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Título

ATOLL GARNET FORMATION IN THE ANDEAN HIGH-PRESSURE RASPAS COMPLEX (SW ECUADOR)

Texto do resumo

Uncommon features, atoll microstructures consist of an idioblastic mineral ring such as garnet surrounding a mixture of other phases and/or island-like poikilitic garnet fractions. Despite their rare occurrence, they have been reported in a wide variety of rocks, including metamorphic rocks under blueschist and eclogite facies conditions. Atoll formation has been attributed to preferential dissolution of cores by fluid infiltration, polymetamorphism, subgrain coalescence, and/or kinetic control associated with rapid growth. As garnet is a key mineral in reconstructing metamorphic trajectories, the characterisation of the different occurrences in a lithotype can provide valuable information on processes and stages of metamorphism.
In the context of the high-pressure Raspas Complex (SW Ecuador), atoll garnets are present in eclogites and blueschists, thus possibly recording their subduction and exhumation history. In this work, we present the microstructural and chemical characterisation of garnet grains from a blueschist, in order to investigate its formation and evolution. The rock is composed of amphibole (34 vol. %; mainly glaucophane, barroisite and winchite), garnet (17%), omphacite (15%), white mica (10%), and quartz (6%). Minor occurrences include rutile (4%), apatite (4%), titanite (3%), zoisite (3%), opaque (2%), feldspar (2%) and chlorite (<1%). The orientation of amphibole and clinopyroxene define the medium-grained foliation. White mica is either paragonite (mostly oriented in the matrix) or phengite (coarser, oriented in the matrix, decussated and in atoll grains).
Garnet porphyroblasts occur as both normal (60%) and atoll (40%) crystals. Normal crystals (0.2-1.2mm) display inclusions of the whole mineral assemblage, mostly in the cores and mantles, defining an internal foliation (Sn-1) that is oblique to the main foliation in the matrix. Small inclusion-free grains are also present. Atoll grains (0.2-1.6mm) are either: partially preserved often fragmented rims with mostly obliterated cores; or remnants of cores, mantles and/or rims, difficult to distinguish due to their strong replacement. Quartz, phengite, amphibole and paragonite are the main replacement phases, which display the same mineral inclusions and chemical compositions as their corresponding matrix occurrences, thus suggesting a coeval formation. Garnet compositional zoning is mostly truncated by the dissolution of cores, as to form the atoll features. In the better preserved grains, spessartine and grossular contents decrease while almandine and pyrope increase from cores (Alm54-57Grs24-29Prp14-16Sps2-4) to rims (Alm58-62Grs14-20Prp18-23Sps1-2). Several grains present more complex variations due to greater replacement during atoll formation.
The formation of the atoll microstructure is understood as a product of fluid-related resorption, possibly facilitated by the presence of microveins and cracks in garnet, which weakened the original grains. This led to element exchange between their dissolving interiors and inclusions and the matrix, resulting in the replacement of the cores with hydrous phases and/or quartz and the re-equilibration of peak assemblage minerals, likely during the exhumation of the complex, given the required amount of fluid for atoll formation. The fluid origin, conditions and full trajectory of the blueschist remains to be further constrained by metamorphic modelling and fluid characterisation.

Palavras Chave

Blueschist; Petrology; Garnet zoning; Compositional mapping; Fluid infiltration