U51A-01
Tectonic Significance of Cretaceous to Tertiary Magmatic and Structural Evolution of the Northern Portion of the Xolapa Complex, Tierra Colorada-Acapulco Area, Southern Mexico
This work presents combined structural and geochronological data from Tierra Colorada to Acapulco area that show evidence of five deformational events and four episodes of arc related magmatism during Mesozoic and Cenozoic times. The oldest sequence correspond to metasedimentary units (paragneisses and marbles) metamorphosed in amphibolite facies that are affected by a penetrative foliation and mineral lineation (D1). This sequence is intruded by a first magmatic pulse (M1) that is represented by ~165 Ma metagranitoids that are affected by metamorphism and migmatization (D2). The second magmatic pulse (M2) corresponds to the intrusion of the El Pozuelo granite (129 +/- 0.5 Ma: concordant U-Pb zircon analysis), which postdates high grade metamorphism and migmatization. The next magmatic episode (M3) is represented by the peraluminous, deformed El Salitre granite (55.3 +/- 3.3 Ma: mineral-whole rock Rb-Sr isochron), and the protomylonitic Las Pi\hbaras I-type granite (54.2 +/- 5.8 Ma: lower intercept U-Pb zircon). Las Pi\hbaras granite is characterized by D3 ductile sheared fabric with normal, top-to-the NNW sense of shear, deformed at 45 - 50 Ma (Rb-Sr and K-Ar ages). D3 also affects the El Pozuelo granite and the lower Cretaceous metavolcanic rocks of the Chapolapa Formation. The 34 Ma undeformed granites (M4) corresponds to the last intrusive pulse in the area, post-dating D4 SSW-verging thrusting of the Cretaceous Morelos Formation over sheared granites and metavolcanic rocks. The last deformational event (D5) corresponds to brittle lateral features that affects all the units. These four pulses of subduction related magmatism in the Tierra Colorada-Acapulco area indicate a regular, northeastward subduction at the Mesoamerican trench since Jurassic time, alternated with contractile and/or extensional deformational events. The gap in magmatic activity at about 90-100 Ma roughly coincides with deposition of platformal limestones of the Morelos Formation during the middle Cretaceous. The stable conditions during deposition of the Morelos Formation may have resulted from a combination of back-arc extension and development of a passive margin during the Middle Cretaceous, which postdated the accretion of an exotic block, either the Guerrero terrane or the Chortís block. Following the Laramide Orogeny in southern Mexico (roughly during the Late Cretaceous) the Paleocene to Miocene tectonic evolution is represented by an alternation of magmatic pulses with contractile and extensional events, which are the result of a combination of several factors such as the geometry of the subducted slab, convergence rate, stress transmission between subducting and overlying plate, as well as to the rate of subduction erosion.
U51A-02
Gravity and Magnetic Survey of the Oaxaca-Juarez Terrane Boundary (Oaxaca Fault), Southern Mexico: Evidence for three Half Grabens
A geophysical survey of the Oaxaca Fault boundary between the Oaxaca (Oaxaquia) (Zapoteco) and Juarez (Cuicateco) terranes along the Etla and Zaachila valleys area, southern Mexico shows a series of NW-SE Bouguer and magnetic anomalies with stronger gradients towards the east. The basement from the Oaxaca terrane has a high density (2.8 gr/cm3 ) and magnetic susceptibility of up to 0.0051 cgs units, which contrast with the Juarez basement that has a lower density (2.67 gr/cm3) and a higher magnetic susceptibility (values ranging between 0.0025 to 0.0045 cgs units). The magnetic susceptibility is similar south of the Donaji fault. Interpretation of six combined gravity and magnetic NE-SW profiles perpendicular to the valleys indicates the presence of a composite depression comprising three N-S sub-basins with the Etla and Zachila sub-basins located at the northern and southern portions, respectively, separated by a third sub-basin relatively displaced westwards. They are bounded on the east by the steeply W-dipping Oaxaca master fault, and on the west by the gently E-dipping Huitzo-Zimatlan fault. Two interpretations are suggested for the southward continuation of the Oaxaca Fault: 1) it continues southwards at depth with the same strike. Together the Bouguer and total field magnetic anomalies suggest that the Oaxaca fault is continuous from Etla via Oaxaca City and Ocotlán de Morelos probably to Miahuatlán de Porfirio Díaz, and 2) it continues with the same strike but is displaced eastwards ~20 km along a sinistral transfer fault, which forms the northern boundary of the Zaachila sub-basin.
