2021 Vol. 12, No. 4
Turbidites from the Shiquanhe-Namco Ophiolite Mélange Zone (SNMZ) record critical information about the tectonic affinity of the SNMZ and the evolutionary history of the Meso-Tethys Ocean in Tibet. This paper reports sedimentologic, sandstone petrographic, zircon U-Pb geochronologic, and clastic rocks geochemical data of newly identified turbidites (Asa Formation) in the Asa Ophiolite Mélange. The youngest ages of detrital zircon from the turbiditic sandstone samples, together with ~115 Ma U-Pb concordant age from the tuff intercalation within the Asa Formation indicate an Early Cretaceous age. The sandstone mineral modal composition data show that the main component is quartz grains and the minor components are sedimentary and volcanic fragments, suggesting that the turbidites were mainly derived from a recycled orogen provenance with a minor addition of volcanic arc materials. The detrital U-Pb zircon ages of turbiditic sandstones yield main age populations of 170-120 Ma, 300-220 Ma, 600-500 Ma, 1000-700 Ma, 1900-1500 Ma, and ~2500 Ma, similar to the ages of the Qiangtang Terrane(age peak of 600-500 Ma, 1000-900 Ma, ~1850 Ma and ~2500 Ma) and the accretionary complex in the Bangong-Nujiang Ophiolite Zone (BNMZ) rather than the age of the Central Lhasa Terrane (age peak of ~300 Ma, ~550 Ma and ~1150 Ma). The mineral modal compositions, detrital U-Pb zircon ages, and geochemical data of clastic rocks suggest that the Asa Formation is composed of sediments primarily recycled from the Jurassic accretionary complex within the BNMZ with the secondary addition of intermediate-felsic island arc materials from the South Qiangtang Terrane. Based on our new results and previous studies, we infer that the SNMZ represents a part of the Meso-Tethys Suture Zone, rather than a southward tectonic klippe of the BNMZ or an isolated ophiolitic mélange zone within the Lhasa Terrane. The Meso-Tethys Suture Zone records the continuous evolutionary history of the northward subduction, accretion, arc-Lhasa collision, and Lhasa-Qiangtang collision of the Meso-Tethys Ocean from the Early Jurassic to the Early Cretaceous.
As a GIS tool, visibility analysis is used in many areas to evaluate both visible and non-visible places. Visibility analysis builds on a digital surface model describing the terrain morphology, including the position and shapes of all objects that can sometimes act as visibility barriers. However, some barriers, for example vegetation, may be permeable to a certain degree. Despite extensive research and use of visibility analysis in different areas, standard GIS tools do not take permeability into account. This article presents a new method to calculate visibility through partly permeable obstacles. The method is based on a quasi-Monte Carlo simulation with 100 iterations of visibility calculation. Each iteration result represents 1% of vegetation permeability, which can thus range from 1% to 100% visibility behind vegetation obstacles. The main advantage of the method is greater accuracy of visibility results and easy implementation on any GIS software. The incorporation of the proposed method in GIS software would facilitate work in many fields, such as architecture, archaeology, radio communication, and the military.
Few global syntheses of oxygen and carbon isotope composition of pedogenic carbonates have been attempted, unlike marine carbonates. Pedogenic carbonates represent in-situ indicators of the climate conditions prevailing on land. The δ18O and δ13C values of pedogenic carbonates are controlled by local and global factors, many of them not affecting the marine carbonates largely used to probe global climate changes. We compile pedogenic oxygen and carbon isotopic data (N=12,167) from Cretaceous to Quaternary-aged paleosols to identify potential trends through time and tie them to possible controlling factors. While discrete events such as the Paleocene-Eocene Thermal Maximum are clearly evidenced, our analysis reveals an increasing complexity in the distribution of the δ18O vs δ13C values through the Cenozoic. As could be expected, the rise of C4 plants induces a shift towards higher δ13C values during the Neogene and Quaternary. We also show that the increase in global hypsometry during the Neogene plays a major role in controlling the δ18O and δ13C values of pedogenic carbonates by increasing aridity downwind of orographic barriers. Finally, during the Quaternary, an increase of 3‰ in δ18O values is recorded both by the pedogenic carbonates and the marine foraminifera suggesting that both indicators may be used to track global climate signal.
