Petrology and geochemistry of meta-ultramafic rocks in the Paleozoic Granjeno Schist, northeastern Mexico: Remnants of Pangaea ocean floor

The Granjeno Schist is a meta-volcanosedimentary upper Paleozoic complex in northeastern Mexico. We suggest different tectonic settings for metamorphism of its serpentinite and talc-bearing rocks based on petrographic and geochemical compositions. According to the REE ratios (La /Yb = 0.51 –20.0 and...

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Bibliographic Details
Published in:Open Geosciences 2017-01, Vol.9 (1), p.361-384
Main Authors: Torres-Sánchez, Sonia Alejandra, Augustsson, Carita, Jenchen, Uwe, Rafael Barboza-Gudiño, J., Alemán Gallardo, Eduardo, Ramírez Fernández, Juan Alonso, Torres-Sánchez, Darío, Abratis, Michael
Format: Article
Language:English
Subjects:
Chromium
Continental margins
Geochemistry
Gondwana
Granjeno Schist
Metamorphism
northeastern Mexico
Ocean floor
Paleozoic
Pangea
Petrology
Rocks
serpentinite
ultramafic rocks
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Summary:The Granjeno Schist is a meta-volcanosedimentary upper Paleozoic complex in northeastern Mexico. We suggest different tectonic settings for metamorphism of its serpentinite and talc-bearing rocks based on petrographic and geochemical compositions. According to the REE ratios (La /Yb = 0.51 –20.0 and La /Sm = 0.72–9.1) and the enrichment in the highly incompatible elements Cs (0.1 ppm), U (2.8 ppm), and Zr (60 ppm) as well as depletion in Ba (1 – 15 ppm), Sr (1 –184 ppm), Pb (0.1 –14 ppm), and Ce (0.1 –1.9 ppm) the rocks have mid-ocean ridge and subduction zones characteristics. The serpentinite contains Al-chromite, ferrian chromite and magnetite. The Al-chromite is characterized by Cr# of 0.48 to 0.55 suggesting a MORB origin, and Cr# of 0.93 to 1.00 for the ferrian chromite indicates a prograde metamorphism. We propose at least two serpentinization stages of lithospheric mantle for the ultramafic rock of the Granjeno Schist, (1) a first in an ocean-floor environment at sub-greenschist to greenschist facies conditions and (2) later a serpentinization phase related to the progressive replacement of spinel by ferrian chromite and magnetite at greenschist to low amphibolite facies conditions during regional metamorphism. The second serpentinization phase took place in an active continental margin during the Pennsylvanian. We propose that the origin of the ultramafic rocks is related to an obduction and accretional event at the western margin of Pangea.
ISSN:2391-5447
2391-5447
DOI:10.1515/geo-2017-0029