Petrogenesis of a zirconolite-bearing Mediterranean-type lamproite from the Peruvian Altiplano (Andean Cordillera)

Phlogopite lamproites occur in the Inner Arc Tectono-Magmatic Domain of the Eastern Altiplano (southern Peru, Andean Cordillera), a geotectonic setting characterizing lamproites at destructive plate margins. One sample free from contamination by crustal felsic melts displays many diagnostic features of <<Mediterranean>> lamproites as regards mineral assemblages and the bulk-rock composition [56.6 Wt-% SiO2, 12.9 wt.% Al2O3, K2O/Al2O3 < 0.8, MgO < 8 wt.%, CaO < 3.5 wt.%, very strong LILE enrichment (La/Yb-N = 27) and Sr, Nb, Ta and Ti negative anomalies (Sr/Nd-N = 0.49, La/Nb-N = 0.38)]. In thin sections, the rock shows a volumetrically dominant fluidal groundmass (82 vol.%) enclosing 1-3 mm olivine-rich polycrystalline aggregates [Mg-Ni-rich olivine, (88 < Mg# < 91), phlogopite (81 < Mg# < 88), Al-poor enstatite (70 < Mg# < 76) and a sanidine + phlogopite + apatite-bearing glassy mesostasis] and a few percent of mafic xenocrysts (olivine, phlogopite, enstatite). The crystallization order and chemical trends in groundmass minerals fit the low-pressure liquid line of descent of phlogopite (silica-rich) lamproites quite well. However, the limited decrease of Al2O3 at increasing FeO and TiO2 and the lack of tetraferriphlogopite end-members in the phlogopite, coupled with the high modal contents (32%) of Ba-rich K-Na sanidine, the augite composition and the late-stage crystallization of Ti-Zr-rich oxides (ilmenite, zirconolite) instead of wadeite, priderite, jeppeite, all argue for Al-Si-rich parental melt, coupled with low water and oxygen activities compared to intraplate lamproites. No simple genetic model (cognate inclusions, mantle xenoliths or xenocrysts) can explain the olivine-rich polycrystalline aggregates nor the mafic xenocrysts. These minerals are tentatively interpreted as liquidus phases from a deeper, olivine-richer lamproite en route to the surface that incompletely mixed up with the phlogopite lamproite. (C) 2003 Elsevier Science B.V. All rights reserved.