Terrane Research Articles

Meso- to Neoproterozoic terrane accretion: Insights from juvenile mafic magmatism from the Votuverava Group and Embu Complex, southern Ribeira Belt, Brazil

Terranes of the Neoproterozoic Ribeira Belt record processes related to West Gondwana assembly. However, the evolutionary history of the belt in the preceding Rodinia supercontinent cycle and its role in the transition to West Gondwana assembly is poorly known. New geochemical and geochronological data of mafic magmatism from Meso- to early Neoproterozoic rock units (1500–900 Ma) integrated with published data provides new constraints to unravel the tectonic setting of the Ribeira Belt. Geochemical and isotopic data from the Votuverava Group and Embu Complex from the southern Ribeira Belt record three distinct periods of juvenile magmatism. Mafic rocks from the Votuverava Group define the oldest magmatism as Calymmian (1490–1475 Ma) and Ectasian (1300–1260 Ma). Both age groups display Th/Yb ratios greater than N-MORB, low Ta/Yb and Nb/La ratios, low Nb and Zr concentrations, negative Nb anomaly, whole-rock ɛNd(T) from +2.25 to +4.01, an Nd two-stage TDM model age spanning 1629–1471 Ma, and zircon εHfT mainly between +5 and +10, characteristics of juvenile mantle source arc-related magmatism. The third event occurred during early Tonian (900 Ma) and is recorded in the mafic rocks from the Embu Complex. Geochemical data indicate that these mafic rocks belong to the tholeiitic series with low LREE enrichment, negative Nb and Ta anomalies, low Nb and Zr contents and low Nb/La and Zr/Y ratios, consistent with arc-related basalts. Whole-rock Sm–Nd isotopic data yielded ɛNd(T) of +0.72 and Nd two-stage TDM model age of 1921 Ma, with zircon Hf isotopes indicating juvenile mantle sources to the mafic magmatism with ɛHfT between +4.5 and +17.2. Thus, mafic rocks from the southern Ribeira Belt record multiple periods of accretion and convergence settings associated with assembly of Rodinia and Gondwana.

Post-collisional magmatism associated with the final closure of the Rushan-Pshart Meso-Tethys Ocean in Pamir, Tajikistan: Inference from Cretaceous igneous rocks of the Pshart accretionary complex

The Pamir orogen was formed by the subducted accretion and amalgamation of Cimmerian terranes from the northern margin of Gondwana with the southern margin of Eurasia. The Mesozoic magmatic rocks are widespread in Pamir and record the tectonic evolution in different stages. The Rushan–Pshart suture zone represents an ancient ocean between Central and Southern Pamir. This paper reports the petrography, geochronology, and geochemistry of Cretaceous granites and diabase dikes that intrude into the Pshart complex. The granites were emplaced between 124 and 118 Ma, based on their zircon U-Pb ages. These granites are characterized by high-K calc-alkaline, low magnesian, and high SiO2, A/CNK, and K2O+Na2O values. They also display strong depletion of Ba, Sr, Eu, and Ti and comparatively weak negative Nb anomalies in spidergrams. Thus, we proposed in this study that these are highly fractionated, strongly peraluminous S-type granites. They were generated by the partial melting of the metasedimentary rocks in the plagioclase stability field and underwent subsequent fractional crystallization during their ascent. The diabase dikes contain low SiO2, and high MgO levels and negative Nb and Ta anomalies, which were interpreted to form in an extensional environment. Late Jurassic–Early Cretaceous closure of the Rushan–Pshart Ocean and subsequent foundering of its oceanic lithosphere caused local extension and upwelling of the asthenospheric mantle. The underplating of mafic magma provided a heat source to melt the metasedimentary-derived granitic that formed in the initial post-collisional environment. The subsequent local extension caused the emplacement of diabase dikes. Based on our new data and combined with data from previous studies, we concluded that the Rushan–Pshart suture zone is the remnant of the Meso-Tethys Ocean and may represent the western continuation of the Bangong–Nujiang suture of the Tibetan Plateau.

Origin of the southern Ryukyu Forearc Terrace: A seaward-protrusion of arc crust elevated by subducted asperities?

