CRUSTAL EVOLUTION OF ARCHEAN ROCKS FROM THE MINNESOTA RIVER VALLEY: CONSTRAINTS FROM HF ISOTOPES

Ancient rocks in the Minnesota River Valley, southwestern Minnesota, occur in the Morton and Montevideo structural blocks, separated by the Yellow Medicine shear zone.  Major units in the Morton block are the ca. 3500 Ma Morton gneiss and the 2604 Ma Sacred Heart granite, but a small 3385 Ma granodiorite intrusion also occurs.  The Montevideo block mainly consists of the 3500 Ma Montevideo gneiss, but a 3380 Ma granodiorite and a 3140 Ma mafic intrusion also occur. In the Montevideo block a garnet-biotite paragneiss contains only 2600 Ma zircons, similar in age to zircon overgrowths found in both blocks. We obtained LA-MC-ICPMS Hf isotopic compositions from zircons previously dated by SHRIMP U-Pb methods. Morton and Montevideo gneisses have similar initial ε_Hf values at 3500 Ma, with model ages ranging from 3760 to 3400 Ma, suggesting they  were mainly mantle derived at ca. 3500 Ma, but could have incorporated components of crust older than 3760 Ma.  Zircons from younger intrusions into both the Morton and Montevideo gneisses, including one from the Sacred Heart granite, have ε_Hf values that plot on the same isotopic evolution path as the gneisses themselves, indicating derivation from ca. 3500 Ma lithosphere similar to the Morton and Montevideo gneisses. Thus the entire terrane remained a closed system for approximately 1 b.y. Zircons from the 2604 Ma Sacred Heart granite of the Morton block and the ca. 2600 Ma garnet-biotite paragneiss of the Montevideo block display less negative, more juvenile initial ε_Hf values. The Sacred Heart granite is likely a mixture of juvenile mantle melt and recycled Morton gneiss.  The garnet-biotite paragneiss, with ε_Hf values similar to those of the Sacred Heart granite, likely had a greywacke protolith derived from an extrusive equivalent of the Sacred Heart granite. These observations are consistent with generation of the Sacred Heart granite and the protolith of the garnet-biotite paragneiss during suturing of the Minnesota River Valley terrane to the southern margin of the Superior Province about 2600 Ma. The similar crystallization ages of the Morton and Montevideo gneisses combined with similar initial ε_Hf values suggest that Yellow Medicine shear zone is not a structural boundary between two terranes, but likely represents localized shear during the construction of Superior Province.