Displacement is vital in the evaluations of tunnel excavation processes, as well as in determining the post-excavation stability of surrounding rock masses. The prediction of tunnel displacement is a complex problem because of the uncertainties of rock mass properties. Meanwhile, the variation and the correlation relationship of geotechnical material properties have been gradually recognized by researchers in recent years. In this paper, a novel probabilistic method is proposed to estimate the uncertainties of rock mass properties and tunnel displacement, which integrated multivariate distribution function and a relevance vector machine (RVM). The multivariate distribution function is used to establish the probability model of related random variables. RVM is coupled with the numerical simulation methods to construct the nonlinear relationship between tunnel displacements and rock mass parameters, which avoided a large number of numerical simulations. Also, the residual rock mass parameters are taken into account to reflect the brittleness of deeply buried rock mass. Then, based on the proposed method, the uncertainty of displacement in a deep tunnel of CJPL-II laboratory are analyzed and compared with the in-situ measurements. It is found that the predicted tunnel displacements by the RVM model closely match with the measured ones. The correlations of parameters have significant impacts on the uncertainty results. The uncertainty of tunnel displacement decreases while the reliability of the tunnel increases with the increases of the negative correlations among rock mass parameters. When compared to the deterministic method, the proposed approach is more rational and scientific, and also conformed to rock engineering practices.
Multiple source provenance of sediments and submarine fan formation are herein investigated based on Quaternary sandy barriers of the Brazilian Southernmost Coastal Plain. LA-ICP-MS dating on 1625 detrital zircons from marine and aeolian facies sands of four successive lagoon-barrier systems are analyzed. The characterization of Archean to Pleistocene zircons into a younger Andean (22 ±4 Ma to 1 ±1 Ma, 48 from 1625), Mesozoic to Paleozoic, and Mesoproterozoic (479 from 1622) populations suggest that the main feeding of sediments to the coastal plain occurred through the La Plata River drainage system. The significant contribution of sediments is transported from the mouth of La Plata River northward by longshore circulation (littoral drift). Minor contributions are also recognized as. a farther source associated with the Patagonia drainage and nearby source related to the Uruguay/Rio Grande do Sul Shield and the Paraná Basin, drained by the Camaquã and Jacuí rivers. The latter one is recognized by the contribution from Neoproterozoic to Early Paleozoic, and some Paleoproterozoic and Archean zircon grains. The definition of the sources of clastic sediments allows inferences about the origination of Rio Grande Fan where both the cold Falkland and the warm Brazil currents played a major role.
New isotopic, geochemical and geochronological data justify the widespread occurrence of middle Paleocene to early Eocene (60-53 Ma) post-subduction felsic magmatism across the entire Sikhote-Alin territory (southeastern Russia), conform with previous observations in Northeast China, the southern Korean Peninsula, and the Inner Zone of Japan. This igneous activity in East Asia coincided with the reactivation (after tectonic quiescence between ~93-60 Ma) of left-lateral strike-slip displacements along the Tan-Lu and Central Sikhote-Alin faults and with the post-60 Ma cessation of subduction/accretion recorded in the Shimanto belt of SW Japan. The Sikhote-Alin post-subduction igneous A-type rocks present diverse mineralogical and geochemical features that suggest interactions of the subducting plate with anhydrous mantle upwelling through slab tears in the continental margin. The middle Paleocene-early Eocene magmatism is not related to subduction but is synchronous with strike-slip tectonics and the termination of accretionary prism development, suggesting a shift in tectonic regime from oceanic plate subduction at a convergent margin to parallel sliding and initiation of a transform continental margin. These new observations are inconsistent with the current tectonic model of 60-50 Ma Izanagi-Pacific ridge subduction beneath East Asian continental margin.
