We report new zircon U-Pb ages, Hf isotopic and geochemical results for the Tongling granitic plutons of Southeast China. SHRIMP U-Pb ages for the Miaojia quartz monzodiorite porphyrite，the ...Tianebaodan and Tongguanshan quartz monzodiorites, the Xinqiaotou granodiorite porphyry, and the Shatanjiao and Nanhongchong granodiorite are 143 ± 2, 141 ± 1 and 142 ± 1, 147 ± 1, and 145 ± 1 and 139 ± 1 Ma, respectively. Combined with previous geochronological data, our results indicate that the porphyritic rocks are older than rocks of the same type lacking porphyritic texture. Geochemically, these high-K calc-alkaline intrusive rocks are characterized by arc-like trace element distribution patterns, with significant enrichment in LILE and LREE but depletion in HFSE. Lu-Hf isotopic compositions of zircons from the high-K calc-alkaline (HKCA) rocks have ε
Hf
(t) values of magmatic 139-147 Ma zircons from −8.1 to −25.6, with two-stage model ages (t
DM2
) of 1.71-2.67 Ga, whereas ε
Hf
(t) values of inherited 582-844 Ma zircons range from 5.4 to −9.5, with t
DM2
of 1.39-2.22 Ma, younger than t
DM2
values of igneous zircon, indicating that newly formed mantle material was added to the continental crust of the Yangtze Block. Moreover, ε
Hf
(t) values of inherited zircon cores older than 1000 Ma are from −7.8 to −26, similar to magmatic zircons, and the t
DM2
values are all greater than 3.0 Ga (3.16-3.75 Ga), reflecting partial melting of ancient sialic material. We conclude that the plutonic melts were derived from both the enriched mantle and the ancient crust. The HKCA Tongling intrusions coincide temporally with the J
3
-K
1
magmatic event that was widespread in Southeast China. This igneous activity may have accompanied sinistral slip along the Tan-Lu fault due to oblique subduction of the Palaeo-Pacific plate.
The development of fracture process zones (FPZ) in the Cracked Chevron Notched Brazilian Disc (CCNBD) monsonite and Brisbane tuff specimens was investigated to evaluate the mechanical behaviour of ...brittle rocks under static and various cyclic loadings. An FPZ is a region that involves different types of damage around the pre-existing and/or stress-induced crack tips in engineering materials. This highly damaged area includes micro- and meso-cracks, which emerge prior to the main fracture growth or extension and ultimately coalescence to macrofractures, leading to the failure. The experiments and numerical simulations were designed for this study to investigate the following features of FPZ in rocks: (1) ligament connections and (2) microcracking and its coalescence in FPZ. A Computed Tomography (CT) scan technique was also used to investigate the FPZ behaviour in selected rock specimens. The CT scan results showed that the fracturing velocity is entirely dependent on the appropriate amount of fracture energy absorbed in rock specimens due to the change of frequency and amplitudes of the dynamic loading. Extended Finite Element Method (XFEM) was used to compute the displacements, tensile stress distribution and plastic energy dissipation around the propagating crack tip in FPZ. One of the most important observations, the shape of FPZ and its extension around the crack tip, was made using numerical and experimental results, which supported the CT scan results. When the static rupture and the cyclic rupture were compared, the main differences are twofold: (1) the number of fragments produced is much greater under cyclic loading than under static loading, and (2) intergranular cracks are formed due to particle breakage under cyclic loading compared with smooth and bright cracks along cleavage planes under static loading.
The power of explosive volcanic eruptions is reflected in the grain size distribution and dispersal of their pyroclastic deposits. Grain size also forms part of lithofacies characteristics that are ...necessary to determine transport and depositional mechanisms responsible for producing pyroclastic deposits. However, the common process of welding and rock lithification prevents quantification of grain size by traditional sieving methods for deposits in the rock record. Here we show that functional stereology can be used to obtain actual 3D volume fractions of clast populations from 2D cross-sectional images. Tests made on artificially consolidated rocks demonstrate successful correlations with traditional sieving method. We show that the true grain size distribution is finer grained than its representation on a random 2D section. Our method allows the original size of vesicular pumice clasts to be estimated from their compacted shapes. We anticipate that the original grain-size distribution of welded ignimbrites can also be characterized by this method. Our method using functional stereology can be universally applied to any type of consolidated, weakly to non-deformed clastic material, regardless of grain size or age and therefore has a wide application in geology.
► The conventional sieving method does not allow calculation of grain size distribution for any clastic rocks. ► The grain size distribution of volcaniclastic rocks can be calculated by functional stereology. ► Very good fit was found between the grain size distributions calculated by functional stereology and sieving methods. ► Compaction of pumice clast into fiamme can be estimated in the calculation of the grain size distribution.