U51A-03
Petrological, Structural and Geochemical Characteristics of the Cuicateco Terrane, Southern Tehuantepec Isthmus, Oaxaca, Mexico
The southeastern portion of Cuicateco terrane in southern Mexico is composed of a low grade volcanosedimentary sequence interbedded with phyllites, and are generally considered be pre-Cretaceous. This sequence is overlain by Albian, rudist-bearing limestones. The volcanic rocks mainly correspond to amphibolites with fine-grained porphyritic to phaneritic textures made of plagioclase, hornblende, epidote and quartz phenocrysts set in a microlitic matrix. The phyllites display lepidoblastic texture made of muscovite, quartz, graphite, zircon and iron oxides, as well as lithic fragments. The rocks of the Cuicateco terrane display at least three deformational events: D1) Reclined to isoclinal F1 folding of the S0 bedding surface, with a general vergence toward northeast and producing a penetrative, moderately SW-dipping S1 axial plane foliation with development of hornblende, epidote, chlorite, muscovite and quartz. D2) Moderately dipping, NE-directed thrusting of the entire sequence, and F2 folding of S1 axial surfaces without development of new minerals along the axial plane. D3) Left-lateral to normal faulting, affecting all rocks. We consider that the D1 event predates the deposition of the Cretaceous fossiliferous limestones, whereas D2 also affects such rocks and it can be associated to the Laramide Orogeny. D3 is possibly a Tertiary feature, similarly to what is observed farther to W along the Chacalapa - La Venta fault system. The chemical compositions of amphibolite rocks of the Cuicateco terrane are similar to oceanic tholeiitic basalts (SiO2 of 45 to 50 wt%, light REE depleted and flat HREE patterns). A possible interpretation of these results suggests a basic volcanism in a backarc environment during the Mesozoic, which proposes an interesting debate on the tectonic implications of the Cuicateco terrane in the evolution of southern Mexico.
U51A-04
Petrogenesis and P-T Conditions of Metamorphic Rocks From the Chiapas Massif Complex in the Custepec Area, Chiapas, Mexico
The Chiapas Massif Complex (CMC), located in the southern Maya block, is primarily composed of igneous and metaigneous rocks of late Permian age. Within the CMC, two metasedimentary basement units have been described: (1) The ‘La Sepultura Unit' is located in the NW part of the CMC and is composed of metapsammites, metapelites and calcsilicate rocks metamorphosed under high-T/low-P conditions and (2) the ‘Custepec Unit' located in the SE part of the CMC. The Custepec Unit is mainly composed of anatectic amphibolites with or without garnet, intercalated with quartz-feldspar and pelitic gneisses, marbles, and calcsilicates. The main foliation trend in the Custepec area is E-W to NW-SE mostly dipping to the north. Stretching lineations and fold axes of varied orientations indicate that the D1 deformation was folded by a subsequent D2 event. We applied the garnet-biotite geothermometer, the GASP, and GRAIL geobarometers in metapelites and the garnet-plagioclase-hornblende-quartz geobarometer in amphibolites. The results are consistent, yielding peak metamorphic conditions above 800°C and 9 Kbar. These data, along with petrographic observations place the metamorphic peak in the high amphibolite facies to granulite facies transition at 25-30 km depth. Relics of bluish-green (low-T) amphiboles yielded similar pressures than the high-T brownish hornblende, indicating a clockwise P-T path with isobaric heating at the metamorphic pressure peak. The high-grade event was followed by greenschist facies retrogression, which is probably contemporaneous with the formation of E-W trending dextral mylonite zones. On the basis of our field observations, chemical composition, and the presence of detritic zircons in the amphibolites, we interpret the Custepec Unit as a volcanosedimentary sequence. Our data favor a model in which peak metamorphism in the CMC during the late Permian is the result of stacking in an orogenic wedge with the Sepultura Unit as the upper plate thrusted over the Custepec Unit from present day north to south, followed by dextral transtension during the early Triassic.
U51A-05
Can the Metamorphic Basement of Northwestern Guatemala be Correlated with the Chuacús Complex?
