Timing and kinematics of the Variscan orogenic cycle at the Moldanubian periphery of the central Bohemian Massif

2021 ◽  
pp. jgs2021-096
Author(s):  
M. Machek ◽  
I. Soejono ◽  
J. Sláma ◽  
E. Žáčková
Keyword(s):  
Bohemian Massif ◽  
Large Scale ◽  
Content Type ◽  
Crustal Rocks ◽  
Tectonic Events ◽  
Dextral Shear ◽  
Supplementary Material ◽  
Orogenic Cycle ◽  
Back Arc ◽  
Lower Crustal

The high-grade complexes along the northern Moldanubian periphery of the central Bohemian Massif provide an outstanding structural record of all episodes of the Variscan collisional evolution. Kinematics and timing of orogenic processes have been examined by structural and microstructural study of middle and lower crustal rocks combined with xenotime and monazite geochronology. Four distinct tectonic events have been identified in the studied units. A first relict sub-horizontal fabric S1 associated with the HP/HT metamorphism is developed only in the lower crustal rocks and was related to back-arc extension or lower crustal flow in a supra-subduction domain. This fabric was at c. 340 Ma completely reworked to the sub-vertical foliation S2 by the major collisional thickening leading to the lower and middle crust juxtaposition. Thereafter, the extensional collapse of thickened orogenic system caused strong refolding to the HT sub-horizontal fabric at c. 325 Ma. The region was subsequently affected by the NNE–SSW oriented horizontal shortening related to the dextral shearing and clockwise rotation of crustal blocks adjacent to the large scale dextral shear zone, the Elbe Zone. It led to the fragmentation and reorientation of the Moldanubian margin to the current position.Supplementary material:https://doi.org/10.6084/m9.figshare.c.5708800.v1

Molecular and petrographical evidence for lacustrine environmental and biotic change in the palaeo-Sichuan mega-lake (China) during the Toarcian Oceanic Anoxic Event

2021 ◽  
pp. SP514-2021-2
Author(s):  
Weimu Xu ◽  
Johan W. H. Weijers ◽  
Micha Ruhl ◽  
Erdem F. Idiz ◽  
Hugh C. Jenkyns ◽  
...  
Keyword(s):  
Organic Matter ◽  
Sichuan Basin ◽  
Large Scale ◽  
Algal Growth ◽  
Black Shales ◽  
Content Type ◽  
Oceanic Anoxic Event ◽  
Anoxic Event ◽  
Supplementary Material ◽  
The Sichuan Basin

AbstractThe organic-rich upper Lower Jurassic Da'anzhai Member (Ziliujing Formation) of the Sichuan Basin, China is the first stratigraphically well-constrained lacustrine succession associated with the Toarcian Oceanic Anoxic Event (T-OAE; ∼183 Ma). The formation and/or expansion of the Sichuan mega-lake, likely one of the most extensive fresh-water systems to have existed on the planet, is marked by large-scale lacustrine organic productivity and carbon burial during the T-OAE, possibly due to intensified hydrological cycling and nutrient supply. New molecular biomarker and organic petrographical analyses, combined with bulk organic and inorganic geochemical and palynological data, are presented here, providing insight into aquatic productivity, land-plant biodiversity, and terrestrial ecosystem evolution in continental interiors during the T-OAE. We show that lacustrine algal growth during the T-OAE accounted for a significant organic-matter flux to the lakebed in the palaeo-Sichuan mega-lake. Lacustrine water-column stratification during the T-OAE facilitated the formation of dysoxic-anoxic conditions at the lake bottom, favouring organic-matter preservation and carbon sequestration into organic-rich black shales in the Sichuan Basin. We attribute the palaeo-Sichuan mega-lake expansion to enhanced hydrological cycling in a more vigorous monsoonal climate in the hinterland during the T-OAE greenhouse.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5433544

scholarly journals The Atud gabbro–diorite complex: glimpse of the Cryogenian mixing, assimilation, storage and homogenization zone beneath the Eastern Desert of Egypt

