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1.
《Applied Geochemistry》2006,21(10):1626-1634
Mineral waters in Britain show a wide range of 87Sr/86Sr isotope compositions ranging between 87Sr/86Sr = 0.7059 from Carboniferous volcanic rock sources in Dunbartonshire, Scotland to 87Sr/86Sr = 0.7207 in the Dalradian aquifer of Aberdeenshire, Scotland. The 87Sr/86Sr composition of the waters shows a general correlation with the aquifer rocks, resulting in the waters from older rocks having a more radiogenic signature than those from younger rocks. This wide range of values means that the Sr isotope composition of mineral water has applications in a number of types of studies. In the modern commercial context, it provides a way of fingerprinting the various mineral waters and hence provides a method for recognising and reducing fraud. From an environmental perspective, it provides the first spatial distribution of bio-available 87Sr/86Sr in Britain that can be used in modern, historical and archaeological studies.  相似文献   

2.
The Yinchanggou Pb-Zn deposit, located in southwestern Sichuan Province, western Yangtze Block, is stratigraphically controlled by late Ediacaran Dengying Formation and contains >0.3 Mt of metal reserves with 11 wt% Pb + Zn. A principal feature is that this deposit is structurally controlled by normal faults, whereas other typical deposits nearby (e.g. Maozu) are controlled by reverse faults. The origin of the Yinchanggou deposit is still controversial. Ore genetic models, based on conventional whole-rock isotope tracers, favor either sedimentary basin brine, magmatic water or metamorphic fluid sources. Here we use in situ Pb and bulk Sr isotope features of sulfide minerals to constrain the origin and evolution of hydrothermal fluids. The Pb isotope compositions of galena determined by femtosecond LA-MC-ICPMS are as follows: 206Pb/204Pb = 18.17–18.24, 207Pb/204Pb = 15.69–15.71, 208Pb/204Pb = 38.51–38.63. These in situ Pb isotope data overlap with bulk-chemistry Pb isotope compositions of sulfide minerals (206Pb/204Pb = 18.11–18.40, 207Pb/204Pb = 15.66–15.76, 208Pb/204Pb = 38.25–38.88), and both sets of data plotting above the Pb evolution curve of average upper continental crust. Such Pb isotope signatures suggest an upper crustal source of Pb. In addition, the coarse-grained galena in massive ore collected from the deep part has higher 206Pb/204Pb ratios (18.18–18.24) than the fine-grained galena in stockwork ore sampled from the shallow part (206Pb/204Pb = 18.17–18.19), whereas the latter has higher 208Pb/204Pb ratios (38.59–38.63) than the former (208Pb/204Pb = 38.51–38.59). However, both types of galena have the same 207Pb/204Pb ratios (15.69–15.71). This implies two independent Pb sources, and the metal Pb derived from the basement metamorphic rocks was dominant during the early phase of ore formation in the deep part, whereas the ore-hosting sedimentary rocks supplied the majority of metal Pb at the late phase in the shallow part. In addition, sphalerite separated from different levels has initial 87Sr/86Sr ratios ranging from 0.7101 to 0.7130, which are higher than the ore formation age-corrected 87Sr/86Sr ratios of country sedimentary rocks (87Sr/86Sr200 Ma = 0.7083–0.7096), but are significantly lower than those of the ore formation age-corrected basement rocks (87Sr/86Sr200 Ma = 0.7243–0.7288). Again, such Sr isotope signatures suggest that the above two Pb sources were involved in ore formation. Hence, the gradually mixing process of mineralizing elements and associated fluids plays a key role in the precipitation of sulfide minerals at the Yinchanggou ore district. Integrating all the evidence, we interpret the Yinchanggou deposit as a strata-bound, normal fault-controlled epigenetic deposit that formed during the late Indosinian. We also propose that the massive ore is formed earlier than the stockwork ore, and the temporal-spatial variations of Pb and Sr isotopes suggest a certain potential of ore prospecting in the deep mining area.  相似文献   

