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Journal of Petrology | Volume 43 | Number 9 | Pages 1617-1650 | 2002
© Oxford University Press 2002

Water-deficient Calc-alkaline Plutonic Rocks of Northeastern Superior Province, Canada: Significance of Charnockitic Magmatism

JOHN A. PERCIVAL1,* and JAMES K. MORTENSEN2

1GEOLOGICAL SURVEY OF CANADA, 601 BOOTH STREET, OTTAWA, ONTARIO, K1A 0E8, CANADA
2DEPARTMENT OF EARTH AND OCEAN SCIENCES, UNIVERSITY OF BRITISH COLUMBIA, 6339 STORES ROAD, VANCOUVER, B.C., V6T 1Z4, CANADA

Calc-alkaline batholiths of the Archaean Minto block, northeastern Superior Province, Canada, have pyroxene- and hornblende-bearing mineral assemblages inferred to have crystallized from hot, water-undersaturated magmas at 2·729–2·724 Ga. A regional amphibolite- to granulite-facies tectonothermal event at 2·70 Ga resulted in mild to negligible metamorphic effects on the dominantly granodioritic units. Geochemical, textural and thermobarometric studies define the crystallization history in compositions ranging from cumulate pyroxenite through quartz diorite, granodiorite, granite, and syn-magmatic gabbroic dykes. Early magmatic assemblages include orthopyroxene, clinopyroxene, plagioclase, biotite, Fe–Ti oxides and ternary feldspar, indicating crystallization from magmas containing <2 wt % H2O at 1100–900°C. Water enrichment in the residual melt induced hornblende crystallization at 5 ± 1 kbar, 800–600°C. Characterized by a continuum of large ion lithophile element (LILE)-enriched, high field strength element (HFSE)-depleted compositions, the I-type suite resembles modern continental arc batholiths in composition and size but not primary mineralogy. Magmatic arcs produced between 2·75 and 1·85 Ga commonly have charnockitic components, possibly because slab-derived fluids interacted with mantle wedges at ambient temperatures higher by ~100°C than at present, producing large volumes of water-deficient magma.

KEY WORDS: granitoid rocks; igneous pyroxenes; water-undersaturated magma; charnockite


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