A REVIEW OF KIMBERLITIC AND ULTRAMAFIC LAMPROPHYRE INTRUSIVES FROM NORTHERN LABRADOR

Four ultramafic lamprophyre to kimberlite intrusive sites have been recognized in the Archean Nain Province of northern Labrador. Two sites are located within the Hopedale Block, at (1) capes Aillik–Makkovik and (2) Ford's Bight, and two are located within the Saglek Block, in the (3) Saglek and (4) northern Torngat Mountains areas. To date, no diamonds have been found in either intrusive site, but geochemical analysis has been, at best, rudimentary. Group 1 has been previously described as kimberlitic, at least in part, but, to date, no detailed geochemical evaluation of diamond indicator minerals has been completed. The intrusive rocks of Group 2 have only recently been identified as part of a possible kimberlite diatreme; there are no whole-rock or mineral geochemical data yet derived for this suite. The Group 3 suite has been defined as kimberlitic for some time, but spatial and geochemical data have been restricted to unpublished reports. Group 4 dykes were originally classified as ultramafic lamprophyres, but mineral assemblages of some dykes indicate that they are macrocrystal hypabyssal phlogopite–perovskite archetypal kimberlites, whereas the remainder are olivine–phlogopite–calcite ultramafic lamprophyres, possibly melilitites. Sample size, however, was very small for definitive determinations, and one kimberlite has a strike extension classified as lamprophyre. Within the kimberlites, the following minerals were identified: (1) lherzolitic, eclogitic and megacrystal garnets (megacrystal garnets were also found in some of the lamprophyres), (2) clinopyroxenes, which plot in Fipke's diamondiferous CPX domain (as does one lamprophyre), (3) orthopyroxenes with < 1% to 1.5 % Al2O3, and (4) olivines with Fo contents of ~ 88-92 (some of the lamprophyres have two populations of olivine with clusters of similar high Fo values). Orthopyroxene and clinopyroxene geothermobarometers suggest a possible harzburgite chemistry for some mineral separates, and also that the postulated geotherm for the intrusive history of the dykes is permissive of diamond stability.