3.
This study provides 87Sr/86Sr, δ13C and δ18O data from the best-preserved limestone and dolomite of the Ediacaran carbonate-dominated Khorbusuonka Group of the Olenek Uplift, NE Siberian Craton, as well as detrital zircon geochronological data from both underlying and overlying sandstones. The Maastakh Formation is characterized by 87Sr/86Sr ratios of ca. 0.70822 and δ13C values between + 4.8 and + 6.0‰. 87Sr/86Sr ratios in limestones of the Khatyspyt Formation are fairly uniform, ranging from 0.70783 to 0.70806. The carbon isotopic composition slowly decreases from bottom (+ 3.7‰) to top (− 0.2‰) of section. The Sr isotopic composition of the Turkut Formation varies from 0.70824 to 0.70914, value of δ13C is about zero: − 0.7…+0.7 ‰. The youngest population of detrital zircons from Maastakh Formation indicates that these rocks were formed not later than 630 Ma. U–Pb detrital zircons data of Kessyusa Group has a single peak at about 543 Ma, which is almost identical to the earlier dating. Based on biostratigraphy and isotopic data, the Sr isotopic compositions from the Khatyspyt Formation (87Sr/86Sr = 0.70783–0.70806) represent the composition of seawater at 560–550 Ma. Such low values of 87Sr/86Sr ratio in Ediacaran water were probably caused by the quick opening of Iapetus Ocean.  相似文献   

4.
In this paper we report the Sr isotope signatures, and Sr, Al and Na concentrations of 30 surface waters (lakes/ponds and rivers/creeks) and 19 soil sample extracts from the island of Bornholm (Denmark) and present a categorized 87Sr/86Sr value distribution map that may serve as a base for provenance studies, including archaeological migration and authenticity proof for particular food products. The Sr isotopic compositions of surface waters range from 87Sr/86Sr = 0.7097–0.7281 (average 0.7175 ± 0.0049; 1σ), whereas 0.1 M HNO3, 0.05 M HNO3, and 0.01 M CaCl2 soil extracts range from 87Sr/86Sr = 0.7095–0.7197 and define somewhat lower but statistically indistinguishable averages of 0.7125 ± 0.003 (1s). These compositions are lower than the values expected from the Precambrian granitoid basement (87Sr/86Sr = 0.758–0.944), and from the overlying, mainly clastic Paleozoic sediments. Combined Sr isotope composition vs. Sr, Na and Al concentration relationships of soil extracts imply that lowering of the isotopic composition of leachable Sr on Bornholm results as a consequence of significant admixture to this fraction of Sr deposited as marine salts (aerosols), and that rainwater only has a minor influence on the Sr budget of the surface waters. Positively correlated Al/Na and [1/Sr] vs. 87Sr/86Sr relationships in soil extracts and surface waters indicate that the surface run-off on Bornholm is characterized by two predominant sources, namely marine aerosols (sea salts) with high Sr and low 87Sr/86Sr values, and a source with lower [Sr] delivering radiogenic Sr to the surface waters, which we equate with Sr leached from the products of mineral weathering (soils).A feasibility study for using Sr isotopic compositions of surface waters and soil extracts as a proxy for bioavailable Sr signatures was performed with a few samples collected in the vicinity of the eleventh century AD Ndr. Grødbygård cemetery site in SW Bornholm, from where Sr isotope compositions of modern fauna samples and tooth enamel of humans buried in the cemetery have been reported. Waters and soil extracts studied herein from around this site range from 87Sr/86Sr = 0.7104–0.7166 and correspond to Sr compositions extracted from snail shells in this area which span a range of 87Sr/86Sr = 0.7095–0.7160. Some human tooth enamel is characterized by more radiogenic values (87Sr/86Sr up to 0.718) which points to a possible provenance of these humans from the granite–gneiss terrain in the north of the island and/or to immigration of these humans in their childhood from other places (for example from mainland Sweden) to Bornholm. If the total compositional range of 87Sr/86Sr = 0.709–0.718 (n = 44) recorded in human enamel from the Ndr. Grødbygård site is considered representative for the variation of bioavailable Sr on Bornholm, then our soil leachate and surface water data entirely covers this range. We therefore propose that the combination of Sr isotope analyses of surface waters and soil leachates are an easy, fast and relatively cost efficient way to characterize a local bioavailable 87Sr/86Sr signature, and consequently propose that the overall average of 87Sr/86Sr = 0.7153 ± 0.0048 (1σ; n = 50) can be taken as a band for bioavailable Sr fractions suitable to discriminate between local and non-local signatures in provenance studies in the field of archaeology and for food and plant authenticity control in agricultural applications.  相似文献   

