NOTE TO USERS The original manuscript received by UMI contains pages with indistinct print. Pages were microfilmed as received. This reproduction is the best copy available UMI GEOCHEMISTRY OF THE UCHI SUBPROVINCE, NORTHERN SUPERIOR PROVINCE: AN EVALUATION OF THE GEODYNAMIC EVOLUTION OF THE NORTHERN MARGIN OF THE SUPERIOR PROVINCE OCEAN BASIN A thesis submitted to the College of Graduate Studies and Research in partial fulfilment of the requirements for the degree of Doctor of Philosophy in the Department of Geol~gicalSciences University of Saskatchewan Saskatoon BY Peter Nigel Hollings Fall, 1998 O Copyright Peter Nigel Hollings. 1998. AU rights resewed. 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Neither the droit d'auteur qui protege cette these. thesis nor substantial extracts fiom it Ni la these ni des extraits substantiels may be printed or otherwise de celle-ci ne doivent Gtre imprimes reproduced without the author's ou autrement reproduits sans son permission. autorisation. UNIVERSITY OF SASKATCHEWAN College of Gladuate Studies and Research SUMlMARY OF DISSERTATION Submitted in partial fulfilment of the requirements for the DEGREE OF DOCTOR OF PHILOSOPEW by Peter Hollings Department of Geological Sciences University of Saskatchewan Examining Committee: Dr. G.J. Sofko m$Uf/AM&W= Dean's Designate. Chair College of Graduate Studies and Research Dr. J.F. Basinger Chair of Advisory Committee. Department of Geological Sciences Dr. R. Kemch Supervisor, Department of Geological Sciences Dr. K.. Ansdell Department of Geological Sciences Dr. C. Holmden Department of Geological Sciences Dr. J. Merriam Department of Geological Sciences a+,& Dr. S. Reid Department of External Exarniner Dr. D. Cad School of Earth and Ocean Sciences University of Victoria P.O.Box 3055, Victoria, BC V8W 3P6 Geochemistry of the Uchi subprovince, northern Superior Province: an evaluation of the geodynamic evolution of the northern margin of the Superior Province ocean basin Older assemblages of the Uchi subprovince, northern Superior Province, Canada, ranging in age from 2.9 to 3.0 Ga, are dominated by an association of komatiite-tholeiite sequences intercalated with felsic volcanic rocks. Komatiites vary from weakly LFEE enriched to strongly depleted. The majority of ultramafic rocks display the conjunction of negative Nb anomalies with trends of increasing SiOz and La/Smn, indicative of contamination by a felsic component. Many of the komatiites and komatiitic basalts are characterised by high VrYb and ScrYb ratios which are unique amongst Archean komatiites. Tholeiitic flows intercalated with the komatiites typically display flat FEE patterns with variable Nb anomalies, consistent with minor degrees of contamination. Two suites of felsic volcanic rocks are intercalated with the komatiite-tholeiite sequences of the older assemblages. Both types display pronounced LREE enrichment and negative Nb and Ti anomalies, but Type I has strongly fractionated HREE patterns. whereas Type 2 HREE are generally flat in conjunction with elevated compatible element contents. Collectively, the geochemical, geochronological and stratigraphic evidence from the northern Superior Province greenstone belts provide evidence for both a spatial and temporal association of mantle plumes and subduction zones during generation of the proto-continent nucleus to the Superior hvince. -_ The 2.8-2.9 Ga assemblages of the Uchi subprovince comprise mafic tholeiites characterised by predominantly flat REE, along with volumetrically significant units of felsic to intermediate volcanic rocks, that display enriched LREE and moderately fractionated HREE in conjunction with pronounced negative Nb anomalies. The geochemistry and geochronology of these assemblages is consistent with the fonnation of ocean plateau fragments which were accreted to the Uchi paleo continental margin where they subsequently formed the basement for a subduction related arc complex. Five distinct suites have been recognised in the -2.7 Ga Confederation assemblage: (1) thoieiitic basalts with flat to smoothly depleted REE; (2) tholeiites with flat to weakly depleted LREE in conjunction with pronounced negative Nb anomalies; (3) Fe-rich basalts with elevated Ti and P contents, enriched LREE adfractionated HREE; (4) LREE enriched basalts and andesites with negative Nb anomalies; and (5) dacites and rhyolites with variably enriched LEE,moderately