2019 Vol. 10, No. 2

Announcement
Editorial
Research Papers
Modelling the impact of sea-level rise on urban flood probability in SE China
James Andrew Griffiths, Fangfang Zhu, Faith Ka Shun Chan, David Laurence Higgitt
2019, 10(2): 363-372. doi: 10.1016/j.gsf.2018.02.012
Abstract:
Urban drainage systems in coastal cities in SE China are characterized by often complex canal and sluicegate systems that are designed to safely drain pluvial flooding whilst preventing tidal inundation. However, the risk of coastal flooding in the region is expected to increase over the next 50-100 years, as urban areas continue to expand and sea-levels are expected to rise. To assess the impact of projected sealevel rise on this type of urban drainage system, a one-dimensional model and decision support tool was developed. The model indicated that although sea-level rise represents a significant challenge, flood probability will continue to be most influenced by rainfall. Events that are significant enough to cause flooding will most likely be minimally impacted by changes to the tidal frame. However, it was found that a sea-level rise of up to 1.2 m by 2010 would result in increased drainage times and higher volumes of over-topping when flooding occurs.
Brief review on climate change and tropical peatlands
Lee Yit Leng, Osumanu Haruna Ahmed, Mohamadu Boyie Jalloh
2019, 10(2): 373-380. doi: 10.1016/j.gsf.2017.12.018
Abstract:
In 2008, the very extensive tropical peats were estimated to be about 182 million ha spanning South America, Asia and Africa. About 20.3% (36.9 million ha) of this area exist in Asia. Peats are classified based on their degree of decomposition, namely Fibrists, Hemists, Saprists and Folists. This makes them different in characteristics. The activities of microorganisms vary in different types of peat due to, for example, the sapric layer of well humified peat can provide water and food to microorganisms during heat stress. In another scenario, deeper peat is older and typically has lower levels of labile carbon to provide substrate for microbes compared to surface peat. A complete understanding of the microbial communities in different layers of peat is essential as microorganisms play major roles in peat decomposition and are important to ecosystem processes. These peats are a very important global carbon (C) store or reserve and could severely impact climate change if not managed well. Peatlands can store as much as 40 to 90 Gt C. Mis-management of peats could severely impact the environment particularly the emission of carbon into the atmosphere. For instance, clearing of peatlands using fire has been reported to release an estimated 88 t C ha-1 to the atmosphere. There are several factors which influence the environmental consequences of tropical peat especially in relation to climate change. The main influences are: (i) changes in temperature, (ii) changes in precipitation or rainfall, (iii) changes in atmospheric composition, and (iv) fire and haze. This paper is a brief review on these four influences in relation to climate change. It is apparent from the brief review that there is a need for continued short and long-term research to better understand tropical peats and how they affect our climate. This will hopefully provide the basis for predicting better what could happen under various scenarios.
Nematode distribution in cultivated and undisturbed soils of Guinea Savannah and Semi-deciduous Forest zones of Ghana
Obed Asiedu, Charles Kodia Kwoseh, Haddish Melakeberhan, Thomas Adjei-Gyapong
2019, 10(2): 381-387. doi: 10.1016/j.gsf.2017.07.010
Abstract:
Climate change affects air temperature, sea levels as well as the soil and its ecosystem. The Guinea Savannah and Semi-deciduous Forest zones of Ghana are characterized by different climatic conditions and vegetative cover. Annual average temperature has been steadily increasing whilst annual total rainfall has been decreasing in both zones, and this has been causing a southward shift of the Savannah into the Forest zone. Soil organisms provide crucial ecosystem services which are required for sustainable agriculture and food production yet crop cultivation disturbs the soil ecosystem. The harsh conditions associated with the Savannah further expose the soil ecosystem to disturbance and loss of biodiversity which threatens food production and security. Soil nematodes are the most abundant animals in the soil and play a central and critical role in the soil food web complex. Studying the nematode community structure gives a reflection of the status of the entire soil ecosystem. Soil samples were taken from cultivated and natural landscapes in the Guinea Savannah and Semi-deciduous Forest agroecological zones to analyse the nematode community. Results from the study showed the Guinea Savannah zone recording warmer soil temperatures, lower organic matter percentage and lower nematode diversity (Genus Richness) as compared to the Semi-deciduous Forest zone. If the Savannah continues to shift southward, the Forest zone soil ecosystem risks disturbance and loss of biodiversity due to the harsh Savannah conditions. Our findings indicate that prevailing crop cultivation practices also disturb soil ecosystem in the two ecological zones which span across West Africa. A disturbed soil ecosystem endangers the future of food production and food security.
Prediction of spatial soil loss impacted by long-term land-use/land-cover change in a tropical watershed
J. H. Abdulkareem, B. Pradhan, W. N. A. Sulaiman, N. R. Jamil
2019, 10(2): 389-403. doi: 10.1016/j.gsf.2017.10.010
Abstract:
The devastating effect of soil erosion is one of the major sources of land degradation that affects human lives in many ways which occur mainly due to deforestation, poor agricultural practices, overgrazing, wildfire and urbanization. Soil erosion often leads to soil truncation, loss of fertility, slope instability, etc. which causes irreversible effects on the poorly renewable soil resource. In view of this, a study was conducted in Kelantan River basin to predict soil loss as influenced by long-term land use/land-cover (LULC) changes in the area. The study was conducted with the aim of predicting and assessing soil erosion as it is influenced by long-term LULC changes. The 13,100 km2 watershed was delineated into four sub-catchments Galas, Pergau, Lebir and Nenggiri for precise result estimation and ease of execution. GIS-based Universal Soil Loss Equation (USLE) model was used to predict soil loss in this study. The model inputs used for the temporal and spatial calculation of soil erosion include rainfall erosivity factor, topographic factor, land cover and management factor as well as erodibility factor. The results showed that 67.54% of soil loss is located under low erosion potential (reversible soil loss) or 0-1 t ha-1 yr-1 soil loss in Galas, 59.17% in Pergau, 53.32% in Lebir and 56.76% in Nenggiri all under the 2013 LULC condition. Results from the correlation of soil erosion rates with LULC changes indicated that cleared land in all the four catchments and under all LULC conditions (1984-2013) appears to be the dominant with the highest erosion losses. Similarly, grassland and forest were also observed to regulate erosion rates in the area. This is because the vegetation cover provided by these LULC types protects the soil from direct impact of rain drops which invariably reduce soil loss to the barest minimum. Overall, it was concluded that the results have shown the significance of LULC in the control of erosion. Maps generated from the study may be useful to planners and land use managers to take appropriate decisions for soil conservation.
The role of land use on the local climate and air quality during calm inter-monsoon in a tropical city
M. C. G. Ooi, A. Chan, M. J. Ashfold, M. Y. Oozeer, K. I. Morris, S. S. K. Kong
2019, 10(2): 405-415. doi: 10.1016/j.gsf.2018.04.005
Abstract:
The modification of land use is known to be a major climate change driver to the local warming and air quality in cities. Despite the reduction of NOx over the years, the Selangor state has captured a higher level of O3 in year 2011. The measurement result has shown that the surge in O3 level was attributed to the reduction of NOx/NMHC ratio. This paper hence attempted to identify the role of land use change from 1999 to 2011 on the ground ozone air quality in the tropical urban conurbation, Greater Kuala Lumpur (GKL), Selangor, Malaysia. With the state-of-the-art chemical weather prediction tool, WRFChem, the external synoptic factors and emission inventory were controlled when comparing the chronological land use changes. The results showed that the urban-induced temperature and wind bias in the tropical region has induced stronger wind to disperse the NOx and carries the TVOC from the suburban to the downwind urban region. The reduction of NOx/TVOC has gradually shifted towards the optimum O3 formation regime in 2011. The formation of highly concentrated ozone becomes more sensitive to the increment of TVOC as the NOx level reduces in the urban. This highlights the essential involvement of TVOC in the ozone formation in lieu of the NOx reduction in the tropical city, a region with growing emitter of reactive biogenic ozone precursors.
