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1.
This paper presents a detailed survey of the activities of selected man-made radionuclides in peat deposits located in SW Spitsbergen. Peat cores from the High Arctic (SW Spitsbergen) were analyzed by gamma spectrometry (137Cs), alpha spectrometry (238Pu, 239,240Pu, 241Am activities) and by ICPMS (240Pu/239Pu atom ratios). Maximum activities evident in the peats correspond to the 1963/1964 global maximum fallout from atmospheric testing of nuclear weapons; some of the activity profiles have been altered post-deposition by water infiltration. Activity ratios of 238Pu/239+240Pu, 241Am/239+240Pu, 239+240Pu/137Cs and 240Pu/239Pu atom ratios indicate mixing between global (stratospheric) and regional (tropospheric) sources of these radionuclides in the Svalbard area. The 238Pu/239+240Pu activity ratios varied from 0.02 ± 0.01 to 0.09 ± 0.03, suggesting global fallout as the dominant source of Pu. The 239+240Pu/137Cs activity ratios varied from 0.01 ± 0.01 to 0.42 ± 0.11, which apparently arises from the post-depositional mobility of 137Cs. The 241Am/239+240Pu activity ratios ranged between 0.10 ± 0.02 and 1.5 ± 0.3 and exceed the published global fallout ratio for Svalbard of 0.37 due to the relatively higher geochemical mobility of Pu vs. Am and/or ingrowth of Am from the decay of 241Pu. The atom ratio 240Pu/239Pu ranged from 0.142 ± 0.006 to 0.241 ± 0.027; however, the vast majority of peat samples exhibited 240Pu/239Pu atom ratios similar to the stratospheric fallout (∼0.18).  相似文献   

2.
It is generally accepted that the low-Mg adakitic rocks were derived from the partial melting of metabasalts/eclogites. In this study, we demonstrate that the early Cretaceous low-Mg adakitic granites in the North Dabie Complex (NDC) were generated by the partial melting of the NDC orthogneisses. Here we present in-situ U–Pb and Lu–Hf isotopes in zircon with whole-rock geochemical and Sr–Nd isotopic compositions were carried out for the Tiantangzhai porphyritic monzogranites from the Dabie orogen, eastern China. The monzogranites are characterized by high Sr (576–988 ppm), low Y (7.3–19.0 ppm), and depletion in HREE (Yb: 0.50–1.78 ppm) (thus resulting in high Sr/Y (34.3–135.2) and (La/Yb)N (17.0–105.2) ratios) without a negative Eu anomaly. They also exhibit high SiO2 (66.5–73.5 wt.%) and K2O (2.7–4.7 wt.%), and low MgO (0.4–1.6 wt.%) or Mg# (28.2–45.3, mostly < 40) values. Whole-rock geochemical compositions suggest that the monzogranites represent low-Mg adakitic rock with high-Si and rich-K features equilibrated with residues rich in garnet. Sr–Nd isotopic compositions (εNd (t) = ? 16.2 to ? 20.3, (87Sr/86Sr)i = 0.707798–0.708804, tDM2(Nd) = 2.3–2.6 Ga) of the monzogranites are distinct from that of the eclogites and amphibolites in the Dabie orogen, but similar to that of the Neoproterozoic (700–800 Ma) gneisses in the NDC. U–Pb dating of zircons gives a consistent age of 130.0 ± 3.4 Ma with discordia upper intercept age of 716 ± 34 Ma for inherited cores identified by CL imaging. Correspondingly, in-situ Lu–Hf analyses of early Cretaceous young age-spots from zircons yield initial 176Hf/177Hf ratios from 0.281898 to 0.282361, εHf(t) values from ? 28.1 to ? 17.6 and two-stage “crust” Hf model ages (tDM2) from 2293 ± 89 to 2949 ± 108 Ma, which are generally in agreement with values of 0.281891 to 0.282218, ? 28.2 to ? 11.7 and 1927 ± 87 to 2963 ± 92 Ma for the pre-Mesozoic inherited cores, respectively. As for individual core-rim pairs in zircon, Th/U ratios increase from the inherited cores to the young growth rims possibly due to variable degrees of partial melting, whereas 176Lu/177Hf ratios greatly decrease because of the garnet effect in residues. Thus, we suggest that the early Cretaceous low-Mg adakitic granites were derived from the partial melting of the NDC Neoproterozoic (700–800 Ma) gneisses, and the foundering of the garnet-bearing residues could have caused the destruction of the over-thickened lower continental crust.  相似文献   

3.
We present results of combined in situ U–Pb dating of detrital zircons and zircon Hf and whole-rock Nd isotopic compositions for high-grade clastic metasedimentary rocks of the Slyudyansky Complex in eastern Siberia. This complex is located southwest of Lake Baikal and is part of an early Paleozoic metamorphic terrane in the eastern part of the Central Asian Orogenic Belt (CAOB). Our new zircon ages and Hf isotopic data as well as whole-rock Nd isotopic compositions provide important constraints on the time of deposition and provenance of early Paleozoic high-grade metasedimentary rocks as well as models of crustal growth in Central Asia. Ages of 0.49–0.90 Ga for detrital zircons from early Paleozoic high-grade clastic sediments indicate that deposition occurred in the late Neoproterozoic and early Paleozoic, between ca. 0.62–0.69 and 0.49–0.54 Ga. Hf isotopic data of 0.82–0.69 Ga zircons suggest Archean and Paleoproterozoic (ca. 2.7–2.8 and 2.2–2.3 Ga; Hfc = 2.5–3.9 Ga) sources that were affected by juvenile 0.69–0.82 Ga Neoproterozoic magmatism. An additional protolith was also identified. Its zircons yielded ages of 2.6–2.7 Ga, and showed high positive εHf(t) values of +4.1 to +8.0, and Hf model ages tHf(DM) = tHfc = 2.6–2.8 Ga, which is nearly identical to the crystallization ages. These isotopic characteristics suggest that the protolith was quite juvenile. The whole-rock Nd isotopic data indicate that at least part of the Slyudyansky Complex metasediments was derived from “non-Siberian” provenances. The crustal development in the eastern CAOB was characterized by reworking of the early Precambrian continental crust in the early Neoproterozoic and the late Neoproterozoic–early Paleozoic juvenile crust formation.  相似文献   

