Xolapa Complex Research Articles

Sediment composition and U–Pb ages of detrital zircons in the Salina Cruz and Puerto Ángel beaches along the Gulf of Tehuantepec, Mexican Pacific

The Salina Cruz and Puerto Ángel beach areas in the Gulf of Tehuantepec, Mexican Pacific coast represent an important economic sector of the region. In this study, the mineralogy and geochemistry of bulk sediments, and geochronology of 400 detrital zircons recovered from the beach sediments were analysed to investigate their origin. The sediments are abundant in quartz, feldspar, ilmenite, cordierite, aragonite and anorthite. The chemical index of weathering revealed a moderate to intense weathering in the source area. The chondrite normalized REE patterns of bulk sediments are similar to the found in the Upper Continental Crust, suggesting the derivation of sediments from felsic igneous rocks. The REE patterns of zircons and the trace elemental ratios reveal a continental crust origin. Zircon U–Pb ages in the Salina Cruz beach were represented by Proterozoic (~545.1–1314.1 Ma; n = 170) and Cenozoic (~0.01–66 Ma; n = 20). The Puerto Ángel beach was abundant in Proterozoic zircon grains (~600.9–1171.4 Ma; n = 109) and followed by Mesozoic grains (~73.78–246.9 Ma; n = 40). The comparison of zircon U–Pb ages of this study with probable source rocks reveals that the Oaxaquia Terrane and Chiapas Massif Complex were the major contributors of Proterozoic zircons to the coastal areas. Similarly, the results indicate that the Cenozoic zircons were contributed by the Chiapas Massif Complex, coastal batholith and Cuicateco Terrane. The Mesozoic zircons are very few, derived from the nearby Xolapa Complex and the Chuacús Terrane.

Multi-stage, Upper Eocene-Oligocene anatexis in the Xolapa metamorphic belt (Puerto Escondido, Mexico): Dynamics of the Xolapa Complex as the decoupled lower crust of the Chortís Block upper crust during its tectonic migration

Three successive strain regimes spanning up to ~16 million years were identified from the field, structural and geochronological analyses in Upper Eocene-Oligocene migmatites of the Xolapa Complex around the Puerto Escondido longitude (~97°W), in southern Mexico. A west-facing asymmetric folding affecting diatexites at ca. 38–31 Mad defines Regime A. Regimes B and C affected younger metatexites. Regime B recorded an outward extension at ca. 31–25 Ma, whereas Regime C occurred at ca. 25–22 Ma as NE-directed thrusting. By coupling the Upper Cretaceous-Oligocene tectonic framework of southern Mexico with the dynamics and timing of the identified strain regimes, we interpret that Regimes A to C recorded distinct stages of the separation and eastward migration of the Chortís Block, viewed as a continuous geodynamic episode. According to our proposal, the upper and lower crust of Chortís adjacent to nuclear Mexico were vertically decoupled before its separation by the presence of pre-Eocene crustal anisotropies. Regime A occurred when the Chortís Block rode over the Xolapa Complex during its initial eastward escape. Since this episode, the Chortís Block (upper crust) was captured by the Caribbean plate, whereas the Xolapa Complex (lower crust) remained as the southernmost North America plate. Chortís eastward migration and the consequent unloading of its decoupled lower crust triggered anatexis, isostatic adjustments, and diachronous uplift of the Xolapa Complex (Regime B). The progressive substitution of Chortís by its thinner lower crust in the tectonic configuration next to nuclear Mexico gradually enhanced the local convergence rate between the Farallon/Cocos plate and just-severed North America. The Farallon/Cocos plate was accreted to North America (Regime C) after a critical lengthening of the uprising lower crust along the southern Mexican margin. The local increase of the convergence rate promoted the landward consumption of the Xolapa Complex by subduction erosion since 22 Ma.

Paleocene to Miocene migmatization and kinematics of the deformation at the northern boundary of the Xolapa Complex: implications for the Chortís Block-southern Mexico connection

