Volcano-sedimentary Succession Research Articles

Sedimentary provenance and role of tectonic inheritance on the control of the Macaúbas group, eastern margin of São Francisco Craton (SE Brazil)

The Neoproterozoic Macaúbas Group represents the precursor basin system of the Araçuaí–West Congo orogen, located in the southern margin of São Francisco Craton, and records a Tonian volcano-sedimentary succession, at the base, overlain by a Cryogenian sequence related to the Neoproterozoic global glacial event. Detailed stratigraphic surveys along the Tonian units allowed the identification of two lithofacies associations related to alluvial fan and braided river environments, while the three Cryogenian lithofacies associations indicate a glaciomarine environment associated with extensive tectonics that has evolved into a marine environment. The results show that the age spectra of the Tonian and Cryogenian units are similar, differing mainly for an increased peak in the Cryogenian sample in ca. 1.0 Ga, where the primary source is probably related to those magmatic rocks associated with the Stenian-Tonian extensional breakup of the São Francisco-Congo paleocontinent. The KS-Test shows that not all the Tonian samples are similar to each other, as in all the Cryogenian samples. The detailed stratigraphic and structural surveys associated with the geochronological U–Pb data allow us to interpret that the Tonian and the basal Cryogenian sequences were deposited in two episodes of an active intracontinental tectonic context related to fault reactivations along the WNW-ESE Pirapora aulacogen structures in superimposed basin-cycles, the middle-to late-Cryogenian sequences extrapolated the WNW-ESE boundaries while the basin local depocenter and its surroundings expanded to a large depocenter during the basin evolution.

Post-collisional volcanism with adakitic signatures in the Arabian-Nubian Shield: A case study of calc-alkaline Dokhan volcanics in the Eastern Desert of Egypt

A volcano-sedimentary succession, including the Guruf volcanic series, is exposed at Gabal Um Guruf in the north Eastern Desert of Egypt. A series of intermediate to silicic volcanic flows and associated pyroclastic deposits unconformably overlie quartz-diorite and granodiorite plutons and are intruded by high-level alkali feldspar granites. The metavolcanic xenoliths in the granodiorite are shown to be unrelated to the Guruf volcanics. Two volcanic cycles can be recognized. The lower unit is a high-K calc-alkaline series with minor basaltic andesite and mostly andesite through dacite. The upper unit consists of rhyolite and rhyodacite that is transitional towards alkaline, A-type magma. Both units are assigned to the Dokhan volcanics and interpreted as post-collisional. REE patterns are characterized by slow progressive enrichment throughout the suite, with heavy REE apparently acting more incompatibly than light REE; concave-upwards REE patterns in basalt andesite suggest residual amphibole in the source. Only the upper rhyolite displays prominent negative Eu anomalies (Eu/Eu* = 0.35–0.73). Apparent crystallization temperatures of primary pyroxene are 800–1000 °C and of primary amphiboles 781–944 °C. The major and trace element chemistry of the suite can be modeled as a single liquid line of descent, with pressure decreasing as the magma cools and evolves, and a prominent role for apatite in sequestering CaO, P2O5, and light REE. Although upper crustal contamination and magma mixing cannot be ruled out, the data do not require them. The suite includes pyrophanite, an unusual Mn-rich ilmenite-group solid solution, not previously recorded in volcanic rocks in the Arabian-Nubian Shield (ANS). The Guruf volcanic series is undeformed and unmetamorphosed, unlike the typical subduction-related volcanic units of the ANS. The geochemical characteristics are consistent with those of many post-collisional ANS rocks. The mostly calc-alkaline character and other traits previously interpreted to indicate an active arc setting instead more likely reflect remelting of earlier arc-related material from the pre-collisional stage (850–740 Ma). The lava flows of the lower succession have adakitic characteristics, including high Sr, low Y, low Yb, high Sr/Y, and high (La/Yb)n. This likely reflects the influence of residual or fractionated amphibole in the ANS crust, rather than slab melting during active subduction. A post-collisional episode of delamination and heating of tonalitic lower crustal material, followed by mixing with asthenospheric mantle, created a primary magma with the characteristics of the Guruf basaltic andesite, including residual amphibole. There is no need for either late subduction in the area or long-term persistence of subduction-influenced material in the local convecting mantle.

The Ediacaran volcano-sedimentary succession in the Western Skoura inlier (Central High Atlas, Morocco): facies analysis, geochemistry, geochronology and geodynamic implications

The Skoura inlier in the Moroccan High Atlas consists of thick Precambrian–Paleozoic formations. It displays, in the western part near Aguerzega village, a volcano-sedimentary succession of about 400 m thickness. It includes a variety of welded and non-welded ignimbrites, lapilli-tuff fallout deposit, coherent lava, sedimentary mass flow deposit and fluvial sediments, which are grouped into three lithofacies associations, corresponding to different depositional environments. The volcanic activity in the studied area represents successive volcanic eruptions, alternating with phases of quiescence (sedimentary beds). The Ediacaran volcano-sedimentary succession of the Skoura inlier represents an evolution in four stages in a subaerial environment. The geochemical results obtained from Skoura volcanic rocks display high silica-rich composition ranging from dacite to rhyolite. Major and trace elements show high-K calc-alkaline and alkali-calcic to alkaline affinities and reflect an active continental margin signature. In addition, the studied rocks show enriched LILE compared to HFSE with negative Nb-, Ta- and Ti anomalies. This is consistent with a post-collisional setting, documented for the final phase of the Pan-African orogeny. Laser ablation U–Pb zircon dating on three rhyolitic ignimbrites from the base and top of two profiles, yielded ages of 580 ± 7 Ma, 574 ± 14 Ma and 571 ± 4 Ma. These ages compare well with the lower Ouarzazate Supergroup, widespread in the Anti-Atlas, as well as in the High Atlas and with rare exposures in the Meseta domain in northern Morocco.