Crustal reworking and orogenic styles inferred from zircon Hf isotopes: Proterozoic examples from the North Atlantic region
C. J. Spencer, C. L. Kirkland, A. R. Prave, R. A. Strachan, V. Pease
2019, 10(2): 417-424. doi: 10.1016/j.gsf.2018.09.008
Abstract:
Zircon Hf evolutionary patterns are powerful tools to investiage magma petrogenesis and crustal evolution. The 176Hf/177Hf isotopic signature of a rock is particularly informative and can be used to derive an estimation of the time when mantle extraction and diagnose closed system reworking where successive samples through time define an Hf evolution array dependant on the source Lu/Hf ratio. However, many magmatic events require new mantle addition as the thermal impetus for melting pre-existing crust. In this situation, rather than simply reflecting reworking, the isotopic signature indicates mixing with contributions from both reworked crust and new radiogenic input. Different geodynamic settings have different propensities for either reworking or addition of new mantle-derived magma. Hence, Hf-time trends carry within them a record, albeit cryptic, of the evolving geodynamic environment as different tectonic configurations recycle and add new crust at different rates, magnitudes, and from different sources. As an example of the difference in apparent Hf evolution slopes, we present Hf-time compilations from three geographically distinct Meso-to Neoproterozoic orogenic belts in the North Atlantic Region whose geodynamic configurations remain a subject of debate. We use the εHf/Ma trajectory to assist in understanding their evolution. The εHf/Ma trajectory of the Sveconorwegian Orogen corresponds to a 176Lu/177Hf ratio of 0.012, which implies a process driven primarily by reworking of preexisting crust that is balanced with input from the depleted mantle resulting in a relatively shallow Hf/Ma path. In stark contrast εHf/Ma slope. The Valhalla Orogen reveals a similar comparatively shallow 3 to these patterns is the steep εHf/Ma trajectory of the Grenville Orogen that requires a mixing process involving a greater contribution of old crust of at least ~1.8 Ga age. The degree of reworking required to produce the εHf/Ma trend of the Grenville Orogen is consistent with a continent-continent collisional orogeny whereas both Sveconorwegian and Valhalla orogens appear more consistent with accretionary margins.
Mineral association and graphite inclusions in nephrite jade from Liaoning, northeast China: Implications for metamorphic conditions and ore genesis
Cun Zhang, Xiaoyan Yu, Tianlong Jiang
2019, 10(2): 425-437. doi: 10.1016/j.gsf.2018.02.009
Abstract:
The Liaoning Province in the northeastern part of the North China Craton (NCC) hosts several tremolite jade (nephrite) deposits. Here we investigate the Sangpiyu tremolite jade deposit where the relationship between abundant graphite inclusions within the jade remains enigmatic. We employ petrography, electron probe microanalysis, X-ray-diffraction, and Raman spectroscopy to characterize the tremolite jade and its inclusion minerals. The Sangpiyu jade is predominately composed of tremolite with minor calcite, dolomite, serpentine, titanite, zoisite, allanite, chlorite, apatite, chromite and graphite. Raman spectroscopy of graphite inclusions shows that the D1/G intensity ratio ranges from 0.78 to 0.88 in deep green samples and from 0.05 to 0.23 in dark green samples. The ranges of D1/(D1 + G) integral area ratio for these types are from 0.0548 to 0.3037 and 0.5528 to 0.7355 respectively. The formation temperature of graphite inclusions in the dark green tremolite jade is computed as 549.8 ℃, whereas that for the deep green sample is about 343.2 ℃. Our results suggest that the jade formation occurred in a multi-stage process through the action of hydrothermal fluids and metamorphism possibly in a subduction-related setting at moderate to high temperatures.
Magmatic and hydrothermal zircon growth during multiple orogenic cycles in an evolving mantle wedge
Jingyi Wang, M. Santosh, Shanshan Li, Sung Won Kim
2019, 10(2): 439-452. doi: 10.1016/j.gsf.2018.07.003
Abstract:
The Hongseong area of the Hongseong-Imjingang Belt in the central-western Korean Peninsula forms part of a subduction-collision system that is correlated with the Qinling-Dabie-Sulu Belt in China. Several serpentinized ultramafic bodies carrying blocks of metamorphosed mafic rocks occur in this area. Here we investigate zircon grains in serpentinites from Bibong (BB) and Wonnojeon (WNJ), and high-pressure (HP) mafic granulite from Baekdong (BD) localities based on U-Pb, REE and Lu-Hf analyses. The zircons from BD HP mafic granulite show distinct age peaks at 838 Ma, 617 Ma and 410 Ma, with minor peaks at 1867 Ma, 1326 Ma and 167 Ma. The Neoproterozoic age peaks in these rocks as well as in the serpentinites suggest subduction-related melt-fluid interaction in the mantle wedge at this time. The older zircon grains ranging in age from the Early to Middle Paleoproterozoic might represent detrital grains from the basement rocks transferred to the wedge mantle through sediment subduction. The BD HP mafic granulite shows a Middle Paleozoic age peak (Devonian; 410 Ma). The 242-245 Ma age peaks in the compiled age data of zircon grains serpentinites from BB and WNJ correspond to a major Triassic event that further added melts and fluids into the ancient mantle wedge to crystallize new zircons. In the chondrite normalized rare earth element diagram, the magmatic zircon grains from the studied rocks show LREE depletion and HREE enrichment with sharply negative Eu and Pr anomalies and positive Ce and Sm anomalies. The REE patterns of hydrothermal zircons show LREE enrichment, and relatively flat patterns with negative Eu anomaly. Zircon Hf signature from the WNJ serpentinite show negative εHf(t) (-18.5 and -23.5) values indicating an enriched mantle source with TDM in the range of 1614 Ma and 1862 Ma. Zircons from the BD HP mafic granulite also show slightly negative εHf(t) (average -4.3) and TDM in the range of 1365-1935 Ma. Our study provides evidence for multiple zircon growth in an evolving mantle wedge that witnessed melt and fluid interaction during different orogenic cycles.