4.
Granitic leucosome and pegmatite are widely distributed within biotite-bearing orthogneiss in the northern part of the Sulu ultrahigh-pressure (UHP) metamorphic terrane, eastern China. A combined study of mineral inclusions, cathodoluminescence (CL) images, U–Pb SHRIMP dates, and in situ trace element and Lu–Hf isotope analyses of zircons provided insight into the nature and timing of partial melting in these rocks. Zircon grains separated from biotite-bearing orthogneiss typically have three distinct domains: (1) pre-metamorphic (magmatic) cores with Qtz + Kfs + Pl + Ap inclusions, which record a Neoproterozoic protolith age of ～ 790 Ma, (2) mantles with Coe + Phe + Ap inclusions that record Triassic UHP age at 227 ± 3 Ma, and (3) narrow rims with quartz inclusions that record HP granulite-facies retrograde metamorphism at ～ 210 ± 3 Ma. In contrast, zircons separated from granitic leucosome have only two distinct domains: (1) the central UHP areas with Coe + Phe + Ap inclusions record Triassic UHP age of 227 ± 3 Ma, and (2) outer magmatic areas with Qtz + Kfs + Ab + Ap inclusions that record partial melting time of 212 ± 2 Ma. Zircons separated from pegmatite contain mineral inclusions of Qtz + Kfs + Ap and show regular magmatic zoning from centre to edge. The centres record partial melting time of 212 ± 2 Ma in line with the outer domains of granitic leucosome, whereas the edges give a younger age of 201 ± 2 Ma related to Pb loss and partial recrystallization during late Triassic regional amphibolite-facies retrogression. These data indicate that partial melting in the north Sulu UHP gneissic rocks took place during post-UHP, retrograde HP granulite-facies metamorphism.Pre-metamorphic (magmatic) zircon cores from biotite-bearing orthogneiss give uniform 176Hf/177Hf of 0.28187 ± 0.00003 (2 SD; standard deviation) corresponding to εHf(790) and Hf model ages (TDM2) of about ? 16.3 and 2.41 Ga, respectively. This is consistent with the generation of its protolith by reworking of Paleoproterozoic to late Archean crust. In contrast, UHP zircon domains from biotite-bearing orthogneiss and granitic leucosome are characterized by distinct trace element composition with low Lu/Hf (< 0.006), low Th/U (< 0.1) and considerably higher, 176Hf/177Hf (0.28233 ± 0.00002; 2 SD) than the pre-metamorphic cores. The uniform but significantly different Hf isotope composition between the UHP (εHf(227) = ? 14.6 ± 0.8; 2 SD) and pre-metamorphic (εHf(227) = ? 27.7) domains indicates equilibration of the Lu–Hf isotope system only within the UHP metamorphic mineral assemblage. The disequilibrium between whole rock and UHP zircon suggests that about two thirds of the whole rock Hf retained in the pre-metamorphic zircon domains. Zircon domains crystallized during partial melting at 212 Ma in granitic leucosome and pegmatites have a Hf isotope composition indistinguishable from that of the UHP zircon domains. This suggests that only Hf (and Zr) equilibrated during UHP metamorphism was remobilized during partial melting while pre-metamorphic zircon remained stable or was not accessible. In contrast, the magmatic zircon edges from pegmatite have somewhat lower 176Hf/177Hf (～ 0.28216) and εHf(t) (? 17.6 ± 1.2; 2 SD) indicating some release of less radiogenic Hf for instance by dissolution of pre-metamorphic zircon during late regional amphibolite-facies retrogression.  相似文献   

5.
The Mombi bauxite deposit is located in 165 km northwest of Dehdasht city, southwestern Iran. The deposit is situated in the Zagros Simply Fold Belt and developed as discontinuous stratified layers in Upper Cretaceous carbonates (Sarvak Formation). Outcrops of the bauxitic horizons occur in NW-SE trending Bangestan anticline and are situated between the marine neritic limestones of the Ilam and Sarvak Formations. From the bottom to top, the deposit is generally consisting of brown, gray, pink, pisolitic, red, and yellow bauxite horizons. Boehmite, diaspore, kaolinite, and hematite are the major mineral components, while gibbsite, goethite, anatase, rutile, pyrite, chlorite, quartz, as well as feldspar occur to a lesser extent. The Eh–pH conditions during bauxitization in the Mombi bauxite deposit show oxidizing to reducing conditions during the Upper Cretaceous. This feature seems to be general and had a significant effect on the mineral composition of Cretaceous bauxite deposits in the Zagros fold belt. Geochemical data show that Al2O3, SiO2, Fe2O3 and TiO2 are the main components in the bauxite ores at Mombi and immobile elements like Al, Ti, Nb, Zr, Hf, Cr, Ta, Y, and Th were enriched while Rb, Ba, K, Sr, and P were depleted during the bauxitization process. Chondrite-normalized REE pattern in the bauxite ores indicate REE enrichment (ΣREE = 162.8–755.28 ppm, ave. ∼399.36 ppm) relative to argillic limestone (ΣREE = 76.26–84.03 ppm, ave. ∼80.145 ppm) and Sarvak Formation (ΣREE = 40.15 ppm). The REE patterns also reflect enrichment in LREE relative to HREE. Both positive and negative Ce anomalies (0.48–2.0) are observed in the Mombi bauxite horizons. These anomalies are related to the change of oxidation state of Ce (from Ce3+ to Ce4+), ionic potential, and complexation of Ce4+ with carbonate compounds in the studied horizons. It seems that the variations in the chemistry of ore-forming solutions (e.g., Eh and pH), function of carbonate host rock as a geochemical barrier, and leaching degree of lanthanide-bearing minerals are the most important controlling factors in the distribution and concentration of REEs. Several lines of evidences such as Zr/Hf and Nb/Ta ratios as well as similarity in REE patterns indicate that the underlying marly limestone (Sarvak Formation) could be considered as the source of bauxite horizons. Based on mineralogical and geochemical data, it could be inferred that the Mombi deposit has been formed in a karstic environment during karstification and weathering of the Sarvak limy Formation.  相似文献   

