Precambrian Metasediments Research Articles

The role of protolith composition in the formation of tin-enriched granitic melts: A modeling study using the example of the southwest China tin province

Important features of Sn mineralization are the heterogeneous geographic distribution and frequent regional separation from W mineralization in spite some similarities of Sn and W behavior during magmatic processes. Major Sn and W mineralization is often spatially associated with peraluminous granites, which are derived from partial melting of metasediments. Several concepts have been suggested to explain those features, such as a weathering-related Sn-enriched source, Sn redistribution between melts and restite during protolith melting, and extensive fractional crystallization. We demonstrate the importance of protolith composition for the formation of Sn (and W) granites by using a comprehensive bulk-rock composition dataset from Precambrian metasediments of the South China Sn-W province and employing a thermodynamic modeling approach. We used four compositional proxies for phase equilibria calculations, which are the metasediments of the Mengdong, Sibao, Pingbian, and Shuangqiaoshan Groups. It is well documented that those Precambrian metasediments are important protoliths of Sn granites in South China. We present quantitative evaluation of the control of protolith composition in the generation of Sn-enriched granitic melts using South China as example, but our conclusions may also be applicable to worldwide Sn–enriched granites. Our results indicate that the protolith major-element geochemistry controls the anatectic reactions and melt productivity at specific melting conditions, and consequently the partitioning behavior of Sn. Further, pre-enrichment of Sn is crucial to the fertility of granitic melt and may be a prerequisite, particularly for the formation of giant Sn deposits. We propose that the heterogeneous distribution of favorable source rocks is one of the important factors that control the spatial distribution of major Sn (and W) districts in South China and other regions worldwide.

Geology and ore fluids geochemistry of the Hamzeh–Gharanin orogenic gold deposit, NW Iran

The Hamzeh–Gharanin gold deposit (HGD) is located in the Qolqoleh–Kasnazan shear zone in the northwest of Sanandaj–Sirjan Zone (SSZ) and 40 km SW of Saqez. The main rock units are an assemblage of the Precambrian metasedimentary (pelitic) rocks and the Cretaceous phyllite associated with marble interlayers. The post–Cretaceous intrusive granitoids with NW–SE trends intruded within these two units. Mineralization occurs mainly within the NE–SW extensional veines and veinlets in proto–mylonitic, mylonitic and ultra–mylonitic rocks. Hydrothermal alterations consist of sericitization, carbonatization, silicification and chloritization. The main ore minerals include pyrite, chalcopyrite, arsenopyrite, pyrrhotite, sphalerite, galena, gold, magnetite and hematite. The fluid inclusions study from quartz minerals associated with the mineralization shows four different phases: single vapor phase (V; type I), single liquid phase (L; type II), liquid–vapor biphasic (LV; type III) and three phases (LLV; type IV). Microthermometric studies were performed on types III and IV of fluid inclusions. Homogenization temperatures (Th) for Types III and IV inclusions vary between 138 and 310 °C (mean 230 °C) and 250–335 °C (mean 300 °C) and their salinity varies between 0.52 and 13.87 and 4.79 to 9.58 (wt% NaCl), respectively. Moreover, the density of Types III and IV inclusions vary from 0.71 to 0.927 g/cm3 and 0.700–0.870 g/cm3, respectively. The oxygen (in quartz: 4.58–7.80 ‰; mean 5.82 ‰) and δ34S (in pyrite: 5.54–6.28 ‰; mean 5.91‰) isotope compositions of the mineralizing fluids indicate an insignificant meteoric water input during hydrothermal evolution and a magmatic origin of sulfur in the deposit which is broadly similar to those of orogenic gold deposits worldwide.

Challenges of dating metasedimentary successions in collisional orogens: A case study of a Neoproterozoic passive margin in West Gondwana

Metasediments in collisional orogens critically record tectonic conditions during sediment formation and the thermal evolution during orogenesis. However, Precambrian metasediments can be difficult to date to a reasonable precision. Maximum depositional ages based on detrital zircon can vary considerably from sample to sample, even within a single formation. Although associated igneous bodies can yield depositional ages, their relationship with associated metasediments can be hard to constrain in poly-deformed terranes, as thrusting leads to the imbrication of rocks of different origins and ages. We present new U-Pb zircon ages for metasediments and tectonically-intercalated basement rocks from the Neoproterozoic Southern Brasília Orogen, West Gondwana, and explore the challenges of dating metasedimentary sequences in collisional orogens more widely. Our approach has been to constrain a metasedimentary succession based on field and petrographic analyses and to consider the youngest detrital zircon age, of all samples, as the maximum depositional age. Most samples yield youngest ages older than the maximum depositional age and are thus referred to as “apparent” maximum depositional ages. This results from the age distributions in the source regions, where there is major peak magmatism at ∼ 2.2–2.1 Ga, and small volumes of rocks with younger ages. Hence, apparent maximum depositional ages of ∼ 2.1 Ga, common in the studied succession, appear to be related to source characteristics. Five metasedimentary rocks were analyzed here, three yielded apparent maximum depositional ages between 1.8 and 1.6 Ga, and two samples provided ages of ∼ 1.0 Ga, considered the maximum depositional age. The Archean-Paleoproterozoic basement of the Southern Brasília Orogen is the main source region for overlying Neoproterozoic metasedimentary successions, with some material derived from the adjacent Southern São Francisco Craton. P-T obtained for the studied succession is consistent with sedimentary facies and geochronological studies that suggest it is the passive margin of the Southern São Francisco Craton.