Pillar burst is one type of rockburst that occurs in underground mines. Simulating the stress change and obtaining insight into the pillar burst phenomenon under laboratory conditions are essential ...for studying the rock behavior during pillar burst in situ. To study the failure mechanism, a novel experimental technique was proposed and a series of tests were conducted on some granite specimens using a true-triaxial strainburst test system. Acoustic emission (AE) sensors were used to monitor the rock fracturing process. The damage evolution process was investigated using techniques such as macro and micro fracture characteristics observation, AE energy evolution, and
b
value analysis and fractal dimension analysis of cracks on fragments. The obtained results indicate that stepped loading and unloading simulated the pillar burst phenomenon well. Four deformation stages are divided as initial stress state, unloading step I, unloading step II, and final burst. It is observed that AE energy has a sharp increase at the initial stress state, accumulates slowly at unloading steps I and II, and increases dramatically at peak stress. Meanwhile, the mean
b
values fluctuate around 3.50 for the first three deformation stages and then decrease to 2.86 at the final stage, indicating the generation of a large amount of macro fractures. Before the test, the fractal dimension values are discrete and mainly vary between 1.10 and 1.25, whereas after failure the values concentrate around 1.25–1.35.
In rock engineering design, significant advances have occurred in recent years in numerical modelling capability with increasing trends to ever greater use of synthetic rock mass models and discrete ...fracture network (DFN) modelling, but all too often with little comparative improvement in geologic base data. As a consequence, there is even more need that reliable estimates be available of strength and deformation characteristics of the rock masses on which or within which engineering structures are to be created, be it a tunnel, a foundation or a slope. Geological Strength Index (GSI) characterization, linked with Hoek-Brown strength determination as a basis for modelling has been widely adopted by engineers and geologists involved in design and construction of engineering structures. The need for geological definition of rock mass properties required as inputs into numerical analysis, constitutes one of the greatest reasons for application of the GSI chart, allowing characterization of even difficult-to-describe rock masses, including tackling even the most problematic of weak and complex rock masses. Back-analyses of tunnels, slopes and foundation behaviour using GSI and its reliable application in rock engineering designs attest to its reliability. With continuing use worldwide, the GSI system has continued to evolve, but greater understanding is needed in the definition of input constants, for establishing both GSI and intact rock properties. This need for improved evaluation, particularly from a geological perspective, is addressed in this paper. Geological processes of tectonism, weathering and alteration all affect GSI. Evaluation of these factors, which are each critical to proper GSI definition, are analyzed based on real rock mass cases. Suggested ranges in variability of intact rock parameters uniaxial compressive strength σci and material constant mi for common rock masses are presented in the context of a composite new GSI chart. This chart allows selection of appropriate GSI ranges for any specific rock suite. Specific key engineering geological characteristics that differentiate igneous, metamorphic and sedimentary one from each other are highlighted through discussion of various example rock units (including gneisses, granites, ophiolites, limestones, schists, siltstones/mudstones/shales, and molassic and flysch formations). Illustrations are given of how geological differentiation dictates variability in geotechnical properties of most common rock masses.
•GSI permits a wide variety of rock masses to be quantified, enhancing geological logic and reducing geo-uncertainty.•The interaction between GSI and geological processes of tectonism, weathering and alteration are analysed.•GSI system allows the influence of various engineering geological characteristics to be better defined.•Specific key engineering geological characteristics that differentiate various rocks from each other are highlighted.•How geological diversity dictates variability in GSI values and intact rock properties of most common rock units is studied.
Drilling has revealed suites of magnesian granite and diorite emplaced in Early Jurassic time (198–195 Ma) and an arc‐related low‐temperature (678 to 696°C) magmatism in NE South China Sea. These ...rocks have 87Sr/86Sri (0.705494 to 0.706623) and εNdt (−0.9 to +2.2) as evidence of evolved mantle‐derived magmas, coupled with enriched fluid‐mobile elements Cs to K and Pb implying involvement of subduction‐zone fluids. Another Early Jurassic granodiorite (zircon U‐Pb 187 Ma) drilled from the SW East China Sea, a magnesian high‐K calc alkaline, is comparably confined to a range of low‐temperature (~675°C) arc‐related granite, characterized by enrichment of fluid‐mobile elements and Nb‐Ta depletion. Its Sr‐Nd isotopes (87Sr/86Sri = 0.705200, εNdt = 1.1) suggest a product of evolved mantle‐derived melts. Together with detrital igneous zircons from Paleocene sequences, these observations reveal an Early Jurassic arc‐related low‐temperature (600 to 740°C) magmatism in the SW East China Sea. These arc‐related granitoids, along with those from SE Taiwan, could define an Early Jurassic NE‐SW trending Dongsha‐Talun‐Yandang magmatic arc zone along the East Asian continental margin paired with Jurassic accretionary complexes from SW Japan, East Taiwan to the West Philippines. This arc‐subduction complex assembly was associated with oblique subduction of the paleo‐Pacific slab beneath Eurasia, presumably responsible for Early Jurassic lithospheric extension in south China block.