The Chuacús complex constitutes a northward concave metamorphic belt that stretches ca. 150 km south of the Cuilco-Chixoy-Polochic (CCP) fault system in central and central-eastern Guatemala. It represents the basement of the southern edge of the Maya block, being well exposed in the sierra de Chuacús and the sierra de Las Minas. It is composed of high-Al metapelites, amphibolites, quartzofeldspathic gneisses, and migmatites. In central Guatemala the Chuacús complex contains ubiquitous epidote-amphibolite mineral associations, and local relics of eclogite reveal a previous high-pressure metamorphic event. North of the CCP, in the Sierra de Los Cuchumatanes area of western Guatemala, metamorphic rocks have been considered the equivalent of the Chuacús complex and hence been given the name Western Chuacús group, These rocks, which were intruded by granitic rocks and later mylonitized, include chloritic schist and gneiss, biotite-garnet schist, migmatites, and amphibolites. No eclogitic relics have been found within metamorphic rocks in northwestern Guatemala. Petrographic analyses of garnet-biotite schist reveal abundant retrogression and the formation of abundant zeolite-bearing veins associated with intrusion. Although metamorphic conditions in the greenschist and amphibolite facies are similar to those in the sierra de Chuacús, the association with deformed intrusive granites is unique for western Guatemala. Hence a correlation with metasediments intruded by the Rabinal granite in the San Gabriel area of Baja Verapaz seems more feasible than a correlation with the Chuacús complex. This idea is supported by reintegration of the Cenozoic left-lateral displacement along the CCP, which would place the metamorphic basement of western Guatemala north of Baja Verapaz, adjacent to metasediments intruded by granites in the San Gabriel-Rabinal area.
U51A-06
Provenance of the Subinal Formation, Central Guatemala, Based on Point-Counting of Pebbles in Conglomerates
The continental Subinal Formation of Central Guatemala is composed of red conglomerates and sandstones that outcrop in the Motagua valley between the San Agustín and Cabañas fault, and in the southeastern corner of Guatemala. Stratigraphic position implies a post-Cretaceous depositional age, which has not been accurately established. Point counts of pebbles in conglomerates were performed in localities distributed along the Motagua valley from Granados to Los Amates, and in the Jocotán-Esquipulas area near the border with Honduras. Pebble types contained in conglomerates in the Motagua area are very diverse, and include sandstones, limestones, chert, milky quartz, phyllite, marble, chlorite schist, quartz-muscovite schist, amphibolite, eclogite, gneiss, granite, gabbro/diorite, volcanic rocks, and abundant serpentinite. Schist and gneiss resembles rocks of the Chuacús complex. Serpentinite, gabbro, amphibolite, and eclogite clasts are similar to oceanic lithosphere from the Motagua suture. Abundant white mica in interbedded sandstones is ubiquitous, suggesting derivation from the mica-rich Chuacús complex. Unfoliated granites and volcanic clasts were probably derived from the northern edge of the Chortis block or an unknown terrane, as no unmetamorphosed igneous rocks are known from the southern edge of the Maya block. The relative abundance of some clast groups correlates with the rock units exposed immediately adjacent to the north across the San Agustín fault. Serpentinite in conglomerate is most abundant near Juan de Paz; eclogite, amphibolite and gneiss are most abundant in the Granados area; and white mica is most abundant where Chuacús complex schists are most pelitic. This suggests that the Cabañas fault accommodated most of the strike-slip movement of the Motagua fault system. In contrast, conglomerates in southeastern Guatemala are more homogeneous, containing chiefly clasts of volcanic origin, with minor limestone, graphitic phyllite, chert, and milky quartz. Pebbles were probably derived from the underlying Padre Miguel volcanics that occur within the Chortis block, not from rock units in the Motagua suture zone. Pebbles contained in red beds in the Motagua valley were derived from rock units of the Motagua suture and the southern margin of the Maya block, but provenance from rock of the Chortis block is not clear. These findings suggest that red-bed-bearing basins in the Motagua valley and in the southeast of Guatemala were not connected, and may have formed at disparate times.
