Diabase Samples Research Articles

Energy Dissipation During Shear Along Experimental Rough Faults

AbstractThe energy budget and the interplay between stable friction evolution and dynamic stick‐slips are tested here under continuous slip along rough faults. We conducted 34 direct‐shear experiments coupled with precise roughness measurements on diabase and limestone fault samples. The faults broad roughness ranges from highly rough and interlocked fractured interfaces to smooth polished surfaces. The analysis focuses on two slip phases: (a) the evolution of the shear strength of rough sample under stable, cumulative displacement; and (b) the dynamic of unstable stick‐slip sliding. We found that the breakdown work during frictional strength evolution increases with roughness increase across multiple scales. The diabase samples are more sensitive to roughness increase than limestone samples in terms of the breakdown work implied by frictional evolution. We attribute this increased diabase sensitivity with fault roughness to its higher bulk elasticity and not to the fault shear stiffness. The diabase faults displayed multiple periodic system‐size stick‐slips, and the measured stored energy during the preparatory stage were surprisingly independent of the fault roughness. This finding suggests that during the preparatory stage a balance between the intracycle fault stiffness and stress drop govern the stored energy magnitude. Further, this energy balance suggests that some interface conditioning occurs before the spontaneous slip overcomes a sticking barrier.

Formation and hydrothermal alteration of a volcanic center: Melt pooling and mass transfers at Langseth Ridge (Gakkel Ridge, Arctic Ocean)

Volcanic centers are characteristic features of ultraslow-spreading mid-ocean ridges, the least-explored parts of the global ridge system. Volcanic centers can provide insights into deep magmatic and metamorphic processes at these ridges. Here, we present data from the largest volcanic center on the Gakkel Ridge, the Langseth Ridge, situated at 60–62°E. Langseth is ∼10 km wide, consisting of three peaks that rise to 585 m water depth, some 3–4 km above the surrounding seafloor. It strikes perpendicular to Gakkel's spreading direction and can be traced for ∼40 km, which translates to an age of ∼8 Myr. Seafloor imaging revealed abundant (pillow) basalt but also fissures and geologic faults across the Langseth Ridge. Basaltic rocks were sampled at all summits and diabase at the slope of the northern summit that dips into the rift valley.Our samples are of normal to depleted mid-ocean ridge basalt composition and exhibit a wide range of major and trace element contents, due to magmatic processes, accumulation of macrocrysts, and hydrothermal alteration. Radiogenic isotope ratios, most notably 143Nd/144Nd and 208Pb/206Pb, trend from typical rift valley compositions to isotopically enriched values with increasing distance to the rift valley. This trend may imply melt pooling from different sources, potentially representing a shift from shallow melting beneath the rift valley to deeper melting of enriched sources and higher degrees of melting underneath Langseth. Mineral compositions and plagioclase sieve textures imply prolonged storage of magma at depth prior to eruption. Hydrothermal alteration occurred over a range of conditions. Basalt from the summits is weakly altered at temperatures ≪100 °C, which likely occurred in situ at the summit sites. Diabase samples experienced chloritization and albitization and display epidote and quartz veins, which formed at >300 °C. These assemblages and temperatures are typical for lower crustal levels and imply uplift of the samples of >1 km. Diabase samples from the Afanasenkov Seamount, another volcanic center on the Gakkel Ridge that we investigated for comparison, were altered under comparable conditions.Our findings suggest a combined volcanic–tectonic origin of the studied volcanic centers, potentially implying that such complexes may generally form due to the interplay of magmatism and tectonics. Researching volcanic centers has the potential to further our understanding of both deep and shallow crustal processes at ultraslow-spreading ridges, providing further insights into the role of these centers as linkages between lithosphere and hydrosphere and the (deep) biosphere they sustain.

Southward continuation and slab rollback of the Neotethyan arc−back-arc system: Insights from Eocene mafic intrusions from North Sumatra, SE Asia