Tectonic Setting of the Kenya Rift in the Nakuru Area, Based on Geophysical Prospecting

In this paper, we present results of tectonic and geophysical investigations in the Kenya Rift valley, in the Nakuru area. We compiled a detailed geological map of the area based on published earlier works, well data and satellite imagery. The map was then integrated with original fieldwork and cross sections were constructed. In key areas, we then performed geophysical survey using Electrical Resistivity Tomography (ERT), Hybrid Source Audio MagnetoTelluric (HSAMT), and single station passive seismic measurements (HVSR). In the study area, a volcano-sedimentary succession of the Neogene-Quaternary age characterized by basalts, trachytes, pyroclastic rocks, and tephra with intercalated lacustrine and fluvial deposits crops out. Faulting linked with rift development is evident and occurs throughout the area crosscutting all rock units. We show a rotation of the extension in this portion of the Kenya rift with the NE–SW extension direction of a Neogene-Middle Pleistocene age, followed by the E–W extension direction of an Upper Pleistocene-Present age. Geophysical investigations allowed to outline main lithostratigraphic units and tectonic features at depth and were also useful to infer main cataclasites and fractured rock bodies, the primary paths for water flow in rocks. These investigations are integrated in a larger EU H2020 Programme aimed to produce a geological and hydrogeological model of the area to develop a sustainable water management system.

Internal Stratigraphy of the Mesoarchean Keonjhar Siliciclastics, Singhbhum Craton, Eastern India: Paleogeographic Implications

In comparison to the greenstone belt volcano-sedimentary successions, mature arenite dominated sedimentary deposits are very rarely preserved in Paleo-Mesoarchean rock record. Such arenitic deposits may be important for the recognition of extensive development of continental freeboard and onset of Phanerozoic style plate tectonics. The Keonjhar siliciclastics are ∼715-m thick mature arenite-dominated Mesoarchean succession that unconformably overlies Paleo-Mesoarchean granite-greenstone core of the Singhbhum craton. The succession is mapped on 1:50,000 scale, and is designated as the Keonjhar Quartzite. It is subdivided into conglomerate dominated lower unit as the Asurkhol Member and mature arenite dominated upper unit as the Kamalpur Member. A lensoid body of iron ore clast bearing conglomerate unconformably overlies the Keonjhar Quartzite, and is designated here as the Chamakpur Conglomerate. The succession is repeated by three westerly dipping reverse faults that have upthrown the deeper parts of the basinal transects against shallower parts. The Asurkhol Member represents subaerial part of a coastal alluvial fan. The Kamalpur Member represents shallow-shelf depositional setting. Detailed mapping and recognition of the fault slices led to the reconstruction of the paleogeographic setting. The present study also highlights the importance for the stratigraphic correlation of the Keonjhar Quartzite with underlying metabasics of Malangtaoli-Jagannathpur lava and the overlying western IOG succession. The internal stratigraphic classification proposed here is also significant from economic point of view with regard to the occurrences of uraniferous QPCs in the Asurkhol Member and detrital iron ore deposits in the Chamakpur Conglomerate.

Unravelling depositional setting, age and provenance of the Simlipal volcano-sedimentary complex, Singhbhum craton: Evidence for Hadean crust and Mesoarchean marginal marine sedimentation

The Singhbhum craton of eastern India preserves an extensive record of basin formation spanning the Paleoarchean to Neoarchean. Although spatially extensive and well exposed, the absolute age, depositional environment, and regional correlations of many of the purportedly Archean basins of this craton remain poorly resolved. We present a detailed lithostratigraphic analysis of siliciclastic strata in the (lower) Simlipal volcano-sedimentary succession and report the first detrital zircon U-Pb ages from this succession. The studied section is dominated by fine- to medium-grained, quartz-rich sandstones, which preserve herringbone stratification, tidal bundles, soft sediment deformation structures along with ubiquitous trough cross-stratification. Petrographic study confirms these sandstones as quartz arenites and reveal their high textural and mineralogical maturity. These features are consistent with deposition of the lower siliciclastic succession of the Simlipal volcano-sedimentary complex within a tidally-influenced marginal marine setting. The new detrital zircon data support a ~3.08 Ga maximum depositional age for the succession and reveal a provenance with age peaks at ca. 3.55–3.45 Ga, 3.38–3.24 Ga, and 3.10–3.08 Ga. These ages likely correspond to local basement sources including the Older Metamorphic Tonalite Gneisses, the Singhbhum Granitoid Complex, and the Mayurbhanj Granite Suite. Two detrital zircons from our dataset have concordant ages of ~4.02 Ga and ~4.03 Ga. They represent the first Hadean detrital zircons recovered from any Archean strata in the Singhbhum craton, documenting the involvement of Hadean crust in the early development of the craton. The detrital zircons show prominent Pb-loss at ~1.2–1.0 Ga that exemplifies the tectonothermal imprint of a late Mesoproterozoic to early Neoproterozoic orogeny (likely related to the assembly of Rodinia) on the Singhbhum craton. Our findings further support the interpretation that the lower siliciclastic strata in the Simlipal succession are the lateral facies equivalents of alluvial fan deposits at the base of the Dhanjori Formation, exposed to the north.