• The southern Ryukyu Forearc Terrace exhibits an extremely high free-air gravity anomaly and a thick upper crust. • The rest of the southern Ryukyu forearc region is characterized by prominent forearc basins and a large-volume accretionary prism. • The forearc terrace represents a remnant seaward-protruded arc crust. • The bulged and uplifted basement of the forearc terrace is most probably caused by the subduction of asperities. A forearc terrace with distinct topography and gravity anomalies lies within the Ryukyu forearc between 125°E and 127°E. The origin and tectonic affinity of this terrace remain uncertain. This study attempts to depict the structural characteristics of the forearc terrace and to trace its origin and formation mechanisms through a comprehensive examination of newly acquired geophysical data combined with the results of previous surveys. The forearc terrace exhibits shallow bathymetry, an extremely high free-air gravity anomaly, a shallow buried acoustic basement, a thin sedimentary cover, and a thick upper crust. In contrast, the rest of the southern Ryukyu forearc region is characterized by a steep upper forearc slope, prominent forearc basins, a deeply buried acoustic basement, and a large-volume accretionary prism. The forearc terrace represents a remnant seaward-protrusion of arc crust created after the frontal portion of the arc crust had been removed by subducted ridges or other types of linear bathymetric highs. This is in contrast to other formation mechanisms such as a block sliding seaward along strike-slip faults or an accretion of exotic terrane from the outer margin of the original arc crust after being scraped off from the subducting oceanic crust. The bulged and uplifted basement of the forearc terrace is most likely associated with subducted seamounts. Further studies, especially deep-sea drilling, are necessary to verify the nature and origin of the forearc terrace.

A first attempt at a provenance study in the Jadar block (Serbia) by means of U-Pb zircon geochronology

U-Pb geochronology of zircon grains retrieved from magmatic rocks intruding the Jadar block terrane in the central Balkans is used here to add new constraints on the terrane accretion processes and the provenance of crustal sources of this potentially exotic crustal block. Using an unorthodox approach, we analyzed zircons extracted from the products of Cenozoic (Cer and Boranja granitoid massifs) and Triassic magmatism (Bobija andesitic tuff - Pietra Verde). In fourteen samples of granites and epiclastites, we analyzed about 600 grains, and of these, about 30-40% were derived from the basement and were used further for the geological interpretation. Most samples show a similar Precambrian and Paleozoic age spectrum, including ubiquitous Neoproterozoic and well-defined Silurian-Ordovician populations. Only a few older zircons are present, composing minor populations at c. 1.2 Ga and 3.2 Ga. The younger zircons represent a ubiquitous Triassic population that is the strongest in all samples. This age population is most likely associated with local Permo-Triassic magmatism generated due to the opening of the Neotethys. In contrast to the magmatic rocks of Boranja and Bobija, the zircon age spectrum of the Cer polyphase pluton shows a strong Carboniferous peak, indicating a potentially important link to the Variscan margin of Eurasia. This supports opposing interpretations that either this part of the Jadar block terrane represents a southern continuation of the ?Bukkium? and Sana-Una terranes comprising displaced fragments of the southern European Variscan foreland, or, more likely, that it has an Adria affinity and that these zircons are derived from Cretaceous sediments of the Sava Zone, i.e., the suture that separates European and Adriatic domains, which were assimilated during the intrusion of the Cer granitic magmas.

Strain Partitioning within the Caribbean–North America Transform Plate Boundary in Southern Haiti, Tectonic and Hazard Implications

ABSTRACT Global Positioning System measurements and high-resolution offshore seismic data within the transform Caribbean–North American plate boundary in southern Haiti, Greater Antilles, show 6–7 mm/yr of plate boundary-normal shortening within a crustal sliver bounded to the south by the Enriquillo left-lateral strike-slip fault and to the north by a south-dipping reverse fault system offshore the northern coast of the Southern Peninsula of Haiti. This overlooked fault system, which we name “Jérémie–Malpasse” (JM), marks the overthrusting of the Cretaceous oceanic crust of the Caribbean basin of the Southern Peninsula over accreted terranes of island arc crust to the north. Geological and geodetic data are consistent with a tectonic model that reconciles recent observations of plate boundary-perpendicular shortening with ample geological evidence for purely strike-slip motion on the Enriquillo fault. This model also provides a framework to interpret the 2010 Mw 7.0 and 2021 Mw 7.2 earthquakes in southern Haiti, whose oblique slip mechanisms are indicative of a localized transpressional strain regime. These findings imply that regional seismic hazard maps must be updated to account for both the existence of the ∼350 km long JM fault system and for the possibility that future earthquakes in southern Haiti may occur on nonvertical faults with a significant component of reverse slip.

The Lower Cretaceous sequence of western Alaska—demise of the Koyukuk terrane?