The Neotethyan subduction and collision with the Asian, Indian, and Australian plates is evidenced by a giant Andean-type arc system and a remarkable Neotethyan igneous belt along the southwestern Eurasian margin. However, the southward extent, and cause, of Neotethyan subduction is poorly understood. Here, a zircon U-Pb dating and whole-rock geochemical study of two groups of Eocene (50−48 Ma) mafic intrusions from Tangse, North Sumatra, helps to unravel the Neotethyan subduction story. Group 1 consists of diabase samples, which have mid-oceanic-ridge− and arc-like geochemical affinities, with low chondrite-normalized La/Yb(cn) (∼0.6) ratios and a high measured εNd(t) value of +8.7. Such geochemical signatures indicate that the magmatic rocks were derived from a refractory depleted mantle source that was metasomatized by slab melts. Group 2 consists of gabbro-diorites, which are enriched in the large-ion lithophile elements and depleted in some high field strength elements. These samples have high La/Yb(cn) (2.9−6.6), low Nb/La(cn) (0.2−0.3), εNd(t) values of +1.1 to +5.0, and εHf(t) values of +8.8 to +15.3. Group 2 samples were produced by the melting of a depleted mantle wedge metasomatized by slab fluids. The magmatism provides evidence for the southern continuation of the Neotethyan igneous belt and tectonic system, whereby slab rollback triggered melting and upwelling in an arc−back-arc geodynamic system from South Tibet to SE Asia.

The Effect of the Microstructure of Diabases from Greece and Cyprus on Their Engineering Characteristics and the Mechanical Behaviour of Concrete

This article presents, for the first time, the study of diabases from various ophiolite complexes from Greece and Cyprus to identify their performance as concrete aggregates. Within this scope, in the first step, diabase samples from four selected ophiolite complexes of Greece (Veria-Naousa, Edessa and Guevgueli) and Cyprus (Troodos) were collected in order to estimate their suitability as concrete aggregates by means of their petrographic and physicochemical characteristics. In the next step, concrete specimens were prepared and their mechanical strength was measured in order to investigate their mechanical performance. Additionally, their petrographic characteristics in relation to the mechanical strength of the created concretes were investigated for the first time. Concrete specimens prepared by employing diabases from the Veria-Naousa and Guevgueli complexes as aggregates were reckoned as the most durable ones in contrast to those derived from the Edesssa complex and even more so than those from Cyprus, with the latter containing the most altered diabases (rodingitised). The overall conclusion of this research is that the engineering properties of the aggregates were dependent on their petrographic characteristics and hence they influenced the final mechanical behaviour of each produced concrete.

Nonlinear Soliton-like Oscillations and Waves during Geomaterial Destruction Based on Electromagnetic Radiation Signals

The work is devoted to the results of processing electromagnetic radiation signals obtained during laboratory loading of marble and diabase samples using a technique for determining the parameters of microcracks, developed and published by the author earlier. As a result of such processing, certain patterns were found in the nature of the evolution of the oscillatory process ensemble of microcracks. For example, solitary non-linear waves almost always preceded a sequence of High Frequency traces. Equations for straight lines approximating High Frequency traces in logarithmic coordinates, close to the equation of the Gutenberg–Richter law. Due to the similarity of seismic processes at different scale levels, the results of modeling at the microscale level can be used to describe seismic processes at the macroscale level, for example, to study the processes occurring immediately before destruction and at the time of destruction in order to search for repeatability and regularities. The regularities obtained can be used in the development of a predictive criterion that makes it possible to predict the time of one or another geophysical (seismic) event.

Mechanical activation of diabase and its effect on the properties and microstructure of Portland cement

Many volcanic rocks with pozzolanic properties are used as additives in cement production. Although diabase is a volcanic rock, it has not been used as an additive material in cement production until now. In this study, pozzolanic reaction ability, mechanical, hydration and microstructure properties were investigated before and after mechanical activation in order to increase the usability of diabase in cement production. Firstly diabase samples were analyzed for chemical, mineralogical compositions and pozzolanic activity. The diabase was first subjected to dry grinding separately from clinker to increase the amorphous structure by decreasing its crystalline structure and mechanical activation was achieved.The pozzolanic activity of the ground diabase gradually increased depending on the grinding time. The effects of 10%, 30% and 50% addition of natural diabase by weight on the characteristics of cement pastes and cement mortars were researched. Zeolite containing cliptonolite mineral was used as a comparison material with diabase. The Blended cement specimens were tested for particle size, setting time, water demand, soundness, heat of hydration, and compressive strength. The compressive strength proportion of the cement mortar samples containing large quantities of natural diabase were lower than that of the Portland control cement at all tested ages up to 28 days, but it was a higher compressive strength than zeolite blended cements. 10%, 30% and 50% diabase blended cements and 10%, 30% and 50% zeolite blended cements give results above the Cem IV 32.5 R standards. 10%, 30% and 50% diabase blended cements have achieved high early compressive strength than zeolite blended cements. 10%, 30% and 50% diabase blended cements have achieved low final compressive strength than zeolite blended cements. In addition, diabase blended cements may produced at approximately 5% lower cost than zeolite blended cements. All results proved that diabase may be a good alternative to new generation mineral additives in producing cement.