5.
High-Ti melanephelinite (3.8–5.9 wt% TiO2), medium-Ti (phono)tephrite (2.7–3.1 wt% TiO2), and low-Ti olivine melanephelinite/basanite (1.9–2.3 wt.% TiO2) are subordinate rock types in the central European Cenozoic Volcanic Province. A contrasting melanephelinite to (phono)tephrite series occurs in the Lou?ná–Oberwiesenthal Volcanic Centre (37–28 Ma) and also as satellite volcanic bodies (26–12 Ma) together with olivine melanephelinite/basanite (17–20 Ma) on the southwestern periphery of the Kru?né hory mountains (Erzgebirge). The volcanic rocks intrude the Variscan basement of the uplifted shoulder of the Oh?e/Eger Rift in the Kru?né hory mountains of the Bohemian Massif. Low Mg# (44–59) and Cr, Ni contents and enrichment of LILE, Zr, Hf, Nb, Ta, U, Th and LREE in the high-Ti melanephelinites contrast with the composition of primitive low-Ti olivine melanephelinites/basanites displaying high Mg# (63–74) and high contents of compatible elements. The high-Ti melanephelinites reveal a wide range in initial 87Sr/86Sr of ca. 0.7034–0.7038 and εNd of 2.4–4.9. The low-Ti melanephelinites show an overlapping range of initial 87Sr/86Sr of ca. 0.7035–0.7036 and εNd of 4.3–5.5. The large variation in initial 87Sr/86Sr ratios at similar εNd values in those rock types is interpreted as evidence for melting of metasomatized lithospheric mantle sources comprising K-bearing phases with radiogenic Sr. Modification of the olivine-free alkali basaltic magmas by differentiation or crustal contamination could give rise to the medium-Ti (phono) tephrites. The initial isotope ratios of all samples are consistent with HIMU-mantle sources and contributions from lithospheric mantle. The olivine-free melanephelinitic rocks often contain alkali pyroxenite–ijolite xenoliths with initial 87Sr/86Sr ratios of ca. 0.7036 and εNd of 3.0. We interpret these xenoliths as samples of an intra-crustal alkali complex derived from similar mantle sources as those for the basaltic volcanic rocks.  相似文献   

6.
C and Sr isotope compositions of carbonate rocks from the intracontinental São Francisco basin can track ocean connections and restriction. The lower three formations of the Bambuí Group can be grouped into three chemostratigraphic intervals (CI), recording different evolution stages of the basin. Lowermost CI-1 comprises the basal cap carbonates of the Sete Lagoas Formation displaying an initial C negative excursion, followed by a coeval C and Sr positive excursions (δ13C values from − 5 to 0‰ and 87Sr/86Sr ratios from 0.7074 to 0.7082) in 10 m of stratigraphic record. It marks a change from a restricted shallow basin influenced by freshwater to a basin connected to external seawaters due to marine transgression. CI-2 comprises carbonates of the middle portion of the Sete Lagoas Formation with δ13C values around 0‰ and 87Sr/86Sr ratios around 0.7082 that matches those observed worldwide for the Late Ediacaran. It records the onset of a Gondwana sea pathway connecting several epicontinental basins, allowing migration of index-fossil Cloudina sp. Uppermost CI-3 starts after a major positive excursion in the δ13C values reaching + 16‰ and a steepened decrease of 87Sr/86Sr ratios to 0.7075 which are lower than those expected for the Ediacaran-Cambrian boundary. This interval comprises the upper Sete Lagoas, Serra de Santa Helena and Lagoa do Jacaré formations and records the end of the connection of the São Francisco basin to the Gondwana sea pathway setting a restricted epeiric sea. Restriction was probably caused by Late Ediacaran uplifting of orogenic belts surrounding the basin. Other West Gondwana Cloudina bearing units also display the same mismatch in the Sr isotope ratios, suggesting that the establishment of intracontinental basins inside large continental masses may challenge the use of isotope chemostratigraphy for interbasinal correlations.  相似文献   

