Adakitic magmatism at the birth of the large Achala Intrusive Complex: Evidence of a thickened crust at the onset of Devonian foreland magmatism of SW Gondwana margin

Author(s)

Camera, Matias M. Morales ; Dahlquist, Juan A. ; Baldo, Edgardo G. ; Alasino, Pablo H. ; Moreno, Juan A. ; Fuenlabrada, Jose M. ; Basei, Miguel A. S.

• Document type: Journal article
• Source: LITHOS; v. 510, p. 30-pg., 2025-10-01.

Abstract

The San Clemente satellite pluton (SCP), located on the eastern flank of the Achala Intrusive Complex (AIC) or Achala Batholith in southwestern Gondwana, represents a crucial record of Devonian foreland magmatism. This study integrates petrological and geochemical data, including whole-rock Sr-Nd and zircon Hf isotopes, as well as geochronological data (U-Pb in zircon and titanite), to constrain the petrogenesis of the SCP. The pluton consists of two main units: the San Clemente granodioritic unit (SCU) and the La Herradura monzogranitic unit (LHU), along with quartz-dioritic and tonalitic enclaves. Petrographic and field evidence suggest an incremental, east-to-west magmatic construction history, involving sequential and nearly synchronous emplacement of quartz-dioritic and tonalitic magmas, followed by the SCU and subsequently the LHU. U-Pb geochronological data indicate a protracted magmatic history for the satellite pluton and its enclaves, spanning from the Early to Middle Devonian. This activity is recorded in both antecrysts (similar to 404 Ma) and autocrysts (390-384 Ma), marking the onset of construction of the Achala Intrusive Complex. The presence of antecrysts points to early magmatic processes occurring in a deep crustal mush reservoir, whereas the autocryst ages reflect the development of a magma chamber at or near the final emplacement level. Petrological, geochemical, and isotopic data classify the SCU as High-SiO2 Adakites (HSA) (SiO2 = 68.7-70.7 %, K2O/Na2O = 0.4-0.6, A/CNK (molar percent Al2O3/[CaO + Na2O + K2O]) = 1.04-1.05, with high Sr/Y and La-N/Yb-N ratios, suggesting a source derived from the partial melting of a thickened mafic lower crust with limited input from the mantle (epsilon Nd-t = -2.3 to -1.8; Sr-87/Sr-86(t) = 0.7047-0.7043; epsilon Hf-t = -9.0 to +2.2). The LHU exhibits hybrid compositions, combining peraluminous two-mica granites with adakitic affinities (SiO2 = 71.9 %, K2O/Na2O = 1.12, ASI = 1.07, with high Sr/Y and La-N/Yb-N ratios), indicative of a dominant felsic crustal source with minor mafic contributions (epsilon Nd-t = -4.6; Sr-87/Sr-86(t) = 0.7057; epsilon Hf-t = -12.0 to +2.2). The enclaves show geochemical affinities to primitive andesites and sanukitoids (SiO2 = 56.1-57.4 %, MgO = 5.2-7.0 %, Mg# (molar percent 100 x MgO/(MgO+FeOt)) = 59-65, Cr = 124-151 ppm, Ni = 121-124 ppm, Ba = 235-1323 ppm, Sr = 501-753 ppm, Ce = 47-275 ppm), reflecting a metasomatized lithospheric mantle source with crustal contamination (epsilon Nd-t = -2.8 to -3.0; Sr-87/Sr-86(t) = 0.7066-0.7091; epsilon Hf-t = -3.5 to +3.8). These findings indicate that the San Clemente pluton preserves key evidence that the formation of the AIC began with partial melting near the base of a thickened crust, followed by more extensive melting of the lower to middle crust, resulting in the formation of the voluminous, peraluminous A-type magmatism characteristic of the AIC. (AU)

• FAPESP's process: 18/06837-3 - U-Pb zircon geochronology and HF data from magmatic zircon in devonian-carboniferous granites as monitors of crustal growth and geodynamic setting, pre-andean continental margin of SW Gondwana
• Grantee: Miguel Angelo Stipp Basei
• Support Opportunities: Research Grants - Visiting Researcher Grant - International
• FAPESP's process: 15/03737-0 - The Rio de La Plata Craton and the Western Gondwana Assembly
• Grantee: Miguel Angelo Stipp Basei
• Support Opportunities: Research Projects - Thematic Grants