2020 ◽  
Vol 177 (5) ◽  
pp. 965-980
Author(s):  
Robert J. Stern ◽  
Kamal Ali ◽  
Paul D. Asimow ◽  
Mokhles K. Azer ◽  
Matthew I. Leybourne ◽  
...  
Keyword(s):  
Fractional Crystallization ◽  
Mafic Magma ◽  
Eastern Desert ◽  
Content Type ◽  
Igneous Complex ◽  
Nubian Shield ◽  
Partial Melts ◽  
Supplementary Material ◽  
Lower Crustal ◽  
Arabian Nubian Shield

We analysed gabbroic and dioritic rocks from the Atud igneous complex in the Eastern Desert of Egypt to understand better the formation of juvenile continental crust of the Arabian–Nubian Shield. Our results show that the rocks are the same age (U–Pb zircon ages of 694.5 ± 2.1 Ma for two diorites and 695.3 ± 3.4 Ma for one gabbronorite). These are partial melts of the mantle and related fractionates (εNd690 = +4.2 to +7.3, 87Sr/86Sri = 0.70246–0.70268, zircon δ18O ∼ +5‰). Trace element patterns indicate that Atud magmas formed above a subduction zone as part of a large and long-lived (c. 60 myr) convergent margin. Atud complex igneous rocks belong to a larger metagabbro–epidiorite–diorite complex that formed as a deep crustal mush into which new pulses of mafic magma were periodically emplaced, incorporated and evolved. The petrological evolution can be explained by fractional crystallization of mafic magma plus variable plagioclase accumulation in a mid- to lower crustal MASH zone. The Atud igneous complex shows that mantle partial melting and fractional crystallization and plagioclase accumulation were important for Cryogenian crust formation in this part of the Arabian–Nubian Shield.Supplementary material: Analytical methods and data, calculated equilibrium mineral temperatures, results of petrogenetic modeling, and cathodluminesence images of zircons can be found at https://doi.org/10.6084/m9.figshare.c.4958822

An assessment of the Upper Succession and the related secondary reservoirs in the Welton Field, onshore UK

2016 ◽  
Vol 8 (1) ◽  
pp. 595-609 ◽  
Author(s):  
James Steventon ◽  
Mike Bowman
Keyword(s):  
Large Scale ◽  
Water Injection ◽  
Soft Rock ◽  
Oil Field ◽  
Content Type ◽  
Fine Grained ◽  
Delta Plain ◽  
Supplementary Material ◽  
Petrophysical Logs ◽  
Crevasse Splay

AbstractThe Welton oil field has produced nearly 20 MMBO (million barrels of oil) since discovery in 1981. Now in post-plateau decline, there is increasing reliance on a series of secondary reservoirs. Production has been from a suite of stacked reservoirs deposited by large-scale prograding delta-plain systems of early Westphalian age. Whilst the bulk of production has been from the Basal Succession, a considerable upside is considered to exist in the less well-studied Upper Succession that comprises predominantly distributary channel and crevasse splay deposits which have produced in excess of 3 MMBO. These accumulations occur within the Deep Soft Rock, Deep Hard Rock and Tupton reservoirs.This paper focuses on a sedimentological analysis of cored intervals, integrated with petrophysical logs and detailed production data to enable further recommendations to identify areas of undrained pay, along with identifying additional reservoir management activities that could optimize future offtake from the field. These reservoirs consist predominantly of very fine-grained sandstone, with permeability values rarely attaining 100 mD and average porosity values of 10–12%.Recommendations include executing tracer communication tests and building a detailed field model, as well as a pilot water-injection scheme to increase production from some of Welton's secondary reservoirs.Supplementary material: A full set of detailed sedimentological logs for each of the cored wells in this study is available at https://doi.org/10.6084/m9.figshare.c.3593984

Transcrustal Magmatic Systems: Evidence from Andesites of the Southern Taupo Volcanic Zone

