Luigi Solari

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.

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.

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.

Polyphase, High-Temperature Eclogite-Facies Metamorphism in the Chuacús Complex, Central Guatemala: Petrology, Geochronology, and Tectonic Implications

This paper describes the first discovery of eclogite-facies rocks in the Paleozoic Chuacús basement complex of north-central Guatemala. In this area, the complex comprises a thick, polydeformed sequence of high-Al metapelite, amphibolite, and quartzofeldspathic banded gneisses and schists characterized by garnet, phengite, and kyanite. Detailed petrographic, electronprobe microanalyses, and a late Carboniferous U-Pb zircon apparent age indicate that this deeply rooted orogenic terrane may be related to the Alleghenian suturing between Gondwana and Laurentia. Eclogite-facies metamorphism is established by assemblages with omphacite-garnet-rutile ± phengite ± zoisite in mafic rocks, which are consistent with garnet-kyanite-zoisite-rutile-quartz-phengite ± staurolite ± chloritoid assemblages in pelitic rocks, and amphibole-calcite/dolomite/aragonite?- rutile-quartz-zoisite ± clinochlore ± diopside in marbles. Moreover, various textural and mineralogical features (such as radial cracks in garnet and kyanite around quartz inclusions; palisade-like coronas of a silica mineral around quartz in some carbonates; lamellar inclusions of a titaniferous phase in garnet, zoisite, and phengite; and plagioclase or white mica in some omphacite; as well as the relatively high Na2O content of garnet [up to 0.12 wt%]), suggest relict ultrahigh-pressure metamorphism (UHPM). These conditions predated high-temperature-high-pressure hydration and decompression melting that occurred between 18 and 23 kbar and 700-770°C. This decompressional melting event of eclogitic rocks is dated as late Carboniferous by U-Pb on discordant zircons from a leucocratic neosome, and may be associated with the initial closure of Pangea. K-Ar ages of ~70-75 Ma on micas and amphibole, stable at 14 kbar and 597°C, are interpreted to record the Cretaceous obduction of Caribbean ophiolites and arc assemblages onto the Chuacúús complex and the southern edge of the Maya block, along the paleo-Motagua fault zone.

Late Oligocene to Middle Miocene rifting and synextensional magmatism in the southwestern Sierra Madre Occidental, Mexico: The beginning of the Gulf of California rift

Although Basin and Range–style extension affected large areas of western Mexico after the Late Eocene, most consider that extension in the Gulf of California region began as subduction waned and ended ca. 14–12.5 Ma. A general consensus also exists in considering Early and Middle Miocene volcanism of the Sierra Madre Occidental and Comondu Group as subduction related, whereas volcanism after ca. 12.5 Ma is extension related. Here we present a new regional geologic study of the eastern Gulf of California margin in the states of Nayarit and Sinaloa, Mexico, backed by 43 new Ar-Ar and U-Pb mineral ages, and geochemical data that document an earlier widespread phase of extension. This extension across the southern and central Gulf Extensional Province began between Late Oligocene and Early Miocene time, but was focused in the region of the future Gulf of California in the Middle Miocene. Late Oligocene to Early Miocene rocks across northern Nayarit and southern Sinaloa were affected by major approximately north-south– to north-northwest–striking normal faults prior to ca. 21 Ma. Between ca. 21 and 11 Ma, a system of north-northwest–south-southeast high-angle extensional faults continued extending the southwestern side of the Sierra Madre Occidental. Rhyolitic domes, shallow intrusive bodies, and lesser basalts were emplaced along this extensional belt at 20–17 Ma. Rhyolitic rocks, in particular the domes and lavas, often show strong antecrystic inheritance but only a few Mesozoic or older xenocrysts, suggesting silicic magma generation in the mid-upper crust triggered by an extension-induced basaltic influx. In northern Sinaloa, large grabens were occupied by huge volcanic dome complexes ca. 21–17 Ma and filled by continental sediments with interlayered basalts dated as 15–14 Ma, a stratigraphy and timing very similar to those found in central Sonora (northeastern Gulf of California margin). Early to Middle Miocene volcanism occurred thus in rift basins, and was likely associated with decompression melting of upper mantle (inducing crustal partial melting) rather than with fluxing by fluids from the young and slow subducting microplates. Along the eastern side of the Gulf of California coast, from Farallon de San Ignacio island offshore Los Mochis, Sinaloa, to San Blas, Nayarit, a strike distance of >700 km, flat-lying basaltic lavas dated as ca. 11.5–10 Ma are exposed just above the present sea level. Here crustal thickness is almost half that in the unextended core of the adjacent Sierra Madre Occidental, implying significant lithosphere stretching before ca. 11 Ma. This mafic pulse, with subdued Nb-Ta negative spikes, may be related to the detachment of the lower part of the subducted slab, allowing an upward asthenospheric flow into an upper mantle previously modified by fluid fluxes related to past subduction. Widespread eruption of very uniform oceanic island basalt–like lavas occurred by the late Pliocene and Pleistocene, only 20 m.y. after the onset of rifting and ∼9 m.y. after the end of subduction, implying that preexisting subduction-modified mantle had now become isolated from melt source regions. Our study shows that rifting across the southern-central Gulf Extensional Province began much earlier than the Late Miocene and provided a fundamental control on the style and composition of volcanism from at least 30 Ma. We envision a sustained period of lithospheric stretching and magmatism during which the pace and breadth of extension changed ca. 20–18 Ma to be narrower, and again after ca. 12.5 Ma, when the kinematics of rifting became more oblique.

