John Duncan Keppie

990 and 1100 Ma Grenvillian tectonothermal events in the northern Oaxacan Complex, southern Mexico: roots of an orogen

Inliers of f1.0–1.3 Ga rocks occur throughout Mexico and form the basement of the Oaxaquia microcontinent. In the northern part of the largest inlier in southern Mexico, rocks of the Oaxacan Complex consist of the following structural sequence of units (from bottom to top), which protolith ages are: (1) Huitzo unit: a 1012F12 Ma anorthosite–mangerite– charnockite–granite (AMCG) suite; (2) El Catro´n unit: z1350 Ma orthogneiss migmatized at 1106F6 Ma; and (3) El Marquez unit: z1140 Ma para- and orthogneisses. These rocks were affected by two major tectonothermal events that are dated using U–Pb isotopic analyses of zircon: (a) the 1106F6 Ma Olmecan event produced a migmatitic or metamorphic differentiation banding folded by isoclinal folds; and (b) the 1004–978F3 Ma Zapotecan event produced at least two sets of structures: (Z1) recumbent, isoclinal, Class 1C/3 folds with gently NW-plunging fold axes that are parallel to mineral and stretched quartz lineations under granulite facies metamorphism; and (Z2) tight, upright, subhorizontal WNW- to NNE-trending folds accompanied by development of brown hornblende at upper amphibolite facies metamorphic conditions. Cooling through 500 jC at 977F12 Ma is documented by 40 Ar/ 39 Ar analyses of hornblende. Fold mechanisms operating in the northern Oaxacan Complex under Zapotecan granulite facies metamorphism include flexural and tangential–longitudinal strain accompanied by intense flattening and stretching parallel to the fold axes. Subsequent Phanerozoic deformation includes thrusting and upright folding under lower-grade metamorphic conditions. The Zapotecan event is widespread throughout Oaxaquia, and took crustal rocks to a depth of f25–30 km by orogenic crustal thickening, and is here designated as Zapotecan Orogeny. Modern analogues for Zapotecan granulite facies metamorphism and deformation occur in middle to lower crustal portion of subduction and collisional orogens. Contemporaneous tectonothermal events took place throughout Oaxaquia, and in various parts of the Genvillian orogen in Laurentia and Amazonia. D 2003 Elsevier Science B.V. All rights reserved.

The Maya-Chortís Boundary: A Tectonostratigraphic Approach

This work presents an updated revision of the complex stratigraphic and tectonic relationships that characterize the geologic boundary between the Chortís and Maya continental blocks of the Caribbean region. Based on field, petrologic, structural and geochronological work in key areas of central Guatemala, as well as analysis of the relevant literature, we propose a new tectonostratigraphic structure that more fully appraises the fundamental tectonic role played by major faults that cut across the continental isthmus between the Americas, and bound separate tectonostratigraphic terranes (or fault blocks according to author JDK). Accordingly, we subdivide the area into seven of these units, from south to north: Chortís, Yoro, Sula, El Tambor, Jacalteco, Achí, and Maya, bounded respectively by the Agúan-La Ceiba, Jocotán-Chamelecón, Motagua, Baja Verapaz (defined in this work), and Chixoy-Polochic fault zones. Unfortunately, the extreme paucity of modern geologic data bearing on the pre-Cretaceous cover and basement units in the entire region constitutes a major obstacle for building convincing paleogeographic models to explain the complex tectonic evolution of the area from Precambrian to Cenozoic time. Consequently, this work should be taken as an attempt line to understand more clearly the nature and contact relationships between deep crustal blocks in nuclear Central America, and as a contribution to interpret their geologic evolution in plate tectonic terms.