2021 ◽  
pp. jgs2020-204
Author(s):  
Christopher Svoboda ◽  
Tyrone O. Rooney ◽  
Guillaume Girard ◽  
Chad Deering
Keyword(s):  
Field Work ◽  
Scoria Cone ◽  
Taupo Volcanic Zone ◽  
Volcanic Zone ◽  
Content Type ◽  
Supplementary Material ◽  
Temperature And Pressure ◽  
Compositional Diversity ◽  
Lower Crustal ◽  
Hot Zone

Studies synthesizing field work, numerical simulations, petrology, geochemistry, and geophysical observations indicate that the compositional diversity of arc lavas results from evolution of mantle-derived magmas by mixing, assimilation, and fractional crystallization. This evolution occurs within complexes called transcrustal magmatic systems. The mafic lower parts of such zones, called hot zones, are difficult to probe. However, a satellite vent near the stratovolcano Ruapehu in the southern Taupo Volcanic Zone (New Zealand) comprises materials that may originate from a hot zone. Magnesian andesites (Mg#64-69) from the Ohakune scoria cone contain primitive olivine (Fo85-91), high Mg# clinopyroxene (Mg#81-88), and orthopyroxene (Mg#76-83), but lack plagioclase. Disequilibrium of Ohakune crystals and groundmass suggests that the crystal cargo of Ohakune andesites was scavenged from deeper and more primitive levels of the magmatic system. Mineral constraints on temperature and pressure indicate that the hot zone initially formed at mid- to lower-crustal pressures (3.5-7.0±2.8 kbar). We interpret the mafic mineralogy and presence of disequilibrium features as evidence that these andesites and their crystal cargo are products of a hot zone in the middle to lower crust. Products of the hot zone may appear before products of the systems that form the bases of mature stratovolcanoes such as Ruapehu.Supplementary material:https://doi.org/10.6084/m9.figshare.c.5494984

Eocene thickening without extra heat in a collisional orogenic belt: A record from Eocene metamorphism in mafic dike swarms within the Tethyan Himalaya, southern Tibet

2021 ◽  
Author(s):  
Yuhua Wang ◽  
Lingsen Zeng ◽  
Li-E Gao ◽  
Zhenyu Chen ◽  
Sanzhong Li
Keyword(s):  
Large Scale ◽  
High Mountain ◽  
Southern Tibet ◽  
Crustal Rocks ◽  
Flow Estimation ◽  
Environmental Consequences ◽  
Slab Breakoff ◽  
Orogenic Belts ◽  
Lower Crustal ◽  
Tethyan Himalaya

Knowledge of the nature of the earliest metamorphism experienced by collisional orogenic belts is essential for reconstruction of tectonic processes that build high mountain chains and their environmental consequences. Understanding the metamorphic nature of Eohimalayan-phase orogeny of the Himalayan orogen, one of the typical examples of orogenic belts worldwide, could provide some important constraints to test different tectonic models (shallow continental subduction vs. slab breakoff) for the early phases of the development of large-scale orogenic belts. As exhumed middle- to lower-crustal rocks in the Kangmar gneiss dome, the garnet amphibolites with a protolith age of 176.4 ± 3.6 Ma experienced a phase of metamorphism at 47.2 ± 1.8 Ma with an increase in pressure as well as temperature from 3−5 kbar and 550−600 °C to over ∼11 kbar and 650 °C. This suggests that the middle- to lower-crustal rocks experienced heating at least by ∼50 °C while they underwent compression and thickening. Heat-flow estimation further demonstrates that the self-produced heat was high enough to achieve the observed pressure-temperature conditions recorded by the garnet amphibolite. Therefore, an additional heat supply is not required during early Eocene metamorphism. A breakoff of the leading part of the subducting Indian continental slab, if it occurred, should be younger than ca. 47 Ma.