fractionated HREE and variable HFSE anomalies. The occurrence of this varied range of rocks in a single assemblage is comparable to modem back arc basins. BIOGRAPHICAL September, 1970 Born in London, England June, 1992 B.Sc (Hons) Geology, Royal Holloway and Bedford New College, University of London HONOURS Graduate Student Scholarship, University of Saskatchewan, 1996- L998 Mineralogical Society Award, University of London, June 199 1 Permission to Use In presenting this thesis in partial fulfilment of the requirements for a Doctor of Philosophy degree from the University of Saskatchewan. I agree that the libraries of this university may make it freely available for inspection. I further agree that permission for copying of this thesis in any manner, in whole or in part. for scholarly purposes may be granted by my supervisor Dr. R. Kerrich or, in his absence, by the head of the Department of Geological Sciences or Dean of Arts and Science. It is understood that any copying of this thesis, or parts thereof, shall not be allowed without my written permission. It is also understood that due recognition shall be given to me and the University of Saskatchewan in any scholarly use which may be made of any material in this thesis. Requests for permission to copy or make other use of material in this thesis, in whole or in part, should be addressed to: Head of the Department of Geological Sciences 1 14 Science Place. University of Saskatchewan Saskatoon, Saskatchewan, S7N 5E2. Abstract Older assemblages of the Uchi subprovince. northern Superior Province. Canada. ranging in age from 2.9 to 3.0 Ga. are dominated by an association of komatiite-tholeiite sequences intercalated with felsic volcanic rocks. Komatiites vary from weakly light rare eanh element (LREE)enriched to strongly depleted (La/Srn, = 0.36- 1.6). The majority of ultramafic rocks display the conjunction of negative Nb anomalies with trends of increasing SiOz, La. LalSm, and Th/Ce with decreasing Mg# and Ni. indicative of contamination by a felsic component. Many of the komatiites and komatiitic basalts are characterised by high VRb and Sc/Yb ratios (277-360 and 58-76 respectively) which are unique amongst Archean komatiites. Normalized V-Sc enrichments versus the heavy rare earth elements (WE)are characteristic of mafk rocktypes derived from previously depleted mantle sources and occur, to some degree, in most mafic arc volcanic rocks. Tholeiitic flows intercalated with the komatiites typically display flat REE patterns with variable Nb anomalies and high TWCe ratios, consistent with minor degrees of contamination. Two suites of felsic volcanic rocks are intercalated with the komatiite-tholeiite sequences of the older assemblages. Both types display pronounced LREE enrichment and negative Nb and Ti anomalies, but Type L has strongly fractionated KREE patterns. whereas Type 2 HREE are generally flat in conjunction with elevated compatible element contents. The predominant Type 1 rhyolite is directly comparable to southern Superior Province examples associated with oceanic arc sequences, and are comparable to Archean high Al, high La/Yb, 'ITGs; they indicate a subduction-related origin for the northern examples. Type 2 rhyolite geochemical signatures may result from mixing of Type 1 rhyolites with tholeiitic magmas or variations in the mantle source. Komatiite-tholeiite associations within Archean terranes are interpreted as the result of plume-relared magmatism comparable to modem oceanic plateaux. In contrast intermediate and felsic rocks are typical of calc alkaline suites generally attributed to Archean subduction related environments. Collectively. the geochemical, geocbronological and stratigraphic evidence from the northern Superior Province greenstone belts provide evidence for both a spatial and temporal association of mantle plumes and subduction zones during generation of the proto-continent nucleus to the Superior Province. The 2-8-29 Ga assemblages of the Kchi subprovince comprise rnafic tholriites characterised by predominantly flat REE (La/Sm, = 0.8- 1.1). These are comparable to modem ocean plateau basalts. along with with volumeuically significant units of felsic to intermediate volcanic rocks. that display enriched LREE (La/Sm, = 3.7-7.2) and moderately fractionated HREE in conjunction with pronounced negative Nb anomalies. The geochemistry and geochronology of these assemblages is consistent