A high-pressure folded klippe at Tehuitzingo on the western margin of an extrusion zone,Acatlán Complex,southern México

High-pressure (HP) rocks at Tehuitzingo, on the western margin of the HP belt within the Paleozoic Acatlán Complex (southern México), occur in a klippe that was thrust over low-grade clastic rocks. The youngest detrital zircon cluster in the low-grade rocks yielded U-Pb ages of 481 ± 16 Ma, which provide an older limit for deposition. The HP rocks are composed of metabasites, serpentinite, granite (482 ± 3 Ma) and mica schist (youngest concordant detrital zircon: 433 ± 3 Ma). The schist and granite are inferred to be high-grade equivalents of lower Paleozoic, low-grade rocks exposed elsewhere in the Acatlán Complex, from which they are inferred to have been removed by subduction erosion. Mineral analyses indicate that the subducted rocks underwent HP metamorphism and polyphase deformation at depths of ~ 50 km (~ 16 kbar and 750 °C: eclogite facies). Subsequent retrogression passed through epidote-amphibolite to greenschist facies, which was synchronous with W-vergent thrusting over the low-grade clastic rocks. Deposition of the low-grade rocks and thrusting are bracketed between either 481–329 Ma (Ordovician-Mississippian), and was followed by F₃ synformal folding. Cooling through ca. 385 °C is indicated by 329 ± 1 and 316–317 ± 2 Ma, ⁴⁰Ar/³⁹Ar muscovite plateau ages in HP rocks, which are 5–17 my younger than those of the adjacent Piaxtla eclogites suggesting younger exhumation. The petrology, P-T conditions and ages of the Piaxtla Suite is consistent with an extrusion channel within the Acatlán Complex along the active western margin of Pangea during the Carboniferous. Detrital zircon populations in the low-grade psammite (ca. 481, 520–650, 720, 750, 815, 890, 1050 and 2750 Ma) and the HP schist (ca. 457–480, 534, 908, 954–1150, 1265, 1845 and 2035 Ma) indicate derivation from the Ordovician Acatlán granitoids, Neoproterozoic Brasiliano orogens, 900–750 Ma Goiás arc (Amazonia), 1–1.3 Ma Oaxaquia, and more ancient sources in Oaxaquia/Amazonia.     

Mafic forearc cumulates and associated rocks in the central high-pressure belt of the Acatlán Complex of southern México: geochemical constraints

The high-pressure (HP) Piaxtla Suite at Tehuitzingo contains peridotites, gabbros, and serpentinized peridotites, as well as granitoids and metasedimentary rocks. The HP mafic rocks are characterized by low SiO₂ (38–52 wt.%) and high Mg# (～48–70), Ni (100–470 ppm), and Cr (180–1750 ppm), typical of cumulate compositions. Trace elements and rare earth element (REE) primitive mantle-normalized patterns display generally flat profiles, indicative of derivation from a primitive mantle with two distinct patterns: (1) gabbroic patterns are characterized by a positive Eu anomaly, low REE abundances, and slightly depleted high REE (HREE) relative to low REE (LREE), typical of cumulus olivine, pyroxene, and plagioclase; and (2) mafic-intermediate gabbroic patterns exhibit very flat profiles characteristic of olivine and clinopyroxene as cumulus minerals. Their Nb/Y and Zr/TiO₂ ratios suggest a subalkaline character, whereas low Ti/V ratios indicate that the Tehuitzingo cumulates are island arc tholeiitic basalts that resemble modern, immature oceanic, forearc magmas. These cumulates have high values of ? _Nd(t) = 5.3–8.5 and ¹⁴⁷Sm/¹⁴⁴Nd = 0.18–0.23, which renders calculations of model ages meaningless. Our data are consistent with the Tehuitzingo arc rocks being part of a tectonically extruded Devonian–early Carboniferous arc developed along the west margin of Gondwana.     

