Petrofabric of forsterite marbles and related rocks from a low-pressure metamorphic terrain (Almadén de la Plata massif, Ossa-Morena Zone, SW Spain) and its kinematic interpretation

Autor: J.I. Gil Ibarguchi, P. Puelles, S. Fernández-Armas, Benito Ábalos
Rok vydání: 2018
Předmět:
Zdroj: Journal of Structural Geology. 117:58-80
ISSN: 0191-8141
DOI: 10.1016/j.jsg.2018.09.006
Popis: A fabric study of forsterite marble and calc-silicate rock paragenetic minerals is presented for samples collected in the Almaden de la Plata massif of SW Spain, that experienced Variscan low-pressure metamorphism related to the tectonic evolution of the suture between the Ossa-Morena and South-Portuguese zones of the Iberian Massif. Two tectonically stacked metamorphic units contain marble, metacarbonate and variably impure dolomitic rocks that record contrasting thermal conditions of the upper greenschist (structuraly upper unit) and the upper amphibolite/granulite facies (structuraly lower unit). Different reactional paths lead to formation of forsterite marbles in both units. Dolomite and calcite petrofabrics disclose their contrasting rheological behaviour during syn-metamorphic ductile deformation under temperatures between 500 and 700 °C. Dolomite formed (harder) porphyroclast systems and boudinaged bands, whereas weaker calcite underwent dynamic recrystallization. In both cases deformation was facilitated by activation of various intracrystalline slip systems, complemented by other deformation mechanisms. The fabrics of other paragenetic metamorphic minerals also denote crystal plasticity and exhibit close geometrical relationships with respect to the macroscopic foliation and mineral/stretching lineation, including a systematic asymmetry suggestive of non-coaxial deformation components. Foliations and lineations are interpreted as solid-state flow planes and directions bearing a kinematic significance. In particular, regional sinistral shear at mid-deep crustal realms coeval with low-pressure metamorphism explains the imbrication of metamorphic units and their exhumation, concomitant with the overall wrench tectonic framework ruling the tectonic evolution of the Ossa-Morena/South-Portuguese suture between the late Devonian (∼360 Ma) and the latest Carboniferous (∼300 Ma).
Databáze: OpenAIRE
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