The impact of crustal rheology on natural seismicity: Campi Flegrei caldera case study
R. Castaldo, L. D'Auria, S. Pepe, G. Solaro, V. De Novellis, P. Tizzani
2019, 10(2): 453-466. doi: 10.1016/j.gsf.2018.02.003
Abstract:
We analyze the crustal rheology beneath the active resurgent Campi Flegrei caldera (CFc) in Southern Italy by modelling the 3D brittle-ductile (B/D) transition, based on available thermal, geological and geophysical data. Firstly, the thermal field in the conductive physical regime is modeled using a finite element method; based on an optimization tool, this method is applied to evaluate the location and dimensions of the deep thermal source beneath the caldera. A horizontally-extended thermal anomaly located at about 5000 m depth below sea level is identified beneath Pozzuoli Bay, a part of the CFc. The same isotherm is located at a depth of 20,000 m beyond the caldera. This indicates a higher horizontal temperature gradient in the caldera with respect to the surrounding area. Next, we utilize this thermal model to image the 3D rheological stratification of the shallow crust below the caldera with two different values of strain rates. Within the caldera, the B/D transitions with ε equal to 10-12 s-1 and 10-8 s-1 are located at 3000 m and 5000 m depths, respectively. Outside the caldera, the transition is very deep (15,000-20,000 m), seemingly uninfluenced by the thermal state of the CFc volcanism. Finally, we compare these results with the spatial distribution of earthquake hypocenters, Benioff strain release and b-value distribution to investigate the relationship between crustal rheology and seismicity characteristics. Our analysis reveals that the image of the B/D transition is in agreement with the distribution of earthquake hypocenters, constraining the potential seismogenic volume of the region. Our study demonstrates that knowledge of the rheological state of a volcanic system is an important element to interpret its dynamic, forecast future activity and improve evaluation of the associated seismic hazard.
Orbital cyclicity in sedimentary sequence and climatic indications of C-O isotopes from Lower Cretaceous in Qingxi Sag, Jiuquan Basin, NW China
Siding Jin, Haiyang Cao, Hua Wang, Michael Wagreich, Sylvain Richoz
2019, 10(2): 467-479. doi: 10.1016/j.gsf.2018.01.005
Abstract:
Sedimentary deposits of the Lower Cretaceous Xiagou Formation form the most significant potential hydrocarbon reservoirs in the Qingxi Sag, Jiuquan Basin (NW China). Zircon U-Pb ages of the dated basalts at the top of the Xiagou Formation give an isochron age of 115.6 Ma, and the sedimentation interval of the Xiagou Formation was speculated to range from about 125/124 Ma to 115 Ma based on paleontological research and stratigraphic correlation analysis. Here we use GR logging data as a palaeoenvironmental and palaeoclimatic proxy to conduct a detailed cyclostratigraphic study of five selected wells. Power spectra, evolutionary fast Fourier transformation and wavelet analysis all reveal significant sedimentary cycles in the Xiagou Formation. The ratios of cycle wavelengths in these stratigraphic units are 33.82 m:7.91 m:3.06 m:1.79 m, which is similar to the ratio of orbital targets of 20:5:2:1. The ratio of 20:5:2:1 is interpreted as Milankovitch cycles of 405 kyr long eccentricity, 100 kyr short eccentricity, 37 kyr obliquity, and 22 kyr precession cycles respectively. A high-resolution astronomical time scale is constructed by tuning the stratigraphy into target curves of orbital cycles respectively. Based on the astronomical time scale, the absolute ages of 55 samples were estimated, which are used for subsequent stable carbon and oxygen isotope stratigraphy analysis. The analysis results of the five studied wells in the Qingxi Sag indicate: (1) a negative trend of δ13C values upwards in the Xiagou Formation, and (2) negative δ18O values with a positive trend upwards. Both relatively heavy values and pronounced covariances of δ13C values and δ18O values indicate an arid-evaporation-controlled climate during the sedimentary period of the Lower Cretaceous Xiagou Formation, Qingxi Sag, Jiuquan Basin. Moreover, positive covariances of SQK1g2+3 indicate extremely high temperature, and negative covariances of SQK1g1 indicate a relatively low temperature.
TEMPORARY REMOVAL: Why would sea-level rise for global warming and polar ice-melt?
Aftab Alam Khan
2019, 10(2): 481-494. doi: 10.1016/j.gsf.2018.01.008
Abstract:
Two major causes of global sea level rise such as thermal expansion of the oceans and the loss of landbased ice for increased melting have been claimed by some researchers and recognized by the IPCC. However, other climate threat investigators revealed that atmosphere-ocean modeling is an imperfect representation, paleo-data consist of proxy climate information with ambiguities, and modern observations are limited in scope and accuracy. It is revealed that global warming and polar ice-melt although a reality would not contribute to any sea level rise. Floating-ice of the polar region on melting would reoccupy same displaced volume by floating ice-sheets. Land-ice cover in the polar region on melting can reduce load from the crust to activate elastic rebound that would raise land for its isostatic equilibrium. Such characteristics would not contribute to sea level rise. Equatorial bulge, polar flattening, elevation difference of the spheroidal surface between equator and pole with lower in the pole, strong gravity attraction of the polar region and week gravity attraction of the equatorial region, all these phenomena would play dominant role in preventing sea level rise. Palaeo-sea level rise and fall in macro-scale (10-100 m or so) were related to marine transgression and regression in addition to other geologic events like converging and diverging plate tectonics, orogenic uplift of the collision margin, basin subsidence of the extensional crust, volcanic activities in the oceanic region, prograding delta buildup, ocean floor height change and sub-marine mass avalanche. This study also reveals that geophysical shape, gravity attraction and the centrifugal force of spinning and rotation of the earth would continue acting against sea level rise.
Chemical, isotopic and amino acid composition of Mukundpura CM2.0 (CM1) chondrite: Evidence of parent body aqueous alteration
N. G. Rudraswami, A. K. Naik, R. P. Tripathi, N. Bhandari, S. G. Karapurkar, M. Shyam Prasad, E. V. S. S. K. Babu, U. V. R. Vijaya Sarathi
2019, 10(2): 495-504. doi: 10.1016/j.gsf.2018.02.001
Abstract:
The carbonaceous chondrites are intriguing and unique in the sense that they are the only rocks that provide pristine records of the early solar nebular processes. We report here results of a detailed mineralogical, chemical, amino acid and isotopic studies of a recently observed fall at Mukundpura, near Jaipur in Rajasthan, India. Abundance of olivines in this meteorite is low and of serpentine minerals is high. FeO/SiO2=1.05 in its Poorly Characterized Phases (PCP) is similar to that observed in other CM2.0 chondrites. The water content of ~9.8 wt.% is similar to that found in many other CM chondrites. Microscopic examination of matrix shows that its terrestrial weathering grade is W0 but aqueous parent body alteration is high, as reflected in low abundance of identifiable chondrules and abundant remnants of chondrules (~7%). Thus, most of the chondrules formed initially have been significantly altered or dissolved by aqueous alterations on their parent bodies. The measured bulk carbon (2.3%) and nitrogen content and their isotopic (δ13C=-5.5‰, δ15N=23.6‰) composition is consistent with CM2.0 classification probably bordering CM1. Several amino acids such as Alanine, Serine, Proline, Valine, Threonine, Leucine, Isoleucine, Asparagine and Histamine are present. Tyrosine and Tryptophan may occur in trace amounts which could not be precisely determined. All these data show that Mukundpura chondrite lies at the boundary of CM2.0 and CM1 type carbonaceous chondrites making it one of the most primitive chondrites.