Lower Cretaceous marine sedimentary rocks, deposited in shallow shelf and basin settings and unconformity-bound, are well exposed in southwest Alaska. Collections of Early Cretaceous fossils from across western Alaska show that similar and coeval Lower Cretaceous clastic rocks are widely distributed though only locally exposed. Volcanic rocks become an important part of the Lower Cretaceous sequence in the Yukon-Koyukuk basin, where they have been interpreted to represent a mobile intra-oceanic island arc, the Koyukuk terrane, that collided with Arctic Alaska to form the Brooks Range orogen. The volcanic rocks are chemically unlike Aleutian arc rocks but share compositional characteristics with spatially related, mid-Cretaceous alkaline intrusive rocks. The volcanic-bearing sequence was also deposited on an angular unconformity, includes both shallow shelf and basin depositional settings, and is unconformably overlain by mid-Cretaceous clastic rocks. The volcanic rocks are therefore considered part of the Lower Cretaceous sequence now identified across western Alaska. In this interpretation, the Lower Cretaceous volcanic rocks are an initial expression of the mid-Cretaceous tectonic regime that included extensional exhumation and subsidence, crustal and upper mantle melting, and high-temperature metamorphism in the hinterland of the Brooks Range orogen. The Cretaceous heating that led to hinterland crust and upper mantle change may have been caused by deep mantle disturbances in a postsubduction setting. This interpretation has implications for the timing of contractional orogenesis, the location and nature of the related continental borderland, and the tectonic setting for development of the Anguyucham and related oceanic terranes.

Complex Late Triassic-Middle Jurassic Subduction-Related Magmatic History from Detritus of Nominal Middle Jurassic Brooks Range Ophiolite, Northern Alaska

Abstract In northern Alaska, the Early Cretaceous sedimentary Yukon-Koyukuk basin documents the progressive unroofing of the adjacent Brooks Range orogen. Igneous clasts in the lower conglomerate are believed to originate from ophiolitic rocks of the two uppermost allochthons in the Brooks Range, the Brooks Range ophiolite and the Angayucham terrane. The emplacement of these oceanic terranes onto the continental margin of the Arctic Alaska terrane documents the initiation of Brookian orogenesis. While most agree that the Angayucham terrane represents a widespread distribution of Late Devonian oceanic crust and Triassic-Early Jurassic oceanic plateau(s)/island(s), the age and origin of the Brooks Range ophiolite remains controversial. We present new age, whole-rock chemistry, and isotopic data from igneous clasts as well as a few Angayucham terrane outcrop samples from the NE Yukon-Koyukuk basin. Our results show that the igneous clasts are mostly subduction-related and more likely to represent eroded material from the Brooks Range ophiolite rather than the Angayucham terrane. Our Late Triassic, and Early and Middle Jurassic zircon crystallization ages for the igneous clasts, combined with their immobile trace element compositions documenting various stages of oceanic subduction (mature arc and later rifting), suggest a long-lived subduction system that was active in the Late Triassic and throughout the Middle Jurassic. Radiogenic lead and neodymium isotopic results yield juvenile signatures for both the igneous clasts and the Angayucham terrane, pointing to their formation in an intraoceanic setting distal from the continental rocks and sediments of the Arctic Alaska terrane. These new data, combined with the published data of others, allow us to propose a revised tectonic model that integrates Late Triassic island arc formation with the tectonic development and emplacement of the Brooks Range ophiolite.

Crustal Seismic Anisotropy Along the Continental Margin in Western Canada From Receiver Function Analysis

AbstractCoastal British Columbia (BC), Canada, has the highest seismic hazard in the country due to convergent and transpressive deformation at offshore plate boundaries between the Pacific, Juan de Fuca (JdF) and North American (NA) plates. Further landward, the crust of the NA plate is made up of several geologically unique exotic terranes and is unusually thin. Investigating the geophysical features in this area can help us better constrain its tectonic history and the geophysical processes that are currently underway. Here, we conduct an analysis of teleseismic body‐wave scattering data (i.e., receiver functions [RFs]) recorded at stations across western coastal BC including northern Vancouver Island and southeastern Alaska. Using these RFs, we perform a harmonic decomposition with respect to earthquake back‐azimuths to determine the orientation of seismic anisotropy over a series of depth ranges, attributable to either mineral alignment or dipping structures. We find a coherent pattern of margin‐parallel orientations at upper crustal depths that persist onto the mainland at distances 420 km from the margin. Furthermore, dominant receiver function orientations at depth are attributed to dipping faults and interfaces, and fabrics due to lower crustal shearing or inherited from tectonic assembly along the margin. This work supports models for the tectonic assembly of this region that involve a combination of plate subduction and transpressive motion along crustal scale faults that pervade a wide portion of the margin. This work also helps to constrain the current geometries of the subducting Pacific and JdF slabs.

Metallogeny of the state of Oaxaca, Mexico

ABSTRACT The state of Oaxaca in southern Mexico includes copious metallic deposits of distinct types that were formed in several tectonothermal events since the Proterozoic. This work integrates a large database of mineral occurrences and deposits in Oaxaca. The 1:850,000 scale metallogenetic map depicts the location of deposits, mining regions, and mining districts on a lithological base, along with a summarized metallogenic history of the state and the distribution of the tectonostratigraphic terranes. We briefly describe the most relevant features of each mineralized area and the characteristics of the metallogenic epochs of Oaxaca. All the geolocation information was organized in GIS format.

