| Popis: |
The 3.5–2.6 Ga rocks of the Minnesota River Valley (MRV) provide a unique opportunity to examine rocks that span the now recognized crucial time (3.2–3.0 Ga) on the Earth when Wilson style plate tectonic processes likely started. To elucidate the crustal evolutionary history of these ancient rocks we obtained Hf isotopic compositions by LA-MC-ICPMS from zircon, previously dated by SHRIMP U–Pb methods; whole-rock Nd isotopic compositions by TIMS; and whole-rock major and trace element concentrations. The oldest, ca. 3500 Ma rocks have major and trace element compositions similar to other Archean tonalite–trondhjemite–granodiorite (TTG) suites, which are consistent with derivation from a mafic source. The ca. 3500 Ma Morton and Montevideo gneisses have similar initial ɛ Hf values that range from 1.9 to −2.6 at 3500 Ma, with model ages ranging from 3.83 to 3.64 Ga. The model ages are interpreted as resulting from melting of a source that is >100 Ma older than the ca. 3500 Ma gneisses. Elevated Nb/Ta ratios and large negative Nb, Ta and Ti anomalies suggest that the source may have been eclogite, consistent with melt formation in an overthickened basaltic lithosphere. Younger (3380 Ma), more chemically evolved, intrusions into both the Morton and Montevideo gneisses plot on a closed-system isotopic evolution path (Nd, Hf), indicating that they were in situ melts of the older ca. 3500 Ma rocks. These intrusions contrast with a more mafic 3140 Ma intrusion, which has zircon with more positive ɛ Hf than the closed-system isotopic evolution defined by the older gneisses. The last major event was the emplacement of the 2604 Ma Sacred Heart granite and formation of a garnet-biotite paragneiss. Isotopic values for the Sacred Heart granite show a large juvenile component relative to all other rocks, suggesting it is a mix of juvenile material and recycled Morton or Montevideo gneiss. The transition from closed to open system evolution of rocks in the MRV at 3140 Ma, and continuing at 2600 Ma, is consistent with a proposed global change in continental crust formation and evolution that began at 3.2–3.0 Ga. |
Full Text from ScienceDirect