Fluid flow and water/rock interaction during the Early Triassic evolution of the western Canada sedimentary basin as revealed by carbonate diagenesis

Diagenetic evolution of sedimentary basins, which is crucial to resource exploration and development, and carbon capture, utilization, and storage (CCUS), is mainly controlled by fluid influx associated with the regional tectonic and hydrogeologic setting of these basins. Therefore, petrographic and isotope geochemical studies integrated with geodynamics of sedimentary basins, fluid flow and transport hydrodynamics are insightful tools to reconstruct the regional history of paleofluid flow and their driving mechanisms. This study investigates carbonate diagenesis in the Lower Triassic Montney Formation of the Western Canadian Sedimentary Basin (WCSB) to constrain the origin of diagenetic fluids, their major flow systems, and their association with the tectonics and hydrogeology of the WCSB. Petrographic analysis shows various generations of early-to late-stage non-ferroan to ferroan calcite (C1–C3) and dolomite (D1-D4) cement. Bulk δ13C and δ18O values of carbonate cements range from −7.1 to +0.4‰ and −9.5 to −2.8‰ (VPDB), respectively, noticeably more depleted than those estimated for Early Triassic seawater. Their 87Sr/86Sr isotope ratio, 0.7108 to 0.7128, is significantly more radiogenic than Early Triassic seawater. These isotopic values confirm the extensive interaction of the Montney Formation pore water with hot basinal brines of Precambrian metasediments and siliciclastic strata, the influx of hydrothermal fluids, and high burial temperature in the basin. The 13C-depleted signature of these cements supports the contribution of organic carbon, from the breakdown and oxidation of organic matter and hydrocarbons via microbial and thermochemical sulfate reduction processes. The results of this study suggest that Montney Formation diagenesis was influenced by evolved Montney pore water interacting with fluids sourced from metasediments of the Cordillera thrust belt and topography-driven fluids from Columbian and Laramide tectonism. This study highlights the importance of an integrated approach to better understand changes in fluid compositions during diagenesis and reconstruction of basin-scale paleofluid flow systems in sedimentary basins.

Saline fluids drive Cu mineralization in Precambrian metasediments: Evidence from the Trans-North China Orogen

Epigenetic hydrothermal Cu-(Co) deposits are widely distributed in the Zhongtiao Mountains within the Paleoproterozoic Trans-North China Orogen in the North China Craton. Here we investigate the Henglingguan Cu deposit hosted in metasediments to gain insights on the fluid evolution characteristics associated with the ore formation. This Cu deposit is characterized by early disseminated mineralization and late quartz-vein type mineralization. The negative sulfur isotope values (about −10‰) of the vein sulfides suggests a sedimentary rock source for sulfur associated with the hydrothermal Cu mineralization. The coexistence of CO2-rich carbonic inclusions and halite-bearing inclusions within a single fluid inclusion assemblage suggests fluid immiscibility in the H2O-NaCl-CO2 system. Based on the fluid phase analysis, our data show that the parent ore fluid was characterized by X(NaCl) of 10 to 20 mol.% (salinity of around 23 to 41 wt% NaCl) and X(CO2) of 10 to 15 mol.% before fluid immiscibility. The estimated temperature–pressure conditions of 320 to 340 °C and 3.5 to 4.0 kbar based on the carbonic fluid inclusions is suggested to be close to the conditions of fluid immiscibility. In-situ composition analyses of fluid inclusions by LA-ICP-MS show that the ore fluids contained Na, Ca, Fe and Mn, and evolved towards Sr-Ba enrichment. The data are consistent with a metamorphic brine derived from the sedimentary rocks. The brine-rich metamorphic fluids were likely derived from the dehydration and decarbonation of evaporitic carbonate rocks developed on the basin margin or a passive continental margin. The decompression during post-collisional exhumation triggered fluid immiscibility resulting in the quartz vein-type Cu mineralization.