6.
The Karoo volcanic sequence in the southern Lebombo monocline in Mozambique contains different silicic units in the form of pyroclastic rocks, and two different basalt types. The silicic units in the lower part of the Lebombo sequence are formed by a lower unit of dacites and rhyolites (67–80 wt.% SiO2) with high Ba (990–2500 ppm), Zr (800–1100 ppm) and Y (130–240 ppm), which are part of the Jozini–Mbuluzi Formation, followed by a second unit, interlayered with the Movene basalts, of high-SiO2 rhyolites (76–78 wt.%; the Sica Beds Formation), with low Sr (19–54 ppm), Zr (340–480 ppm) and Ba (330–850 ppm) plus rare quartz-trachytes (64–66 wt.% SiO2), with high Nb and Rb contents (240–250 and 370–381 ppm, respectively), and relatively low Zr (450–460 ppm). The mafic rocks found at the top of the sequence are basalts and ferrobasalts belonging to the Movene Formation. The basalts have roughly flat mantle-normalized incompatible element patterns, with abundances of the most incompatible elements not higher than 25 times primitive mantle. The ferrobasalt has TiO2 ∼ 4.7 wt.%, Fe2O3t = 16 wt.%, and high Y (100 ppm), Zr (420 ppm) and Ba (1000 ppm). The Movene basalts have initial (at 180 Ma) 87Sr/86Sr = 0.7052–0.7054 and 143Nd/144Nd = 0.51232, and the Movene ferrobasalt has even lower 87Sr/86Sr (0.70377) and higher 143Nd/144Nd (0.51259). The silicic rocks show a modest range of initial Sr-(87Sr/86Sr = 0.70470–0.70648) and Nd-(143Nd/144Nd = 0.51223–0.51243) isotope ratios. The less evolved dacites could have been formed after crystal fractionation of oxide-rich gabbroic cumulates from mafic parental magmas, whereas the most silica-rich rhyolites could have been formed after fractional crystallization of feldspars, pyroxenes, oxides, zircon and apatite from a parental dacite magma. The composition of the Movene basalts imply different feeding systems from those of the underlying Sabie River basalts.  相似文献   

7.
Elemental and Sr–Nd isotopic data on metatexites, diatexites, orthogneisses and charnockites from the central Ribeira Fold Belt indicate that they are LILE-enriched weakly peraluminous granodiorites. Harker and Th–Hf–La correlation trends suggest that these rocks represent a co-genetic sequence, whereas variations on CaO, MnO, Y and HREE for charnockites can be explained by garnet consumption during granulitic metamorphism.Similar REE patterns and isotopic results of ?Nd565 = ?5.4 to ?7.3 and 87Sr/86Sr565 = 0.706–0.711 for metatexites, diatexites, orthogneisses and charnockites, as well as similar TDM ages between 2.0 and 1.5 Ga are consistent with evolution from a relatively homogeneous and enriched common crustal (metasedimentary) protolith.Results suggest a genetic link between metatexites, diatexites, orthogneisses and charnockites and a two-step process for charnockite development: (a) generation of the hydrated igneous protoliths by anatexis of metasedimentary rocks; (b) continuous high-grade metamorphism that transformed the “S-type granitoids” (leucosomes and diatexites) into orthogneisses and, as metamorphism and dehydration progressed, into charnockites.  相似文献   

8.
The NW–SE trending Longshoushan is in the southwestern margin of the Alxa Block, which was traditionally considered the westernmost part of the North China Craton (NCC). Precambrian crystalline basement exposed in the Longshoushan area was termed the “Longshoushan Complex”. This complex's formation and metamorphism are significant to understand the geotectonics and early Precambrian crustal evolution of the western NCC. In this study, field geology, petrology, and zircon U–Pb and Lu–Hf isotopes of representative orthogneisses and paragneisses in the Longshoushan Complex were investigated. U–Pb datings reveal three Paleoproterozoic magmatic episodes (ca. 2.33, ca. 2.17 and ca. 2.04 Ga) and two subsequent regional metamorphic events (ca. 1.95–1.90 Ga and ca. 1.85 Ga) for metamorphic granitic rocks in the Longshoushan Complex. U–Pb dating of the detrital magmatic zircons from two paragneisses yields concordant 207Pb/206Pb ages between 2.2 Ga and 2.0 Ga, and a small number of metamorphic zircon rims provide a ca. 1.95 Ga metamorphic age, suggesting that the depositional time of the protolith was between 2.0 and 1.95 Ga and that the sedimentary detritus was most likely derived from the granitic rocks in the Longshoushan Complex itself. Zircon Lu–Hf isotopic analyses indicate that nearly all magmatic zircons from ca. 2.0 Ga to ca. 2.17 Ga orthogneisses have positive εHf(t) values with two-stage Hf model ages (TDMC) ranging from 2.45 to 2.65 Ga (peak at ca. 2.5 Ga), indicating that these Paleoproterozoic granitic rocks were derived from the reworking of the latest Neoarchean–early Paleoproterozoic juvenile crust. Detrital magmatic zircons from two paragneisses yield scattered 176Hf/177Hf ratios, εHf(t) and TDMC values, further indicating that the sedimentary detritus was not only derived from these plutonic rocks but also from other unreported or denuded Paleoproterozoic igneous rocks. The ca. 2.15 Ga detrital magmatic zircons from one paragneiss have negative εHf(t) values with TDMC ranging from 2.76 to 3.04 Ga, indicating another important crustal growth period in the Longshoushan region. These data indicate that the Longshoushan Complex experienced Neoarchean–Early Paleoproterozoic crustal growth, approximately ca. 2.3–2.0 Ga experienced multiphase magmatic events, and approximately ca. 1.95–1.90 Ga and ca. 1.85 Ga experienced high-grade metamorphic events. The sequence of tectonothermal events is notably similar to that of the main NCC. Together with the datasets from an adjacent area, we suggest that the western Alxa Block was most likely an integrated component of the NCC from the Neoarchean to the Paleoproterozoic.  相似文献   