The closure of the Rocas Verdes Basin and early tectono-metamorphic evolution of the Magallanes Fold-and-Thrust Belt, southern Patagonian Andes (52–54°S)

The Western Domain of the Magallanes Fold-and-Thrust Belt (MFTB) between 52°-54°S is part of a poorly studied hinterland region of the southernmost Andean Cordillera. This domain consists of NNW-SSE trending tectonic slices of pre-Jurassic basement units and Upper Jurassic-Lower Cretaceous ophiolitic complexes and volcano-sedimentary successions of the Rocas Verdes Basin (RVB). New detrital zircon U–Pb ages of metatuffs and metapsammopelites constrain episodes of Late Jurassic rift-related volcanism (ca. 160 Ma) followed by Early Cretaceous sedimentation (ca. 125 Ma) during the opening of the RVB. Shear bands developed in the RVB units further record the initial phases of the Andean Orogeny. The 30-km wide thrust stack located on top of the Eastern Tobífera Thrust consists of mylonitic metatuffs, metapelites and metabasalts with a NE-verging brittle-ductile S1* foliation. Phengite-bearing metatuffs commonly record pressure-temperature (P-T) conditions between ~3–6 kbar and ~210–460 °C, consistent with underthrusting of the RVB beneath the parautochthonous magmatic arc in the west. Peak metamorphic conditions of ~6 kbar and 460 °C are derived from a metapsammopelite with textures of contact metamorphism overprinting early mylonitic structures (at least S1*). A back-arc quartz-diorite, intruded at ca. 83 Na, is in contact with the metapsammopelite and constrain the minimum age of deformation at deep crustal depths. Campanian-Maastrichtian (ca. 70–73 Ma) 40Ar/39Ar phengite dates from a mylonitic metapelite indicate the timing of thrusting and backthrusting during the initial uplift of the underthrusted crustal stack. These findings reveal a ~400 km along-strike connection of mylonite belts in a continent-verging thrust structure that became active at the onset of the Andean orogeny during the closure of the Rocas Verdes back-arc marginal basin.

Distinguishing lavas from intrusions in an Early Paleozoic submarine backarc sequence, Puna, Argentina

In ancient volcano-sedimentary successions, the reconstruction of the facies architecture may be difficult because of incomplete exposure, deformation, metamorphism and hydrothermal alteration. The distinction of lavas or domes from shallow intrusions is critical for the ordering of volcanic, intrusive, sedimentary and hydrothermal events. Both lavas and shallow intrusions typically have porphyritic textures and may have breccia along their top contacts. Detailed study of these breccias and their contact relationships is a reliable means of distinguishing lavas or domes from shallow intrusions. In this contribution, we describe rhyolites in an Early Paleozoic, submarine, volcano-sedimentary succession in northwestern Argentina, known as the Chiquero Formation, formed in a rapidly subsiding backarc setting. This succession is mainly composed of turbidite interbedded with porphyritic rhyolite-dacite units.Many of the rhyolites have breccias along their upper contacts. Rhyolite clasts in the breccias match the underlying rhyolites in terms of phenocryst populations and texture. The main breccia facies along upper contacts are monomictic rhyolite breccia, mudstone-matrix rhyolite breccia and siliceous mudstone-matrix rhyolite breccia. The monomictic rhyolite breccia is interpreted to be in situ hyaloclastite formed by quench fragmentation of the tops of rhyolite lavas extruded onto the sea floor. The mudstone-matrix rhyolite breccia and the siliceous mudstone-matrix rhyolite breccia are also interpreted as hyaloclastite but the spaces between the rhyolite clasts have been infilled by the locally available sediment and hydrothermal silica, respectively. The mudstone-matrix rhyolite breccia and the siliceous mudstone-matrix rhyolite breccia superficially resemble peperite in being composed of igneous and sedimentary components. However, the infilling sediment is laminated and the laminae are conformable with bedding in the overlying turbidite sequence, implying that the sediment passively infilled pore spaces in pre-existing rhyolite breccia. Hence, the studied rhyolites of the Chiquero Formation were submarine lavas emplaced on the sea floor. Those lavas were probably emplaced during the Middle Ordovician based on correlation with a dated rhyolite interpreted to be syn-sedimentary. However, at some localities, the turbidite sequence contains Tremadocian/Arenigian (Early Ordovician) fossils and a discrepancy exists between the isotopic age of the rhyolites and the age of the host sedimentary sequence based on fossils. It is likely that the Chiquero Formation spans a wider age range than previously thought, from Early Ordovician to Late Ordovician, and that at least four episodes of rhyolitic-dacitic magmatism contributed to this backarc succession.

