首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 15 毫秒
1.
A series of consolidated drained triaxial test was performed on weakly bonded soils that were artificially prepared by mixing sand (87%) and kaolin (13%). To create weakly bond strength, samples were fired at 500°C for 5 h. A critical state (CS) concept was used to interpret the results in order to describe the behaviour of the studied soil. Defining the CS is not always straightforward. Several tests showed changing in deviator stress q and volumetric strain εv up to the end of tests. Hence, defining the CS requires careful assessments on q–εa, Δu–εa, q–p′ and v–ln p′ spaces which were performed. A ‘discontinuity’ approach was applied to position the CS on dilatant path in v–ln p′ spaces after considering stress-strain and volumetric strain curves. The critical state line in a v–ln p? space from bonded samples exhibited differences to the destructured samples. This suggests that the influence of cementation bond is substantially clear at lower stresses but as stresses increase beyond the stress yield, cementation degradation becomes more significant. The effect of bonding can be clearly visualised from the normalisation of the stress paths of destructured and bonded samples.  相似文献   

2.
Soil hydraulic properties such as soil infiltration rate and hydraulic conductivity are closely linked to runoff generation and infiltration processes but little is known about them on karst hillslopes. The objectives of this paper were to investigate the change in soil stable infiltration rate (q s) and near-saturated hydraulic conductivity (K ns) in different slope positions and to understand their relationship with rock fragment content and soil texture within the topsoil in subtropical karst regions of southwest China. Tension infiltrometers (20 cm in diameter) were used to measure q s and K ns at pressure head of −20 mm on hillslopes 1 (a disintegrated landslide failure) and 2 (an avalanche slope). The change of q s and K ns was great and they mostly had a moderate variability with coefficient of variations (CV) between 0.1 and 1.0 in the different slope positions. On average, q s ranged from 0.43 to 4.25 mm/min and K ns varied from 0.75 to 11.00 mm/min. These rates exceed those of most natural rainfall events, confirming that overland flow is rare on karst hillslopes. From bottom to top, q s and K ns had a decrease–increase–decrease trend due to the presence of large rock outcrops (>2 m in height) on hillslope 1 but had an increasing trend on hillslope 2 with less complex landform. They tended to increase with increase in total rock fragment content (5–250 mm) within the topsoil as well as slope gradient on both hillslopes. Pearson correlation analysis suggested that higher coarse pebble (20–75 mm), cobble (75–250 mm), and sand (2–0.05 mm) contents as well as total rock fragment content could significantly facilitate water infiltration into soils, but higher clay (<0.002 mm) content could restrict water movement. This result indicated that rock fragment, sand, and clay contents may remarkably affect water flow in the topsoil layers, and should be considered in hydrological modeling on karst hillslopes in subtropical regions.  相似文献   

3.
《International Geology Review》2012,54(13):1735-1754
Widespread granitic intrusions in the northeast part of the Wulonggou area were previously thought to be emplaced into the Palaeoproterozoic Jinshuikou Group during the Neoproterozoic. This contribution presents detailed LA-ICP-MS zircon U–Pb geochronology, major and trace element geochemistry, and zircon Hf isotope systematic on the Wulonggou Granodiorite and Xiaoyakou Granite from the Wulonggou area. Three granodiorite samples yielded U–Pb zircon ages of 247 ± 2, 248 ± 1, and 249 ± 1 Ma, and one granite sample yielded U–Pb zircon age of 246 ± 3 Ma. The granodiorite samples are metaluminous with an alumina saturation index of 0.90–0.96, as well as intermediate- to high-alkali contents of 5.49–6.14 wt.%, and low Zr+Nb+Ce+Y contents, and low Fe2O3T/MgO ratios, which suggest an I-type classical island arc magmatic source. The granite samples are peraluminous with an alumina saturation index of 1.02–1.03, Sr content of 305.00–374.00 ppm, Sr/Y ratios of between 17.68 and 28.77, (La/Yb)N values of 16.98–25.07, low HREEs (Yb = 1.10–2.00 ppm), and low Y (13.00–21.10 ppm), which suggest adakite-like rocks. All granodiorite samples have zircons εHf(t) values ranging from ?2.9 to +3.9, and granite samples have zircon εHf(t) values ranging from ?7.8 to +3.2. These Hf isotopic data suggest that the Early Triassic granites were derived from the partial melting of a mafic Mesoproterozoic lower crust, although the degree of ancient crustal assimilation may be higher for the Xiaoyakou Granite. It is suggested here that the ca. 246–248 Ma magma was generated during the northward subduction of the Palaeo-Tethys oceanic plate.  相似文献   

