Electrical Structure and Fault Features of Crust and Upper Mantle beneath the Western Margin of the Qinghai-Tibet Plateau： Evidence from the Magnetotelluric Survey along Zhada-Quanshui Lake Profile

The magnetotelluric (MT) survey along the Zhada (札达)-Quanshui (泉水) Lake profile on the western margin of the Qinghai (青海)-Tibet plateau shows that the study area is divided into three tectonic provinces by the Yalung Tsangpo and Bangong (班公)-Nujiang (怒江) sutures. From south to north these are the Himalayan terrane, Gangdise terrane, and Qiangtang (羌塘) terrane. For the study area, there are widespread high-conductivity layers in the mid and lower crust, the top layers of which fluctuate intensively. The high-conductivity layer within the Gangdise terrane is deeper than those within the Qiangtang terrane and the Himalaya terrane, and the deepest high-conductivity layer is to the south of the Bangong-Nujiang suture. The top surface of the high-conductivity layer in the south of the Bangong-Nujiang suture is about 20 km lower than that in the north of it. The high-conductivity layer within the Gangdise terrane dips toward north and there are two high-conductivity layers within the crust of the southern Qiangtang terrane. In the upper crust along the profile, there are groups of lateral electrical gradient zones or distortion zones of different scales and occurrence indicating the distribution of faults and sutures along the profile. According to the electrical structure, the structural characteristics and space distribution of the Yalung Tsangpo suture,Bangong-Nujiang suture, and the major faults of Longmucuo (龙木错) and Geerzangbu are inferred.     

Neogene–Quaternary Magmatism in Eastern Balkaria (North Caucasus,Russia): Evidence from the Isotope–Geochronological Data

Doklady Earth Sciences - An isotope-geochronological study for a number of young volcanic localities in the eastern part of Balkaria (North Caucasus, Russia), referred to the...     

Neoproterozoic arc–back-arc system in the Central Eastern Desert of Egypt: Evidence from supra-subduction zone ophiolites

Ophiolites are widely distributed in the Central Eastern Desert (CED) of Egypt, occurring as clusters in the northern (NCEDO) and southern (SCEDO) segments. Mineralogical and geochemical data on the volcanic sections of Wizer (WZO) and Abu Meriewa (AMO) ophiolites as representatives of the NCEDO and SCEDO, respectively, are presented.The WZO volcanic sequence comprises massive metavolcanics of MORB-like compositions intruded by minor boninitic dykes and thrust over island-arc metavolcanic blocks in the mélange matrix. Such transitional MORB-IAT-boninitic magmatic affinities for the WZO metavolcanics suggest that they most likely formed in a protoarc–forearc setting. Chemical compositions of primary clinopyroxene and Cr-spinel relicts from the WZO volcanic section further confirm this interpretation. The compositional variability in the WZO volcanic sequence is comparable with the associated mantle rocks that vary from slightly depleted harzburgites to highly depleted harzburgites containing small dunite bodies, which are residues after MORB, IAT and boninite melt formation, respectively. Source characteristics of the different lava groups from the WZO indicate generation via partial melting of a MORB source which was progressively depleted by melt extraction and variably enriched by subduction zone fluids. MORB-like magma may have been derived from ~ 20% partial melting of an undepleted lherzolite source, leaving slightly depleted harzburgite as a residuum. The generation of island-arc magma can be accounted for by partial melting (~ 15%) of the latter harzburgitic mantle source, whereas boninites may have been derived from partial melting (~ 20%) of a more refractory mantle source previously depleted by melt extraction of MORB and IAT melts, leaving ultra-refractory dunite bodies as residuum.The AMO volcanic unit occurs as highly deformed pillowed metavolcanic rocks in a mélange matrix. They can be categorized geochemically into LREE-depleted (La/Yb_CN = 0.41–0.50) and LREE-enriched (La/Yb_CN = 4.7–4.9) lava types that show an island arc to MORB geochemical signature, respectively, signifying a back-arc basin setting. This is consistent, as well, with their mantle section. Source characteristics indicate depleted to slightly enriched mantle sources with overall slight subduction zone geochemical affinities as compared to the WZO.Generally, CED ophiolites show supra-subduction zone geochemical signature with prevalent island arc tholeiitic and minor boninitic affinities in the NCEDO and MORB/island-arc association in the SCEDO. Such differences in geochemical characteristics of the NCEDO and SCEDO, along with the abundance of mature island arc metavolcanics which are close in age (~ 750 Ma) to the ophiolitic rocks, general enrichment in HFSE of ophiolites from north to south, and lack of a crustal break and major shear zones, is best explained by a geotectonic model whereby the CED represents an arc–back-arc system above a southeast-dipping subduction zone.     

