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.A.; PANKHURST, R.J; GASCHNIG, R.M.; RAPELA, C.W.; CASQUET, C.; ALASINO, P.H.; GALINDO, C.; BALDO, E.G

Medellín

Simposio; VIII South American Symposium on Isotope Geology; 2012

GEMMA

Early-Middle Ordovician granitic magmatism is by far the most voluminous of the Sierras Pampeanas and represents the most significant magmatic event. These granitoids are calc-alkaline and were intruded at an active continental margin. _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 Ga and 1.4 to 1.7 Ga, respectively. Middle–Late Devonian magmatism occurred in the foreland, away from the orogenic front in the west, and included F-U-REE rich A-type granites. The Achala granite (368 Ma) is the largest batholith in the Sierras Pampeanas 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 -4.0 to -6.5, respectively. Rapela et al. (2008) found rather less radiogenic Nd in small tonalite and leucogranite bodies (_Ndt = -1.2 to -1.9, mean = -1.6, and _Ndt = -1.1 to -1.4, respectively); these minor intrusions could represent a more juvenile magma ascending through local conduits. The Early Carboniferous A-type granites were emplaced at ca. 343 Ma. Small scattered plutons were intruded along extensional shear zones, in a dominantly extensional setting. These granites have _Hft and _Ndt values ranging from -6.7 to +2.2 and -0.5 to -3.6, respectively. The average _Ndt for Carboniferous granitoids (data from Dahlquist et al., 2010) is -2.3. Thus our current isotopic data indicate that the Ordovician magmatic episodes in the proto  Andean margin of Gondwana are dominantly related to crustal reworking and stabilization, whereas the Late Paleozoic magmatism involved some (limited) continental growth, especially during the Early Carboniferous.   REFERENCES Dahlquist et al., 2010. Lithos, 115, 65 Rapela et al., 2008. SSAGI, 6th, CD-ROM, Extended Abstract 53.