Relics of ophiolite-bearing accretionary wedges in NE Brazil and NW Africa: Connecting threads of western Gondwana´s ocean during Neoproterozoic times

Neoproterozoic breakup of Rodinia resulted in the formation of several oceanic realms between dispersing cratons, which were later consumed during the assembly of Gondwana. In its western portion, the interior orogenic belts of Gondwana formed during the Brasiliano-Pan African Orogeny in the late Neoproterozoic-early Cambrian. Available geophysical, structural and petrological data suggest that the complex network of shear zones that once connected the Borborema province (NE Brazil), Tuareg shield (Hoggar) and Central African domain (NW Africa) likely represent ancient sutures that mark collisional episodes between Archean-Paleoproterozoic paleocontinents such as Amazonian-West African and São Francisco-Congo. Mafic, ultramafic and sedimentary sequences associated with this set of structures respresent dismembered ophiolite slices interpreted as oceanic remnants (sensu lato) that were emplaced during the late stages of the Gondwana assembly. For instance, the composite Transbrasiliano-Khandi-In-Tedeini-Silet shear system crosscuts rock assemblages preserving a complex history of oceanic-crust-transition development (Novo Oriente complex) in association with primitive to evolved magmatic arcs and UHP rocks both in the Borborema province and NW Africa. In the central Borborema province, preserved ophiolitic slices are strongly overprinted by ductile and brittle deformation events, but partially preserved MORB-like amphibolites are akin to subduction-related-types that crystallized in early- and late Neoproterozoic times docked via terrane accretion and dispersed by strike-slip shear zones. In the southern Borborema province, an example of a Neoproterozoic ophiolitic assemblage is the Monte Orebe complex, that encompasses T-MORB mafic rocks, ultramafic lenses, and exhalative sedimentary rocks akin to early to late stages of oceanic basin spreading, emplaced during convergent plate motions between the Pernambuco-Alagoas superterrane and the São Francisco craton. Correlative units are found in Cameroon, including the strongly hydrotermalized ultramafic rocks of the Lomié and Boumnyebel complexes, that are structurally controlled by top-to-the-south verging nappes found in the N-NW margin of the Congo craton. In all scenarios, the ophiolitic complexes are related to intra-oceanic and continental magmatic arcs as well as to geophysical signatures comparable to Phanerozoic suture zones. Although strongly dismembered, scrapped off Neoproterozoic oceanic crust partially preserved within the major belts of western Gondwana demonstrate the role of accretion-collisional orogenesis during its assembly.

Polybaric magmatic evolution of the eocene penticton group alkaline rocks, south-central British Columbia

Volcanic rocks of the Challis-Kamloops belt erupted during the Eocene (55–45 Ma) throughout south-central British Columbia, Idaho, and Wyoming of the North American Cordillera. The Eocene volcanic rocks of southern British Columbia can be subdivided into three distinct groups (Kamloops, Princeton, Penticton) based on geography and composition. The southernmost Penticton Group is unique as it is composed of volcanic and intrusive alkaline rocks (e.g., Marron Formation, Terrace Mountain outlier, Allendale Lake pluton) as opposed to calc-alkaline rocks that are typical of the Kamloops and Princeton groups to the north. The rocks of this study are from the Penticton Group, are compositionally variably (SiO 2 = 51.2–64.7 wt%), and have highly fractionated chondrite-normalized REE patterns (La N /Yb N = 71–104) without noticeable negative Eu-anomalies (Eu/Eu* = 0.81–1.00). The rocks have similar Sr-Nd isotopic ( 87 Sr/ 86 Sr i = 0.706422–0.706563; ε Nd ( t ) = −4.3–6.1) ratios and Neoproterozoic depleted mantle model ages (T DM = 794–873 Ma). Petrological modeling demonstrates that the primary process of chemical differentiation within the Penticton Group is fractional crystallization. However, it appears that the Marron, Allendale Lake, and Terrace rocks underwent fractional crystallization at different pressures (i.e., 4 GPa, 1.5 GPa, and 1 GPa). The polybaric differentiation of the Penticton Group indicates that the alkaline system evolved at different crustal depths in a similar manner as magmas related to the Yellowstone hotspot and markedly different from the Kamloops and Princeton groups. We suggest that the Penticton Group is underlain by ancient lithospheric mantle that was modified by Precambrian subduction processes and was isolated from the convecting mantle whereas the northern part of the Eocene volcanic belt is underlain by exotic accreted terranes (Intermontane Superterrane) and was derived from a different, younger lithospheric mantle. • The Penticton Group alkaline rocks were emplaced during the Eocene (∼50 Ma). • The primary process of chemical differentiation is fractional crystallization. • Differentiation occurred under variable pressure at a post-collisional setting. • The magmas were derived from the ancient lithospheric mantle of North America.