Key Points
Drilling from South and East China Seas reveals an Early Jurassic arc‐related magmatism
The igneous suites are Nb‐Ta depleted, low temperature, and evolved mantle‐derived rocks
Slab subduction‐related architecture along the East Asian continental margin is proposed
In this study, Neoproterozoic Halaban ophiolite situated in the eastern part of the Arabian shield, Saudi Arabia, was investigated. The Halaban ophiolite represents a section of a dismembered ...ophiolite, tectonically emplaced over a volcano-sedimentary succession of island arc assemblages. It is variably, altered, deformed and metamorphosed to serpentinized rocks. It includes serpentinized peridotite and metagabbro with rare metabasalt, chromitite pods, listvenite and rodingite. The contact between the lower mantle unit (serpentinized peridotite) and the upper crustal unit (metagabbro) is originally magmatic, although it is disrupted by tectonism. Nevertheless, because of the high degree of alteration and metamorphism, few fresh relicts of primary minerals (chromian spinel, olivine and pyroxenes) were recorded. The mineral assemblages in the Halaban ophiolite are comparable to greenschist, to lower amphibolite facies metamorphism. The fresh olivine relics have high Fo and Ni content, while the fresh chromian spinel cores have high Cr# accompanied by a low TiO2. The preserved texture (mesh and bastite), fresh relict minerals, and the whole-rock compositions (Mg# = molar Mg/ (Mg + Fe2+) = 0.91–0.93 with low CaO and Al2O3 content) suggest that the protolith is mainly harzburgite with minor dunite. The chemical composition of Halaban metagabbro is indicative of a tholeiitic affinity. In addition, the very low modal abundance of clinopyroxene and the high Ni, Cr and Co content of the serpentinized peridotite, and high-Cr# (av. 0.66) with low-Ti (mostly ≤0.1 wt%) of chromian spinel, suggest significant partial melting. The serpentinized peridotite (harzburgite and dunite) undergone different degrees of partial melting, ranging from approximately 32% to 37% for harzburgite and approximately 37% to 42% for dunite. The variation of the degree of partial melting between harzburgite and dunite is indicative of mantle heterogeneity beneath the Arabian Shield. The general petrological characteristics of the Halaban ophiolite are substantially similar to those formed by melt-peridotites interaction and subsequent melt mixing in a fore arc setting within a supra-subduction zone system. Halaban chloritite is formed as a secondary product due to the metasomatic interaction of the ophiolitic ultramafic rocks. The content of Cr, Co and Ni of the chloritite sample was consistently close to that of the serpentinites.
Display omitted
•Serpentinized ultramafics of Halaban have relict primary mantle minerals such as olivine and chrome spinel.•Fresh relics of olivine have high Fo and NiO contents, similar mantle olivines.•Protoliths of the serpentinized ultramafics of Halaban have high partial melt degrees•The Halaban ultramafics are remnants of Neoproterozoic forearc depleted mantle emplaced in a suprasubduction zone
High-energy laser technology is expected to be applied into deep drilling and tunnel excavation due to its attractive prospects for enhanced hard rock breakage. However, there is still a lack of ...proper experimental methods and numerical tools to quantitatively evaluate the influence of laser radiation on rock fracture. Combined with our specially designed experimental tests, this work aims to provide a numerical tool to quantitatively investigate laser-induced rock damage. Using a specially designed experimental platform, we conducted direct tensile tests on rock samples in the shape of dog-bone to determine rock damage under the laser radiation of different parameters. The experimental results show that a high-energy laser beam can cause a significant loss in rock tensile strength. Then, a mathematical model was established to describe the temporal and spatial evolution of rock damage in a given path of laser scanning, which was implemented in a four-dimensional lattice spring model (4D-LSM) to simulate our experimental tests. Through a comparison between the numerical and experimental results, the multiparameter damage model was proven to be more suitable for the numerical simulation of laser-induced rock damage. Finally, by introducing a laser damage model, an orthogonal experimental design was used to analyse the influencing significance of different factors on the efficiency of rock breaking and the wear of cutter tools in laser-assisted rock cutting, showing the prospects for application of the proposed numerical model to laser-assisted tunnel boring machine (TBM) tunnelling in the future.
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