9.
The northeastern Gangdese Pb–Zn–Ag–Fe–Mo–W polymetallic belt (NGPB), characterized by skarn and porphyry deposits, is one of the most important metallogenic belts in the Himalaya–Tibetan continental orogenic system. This belt extends for nearly four hundred kilometers along the Luobadui–Milashan Fault in the central Lhasa subterrane, and contains more than 10 large ore deposits with high potential for development. Three major types of mineralization system have been identified: skarn Fe systems, skarn/breccia Pb–Zn–Ag systems, and porphyry/skarn Mo–Cu–W systems. In this study, we conducted a whole-rock geochemical, U–Pb zircon geochronological, and in situ zircon Hf isotopic study of ore-forming rocks in the NGPB, specifically the Jiangga, Jiaduopule, and Rema skarn Fe deposits, and the Yaguila Pb–Zn–Ag deposit. Although some of these deposits (porphyry Mo systems) formed during the post-collisional stage (21–14 Ma), the majority (these three systems) developed during the main (‘soft collision’) stage of the India–Asia continental collision (65–50 Ma). The skarn Fe deposits are commonly associated with granodiorites, monzogranites, and granites, and formed between 65 and 50 Ma. The ore-forming intrusions of the Pb–Zn–Ag deposits are characterized by granite, quartz porphyry, and granite porphyry, which developed in the interval of 65–55 Ma. The ore-forming porphyries in the Sharang Mo deposit, formed at 53 Ma. The rocks from Fe deposits are metaluminous, and have relatively lower SiO2, and higher CaO, MgO, FeO contents than the intrusions associated with Mo and Pb–Zn–Ag mineralization, while the Pb–Zn–Ag deposits are peraluminous, and have high SiO2 and high total alkali concentrations. They all exhibit moderately fractionated REE patterns characterized by lower contents of heavy REE relative to light REE, and they are enriched in large-ion lithophile elements and relatively depleted in high-field-strength elements. Ore-forming granites from Fe deposits display 87Sr/86Sr(i) = 0.7054–0.7074 and εNd(t) = − 4.7 to + 1.3, whereas rocks from the Yaguila Pb–Zn–Ag deposit have 87Sr/86Sr(i) = 0.7266–0.7281 and εNd(t) = − 13.5 to − 13.3. In situ Lu–Hf isotopic analyses of zircons from Fe deposits show that εHf(t) values range from − 7.3 to + 6.6, with TDM(Hf)C model ages of 712 to 1589 Ma, and Yaguila Pb–Zn–Ag deposit has εHf(t) values from − 13.9 to − 1.3 with TDM(Hf)C model ages of 1216 to 2016 Ma. Combined with existing data from the Sharang Mo deposit, we conclude that the ore-forming intrusions associated with the skarn Fe and porphyry Mo deposits were derived from partial melting of metasomatized lithospheric mantle and rejuvenated lower crust beneath the central Lhasa subterrane, respectively. Melting of the ancient continental material was critical for the development of the Pb–Zn–Ag system. Therefore, it is likely that the source rocks play an important role in determining the metal endowment of intrusions formed during the initial stage of the India–Asia continental collision.  相似文献   

10.
Late Silurian–early Devonian magmatism of the NW Junggar region in the Central Asian Orogenic Belt provides a critical geological record that is important for unraveling regional tectonic history and constraining geodynamic processes. In this study, we report results of Zircon U–Pb ages and systematic geochemical data for late Silurian–early Devonian largely granitic rocks in NW Junggar, aiming to constrain their emplacement ages, origin and geodynamic significance. The magmatism consists of a variety of mafic to felsic intrusions and volcanic rocks, e.g. adakitic granodiorite, K-feldspar granite, syenitic granite, gabbro and rhyrolite. U–Pb zircon ages suggest that the granitoids and gabbros were emplaced in the late Silurian–early Devonian (420–405 Ma). Adakitic granodiorites are calc-alkaline, characterized by high Sr (407–532 ppm), low Y (12.2–14.7 ppm), Yb (1.53–1.77 ppm), Cr (mostly < 8.00 ppm), Co (mostly < 11.0 ppm) and Ni (mostly < 4.10 ppm) and relatively high Sr/Y (31–42) ratios, analogous to those of modern adakites. K-feldspar granites and rhyolites are characterized by alkali- and Fe-enriched, with high Zr, Nb and Ga/Al ratios, geochemically similar to those of A-type granites. Syenitic granites show high alkaline (Na2O + K2O = 8.39–9.34 wt.%) contents, low Fe# values (0.73–0.80) and are weakly peraluminous (A/CNK = 1.00–1.07). Gabbros are characterized by low MgO (6.86–7.15 wt.%), Mg# (52–53), Cr (124–133 ppm) and Ni (84.7–86.6 ppm) contents. The geochemical characteristics of the gabbroic samples show affinity to both MORB- and arc-like sources. All granitoids have positive εNd(t) (+ 3.9 to + 6.9) and zircon εHf(t) (+ 9.8 to + 15.2) values and low initial 87Sr/86Sr ratios (0.7035–0.7043), with young TDM(Nd) (605–791 Ma) and TDM(Hf) (425–773 Ma) ages, suggesting significant addition of juvenile material. The adakitic granodiorites probably resulted from partial melting of mafic lower crust, leaving an amphibolite and garnet residue. The K-feldspar granites, rhyolites and syenitic granites probably formed from partial melting of the Xiemisitai mid-lower crust, while the gabbroic intrusion was probably generated by interactions between asthenospheric and metasomatized lithospheric mantle. Voluminous plutons of various types (adakites, A-type granites, I-type granites, and gabbros) formed during 420–405 Ma, and their isotopic data suggest significant additions of juvenile material. We propose that a slab roll-back model can account for the 420–405 Ma magmatic “flare up” in NW Junggar as well as an extensional setting.  相似文献   