U–Pb zircon geochronology of the Ediacaran volcano-sedimentary succession of the NE Saghro inlier (Anti-Atlas, Morocco): Chronostratigraphic correlation on the northwestern margin of Gondwana

We report a new regional correlation for the Ediacaran succession in the Anti-Atlas belt on the northwestern margin of Gondwana, based on U-Pb LA-ICP-MS zircon geochronology of volcanic rocks in the NE edge of the Saghro inlier. The thick volcano-sedimentary succession comprises a diverse suite of rhyolitic-ignimbrite, basaltic to andesitic lava fields, rhyolitic lava, mafic hydroclastic complex, fallout and surge deposits, pyroclastic dyke, interbedded clastic sediment and subvolcanic bodies.Ten volcanic rocks yield crystallization ages ranging from 573 to 547 Ma, consistent with a lower and upper Ouarzazate Supergroup affinity respectively. Inherited zircon ages range from 623 Ma to 600 Ma, analogous to zircon peaks in the older volcano-sedimentary rocks of the Bou Salda, and Saghro groups in the Anti-Atlas, suggesting the continuity of the Saghro Group beneath the Ouarzazate Supergroup at the NE edge of the Saghro inlier.Rocks with a lower Ouarzazate Supergroup affinity include lithic-poor ignimbrites which yield ages of 573.6 ± 1.9 Ma, 571.8 ± 4.2 Ma, 571.3 ± 2.6 Ma, and 567.4 ± 2.9 Ma, two fallout deposits which yield ages of 563.5 ± 2.1 Ma and 569.2 ± 1.9 Ma, a surge deposit dated at 571.6 ± 2.8 Ma and a rhyolite lava dated at 562.5 ± 3.1 Ma. Two lithic-poor ignimbrites from the upper Ouarzazate Supergroup are dated at 557.3 ± 2.6 Ma and 547.9 ± 3.1 Ma.Volcanic activity at the NE edge of the Saghro inlier is related to West African Cadomian orogenic (WACadomian) activity between 620 and 560 Ma. During this period the Saghro and Bou Salda groups were deposited, followed by the lower Ouarzazate Supergroup. Later extension along the Gondwanan margin took place close to Ediacaran – Cambrian boundary, contemporaneous with upper Ouarzazate Supergroup deposition.

Post-volcanic activities in the Early Miocene Kırka-Phrigian caldera, western Anatolia – caldera basin filling and borate mineralization processes

ABSTRACT The formation of large, economic borate deposits requires a boron-rich source, the means of transporting and concentrating the boron in a restricted environment, and mechanisms for the preservation of the deposit. There are several Miocene basins in western Turkey containing world-class borate reserves, with mineralization present as stratabound deposits in volcano-sedimentary successions. Although it is well-documented that the conditions required to form and preserve large borate deposits are most common in post-collisional tectonic settings (of which western Anatolia is a prime example), recent advances in the understanding of extensional tectonics and volcanism in this region, make it possible to gain fresh insights into their formation. Here, we suggest that formation of one of the largest borate deposits in the world was intimately related to the recently recognized Kırka-Phrigian caldera that lies in the northernmost part of the Miocene Eskişehir–Afyon volcanic field. Following caldera collapse, the basin filled with lacustrine sediments and volcaniclastic deposits with the boron mineralization concentrated in two main sub-basins: Sarıkaya and Göcenoluk. The close spatial and temporal relationship between borate deposition and the vast Early Miocene ignimbrite deposits that surround the caldera (and contain high levels of elements associated with mineralization) strongly suggest that the ignimbrites were the major source of boron. The boron was transported by geothermal fluids and post-volcanic gases that vented into warm water at the base of the caldera-paleolake system and was then concentrated during cycles of sedimentation and evaporation, with most of the mineralization concentrated along a N-S striking fault system.

U-Pb Zircon Dating of Migmatitic Paragneisses and Garnet Amphibolite from the High Pressure Seve Nappe Complex in Kittelfjäll, Swedish Caledonides

The Seve Nappe Complex exposed in the Kittelfjäll area of the northern Scandinavian Caledonides comprises a volcano-sedimentary succession representing the Baltica passive margin, which was metamorphosed during the Iapetus Ocean closure. Garnet amphibolites, together with their host migmatitic paragneisses, record a potential (U)HP event followed by decompression-driven migmatization. The garnet amphibolites were originally thought to represent retrogressively altered granulites. The petrological and geochemical features of a studied garnet amphibolite allow for speculation about a peridotitic origin. Zirconium (Zr) content in rutile inclusions hosted in garnet in paragneisses points to near-peak temperatures between 738 °C and 780 °C, which is in agreement with the c. 774 °C obtained from the matrix rutile in the garnet amphibolite. The matrix rutile in multiple paragneiss samples records temperatures below 655 °C and 726 °C. Whereas the LA-ICP-MS U-Pb dating of zircon cores revealed the age spectrum from Paleoproterozoic to early Paleozoic, suggesting a detrital origin of zircon cores in paragneisses, the metamorphic zircon rims show an Early Ordovician cluster c. 475–469 Ma. Additionally, zircon cores and rims from the garnet amphibolite yielded an age of c. 473 Ma. The REE patterns of the Caledonian zircon rims from the paragneisses show overall low LREE concentrations, different from declining to rising trends in HREE (LuN/GdN = 0.49–38.76). Despite the textural differences, the cores and rims in zircon from the garnet amphibolite show similar REE patterns of low LREE and flat to rising HREE (LuN/GdN = 3.96–65.13). All zircon rims in both lithologies display a negative Eu anomaly. Hence, we interpret the reported ages as the growth of metamorphic zircon during migmatization, under granulite facies conditions related to exhumation from (U)HP conditions.

