U–Pb zircon dating of the Gruf Complex: disclosing the late Variscan granulitic lower crust of Europe stranded in the Central Alps

Abstract

Permian granulites associated with noritic intrusions and websterites are a common feature of the post-Variscan European crust. Such granulites are common in the Southern Alps (e.g. Ivrea Zone), but occur only in the Gruf Complex in the Central Alps. To understand the geotectonic significance of these granulites, in particular in the context of Alpine migmatisation, zircons from 15 highgrade samples have been U-Pb dated by SHRIMP II analysis. Oscillatory zoned zircons from charnockite sheets, interpreted as melts generated through granulite facies fluid-absent biotite melting at 920-940°C, yield ages of 282-260 Ma. Some of these zircons contain inclusions of opx, unequivocally attributable to the granulite facies, thus confirming a Permian age for the charnockites and associated granulites. Two samples from an enclave-rich orthogneiss sheet yield Cambrian and Ordovician zircon cores. Two deformed leucogranites and six ortho-and augengneisses, which compose two-thirds of the Gruf Complex, give zircon ages of 290-260 Ma. Most zircons have milky rims with ages of 34-29 Ma. These rims date the Alpine amphibolite facies migmatisation, an interpretation confirmed by directly dating a leucosome pocket from upper amphibolite facies metapelites. The Gruf charnockites associated with metre-scale schlieren and boudins of opx-sapphirine-garnet-granulites, websterites and gabbronorites can thus be identified as part of the postVariscan European lower crust. A geotectonic reconstruction reveals that this piece of lower crust stranded in the (European) North upon rifting of the Neotethys, such contrasting the widespread granulite units in the Southern Alps. Emplacement of the Gruf lower crust into its presentday position occurred during migmatisation and formation of the Bergell Pluton in the aftermath of the breakoff of the European slab.

Tertiary Pegmatite Dikes of the Central Alps

“…Uranium-Pb dating of monazite from orthogneisses and migmatites from the SSB of the Bodengo valley yielded an age of 23-24 Ma (Hänny et al 1975, Köppel et al 1980, which is slightly older than the Rb-Sr whole rock isochron age of 22 Ma. Along the western boundary of the Bergell pluton, within the Gruf complex, recent SHRIMP U-Pb dating has yielded ages in the range of 34-29 Ma for the outer rim of pre-Alpine zircons (Galli et al 2012). These ages are consistent with field observations that migmatization at 720-740 °C in the Gruf complex was coeval with intrusion of the Bergell pluton (Davidson et al 1996, Berger et al 1996, Galli et al 2012.…”

“…Therefore, in the SSB the leucocratic dikes (including pegmatite and aplite) are often associated with small granitoid bodies (meter to decameter in size) and in situ leucosome segregates of the host rock (Burri et al 2005). The interpretation of this structural assemblage is locally difficult due to the occurrence of pre-Alpine leucocratic dikes, migmatites, and granitoids (Hänny et al 1975, Romer et al 1996, Galli et al 2012. The age of Alpine migmatization and of emplacement of leucocratic dikes in the Central Alps apparently spans approximately the same age bracket as the Tertiary intrusions, whose endmembers are represented by the large Bergell intrusion (32-28 Ma, von Blanckenburg et al 1992, Berger et al 1996, Oberli et al 2004) and the younger and smaller Novate stockwork intrusion (24.0 ± 1.2 Ma, Liati et al 2000).…”

“…Along the western boundary of the Bergell pluton, within the Gruf complex, recent SHRIMP U-Pb dating has yielded ages in the range of 34-29 Ma for the outer rim of pre-Alpine zircons (Galli et al 2012). These ages are consistent with field observations that migmatization at 720-740 °C in the Gruf complex was coeval with intrusion of the Bergell pluton (Davidson et al 1996, Berger et al 1996, Galli et al 2012. SHRIMP U-Pb dating of multiple zircon domains from migmatite leucosomes of the SSB close to Bellinzona (Rubatto et al 2009) suggests protracted melting over a time span ranging from 32 to 22 Ma.…”

Tectonometamorphic history of the Gruf complex (Central Alps): exhumation of a granulite–migmatite complex with the Bergell pluton

“…Droop and Bucher-Nurminen (1984), and Schmitz et al (2009) suggested an Alpine age for the granulites, whereas Huber (1999), Burri et al (2005) and Galli et al (2011) envisaged a pre-Alpine age. Subsequently, the charnockites have been dated at 282-260 Ma (U-Pb on zircon) placing granulite facies conditions in the Permian (Galli et al 2012).…”

“…The Gruf complex is dominated by migmatitic orthogneisses, paragneisses and micaschists. Upper amphibolite-facies migmatisation (Bucher-Nurminen and Droop 1983;Nagel et al 2002) occurred between 34 and 29 Ma Galli et al 2012). Highpressure rocks known from the adjacent Adula nappe are absent in the Gruf complex.…”