations. Most lamprophyres are strongly porphyritic, with large (up to cm) crystals of clinopyroxene, amphibole, olivine pseudomorphs and opaque minerals embedded in a microcrystalline groundmass primarily composed of amphibole and feldspars. Detailed petrography and chemical analysis of crystal populations reveals complex compositional zoning patterns in large crystals, most of which are antecrysts recycled from earlier stages of the magmatic system at depth. The growth stratigraphy of clinopyroxene and amphibole antecrysts requires an intricate succession of magmatic processes in the plumbing system, including magma recharge, magma mixing and polybaric fractionation during magma ascent and emplacement. In particular, significant leaps in MgO with crystal growth indicate events of recharge of more primitive magma into the system. The groundmass composition is similar in the whole lamprophyre suite, but the accumulation of antecrysts triggers whole rock enrichments in MgO (up to 14. wt.%) and compatible elements that could be erroneously interpreted as parental magma compositions. This way, the variable entrainment of antecrysts in ascending lamprophyre magmas modulates whole rock compositional variability in the lamprophyre suite. The present study widens our previous results from one of the lamprophyres and shows that i) much information on magmatic processes may be derived from detailed mineral chemistry in addition to whole rock investigations, and ii) interpretation of whole rock evolutionary trends should be preceded by a close textural understanding of the rocks and their crystal cargo.