Geochronology, petrogenesis and tectonic significance of two episodes of Neoproterozoic diabasic magmatism in South China: from orogenesis to intracontinental rifting

ABSTRACT Neoproterozoic mafic magmatic rocks are widely distributed in the Jiangnan Orogenic Belt. Understanding the petrogenesis and tectonic settings of these mafic rocks has significant implications for the regional tectonic evolution and the assembly and breakup of supercontinent Rodinia. In this research, two Neoproterozoic diabase samples (Baotan and Huaihua) were investigated by zircon and baddeleyite U–Pb dating with the ages of 805 Ma and 788 Ma, respectively. The Baotan samples are depleted in HFSE, enriched in LILE and have enriched Sr–Nd isotope compositions (87Sr/86Sr(i) = 0.731620 to 0.745440, εNd(t) = – 3.7 to – 7.6). We suggest that they were derived from an enriched lithospheric mantle in a convergent tectonic setting, which was metasomatized by contemporaneous slab–derived hydrous fluids. On the contrary, the Huaihua diabase samples have trace element patterns similar to oceanic island basalts but little HFSE depletion and slightly enriched Sr–Nd isotopes (87Sr/86Sr(i) = 0.703101 to 0.705006, εNd(t) = – 0.1 to – 1.6). We infer that they originated from partial melting of a convective asthenospheric mantle under an extensional environment, which was previously metasomatized by slab–derived melts. In combination with other geological observations, we proposed a geodynamic evolution model for the South China Block from the mid–Neoproterozoic orogenesis to the late Neoproterozoic intracontinental rifting.

Two episodes of Mesozoic mafic magmatism in the Nansha Block: Tectonic transition from continental arc to back-arc basin

It has been proposed that the proto-South China Sea (PSCS) existed in the southeastern continental margin of the South China Block (SCB) before the opening of the present South China Sea (SCS). However, the age of initial spreading and the geodynamics of the opening of the PSCS remain enigmatic. Here we present geochronological and geochemical data for diorite and diabase samples from Well NK-1, which was drilled into the Nansha Block in the Dangerous Grounds area of the southern SCS, to investigate their petrogenesis and their association with the formation of the PSCS. The Early Jurassic diorites (ca. 177 Ma) are geochemically similar to continental arc basalt, with enrichment in Rb, Ba, Pb, and Sr, and pronounced negative Nb, Ta, and Ti anomalies. They were originated from a mantle source metasomatized by subducted-sediment-derived melts in a continental arc setting related to subduction of the Paleo-Pacific plate beneath the SCB. The Early Cretaceous diabases (ca. 124 Ma) are compositionally similar to back-arc basin basalt, with enrichment in Rb, Ba, and Pb, and negligible Nb, Ta, Ti, Zr, and Hf anomalies. The mantle source of the diabases was metasomatized by aqueous fluids derived from the subducted slab in a back-arc basin associated with subduction of the Paleo-Pacific plate. Furthermore, the diabase samples are geochemically similar to mafic rocks and ophiolite complexes previously regarded as remnants of the PSCS oceanic crust. Therefore, the Early Cretaceous diabases in the Nansha Block are interpreted as representing coeval magmatism of the nascent ocean of the PSCS, indicating that the PSCS during the Early Cretaceous was a spreading back-arc basin associated with retreat of the subducting Paleo-Pacific plate.

Thermal history of the East European Platform margin in Poland based on apatite and zircon low-temperature thermochronology

