Hf and Nd isotopes in Early Ordovician to Early Carboniferous granites as monitors of crustal growth in the proto-andean margin of Gondwana

DAHLQUIST, J.; PANKHUSRT, R.; GASCHNING, R.; RAPELA, C.W.,; CASQUET, C.; ALASINO, P.; GALINDO, C; BALDO, E.,

Simposio; VIII Simposio Sudamericana de Geología Isotópica; 2012

The Early-Middle Ordovician granitic magmatism is by far the most voluminous of the Sierras Pampeanas and thus represents the most significant magmatic event. These granitoids are calc-alkaline and were intruded at an active continental margin. For these samples Hft and Ndt values range from -3.3 to -14.7 and -3.3 to -6.3 (t = 473 Ma), respectively, with average TDM Hf and TDM Nd ranging from 1.5 to 2.2 and 1.4 to 1.7, respectively. During the Middle-Late Devonian, magmatism took place in a foreland position away from the orogenic front in the west. F-U-REE rich A-type granites formed at this time. The Achala granite, the largest batholith in the Sierras Pampeanas has ca. 368 Ma, and intruded rocks formed during Cambrian (Pampean) and Ordovician (Famatinian) events. It has Hft and Ndt values ranging from -3.6 to -5.8 and -4.o to -6.5, respectively. Remarkably, Rapela et al. (2008) found rather less radiogenic Nd in small-scale tonalite and leucogranites bodies (Ndt = -1.2 to -1.9, mean = intrusions could represent a more juvenile magma ascending through local conduits. The Early Carboniferous A plutons were intruded along extensional she These granites have Hft and respectively. The average Ndt (2010) is -2.3. Thus the magmatic episodes in the proto Ordovician are dominantly related to crustal reworking and stabilization, rather than the formation of new continental crust by juvenile material accretion, whereas the Late Paleozoic magmatism involved some (although limited) continental growth, especially during the Early Carboniferous