U51A-07
Petrographic and Microstructural Characterization of Greenschist Facies Rocks in Northern Nicaragua
Low-grade basement rocks mapped south of the Guayape fault in the Macuelizo and San Fernando areas of Nueva Segovia in Nicaragua include quartz-chlorite-muscovite +/- andalusite schist, metalimestone, quartzite, metaconglomerate, and minor graphite-bearing schist. Two bands of blue amphibole-bearing schist were mapped adjacent to the Ocotal-Macuelizo road. A second localized metamorphic event at higher grade formed garnet-biotite-bearing hornfels that defines an aureole around the intrusive Dipilto batholith. Microstructurally the schists show up to three phases of deformation. A first foliation defined by phylosilicate schistosity is deflected by cleavage domains spaced between 1mm and 2 cm. Opaque content in cleavage domains is high, and quartz is relatively scarce, suggesting that pressure solution was an important mechanism for the formation of the second foliation. Some samples showed crenulation within the cleavage domains, probably due to a third deformation phase. Quartz in schist invariably shows signs of intracrystalline deformation as strong undulatory extinction and deformation lamellae. Fine grained aggregates of recrystallized quartz are common in some of the samples. Mapped metaconglomerates contain stretched pebbles with aspect ratio of ca.7:1. In Telpaneca, greenschists with structures resembling deformed pillows were found associated with chlorite-muscovite schists, which suggest an oceanic component in the metamorphic basement. Reconnaissance of low-grade rocks north of the Guayape fault in Olancho, Honduras, revealed that the most abundant rock type is likewise quartz-muscovite chlorite schist and quartzite, with minor greenschist. The later contained albite + epidote + chlorite + actinolite, indicative of similar greenschist facies conditions. Microstructurally the schist is strongly microfolded, and quartz rods define a strong lineation. Neither prototith type, metamorphic facies, nor ductile deformation features point to the existence of two disparate metamorphic units on either side of the Guayape fault.
U51A-08
New Ages for Gorgona Island, Colombia: Implications for Previous Petrogenetic and Tectonic Models
The Gorgona Island, located 50 km to the west of the Colombian Pacific coast, is the only known site with Phanerozoic komatiites in the world besides a key element in several reconstruction of the interaction between the Caribbean and the South America Plate. The Gorgona komatiites are part of an igneous complex that also includes picritic basalts and breccias, gabbros and peridotites (dunites and wherlites), and is covered by deformed mid-Eocene and younger underformed marine sediments. Datings of the igneous rocks were only performed on basalts and include an 86 Ma K-Ar age, an 88.9 ± 1.2 Ma weighted mean of four Ar-Ar ages and an 89.2 ± 5.2 Ma Re-Os isochron age from basalts. Gorgona rocks are affected by reverse faulting with a general eastward vergence. The island is the only subaerially exposed part of a NE elongated sliver accreted in a dextral transpressional regime to the South America continental margin between the Late Eocene and the Early Miocene. Petrologic studies found large spread in radiogenic isotopes and incompatible trace element ratios in Gorgona ultramafic rocks, which have been interpreted as requiring at least two different sources of: 1) a depleted mantle responsible for the generation of the komatiites and most basalts, and 2) an enriched mantle responsible for some rarer enriched basalts and picrites. Despite the large compositional and isotopic heterogeneity the most common interpretation is that the Gorgona ultramafic rocks are the product of a single mantle plume, although it has recently proposed that this would be a separate plume from that generating the bulk of the Caribbean plateau at ~90 Ma. Our new study focused on the geochronology of the Gorgona igneous suite as we consider that this tectonically and petrologically complex island is unlike to have such a narrow age range. We attempted to date eight samples of komatiites, basalts and gabbros by Ar-Ar laser step heating. For four of these samples we successfully obtain reliable plateau and/or isochron ages. Only one basaltic sample, located in the western coast, yielded an age comparable with those previously reported in the literature. For two basalts intercalated with komatiites and a gabbro exposed in the north-eastern coast of the island we obtained younger ages, similar to those reported for some mafic and ultramafic rocks along the Pacific coast of Colombia. The two sets of ages for the ultramafic suite of Gorgona also correspond to different petrologic types. The depleted rocks in the eastern coast are younger than the enriched basalts and picrites located in the southern and western part of the island with ages around 90 Ma, suggesting a more complex tectonic evolution with the accretion of at least two different blocks. This eventually questions the "single plume" model for the formation of the Gorgona Island plateau.