Jurassic Volcanism Research Articles

High-precision U[sbnd]Pb geochronology of an ignimbrite flare-up in a silicic large igneous province (Chon Aike, Patagonia)

Silicic large igneous provinces (SLIPs) are periods of particularly voluminous felsic volcanism in the geologic record. Previous work has suggested an overall long lifespan for SLIPs of 20 to 40 Myr, commonly punctuated by shorter-lived ‘flare-ups’ of higher volcanic productivity (1–5 Myr), but detailed studies of individual flare-up events are lacking. The Jurassic Chon Aike SLIP (CASP) is the product of an exceptional geological event wherein voluminous felsic volcanism (ca. 219,000 km3) was generated predominately via crustal anatexis over 45 Myr. We focus on the Late Jurassic El Quemado Complex (EQC), which marked the final stages of felsic volcanism for the CASP. In-situ U-Pb ages for the EQC previously suggested a duration of ∼5 Myr; however, new high-precision CA-ID-TIMS ages indicate the total durations of ignimbrite successions were shorter than 350 kyr. A compilation of zircon U-Pb ages for the eight CASP formations in Patagonia and the Antarctic Peninsula reveals changes in volcanic duration between formations deposited in the intraplate relative to the continental margin, suggesting a spatial control over the magmatic lifespans of geographically restricted systems. We suggest that the rate of magmatism in the CASP was primarily controlled by heterogeneities in the crust, largely between Proterozoic igneous crust and younger, metasedimentary crust that was recently accreted relative to the timing of Jurassic volcanism. The observed duration of volcanism was modulated by these differences in crustal properties from the injection of mafic magmas into the lower crust during extension and mantle upwelling, following rollback of the subducting oceanic slab towards the Paleo-Pacific margin of Gondwana. The short duration in the EQC is the product of unique overlapping conditions that favored crustal melting and resulted in the voluminous ignimbrite flareup (104 km3) of some of the Earth's highest δ18O magmas measured.

Mid-Jurassic volcanism at Bokhara River and insights into metasomatism in the lithospheric mantle of the Thomson Orogen, eastern Australia

Eastern Australia is covered by extensive, thick regolith, which obfuscates much of its basement geology, making geological sampling difficult. The Bokhara River diatremes erupted through the Thomson Orogen in eastern Australia and are covered by ∼300 m of Cretaceous regolith cover. Hitherto, these diatremes have only been characterised by magnetic anomaly surveys and private exploration drilling. The melts were assumed to be leucititic, owing to their proximity to Miocene leucitite centres to the south, while their position along the Cosgrove hotspot track led to the assumption that the diatremes erupted at ca 20 Ma. However, whole-rock chemistry shows that the diatremes are basanites and, according to 40Ar/39Ar dating, erupted during the mid-Jurassic (ca 180 Ma; estimated by reproducible slightly discordant ages), substantially older than the assumed age and coincident with widespread Mesozoic flood basalts across Gondwana. The basanites entrained abundant mantle material during accent, which contaminated the basanite with ∼12.8 wt% xenocrystic material. The xenoliths are all fertile spinel lherzolites with Mg# 87–89, CaO 2.64–5.90 wt% and Al2O3 2.87–3.83 wt% that have been cryptically metasomatised by a mafic silicate melt, which resulted in chromatographic rare-earth element patterns. This demonstrates that mantle metasomatism occurred during the Mesozoic in this part of eastern Australia, revealing another expression of intraplate volcanism prior to Australia rifting away from Gondwana. The identification and characterisation of this Jurassic volcanism hosting evidence for mantle metasomatism suggest that intraplate volcanism and mantle metasomatism are synonymous within orogenic eastern Australia, and not restricted to the Cenozoic.