ABSTRACT The Xolapa Complex of southern Mexico preserves evidence of its connection with the Chortís Block of Central America. Orthogneisses yielded U-Pb protolith ages from 172 to 169 Ma and paramigmatites are dated at 217 Ma. El Salitre and the Amatlán granitoids show crystallization ages of 65 and 59–57 Ma, respectively. In central Xolapa, diatexites have ages from 38 to 31 Ma, and granites were dated from 30 to 29 Ma. In the southeastern Xolapa, crystallization ages of 45 Ma and 23 to 21 Ma were obtained from migmatites and anatectic granites, respectively. Previous data indicate three diachronous stages of crustal melting along the Xolapa block. Structural data reveal that the Xolapa Complex is tectonically juxtaposed against the Acatlán Complex through ductile zones. Two mylonitic foliations are predominant in the northern boundary of the Xolapa Complex: (i) a sub-vertical NW-SE foliation with subhorizontal or low-plunging stretching lineation and left-lateral kinematics; and (ii) a subhorizontal NE-SW foliation with normal plunging stretching lineation and top-to-NE kinematics. The first mylonitic event related to the sinistral eastward motion of the Chortís Block occurred between 58 and 54–50 Ma. The second event took place in central Xolapa after or synchronous with the 38–31 Ma emplacement of diatexites. This event is related to an extensional phase that uplifts the Xolapa Complex with concerning the Acatlán Complex. In the southeastern region, deformation occurred after or synchronous with the emplacement of anatectic granites dated at 23–22 Ma. Our geochronological and structural data delineated an eastward decreasing age trend of migmatization, whereas the mylonitic deformation reported through the Xolapa Complex is consistent with a diachronous migration and detachment of the Chortís Block relative to the Acatlán Complex from Paleocene to early Miocene times.

Petrology and U–Pb geochronology of high-grade metavolcano-sedimentary rocks from central Xolapa Complex, southern Mexico

The Xolapa Complex (XC) is a high-grade metamorphic belt exposed along the southern margin of the North American plate in Mexico. Its evolution includes an episode of widespread anatexis related to crustal thickening and subsequent uplift to upper crustal levels, which culminated in the Paleocene. On the basis of field and petrographic work, this study integrates a petrological modelling approach with LA-ICP-MS U–Pb zircon and monazite geochronology to elucidate the tectonothermal evolution of this metamorphic belt. The study area in central XC includes a suite of alternating migmatitic paragneiss and garnet-bearing mafic schist. Petrological evidence and phase equilibria calculations demonstrate that a staurolite–kyanite grade metamorphism occurred at ~640–670 °C and 8–9 kbar before extensive migmatization took place. Differential anatexis of fertile and refractory layers occurred progressively via biotite and amphibole dehydration-melting reactions, and continued during peak metamorphism at granulite-facies conditions of ~800–820 °C and ~5–7 kbar. According to Ti-in-zircon thermometry, melt generation/mobilization progressed during cooling until crystallize at ~700 °C. The age of prograde metamorphism was likely recorded by monazite cores that give mid- to Late Cretaceous ages. Cooling and melt crystallization could be constrained by leucosome zircon dated at 61.8±0.6 Ma. Both zircon and low-Y monazite from whole rock and melanosome portions, respectively, are consistent with an episode of growth around 60.9±0.5 Ma. Monazite, however, would have continued its crystallization process for a period of ca. 10 Ma. Thin zircon and high-Y monazite rims show ages between ~34–25 Ma and are interpreted to reflect a stage of reheating by magmatic advection related to large Oligocene plutonism in the region. Age and nature of central XC basement show good correlation with other Early-Cretaceous volcano-sedimentary basins of central and southern Mexico. A period of crustal thickening would have buried the sequence at depths of ~30 km, causing Barrovian-type metamorphism during mid- to Late Cretaceous time and eventually, anatexis that evolved at granulite-facies conditions during orogenic collapse in the Paleocene (≥62 Ma). The time span from leucosome crystallization to late-stage reheating in central XC would imply a protracted high-temperature evolution indicating a middle- to upper-crustal residence time of at least ca. 30 Ma.

The opening and closure of the Jurassic-Cretaceous Xolapa basin, southern Mexico

Structural data and cross-cutting relationships in the Xolapa Complex around Puerto Escondido, southern Mexico, indicate the development of two pre-Cenozoic folding systems recorded, in part, by migmatites. The older system is defined by gently SW-plunging, vertical open folds (FS1), related to a southwestward extension. A subsequent FS2 folding system is defined in turn by slightly SE-plunging, northwest-facing asymmetric folds, related to a northeastward transpression. The comparison between our structural data and those available in the literature indicate that both FS1 and FS2 are pervasive in the whole Xolapa Complex. In addition to the reciprocal link between orientation, lineation distribution, and kinematics between folding systems, higher strain components of FS2 and FS1 yield a strikingly similar direction but opposite sense, suggesting a paired tectonic cycle of opening (FS1) and closure (FS2) of a Xolapa Complex basin. From the comparison between crosscutting relationships in the study area and a compilation of U-Pb zircon data from the literature, we deduce that the opening of the Xolapa basin occurred sometime between 158 and 129 Ma; this is between the termination of the continental Nazas arc and the early stages of the extensional Guerrero arc. On the other hand, the closure of the Xolapa basin (FS2) took place between 94 and 72 Ma.