Abstract. The Phanerozoic tectonothermal evolution of the SW slope of the East European Platform (EEP) in Poland is reconstructed by means of thermal maturity, low-temperature thermochronometry, and thermal modelling. We provide a set of new thermochronometric data and integrate stratigraphic and thermal maturity information to constrain the burial and thermal history of sediments. Apatite fission track (AFT) analysis and zircon (U-Th)/He (ZHe) thermochronology have been carried out on samples of sandstones, bentonites, diabase, and crystalline basement rocks collected from 17 boreholes located in central and NE Poland. They penetrated sedimentary cover of the EEP subdivided from the north to south into the Baltic, Podlasie, and Lublin basins. The average ZHe ages from Proterozoic basement rocks as well as Ordovician to Silurian bentonites and Cambrian to lower Carboniferous sandstones range from 848 ± 81 to 255 ± 22 Ma with a single early Permian age of 288 Ma, corresponding to cooling after a thermal event. The remaining ZHe ages represent partial reset or source ages. The AFT ages of samples are dispersed in the range of 235.8 ± 17.3 Ma (Middle Triassic) to 42.1 ± 11.1 Ma (Paleogene) providing a record of Mesozoic and Cenozoic cooling. The highest frequency of the AFT ages is in the Jurassic and Early Cretaceous prior to Alpine basin inversion. Thermal maturity results are consistent with the SW-ward increase of the Paleozoic and Mesozoic sediments thickness. An important break in a thermal maturity profile exists across the base Permian–Mesozoic unconformity. Thermal modelling showed that significant heating of Ediacaran to Carboniferous sedimentary successions occurred before the Permian with maximum paleotemperatures in the earliest and latest Carboniferous for Baltic–Podlasie and Lublin basins, respectively. The results obtained suggest an important role of early Carboniferous uplift and exhumation at the SW margin of the EEP. The SW slope of the latter was afterward overridden in the Lublin Basin by the Variscan orogenic wedge. Its tectonic loading interrupted Carboniferous uplift and caused resumption of sedimentation in the late Viséan. Consequently, a thermal history of the Lublin Basin is different from that in the Podlasie and Baltic basins but similar to other sections of the Variscan foreland, characterized by maximum burial at the end of Carboniferous. The Mesozoic thermal history was characterized by gradual cooling from peak temperatures at the transition from Triassic to Jurassic due to decreasing heat flow. Burial caused maximum paleotemperatures in the SW part of the study area, where the EEP was covered by an extensive sedimentary pile. However, further NE, due to low temperatures caused by shallow burial, the impact of fluids can be detected by vitrinite reflectance, illite/smectite, and thermochronological data. Our new results emphasize the importance of using multiple low-temperature thermochronometers and thermal modelling in connection with thermal maturity analysis to elucidate the near-surface evolution of platform margins.

Multiphase ophiolite formation in the Northern Altyn Tagh Orogen, southeastern Tarim

Early Paleozoic ophiolitic mélanges in the Altyn Tagh Orogen, southeastern Tarim, yield a large range of formation ages and geochemical affinities. This study focused on the Hongliugou ophiolitic mélange in the North Altyn Tagh subduction-accretion belt and involved mineral chemical, zircon geochronological, and whole-rock elemental and isotopic investigations of the ultramafic and mafic rocks. In the studied lherzolite samples, Cr-spinel, olivine, and pyroxene show mineral chemistry akin to that of abyssal peridotite. Subhedral-anhedral Cr-spinel grains with high Cr# values (100*Cr/[Cr+Al] of 30–40) and negative oxygen fugacity (fO₂) values represent original spinel formed during mid-ocean ridge basalt (MORB) melt extraction. Other subhedral-anhedral and euhedral Cr-spinel with low Cr# values of &lt; 30 to 20 display a wide range of negative to positive fO₂ values, indicating interaction of peridotite with MORB magma. New zircon U-Pb dating results record ages of <i>ca.</i> 490 Ma for gabbro and <i>ca.</i> 486 Ma for diabase in the Hongliugou ophiolitic mélange. Whole-rock geochemical compositions suggest that the gabbro and diabase samples are tholeiitic and show MORB affinities or transitional affinities between MORB and island arc tholeiite. These mafic rocks might have been derived from a depleted MORB-source mantle that was variably metasomatized by subduction-induced fluids based on their variable Th/Yb, high La/Nb, low Th/Nb, and depleted isotopes (initial ⁸⁷Sr/⁸⁶Sr ratios of 0.703073 to 0.703385 and <i>ε</i>_Nd(<i>t</i>) values of +6.2 to +6.3). New results, when integrated with previous work, clarify that <i>ca.</i> 520 to 510 Ma ophiolites formed in the initial subduction setting and <i>ca.</i> 490 to 480 Ma ophiolites in a back-arc setting. Minor <i>ca.</i> 450 Ma ophiolite probably represents a late phase of ophiolite formation due to the presence of a very minor remnant ocean.