Geochronology, Eruption Sequence and Geochemistry of Mid–Late Jurassic Volcanics South of Manzhouli: Petrogenesis and Implications for Mesozoic Tectonic Regime Transformation

AbstractTo the south of Manzhouli, Hulunbuir, Inner Mongolia, experienced a tectonic regime transformation from compression to extension in the mid‐Mesozoic. Based on systematic research of the volcanics, petrology, volcanic facies, chronology and geochemistry of rocks in the Buridun area, two stages of volcanics are identified. The first stage named the trachyte series was formed in the late Middle Jurassic (167–163 Ma), its eruption rhythm is pyroxene trachyandesite–trachyandesite–trachyte, and its origin rock is basic volcanics from thickened lower crust, with a tectonic setting in the collision orogeny after the closure of the Mongolia Okhotsk Ocean (MOO). The second stage is a bimodal volcanic rock, formed in the early Late Jurassic (163–160 Ma). The eruption rhythm of basic volcanics in this stage is basaltic andesite–basalt–olivine basalt, which comes from the metasomatized lithospheric mantle, the acidic volcanics of which being characterized by the eruption rhythm of sedimentary‐explosive‐overflow facies, which came from the partial melting of newly formed lower crust, and this shows the characteristics of A‐type granite; the tectonic setting is extension of the lithosphere after collision and closure of the MOO. The changes in the formation age and tectonic setting of the two stages of volcanics demonstrate that the transition time from the compressive system to the extensional system south of Manzhouli is about 163 Ma.

Late Jurassic Volcanism Deduced from Geochemical, Geochronological, and Sr‐Nd‐Hf Isotopic Composition Characteristics of the Nanyuan Formation, South China

AbstractThe Nanyuan Formation contains information related to the Mesozoic tectonic transformation. In this study, three representative profiles were surveyed from the Nanyuan Formation, and multiple analyses were conducted. Zircon U‐Pb dating yielded their ages as approximately 158–146 Ma. The volcanic rocks are enriched in Rb, Th, U, K, and Pb and depleted in Nb, Ta, P, and Ti, implying their affinity for I‐type granites. The εNd(t) values (–8.3 to −6.0), (87Sr/86Sr)i values (0.7077–0.7094) of the volcanic rock, and εHf(t) values (–8.71 to 0.12) of the Mesozoic zircons suggest that the Nanyuan Formation magma originated in the lower crust with the involvement of depleted mantle materials. The parent rocks of the rhyolitic and dacitic volcanic rocks formed by partial melting of basement rocks in South China and the andesitic volcanic rocks were derived from partial melting of the metasomatites generated by slab‐mantle interaction. The fractional crystallization also played an important role in later stage. Discrimination diagrams of the volcanic rocks indicated that they formed in a volcanic arc environment. Combined with previous data, the Nanyuan Formation recorded subduction of the Paleo‐Pacific Plate before regional tectonic transformation. The compressive stress field endured until the end of the Late Jurassic.

Petro-geochemical features of the Bajocian island-arc volcanism in the Lesser Caucasus (Azerbaijan)


 
 
 This article discusses the petro-geochemical features of the Bajocian volcanism in the Azerbaijan Lesser Caucasus. Jurassic volcanism, manifested with varying intensity in the Lok-Karabakh zone, took place in various geodynamic settings, depending on the alternation of extension and compression processes in the island arc. Under these conditions, in the Lesser Caucasus during Middle Jurassic Epoch, two volcanic associations were formed: the Bajocian basalt-rhyolite and the basalt-andesite-dacite-rhyolite of the Bathonian age. It was found that the volcanic rocks of the Lower Bajocian complex belong to the tholeiitic series, and the Upper Bajocian rocks belong to the calc-alkaline series. In the rocks of the association, light REE slightly prevail over heavy ones and form almost flat spectra, the normalized plots are characterized by the chondritic nature of the distribution of rare earth elements, and the lines are parallel to the spectrum of the distribution of rare earth elements in MORB. In such rocks particularly the europium ratio (Eu / Eu * = 0.81–1.21) approaches 1 and low La / Yb ratios are observed. In some samples of more basic rocks, the content of heavy REE increases. Such a distribution of REE in the studied volcanic rocks is common for basic rocks of the tholeiitic series in typical island arcs. In the analyzed single rhyolite sample, a negative Eu anomaly is observed (Eu / Eu * = 0.56). The volcanic rocks on the primitive mantle normalized multi-element plots are characterized by depletion in Ta, Nb and enrichment in LILE (Rb, Ba, Pb, U, Th), which is characteristic of island arc-related volcanic rocks of supra-subduction zones (SSZ). The rocks are also depleted in titanium, potassium, and phosphorus. It was concluded that, in terms of geochemical features, the Middle Jurassic igneous rocks were formed at the ensimatic island arc, which was the initial stage of the development of the island arc tectonic setting, replaced in the Upper Jurassic by ensialic subduction.
 
