Granitoid geochemistry and Late Archean crustal evolution in the central Slave Province

Davis, William James (1991) Granitoid geochemistry and Late Archean crustal evolution in the central Slave Province. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The Contwoyto-Nose Lakes area in the central Slave Province is a Late Archean granite-greenstone terrain consisting of metavolcanic (Central Volcanic Belt) and metasedimentary rocks extensively intruded by syn- to post-deformation granitoid rocks. The latter, representing over 65% of exposed crust in the area, were emplaced between 2616 and 2585 Ma, post-dating the earlier (ca. 2670-2650 Ma) assemblage by more than 35 m.y. -- Plutonic rocks in the area are subdivided into 8 suites. Four of the suites are strongly deformed and recrystallized, and are temporally linked to the supracrustal assemblage (van Breemen et al., 1990). These include: 1) hornblende diorites of the Central Volcanic Belt (CVB) Suite; 2) quartz porphyritic trondhjemitic plutons of the Gondor Suite; 3) biotite tonalites to monzogranites of the Wishbone Suite; and, 4) biotite tonalites of the Olga Suite. Two suites, the Siege and Concession, are interpreted to have been emplaced syn- to late- during the regional metamorphism and deformation, whereas the Yamba and Contwoyto Suites post-date this event. The Siege Suite consists of leucocratic biotite tonalite. The Concession Suite is predominantly tonalite but ranges from hornblende diorite to biotite granodiorite compositions. These rocks define prominent aeromagnetic highs and commonly form composite bodies. In many cases, plutons are tabular-shaped, with long dimensions parallel to the regional foliation (S₂). Two samples have yielded U-Pb ages of 2608±1 and 2608+5/-3 Ma. The Yamba Suite consists dominantly of red weathering, biotite monzogranites and associated pegmatites. The Contwoyto Suite, on the other hand, contains primary muscovite and biotite (± tourmaline) and is grey-green weathering. Pegmatites are a ubiquitous feature of the suite. One sample of the Yamba and 3 samples of the Contwoyto Suite have yielded U-Pb ages within error of ca. 2582-2585 Ma. -- Rocks of the CVB are calc-alkaline and most have trace element characteristics diagnostic of rocks formed in modern supra-subduction zone settings. The felsic plutonic suites include low-Al₂O₃ (Gondor, Wishbone Suites) and high-Al₂O₃ (Olga Suite) trondhjemites. The former have high abundances of HFSE and REE and are compositionally similar to rocks from Phanerozoic ophiolites, ridges and island arc settings. They are interpreted to have formed by low pressure, partial melting of mafic protoliths leaving a pyroxene- and plagioclase-dominated residue. In contrast, the high-Al₂O₃ trondhjemites have steep REE patterns with low abundances of the HREE and are interpreted to have formed by melting of garnet-bearing mafic protoliths. -- The Siege Suite has characteristics of high-Al₂O₃ trondhjemites and may also have formed by partial melting of garnet-bearing mafic crust. The Concession Suite is calc-alkaline and varies from metaluminous to weakly peraluminous with increasing SiO₂ content (50-72 wt%). The suite is characterized by high contents of Sr (up to 1600 ppm), Ba (up to 2000 ppm), and Ce (up to 170 ppm), high (Ce/Yb)N, low Rb/Sr (<0.3). no or small negative Eu anomalies and pronounced negative Nb anomalies. Mafic samples are most enriched in trace elements; Sr, Ba, Y and the REE show a continuous decrease with increasing SiO₂. Although the REE characteristics are consistent with those predicted in melts of garnet-bearing mafic crust, the chemical variation within the suite cannot be accounted for by variable degrees of melting of basalt. The tonalites can however, be related to the mafic end members by assimilation and fractional crystallization (amphibole-plagioclase dominated) processes. The composition of potential parental magmas is similar to some high-Mg andesites (HMA; e.g. sanukitoids, bajaites) derived from melting hydrated enriched peridotite. The juvenile εNd(I) values measured for the suite limit the mantle enrichment event to the 200 m.y. preceding magmatism. The HFSE characteristics suggest mantle enrichment may have been subduction-related. -- Post-deformation plutonic suites are high SiO₂ (68 to 78 wt%) and peraluminous. The Yamba Suite is K₂O-rich with high Rb/Sr ratios and strongly negative Eu and Sr anomalies relative to enriched REE patterns. The Contwoyto Suite is, in general, less potassic and more strongly peraluminous. REE patterns are variable, probably reflecting the evolution of fluid phases and pegmatite formation, εNd(I) values for both suites exhibit a wide range from positive (+3.7) to negative values (-5.1). The Contwoyto Suite has been modelled as partial melts of dominantly metasedimentary protoliths, possibly similar to the exposed turbidite sequences. The origin of the Yamba Suite is more equivocal. The spatial association with the earlier mantle-derived Concession Suite and their compositional similarity to granites of hybrid origin suggest that they may be products of the mixing and homogenization of mantle-and crust-derived materials. The negative εNd(I) values for some of these rocks require that the crustal component had a significant pre-history (> 400 m.y.). -- Nd isotopic data from representative rock types along an E-W traverse at 65° N across the central pan of the province reveal distinctly different characteristics in the east compared to the west. Supracrustal and plutonic rocks from the east have positive εNd(I) values consistent with juvenile sources and formation remote from significantly older crust. In contrast, samples of post-deformation granites west of 110°30'W have negative εNd(I) values. The Nd isotopic data for these granitoid rocks reflect the presence of mixed crustal sources dominated by Mid to Early Archean crust or derivative sediments. The asymmetric pattern defined by the Nd isotopic data suggests the presence of distinct crustal blocks beneath the Slave Province, as predicted by models proposing tectonic assembly of the province through accretion of juvenile crust to an older continental mass. -- The secular evolution in the mineralogy and geochemistry of the plutonic suites reflects a change from dominantly mantle- to crustal-derived plutonism. Igneous rocks of the early assemblage are interpreted to be remnants of an allocthonous island arc terrane, which was accreted to a continental block during the late Archean deformation. Crustal shortening and thickening during collision caused melting of previously subduction-modified mantle to generate HMA magmas, possibly by the detachment of the lower part of the lithosphere (e.g. Houseman et al., 1981). Intrusion of these mantle-derived magmas into the crust, in combination with crustal thickening, caused crustal melting and the generation of peraluminous granites. The nature and evolution of the granitoid rocks is comparable to features of more recent collisional orogenic belts. This interpretation suggests that the tectonic and related igneous processes leading to crustal stabilization in the Late Archean Slave Province were relatively similar to those in modern orogenic belts, which implies a continuity in process for the last 2.7 Ga.

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