4.
ABSTRACT

Southeastern China is characterized by an extensive Late Mesozoic (Yanshanian) tectono-magmatic-metallogenic event. Although Late Cretaceous volcanism gradually weakened during the epilogue of the Yanshanian event, its petrogenesis and geodynamic processes remain unclear. In this study, we present new zircon U–Pb–Hf isotopic, whole-rock elemental, and Sr–Nd isotopic compositions data, for volcanic rocks from the Zhaixia Formation of the Shimaoshan Group in Fujian Province. The lower member of the Zhaixia Formation consists of basalts and rhyolites, and the upper member is only rhyolites. These volcanic rocks erupted in the early stage of Late Cretaceous, with basalts erupting earlier (ca. 99–98 Ma) than rhyolites (ca. 98–94 Ma). These basalts record high-K calc-alkaline to shoshonitic, light rare earth element (LREE)- and LILE-enrichment, high field strength element (HFSE)depletion with negligible Eu anomalies, and uniform whole-rock εNd(t) (–3 to –6) and zircon εHf(t) (–3.3 to –14.1) values. The overlying rhyolites record peraluminous and high-K calc-alkaline characteristics, LREE- and LILE-enrichment with negative Eu anomalies, and Nb–Ta depletion. The whole-rock εNd(t) and zircon εHf(t) values of these rhyolites both increase from the lower member (εNd(t), –1.5 to –4.7; εHf(t), –5.1 to –16.1) to the upper member (εNd(t), –0.5 to 0.1; εHf(t), –0.3 to –4.3). The features imply that these basalts were derived from the partial melting of the enriched lithospheric mantle and the overlying rhyolites from the melting of the crustal components, respectively. Data from the rhyolites in the upper member indicate that more juvenile, Nd–Hf isotopically depleted materials were injected into their source. During the Late Cretaceous, the new, fast rollback of the subducting slab triggered lithospheric extension and asthenospheric upwelling beneath the coastal regions, which induced the melting of lithospheric mantle and crustal components. As continued, the new round of basaltic underplating provided necessary heat to cause partial melting of the deep crust, including the younger, juvenile, and isotopically depleted crustal components.  相似文献   

5.
The Wunugetushan porphyry Cu–Mo deposit is located in northeastern China. The deposit lies within the Mongolia–Erguna metallogenic belt, which is associated with the evolution of the Mongol–Okhotsk Ocean. The multiple episodes of magmatism in the ore district, occurred from 206 to 173 Ma, can be divided into pre-mineralization stage (biotite granite), mineralization stage (monzogranitic porphyry and rhyolitic porphyry), and post-mineralization stage (andesitic porphyry). The biotite granite has (87Sr/86Sr)i values of 0.704105–0.704706, εNd(t) values of ?0.67 to ?0.07, and εHf(t) values of ?0.4 to 2.8, yielding Hf two-stage model ages (TDM2) 1250–1067 Ma, and Nd model ages of 1.04–0.96 Ga, indicating that the pre-mineralization magmas were generated by the remelting of Neoproterozoic juvenile crustal material. The monzogranitic porphyry has (87Sr/86Sr)i values of 0.704707–0.706134, εNd(t) values of 0.29–1.33, and εHf(t) values of 1.0–2.9, yielding TDM2 model ages of 1173–1047 Ma. The rhyolitic porphyry has (87Sr/86Sr)i ratio of 0.702129, εNd(t) value of ?0.21, and εHf(t) values of ?0.5 to 7.1, TDM2 model ages from 1269 to 782 Ma. These results show that the magmas of mineralization stage were generated by the partial melting of juvenile crust mixed with mantle-derived components. The andesitic porphyry has (87Sr/86Sr)i ratio of 0.705284, εNd(t) value of 0.82, and εHf(t) values from 4.1 to 7.4, indicating that the post-mineralization magma source contained more mantle-derived material. The Mesozoic Cu–Mo deposits which genetically related to Mongol–Okhotsk Ocean were temporally distributed in Middle to Late Triassic (240–230 Ma), Early Jurassic (200–180 Ma), and Later Jurassic (160–150 Ma) period. The Middle Triassic to Early Jurassic Cu–Mo mineralization was dominated by Mongol–Okhotsk oceanic plate southeast-directed subducted beneath the Erguna massif. The Later Jurassic Cu–Mo mineralization was controlled by the continent–continent collision between Siberia plate and Erguna massif.  相似文献   