 

Migration of Middle-Late Jurassic volcanism across the northern North China Craton in response to subduction of Paleo-Pacific Plate

The primary driver of magmatism, crustal deformation and metallogeny of the North China Craton in the Jurassic remains actively debated, either attributed to the subduction of the Paleo-Pacific (i.e., Izanagi) plate in the east or to the closure of the Mongol-Okhotsk Ocean in the north. This issue is addressed here by examining temporal and spatial variation of the contemporary volcanic rocks (i.e., Tiaojishan Formation) of the Yanshan belt in the northern part of the craton. These rocks are mainly calc-alkaline and intermediate-felsic in composition and show a highly coherent geochemical composition and unradiogenic zircon Hf isotope, indicating that they have similar origins and formed in a subduction-related setting. The eruption age of the Jurassic volcanic rocks decreases westward (i.e., inland-ward) from Western Liaoning (166–153 Ma), through Pingquan-Chengde-Luanping (162–153 Ma) and Jingxi-Xuanhua (158–149 Ma), to Yuxian (157–142 Ma). This temporal and spatial variation is consistent with the motion of the subducted Paleo-Pacific plate over that time. In addition to the westward migration, the earliest volcanic rocks in the Yanshan belt formed earlier than those in the adjacent southern Great Xing'an Range in the north. These observations suggest that the southward subduction and closure of the Mongol-Okhotsk plate played a limited role in generation of these volcanic rocks. Two episodes of Middle-Late Jurassic volcanism (166–153 Ma and 150–139 Ma) are observed in the northern Great Xing'an Range. They are interpreted as results of closure of the Mongol-Okhotsk Ocean and subduction of the Paleo-Pacific plate, respectively. Temporal-spatial variation of volcanic activity in the northern North China Craton and adjacent region demonstrates that the Paleo-Pacific subduction was the principal cause of the evolution of Eastern China during the late Mesozoic, whereas the Mongol-Okhotsk tectonic regime is largely confined to the northern Great Xing'an Range and the area north to it.

Late Jurassic epithermal mineralization of Shitouban Au deposit, Dehua-Youxi area, southeast China: Quartz Rb[sbnd]Sr dating, fluid inclusions, and H-O-S-Pb-Sr isotope analyses

Shitouban Au deposit is a vein-type deposit, which located in the Dehua-Youxi area (Fujian Province, southeast China) and hosted by late Jurassic volcanic-sedimentary rocks. In this study, a detailed investigation on fluid inclusions, multiple isotopic compositions (H-O-S-Pb-Sr) and RbSr isotope dating of quartz have been carried out in an attempt to constrain the ore-forming processes and ore genesis. The deposit shares a characteristic of low-sulfidation epithermal Au deposit, such as low-temperature (peak Th 160–180 °C, average 168 °C) and low-salinity (2-3 wt% NaCl equiv., average 2.47). The hydrogen and calculated oxygen isotope values for fluids are −64 to −54‰ (average − 60‰) and - 0.95 to 0.76‰ (average 0.15‰) respectively, indicating that the ore-forming fluid was a mixture of dominant meteroric water and subsidiary magmatic fluid. Pyrite from the deposit shows a δ34S range from - 4.5 to - 1.2‰ (average −2.4‰), indicating a magmatic sulfur. The lead isotope ratios of pyrite, 206Pb/204Pb (18.399–18.494), 207Pb/204Pb (15.695–15.841), 208Pb/204Pb (38.842–39.297), are more radiogenic than the Jurassic magmatic rocks in the area, indicating a mixing metal source of magmatic and basement rocks. Based on the RbSr isotopic isochron age of Au-bearing quartz, the metallogenic epoch of Shitouban Au deposit is 157 ± 1 Ma, and consistent with the Jurassic volcanism in the Dehua-Youxi area. It is proposed that the primary hydrothermal fluids may have exsolved from the late Jurassic magma chamber and subsequently ascent into the shallower crust level near paleosurface to mix with meteoric water, which cause precipitation of Au and sulfides in the epithermal environment due to a decrease in metal solubility triggered by fluid boiling, cooling, and mixing.