11.
Ore-forming porphyries and barren granitoids from porphyry Cu deposits differ in many ways, particularly with respect to their adakitic affinity and calc-alkaline characteristics. In this study, zircon U–Pb and molybdenite Re–Os dating, whole rock geochemistry, whole rock Sr–Nd–Pb and zircon O–Hf isotopic analyses were carried out on the ore-forming granitoids from the Kounrad, Borly and Sayak deposits, and also on pre-ore and post-ore granitoids in adjacent regions of Central Kazakhstan. Geochronology results indicate that pre-ore magmatism occurred in the Late Devonian to Early Carboniferous (361.3–339.4 Ma), followed by large scale Cu mineralization (325.0–327.3 Ma at Kounrad, 311.4–315.2 Ma at Borly and 309.5–311.4 Ma at Sayak), and finally, emplacement of the Late Carboniferous post-ore barren granitoids (305.0 Ma). The geochemistry of these rocks is consistent with calc-alkaline arc magmatism characterized by strong depletions in Nb, Ta and Ti and enrichments in light rare earth elements and large ion lithophile elements, suggesting a supra-subduction zone setting. However, the ore-forming rocks at Kounrad and Sayak show adakitic characteristics with high Sr (517.5–785.3 ppm), Sr/Y (50.60–79.26), (La/Yb)N (9.37–19.62) but low Y (6.94–11.54 ppm) and Yb (0.57–1.07 ppm), whereas ore-forming rocks at Borly and barren rocks from northwest of Borly and Sayak have normal arc magma geochemical features. The Sr–Nd–Hf–O isotopic compositions show three different signatures: (1) Sayak granitoids have very young juvenile lower crust-derived compositions ((87Sr/86Sr)i = 0.70384 to 0.70451, ɛNd (t) = + 4.9 to + 6.0; TDM2 (Nd) = 580 to 670 Ma, ɛHf (t) = + 11.3 to + 15.5; TDMC (Hf) = 330 to 600 Ma, δ18O = 6.0 to 8.1‰), and were probably generated from depleted mantle-derived magma with 5–15% sediment melt addition in the magma source; (2) the Kt-1 granite from northwest of Sayak shows extremely enriched Sr–Nd isotopic compositions ((87Sr/86Sr)i = 0.71050, ɛNd (t) = − 7.8, TDM2 (Nd) = 1700 Ma), likely derived from partial melting of ancient continental crust; (3) other granitoids have transitional Sr–Nd compositions between the Sayak and Kt-1 samples, indicating a juvenile lower crust source with the addition of 10–30% of ancient crustal material. The pre-ore magmatism was probably related to partial melting of juvenile lower crust due to northward subduction of the Junggar–Balkhash Ocean, whereas the ore-forming adakitic rocks at Aktogai, Kounrad and Sayak formed by partial melting of thickened lower crust which subsequently delaminated. The ore-forming rocks at Borly, and the later post-ore barren granites, formed by partial melting of juvenile lower crust with normal thickness. This tectonic setting supports the existence of an Andean-type magmatic arc in the Devonian to the Late Carboniferous, resulting from the subduction of the Junggar–Balkhash oceanic plate. The link between whole rock geochemistry and scale of mineralization suggests a higher metallogenic potential for adakitic rocks than for normal arc magmatism.  相似文献   

12.
The Xishan deposit, located in the western Guangdong Province in South China, is a quartz-vein type W-Sn deposit with an average Sn grade of 0.1–0.4 wt%. The deposit is temporally and spatially associated with Xishan alkali feldspar granite. The W–Sn mineralization is present mainly as veins that are hosted by the granite. In this paper we present new zircon U–Pb age, whole-rock geochemical data, Sr–Nd–Pb–Hf isotopic data and Re–Os age in order to constrain the nature and timing of magmatism and mineralization in the Xishan mining district with implications on geodynamic settings. LA–ICP–MS zircon U–Pb analyses yielded an age of 79.14 ± 0.31 Ma for the alkali feldspar granite, consistent with the molybdenite Re–Os age of 79.41 ± 1.11 Ma. The alkali feldspar granite shows high contents of SiO2 (71.52–76.25 wt%), high total alkalis (Na2O + K2O = 9.35–13.51 wt%), high field strength elements (e.g. Zr = 95.4–116 ppm, Y = 97.1–138 ppm, Nb = 36.1–55.5 ppm, Ga = 97.1–138 ppm), and rare earth elements (total REE = 171.8–194.0 ppm) as well as high Ga/Al ratios (10,000 × Ga/Al = 3.23–3.82) suggesting that it has the geochemical characteristics of A-type granite and shows an A2 subtype affinity. Sr–Nd isotopes of the alkali feldspar granite show that (87Sr/86Sr)i values range from 0.7111 to 0.7183, and the εNd(t) values and Nd model ages (T2DM) vary from −6.8 to −6.5 and 1414 to 1433 Ma, respectively. The Pb isotopic compositions are variable, with 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb values ranging from 18.783 to 18.947, 15.709 to 15.722 and 38.969 to 39.244, respectively, indicating that the alkali feldspar granite was derived from a mantle-crust mixed source. In situ Hf isotopic analyses reveal that the alkali feldspar granite has εHf(t) values ranging from −9.69 to −0.04 and two-stage Hf model ages from 1145 Ma to 1755 Ma, indicating that the alkali feldspar granite was formed by the partial melting of Mesoproterozoic crusts of the Cathaysia Block with additions of mantle-derived materials. These results, together with previously presented regional geological relationships, suggest that the formation of the Xishan granite and associated W–Sn mineralization is related to lithospheric extension and asthenospheric upwelling that are attributed to a directional change of Pacific plate motion.  相似文献   

13.
To better understand the formative mechanism of the Cretaceous Gyeongsang Basin in South Korea, we determined the geochemical compositions of Early Cretaceous syntectonic basaltic rocks intercalated with basin sedimentary assemblages. Two distinct compositional groups appeared: tholeiitic to calc-alkaline basalts from the Yeongyang sub-basin and high-K to shoshonitic basaltic trachyandesites from the Jinju and Uiseong sub-basins. All collected samples exhibit patterns of light rare earth element enrichment and chondrite-normalized (La/Yb)N ratios ranging from 2.4 to 23.6. In a primitive-mantle-normalized spidergram, the samples show distinctive negative anomalies in Nb, Ta, and Ti and a positive anomaly in Pb. The basalts exhibit no or a weak positive U anomaly in a spidergram, but the basaltic trachyandesites show a negative U anomaly. The basalts have highly radiogenic Sr [(87Sr/86Sr)i = 0.70722–0.71145], slightly negative εNd, positive εHf [(εNd)i = −2.7 to 0.0; (εHf)i = +2.9 to +6.4], and radiogenic Pb isotopic compositions [(206Pb/204Pb)i = 18.20–19.19; (207Pb/204Pb)i = 15.60–15.77; (208Pb/204Pb)i = 38.38–39.11]. The basaltic trachyandesites are characterized by radiogenic Sr [(87Sr/86Sr)i = 0.70576–0.71119] and unradiogenic Nd, Hf, and Pb isotopic compositions [(εNd)i = −14.0 to −1.4; (εHf)i = −17.9 to +3.7; (206Pb/204Pb)i = 17.83–18.25; (207Pb/204Pb)i = 15.57–15.63; (208Pb/204Pb)i = 38.20–38.70]. The “crust-like” signatures, such as negative Nb–Ta anomalies, elevated Sr isotopic compositions, and negative εNd(t) and εHf(t) values, of the basaltic trachyandesites resemble the geochemistry of Early Cretaceous mafic volcanic rocks from the southern portion of the eastern North China Craton. Considering the lower-crust-like low U/Pb and high Th/U ratios and the unradiogenic Pb isotopic compositions, the basaltic trachyandesites are considered to be derived from lithospheric mantle modified by interaction with melts that originated from foundered eclogite. Basaltic volcanism in the Yeongyang sub-basin is coeval with the basaltic trachyandesite magmatism, but it exhibits an elevated 87Sr/86Sr ratio at a given 143Nd/144Nd and highly radiogenic Pb isotopic compositions, which imply an origin from an enriched but heterogeneous lithospheric mantle source. Melts from subducted altered oceanic basalt and pelagic sediments are considered to be the most likely source for the metasomatism. An extensional tectonic regime induced by highly oblique subduction of the Izanagi Plate beneath the eastern Asian margin during the Early Cretaceous might have triggered the opening of the Gyeongsang Basin. Lithospheric thinning and the resultant thermal effect of asthenospheric upwelling could have caused melting of the metasomatized lithospheric mantle, producing the Early Cretaceous basaltic volcanism in the Gyeongsang Basin.  相似文献   