Sandstone Provenance of the Arperos Basin (Sierra de Guanajuato, Central Mexico): Late Jurassic-Early Cretaceous Back-Arc Spreading as the Foundation of the Guerrero Terrane

Three paleogeographic scenarios have been proposed for the Mesozoic volcano-sedimentary successions that compose the Guerrero terrane, western Mexico. In the type 1 scenario, the Guerrero terrane is an exotic Pacific arc accreted to nuclear Mexico by the consumption of a pre-Cretaceous oceanic basin, named Arperos Basin. The type 2 scenario considers the Guerrero terrane as a fringing multiarc system, accreted by the closure of pre-Cretaceous oceanic basin substrates at multiple subduction zones with varying polarities. In the type 3 scenario, the Guerrero terrane is interpreted as a North American west-facing para-autochthonous arc, which was drifted in the paleo-Pacific domain by the opening of the Cretaceous back-arc oceanic Arperos Basin. To test these reconstructions, we present here a combined study that includes geologic mapping, stratigraphy, U-Pb geochronology, and sandstone provenance data from the Arperos Basin in the Sierra de Guanajuato, central Mexico. Our data document that the Arperos Basin developed in a back-arc setting and evolved from continental to open oceanic conditions from the Late Jurassic to the Early Cretaceous. Sandstone provenance analysis shows an asymmetric distribution of the infill sources for the Arperos Basin: continent-recycled sedimentary rocks were deposited along its northeastern side, whereas magmatic arc-recycled clastic rocks developed at its southwestern side. Such asymmetric distribution closely fits with sedimentological models proposed for present-day continent-influenced back-arc basins. On the basis of this evidence, we favor a type 3 scenario for the Guerrero terrane, which is then considered to represent a detached slice of the Mexican leading edge that drifted in the paleo-Pacific domain during back-arc extension and subsequently accreted back to the Mexican craton.

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.

Stratigraphy, geochronology, and geochemistry of the Laramide magmatic arc in north-central Sonora, Mexico