Nature and Preservation of Late Jurassic Breakup-Related Volcanism in the Carnarvon Basin, North West Shelf, Australia: Implications for Extrusive to Intrusive Ratios in Large Igneous Provinces

Nature and Preservation of Late Jurassic Breakup-Related Volcanism in the Carnarvon Basin, North West Shelf, Australia: Implications for Extrusive to Intrusive Ratios in Large Igneous Provinces

Lower to Middle Jurassic volcanism and Au–Ag mineralization at Cerro Moro District, Deseado Massif, Argentine Patagonia

In the Cerro Moro Mine area, a horst block exposes a volcanic sequence with ages ranging from 191 Ma to 167 Ma, extending the Jurassic volcanic history of the Deseado Massif back to the Lower Jurassic (Sinemurian-Pleinsbachian boundary). Three volcanic sequences are recognized: a lower volcanic complex comprising welded tuffs, rhyolitic volcanic breccias, andesite lavas and intercalated continental sediments from 191 to 186 Ma, an intermediate sequence of welded ignimbrites, block-and-ash flow deposits and epiclastic sediments with ages ranging from 182 to 181 Ma, and an upper sequence of rhyolitic domes and welded ignimbrites from 180 to 167 Ma. Epithermal vein mineralization was formed in a series of pulses beginning at 180 Ma and culminating in a bonanza event at 171 Ma, coincident with the eruption of the upper volcanic sequence. Increasing grades of precious and base metals with time leading up to the bonanza event are interpreted as the result of repeated fertilization of the hydrothermal system by successive fluid pulses. Detrital, entrained and inherited zircons in the Jurassic rocks have ages ranging from the Triassic to Paleoproterozoic.

Geology and epithermal silver-gold vein mineralization of the San José mining district, Santa Cruz province, Argentina

The San José mining district in the northwestern Deseado Massif of southern Argentina produces silver and gold from intermediate sulfidation style epithermal veins.The district geology is characterized by a subhorizontal sequence of Jurassic volcanic rocks, dominated by volcanogenic agglomerates and lava flows of andesitic to basaltic andesitic composition of the Bajo Pobre Fm, that are locally overlain by felsic ignimbrites and air fall tuffs of the Chon Aike and La Matilde Fms. Several small hornblende-porphyritic subvolcanic stocks, as well as polymictic and matrix-supported breccias cross-cut the andesite sequence. Geochronology indicates that Jurassic volcanism was active in the district between 151.3 ± 0.7 and 144.7 ± 0.1 Ma, and mineralization formed from 151 to 141 Ma. More than 50% of the San José district is covered by post-Jurassic deposits including Cretaceous sedimentary and volcano-sedimentary rocks, Tertiary plateau basalts and modern periglacial gravel deposits.The Huevos Verdes-Frea vein cluster is the economic heart of the San José district and source of its production with more than 20 km of northwest-striking veins hosted by andesite lava of the Bajo Pobre Fm.West-northwest-striking veins (~N280° and ~N320°) display dextral strike-slip motion and contain ore shoots over strike lengths of up to 1200 m (Kospi, Odin, Frea, Ayelen). By contrast, north-northwest striking veins (>N320°) (Huevos Verdes) show sinistral strike-slip motion and host more discontinuous ore shoots, with strike length up to ~400 m separated by transverse syngenetic faults. Vein mineralization is characterized by three paragenetic stages. Pre-ore barren quartz and chalcedony is followed by the ore stage, consisting of mottled and crustiform quartz-sulfides and late sulfide-rich veinlets. Sulfide paragenesis shows repetitive cycles starting with pyrite, sphalerite, galena, chalcopyrite, minor bornite, and covellite, followed by precious metal-dominated pulses with electrum, acanthite/argentite, and rare silver sulfosalts. Gangue mineralogy is mainly characterized by quartz intergrown with minor adularia, illite, illite/smectite, and preferentially at deeper levels Fe-chlorite (±epidote), indicating precipitation from near-neutral pH hydrothermal fluids at temperatures of >220 °C. Kaolinite gangue accompanies illite/smectite with increasing presence towards higher levels and in later stages suggest fluid mixing with steam-heated fluids resulting in decreased fluid temperatures and pH towards shallower levels and over time.Other mineralized sectors of the district lack economic significance. The El Pluma-La Sorpresa vein cluster comprises narrow quartz veins and silicic ribs hosted by andesitic volcanic rocks of the Bajo Pobre Fm. with illite, illite/smectite and kaolinite alteration. Cerro Saavedra is an alteration system in the vent area of a partially eroded andesitic volcanic edifice with heterolithic explosion breccias where proximal residual quartz with vuggy texture is surrounded by a halo of quartz-alunite and distal illite alteration. At Saavedra Oeste a swarm of northwest-striking, short vein and breccia lenses is hosted by ignimbrites of the Chon Aike Fm. Hydrothermal alteration ranges from proximal silicification grading out to illite, illite/smectite and minor kaolinite.