A Carboniferous high-pressure klippe in the western Acatlán Complex of southern México: implications for the tectonothermal development and palaeogeography of Pangea

High-pressure (HP) rocks are critical for palinspastic restorations because they mark inferred subducted/extruded oceanic crust; knowledge of their geometric, geodynamic, and age relationships provide essential constraints on palaeogeographic reconstructions. The westernmost HP belt (Ixcamilpa) in the Acatlán Complex of southern Mexico has been inferred to be a mid-Late Ordovician backarc basin on the southern Iapetan margin that was subducted beneath eastern Laurentia and extruded up the subduction zone during the Early Silurian. Re-examination of Ixcamilpa HP rocks has revealed that they comprise lower Palaeozoic rift-passive margin protoliths and occur in a W-vergent klippe (not a suture) formed during polyphase deformation. Peak metamorphic mineral assemblages of blueschist-amphibole eclogite facies underwent retrogression through epidote amphibolite to greenschist facies. ⁴⁰Ar/³⁹Ar dating of various rocks yielded plateau ages of 344–339 Ma for calcic amphibole, 318 ± 4 Ma for glaucophane, and 329–325 ± 2 Ma for muscovite (excess argon), which clearly indicate a Carboniferous tectonothermal event. We interpret the 20 million years range in amphibole ages as reflecting progressive unroofing. The terminal stage of progressive thrusting placed the HP rocks above the middle Mississippian Zumpango Unit, during which a single penetrative sub-greenschist fabric was produced. Subsequent Permian or Laramide deformation refolded all the rocks about NE-trending upright folds. We postulate that the root zone of the HP nappe lies to the east in the median HP belt, which has a structure consistent with an extrusion zone. Inasmuch as similar units of the Acatlán Complex bound this HP root zone on either side, it is inferred to have been extruded into the upper plate above the subduction zone, and thus is not an oceanic suture. Our new data provide constraints for a Carboniferous palaeogeographic reconstruction, whereby subduction erosion of passive margin rocks took place along the western margin of Pangea and were subsequently extruded into the upper (Acatlán) plate.     

Arc plutonism in a transtensional regime: the late Palaeozoic Totoltepec pluton,Acatlán Complex,southern Mexico

The ENE-trending, ca. 306–287 Ma, Totoltepec pluton is part of a Carboniferous–Permian continental magmatic arc on the western Pangaean margin. The 15 km?×?5 km pluton is bounded by two N–S Permian dextral faults, an E–W thrust to the south, and an E–W normal fault to the north. Thermobarometric data indicate that the main, ca. 289–287 Ma, part of the pluton was emplaced at ≤20 km depth and ≥700°C and was exhumed to 11 km and 400°C in 4 ± 2 million years. We have documented the following intrusive sequence: (1) the 306 Ma northern marginal mafic phase; (2) the 287 Ma main trondhjemitic phase; and (3) ca. 289–283 Ma sub-vertical dikes that vary from (a) N39E, undeformed with crystal growth perpendicular to the margins, through (b) ca. N50–73E, foliated and folded with sinistral shear indicators, to (c) N73–140E and boudinaged. The obliquity of the boundary between the folded and stretched dikes relative to the N–S dextral faults suggests sequential emplacement in a transtensional regime (with 20% E–W extension), followed by different degrees of clockwise rotation passing through a shortening field accompanied by sinistral shear into an extensional field. The ca. 289–287 Ma intrusion also contains a steep ENE-striking foliation and hornblende lineations varying from sub-horizontal to steeply plunging, probably the result of emplacement in a triclinic strain regime. We infer that magmatism ceased when some of the dextral motion was transferred from the western to the eastern bounding fault, causing thrusting to take place along the southern boundary of the pluton. This mechanism is also invoked for the rapid uplift and exhumation of the pluton between ca. 287 Ma and 283 Ma. The distinctive characteristics of the Totoltepec pluton should prove useful in identifying similar tectonic settings within continental arcs.     

