Eclogite boudins in a Sierra de los Filabres road cut
A road cut at 1,200 m in the Sierra de los Filabres (Betic Cordillera, Almería) exposes a 60 m section. At the base, serpentinite with antigorite and talc-carbonate veins passes upward into garnet-mica schist containing metre-scale eclogite boudins. The schist foliation strikes N30°E and dips 45°NW; boudin long axes lie within the foliation, and the foliation wraps the boudins. The eclogite contains garnet, omphacite and rutile, with no primary plagioclase; retrograde chlorite and actinolite form symplectites around omphacite. A 19 Ma calc-alkaline dyke cuts both foliation and boudins but is truncated by an unconformity. Above the unconformity, Tortonian conglomerate contains schist and eclogite clasts. A student argues that the eclogite proves Alpine subduction; a colleague says crustal thickening alone can produce high-pressure rocks. You have one field season and a petrology budget.
1. Establish the relative chronology of eclogite metamorphism, boudinage, schist foliation, dyke intrusion, erosion and conglomerate deposition. State which relationships are definitive and which are ambiguous.
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Definitive relationships: the dyke cuts both foliation and boudins, so the dyke postdates both; the unconformity truncates the dyke, so erosion and Tortonian conglomerate deposition are youngest; conglomerate clasts include schist and eclogite, confirming exhumation before deposition. Because the foliation wraps the boudins and boudin long axes lie in the foliation, boudinage was syn- to late-foliation, not a later brittle event. The ambiguous point is whether the eclogite-facies peak preceded or accompanied foliation. If garnet porphyroblasts contain inclusion trails continuous with the external foliation, peak high-P metamorphism was pre- to syn-foliation. If garnet grew post-kinematically and is wrapped by a later fabric, the peak may postdate early foliation. The 19 Ma dyke gives only a minimum age for eclogite and foliation. A defensible sequence is: eclogite-facies peak (pre- or syn-foliation) → boudinage during continuing extension → foliation development and retrogression → dyke at 19 Ma → uplift, erosion and Tortonian conglomerate. Petrography is needed to resolve peak versus fabric timing.
2. A colleague argues that crustal thickening alone can produce the eclogite. Assess this claim using the mineral assemblage, geothermal gradient and geochronology. Which two analyses would you prioritise?
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Garnet + omphacite + rutile without primary plagioclase indicates pressures of roughly 1.5–2.0 GPa and temperatures near 500–600 °C, corresponding to a low geothermal gradient of about 5–10 °C/km. Crustal thickening alone normally produces Barrovian sequences at 25–35 °C/km; at 1.5 GPa that would give temperatures closer to 700–800 °C unless burial was extremely rapid and followed by unusual cooling. The association with serpentinite and with lawsonite- or glaucophane-bearing rocks elsewhere in the Nevado-Filábre complex supports a subduction setting. However, high pressure alone is not proof: a cold, thick arc root or rapid burial in a collision zone can also generate eclogite. The decisive evidence is a cold, clockwise pressure–temperature–time path, ideally with isothermal decompression. Prioritise garnet Lu–Hf or Sm–Nd geochronology to date prograde growth, and pseudosection modelling using measured mineral compositions and inclusions to obtain peak pressure–temperature conditions and the shape of the path. Zircon U–Pb and trace-element analyses can date later fluid or retrograde events. If the peak age is Alpine and the path is cold, subduction is strongly supported.