6.
Both oceanic and continental HP rocks are juxtaposed in the Huwan shear zone in the western Dabie orogen, and thus provide a window for testing the buoyancy‐driven exhumation of dense oceanic HP rocks. The HP metamorphic age of the continental rocks in this zone has not been well constrained, and hence it is not known if they are of the same age as the exhumation of the HP oceanic rocks. In situ laser ablation (multiple collector) inductively coupled plasma mass spectrometry (LA‐(MC‐)ICP‐MS), U–Pb, trace element and Hf isotope analyses were made on zircon in a granitic gneiss and two eclogites from the Huwan shear zone. U–Pb age and trace element analysis of residual magmatic zircon in an eclogite constrain its protolith formation at 411 ± 4 Ma. The zircon in this sample displays εHf (t) values of +6.1 to +14.4. The positive εHf (t) values up to +14.4 suggest that the protolith was derived from a relatively depleted mantle source, most likely Palaeotethyan oceanic crust. A granitic gneiss and the other eclogite yield protolith U–Pb ages of 738 ± 6 and 700 ± 14 Ma, respectively, which are both the Neoproterozoic basement rocks of the Yangtze Block. The zircon in the granitic gneiss has low εHf (t) values of ?14.2 to ?10.5 and old TDM2 ages of 2528–2298 Ma, suggesting reworking of Palaeoproterozoic crust during the Neoproterozoic. The zircon in the eclogite has εHf (t) values of ?1.0 to +7.4 and TDM1 ages of 1294–966 Ma, implying prompt reworking of juvenile crust during its protolith formation. Metamorphic zircon in both eclogite samples displays low Th/U ratios, trace element concentrations, relatively flat heavy rare earth element patterns, weak negative Eu anomalies and low 176Lu/177Hf ratios. All these features suggest that the metamorphic zircon formed in the presence of garnet but in the absence of feldspar, and thus under eclogite facies conditions. The metamorphic zircon yields U–Pb ages of 310 ± 3 and 306 ± 7 Ma. Therefore, both the oceanic‐ and continental‐type eclogites share the same episode of Carboniferous eclogite facies metamorphism. This suggests that high‐pressure continental‐type metamorphic rocks might have played a key role in the exhumation and preservation of oceanic‐type eclogites through buoyancy‐driven uplift.  相似文献   

7.
实验变形煤的光性组构分析   总被引：4，自引：1，他引：4       下载免费PDF全文
三种Rmaxo分别为0.67%,3.41%和4.90%的煤样,在t=350-700℃、Pc=400-600MPa、ε=10%-30%、ε=3.63×10-4-2.59×10-5s-1条件下的变形实验表明:(1)煤光性组构的成因是芳环层片在构造应力作用下重新定向所致,重新定向的主要机制是煤化过程中芳环层片的择优成核生长,同时存在物理转动定向机制的作用;(2)芳环层片的重新定向主要发生在煤级相对较低阶段,VRI的Z轴主要反映这一阶段的构造应力方向;(3)YRI的形态特征并非仅与有限应变有关,它不能直接作为有限应变分析的标志。  相似文献   

8.
1 Introduction A high-level generalization of structures in the earth crust has been given by Ramsay (1980): low-angle thrusts in the brittle upper crust and high-angle reverse shear zones in the ductile middle-lower crust are formed in contractional regimes; high-angle normal faults in the brittle upper crust and low-angle normal shear zones in the ductile middle- lower crust are formed in extensional regimes. The formation of low-angle thrusts and high-angle normal faults in brittle domains …  相似文献   