Middle-Late Ordovician magmatism and Late Cretaceous collision in the southern Maya block, Rabinal-Salamá area, central Guatemala: Implications for North America-Caribbean plate tectonics

The Rabinal-Salama area in central Guatemala provides critical data bearing on the relationships between the North American and Caribbean plates because it lies within the Polochic-Motagua fault zone that separates the two plates. The cumulative Cenozoic sinistral displacement across this zone that separates the Maya and Chortis terranes has been variously estimated to be ~125 km or ~1100 km, evidence for which should be recorded in the rocks of the studied area. The Rabinal-Salama area lies between two of the east-west faults within the Motagua fault zone, the Polochic fault, and the Baja Verapaz shear zone. The shear zone separates the Maya block from eclogitic rocks of the Chuacus Complex that pass southward into ophiolitic rocks and melanges that define a suture between the Chuacus Complex and the Chortis block. The following sequence of events is recorded in the Rabinal-Salama area: (1) low-grade, pre-Silurian siliciclastic metasedi-mentary rocks (San Gabriel unit), that are intruded by (2) ca. 462–453 Ma calc-alkaline, peraluminous, S-type Rabinal granite suite, and unconformably overlain by (3) very low grade clastic and calcareous metasedi-mentary rocks (Santa Rosa Group) containing Mississippian conodonts and pebbles of granite, sandstone, and phyllite derived from the older units. The Rabinal granite suite is inferred to be rift related, inheriting its calc-alkaline signature from its source, along with the ca. 1 Ga xenocrystic zircons (upper intercept U-Pb data). Deformation in all these Paleozoic rocks produced a steeply south-southwest–dipping cleavage (chlorite and sericite) and a stretched quartz lineation. These fabrics become more intense adjacent to the Baja Verapaz shear zone, where C-S fabrics and rotated porphyroclasts indicate a reverse sense of motion with a sinistral component. White mica in the shear zone yields 74–65 Ma K-Ar ages, which are inferred to closely postdate the time of crystallization. Thus, although evidence for major sinistral displacement is absent, the kinematics are consistent with uplift and exhumation of the Chuacus Complex during obduction of the Baja Verapaz ophiolite onto the Paleozoic rocks of the Rabinal-Salama area in latest Mesozoic-Paleocene. This is inferred to have been produced during collision of the Cuban arc and Chortis block with the southern Maya block. Restoration of the Early Mesozoic ~70° anticlockwise rotation of the Maya block places the Rabinal-Salama area adjacent to northeastern Mexico, where comparable continental-shallow marine, Paleozoic rocks occur near Ciudad Victoria overlying the ca. 1 Ga Oaxaquia basement.

Geochronology and Geochemistry of the ~917 Ma, Calc-alkaline Etla Granitoid Pluton (Oaxaca, Southern Mexico): Evidence of Post-Grenvillian Subduction along the Northern Margin of Amazonia

The post-tectonic Etla pluton intrudes the ~1 Ga granulitic Oaxacan Complex that cooled through 450°C by ~945 Ma. The Etla pluton consists of massive, coarse, porphyritic granodiorite-monzogranite (plagioclase, K-feldspar, quartz, biotite ± hornblende) with fine-grained felsic rocks along the margin. Geochemistry indicates that it is a peraluminous, I-type, medium-K, calc-alkaline, volcanic-arc granite-trondjemite with relatively low contents of high-field-strength elements and flat REE patterns. U-Pb zircon isotopic analyses fall on a chord with intercepts at 180 ± 50 Ma and 920 ± 25 Ma: the latter is similar to the 207Pb/206Pb age of 917 ± 6 Ma of the least discordant (1%) analysis and is inferred to date the time of intrusion. This pluton is synchronous with similar igneous activity in Avalonia (eastern Appalachians) and in Tocantins Province of central Brazil, which may form parts of a peri-Amazonian magmatic arc. 40Ar/39Ar laser step-heating analyses of biotite and K-feldspar yielded plateau ages of 207 ±5 Ma and 221 ± 3 Ma, respectively, that may be related to Phanerozoic reheating.