Geochemical characteristics from tests of four modern planktonic foraminiferal species in the Indonesian Throughflow region and their implications
Peng Zhang, Rina Zuraida, Yair Rosenthal, Ann Holbourn, Wolfgang Kuhnt, Jian Xu
2019, 10(2): 505-516. doi: 10.1016/j.gsf.2018.01.011
Abstract:
Test geochemistry of planktonic foraminifera is an indispensable tool in reconstructing past ocean hydrological changes. It is essential to investigate region-specific implications of test geochemistry, although those established from other regions can be broadly applied. In this study, characteristics of δ18O and Mg/Ca from tests of four planktonic foraminiferal species, Globigerinoides ruber sensu stricto (s.s.), Globigerinoides sacculifer, Pulleniatina obliquiloculata and Neogloboquadrina dutertrei, from 60 core-top sediment samples retrieved from the Indonesian Throughflow (ITF) region were studied. These geochemical data were compared with modern hydrographic profiles in order to assess their relations and to investigate potential implications of test geochemical parameters in reconstructing past oceanographic change in the ITF region. Calcification depths of these four species were first estimated based on comparison between measured test δ18O and predicted calcite δ18O that was calculated from modern temperature and salinity. The results indicate that G. ruber s.s. and G. sacculifer calcify within the mixed-layer at 0-50 m and 20-75 m, respectively, whereas P. obliquiloculata and N. dutertrei calcify within the thermocline at around 75 to 125 m. A combined study of excess Mg/Ca (difference between measured and predicted Mg/Ca) and salinity suggests that salinity exerts a negligible impact on test Mg/Ca of these foraminiferal species in the ITF region. Comparison of test Mg/Ca-derived temperatures with temperature profiles of the upper 200 m of the water column from the seas of the ITF region also indicate calcification depths of these species, which match well with the above estimations using test δ18O. It further indicates that G. sacculifer may be more sensitive in reflecting changes in the depth of the mixed-layer, highlighting a potential use of Mg/Ca temperature difference between G. ruber s.s. and G. sacculifer in reconstructing the depth of the mixed-layer in the ITF region.
Reintegrating nanogranitoid inclusion composition to reconstruct the prograde history of melt-depleted rocks
Omar Bartoli
2019, 10(2): 517-525. doi: 10.1016/j.gsf.2018.02.002
Abstract:
A recent fascinating development in the study of high-grade metamorphic basements is represented by the finding of tiny inclusions of crystallized melt (nanogranitoid inclusions) hosted in peritectic phases of migmatites and granulites. These inclusions have the potential to provide the primary composition of crustal melts at the source. A novel use of the recently-published nanogranitoid compositional database is presented here. Using granulites from the world-renowned Ivrea Zone (NW Italy) on which the original melt-reintegration approach has been previously applied, it is shown that reintegrating melt inclusion compositions from the published database into residual rock compositions can be a further useful method to reconstruct a plausible prograde history of melt-depleted rocks. This reconstruction is fundamental to investigate the tectonothermal history of geological terranes.
The role of carbonate-fluoride melt immiscibility in shallow REE deposit evolution
Jindrich Kynicky, Martin P. Smith, Wenlei Song, Anton R. Chakhmouradian, Cheng Xu, Antonin Kopriva, Michaela Vasinova Galiova, Martin Brtnicky
2019, 10(2): 527-537. doi: 10.1016/j.gsf.2018.02.005
Abstract:
The Lugiin Gol nepheline syenite intrusion, Mongolia, hosts a range of carbonatite dikes mineralized in rare-earth elements (REE). Both carbonatites and nepheline syenite-fluorite-calcite veinlets are host to a previously unreported macroscale texture involving pseudo-graphic intergrowths of fluorite and calcite. The inclusions within calcite occur as either pure fluorite, with associated REE minerals within the surrounding calcite, or as mixed calcite-fluorite inclusions, with associated zirconosilicate minerals. Consideration of the nature of the texture, and the proportions of fluorite and calcite present (~29 and 71 mol%, respectively), indicates that these textures most likely formed either through the immiscible separation of carbonate and fluoride melts, or from cotectic crystallization of a carbonate-fluoride melt. Laser ablation ICP-MS analyses show the pure fluorite inclusions to be depleted in REE relative to the calcite. A model is proposed, in which a carbonate-fluoride melt phase enriched in Zr and the REE, separated from a phonolitic melt, and then either unmixed or underwent cotectic crystallization to generate an REE-rich carbonate melt and an REE-poor fluoride phase. The separation of the fluoride phase (either solid or melt) may have contributed to the enrichment of the carbonate melt in REE, and ultimately its saturation with REE minerals. Previous data have suggested that carbonate melts separated from silicate melts are relatively depleted in the REE, and thus melt immiscibility cannot result in the formation of REE-enriched carbonatites. The observations presented here provide a mechanism by which this could occur, as under either model the textures imply initial separation of a mixed carbonate-fluoride melt from a silicate magma. The separation of an REE-enriched carbonate-fluoride melt from phonolitic magma is a hitherto unrecognized mechanism for REE-enrichment in carbonatites, and may play an important role in the formation of shallow magmatic REE deposits.
Attribution of the Langshan Tectonic Belt: Evidence from zircon U-Pb ages and Hf isotope compositions
Chuang Bao, Yuelong Chen, Xiangkun Zhu, Junxiang Zhao
2019, 10(2): 539-551. doi: 10.1016/j.gsf.2018.03.001
Abstract:
This study describes a previously unidentified Neoproterozoic mafic dyke emplaced in the northern flank of the Langshan Tectonic Belt. This dyke intruded into the micaquartz schist of the Zhaertaishan Group, and yielded an age of 908±8 Ma. The youngest U-Pb ages of micaquartz schist from the Zhaertaishan Group in the Langshan area were 1118±33 Ma, 1187±3 Ma and 1189±39 Ma, suggesting that the depositional age of the protolith of the schist was between 908±8 Ma and 1118±33 Ma. In addition, 436 U-Pb age data and 155 Lu-Hf isotopic data from six samples in the Langshan Tectonic Belt and one Permian greywacke from the Wuhai area show distinct differences between the northern and southern flanks of the Main Langshan area. The U-Pb ages of the northern flank are primarily Meso-Neoproterozoic; similar ages have not been identified in the southern flank to date. Moreover, two-stage Hf model ages of the northern flank feature three age peaks at ~900 Ma, ~1700 Ma and ~2600 Ma; this differs from Hf model ages of the southern flank, which feature one strong age peak at ~2700 Ma. These results suggest that the northern and southern flanks of the Main Langshan area have different geochronologic characteristics and should be divided further. Based on the U-Pb ages and Hf model ages, the northern and southern flanks of the Main Langshan area are named the North and South Langshan Tectonic Belts. Comparison of the U-Pb age and two-stage Hf model age distributions from the North Langshan Tectonic Belt, South Langshan Tectonic Belt, Alxa Block and the North China Craton (NCC) reveal that the North Langshan Tectonic Belt is similar to the Alxa Block and that the South Langshan Tectonic Belt is similar to the NCC. In addition, the zircon U-Pb age of 860±7 Ma commonly observed in the Alxa Block was detected in the Permian greywacke from the Wuhai area of the NCC, which suggests that the amalgamation of the North and South Langshan Tectonic belts (i.e., the amalgamation of the Alxa Block and the NCC), occurred between Devonian and late Permian.