The Juchatengo complex: an upper-level ophiolite assemblage of late Paleozoic age in Oaxaca, southern Mexico

The Juchatengo complex (JC) suite is located between the Proterozoic Oaxacan complex to the north and the Xolapa complex to the south, and was amalgamated by late Paleozoic magmatism. It consists of mafic and sedimentary rocks that have oceanic affinities, with internal pseudostratigraphic, structural and metamorphic characteristics, which resemble a typical upper-level ophiolite assemblage. New U–Pb zircon and previous hornblende K–Ar analyses yield ages of ca. 291–313 Ma (U–Pb) for plagiogranites and ca. 282–277 Ma for tonalites intruding the entire sequence, including pelagic sediments at the top, with a maximum deposition age of ca. 278 Ma and noteworthy local provenance. These data constrain the age of the JC to the Late Pennsylvanian–Early Permian period. Hf isotopic analyses obtained from zircons in the JC plagiogranite and tonalite show that they come from a similar primitive mantle source (176Hf/177Hf: 0.282539–0.283091; ƐHf(t): + 3.2 to + 15.0). ƐHf(t) values from near 0 to − 2.8 in the tonalites indicate a contribution from the continental crust. Trace elements and REE patterns in whole rock and zircons point to a primitive mantle source for differentiated mafic, plagiogranite dykes and tonalitic plutons. Geochronological and geochemical data address the generation of new oceanic crust above the subduction zone, probably in a backarc setting. In this tectonic scenario, the JC ophiolite originated due to the convergence of the paleo-Pacific plate below the already integrated Oaxacan and Acatlan complexes in western Pangea. The dextral displacement places the deformation in a transtensional regime during the late Paleozoic age.

Early Tertiary extensional magmatism in southern Mexico and its relationship to exhumation of the Xolapa complex and detachment of the Chortis block

Early Tertiary extensional magmatism in southern Mexico and its relationship to exhumation of the Xolapa complex and detachment of the Chortis block

Lateral spreading of the middle to lower crust inferred from Paleocene migmatites in the Xolapa Complex (Puerto Escondido, Mexico): Gravitational collapse of a Laramide orogen?

Field and laboratory studies including textural, petrological and geochronological analysis of migmatites of the Xolapa Complex affected by the same deformational event suggests that the deformation occurred during crystallization of neosome. The deformation represents sub-horizontal spreading of middle to lower crust according to structural data, and occurred during the Paleocene as inferred from U-Pb laser ablation dating of zircon. According to a compilation of stratigraphic, magmatic and deformational data reported in southern Mexico, the sub-horizontal spreading occurred after the culmination of the Upper Cretaceous Laramide orogeny. Compiled regional data indicate (a) coast-to-coast mass transfer in the upper crust from Zihuatanejo to Veracruz from the Cretaceous–Tertiary boundary until the mid-Eocene; (b) contemporaneous adakite-like magmatism; and (c) extension in the Mixteco and Oaxaca terranes of southern Mexico coeval to compression in the surrounding crust. Because all these observations are spatially associated to areas of the Mixteco and Oaxaca terranes, we propose that these areas experienced a post-orogenic gravitational collapse since the Cretaceous–Tertiary boundary until the mid-Eocene. This proposal implies that the gravitational collapse was recorded in the Xolapa Complex by migmatite formation in response to ascent of the brittle-ductile transition, decompression of the orogenic root, and/or shear-driven asthenospheric upwelling. Gravitational collapse deformation was accommodated by the subsequent flow of the migmatites towards the foreland, preceding the eastward escape of the Chortis Block.