Palaeozoic structures in the Xayacatlán area,Acatlán Complex,southern Mexico: transtensional rift‐ and subduction‐related deformation along the margin of Oaxaquia

The Xayacatlán area (eastern Mixteca terrane, southern Mexico) was previously inferred to preserve the Ordovician‐Silurian thrust contact between vestiges of the Iapetus Ocean and the para‐autochthon bordering Oaxaquia. Detailed remapping indicates that the rocks occur in four vertically‐bounded, NS fault blocks. The latter record the following tectonothermal events that post‐date Iapetus and occurred along the margins of the Rheic (1) and Pacific (2 and 3) oceans: (1) dextral transtension accompanying intrusion of an NS, tholeiitic dike swarm at ～442 Ma; (2) penetrative, greenschist‐facies deformation during the Mississippian related to extrusion of high‐pressure rocks; and (3) subgreenschist‐facies dextral transtension on NS faults during the generation of Middle Permian fabrics.     

Geochemistry and Sm–Nd isotopic systematics of Ediacaran–Ordovician,sedimentary and bimodal igneous rocks in the western Acatlán Complex,southern Mexico: Evidence for rifting on the southern margin of the Rheic Ocean

Ordovician igneous rocks in the western Acatlán Complex (Olinalá area) of southern Mexico include a bimodal igneous suite that intrudes quartzites and gneisses of the Zacango Unit, and all these rocks were polydeformed and metamorphosed in the amphibolite facies during the Devono-Carboniferous. The Ordovician igneous rocks consist of the penecontemporaneous amphibolites, megacrystic granitoids and leucogranite, the latter dated at ca. 464 Ma. Geochemical and Sm–Nd data indicate that the amphibolites have a differentiated tholeiitic signature, and that its mafic protoliths formed in an extensional setting transitional between within-plate and ocean floor. The amphibolites are variably contaminated by a Mesoproterozoic crustal source, inferred to be the Oaxacan basement exposed in the adjacent terrane. The most primitive samples have εNdt (t = 465 Ma) values significantly below that of the contemporary depleted mantle and were probably derived from the sub-continental lithospheric mantle. The megacrystic granites were most probably derived by partial melting of an arc crustal source (similar to the Oaxacan Complex) and triggered by the ascent of mafic magma from the lithospheric mantle. Sm–Nd isotopic signatures suggest that metasedimentary rocks from Zacango Unit were derived from adjacent Oaxacan Complex. Trace elements relationships (e.g. La/Th vs. Hf) and REE patterns suggest provenance in felsic-intermediate igneous rocks with a calc-alkaline signature. The Ordovician bimodal magmatism is inferred to have resulted from rifting on the southern flank of the Rheic Ocean and is an expression of a major rifting event that occurred along much of the northern Gondwanan margin in the Ordovician.     

U–Pb geochronology of two discrete Ordovician high-pressure metamorphic events in the Seve Nappe Complex, Scandinavian Caledonides

U–Pb geochronology for eclogites in two different areas of the Seve Nappe Complex (SNC) in Sweden confirms previous indications of discrete Ordovician high-pressure events affecting various parts of the complex. In Norrbotten, just north of the Arctic circle, eclogites from the Ts?kkok and Vaimok Lenses yield identical metamorphic zircon ages of 482 ± 1 Ma. Titanite in a metagabbro from the Vaimok Lens retains an older age of 607 ± 2 Ma, which may date a protolith coeval with mafic dikes in the overlying Sarek Lens; high-U zircon cores in one of the eclogites also indicate a similar age. Farther south, in J?mtland, the Tjeliken eclogite yields a significantly younger metamorphic age of 446 ± 1 Ma. Although they support the age discrepancy between the Norrbotten and J?mtland eclogites, the U–Pb ages of both eclogite suites are ca. 20 m.y. younger than previously reported Sm–Nd ages. The latter may either represent early prograde growth or be spuriously too old due to isotopic disequilibrium. The SNC has traditionally been taken to represent the outermost margin of Baltica, linking the Early Caledonian eclogite–forming events to subduction of Baltica below an offshore arc. Alternatively, the coincidence of these eclogite-forming events with orogenic phases recorded on the Laurentian margin may point to an origin from other regions of the Iapetus Ocean.     

