Jurassic Volcanic Rocks Research Articles

Discovery of the Mid–Late Jurassic volcanic rocks (163.4 Ma) in Nanhuangcheng Island, Shandong Province, China

Discovery of the Mid–Late Jurassic volcanic rocks (163.4 Ma) in Nanhuangcheng Island, Shandong Province, China

Tectonic Structures in the Southeastern Flank of the Junction Zone Between the In’yali-Debin Synclinorium and the Omulevka Terrane (Kolyma-Omolon Microcontinent, NE Asia)

The study area is located in the zone of junction of the In’yali-Debin synclinorium and the Omulevka terrane (Kolyma-Omolon microcontinent of the Verkhoyansk-Kolyma fold belt). The main tectonic structure is the Momontai syncline made of Middle Jurassic clastic rocks overlain by Upper Jurassic volcanic and volcaniclastic rocks of the Uyandina-Yasachnaya volcanic arc. The Au and Au-Ag occurrences known here are confined to NE-trending faults and hosted in subvolcanic and volcanogenic rocks, mainly rhyolites and dacites. The studied Middle Jurassic rocks are found to be characteristic of near-shore shelfal environments changing, in certain time intervals, to deltaic conditions of sedimentation. The presence in the conglomerates and sandstones of large poorly rounded quartzite and carbonate fragments and of mudstone and volcanite pebbles indicates a proximal provenance of the clastics. For the first time we established a sharp angular unconformity between the intensely deformed Middle Jurassic and Upper Jurassic rocks in the region. Relations between the Late Jurassic subvolcanic deposits and the host rocks were studied. The carried out structural and tectonophysical investigations showed that folding occurred in two deformation stages. During the first stage, the Middle Jurassic clastic rocks were draped into small recumbent to isoclinal folds, cylindrical and parallel-type, with a NW strike. Cleavage is rare. The structural paragenesis of bedding-plane detachment faults, thrusts, normal faults, and strike slip faults is found to have formed in a single stress field together with the development of folds of the first deformation stage. The intensity of the first-stage folding increases from west to east. Restored are axes of paleostresses responsible for the formation of fold-and-thrust structures of the first deformation stage. The Upper Jurassic volcanogenic-sedimentary strata were deformed into superposed large, simple, open folds of the second stage, which exhibit sublatitudinal orientation. They associate with small-scale thrusts and, rarely, strike-slip faults. It is recognized that in the late Middle Jurassic or the early Late Jurassic the region was affected by intense folding which produced tectonic structures of NW strike. Accumulation of the Late Jurassic volcanogenic rocks and intrusion of subvolcanic deposits occurred on/into the earlier deformed Middle Jurassic rocks.

Tectonic transition from subduction to retreat of the palaeo-Pacific plate: new geochemical constraints from the late Mesozoic volcanic sequence in eastern Fujian Province, SE China

AbstractThe widespread occurrence of late Mesozoic volcanic rocks in SE China is associated with widespread mineralization. Most geologists have accepted the model of Pacific plate subduction beneath the eastern Asian continent, but there are still controversies, especially the initial timing of the tectonic transition. In order to understand the tectonic evolution of the palaeo-Pacific plate subduction, an integrated study of zircon U–Pb, Lu–Hf isotope compositions and whole-rock geochemistry was carried out for Mesozoic volcanic rocks in eastern Fujian Province. According to the field observations and zircon U–Pb ages, these volcanic rocks can be divided into three phases: an early stage of Late Jurassic volcanic rocks (Phase 1, 159–153 Ma), a late stage of Late Jurassic volcanic rocks (Phase 2, 152–146 Ma) and an early stage of Early Cretaceous volcanic rocks (Phase 3, 143–139 Ma). Overall, these volcanic rocks are composed of intermediate–acid pyroclastic rocks and lava, with high SiO2, Na2O, K2O and Al2O3 contents, belonging to the high-K calc-alkaline and peraluminous series, with enrichment in large ion lithophile and light rare earth elements but depletion in high-field-strength elements and heavy rare earth elements. However, the Phase 1 and 2 volcanic rocks contain different zircon Hf isotopic compositions and whole-rock geochemistry to the Phase 3 volcanic rocks, implying that they have a different petrogenesis. Our study combined with previous research shows that the decreasing zircon ϵHf(t) values of the Middle–Late Jurassic volcanic rocks indicate a decreasing mantle-derived material contribution. On the contrary, the majority of the Early Cretaceous volcanic rocks have variable ϵHf(t) values, requiring varied degrees of involvement of juvenile components in their origin. Thus, we consider that the Late Jurassic volcanic rocks were generated in a compressional tectonic environment during the early stage (> 146 Ma) of palaeo-Pacific subduction. In contrast, the Cretaceous volcanic rocks were formed in an extensional tectonic setting during a later stage (< 143 Ma) of subduction.