Geochemical Characteristics of Mid-oceanic Ridge Volcanic Glass from Lau Basin: Evidence from the Major Elements Variation

Orientale size craters are not recognized on Earth nor expected for Phanerozoic and Proterozoic eons from conventional crater size frequency distributions (Ivanov et al., 2002). Here suggested are three such Phanerozoic craters, modified by plate tectonics, and tentatively correlated with extinction and “ophiolite obduction” events. Hypothesis testing is proposed and plate tectonics implications are discussed. Such basins might manifest:

• circular to elliptical rims (or rim segments), with exposed lithospheric mantle, as strain markers for plate boundary motion;
• thick ejecta near rim expressed as “ophiolitic melange”;
• power law decay of ejecta thickness with radial distance from rim (McGetchin et al., 1973) and/or systematic azimuthal variation of ejecta thickness for low angle impacts (Schultz, 1999);
• weathering resistant shocked mantle minerals (Bohor et al., 1990) in ejecta;? global spherule layer with PGE anomalies (Alvarez et al., 1980);
• rim structures consistent with cratering mechanics (Melosh, 1989; Kenkmann, 2014);
• impact melt basement (Grieve et al., 1992; Pierazzo et al. 2000) recording uniform cooling age and Earth's magnetic polarity of the time. Tentatively suggested Phanerozoic impact basins:
• Yucatan Basin: Greater Antilles ophiolite rim – KPg Boundary? Maastrichtian ophiolite obduction in southeast Cuba (Iturralde‐Vinent et al., 2006).
• Sulu Sea Basin: Palawan, Sabah etc. ophiolite rim – Middle Miocene Disruption? MM ophiolitic mélange emplacement in Sabah (Clennell, 1991).
• Loyalty Basin: New Caledonia ophiolite and d'Entrecasteaux ridge rim – EO Boundary? EO ophiolite obduction in New Caledonia (Cluzel et al., 2012).

     

Formation of adakitic granitoids in the collisional orogens: Evidence from the Early Paleozoic granitoids of the Munku–Sardyk Range,Eastern Sayan

The classical models of adakite formation by melting of basaltic layer of oceanic lithosphere in the subduction zone were verified using geochemical and Sr–Nd isotope data on the Early Paleozoic granitoids of Eastern Sayan. The presence of adakites in fold belts is usually regarded as geochemical proxy for paleogeodynamic reconstruction. The formation of felsic derivatives with adakitic signatures in the collisional orogens is inconsistent with these models and requires their revision. It is shown that the composition of the granitoids and their evolution cannot be described with these models. In order to solve this problem, two hypotheses of granitoid formation by mixing of two geochemically contrasting reservoirs were proposed and verified. According to the first hypothesis, the granitoids represent the mixing products between alkaline olivine basalts and partial melts of the gray gneiss basement of this region. The second model relates the formation of the granitoids with melting of geochemically 2700 Ma-old enriched source in the subcontinental lithospheric mantle. In spite of differences, both these hypotheses are based on the remobilization of sources formed at the previous stages of the geological evolution of the region. In both cases, adakitic geochemical characteristics of forming felsic magmas are determined by the composition of protolith rather than by their geodynamic position. Obtained preliminary results place constraints on genetic models and geochemical reservoirs participating in the formation of the granitoids.     

Specific Features in the Deep Structure of the Naryn Basin–Baibichetoo Ridge–Atbashi Basin System: Evidence from the Complex of Geological and Geophysical Data

New magnetotelluric data were obtained for the Karabuk profile crossing the Naryn basin–Baibichetoo Ridge–Atbashi basin geodynamic system (Central Tien-Shan). The complex geological–geophysical cross section along the profile provides a good agreement between the surface tectonic structures and the deep geoelectric model. The electric conductivity anomalies revealed as subvertical conductors striking along the flanks of basins may be explained by the zones of dynamic influence of faults and cataclasis of granite.     

