Geochemical evolution of high-pressure mafic granulites from the Bacariza formation (Cabo Ortegal complex, NW Spain): an example of a heterogeneous lower crust

Abstract

The Cabo Ortegal complex (northwestern Iberian massif ) is a klippen formed of several structural units stacked during the Hercynian collision. All these units include ultramafic rocks, metabasites and quartzfeldspathic gneisses affected by different metamorphic conditions. The Bacariza formation is heterogeneous showing a conspicuous layering mainly defined by alternate high-pressure ultrabasic-to-basic granulites, retrogressed garnet amphibolites of intermediate composition and rare acid rocks forming garnet trondhjemitic gneisses. This layering is inherited from a gabbroic protolith showing a composition rich in Fe and Ti. Major and trace elements of these rocks can be correlated to continental tholeiitic series of extensional settings. These high-pressure granulites are situated in normal contact between$serpentinised ultramafic rocks and other high-grade metabasites with lessevolved and more-depleted composition comparable to T-type and N-type MORB. It is suggested that the layered gabbro-type protolith was part of a continuous mafic crust. This crust was initially formed during Early Ordovician in a continental extensional setting and progressively evolved to oceanic spreading.

Tectonic framework of the Cabo Ortegal Complex: A slab of lower crust exhumed in the Variscan orogen (northwestern Iberian Peninsula)

“…Moreno (1999) interpreted this ultramaµc-layered complex as the root of an oceanic arc similar to the Himalayan Jijal Complex. The contact with the overlying maµc rock unit (II) is gradational (Galán and Marcos, 1997).…”

“…Some of these rocks lack Pl in equilibrium with Grt-Cpx and others show relict Ky and/or Zo, suggesting an earlier eclogite facies metamorphism (Galán and Marcos, 2000). Their composition is essentially ultrabasic-basic and equivalent to Fe-Ti rich (picro) basalts-(mela) gabbros (Galán and Marcos, 1997). In the middle member (400 m thick), these rocks alternate with garnet amphibolites (Grt, Hbl, Pl, Qtz, ± Bi, Ilm, Ttn, Czo, Ep, ± Aln) retrogressed from granulites, the composition of which is basic to intermediate.…”

“…This sequence is considered the relic of an earlier thinned lower crust (probably with a portion of lithospheric mantle) (Galán and Marcos, 1997), although some authors considered some of the maµc rocks related to a volcanic arc close to a continental margin. Deformation started in shear zones under eclogite facies conditions, but was reequilibrated during their exhumation under retrograde granulite and amphibolite facies conditions.…”

Insights on high-grade deformation in quartzo-feldspathic gneisses during the early Variscan exhumation of the Cabo Ortegal nappe, NW Iberia

“…This could point to the presence of an early Ordovician metamorphic event similar to the one described by Abati et al (1999) in equivalent units of the Órdenes complex. Such a pre-Variscan event would be consistent with the presence of coronitic textures in the Cabo Ortegal metagabbros (Galán and Marcos, 1997;Marcos et al, 2002). However, to be able to tie these metastable early Ordovician monazites from Cabo Ortegal to a particular set of metamorphic conditions remains to be conclusively demonstrated.…”

The Corredoiras orthogneiss (NW Iberian Massif): Geochemistry and geochronology of the Paleozoic magmatic suite developed in a peri-Gondwanan arc

“…The ages of magmatism vary between 520 and 500 Ma (Abati et al, 1999Castifieiras et al, 2010;Fernandez Suarez et al, 2007;Ord6fiez Dsado, 1998;Santos et al, 2002). There are a few studies on the geo chemistry of the igneous rod{s in the HP-HT units (Galan and Marcos, 1997;Gillbarguchi et al, 1990;Mendia Aranguren, 2000) and the mafic rocks in the IP units ; however, no geo chemical study has been carried out nowadays in the felsic rocks from the IP upper units.…”