Subduction-related Jurassic volcanism in the Mesa Central province and contemporary Gulf of Mexico opening

Jurassic volcanic successions in the Mesa Central (central Mexico) are considered as remnants of the Early-Middle Jurassic Nazas volcanic province, which is interpreted either, as a volcanic arc, result of the eastward subduction of paleo-Pacific plate underneath north-central Mexico or as a volcanic belt influenced by transtension related to the breakup of Pangea and subduction from the paleo-Pacific domain. We present new geochemical and geochronological data of Jurassic volcanic rocks exposed in Durango, Coahuila, Zacatecas, San Luis Potosí and Nuevo León that confirm observations from previously reported localities in northeastern Mexico, a subduction signature for all studied rocks and ages of volcanism between 191 and 165 Ma (U–Pb, zr, LA-ICP-MS). Initial εNd values (t = 180 Ma) lye by −4.56 to 0.06 for samples from Real de Catorce and Charcas, in San Luis Potosí, Villa Juarez and Sierra de Ramirez, Durango and La Ballena, Zacatecas. Such results are indicative of a depleted mantle source and partial mixing of juvenile mantle and crustal melts. Nd model ages between 695 and 1125 Ma suggest a Precambrian underlying crust. On another hand, volcanic rocks interbedded with red-beds, which are interpreted as basal deposits related to the Gulf of Mexico initial rifting, and their associated extensional sub-basins, provide with stratigraphic evidence that links early Jurassic volcanism and the opening of the Gulf of Mexico. In any case, the subduction process may have influenced NW-SE oriented extension in the Gulf of Mexico area, as an atypical back arc inland, whose extension axis made a high angle with the ancient Pacific trench. Whereas the classical model of a continental arc-back arc setting, placed close to and parallel to the trench, might be exclusive of continental margins of thick, vast and well-consolidated landmasses, variants in the geometry of such volcanic and extensional settings could be controlled by heat flow patterns in the mantle, crustal thickness and composition, as well as tectonic plates dynamics.

Depositional Environments in the Eastern Paratethys during the Final Middle Miocene Transgression (Kura Basin, Eastern Georgia)

The mineral composition and depositional environments of the Konkian in eastern Georgia (Kura trough, eastern Paratethys) corresponding to the Kosovian (13.65–12.829 Ma) in the Central Paratethys and the final Middle Miocene marine transgression in Paratethys are considered. Jurassic and Cretaceous volcanic rocks of the Lesser Caucasus (Adzhar–Trialet and Artvin–Bolnisi zones) were identified as the main provenances for the terrigenous material. Eight lithofacies types distinguished for the studied rocks characterize depositional environments with significant wave or submarine-fluvial influence. The change of depositional environment was controlled by the phases of transgression, as well as by the progradation and lateral migration of the prodelta. The development of depositional environment corresponds to the Konkian stratigraphic beds and reflects the main stages of the final Middle Miocene transgression in the Eastern Paratethys.

