Birth of the Achala Intrusive Complex: Middle Devonian Adakitic magmatism in the pre-Andean margin of SW Gondwana (Sierras Pampeanas, Argentina)

One of the major exponents of Middle Paleozoic magmatism on the SW margin of Gondwana is the Achala Batholith (AB). This large intracontinental intrusive complex (> 2,500 km²), emplaced during the Late Devonian (359-379 Ma), is predominantly composed of felsic two-mica granitic units (mostly ≥70% SiO₂). These rocks are F-rich peraluminous A-type granites, derived from dominant crustal sources (εNd_t (whole-rock): -6.6 to -3.9; εHf_t (Zrn): -6.3 to -3.6) (Dahlquist et al., 2021).

New data indicate that the origin of the rocks that form the central-eastern margin of the AB is different from that of the rocks mentioned above. Two main granitic units were recognized in this region: La Herradura porphyritic monzogranite (LHU) and San Clemente granodiorite (SCU) (Pérez and Baldo, 1994). The SCU includes tonalitic and quartz-dioritic enclaves (QDU).

SCU consists of SiO₂-rich granodiorite (68.7-70.7% wt.) with geochemical characteristics of “High-Silica Adakite” HSA) (Martin et al., 2005) (i.e., high Na₂O contents (~4.19% (HSA); 4.92% (SCU)), low K₂O/Na₂O (~0.47; 0.50), high Mg# (~0.48, 0.46), and high ratios of Sr/Y (~55.65; 48.2) and La/Yb (~21.8; 24.8)). LHU exhibits geochemical features similar to the typical granites of the AB mentioned above. However, these features show slight differences pointing to HSA compositions, such as higher Na₂O content (3.88% (LHU) vs. 3.13% av. (AB (n=41)), lower K₂O/Na₂O (1.12 vs. 1.68 av.), higher Mg# (0.42 vs. 0.30 av.), as well as, lower ISA value (1.11 vs. 1.28 av.), and higher ratios of Sr/Y (22.0 vs. 4.4 av.) and La/Yb (44.6 vs. 30.9 av.).

The individual ²³⁸U/²⁰⁶Pb zircon ages from the main units and QDU show a wide range from ~380 to ~400 Ma, indicating the presence of zircon autocrysts and antecrysts crystallizing over a prolonged magmatic activity. Using the youngest age populations (i.e., zircon autocrysts), a weighted mean crystallization age of 388 ± 2 Ma (MSWD = 1.2, n = 11) is calculated for SCU, 388 ± 3 Ma (MSWD = 0.38, n = 10) for LHU, and 384 ± 2 Ma (MSWD = 0.76) for QDU. These ages suggest quasi-synchronous magmatism for the different units and extend the onset of plutonic activity of the AB, at least, into the Middle Devonian.

The Hf isotopes in zircon (εHf_t) show that: i) SCU has a range from −7 to +1 (with an outlier value of +7) and a median of −3; ii) LHU has a range from −12 to +2 and a median of −5; and iii) QDU has a range from −4 to +4 and a median of 0. The variation in εHf_t within each unit, in addition to that identified among units, suggests variable interaction between a cortical source and juvenile mafic magmas occurred during the prolonged magmatic activity. This source variability is not recorded by the Sm-Nd isotopic system in whole rock, where the SCU and QDU have a similar value of εNd_t (−2.3 and −2.8, respectively).

Recently, Dahlquist et al. (2021) identified small bodies of HSA in the central sector of the AB (with two age peaks at 373-381 Ma), suggesting that the HSA were originated from the interaction between a melt derived from the oceanic slab and the continental crust. We suggest a similar origin for these oldest units of the AB.

References

Dahlquist, J.A., Morales Cámera, M.M., Alasino, P.H., Pankhurst, R.J., Basei, M.A.S., Rapela, C.W., Moreno, J.A., Baldo, E.G., Galindo, C., 2021. A review of Devonian-Carboniferous magmatism in the central region of Argentina, pre-Andean margin of SW Gondwana. Earth Sci. Rev. 221, 103781.

Martin, H., Smithies, R.H., Rapp, R., Moyen, J.F., Champion, D., 2005. An overview of adakite, tonalite–trondhjemite–granodiorite TTG, and sanukitoid: relationships and some implications for crustal evolution. Lithos 79, 1–24.

Pérez, E.M.B., Baldo, E.G., 1994. Petrofacies graníticas del sector centro-oriental del batolito de Achala. Córdoba, Argentina. 7th Chilean Geological Congress, vol. 2: 1146–1150. University of Concepción, Chile.