Rj. Theriault et I. Ermanovics, SM-ND ISOTOPIC AND GEOCHEMICAL CHARACTERIZATION OF THE PALEOPROTEROZOIC TORNGAT OROGEN, LABRADOR, CANADA, Precambrian research, 81(1-2), 1997, pp. 15-35
The Torngat orogen is a Paleoproterozoic, granulite-facies belt that w
elds the Archean components of the Nain and Rae provinces in eastern L
abrador. Sm-Nd isotopic and supporting bulk rock major, trace and rare
earth element data help elucidate the sources of the main Paleoproter
ozoic lithologies that comprise the orogen. In the foreland, Paleoprot
erozoic Ramah Group metasediments intercalated with reworked Nain Prov
ince gneiss show variable trace element and epsilon(Nd)(1.9 Ga) from -
2.9 to -12.0, ranging between Nain Province crust and Paleoproterozoic
Mugford Group basalts compositions. The Ramah Group data suggest deri
vation of Ramah detritus from Archean Nain Province gneisses and from
the Mugford Group. In the Lac Lomier complex (Rae Province hinterland)
, metasediments with variable trace element distribution patterns akin
to Rae Province crust, and epsilon(Nd)(1.9 Ga) from -2.9 to -10.5 sug
gest derivation from Rae Province crust and from an unspecified juveni
le component. In the core of the orogen, pelitic and psammitic protoli
ths of the Tasiuyak gneiss (epsilon(Nd)(1.9 Ga) from -1.9 to -2.6) wer
e derived from crustal sources unknown in both Nain and Rae provinces,
and may have been deposited in a trench environment starved of adjace
nt cratonic debris. Dioritic to granitic eastern and western metapluto
nic suites intruding the Tasiuyak gneiss and the Lac Lomier complex di
splay contrasting geochemical and Sm-Nd isotopic compositions. The eas
tern metaplutonic suite has a tendency toward lower SiO2, total alkali
s, less fractionated REE and higher MgO, CaO and TiO2 relative to the
western metaplutonic suite. The eastern suite yields epsilon(Nd)(1.9 G
a) from +3.8 to -6.7 and negative f(Sm/Nd) suggesting igneous precurso
rs from Rae crust and a LREE-enriched juvenile component. The western
suite yields epsilon(Nd)(1.9 Ga) from -1.5 to -8.5, suggesting an impo
rtant contribution from Rae crustal sources in the petrogenesis of the
igneous protoliths. The presence of Rae Province Nd isotopic composit
ions, and the absence of Nain Province signatures in these intrusions
favors the eastern Rae Province as the active margin during Nain-Rae c
onvergence.