U-Pb dating of detrital zircons from basal Post Paleoproterozoic metasediments in the Subpolar and Polar Urals: evidence for a Cryogenian, not Mesoproterozoic age

ABSTRACT In the last ten years, U-Pb ages of detrital zircons has been obtained from Precambrian metasediments of some regions of Timan and the northern part of the Urals, which lie at the base of the visible Post Paleoproterozoic section of the Timan-Ural region. However, the basal status of these stratons remains unproven due to the lack of their relationship with the underlying formations. This article presents the first U-Pb-dating of detrital zircons from terrigenous metasediments of the Puyva suite (Subpolar Urals) and the Miniseyshor suite (Polar Urals) which reliably represents the basis of the Mesoneoproterozoic of the Timan-Northern Urals. The maximum age of deposition (MAD) detrital zircon the onset of sediment deposition is about 870 Ma for the Puyva suite and 660 Ma for the Nyarovey series, which corresponds to the Cryogenian. Therefore, the widespread opinion that the Puyva suite and the Nyrovey series are Mesoproterozoic in age is incorrect. Theis geochronological information integrated with MADs from other Precambrian complexes of the Timan- Urals region, suggests that the Timanian passive continental margin probably formed Tonian-Cryogenian.

A preliminary investigation for the assessment of geothermal potential at Eastern Peninsular India

Eastern part of peninsular India has exposed Precambrian metasediments, granitic gneissic rocks, basic and ultrabasic intrusives. More than 30 thermal springs issuing through these rocks have been located in the states of Bihar, Jharkhand, and West Bengal. Based on chemical geothermometry of these thermal waters, the reservoir temperatures are estimated to be in the range of 100–195 °C. The area has high heat producing granites with the heat flow values ranging from 59.8 to 119.5 mW/m2. The area is suitable for hosting wet as well as a dry geothermal system. Using Organic Rankin Cycle, a minimum potential of Eastern Peninsular India is estimated to be of 2566 MWe capacity. The exploitation of these geothermal systems in the area can help reduce greenhouse gases. Approximately 6 × 105 tons of CO2 emissions per year can be reduced as well as carbon equivalent revenue of 106 million USD per year can be generated with the installation of geothermal power plants. Furthermore, geothermal fields in the region can also be utilized for fish farming, cold storage, greenhouses, etc.

Distribution and Economic Potential of Manganese Deposits in Nigeria: A Review

The Northern basement complex of Nigeria contains a large number of manganese deposits. So far, ten deposits of manganese have been reported by previous workers. These deposits occur within Precambrian metasediments (schist belt), mafic and ultramafic rocks which are Proterozoic in age and folded into synclinorial belts within the crystalline basement complex. Considering their widespread distribution in space, time and tectonic setting, some are considered to be of ensilalic mode of evolution while others are ensimatic. However, the mineralizations are mostly of poor grade, and thus require beneficiation processing. The local steel industries within the country have continued to depend on manganese ore. No satisfactory substitute for manganese in steel production has been identified as at present. The applications of manganese deposits by geologists, chemical and environmental engineers, ceramicists, soil scientists and microbiologists shows a bright future for manganese ore in Nigeria. The ever increasing demand for steel products has continued to put pressure on Federal Government of Nigeria to complete the construction work at the nation’s steel producing plant which, will result in the need for a manganese concentrates for economic growth and development.

Mixing processes in modern estuarine sediments from the Gulf of Khambhat, western India

Sandy and muddy detritus from two distinctive end-member provenance terrains, basaltic and granitic, are transported to the estuaries of the Gulf of Khambhat by fluvial discharge and opposing tidal currents making this embayment ideal for investigating the effects of mineralogical and geochemical mixing on sediment composition in estuarine depositional systems. Rivers flowing predominantly from the north over Precambrian metasediments and granites supply a felsic plutoniclastic sand to the gulf, whilst a mafic volcaniclastic sand, sourced by Deccan basalts, is transported by rivers draining the Western Ghats and from the local coastal erosion in the southern part of the gulf.Sediments from the Tapti estuary are rich in basaltic rock fragments (∼50%) and are characterised by a distinctly basaltic source composition (La/Sc∼0.25–0.86, Th/Sc∼0.12–0.40, Cr/Th∼13–121, Eu/Eu*∼0.93–1.68 and LREE/HREE∼2.96–6.57). By contrast, the sediments of the Sabarmati inner estuary are rich in quartz grains (80%) as evident from the major and trace element compositions (La/Sc∼1.38–6.63, Th/Sc∼0.58–2.98, Cr/Th∼0.58–19.39, Eu/Eu*∼0.46–0.86 and LREE/HREE∼2.90–11.40). The detrital compositions of the remaining major estuaries, the Narmada, Dhadhar and Mahi, scatter across a broad range in a QFR diagram and contain varying amounts of quartz, feldspar and rock fragments. In the Narmada Estuary the mixed signature characterised by REE values including Eu/Eu* (∼0.42–2.28) and LREE/HREE (∼4.37–14.80) is caused by the presence of diverse rock types that crop out in the source area. In the Mahi estuary, which largely flows across felsic source terrains the compositional signature is controlled by mixing of felsic detritus from riverine input with mafic sediments from seaward side in the estuary.The two end-member detrital compositions, plutoniclastic and volcaniclastic, are mixed in the macro-tidal estuaries. Mixing of sediment takes place where fluvial currents and tidal flows interact and the proportion of mixing varies systematically from outer to central to inner estuary. A high degree of mixing of felsic sediments with Deccan basalt derived sediments takes place in the outer estuary resulting in felsic:mafic ratios of ∼40:60. Sediments in the central estuaries (∼65:35) and in the fluvial-dominated inner estuaries (∼90:10) are characterised by moderate to minor degrees of mixing. The mixing in estuaries is systematic and is reflected in both the mineralogy and geochemistry of the sediments. Provenances of ancient estuarine successions should therefore be interpreted with great care incorporating awareness of the mixing processes which influence sediment compositions to different degrees in estuaries, especially those characterised by strong tidal currents.

