Old and juvenile source of Paleozoic and Mesozoic basaltic magmas in the Acatlán and Ayú complexes, Southern Mexico: Nd isotopic constraints

• Three episodes of 600–190 Ma rift tholeiites: lower and upper Paleozoic and Mesozoic.
• 1 Ga subcontinental lithospheric mantle source for Neoproterozoic–Ordovician.
• 1 Ga + juvenile mantle source for 365–330 Ma and 210–190 Ma mafic rocks.
• Alternation of rifting and underthrusting related to variations in Beniof zone dip.

The Neoproterozoic–Paleozoic Acatlán Complex and the Mesozoic Ayú Complex of southern Mexico consist of clastic rocks and rift-related igneous rocks inferred to have originated along a rifted passive margin in southwestern Mexico, either in an inactive (Neoproterozoic–Ordovician) or an active (Devonian–Carboniferous and Triassic–Early Jurassic) tectonic setting. The latter formed on the inner margin of a backarc basin. These passive margin rocks were partly underthrust beneath the Acatlán Complex, into which they were subsequently extruded: extrusion was synchronous with the backarc basin development. Thus, (i) the Neoproterozoic–Ordovician rocks underwent underthrusting, high-pressure metamorphism, and extrusion during the Late Devonian–Carboniferous (365–330 Ma); (ii) the Carboniferous rocks underwent underthrusting, amphibolite facies metamorphism, and extrusion during the Permian and Triassic; and (iii) the Triassic–Lower Jurassic rocks underwent underthrusting, amphibolite facies metamorphism, and extrusion during the Jurassic. Nd isotopic data from tholeiitic mafic rocks on either side of the HP extrusion zone reveal that both were underlain by similar peri-Rodinian subcontinental lithospheric mantle in the Neoproterozoic–Ordovician, which was supplemented in the Devonian–Carboniferous and Triassic–Early Jurassic by a juvenile depleted mantle source. The alternation of underthrusting and backarc rifting accompanied by extrusion may be related to flattening and steepening of the Beniof zone, respectively.

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