Massive gas emissions (e.g., CO2, CH4 and SO2) during the formation of large igneous provinces (LIPs) have been suggested as the primary cause of dramatic climatic change and the consequent ecological collapses and biotic crises. Thermogenic carbon of crustal sediments induced by intrusive magmatism throughout the LIPs is considered as the primary trigger for environmental catastrophe including mass extinction, as illustrated in the case of the Emeishan LIP in Southwest China. Here we evaluate the Emeishan LIP to address the causal link between carbon degassing and environmental crises during the end-Guadalupian of Middle Permian. An assessment of the carbon flux degassed from recycled oceanic crust in the Emeishan plume shows that recycled oceanic crust contributed significantly to the carbon flux. Using evidence from carbonate carbon isotopic records at the Gualupian-Lopingian (G-L) boundary stratotype at Penglaitan of South China, our study suggests that carbon degassed from massive recycled components in the Emeishan plume served as a major end-Guadalupian (Middle Permian) carbon isotope excursion. The model based on the Emeishan LIP also offers new insights into the important role of recycled carbon released from other LIPs in climatic change and mass extinctions, as in the cases of the end-Permian Siberian and end-Cretaceous Deccan Traps. Our work highlights that carbon released from subducted slabs is returned to the atmosphere via upwelling mantle plumes, which could drive global climatic change and mass extinction.
The discovery of unconventional hydrocarbon resources since the late 20th century changed geologists' understanding of hydrocarbon migration and accumulations and provides a solution to energy shortage. In 2016, unconventional oil production in the USA accounted for 41% of the total oil production; and unconventional natural gas production in China accounted for 35% of total gas production, showing strong growth momentum of unconventional hydrocarbons explorations. Unconventional hydrocarbons generally coexist with conventional petroleum resources; they sometimes distribute in a separate system, not coexisting with a conventional system. Identification and prediction of unconventional resources and their potentials are prominent challenges for geologists. This study analyzed the results of 12,237 drilling wells in six representative petroliferous basins in China and studied the correlations and differences between conventional and unconventional hydrocarbons by comparing their geological features. Migration and accumulation of conventional hydrocarbon are caused dominantly by buoyance. We propose a concept of buoyance-driven hydrocarbon accumulation depth to describe the deepest hydrocarbon accumulation depth driven dominantly by buoyance; beyond this depth the buoyance becomes unimportant for hydrocarbon accumulation. We found that the buoyance-driven hydrocarbon accumulation depth in petroliferous basins controls the different oil/gas reservoirs distribution and resource potentials. Hydrocarbon migration and accumulations above this depth is dominated by buoyancy, forming conventional reservoirs in traps with high porosity and permeability, while hydrocarbon migration and accumulation below this depth is dominated by non-buoyancy forces (mainly refers to capillary force, hydrocarbon volume expansion force, etc.), forming unconventional reservoirs in tight layers. The buoyance-driven hydrocarbon accumulation depths in six basins in China range from 1200 m to 4200 m, which become shallower with increasing geothermal gradient, decreasing particle size of sandstone reservoir layers, or an uplift in the whole petroliferous basin. The predicted unconventional resource potential below the buoyance-driven hydrocarbon accumulation depth in six basins in China is more than 15.71×109 t oil equivalent, among them 4.71×109 t reserves have been proved. Worldwide, 94% of 52,926 oil and gas reservoirs in 1186 basins are conventional reservoirs and only 6% of them are unconventional reservoirs. These 94% conventional reservoirs show promising exploration prospects in the deep area below buoyance-driven hydrocarbon accumulation depth.
Serpentinized peridotites in the Yangkou (YK), Suoluoshu (SLS) and Hujialin (HJL) areas in the Sulu ultrahigh-pressure terrane represent the relic of ancient subcontinental lithospheric mantle below the North China Craton. Their protoliths, harzburgite and dunite, were variably hydrated by aqueous fluids released from subducting Yangtze continent. The rocks are enriched in fluid-mobile elements (FME) including Sb (42-333 times the depleted mantle value) and Pb (30-476 times). The degrees of the FME enrichment are comparable to that of the Himalayan forearc serpentinites, and greater than forearc mantle serpentinites from Marianas, suggesting that the degrees of FME enrichment in the forearc serpentinites are greater in continental subduction zones than those in the oceanic subduction zones. Lizardite after olivine in the SLS serpentinite shows higher degrees of enrichment in Sb and As than those for antigorite after both olivine and orthopyroxene in the YK area. The antigorite has highly enriched in Pb, U, Cs, and LREE, but not for the lizardite. The abundance of FME in two different species of serpentine reflects the different temperature of hydration. At temperature lower than 300℃, formed lizardite at shallow depths of the mantle wedge incorporates elements that are fluid mobile at low temperatures, such as Sb and As. When the temperature greater than 300℃, formed antigorite at a relatively deep mantle wedge incorporate more FME from the subducting continental slab (or fragments), including Pb, U, Cs, LREE as well as Sb and As. The eventual breakdown of antigorite (600-700℃) in prograde metamorphism would discharge water as well as FME into the subducting channel and/or the overlying mantle.