Seismicity distribution in the vicinity of the Chile Triple Junction,Aysén Region,southern Chile

The Aysén Region, southern Chile, is the area located at the southern end of the Nazca-South America subduction zone, to the east of the Chile Triple Junction. This region has historically presented low levels of seismicity mostly related to volcanism. Nonetheless, a seismic sequence occurred in 2007, related to the reactivation of the strike-slip Liquiñe-Ofqui Fault System (LOFS), confirmed that this region is not exempt from major seismic activity M ∼ 7. Here we present results from background local seismicity of two years (2004–2005) preceding the sequence of 2007. Event magnitudes range between 0.5 and 3.4 M_L and hypocenters occur at shallow depths, mostly within the upper 10 km of crust, in the overriding South American plate. No events were detected in the area locus of the 2007 sequence, and the Wadati–Benioff (WB) plane is not observable given the lack of subduction inter-plate seismicity in the area. A third of the seismicity is related to Hudson volcano activity, and sparse crustal events can be spatially associated with the trace of the Liquiñe-Ofqui fault, showing the largest detected magnitudes, in particular at the place where the two main branches of the LOFS meet. Other minor sources of seismicity correspond to glacial calving in the terminal zones of glaciers and mining explosions.     

Geochemical distribution of arsenic,cadmium, lead and zinc in river sediments affected by tailings in Zimapán,a historical polymetalic mining zone of México

High arsenic (As) groundwater is widely distributed in northwestern Hetao Plain, an arid region with sluggish groundwater flow. Observed As concentration in groundwater from wells ranges from 76 to 1,093 μg/l. Most water samples have high total dissolved solids, with Cl and HCO₃ as the dominant anions and Na as the dominant cation. The major hydrochemical types of most saline groundwaters are Na–Mg–Cl–HCO₃ and Na–Mg–Cl. By contrast, fresh groundwaters generally belong to the Na–Mg–HCO₃ type. High concentrations of arsenic in shallow aquifers are associated with strongly reducing conditions, as evidenced by high concentrations of dissolved organic carbon, ammonium, as well as dissolved sulfide and Fe, dominance of arsenite, relatively low concentrations of nitrate and sulfate, and occasionally high content of dissolved methane (CH₄). High As groundwaters from different places at Hetao Plain experienced different redox processes. Fluoride is also present in high As groundwater, ranging between 0.40 and 3.36 mg/l. Although fluorosis poses an additional health problem in the region, it does not correlate well with As in spatial distribution. Geochemical analysis indicates that evapotranspiration is an important process controlling the enrichment of Na and Cl, as well as trace elements such as As, B, and Br in groundwater. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Palaeomagnetic assessment of plutons from the southern Peninsular Ranges batholith and the Jurassic Vizcaíno igneous suites,Baja California,México

ABSTRACT

We report geological and palaeomagnetic data from five discrete plutons in the southern part of the Peninsular Ranges batholith (PRB) and one pluton that is part of the Jurassic plutonic suite in the Vizcaíno peninsula. The PRB plutons are Cretaceous and belong to the Alisitos island arc. The Jurassic pluton intrudes a Triassic-Jurassic ophiolite.

