Journal of Petrology | Volume 43 | Number 8 | Pages 1551-1570 | 2002
© Oxford University Press 2002
A Mainly Crustal Origin for Tonalitic Granitoid Rocks, Superior Province, Canada: Implications for Late Archean Tectonomagmatic Processes
1GEOLOGICAL SURVEY OF CANADA, 601 BOOTH STREET, OTTAWA, ON, CANADA K1A OE8
2DEPARTMENT OF EARTH SCIENCES, UNIVERSITY OF WESTERN ONTARIO, LONDON, ON, CANADA N6A 5B7
The central Wabigoon subprovince of the Superior Province, like most plutonic domains within Archean cratons, is dominated by granitoid rocks of the tonalite–trondhjemite–granodiorite (TTG) series. Heterogeneous <2·83–2·74 Ga tonalite gneisses and foliated tonalite to granodiorite units, emplaced at 2·722–2·709 Ga, exhibit initial 
Nd values (–3·1 to +3·3) indicative of variable input from light rare earth element enriched older (3·2–3·4 Ga) crustal materials. Their 
18O (VSMOW) range (+7·1 to +8·9
), which overlaps closely that of average upper Superior Province crust, indicates input from high-18O crustal materials. The preferred petrogenetic model for Wabigoon tonalitic rocks involves partial melting of overthickened amphibolite-dominated lower-crustal materials within a Cordilleran-type arc. Assimilation of >2·74 Ga tonalite gneiss crust by younger tonalite magmas was probably an important process. Unlike the model of TTGs representing direct partial melts of subducting slabs in an arc setting, this model requires no direct tie to subduction. Careful re-evaluation of the TTG classification is required for it can mistakingly ‘pigeon-hole’ temporally and genetically unrelated rocks and perhaps assume an incorrect petrogenetic or tectonic model. An important role of crustal recycling processes in central Wabigoon tonalite petrogenesis is in keeping with evidence that supports substantial rates of continental recycling as far back as the earliest Archean.
KEY WORDS: Archean; crustal evolution; Nd–O isotopes; TTG
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