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1.
《地学前缘(英文版)》2020,11(5):1593-1608
The Gejiu-Bozushan-Laojunshan W-Sn polymetallic metallogenic belt(GBLB) in southeast Yunnan Province is an important part of the southwestern Yangtze Block in South China.Tin polymetallic mineralization in this belt includes the Niusipo,Malage,Songshujiao,Laochang and Kafang ore fields in the Gejiu area which are spatially and temporally associated with the Kafang-Laochang and Songshujiao granite plutons.These granites are characterized by variable A/CNK values(mostly 1.1,except for two samples with 1.09),high contents of SiO_2(74.38-76.84 wt.%) and Al_2 O_3(12.46-14.05 wt.%) and variable CaO/Na_2 O ratios(0.2-0.65) as well as high zircon δ~(18)O values(7.74‰-9.86‰),indicative of S-type affinities.These rocks are depleted in Rb,Th,U,Ti,LREE[(La/Yb)N=1.4-20.51],Ba,Nb,Sr,and Ti and display strong negative Eu and Ba anomalies.The rocks possess high Rb/Sr and Rb/Ba ratios,relatively low initial ~(87)Sr/~(86)Sr ratios(0.6917-0.7101),and less radiogenic εNd(t)values(-8.0 to-9.1).The zircon grains from these rocks show negative ε_(Hf)(t) values in the range of-3.7 to-9.9 with mean T_(DM2)(Nd) and T_(DM2)(Hf) values of 1.57 Ga and 1.55 Ga.They show initial ~(207)Pb/~(204)Pb ranging from15.69 to 15.71 and ~(206)Pb/~(204)Pb from 18.36 to 18.70.Monazite from Songshujiao granites exhibits higher U and lower Th/U ratios,lower δ~(18)O values and higher ε_(Hf)(t) values than those of the zircon grains in the KafangLaochang granites.The geochemical and isotopic features indicate that the Laochang-Kafang granites originated by partial melting of Mesoproterozoic crustal components including biotite-rich metapelite and metagraywacke,whereas the Songshujiao granites were derived from Mesoproterozoic muscovite-rich metapelite crustal source.Most zircon grains from the Songshujiao,Laochang and Kafang granites have high-U concentrations and their SIMS U-Pb ages show age scatter from 81.6 Ma to 88.6 Ma,80.7 Ma to 86.1 Ma and 82.3 Ma to 87.0 Ma,suggesting formation earlier than the monazite and cassiterite.Monazite SIMS U-Pb ages and Th-Pb ages of three same granite samples are consistent and show yielded 206 Pb/~(238)U ages of 83.7 ± 0.6 Ma,83.7±0.6 Ma,and 83.4±0.6 Ma,and ~(208)Pb/~(232)Th ages of 83.2 ± 0.5 Ma,83.8 ± 0.4 Ma,and 83.5±0.9 Ma,which are within the range of the SIMS zircon U-Pb ages from these rocks.The data constrain the crystallization of the granites at ca.83 Ma.In situ U-Pb dating of two cassiterite samples from the cassiterite-sulfide ore in the Songshujiao ore field and Kafang ore field,and two from the cassiterite-oxide+cassiterite bearing dolomite in the Laochang ore field yielded weighted mean 206 Pb/~(238)U ages of 83.5±0.4 Ma(MSWD=0.6),83.5 ± 0.4 Ma(MSWD=0.5),83.6 ±0.4 Ma(MSWD=0.6) and 83.2 ±0.7 Ma(MSWD=0.6),respectively.Combined with geological characteristics,the new geochronological data indicate that the formation of the granites and Sn polymetallic deposits are coeval.We correlate the magmatic and metallogenic event with lithospheric thinning and asthenosphere upwelling in continental extension setting in relation to the eastward subduction of the Neo-Tethys beneath the Sanjiang tectonic domain during Late Cretaceous. 相似文献
2.
《地学前缘(英文版)》2020,11(5):1621-1634
The Izu-Bonin arc system is sediment-poor(～400 m thick with no accretionary prism) and,therefore,the influence of the altered oceanic crust(AOC) is most likely the source of the documented along-arc lava compositional variations,especially in Pb isotopes.Izu-Bonin arc lava geochemistry suggests an influx of subduction component from an Indian-type AOC.However,samples drilled from the western Pacific geochemical reference site at Integrated Ocean Drilling Program Site 1149 implies subduction of a Pacific-type AOC.To solve the apparent discrepancy of slab input versus arc output in this arc system,samples of the AOC were dredged from vertical fault scarps of the subducting Pacific Plate along a transect from 27.5°N to 34.50 N.Samples range from tholeiitic to mildly alkalic mid-ocean ridge basalts as well as trachybasalts,basaltic trachyandesites,tephrites,and phono-tephrites.Isotope ratios also exhibit a range of values(~(87)Sr/~(86)Sr=0.70282-0.70673,143 Nd/144 Nd=0.512552-0.513174,~(206)Pb/204 Pb=18.43-20.00,207 Pb/204 Pb=15.40-15.67,~(208)Pb/~(204)Pb=37.75-39.55).Our results suggest that there is a geochemical variation in the AOC that is neither completely due to seawater or hydrothermal alteration,nor to petrogenetic processes.Rather,this variation is the result of the Pacific-Izanagi Ridge system tapping into a heterogeneous,plume-polluted mantle source during the Mid-Cretaceous volcanic event.The observed Pacific-type AOC is not responsible for the Indian-type Pb isotopic signature of Izu-Bonin arc lavas.This leads us to propose an alternative scenario where the Izu-Bonin arc lava Indian-type Pb isotopic signature originates from slab-derived fluids interacting and adsorbing Pb from an Indian-type mantle wedge through zone-refining. 相似文献
3.
