Journal of Petrology Advance Access originally published online on April 10, 2008
Journal of Petrology 2008 49(5):999-1025; doi:10.1093/petrology/egn014
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Origin of Pyroxenite–Peridotite Veined Mantle by Refertilization Reactions: Evidence from the Ronda Peridotite (Southern Spain)
1Géosciences Montpellier, Cnrs-Université De Montpellier II, CC 60, Place E. Bataillon, 34095 Montpellier, France
2Instituto Andaluz De Ciencias De La Tierra (IACT), CSIC & Universidad de Granada, Facultad De Ciencias, 18002 Granada, Spain
RECEIVED FEBRUARY 10, 2007; ACCEPTED FEBRUARY 19, 2008
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Abstract |
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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 ‘asthenospherized’ part of the Ronda peridotite contains abundant layered websterites (‘group C’ pyroxenites), without significant deformation, that occur as swarms of layers showing gradual modal transitions towards their host peridotites. Previous studies have suggested that these layered pyroxenites formed by the replacement of refractory spinel peridotites. Here, we present a major- and trace-element, and numerical modelling study of a layered outcrop of group 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 [= Mg/(Mg + 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 (>0·1 and > 1, 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 convex-upward. 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 latter are probably volatile-rich, small-volume melt fractions residual after the refertilization reactions that generated group C pyroxenites, which migrated throughout the massif—including 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 more fertile than the residual, ‘coarse granular’ 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.
KEY WORDS: subcontinental mantle; refertilization; pyroxenite; peridotite; websterite; melt–rock reaction; plagioclase; trace elements
*Corresponding author. Telephone: +34 958 240 473. Fax: +34 958 243 368. E-mail: carlosg{at}ugr.es