Petrochronology of the migmatization event of the Xolapa Complex, Mexico, microchemistry and equilibrium growth of zircon and garnet

ABSTRACTThe number of migmatization events in the Xolapa Complex and their absolute age are controversial. U–Pb dating by laser ablation–inductively coupled plasma–mass spectrometry was performed on zircon grains from migmatites to investigate the age of different textural domains. Rare-earth element (REE) partition coefficients between zircon and garnet were compared with those established for different temperatures in order to test for equilibrium growth. Two age domains were identified. In one sample where zircon and garnet coexist, the outer zircon overgrowths yield a mean age of 54.16 ± 0.29 Ma (mean square weighted deviation (MSWD) = 3.5), whereas intermediate zones, between the core and outer overgrowths, yield an age of 122.7 ± 1.8 Ma (MSWD = 2.5). Partition coefficients were calculated for REEs between coexisting garnet (two different populations) and zircon using (1) the composition of ca. 54 Ma zircon overgrowths and garnet rims and (2) zircon intermediate zones together with garnet cores. The cores of small garnet grains (garnet A) may have grown in equilibrium with zircon domains of ca. 122 Ma. Both garnet cores and rims of the larger porphyroblasts (garnet B) seem to be in equilibrium with ca. 54 Ma zircon overgrowths. Petrographic observations suggest that crystallization of garnet A occurred during partial melting, placing equilibrium growth and therefore a first migmatitic event during the Early Cretaceous at ca. 122 Ma. This migmatitic event may be related to the collision of the Chortís Block with western Mexico. A second migmatitic event of ca. 54 Ma is suggested by equilibrium growth of large garnets (group B) and the outer zircon overgrowths. The high geothermal gradient necessary for this second migmatitic event might be related to the exhumation of the Xolapa Complex, as a result of the transpression and tectonic transport of the Chortís Block to the southeast from the end of the Mesozoic to most of the Cenozoic.

Origin and provenance of basement metasedimentary rocks from the Xolapa Complex: New constraints on the Chortis–southern Mexico connection

The U–Pb (LA-MC-ICPMS) geochronology of Xolapa metasedimentary rocks from Tierra Colorada, Guerrero to Puerto Ángel, Oaxaca in southern Mexico reveals that their protoliths accumulated in two distinctive cycles of sedimentation, one of Early Jurassic age and another of Late Cretaceous age. These ages are younger than thought and demonstrate that Xolapa metasedimentary rocks are not rocks from the Acatlán or Oaxacan complexes or their Paleozoic sedimentary covers as claimed. However, detrital zircon ages indicate that Xolapa sediments received contemporaneous detritus most likely from these assemblages suggesting a probably (para-)autochthonous origin for the Xolapa terrane. Xolapa rocks record two major tectonothermal events of 64–59Ma and ~34Ma; the first event produced the high-grade metamorphism and widespread migmatization that characterize Xolapa and the second event is likely related to extended heating produced by coeval arc plutonism.Pre-Jurassic assemblages of the Chortis block of Central America contain zircon populations that significantly coincide with those recorded in both, the Acatlán and Oaxacan complexes and their Paleozoic sedimentary covers as well as with those recorded in Xolapa metasediments, which suggests a spatial connection among these petrotectonic assemblages during much of the Mesozoic.It is proposed that Xolapa was generated in a basin floored by Permian rocks flanked on one side by southern Mexico terranes and on the other side by the Chortis block. Contraction of the basin tied to the approach and accretion of the Guerrero terrane arc assemblages during Late Cretaceous time produced crustal thickening and high-grade metamorphism and migmatization at mid-crustal levels. Diachronic exhumation of Xolapa began during Early Paleogene time very likely promoted by the detachment and migration of the Chortis block. The migration would additionally produce slicing of Xolapa assemblages generating its elongated and juxtaposed structure and the margin truncation that characterizes southern Mexico.

Petrogenesis and thermobarometry of the ∼50 Ma rapakivi granite-syenite Acapulco intrusive: Implications for post-Laramide magmatism in southern Mexico