《地学前缘(英文版)》2020,11(5):1533-1548
The Chinese North Tianshan(CNTS) in the southern part of the Central Asian Orogenic Belt(CAOB) has undergone multistage accretion-collision processes during Paleozoic time,which remain controversial.This study addresses this issue by tracing the provenance of Late Paleozoic sedimentary successions from the Bogda Mountain in the eastern CNTS through U-Pb dating and Lu-Hf isotopic analyses of detrital zircons.New detrital zircon U-Pb ages(N=519) from seven samples range from 261±4 Ma to 2827±32 Ma.The most prominent age peak is at 313 Ma and subordinate ages vary from 441 Ma to 601 Ma,with some Precambrian detrital zircon ages(～7%) lasting from 694 Ma to 1024 Ma.The youngest age components in each sample yielded weighted mean ages ranging from 272±9 Ma to 288±5 Ma,representing the maximum depositional ages.These and literature data indicate that some previously-assumed "Carboniferous"strata in the Bogda area were deposited in the Early Permian,including the Qijiaojing,Julideneng,Shaleisaierke,Yangbulake,Shamaershayi,Liushugou,Qijiagou,and Aoertu formations.The low maturity of the sandstones,zircon morphology and provenance analyses indicate a proximal sedimentation probably sourced from the East Junggar Arc and the Harlik-Dananhu Arc in the CNTS.The minor Precambrian detrital zircons are interpreted as recycled materials from the older strata in the Harlik-Dananhu Arc.Zircon E_(Hf)(t) values have increased since ～408 Ma,probably reflecting a tectonic transition from regional compression to extension.This event might correspond to the opening of the Bogda intraarc/back arc rift basin,possibly resulting from a slab rollback during the northward subduction of the North Tianshan Ocean.A decrease of zircon ε_(Hf)(t) values at ～300 Ma was likely caused by the cessation of oceanic subduction and subsequent collision,which implies that the North Tianshan Ocean closed at the end of the Late Carboniferous. 相似文献
4.
《地学前缘(英文版)》2020,11(6):2221-2242
We present the first evidence of Archean oceanic crust submitted to Proterozoic high-pressure (HP) metamorphism in the South American Platform. Sm–Nd and Lu–Hf isotopic data combined with U–Pb geochronological data from the Campo Grande area, Rio Grande do Norte domain, in the Northern Borborema Province, reflect a complex Archean (2.9 Ga and 2.6 Ga) and Paleoproterozoic (2.0 Ga) evolution, culminating in the Neoproterozoic Brasiliano/Pan-African orogeny (ca. 600 Ma). The preserved mafic rocks contain massive poikiloblastic garnet and granoblastic amphibole with variable proportions of plagioclase + diopside in symplectitic texture, typical of high-pressure rocks. These clinopyroxene-garnet amphibolites and the more common garnet amphibolites from the Campo Grande area are exposed as rare lenses within an Archean migmatite complex. The amphibolite lenses represent 2.65 Ga juvenile tholeiitic magmatism derived from depleted mantle sources (positive εHf(t) values of +3.81 to +30.66) later enriched by mantle metasomatism (negative εNd(t) values of –7.97). Chondrite and Primitive Mantle-normalized REE of analyzed samples and discriminant diagrams define two different oceanic affinities, with E-MORB and OIB signature. Negative Eu anomalies (Eu/Eu1 = 0.75–0.95) indicate depletion of plagioclase in the source. Inherited zircon cores of 3.0–2.9 Ga in analyzed samples indicate that the Neoarchean tholeiitic magmatism was emplaced into 2923 ± 14 Ma old Mesoarchean crust (εNd(t) = –2.58 and Nd TDM = 3.2 Ga) of the Rio Grande do Norte domain. The age of retro-eclogite facies metamorphism is not yet completely understood. We suggest that two high-grade metamorphic events are recognized in the mafic rocks: the first at 2.0 Ga, recorded in some samples, and the second, at ca. 600 Ma, stronger and more pervasive and recorded in several of the mafic rock samples. The Neoproterozoic zircon grains are found in symplectite texture as inclusions in the garnet grains and represent the age of HP conditions in the area. These zircon grains show a younger cluster of concordant analyses between 623 ± 3 Ma and 592 ± 5 Ma with εHf(t) values of +0.74 to –65.88. Thus, the Campo Grande rock assemblage is composed of Archean units that were amalgamated to West Gondwana during Neoproterozoic Brasiliano orogeny continent-continent collision and crustal reworking. 相似文献
5.