Paleoenvironmental conditions over the Caribbean Large Igneous Province during the Late Cretaceous in NW of South American Margin: A sedimentological and ichnological approach

The Late Cretaceous in northwest South America was a period marked by synchronous regional geological processes (e.g., accretion of oceanic terranes, arc magmatism, sea-level fluctuations, oceanographic dynamics), related to the interaction of the Caribbean Large Igneous Province (CLIP) with the NW margin of South America. This synchroneity makes it difficult to reconstruct the complex geological evolution of this region. In Colombia, Upper Cretaceous marine sedimentary and low-grade metamorphic rocks are commonly exposed in both the Western Cordillera and the Sinú-San Jacinto Basin in the Pacific and Caribbean regions respectively. Significant deformation and structural complexity are the main limiting factors for sedimentological and paleontological studies of these deposits.Detailed sedimentological and ichnological analyses were carried out in eight sections, two in the Caribbean region and six in the Pacific region. The Caribbean sections are composed of limestones, mudrocks, siliceous and calcareous mudrocks, and cherts, associated with pelagic and hemipelagic deposits (E2.2 and G2.1 facies). The Pacific sections are characterized by two facies associations, one composed of mudrocks, limestones, as well as chert associated with pelagic and hemipelagic deposits (E2.2 and G2.1 facies) and the other of interlayered siliceous mudrocks and sandstones associated with turbiditic systems (C2 facies). Upper Cretaceous deposits in both regions are characterized by relatively low to moderate abundant and diverse trace fossils assemblages, consisting of Chondrites, Planolites, and Zoophycos in the Caribbean region and Chondrites, ?Nereites, Phycodes, Phycosiphon, Planolites, Thalassinoides, and Zoophycos in the Pacific region, assigned to the Zoophycos ichnofacies. Sedimentological and ichnological features suggest deposition in deep-marine environments characterized by pelagic, hemipelagic, and turbiditic sedimentation. Integrated with the biostratigraphical framework, this allows us to establish a regional correlation between both regions. High nutrient supply, poor oxygenation, and low hydrodynamic energy and sedimentation rates interrupted by episodic increases associated with turbiditic deposits are main paleoenvironmental factors controlling the sedimentation over the CLIP during Late Cretaceous.

Mesozoic–Cenozoic multistage tectonic evolution of the Pamir: Detrital fission‐track constraints from the Tajik Basin

AbstractKnowledge of the tectonic history of the Pamir contributes to our understanding of both the evolution of collisional orogenic belts as well as factors controlling Central Asian aridification. It is, however, not easy to decipher the Mesozoic–Cenozoic tectonics of the Pamir due to extensive Neogene deformation in an orogen that remains largely understudied. This study reports detrital apatite and zircon fission‐track (FT) ages from both the eastern Tajik Basin sedimentary rocks and Pamir modern river sands. These FT data, supported by vitrinite reflectance and zircon and apatite U–Pb double dating, suggest that the majority of the FT ages are unreset and record exhumation stages of the Pamir, which has served as the source terrane of the Tajik Basin since the Cretaceous. Furthermore, we combine the new data with a compilation of published detrital apatite and zircon FT data from both the Tajik Basin sedimentary rocks and Pamir modern river sands, to explore the Mesozoic–Cenozoic tectonic history of Pamir. Deconvolved FT Peak Ages document two major Mesozoic exhumation events associated with the Late Triassic–Early Jurassic Cimmerian orogeny that reflects accretion of the Pamir terranes, as well as the Early–early Late Cretaceous deformation associated with the northward subduction of the Neo‐Tethys Ocean beneath Pamir. The compiled data also show significant Late Eocene–Neogene exhumation associated with the ongoing formation of the Pamir, which peaks at ca. 36, 25, 14 and 7 Ma.