Controls on the preservation of Jurassic volcanism in the Northern Carnarvon Basin

The Northern Carnarvon Basin (NCB) forms part of the North West Australian margin. This ‘volcanic’ rifted margin formed as Greater India rifted from the Australian continent through the Jurassic, culminating in breakup in the Early Cretaceous. Late Jurassic to Early Cretaceous syn-rift intrusive magmatism spans 45000km2 of the western Exmouth Plateau and the Exmouth Sub-basin; however, there is little evidence of associated contemporaneous volcanic activity, with isolated late Jurassic volcanic centres present in the central Exmouth Sub-basin. The scarcity of observed volcanic centres is not typical of the extrusive components expected in such igneous provinces, where intrusive:extrusive ratios are typically 2–3:1. To address this, we have investigated the processes that led to the preservation of a volcanic centre near the Pyrenees field and the Toro Volcanic Centre (TVC). The volcanic centre near the Pyrenees field appears to have been preserved from erosion associated with the basin-wide KV unconformity by fault-related downthrow. However, the TVC, which was also affected by faulting, is located closer to the focus of regional early Cretaceous uplift along the Ningaloo Arch to the south and was partly eroded. With erosion of up to 2.6km estimated across the Ningaloo Arch, which, in places, removed all Jurassic strata, we propose that the ‘Exmouth Volcanic Province’ was originally much larger, extending south from the TVC into the southern Exmouth Sub-basin prior to regional uplift and erosion, accounting for the ‘missing’ volume of extrusive igneous material in the NCB.

Chapter 3.1b Antarctic Peninsula and South Shetland Islands: petrology

Abstract The Antarctic Peninsula contains a record of continental-margin volcanism extending from Jurassic to Recent times. Subduction of the Pacific oceanic lithosphere beneath the continental margin developed after Late Jurassic volcanism in Alexander Island that was related to extension of the continental margin. Mesozoic ocean-floor basalts emplaced within the Alexander Island accretionary complex have compositions derived from Pacific mantle. The Antarctic Peninsula volcanic arc was active from about Early Cretaceous times until the Early Miocene. It was affected by hydrothermal alteration, and by regional and contact metamorphism generally of zeolite to prehnite–pumpellyite facies. Distinct geochemical groups recognized within the volcanic rocks suggest varied magma generation processes related to changes in subduction dynamics. The four groups are: calc-alkaline, high-Mg andesitic, adakitic and high-Zr, the last two being described in this arc for the first time. The dominant calc-alkaline group ranges from primitive mafic magmas to rhyolite, and from low- to high-K in composition, and was generated from a mantle wedge with variable depletion. The high-Mg and adakitic rocks indicate periods of melting of the subducting slab and variable equilibration of the melts with mantle. The high-Zr group is interpreted as peralkaline and may have been related to extension of the arc.

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.

