ABSTRACT The supercontinent of Pangea formed through the diachronous collision of Laurussia and Gondwana during the late Paleozoic. While magmatism associated with its formation is well documented in the Variscan orogeny of Europe and Alleghanian orogeny of the United States, little is known about the Sonora orogeny of northern Mexico. This paper reports geochronology (U-Pb zircon), whole-rock geochemistry, and Lu-Hf zircon isotope data on basement cores from the western Gulf of Mexico, which were used to develop a tectonomagmatic model for pre- to post-Pangea amalgamation. Our results suggest the existence of three distinct phases of magmatism, produced during different stages of continental assembly and disassembly. The first phase consists of Early Permian (294–274 Ma; n = 3) granitoids with geochemical signatures indicative of a continental arc tectonic setting. This phase formed on the margins of Gondwana during the closure of the Rheic Ocean, prior to the final amalgamation of Pangea. It likely represents a lateral analogue of late Carboniferous–Early Permian granitoids that intrude the Acatlán and Oaxacan Complexes. The second phase of magmatism includes Late Permian–Early Triassic (263–243 Ma; n = 13) granitoids with suprasubduction geochemical affinities. However, Lu-Hf isotope data indicate that these granitoids formed from crustal anatexis, with ε Hf values and two-step Hf depleted mantle model ages (T DM[Hf] ) comparable to the Oaxaquia continental crust into which they intrude. This phase of magmatism is likely related to coeval granitoids in the Oaxaca area and Chiapas Massif. We interpret it to reflect late- to postcollisional magmatism along the margin of Gondwana following the assembly of Pangea. Finally, the third phase of magmatism includes Early–Middle Jurassic (189–164 Ma; n = 2) mafic porphyries, which could be related to the synchronous suprasubduction magmatism associated with the Nazas arc. Overall, our results are consistent with Pangea assembly through diachronous collision of Laurussia and Gondwana during subduction of the Rheic Ocean. They suggest that postorogenic magmatism in the western termination of the Rheic suture occurred under the influence of a Panthalassan subduction zone, before opening of the Gulf of Mexico.

The Acatlán Complex of southern México comprises metasedimentary and metaigneous rocks that represent the vestige of a Paleozoic ocean. Juxtaposed against granulite-facies gneisses of Mesoproterozoic (ca. 1 Ga) age, the complex has previously been related to the Iapetus Ocean and interpreted to preserve a tectonostratigraphic record linked to that of the Appalachian orogen: (1) Cambro-Ordovician deposition of a trench or forearc sequence (the Petlalcingo Group: the Magdalena, Chazumba, and Cosoltepec Formations) and an oceanic assemblage (the Piaxtla Group), (2) polyphase Late Ordovician–Early Silurian deformation (the Acatecan orogeny) during which the Piaxtla Group underwent eclogite-facies metamorphism synchronous with megacrystic granitoid emplacement, (3) deposition of the arc-related Tecomate Formation and intrusion of megacrystic granitoid plutons during the Devonian, and (4) deformation under greenschist-facies conditions during the Late Devonian Mixtecan orogeny. However, recent structural, geochronological, and geochemical studies have shown that (1) the Cosoltepec Formation is bracketed between ca. 455 Ma and the latest Devonian and may be part of a continental rise prism with slivers of oceanic basalt; (2) the Magdalena and Chazumba Units represent a clastic wedge assemblage of Permo-Triassic age; (3) the eclogitic metamorphism is locally Mississippian in age; (4) the Tecomate Formation is an arc complex of latest Pennsylvanian–Middle Permian age; (5) the megacrystic granitoid rocks span the Ordovician and have a calc-alkaline geochemistry, whereas accompanying mafic units have mixed continental arc–tholeiitic affinities and are locally as young as the earliest Silurian; (6) the greenschist-facies tectonothermal event occurred in the Permo-Triassic; and (7) the complex records a Jurassic tectonothermal event that resulted in local high-grade metamorphism and migmatization. This revised geological history precludes any linkage to Iapetus, but is consistent with that of the Rheic and paleo-Pacific Oceans and is interpreted to record (1) development of a rift or passive margin on the southern flank of the Rheic Ocean in the Cambro-Ordovician, (2) formation of either an arc or an extensional regime along the formerly active northern margin of Gondwana throughout the Ordovician, (3) ocean closure documented by subduction-related eclogite-facies metamorphism and exhumation during the Late Devonian–Mississippian, (4) Permo-Triassic convergent tectonics on the paleo-Pacific margin of Pangea, and (5) overriding of a Jurassic plume.