This study examines the Ga isotopic compositions of sulfides in the Yuhuang and Duanqiao hydrothermal fields on the Southwest Indian Ridge, mid-ocean ridge basalts (MORB), and calcareous sediments around the hydrothermal fields. The δ71/69GaNIST-994 values of the MORB samples vary little (+1.20‰ to +1.23‰, with an average of +1.22‰) and are consistent with the δ71/69GaNIST-994 values of two standard basalt samples (BCR-2 and BHVO-2), indicating that Ga isotopes may either not fractionate or fractionate only slightly under high-temperature geological processes; therefore, the δ71/69GaNIST-994 value of oceanic crust may be +1.22‰. The sediments (+1.28‰ to +1.47‰, with an average of +1.38‰) are rich in heavier Ga isotopes than the basalts, and the Ga present in the sediments may have originated from soluble Ga present in the seawater that was adsorbed by (Mn, Fe) oxides/hydroxides. The Ga contribution of basaltic debris to the sediments was almost negligible. Thus, we speculate that the δ71/69GaNIST-994 value of seawater in the study area fell within a range from +1.92‰ to +2.36‰. The δ71/69GaNIST-994 values of the sulfides in the Yuhuang hydrothermal field range from +0.99‰ to +1.57‰, with an average of +1.25‰, and the δ71/69GaNIST-994 values of the sulfides in the Duanqiao hydrothermal field range from +0.93‰ to +1.55‰, with an average of +1.19‰. The δ71/69GaNIST-994 ranges of the sulfides in the Yuhuang and Duanqiao hydrothermal fields are similar, with the Ga isotopic fractionation reaching 0.58‰ and 0.62‰, respectively. The average δ71/69GaNIST-994 values in the sulfides are close to those in the MORBs. This suggests that Ga within the sulfides in the Yuhuang and Duanqiao hydrothermal fields mainly originated from MORBs, with seawater and sediments making only small contributions. The Ga isotopic fractionation in the sulfides may be related to processes associated with the formation of sulfides, such as rapid precipitation or the admixture of different stages of sulfide. This study is of great significance for understanding the global distribution of Ga isotopes and the Ga cycle in submarine hydrothermal systems.
We present here for the first time, the Raman and infrared spectroscopic investigation of amphiboles from the World's deepest borehole, the Kola super-deep borehole, at the depth of 11.66 km. The Kola Super-deep borehole (SG-3) (henceforth referred as KSDB) is located in the northwest of the Kola Peninsula in the northern frame of the Pechenga structure, Russia. It was drilled in the north-eastern part of the Baltic Shield (69о5'N, 30о44'E) and reached a depth of 12.262 km. It has been drilled in the northern limb of the Pechenga geosyncline composed of rhythmically inter-bedded volcanogenic and tuffaceous-sedimentary strata extending to the NW at 300°-310° and dipping to SW at angles of 30°-50°. The SG-3 geological section is represented by two complexes-Proterozoic and Archaean. Amphibolite facies is dominant in the depth region from 6000 m to 12,000 m to the deepest.The Raman spectra of the sample reveal abundant presence of plagioclase and amphiboles. The most distinct Raman peak in this study indicates the tremolite-ferro-actinolite rich enrichment of the borehole samples at this depth corroborating earlier conventional petrographic studies.