Jurassic volcanism of the Chon Aike Silicic LIP in the northeastern Deseado Massif

Two compositionally different volcanic cycles are documented by geological mapping, lithofacies and petrographic analyses of Jurassic volcanic rocks in the northeastern sector of the Deseado Massif, constituting the western counterpart of the Karoo-Ferrar LIPs in Patagonia. The first cycle is of intermediate composition and is represented by andesitic lava flows, phenoandesitic moderate-grade ignimbrites and volcaniclastic deposits. These rocks constitute a previously unknown large outcrop (~100 km2) of the Bajo Pobre Formation. The second cycle (Chon Aike Formation) is acidic in composition and constituted by several pyroclastic deposits (ignimbrites and co-ignimbrite lag breccias), lavic bodies (rhyolitic lava domes, dykes, and coulees) and rhyolitic pyroclastic dykes. Structural measurements collected in the studied area suggest an extensional tectonic control on the emplacement of, at least, the Chon Aike Formation rocks. We document two major NE-striking (SE-dipping) normal faults, which constitute direct field evidence of the extensional tectonic control on the Jurassic Chon Aike Silicic LIP in the Deseado Massif at the time when southern Gondwana broke up.

Tectono-magmatic implications of the Jurassic volcanic and volcanoclastic record of the Santa Marta Massif (Colombia)

The Lower-Middle Jurassic volcanic and volcanoclastic rocks of the Santa Marta Massif (SMM) record the magmatic evolution of a continental arc developed on the NW margin of Gondwana. However, the lack of a comprehensive description of these successions has made it difficult to understand the geologic processes that occurred during the onset, development, and termination of the volcanic activity. Our new U–Pb zircon ages, whole-rock geochemistry, and petrographic analyses suggest that volcanism started at ca. 191 Ma, similar to plutonic activity, likely after the slab rolled-back from a previous flat-slab subduction zone. During this initial stage, the volcanic deposits consisted of andesite-basaltic andesite and trachy-andesite rocks, of calc-alkaline to high-K calc-alkaline character and dominantly metaluminous affinity. At ca. 177–168 Ma, the geologic record included andesite-basaltic andesites, dacites and, rhyolites of high-K calc-alkaline to calc-alkaline character, and peraluminous to metaluminous affinity. Finally, before cessation, at ca. 167–163 Ma, the volcanism presented andesite-basaltic andesite, trachy-andesite, trachyte and rhyolite compositions, with dominantly high-K calc-alkaline character, and peraluminous to metaluminous affinity. The geochemical comparison with coeval volcanic units deposited in La Guajira, Middle Magdalena Valley, Perijá, and San Lucas ranges indicate a similar magmatic evolution in the north of Colombia. The causes of the cessation of magmatic activity are still unknown, however, these may be associated with the generalized extensional event that was triggered by the breakup of Pangea.

Volcanic geological features and geochemical implications of Late Jurassic volcanoes in Duolun volcanic‐eruption basin of Inner Mongolia, China

Duolun volcanic‐eruption basin is part of the Mesozoic Tectono‐Magmatic Belt in the Great Khingan–Taihang Mountains. A lot of previous studies about this region have been made in geochronology, geomorphic genesis, magmatic origin, and Mesozoic Plate tectonic evolution. This article summarizes petrological features of volcanic rocks, lithofacies, volcanic sedimentary sequences, volcanic mechanism, and characters of volcanic eruptions in Duolun volcanic‐eruption basin. The volcanic sedimentary strata in this region are distributed along a series of NE and NNE tectonic belts, and accumulation thicknesses are controlled by volcanic edifices. We identified a total of seven volcanic edifices in Duolun volcanic‐eruption basin. For some typical volcanic edifices, we had studied their compositions, structures, geometries, and spatial distributions in detail, setting up regional volcanic tectonic framework. In the Manketouebo (J3m) cycle, the intensity of the eruption was large, characterized by large broken volcanoes in the early stage, and small dome and cone volcanoes in the late stage. In the Baiyingaolao (J3b) cycle, the strong volcanic activities manifested as Plinian‐type, are characterized by a large circular or nearly circular caldera at the top, moreover, late extrusive and intrusive domes in different cycles, ring faults and radial faults were also developed. Geochemical researches indicate that Duolun volcanic rocks have the basic characteristics of crust–mantle mixed source, and the tectonic environment for the Late Jurassic volcanic rocks is related to the collisional orogeny. Our work has great significance for further research on genesis of volcanic rocks, magma evolution, deep dynamics mechanism and their relationship between mineralization.

