Mantle heterogeneity and crustal recycling in Archean granite-greenstone belts: Evidence from Nd isotopes and trace elements in the Rainy Lake area, Superior Province, Ontario, Canada
Abstract
The ≧2685-2752 Ma old granite-greenstone crust in the Rainy Lake area, Ontario, consists of metaigneous and metasedimentary rocks that range in composition from tholeiite to monzogranite and include anorthosite, trachyandesite, monzodiorite and high-silica rhyodacite. Major element, rare earth and other trace element data are the basis for modelling the formation of the crust by melting of large-ionlithophile element enriched and unenriched mantle, by melting of basalt at mantle to crustal levels and by melting of monzodiorite and tonalite at crustal levels. All metaigneous rocks lie on a 143Nd/144Nd vs. 147Sm/144Nd isochron with an age of 2737 ±42Ma and an initial 143Nd/144Nd of 0.509178 ±33 (ɛNd = +1.9). This age is consistent with U-Pb zircon ages, which suggests the Nd isotopic system has been unaffected since the crust-forming events. The positive initial ɛNd's are further evidence for time-averaged depletion in Sm/Nd relative to CHUR for the Archean mantle. The similarity of the initial Nd isotopic composition for both mantle-derived and crustally-derived rocks suggests rapid recycling of crustal components, which were previously derived from depleted mantle sources. Initial 143Nd/144Nd ratios on individual rocks range from ɛNd = +3.3 to ɛNd = -0.4. Younger granitoids have lower ɛNd values (+1.5 to -0.1) relative to tholeiites and monzodiorites crystallized from mantle-derived melts (+3.3 to +1.0). Thus, incorporation of slightly older crust (ca. 100-200 Ma) in some of the granitoid source areas is possible. Mantle-derived rocks form an isochron of 2764 ±58Ma that represents a minimum age for enrichment processes in the mantle sources for the Rainy Lake area. Consideration of data from the Abitibi belt suggests such enrichment processes in the mantle may have preceded crust-forming events in a wide area of the Superior Province, perhaps by as much as 50-70 Ma.
- Publication:
-
Geochimica et Cosmochimica Acta
- Pub Date:
- December 1986
- DOI:
- 10.1016/0016-7037(86)90215-2
- Bibcode:
- 1986GeCoA..50.2631S
- Keywords:
-
- Earth Crust;
- Earth Mantle;
- Granite;
- Neodymium Isotopes;
- Trace Elements;
- Ontario;
- Petrology;
- Rocks;
- Samarium Isotopes