The basement granite gneisses from the north-central Aravalli Craton in NW India were investigated for geochemistry and geochronology. In a peneplain terrain, the granite gneiss outcrops are scanty and samples were collected mainly from two small hills and several ground-level exposures in the Sakhun-Ladera region. Well-foliated granite gneiss is the dominant lithology that also hosts dark, lenticular enclaves, and is in turn, intruded by mafic dykes. The granite gneiss has silica content ranging from 61.37 wt.% to 68.27 wt.% that marks a slight overlap with the enclaves (54.32 wt.% to 62.17 wt.%). Both groups have a high K2O/Na2O ( ~2 or higher) ratio. Geochemically, the granite gneiss classify as granite-granodiorite, and enclaves as granodiorite-diorite. The In-situ LA-ICP-MS zircon U-Pb geochronology of granite gneiss has yielded a statistically valid 1721 ±9 Ma age that we interpret as the emplacement age for the granitic protolith. Geochemical characteristics of granite gneiss underline fractional crystallization of an I-type melt as the main process, and continuity of trends in enclaves underlines their mutual genetic link. The genetic association is further verified by a consistency in the trace element characteristics and REE patterns. The Nd-isotope signatures define a single grouping for both granite gneiss and enclaves, with εNd(t) values ranging from -6.38 to -6.61, further substantiating a common source. The geochemical tectonic discrimination schemes consistently point toward an extensional setting and A-type characteristics for granite gneiss and enclaves. These are analogous to the coeval (1.72-1.75 Ga), A-type granitoids from the Khetri and Alwar basin in the North Delhi Fold Belt, implying a much larger areal extent for the Paleoproterozoic anorogenic magmatism in the northern segment of the Aravalli Craton. The Paleoproterozoic age for the presumed ‘Archean’ basement in this region offers tacit evidence that the BGC-II is a stratigraphically younger terrane as compared to the Archean age, BGC-I.
Since incorrect site selection has sometimes led to the failure of artificial recharge projects, it is necessary to increase the effectiveness of such projects and minimize their failure by employing new techniques. Therefore, the present research used a combination of decision-making models, numerical groundwater modeling and clustering technique to determine suitable sites for implementation of an artificial recharge project. This hybrid approach was employed for the Yasouj aquifer located in southwestern Iran. In the first stage, by employing an AHP decision-making model, hydraulic conductivity, specific yield, slope, land use, depth to groundwater, and aquifer thickness were selected from 21 criteria used in previous research. The selected criteria were then entered as input into the classical k-means clustering model. Using the output, aquifer was divided into seven different regions or clusters. These clusters were then matched with the land use map, and some of the abandoned land areas were selected as the final option for implementing the artificial recharge project. Finally, the MODFLOW code in the GMS software was used to simulate the groundwater level and cluster the sites selected, with regards to increase in groundwater level. Results indicated that the most significant increases in groundwater level (43 and 27 cm) were those of Clusters 2 and 6 in the northern and western parts of the aquifer, respectively. Therefore, this approach can be used in other similar aquifers in arid and semi-arid regions to select the best sites for artificial recharge and to prevent loss of floodwaters.
Dating of fracture-filling calcite with U-Pb geochronology is becoming a rapidly adopted technique for determining the absolute timing of brittle deformation in the upper crust. Slickenfibre calcite is a desirable target, as it precipitates between individual fault slip displacement events, and provides additional kinematic information. Here we present a case study of slickenfibres formed on the Očkov thrust in the Lower Palaezoic Prague Basin, Bohemian Massif, utilising a combination of petrographic and in situ methods. We demonstrate that slickenfibre external textures can be preserved, whilst internally primary textures are removed by fluid infiltration and recrystallization, leading to variable U and Pb mobilisation. One slickenfibre yielded a date of ca. 250 Ma, which we interpret as recording fault slip along the Očkov thrust. Another cross-cutting slickenfibre yielded more scattered U-Pb data, with an imprecise apparent age around ca. 95 Ma. This slickenfibre is recrystallised, destroying the primary textures, and exhibits element mobility. The meaning of this younger apparent age is therefore questionable; whereas it likely reflects Cretaceous U and Pb mobility assisted by fluid-flow along the fault plane, it may not reflect a period of fault slip. Our results demonstrate that slickenfibre-based U-Pb dates do not unequivocally relate to fault motion, and that petrographic and elemental analyses are important requirements for interpreting calcite U-Pb data.
