STRUCTURAL AND MINERAL RELICS IN MANTLE ROCKS OF THE MONASHEE COMPLEX: VESTIGES OF THE PRE-CORDILLERAN HISTORY?

Multiscale structural analysis has been peformed on peridotite boudins surfacing in the Northern portion of the Thor-Odin dome (JOHNSTON, 1998), in the upper part of the high strain zone (e.g. JOHNSTON et al., 2000, KRUSE & WILLIAMS 2007) at the contact between two tectonic units (“Selkirk allochthon” and Monashee complex) interpreted as a thrust (e.g. READ & BROWN 1981; BROWN et al. 1986; GIBSON et al. 1999; GIBSON et al. 2008) or as a normal shear zone (KRUSE & WILLIAMS 2007). Peridotites are embodied in HT-gneisses, with minor metabasics, and are aligned in the regional scale transposition foliation (St), locally containing an internal foliation (Sp) at a high angle to St, marked by granulite-amphibolite facies minerals (SPALLA et al., 2011). Peridotites show two textures: granoblastic (polygonal aggregate with medium grain size) and poikiloblastic (with pyroxene poikiloblasts, partially replaced by amphibole). The granoblastic type, occurring where the ultramafics are foliated (Sp), contains Ol, Opx, Spl, minor Amp, Phl and opaque minerals. The poikiloblastic type is not foliated and comprises Ol, Amp, Opx, Spl, Phl, opaque and rare Cpx. Amp and Phl occur as porphyroblasts or in veins. This texture characterizes undeformed domains preserved in the granoblastic type (Sp). Two kinds of Amp and Phl have been detected: (1) brown Amp, which replaces Px and is partially replaced by a thin symplectite of green Amp and Spl and (2) green Amp, which occurs where brown Amp recrystallizes, showing a foam texture. Whole rock chemical composition data indicate that these rocks are metasomatized mantle peridotites.

The applied multiscale structural analysis, integrated with phase-transformation investigations, allowed to exploit a long textural and mineral-chemical memory of granulites and peridotites also in high strain domains, as the regional scale Greenbush Lake shear zone, and help to unravel the emplacement history of mantle peridotites in the high-grade continental crust of the Thor-Odin dome. In particular peridotites still preserve mineral relics marking the Ol internal foliation indicating that these rocks underwent a T-prograde path predating the PT climax preceding the Cordilleran crustal evolution of these rocks.