Our study was designed to evaluate the palaeomagnetic homogeneity of the batholith from the Sierra San Pedro Mártir, at ~31°N, to about ~28.3°N. The Punta Prieta, Nuevo Rosarito, San Jerónimo, and La Rinconada plutons in the western zone of the PRB are characterized by magnetizations residing in magnetite. The Compostela pluton is emplaced in a transition zone and has a magnetization that resides in haematite. The five Cretaceous plutons yield a combined palaeopole at 80.3°N, 162.1°E, A₉₅ = 9.8°, N = 5 that after correcting for the opening of the Gulf of California rotates to 77.6°N, 173.6°E, the rotated pole being in angular distance of only 4.4° from the North America reference pole. The Jurassic San Roque pluton yields a mean 0.6°N, 306.1°E, A₉₅ = 9.2°, N = 10, which is discordant, showing a clockwise rotation of about 131° ± 16° and flattening of 9.5° ± 12.9° with respect to the 150 Ma cratonic reference palaeopole. The results suggest that the intrusion of the undeformed Cretaceous Punta Prieta to Compostela plutons (128.1 ± 1.4 and 100.5 ± 2.7 Ma, respectively) restricts tectonic accretion of the Jurassic-Early Cretaceous sequences to the North America margin to the time before mid-Cretaceous magmatism (~100 Ma) in the PRB near present latitude 28°N. Mesozoic and Cenozoic strike-slip faulting along the Vizcaíno margin can account for the 131° clockwise rotation of the San Roque pluton. Our results do not support significant latitudinal movement between Vizcaíno, the PRB, and mainland Mexico with the exception of the Neogene San Andreas Fault-related right lateral movement.     

New stratigraphic,geochronological, and structural data from the southern Guanajuato Mining District,México: implications for the caldera hypothesis

Abstract

The Cenozoic stratigraphy of the southern Guanajuato Mining District (GMD) was established 40 years ago. The existence of a caldera structure that produced the Cenozoic volcanic cover was postulated and the world-class silver ore deposit of the Oligocene age has been closely related to magmatism. In this context, we present a new geological map of the southern GMD, U–Pb and Ar–Ar ages of the volcanic units, and structural data for the Cenozoic faults. Our results document that the volcanic centre was active between ca. 33.5 Ma and ca. 31.3 Ma, coeval with NW–SE normal faulting. We propose that the Bufa, Calderones, and Cedro formations are stratigraphic units directly related to the volcanic centre. Although the younger Chichíndaro Rhyolite scarcely crops out within the study area, it appears to be more extensive outside of the study area, forming part of the rhyolitic volcanism of the Mesa Central of Mexico. In the study area, the Chichíndaro Rhyolite buries major faults, demonstrating that it was emplaced after the peak of faulting. The two main structures are the El Cubo and Veta Madre grabens; also there are several faulted and brecciated zones where silver–gold mineralization was emplaced. The extension direction changed from NE to NW producing normal faulting, reactivating older structures and allowing dike intrusion. The extensional phase continued to be active throughout the Oligocene. The age of the volcanic event and a new K–Ar age of the Veta Madre vein of 29.8 ± 0.8 Ma (K–Ar in adularia) indicate that the hydrothermal event began immediately after the emplacement of the Cedro Formation. The emplacement of the Chichíndaro Rhyolite allowed hydrothermal activity to be active for two million years or more.     

Jade in the Serpentinite Mélanges of the Rio San Juan Complex from the Dominican Republic

<正>New occurrences of jadeitite,jadeite quartzites,and jadeite-lawsonitequartzites have recently been discovered in the Rio San Juan Complex(RSJC)of the northern Dominican Republic.These rocks are found in serpentinite mélanges associated with a former intra-oceanic     

The structure and metamorphism of the Red Point Metamorphic Complex—A newly discovered high-pressure metamorphic complex from the south coast of Tasmania

This study presents new data on the deformational and metamorphic history of previously unstudied Cambrian high-pressure metamorphic rocks exposed on the remote south coast of Tasmania. The Red Point Metamorphic Complex consists of two blocks of high-pressure, medium-grade metamorphic rocks including pelitic schist and minor garnet-bearing amphibolite, which are faulted against a sequence of low-grade phyllite and quartzite. The Red Point Metamorphic Complex records five phases of deformation, all of which except the first are expressed at a mesoscopic scale in both the medium- and low-grade rocks. Peak metamorphic conditions in the high-pressure epidote–amphibolite facies is recorded by medium-grade schist and amphibolite and was synchronous with the second major deformation event, which produced a pervasive schistosity and mesoscale isoclinal folds. The juxtaposition of the low- and medium-grade rocks is interpreted to have first occurred prior to the development of upright, opening folding associated with the third deformation. However, the present contacts between the two contrasting metamorphic sequences formed during widespread faulting and ductile-shear zone development associated with the fourth and fifth deformation events. The new data from the Red Point Metamorphic Complex provide insights into the structural and metamorphic history experienced by the medium-grade rocks of Tasmania during the Cambrian Tyennan Orogeny. This study demonstrates that Cambrian medium-grade metamorphic rocks are more widespread throughout Tasmania than previously realised, which represents an important step towards understanding the large-scale architecture of the Tyennan Orogen.     

