Low-grade Metasedimentary Rocks Research Articles

Xenoliths of the bode dike system: evidence for early Devonian arc-type magmatism and late Carboniferous–Permian crust reworking beneath the eastern Harz Mountains (Germany)

Xenoliths recovered from the post-Variscan Bode dike system of the eastern Harz Mountains provide evidence for the existence of an Early Devonian magmatic arc system hidden beneath very low-grade metasedimentary rocks of the Rhenohercynian Zone, but also for Late Carboniferous–Early Permian crust reworking. This interpretation is based on petrographic observations and whole-rock geochemical analyses of granite xenoliths, in addition to results of zircon U–Pb dating and Hf isotope analyses. Zircon grains recovered from variably deformed granite xenoliths yield ages between 419 and 393 Ma, interpreted to reflect the timing of granite intrusion. Rare zircon xenocrysts of Archean (ca. 2.92–2.65 Ga) and Proterozoic age (ca. 1.5 to 0.56 Ga), all with subchondritic εHf420 Ma values (− 0.8 to − 5.5) indicate reworking of older crust. Compilation of age-Hf isotope data further suggests that the pre-Variscan granitoids beneath the Harz Mountains belong to the same magmatic arc system exposed widespread in the adjacent Mid-German Crystalline Zone, and interpreted to result from NW-ward subduction of the Rheic Ocean beneath Avalonia-Baltica. Zircon in xenoliths with granophyric texture yields ages at 400 Ma and 295–310 Ma, indicating re-melting of Devonian granitoid basement during post-Variscan rift-related magmatism, immediately prior to Bode dike intrusion.Graphical abstractLeft: Late Devonian intrusion of magmatic arc granites during NW-ward subduction of the Rheic ocean beneath Avalonia-Baltica. Right: Formation of the Bode dike system and Harz granites during Late Carboniferous-Permian extension.

Bhumichula plateau: A remnant high-elevation low-relief surface in the Himalayan thrust belt of western Nepal

The Himalaya is known for dramatically rugged landscapes including the highest mountains in the world. However, there is a limited understanding of the timing of attainment of high elevation and relief formation, especially in the Nepalese Himalaya. Anomalous high-elevation low-relief (HELR) surfaces, which exhibit geomorphic antiquity and are possibly remnants of formerly widespread high-elevation paleosurfaces, provide a unique opportunity to assess the attainment of regional high elevation in the Himalaya. The Bhumichula plateau is one such HELR surface (4300−4800 m) in the western Nepalese Himalayan fold-thrust belt. The Bhumichula plateau is situated in the Dadeldhura klippe (also called the Karnali klippe), an outlier of Greater Himalayan Sequence high-grade metasedimentary/igneous rocks surrounded by structurally underlying Lesser Himalayan Sequence low-grade metasedimentary rocks. We assess the origin of the Bhumichula plateau by combining regional geological relationships and zircon and apatite (U-Th-Sm)/He and apatite fission track thermochronologic ages. The HELR surface truncates pervasive west-southwestward dipping foliations, indicating that it post-dates tilting of rocks in the hanging wall of the Main Central thrust above the Lesser Himalayan duplex. This suggests that the surface originated at high elevation by erosional beveling of thickened, uplifted crust. Exhumation through the ∼180−60 °C thermal window occurred during middle Miocene for samples on the plateau and between middle and late Miocene for rocks along the Tila River, which bounds the north flank of the Bhumichula plateau. Cooling ages along the Tila River are consistent with erosional exhumation generated by early Miocene emplacement of the Main Central (Dadeldhura) thrust sheet, middle Miocene Ramgarh thrust emplacement, and late Miocene growth of the Lesser Himalayan duplex. The most recent middle-late Miocene exhumation took place as the Tila River and its northward flowing tributaries incised upstream, such that the Bhumichula plateau is a remnant of a more extensive HELR paleolandscape. Alpine glaciation lowered relief on the Bhumichula surface, and surface preservation may owe to its relatively durable lithology, gentle structural relief, and elevation range that is above the rainier Lesser Himalaya.