Terrestrial organic carbon isotopic composition (δ13Corg) and environmental perturbations linked to Early Jurassic volcanism: Evidence from the Qinghai-Tibet Plateau of China

Abstract The Early Jurassic break-up of Pangea and the formation of the Central Atlantic Ocean are associated with changes to the atmosphere, climate, and environment, as recorded in marine carbonates from the western Tethys Ocean. However, the expression of these global changes in terrestrial successions is less well known. To better understand terrestrial-response, we investigated organic carbon isotope composition (δ13Corg), weathering trends and geochemistry in Lower Jurassic lacustrine strata in the Qaidam Basin, China, on the northern margin of the eastern Tethys Ocean. Four negative δ13Corg carbon isotope excursions are identified in lacustrine black shales during the Pliensbachian to early Toarcian stages. These negative excursions correspond to the early Pliensbachian Warming Interval, the Subnodonus Negative Event at the beginning of the late Pliensbachian, the Pliensbachian–Toarcian Boundary Event, and the early Toarcian Oceanic Anoxia Event (T-OAE), respectively, well documented in western Tethys Ocean successions. In our terrestrial successions from the Qaidam Basin, the first three events occurred under relatively warm and humid climates associated with enhanced weathering, lake transgression, and the prevalence of swamp environments. In contrast, the T-OAE was a period of intense warming and arid climate, expressed by the deposition of red beds, a hiatus in coal deposition, a sharp reduction in plant diversity, and a floristic turnover in palynoflora. Our findings show that each negative excursion is associated with peaks in mercury and nickel concentrations, supporting the hypothesis that magma intrusion into organic-rich rocks and accompanying the volcanic eruption was responsible.

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.

Revised Timing of Cenozoic Atlantic Incursions and Changing Hinterland Sediment Sources during Southern Patagonian Orogenesis

Abstract New detrital zircon U-Pb geochronology data from the Cenozoic Magallanes-Austral Basin in Argentina and Chile ~51° S establish a revised chronostratigraphy of Paleocene-Miocene foreland synorogenic strata and document the rise and subsequent isolation of hinterland sources in the Patagonian Andes from the continental margin. The upsection loss of zircons derived from the hinterland Paleozoic and Late Jurassic sources between ca. 60 and 44 Ma documents a major shift in sediment routing due to Paleogene orogenesis in the greater Patagonian-Fuegian Andes. Changes in the proportion of grains from hinterland thrust sheets, comprised of Jurassic volcanics and Paleozoic metasedimentary rocks, provide a trackable signal of long-term shifts in orogenic drainage divide and topographic isolation due to widening of the retroarc fold-thrust belt. The youngest detrital zircon U-Pb ages confirm timing of Maastrichtian-Eocene strata but require substantial age revisions for part of the overlying Cenozoic basinfill during the late Eocene and Oligocene. The upper Río Turbio Formation, previously mapped as middle to late Eocene in the published literature, records a newly recognized latest Eocene-Oligocene (37-27 Ma) marine incursion along the basin margin. We suggest that these deposits could be genetically linked to the distally placed units along the Atlantic coast, including the El Huemul Formation and the younger San Julián Formation, via an eastward deepening within the foreland basin system that culminated in a basin-wide Oligocene marine incursion in the Southern Andes. The overlying Río Guillermo Formation records onset of tectonically generated coarse-grained detritus ca. 24.3 Ma and a transition to the first fully nonmarine conditions on the proximal Patagonian platform since Late Cretaceous time, perhaps signaling a Cordilleran-scale upper plate response to increased plate convergence and tectonic plate reorganization.

Lower Jurassic felsic diatreme volcanism recognized in central Patagonia as evidence of along-strike rift segmentation