Genesis and geodynamic evolution of serpentinized ultramafics and associated magnesite deposits in the Al-Wask ophiolite, Arabian Shield, Saudi Arabia

Situated along the Yanbu Suture Zone, the Al-Wask ophiolite is one of the largest and best-preserved ophiolite sequences in the Proterozoic Arabian shield. A mantle section of serpentinized ultramafics is structurally overlain by a crustal section of gabbros and pillow lavas. The whole ophiolite sequence is capped by pelagic sedimentary cover, and tectonically emplaced over a metamorphosed island-arc volcano-sedimentary succession. The Al-Wask ultramafic rocks are strongly deformed, metamorphosed, and altered by carbonatization and silicification. Samples dominated by antigorite indicate upper greenschist to lower amphibolite facies peak metamorphic grade, whereas samples dominated by lizardite and magnesite preserve lower grade conditions that we interpret as a cooling path buffered to low CO₂ activity by the increasing stability of magnesite with decreasing temperature. Nearly all the primary silicate minerals have been replaced by serpentine minerals, leaving only relics of primary olivine and chromian spinel. Petrographic observation of relict olivine and spinel and of mesh and bastite textures in the serpentines suggest that the peridotite protoliths were mainly harzburgite with minor dunite. Whole-rock compositions of serpentinites show low CaO (<0.1 wt.%), Al₂O₃ (<1.5 wt.%), and Y (<0.4 ppm) combined with high Mg# (0.90–0.92), Ni, Co, and Cr contents; all these indicate a highly refractory mantle protolith. The mineral chemistry of relict primary spinel and olivine provides additional petrogenetic and geodynamic indicators. The high Cr# (> 60) and low TiO₂ (≤0.2 wt. %) of spinel and high forsterite contents (90–92) of associated olivine indicate residual mantle that underwent extensive partial melt extraction. The whole-rock and mineral chemistry of the serpentinized ultramafic rocks are both consistent with extracted melt fractions from ∼32 to 38 percent. This extent of melting is typical of fore-arc supra-subduction zone settings, which is the most likely tectonic environment for formation and preservation of the Al-Wask ophiolite. Two types of magnesite deposits can be distinguished in the Al-Wask mantle section: an early generation of massive magnesite and a later generation of magnesite veins. Hence the Al-Wask ophiolite underwent multiple stages of carbonatization, likely involving different sources of CO₂-bearing fluids. The massive magnesite likely formed at relatively high temperature during cooling from peak metamorphic condition from CO₂-bearing fluid probably derived from decomposition of subducted carbonates. Using thermodynamic calculations in the simple MgO-SiO₂-H₂O-CO₂ system, we constrain the path of the reaction boundary where lizardite and magnesite can coexist at equilibrium. On the other hand, the cryptocrystalline magnesite veins fill tectonic fractures and likely formed at low temperature and shallow levels, after serpentinization and ophiolite obduction.

Multistage structural evolution of the end-Cretaceous–Cenozoic Wleń Graben (the Sudetes, NE Bohemian Massif) – a contribution to the post-Variscan tectonic history of SW Poland

Results of a new mapping and structural field study of the Wleń Graben (North Sudetic Synclinorium, SW Poland), made up of a post-Variscan volcano-sedimentary succession, were used to set up a new model of its multiphase tectonic evolution. The Wleń Graben constitutes a narrow tectonic trough, ca. 17.5 km long and up to 3.5 km wide, superimposed on the low-grade metamorphic rocks of the Kaczawa Metamorphic Unit and bounded by steep, NW−SE-oriented, normal and reverse faults. Previously, a simple, one-stage evolution of the graben was considered, with a single Alpine age intraplate compressional event responsible for the formation of the unit. The present study shows that the Late Cretaceous (post-Santonian?) evolution of the Wleń Graben was dominated by NW−SE-oriented, normal faults during the first, extensional stage of its formation. The central and southern parts of the graben were strongly affected by NW−SE-trending reverse faults and overthrusts, which reflect the second, probably latest Cretaceous to early Palaeogene(?) compressional event of tectonic deformation. Moreover, the whole area of the graben is dissected by sinistral strike-slip faults oriented perpendicular to the graben margins, representing the third stage of deformation (late Palaeogene–Neogene). The latest stage of evolution of the Wleń Graben includes a possible Neogene to Quaternary development of normal faults, interpreted here as gravitational collapse structures related to present-day morphology, rather than tectonically induced ones.

Reconnaissance geology and geophysics of the Mercurio structural dome, Chihuahua, Mexico

