Abstract

The Lacorne Block in the Southern Volcanic Zone of the Abitibi Subprovince is composed of interleaved metavolcanic and metasedimentary rocks that are intruded by syn- to posttectonic diorites, granodiorites, and granites. These rocks form the Lacorne, Lamotte, and Preissac plutons, which can be divided into an early suite of dioritic–granodioritic rocks and a later suite of S-type, leucocratic granites with an estimated age of 2640 Ma. This study presents Sm–Nd data and U–Pb monazite and titanite ages for the late leucocratic granites of the Preissac and Lamotte plutons. A biotite–muscovite monzogranitic phase of the Lamotte pluton is dated at 2647 ± 2 Ma, but similar phases of the Preissac pluton are dated at 2681–2660 Ma. These ages extend the period of leucogranitic plutonism for this area to 40 Ma and suggest that the age of collision of the Abitibi and the Pontiac subprovinces occurred before 2685 Ma. The εNd values for the leucogranites range from −1 to +3 and suggest an origin largely through melting of sediments having a juvenile isotopic signature (i.e., a short crustal residence time). Possible sources of the leucogranites include metasedimentary rocks of the Pontiac Subprovince, the Lacorne Block, and the Southern Abitibi Volcanic Zone, but the εNd values of the granites are most consistent with melting of metasediments of the Southern Volcanic Zone. We suggest that sediments of the Southern Volcanic Zone formed an accretionary prism along the southern continental margin of the Abitibi before collision with the Pontiac Subprovince. This prism was subsequently trapped between the two colliding margins, subducted, and partially melted to produce the Lamotte, Preissac, and Lacorne leucogranites.

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