Cr-Zr-Armalcolite-Bearing Lamproites of Cancarix, SE Spain

Simona Contini, Gianpiero Venturelli, Lorenzo Toscani, Silvio Capedri and Mario Barbieri
Istituto di Petrografia Università, viale delle Scienze, 43100 Parma, Italy
Istituto di Mineralogia e Petrologia Università, via S. Eufemia, 41100 Modena, Italy
Dipartimento di Scienze della Terra, Università ‘La Sapienza’, Piazzale A. Moro, 00185 Roma, Italy

Abstract: Lamproites with high MgO, high SiO2 affinity are abundant only in SE Spain where the Cancarix outcrop (in the province of Albacete) occurs. The rocks of Cancarix are peralkaline, saturated to oversaturated in silica, very high in K2O, Th and LREE. The mineralogy and petrography show some variations which depend on the conditions of emplacement and rate of cooling of the magma. The following phases may be present: olivine, phlogopite, K-amphibole, clinopyroxene, sanidine, orthopyroxene, apatite, and, in minor amounts, Cr-spinel, minerals of the pseudobrookite group, ilmenite, roedderite, dalyite, carbonate, analcime (probably pseudomorphous on leucite), a silica phase, rutile, pyrochlore (?), britholite (?) and glass. The lamproites of Cancarix contain Cr-Zr-armalcolite, which is typical of lunar basalts and which has been found also in kimberlites. The composition of early magmatic spinel and the occurrence of Cr-Zr-armalcolite indicate low oxygen fugacity for the primitive magma and related mantle source, in agreement with recent experimental data on lamproitic systems. The redox conditions changed during crystallisation, leading to increase of the Fe3+/Fe2+ ratio in the system. During the later stages of crystallisation, the residual melts/fluids were depleted in alumina and enriched in several components, e.g. Na, Zr, Fe, REE, Cl, etc. stabilising Na-Fe-clinopyroxene, dalyte, arfvedsonitic rims around K-richterite and other alumina-free phases which, on a chemical basis, have been identified as britholite and pyrochlore. Rough comparison with experimental systems and the geochemistry of the rocks suggests that the magma was generated at shallow depth (≪20 kbar) in the lithospheric mantle, which, after early depletion, underwent strong enrichment in many ‘incompatible’ elements.

Keywords: lamproitic magma • oxygen fugacity • crystallisation • Cancarix • SE Spain

Mineralogical Magazine; June 1993 v. 57; no. 387; p. 203-216; DOI: 10.1180/minmag.1993.057.387.02
© 1993, The Mineralogical Society
Mineralogical Society (