New geochronological and geochemical data of the Longsheng mafic-ultramafic suite in northern Guangxi, China, and their implications in Rodinia breakup

The Longsheng mafic-ultramafic suite consists of more than 100 mafic and ultramafic intrusions and are extensively distributed within the Neoproterozoic Danzhou Group along the Jiangnan Orogenic Belt in northern Guangxi, China. The suite has been a hotly debated subject in the context of emplacement age, source of magmas, and tectonic setting for several decades. In this article, we present newly obtained LA-ICP-MS zircon U-Pb age data, whole-rock major and trace elemental data, and Sr-Nd and zircon Lu-Hf isotopic data. These data support a middle Neoproterozoic intrusive emplacement mechanism as a result of asthenospheric underplating and subsequent lithospheric thinning, extension, and rifting. This integrated process might well correspond to the early stage of the Rodinia breakup. For example, five mafic-ultramafic rock samples yielded an emplacement age of 774 ± 2 Ma. The samples contain a large number of inherited zircon xenocrysts dated at 821 ± 3 Ma, 1.4–2.1 Ga, and 2.3–2.8 Ga, indicative of crustal contamination. The mafic-ultramafic rocks are characterized by calc-alkaline to shoshonitic associations; their Zr/Nb, Th/Yb, La/Nb, Ba/La ratios, trace, and REE patterns show an OIB affinity. Their initial 87Sr/86Sr values range from 0.697721 to 0.708579 with an average of 0.702352. They have relatively low initial 143Nd/144Nd values of 0.511530–0.511859 with an average of 0.511661. Sixteen magmatic zircon grains from diabase samples show 176Hf/177Hf values of 0.281326–0.282638, eHf(t) values of − 34.48–11.89, and TDM2 of 925–3819 Ma. Their Nb/U and Th/Ta ratios support crustal contamination as well. It is concluded that the Longsheng mafic-ultramafic suite was originated from partial melting of the enriched upper mantle (EM-I type) due to asthenospheric underplating and lithospheric extension during the incipient Rodinia breakup, and that the magmas were contaminated with crustal material during ascent.

Experimental and numerical investigations of microwave-induced damage and fracture formation in rock

Microwave technology is increasingly used in laboratory tests and field applications as an effective rock breaking technique. The underlying mechanism of rock fracturing induced by microwave energy, however, has not been well addressed. In this study, we employ experimental and numerical methods to investigate the global and local damage of rock in microwave radiation direction, including the fracture formation process. Our analysis focuses on three damage indicators under microwave irradiation, namely P-wave velocity attenuation, temperature distribution and fracture pattern. For a diabase sample, our experimental results indicate that the microwave power and irradiation time has a substantial influence on these three indicators. With increasing power level and irradiation time, we measured a lower P-wave velocity, higher temperature, and more fractures within the rock substrate. We observed that under the same energy, heating at higher power levels for shorter durations has a better weakening effect. Our numerical results show that thermal stress mismatch in the local high-temperature area is the main reason for crack initialization. Nucleation and propagation of microcracks depend on the thermal stress induced by global temperature increase, the geometry of the sample and existing fractures.

The 1.14 Ga mafic intrusions in the SW Yangtze Block, South China: Records of late Mesoproterozoic intraplate magmatism

Abstract The late Mesoproterozoic geodynamic setting of the Yangtze Block, South China remains largely unknown due to the scarce geological records of this age. This study presents a new dataset consisting of petrology, whole-rock geochemistry, zircon U-Pb geochronology and Hf isotopes of newly identified late Mesoproterozoic diabases in the southwestern Yangtze Block. Two diabase samples give zircon U-Pb ages of 1138 ± 4 Ma and 1141 ± 4 Ma. The diabases are tholeiitic in composition and have consistent high TiO2 contents (2.18–2.93 wt%) but low Mg# values (41–47). They are slightly enriched in LREEs with (La/Yb)N ratios of 2.87–3.99 and show E-MORB-like trace element patterns with positive Nb-Ta anomalies. The diabase samples exhibit positive zircon eHf(t) values from + 9.4 to + 13.7 and whole-rock eNd(t) values from + 3.4 to + 4.0. These results suggest that they may have undergone negligible crustal contamination during the magma evolution and were most likely generated from partial melting of a depleted asthenosphere mantle source. The diabases possess consistent Nb/Yb (4.87–5.91) and Th/Yb (0.031–0.59) ratios, ruling out the possibility of interaction with subduction-related materials. They have high Ti/V (38–46) and Zr/Y (3.36–5.23) ratios, typical of within-plate basalts. Considering the contemporary A-type granites and alkaline basalts in the same region, intense continental rifting most likely occurred in the southwestern Yangtze Block during the late Mesoproterozoic. Widespread intraplate magmatism of 1.2–1.1 Ga in the Yangtze Block indicates that it may not have been involved in the Rodinia assembly at the time.

