DO HIGH-GRADE BLOCKS RECORD THE COOLING OF THE FRANCISCAN SUBDUCTION ZONE?
The ages and petrological context of zircon in one of the oldest known Franciscan eclogite/amphibolite blocks demonstrate long residence under metamorphic conditions beginning with the earliest known Franciscan blueschist assemblage. Zircon in garnet porphyroblasts is ~176 Ma and has eclogitic trace element signatures, whereas zircon in the matrix of Na-Ca-amphibole and relict omphacite is ~160 Ma and lacks those signatures. Significant dispersion within the two age populations suggests that the zircon evolved over tens of m.y. Garnet and amphibole show complex, intertwined growth histories that support this interpretation. Garnet cores contain the blueschist assemblage: Gln + Ab + Chl + Phg + Qtz + Rt + Ap ± Kfs ± Ttn. Complex mantles and rims that contain the older population of zircon surround the cores. Matrix amphibole is clearly later than most of the garnet growth and replaces some of the omphacite. Rare inclusions of Na-Ca amphibole occur only in the outermost parts of the porphyroblasts, but small euhedral garnets are included in and crystallized with large amphibole grains. Late garnet veins cut the amphibole foliation. The youngest assemblage in the matrix is Gln rims on Na-Ca amphibole associated with patches of Phg +Chl +Ttn. Thus, the block records prograde blueschist (>176 Ma), peak eclogite (~176 Ma), later garnet barroisite amphibolite (~160 Ma) and retrograde blueschist (<160 Ma) conditions.
The mélange carrying this block was emplaced against Jd-bearing metasandstone that is younger than 112 Ma, so the block spent at least 64 m.y. in the metamorphic realm of the subduction zone. Blueschist-forming conditions existed throughout the evolution of subduction zone and therefore the high-grade blocks likely record a thermal history much more complicated than a single stage of cooling following subduction initiation.