《地学前缘(英文版)》2020,11(6):2347-2364
The Late Cretaceous Sabzevar ophiolite represents one of the largest and most complete fragments of Tethyan oceanic lithosphere in the NE Iran. It is mainly composed of serpentinized mantle peridotites slices; nonetheless, minor tectonic slices of all crustal sequence constituents are observed in this ophiolite. The crustal sequence contains a well-developed ultramafic and mafic cumulates section, comprising plagioclase-bearing wehrlite, olivine clinopyroxenite, olivine gabbronorite, gabbronorite, amphibole gabbronorite and quartz gabbronorite with adcumulate, mesocumulate, heteradcumulate and orthocumulate textures. The crystallization order for these rocks is olivine ± chromian spinel → clinopyroxene → plagioclase → orthopyroxene → amphibole. The presence of primary magmatic amphiboles in the cumulate rocks shows that the parent magma evolved under hydrous conditions. Geochemically, the studied rock units are characterized by low TiO2 (0.18–0.57 wt.%), P2O5 (<0.05 wt.%), K2O (0.01–0.51 wt.%) and total alkali contents (0.12–3.04 wt.%). They indicate fractionated trends in the chondrite-normalized rare earth element (REE) plots and multi-element diagrams (spider diagrams). The general trend of the spider diagrams exhibit slight enrichment in large ion lithophile elements (LILEs) relative to high field strength elements (HFSEs) and positive anomalies in Sr, Pb and Eu and negative anomalies in Zr and Nb relative to the adjacent elements. The REE plots of these rocks display increasing trend from La to Sm, positive Eu anomaly (Eu/Eu1 = 1.06–1.54) and an almost flat pattern from medium REE (MREE) to heavy REE (HREE) region [(Gd/Yb)N = 1–1.17]. Moreover, clinopyroxenes from the cumulate rocks have low REE contents and show marked depletion in light REE (LREE) compared to MREE and HREE [(La/Sm)N = 0.10–0.27 and (La/Yb)N = 0.08–0.22]. The composition of calculated melts in equilibrium with the clinopyroxenes from less evolved cumulate samples are closely similar to island arc tholeiitic (IAT) magmas. Modal mineralogy, geochemical features and REE modeling indicate that Sabzevar cumulate rocks were formed by crystal accumulation from a hydrous depleted basaltic melt with IAT affinity. This melt has been produced by moderate to high degree (~15%) of partial melting a depleted mantle source, which partially underwent metasomatic enrichment from subducted slab components in an intra-oceanic arc setting. 相似文献
6.
《地学前缘(英文版)》2020,11(6):1915-1934
The first data on P-T metamorphic conditions coupled with U-Pb monazite and zircon age obtained for the Neoarchean Kitoy granulite-gneiss terrane (SW Siberian Craton). Alumina gneisses of the Kitoy terrane indicate two-staged metamorphic evolution. The first stage of regional metamorphism (M1) occurred at high-amphibolite facies conditions at T = 780–800 °C and P = 8–9 kbar. The second stage (M2) belongs to MT-HT/LP type of metamorphism with the wide temperature interval 600–750 °C and pressure 2–4 kbar. Two age peaks were established on the basis of U-Pb monazite and zircon dating in garnet-anthophyllite gneisses. Both of them correspond to the Neoarchean age: the age of M1 falls into the interval of ca. 2489–2496 Ma, the age of M2 – ca. 2446–2456 Ma. The high-temperature metamorphism of the Kitoy block and nearly coeval granitoid magmatism can be an evidence for the Neoarchean collision in SW Siberian craton. 相似文献
7.
《地学前缘(英文版)》2020,11(6):2083-2102
Chromitite bodies hosted in peridotites typical of suboceanic mantle (s.l. ophiolitic) are found in the northern and central part of the Loma Caribe peridotite, in the Cordillera Central of the Dominican Republic. These chromitites are massive pods of small size (less than a few meters across) and veins that intrude both dunite and harzburgite. Compositionally, they are high-Cr chromitites [Cr# = Cr/(Cr + Al) atomic ratio = 0.71–0.83] singularly enriched in TiO2 (up to 1.25 wt.%), Fe2O3 (2.77–9.16 wt.%) as well as some trace elements (Ga, V, Co, Mn, and Zn) and PGE (up to 4548 ppb in whole-rock). This geochemical signature is unknown for chromitites hosted in oceanic upper mantle but akin to those chromites crystallized from mantle plume derived melts. Noteworthy, the melt estimated to be in equilibrium with such chromite from the Loma Caribe chromitites is similar to basalts derived from different source regions of a heterogeneous Caribbean mantle plume. This mantle plume is responsible for the formation of the Caribbean Large Igneous Province (CLIP). Dolerite dykes with back-arc basin basalt (BABB) and enriched mid-ocean ridge basalt (E-MORB) affinities commonly intrude the Loma Caribe peridotite, and are interpreted as evidence of the impact that the Caribbean plume had in the off-axis magmatism of the back-arc basin, developed after the Caribbean island-arc extension in the Late Cretaceous. We propose a model in which chromitites were formed in the shallow portion of the back-arc mantle as a result of the metasomatic reaction between the supra-subduction zone (SSZ) peridotites and upwelling plume-related melts. 相似文献
8.