Remnants of a Middle Triassic Oceanic Lithosphere in the Western Yarlung Zangbo Suture Zone, Southern Tibet

AbstractDiscontinuous ophiolite suites along the Yarlung Zangbo suture zone (YZSZ) in southern Tibet representing remnants of the Neo‐Tethyan oceanic lithosphere are considered to be formed between Late Jurassic and Early Cretaceous. Older ophiolite or ophiolitic mélange (e.g. Triassic) to reveal the initial evolution the Neo‐Tethyan ocean within the YZSZ have rarely been documented so far. The western YZSZ extending from the Saga to Ladakh area are composed of the northern ophiolitic sub‐belt, the Zhongba terrane and the southern ophiolitic sub‐belt. In this study, we document structural, petrological and geochronological data of mafic intrusions from the Mayoumu massif in the southern ophiolitic sub‐belt of the western YZSZ. Two lithos‐tectonic sub‐units, the southern ophiolitic complex and the northern ophiolitic mélange, are recognized in the Mayoumu massif in terms of structural pattern and petrology. LA‐ICP‐MS zircon U‐Pb dating of gabbro from the ophiolitic mélange yields an age of 243 ± 1 Ma with zircon ∊Hf(t) values of +7.9 to +13.2. Two samples of diabase samples from the ophiolite yield ages of 131±1 Ma and 124±1 Ma with zircon ∊Hf (t) values ranging from +10.8 to +15.0 and +12.3 to +15.4, respectively. Geochemically, rare earth elements (REEs) patterns show that these mafic intrusions are similar to those of the normal mid‐ocean‐ridge basalt (N‐MORB). Enrichment of fluid‐mobile elements (e.g. Rb and Ba) and depletion of Nb, and Ta suggest that these intrusions were possibly originated from melting of a depleted mantle source influenced by subducted slab. Our data strongly confirms that the Neo‐Tethyan ocean between the Lhasa terrane and Gondwana had been existed since the Middle Triassic at least. The ages of the two diabases from the ophiolitic complex reveal that mafic magma activities within the Neo‐Tethyan ocean could be as a continuing process during 120–130 Ma. Emplacement and preservation of older ophiolitic recorder during evolution of the Neo‐Tethyan Ocean may be closely related to the occurrence of the Zhongba micro‐terrane within the western YZSZ.

Geology and genesis of the Cihai mafic intrusions in Beishan Terrane, Xinjiang, Northwest China: Implication for iron mineralization and tectonic setting

The Cihai Fe deposit (100 Mt @ 48% Fe) represents an economically important iron deposit in Beishan Terrane, on the southern margin of the Central Asian Orogenic Belt, Xinjiang, northwest China. Many Late Carboniferous to Permian Fe(-Cu) deposits (e.g., Yamansu, Shaquanzi, and Weiya) are also present in adjacent Eastern Tianshan orogenic belt. The Cihai deposit is hosted in mafic dykes (i.e., diabase) and in the contact zone between the mafic intrusion and Mesoproterozoic schist. New zircon U–Pb isotope dating in this study gives a crystallization age of 306.9 ± 3.2 Ma for the ore-bearing diabase, which is the first age-constrained Late Carboniferous mafic magmatic activity at Cihai district. In combination with previous Ar–Ar amphibole dating (281.9 ± 2.2 Ma) for the massive ores, it is suggested that the main iron mineralization and associated mafic magmatic activities occurred at ca. 306–282 Ma, coeval with the large-scale iron mineralization epoch (ca. 324–280 Ma) in Eastern Tianshan. The Cihai diabase samples are LILEs-rich (e.g., K, Rb, and Sr) and HFSEs-depleted (e.g., Nb, Ta, and Ti) calc-alkaline series, and exhibit high Mg# values (51–63), as well as low compatible element concentrations (Ni = 14–53 ppm; Co = 18–43 ppm; and Cr = 26–219 ppm), indicating a mantle wedge source. They have elevated Ba/La and Sr/Nd values with limited Th/Yb ratios (<1), and display slightly Eu anomalies (0.90–1.19). Isotopically, they are characterized by positive εHf(t) values (+13.0 to + 16.1) and young Hf model ages (300 to 420 Ma), which are broadly similar to the isotopic compositions of the depleted mantle and within the ranges of most other mafic–ultramafic intrusions in Beishan-Tianshan orogen. These features suggest that the Cihai diabase intrusions were most likely formed by partial melting of a mantle wedge with some degree of metasomatism by slab-derived fluids. Combined our new geochemical and isotopic results with previous regional investigations, we suggest that the earliest generation of mafic dykes formed in an arc setting related to southward subduction of the paleo-Tianshan Ocean at ca. 306 Ma, and the main iron mineralization and intensive mafic magmatism occurred in a subsequent post-collisional regime during the Early Permian.

