MULTI-STAGE EXHUMATION OF AN ECLOGITE/BLUESCHIST IN THE COASTAL RANGE OF CHILE: A COMPARISON TO FRANCISCAN TECTONIC BLOCKS

A coarse crystalline eclogite/blueschist residual boulder from the Coastal Range of Chile (latitude 41^oS), contains replacement textures indicating retrograde metamorphism from eclogite (omphacite + garnet + hornblende) to blueschist (glaucophane + epidote + white mica) to greenschist conditions, defining an episodic exhumational PT path. Associated amphibolite/blueschist blocks contain cumulative petrologic, chronologic, and structural evidence consistent with this sequence. ⁴⁰Ar-³⁹Ar analyses on hornblende and white mica separates from a sample of retro-garnet amphibolite (blueschist), combined with mineral analyses of the eclogite-blueschist, suggest eclogite grade metamorphism (T=553 ± 30 ^oC, P>13.2 kb) prior to 361 Ma (late Devonian), followed by decreasing thermal gradient to blueschist grade between 325 Ma and 304 Ma (Carboniferous). Incipient foliation and growth of finer grained, lower greenschist minerals in the retro-eclogite sample suggests upward emplacement, with increasing geothermal gradient, into the regional lower greenschist grade complex by ≈ 285-210 Ma (Permian-Triassic), when the penetrative planar fabric of the complex was developed.

Although the metamorphism recorded in the Chilean eclogite/blueschist predates those of the Franciscan Complex by about 200 Ma, the tectonic sequence of subduction inception related to eclogite metamorphism, retrograde blueschist metamorphism, and later structural emplacement to higher crustal levels, aided by serpentinite diapirism, in a dextral transpressional setting, bears similarity to processes often proposed for tectonic blocks incorporated in sub-greenschist grade mélanges of the Franciscan. The presence of latest stage planar foliation and incipient greenschist overprint in some of the high grade Chilean blueschists, not evident in the Franciscan tectonic blocks, may indicate incorporation into a deeper crustal level than is presently exposed in the California Coastal Range.