Late Jurassic to Early Cretaceous volcanism of Hong Kong: Insights from the Ping Chau Formation
Lulin Wang, Longlong Zhao, Mingzhong Tian, Fadong Wu, Xujiao Zhang
2019, 10(2): 553-568. doi: 10.1016/j.gsf.2018.03.012
Abstract:
The vast expanse of Mesozoic igneous rocks in Hong Kong contain important geological records of late Mesozoic magmatic events and tectonic processes from the coastal region of Southeast China. Of these, the Ping Chau Formation in the northwestern New Territories is the youngest known stratum. We perform a detailed study of the volcanic rocks of the Ping Chau Formation utilizing zircon U-Pb dating, with major and trace elements geochemistry. LA-ICP-MS zircon U-Pb data reveal Early Cretaceous age from two volcanic rock samples, with zircon crystallization from magmas at 140.3±0.8 Ma and 139.3±0.9 Ma, respectively. These rocks have high contents of total alkalis (Na2O + K2O=5.58-9.45 wt.%), high-field-strength elements and light rare earth elements, conspicuous negative Eu anomalies, and depletions in Nb, Ta, Ti, Sr, Ba and P. Using this data, in combination with previous studies on the late Mesozoic volcanic belt in Southeast China, we propose that the volcanic rocks of the Ping Chau Formation probably originated from deep melting of the crust in a back-arc extensional setting induced by the subduction of the paleo-Pacific Plate. This formation represents the final stages of Early Cretaceous volcanic activity in Hong Kong, as associated with large-scale lithospheric extension, thinning and magmatism. Our results provide new information that can be used in evaluating the significance of Early Cretaceous volcanism and tectonics in Southeast China.
Tourmaline geochemistry and boron isotopic variations as a guide to fluid evolution in the Qiman Tagh W-Sn belt, East Kunlun, China
Zhen Zheng, Yanjing Chen, Xiaohua Deng, Suwei Yue, Hongjin Chen, Qingfei Wang
2019, 10(2): 569-580. doi: 10.1016/j.gsf.2018.04.007
Abstract:
The Qiman Tagh W-Sn belt lies in the westernmost section of the East Kunlun Orogen, NW China, and is associated with early Paleozoic monzogranites, tourmaline is present throughout this belt. In this paper we report chemical and boron isotopic compositions of tourmaline from wall rocks, monzogranites, and quartz veins within the belt, for studying the evolution of ore-forming fluids. Tourmaline crystals hosted in the monzogranite and wall rocks belong to the alkali group, while those hosted in quartz veins belong to both the alkali and X-site vacancy groups. Tourmaline in the walk rocks lies within the schorl-dravite series and becomes increasingly schorlitic in the monzogranite and quartz veins. Detrital tourmaline in the wall rocks is commonly both optically and chemically zoned, with cores being enriched in Mg compared with the rims. In the Al-Fe-Mg and Ca-Fe-Mg diagrams, tourmaline from the wall rocks plots in the fields of Al-saturated and Ca-poor metapelite, and extends into the field of Li-poor granites, while those from the monzogranite and quartz veins lie within the field of Li-poor granites. Compositional substitution is best represented by the MgFe-1, Al(NaR)-1, and AlO(Fe(OH))-1 exchange vectors. A wider range of δ11B values from -11.1‰ to -7.1‰ is observed in the wall-rock tourmaline crystals, the B isotopic values combining with elemental diagrams indicate a source of metasediments without marine evaporates for the wall rocks in the Qiman Tagh belt. The δ11B values of monzogranite-hosted tourmaline range from -10.7‰ and -9.2‰, corresponding to the continental crust sediments, and indicate a possible connection between the wall rocks and the monzogranite. The overlap in δ11B values between wall rocks and monzogranite implies that a transfer of δ11B values by anataxis with little isotopic fractionation between tourmaline and melts. Tourmaline crystals from quartz veins have δ11B values between -11.0‰ and -9.6‰, combining with the elemental diagrams and geological features, thus indicating a common granite-derived source for the quartz veins and little B isotopic fractionation occurred. Tourmalinite in the wall rocks was formed by metasomatism by a granite-derived hydrothermal fluid, as confirmed by the compositional and geological features. Therefore, we propose a single B-rich sedimentary source in the Qiman Tagh belt, and little boron isotopic fractionation occurred during systematic fluid evolution from the wall rocks, through monzogranite, to quartz veins and tourmalinite.
Age and metal source of orogenic gold deposits in Southeast Guizhou Province, China: Constraints from Re-Os and He-Ar isotopic evidence
Jiasheng Wang, Hanjie Wen, Chao Li, Jinrang Zhang, Wei Ding
2019, 10(2): 581-593. doi: 10.1016/j.gsf.2018.05.012
Abstract:
The orogenic gold deposits in Southeast Guizhou are an important component of the Xuefeng polymetallic ore belt and have significant exploration potential, but geochronology research on these gold deposits is scarce. Therefore, the ore genetic models are poorly constrained and remain unclear. In the present study, two important deposits (Pingqiu and Jinjing) are investigated, including combined Re-Os dating and the He-Ar isotope study of auriferous arsenopyrites. It is found that the arsenopyrites from the Pingqiu gold deposit yielded an isochron age of 400±24 Ma, with an initial 187Os/188Os ratio of 1.24±0.57 (MSWD=0.96). An identical isochron age of 400±11 Ma with an initial 187Os/188Os ratio of 1.55±0.14 (MSWD=0.34) was obtained from the Jinjing deposit. These ages correspond to the regional Caledonian orogeny and are interpreted to represent the age of the main stage ore. Both initial 187Os ratios suggest that the Os was derived from crustal rocks. Combined with previous rare earth element (REE), trace elements, Nd-Sr-S-Pb isotope studies on scheelite, inclusion fluids with other residues of gangue quartz, and sulfides from other gold deposits in the region, it is suggested that the ore metals from Pingqiu and Jinjing were sourced from the Xiajiang Group. The He and Ar isotopes of arsenopyrites are characterized by 3He/4He ratios ranging from 5.3×10-4 Ra to 2.5×10-2 Ra (Ra=1.4×10-6, the 3He/4He ratio of air), 40Ar*/4He ratios from 0.64×10-2 to 15.39×10-2, and 40Ar/36Ar ratios from 633.2 to 6582.0. Those noble gas isotopic compositions of fluid inclusions also support a crustal source origin, evidenced by the Os isotope. Meanwhile, recent noble gas studies suggest that the amount of in situ radiogenic 4He generated should not be ignored, even when Th and U are present at levels of only a few ppm in host minerals.
