3.3. Retrograde metamorphic mineral assemblages
Metamorphic overprint is variably developed and depends on the intensity
of the retrograde deformation. In less deformed rocks, it is defined by
formation of coronitic shells of orthopyroxene ± Fe-oxide around olivine
and clinopyroxene + spinel and calcic amphibole + spinel symplectites
between olivine and plagioclase. These replacement microstructures
record subsolidus cooling in granulite and amphibolite-facies
metamorphic conditions. However, retrograde metamorphism and hydrous
assemblages becomes more pervasive structurally downward the Rio Boba
plutonic sequence, i.e. toward the Jobito basal detachment zone. In
these 250 m-thick lower structural levels, retrograde metamorphism is
related to development of a network of amphibolite to upper
greenschist-facies mylonitic shear zones and veins, where pyroxene is
extensively replaced by green-brown and green calcic amphibole and
plagioclase by epidote/clinozoisite, albite and chlorite. In the shear
zones, the gabbronorites have been completely recrystallized and
transformed into amphibolites, characterized by a well-developed
penetrative plane-linear fabric. This metamorphic fabric is sub-parallel
to the foliation in the Jobito amphibolites (91-85 Ma), the magmatic
foliation in the La Manaclá suite of hornblende gabbro-diorite-tonalite
(89-83 Ma), the Jobito basal detachment zone (75-71 Ma), and the
elongation of the lenticular bodies of sheared and pervasive
serpentinized peridotites, suggesting that it is a late feature (Fig.
1c; Escuder-Viruete et al., 2013a; Escuder-Viruete & Castillo-Carrión,
2016).