Strontium isotopes are consistent with low‐elevation foraging limits for Henst's goshawk

ABSTRACTApex predators play critical roles in the ecosystems they inhabit. Unfortunately, little is known about movement patterns for many species. This information is critical for evaluating vulnerability to habitat loss and the adequacy of existing or proposed protected areas. Strontium isotope ratios (87Sr/86Sr) in prey remains reflect the geology where individuals lived and were killed, and can be used to identify foraging ranges for predators. We tested the degree to which 87Sr/86Sr in consumed lemurs (Lemuroidea) can be used to track foraging ranges for Henst's goshawk (Accipiter henstii) at Ranomafana National Park (RNP), eastern Madagascar, Africa. This large‐bodied endemic accipiter is sparsely distributed and rarely observed outside of forest or at elevations >1,200 m. A geologic boundary divides lower and higher elevations at RNP: Precambrian migmatite is at lower elevations within and to the east of RNP and Precambrian metasediments are limited to higher elevations in southwestern RNP. We collected foliage from trees and understory plants to establish 87Sr/86Sr for both geologies, and remains from 19 depredated lemurs from 4 hawk nests located along the eastern edge of RNP. Leaves from metasediments have greater 87Sr/86Sr than leaves from migmatite. 87Sr/86Sr for lemur bones suggests that 18 of the 19 predated individuals came from forests underlain by migmatite. Thus, strontium isotope data suggest that Henst's goshawk primarily hunted on migmatite at RNP, which supports a growing body of evidence that this species forages at elevations below approximately 1,200 m. Most reserves in eastern Madagascar protect forest at higher elevations. Conserving remaining forest and revegetating denuded land at lower elevations will likely be critical for survival of this species. Understanding mobility patterns is crucial for managing predator populations. Isotopic analysis of prey is a noninvasive method for monitoring foraging ranges that complements existing approaches. This geochemical tool is readily adaptable to other systems and species. © 2017 The Wildlife Society.

Abandoned mine tailings in cultural itineraries: Don Quixote Route (Spain)

Metallic mining wastes are a crucial environmental concern because of the accumulation and emission of toxic elements. The abandoned San Quintín mine tailings, from the exploitation of galena and sphalerite (Puertollano mining district, Spain), are surrounded by farmland and are crossed by the European Cultural Itinerary known as the “Don Quixote Route”. Geophysical, mineralogical and geochemical techniques have been applied to determine the geometry and composition of mine ponds and the possible occurrence of AMD that could entail ecosystem and human risks. Seven electrical resistivity tomography profiles were surveyed, and re-worked tailing samples of two ponds, five soil samples, nine water samples and gaseous mercury emissions were analyzed. Both mine ponds are ~8m deep and overlie Quaternary colluvial sediments deposited over Precambrian metasediments. The pond infilling is mainly composed of quartz and clay minerals, and minor amounts of gypsum and sulfides, with significant As, Cd, Hg, Pb, Sb and Zn contents. This metal emission source affects the underlying colluvial sediments and high Hg and Pb contents have also been found in nearby agricultural soils. AMD has been identified, but metal contamination in the stream that borders the tailings is below the USEPA's recommended limits. Although gaseous mercury emissions from a cinnabar stockpile increase in summer, the measured levels do not reach the WHO's recommended limit. The Enrichment Factor and Igeo indexes for As, Cd, Hg, Pb, Sb and Zn indicate strong to extreme contamination in the mine ponds and moderate contamination in the colluvial sediments/metasediments. Aeolian dispersion is the most important contaminant agent. The surrounding soils show strong to extreme levels of Hg and moderate to extreme levels of As, Cd, Pb and Sb. Restoration of these abandoned wastes is recommended to reduce the health risks for residents and tourists due to this aeolian contaminant dispersion.