The early Jurassic volcanism of Central Patagonia covers an extensive area of 50,000 km2 where the volcanic deposits occur as isolated systems. The volcanic records form an elongated belt in which the composition, depositional and genetic features, show several differences along with its distribution. The Cañadón Chileno Complex (CCHC), located in the Río Negro province, provides the opportunity to evaluate and improve the knowledge about the Lower Jurassic volcanic stratigraphy and the Lower Jurassic regional setting of Central Patagonia.Based on the field and laboratory data developed in the present work, 22 lithofacies were recognized and grouped into eight facies associations. The continental sedimentary environments include alluvial fan deposits (FA 1), ephemeral deposits (FA 2), braided fluvial deposits (FA 3), alluvial plains, or over banks (FA 4), and lacustrine deposits (FA 9). On the other hand, the pyroclastic facies were separated according to the pyroclastic currents involved during the deposition; into fall-out, dilute, and density currents (FA 5, FA 6, and FA 7). Effusive andesitic feeders and lava-flows (FA 8) were also recognized.The stratigraphic data obtained in the present work allow proposing the existence of two felsic diatreme volcanoes in the CCHC records - described here as Southern and Northern Zone-, developed over a local subsided area, represented by an asymmetrical basin.Seven units were recognized and described in the CCHC (Units a, b, c, d, e, f, and g: unit (a) represents an initial stage of continental sedimentation recorded throughout the entire Complex. Unit (b) represents the upper felsic diatreme facies, in Southern Zone of the Complex, where debris flows are interbedded with massive lapilli-tuff deposits. Unit (c) consists of local andesitic lavas flows and feeder dikes, and the unit (d) consists of welded lapilli tuff deposits interpreted as the growth of the volcanic system. The Northern Zone evolves similarly with the felsic intra-diatreme deposits of unit (e). The unit (f) consisting of an effusive stage represented by extended andesitic lava flows. Finally, a deep lacustrine system is installed (unit g) that includes shallow and deep facies, with intercalations of local pyroclastic deposits.Also, new geochronological data (U-Pb zircon age of 188 ± 3 Ma) was determined to confirm and reinforce the correlation criteria between the different volcanic areas in Central Patagonia, indicating that the volcanism described here is synchronous with the Northern silica-rich calderas of the Garamilla Formation as well as the large andesitic volcanoes of the Lonco Trapial Formation located southward to the CCHC. The regional volcanism changes described, in the present work, are connected with the N-S rift segment limited by E-W regional transfer fault systems developed in continuity to the strike-slip structures of the La Esperanza area.

El Complejo Volcánico Bahía Laura en la zona del distrito minero Manantial Espejo

El levantamiento geológico detallado junto con el análisis petrográfico y geoquímico de las unidades litológicas relevadas en la zona del distrito minero Manantial Espejo, permitió plantear la evolución del volcanismo Jurásico en esa zona del Macizo del Deseado (Provincia de Santa Cruz, Patagonia argentina). Se interpretaron tres ciclos de actividad volcánica, favorecidos por la tectónica distensiva que dominó esta provincia geológica durante el Jurásico Medio a Superior. El primero, el Ciclo Volcánico Inferior, corresponde a la efusión de lavas de composición andesítica (Formación Bajo Pobre) con una intercalación de areniscas tufíticas y niveles de aglomerados volcánicos producto de remoción en masa. La efusión de estas lavas se produjo sobre un basamento conformado por esquistos y granitos. El segundo, Ciclo Volcánico Medio, se inicia con una ignimbrita de composición dacítica a la que le siguen dos unidades ignimbríticas riolíticas, denominadas 1 y 2. La Ignimbrita dacítica y la Ignimbrita riolítica 2, son rocas con fuerte soldamiento, texturas reomórficas y muy alta participación de cristaloclastos, características que indican una elevada temperatura de depositación a partir del colapso de columnas eruptivas de baja altura, de relativa baja energía, elevada densidad y un bajo contenido de gases, originadas a partir de fontanas tipo ?boiling-over?. La Ignimbrita riolítica 1, intercalada entre las anteriores, se caracteriza por incluir grandes fragmentos líticos de naturaleza polimíctica, se correlaciona con las ignimbritas de extra-caldera formadas durante el colapso y erupción de la Caldera Cerro Primero de Abril, ubicada al norte del distrito Manantial Espejo, en las inmediaciones de Mina Martha. Continúa un período inter-eruptivo, durante el cual se depositan tufitas y travertinos que evidencian una actividad hidrotermal del tipo ?hot spring? controlada por estructuras de rumbo NNE y NNO a NO. El aporte de calor y la fuente de CO2 del sistema geotermal habrían estado vinculados a la desgasificación de un nuevo pulso de magma riolítico que marca el inicio del Ciclo Volcánico Superior, y que habría ascendido a través del mismo sistema extensional, evidenciado por diques riolíticos de rumbo NNO. El ascenso de lavas riolíticas dio lugar a la formación de dos centros volcánicos, el Centro Volcánico Norte, compuesto por un complejo extrusivo de domos riolíticos con depósitos de colapso tipo block and ash, y el Centro Volcánico Sur con el desarrollo de una compleja asociación de facies que sugieren que se trató de una erupción de tipo hidromagmática, con la formación de un aparato volcánico de tipo maar-diatrema al que se asocia un depósito de brecha constituida por fragmentos de lava acompañados por cantidades subordinadas de litoclastos de ignimbritas, tufitas y rocas laminadas silicificadas. Asociado a este evento explosivo se reconoció un depósito de tobas de caída con lapilli acrecionales que apuntan también a un probable origen hidromagmático. La interacción entre el magma y agua líquida sobrecalentada habría sido el responsable para la generación de esta erupción. Luego de un corto periodo de calma volcánica, la continuidad en el régimen deformacional distensivo permite nuevos asensos de magmas con la generación de nuevas unidades ignimbríticas riolíticas de similares características a las del segundo ciclo. El volcanismo andesítico a riolítico de la zona pertenece a una serie calcoalcalina de moderado a alto potasio, con afinidades de arco, mayoritariamente peraluminosas y subalcalinas. Dentro de este esquema, el Ciclo Volcánico Inferior es de composición netamente andesítica, el Ciclo Volcánico Medio registra la evolución de los magmas desde dacitas a riolitas y, por último, el Ciclo Volcánico Superior es típicamente riolítico en composición.

