Depositional and tectonic setting of the Paleoproterozoic Lower Aillik Group, Makkovik Province, Canada: evolution of a passive margin-foredeep sequence based on petrochemistry and U-Pb (TIMS and LAM-ICP-MS) geochronology

The Paleoproterozoic Lower Aillik Group is a deformed metasedimentary-metav olcanic succession located in the Makkovik Province of Labrador, eastern Ca nada. The group is situated near the boundary between reworked Archaean gne iss of the Nain (North Atlantic) craton and juvenile Paleoproterozoic crust that was both tectonically accreted and formed on or adjacent to this crat on during the ca. 1.9-1.78 Ma Makkovikian orogeny. The Lower Aillik Group i s structurally underlain by Archaean gneiss and structurally overlain by ca . 1860-1807 Ma bimodal, dominantly felsic volcanic and volcaniclastic rocks of the Upper Aillik Group. We present geochemical data from metavolcanic r ocks and U-Pb geochronological data from several units of the Lower Aillik Group in order to address the depositional and tectonic history of this gro up. U-Pb data were obtained using both thermal ionization mass spectrometry (TIMS) and laser ablation microprobe-inductively coupled plasma-mass spect rometry (LAM-ICP-MS). Two quartzite units near the structural base of the L ower Aillik Group contain detrital zircons only of Archaean age, and are in terpreted to have been deposited on the Nain craton during post-2235 Ma rif ting and initiation of a passive continental margin. Overlying mafic metavo lcanic rocks contain thin horizons of intermediate tuff, one of which is da ted at 2178 +/- 4 Ma. This relatively old age, and an inferred stratigraphi c relationship with underlying sedimentary units. suggest that the volcanic rocks represent transitional oceanic crust. consistent with their geochemi cal similarity to tholeiitic rifted margin sequences of Mesozoic age in eas tern North America. A package of interlayered psammitic and semipelitic met asedimentary rocks that appears to stratigraphically overlie the mafic volc anic unit is dominated by Paleoproterozoic detrital zircons but also contai ns Archaean grains. This package was deposited after 2013 Ma, the age of th e youngest concordant zircon. The U-Pb data imply a minimum 165 m.y. time g ap between mafic volcanism and sedimentation, and are consistent with depos ition of the psammite-semipelite unit in an evolving foredeep that heralded the approach of a Paleoproterozoic are terrane. Accretion of this terrane to the Nain cratonic margin at ca. 1.9 Ga initiated the Makkovikian orogeny . Although the Lower Aillik Group is highly deformed and may contain intern al tectonic boundaries or be incomplete, the U-Pb and geochemical data allo w quantitative assessment of a prolonged rift-drift-basin closure cycle tha t characterized the Early Paleoproterozoic evolution of the southern Nain c ratonic margin. (C) 2001 Elsevier Science B.V. All rights reserved.