Reconstructing the Physical and Chemical Development of a Pluton-Porphyry Complex in a Tectonically Reorganized Arc Crustal Section, Tioga Pass, Sierra Nevada

Abstract In ancient or partially eroded arc sections, a protracted history of tectonism and deformation makes interpretation of local volcanic-plutonic relationships challenging. The fragmentary preservation of volcanic rocks relative to the extensive plutonic record in upper-crustal arc sections also suggests that a broader-scale approach that includes volcanic-hypabyssal-plutonic “fields” is useful. In this context, studies of hypabyssal intrusions emplaced at the intersection of volcanic and plutonic fields provide additional physical and chemical constraints on shallow-level magmatic processes. New mapping, U-Pb zircon geochronology, and geochemistry at Tioga Pass, in the central Sierra Nevada arc section, document the physical and chemical evolution of the Tioga Pass hypabyssal complex, a ca. 100 Ma system that includes an intrusive dacite-rhyolite porphyry unit and comagmatic Tioga Lake quartz monzodiorite. We interpret these units as a Cretaceous subvolcanic magma feeder system intruding a package of tectonically displaced Triassic and Jurassic volcanic and sedimentary rocks, rather than the previous interpretation of a Triassic caldera. The Tioga Pass magmatic system is a well-exposed example of a hypabyssal complex with meso- to micro-scale structures that are consistent with rapid cooling and emplacement between 0–6 km depth and compositions suggestive of extensive fractionation of largely mantle-derived magma. The Tioga Pass porphyry unit is one of many hypabyssal intrusions scattered along a ~50-kilometer-wide belt of the east-central Sierra Nevada that are spatially associated with coeval volcanic and plutonic rocks due to tectonic downward transfer of arc crust. They provide a valuable perspective of shallow magmatic processes that may be used to test upper-crustal plutonic-volcanic links in tectonically reorganized arc sections.

Volcanism of the La Quinta Formation in the Perijá mountain range

This study reports new data on the petrography, total rock chemistry and U-Pb zircon geochronology of volcanic rocks of the La Quinta Formation that outcrop the western flank of the Perijá mountain range and the Cesar and La Guajira departments. The volcanic rocks consist of basaltic, andesitic, dacitic and rhyolitic lavas, and the volcaniclastic rocks consist of crystal-vitric and crystal-lithic tuffs and agglomerates of calc-alkaline affinity, formed in a continental margin arc setting. Geochronological data suggest that the La Quinta Formation was volcanically active for approximately 25 Ma, during which its composition varied from basaltic trachyandesites to rhyolites. U-Pb dating suggests that the volcanism began in approximately 191 Ma (Sinemurian age) and continued until approximately 164 Ma, with at least three periods of increased volcanic activity. The inherited zircons contain Triassic, Permian, Neoproterozoic and Mesoproterozoic populations, indicating that this arc was emplaced on rocks of the Chibcha Terrane along the South American paleomargin and that it is part of the same arc that formed the Jurassic volcanic rocks of the Sierra Nevada de Santa Marta, Cocinas and San Lucas mountain ranges and the Upper Magdalena Valley.