Although δ13C data (either δ13Ccarb or δ13Corg) of many Triassic-Jurassic (T-J) sections have been acquired, paired δ13Ccarb and δ13Corg from continuous T-J carbonate sections, especially in eastern Tethys, have been scarcely reported. This study presents paired and decoupled δ13Ccarb and δ13Corg data from a continuous T-J carbonate section in Wadi Naqab. The T-J Wadi Naqab carbonate section, located in United Arab Emirates, Middle East, represents tropical and shallow marine sedimentation in eastern Tethys. At the T-J boundary interval, an initial carbon isotope excursion (CIE) is observed with different magnitude of isotope excursion and timing in δ13Ccarb and δ13Corg, while subsequently a positive CIE is only distinct in δ13Ccarb. Based on petrological, carbon isotope, Rock-Eval and elemental analyses, the δ13Ccarb is thought to record marine inorganic carbon, and the δ13Corg to record terrigenous organic carbon. Therefore, the paired δ13Ccarb and δ13Corg herein potentially document simultaneous changes in T-J atmospheric and marine settings of eastern Tethys. Their decoupled behavior may likely be caused by different changes or evolution of carbon pool between marine and atmospheric settings. The initial CIE present in both δ13Ccarb and δ13Corg may indicate influence of isotopically light carbon release related to CAMP activity in both atmospheric and marine settings. The following positive CIE only in δ13Ccarb suggests relatively steady carbon isotope composition in atmosphere, but enhanced burial of isotopically light carbon in marine settings. Furthermore, the T-J carbonates in the studied section were possibly deposited in normal and oxic shallow marine conditions. Global correlation based on the Wadi Naqab section and other T-J sections suggests spatially different T-J environmental parameters:in eastern Tethys and western Panthalassa, oxic condition, lacking organic-rich sediment, weaker ocean acidification and less influence of isotopically light carbon are more prevalent; in western Tethys and eastern Panthalassa, oxygen-depleted condition, black shales, stronger acidification and heavier influence of isotopically light carbon are more prevalent. These differences may be related to spatial distance from the CAMP or to different paleogeography.
Zircon U-Pb and Hf isotope data integrated in this study for magmatic and metamorphic rocks from the Hida Belt, southwest Japan, lead to a new understanding of the evolution of the Cordilleran arc system along the ancestral margins of present-day Northeast Asia. Ion microprobe data for magmatic zircon domains from eight mafic to intermediate orthogneisses in the Tateyama and Tsunogawa areas yielded weighted mean 206Pb/238U ages spanning the entire Permian period (302-254 Ma). Under cathodoluminescence, primary magmatic growth zones in the zircon crystals were observed to be partially or completely replaced by inward-penetrating, irregularly curved featureless or weakly zoned secondary domains that mostly yielded U-Pb ages of 250-240 Ma and relatively high Th/U ratios (> 0.2). These secondary domains are considered to have been formed by solid-state recrystallization during thermal overprints associated with intrusions of Hida granitoids. Available whole-rock geochemical and Sr-Nd isotope data as well as zircon age spectra corroborate that the Hida Belt comprises the Paleozoic-Mesozoic Cordilleran arc system built upon the margin of the North China Craton, together with the Yeongnam Massif in southern Korea. The arc magmatism along this system was commenced in the Carboniferous and culminated in the Permian-Triassic transition period. Highly positive εHf(t) values (> +12) of late Carboniferous to early Permian detrital zircons in the Hida paragneisses indicate that there was significant input from the depleted asthenospheric mantle and/or its crustal derivatives in the early stage of arc magmatism. On the other hand, near-chondritic εHf(t) values (+5 to -2) of magmatic zircons from late Permian Hida orthogneisses suggest a lithospheric mantle origin. Hf isotopic differences between magmatic zircon cores and the secondary rims observed in some orthogneiss samples clearly indicate that the zircons were chemically open to fluids or melts during thermal overprints. Resumed highly positive zircon εHf(t) values (>+9) shared by Early Jurassic granitoids in the Hida Belt and Yeongnam Massif may reflect reworking of the Paleozoic arc crust.
