Evidence for Paleoproterozoic anatexis and crustal reworking of Archean crust in the São Francisco Craton, Brazil: A dating and isotopic study of the Kinawa migmatite

Abstract

New SHRIMP U-Pb zircon dating indicates that, contrary to previous thought, part of the Archean crust in the southern São Francisco Craton (SFC), NW of the Jeceaba Bom Sucesso Lineament was affected by the Paleoproterozoic Transamazonian orogeny. The Kinawa migmatite is a reworked part of the Archean crust in the southern SFC, and occurs within the Cláudio Shear Zone, a NE-NW crustal-scale structure. The protolith of the migmatite is a metagranodiorite that has an U-Pb zircon crystallization age of 2704±3Ma coeval with the voluminous potassic magmatism in the region. Moreover, the εHf(t=2704) data suggests a slightly evolved crust (−1.5 to −6.1) and thus a relatively long crustal residence (∼700Ma). Surprisingly, the migmatite itself yielded much younger ages of 2034±32Ma and 2048±25Ma, which coincide with the ages of the Mineiro Belt and Mantiqueira Complex. The distribution of ages using data from the literature and this study show that within the Archean domain the occurrence of Paleoproterozoic ages is restricted to the Cláudio Shear Zone. Inherited zircon crystals from all samples yield Neoarchean ages (∼2.7Ga), and similar Hf signatures to the metagranodiorite protolith (εHf(t=2704)=−5.5 to −0.5). The εHf(t) signatures of the Kinawa migmatite are heterogeneous and record two events, one is interpreted to be the formation of the protolith with lower 176Hf/177Hf (at ∼2.7Ga) and the other with higher 176Hf/177Hf (at ∼2.05Ga) as the anatexis that produced the migmatite. An important result is that the migmatite had two sources. Two samples are consistent with derivation from an evolved crust comparable with the metagranodiorite protolith (e.g. 176Lu/177Hf ∼0.01 and εHf(t=2034) −13.4 to −17), but the other sample has much less negative values (εHf(t=2048)=−4.6 to −8.7), which requires a different source with much lower 176Lu/177Hf, and therefore younger, possibly Post-Archean material.