Guerrero ferrane of Mexico: Its role in the Southern Cordillera from new geochemical data COMMENT Olivier Monod, Michel Faure Département Sciences de la Terre, URA CNRS 1366, GDR 069, Université d'Orléans, B.P. 6759, 45067 Orléans, FRANCE Denis Thiéblemont SGN/GEO, BRGM, B.P. 6009, 45060 Orléans Cédex, FRANCE Centeno-Garcia et al. (1993) in presenting newly acquired geochemical data from several areas in southwest Mexico, extended the conclusions to the entire Guerrero terrane along the Pacific mar- gin of Mexico. As they pointed out, very little is known about the structure of the Guerrero terrane itself. From our knowledge of the deformational history of a significant part of this region, it appears that their geodynamic interpretation, based mainly on geochemical data, is not supported by structural arguments. We point out some questionable aspects of this interpretation. 1. Considering the complex structure of the area, the precise location of the samples is necessary to evaluate their attribution to specific tectonic units. Unfortunately, the origin of the samples is unclear from Figure 2 of Centeno-Garcia et al. (1993). For instance, the Huetamo subterrane is represented by only one sample, whereas two distinct sample localities, 30 km apart, are shown on the map. Conversely, three samples are said to come from the Teloloapan subterrane, but none is located on the map. 2. Centeno-Garcia et al.'s (1993) interpretation of the data dis- regards some of the main tectonic boundaries within the different subterranes grouped into the so-called Jurassic-Cretaceous arc-as- semblage. For example, from a detailed sampling map provided to us by Centeno-Garcia et al., it appears that the three samples from the Teloloapan subterrane come from two distinct tectonic units: the calc-alkalic Teloloapan unit (sample G9-89) and the tholeiitic Arcelia unit (G13-89, G14-89) (Campa and Ramirez, 1979; Monod and Faure, 1992; Talavera, 1993). Likewise, Centeno-Garcia et al. (1993) considered the Taxco area of the Teloloapan subterrane as "excep- tional" because a rhyolitic tuff from the Taxco Viejo schist yielded an isotopie signature that suggests continental contamination. How- ever, such a result was predictable because the formation does not belong to the Jurassic-Cretaceous arc assemblage, as stated by Cen- teno-Garcia et al. (1993), but to the underlying Tejupilco schists (de Cserna, 1982; Elias-Herrera and Sanchez-Zavala, 1992; Monod et al., 1993; Salinas et al., 1993). 3. From the geochemical analyses of two basaltic pillows of unknown age, Centeno-Garcia et al. (1993) concluded that the floor of the Arteaga complex was an oceanic crust. However, it would appear that these basaltic pillows occur as blocks reworked in a formation having a "block-in-matrix" aspect. Moreover, the asso- ciated sandstone and shale show isotopie characteristics that suggest an "evolved continental source," not an oceanic one. 4. The geodynamic interpretation of the geochemical data on the volcanic rocks from the Jurassic-Cretaceous arc assemblage is not unequivocal. For instance, Centeno-Garcia et al. (1993) inter- preted the rare earth element (REE) patterns of these rocks as "typ- ical of evolved island arcs." In fact, similar REE patterns are also found in lavas from nonconvergent settings. Examples include light- REE-enriched (E-type) mid-ocean ridge basalts (Basaltic Volcan- ism Study Project, 1981, Fig. 1.2.5.15b) or continental tholeiites (Ba- saltic Volcanism Study Project, 1981, Figs. 1.2.4.5a and 1.2.3.4). Likewise, similar isotopic characteristics as those of the rocks from the Jurassic-Cretaceous arc assemblage are known in continental tholeiites from the Columbia River or Karoo provinces (Basaltic Volcanism Study Project, 1981, Fig. 1.2.3.7; Hawkesworth et al., 1984). For us, one of the major issues in southwest Mexico remains the nature of the basement of each of the different Guerrero tectonic units. In contrast with Centeno-Garcia et al.'s (1993) assertion that the Guerrero lacks older basement other than the Arteaga complex, we maintain that large areas of continental basement are present, both in the Teloloapan unit (de Cserna, 1982; Monod et al., 1993) and in the Zihuatanejo area (Stein et al., 1993). This accounts for the negative e Nd values reported for the Cretaceous Puerto Vallarta granitoids and is consistent with the dominantly calcareous nature of the sediments associated with the calc-alkalic arc volcanic rocks in both areas. In contrast, the tholeiitic volcanics of the Arcelia unit rocks are associated with deep-seated sedimentaiy rocks (radiolar- ian chert), and this suggests an entirely different environment, such as a marginal sea (Monod et al., 1993). To summarize, detailed trace element and isotopic data may provide important constraints on models for the evolution of the Mexican Pacific margin, provided that standard geological and structural analyses have been carried out before sampling rocks for the geochemical analyses. REFERENCES CITED Basaltic Volcanism Study Project, 1981, Basaltic volcanism on the terrestrial planets: New York, Pergamon Press, 1286 p. Campa, M.F., and Ramirez, J., 1979, La evolución geológica y la metalo- genesis de Guerrero: Universidad Autónoma de Guerrero, Serie Tec- nico-Cientifica, v. 1, 84 p. 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