9.
Mafic intrusive rocks (1.79–1.78 Ga) of the Transscandinavian Igneous Belt (TIB) and the c. 1.87 Ga Hedesunda Igneous Complex in the Fennoscandian Shield of south‐central Sweden were studied using whole‐rock and isotope geochemistry. Rock types vary from gabbros/norites (and leucogabbros) to quartz diorites, with Mg# between 76 and 49, and wt% SiO2 between 43.6 and 59.7, indicating some variation in evolutionary levels and variable cumulus components. Geochemical signatures are calc‐alkaline to shoshonitic, large ion lithophile elements and light rare earth elements enriched and high‐field strength elements depleted of continental‐arc type. εNd(t) ranges between +1.0 and +2.7, and 87Sr/86Sr(t) between 0.7020 and 0.7038. There is no systematic correlation between chemical parameters and isotope ratios. These isotopic data overlap with other mafic plutonic TIB rocks; samples from the Dala Province (DP) tend to overlap with the c. 1.7 Ga basic Dala lavas of TIB at slightly elevated relative Sr/Nd ratios. With two exceptions, the εNd(t) of +1 to +2 conform to an isotopically ‘mildly depleted’ source, typical for mafic TIB rocks and many Svecofennian rocks in the region. Reported values above εNd(t) +2.0 are scarce in the TIB. Mantle sources represent depleted mantle wedge material that was enriched by fluids/melts not long before (TDM c. 2.0 Ga), that is during subduction in the preceding Svecofennian (2.0–1.87 Ga) and/or during the TIB‐0&1 event (1.85–1.78 Ga). The palaeotectonic settings inferred are active continental margins; N–S‐directed convergence at 1.87 Ga and E–W‐directed at 1.79–1.78 Ga. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

10.
Any knowledge about Archaean to Palaeoproterozoic magmatic and metamorphic events in North Korea has the potential to make a significant difference to our understanding of the early tectonic configuration and evolution of East Asia. This zircon U–Pb dating and Hf isotopic study documents multiple Neoarchaean to Palaeoproterozoic tectonothermal events from the meta-igneous complex in the Machollyong ‘Group’ of the Rangnim Massif. Two tonalitic-trondjemitic gneiss samples record a crystallization age of meta-igneous protoliths at ca. 2.56 Ga and multiple migmatization and metamorphism from 2.52 to 1.85 Ga. A meta-dolerite sample yields a magmatic emplacement age of ca. 1.83 Ga. In situ zircon Hf isotopic data indicate that most zircons from the gneiss samples have εHf(t) values from –16.9 to + 3.1 and crustal model ages from 2.84 to 3.73 Ga, whereas magmatic zircons from the meta-dolerite dike record εHf(t) values from –5.2 to + 5.2 and model ages of 2.05–2.44 Ga. The first-recognized Neoarchaean tonalitic-trondjemitic migmatite complex in the Rangnim Massif, together with previously identified tonalitic-trondhjemitic-granodioritic (TTG) rocks in the Rimjingang Belt and the coeval counterparts in western Gyeonggi massif, represents the oldest crustal nuclei in the Korean Peninsula. The multiple tectonothermal events in this study present reliable evidence not only for attesting to consanguinity of the basement between the Korean Peninsula and the North China Craton but also for defining the influence scope of the late Palaeoproterozoic orogeny in the Korean Peninsula.  相似文献   