The Acapulco intrusion is a composite pluton that belongs to the coastal batholithic belt of southern Mexico, intruding the Xolapa metamorphic complex and cropping out in the neighboring area of Acapulco city. The Acapulco intrusion has been considered as an anomaly based on its age, which contrasts with the surrounding plutons and the general age trend from the coastal batholithic belt and corresponds to an Eocene–Oligocene age. It ranges in composition from granite (sensu stricto) to syenite and diorite. The most distinctive characteristic of the Acapulco intrusion is the rapakivi texture developed in the granites, which are characterized by biotite, amphibole, allanite, and fl uorite as distinctive minerals, plus titanite, zircon, and apatite as accessory phases. Geochemically, the Acapulco intrusion varies from metaluminous to peraluminous, and displays the distinctive signatures of arc-related magmas. The studied rocks show strong negative Sr, Ba, and Eu anomalies, coupled with incompatible element enrichments and high Ga/Al ratios, which are typical characteristics of A-type granites that underwent strong plagioclase fractionation from a formerly metaluminous magma. Initial isotopic ratios ( 87 Sr/ 86 Sr from 0.7035 to 0.7100, and eNd from +5.50 to +1.78) indicate a range from depleted mantle compositions to compositions consistent with crustal contamination by continental crust, particularly from the surrounding Xolapa Complex. U-Pb geochronology in zircons by laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) established crystallization ages of 49.40 ± 0.40 Ma, 50.20 ± 1.0 Ma, 50.42 ± 0.39 Ma, and 50.56 ± 0.39 Ma for different lithologies of the Acapulco intrusion. These geochronological data, together with previous published works, confi rm that post-Laramide plutonism between 50 and 60 Ma is widespread in the southern continental margin of Mexico as a major magmatic event. Finally, new thermobarometric determinations established emplacement conditions of ~700 °C at 8–10 km depth (2.08–2.8 kbar), indicating an exhumation rate of ~0.21 km/m.y. between 50 and 20 Ma, which is slower than the previous estimated rate of 0.44 km/m.y. These results call for a review on models suggesting fast and/or slow exhumation of the southern Mexico coastal batholitic belt.

The North American-Caribbean Plate boundary in Mexico-Guatemala-Honduras

Abstract New structural, geochronological, and petrological data highlight which crustal sections of the North American–Caribbean Plate boundary in Guatemala and Honduras accommodated the large-scale sinistral offset. We develop the chronological and kinematic framework for these interactions and test for Palaeozoic to Recent geological correlations among the Maya Block, the Chortís Block, and the terranes of southern Mexico and the northern Caribbean. Our principal findings relate to how the North American–Caribbean Plate boundary partitioned deformation; whereas the southern Maya Block and the southern Chortís Block record the Late Cretaceous–Early Cenozoic collision and eastward sinistral translation of the Greater Antilles arc, the northern Chortís Block preserves evidence for northward stepping of the plate boundary with the translation of this block to its present position since the Late Eocene. Collision and translation are recorded in the ophiolite and subduction–accretion complex (North El Tambor complex), the continental margin (Rabinal and Chuacús complexes), and the Laramide foreland fold–thrust belt of the Maya Block as well as the overriding Greater Antilles arc complex. The Las Ovejas complex of the northern Chortís Block contains a significant part of the history of the eastward migration of the Chortís Block; it constitutes the southern part of the arc that facilitated the breakaway of the Chortís Block from the Xolapa complex of southern Mexico. While the Late Cretaceous collision is spectacularly sinistral transpressional, the Eocene–Recent translation of the Chortís Block is by sinistral wrenching with transtensional and transpressional episodes. Our reconstruction of the Late Mesozoic–Cenozoic evolution of the North American–Caribbean Plate boundary identified Proterozoic to Mesozoic connections among the southern Maya Block, the Chortís Block, and the terranes of southern Mexico: (i) in the Early–Middle Palaeozoic, the Acatlán complex of the southern Mexican Mixteca terrane, the Rabinal complex of the southern Maya Block, the Chuacús complex, and the Chortís Block were part of the Taconic–Acadian orogen along the northern margin of South America; (ii) after final amalgamation of Pangaea, an arc developed along its western margin, causing magmatism and regional amphibolite–facies metamorphism in southern Mexico, the Maya Block (including Rabinal complex), the Chuacús complex and the Chortís Block. The separation of North and South America also rifted the Chortís Block from southern Mexico. Rifting ultimately resulted in the formation of the Late Jurassic–Early Cretaceous oceanic crust of the South El Tambor complex; rifting and spreading terminated before the Hauterivian ( c . 135 Ma). Remnants of the southwestern Mexican Guerrero complex, which also rifted from southern Mexico, remain in the Chortís Block (Sanarate complex); these complexes share Jurassic metamorphism. The South El Tambor subduction–accretion complex was emplaced onto the Chortís Block probably in the late Early Cretaceous and the Chortís Block collided with southern Mexico. Related arc magmatism and high- T /low- P metamorphism (Taxco–Viejo–Xolapa arc) of the Mixteca terrane spans all of southern Mexico. The Chortís Block shows continuous Early Cretaceous–Recent arc magmatism.

