Abstract

Quartzofeldspathic xenoliths representing the probable structural basement to the northern Dunnage Zone occur in lamprophyre dykes on Alcock Island and vicinity, Notre Dame Bay. The dykes occur close to the boundary between seismically defined lower crustal blocks (LCB) below the Dunnage Zone, so the assignment of the xenoliths to a specific subsurface source is equivocal. The xenoliths include (i) clinopyroxene-bearing meta-(quartz) diorite to tonalite; (ii) plagioclase + quartz-rich rocks; and (iii) garnetiferous gneiss, including granulite. Paragneiss is absent, so the gneissic xenoliths do not appear to be direct analogues of Gander Zone metasediments thought to underlie parts of the Dunnage Zone farther to the southeast. The xenoliths also differ from Grenvillian crust exposed in western Newfoundland: the granulites are mineralogically distinct (contain clinopyroxene + garnet rather than orthopyroxene + garnet) and yield higher metamorphic P (approx. 8–10 kbar versus approx. 3–7 kbar) (1 kbar = 100 MPa), and the granitoid xenoliths are dioritic–tonalitic rather than granitic. Consequently, we propose that the xenoliths are derived from a non-Gander, non-Grenville source that may include Central LCB and (or) exotic basement material. We cannot, however, exclude the possibility that the xenoliths represent deeper structural levels of Grenvillian or Gander Zone crust than are presently exposed in outcrop.

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