New insights into the generation of the 2013 Lushan Earthquake (Ms 7.0), China

AbstractGlobal seismic waveform inversion can provide information on where the 2013 Ms 7.0 Lushan earthquake occurred in Sichuan Province, China, and how its aftershock sequence expanded. To investigate the generation mechanism of the Lushan earthquake and its relation to the 2008 Wenchuan earthquake (Ms 8.0), 50 temporary seismic stations were installed in the source area following the Lushan earthquake. Crustal stress data were also collected along the Longmen‐Shan Fault zone (LMFZ) to reveal its influence on the generation of the Lushan earthquake. Seismic imaging and crustal stress analysis have revealed that the Lushan earthquake occurred in a distinct area with high velocity (Vp, Vs), low Poisson's ratio (σ), and high crustal stress. The high‐velocity zone at the Lushan source may reflect Precambrian metasedimentary or igneous rock in the seismogenic layer, which enables the accumulation of high crustal stress to generate large earthquakes. However, a sharply contrasting gap zone with low‐velocity, high‐σ anomalies is clearly imaged in the upper crust under the concatenated area between the Lushan and Wenchuan earthquakes. Seismic images indicate that the low‐velocity gap zone is associated with fluid‐bearing ductile flow from the lower crustal materials of Tibet being pushed into a weakened segment of the LMFZ. This study suggests that the 2013 Lushan earthquake may have been triggered by high crustal stress accumulation together with high coseismic stress further increased by the Wenchuan earthquake in the metamafic seismogenic layer. Contrasting rheological variations in the crust and crustal stress changes along the LMFZ control the rupture processes that generated the Lushan and Wenchuan earthquakes, as well as the generation of new earthquakes in the future.

Origin of Late Triassic high-K calc-alkaline granitoids and their potassic microgranular enclaves from the western Tibet Plateau, northwest China: Implications for Paleo-Tethys evolution

The western Tibet Plateau comprises a series of crustal terranes that were successively accreted to the southern margin of Eurasia associated with the evolution of Paleo- and Neo-Tethys. This paper presents the first detailed SHRIMP zircon U–Pb chronology, major and trace element, and Sr–Nd–Hf isotope geochemistry of three Mesozoic plutons (South Kudi, Arkarz and Mazha) and their microgranular enclaves in the western Kunlun and Tianshuihai terranes. SHRIMP zircon U–Pb dating shows that the three plutons were emplaced in the Late Triassic (215–209Ma) and show a southward-younging trend. The South Kudi pluton (215Ma) is composed of high-K calc-alkaline granodiorite and monzogranite, with initial 87Sr/86Sr ratios of 0.7093–0.7099, εNd (T) of −4.9 to −5.4, and εHf (T) (in-situ zircon) of 0.3. Elemental and isotopic data suggest that the granitoids were generated by partial melting of the Precambrian metasedimentary–igneous basement in the normal lower-crust (<40km) of the western Kunlun terrane triggered by underplating of basaltic magma. The Arkarz pluton (213Ma) consists of high-K calc-alkaline monzogranite, syenogranite and alkali-feldspar granite and contains abundant microgranular enclaves. The host granites have initial 87Sr/86Sr ratios of 0.7071–0.7085, εNd (T) of –3.7 to −4.8, and εHf (T) of −0.7, and were also generated by partial melting of the Precambrian metasedimentary–igneous basement in the normal lower-crust (<40km) of the western Kunlun terrane triggered by underplating of enclave-forming potassic magma. Fractional crystallization of these pure crustal melts may explain the more felsic end-member granite. The enclaves are mainly basic (SiO2 48.0–54.9wt.%) with high K2O (1.4–3.8wt.%). They have initial 87Sr/86Sr ratios of 0.7071–0.7080 and εNd (T) of −2.6 to −4.5. We interpret the enclave magmas as having been derived by partial melting of the metasomatized mantle wedge in the spinel–garnet transition zone (~60–80km). The Mazha pluton (209Ma) is composed of high-K calc-alkaline tonalite and granodiorite, with initial 87Sr/86Sr ratios of 0.7087–0.7097, εNd (T) of −6.3 to −6.5, and εHf (T) of −2.8. Elemental and isotopic data suggest that the granitoids were generated by mixing of mantle-derived basaltic magma with felsic melt derived by partial melting of lower crust of the Tianshuihai terrane. Our new data suggest that during the Late Triassic (215–209Ma) the Tianshuihai terrane and the southern part of the western Kunlun terrane were a continental arc in response to the northward subduction of the Paleo-Tethys between the Tianshuihai and Karakorum terranes; the collision of the Karakorum terrane with the Tianshuihai terrane is not earlier than the age of 209Ma, and most likely occurred during the Early Jurassic.