Using monazite U-Pb geochronology of Precambrian shales to constrain fluid flow and hydrocarbon accumulation events of a petroliferous basin

Obtaining precise ages of the Phanerozoic fluid flow events of petroliferous basins is essential for understanding the tectonothermal histories, petroleum- and ore-forming processes. Conventional isotopic dating methods are often compromised by inheritance from detrital constituents and/or subsequent thermal resetting. Monazite is a trace mineral widely present in low-grade metasedimentary rocks, and as an excellent U–Th–Pb geochronometer it has great potential in dating fluid flow activities related to basin evolution. We present results of an in-situ monazite U-Th-Pb geochronology study on shale samples from the Neoproterozoic (651 Ma) Datangpo Formation intersected by a drill-hole in the Sichuan Basin, a typical petroliferous basin in western South China. We recognized two fluid flow events at ca. 450 Ma and ca. 263 Ma, and relate the former to the far-field effect of the Proto-Pacific Ocean subduction and the latter to emplacement of the Emeishan large igneous province. We infer that the ca. 450 Ma fluid flow event induced the first stage of oil charging in the paleo oil pools in the Sichuan Basin, as also recorded by the bitumen samples from the Anyue gas field which yielded re-Os isotopic ages of 458–492 Ma, whereas the ca. 263 Ma event triggered the gas generation of the Paleozoic marine strata in the Sichuan Basin. The results and findings of this study have important implications to reconstructing the complex histories of fluid flow, oil charging and gas generation events of petroliferous basins.

Formation of the Banxi Sb deposit in Eastern Yangtze Block: Evidence from individual fluid inclusion analyses, trace element chemistry, and He-Ar-S isotopes

The Banxi Sb deposit (>100,000 t Sb metal with a grade of 0.02%–64.5% Sb (typically 15.3%–25.9% Sb) in the Eastern Yangtze Block is hosted in Neoproterozoic low-grade metamorphic clastic rocks of the Banxi Group. The deposit is characterized by vein-type Sb-only mineralization. Analyses of individual quartz-hosted fluid inclusions obtained by LA-ICP-MS identified Na as the most abundant cation (thousands to tens of thousands of ppm) in the ore-forming fluids of the Banxi Sb deposit, together with variable contents of K, Ca, Al, As, Fe and Sb (tens to thousands of ppm). Multiple chemical criteria, including high Na+/K+ (typically 3–15) and low Mn2+/Na+ (0.003–0.2) and Sr2+/Na+ (0.001–0.01) ratios of individual fluid inclusions, suggest that the fluids responsible for quartz and stibnite precipitation in the Banxi Sb deposit were likely dominated by basinal brines within low-grade metasedimentary rocks. Moreover, the extremely low 3He/4He ratios (0.001–0.048Ra) but relatively high 3He/36Ar (0.5 × 10−5–5.8 × 10−4) and 40Ar/36Ar ratios (323–1111 with an average of 737) of fluid inclusions trapped in ore-stage quartz further support a sedimentary origin of ore-forming fluids in this deposit. The positive and homogeneous sulfur isotopic values (δ34S = 8.0–9.5‰) of stibnite and arsenopyrite are comparable to those of sulfides in Proterozoic metasedimentary rocks (δ34S = 5.6–11.5‰), indicating these metasedimentary rocks may have served as the primary source of S for the Banxi Sb deposit. Combined with geological features and new trace element chemistry data for quartz and stibnite, we envisaged a new formation model for the Banxi Sb deposit that assumes that circulation of weakly alkaline basinal brines through faults under an extensional tectonic setting during the Early Cretaceous (∼130 Ma), mobilization of Sb and S from the underlying and host metasedimentary rocks and ascent of ore- forming fluids along faults, and ultimately precipitation of Sb minerals in or around fault/fracture zones as a result of the decreasing pH of the ore-forming fluids as well as sulfidation during fluid-rock reaction.

Variscan structures and their control on latest to post-Variscan basin architecture: insights from the westernmost Bohemian Massif and southeastern Germany