The Jurassic volcanic–intrusive complex in the Dehua gold orefield, coastal region of SE China: Implications for the tectonic setting and epithermal mineralization

The Dehua gold orefield is an important epithermal gold metallogenic district closely related to volcanic–subvolcanic rocks in the coastal region of the Cathaysia Block, southeastern China. This paper presents new LA-ICP-MS zircon U–Pb ages, elemental and Hf isotopic data for volcanic and intrusive rocks in the Dehua gold orefield. Our results show that the volcanic rocks were produced during the Middle Jurassic (ca. 169–164 Ma), whereas the intrusive rocks were formed in the Late Jurassic (ca. 163–157 Ma). Geochemically, these rocks are characterized by high-K calc-alkaline series and are commonly enriched in LILEs and LREE, depleted in HFSE with negative Nb, Ta, and Ti anomalies, and have lower Ta, Yb, Y and Rb contents, suggesting an arc magma geochemical affinities. Hence, this study firstly reports the Middle Jurassic high-K continental arc volcanic rocks in SE China, implying that the initial subduction of the paleo-Pacific plate began at least in the Middle Jurassic. Besides, in situ Hf isotopic analyses for dated zircon from the volcanic and intrusive rocks yield εHf(t) values range from −9.03 to +1.46 and the Paleo- to Mesoproterozoic two stage Hf model ages. The partial melting of the mixture of ancient continental crust materials and newly underplated potassic-rich mantle-derived mafic magmas likely explains the genesis of volcanic and intrusive rocks in the Dehua gold orefield. In addition, the exposed areas of the Jurassic volcanic rocks in the coastal region of SE China may have great potential to prospect the epithermal deposits.

An Early Cretaceous Ag-Pb-Zn mineralization at Halasheng in the South Erguna Block, NE China: Constraints from U-Pb and Rb-Sr geochronology, geochemistry and Sr-Nd-Hf isotopes

The Halasheng deposit, located in the southern Erguna Block, is a vein-type Ag-Pb-Zn deposit in the Derbugan metallogenic belt. The orebodies, hosted in Middle-Late Jurassic volcanic rocks, show a close spatial and temporal relationship with Early Cretaceous granites. Zircon U-Pb dating suggests that the granites were emplaced at 134–128 Ma. Rb-Sr dating of ore-related sphalerite gives an imprecise age of 122 ± 18 Ma, indicating that the Halasheng deposit formed in the Early Cretaceous. Whole-rock major and trace element analyses show that the Cretaceous granites belong to the metaluminous to peraluminous, shoshonite to high-K calc-alkaline series, and can be classified as A-type granites. They are enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE), but depleted in high field-strength elements (HFSE) and heavy rare earth elements (HREE). The quartz monzonite, which has a spatial relationship with mineralization, has low initial 87Sr/86Sr (0.7058–0.7069), and positive εNd (t) (0.42–0.61) and εHf (t) (3.47–6.45), with Nd and Hf two-stage model ages of 0.81–0.85 Ga and 0.69–0.86 Ga, respectively. These geochemical features are similar to those of major Mesozoic granite intrusions in the Erguna Block. We propose that these graintes originated from partial melting of juvenile crust in an intraplate extensional setting. Taken together, the Halasheng Ag-Pb-Zn deposit formed in the Early Cretaceous after the orogeny related to closure of the Mongol-Okhotsk Ocean. The relatively low-temperature of Ag-Pb-Zn ore and alteration mineralogy suggest that there is only a distal relationship, if any, to nearby potentially-causative granite intrusions.