《地学前缘(英文版)》2020,11(5):1581-1592
The newly-discovered supergiant Huayangchuan uranium(U)-polymetallic(Sr,Se,REEs,Ba,Nb and Pb) deposit is located in the Qinling Orogen,central China.The deposit underwent multistage mineralization,with the main carbonatite ore stage being the most important for the U,Nb,REE,Sr and Ba endowments.According to the mineral assemblages,the main carbonatite ore stage can be divided into three substages,i.e.,sulfate(Ba-Sr),alkali-rich U and REE-U mineralization.Main-stage titanite from the Huayangchuan igneous carbonatite are rich in high field strength elements(HFSEs,e.g.,Zr,Nb and REEs),and show clear elemental substitutions(e.g.,Ti vs.Nb+Fe+Al and Ca+Ti vs.Fe+Al+REE).High-precision LA-ICP-MS titanite dating yielded a U-Pb age of 209.0 ± 2.9 Ma,which represents the mainstage mineralization age at Huayangchuan,and is coeval with the local carbonatite dyke intrusion.This mineralization age is further constrained by the Re-Os dating of molybdenite from the Huayangchuan carbonatite,which yielded a weighted mean age of 196.8 ± 2.4 Ma.Molybdenite Re contents(337.55-392.75 ppm) and C-OSr-Nd-Pb isotopic evidence of the Huayangchuan carbonatite both suggest a mantle origin for the carbonatite.Our study supports that the Late Triassic carbonatite magmatism was responsible for the world-class U-Mo-REE mineralization in the Qinling Orogen,and that the regional magmatism and ore formation was likely caused by the closure of the Mianlue ocean and the subsequent North China-South China continent-continent collision. 相似文献
9.
《地学前缘(英文版)》2020,11(6):2339-2346
A new cerite group mineral species, taipingite-(Ce), ideally (Ce73+, Ca2)Σ9Mg(SiO4)3[SiO3(OH)]4F3, has been found in the Taipingzhen rare earth element (REE) deposit in the North Qinling Orogen (NQO), Central China. It forms subhedral grains (up to approximately 100 μm × 200 μm) commonly intergrown with the REE mineral assemblages and is closely associated with allanite-(Ce), gatelite-(Ce), törnebohmite-(Ce), fluocerite-(Ce), fluocerite-(La), fluorite, bastnäsite-(Ce), parisite-(Ce) and calcite. Taipingite-(Ce) is light red to pinkish brown under a binocular microscope and pale brown to colorless in thin section, and it is translucent to transparent with a grayish-white streak and vitreous luster. This mineral is brittle with conchoidal fracture; has a Mohs hardness value of approximately 5½ and exhibits no cleavage twinning or parting. The calculated density is 4.900(5) g/cm3. Optically, taipingite-(Ce) is uniaxial (+), with ω = 1.808(5), ε = 1.812(7), c = ε, and a = b = ω. Furthermore, this mineral is insoluble in HCl, HNO3 and H2SO4. Electron microprobe analysis demonstrated that the sample was relatively pure, yielding the empirical formula (with calculated H2O): (Ce4.02La1.64Nd1.49Pr0.41Sm0.10Gd0.02Ho0.02Tm0.01Lu0.02Y0.03Ca0.66Mg0.05Th0.01–0.51)Σ9(Mg0.75Fe0.253+)Σ1(SiO4)3{[SiO3(OH)]3.98[PO3(OH)]0.02}Σ4(F1.81OH1.17Cl0.02)Σ3. Taipingite-(Ce) is trigonal and exhibits space group symmetry R3c with unit cell parameters a = 10.7246(3) Å, c = 37.9528(14) Å, V = 3780.39(20) Å3 and Z = 6. The strongest eight lines in the X-ray diffraction pattern are [d in Å(I)(hkl)]: 4.518(50)(202), 3.455(95)(122), 3.297(85)(214), 3.098(35)(300), 2.941(100)(02,10), 2.683(65)(220), 1.945(40)(238) and 1.754(40)(30,18). The crystal structure has been refined to a R1 factor of 0.025, calculated for the 2312 unique observed reflections (Fo ≥ 4σ). The mineral is named after its discovery locality and is characterized as the F-dominant analogue of cerite-(Ce). 相似文献
10.
