Nance et al. raise three issues regarding the geochronology and tectonic interpretations of the Acatlán Complex of southern Mexico presented in our paper . They specifically inquire about: (1) the Mesoproterozoic crystallization age reported for megacrystic granites and tonalitic gneisses of the Tecolapa suite (formerly part of the Esperanza Granitoids); (2) the Taconian-age of the high-pressure (HP) metamorphism of Xayacatlán Formation; and (3) the provenance of Xayacatlán rocks. They propose an alternative model for the evolution of the Acatlán Complex. We welcome the comment and the opportunity to reiterate the main points of our work. We take also the opportunity to give the precise locations of the samples dated in our work (Table 1). The geochronological and geological data presented in our paper underscore the conclusions presented by several authors (including some of Nance's co-authors) indicating that the stratigraphy of the complex as originally defined is oversimplified and that it should not be further used to avoid confusion and misinterpretations [2,3]. Unfortunately, much of the discussion by Nance et al. is based on the old stratigraphic scheme. Furthermore, the comment does not provide geographic, petrographic or geological information of dated samples (or they refer to in press papers) making it hard to determine if we are discussing the same rocks or units. Below, we address the three main points in the Nance et al. comment. Age of the Esperanza Granitoids and the Tecolapa suite: Nance et al. use U/Pb ages of some granitic bodies (La Noria, Los Hornos, Palo Liso), which they include in the Esperanza Granitoids unit to propose a Middle Ordovician (∼ 460 ± 10 Ma) crystallization age for the whole unit. However, geochronological data and critical geological evidence presented in our paper and in other studies [4,5] demonstrate that the Esperanza Granitoids as defined by them, include granitic bodies formed by different magmatic events of a different age. The evidence for this assertion follows. Firstly, megacrystic granites from La Noria-Los Hornos were dated by Yañez et al.  (by TIMS) at 371 ± 34 Ma. We have recently confirmed this age with new TIMS U/Pb geochronology yielding an identical age of 371 ± 17 Ma, thus confirming the existence of a Devonian magmatic event. Furthermore, the classic Esperanza Granitoids are affected by eclogitic metamorphism . La Noria-Los Hornos, on the other hand, are locally deformed but do not show any evidence of eclogite metamorphism. Ortega-Gutierrez et al.  demonstrate that La Noria-Los Hornos granites and the Esperanza Granitoids are unrelated plutons, a statement that is supported by our data. Secondly, megacrystic granites belonging to the classic Esperanza Granitoids yield an Early Silurian TIMS age (440 ± 18 Ma) , which was confirmed by LA-ICPMS ages (442 ± 5 Ma) in our paper . An older, 471 Ma SHRIMP age was recently reported for granites from the same area based on a single concordant zircon . This latter age is identical within analytical uncertainty to the age (470 ± 16 Ma) reported in our work for petrographically identical granites from the Piaxtla area. The combined data therefore indicate that the classic Esperanza Granitoid magmatism ranged from 471 to 440 Ma. Thirdly, there is a suite of K-feldspar megacrystic granitoids and tonalitic to grabbroic gneisses outcropping in the Olinalá region, which yielded Mesoproterozoic (1163-1043 Ma) crystallization ages, which we named the Tecolapa suite. Rocks of this suite have been confused with the Esperanza Granitoids as defined by Ortega-Gutiérrez et al.  and were long considered as part of this unit. The ages reported in our paper were based on the analysis of twenty-five to fifty bipyramidal zircons from two distinctive samples (CU-920 and ACA-505) using a spot size of 50 μm in one sample and 35 μm in the other. We analyzed tips as close as possible from borders to determine the crystallization age. Our reaported magmatic ages are defined by zircon clusters composed of 17 grains in sample CU-920 and 19 in sample ACA-505 and not on a single or a few zircons even if concordant. Because Pb-loss commonly produces analyses that are younger than the crystallization age but are still analytically concordant, we rely on clustering as an indicative that an age is robust. CL imaging revealed, however, that some zircons do contain thin rims (generally less than 15 μm wide), which were originally not possible to analyze accurately with the size of our laser beam. In order to determine the age and nature of these rims, we performed new analyses using a smaller spot size of 10 μm. The new analyses show that the rims are ∼ 465 My old as claimed by Nance et al., and as we interpreted in our paper as the time of Pb-loss. However, the