Mesoproterozoic (~1.32 Ga) modification of lithospheric mantle beneath the North China craton caused by break-up of the Columbia supercontinent

The Mesoproterozoic Yanliao diabase sills are widespread at the northern margin of the eastern North China Craton (NCC) and provide a means to examine the evolution of the NCC lithospheric mantle from amalgamation to breakup and dispersal of the Columbia supercontinent. Here, we report new baddeleyite U–Pb ages, whole-rock geochemical and Sr–Nd–Hf isotopic data, and baddeleyite Hf isotopes for diabase rocks from different sills, which provide insights into the source and petrogenesis of the magmas and the Mesoproterozoic evolution of the NCC. In situ baddeleyite U–Pb dating yields consistent ages of ca. 1320 Ma for these sills, suggesting coeval emplacement. The diabase samples are all tholeiitic and have high TiO2 and total Fe2O3, and low alkali contents. They have variable Mg# values and major- and trace-element contents but uniform primitive-mantle-normalized trace-element patterns, with weak enrichment in light rare-earth elements (LREEs) and large-ion lithophile elements (LILEs), and depletion in high-field-strength elements (HFSEs). The whole-rock and in situ isotopic contents of the diabase samples are homogeneous, with initial 87Sr/86Sr ratios of 0.7032–0.7049, εNd(t) values of −0.7 to +3.1, εHf(t) values of +0.8 to +3.7, and baddeleyite εHf(t) values of +1.9 to +7.2. All of these geochemical features suggest that these rocks were derived from high-degree partial melting of a fertile lithospheric mantle source at the depth of spinel stability, with subsequent olivine, clinopyroxene, and plagioclase fractionation. This lithospheric mantle was distinct from the previous Archean to Paleoproterozoic cratonic lithospheric mantle beneath the NCC, indicating a refertilization process by upwelling asthenospheric materials during the Mesoproterozoic. This upwelling, together with other Mesoproterozoic magmatism at the craton margin, would have significantly modified the subcontinental lithospheric mantle of the eastern NCC during the Mesoproterozoic, thereby increasing the susceptibility of the craton to subsequent Mesozoic decratonization.

Middle–Late Triassic bimodal intrusive rocks from the Tethyan Himalaya in South Tibet: Geochronology, petrogenesis and tectonic implications

This paper presents new geochronological and geochemical (whole-rock major and trace elements and Sr–Nd–Pb isotopes) data on intrusive rocks exposed in the Qiongduojiang area, the northern part of the Tethyan Himalaya. The Qiongduojiang intrusive rocks are mainly composed of mafic rocks (diabase and gabbro) with subordinate felsic rocks (monzonite), indicating bimodal component characteristics. In zircon U-Pb dating of monzonite, diabase and gabbro samples, the weighted mean ages of magma emplacement were determined to be 230.5 ± 2.9 Ma to 227.4 ± 4.7 Ma, showing Middle–Late Triassic magmatic activity in the northern part of the Himalayan belt. The monzonitic rocks share most of the geochemical features of A-type granites, and their Nd isotopic compositions are consistent with those of coeval diabase rocks. This suggests that they are likely generated by mantle-derived magmas, which had assimilated significant amounts of continental crust. The gabbro samples exhibit the geochemical composition of ocean island basalt (OIB) features and therefore assumingly originate from a depleted mantle source with insignificant crustal contamination. In contrast, the diabase samples display enriched mid-ocean ridge basalt (E-MORB) features and were derived from spinel bearing peridotite that was metasomatised by OIB-like components with limited crustal contamination. The Qiongduojiang bimodal magmas can be considered as the products of the opening of the Neo-Tethyan ocean. Considering the magmatic petrology, paleomagnetism and paleontology data, we further propose that the Neo-Tethyan ocean began opening before ~230 Ma.