11.
《International Geology Review》2012,54(13):1630-1657
New geological, geochronological, and geochemical results on volcanic rocks and cobbles from early Mesozoic sedimentary rocks identify two contrasting latest Permian–Triassic volcanic rock suites in the northern North China Craton (NCC). The early rock suite erupted during the latest Permian–Early Triassic at ca. 255–245 Ma and was probably widely distributed in the northern NCC prior to the Early Jurassic. It comprises rhyolitic welded tuff, rhyolite, and tuffaceous sandstone and is characterized by high contents of SiO2 and K2O, moderate initial 87Sr/86Sr, low negative εNd(t) and εHf(t) values, and old Nd-Hf isotopic model ages. It was likely produced by fractional crystallization of lower crustal-derived magmas due to underplating by lithospheric mantle-derived magmas near the crust–mantle boundary in syncollisional to post-collisional/post-orogenic tectonic settings. The late rock suite, erupted during the Middle–Late Triassic at ca. 238–228 Ma, displays adakitic geochemical signatures and consists of intermediate volcanic rocks such as andesite, trachyandesite, and autoclastic trachyandesite breccia, with minor felsic rocks. This suite is characterized by high Al2O3, MgO, Sr, Ba, Cr, V, and Ni concentrations; high Mg# values; low Y and Yb concentrations and high Sr/Y ratios; low initial 87Sr/86Sr; high negative εNd(t) and εHf(t) values; and young Nd-Hf isotopic model ages. The younger suite was generated by mixing of magmas derived from melting of upwelling asthenosphere, with melts of ancient lower crust induced by underplating of basaltic magmas in an intraplate extensional setting. Strong upwelling of asthenospheric mantle and significant involvement of the asthenospheric mantle materials indicate that the lithospheric mantle beneath the northern NCC was partially delaminated during Middle–Late Triassic time, representing the initial destruction and lithospheric thinning of the northern NCC. Lithospheric thinning and delamination are likely the most important reasons for the Triassic tectonic transition and change of magmatism and deformation patterns in the northern NCC.  相似文献   

12.
ABSTRACT

We report new zircon U–Pb ages, Hf isotopic and geochemical results for the Tongling granitic plutons of Southeast China. SHRIMP U–Pb ages for the Miaojia quartz monzodiorite porphyrite,the Tianebaodan and Tongguanshan quartz monzodiorites, the Xinqiaotou granodiorite porphyry, and the Shatanjiao and Nanhongchong granodiorite are 143 ± 2, 141 ± 1 and 142 ± 1, 147 ± 1, and 145 ± 1 and 139 ± 1 Ma, respectively. Combined with previous geochronological data, our results indicate that the porphyritic rocks are older than rocks of the same type lacking porphyritic texture. Geochemically, these high-K calc-alkaline intrusive rocks are characterized by arc-like trace element distribution patterns, with significant enrichment in LILE and LREE but depletion in HFSE. Lu–Hf isotopic compositions of zircons from the high-K calc-alkaline (HKCA) rocks have εHf(t) values of magmatic 139–147 Ma zircons from ?8.1 to ?25.6, with two-stage model ages (tDM2) of 1.71–2.67 Ga, whereas εHf(t) values of inherited 582–844 Ma zircons range from 5.4 to ?9.5, with tDM2 of 1.39–2.22 Ma, younger than tDM2 values of igneous zircon, indicating that newly formed mantle material was added to the continental crust of the Yangtze Block. Moreover, εHf(t) values of inherited zircon cores older than 1000 Ma are from ?7.8 to ?26, similar to magmatic zircons, and the tDM2 values are all greater than 3.0 Ga (3.16–3.75 Ga), reflecting partial melting of ancient sialic material. We conclude that the plutonic melts were derived from both the enriched mantle and the ancient crust. The HKCA Tongling intrusions coincide temporally with the J3–K1 magmatic event that was widespread in Southeast China. This igneous activity may have accompanied sinistral slip along the Tan-Lu fault due to oblique subduction of the Palaeo-Pacific plate.  相似文献   

