1.a High-pressure facies series

Tectonic melange, Chama, Venezuela
Image Credit: Dorothee Mertmann
Eclogite: abundant garnet, with pale green omphacitic pyroxene.
Image Credit: Yardley et al. (1990)

Metamorphism along low geothermal gradients results in a series of rocks that pass through the zeolite, prehnite-pumpellyite, blueschist, and eclogite facies of regional metamorphism. This is the classic case in the metamorphic conditions encountered in accretionary wedges. The best-studied example of this type of metamorphism occurs within the Cretaceous Franciscan Complex of California. The Franciscan Complex contains highly folded and faulted blocks and slabs of both unmetamorphosed or weakly metamorphosed rocks, and smaller tectonic blocks bounded by faults. It is structurally extremely complex, so that continuous structures can be mapped over large areas only in a few places ("coherent terranes"). Between the coherent terranes, mélange units occur which may extend for hundreds of kilometers along strike and reach as much as tens of kilometers in width. Melange is the term for a chaotic terrane of tectonically affected rocks. Non- or weakly metamorphosed rocks typically found in the melanges are mostly of oceanic or deep-water continental-margin association, such as pillow basalts, shales, radiolarian cherts, and limestones, along with clastic sedimentary rocks, typically found in oceanic trenches, such as turbiditic litharenites. All these rock types may be metamorphosed in the zeolite, prehnite-pumpellyite, blueschist, and eclogite facies field. The intimate association of sheared serpentinite bodies (rocks formed by hydratation of ultramafic rocks from different sources in one or more environments) with melange and the common occurrences of serpentinite in the sheared matrix strongly suggest that the ductile, rheologically weak serpentinite may play a role in the origin and emplacement of melange.

P-T paths at Convergent Plate Margins, showing a prograde high pressures metamorphism, from blueschist to eclogite facies. Source: MBG.

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