Eocene Terrane Accretion in Northern Cascadia Recorded by Brittle Left‐Lateral Slip on the San Juan Fault

AbstractThe San Juan fault (SJF), on southern Vancouver Island, Canada, juxtaposes the oceanic Wrangellia and Pacific Rim terranes in the northern Cascadia forearc, and has been suggested to play a role in multiple Mesozoic‐Cenozoic terrane accretion events. However, direct observations of the SJF's kinematics have not been documented and its exact role in accommodating strain arising from terrane accretion is unknown. To test if, how, and when the SJF accommodated accretion‐related strain, we use geologic mapping, kinematic inversion of fault‐plane slickenlines, and dating of marine sediments to constrain the timing and direction of brittle slip of the SJF. P‐ and T‐axes from kinematic inversions indicate predominantly left‐lateral slip. Left‐lateral brittle faulting cross‐cuts ∼51 Ma magmatic intrusions and foliation, providing a maximum age of brittle deformation. The fault zone is non‐conformably overlain by a >300 m‐thick sequence of clastic marine shelf and slope sediments that are not left‐laterally offset. A strontium isotope age of foraminifers helps constrain the depositional age of the sediments to late Eocene–early Oligocene, bracketing left‐lateral slip to the Eocene. Eocene left‐lateral slip is temporally and kinematically consistent with regional southwest‐northeast compression during accretion of the Siletzia ocean island plateau, suggesting brittle slip on the SJF accommodated strain resulting from the accretion of this terrane. This result does not support hypotheses that brittle slip along the SJF directly accommodated earlier accretion of the Pacific Rim terrane to Wrangellia, instead it offsets the older accretionary boundary between these two terranes.

Continental growth during migrating arc magmatism and terrane accretion at Sikhote-Alin (Russian Far East) and adjacent northeast Asia

Net additions to the continental landmasses, or continental growth, is a process that has irrevocably shaped our environment throughout the Earth history. Studies of ancient orogens suggest that continental growth can be facilitated by plate tectonics-driven arc magmatism and material accretion along convergent margins. However, such details are not well-understood due to the geological complexity of ancient orogens. Specifically, the role of the magmatic arc and its contributions to growing an accretionary margin remains unclear. This study explores an under-studied, long-lived magmatic arc along a 1500 km-wide swath of the NE Asian continental margin between 42° to 52°N (i.e., Sikhote-Alin and adjacent NE China, Sakhalin, and Hokkaido). We present new igneous rock zircon U-Pb ages (n = 93) and geochemistry (n = 61) from Sikhote-Alin that complement limited published data (n > 60). We synthesize these data to reveal four episodes of arc magmatism at Sikhote-Alin, including (1) 134 to 120 Ma S-type, (2) 110 to 100 Ma I-type, (3) 95 to 52 Ma highly-evolved I-type, and (4) <46 Ma I-type. We then combine our new results with published data (n > 600) and geology from adjacent NE Asia to analyze overall magmatic trends during a ∼ 1500 km trenchward migration of arc magmatism driven by Jurassic to Cenozoic juvenile terrane accretion. The young, growing continental margin was modified by arc magmatism with chemical compositions determined by: (1) generation from a mantle wedge mixed with 1.3– 1.4% sediment component from the downgoing slab; and, (2) silica-enrichment from crustal magmatic differentiation. Here, convergent margin continental growth that involved crustal accretion and trenchward migrating arc magmatism show magma compositions that evolve from initial mantle source mixing followed by crustal differentiation.