13.
The Jurassic magmatic and volcanic rocks are widespread along the west central Lhasa subterrane. However, the petrogenesis of these rocks is poorly understood because of lacking high-quality geochronology and geochemical data. Here, we present new zircon U–Pb age and Hf isotopic data, whole-rock geochemical and Sr–Nd–Pb isotopic data for the Songduole and Qiangnong plutons in Geji area. LA-ICP-MS dating of zircon yield crystallization ages of 172.1 ± 1.9 and 155.9 ± 1.2 Ma for the Songduole and Qiangnong plutons, respectively. Geochemically, Songduole and Qiangnong granodiorite are characterized by high MgO (2.63–3.49 wt%), high Mg# (49–50), and low TiO2 (0.48–0.57 wt%). Besides, all rocks show metaluminous, calc-alkaline signatures, with strong depletion of Nb, Ta, and Ti, enrichment of large-ion lithophile (e.g. Rb, Th, K), and a negative correlation between SiO2 and P2O5. All these features are indicative of arc-related I-type magmatism. Five samples from the Songduole granodiorite have whole rock (87Sr/86Sr)i of 0.71207–0.71257, εNd(t) values of ?15.1 to ?13.9, zircon εHf(t) values of ?17.4 to ?10.5, (206Pb/204Pb)t ratios of 18.402–18.854, (207Pb/204Pb)t ratios of 15.660–15.736, and (208Pb/204Pb)t ratios of 38.436–39.208. Samples from the Qiangnong granodiorite have (87Sr/86Sr)i of 0.71230–0.71252, εNd(t) values of ?15.1 to ?14.2, zircon εHf(t) values of ?12.6 to ?6.4, (206Pb/204Pb)t ratios of 18.688–18.766, (207Pb/204Pb)t ratios of 15.696–15.717, and (208Pb/204Pb)t ratios of 38.546–39.083. These geochemical signatures indicate that the two plutons most likely originated from partial melting of the ancient Lhasa lower crust with obvious inputs of mantle-derived melts. Combined with regional geology, our results indicate that the Jurassic magmatism in the west central Lhasa subterrane most likely resulted from the southward subduction of the Bangong Ocean lithosphere beneath the central Lhasa terrane.  相似文献   

14.
《International Geology Review》2012,54(11):1413-1434
We present new zircon ages and Hf-in-zircon isotopic data for plutonic rocks and review the crustal evolution of the Chinese Central Tianshan (Xinjiang, northwest China) in the early to mid-Palaeozoic. The Early Ordovician (ca. 475–473 Ma) granitoid rocks have zircon εHf(t) values either positive (+0.3 to +9.5) or negative (?6.0 to ?12.9). This suggests significant addition of juvenile material to, and coeval crustal reworking of, the pre-existing continental crust that is fingerprinted by numerous Precambrian zircon xenocrysts. The Late Ordovician–Silurian (ca. 458–425 Ma) rocks can be assigned to two sub-episodes of magmatism: zircon from rocks of an earlier event (ca. 458–442 Ma) has negative zircon εHf(t) values (?6.3 to ?13.1), indicating a predominantly crustal source; zircon from later events (ca. 434–425 Ma) has positive zircon εHf(t) values (+2.6 to +8.9) that reveal a predominantly juvenile magma source. The Early Devonian (ca. 410–404 Ma) rocks have near-zero zircon εHf(t) values, either slightly negative or positive (?1.4 to +3.5), whereas the Mid-Devonian rocks (ca. 393 Ma) have negative values (?11.2 to ?14.8). The Late Devonian (ca. 368–361 Ma) granites are undeformed and are chemically similar to adakite but have relatively low negative whole-rock εNd(t)values (?2.4 to ?5.3). We interpret the Early Ordovician to Mid-Devonian magmatic event to reflect combined juvenile crustal growth and crustal reworking processes via episodic mafic underplating and mantle–crust interaction. The Late Devonian episode may signify delamination of the over-thickened Chinese Central Tianshan crust.  相似文献   

15.
 Physical properties including the equation of state, elasticity, and shear strength of pyrite have been measured by a series of X-ray diffraction in diamond-anvil cells at pressures up to 50 GPa. A Birch–Murnaghan equation of state fit to the quasihydrostatic pressure–volume data obtained from laboratory X-ray source/film techniques yields a quasihydrostatic bulk modulus K 0T =133.5 (±5.2) GPa and bulk modulus first pressure derivative K ′ 0T =5.73 (±0.58). The apparent equation of state is found to be strongly dependent on the stress conditions in the sample. The stress dependency of the high-pressure properties is examined with anisotropic elasticity theory from subsequent measurements of energy-dispersive radial diffraction experiments in the diamond-anvil cell. The calculated values of K 0T depend largely upon the angle ψ between the diffracting plane normal and the maximum stress axis. The uniaxial stress component in the sample, t=σ3−σ1, varies with pressure as t=−3.11+0.43P between 10 and 30 GPa. The pressure derivatives of the elastic moduli dC 11/dP=5.76 (±0.15), dC 12/dP=1.41 (±0.11) and dC 44/dP=1.92 (±0.06) are obtained from the diffraction data assuming previously reported zero-pressure ultrasonic data (C 11=382 GPa, C 12=31 GPa, and C 44=109 GPa). Received: 21 December 2000 / Accepted: 11 July 2001  相似文献   