《地学前缘(英文版)》2020,11(6):2323-2337
Petrological characterization, U–Pb geochronology, Lu–Hf analyses and major and trace element data from mafic intrusions in the Central Espinhaço (central portion of the Brazilian shield) are used here to investigate the geological significance of the Early Neoproterozoic magmatism in the context of the São Francisco-Congo paleocontinent. These mafic bodies are represented by medium to coarse-grained metagabbros with plagioclase, amphibole and clinopyroxene. Zircon U–Pb isotopic data from two samples yielded weighted mean 206Pb/238U ages of 895 ± 3.4 Ma (MSWD = 1.7) and 896 ± 2.4 Ma (MSWD = 0.64), regarded as the best estimates for the crystallization age of these mafic rocks. Major and trace element data (including REEs) show that the gabbros originated from a subalkaline tholeiitic magma, typical of intraplate magmatism. Such rocks are slightly enriched in LREEs and LILEs and depleted in HFSEs. Our new isotope and geochemical data, along with regional knowledge, indicate that these metagabbros mark the beginning of an important Tonian-age extensional tectonic event of the landmass of which the São Francisco-Congo paleocontinent was part (Rodinia supercontinent or Central African block?). We furthermore suggest that these rocks belong to a prominent suite of Tonian-age mafic rocks that mark a diachronic breakup attempt of this landmass which may have occurred from south to north along the Espinhaço mountain range. 相似文献
11.
《地学前缘(英文版)》2020,11(4):1189-1201
Numerous lenses of garnet amphibolite occur in the garnet-bearing biotite-plagioclase gneiss belt in the Baishan area of the Beishan Orogen,which connects the Tianshan Orogen to the west and the Mongolia-Xing'anling Orogen to the east.The study of metamorphism in Beishan area is of great significance to explain the tectonic evolution of Beishan orogen.According to the microstructures,mineral relationships,and geothermobarometry,we identified four stages of mineral assemblages from the garnet amphibolite sample:(1) a pre-peak stage,which is recorded by the cores of garnet together with core-inclusions of plagioclase(Pl_1);(2) a peak stage,which is recorded by the mantles of garnet together with mantle-inclusions of plagioclase(Pl_2)+amphibole(Amp_1)+Ilmenite(Ilm_1)+biotite(Bt_1),developed at temperature-pressure(P-T) conditions of 818.9-836.5℃ and7.3-9.2 kbar;(3) a retrograde stage,which is recorded by garnet rims + plagioclase(Pl_3)+amphibole(Amp_2)+orthopyroxene(Opx_1)+biotite(Bt_2)+Ilmenite(Ilm_2),developed at P-T conditions of 796.1-836.9℃ and5.6-7.5 kbar;(4) a symplectitic stage,which is recorded by plagioclase(Pl_4)+orthopyroxene(Opx_2)+amphibole(Amp_3)+biotite(Bt_3) symplectites,developed at P-T conditions of 732 ±59.6℃ and 6.1 ±0.6 kbar.Moreover,the U-Pb dating of the Beishan garnet amphibolite indicates an age of 301.9 ±4.7 Ma for the protolith and 281.4±8.5 Ma for the peak metamorphic age.Therefore,the mineral assemblage,P-T conditions,and zircon U-Pb ages of the Beishan garnet amphibolite define a near-isothermal decompression of a clockwise P-T-t(Pressure-Temperature-time) path,indicating the presence of over thickened continental crust in the Huaniushan arc until the Early Permian,then the southern Beishan area underwent a process of thinning of the continental crust. 相似文献
12.
《地学前缘(英文版)》2020,11(5):1755-1761
Thermal conductivity(k) of iron is measured up to about 134 GPa.The measurements are carried out using the single sided laser heated diamond anvil cell,where the power absorbed by a Fe metal foil at hotspot is calculated using a novel thermodynamical method.Thermal conductivity of fee(γ)-Fe increases up to a pressure of about46 GPa.We find thermal conductivity values in the range of 70-80 Wm~(-1)K~(-1)(with an uncertainty of 40%),almost constant with pressure,in the hcp(e) phase of Fe.We attribute the pressure independent k above 46 GPa to the strong electronic correlation effects driven by the electronic topological transition(ETT).We predict a value of thermal conductivity of ε-Fe of about 40±16 Wm~(-1)K~(-1) at the outer core of Earth. 相似文献
13.