Timing of high-pressure metamorphic events in the Bulgarian Rhodopes from Lu–Hf garnet geochronology

Within the Mediterranean realm, the Rhodopes represent a nappe stack of oceanic and continental fragments assembled along the Eurasian continental margin during the Alpine orogeny. The timing of the high-pressure (HP) metamorphism has long been ambiguous, lacking detailed geochronological and geochemical control on subduction-exhumation and nappe stacking processes. Here we apply the Lu–Hf and Sm–Nd chronometers to a suite of representative eclogite samples covering two different key units of the Rhodopean nappe stack: (1) the Kimi Complex (Upper Allochthon) and (2) the Middle Allochthon. In addition to geochronology, we also determined whole rock Hf and Nd isotope compositions as well as major and trace element concentrations in order to constrain the nature of the eclogite protoliths. Two HP metamorphic events were revealed by Lu–Hf geochronology: (1) a Lower Cretaceous event in the Upper Allochthon (126.0 ± 1.7 Ma) and (2) an Eocene event in the Middle Allochthon (44.6 ± 0.7 Ma; 43.5 ± 0.4 Ma; 42.8 ± 0.5 Ma), at conditions of ca. 700°C/20–25 kbar. Our new data provide direct evidence for multiple subduction events in the Rhodopes. Exhumation and subsequent thrusting of the Middle Allochthon on the Lower Allochthon can be narrowed down to the time span between 42 and 34 Ma. In a broader tectonic context, the Eocene ages for the HP metamorphism support the view that the Rhodopes represent a large-scale tectonic window, exposing the deepest nappe units of the Hellenides.     

Relationships between magmatism,metamorphism and deformation in the Fraser Complex,Western Australia: Constraints from new SHRIMP U–Pb zircon geochronology

SHRIMP U–Pb zircon isotopic data have been obtained for four samples collected from granitoids and paragneisses in the Fraser Complex, a large composite metagabbroic body cropping out in the Mesoproterozoic Albany‐Fraser Orogen of Western Australia. The data are combined with the results of field mapping and petrographic analysis to revise a model for the geological evolution of the Fraser Complex. Three main phases of deformation are recognised in the Fraser Complex (D_1–3) associated with two metamorphic events (M_1–2), which involve four distinguishable episodes of recrystallisation. The first metamorphic event recognised (M_1a/D₁) reached granulite facies and is characterised by peak T ≥800°C and P = 600–700 MPa. A syn‐M_1a/D₁ charnockite has a U–Pb SHRIMP zircon age of 1301 ± 6 Ma, which also provides an estimate for the age of intrusion of Fraser Complex gabbroic rocks. Disequilibrium textures comprising randomly oriented minerals (M_1b), consistent with approximately isobaric cooling, formed in various lithologies in the interval between D₁ and D₂. Post‐D₁, pre‐D₂ granites intruded at 1293 ± 8 Ma and were foliated during the D₂ event, which culminated in the burial of the Fraser Complex to depths equivalent to 800–1000 MPa. Following burial, pyroxene granulites on the western boundary of the complex were pervasively retrogressed to garnet amphibolite (M_2a). An igneous crystallisation age of 1288 ± 12 Ma from a syn‐M_2a aplite dyke suggests that retrogression may have occurred only a few millions of years after the peak of granulite facies metamorphism. Exhumation to depths of less than ～400 MPa occurred within ～20–30 million years of the M_2a pressure peak. Associated deformation (D₃) is characterised by the development of mylonite and transitional greenschist/amphibolite facies disequilibrium textures (M_2b).     