16.
The medium-tonnage Sarsuk polymetallic Au deposit is located in the Devonian volcanic–sedimentary Ashele Basin of the south Altay Orogenic Belt (AOB), Northwest China. Within the deposit, the rhyolite porphyries and diabases are widespread, emplaced into strata. The orebodies are hosted by the rhyolite porphyries. We studied the petrography, geochemistry, and Sr–Nd–Hf isotopes of the rhyolite porphyries and diabases, in order to understand the petrogenesis of these rocks and their tectonic significance. They display typical bimodality in geochemistry compositions. The diabases are characterized by SiO2 contents of 44.84–59.77 wt.%, high Mg# values (43–69), enrichment in large ion lithophile elements (LILE) and light rare earth elements (LREE), depletion in Nb and Ta, low (87Sr/86Sr)i (0.706687–0.707613) values, positive εNd(t) (4.8–6.8) values, and positive and high εHf(t) (7.15–15.19) values, suggesting a depleted lithosphere mantle source that might have been metasomatized by subduction-related components. The rhyolite porphyries show affinity to sanukitoid magmas contents [high SiO2 (78.6–81.82 wt.%) and MgO (3.38–5.94 wt.%, one sample at 0.61 wt.%), and enrichments in LILE and LREE], they were derived from the equilibrium reactions between a mantle source and subducted oceanic crust materials. Those characteristics together with the positive εNd(t) (4.1–8.4) and εHf(t) (2.88–15.17) values indicate that the diabases and rhyolite porphyries were generated from the same mantle peridotite source. But the rhyolite porphyries underwent fractional crystallization of Fe–Ti oxides, plagioclase, and apatite due to their negative Eu (δEu = 0.21–0.28) and P anomalies. According to the geochemical and isotopic data, the Sarsuk Middle Devonian igneous rocks are considered to be the products of the juvenile crustal growth in an island arc setting. The Sarsuk polymetallic Au deposit formed slightly later than the Ashele Cu–Zn deposit in the Ashele Basin, but they have the same tectonic setting, belonging to the trench–arc–basin system during extensional process in the south AOB.  相似文献   

17.
Here we first present samarium (Sm)–neodymium (Nd) isotopic data for the ∼2.5 Ga Wangjiazhuang BIF and associated lithologies from the Wutai greenstone belt (WGB) in the North China Craton. Previous geochemical data of the BIF indicate that there are three decoupled end members controlling REE compositions: high-T hydrothermal fluids, ambient seawater and terrigenous contaminants. Clastic meta-sediment samples were collected for major and trace elements studies in an attempt to well constrain the nature of detrital components of the BIF. Fractionated light rare earth elements patterns and mild negative Eu anomalies in the majority of these meta-sedimentary samples point toward felsic source rocks. Moreover, the relatively low Th/Sc ratios and positive εNd(t) values are similar to those of the ∼2.5 Ga granitoids, TTG gneisses and felsic volcanics in the WGB, further indicating that they are derived from less differentiated terranes. Low Chemical Index of Weathering (CIW) values and features in the A-CN-K diagrams for these meta-sediments imply a low degree of source weathering. Sm–Nd isotopes of the chemically pure BIF samples are characterized by negative εNd(t) values, whereas Al-rich BIF samples possess consistently positive εNd(t) features. Significantly, the associated supracrustal rocks in the study area have positive εNd(t) values. Taken together, these isotopic data also point to three REE sources controlling the back-arc basin depositional environment of the BIF, the first being seafloor-vented hydrothermal fluids (εNd(t) < −2.5) derived from interaction with the underlying old continental crust, the second being ambient seawater which reached its composition by erosion of parts of the depleted landmass (likely the arc) (εNd(t) > 0), the third being syndepositional detritus that received their features by weathering of a nearby depleted source (likely the arc) (εNd(t) > 0).  相似文献   

