Abstract ID/ No de résumé: 458

PALEOPROTEROZOIC ANORTHOSITES AMONG MESOPROTEROZOIC ANORTHOSITES : THE FIELD IDENTIFICATION OF TWO SIMILAR PLUTONIC TERRANES IN NORTHERN LABRADOR

RYAN, Bruce, Newfoundland Geological Survey, Department of Mines and Energy, P.O. Box 8700, St John's, NF, A1B 4J6, abr@zeppo.geosurv.gov.nf.ca; HAMILTON, Mike A., Geological Survey of Canada, 601 Booth St., Ottawa, ON K1A 0E8, hamilton@nrn1.NRCan.gc.ca

Room for the session: 303a
Board if it is a poster:
Date of the presentation (mm/dd): 05/20
Beginning of the presentation: 14:20
Number of symposium, special session or general session: SS3

Lieu de la présentation: 303a
# du tableau si c'est un poster :
Date de la présentation (mm/dd): 05/20
Début de la présentation: 14:20
No du symposium, de la session spéciale ou de la session générale: SS3

It has traditionally been believed that all the igneous rocks between Nain and Okak Bay in northern Labrador belong to the Nain Plutonic Suite (NPS), a 1350 - 1290 Ma anorogenic Mesoproterozoic plutonic welt straddling a Paleoproterozoic (ca. 1860 Ma) collisional suture (Torngat Orogen). The main rock types of the NPS are anorthositic and granitic. Recent mapping by the Newfoundland Geological Survey and the Geological Survey of Canada in the Okak Bay - Webb's Bay area, north of Nain, has shown that a significant part of the northern extent of this welt consists, in fact, of a Paleoproterozoic (>2000 Ma) group of plutons that is mesoscopically and compositionally similar to the NPS. This group of Paleoproterozoic intrusions extends at least as far south as Nain. Field characteristics were instrumental in initially subdividing the two groups, and this first-order subdivision has been verified by age-dating. The older group is intruded by (altered) mafic dykes, is regionally - though inhomogeneously - deformed and slightly metamorphosed, and is intruded by (foliated) aplitic granitic dykes. The NPS lacks such regional characteristics, being, for the most part, a pristine igneous terrane preserving delicate primary structures. The Paleoproterozoic group also exhibits mesoscopic and microscopic textural characteristics that set it apart from the NPS. The presence of two nearly identical bimodal suites of rocks, separated in time by over 700 million years, dispels the belief that the rock-types of the NPS represent a singularly unique magmatic event, clearly establishes the repetitive nature of this type of magmatism in the crust of northern Labrador, and raises more far-reaching questions about the genesis of "massif-type" anorthosites in general. The anorogenic setting ascribed to bimodal plutonic terranes such as the Mesoproterozoic NPS, based on their temporal and spatial relation to prior and subsequent local events, cannot be rigorously advocated as being applicable to the genesis of the Paleoproterozoic group of rocks in northern Labrador. The older group seems more amenable to interpretation as a pre-tectonic assemblage intruded during continental extension allied with fragmentation of the Archean North Atlantic craton. The tectonic effects evident in the Paleoproterozoic plutons can be ascribed to their location within the foreland of the Torngat Orogen collision.



Abstract ID/ No de résumé: 736

U-PB EVIDENCE FOR PALEOPROTEROZOIC ANORTHOSITIC AND GRANITOID MAGMATISM PREDATING EMPLACEMENT OF THE MESOPROTEROZOIC NAIN PLUTONIC SUITE, LABRADOR

HAMILTON, Mike A., and EMSLIE, Ronald F., Geological Survey of Canada, 601 Booth St., Ottawa, ON K1A 0E8, hamilton@nrn1.NRCan.gc.ca; RYAN, Bruce, Newfoundland Department of Mines and Energy, P.O. Box 8700, St. John’s, NF A1B 4J6

Room for the session: 303a
Board if it is a poster:
Date of the presentation (mm/dd): 05/20
Beginning of the presentation: 14:40
Number of symposium, special session or general session: SS3

Lieu de la présentation: 303a
# du tableau si c'est un poster :
Date de la présentation (mm/dd): 05/20
Début de la présentation: 14:40
No du symposium, de la session spéciale ou de la session générale: SS3

Discovery in recent years of a world-class Ni-Cu-Co sulphide deposit in troctolitic rocks of the Nain Plutonic Suite (NPS) at Voisey’s Bay has prompted more detailed mapping of the less-well-known northeastern quadrant of the complex and its earlier Proterozoic and Archean gneissic country rocks.

New U-Pb ages are presented for two remote northern members of the Mesoproterozoic Nain Plutonic Suite (NPS), which are shown to have crystallized at 1322 +/- 1 Ma (leuconorite, Puttuaalu Brook; hosting blocks of older pale anorthosite) and 1330 +/- 1 Ma (quartz monzodiorite sheet; west of Iglusuataliksuak Lake). Both were emplaced and cooled within the range of known ages of NPS magmatism (i.e., ca. 1350 - 1290 Ma).

Along the northeast margin of the NPS, an important suite of locally deformed massif-type anorthositic rocks showing variable amounts of secondary alteration, and intruded by a series of metadiabase dykes, has also been recognized. U-Pb zircon dating of one of these anorthositic rocks from near Aupalukitak Mountain reveals a Paleoproterozoic emplacement age of 2112 +5/-4 Ma. This represents a newly recognized period of earlier Proterozoic anorthosite formation, now known to coincide with established ages of nearby 2135-2110 Ma granitic plutons in Nain craton. Additional Paleoproterozoic emplacement ages have been determined for other variably foliated granitic - monzonitic - dioritic rocks in the region (Halbach, 2128 Ma; Illuilik, 2124 Ma; Alliger Lake, 2109 Ma), and together with the basic rocks belie the general lithological similarities shared with the younger anorogenic NPS suite. Emplacement of diabase dyke swarms at 2121 +/- 1.5 Ma and ca. 2045 Ma further substantiate the regional importance of Paleoproterozoic basic magmatic activity.

Recognition of significant Paleoproterozoic anorthositic rocks within this dominantly Mesoproterozoic magmatic province imposes new constraints on massif anorthosite petrogenesis. We speculate that the overlapping Paleoproterozoic anorthositic and associated monzonitic-granitic magmatism occurred during a period of relative tectonic quiescence between late Archean tectonic assembly of western Nain craton (ca. 2550 Ma) and the onset of late Paleoproterozoic Nain-Rae collision during Torngat Orogen (ca. 1860 Ma). Taken together, the evidence of bimodality within the major 2135-2109 Ma magmatic suites, and the profusion of regional diabase dyke swarms points towards a model of continental extension and perhaps fragmentation. Both Meso- and Paleoproterozoic episodes of anorthositic (and associated granitoid) plutonism can be accomodated in general models of lithospheric thinning, though it is unclear why a spatial-petrological relationship persisted over a considerable interim of 800 Ma.



Total number: 2