Abstract. The Bohemian Massif exposes structures and metamorphic rocks remnant from the Variscan orogeny in central Europe and is bordered by the Franconian Fault System (FFS) to the west. Across the FFS, Variscan units and structures are buried by Permo-Mesozoic sedimentary rocks. We integrate existing DEKORP 2D seismic reflection, well, and surface geological data with the newly acquired FRANKEN 2D seismic survey to investigate the possible westward continuation of Variscan tectonostratigraphic units and structures and their influence on latest to post-Variscan basin development. Subsurface Permo-Mesozoic stratigraphy is obtained from available wells and tied to seismic reflection profiles using a synthetic seismogram calculated from density and velocity logs. Below the sedimentary cover, three main basement units are identified using seismic facies descriptions that are compared with seismic reflection characteristics of exposed Variscan units east of the FFS. Our results show upper Paleozoic low-grade metasedimentary rocks and possible Variscan nappes bounded and transported by Variscan shear zones ca. 65 km west of the FFS. Basement seismic facies in the footwall of the Variscan shear zones are interpreted as Cadomian basement and overlaying Paleozoic sequences. We show that the location of normal fault-bounded latest to post-Variscan late Carboniferous–Permian basins are controlled by the geometry of underlying Variscan shear zones. Some of these late Carboniferous–Permian normal faults reactivated as steep reverse faults during the regional Upper Cretaceous inversion. Our results also highlight that reverse reactivation of normal faults gradually decreases west of the FFS.

Geochemistry of low-grade metasedimentary rocks from the Biga Peninsula, NW Turkey: Implications for provenance and tectonic setting

. Simplified tectonic map of the Eastern Mediterranean region indicating the major geotectonic units and the bounding sutures (modified from .

In situ U-Pb and geochemical evidence for ancient Pb-loss during hydrothermal alteration producing apparent young concordant zircon dates in older tuffs

Zircon U-Pb geochronology is vital for providing age constraints for the calibration of the geologic timescale. The presence of zircon grains that yield concordant ages younger than the host sedimentary rock is often ignored, attributed to young outliers and possible contaminants. Using in situ U-Pb dating techniques, which analyze zircon crystals drilled directly from polished thin sections, two tuff beds from the Brockman Iron Formation, Pilbara Craton, Australia, yielded weighted mean 207Pb/206Pb ages of 2466 ± 6 Ma and 2469 ± 5 Ma (95% confidence level), consistent with independent age constraints. The tuffs also contain zircons that gave younger, yet concordant, U-Pb dates (2450–2270 Ma) as well as some with highly discordant dates. The “young” zircons have low or negligible common Pb, and display magmatic-type euhedral outlines, oscillatory zonation and Th/U ratios, indistinguishable from syn-depositional zircons, and cannot be readily dismissed on the grounds of high discordance or common-Pb contents. The “young” zircon dates are statistical outliers that overlap with the age of hydrothermal xenotime in deformed, bedding-parallel veins in shale that underlies the iron formation, which yields an age of 2383 ± 4 Ma (95% confidence level). The xenotime in overpressure veins provides independent evidence for hydrothermal fluid flow broadly coeval with isotopic resetting of “young” zircon crystals. Although SEM-CL imaging found no evidence for alteration, element mapping shows that most zircon grains have been altered by fluid infiltration, which proceeded along fractures or radiation-damaged growth zones leading to elevated Fe, Ca and Y contents in the altered domains. Total and partial Pb loss caused by hydrothermal alteration can explain the young concordant and discordant ages, respectively. These results have implications for the use of U-Pb zircon geochronology, particularly in detrital zircon studies, which routinely use the age of the youngest zircon grains to constrain the age of deposition. The erosion and re-deposition or in situ growth of young zircons in low-grade metasedimentary rocks may lead to erroneously young depositional ages.

Evolution of Neoproterozoic Shillong Basin, Meghalaya, NE India: implications of supercontinent break-up and amalgamation

AbstractFragmentation and amalgamation of supercontinents play an important role in shaping our planet. The break-up of such a widely studied supercontinent, Rodinia, has been well documented from several parts of India, especially the northwestern and eastern sector. Interestingly, being located very close to the Proterozoic tectonic margin, northeastern India is expected to have had a significant role in Neoproterozoic geodynamics, but this aspect has still not been thoroughly studied. We therefore investigate a poorly studied NE–SW-trending Shillong Basin of Meghalaya from NE India, which preserves the stratigraphic record and structural evolution spanning the Neoproterozoic Era. The low-grade metasedimentary rocks of Shillong Basin unconformably overlie the high-grade Archean–Proterozoic basement and comprise a c. 4000-m-thick platform sedimentary rock succession. In this study, we divide this succession into three formations: lower Tarso, middle Ingsaw and upper Umlapher. A NW–SE-aligned compression event later caused the thrusting of these sedimentary rocks over the basement with a tectonic contact in the western margin, resulting in NE–SW-trending fold belts. The rift-controlled Shillong Basin shows a comparable Neoproterozoic evolution with the equivalent basins of peninsular India and eastern Gondwana. The recorded Neoproterozoic rift tectonics are likely associated with Rodinia’s break-up and continent dispersion, which finally ended with the oblique collision of India with Australia and the intrusion of Cambrian granitoids during the Pan-African Orogeny, contributing to the assembly of Gondwana. This contribution is the first to present a complete litho-structural evolution of the Shillong Basin in relation to regional and global geodynamic settings.

