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Origin of Early Carboniferous pseudo‐adakites in northern Brittany (France) through massive amphibole fractionation from hydrous basalt
Author(s) -
Barboni Mélanie,
Bussy François,
Chiaradia Massimo
Publication year - 2011
Publication title -
terra nova
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.353
H-Index - 89
eISSN - 1365-3121
pISSN - 0954-4879
DOI - 10.1111/j.1365-3121.2010.00974.x
Subject(s) - adakite , geology , geochemistry , mafic , continental crust , basalt , amphibole , subduction , underplating , oceanic crust , context (archaeology) , crust , petrology , tectonics , paleontology , quartz
Terra Nova, 23, 1–10, 2011 Abstract The first Variscan pseudo‐adakites were identified in close association with the Saint‐Jean‐du‐Doigt (SJDD) mafic intrusion (Brittany, France) in a geodynamic context unrelated to subduction. These rocks are trondhjemites emplaced 347 ± 4 Ma ago as 2–3 km 2 bodies and dykes. Trace‐element concentrations and Sr–Nd–Pb isotope ratios indicate that the SJDD pseudo‐adakites probably resulted from extreme differentiation of an SJDD‐type hydrous basaltic magma in a lower continental crust of normal thickness (0.8 GPa). Modelling shows that garnet is not a required phase, which was commonly believed to be the case for continental arc‐derived adakite‐like rocks. A massive fractionation of amphibole fits the data much better and does not require high pressures, in agreement with the inferred extensional tectonic regime at the time of pluton emplacement. Alternatively, the SJDD pseudo‐adakites could have resulted from the melting of newly underplated SJDD mafic precursors, but thermal considerations lead us to believe that this was not the case.