Geochemistry of the granulitic Bondy gneiss complex: a 1.4 Ga arc in the Central Metasedimentary Belt, Grenville Province, Canada

Abstract

The 1.4 Ga Bondy gneiss complex (BGC) is one of a series of gneiss complexes that outcrops as tectonic domes structurally below Grenvillian, 1.3/1.25 Ga, marble and quartzite assemblages in the Central Metasedimentary Belt (CMB) of Quebec, western Grenville Province. The complex comprises a variety of tonalitic to granitic orthogneiss intercalated with thin units of layered metabasite and laminated felsic gneiss that host a Cu/Au/Fe oxides hydrothermal system. A metatonalite pluton outcrops at the southern end of the complex. These rocks were metamorphosed at granulite-facies at 1.20 Ga. In spite of this metamorphism and variable Rb, U and/or Th loss, LILE depletion in the mafic and felsic gneisses is subordinate, and was mainly controlled by the mineralogy of the protoliths during percolation of metamorphic fluids. In most cases, trace element signatures of the granulites do not show evidence for REE and HFSE mobility. Their major, trace element and Nd-isotope geochemistry displays systematic and highly reproducible signatures attributable to volcanic and plutonic rocks formed in a mature island-arc setting. The arc event is inferred from the association of mafic granulite, intermediate gneiss, and tonalite with similar geochemistry to that of modern mature calc-alkaline island-arc rocks. Some of the mafic granulites and laminated quartzofeldspathic gneisses display geochemical signatures characteristic of modern tholeiitic basalts and high-silica rhyolites related to back-arc rifting. The NdT values suggest a depleted mantle source with the addition of a crustal component to form the felsic and intermediate magmas. Taking as an example the Sunda, Lesser Antilles and New Zealand island-arcs, we advocate that 1.4 Ga magmatism occurred in mature island-arc, back-arc system along the Laurentian margin. # 2002 Elsevier Science B.V. All rights reserved.