Low-Ti gabbroic pluton in Dali, SW China: new evidence for back-arc lithospheric melting inducing the early-stage magmatism of the Emeishan large igneous province

2021 ◽  
pp. jgs2020-224
Author(s):  
Bei Zhu ◽  
Zhaojie Guo ◽  
Shaonan Zhang ◽  
Ning Ye ◽  
Ziye Lu ◽  
...  
Keyword(s):  
Early Stage ◽  
Large Igneous Province ◽  
Spinel Peridotite ◽  
Heavy Rare Earth Element ◽  
Content Type ◽  
Igneous Province ◽  
Supplementary Material ◽  
Back Arc ◽  
Emeishan Lip ◽  
Yangtze Plate

The latest studies proved contribution of the Emeishan mantle plume (the widely-regarded origin of the Emeishan LIP in the western Yangtze Plate. LIP: large igneous province) to the Palaeo-Tethys subduction. However, whether the Palaeo-Tethys subduction oppositely affected the formation of the Emeishan LIP remains poorly understood. Here, we report geochronological, petrological, geochemical and isotopic studies of a gabbroic intrusion in this LIP, located in Jiangwei, the Dali area. The gabbro has a weighted mean SHRIMP U-Pb age of ∼262 Ma. Key geochemical features include Nb, Ta and Ti depletion; Th, U and Sr enrichment, low light/heavy rare earth element ratios and ∼0.707 87Sr/86Sr(t) and ∼-0.21 εNd(t) values. We conducted pMELTS thermodynamic modeling and batch melting calculations to evaluate the origin and evolution of the gabbro, based on real components of low-Ti picrites and xenolith of the Yangtze lithosphere. The results support 3% melting of a hydrated spinel peridotite source from the Yangtze lithosphere can produce magma equivalent to the gabbro components. Integrating this conclusion with tectonic background of the western Yangtze Plate and volcano-stratigraphic record of the Emeishan LIP, we infer the early-stage magmatism of the Emeishan LIP was triggered by Paleo-Tethys back-arc extension with fluid modification from subductional slab.Supplementary material:https://doi.org/10.6084/m9.figshare.c.5433267

Utilization of continental transforms in break-up: observations, models, and a potential link to magmatism

2021 ◽  
pp. SP524-2021-119
Author(s):  
E. R. Lundin ◽  
A. G. Doré ◽  
J. Naliboff ◽  
J. Van Wijk
Keyword(s):  
Stress Field ◽  
Numerical Modelling ◽  
Thermal Gradients ◽  
Transform Faults ◽  
Content Type ◽  
Lithospheric Thinning ◽  
Potential Link ◽  
Supplementary Material ◽  
Break Up ◽  
Lower Crustal

AbstractReactivation of continental transform faults (hereafter; transforms) is identified herein as a significant factor in continental break-up, based on a global review of divergent margins and numerical modelling. Divergent margins that have reactivated transforms are characterized by linear and abrupt terminations of thick continental crust. Transforms represent some of the largest structures on Earth, and these megastructures represent major lithospheric weaknesses and are therefore prone to reactivation upon changes in the stress field, which typically occur during plate break-up. The blunt termination of the margins is consistent with observations of very limited pre-breakup lithospheric thinning of such margins. This mode of break-up appears to occur abruptly, and contrasts notably with highly tapered and slowly extended divergent margins. Magma leakage along transforms is well-known worldwide where divergence occurs across such features. This leakage may evolve to dike injections, further reducing the plate strength. We observe that many of the blunt margins we attribute to transform reactivation have been prone to above-normal magmatism and are marked by seaward dipping reflectors underlain by high-velocity lower crustal intrusions. The magmatism may be directly related to the separation of abruptly terminated margins, whereby the large resulting lateral thermal gradients trigger edge-driven convection and melt addition.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5756724

Carboniferous sedimentary provenance and tectonic setting in the Darbut region of Western Junggar (NW China): evidence from mineralogy, geochemistry and detrital zircon U-Pb dating

2021 ◽  
pp. jgs2020-132
Author(s):  
Kai Weng ◽  
Yunpeng Dong ◽  
Xueyi Xu ◽  
Zhongping Ma ◽  
Bo Chen
Keyword(s):  
Detrital Zircon ◽  
Tectonic Setting ◽  
Chemical Compositions ◽  
Content Type ◽  
Clastic Rocks ◽  
Magmatic Rocks ◽  
Central Asian ◽  
Supplementary Material ◽  
Back Arc ◽  
T Values