A new Carboniferous–Permian intra‐oceanic subduction system in the North Tianshan (NW China): Implications for multiple accretionary tectonics of the southern Altaids

The North Tianshan in NW China is a key to understanding the accretionary history of the southern Altaids. However, the tectonic setting of the North Tianshan in the Carboniferous‐Permian is debatable, and its amalgamation history is ambiguous. We report new data on the field relations, geochronology and geochemistry of Carboniferous–Jurassic volcanic rocks and sedimentary rocks from the Shichang area in order to constrain the late Palaeozoic–Early Mesozoic tectonic evolution of the North Tianshan. New LA‐ICP‐MS zircon U–Pb dates demonstrate that the Carboniferous–Permian stratigraphy is a tectonic imbrication of Carboniferous–Permian rocks and Carboniferous volcanic rocks were emplaced in Permian volcano‐sedimentary rocks. The 311 ± 2 Ma volcanic rocks are calc‐alkaline, enriched in Light Rare Earth Elements and large‐ion lithophile elements, depleted in high‐field‐strength elements, and have low initial87Sr/86Sr ratios with positiveεNd(t)(+0.3 to +8.6)values. The geochemical‐isotopic data suggest the volcanic rocks were generated in an intra‐oceanic island arc. The turbidite samples 18SC02 (~267 Ma to ~349 Ma) and 18SC09 (~299 Ma to ~369 Ma) have a relatively narrow age range with no Devonian or Precambrian peaks. Our detrital zircon data suggest the presence of a so far unrecognized intra‐oceanic arc, which was isolated from the main Yili‐Central Tianshan and Mosuowan arcs in the Shichang area of the North Tianshan during the Late Carboniferous to Permian. The compiled detrital zircon data from Carboniferous to Jurassic indicates that the North Tianshan intra‐oceanic arc was probably amalgamated with the Mosuowan arc by the Early Triassic, and then accreted to the Yili‐Central Tianshan arc by the Middle‐Late Triassic, as revealed by the changes of zircon provenance of Carboniferous, Permian, Triassic and Jurassic sedimentary rocks. All integrated data suggest that the southern Altaids most likely formed by processes of multiple subduction–accretion and convergence similar to that in the ongoing southwest Pacific.

A true polar wander trigger for the Great Jurassic East Asian Aridification

AbstractA drastic environmental change occurred during the Middle to Late Jurassic as much of East Asia transitioned from a wet seasonal to an extremely arid climate. The timing, scope, and especially mechanism for this aridification are contentious. In this study, we report paleomagnetic data and ages from Jurassic volcanic rocks in North China and for the first time reveal a large-scale southward displacement of ∼25° for the Eastern Asian blocks (EABs) sometime between 174 ± 6 Ma and 157 ± 4 Ma. We suggest that the rapid motion documented by our paleomagnetic studies resulted from large-scale true polar wander (TPW). The TPW rotation displaced the EABs from the Northern Hemisphere humid-temperate belt into the subtropical/tropical arid zone. The resultant latitudinal motion coincided with a remarkable environmental change recorded over 10,000,000 km2 in East Asia between ca. 165 Ma and 155 Ma. We call the climate transition the “Great Jurassic East Asian Aridification” and argue that TPW-induced climatic shifts were also responsible for the demise of the Yanliao Biota and subsequent radiation of the Jehol Biota during the Late Jurassic and Early Cretaceous.

Contrasting tectonic settings in Northern Chon Aike Igneous Province of Patagonia: subduction and mantle plume-related volcanism in the Marifil formation

ABSTRACT The northern zone of the Chon Aike Igneous Province, located in the North Patagonian Massif, exhibits extensive outcrops of Jurassic volcanic rocks of the Marifil Formation. In the Arroyo Verde area, the initial volcanic stage of the Marifil Formation, that we denominate V0 (192.6 ± 2.5 Ma), includes coulées, megabreccias and lapilli tuffs assigned to plinian-type volcanism. This magmatism was generated by cold-wet-oxidized magmas that resemble those produced in active continental margins and volcanic arcs. The second stage, located unconformably over the first, includes welded lava-like ignimbrites, massive lapilli tuff and rhyolitic lava flow that resemble the Snake River-type volcanism. The magma that produces this volcanic stage exhibit the characteristics of hot-dry-reduced magmas emplaced in intraplate continental environments associated with continental rifting. This stage coincides with the V1 volcanic episode early recorded in the Marifil Formation.

