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

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 (∼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 (εNd_t = −2.3 to −1.8; ⁸⁷Sr/⁸⁶Sr_t = 0.7047–0.7043; εHf_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 (εNd_t = −4.6; ⁸⁷Sr/⁸⁶Sr_t = 0.7057; εHf_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 × 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 (εNd_t = −2.8 to −3.0; ⁸⁷Sr/⁸⁶Sr_t = 0.7066–0.7091; εHf_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.