Origin of Middle Triassic high-K calc-alkaline granitoids and their potassic microgranular enclaves from the western Kunlun orogen, northwest China: A record of the closure of Paleo-Tethys

The western Kunlun orogen, the northwest Tibet Plateau, occupies a key tectonic position at the junction between the Pan-Asia and the Tethyan tectonic domains. It is related to subduction of Proto- and Paleo-Tethys and terrane collision from early Paleozoic to early Mesozoic. This paper presents the first detailed SHRIMP zircon U–Pb chronology, mineral chemistry, major and trace element, and Sr–Nd–Hf isotope geochemistry of three Middle Triassic plutons (Yuqikapa, Muztaga and Taer) and their microgranular enclaves in the western Kunlun orogen. SHRIMP zircon U–Pb dating shows that the Yuqikapa pluton was emplaced in the Anisian (~243Ma) and the Muztaga and Taer plutons that contain microgranular enclaves were emplaced in the Ladinian (234–227Ma). The Yuqikapa pluton is composed of peraluminous high-K calc-alkaline two-mica granites (monzogranite–syenogranite–alkali-feldspar granite), with initial 87Sr/86Sr ratios of 0.7071–0.7077, εNd (T) of −4.4 to −6.3 and εHf (T) (in-situ zircon) of −3.3. Elemental and isotopic data suggest that they were formed by “wet” anatexis of the Precambrian metasedimentary and metaigneous rocks in the continental collision (overthrust) zone. The Muztaga and Taer plutons consist of metaluminous to weakly peraluminous high-K calc-alkaline granitoids (granodiorite–monzogranite), with initial 87Sr/86Sr ratios of 0.7052−0.7104, εNd (T) of −3.6 to −7.3 and εHf (T) (in-situ zircon) of −3.1 to −1.0. Elemental and isotopic data suggest that they were formed by partial melting of the Precambrian metasedimentary and metaigneous rocks in the normal lower-crust (<40km) for the Muztaga pluton and in the thickened lower-crust (~40–50km) for the Taer pluton, triggered by underplating of coeval (233–226Ma) mantle-derived enclave-forming magmas. The enclaves are basic (SiO2 48.0–51.6wt.%) to intermediate (SiO2 53.0–54.6wt.%) with high K2O (1.4–4.2wt.%). They generally have lower initial 87Sr/86Sr ratios (0.7034–0.7084) and high initial εHf (−1.1 to +1.2) than their respective host granitoids. The enclave magmas are considered to have been derived from the partial melting of subduction-modified mantle in the spinel stability field (<60km) (enclaves in the Muztaga pluton) and the spinel-garnet transition zone (~60–80km) (enclaves in the Taer pluton). Final closure of the Paleo-Tethys occurred in the Middle Triassic Anisian (~243Ma). The Middle Triassic Ladinian (234–227Ma) dates the post-collisional regime caused by Paleo-Tethyan oceanic slab break-off.

Neoproterozoic–Devonian stratigraphic evolution of the eastern Murzuq Basin, Libya: a tale of tilting in the central Sahara

AbstractThe Murzuq Basin is one of the most petroliferous basins of North Africa. Its remote eastern flank has been largely ignored since early reconnaissance work in the 1950s and 1960s. This article presents new stratigraphic and sedimentological data on the Neoproterozoic through Devonian succession from the Mourizidie and Dor el Gussa regions. The Neoproterozoic to Cambrian Mourizidie and Hasawnah formations in the eastern part of the Mourizidie region dip to the east and north‐east, resting directly on late Precambrian metasediments and granitoids. These strata record the initial progradation of sand‐dominated braidplain systems upon peneplained Precambrian basement. Rhyolite clasts in the Hasawnah Formation may record tectonically driven uplift and unroofing in the southern Tibesti Massif or tectonomagmatic rejuvenation to the south of this massif. In the western part of the Mourizidie region, Late Ordovician through Silurian strata (Mamuniyat and Tanezzuft–Akakus formations) directly overlie late Precambrian metasediments and granitoids, and dip at a low angle towards the west into the Murzuq Basin. Elsewhere at the eastern Murzuq Basin flank, in Dor el Gussa, Late Ordovician glaciogenic sediments rest with angular unconformity upon shallow marine sandstones of Cambrian–Ordovician age. This angular unconformity may also occur in the Mourizidie region and indicates widespread tectonism, either as a result of a Middle–Late Ordovician orogenic event, far‐field tectonism related to the opening of the Rheic Ocean along the northern margin of Gondwana or alternatively crustal depression associated with the growth of Late Ordovician ice sheets. Unconformity development was also probably associated with glacial incision. Following ice sheet retreat, isostatic rebound during deglaciation resulted in uplift of tens to hundreds of metres, locally removing all Cambrian and Ordovician formations. Rising sea levels in the Silurian led to deposition of the Tanezzuft Formation on Precambrian basement in the northwestern Mourizidie region.

