Garnet variety and zircon ages in UHP meta-sedimentary rocks from the Jubrique zone (Alpujarride Complex, Betic Cordillera, Spain): evidence for a pre-Alpine emplacement of the Ronda peridotite

The Ronda peridotite is a group of lherzolite slabs (1.5 to 2km thick) in southern Spain. Despite clear evidence that pre-Alpine events affected pre-Permo-Triassic units from the Alboran domain (internal zone of the Betic-Rif Cordillera, Spain, and Morocco), numerous papers continue to emphasize Alpine metamorphic and structural evolution. Here, we evaluate the pre-Cenozoic evolution of the Ronda peridotite by reporting new petrographic and U-Pb SHRIMP zircon dating of meta-sedimentary rocks from the Jubrique zone (Alpujarride Complex, Betic Cordillera, Spain) directly overlying the Ronda peridotite. Field inspection and petrographical study revealed generalized migmatitic textures and a gradual transition mainly defined by garnet content (from ~30 to <3wt.%) and size (from 1.5cm to <0.5mm) in the overlying granulite-gneiss sequence, suggesting that most garnet grew as a consequence of the peridotite emplacement. Garnet shows notable variations in composition and inclusion types, which are interpreted as reflecting different stages of garnet growth. Diamond-bearing garnets are only well-preserved in gneisses from the uppermost part of the sequence, whereas the large garnets from rocks overlying the peridotite mainly record later thermal events. SHRIMP zircon dating indicates two age peaks at 330 +/- 9 and 265 +/- 4 Ma. The oldest age characterizes rims overgrowing detrital cores and reflects an early Hercynian metamorphism; the younger age characterizes zircon with magmatic oscillatory zoning, reflecting anatexis. On the basis of these data and of previous dating of monazite included in the large garnets, we conclude that the peridotite was emplaced either shortly before or during early Hercynian times, ~330 Ma.