Seismic structure and composition of the southern central Iberian crust: The ALCUDIA wide angle seismic reflection transect

The nature of the crust beneath central Iberia was estimated by a wide-angle seismic reflection/refraction transect, ALCUDIA-WA, which sampled the southern half of the Variscan Central Iberian Zone, covered in the north by the Cenozoic Tajo Basin. The shot gathers recorded by vertical component sensors revealed well defined P- and S-wave phases. These arrivals were modeled by an iterative forward approach providing 2D crustal models showing variations in the velocity distribution with upper crustal P- and S-wave velocities increasing northwards. The lower crust P-wave velocities are homogeneous along the profile while the S-wave velocities slightly increase northwards. The Moho is placed at 32 km depth in the southern edge of the profile, deepening northward down to 35 km beneath the Tajo Basin. The Poisson's ratio, calculated from P- and S-wave velocities, varies along the profile at upper crustal depths. The highest values are located below the Mora and Pedroches batholiths. These resulting physical properties can serve to constrain the crustal composition by comparing them with laboratory measurements on rock samples. Our results suggest that the upper crust in the southern and central segments of the ALCUDIA profile is made up of low-grade metasedimentary rocks, while the northern segment is dominated by igneous rocks, in agreement with the surface geology. Separated by a sharp boundary located between 12 km (south) and 18 km (north) depth, the lower crust is more homogeneous and shows low Poisson’ ratios compatible with a rather felsic composition. However, outstanding lamination described in coincident vertical incidence data indicates some degree of intercalation with mafic components.

Late Devonian transition from advancing to retreating subduction in the SW Central Asian Orogenic Belt: Insights from multiple deformation and magmatic events in the southern Yili Block, NW China

In order to better constrain the Late Paleozoic tectonic evolution of the SW Central Asian Orogenic Belt, we conducted field structural investigations, zircon U-Pb dating, whole-rock geochemical and Sr-Nd isotopic studies on the meta-sedimentary rocks and intruding granites and mafic dykes in the Dahalajunshan area, southern Yili Block of NW China. Our results allow us to recognize five tectonic/magmatic events. Field-scale and microscopic structures of the low-grade meta-sedimentary rocks showed a regional top-to-the-SE thrusting (Event 1). Undeformed granites that contain dioritic enclaves (Event 2) crosscut the regional foliations and thus post-date Event 1 after ca. 379–369 Ma. Mafic dykes intruded all these rocks after ∼328 Ma (Event 4–1), posterior to brittle-ductile thrusting (N-S contraction) (Event 3) and prior to normal faulting (N-S extension) (Event 4–2). All the magmatic rocks formed in a subduction-related setting. The granites and dioritic enclaves derived from a mixing of crustal- and mantle-derived sources variously modified by slab-derived hydrous melts. The primary magma of the diabase dykes derived from a mantle source modified by the slab fluids with 5% crustal contamination. Variation from regional compression (ductile and brittle-ductile thrusting) to extension (normal faulting) and increasing mantle input in arc-related magmatism reveals a transition from Early Paleozoic advancing subduction (events 1 to 3) to Late Paleozoic retreating subduction (events 4–1 and 4–2) of the Junggar oceanic plate beneath the Yili Block.

Contribution of Precambrian basements to the Mesozoic ore-fluid system: An illustration using the Majiayao gold deposit, Jiaodong, China