The Granjeno Schist of northeastern México is the oldest component of the Sierra Madre terrane and comprises polydeformed, pelitic metasedimentary and metavolcaniclastic rocks that enclose lenses of serpentinite-metagabbro. This low-grade Paleozoic assemblage is exposed in the core of a NNW-trending frontal anticline of the Laramide fold-thrust belt where it is tectonically juxtaposed against ca. 1 Ga granulites of the Novillo Gneiss. Silurian strata that unconformably overlie the Novillo Gneiss are unmetamorphosed and contain fauna of Gondwanan affinity. LA-ICPMS (laser ablation inductively coupled plasma mass spectroscopy)U-Pb ages for detrital zircons from a Granjeno phyllite yield age populations that cluster in the ranges ca. 1375–880 Ma, ca. 650–525 Ma, and ca. 460–435 Ma and slightly discordant grains with individual ages of ca. 1435 Ma, ca. 1640 Ma, ca. 2105 Ma, and ca. 2730 Ma. The youngest detrital zircon indicates a maximum depositional age for the Granjeno Schist of ca. 435 Ma (Lower Silurian). Detrital zircons of Neoproterozoic–Cambrian age suggest a provenance in the Maya terrane beneath the Yucatan Peninsula or the Brasiliano orogens of South America, and a source for the detrital zircons of Ordovician–Silurian age is present in the Acatlán Complex of southern México. Provenance of the Mesoproterozoic detrital zircons is likely to have been the adjacent Novillo Gneiss, which has yielded ages of ca. 990–980 Ma, ca. 1035–1010 Ma, and ca. 1235–1115 Ma. These detrital ages closely match those recorded from the Cosoltepec Formation of the Acatlán Complex and support correlation of the two units, which are both interpreted to be vestiges of the southern margin of the Rheic Ocean.

U-Pb zircon data from three undeformed to slightly deformed, megacrystic, granitoid plutons in the northern Acatlán Complex of southern México has indicated that all three are part of a larger suite of late Ordovician plutons. 40 Ar/ 39 Ar data from hornblende and biotite show mainly disturbed spectra, but biotite from the Palo Liso and Los Hornos plutons yields plateaus with ages of 305 ± 26 Ma and 157 ± 12 Ma, respectively. These thermal events may be correlated, respectively, with Permo-Triassic and Jurassic tectonothermal events recorded elsewhere in the Acatlán Complex. All three plutons are peraluminous with calc-alkaline affinities, characteristics that are consistent with inherited zircon ages and together suggest a source in Mesoproterozoic calc-alkaline rocks similar to those exposed in the neighboring Oaxaca terrane. We interpret these granites to be related to the early Ordovician separation of peri-Gondwanan terranes from Gondwana during the opening of the Rheic Ocean. Elsewhere in the Acatlán Complex, Ordovician megacrystic granitoids of the Piaxtla Suite were subjected to high-grade metamorphism, which we infer to be related to subduction along the Gondwanan margin during the Devonian–Carboniferous. The three plutons reported here were not affected by Devono-Carboniferous metamorphism and thus are inferred to have remained outside the subduction zone.

The Cosoltepec Formation consists of unfossiliferous phyllites and psammites with tectonic slices of basalt and forms a major part of the Acatlán Complex of southern México. It has traditionally been interpreted as part of either a Cambro-Ordovician accretionary prism or a passive margin deposited in the Iapetus Ocean. Although no reliable age data are available from the basaltic slices, their widespread tectonic interleaving with the Cosoltepec Formation suggests that some of the sedimentary rocks were originally deposited directly on the ocean-floor basalts. The age of the Cosoltepec metasediments ranges from Ordovician to uppermost Devonian (oldest unconformably overlying sediments: uppermost Devonian). The mafic rocks have been affected by greenschist- to sub-greenschist-facies metamorphism. The geochemistry indicates that they are mainly mid-ocean ridge basalt (MORB) tholeiites with flat or depleted rare earth element (REE) patterns and resemble MORB derived from heterogeneous sources. Another minor group consists of ocean island basalts and andesites, which have distinctly fractionated REE patterns, whereas their mantle-normalized trace element patterns do not show Nb-Ta or Ti negative anomalies. Thus, these mafic rocks appear to represent oceanic lavas that were tectonically incorporated into the clastic rocks of the Cosoltepec Formation during deformation that started immediately prior to deposition of late Fammenian sedimentary rocks and continued into the Mis-sissippian. Tectonic juxtaposition of these rocks with eclogitic rocks suggests that the deformation was related to exhumation following subduction: the Cosoltepec Formation and its mafic lenses derived from the overriding plate, whereas the eclogitic rocks represent parts of the subducting plate. The probable Middle Ordovician–Middle Devonian age of the oceanic rocks together with their initial deformation during the Devonian–Carboniferous suggest that they represent vestiges of the Rheic Ocean.