The Carboniferous tectonic evolution of Western Junggar is crucial to understanding the subduction-accretion process of the Central Asian Orogenic Belt (CAOB), but the nature of this setting is still controversial. In this work, composite mineralogical, geochemical and detrital zircon U-Pb geochronological investigations have been conducted on Carboniferous clastic rocks in the Darbut region. The chemical compositions and sedimentary features show low sediment maturity and limited recycling, suggesting short-distance transportation and rapid accumulation. The samples contain igneous rock debris, mainly andesite and small amounts of basalt and granite, and a heavy mineral assemblage of Zr + Ap + Aug + Hbl + iron-bearing minerals (Hem-Lm, Ilm, Mag, and Py). The samples feature moderate ratios of Zr/Sc (average 15.47) and Th/Sc (average 0.61), and low ratios of La/Sc, Co/Th, and La/Th, as well as low Hf contents, suggesting intermediate to felsic arc-related igneous provenances. Detrital zircon grains from the clastic rocks show prominent age peaks in the Devonian and Carboniferous with positive εHf(t) values, indicating a consistent provenance associated with the Tiechanggou–Halaalate island arc. Combining the petrology, geochemistry and geochronology of the sedimentary and magmatic rocks, we conclude that the Darbut Carboniferous volcanic-sedimentary strata were deposited in a back-arc basin during ∼327-311Ma.Supplementary material:https://doi.org/10.6084/m9.figshare.c.5357293

Tectonic significance of the Variscan suture between Brunovistulia and the Bohemian Massif

2020 ◽  
pp. jgs2020-176
Author(s):  
S. Collett ◽  
P. Štípská ◽  
K. Schulmann ◽  
J. Míková ◽  
A. Kröner
Keyword(s):  
Bohemian Massif ◽  
Inductively Coupled Plasma ◽  
Sensu Stricto ◽  
Geochemical Data ◽  
Magmatic Zircon ◽  
Analytical Methodology ◽  
Content Type ◽  
Inductively Coupled ◽  
Single Zircon ◽  
Supplementary Material

The Velké Vrbno Dome crops-out at the boundary between the Brunovistulian Terrane and the internal parts of the Bohemian Massif. Here, eclogite boudins occur within an Ediacaran volcano-sedimentary sequence. Strong Nb depletion (Nb/Nb* = 0.19 – 0.82) combined with moderately positive Nd isotopic compositions (εNd(i) = +3.89 – +5.77) are used to argue for emplacement of the eclogite protoliths in a transitional supra-subduction to continental-rift setting. Conversely, heterogeneously enriched large ion lithophile elements and highly radiogenic Sr isotopic ratios (87Sr/86Sr = 0.705–0.720) are interpreted to have been modified following fluid infiltration subsequent to eclogite-facies metamorphism.U-Pb laser ablation inductively coupled plasma mass spectrometry dating of magmatic zircon from the rift-type eclogite indicates Early Cambrian emplacement (c.535 Ma) following episodic Ediacaran volcanic arc activity. Moreover, a continental setting is emphasised by zircon dating of a mylonitic orthogneiss, revealing a fragment of Palaeoproterozoic (c.2000 Ma) basement, the first such finding within the Brunovistulian Terrane sensu stricto.The new data from eclogite confirm that rifting in this segment of Gondwana pre-dated the Ordovician opening of the Rheic Ocean and therefore that the suture between Brunovistulia and the rest of the Bohemian Massif likely represents the vestige of an older hyperextended basin or oceanic tract.Supplementary material: Previously unpublished single zircon evaporation ages from Ediacaran orthogneiss from the Velké Vrbno Dome (supplement A); detailed analytical methodology (supplement B); whole rock geochemical data (supplement C); and U-Pb LA-ICP-MS zircon data (supplement d). https://doi.org/10.6084/m9.figshare.c.5233079

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