Late Mesozoic tectonostratigraphic division and correlation of the Bohai Bay basin: Implications for the Yanshanian Orogeny

We present the results of Mesozoic sequences of the Bohai Bay basin in North China, based mainly on geochronology and interpretations of seismic profiles and logging data. Five tectono-stratigraphic sequences are defined: Lower-Middle Triassic, Lower-Middle Jurassic, Upper Jurassic, Lower Cretaceous and Upper Cretaceous. Based on an analysis of detrital zircons, the clastic rocks recorded two intervals of Jurassic magmatic activity, during 180–175 and 160–152 Ma, which can be correlated to the Nandaling and Tiaojishan Formations in the Yanshan area, respectively. However, since Jurassic volcanic rocks have not yet been found in the Bohai Bay basin, we speculate that these two stages of activity were mainly concentrated around the periphery of the North China Craton (NCC) rather than within. Based on an analysis of zircons from volcanic rocks and pyroclastics, the early Cretaceous magmatism in the Bohai Bay basin can be divided into two stages, 125–120 and 110–100 Ma, which can be correlated to magmatism in the eastern part of the NCC. The zircon ages indicate an absence of volcanic activity during the late Tuchengzi and Zhangjiakou periods which may correlate to the uplift of the Bohai Bay basin in the late Late Jurassic. Comparison of the development of Mesozoic basins and sedimentary strata in the central-eastern part of Yanshan tectonic belt and the the Bohai Bay basin indicates that the two areas are generally comparable, but with substantial differences. The central-eastern part of the Yanshan structural belt lacks Early-Middle Triassic strata, and the Bohai Bay basin lacks late Jurassic-early Cretaceous strata. Based on research results from late Mesozoic sedimentary structures in the central and eastern parts of the Yanshan tectonic belt, we infer that episode A of the Yanshanian Orogeny was weak in the Bohai Bay basin and its periphery, while episode B of the Yanshanian Orogeny had a strong influence on the Bohai Bay basin and its periphery. The available data reveal differences in the expression of these episodes: the Bohai Bay basin is characterized by vertical uplift, and the northern margin of NCC is characterized by horizontal compression uplift.

Early Jurassic volcanic rocks in the Xiongcun district, southern Lhasa subterrane, Tibet: Implications for the tectono-magmatic events associated with the early evolution of the Neo-Tethys Ocean

Early Mesozoic (Late Triassic to Middle Jurassic) igneous rocks in the southern Lhasa subterrane, Tibet, record important information on the early tectono-magmatic evolution of the Neo-Tethys Ocean. This paper presents petrological, geochronological, and geochemical data for the Xiongcun Formation ignimbrites, to constrain the petrogenesis and tectonic setting of these rocks in the southern Lhasa subterrane. The Xiongcun Formation is dominated by andesitic and dacitic rocks. These rocks yield Early Jurassic (195–176 Ma) zircon UPb ages and display typical arc-like geochemical signatures, being enriched in large-ion lithophile elements (e.g., Rb, Ba, and U) and depleted in high-field-strength elements (e.g., Nb, Ta, and Ti). Sr–Nd–Pb–Hf isotopic compositions are highly depleted. These chemical and isotopic data suggest that the Xiongcun Formation ignimbrites probably formed in an intra-oceanic island arc setting related to the northward subduction of the Neo-Tethys oceanic slab. The parental magmas were derived from a depleted mantle source modified by fluids released from the Neo-Tethys oceanic slab. Combined with previously reported data, we suggest that the volcanic rocks in the Early Jurassic Xiongcun and Yeba formations of the southern Lhasa subterrane were generated by a single subduction system, but developed on oceanic and continental crust, respectively.

