Mid-Jurassic Tectonothermal Event Superposed on a Paleozoic Geological Record in the Acatlán Complex of Southern Mexico: Hotspot Activity During the Breakup of Pangea

U–Pb geochronological constraints on the Triassic–Jurassic Ayú Complex, southern Mexico: Derivation from the western margin of Pangea-A

“…160 and 171 Ma, which are broadly similar to the Sm-Nd and Rb-Sr whole rock garnet and biotite ages (172 ± 1 Ma and 175 ± 3, respectively; Yáñez et al, 1991) as well as the 161 ± 2 Ma 40 Ar/ 39 Ar muscovite cooling age . The U-Pb data are also similar to the age of migmatization as dated in a leucosome in the southern part of the area (171 ± 1 Ma -single grain thermal ionization mass spectrometry age; Keppie et al, 2004), suggesting that the genesis of granitic dikes and the migmatites may be related. The rest of the dated zircons in the igneous rocks are interpreted as inherited xenocrysts as they are also found in the metasedimentary host rocks (compare histograms in Figs.…”

“…The area is underlain by a poly-deformed and metamorphosed flysch-or turbiditic siliciclastic sequence consisting of micaceous pelites interlayered with metapsammites, metagreywackes, and amphibolites (Chazumba Formation: Ortega-Gutiérrez, 1978; Chazumba Lithodeme: Keppie et al, 2006b). Similar metasedimentary rocks in the southern study area (Magdalena Lithodeme: Keppie et al, 2006b) underwent partial melting at 171 ± 1 Ma (Magdalena Migmatite; U-Pb concordant zircon age: Keppie et al, 2004) accompanied by the intrusion of dioritic, granodioritic, granitic, pegmatitic as well as leucogranitic sheets and dikes of the same age (San Miguel intrusives: Yáñez et al, 1991;Ortega-Gutiérrez et al, 1999;Keppie et al, 2004;Fig. 4).…”

“…4). The Devonian-Carboniferous Cosoltepec Formation (Talavera-Mendoza et al, 2005), the Permian Tecomate Formation and the Carboniferous-Permian Totoltepec pluton are exposed in the hanging wall of this shear zone (Malone et al, 2002;Keppie et al, 2004;Kirsch et al, in press;Fig. 3).…”

“…350°C took place by 161 ± 2 Ma and 154 ± 6 Ma in the Tepejillo area and through ca. 280°C by 150 ± 2 Ma in the Ayú area ( 40 Ar/ 39 Ar muscovite and biotite cooling ages; Keppie et al, 2004) implying a cooling rate of 20-35°C/ Ma; and (v) extrapolation of this cooling rate suggests that the Ayú Complex would not have reached the surface until ca. 152-147 Ma (latest Jurassic/earliest Cretaceous).…”

“…Comparison of different stratigraphies by several authors(Ortega-Gutiérrez et al, 1999;Keppie et al, 2004;Talavera-Mendoza et al, 2005;Keppie et al, 2006bKeppie et al, , 2008bMorales-Gámez and Keppie, 2008; Kirsch et al, in press) for the eastern units in the Acatlán Complex. This work proposes the exclusion of the Chazumba Lithodeme (including the Magdalena Migmatite), the ultramafic Tepejillo and Tultitlán lenses (black), the ortho-amphibolites, and the San Miguel intrusives from the Acatlán Complex and their placement into a new complex -the Early Mesozoic Ayú Complex.…”