Petrogenesis of the Late Triassic shoshonitic Shadegai pluton from the northern North China Craton: Implications for crust-mantle interaction and post-collisional extension
Liqiong Jia, Liang Wang, Genhou Wang, Shibin Lei, Xuan Wu
2019, 10(2): 595-610. doi: 10.1016/j.gsf.2018.08.002
Abstract(130) HTML PDF(1)
Abstract:
Latest Permian to Triassic plutons are widespread in the northern North China Craton (NCC); most of them show calc-alkaline, high-K calc-alkaline, or alkaline geochemical features. The Shadegai pluton in the Wulashan area has shoshonitic affinity and I-type character, and is composed of syenogranites containing abundant mafic microgranular enclaves (MMEs). LA-MC-ICP-MS U-Pb data yield weighted mean 206Pb/238U ages of 222±1 Ma and 221±1 Ma for the syenogranites and MMEs, respectively, suggesting their coeval formation during the Late Triassic. The syenogranites have high SiO2 (70.42-72.30 wt.%), K2O (4.58-5.22 wt.%) and Na2O (4.19-4.43 wt.%) contents but lower concentrations of P2O5 (0.073-0.096 wt.%) and TiO2 (0.27-0.37 wt.%), and are categorized as I-type granites, rather than A-type granites, as previously thought. These syenogranites exhibit lower (87Sr/86Sr)i ratios (0.70532-0.70547) and strongly negative whole-rock εNd(t) values (-12.54 to -11.86) and zircon εHf(t) values (-17.81 to -10.77), as well as old Nd (1962-2017 Ma) and Hf (2023-2092 Ma) model ages, indicating that they were derived from the lower crust. Field and petrological observations reveal that the MMEs within the pluton probably represent magmatic globules commingled with their host magmas. Geochemically, these MMEs have low SiO2 (53.46-55.91 wt.%) but high FeOt (7.27-8.79 wt.%) contents. They are enriched in light rare earth elements (LREEs) and large ion lithophile elements (LILEs), and are depleted in heavy rare earth elements (HREEs) and high field strength elements (HFSEs). They have whole-rock (87Sr/86Sr)i ratios varying from 0.70551 to 0.70564, εNd(t) values of -10.63 to -9.82, and zircon εHf(t) values of -9.89 to 0.19. Their geochemical and isotopic features indicate that they were derived from the subcontinental lithospheric mantle mainly metasomatized by slab-derived fluids, with minor involvement of melts generated from the ascending asthenospheric mantle. Petrology integrated with elemental and isotopic geochemistry suggest that the Shadegai pluton was produced by crust-mantle interactions, i.e., partial melting of the lower continental crust induced by underplating of mantle-derived mafic magmas (including the subcontinental lithospheric mantle and asthenospheric mantle), and subsequent mixing of the mantle- and crust-derived magmas. In combination with existing geological data, it is inferred that the Shadegai pluton formed in a post-collisional extensional regime related to lithospheric delamination following the collision between the NCC and Mongolia arc terranes.
São Francisco-Congo Craton break-up delimited by U-Pb-Hf isotopes and trace-elements of zircon from metasediments of the Araçuaí Belt
Mathias Schannor, Cristiano Lana, Marco A. Fonseca
2019, 10(2): 611-628. doi: 10.1016/j.gsf.2018.02.011
Abstract:
Detrital zircon U-Pb geochronology combined with Hf isotopic and trace element data from metasedimentary rocks of the Araçuaí Belt in southeastern Brazil provide evidence for break-up of the Congo-São Francisco Craton. The U-Pb age spectra of detrital zircons from metasediments of the Rio Doce Group (RDG) range from 900-650 Ma and define a maximum depositional age of ca. 650 Ma. Zircon trace element and whole rock data constrain an oceanic island arc as source for the deposition setting of the protoliths to the metasediments. Zircon εHf(t) values from these rocks are positive between +1 and +15, supporting previous evidence of a Neoproterozoic extensional phase and oceanic crust formation in a precursor basin to the Araçuaí Belt. Recrystallization of detrital zircon at ca. 630 Ma is compatible with a regional metamorphic event associated with terrane accretion to the Paleoproterozoic basement after transition from an extensional to a convergent regime. The juvenile nature, age spectra and trace element composition recorded in detrital zircons of metasediments from the Araçuaí Belt correspond with zircons from metasedimentary rocks and oceanic crust remnants of other orogenic belts to its south. This suggests that rifting and oceanic crust formation of the entire orogenic system, the so-called Mantiqueira Province, was contemporaneous, most likely related to the opening of a large ocean. It further indicates that the cratonic blocks involved in the orogenic evolution of the Mantiqueira Province were spatially connected as early as 900 Ma.
Hydrothermally induced diagenesis: Evidence from shallow marine-deltaic sediments, Wilhelmøya, Svalbard
Beyene G. Haile, Urszula Czarniecka, Kelai Xi, Aleksandra Smyrak-Sikora, Jens Jahren, Alvar Braathen, Helge Hellevang
2019, 10(2): 629-649. doi: 10.1016/j.gsf.2018.02.015
Abstract:
Sedimentary basins containing igneous intrusions within sedimentary reservoir units represent an important risk in petroleum exploration. The Upper Triassic to Lower Jurassic sediments at Wilhelmøya (Svalbard) contains reservoir heterogeneity as a result of sill emplacement and represent a unique case study to better understand the effect of magmatic intrusions on the general burial diagenesis of siliciclastic sediments. Sills develop contact metamorphic aureoles by conduction as presented in many earlier studies. However, there is significant impact of localized hydrothermal circulation systems affecting reservoir sediments at considerable distance from the sill intrusions. Dolerite sill intrusions in the studied area are of limited vertical extent (~12 m thick), but created localized hydrothermal convection cells affecting sediments at considerable distance (more than five times the thickness of the sill) from the intrusions. We present evidence that the sedimentary sequence can be divided into two units: (1) the bulk poorly lithified sediment with a maximum burial temperature much lower than 60-70 ℃, and (2) thinner intervals outside the contact zone that have experienced hydrothermal temperatures (around 140 ℃). The main diagenetic alteration associated with normal burial diagenesis is minor mechanical plastic deformation of ductile grains such as mica. Mineral grain contacts show no evidence of pressure dissolution and the vitrinite reflectance suggests a maximum temperature of ~40 ℃. Contrary to this, part of the sediment, preferentially along calcite cemented flow baffles, show evidence of hydrothermal alteration. These hydrothermally altered sediment sections are characterized by recrystallized carbonate cemented intervals. Further, the hydrothermal solutions have resulted in localized sericitization (illitization) of feldspars, albitization of both K-feldspar and plagioclase and the formation of fibrous illite nucleated on kaolinite. These observations suggest hydrothermal alteration at T > 120-140 ℃ at distances considerably further away than expected from sill heat dissipation by conduction only, which commonly affect sediments about twice the thickness of the sill intrusion. We propose that carbonate-cemented sections acted as flow baffles already during the hydrothermal fluid mobility and controlled the migration pathways of the buoyant hot fluids. Significant hydrothermally induced diagenetic alterations affecting the porosity and hence reservoir quality was not noted in the noncarbonate-cemented reservoir intervals.
Re-folded structure of syn-orogenic granitoids (Padrón dome, NW Iberia): Assessing rheological evolution of cooling continental crust in a collisional setting
Rubén Díez Fernández, Francisco J. Rubio Pascual, Luis Miguel Martín Parra
2019, 10(2): 651-669. doi: 10.1016/j.gsf.2018.03.007
Abstract:
This contribution discusses about the rheological, kinematic and dynamic frameworks necessary to produce recumbent and upright folds from syn-orogenic granitic massifs that were formed during an early stage of magma genesis related to the onset of a migmatitic dome. Syn-kinematic granitoids occurring within the high-grade infrastructure of the Padrón migmatitic dome (NW Iberia) are deformed into large-scale recumbent folds (D2) that are later affected by upright folds (D3). Petrostructural analysis of a selected area of this dome reveals that after a period of crustal thickening (D1), NNW-directed extensional flow gave way to recumbent folds and penetrative axial plane foliation (S2). Superimposed subhorizontal compression resulted in upright folds (D3). A closer view into the dynamics of the dome allows exploring the factors that may condition the nucleation of folds with contrasting geometries during progressive deformation of molten continental crust. The formation of folds affecting syn-kinematic granitoids suggests a cooling metamorphic path in migmatitic domes. Active and passive folding mechanisms require a crystallizing (cooling) magma to nucleate folds. A more competent metamorphic host inhibits fold nucleation from much less competent magmas. As it crystallizes, magma becomes more rigid (competent), and approaches viscosity values of its host. Passive folding is favored when no significant competence contrast exists between magma and host, so this folding mechanism is more likely shortly after magma genesis and emplacement. In such conditions, and under dominant subhorizontal flow accompanied by flattening (D2), passive folding would produce isoclinal recumbent geometries. Further magma cooling introduces a shift into the rheological behavior of partially molten crust. Thereon, crystallizing magma bodies would represent significant competence contrasts relative to their host. At this point, buckling is a more likely folding mechanism, and more regular, buckle folds re-fold previous structures after significant cooling. The geometry of resulting folds is upright due to dominant subhorizontal compression (D3) at this stage.