Analogue models of an Early Cenozoic transpressive regime in southern Mexico: implications on the evolution of the Xolapa complex and the North American-Caribbean Plate boundary

Abstract We present analogue models that illustrate the tectonic evolution of the continental margin of southwestern Mexico and the Early Cenozoic deformation of the Xolapa complex. Together with geological data they suggest that oblique convergence caused distributed deformation and mountain building near the present-day margin of southern Mexico in a general left-lateral transpressional regime. A similar deformation is also observed north of the Xolapa complex in Maastrichtian to Paleocene sedimentary and volcanic rock units. Since post-Oligocene exhumation of middle crust does not significantly affect Late Eocene to Oligocene volcanic rocks, we infer that the evolution of the transform margin led to the formation of discrete boundaries that eventually decoupled exhumed mid-lower crust from the onshore upper-crust sequences since the Late Eocene.

Tectonic significance of Cretaceous-Tertiary magmatic and structural evolution of the northern margin of the Xolapa Complex, Tierra Colorada area, southern Mexico

The Tierra Colorada area sits along the northern limit of the Xolapa Complex, where it is juxtaposed against the Mixteco (Paleozoic) and Guerrero (Mesozoic) terranes of southern Mexico, just north of Acapulco. This paper presents combined structural and geochronological data from Tierra Colorada area that show evidence of four deformational events and several episodes of arc magmatism during Mesozoic and Cenozoic time. The oldest magmatism is represented by ca. 165 Ma granitoids and was followed by intrusion of the foliated El Pozuelo granite (129 ± 0.5 Ma; concordant U-Pb zircon analysis). This intrusion postdates D 1 metamorphism and migmatization in the Xolapa Complex. The next magmatic episode is represented by the peraluminous, foliated El Salitre granite (55.3 ± 3.3 Ma; mineral–whole-rock Rb-Sr isochron) and the protomylonitic Las Pinas I-type granite (54.2 ± 5.8 Ma; lower intercept U-Pb zircon). Las Pinas granite is characterized by D 2 ductile fabric with normal, top-tothe north-northwest sense of shear, deformed at 45–50 Ma (Rb-Sr and K-Ar ages). The ca. 34 Ma undeformed granites correspond to the last intrusive pulse in the area, postdating both D 3 south-southwest–verging thrusting of the Cretaceous Morelos Formation over sheared granites and Lower Cretaceous volcanic rocks, and open folding during D 4 . These four pulses of subduction-related magmatism in the Tierra Colorada area indicate a regular northeastward subduction at the Mesoamerican trench since Jurassic time, and alternate with contractile and/or extensional tectonic events. The gap in magmatic activity ca. 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 backarc extension and development of a passive margin during the Early–middle Cretaceous, which postdated the accretion of an exotic block, either the Guerrero terrane or the Chortis block. Following the Laramide orogeny in southern Mexico (roughly during the Late Cretaceous) the Paleocene–Miocene tectonic evolution in the Tierra Colorada area involved an alternation of magmatic pulses with extensional and contractile events. This was the result of a combination of several factors, including the geometry of the subducted slab, convergence rate, stress transmission between the subducting and overlying plates, and the rate of subduction erosion.

Geologic evolution of the Xolapa Complex, southern Mexico: Evidence from U-Pb zircon geochronology: Discussion

The Xolapa Complex of southern Mexico is composed of mid-crustal arc-related gneisses of poorly resolved ages, intruded by undeformed Cenozoic calc-alkaline plutons. Twelve undeformed and deformed tonalitic/ granodioritic samples from three transects across the Sierra Madre del Sur (Acapulco, Puerto Escondido, and Puerto Angel) were chosen for U-Pb zircon analysis. The measurements were performed on single crystals of zircons, using a multiple-collector laserablation inductively coupled plasma–mass spectrometer (MC-LA-ICP-MS). About 20–30 crystals were measured from each sample. Three gneisses and migmatites from the eastern transect (Puerto Angel), located 30–42 km from the coast yielded Grenvilleaged zircons (970–1280 Ma), suggesting that the samples represent Oaxacan basement, not deformed Xolapa Complex. The central transect (Puerto Escondido) yielded Oligocene ages (25–32 Ma) on undeformed plutons as well as mid-Mesozoic and Permian ages on gneisses. Most samples along the Puerto Escondido transect contain inherited ca. 1.1 Ga xenocrystals of zircons. The western transect (Acapulco) yielded Late Jurassic–Early Cretaceous ages (160–136 Ma) on gneisses, and Paleocene (55 Ma) and Oligocene (34 Ma) ages on undeformed plutons, with no inherited Grenville ages. The older ages and xenocrystic zircons in arc-related Xolapa Complex mirror the crustal ages found in neighboring terranes (Mixteca and Oaxaca) to the north of the Xolapa Complex, suggesting an autochthonous origin of Xolapa with respect to its neighboring northbounding terranes. The new data and previously published ages for Xolapa suggest that metamorphism and migmatization of the deformed arc rocks took place prior to the Cenozoic. Eocene and Oligocene plutons representing renewed arc-related magmatism in the area are common throughout Xolapa, and probably represent the more deeply exposed continuation of the Sierra Madre Occidental arc to the northwest. The available U-Pb data argue against the previously proposed eastward migration of magmatism between Acapulco and Puerto Angel during the Oligocene.

