Unravelling source variations in arc and back-arc magmas with strong crustal signature: A case study in the Famatinian Orogen

DE ABREU BARBOSA ARAUJO, LUCAS EDUARDO ; WEINBERG, ROBERTO; ORTIZ JOYA, GUILLERMO; MORFULIS, M.; SOLA, ALFONSO; BECCHIO, R. A.; SUZAÑO, NÉSTOR O.

Baveno

Simposio; 10th Hutton Symposium on Granites and Related Rocks; 2023

The genesis of granitic magmas has been extensively discussed in the literature. In this work, we aim to contribute to this discussion using field-based evidence and zircon studies in arc and back-arc granitoids from Famatinian Arc, Puna Plateau, Argentina. The arc rocks are represented by a mafic enclave-bearing tonalite (MAC – Graphic 1a), a hybrid granite (TT – Graphic 1b) and a dacite (CAL). The back-arc rocks are represented by a leucocratic garnet-muscovite-biotite granite (LB – Graphic 1c) with igneous mme and xenoliths of metasedimentary rocks, and a biotite-granite (LA – Graphic 1d). All these rocks present a widespread range on U-Pb zircon crystallization ages, ranging from ca. 510 to 440 Ma. For the arc rocks, U-Pb titanite ages constrains the magmatic crystallization at around ca. 470 Ma. All of these granitoids present U-Pb apatite magmatic ages constrained at ca. 460-440 Ma. All these granitoids have zircon crystals presenting similar mean εHf(t) values (around -3), except for the LB granite in the back-arc domain which has mean εHf(t) values around -0.3 (Graphic 2a). This similarity could be explained by a significant crustal input that homogenizes the signature of these granitoid magmas. The significant proportion of inherited zircon crystals also corroborates with that assumption. Based on trace elements in zircon, the arc granitoids are less fractionated (Eu/Eu*=0.23 to 1.0 and lower Zr/Hf – Graphic 2b) and more oxidized (mean ΔFMQ= +0.9 to +1.3 – Graphic 2c) than the CA arc dacite and back-arc granites (Eu/Eu*= 0.11 to 0.35; higher Zr/Hf; mean ΔFMQ= -1.5 to +0.3 – Graphics 2b and c). The back-arc grt-bt-musc granite (LB) presenting a cluster of high oxidized zircon crystals (ΔFMQ: +3.0 to +6.6) and more juvenile zircon crystals (εHf(t) ~ -0.3 – Graphic 2c) possibly indicate a mafic input in the back-arc. Heterogeneities on sources from arc magmas have also been identified using molar proportions of P, Y and REE in zircon crystals (Graphic 2d). MAC tonalite crystallizes from typical I-type sources, whereas CA dacite has dominantly S-type sources, and the TT hybrid granite presents grains from both I- and S-type sources. This bimodality in the hybrid granite is corroborated by strong evidence of magma mixing in the field.