Geochemical characterization of black shales from the Río Mayer Formation (Early Cretaceous), Austral-Magallanes Basin, Argentina: Provenance response during Gondwana break-up

During the Early Cretaceous important changes occurred in South-western Gondwana related to its fragmentation, and South Atlantic Ocean opening. Then, two basins developed in southernmost Patagonia: the Austral-Magallanes Basin and the Rocas Verdes-Magallanes Basin. The Río Mayer Formation corresponds to the initial sedimentary stages of the Austral Basin including black shales, marls and fine-grained sandstones deposited in an outer shelf environment. We studied and sampled three detailed sedimentary logs at Río Guanaco Locality (S 49° 57′ 11"; W 72° 04′ 56″), dividing this unit into three informal sections (lower, middle and upper). Whole rock geochemical analyses were carried out over the three sections with the focus on trace elements, in particular, rare earth elements (REE). While, the lower section of the unit shows an affinity to acid volcanic source areas, the middle and upper sections show an affinity to intermediate source areas. These changes through the Río Mayer Formation can be correlated with the initial magmatic activity hosted in the Southern Patagonian Batholith during the Neocomian. Considering the REE patterns, the lower section of the Río Mayer Formation shows an affinity to the felsic compositions of the Tobífera Formation, while the upper section would be related to the erosion of the Punta Barrosa/Cerro Toro formations of the Rocas Verdes-Magallanes Basin. The clear distinction between the REE patterns of the Río Mayer and Zapata formations is considered evidence of the compartmentalization of these basins during the Early Cretaceous. Finally, the absence of correlation between the geochemistry of the studied unit and the Jurassic volcanic rocks of the Deseado Massif indicates that the north-eastern sector of the basin was not a source area for the Austral Basin, at least during the deposition of the Río Mayer Formation in the study area.

Geochemical Evidence for the Depositional Environment of the Esfandaghe Manganese Deposit, Kerman Province, Iran

The Esfandaghe manganese deposit is located in the east of Esfandaghe village, Kerman province in Iran. Late Jurassic basalt, lithic tuff, radiolarian chert, shale, flysch turbidite, sandstone, mudstone and dark shale as well as mid-early Jurassic volcanic rocks of rhyolitic crystal tuff and tuffaceous sandstone are the typical stratigraphic units in the study area. Mn ore occurs as interlayers with radiolarian chert and shale overlaying the late Jurassic volcanic rocks. Mn/Fe ratio in manganese ore bodies varies from 0.10 to 43.82 and the position of samples on Fe–Si × 2-Mn, Mn–Fe–(Co + Ni + Cu) × 10, Ni–Zn–Co and (Co + Ni)–(Zn + Cu + Pb + V) diagrams reveals that hydrothermal activity was the main process responsible for mineralization in the Esfandaghe manganese deposit. Cu, Zn, Ni, Co and HREEs are enriched in Esfandaghe manganese relative to Post-Archean Australian Shale. REE contents in the Esfandaghe Mn deposit vary from 22 to 313 ppm, conformable with REE concentration ranges of hydrothermal ferromanganese deposits.

Early Jurassic Magmatism of the Kobyumé Graben System (Verkhoyansk Framing of the Siberian Craton) and Its Geodynamic Nature

Data on the Early Jurassic volcanic rocks of the Kobyume graben system, which formed at the Verkhoyansk passive margin of the Siberian Craton, are presented. The volcanic complex contains products of basaltic eruptions that occurred under conditions of a shallow sea. The basalts have elevated TiO2 contents (>1.8 wt%), and variations of their composition were controlled by fractionation processes. By their geochemical characteristics, the volcanic rocks are transitional between ocean island basalts and enriched basalts of mid-oceanic ridges. Taking into account the remote location of the Kobyume graben region from the convergent boundaries of lithospheric plates and from the nearest areas of contemporaneous magmatism, the data obtained support the conclusion that the Kobyume tectono-magmatic areal is a result of a small mantle plume effect onto the passive margin of the Siberian continent.