The Piaxtla Suite of the Acatlán Complex (southern México) has previously been considered a vestige of the Iapetus Ocean that underwent eclogite-facies metamorphism during Late Ordovician subduction and exhumation. Study of granitoid, mafic, and metasedimentary rocks of the Asis Lithodeme of the Piaxtla Suite reveals a complex tectonothermal history involving: (1) eclogite-facies syntectonic metamorphism preserved as aligned omphacite in mafic lenses dated at 346 ± 3 Ma (concordant U-Pb zircon age), which is inferred to result from subduction; (2) polyphase deformation involving WSW-ENE tectonic transport under amphibolite-facies conditions accompanied by migmatization due to decompression melting dated at ca. 347–330 Ma (SHRIMP [sensitive high-resolution ion microprobe] zircon ages); (3) continued deformation under greenschist facies; and (4) development of several phases of late folds and crenulation cleavage. Pressure, temperature, and time ( P-T-t ) data suggest rapid isothermal decompression from eclogite to upper amphibolite facies during the Visean (Middle Mississippian) followed by cooling. In the absence of age data for the latter stage, nearby unmetamorphosed latest Upper Devonian sedimentary rocks contain metamorphic, Piaxtla Suite clasts suggesting either diachronism in the exhumation process or synchronous exhumation and subsidence in adjacent areas. Such rapid exhumation of eclogites is typical of continent-continent collision zones, and the subhorizontal, WSW-ENE kinematics is compatible with either lateral thrust ramping or extension in the orogen. Devonian–Carboniferous subduction and exhumation are incompatible with an origin within the Iapetus Ocean, because that ocean had closed by Silurian times. However, they are consistent with oblique subduction of the leading edge of Gondwana along the southern flank of the Rheic Ocean during the amalgamation of Pangea.

The Caltepec shear zone is a dextral transpressional tectonic boundary between the Oaxacan and Acatlán Complexes, which are crystalline basements of the Zapo-teco and Mixteco terranes in southern México, respectively. The terrane boundary (2–6 km wide) reveals protracted and polyphase tectonic activity from at least Early Permian to the present. The major tectonothermal event in the Caltepec fault zone was related to the oblique collision of the metamorphic complexes during the amalgamation of Pangea. An anatectic leucosome and the resulting syntectonic granite (Cozahuico Granite) in the fault zone yielded U-Pb zircon ages of 275.6 ± 1.0 Ma and 270.4 ± 2.6 Ma, respectively. The initial 87 Sr/ 86 Sr ratios (0.70435–0.70686) and Sm-Nd model ages (T DM ) (1.0–1.6 Ga) for the Cozahuico Granite and leucosome indicate a magmatic mixture that originated from melted Proterozoic crust and a component of depleted mantle. The Leonardian age of the cover (Matzitzi Formation) and a 40 Ar/ 39 Ar cooling age (muscovite) of 268.59 ± 1.27 Ma for mylonitic mica schist at the base of the thrust imply high cooling rates (∼180 °C/Ma) and uplift during the Permian. The adjacent sedimentological record indicates intense tectonic reactivation during Early Cretaceous, Paleogene, and Neogene along the long-lived Caltepec fault zone, alternating with periods of relative tectonic quiescence during Triassic, Jurassic, and Mid-Cretaceous times. The trend of the Caltepec fault zone parallel to the Oaxaca fault, 50 km to the east, is interpreted as part of a synchronous and dynamically coupled tectonic system that has been releasing tectonic stresses associated with the rupture of Pangea and the evolution of the Pacific margin of southern México from Jurassic to Holocene times.