Metallogeny of the state of Oaxaca, southern Mexico: A review

The state of Oaxaca is one of the most geologically complex and diverse in Mexico. It comprises five tectonostratigraphic terranes of contrasting nature that form a sort of tectonic collage where several magmatic pulses were developed. The different lithological assemblages of Oaxaca favored the generation of a variety of ore deposit types, some of which have been a prime source of several commodities for centuries. This paper addresses the time-space relationship between ore deposits and tectono-magmatic events, based upon the available descriptions and radiometric ages. Most Meso-to Neoproterozoic ore deposits of Mexico occur in this region, and were formed during the regional granulite-facies metamorphic stage(s) recorded in the Oaxacan Complex. Such deposits are rare element-bearing pegmatites, nelsonites (ilmenite-apatite rich rocks), Ti-rich anorthosites, and pegmatites. The Oaxacan Complex also hosts a number of orogenic gold lodes that were likely formed in more than a single metamorphic episode and still require additional research to clarify their age and geological affiliation.The onset of arc-related magmatism in the Late Carboniferous-Permian is recorded by the intrusion of the calc-alkaline Zaniza, Cuanana and Honduras batholiths, along with several minor deformed and undeformed bodies enclosed in the Oaxacan Complex. Little is known about the metallogeny associated with such magmatism. Although supra-subduction magmatism re-initiated in the Jurassic and is probably responsible for the San Juan Mazatlán Cu porphyry, most of the metallogeny of Oaxaca is tightly linked to the Sierra Madre del Sur (SMS) igneous province, which began in Jalisco by the Late Cretaceous and migrated to the SE until it reached the Isthmus of Tehuantepec by the Middle Miocene. In Oaxaca, the Oligocene-Miocene coastal belt of granitic-granodioritic intrusions accounts for some IOCG deposits, especially in its easternmost portion, which then continues to the Coastal Chiapanecan Neogene batholithic belt.In W-NW Oaxaca, the intracontinental igneous province of the SMS marked the beginning of Cenozoic magmatism and is represented by thick volcanic-hypabyssal successions along with minor intrusions, with which copious magmatic-hydrothermal deposits are associated. Early Eocene and mid-Oligocene volcanism is restricted to the western half of the state, where basic-to intermediate-composition effusive products occur in the Mixteca and western Oaxaca tectonostratigraphic terranes. During this time span, many low-to intermediate-sulfidation epithermal, Fe-rich (IOCG) and polymetallic skarn deposits were produced. However, the “boom” in magmatic-hydrothermal mineralizations occurred during the terminal pulses of the SMS volcanism (Miocene), which comprise profuse intermediate volcanic rocks capped by the outburst of silicic magmatism. The early-to mid-Miocene event constitutes the most prominent in terms of metal endowment and diversity of ore deposit types, accounting for about 250 deposits and occurrences in the central and near-eastern portions of the state, which are ascribed to low-to intermediate-sulfidation epithermal, sulfide skarns, IOCG “clan” and Cu(–Mo) porphyry types. Among these, epithermal deposits are, by far, the most abundant type.Recent Ti-rich beach deposits and Au alluvial placers are present in southern and easternmost Oaxaca, respectively. In addition, supergene enrichment zones and uneconomic laterites occurrences are found in some localities.Oaxacan ore deposits can be grouped into five major metallogenic epochs: 1) Meso-Neoproterozoic REE-Th-U-Ti deposits and, possibly, orogenic gold lodes during the Olmecan-Zapotecan events (assembly of Rodinia); 2) feasible late Carboniferous-Permian (?) orogenic gold deposits; 3) early Eocene Ag–Au–Pb–Zn–Fe deposits during the onset of the SMS volcanic activity; 4) mid-Oligocene Ag–Pb–Zn–Sb–Hg–Fe(-Au) deposits during effusive-dominated pulses of the SMS volcanism; and 5) early-to mid-Miocene Au–Ag–Pb–Zn–Cu–Fe(-Mo-Sb) deposits during the termination of the SMS magmatism. The existence of a wealth of ore deposits with varying features underlines the economic potential of metallogenetic provinces and epochs in Oaxaca and exalts the necessity to expand the knowledge regarding the tectonic-magmatic-metallogenic evolution of the state.

Warm-waterTcherskidiumfauna (Brachiopoda) in the Late Ordovician Northern Hemisphere of Laurentia and peri-Laurentia

AbstractThe Late Ordovician (late Katian)Tcherskidiumfauna consisted of large- and thick-shelled virgianid pentamerid brachiopods characterized by large and ribbed shells ofTcherskidiumandProconchidiumand usually associated withHolorhynchus,Deloprosopus, andEoconchidium. This unique fauna was widely distributed across several tectonic plates, largely confined to the paleoequatorial and especially the northern paleotropical zones, such as northern Laurentia, accretionary terranes of Alaska, Kolyma, Baltica, Siberia, Kazakh and adjacent terranes, and South China. In Laurentia, the eponymous genusTcherskidiumwas predominant in regions north of the paleoequator and, in sharp contrast, was absent south of the paleoequator. In this study,Tcherskidium lonein. sp. andProconchidium schleyin. sp. are described from Alaska and North Greenland, respectively, adding new data on theTcherskidiumfauna of the Late Ordovician Northern Hemisphere. Shell gigantism, together with the sharp paleobiogeographic division, suggests that the Late Ordovician (late Katian) Northern Hemisphere had a prevailing warm-water mass, probably due to the lack of large landmass beyond the northern tropics. This was in sharp contrast to the Southern Hemisphere, which was frequently influenced by cold-water invasions from the ice-bearing Gondwana supercontinent centered on the South Pole.UUID:http://www.zoobank.org/25d9b772-bd7d-4ad6-bfc6-ba02b1567cf3

Geochemical and detrital zircon age constraints on Meso- to Neoproterozoic sedimentary basins in the southern Yangtze Block: Implications on Proterozoic geodynamics of South China and Rodinia configuration