A Reconstruction of a Vendian–Cambrian Active Continental Margin within the Southern Urals: Results of Detrital Zircons Studying from Ordovician Terrigenous Rocks

Geotectonics - Detrital zircons of Ordovician terrigenous sequences are studied in various Southern Uralian tectonic units. The age of detrital zircons of the West Uralian and Transuralian...     

Genesis of Diamond in Metal–Carbon and Metal–Sulfur–Carbon Melts: Evidence from Experimental Data

Doklady Earth Sciences - The experimental data on diamond growth in the Fe–Ni–S–C and Fe–S–C systems with a sulfur content of 5–14 wt % at 5.5 GPa and...     

Genesis of the Dalingzi Pb-Zn Deposit in the Western Margin of Yangtze Plate: Constraints from Fluid Inclusions and Isotopic EvidenceEI北大核心CSCD

The Daliangzi Pb-Zn deposit is a large deposit hosted in the Sinian Dengying Formation dolostone, located in the Sichuan-Yunnan-Guizhou ore concentration area. Ore minerals are mianly sphalerite, galena, and gangue minerals consist of dolomite, quartz and calcite. The metallogenic stages may be divided into sphalerite-pyrite-carbon stage, sphalerite-galena stage and galena-chalcopyrite-carbonate stage. The ore-forming fluid is basin brine, which is characterized by medium-low temperature of 117.5 ℃ to 320.3 ℃ and medium salinity of 5.11% NaCleqv to 18.96% NaCleqv, moreover, the abundant CH4 and pitch in the fluid inclusions indicate that the participation of organic matter in the mineralization. The δ13CV-PDB and δ18OSMOW values of the Dengying Formation dolostone are similar to that of marine carbonate, revealing that the dolostone belongs to marine carbonate. Both the δ13CV-PDB and δ18OSMOW values of hydrothermal calcites are lower than that of the Dengying Formation dolostone, which may result from dissolution of the Dengying Formation dolostone. The δ34S values of ore minerals are mainly in the range of 9.8‰-20.8‰, indicating the sulfur may come from thermochemical reduction of marine sulfate in the Dengying Formation. The 207Pb/204Pb versus 206Pb/204Pb diagram manifests that Pb is crustal origin, and likely comes mainly from the wall rocks and less from the basement. (87Sr/86Sr)i ratios of sphalerites and hydrothermal calcite are higher than that of the Dengying Formation dolostone, indicating that the ore-forming fluid flew through the basement. In conclusion, the ore-forming fluid was basin brine, which extracted the metallogenic materials, Pb and Zn, from the basement and wall rocks. When the ore-forming fluid reached the "black fractured zones", carbonized tectonic breccia zone, S2- was produced by the thermochemical reduction reaction under the influence of the organic matter, and interaction between the S2- and Pb2+, Zn2+, resulted in the precipitation of ore metals. © 2018, Science Press. All right reserved.     

Relationship between Granite and Eclogite on the Southern Margin of the Qilian Mountains: Evidence from Zircon SHRIMP Agesof the Aolaoshan Granite

Zircon SHRIMP ages of the Aolaoshan granite on the south margin of the Qilian Mts. range from 445±15.3 to 496±7.6 Ma (averaging 473 Ma), belonging to the Early Ordovician. Geochemically, the granite is similar to I-type granite and, tectonically, was formed in an island-arc environment based on relevant diagrams for structural discriminations. Considering also the regional geology, the authors suggest that the granite is part of an ultrahigh-pressure belt on the south margin of the Qilian Mts. and that its formation bears a close relationship to this belt.     