Минеральные отложения пещеры Холодный грот (Бзыбское ущелье, Абхазия) и изотопный состав углерода d13С и кислорода d18О карстогенных карбонатов

The article is devoted to the results of the study of the mineral and isotopic composition of speleothems of the of the Kholodnyy Grotto cavein the Bzyb gorge of Abkhazia. The cavity was mapped and various types of secondary mineral deposits were recorded. The following mineral formations (speleothems) have been established in the cave: Mondmilch, corallites, guhrs, carbonate breccias, biogenic phosphates. Attention is drawn to the abundance of allogeneic mineral inclusions in drip calcite associated with the weathering products of igneous rocks (zircon, potassium feldspar, ilmenite, rutile, monazite, apatite). The source of these minerals is presumably the weathering crust of the Middle Jurassic volcanics. The oxygen isotope composition of all studied carbonates reflects the participation of meteoric waters. Moreover, the isotopic composition of carbon is subject to significant variations associated with kinetic fractionation (CO2 degassing). Based on the results of petrographic and isotopic studies, an attempt was made to typify speleothems according to the nature of their feeding and the characteristics of crystallization. Also, the cave revealed phosphate mineralization in the form of dark brown thin-layered crusts of hydroxylapatite, formed in place of old guano deposits.

Titanite Mineralization of Microbial Bioalteration Textures in Jurassic Volcanic Glass, Coast Range Ophiolite, California

Volcanic glasses are rarely preserved in the rock record, and the quality of preservation generally declines with increasing age. Records preserved in ancient basaltic glasses therefore provide important links between processes operating in the distant past, and those that are active on the Earth today. Microbial colonization has been linked to the formation of characteristic structures in basaltic glass, including tubules and granule-filled tubules, which are thought to be produced by microbially mediated glass dissolution. Structures of similar occurrence and morphology but filled almost entirely with fine-grained titanite have been documented in some ancient metabasalts. It has been suggested that the ancient titanite-mineralized structures are mineralized equivalents of hollow tubules in modern glassy basaltic rocks, but a direct link has not been firmly established. We report the discovery of tubular bioalteration structures in fresh and minimally altered basaltic glasses of middle Jurassic (164 Ma) age from the Stonyford Volcanic Complex (SFVC), Coast Range Ophiolite, California. Tubular structures hosted in unaltered basaltic glass are typically hollow, whilst those in zones of zeolitic alteration are mineralized by titanite. Tubules are continuous across zeolite-glass interfaces, which mark an abrupt change from titanite-filled to hollow tubules, demonstrating that titanite growth occurs preferentially within pre-existing tubular structures. Titanite mineralization in the SFVC represent a link between tubular structures in modern basaltic glass and titanite-mineralized features of similar morphology and spatial distribution in ancient metabasalts. Our observations support a link between textures in modern glassy basaltic rocks and some of the oldest-known putative ichnofossils.