The Mercurio structural dome is a poorly exposed and complex structure located in the transitional region between the Coahuila Calcareous Platform and the San Pedro El Gallo sector of the Sierra Madre Oriental, Mexico. It is located in the State of Chihuahua, close to the limits with Coahuila and Durango, Mexico. The dome is a circular structure, ~16 km in diameter, that can be seen in air-photos, satellite images, and shaded relief maps, but that has a subtle topographic expression on the ground. As seen in the field, the most conspicuous topographic features in the area are several hills with the morphology of volcanic necks that rise up to 250 m above the surrounding terrain. The deformation fringe of the dome is a series of cuesta-like low hills, less than 30 m high, where a poorly lithified volcano-sedimentary succession (litharenites, polymictic conglomerates, and ignimbrites) is almost completely masked by desert pavement, which is mainly constituted by well-rounded calcareous clasts derived from the Mesozoic sedimentary marine rocks and by less abundant Paleogene volcanic rocks exposed in the region. Inside the dome the following units are exposed: 1) the pre-volcanic basement in a NW-trending, upright, open anticline developed in limestone of the Aurora Formation, 2) a series of hills where is exposed a succession of epiclastic and volcanic rocks, which are similar, in age and lithology, to some facies of the Ahuichila Formation, and 3) a NW-trending dike, exposed at Cerro Dinamita, which is interpreted as an offshoot of the buried subvolcanic body that created the dome. The deformation fringe around the buried intrusive has a quaquaversal array in the bedding and forms a simple monocline-like structure in the NE part of the dome. A set of SE- and NW-trending plunging folds forms the SE and SW portions of the dome, respectively. The NW part of the fringe is nearly completely masked by volcanic rocks, but there is a ~W plunging syncline in the area. Geophysical data show a broad gravimetric high in the region, and there is a distinct aeromagnetic anomaly inside the dome. The morphological expression of the dome lies just east of a NW-trending lineament of gravity and magnetic anomalies, which may be the buried portion of a normal fault shown in geologic maps of the region northwest of the studied area. Another possible cause is an alignment of buried intrusive bodies suggested by the regional aeromagnetic data, a small diorite outcrop south of Sierra El Diablo, and presence of volcanic necks in the northern portion of Sierra Los Alamos. Available geological and geophysical information was used to model a near-surface, irregular intrusive body with variable magnetic susceptibilites. This variation in susceptibilities is consistent with observed differences in rock composition in the exposed volcanic rocks and with evidence that the structure was formed by a bimodal (andesite-rhyolite) magmatic system where mixing/mingling occurred. As a whole, the set of structures is interpreted as a dome formed by forceful magma injection into a previously folded Paleogene volcano-sedimentary succession. U-Pb zircon ages were used to bracket the age of the deformation pulses registered in the rocks. Litharenites from the deformed volcano-sedimentary succession yielded an Ypresian zircon age of ~51 Ma. A tilted, lithic-rich ignimbrite collected near the top of the exposed volcano-sedimentary succession has mean age of 46.4 +0.8/-1.6 Ma, and the Cerro Dinamita dike has a mean age of 29.37 ± 0.24 Ma. Thus, the youngest pulse of Laramide deformation in the area is younger than ~46 Ma and the re-folding, associated with emplacement of the dome occurred at ~29 Ma. Detrital zircon U-Pb ages from Mercurio sandstones suggest dominant sediment sources from plutonic and/or volcanic rocks exposed along western Mexico. Likely subordinate sources are Mesozoic sedimentary rocks in northern and central Mexico. Distribution of detrital zircon U-Pb ages in the studied samples is similar to that documented in sandstones of the Difunta Group at the Parras and La Popa basins, except that older grains (>1.0 Ga), documented in the clastic rocks of these basins, are scarce in the sandstones of the Mercurio area.

Petrogenetic evolution of Cryogenian Halaban ophiolite, Saudi Arabia: A fragment of fore-arc oceanic lithosphere mantle

In this study, Neoproterozoic Halaban ophiolite situated in the eastern part of the Arabian shield, Saudi Arabia, was investigated. The Halaban ophiolite represents a section of a dismembered ophiolite, tectonically emplaced over a volcano-sedimentary succession of island arc assemblages. It is variably, altered, deformed and metamorphosed to serpentinized rocks. It includes serpentinized peridotite and metagabbro with rare metabasalt, chromitite pods, listvenite and rodingite. The contact between the lower mantle unit (serpentinized peridotite) and the upper crustal unit (metagabbro) is originally magmatic, although it is disrupted by tectonism. Nevertheless, because of the high degree of alteration and metamorphism, few fresh relicts of primary minerals (chromian spinel, olivine and pyroxenes) were recorded. The mineral assemblages in the Halaban ophiolite are comparable to greenschist, to lower amphibolite facies metamorphism. The fresh olivine relics have high Fo and Ni content, while the fresh chromian spinel cores have high Cr# accompanied by a low TiO2. The preserved texture (mesh and bastite), fresh relict minerals, and the whole-rock compositions (Mg# = molar Mg/ (Mg + Fe2+) = 0.91–0.93 with low CaO and Al2O3 content) suggest that the protolith is mainly harzburgite with minor dunite. The chemical composition of Halaban metagabbro is indicative of a tholeiitic affinity. In addition, the very low modal abundance of clinopyroxene and the high Ni, Cr and Co content of the serpentinized peridotite, and high-Cr# (av. 0.66) with low-Ti (mostly ≤0.1 wt%) of chromian spinel, suggest significant partial melting. The serpentinized peridotite (harzburgite and dunite) undergone different degrees of partial melting, ranging from approximately 32% to 37% for harzburgite and approximately 37% to 42% for dunite. The variation of the degree of partial melting between harzburgite and dunite is indicative of mantle heterogeneity beneath the Arabian Shield. The general petrological characteristics of the Halaban ophiolite are substantially similar to those formed by melt-peridotites interaction and subsequent melt mixing in a fore arc setting within a supra-subduction zone system. Halaban chloritite is formed as a secondary product due to the metasomatic interaction of the ophiolitic ultramafic rocks. The content of Cr, Co and Ni of the chloritite sample was consistently close to that of the serpentinites.