《地学前缘(英文版)》2020,11(6):2157-2168
Mount Cameroon volcano has erupted several times in the 20th Century with documented eruptions in 1909, 1922, 1954, 1959, 1982, 1999 and 2000. Evidence of historic volcanism is represented by several older lava flows and lahar deposits around the flanks of the volcano. This study aims to assess the evolution of Mount Cameroon volcanism through its eruptive history via interpretation of mineralogical, whole rock geochemical and Pb, Sr, Nd isotope data generated from historic and recent lava flows. In this study, samples were collected from the 1959, 1982, 1999 and 2000 eruptions and from several historic eruption sites with unknown eruption dates.Evaluation of major and trace element data demonstrates that Mount Cameroon is geotectonically associated with within-plate Ocean Island Basalt Settings. More than 90% of the studied historic lavas (n = 29) classify as tephrites and basanites whereas the modern lavas (n = 38) are predominantly trachybasalts, demonstrating evolution from primitive to evolved lavas over time typically in response to fractional crystallization. Petrographically, the lavas are porphyritic with main mineral phases being olivine, clinopyroxene, plagioclase feldspars and Fe–Ti–Cr oxides. The 1982 lavas are predominantly aphyric and dominated by lath-shaped flow-aligned plagioclase in the groundmass. Olivine chemistry shows variable forsterite compositions from Fo60–89. Clinopyroxenes vary from diopside through augite to titanaugite with chemical composition ranges from Wo45En32Fs7 to Wo51En47Fs17. Plagioclase feldspars vary from labradorite (An56–70) to bytownite (An80–87). For the Fe–Ti–Cr oxides, calculated ulvöspinel component shows a wide variation from ulv38–87. CIPW-normative classification on the Di-Ol-Hy-Qz-Ne system shows that all Mount Cameroon lavas are nepheline-normative (Ne ranges from 4.20 wt.% to 11.45 wt.%).Radiogenic isotope data demonstrate that Mount Cameroon lavas are HIMU (or high μ = 238U/204Pb), characterized by 206Pb/204Pb = 20.19–20.46, 207Pb/204Pb = 15.63–15.69, 208Pb/204Pb = 40.01–40.30, 87Sr/86Sr = 0.70322–0.70339 (εSr = −21.37 to −18.96) and 143Nd/144Nd = 0.51276–0.51285 (εNd = +2.29 to +4.05). The historic lavas show stronger HIMU signature relative to the modern lavas, suggesting evolution towards less HIMU signatures over time. This study has revealed that Mount Cameroon volcanism has evolved from primitive magmas characterized by stronger HIMU signatures with high 206/204Pb and 208/204Pb isotopes, low SiO2 and high Mg, Ni, Cr content towards lower HIMU signatures with relatively higher SiO2, lower Mg, Cr and Ni compositions. The geochemical and isotopic changes, which account for the evolution of magmatism on Mount Cameroon occur over long periods of time because all the modern lavas erupted within the last 100 years are isotopically homogeneous, with very limited variation in SiO2 compositions. 相似文献
14.
《地学前缘(英文版)》2020,11(4):1345-1352
One of the clocks that record the Earth history is(quasi-) periodic astronomical cycles.These cycles influence the climate that can be ultimately stored in sedimentary rocks.By cracking these(quasi-) periodic sedimentation signals,high resolution astronomical time scale(ATS) can be obtained.Paleoclimate proxies are widely used to extract astronomical cycles.However different proxies may respond differently to astronomical signals and nonastronomical noises including tectonics,diagenesis,and measurement error among others.Astronomical time scale constructed based on a single proxy where its signal-to-noise ratio is low may have uncertainty that is difficult to evaluate but can be revealed by utilizing other proxies.Here,we test eight astronomical age models using two astrochro no logical methods from four paleoclimate proxies(i.e.,color reflection L~* and b~*,natural gamma radiation,and bulk density) from the Turonian to the Coniacian of the Cretaceous Period at the Demerara Rise in the equatorial Atlantic.The two astrochronological methods are time calibration using long eccentricity bandpass filtering(E1 bandpass) and tracking the long eccentricity from evolutive harmonic analysis(tracking EHA).The statistical mean and standard deviation of four age models from the four proxies are calculated to construct one integrated age model with age uncertainty in each method.Results demonstrate that extracting astronomical signals from multiple paleoclimate proxies is a valid method to estimate age model uncertainties.Anchored at the Cenomanian/Turonian boundary with an age of 93.9 ± 0.15 Ma from biostratigraphy,the ages for CC11/CC12(calcareous nannofossil zones),Turonian/Coniacian(CC12/CC13),CC13/CC14,and Coniacian/Santonian boundaries are 91.25±0.20 Ma,89.87±0.20 Ma,86.36±0.33 Ma,and 86.03±0.32 Ma in E1 bandpass method,compared with 91.17±0.36 Ma,89.74±0.38 Ma,86.13±1.31 Ma,and 85.80±1.33 Ma respectively in tracking EHA method.These results are consistent with previous studies within error and provide a reliable estimation of uncertainties of the ages. 相似文献
15.