In contrast to most Mesozoic granite-hosted gold deposits in the giant Jiaodong gold province (China), the Majiayao deposit represents a significant example hosted in the Archean high-grade metamorphic terrane. The mineralization is characterized by auriferous quartz-sulfide veins together with notable Fe-bearing carbonates (ankerite and siderite), iron oxides and barite. In this study, the comprehensive in-situ analyses of vein-hosted hydrothermal minerals (such as: xenotime U-Pb dating, ankerite Rb-Sr isotopes and monazite Sm-Nd isotopes) and sulfur isotopic analyses of gold-bearing sulfides are used to constrain the timing of mineralization and to better understand the ore genesis. The syn-ore hydrothermal xenotime yielded a U-Pb age of 120.0 ± 1.2 Ma marking timing of gold mineralization, which is much younger than wallrocks’ latest metamorphism at 1950 to 1850 Ma. The relatively higher and variable initial 87Sr/86Sr ratios (0.715809–0.718753) of ore-related ankerite may be leached from ore-hosting Archean metamorphic rocks and/or regional Neoproterozoic Penglai Group low-grade metasedimentary rocks via extensive fluid-rock interactions. This inference is also supported by the newly reported near-zero δ34S values (–1.2 to 1.4‰) of pyrite from altered wallrocks, which are significantly comparable to the surrounding Archean plagioclase amphibolite. The monazite has relatively homogenous εNd(t=120Ma) values ranging from –19.3 to –17.6, which are much higher than the Nd isotope compositions of the ore-hosting Archean high-grade metamorphic rocks. Combining the widespread mafic dykes and previous relevant geochemical studies, it is proposed that the ore-forming fluids were probably derived from deeply mantle-related contents, followed by intense interaction with Archean basements at the shallow crustal level. In addition, the progressive decrease of δ34S values of pyrite and more appearances of iron oxides and barite from early to late mineralization stages, are ascribed to gradual fluid oxidation by extensive carbonatization of Fe-rich Archean wallrocks.

Rhabdophane Th-Pb ages indicate reactivation of Mesoarchean structures in west Pilbara Craton during breakup of Greater India and Australia-Antarctica

Abstract Uranium-Th-Pb dating of phosphate minerals in very low-grade metasedimentary rocks from the Archean Pilbara Craton, Western Australia, has revealed a long history of deformation and fluid flow during the Paleoproterozoic. However, this technique has not detected evidence for fluid flow along craton margins during Phanerozoic rifting and breakup. We report the use of in situ Th-Pb geochronology of rhabdophane, a hydrous light rare earth element phosphate, to date fluid flow in shale from the 2.76 Ga Mount Roe Basalt from drill hole number 6 of the Archean Biosphere Drilling Program (ABDP6), northwestern Pilbara Craton. Thorium-Pb dating of rhabdophane in carbonaceous shale yields three main populations with weighted mean 208Pb/232Th ages of 152 ± 6 Ma, 132 ± 4 Ma, and 119 ± 4 Ma, which indicates phosphate growth up to 2.64 b.y. after deposition. The rhabdophane ages are coeval with three major breakup events in eastern Gondwana: separation of Southwest Borneo and Argoland from Australia (ca. 156–152 Ma), breakup of Greater India from Australia (ca. 140–135 Ma), and separation of Greater India/India from Antarctica (ca. 123 Ma). The proximity of drill hole ABDP6 to major Mesoarchean faults and shear zones on the craton margin, which are parallel to rift propagation and basin development, points to episodic reactivation of ancient crustal structures >2.8 b.y. after their formation. Our results also highlight the potential of rhabdophane as a U-Th-Pb geochronometer for dating low-temperature (<200 °C) fluid flow and hydrous alteration. The migration of Mesozoic fluids through Archean shales adds weight to questions about the origin of geochemical signals in ancient altered rocks and how to extract information about the early environment and biosphere.

Paleozoic-early Mesozoic structural evolution of the West Gondwana accretionary margin in southern Patagonia, Argentina

The Upper Devonian-Upper Triassic low-grade metasedimentary rocks exposed along the South Patagonian Andes preserve the structural record of deformation associated with pre-Andean orogenic phases that predated the inception of the Jurassic rift during Gondwana fragmentation. Through the structural analysis at different scales, we identified deformation structures mainly with cratonward vergence, produced at two stages of structure development for the Bahía de la Lancha (early Carboniferous) and Río Lácteo (Middle Devonian-early Carboniferous) formations, and in a single one, for the Nunatak Viedma Unit (Late Triassic). The structural features were integrated with available U–Pb detrital zircon ages and fission-track cooling ages from those units. It leads us to identify a westward younging of sedimentation (i.e. protolith deposition) and widening of the continental margin throughout Paleozoic-early Mesozoic times, tectonically stabilized during the late Carboniferous-middle Permian Gondwanide orogeny and the Late Triassic-Early Jurassic Chonide orogeny. The structural style holds features of fold-and-thrust belts, which is discussed in a context of accretionary tectonics inherent to the Terra Australis exterior orogen.