Advanced reliability analysis of slopes in spatially variable soils using multivariate adaptive regression splines
Leilei Liu, Shaohe Zhang, Yung-Ming Cheng, Li Liang
2019, 10(2): 671-682. doi: 10.1016/j.gsf.2018.03.013
Abstract(119) HTML PDF(7)
Abstract:
This study aims to extend the multivariate adaptive regression splines (MARS)-Monte Carlo simulation (MCS) method for reliability analysis of slopes in spatially variable soils. This approach is used to explore the influences of the multiscale spatial variability of soil properties on the probability of failure (Pf ) of the slopes. In the proposed approach, the relationship between the factor of safety and the soil strength parameters characterized with spatial variability is approximated by the MARS, with the aid of Karhunen-Loève expansion. MCS is subsequently performed on the established MARS model to evaluate Pf. Finally, a nominally homogeneous cohesive-frictional slope and a heterogeneous cohesive slope, which are both characterized with different spatial variabilities, are utilized to illustrate the proposed approach. Results showed that the proposed approach can estimate the Pf of the slopes efficiently in spatially variable soils with sufficient accuracy. Moreover, the approach is relatively robust to the influence of different statistics of soil properties, thereby making it an effective and practical tool for addressing slope reliability problems concerning time-consuming deterministic stability models with low levels of Pf. Furthermore, disregarding the multiscale spatial variability of soil properties can overestimate or underestimate the Pf. Although the difference is small in general, the multiscale spatial variability of the soil properties must still be considered in the reliability analysis of heterogeneous slopes, especially for those highly related to cost effective and accurate designs.
Geochronology and geochemistry of Permo-Triassic sandstones in eastern Jilin Province (NE China): Implications for final closure of the Paleo-Asian Ocean
Qingxiang Du, Zuozhen Han, Xiaoli Shen, Chao Han, Zhigang Song, Lihua Gao, Mei Han, Wenjian Zhong
2019, 10(2): 683-704. doi: 10.1016/j.gsf.2018.03.014
Abstract:
In the eastern part of the Central Asian Orogenic Belt (CAOB) in northeastern (NE) China, scattered outcrops of molasse deposits mark the ending of an orogeny and are crucial for understanding the evolution of the Paleo-Asian Ocean (PAO). However, the timing of tectonic events and the relationships among these strata remain controversial. To better constrain these geologic events, a comprehensive study of the detrital zircon U-Pb geochronology and geochemistry of the sandstones of the Kaishantun (KST) Formation and Kedao (KD) Group in eastern Jilin Province, NE China, was conducted. The KST Formation is traditionally considered a molasse deposit. The sandstones display low CIA, PIA and high ICV values and low Th/U and Rb/Sr ratios, which suggest that the rocks were derived from an immature intermediate-felsic igneous source and experienced a simple sedimentary recycling history with relatively weak chemical weathering. LA-ICP-MS U-Pb dating of detrital zircons from two samples of the KST Formation yields ages of 748-252 Ma, suggesting that the KST Formation was deposited between 254.5 Ma and 252 Ma in Late Permian. The zircons were mainly derived from the continental northern part of the North China Craton (NCC). In contrast, the U-Pb dating of detrital zircons from five samples of the KD Group yields ages of 2611-230 Ma, suggesting that the KD samples were deposited in the Early to Middle Triassic (ca. 248-233 Ma). The detrital zircon ages for the KD samples can be divided into groups with peaks at 2.5 Ga, 1.8 Ga, 800-1000 Ma, 500 Ma and 440-360 Ma, which suggest that the samples were derived from bidirectional provenances in the Jiamusi-Khanka Block and the NCC. These new data, combined with previously published results, suggest that at least three orogenic events occurred in central-eastern Jilin Province during the Early Permian (270-262 Ma), Early Triassic (254-248 Ma) and Middle-Late Triassic (242-227 Ma). The final closure of the PAO occurred during 242-227 Ma in the Middle-Late Triassic along the Changchun-Yanji suture zone. The detrital zircon geochronological data clearly record plate convergence and the scissor-like closure of the PAO in the eastern CAOB.
Petrogenesis of Middle-Eocene granitoids and their Mafic microgranular enclaves in central Urmia-Dokhtar Magmatic Arc (Iran): Evidence for interaction between felsic and mafic magmas
Kazem Kazemi, Ali Kananian, Yilin Xiao, Fatemeh Sarjoughian
2019, 10(2): 705-723. doi: 10.1016/j.gsf.2018.04.006
Abstract(102) HTML PDF(4)
Abstract:
Whole rock major and trace element geochemistry together with zircon U-Pb ages and Sr-Nd isotope compositions for the Middle Eocene intrusive rocks in the Haji Abad region are presented. The granitoid hosts, including granodiorite and diorite, yielded zircon U-Pb ages with a weighted mean value of 40.0±0.7 Ma for the granodiorite phase. Mafic microgranular enclaves (MMEs) are common in these plutons, and have relatively low SiO2 contents (53.04-57.08 wt.%) and high Mg# (42.6-60.1), probably reflecting a mantle-derived origin. The host rocks are metaluminous (A/CNK=0.69-1.03), arc-related calc-alkaline, and I-type in composition, possessing higher SiO2 contents (59.7-66.77 wt.%) and lower Mg# (38.6-52.2); they are considered a product of partial melting of the mafic lower crust. Chondrite-normalized REE patterns of the MMEs and granitoid hosts are characterized by LREE enrichment and show slight negative Eu anomalies (Eu/Eu*=0.60-0.93). The host granodiorite samples yield (87Sr/86Sr)i ratios ranging from 0.70498 to 0.70591, positive εNd(t) values varying from +0.21 to +2.3, and TDM2 ranging from 760 to 909 Ma, which is consistent with that of associated mafic microgranular enclaves (87Sr/86Sr)i=0.705111-0.705113, εNd(t)=+2.14 to +2.16, TDM2=697-785 Ma). Petrographic and geochemical characterization together with bulk rock Nd-Sr isotopic data suggest that host rocks and associated enclaves originated by interaction between basaltic lower crust-derived felsic and mantle-derived mafic magmas in an active continental margin arc environment.