14.
The Dongping gold deposit, located in Chongli County (Hebei Province) about 200 km northwest of Beijing, is one of the largest gold-producing areas along the northern margin of the North China Craton. It is located in the of Shuiquangou alkaline igneous complex of Middle Devonian age (394.3 ± 3.2 Ma), composed chiefly of highly alkaline syentite and quartz syenites. This study reveals the age of the Carboniferous in the deposit at 351.7 ± 2.8 Ma (MSWD = 1.9). The Dongping deposit is locally hosted in Cretaceous (~143 ± 1 Ma) alkali granites that intruded the older and the gold mineralization is closely associated genetically with this event. Hydrothermal zircons in the alkali granites have Th/U ratios mostly ranging between 0.01 and 0.7 indicating oscillatory zoning. A few grains with high Th/U ratios (1.31–2.07) may be from metamorphic domains. Negative εHf(t) values of the zircon mainly range between −19.75 and −16.93, suggesting that they originated principally by the melting of recycled continental crust. Less abundant zircons with εHf(t) ranging from −25.76 to −23.46, with Hf model ages (TDM2) of 2.54 to 2.67 Ga, (mainly 2.2 to 2.3 Ga) suggest that recycled Neoarchean basement was also present in the source region. The Devonian syenites and quartz syenites have TDM1 ages ranging from 1.96 to 2.08 Ga. Zircons from these rocks have εHf(t) values of −11.9 to −18.9. Certain zircons from the gold-bearing granite of Paleozoic age have an initial 176Hf/177Hf ratio of 0.281816 to 0.282058 and 0.282147 to 0.282348, reflecting a homogenous distribution of hafnium isotopes typical of magmatic sources. The TDM1 and TDM2 of the latest intrusion varying 1.33 to 1.59 Ga and 1.72 to 2.11 Ga respectively, indicating that the Neoproterozoic to Mesoproterozoic rocks of this area are an important source for the younger magma which are important to forming ore deposits. The TDM2 indicate that the magma may be derived from a very old crustal basement (~2.67 Ga) in the northern margin of North China Craton by partial melting.  相似文献   

15.
The Jinchang gold deposit is located in the easternmost portion of the Central Asian Orogenic Belt (CAOB), and represents one of the major gold districts in eastern Jilin–Heilongjiang provinces of China. The gold ore bodies are hosted mainly in altered Mesozoic granitoids, breccia pipes and ring and radial faults. Gold mineralization consists of alteration (stockwork in hydrothermally altered granites), breccia, and quartz-sulfide vein-types. Alteration assemblages around the alteration-style ore body show a vertical sequence of potassic, phyllic, and propylitic zones.In this study, we present U–Pb and Lu–Hf isotope data on zircons derived from mineralized granophyric granite, biotite monzogranite, granodiorite, and granite porphyry, and sericite Rb–Sr ages from the Jinchang gold deposit. The results show 206Pb/238U ages of 201 ± 3 Ma (MSWD = 1.1), 203 ± 4 Ma (MSWD = 1.4), 201 ± 5 Ma (MSWD = 2.1), and 110 ± 3 Ma (MSWD = 1.6), respectively. Sericite from the gold-mineralized phyllic-altered granodiorite and granite porphyry returns Rb–Sr isochron ages of 110 ± 4 Ma (MSWD = 1.04) and 107 ± 5 Ma (MSWD = 0.91), respectively. Our new data indicate that the gold mineralization at Jinchang took place at ca. 110 Ma and was temporally related to intrusion of the granite porphyry. Zircon ε Hf (200 Ma) values of the ca. 200 Ma granites vary from − 4.8 to + 8.1, with TCDM model ages of 727–1535 Ma, reflecting their derivation mainly by partial melting of juvenile Proterozoic crust. The gold-bearing 110 Ma granite porphyry returns ε Hf (110 Ma) values in the range of − 1.6 to + 9.8, with TCDM model ages of 542–1069 Ma, suggesting partial melts of juvenile Proterozoic crust with notable input of mantle components as compared to the ca. 200 Ma granites. Compiled oxygen (δ18OSMOW = − 0.7–10.1) and hydrogen (δDSMOW = − 99 to − 70) stable isotopic values of quartz from ores indicate that the ore-forming fluids were predominantly exsolved from magmas with minor amount of meteoric water in quartz-sulfide veins at the late stage. The Hf isotope data from the granite porphyry, integrated with the results from previous data on S and Pb isotopic composition of ores, constrain the source of ore-forming components as lower crust with discernible mantle inputs and wall rock assimilation. Our results have implications bearing on the widespread magmatism and metallogenic event during the Early Cretaceous time in East China, and link them to mantle upwelling that contributed both heat and volatiles for crustal melting and scavenging of metals which in turn were concentrated in upper crustal levels through exsolution for the magmas.  相似文献   