Application of Electromagnetic and Electrical Resistivity Methods for Borehole Siting in Low-Grade Metavolcanic and Metasedimentary Rocks, Sunyani West, Ghana

This paper employs the electromagnetic (EM) and the electrical resistivity methods for siting boreholes for communities in the Sunyani-West district of Ghana, a terrane comprising a mixture of granites and low-grade metavolcanic and metasedimentary rocks. These communities which depend on groundwater for livelihood have over the past few decades been hit by drastic seasonal irregularities in rainfall, making it imperative to seek an alternative source of potable water (preferably groundwater). Measurements to locate sites for boreholes were made by employing the Geonics EM-34 conductivity meter. In addition, co-located measurements were made with the ABEM SAS 1000C Terrameter in the Schlumberger electrode array configuration. Results of the measurements showed that each community could have at least two borehole sites with a high potential for groundwater extraction. The overburden of selected sites varied for the communities and ranged from 40 to 67 m. However, recommended drilling depths at these sites were 60 m for Adwinsa community, 70 m for Akurakan community and 60 m for Nana Ameyaw community. Test boreholes sunk at respective depths of 45 m, 55 m and 68 m in Adwinsa, Nana Ameyaw and Akurakan had yields of 12 m 3 /h (200 l/min), 24 m 3 /h (400 l/min) and 1.14 m 3 /h (19 l/min) respectively. In all three communities the aquifer was intercepted in phyllite bedrocks. Keywords: aquifer, groundwater potential, borehole, electrical resistivity, vertical electrical sounding, conductivity DOI: 10.7176/JEES/10-10-05 Publication date: October 31 st 2020

Role of site effect for the evaluation of attenuation characteristics of P, S and coda waves in Kinnaur region, NW Himalaya

The site effect and attenuation studies are carried out for Kinnaur region of northwest Himalaya, India. A total of 109 local events happened in Kinnaur region of magnitude range 1.6–4.5, are utilized for present work. The earthquake records are influenced by the site effect depending on soft sediment thickness beneath the recording sites. Therefore, in the present study, records are corrected for site effects to estimate P (Qp), S (Qs) and coda (Qc) wave quality factor. The regional frequency dependent attenuation relations, i.e., $$ Q_{p} \left( f \right) = \left( {29 \pm 1} \right)f^{(1.01 \pm 0.05)} $$ , $$ Q_{s} \left( f \right) = \left( {38 \pm 5} \right)f^{(1.1 \pm 0.06)} $$ and $$ Q_{c} \left( f \right) = \left( {74 \pm 11} \right)f^{(1.17 \pm 0.01)} $$ are established for the Kinnaur region. The Kinnaur Himalaya mainly belongs to Higher Himalaya Crystalline (HHC) and Tethys Himalaya, where these two geological units are differentiated by the South Tibetan Detachment System (STDS). The resonance frequencies and attenuation characteristics are estimated for both regions, i.e., HHC and Tethys Himalaya. A comparison is made between HHC and Tethys Himalaya in the form of resonance frequencies and attenuation properties. The low value resonance frequency and high rate of attenuation towards the northern side of STDS, i.e., Tethys Himalaya support the presence of low-grade metasedimentary rocks. It suggests that Tethys Himalaya has high seismic hazard potential zone compared to HHC.

Triassic–Jurassic evolution of the eastern North China Craton: Insights from the Lushun-Dalian area, South Liaodong Peninsula, NE China

AbstractThe Lushun-Dalian area of the South Liaodong Peninsula, in NE China, located in the SE margin of the North China Craton (NCC) exposes a suite of Middle-Late Proterozoic low-grade metamorphic sedimentary rocks which can be divided into a lower competent layer, a middle incompetent layer, and an upper competent layer on the basis of lithology and deformation style. Two stages of deformation recorded both in the metasedimentary rocks and a magmatic complex intruded in them indicate that the Lushun-Dalian area is a key region to decipher the Triassic–Jurassic tectonic evolution of the eastern NCC. The earliest D1 deformation mylonitized the magmatic complex and thrusted it northeastward over the low-grade metasedimentary rocks, in which a series of NE-verging folds and NE-directed brittle thrust faults developed. The D2 deformation erased the D1 fabrics in the incompetent layer by a top-to-the-NW ductile shearing and refolded the D1 fabrics in the lower and upper competent units, producing a series of km-scale SW-plunging folds. New zircon secondary ion mass spectrometry and laser ablation–inductively coupled plasma–mass spectrometry U-Pb ages from the magmatic complex and the granite porphyry dikes intruded in it, combined with the unconformity between the low-grade metasedimentary rocks and the Early Cretaceous volcanic rocks, indicate that D1 and D2 occurred after 211 Ma and before the Early Cretaceous. The decrease of the deformation intensity of D1 and D2 from the Lushun-Dalian area toward the interior of the NCC in the NE and NW directions suggests that D1 was the structural response in the overriding plate to the NCC-South China Block convergence during the Late Triassic to Early Jurassic, and D2 was the structural response to the northwestward subduction of the Paleo–Pacific plate beneath the NCC in the Middle-Late Jurassic. The superimposition of D2 on D1 recorded a significant tectonic transformation from the nearly E-W–trending Tethysian domain to the NE-SW–trending Pacific domain.