The late Mesoproterozoic to early Neoproterozoic sedimentary sequences in the southwestern Yangtze Block, represented by the Julin, Huili, and Kunyang groups, provide vital evidence for deciphering the tectonic evolution of the Yangtze Block that have implications on the reconstruction of the Rodinia supercontinent. Zircon geochronological data reveal that the Julin and Huili groups were deposited at 1167–982 Ma and 1176–958 Ma, respectively. The Haizishao Formation, the conventionally thought the upper most of the Julin Group, was deposited after 982 Ma. The stratigraphicphic position of the Haizishao Formation basal conglomerate and diversity in its detritus source, and tectonic setting, suggest that the Haizishao Formation may not be a part of the Julin Group but represents an independent sedimentary event. Geochemical tectonic discrimination characteristics based on major elements, Th-Sc-Zr/10, Ti/Zr-La/Sc, La/Th-Hf and Sc/Th-La/Sc plots, in combination with detrital zircon age distribution pattern and bimodal magmatism, point toward a transition from continental rift to passive continental margin setting followed by an active continental margin setting for Julin and Huili groups. However, the Huili Group is significantly different from both the Julin and Kunyang groups in terms of lithological composition, depositional environment, detrital zircon age distribution, and tectonic setting, implying that the Julin and Huili basins may have been located in different microcontinents during the late Mesoproterozoic - early Neoproterozoic times.Detrital zircon U-Pb ages and Lu-Hf isotopic characteristics of Julin and Huili groups indicate detritus sourced from the continental interior and exotic terranes, such as the Yudongzi, Douling, and Kongling complexes and magmatic rocks at the southwestern margin of Yangtze Block, the Gawler Craton in Australia, the Transantarctic Mountains in East Antarctica, and the Ongole domain in the Eastern Dharwar Craton of India. Combining provenance analyses and comparison of detrital zircon data, the Yangtze Block was paleogeographically placed in the external position to the Rodinia supercontinent, close to north-central India, Australia, and East Antarctica during the late Mesoproterozoic to early Neoproterozoic.

The origin of Patagonia: insights from Permian to Middle Triassic magmatism of the North Patagonian Massif

AbstractWe conducted a U–Pb–Hf isotope study on zircon crystals from ignimbrites of the Changhsingian to Olenekian (253–248 Ma) Los Menucos Basin in the North Patagonian Massif (NPM), Argentina, in order to evaluate the age and petrogenesis of the magmas. Additionally, a compilation of whole-rock geochemistry and U–Pb–Hf in zircon isotope data for the Permian to Middle Triassic rocks of the NPM, for comparison with our data, was made to assess whether Patagonia would have been an exotic terrane accreted to SW Gondwana during the late Palaeozoic. We interpret the available U–Pb–Hf data to suggest that northern Patagonia experienced eastward arc expansion from the early Permian, about 273 Ma ago. This ∼820 km arc expansion event involved crustal shortening and magmatism with high-silica adakitic affinity, resulting in Hf-isotopic pull-down. At 253 Ma, slab steepening became associated with the coeval emplacement of ignimbrites of the Los Menucos Basin, which involved post-orogenic to intraplate magmatism. During the Middle Triassic, a slab break-off triggered uplift and basaltic underplating, promoting the emplacement of dike swarms with C-type adakitic signature at 246–244 Ma. The Hf isotope data for SW Gondwana for the same period indicate distinct trends that are explained here by differential slab roll-back since the Guadalupian, in a slab-tearing setting. Therefore, Permian to Middle Triassic magmatism is interpreted as having been associated with an eastward-directed proto-Pacific subduction system, which ultimately supports an autochthonous origin for Patagonia.

Tectonometamorphic evolution of the Trivandrum and Southern Madurai blocks in the Southern Granulite Terrane, south India: correlation with south-central Madagascar

AbstractDetailed structural investigations across the Achankovil terrane boundary shear zone (AKSZ) system show distinct differences in the geometry of superposed fold structures between the Trivandrum (TB) and Southern Madurai (SMB) blocks separated by the AKSZ. The metamorphic temperatures and pressures estimated from the SMB and TB have a similar range, that is, 600–880°C and 5–8 kbar. The similar clockwiseP-Tpaths retrieved by phase equilibrium modelling from both the blocks represent the last deformation and metamorphism shared between them during their accretion along the AKSZ. The distinct evolutionary history of the SMB and TB prior to their amalgamation is supported by the contrasting structural, metamorphic and chronological patterns, particularly the lack of prominent middle Neoproterozoic ages and the presence of Palaeoproterozoic ages in the TB, and vice versa in the SMB. The prominent 600–500 Ma monazite ages in the TB, SMB and AKSZ attest to their timing of accretion along the AKSZ. The study corroborates the S-directed subduction model proposed for terrane accretion along the AKSZ, and provides further insight into the subduction–accretion–collision tectonics associated with the late Neoproterozoic – Cambrian evolutionary history of this region.The tectonothermal histories and geochronology of the SMB and TB are compared to the once adjoined crustal domains of Madagascar within Gondwanaland. It is suggested that the TB is equivalent to the Androyan and Anosyan domains of southern Madagascar, the SMB is equivalent to the Antananarivo and Itremo–Ikalamavony blocks of the central Madagascar, and the sinistral AKSZ is contiguous with the Ranotsara Shear Zone in southern Madagascar.