Geologic evolution of the Xolapa Complex, southern Mexico: Evidence from U-Pb zircon geochronology: Reply

Geologic evolution of the Xolapa Complex, southern Mexico: Evidence from U-Pb zircon geochronology: Reply

Syn‐deformational migmatites and magmatic‐arc metamorphism in the Xolapa Complex, southern Mexico

AbstractThe Xolapa Complex (XC) is the largest plutonic and metamorphic mid‐crustal basement unit in Mexico and represents an ancient continental magmatic‐arc. A complete range from metatexite to diatexite migmatitic structures has been produced during a single high‐grade metamorphic event. However, structural relics reveal the existence of early Cpx + Pl + Qtz ± Opx and Grt + Opx + Pl + Qtz ± Cpx pre‐migmatitic metamorphic assemblages. Field relationships and microstructural observations allow us to constrain five pre‐, syn‐ and post‐migmatitic deformational phases. It is argued that migmatitic structures and minor anatectic granites were developed during ductile recumbent folding and shear structures related to the D2–D3 phases. Late post‐migmatitic ductile‐brittle deformation is evidenced by the development of NNE trending transpressional thrusting (D4), and E–W left‐lateral mylonitic shear zones (D5). Biotite‐breakdown melting in felsic rocks and amphibole‐breakdown melting in mafic rocks, as well as geothermobarometric results, indicate that metamorphism took place at temperatures from 830 to 900 °C and pressures ranging from ≥6.3 to 9.5 kbar. Late migmatitic assemblages equilibrated in the highest temperature range along a clockwise P–T path. The relationships between the large diversity of migmatitic structures and the progressive production of melt suggest that feedback relations prevailed as a time‐marker during a contractional regime. Deformation, metamorphism, and plutonism of the XC show that this terrane evolved as a north‐east‐verging thrust system with synkinematic metamorphism and partial melting, during the Late Cretaceous – Palaeogene. The tectonothermal history of XC is analogous to a Cordilleran metamorphic magmatic‐arc formed in an accretionary tectonic framework. This new model provides constraints on the exhumation mechanism and thermal evolution of southern Mexico.

Geologic evolution of the Xolapa Complex, southern Mexico: Evidence from U-Pb zircon geochronology