Insights into post-orogenic extension and opening of the Palaeo-Tethys Ocean recorded by an Early Devonian core complex in South China

The Palaeo-Tethys Ocean formed during the Devonian as a consequence of blocks separating from northern Gondwana. The record of the Palaeo-Tethys Ocean stretches across Eurasia from the European Alps eastward to the southwestern South China block (SCB). Tectonic domes of the SCB record the contraction, extension, decompression, and partial melting of deep crust that occurred as a result of detachment faulting and tectonic evolution of the Palaeo-Tethys Ocean. This study investigates post-orogenic (Kwangsian) extension and opening of the Palaeo-Tethys Ocean as recorded by the Motianling core complex in the southwestern SCB, based on structural analysis and 40Ar/39Ar geochronology. The Motianling core complex in the southernmost part of the Jiangnan fold belt comprises the Sanfang gneissic granite surrounded by low-grade metasedimentary rocks. The core complex records at least three deformation (D) events: pre-D1, D1, and D2. The pre-D1 event produced large-scale NE-striking folds with axial surface cleavages and was coeval with emplacement of the Sanfang batholith. D1 deformation is represented by a top-to-the-NW ductile–brittle normal faulting system with mylonitic foliation and was coincident with greenschist-facies metamorphism, indicating that the core complex formed during NW–SE extension. D2 deformation is dominated by Cenozoic NW–SE-trending normal faults. The timing of D1 deformation and subsequent cooling was constrained by 40Ar/39Ar geochronology. Syn-kinematic muscovite yields an 40Ar/39Ar age of 417 ± 2.3 Ma (1σ) and suggests that the dome was initially developed in the Early Devonian through ductile shear zones. Magmatic biotite from the granitic mylonite yields a weighted mean 40Ar/39Ar age of 363.0 ± 3.1 Ma. Therefore, the Motianling core complex records at least two major stages of tectonic evolution: (1) Kwangsian (Caledonian) orogenic collapse and subsequent first-stage cooling at ∼417 to 363 Ma; and (2) Cenozoic regional extension and final cooling at ca. 45 Ma, as inferred from published apatite fission track data. The present results, combined with previous studies, indicate that the D1 deformation event resulted in the formation and exhumation of the Motianling core complex in a continental rifted margin during the opening of the Palaeo-Tethys Ocean.

Twenty million years of post-orogenic fluid production and hydrothermal mineralization across the external Araçuaí orogen and adjacent São Francisco craton, SE Brazil

The Araçuaí orogen (AO) is a hot orogenic system that is known for its widespread and long-lived magmatism (630–480 Ma). Within the last c. 20 million years of its evolution, the AO saw the widespread development of hydrothermal systems in its lower-grade external fold-and-thrust belt and its Foreland Domain (eastern margin of the adjoining São Francisco craton). UPb crystallization ages for hydrothermal monazite, rutile, and xenotime indicate that fluid circulation along the entire AO occurred between 515 to ca. 495 Ma, largely overlapping with UPb ages of hydrothermal systems and mineralized zones in the adjacent São Francisco craton. Titanium-in-quartz thermometry suggests that most hydrothermal systems closed at temperatures of ca. 360 °C. The SmNd isotope composition of the vein monazite samples ranges from εNd500Ma −16.8 to −17.8, and one less-evolved sample with εNd500Ma of −5.9. The SmNd isotope compositions overlap with the bulk rock SmNd data for the Espinhaço Supergroup, suggesting that the hydrothermal veins in this study are mainly related to metamorphic fluids derived from the prograde dehydration of low-grade metasedimentary rocks. The UPb ages, SmNd isotope composition, and vein mineralogy are all consistent with a 20 My period of channelized fluid flow, characterized by an oxidizing, low pH, REE-, P-, and Ti-bearing aqueous metamorphic fluid. The expression of supercritical fluid flow in the core of the AO is given by the emplacement of large volumes of bare/mineralized pegmatite bodies and regional resetting of monazite ages in host rocks. On a regional context, this widespread fluid flow is associated with a high-T, low-P metamorphism that marks the beginning of the post-collisional decompression of the orogeny, which is associated with lithospheric mantle delamination. The reorganization of mass and heat, due to extensional collapse, resulted in multi-sourced fluid production and mineralization from deep to shallow crustal levels that affected an area of over 400.000 km2 along the eastern border of the São Francisco Craton.

