Origin of Pyroxenite-Peridotite Veined Mantle by Refertilization Reactions: Evidence from the Ronda Peridotite (Southern Spain)

Abstract

The Ronda orogenic peridotite (southern Spain) contains a variety of pyroxene-rich rocks ranging from high-pressure garnet granulites and pyroxenites to low-pressure plagioclase^spinel websterites. The ‘’ part of the Ronda peridotite contains abundant layered websterites (‘group C’pyroxenites), without significant deformation, that occur as swarms of layers showing gradual modal transitionstowardstheirhost peridotites. Previousstudieshave suggested that these layered pyroxenites formed by the replacement of refractory spinel peridotites. Here, we present a major- and trace-element, and numerical modellingstudy ofa layered outcrop ofgroup C pyroxenite near the locality of Tolox aimed at constraining the origin of these pyroxenites after host peridotites by pervasive pyroxene-producing, refertilization melt^ rock reactions. Mg-number [¼ Mg/(Mg þ Fe) cationic ratio] numerical modelling shows that decreasing Mg-number with increasing pyroxene proportion, characteristic of Ronda group C pyroxenites, can be accounted for by a melt-consuming reaction resulting in the formation of mildly evolved, relatively low Mg-number melts (� 0� 65) provided that the melt fraction during reaction and the time-integrated melt/rock ratio are high enough (40� 1and41, respectively) to balance Mg^Fe buffering by peridotite minerals. This implies strong melt focusing caused by melt channelling in high-porosity domains resulting from compaction processes in a partial melted lithospheric domain below a solidus isotherm represented by the Ronda peridotite recrystallization front.The chondrite-normalized rare earth element (REE) patterns of group C whole-rocks and clinopyroxenes are convexupward. Numerical modeling of REE variations in clinopyroxene produced by a pyroxene-forming, melt-consuming reaction results in curved trajectories in the (Ce/Nd)N vs (Sm/Yb)N diagram (where N indicates chondrite normalized). Based on (Ce/Nd)N values, two transient, enriched domains between the light REE (LREE)depleted composition of the initial peridotite and that of the infiltrated melt may be distinguished in the reaction column: (1) a lower domain characterized by convex-upward REE patterns similar to those observed in Ronda group C pyroxenite^peridotite; (2) an upper domain characterized by melts with strongly LREE-enriched compositions.The latterare probably volatile-rich, small-volume melt fractions residual after the refertilization reactionsthatgeneratedgroup C pyroxenites, which migrated throughout the massifcincluding the unmelted lithospheric spinel-tectonite domain. The Ronda mantle domains affected by pyroxenite- and dunite- or harzburgite-forming reactions (the ‘layered granular’subdomain and ‘plagioclase-tectonite’domain) are on average morefertilethan the residual,‘coarsegranular’subdomain at the recrystallization front. This indicates that refertilization traces the moving boundaries of receding cooling of a thinned and partially melted subcontinental lithosphere. This refertilization process may be widespread during transient thinning and melting of depleted subcontinental lithospheric mantle above upwelling asthenospheric mantle.