Petrology of two contrasting Mexican volcanoes, the Chiapanecan (El Chichón) and Central American (Tacaná) volcanic belts: the result of rift- versus subduction-related volcanism

The alkaline El Chichón and calc-alkaline Tacaná volcanoes, located in southern Mexico, form parts of the Chiapanecan Volcanic Belt and Central American Volcanic Arc, respectively. El Chichón has emitted potassium-, sulphur-, and phosphorus-rich trachyandesites and trachybasalts (as mafic enclaves), whereas Tacaná has erupted basalts to dacites with moderate potassium contents, and minor high-Ti magmas (1.5–1.8 wt.% TiO2). The magmatic evolution in the two volcanoes has involved similar fractionating assemblages: Fe-Ti oxides, olivine, plagioclase, pyroxenes, amphibole, and apatite. K2O/P2O5 ratios and isotopic signatures indicate that magmas from both El Chichón and Tacaná have undergone significant crustal contamination. The volcanism at both Tacaná and El Chichón was previously related to northeastward subduction of the Cocos Plate, representing the main arc and the backarc, respectively. Although such an origin is in accord with Tacaná occurring 100 km above the Cocos Benioff Zone, it is inconsistent with: (a) the absence of a calc-alkaline belt between El Chichón and the Middle America Trench; and (b) truncation of the subducted Cocos Plate by the southwesterly dipping Yucatan slab near the Middle America Trench (i.e. the Cocos Plate does not presently underlie El Chichón). On the other hand, El Chichón and the Chiapanecan Volcanic Belt are located on the sinistral Veracruz fault zone that forms the northern boundary of the Southern Mexico block, which has been migrating relatively to the east since ca. 5 Ma. In this context, the anomalous high potassium, sulphur, and phosphorus levels in the El Chichón magmas are explicable in terms of rifting in a pull-apart system with the weak subduction fingerprint inherited from the Yucatan slab.

Sedimentary Origin of Calcareous Intrusions in the ~1 Ga Oaxacan Complex, Southern Mexico: Tectonic Implications

Intrusive calcareous bodies, marbles and calc-silicate rocks, are a distinctive feature of the highgrade metamorphic suites of the ~1 Ga northern Oaxacan Complex. They typically form dike-like intrusions up to 4 m thick which cut across the surrounding high-grade granulite- and upperamphibolite facies metamorphic rocks. Various protoliths are possible for these carbonate bodies: (1) sediments including evaporites; (2) metasomatic skarns; and (3) carbonatites. An evaporitic protolith is supported by the predominance of scapolite, low abundances of incompatible trace elements (including Nb and rare-earth elements) relative to carbonatites, and the presence of a sharp contact with host rocks without a significant contact metamorphic aureole or fenitization. It is inferred that limestones and related rocks were remobilized under granulite-facies conditions and intruded into the host rocks. The widespread distribution of such evaporites in the Oaxacan Complex is consistent with deposition after the worldwide ~1.3 Ga oxygenation event that increased the marine sulfate reservoir. Intrusion of rift-related plutons into the sediments at ~1157-1130 Ma provides a younger limit on the age of protholiths of the metamorphic suites. Modern analogues for such evaporites are rifts associated with passive margins (e.g., Red Sea) and active margins (e.g., Gulf of California). The presence of evaporites implies a paleolatitude of 10-35°, a conclusion consistent with a paleogeographic provenence for the Oaxacan Complex adjacent to either Amazonia or eastern Laurentia in Rodinia reconstructions.

Petrology and Geochemistry of El Chichón and Tacaná: Two Active, yet Contrasting Mexican Volcanoes

El Chichon and Tacana have been widely considered subduction-related volcanoes, although they show differences in mineral assemblage and magma composition. El Chichon emitted potassium- and sulfur-rich trachyandesites and trachybasalts during its eruptive history, whereas Tacana erupted basalts to dacites with moderate potassium contents, and minor high-Ti magmas. The magmatic evolution in both volcanoes involved similar fractionating assemblages of Fe-Ti oxides, olivine, plagioclase, pyroxenes, amphibole and apatite. Both K2O/P2O5 ratios and isotopic signatures, indicate that the melts of El Chichon and Tacana experienced significant crustal contamination. Magma genesis for both volcanoes has been related to the northeastward subduction of the Cocos Plate. Even if such origin agrees with the location of Tacana, situated 100 km above the Cocos Benioff Zone, a subduction origin is at odds with recent tectonic and geophysical data obtained for southern Mexico for El Chichon, located about 400 km from the trench. In this chapter we review the existing petrographic and geochemical data for El Chichon and Tacana volcanoes, in order to understand their magma genesis and evolution.