Tracing sources and melting processes: Hf isotope variability in zircons from the S-type Cupim pluton

Melts resulting from the partial melting of metasedimentary rocks display intricate radiogenic isotope signatures, influenced by both the disequilibrium nature of crustal anatexis at relatively low temperatures and the mixing of isotopically distinct sources. Consequently, the whole-rock and mineral radiogenic isotope compositions (e.g., Sr, Nd, and Hf) exhibit significant dispersion and variation, often exceeding 10 units of normalized ε values. Therefore, their isotopic composition does not solely reflect a single source signature but rather indicates the extent to which accessory phases participated in the partial melting process or the contribution of each sedimentary source end-member.

The Cupim Pluton, situated in the Paleoproterozoic Mineiro Belt, Brazil, consists of sediment-derived granites (two-mica and garnet-bearing) with minor magmatic differentiation. Zircon grains within these granites frequently display core/rim textures, which were dated using the U-Th-Pb system, and their Hf isotopic compositions were measured using LA-(MC)-ICP-MC techniques. The inherited cores predominantly exhibit Neoarchean ages with minor Paleoproterozoic ages, while uniform crystals and rims display ages from 2013 ± 23 to 1977 ± 42 Ma, interpreted as the crystallization age of these granites. This is the youngest magma production stage during the Paleoproterozoic Minas orogeny in the southern São Francisco Craton (see Alkmim and Teixeira, 2017) and marks the last collisional event. The ε_Hf(t) values for the Neoarchean cores are generally positive to sub-chondritic, whereas ~2000 Ma zircons exhibit a wide negative range of values (from -12 to -1).

Different patterns of zircon populations are observed: those with a significant inherited component display more negative and narrower values of ε_Hf(2000) compared to populations lacking inherited zircons. Hence, the Hf composition of newly formed Cupim zircons is influenced by the degree of dissolution and homogenization of inherited zircons, a process known as the ‘core effect’. Conversely, the mineralogy and chemical composition of these leucogranites suggest source heterogeneity, indicating that the Hf isotopic composition of zircons may also reflect this source mixture. Indeed, the isotopic signature of detrital zircons from two surrounding metasedimentary rocks (metagraywacke with ε_Hf(t) ~ -11 and metadiamictite with ε_Hf(t) ~ 0) can account for the widely varied Hf composition of Cupim zircons.

However, garnet-bearing leucogranites, derived from clay-rich sources, do not exhibit more positive ε_Hf(t) values compared to those derived from clay-poor sources. Thus, the isotopic composition of Cupim zircons primarily reflects the ‘core effect’ and secondarily attests to source heterogeneity.

We thank CAPES and Instituto GeoAtlântico for their financial support.

Alkmim, F.F., Teixeira, W., 2017. The Paleoproterozoic Mineiro Belt and the Quadrilátero Ferrífero. pp. 71–94. https://doi.org/10.1007/978-3-319-01715-0_5