Cryogenian glaciomarine megaclasts of the MacDonald Corridor, Bimbowrie Conservation Park, Olary Region, South Australia

Ten glacially related megaclasts, unique perhaps from a global perspective, have been identified in the MacDonald Corridor. Included are perhaps the largest known glacially-related basement-derived megaclasts, the Bimbowrie Megaclast measuring 1.25 km in exposed width. The MacDonald Corridor is the most outward of four half-grabens to the east of the main Adelaide Geosyncline rift complex. All four contain Neoproterozoic low-grade metasedimentary rocks situated between inliers of Paleoproterozoic–Mesoproterozoic basement at the southwestern margin of the Curnamona Province. These corridors originated as extensional structures during the last of three major episodes of rifting that led to the eventual break-up of the supercontinent Rodinia. This rift phase was synchronous with the older (Sturtian-age) of the two major Cryogenian glaciations that are recorded in the rift and sag-basin complex of the Adelaide Geosyncline in South Australia. Those megaclasts selected for description vary in size from 0.1 to 1.25 km in exposed width, and were derived from the eastern rift flank, the Kalabity Inlier They therefore comprise either Paleoproterozoic metamorphic or Mesoproterozoic rocks or a composite of both. The megaclasts fall into two groups, those contained within the sedimentary fill of the rift basin, the Bimbowrie and Adlaweena megaclasts being the largest, and those resting as erratics on the flank. Of these, the 450 m wide Old Boolcoomata Megaclast resides within the remains of a steeply dipping U-shaped glacial valley. Tectonic activity and ice action are considered to have been responsible for creating the high-relief topography of the Kalabity Inlier. Ice was also instrumental in separating the megaclasts from their source and transporting them into the tectonically active MacDonald Corridor half-graben. Additionally, the variation of floating ice-cover controlled iron deposition within the glacigene Benda Siltstone. The present morphology of the MacDonald Corridor is largely due to the Cambrian Delamerian Orogeny, which while causing some structural complexity, exhumed and tilted the infill of the corridor to view.

Geological significance of the former Xiong’er Volcanic Belt on the southwestern margin of the North China Craton

The rock association of low-grade metasedimentary rocks and greenschists located within the Meso-Cenozoic Liupanshan Fault system on the southwestern margin of the North China Craton (NCC) is regarded as part of the Paleoproterozoic Xiong’er Group. These lowgrade rocks are separated by normal faults, with the greenschist located in the hanging walls. Zircon LA–ICP–MS U–Pb ages of the greenschists range from 2455 to 423 Ma, suggesting that they are not Paleoproterozoic in age. The protolith ages (206–194 Ma) of the greenschists were determined by LA–ICP–MS U–Pb dating of zircons from two siltstone interlayers. The petrology and geochemistry of the greenschists reveal that their protolith was continental tholeiitic basalt that formed in an extensional environment such as a continental rift. Thus, it is proposed that the protolith of the greenschists was a mafic volcanic rock of Late Triassic–Early Jurassic age and was metamorphosed during the Jurassic due to tectonism within the Liupanshan tectonic belt. These results show that the greenschists should be reclassified and removed from the Xiong’er Group, and explains why they differ so much from those of typical Xiong’er Group successions in other areas. The formation of the mafic volcanic rocks under conditions of continental rifting differs from that of coeval granitic rocks in the western Qinling Orogen, where the extension occurred during a post-collisional stage in the Late Triassic, which further suggests that the southwestern margin of the NCC became an extensional setting after the Late Triassic.