The ~790 ka Australasian (micro) tektite strewn field is one of the most recent and best-known examples of impact ejecta emplacement as the result of a large-scale cratering event across a considerable part of Earth's surface (>10% in area). The Australasian strewn field is characterized by a tri-lobe pattern consisting of a large central distribution lobe, and two smaller side lobes extending to the west and east. Here, we report on the discovery of microtektite-like particles in sedimentary traps, containing abundant micrometeorite material, in the Sør Rondane Mountain (SRM) range of East Antarctica. The thirty-three glassy particles display a characteristic pale yellow color and are predominantly spherical in shape, except for a single dumbbell-shaped particle. The vitreous spherules range in size from 220 to 570 μm, with an average diameter of ~370 μm. This compares relatively well with the size distribution (75-778 μm) of Australasian microtektites previously recovered from the Transantarctic Mountains (TAM) and located ca. 2500-3000 km from the SRM. In addition, the chemical composition of the SRM particles exhibits limited variation and is nearly identical to the ‘normal-type’ (i.e.,<6% MgO) TAM microtektites. The Sr and Nd isotope systematics for a single batch of SRM particles (n=26) strongly support their affiliation with TAM microtektites and the Australasian tektite strewn field in general. Furthermore, Sr isotope ratios and Nd model ages suggest that the target material of the SRM particles was composed of a plagioclase- or carbonate-rich lithology derived from a Paleo- or Mesoproterozoic crustal unit. The affiliation to the Australasian strewn field requires long-range transportation, with estimated great circle distances of ca. 11,600 km from the hypothetical source crater, provided transportation occurred along the central distribution lobe. This is in agreement with the observations made for the Australasian microtektites recovered from Victoria Land (ca. 11,000 km) and Larkman Nunatak (ca. 12,000 km), which, on average, decrease in size and alkali concentrations (e.g., Na and K) as their distance from the source crater increases. The values for the SRM particles are intermediate to those of the Victoria Land and Larkman Nunatak microtektites for both parameters, thus supporting this observation. We therefore interpret the SRM particles as ‘normal-type’ Australasian microtektites, which significantly extend the central distribution lobe of the Australasian strewn field westward. Australasian microtektite distribution thus occurred on a continent-wide scale across Antarctica and allows for the identification of new, potential recovery sites on the Antarctic continent as well as the southeastern part of the Indian Ocean. Similar to volcanic ash layers, the ~790 ka distal Australasian impact ejecta are thus a record of an instantaneous event that can be used for time-stratigraphic correlation across Antarctica.
The Shaitian granite complex (SGC) spans more than 80 Ma of crustal growth in the Arabian-Nubian Shield in southeast Egypt. It is a voluminous composite intrusion (60 km2) comprising a host tonalite massif intruded by subordinate dyke-like masses of trondhjemite, granodiorite and monzogranite. The host tonalite, in turn, encloses several, fine-grained amphibolite enclaves. U-Pb zircon dating indicates a wide range of crystallization ages within the SGC (800 ±18 Ma for tonalites; 754 ±3.9 Ma for trondhjemite; 738 ±3.8 Ma for granodiorite; and 717 ±3.2 Ma for monzogranite), suggesting crystallization of independent magma pulses. The high positive εNdi (+6-+8) indicate that the melting sources were dominated by juvenile material without any significant input from older crust. Application of zircon saturation geothermometry indicates increasing temperatures during the generation of melts from 745 ±31℃ for tonalite to 810 ±25℃ for trondhjemite; 840 ±10℃ for granodiorite; and 868 ±10℃ for monzogranite. The pressure of partial melting is loosely constrained to be below the stability of residual garnet (<10 kbar) as inferred from the almost flat HREE pattern ((Gd/Lu)N=0.9-1.1), but >3 kbar for the stability of residual amphibole as inferred from the significantly lower NbN and TaN compared with LREEN and the sub-chondrite Nb/Ta ratios exhibited by the granitic phases. The inverse relation between the generation temperatures and the ages estimates of the granitoid lithologies argue against a significant role of fractional crystallization. The major and trace element contents indicate the emplacement of the SGC within a subduction zone setting. It lacks distinctive features for melt derived from a subducted slab (e.g. high Sr/Y and high (La/Yb)N ratios), and the relatively low MgO and Ni contents in all granite phases within the SGC suggest melting within the lower crust of an island arc overlying a mantle wedge. Comparison with melts produced during melting experiments indicates an amphibolite of basaltic composition is the best candidate as source for the tonalite, trondhjemite and granodiorite magmas whereas the monzogranite magma is most consistent with fusion of a tonalite protolith. Given the overlapping Sm-Nd isotope ratios as well as several trace element ratios between monzogranite and tonalite samples, it is reasonable to suggest that the renewed basaltic underplating may have caused partial melting of tonalite and the emplacement of monzogranite melt within the SGC. The emplacement of potassic granite (monzogranite) melts subsequent to the emplacement of Na-rich granites (tonalitetrondhjemite-granodiorite) most likely suggests major crustal thickening prior arc collision and amalgamation into the over thickened proto-crust of the Arabian-Nubian shield. Eventually, after complete consolidation, the whole SGC was subjected to regional deformation, most probably during accretion to the Saharan Metacraton (arc-continent collisions) in the late Cryogenian -Ediacaran times (650-542 Ma).