The Bastar craton, central India: A window to Archean – Paleoproterozoic crustal evolution

The Bastar craton in central India, surrounded by cratonic blocks and Paleoproterozoic to Neoproterozoic orogenic belts, is a window to investigate the Archean-Paleoproterozoic crustal evolution and tectonic processes. Here we propose a new tectonic classification of the craton into the Western Bastar Craton (WBC), Eastern Bastar Craton (EBC), and the intervening Central Bastar Orogen (CBO). We present petrologic, geochemical and zircon U-Pb, REE and Lu-Hf data from a suite of rocks from the CBO and along the eastern margin of the WBC Including: (1) volcanic successions comprising meta-andesite and fine-grained amphibolite, representing arc-related volcanics along a convergent margin; (2) ferruginous sandstone, in association with rhyolite, representing a volcano-sedimentary succession, deposited in an active trench; and (3) metamorphosed mafic-ultramafic suite including gabbro, pyroxenite and dunite invaded by trondhjemite representing the section of sub-arc mantle and arc root adjacent to a long-lasting subduction system. Petrologic studies indicate that the mafic-ultramafic suite crystallized from an island arc tholeiitic parental magma in a suprasubduction zone environment. The chondrite-normalized and primitive mantle normalized diagrams of the mafic and ultramafic rocks suggest derivation from MORB magma. The mixed characters from N-MORB to E-MORB of the studied samples are consistent with subduction modification of a MORB related magma, involving partial melting of the metasomatized mantle wedge. Our zircon U-Pb age data suggest that the cratonic nuclei was constructed as early as Paleoarchean. We present evidence for active subduction and arc magmatism through Mesoarchean to Neoarchean and early Paleoproterozoic, with the trench remaining open until at least 2.3 Ga. Two major crust building events are recognized in the Bastar craton: during Mesoarchean (recycled Paleoarchean subduction-related as well as juvenile/depleted mantle components) and Neoarchean (accretion of juvenile oceanic crust, arc magmatism including granite batholiths and related porphyry mineralization). The final cratonization occurred during latest Paleoproterozoic, followed by collisional assembly of the craton and its incorporation within the Peninsular Indian mosaic during Mesoproterozoic. In the global supercontinent context, the craton preserves the history of Ur, the earliest supercontinent, followed by the Paleo-Mesoproterozoic Columbia, as well as minor thermal imprints of the Neoproterozoic Rodinia and associated Grenvillian orogeny.

Paleoenvironment, sediment provenance and tectonic setting of Tonian basal deposits of the Macaúbas basin system, Araçuaí orogen, southeast Brazil

The Neoproterozoic Macaúbas Group represents the main precursor basin system of the Araçuaí orogen, including a Tonian volcano-sedimentary succession, at the base, overlain by a Cryogenian sequence. Belonging to the Tonian succession, the Matão – Duas Barras Formation, exposed at the southern border of the Porteirinha basement block in the proximal (western) Araçuaí orogen, represents the very first stage of rifting ever recorded in the Macaúbas basin system. Sedimentological analysis on that formation has revealed two distinct and consecutive facies associations, both indicating half-graben sedimentary filling: i) fault-sourced alluvial fans; and ii) fluvial deposits associated with braided rivers. U-Pb ages of detrital zircon grains from the Matão – Duas Barras Formation and its underlain unit (ascribed to the Espinhaço Supergroup) uniformly show a dominant Rhyacian to Orosirian peak at ca. 2182 Ma – 1863 Ma, as well as several minor Archean age peaks from ca. 3186 Ma to ca. 2655 Ma. The Matão – Duas Barras fluvial deposits also show two Mesoproterozoic age clusters at ca. 1520 Ma and ca. 1156 Ma. Together with sedimentological features, the Archean and Paleoproterozoic zircon ages indicate sediment provenance from the granite-gneiss sources located in the neighboring Porteirinha block, which would have acted as a basement high (a large horst) at least during the initial filling of the Early Tonian Macaúbas rift. The ca. 1520 Ma and ca. 1156 Ma peaks are in good agreement with the Calymmian and Stenian development stages (II and III) of the Espinhaço Supergroup, which acted as an important secondary sediment sources for the basal Macaúbas Group. This also suggests that the Mesoproterozoic sources were exposed by the Porteirinha block uplift coeval with the Matão – Duas Barras basin filling. The maximum sedimentary ages given by the youngest zircon grains, together with a thorough literature compilation, bracket a depositional interval for the basal Macaúbas deposits from 1000 Ma to 933 Ma. Changes in sedimentary style and geochronological spectra suggest variations in the tectonic activity and subsidence rates of the Early Tonian Macaúbas basin, marking its first stage as an entirely siliciclastic sedimentation.

