Abstract. The Sobrado unit, within the upper part of the Órdenes Complex (NW
Spain) represents an allochthonous tectonic slice of exhumed high-grade
metamorphic rocks formed during a complex sequence of orogenic processes in
the middle to lower crust. In order to constrain those processes, U–Pb
geochronology and rare-earth element (REE) analyses of accessory minerals in migmatitic
paragneiss (monazite, zircon) and mylonitic amphibolites (titanite) were
conducted using laser ablation
split stream inductively coupled plasma mass spectrometry (LASS-ICP-MS). The youngest metamorphic zircon age obtained
coincides with a Middle Devonian concordia monazite age (∼380 Ma)
and is interpreted to represent the minimum age of the Sobrado high-P
granulite facies metamorphism that occurred during the early stages of the
Variscan orogeny. Metamorphic titanite from the mylonitic amphibolites
yield a Late Devonian age (∼365 Ma) and track the progressive
exhumation of the Sobrado unit. In zircon, cathodoluminescence images and
REE analyses allow two aliquots with different origins in the paragneiss to
be distinguished. An Early Ordovician age (∼490 Ma) was
obtained for metamorphic zircons, although with a large dispersion, related
to the evolution of the rock. This age is considered to mark the onset of
granulite facies metamorphism in the Sobrado unit under intermediate-P
conditions, and related to intrusive magmatism and coeval burial in a
magmatic arc setting. A maximum depositional age for the Sobrado unit is
established in the late Cambrian (∼511 Ma). The zircon
dataset also record several inherited populations. The youngest cogenetic
set of zircons yields crystallization ages of 546 and 526 Ma which are thought
to be related to the peri-Gondwanan magmatic arc. The additional presence of
inherited zircons older than 1000 Ma is interpreted as suggesting a West
African Craton provenance.
Unraveling the origins and P-T-t evolution of the allochthonous Sobrado unit (Órdenes Complex, NW Spain) using combined U–Pb titanite, monazite and zircon geochronology and rare-earth element (REE) geochemistry