《地学前缘(英文版)》2020,11(5):1841-1858
The Admiralty Mountains region forms the northern termination of the northern Victoria Land, Antarctica. Few quantitative data are available to reconstruct the Cenozoic morpho-tectonic evolution of this sector of the Antarctic plate, where the Admiralty Mountains region forms the northern termination of the western shoulder of the Mesozoic–Cenozoic West Antarctica Rift System. In this study we combine new low-temperature thermochronological data (apatite fission-track and (U-Th-Sm)/He analyses) with structural and topography analysis. The regional pattern of the fission-track ages shows a general tendency to older ages (80–60 Ma) associated with shortened mean track-lengths in the interior, and younger fission-track ages clustering at 38–26 Ma with long mean track-lengths in the coastal region. Differently from other regions of Victoria Land, the younger ages are found as far as 50–70 km inland. Single grain apatite (U-Th-Sm)/He ages cluster at 50–30 Ma with younger ages in the coastal domain. Topography analysis reveals that the Admiralty Mountains has high local relief, with an area close to the coast, 180 km long and 70 km large, having the highest local relief of >2500 m. This coincides with the location of the youngest fission-track ages. The shape of the area with highest local relief matches the shape of a recently detected low velocity zone beneath the northern TAM, indicating that high topography of the Admiralty Mountains region is likely sustained by a mantle thermal anomaly. We used the obtained constraints on the amount of removed crustal section to reconstruct back-eroded profiles and calculate the erosional load in order to test flexural uplift models. We found that our back-eroded profiles are better reproduced by a constant elastic thickness of intermediate values (Te = 20–30 km). This suggests that, beneath the Admiralty Mountains, the elastic properties of the lithosphere are different with respect to other TAM sectors, likely due to a stationary Cenozoic upper mantle thermal anomaly in the region. 相似文献
16.
《地学前缘(英文版)》2020,11(4):1337-1344
The Quantougou(QTG) Fauna in central Lanzhou Basin is an important late Mid-Miocene fauna on the northeastern Tibetan Plateau margin,but its numerical age remains a matter of debate.Here,we present a new magnetostratigraphic record for a fluvio-lacustrine section to further constrain the age of the QTG Fauna.Results suggest that the studied section spans from polarity chrons C5Cn.2n to C5 n.2n or C5 An.1n,with ages of ca.16.5 Ma to 10 Ma or 16.5 Ma to 12 Ma.The QTG Fauna is located at the top of polarity chron C5r.3 r or C5 Ar.2 r,which corresponds to an age of 11.7 Ma or 12.8 Ma for the fauna.Accordingly,the associated Myocricetodontinae(a subfamily of Gerbillidae,Rodentia) is suggested to have appeared in the Lanzhou Basin at 11.7 Ma or 12.8 Ma,which is the oldest Myocricetodontinae in East Asia but is still much younger than the ～20 Ma appearance of this subfamily in West and South Asia.Our age data support the interpretation that East Asian Myocricetodontinae originated from South Asia.The QTG fauna further suggest a dry and open grassland environment,which is consistent with global cooling after the Mid-Miocene Climatic Optimum. 相似文献
17.
《地学前缘(英文版)》2020,11(3):895-914
A section from the Linglong gold deposit on the northwestern Jiaodong Peninsula,East China,containing Late Mesozoic magmatic rocks from mafic and intermediate dikes and felsic intrusions,was chosen to investigate the lithospheric evolution of the eastern North China Craton(NCC).Zircon U-Pb data showed that low-Mg adakitic monzogranites and granodiorite intrusions were emplaced during the Late Jurassic(～145 Ma) and late Early Cretaceous(112-107 Ma),respectively;high-Mg adakitic diorite and mafic dikes were also emplaced during the Early Cretaceous at～139 Ma and ～118 Ma,and 125-145 Ma and 115-120 Ma,respectively.The geochemical data,including whole-rock major and trace element compositions and Sr-Nd-Pb isotopes,imply that the mafic dikes originated from the partial melting of a lithospheric mantle metasomatised through hydrous fluids from a subducted oceanic slab.Low-Mg adakitic monzogranites and granodiorite intrusions originated from the partial melting of the thickened lower crust of the NCC,while high-Mg adakitic diorite dikes originated from the mixing of mafic and felsic melts.Late Mesozoic magmatism showed that lithosphere-derived melts showed a similar source depth and that crust-derived felsic melts originated from the continuously thickened lower crust of the Jiaodong Peninsula from the Late Jurassic to Early Cretaceous.We infer that the lower crust of the eastern NCC was thickened through compression and subduction of the Palaeo-Pacific plate beneath the NCC during the Middle Jurassic.Slab rollback of the plate from ～160 Ma resulted in lithospheric thinning and accompanied Late Mesozoic magmatism. 相似文献
18.