18.
《International Geology Review》2012,54(13):1594-1612
ABSTRACT

The mechanism that triggered large-scale Late Mesozoic magmatism in the northeastern Great Xing’an Range (NE GXAR) is strongly controversial. In this paper, we present whole rock geochemistry and zircon trace element, U-Pb and Hf isotopic data on the volcanic rocks in the Longjiang and Guanghua formations in the northeastern Xing’an Block. Zircons with ages of 120–119 Ma indicate that these volcanic rocks were formed in the Early Cretaceous. Combined with previous data, it is clear that volcanic rocks in the NE GXAR erupted between 128 and 108 Ma. The andesite samples of the Longjiang Formation show high contents of Al2O3, CaO, and MgO, significant negative Nb, Ta, and Ti anomalies; εHf (t) values of zircons from the andesite sample vary from +4.13 to +7.67, indicating an enriched mantle source. The rhyolites of the Guanghua Formation show high SiO2 and K2O concentrations, low P2O5, MgO, Cr, and Ni contents and Mg# values. The positive εHf (t) values (+5.72 to +10.58) with two-stage Hf model ages ranging from 939 to 701 Ma indicate that the rhyolites are derived from the partial melting of basaltic lower crust. Combined with the regional geological evolution, we conclude that the generation of the Early Cretaceous volcanic rocks in the NE GXAR might be triggered by the dehydration, disintegration, and foundering of the Mongol-Okhotsk Oceanic flat-slab and the subsequent upwelling of the asthenosphere.  相似文献   

19.
The magnetite-series (I-type) calc-alkaline granitoid suit, ranging from pyroxene monzodiorite to granodiorite, is associated with the porphyry and skarn gold–copper deposits at the Shizishan orefield in Tongling district, Anhui Province. In-situ U–Pb dating and Hf isotope analysis of magmatic and inherited zircons are combined with whole rock Sr–Nd–Pb isotopic data and mineral thermobarometry to interpret the petrogenesis. The magmatic zircons from the quartz monzodiorites yield weighted average 206Pb/238U ages of ca. 139 Ma and mean εHf(t) value of −19.8 ± 3.9 (1σ), while those from the pyroxene monzodiorite show a similar mean age but notably higher mean εHf(t) value (−8.5 ± 1.4). The inherited zircons from the quartz monzodiorite yield ages of 0.8, 2.0 and 2.4 Ga with mean εHf(t) value of −2.9 ± 1.4, while those from the pyroxene monzodiorite show younger ages (165 to 245 Ma) but similar mean εHf(t) value (−5.6 ± 4.5). Whole rock Sr–Nd–Pb isotope data indicate that crustal material significantly contributed to the magma. Mineral thermobarometry results reveal that the depths of the discrete magma chambers were about 23 km, and 10 to 2 km deep.The data above combined with previous studies suggest that: 1) The magma emplacement and crystallization (typically for zircons) mainly occurred at about 139 Ma, consistent with the age of mineralization; 2) The primary pyroxene monzodioritic magma might have mixed with the magma produced by partial melting of the Yangtze lower crust, and accumulated in the magma chamber at ca. 23 km deep in the lower crust level; 3) AFC and magma mixing were the dominate processes for the magmatic evolutions at shallow level (2 to 10 km), where the circumstances were favorable for mineralization.  相似文献   

20.
Strain analysis of the Baraitha conglomerate is attempted by direct measurements on extracted pebbles and by micrometric analysis. The overall deformation is of flattening type, with thek value lower by more than half in the matrix than in the pebbles. The viscosity contrast between pebbles and matrix (μ i/μm) is in the ratio of 2:1 and the bulk deformation appears to be strongly controlled by Ci (concentration of pebbles expressed as percentage). The total shortening (≃35%) in the Baraitha conglomerate is comparable with the shortening accomplished in the folding of the overlying Bijawar Group volcanosedimentary sequence. The bulk strain axesX t, Yt andZ t, as determined from the analysis of the deformed conglomerate, are unsymmetrically oriented with reference to folds formed by oblique flexural-slip with neitherX t norY tcoincident with the fold hinges. The lack of transection of folds by cleavage again suggests flattening deformation. The extension in theY tdirection is greater in the matrix than in the pebbles.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号