nature of this age is uncertain because the U/Th ratios of the rims span from magmatic (U/Th < 7) to clearly metamorphic (U/Th > 10) values. Geological evidence, however, suggest that overgrowths are more probably of metamorphic origin because rocks from the Tecolapa suite are unequivocally intruded by post-tectonic granites dated at 478-472 Ma (Early Ordovician) preventing a younger crystallization age for the Tecolapa rocks. Thus, available geochronological data and geological evidence support a Mesoproterozoic crystallization age for these rocks and justify their separation from the Ordovician-Early Silurian granites of the classic Esperanza Granitoids. The presence of Mesoproterozoic rocks in the Acatlán complex is a new finding that indicates that their origin dates back to the configuration of Rodinia. Nance et al. suggest that this Grenvillian element could be related to the evolution of the Oaxaca Complex, a Mesoproterozoic terrane of South American origin. The age of Tecolapa granites (1163-1043 Ma) match better with the major magmatic pulse (∼ 1100 Ma) in the Grenville province of SW North America than with the main magmatism in Oaxaca and in the Grenville province of South America (∼ 980-990 Ma) . Furthermore, the close relationship of Tecoloapa rocks with those of the Xayacatlán and Ixcamilpa rocks showing Laurentian affinities also point out to a North American origin. Age of the high-pressure metamorphism: Nance et al. propose a Mississippian (∼ 345 Ma) age for the HP metamorphism recorded in some units of the Acatlán Complex based on two U/Pb ages from an eclogite and a migmatite from the Piaxtla group. Following the original definition of the Piaxtla group , these authors assume that rocks of the Xayacatlán and Esperanza Granitoids units share a single HP metamorphic event. As outlined in our paper, there is convincing geological and geochronological evidence to propose the existence of three major events of HP metamorphism affecting different units of the Acatlán Complex.(1)In the Olinalá region, in the western Acatlán Complex, eclogites and garnet-amphibolites of the Xayacatlán Formation are overthrusted by metabasites and schists of El Rodeo Formation. The latter are affected by a greenschist, low-pressure metamorphism and, to our knowledge; there is no evidence indicating that this unit was affected by a HP event. Given the contrasting metamorphic conditions of both assemblages, eclogitic metamorphism in the Xayacatlán Formation and the greenschist metamorphism in the El Rodeo Formation must have occurred before their juxtaposition. The age of juxtaposition is constrained by El Progreso leucogranite dated at 476 ± 8 Ma, which intrudes the contact between the Xayacatlán and El Rodeo formations providing a minimum Early Ordovician age for the HP, eclogitic metamorphism in the Xayacatlán Formation.(2)Detrital zircon geochronology in rocks of the Ixcamilpa suite indicates a maximum Middle Ordovician depositional age. Rocks of this suite underwent blueschist metamorphism of poorly resolved age but it is evident that it took place after Middle Ordovician preventing any linkage with the eclogitic metamorphism of Xayacatlán Formation.(3)Reported ages for the classic Esperanza Granitoids range from 471 to 440 Ma. There are eclogites and migmatites intimately associated with augen gneisses. The age of this eclogitic metamorphism is also poorly constrained but it certainly occurred after 440 Ma, the youngest confirmed age for Esperanza rocks. This metamorphism therefore cannot be related to eclogitic metamorphism of Xayacatlán Formation, thereby confirming the existence of two distinctive phases of eclogitic metamorphism in the Acatlán Complex. Unfortunately, Nance et al. do not provide geographic nor petrographic information of their dated samples making it impossible to be sure if their ages were obtained from rocks of Xayacatlán or Esperanza Granitoids formations, both of which contain eclogites and migmatites. From our own data, we infer that Nance et al. ages are from rocks belonging to the Esperanza Granitoids unit for which we obtained Devonian to Mississippian Ar/Ar cooling ages. If so, their ages do not disagree with the Early Ordovician age we inferred for the Xayacatlán eclogites since metamorphisms in the Esperanza Granitoids and Xayacatlán units are not genetically related. Provenance of Xayacatlán rocks: In our paper, we proposed a Laurentian provenance for the Ixcamilpa blueschist suite based mainly on the presence of a significant population of detrital zircons at ∼ 477 Ma and a smaller but distinctive population at ∼ 1821 Ma, both of exclusive Laurentian affinity.
ASJC Scopus subject areas
- Geochemistry and Petrology
- Earth and Planetary Sciences (miscellaneous)
- Space and Planetary Science