The study of elastic properties and chemical composition of selected rocks

Abstract The aim of the investigation was to study the elastic properties and chemical composition of rock samples based on laboratory measurements. Three cuboid samples of diabase, granite and quartz were tested under laboratory conditions. The shape of samples is a cuboid with dimensions 0.1 × 0.05 × 0.05 m and the measurements were made in three directions. The Pundit Lab+ equipment was used for tests. The signal frequency value was 250 kHz. The propagation times of P- and S-waves were designated that allows to determine the elastic dynamic moduli values. The Japanese JEOL USA Scanning Electron Microscope was also used for tests. The chemical composition of rocks were determined by characteristic X-rays analysis. Obtained results show that the ultrasonic measurements and SEM are useful tools to assign properties of rocks. This knowledge makes easier recognition in preliminary stages during engineering study.

U–Pb baddeleyite crystallization age for a Corson diabase intrusion: possible Midcontinent Rift magmatism in eastern South Dakota

The largely buried basement of the northern Great Plains includes suture zones and terrane boundaries that represent a significant part of the growth of Laurentia in the Proterozoic. Basement exposures in this region east of the Black Hills are rare. In southeastern South Dakota, southwestern Minnesota, northeastern Nebraska, and northwestern Iowa, small outcrops of the Proterozoic Sioux Quartzite occur. In southeastern South Dakota, Corson diabase sills or dykes have intruded the quartzite. U–Pb ID–TIMS baddeleyite data from a Corson diabase sample yield an upper intercept date of 1149.4 ± 7.3 Ma, suggesting the diabase is related temporally to the Midcontinent Rift (MCR). The similarity in age of this diabase to the Inspiration sill, Pigeon River, Kipling, and Abitibi dykes suggests that early Midcontinent Rift development was not localized within the Nipigon Embayment, but extended along a roughly east–northeast zone from the Abitibi dykes to the Corson diabase. The presence of the Corson intrusions 250 km west of the MCR is hypothesized to represent a failed rift arm within the Superior craton. The greater strength of the Superior craton relative to lithosphere south of the Spirit Lake tectonic zone resulted in a shift of the southwestern rift arm in southern Minnesota along the Belle Plaine fault southeastward to the Iowa border. Alternatively, the apparent northeast trend of known occurrences of the Corson diabase is also consistent with a mantle plume centre explanation for early Midcontinent rifting.

Diabase-hosted copper mineralization in the Qunjsai deposit, West Tianshan, NW China: Geological, geochemical and geochronological characteristics and mineralization mechanism

The Qunjsai copper deposit in West Tianshan, Xinjiang, NW China, with a reserve of 7.5 million metric tons of ore grading 0.57 wt% Cu, is characterized by mineralization occurring as sparsely disseminated chalcopyrite, bornite and chalcocite in a diabase dike. The deposit is different from most magmatic-hydrothermal Cu deposits in that the mineralization is mostly constrained to the dike and is not associated with extensive wall rock alteration. On the other hand, the deposit differs from typical orthomagmatic deposits in that it lacks significant Ni and PGE mineralization. Zircon grains from a diabase sample yield a weighted 206Pb/238U age of 289.9 ± 1.4 Ma, which is broadly coeval with the Re–Os isochron age of 307 ± 10 Ma from six Cu concentrate samples (mixture of bornite+chalcopyrite+chalcocite with an average Cu grade of 29%), suggesting that they are coeval. Petrochemical studies suggest that the diabase dikes in the Qunjsai area were derived from a subduction-related, fluid-metasomatized sub-continental lithospheric mantle, with assimilation of crustal materials in a post-collisional extensional setting. Measured δ34S (−3.8 to 0.8‰ CDT) and lead isotopes of sulfides, as well as initial 187Os/188Os ratios of Cu concentrates (0.42 ± 0.19) are consistent with the notion that the sulfur and metals were derived from a mantle source and were contaminated by crustal components. The flat patterns of mantle-normalized chalcophile element diagrams are comparable with those from typical orthomagmatic Ni–Cu sulfide deposits. Based on these data, we propose that the Qunjsai deposit is of orthomagmatic origin, the depletion of Ni and PGE, being due to source rocks originally rich in Cu and to segregation of a Cu-rich sulfide melt from a Ni-rich sulfide melt. This orthomagmatic mineralization was followed by hydrothermal alteration (mainly chlorite and epidote alteration) by magmatic fluids, which may have introduced additional amounts of Cu and S and reworked the magmatic sulfides.