16.
The Shangjiazhuang Mo deposit is located on the Jiaodong Peninsula in eastern China, which is famous for the ca. 120 Ma “Jiaodong-type” Au deposits with total Au endowment of over 3000 t. In this paper, we discuss the deposit geology, mineralization age, and geochemical features of the host granodiorite of the Shangjiazhuang Mo orebody. Using this information, we aim to clarify the time and geodynamic mechanism for the Mo deposit, which is another constraint to understand the genesis of Au deposits. The Mo mineralization generally occurs as quartz–sulfide veins within the medium-grained Yashan granodiorite. The alteration consists of potassic alteration, silicification, sericitization, chloritization, and carbonatization with a weak unclear zonation. The ore minerals mainly include molybdenite, chalcopyrite, and pyrite. We measured Re–Os isotopes of molybdenite grains, which yielded a weighted mean model age of 116.9 ± 0.81 (MSWD = 1.03) and a well-constrained 187Re–187Os isochron age of 117.1 ± 1.4 Ma (MSWD = 1.6). These ages are slightly younger than the age of Au mineralization on the Jiaodong Peninsula. Rhenium contents of 5.84–29.99 ppm with an average of 16.4 ppm in molybdenites indicate a crustal source. Whole-rock geochemical compositions show that the granodiorite is high-K calc-alkaline and metaluminous to peraluminous. The samples show low Y contents from 8.2 to 10.5 ppm and Sr/Y ratios from 48.2 to 58.8, displaying an adakitic affinity. The Yashan granodiorite has high initial 87Sr/86Sr ratios of 0.7101 to 0.7104, low εNd(t) values of − 17.6 to − 16.7, and zircon εHf(t) values from − 24.8 to − 17.1, with corresponding Hf model ages of 2.7 to 2.2 Ga. These isotopic data, together with the adakitic affinity of the granodiorite, indicate that the parental magma was derived from ancient crust. Mafic microgranular enclaves (MME) that are contemporaneous with the host granodiorite show SiO2 contents of 57.98–58.41 wt% and depletion in Nb–Ta. The MMEs show enriched initial 87Sr/86Sr ratios of 0.7102 to 0.7106 and low εNd(t) values of − 17.3 to − 16.3. The MMEs are the products of mixing between the metasomatized lithospheric mantle-derived mafic magma and the ancient crust-derived felsic magma. The Early Cretaceous Mo mineralization (120–110 Ma) is slightly younger than the peak time of Au mineralization (126–120 Ma) on the Jiaodong Peninsula, but have a different spatial distribution which suggests different sources of Au and Mo. The “Jiaodong-type” Au deposits were probably related to the upwelling of metasomatized lithospheric mantle, while the Mo mineralization on the Jiaodong Peninsula may delineate a 120–110 Ma Mo metallogenic belt along the southern margin of the North China Craton with the East Qinling, which is related to the melting of ancient crustal sources. The subduction of the Paleo-Pacific slab and accompanying asthenospheric upwelling triggered upwelling of metasomatized lithospheric mantle, forming “Jiaodong-type” Au deposits. Subsequently, the ponding of mantle-derived magmas resulted in partial melting of ancient crust and associated Mo deposits.  相似文献   

17.
Post-collisional, potassic magmatic rocks widely distributed in the eastern Lhasa terrane provide significant information for comprehensive understanding of geodynamic processes of northward subduction of the Indian lithosphere and uplift of the Tibetan Plateau. A combined dataset of whole-rock major and trace elements, Sr–Nd–Pb isotopes, and in situ zircon U–Pb dating and Hf–O isotopic analyses are presented for the Yangying potassic volcanic rocks (YPVR) in the eastern part of the Lhasa terrane, South Tibet. These volcanic rocks consist of trachytes, which are characterized by high K2O (5.46–9.30 wt.%), SiO2 (61.34–68.62 wt.%) and Al2O3 (15.06–17.36 wt.%), and relatively low MgO (0.47–2.80 wt.%) and FeOt (1.70–4.90 wt.%). Chondrite-normalized rare earth elements (REE) patterns display clearly negative Eu anomalies. Primitive mantle-normalized incompatible trace elements diagrams exhibit strong enrichment in large ion lithophile elements (LILE) relative to high field strength elements (HFSE) and display significantly negative Nb–Ta–Ti anomalies. Initial isotopic compositions indicate relatively radiogenic Sr [(87Sr/86Sr)i = 0.711978–0.712090)] and unradiogenic Nd [(143Nd/144Nd)i = 0.512121–0.512148]. Combined with their Pb isotopic compositions [(206Pb/204Pb)i = 18.615–18.774, (207Pb/204Pb)i = 15.708–15.793, (208Pb/204Pb)i = 39.274–39.355)], these data are consistent with the involvement of component from subducted continental crustal sediment in their source region. The whole-rock Sr–Nd–Pb isotopic compositions exhibit linear trends between enriched mantle-derived mafic ultrapotassic magmas and relatively depleted crustal contaminants from the Lhasa terrane. The enrichment of the upper mantle below South Tibet is considered to result from the addition of components derived from subducted Indian continental crust to depleted MORB-source mantle during northward underthrusting of the Indian continental lithosphere beneath the Lhasa terrane since India–Asia collision at ~ 55 Ma. Secondary Ion Mass Spectrometry (SIMS) U–Pb zircon analyses yield the eruptive ages of 10.61 ± 0.10 Ma and 10.70 ± 0.18 Ma (weighted mean ages). Zircon Hf isotope compositions [ƐHf(t) = −4.79 to −0.17], combined with zircon O isotope ratios (5.51–7.22‰), imply an addition of crustal material in their petrogenesis. Clinopyroxene-liquid thermobarometer reveals pressure (2.5–4.1 kbar) and temperature (1029.4–1082.9 °C) of clinopyroxene crystallization, suggesting that depth of the magma chamber was 11.6–16.4 km. Energy-constrained assimilation and fractional crystallization (EC–AFC) model calculation indicates depth of assimilation and fractional crystallization in the region of 14.40–18.75 km underneath the Lhasa terrane, which is in consistent with depth of the magma chamber as suggested by clinopyroxene-liquid thermobarometer. Based on the whole-rock major and trace elements and Sr–Nd–Pb isotope compositions, combined with EC–AFC modeling simulations and zircon Hf–O isotope data, we propose that the YPVR resulted from assimilation and fractional crystallization (AFC) process of the K-rich mafic primitive magmas, which were caused by partial melting of the Indian continental subduction-induced mélange rocks.  相似文献   