Sources of Magmatic Rocks from the Deep-Sea Floor of the Arctic Ocean and the Central Atlantic: Evidence from Data on the U–Pb Age,Hf Isotopes,and REE Geochemistry of Zircons

The results of geochronological (U–Pb), isotope–geochemical (Lu–Hf), and geochemical (REEs) studies of young (MZ, KZ) and xenogenic (AR, PR) zircons from magmatic rocks of the Central Arctic rises of the Arctic Ocean (AO) and the crest zone of the Mid-Atlantic Ridge (MAR) are presented. The data obtained show that the depleted mantle could be a source of young (KZ) zircons of the MAR, whereas young (MZ) zircons of the MAR and all xenogenic (AR, PR) zircons of the AO and MAR are from crustal rocks of the continental lithosphere.     

Magmatic Evolution of Changbaishan Tianchi Volcano,China–North Korea: Evidence from Mineral-Hosted Melt and Fluid Inclusions

Data on mineral-hosted melt, fluid, and crystalline inclusions were used to study the composition and evolution of melts that produced rocks of Changbaishan Tianchi volcano, China–North Korea, and estimate their crystallization parameters. The melts crystallized within broad ranges of temperature (1220–700°C) and pressure (3100–1000 bar), at a drastic change in the redox potential: Δ log \(f_{O_2}\) from NNO + 0.92 to +1.42 for the basalt melts, NNO –1.61 to –2.09 for the trachybasaltic andesite melts, NNO –2.63 to –1.89 for the comendite melts, and NNO –1.55 to –3.15 for the pantellerite melts. The paper reports estimates of the compositions of melts that produced the continuous rock series from trachybasalt to comendite and pantellerite. In terms of trace-element concentrations, all of the mafic melts are comparable with OIB magmas. The silicic melts are strongly enriched in trace elements and REE. The most strongly enriched melts contain concentrations of certain elements almost as high as in ores of these elements. The paper reports data on H2O concentrations in melts of different composition. It is demonstrated that the variations in the H₂O concentrations were controlled by magma degassing. Data are reported on the Sr and Nd composition of the rocks. The deviations in the Sr isotopic composition are proportional to the ⁸⁷Sr/⁸⁶Sr ratio and could be produced in a melt with a high enough ⁸⁷Sr/⁸⁶Sr ratio during a geologically fairly brief time period. The evolution of melts that produced rocks of the volcano was controlled by crystallization differentiation of the parental basalt magmas at insignificant involvement of melt mixing and liquid immiscibility of silicate and sulfide melts. The alkaline salic rocks were generated in shallow-sitting (13–3.5 km) magmatic chambers in which the melts underwent profound differentiation that gave rise to pantellerites and comendites strongly enriched in trace elements (Th, Nb, Ta, Zr, and REE). Data on the composition of the magmas and parameters of their derivation are used to develop a generalized petrologic–geodynamic model for the origin of Changbaishan Tianchi volcano.     

Metamorphic P–T path of mafic granulites from Eastern Hebei: Implications for the Neoarchean tectonics of the Eastern Block,North China Craton

This study documents the metamorphic evolution of mafic granulites from the Eastern Hebei Complex in the Eastern Block of the North China Craton. Mafic granulites from Eastern Hebei occur as boudins or enclaves within Neoarchean high-grade TTG gneisses. Petrographic observations reveal three characteristic metamorphic mineral assemblages in the mafic granulites: the pre-peak hornblende + plagioclase + ilmenite + quartz + sphene assemblage (M₁) existing as mineral inclusions within coarse-grained peak assemblage (M₂) represented by garnet + clinopyroxene + orthopyroxene + plagioclase + hornblende + ilmenite + quartz, and post-peak assemblage (M₃) marked by garnet + quartz ± ilmenite symplectites surrounding the peak pyroxene and plagioclase. Based on pseudosection modeling calculated in the NCFMASHTO model system using the program THERMOCALC, P–T conditions of the pre-peak (M₁), peak (M₂) and post-peak (M₃) assemblages are constrained at 600–715 °C/6.0 kbar or below, 860–900 °C/9.6–10.3 kbar, and 790–810 °C/9.6–10.4 kbar, respectively. These P–T estimates, combined with their mineral compositions and reaction relations, define an anticlockwise P–T path incorporating isobaric cooling subsequent to the peak medium-pressure granulite-facies metamorphism for the mafic granulites from Eastern Hebei. Such an anticlockwise P–T path suggests that the end-Neoarchean metamorphism of the Eastern Hebei Complex correlated closely with underplating and intrusion of voluminous mantle-derived magmas. In conjunction with other geological considerations, a mantle-plume model is favored to interpret the Neoarchean tectonothermal evolution of the Eastern Hebei Complex and other metamorphic complexes in the Eastern Block. The prograde amphibolite-facies metamorphism (M₁) was initiated due to the upwelling of the relatively cooler mantle plume head, followed by the peak medium-pressure granulite-facies metamorphism (M₂) as triggered by the uprising hotter plume “tail”, and finally when plume activity ceased, the heated metamorphic crust experienced nearly isobaric cooling (M₃).     

