Date of Award
Doctor of Philosophy (PhD)
R. H. McNutt
Twelve Archean granitoid plutons, intrusive into greenstones of the Wabigoon volcanic-plutonic belt in the Superior Province were investigated as to their nature and time of emplacement evidenced from field mapping, petrography, X-ray fluorescence for major and trace elements, in situ gamma ray spectrometry for radioelements and mass spectrometry for Sr isotopes.
Despite common tectonic setting, these plutons range from "homogeneous" granodiori tes and porphyries to concentrically zoned or complex plutons of granite-monzonite-monzodiorite. Homogeneity has been tested in one lithologically monotonous pluton by extensive sampling and trend surface analysis. Tenuous textural and petrochemical phases are defined and attributed to subtle magmatic differentiation overprinted by deuteric autometasomatism. The genetic model proposed is one of pulsating diapirism followed by fluid infiltration, diffusion and cation exchange.
Lithologically zoned plutons are multiple intrusives with younger, more felsic cores and discontinuous petrochemical trends. Cores exhibit homogeneity, euteric metasomatism and felsic dyking similar to the monolithologic plutons. The genetic model proposed is multiple diapiric intrusion from multiple sources through a common conduit.
Quartz-feldspar porphyries show petrographlc and geochemical evidence of extensive alteration, cataclasis and metasomatism more indicative of a synvolcanic hypabyssal origin than affinity with phaneritic granitoids.
Binary plots of concentration versus a chemical index compiled for 247 granitoid samples provide rough estimates of element clarkes. Some plutons are anomalous but as a group these granitoids have high Sr, Ba, K/Rb and low Rb/Sr, Th and U. There is no evidence for large scale magma contamination but ubiquitous lensoid mafic enclaves derived from adjacent supercrustals, experienced basification and alkali metasomatism with enrichment in Rb, Nb, Ce and Y and growth of megacrysts.
There is also no chemical evidence in this study to support either a model of secular evolution of Archean granitoids or to demonstrate a chemical distinction between high level and low level plutons.
Seven stocks and a portion of a batholithic canplex have been dated by Rb/Sr whole-rock isochrons sensu stricto (York I, λ = 1.39 x 10⁻¹¹y⁻¹ 20 errors):
Student's-T tests show only Ottertail is distinct. Weighted pooling of isochrons gives: 2616 ± 43 m.y. for seven stocks while a composite 39 point regression yields 2623 ± 35 m.y. These estimates lie median to the Algoman Orogeny (2400 - 2750 m.y.) and are younger than the proposed Kenoran Orogeny (mean age: 2690 m.y.) and suggest coeval vulcanism, batholithic and intra-belt granitic magmatism spanning less than 35 m.y. (68% confidence). A possible southward juvenescence is beyond Rb/Sr resolution. Initial Sr ratios straddle the single-stage, linear mantle growth and suggest a low (⁸⁷Sr/⁸⁶Sr)i provenance.
Aeromagnetic maps suggest that these stocks are upper crust features with dimensions approximated by their surficial exposures.
Regional distribation and emplaceme.nt was controlled by diapirism or polydiapirism from a lower crust or upper mantle source layer. Upwelling was initiated by deep extensions of faults and superbelt interfaces or by downwarping and crustal thinning at paleobasin centers.
Birk, Wolf Dieter, "The nature and timing of granitoid plutonism in the Wabigoon volcanicplutonic beit, Northwestern Ontario: Geochemistry, Rb/Sr geochronology, Petrography and Field investigation" (1978). Open Access Dissertations and Theses. Paper 3844.