18.
The Gangbian alkaline complex in the southeastern Yangtze Block (South China) is composed of Si-undersaturated pyroxene syenites and Si-saturated to -oversaturated syenites and quartz monzonites. SIMS zircon U–Pb analyses indicate that the complex was emplaced at 848 ± 4 Ma, during a previously-recognized interval of magmatic quiescence between the ca 1.0–0.89 Ga Sibaoan orogenic magmatism and the ca 0.83–0.78 Ga magmatic flare-up. The Gangbian rocks are characterized by wide, coherent variations in major and trace elements (SiO2 = 47.6–68.4%, K2O + Na2O = 4.5–10.5%, K2O/Na2O = 0.4–1.2, MgO = 1.2–8.5%, Cr = 4.5–239 ppm, and Ni = 4.5–143 ppm) and by enrichment in LIL and LREE and depletion in Nb, Ta and P in trace element spidergrams. Their whole-rock εNd(T) (? 6.5 to ? 0.4) and εHf(T) (? 10.7 to 0.4) are positively correlated, suggesting involvement of both metasomatized mantle and continental crust materials in their genesis. In situ zircon Hf–O isotopic measurements for the most evolved quartz monzonite sample yield a binary mixing trend between the mantle- and supracrustal-derived melts. It is suggested that the pyroxene syenites were derived by partial melting of metasomatized, phlogopite-bearing lithospheric mantle, and the parental magma experienced extensive fractionation of pyroxene and olivine associated with varying degrees of crustal contamination. Subsequent fractional crystallization of hornblende and minor amounts of plagioclase from the alkali basaltic magmas, accompanied by crustal contamination, produced the Si-saturated to -oversaturated syenites and quartz monzonites. These ca. 0.85 Ga alkaline rocks and neighboring contemporaneous dolerite dykes are the products of the anorogenic magmatism after the Sibao Orogeny. They post-date the final amalgamation between the Yangtze and Cathaysia Blocks, most likely manifesting the initial rifting of South China within the Rodinia supercontinent.  相似文献   

19.
Generally, arc-related or subduction-related mafic magmas are formed during or slightly postdate subduction, and characterized by depletion in high field strength elements (HFSEs) relative to the large ion lithophile elements (LILEs) and light rare-earth elements (LREEs). Combining with mineral chemistry and Sr–Nd isotopes, these geochemical characteristics were usually used to define an arc setting, especially for some ancient arcs that had been strongly modified by later tectonic activities. However, we report an exceptional case from the northern part of the Triassic Yidun Arc Belt, eastern Tibetan Plateau. The Ganluogou gabbro (∼152 Ma) occurs as several intrusive bodies. Its mineral assemblage is olivine (chrysolite), plagioclase (anorthite), clinopyroxene (diopside), amphibole (edenite and pargasite) and phlogopite. Whole rock geochemistry shows low SiO2 (42.87–46.99 wt.%), total rear earth elements (ΣREE = 22.8–28.4 ppm), Na2O + K2O (0.92–1.34 wt.%), and high Al2O3, MgO and FeO contents. It has small variations of initial 87Sr/86Sr ((87Sr/86Sr)i = 0.7053–0.7055) and εNd(t) values (−4.8 to −1.8). All the samples exhibit enrichment in LILEs including Th and U, but strongly depleted in HFSEs, including Nb, Ta, Zr and Hf. For the mineral chemistry, there are two type amphiboles. Amp(I) show higher V, Sc, Cr, Sr, Nb and Zr contents, but lower Th and U contents than those of Amp(II). Their REE patterns range from convex shape without Eu anomaly to LREE-enriched pattern with weak positive Eu anomaly. We suggest that Amp(I) was crystallized from a liquid that was mainly buffered by olivine, clinopyroxene and plagioclase, while Amp(II) crystallized from later melt that was mainly buffered by olivine. Based on clinopyroxene chemistry, compositions of coexisting olivine and plagioclase, and whole rock Sr–Nd isotopes, the parental magma of the Ganluogou gabbro is interpreted as a tholeiitic arc-affinity magma, which might be derived from an N-MORB mantle that had been metasomatised by slab-derived melts in the late Triassic (237–206 Ma). Thus, the Ganluogou gabbro provides an example that magmas exhibiting arc-affinity could in fact be formed in a post-orogenic extensional setting.  相似文献   

20.
The southern segment of the Eastern Ghats Mobile Belt (EGMB) in India was an active convergent margin during Mesoproterozoic, prior to the final collision in Neoproterozoic during the assembly of the Rodinia supercontinent. Here we present mineralogical, whole-rock geochemical, zircon U–Pb and Hf isotopic data from a granitoid suite in the Bopudi region in the EGGB. The granitoid complex comprises quartz monzodiorite with small stocks of rapakivi granites. The monzodiorite, locally porphyritic, contains K-feldspar megacrysts, plagioclase, quartz, biotite and ortho-amphibole. The presence of mantled ovoid megacrysts of alkali feldspar embaying early-formed quartz, and the presence of two generations of the phenocrystic phases in the rapakivi granites indicate features typical of rapakivi granites. The K-feldspar phenocrysts in the rapakivi granite are mantled by medium-grained aggregates of microcline (Ab7 Or93), which is compositionally equivalent to the rim of Kfs phenocryst and Pl (An23–24 Ab75). The geochemistry of both the granitoids shows arc-like features for REE and trace elements. LA-ICP-MS zircon analyses reveal 207Pb/206Pb ages of 1582 (MSWD = 1.4) for the rapakivi granite 1605 ± 3 Ma (MSWD = 3.9) for the monzodiorite. The zircons from all the granitoid samples show high REE contents, prominent HREE enrichment and a conspicuous negative Eu anomaly, suggesting a common melt source. The zircons from the monzodiorite have a limited variation in initial 176Hf/177Hf ratios of 0.28171–0.28188, with εHf(t) values of −2.2 to +2.8. Correspondingly, their two-stage Hf isotope model ages (TDM2) ranging from 2.15 to 2.47 Ga probably suggest a mixed source for the magma involving melting of the Paleoproterozoic basement and injection of subduction-related juvenile magmas. The prominent Mesoproterozoic ages of these granitoids suggest subduction-related arc magmatism in a convergent margin setting associated with the amalgamation of the Columbia-derived fragments within the Neoproterozoic Rodinia assembly.  相似文献   

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