Emplacement of lobate sheet flows with hyaloclastites onto soft sediment: The Erquy Neoproterozoic lava pile, Armorican Massif (France)

The Neoproterozoic volcano-sedimentary succession of the Heussaye Headland (Erquy) belongs to the Cadomian block located in the north-east of the Armorican Massif (France). The volcanism, dated at approximately 608 ± 7 Ma (zircon U/Pb data), occurred in a back-arc basin. This site exhibits a noteworthy verticalized pile of moderately metamorphosed (greenschist facies) basaltic andesite flows of arc/back-arc tholeiite affinity, alternating with metasedimentary layers (sandstones and siltstones). A large range of volcanic facies, characteristic of an underwater soft-substrate environment, can be continuously studied for over than 200 m parallel to the flow edges. They include pillow lavas, lava lobes, fluidal peperites and sediment-matrix hyaloclastites. The presence of parallel lava lobes in a sector where one lava flow is capped by a variably thick hyaloclastite carapace, together with the occurrence of asymmetric turbiditic folds in interbedded metasedimentary layers, suggests the existence of an easterly-facing gentle paleoslope. The sudden thinning of this hyaloclastite carapace in two spots located approximately 60 m from each other, is attributed to the synvolcanic activity of two conjugate normal paleofaults. This hypothesis is supported by the observation of a hyaloclastite accumulation in one of the two spots close to a strike-slip fault, which may have been extensional at the time of the volcanism. Water entered the lava flow along the faults, resulting in an interstitial network of hyaloclastites by quench fragmentation in the upper part of the hanging wall block. All these observations are consistent with a model of lava emplacement along a subaqueous slope resulting from synmagmatic extensional faulting in a marginal basin context.

Sedimentological responses to the Hunter–Bowen Orogeny (eastern Australia): evidence from the northern Gympie Terrane

The volcano-sedimentary succession of the Gympie Terrane is an important record of the eastern Gondwana subduction boundary during the Permian and Triassic. Previous work in this region has focused mainly on the geology of the Gympie Goldfields in the southern part of the Gympie Terrane, whereas the northern part has remained relatively understudied. Here we present sedimentological and structural observations from the northern Gympie Terrane, complemented by new U–Pb detrital zircon data from key stratigraphic units. The stratigraphic succession comprises deep-marine lower Permian mudstones and volcaniclastic rocks derived from a distal volcanic source, overlain by shallow-marine middle–upper Permian carbonate and siliciclastic rocks. The Lower Triassic rocks are dominated by terrestrial alluvial fan deposits, which are overlain by Middle Triassic shallow-marine facies rocks. Our new detrital zircon data show that the cessation of deep-marine sedimentation had occurred prior to ca 263 Ma, possibly in response to the onset of the Hunter–Bowen Orogeny. Deformation and uplift associated with the Hunter–Bowen Orogeny led to an increasing proportion of recycled detritus in the sediments, as indicated by the detrital zircon data. The second phase of Hunter–Bowen Orogeny, at ca 250 Ma, likely led to the transition to a terrestrial depositional setting, and the increased input of recycled continental material into the sedimentary basin. The uppermost units in the Gympie Terrane are Middle Triassic marine successions, which were deposited in a period of tectonic relaxation prior to the last phase of the Hunter–Bowen Orogeny at 235–230 Ma.HighlightsThe geology of the Gympie Terrane provides an insight into the evolution of the eastern Gondwanan subduction boundary during the Permian and Triassic.A broadly correlative stratigraphy is recognised in the northern and southern parts of the Gympie Terrane.The cessation of deep-water deposition in the Gympie Terrane coincides with the first phase of Hunter–Bowen Orogeny.Changes in detrital zircon provenance in the Gympie Terrane correspond to the timing of tectonic switches along the convergent plate boundary of eastern Gondwana.

Cambrian shallow-marine to emergent alkaline volcanism near Ouinguigui (Ougnat inlier, eastern Anti-Atlas, Morocco): Volcanic facies, geochemistry and geodynamic setting

Various forms of volcanism occurred during the Middle Cambrian southeast of the Ougnat inlier in the eastern Anti-Atlas of Morocco. This study is the first to examine the formation of these volcanic deposits and interrelationships to the sedimentary host and tectonics on local and regional scales. The emplacement history of the volcanosedimentary succession of Ouinguigui has been characterised by a multi-disciplinary approach of field work, facies analysis of the volcanic rocks and their host and by geochemical analysis of coherent volcanic rocks. Additionally, we introduce a generic facies code system for subaqueous to emergent basaltic successions to make related deposits more comparable. The sedimentary host rock is composed of pre-transgressive conglomerates and sandstones of the Tikirt and Akerouz Member, which are interpreted as deposits of shelf-type fan-delta and shoreface deposits. The transgression is reflected by an abrupt decrease in grain size and the presence of packstones similar to the Micmacca Breccia Member. The post-transgressive clastic deposits of the Feijas Internes Group and Tabanit Group are the main host for the volcanic activity. They are composed of coarsening-upward sequences, which reflect gradual transitions from offshore to shoreface and foreshore facies.The alkaline volcanism comprises four main phases, which are induced by the SW-vergent tilting of a basement block during extensional tectonics. Phase I starts with the emplacement of diatremes within the Jbel Wawrmast Formation and Jbel Afraou Formation reaching an estimated depth of ca. 200 m. They are the result of phreatomagmatic eruptions caused by the rise of basaltic magma into the coastal environment of the host. Phase II is characterised by the emplacement of large volumes of basaltic lava through fissure eruptions and the formation of a lava delta in the eastern part as well as subaqueous to emergent phreatomagmatic volcanism in the middle and western part of the Ouinguigui area. Wave activity led to the formation of volcanogenic conglomerates and sandstones. Phase III and IV are reflected by the emplacement of subvolcanic intrusions consisting of trachy andesites and trachytes.Whole rock geochemical data of 13 samples confirm the alkaline character of the Ouinguigui magmatism, which derived from a garnet lherzolite source of within-plate volcanism.