Stream Sediments Geochemical Exploration in the Northwestern Part of Wadi Allaqi Area, South Eastern Desert, Egypt

Geochemical stream sediments survey was conducted in the northwestern part of Wadi Allaqi area, Eastern Desert, Egypt. The area comprises Precambrian metasediments, intermediate metavolcanics, gabbro, and serpentinites, with intrusive masses of granites and quartz-porphyry and invaded by several quartz veins. The −1.0-mm size fraction is analyzed for As, Cu, S, Mo, Pb, Zn, Co, Ni, Rb, Ba, Sr, Nb, V, U, Th, Cr, Zr, La, Ce, Nd, and Y. The geochemical survey is supported by heavy minerals study in the −0.125 + 0.0625-mm fraction. The geochemical data were statistically investigated using Q-mode cluster and R-mode factor analyses as well as the enrichment factor. Factors 1 (Zr, Nb, Nd, La, and Y), 2 (V, Sr, and Zn), and 4 (Ba and Rb) are mainly controlled by the lithological characters of the rocks hosting Au-sulfide mineralizations and their accompanied hydrothermal alteration zones. In the mineralization Factor 3 (Cu, S, As, Ce, and Mo), arsenic, Cu, S, and Mo are direct indicators, while Ce is indirect one for the Au-sulfide mineralizations. The Cu–S–As–Mo association with Pb and Zn anomalies in the stream sediments draining the quartz-porphyry point to its porphyry copper mineralization. Cobalt and Ni (Factor 5) are pathfinders for the Fe- and Cu-sulfides, whereas Zn and Pb of Factor 8 are additional pathfinders for the Au-sulfide mineralizations. The southern stream sediments having high U/Th ratios with U–Mo association and draining granites traversed by pegmatites, as well as the stream sediments draining Um Garayat area and the quartz-porphyry stock with high abundance of monazite, zircon, epidote, sphene, and ilmenite, could signify sources of U and Th (Factor 7). Two watershed areas have distinct enrichment factors for arsenic suggesting unexplored extensions of Au-sulfide mineralization linked to the Allaqi shear-zone. The enrichment of the mineralization Factor 3 in the drainage system is mainly controlled by the prevailed mechanical dispersion for the hosting heavy minerals in such arid region with minor role of hydromorphic dispersion. The chemistry and mineralogy of the stream sediments are evidently allied to the drained bedrocks and their hosted mineralizations that signify a promising area for detailed exploration.

Dating of Precambrian Metasedimentary Rocks and Timing of their Metamorphism in the Soursat Metamorphic Complex (NW IRAN): Using LA–ICP-MS, U–Pb Dating of Zircon and Monazite

Soursat Metamorphic Complex (SMC) in west of Takab city is one of the polyphase metamorphic terrenes in northern Sanandaj-Sirjan metamorphic belt of Iran. The SMC composed mainly of metasedimentary rocks associated with granitic intrusions. Based on metamorphic rocks, two metamorphic phases could be separate in the complex. M1 is representative of regional metamorphisms which varies from greenschist to amphibolite facies and consists of mica-schist, garnet-schist, staurolite-schist, kyanite- schist, fibrolite-schist, garnet amphibolites, marble, gneiss. M2 is contact metamorphism with clear outcrop just in the central of SMC and overprinted on M1. This phase consists of andalusite-schist, cordierite-schist and Actinolite- schist. SMC is in tectonic contact with Precambrian to Paleozoic sedimentary rocks which make it difficult to date it based on stratigraphy. In this study, U-Pb dating of zircons and Monazites used in order to find the ages of deposition and metamorphism of metasediments in SMC. U-Pb dating of zircons from a staurolite-schist in the complex by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) yielded a maximum Precambrian depositional age of 605±43. Monazites were also dated from a garnet-schist using the same technique and yielded a 238 U/ 206 Pb age of 61±8 which is interpreted as the age of the regional metamorphism. Based on these ages, metasedimentary protoliths of the SMC are deposited at the same time as the surrounding essentially unmetamorphosed Precambrian-Cambrian sedimentary rocks and regional metamorphism occurred later in Paleocene and could be related to the orogenesis during the collision of Arabian plate with Iranian block and closure of Neotethys.