Oligocene subduction-related plutonism in the Nodoushan area, Urumieh-Dokhtar magmatic belt: Petrogenetic constraints from U-Pb zircon geochronology and isotope geochemistry
Badieh Shahsavari Alavijeh, Nematollah Rashidnejad-Omran, Fatma Toksoy-Köksal, Wenliang Xu, Jalil Ghalamghash
2019, 10(2): 725-751. doi: 10.1016/j.gsf.2018.03.017
Abstract:
Geochemical data and Sr-Nd isotopes of the host rocks and magmatic microgranular enclaves (MMEs) collected from the Oligocene Nodoushan Plutonic Complex (NPC) in the central part of the Urumieh-Dokhtar Magmatic Belt (UDMB) were studied in order to better understand the magmatic and geodynamic evolution of the UDMB. New U-Pb zircon ages reveal that the NPC was assembled incrementally over ca. 5 m.y., during two main episodes at 30.52±0.11 Ma and 30.06±0.10 Ma in the early Oligocene (middle Rupelian) for dioritic and granite intrusives, and at 24.994±0.037 Ma and 24.13±0.19 Ma in the late Oligocene (latest Chattian) for granodioritic and diorite porphyry units, respectively. The spherical to ellipsoidal enclaves are composed of diorite to monzodiorite and minor gabbroic diorite (SiO2=47.73-57.36 wt.%; Mg#=42.15-53.04); the host intrusions are mainly granite, granodiorite and diorite porphyry (SiO2=56.51-72.35 wt.%; Mg#=26.29-50.86). All the samples used in this study have similar geochemical features, including enrichment in large ion lithophile elements (LILEs, e.g. Rb, Ba, Sr) and light rare earth elements (LREEs) relative to high field strength elements (HFSEs) and heavy rare earth elements (HREEs). These features, combined with a relative depletion in Nb, Ta, Ti and P, are characteristic of subduction-related magmas. Isotopic data for the host rocks display ISr=0.705045-0.707959, εNd(t)=-3.23 to +3.80, and the Nd model ages (TDM) vary from 0.58 Ga to 1.37 Ga. Compared with the host rocks, the MMEs are relatively homogeneous in isotopic composition, with ISr ranging from 0.705513 to 0.707275 and εNd(t) from -1.46 to 4.62. The MMEs have TDM ranging from 0.49 Ga to 1.39 Ga. Geochemical and isotopic similarities between the MMEs and their host rocks demonstrate that the enclaves have mixed origins and were most probably formed by interactions between the lower crust- and mantle-derived magmas. Geochemical data, in combination with geodynamic evidence, suggest that a basic magma was derived from an enriched subcontinental lithospheric mantle (SCLM), presumably triggered by the influx of the hot asthenosphere. This magma then interacted with a crustal melt that originated from the dehydration melting of the mafic lower crust at deep crustal levels. Modeling based on Sr-Nd isotope data indicate that ~50% to 90% of the lower crust-derived melt and ~10% to 50% of the mantle-derived mafic magma were involved in the genesis of the early Oligocene magmas. In contrast, ~45%-65% of the mantle-derived mafic magma were incorporated into the lower crust-derived magma (~35%-55%) that generated the late Oligocene hybrid granitoid rocks. Early Oligocene granitoid rocks contain a higher proportion of crustal material compared to those that formed in the late Oligocene. It is reasonable to assume that lower crust and mantle interaction processes played a significant role in the genesis of these hybridgranitoid bodies, where melts undergoing fractional crystallization along with minor amounts of crustal assimilation could ascend to shallower crustal levels and generate a variety of rock types ranging from diorite to granite.
Evaluation of potential landslide damming: Case study of Urni landslide, Kinnaur, Satluj valley, India
Vipin Kumar, Vikram Gupta, Imlirenla Jamir, Shovan Lal Chattoraj
2019, 10(2): 753-767. doi: 10.1016/j.gsf.2018.05.004
Abstract(133) HTML PDF(4)
Abstract:
This work aims to understand the process of potential landslide damming using slope failure mechanism, dam dimension and dam stability evaluation. The Urni landslide, situated on the right bank of the Satluj River, Himachal Pradesh (India) is taken as the case study. The Urni landslide has evolved into a complex landslide in the last two decade (2000-2016) and has dammed the Satluj River partially since year 2013, damaging ~200 m stretch of the National Highway (NH-05). The crown of the landslide exists at an altitude of ~2180-2190 m above msl, close to the Urni village that has a human population of about 500. The high resolution imagery shows ~50 m long landslide scarp and ~100 m long transverse cracks in the detached mass that implies potential for further slope failure movement. Further analysis shows that the landslide has attained an areal increase of 103,900±1142 m2 during year 2004-2016. About 86% of this areal increase occurred since year 2013. Abrupt increase in the annual mean rainfall is also observed since the year 2013. The extreme rainfall in the June, 2013; 11 June (~100 mm) and 16 June (~115 mm), are considered to be responsible for the slope failure in the Urni landslide that has partially dammed the river. The finite element modelling (FEM) based slope stability analysis revealed the shear strain in the order of 0.0-0.16 with 0.0-0.6 m total displacement in the detachment zone. Further, kinematic analysis indicated planar and wedge failure condition in the jointed rockmass. The debris flow runout simulation of the detached mass in the landslide showed a velocity of ~25 m/s with a flow height of ~15 m while it (debris flow) reaches the valley floor. Finally, it is also estimated that further slope failure may detach as much as 0.80±0.32 million m3 mass that will completely dam the river to a height of 76±30 m above the river bed.
Geochronology and mineralogy of the Weishan carbonatite in Shandong province, eastern China
Chen Wang, Jianchao Liu, Haidong Zhang, Xinzhu Zhang, Deming Zhang, Zhixuan Xi, Zijie Wang
2019, 10(2): 769-785. doi: 10.1016/j.gsf.2018.07.008
Abstract:
The Weishan REE deposit is located at the eastern part of North China Craton (NCC), western Shandong Province. The REE-bearing carbonatite occur as veins associated with aegirine syenite. LA-ICP-MS bastnaesite Th-Pb ages (129 Ma) of the Weishan carbonatite show that the carbonatite formed contemporary with the aegirine syenite. Based on the petrographic and geochemical characteristics of calcite, the REE-bearing carbonatite mainly consists of Generation-1 igneous calcite (G-1 calcite) with a small amount of Generation-2 hydrothermal calcite (G-2 calcite). Furthermore, the Weishan apatite is characterized by high Sr, LREE and low Y contents, and the carbonatite is rich in Sr, Ba and LREE contents. The δ13CV-PDB (-6.5‰ to -7.9‰) and δ13OV-SMOW (8.48‰-9.67‰) values are similar to those of primary, mantle-derived carbonatites. The above research supports that the carbonatite of the Weishan REE deposit is igneous carbonatite. Besides, the high Sr/Y, Th/U, Sr and Ba of the apatite indicate that the magma source of the Weishan REE deposit was enriched lithospheric mantle, which have suffered the fluid metasomatism. Taken together with the Mesozoic tectono-magmatic activities, the NW and NWW subduction of Izanagi plate along with lithosphere delamination and thinning of the North China plate support the formation of the Weishan REE deposit. Accordingly, the mineralization model of the Weishan REE deposit was concluded: The spatial-temporal relationships coupled with rare and trace element characteristics for both carbonatite and syenite suggest that the carbonatite melt was separated from the CO2-rich silicate melt by liquid immiscibility. The G-1 calcites were crystallized from the carbonatite melt, which made the residual melt rich in rare earth elements. Due to the common origin of G-1 and G-2 calcites, the REE-rich magmatic hydrothermal was subsequently separated from the melt. After that, large numbers of rare earth minerals were produced from the magmatic hydrothermal stage.
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