Research Article| July 01, 2004 Geologic evolution of the Xolapa Complex, southern Mexico: Evidence from U-Pb zircon geochronology Mihai N. Ducea; Mihai N. Ducea 1University of Arizona, Department of Geosciences, Tucson, Arizona 85721, USA Search for other works by this author on: GSW Google Scholar George E. Gehrels; George E. Gehrels 1University of Arizona, Department of Geosciences, Tucson, Arizona 85721, USA Search for other works by this author on: GSW Google Scholar Sarah Shoemaker; Sarah Shoemaker 1University of Arizona, Department of Geosciences, Tucson, Arizona 85721, USA Search for other works by this author on: GSW Google Scholar Joaquin Ruiz; Joaquin Ruiz 1University of Arizona, Department of Geosciences, Tucson, Arizona 85721, USA Search for other works by this author on: GSW Google Scholar Victor A. Valencia Victor A. Valencia 1University of Arizona, Department of Geosciences, Tucson, Arizona 85721, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Mihai N. Ducea 1University of Arizona, Department of Geosciences, Tucson, Arizona 85721, USA George E. Gehrels 1University of Arizona, Department of Geosciences, Tucson, Arizona 85721, USA Sarah Shoemaker 1University of Arizona, Department of Geosciences, Tucson, Arizona 85721, USA Joaquin Ruiz 1University of Arizona, Department of Geosciences, Tucson, Arizona 85721, USA Victor A. Valencia 1University of Arizona, Department of Geosciences, Tucson, Arizona 85721, USA Publisher: Geological Society of America Received: 06 Aug 2003 Revision Received: 06 Dec 2003 Accepted: 12 Jan 2004 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (2004) 116 (7-8): 1016–1025. https://doi.org/10.1130/B25467.1 Article history Received: 06 Aug 2003 Revision Received: 06 Dec 2003 Accepted: 12 Jan 2004 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Mihai N. Ducea, George E. Gehrels, Sarah Shoemaker, Joaquin Ruiz, Victor A. Valencia; Geologic evolution of the Xolapa Complex, southern Mexico: Evidence from U-Pb zircon geochronology. GSA Bulletin 2004;; 116 (7-8): 1016–1025. doi: https://doi.org/10.1130/B25467.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The Xolapa Complex of southern Mexico is composed of mid-crustal arc-related gneisses of poorly resolved ages, intruded by undeformed Cenozoic calc-alkaline plutons. Twelve undeformed and deformed tonalitic/granodioritic samples from three transects across the Sierra Madre del Sur (Acapulco, Puerto Escondido, and Puerto Angel) were chosen for U-Pb zircon analysis. The measurements were performed on single crystals of zircons, using a multiple-collector laser-ablation inductively coupled plasma–mass spectrometer (MC-LA-ICP-MS). About 20–30 crystals were measured from each sample. Three gneisses and migmatites from the eastern transect (Puerto Angel), located 30–42 km from the coast, yielded Grenville-aged zircons (970–1280 Ma), suggesting that the samples represent Oaxacan basement, not deformed Xolapa Complex. The central transect (Puerto Escondido) yielded Oligocene ages (25–32 Ma) on undeformed plutons as well as mid-Mesozoic and Permian ages on gneisses. Most samples along the Puerto Escondido transect contain inherited ca. 1.1 Ga xenocrystals of zircons. The western transect (Acapulco) yielded Late Jurassic–Early Cretaceous ages (160–136 Ma) on gneisses, and Paleocene (55 Ma) and Oligocene (34 Ma) ages on undeformed plutons, with no inherited Grenville ages. The older ages and xenocrystic zircons in arc-related Xolapa Complex mirror the crustal ages found in neighboring terranes (Mixteca and Oaxaca) to the north of the Xolapa Complex, suggesting an autochthonous origin of Xolapa with respect to its neighboring north-bounding terranes. The new data and previously published ages for Xolapa suggest that metamorphism and migmatization of the deformed arc rocks took place prior to the Cenozoic. Eocene and Oligocene plutons representing renewed arc-related magmatism in the area are common throughout Xolapa, and probably represent the more deeply exposed continuation of the Sierra Madre Occidental arc to the northwest. The available U-Pb data argue against the previously proposed eastward migration of magmatism between Acapulco and Puerto Angel during the Oligocene. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Carbonate detritus and mylonite zones in Guerrero, Mexico and northern Honduras: new evidence for detachment of the Chortis block from southern Mexico

Brecciated lower Cretaceous carbonates on a collapsed margin in Guerrero add support to the growing evidence for the Chortis block originating from, and later detaching from, southern Mexico. Fault blocks with over 2000 m of breccia have been measured, suggesting breakup of a carbonate platform due first to magmatic uplift and later due to transcurrent faulting during detachment. There is less breccia on the Chortis block; however, magmatism may have been responsible for extensive block faulting which resulted in thick sections of well rounded limestone conglomerate that have been observed in Honduras. Mylonite scars may be evidence of basement rifting and are the basis for dividing the Xolapa complex, created by early Paleogene magmatism, into three sub-blocks. At about the same time Chortis may also have been broken into three sub-blocks by sinistral forces. A tectonic history is proposed that is based on published work by others but supported by data presented in this paper.

Stress transmission across an active plate boundary: an example from southern Mexico

Paleostress tensors calculated from inversion of fault-slip data at 60 stations between Zihuatanejo and Tehuantepec in southern Mexico were separated into four distinct groups and assigned to time intervals between 150 Ma and Recent. The change in orientation of the maximum horizontal stress closely traces the change in direction of convergence between the North American and the Farallon/Cocos plates for the same time interval. Depending on the angle between the convergence direction and the plate margin, both thrust fault and strike-slip stress regimes were active. Based on this comparison, we infer that the stress distribution in the continental crust of southern Mexico has been controlled by interplate stress transmission. Latest Cretaceous-Early Tertiary mylonites at the northern edge of the Xolapa complex portray left-lateral oblique crustal extension and oceanward migration of the upper plate. The orientation of extension at mid-crustal level grossly correspond to the orientation of the minimum compressive stress at upper crustal level. During the last 20 Ma a tensional stress regime as a response to surface uplift has been dominant in southern Mexico.