GACMAC 2004 : Special Session – Evolution of deep to mid-crustal terrains through time.
A Tale of Two Crustal Levels: Probing Neoarchean mid- and lower-crustal
orogenic development utilizing magmatic and inherited zircons from the
Snare River Terrane, southwestern Slave Province.
Venessa Bennett 1, Toby Rivers 1, Valerie Jackson2 and Carolyn Relf 2
1. Memorial University of Newfoundland, St. John's.
2. C.S. Lord Northern Geoscience Centre, Yellowknife
The Snare River terrane (SRT), southwestern Slave Province, which records ~ 90 M.y. of Neoarchean crust
formation and evolution, is a distinct polydeformed and polymetamorphosed orogen exposed at three crustal levels:
(i) upper-crustal, greenschist- to amphibolite-facies supracrustal belts, (ii) a mid-crustal wedge composed of
granulite-facies gneisses and granitoid rocks, and (iii) intervening granitoid and metasedimentary rocks.
Geochronological studies, utilizing U-Pb zircon crystallization ages and inheritance trends, of representative plutons
are used to probe mid- and lower-crustal development of the SRT.
Formation and growth (~2674-2610 Ma) of the mid-crust of the SRT was characterized by episodic
additions of metaluminous magmas derived from the lower-crust. The orogenic peak, (~2600 Ma), which marked a
significant change in the rate, abundance and type of magmatism, was characterized by emplacement of voluminous,
lower- and mid-crustally sourced meta- and peraluminous plutons and associated with granulite-facies
metamorphism, in part driven by magmatic heat advection at mid-crustal emplacement levels. Post-peak (~26002580 Ma) evolution of the mid-crust featured tectonic collapse, mid-crustal exhumation, small-volume meta- and
peraluminous magmatism and high temperature – low pressure metamorphism. The U-Pb data imply that crustal
growth and evolution was not uniform in time or space. In particular, there is a first-order younging of crust with
depth, such that magmas formed at the orogenic peak predominate in the mid-crust.
Inherited zircon age data from magmas that intruded the mid-crust during its evolution provide
constraints on the nature of age diversity in lower-crustal protoliths. Inheritance trends of peraluminous magmas
exhibit similar age trends to detrital zircon populations determined for meta-turbidites from the SRT, supporting a
predominantly metasedimentary origin. Inheritance trends of metaluminous magmas illustrate a progressive
evolution from an ‘isotopically homogeneous’ lower crust (i.e., little to no inheritance) at the onset of crustformation of the SRT to the gradual development of an ‘isotopically heterogeneous’ lower crust comprising multiple
age components ranging in age from ~2770 to 2630 Ma. These trends can be accounted for by the processes of
magmatic addition and tectonic transport of material to the SRT lower-crust during orogenesis.
In summary, mid-crustal growth and evolution principally involved redistribution of lower-crustal
material by metaluminous and peraluminous magmatism, and lower crustal evolution entailed periodic emplacement
of mantle-derived melts and tectonic transport of material into the melt region at the orogenic peak. Collectively, the
zircon crystallization and inheritance datasets reveal two separate, yet closely interrelated tales of Neoarchean midand lower-crustal evolution in the SRT.
Reference: Bennett,V., Jackson., V., Rivers, T. and Relf, C., 2004. A Tale of Two Crustal Levels:
Probing Neoarchean mid- and lower-crustal orogenic development utilizing magmatic and inherited zircons from the
Snare River Terrane, southwestern Slave Province. In Geological Association of Canada – Mineralogical association
of Canada – Society of Economic Geologists(GAC-MAC-SEG) Annual Meeting, St Catherines 2004, Program with
Abstracts, 29: 327