Genesis of the Fuxing porphyry Cu deposit in Eastern Tianshan,China: Evidence from fluid inclusions and C–H–O–S–Pb isotope systematics

The Fuxing porphyry Cu deposit is a recently discovered deposit in Eastern Tianshan, Xinjiang, northwestern China. The Cu mineralization is associated with the Fuxing plagiogranite porphyry and monzogranite, mainly presenting as various types of hydrothermal veins or veinlets in alerted wall rocks, with potassic, chlorite, phyllic, and propylitic alteration developed. The ore-forming process can be divided into four stages: stage I barren quartz veins, stage II quartz–chalcopyrite–pyrite veins, stage III quartz–polymetallic sulfide veins and stage IV quartz–calcite veins. Four types of fluid inclusions (FIs) can be distinguished in the Fuxing deposit, including hypersline (H-type), vapor-rich two-phase (V-type), liquid-rich two-phase (L-type), and trace amounts of pure vapor inclusions (P-type), but only the stage I quartz contains all types of FIs. The stages II and III quartz have two types of FIs, with exception of H- and P-types. In stage IV quartz minerals, only the L-type inclusions can be observed. The FIs in quartz of stages I, II, III and IV are mainly homogenized at temperatures of 357–518 °C, 255–393 °C, 234–322 °C and 145–240 °C, with salinities of 1.9–11.6 wt.% NaCl equiv., 1.6–9.6 wt.% NaCl equiv., 1.4–7.7 wt.% NaCl equiv. and 0.9–3.7 wt.% NaCl equiv., respectively. The ore-forming fluids of the Fuxing deposit are characterized by high temperature, moderate salinity and relatively oxidized condition. Carbon, hydrogen and oxygen isotopic compositions of quartz indicate that the ore-forming fluids were gradually evolved from magmatic to meteoric in origin. Sulfur and lead isotopes suggest that the ore-forming materials were derived from a deep-seated magma source. The Cu mineralization in the Fuxing deposit occurred at a depth of ~ 1 km, and the changes of oxygen fugacity, decompression boiling, and local mixing with meteoric water were most likely critical for the formation of the Fuxing Cu deposit.     

Source region analyses of the morainal detritus from the Grove Mountains: Evidence from the subglacial geology of the Ediacaran–Cambrian Prydz Belt of East Antarctica

The Grove Mountains are the inland exposures of the Prydz Belt in East Antarctica. Although the 550–500 Ma orogenic event was recognized as the latest major magmatic–metamorphic activity in the Prydz Belt, its subduction–collision origin was not confirmed until the discovery of high-pressure (HP) mafic granulite erratic boulders in the glacial moraines from the Grove Mountains. Because no HP metamorphic bedrock is exposed in this area, an understanding the regional geology required a thorough study of the morainal debris mineralogy and detrital zircon U–Pb chronology. Detrital zircon U–Pb age histograms show 550–450 Ma, 900–800 Ma, and 1100–1000 Ma modes from three morainal deposits and one paleosol samples. The oldest ages were 2300 to 2420 Ma. Detailed electron probe microanalyses (EPMA) for the detrital mineral grains were compared with the minerals from the nearby exposed bedrock. The mineral chemistry indicates that the exposed bedrock in the Grove Mountains was not the sole source for morainal materials. This new U–Pb zircon geochronology and microprobe mineral data support the previous interpretation that the 550–500 Ma tectonic activity was the final collisional event that formed the Prydz Belt and amalgamated East Antarctica.