The North Qaidam orogenic belt (NQOB) is generally considered to be an early Paleozoic ultrahigh pressure metamorphic belt, but increasing reports of the Neoproterozoic magmatic and metamorphic events indicate that the NQOB probably also experienced the assembly of the Rodinia. However, the Neoproterozoic evolution of the NQOB is not well constrained due to the sparse records and ambiguous nature of the Neoproterozoic metamorphism. In order to reveal the multi-orogenic history of the NQOB, an integrated study of petrology, phase equilibrium modelling and geochronology was conducted on an epidote eclogite and host garnet mica schist from the Yuka-Luofengpo terrane. New zircon and monazite U-Pb ages show that the protolith of the garnet mica schist was deposited during 994-920 Ma and experienced Neoproterozoic (920-915 Ma) and early Paleozoic (451-447 Ma) polyphase metamorphism together with the enclosed eclogite. Relic omphacite inclusions were first identified in garnet and early Paleozoic zircon domains from the garnet mica schist, which provide solid evidence for the early Paleozoic eclogite facies metamorphism of the mica schist. Similar early Paleozoic peak P-T conditions of >27.4 kbar/613-670℃ and 30.2-30.8 kbar/646-655℃ were obtained for the garnet mica schist and enclosed eclogite, respectively, indicating that eclogites and their host paragneisses in this region underwent continental deep subduction as a coherent metamorphic terrane in early Paleozoic. The peak P-T conditions of the Neoproterozoic metamorphism were roughly constrained at 7.7-12.0 kbar and 634-680℃ for the garnet mica schist, based on stability field of mineral inclusions in Neoproterozoic zircons domains in P-T pseudosection, the relic garnet core composition and Ti-in-zircon thermometer. The high thermal gradients (16-37℃/km) defined by presently our and previously reported P-T conditions indicate that the Neoproterozoic metamorphism likely occurred in continental collision setting at >945-890 Ma. Since the Grenvillian syn-orogenic granitic magmatism and metamorphism (ca. 1.0-0.9 Ga) in the NQOB are much younger than the Grenvillian orogeny in the central part of Rodinia, the Qaidam Block was probably located at the north margin of Rodinia in Neoproterozoic.
Relying on the conceptual DPSIR framework and MODFLOW analysis, this study used a mixed approach to produce groundwater resource management solutions for the Najafabad area in central Iran. According to DPSIR results, agricultural activities put the highest pressure on groundwater resources in this region. The results showed the effectiveness of reducing water withdrawal over 30 years in maintaining the aquifer in a state of equilibrium. The best scenario consisted of cutting down extraction by 10% over the said period. Output maps of the water table rise at the Najafabad aquifer clearly showed that the groundwater management scenario involving a 10% reduction of water withdrawal was the most effective solution, as it would raise the water level by 6.7 m. Regarding other scenarios, reducing cultivated area by 20% was found to raise the water table by 5.03 m on average, while cutting down water withdrawal by 5% increased the water table by 3.6 m, and a 10% reduction of the cultivated area resulted in a 1.85 m rise. The combined model proposed here can be used for similar aquifers and can aid decision-makers and managers.