Structural Setting of the 2008 Mw 7.9 Wenchuan, China, Earthquake

The 2008 M w 7.9 Wenchuan earthquake ruptured an imbricate thrust system in the Longmen Shan range, which forms the eastern boundary of the Tibetan Plateau. We use seismic reflection data and surface geology to construct geologic cross sections and a three-dimensional (3D) fault model in the range front. The rupture was complex, with slip on both the steeply dipping Beichuan fault and the more shallowly dipping Pengguan fault, which appear to merge at depth. In contrast, only the Beichuan fault ruptured in the north, with the transition near a lateral fault offset. Thus, the earthquake involved multiple thrust splays and breached a significant segment boundary. To investigate the tectonic setting of the earthquake, we extend our previous measurements of crustal shortening (Hubbard and Shaw, 2009) into the interior of the range and show that shortening correlates with topography. This suggests that upper-crustal shortening can produce the high topography of the Longmen Shan without calling on other uplift mechanisms. Based on this understanding, we model the thrust belt as a critical taper wedge, to assess what rock and fault strengths are required to explain the topographic slopes within the range front and basin. The low taper within the basin is consistent with a weak detachment ( δ f ≈0.1), while a slightly stronger ( δ f ≈0.3–0.4) detachment is necessary to explain the higher taper of the range front. This difference may relate to the localization of the detachment within a Triassic evaporite sequence within the basin, while the faults beneath the range front are rooted in Precambrian metasedimentary or igneous rocks. Critical taper theory implies that shortening in the range front where the 2008 Wenchuan earthquake occurred is many times greater than in the basin, consistent with our shortening measurements. We examine the implications of these findings for seismic hazard assessment in the region and in other active mountain belts around the world.

Lithological and structural characteristics of the Lake Bosumtwi impact crater, Ghana: Interpretation of acoustic televiewer images

Abstract— Bosumtwi is a very well‐preserved 1.07 Myr old, complex terrestrial impact crater located in south‐central Ghana, West Africa. The impact structure has a diameter of about 10.5 km and was formed in 2.1–2.2 Gyr Precambrian metasedimentary and metavolcanic rocks. Drilling and logging was carried out during the Lake Bosumtwi Drilling Project (BCDP) which was supported by the International Continental Scientific Drilling Program (ICDP). One of the aims of this project is to achieve detailed information on the subsurface structure and crater fill of one of the best preserved large young impact structures.We interpreted the wireline logs and televiewer images. The physical properties including shallow resistivity, p‐wave velocity, magnetic susceptibility, and borehole diameter of the breccia differ significantly from those of the meta‐graywackes and slate/phyllites. Fractures observed in the televiewer images are interpreted to determine their characteristic structural features. The fracture dip angles are steep (50–70°) and the two main dip directions are southeast and southwest. Most fractures observed in the borehole are open. The indicated main stress direction is north‐south.

Two Paleoproterozoic strongly peraluminous granitic plutons (Nonggeori and Naedeokri granites) at the northeastern part of Yeongnam Massif, Korea: Geochemical and isotopic constraints in East Asian crustal formation history

In the northeastern Yeongnam Massif of South Korea, voluminous granitoids were emplaced from Mesozoic to Paleoproterozoic period. Nonggeori (NG) and Naedeokri (ND) granites are known as two of Paleo- to Meso-proterozoic plutons in the northeastern margin of the Yeongnam Massif. They occur as stocks intruding Precambrian metasediments at the Taebaek area in northeastern part of South Korea. Pegmatites occur at adjoining areas either near or between NG and ND granites. The pegmatites have experienced tin mineralization. The NG and ND granites consist of quartz, plagioclase, K-feldspar, biotite and muscovite. The accessory minerals are tourmaline, zircon, apatite, chlorite and calcite. The mineral assemblages and major chemical compositions of the two granites are very similar. Geochemically, the NG and ND granites are strongly peraluminous, with a molar ratio of A/CNK from 1.13 to 1.46 for NG and ND granites and 1.12 to 1.34 for pegmatites. In the chondrite-normalized diagrams, the NG and ND granites have characteristics of LREE-enriched, HREE-depleted patterns and large negative Eu anomalies. However, the pegmatite shows two types of the REE pattern. One is flattened type with highly negative Eu anomalies, the other is slightly LREE enriched with strong positive Eu anomalies. Trace element characteristics of NG, ND granites and pegmatites are generally compatible with those of syn-collisonal granite rather than volcanic arc granite. The granites show Sm–Nd whole rock age of 1865 ± 120 Ma with ɛ Nd (1.87 Ga) = −7.0, suggesting light-REE enriched continental like feature. Based on the initial ɛ Nd value and trace element geochemistry, the NG and ND granites apparently were originated from remelting (or recycling) of crustal materials. However, ultimately, the protolith of NG, ND granites including neighbor pegmatites was derived from depleted mantle sources, which are related with those that had been involved in producing East Asian crust such as the basement of the Sino-Korean Craton during the Archean. Particularly, the Precambrian granitoids from the Yeongnam Massif may provide critical evidence for unraveling Precambrian plate tectonics and crustal evolution in East Asia including Japanese islands.