The supra-ophiolitic sedimentary cover of the Asbestos ophiolite, Québec, Canada: First geochemical evidence of transition from oceanic to continental sediment flux É. Bédard a, ⁎, R. Hébert a , C. Guilmette a , J. Dostal b a Département de géologie et de génie géologique, Université Laval, Québec, Qc., Canada G1K 7P4 b Department of Geology, Saint Mary's University, Halifax, NS, Canada B3H 3C3 ABSTRACT ARTICLE INFO Article history: Received 1 November 2007 Accepted 16 April 2008 Available online 8 May 2008 Keywords: Ophiolites Quebec Appalachians Intraoceanic sedimentary cover Geochemistry The 473 +5/- 3 Myr Asbestos ophiolite complex of the Quebec Appalachians was formed in a forearc basin and obducted on a margin of Laurentia ~460 Myr ago. The complex together with its sedimentary cover is well exposed at Burbank Hill (~130 km SW of Québec City) where eight distinct lithologies have been identified: 1) pyroxenites and wehrlites with minor dunitic layers, (2) fractured gabbros, (3) breccia containing gabbro and diabase fragments, (4) polygenic conglomerates, (5) red mudstone/chert/sandstone succession, (6) tuffs with intercalated grey chert, (7) greenish grey mudstones and (8) slates and sandstones of the Saint-Daniel Mélange. The gabbroic and mafic volcanic rocks show the characteristics of arc tholeiites (TiO 2 ~ 0.8–0.85 wt.%) and boninites (TiO 2 b wt. 0.35% and Mg# N 45; (La/Sm) N ~ 1.9–3.3). These rock-types also occur as mafic fragments in the polygenic conglomerate. The mafic fragments (crystals and rocks) within the conglomerate and the ophiolitic sandstones also show the same greenschist facies metamorphism as the mafic igneous rocks of the complex. The conglomerate was probably formed after the forearc crust was fragmented by pre-obduction normal faults. These fault scarps would have promoted the erosion of the oceanic crust, leading to the accumulation of diverse detritus in grabens. The sedimentary lithologies which overlie the conglomerate exhibit a gradual increase in continent-derived material ((La/Sm) N ~5 and La/ Yb ~20) and a decrease in ophiolitic material moving upward in the stratigraphic sequence. The sedimentary rocks at the bottom of the sequence (group 3) are mostly composed of ophiolite material with only 20% of crustal material. They also have a high content of Cr and MgO with values of 350–480 ppm and 5.7–8%, respectively. The rocks of the uppermost part of the pile (group 1) which contain up to 80% crustal material have low abundances of Cr and MgO (30–100 ppm and 2.7–4.5 wt.%). Continental detritus on the top of the Asbestos ophiolite suggests the complex formed near the Laurentian continental margin and that the eroded continental material had access to the basin where the ophiolite was formed. This is consistent with the obduction of the Asbestos and Thetford–Mines ophiolites on the Laurentian margin shortly after their formation. The proximity of thick Laurentian continental crust near the trench could explain why the subduction zone blocked-up soon after the ophiolites formation. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Sedimentary rock sequences can provide important information about the source of the detrital material and the depositional environment. In particular, the rarely investigated sediment cover of ophiolites can yield critical data on the tectonic settings where ophiolites were formed and their history. This also applies to ophiolitic slivers of Iapetan affinity in the southern Québec Appalachians which have been interpreted to be remnants of a forearc basin crust (Hébert and Bédard, 2000). The Thetford–Mines, Asbestos and Mont–Orford complexes are the main ophiolitic massifs of the Québec Appalachians (Fig. 1) and occur as complete or tectonically dismembered sections which typically contain all the characteristic units of ophiolites: the dynamothermal sole, the upper mantle section, the ultramafic to mafic cumulates, the volcanic rocks and the intraoceanic sedimentary cover (Hébert and Bédard, 2000). This study focuses on the upper crustal section of the Asbestos ophiolite complex outcropping at Burbank Hill, about 130 km south- west of Québec City (Fig. 1). The section is located b 2 km south of the open pit of Jeffrey Mine near the town of Asbestos. Lithological units at Burbank Hill (Fig. 2) were originally described as gabbros and mafic volcanic rocks by Lamarche (1973). However, a detailed investigation reveals the presence of a wide variety of rocks and a complex origin of the sequence. Rocks identified during field work include dunite, pyroxenite, fractured and foliated gabbros and breccia overlain by polygenic conglomerate, well bedded sandstone, volcaniclastic layers, mudstone and chert. This sequence provides a good opportunity to use sedimentary rocks to constrain the tectonic history of the ophiolites. Lithos 105 (2008) 239–252 ⁎ Corresponding author. Tel.: +1 418 656 2131x4917; fax: +1 418 656 7339. E-mail addresses: emilie.bedard.3@ulaval.ca (É. Bédard), rejean.hebert@ggl.ulaval.ca (R. Hébert), carl.guilmette.1@ulaval.ca (C. Guilmette), jarda.dostal@smu.ca (J. Dostal). 0024-4937/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2008.04.005 Contents lists available at ScienceDirect Lithos journal homepage: www.elsevier.com/locate/lithos