Geochemistry and origin of the early Precambrian crust of northeastern Minnesota

Details Geochemistry and origin of the early Precambrian crust of northeastern Minnesota Geochemistry and origin of the early Precambrian crust of northeastern Minnesota

Mar 1975

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1975gecoa..39..325a&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 39, Issue 3, pp.325-362

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79

Scientific paper

Igneous rocks of the Early Precambrian greenstone-granite complex of northeastern Minnesota consist of four major groups: 1. (1) metavolcanic tholeiitic basalt and basaltic andesite; 2. (2) metavolcanic trondhjemite gneiss, intrusive tonalite, extrusive and hypabyssal dacite and rhyodacite; 3. (3) intrusive quartz monzonite and; 4. (4) post-kinematic intrusive syenodiorite and syenite. Major and rare earth elements, Rb, Sr and Ba were precisely determined for rocks of each group. Quantitative trace-element modeling is combined with isotopic and experimentalpetrologio evidence to delimit the origin of the individual rock types: 1. (1) basalt by shallow melting of mantle peridotite followed by fractional crystallization to produce basaltic andesite; 2. (2) trondhjemite, tonalite and dacite by partial melting of eclogite at mantle depths; 3. (3) quartz monzonite by partial melting of short-lived graywacke at crustal depths; and 4. (4) syenodiorite and syenite by partial melting of a mixed peridotite-eclogite source at mantle depths. Initial Sr 87 / Sr 88 ratios of previously determined isochrons for all rock types are less than 0.701, and preclude magma origin by remelting of older granitic crust. The tectonothermal event 2700-2750 m.y. ago in northeastern Minnesota and adjacent Ontario represents addition of new erustal material from the mantle.

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