《地学前缘(英文版)》2020,11(5):1821-1840
New,integrated petrographic,mineral chemistry,whole rock geochemical,zircon and titanite UPb geochronology,and zircon Hf isotopic data from the Montezuma granitoids,as well as new geochemical results for its host rocks represented by the Corrego Tingui Complex,provides new insights into the late-to post-collisional evolution of the northeastern Sao Francisco paleocontinent.U-Pb zircon dates from the Montezuma granitoids spread along the Concordia between ca.2.2 Ga to 1.8 Ga and comprise distinct groups.Group I have crystallization ages between ca.2.15 Ga and 2.05 Ga and are interpreted as inherited grains.Group II zircon dates vary from 2.04 Ga to1.9 Ga and corresponds to the crystallization of the Montezuma granitoids,which were constrained at ca.2.03 Ga by the titanite U-Pb age.Inverse age zoning is common within the ca.1.8 Ga Group III zircon ages,being related to fluid isotopic re-setting during the Espinhaco rifiting event.Zircon ε_(Hf)(t) analysis show dominantly positive values for both Group I(-4 to+9) and Ⅱ(-3 to+8) zircons and T_(DM2) model ages of 2.7-2.1 Ga and 2.5-1.95 Ga,respectively.Geochemically,the Montezuma granitoids are weakly peraluminous to metaluminous magnesian granitoids,enriched in LILES and LREE,with high to moderate Mg#and depleted in some of the HFSE.Their lithochemical signature,added to the juvenile signature of both inherited and crystallized zircons,allowed its classification as a shoshonitic high Ba-Sr granitoid related to a late-to post-collisional lithosphere delamination followed by asthenospheric upwelling.In this scenario,the partial melting of the lithospheric mantle interacted with the roots of an accreted juvenile intra-oceanic arc,being these hybrid magma interpreted as the source of the Montezuma granitoids.The Corrego Tingui Complex host rocks are akin to a syn-to late-collisional volcanic arc granitoids originated from the partial melting of ancient crustal rocks.The results presented in this study have revealed the occurrence of juvenile rocks,probably related to an island arc environment,that are exotic in relation to the Paleo-to Neoarchean crust from the Sao Francisco paleocontinent's core. 相似文献
19.
《地学前缘(英文版)》2020,11(4):1271-1287
The northern Xinjiang region is one of the most significant iron metallogenic provinces in China.Iron deposits are found mainly within three regions:the Altay,western Tianshan,and eastern Tianshan orogenic belts.Previous studies have elaborated on the genesis of Fe deposits in the Altay orogenic belt and western Tianshan.However,the geological characteristics and mineralization history of iron deposits in the eastern Tianshan are still poorly understood.In this paper I describe the geological characteristics of iron deposits in the eastern Tianshan,and discuss their genetic types as well as metallogenic-tectonic settings,Iron deposits are preferentially distributed in central and southern parts of the eastern Tianshan.The known iron deposits in the eastern Tianshan show characteristics of magmatic Fe-Ti-V(e.g.,Weiya and Niumaoquan),sedimentary-metamorphic type(e.g.,Tianhu),and iron skarn(e.g.,Hongyuntan).In addition to the abovementioned iron deposits,many iron deposits in the eastern Tianshan are hosted in submarine volcanic rocks with well-developed skarn mineral assemblages.Their geological characteristics and magnetite compositions suggest that they may belong to distal skarns.SIMS zircon U-Pb analyses suggest that the Fe-Ti oxide ores from Niumaoquan and Weiya deposits were formed at 307.7±1.3 Ma and 242.7±1.9 Ma,respectively.Combined with available isotopic age data,the timing of Fe mineralization in the eastern Tianshan can be divided into four broad intervals:Early Ordovician-Early Silurian(476-438 Ma),Carboniferous(335-303 Ma),Early Permian(295-282 Ma),and Triassic(ca.243 Ma).Each of these episodes corresponds to a period of subduction,post-collision,and intraplate tectonics during the Paleozoic and Mesozoic time. 相似文献
20.
《地学前缘(英文版)》2020,11(6):2197-2205
This study investigates the suitability of statistical techniques for evaluating the fluoride content and the groundwater quality from Robles Department (RD) and Banda Department (BD) in Santiago del Estero (Argentina). For the original statistical study, evaluation of nine parameters (fluoride, pH, conductivity, atmospheric and water temperature, total dissolved solids, chloride, hardness, and alkalinity) of 110 collected underground water samples from 23 dispersed rural areas was proposed. Groundwater samples were obtained by sampling taken from wells at different depths. Fluoride levels were determined by a standard colorimetric method in two seasonal periods, the dry (from April to September) and rainy (from October to March) period. The analytical results obtained for physicochemical parameters such as pH, total dissolved solids (TDS), and temperature does not reveal any notable difference between the rainy and dry seasons studied. In both seasons, the atmospheric temperature average was 22 °C. With respect to fluoride content, approximately 50% of the analysed groundwater samples exceeded the limit established by current legislation (1.0 mg/L), obtaining concentration levels in the range of 0.01–2.80 mg/L. This study demonstrates the usefulness of the univariate statistical method (quartiles calculation, interquartile range IQR), multivariate principal component analysis (PCA), and cluster analysis to establish a better understanding of the state of the contamination of the waters in the region studied. 相似文献