The Laramide magmatic arc in the Arizpe-Mazocahui quadrangle of north-central Sonora, Mexico, is composed of volcanic rocks assigned to the Tarahumara Formation and several granitic plutons that intrude it. The arc was built over juxtaposed crustal basements of the Caborca and Mazatzal provinces. A basal conglomerate of the >4-km-thick Tarahumara Formation overlies deformed Proterozoic igneous rocks and Neoproterozoic to Early Cretaceous strata, thus constraining the age of a contractional tectonic event that occurred between Cenomanian and early Campanian time. The lower part of the Tarahumara Formation is composed of rhyolitic ignimbrite and ash-fall tuffs, andesite flows, and interbedded volcaniclastic strata, and its upper part consists of rhyolitic to dacitic ignimbrites, ash-fall tuffs, and volcaniclastic rocks. The Tarahumara Formation shows marked lateral facies change within the study area, and further to the north it grades into the coeval fluvial and lacustrine Cabullona Group. The age of the Tarahumara Formation is between ca. 79 and 59 Ma; the monzonitic to granitic plutons have ages of ca. 71–50 Ma. The informally named El Babizo and Huepac granites, La Aurora and La Alamedita tonalities, and the Puerta del Sol granodiorite compose the El Jaralito batholith in the southern part of the area. Major and trace element composition of the Laramide igneous rocks shows calc-alkaline differentiation trends typical of continental magmatic arcs, and the isotope geochemistry indicates strong contribution from a mature continental crust. Initial 87Sr/86Sr values range from 0.70589 to 0.71369, and eNd values range from –6.2 to –13.6, except for the El Gueriguito quartz monzonite value, –0.5. The Nd, Sr, and Pb isotopic values of the studied Laramide rocks permit comparison with the previously defined Laramide isotopic provinces of Sonora and Arizona. The El Gueriguito pluton and Bella Esperanza granodiorite in the northeastern part of the study area along with plutons and mineralization of neighboring northern Sonora have isotopic values that correspond with those of the southeastern Arizona province formed over the Mazatzal basement ([Lang and Titley, 1998][1]; [Bouse et al., 1999][2]). Isotopic values of the other Laramide rocks throughout the study area are similar to values of provinces A and B of Sonora ([Housh and McDowell, 2005][3]) and to those of the Laramide Pb boundary zone of western Arizona, while the Rancho Vaqueria and La Cubana plutons in the northernmost part of the area have the isotopic composition of the Proterozoic Mojave province of the southwestern United States. These data permit us to infer that a covered crustal boundary, between the Caborca block with a basement of the Mojave or boundary zone and the Mazatzal province, crosses through the northeastern part of the area. The boundary may be placed between outcrops of the El Gueriguito and Rancho Vaqueria plutons, probably following a reactivated Cretaceous thrust fault located north of the hypothesized Mojave-Sonora megashear, proposed to cross through the central part of the area. [1]: #ref-53 [2]: #ref-13 [3]: #ref-50

Permian–Carboniferous arc magmatism and basin evolution along the western margin of Pangea: Geochemical and geochronological evidence from the eastern Acatlán Complex, southern Mexico

In the Acatlan Complex of southern Mexico, a late Paleozoic assemblage, consisting of a gabbro-diorite-tonalite-trondhjemite suite (Totoltepec pluton) and clastic-calcareous metasedimentary rocks (Tecomate Formation), postdates collisional orogeny that resulted in the amalgamation of Pangea. This region offers a rare opportunity to examine assemblages developed at different crustal levels along the periphery of Pangea at the critical stage between amalgamation and breakup. The Totoltepec pluton consists of minor mafic-ultramafic rocks (306 ± 2 Ma; concordant U-Pb zircon analysis) that are marginal to the main mafic-felsic intrusion (289 ± 2 Ma). Geochemistry of the marginal rocks indicates an arc tholeiitic to calc-alkaline character with high large ion lithophile elements (LILEs)/high field strength elements (HFSEs), flat rare earth element (REE) patterns, and initial ɛ Nd values of +1.3 to +3.3. The younger Totoltepec phase exhibits a calc-alkaline trace-element geochemistry with flat to moderately fractionated light (L) REE–enriched patterns and initial ɛ Nd values of –0.8 to +2.6, which are also consistent with an arc environment. The Sm-Nd isotopic signature is more primitive compared to contemporaneous arc-related igneous rocks in southern Mexico, suggesting the pluton was emplaced in a less mature, outboard part of the arc, and/or along a fault conduit. The Tecomate Formation, as currently defined, is a composite of lithologically similar strata deposited in several fault-bounded basins ranging from Carboniferous to Early Permian in age. To the south of the Totoltepec pluton, the depositional age of the Tecomate Formation is tightly constrained in one section to ca. 300 Ma, but in another section, it is between ca. 288 and ca. 263 Ma. The Tecomate Formation rocks are interpreted to have been derived from a late Paleozoic arc based on (1) arc-related geochemistry, (2) ɛ Nd (t) values ranging from –5.6 to +0.3 (t = 288 Ma) that overlap those of the Totoltepec pluton, and (3) detrital zircons with predominantly Carboniferous–Permian ages. The Totoltepec and Tecomate units in the study area form part of a continental arc extending from Guatemala to California, which necessitates subduction of the paleo-Pacific oceanic lithosphere beneath the western margin of a Pangea-A configuration.