Morphometric and hypsometric analysis in the Tierra Nueva Basin,San Luis Potosí, México

In areas with long periods of drought, it is essential to implement strategies to manage the available water resource. Tierra Nueva Basin is affected by this situation, consequently the farm production and livestock holdings are affected and the people don’t have access to enough water. In this paper, we propose an integrative methodology based on mathematical tools such as hypsometric and morphometric analysis applying geographic information systems. The proposal is strengthened with the aggregation of geological-structural, morphometrical, hypsometrical parameters and climatological information through a precipitation analysis provided by the National Water Commission from 1962 to 2010. According to the available data and the results obtained through the implemented proposal, an acceptable level of reliability is inferred allowing to determine areas structurally suitable for the use of surface water and its uptake. The methodology that we propose facilitates and simplifies the processes of searching and exploring locations suitable for surface water capture in arid and semi-arid zones, identifying in a qualitative and quantitative manner the optimal zone. For the case study, test-and-validation of the methodology the Tierra Nueva Basin in San Luis Potosí, México was selected. Tierra Nueva is a semi-arid site where “La Muñeca” dam is located. The results obtained in this work confirm the location of the “La Muñeca” dam as one of the most suitable areas for water collection. The methodology that we propose is a useful tool for the studies of water capturing purposes in arid and semi-arid zones.     

P–T history of garnet-websterites in the Sharyzhalgai complex,southwestern margin of Siberian craton: evidence for Paleoproterozoic high-pressure metamorphism

The Saramta massif in the Paleoproterozoic Sharyzhalgai complex, the southwestern margin of the Siberian craton, is mainly composed of spinel-peridotites with garnet-websterites; it is enclosed within granitic gneisses and migmatites with mafic intercalations of granulite-facies grade. The garnet-websterites occur as lenses or layers intercalated within spinel-harzburgite and spinel-lherzolite. They consist mainly of clinopyroxene (Cpx), garnet (Grt), and orthopyroxene (Opx): Grt often includes Cpx, Opx, and pargasite (Prg). Opx also occurs as kelyphite with plagioclase (Pl), spinel, olivine, Prg, and biotite. Relationships between textures and chemical compositions of these minerals suggest the following P–T stages: stage 1 (pre-peak), 0.9–1.5 GPa at 640–780 °C; stage 2 (peak), 2.3–3.0 GPa at 920–1030 °C as the minimum estimate; and stage 3 (post-peak), 750–830 °C at 0.5–0.9 GPa. Finally, the garnet-websterites are veined with lower amphibolite- to greenschist-facies minerals (stage 4).These results suggests that the Saramta massif was carried to depths of c. 100 km by subduction, and metamorphosed under eclogite-facies conditions in the Paleoproterozoic, despite the commonly held view that high geothermal gradients in those times would have prevented such deep subduction. Paleoproterozoic plate subduction at the southwestern margin of the Siberian craton might have caused subduction-zone magmatism and mantle metasomatism similar to those in the Phanerozoic.     

Interpretation of gravity profiles across the northern Oaxaca terrane,its boundaries and the Tehuacán Valley,southern Mexico

A gravity study was conducted across the northern Oaxaca terrane and its bounding faults: the Caltepec and Oaxaca Faults to the west and east, respectively. These faults juxtapose the Oaxaca terrane against the Mixteca and Juarez terranes, respectively. The Oaxaca Fault also forms the eastern boundary of the Cenozoic Tehuacán depression. On the west, at depth, the Tehuacán valley is limited by the normal buried Tehuacán Fault. This gravity study reveals that the Oaxaca Fault system gives rise to a series of east tilted basamental blocks (Oaxaca Complex). The tectonic depression is filled with Phanerozoic rocks and has a deeper depocenter to the west. The gravity data also indicate that on the west, the Oaxaca Complex, the Caltepec and Santa Lucia faults continue northwestwards beneath Phanerozoic rocks. A major E–W to NE–SW discontinuity is inferred to exist between profiles 1 and 2.