Open AccessA 19 to 17 Ma amagmatic extension event at the Mid‐Atlantic Ridge: Ultramafic mylonites from the Vema Lithospheric Section
Author(s) -
Cipriani Anna,
Bonatti Enrico,
Seyler Monique,
Brueckner Hannes K.,
Brunelli Daniele,
Dallai Luigi,
Hemming Sidney R.,
Ligi Marco,
Ottolini Luisa,
Turrin Brent D.
Publication year - 2009
Publication title -
geochemistry, geophysics, geosystems
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.928
H-Index - 136
ISSN - 1525-2027
DOI - 10.1029/2009gc002534
Subject(s) - geology , mylonite , amphibole , mid atlantic ridge , mantle (geology) , lithosphere , peridotite , geochemistry , seafloor spreading , paleontology , ridge , shear zone , quartz , tectonics
A >300 km long lithospheric section (Vema Lithospheric Section or VLS) is exposed south of the Vema transform at 11°N in the Atlantic. It is oriented along a seafloor spreading flow line and represents ∼26 Ma of accretion at a single 80 km long segment (EMAR) of the Mid‐Atlantic Ridge. The basal part of the VLS exposes a mantle unit made mostly of relatively undeformed coarse‐grained/porphyroclastic peridotites that were sampled at close intervals. Strongly deformed mylonitic peridotites were found at 14 contiguous sites within a ∼80 km stretch (∼4.7 Ma interval); they are dominant in a time interval of 1.4 Ma, from crustal ages of 16.8 to 18.2 Ma (mylonitic stretch). Some of the mylonites are “dry,” showing anhydrous high‐T deformation, but most contain amphibole. The mylonitic peridotites tend to be less depleted than the porphyroclastic peridotites on the basis of mineral major and trace elements composition, suggesting that the mylonites parent was a subridge mantle that underwent a relatively low degree of melting. The Sr, Nd, and O isotopic composition of the amphiboles is MORB‐like and suggests either that seawater did not contribute to their isotopic signature or that their isotopic ratios re‐equilibrated during fluid circulation in the upper mantle. Four 40 Ar/ 39 Ar ages, on three amphiboles separated from the peridotites, are close to crustal ages predicted from magnetic anomalies, confirming that the amphiboles formed close to ridge axis. We propose that crustal accretion at the EMAR segment has been mostly symmetrical for the 26 Ma of its recorded history, except for the ∼1.4 Ma interval of prevalent ultramafic mylonites (mylonitic stretch) that may record a period of quasi‐amagmatic asymmetric accretion of oceanic lithosphere close to the ridge–Vema transform intersection, possibly with development of detachment faults. This interval may correspond to a thermal minimum of the subridge upwelling mantle, marking the transition from a period of decreasing to one of increasing mantle melting below the EMAR segment.