7.
This work presents an integrated study of zircon U–Pb ages and Hf isotope along with whole-rock geochemistry on Silurian Fengdingshan I-type granites and Taoyuan mafic–felsic intrusive Complex located at the southeastern margin of the Yangtze Block, filling in a gap in understanding of Paleozoic I-type granites and mafic-intermediate igneous rocks in the eastern South China Craton (SCC). The Fengdingshan granite and Taoyuan hornblende gabbro are dated at 436 ± 5 Ma and 409 ± 2 Ma, respectively. The Fengdingshan granites display characteristics of calc-alkaline I-type granite with high initial 87Sr/86Sr ratios of 0.7093–0.7127, low εNd(t) values ranging from −5.6 to −5.4 and corresponding Nd model ages (T2DM) of 1.6 Ga. Their zircon grains have εHf(t) values ranging from −2.7 to 2.6 and model ages of 951–1164 Ma. The Taoyuan mafic rocks exhibit typical arc-like geochemistry, with enrichment in Rb, Th, U and Pb and depletion in Nb, Ta. They have initial 87Sr/86Sr ratios of 0.7053–0.7058, εNd(t) values of 0.2–1.6 and corresponding T2DM of 1.0–1.1 Ga. Their zircon grains have εHf(t) values ranging from 3.2 to 6.1 and model ages of 774–911 Ma. Diorite and granodiorite from the Taoyuan Complex have initial 87Sr/86Sr ratios of 0.7065–0.7117, εNd(t) values from −5.7 to −1.9 and Nd model ages of 1.3–1.6 Ga. The petrographic and geochemical characteristics indicate that the Fengdingshan granites probably formed by reworking of Neoproterozoic basalts with very little of juvenile mantle-derived magma. The Taoyuan Complex formed by magma mixing and mingling, in which the mafic member originated from a metasomatized lithospheric mantle. Both the Fengdingshan and Taoyuan Plutons formed in a post-orogenic collapse stage in an intracontinental tectonic regime. Besides the Paleozoic Fengdingshan granites and Taoyuan hornblende gabbro, other Neoproterozoic and Indosinian igneous rocks located along the southeastern and western margin of the Yangtze Block also exhibit decoupled Nd–Hf isotopic systemics, which may be a fingerprint of a previous late Mesoproterozoic to early Neoproterozoic oceanic subduction.  相似文献   

8.
The period spanning from 825 to 540 Ma is characterized by major changes in the surficial Earth system. This extraordinary interval starts with the breakup of the Rodinia supercontinent and eruption of a series of large igneous provinces and ends with the assembly of Gondwana, giving rise to the Pan-African orogenies. This paleogeographic reorganization is accompanied by a global climatic cooling, including the paroxysmal Cryogenian “snowball” glacial events. The 87Sr/86Sr of seawater displays a major long-term rise over this interval that is punctuated by episodic, smaller declines and inflections. We use a coupled deep time climate-carbon numerical model to explore the complex role of tectonics and climate on this distinct evolution in seawater 87Sr/86Sr. We show that the modulation of the weathering of the erupted large igneous provinces by continental drift explains the changes in seawater 87Sr/86Sr from 800 to 635 Ma. The subsequent sharp rise in seawater 87Sr/86Sr from 635 to 580 Ma is the result of erosion of radiogenic crust exposed in the Pan-African orogens. Coeval evolution of atmospheric CO2 displays a decrease from about 80 times the pre-industrial level around 800 Ma to 5 times just before the beginning of the Phanerozoic.  相似文献   

9.
The Tianqiao Pb–Zn deposit in the western Yangtze Block, southwest China, is part of the Sichuan–Yunnan–Guizhou (SYG) Pb–Zn metallogenic province. Ore bodies are hosted in Devonian and Carboniferous carbonate rocks, structurally controlled by a thrust fault and anticline, and carried about 0.38 million tons Pb and Zn metals grading > 15% Pb + Zn. Both massive and disseminated Pb–Zn ores occur either as veinlets or disseminations in dolomitic rocks. They are composed of ore minerals, pyrite, sphalerite and galena, and gangue minerals, calcite and dolomite. δ34S values of sulfide minerals range from + 8.4 to + 14.4‰ and display a decreasing trend from pyrite, sphalerite to galena (δ34Spyrite > δ34Ssphalerite > δ34Sgalena). We interpret that reduced sulfur derived from sedimentary sulfate (gypsum and barite) of the host Devonian to Carboniferous carbonate rocks by thermal–chemical sulfate reduction (TSR). δ13CPDB and δ18OSMOW values of hydrothermal calcite range from –5.3 to –3.4‰ and + 14.9 to + 19.6‰, respectively, and fall in the field between mantle and marine carbonate rocks. They display a negative correlation, suggesting that CO2 in the hydrothermal fluid was a mixture origin of mantle, marine carbonate rocks and sedimentary organic matter. Sulfide minerals have homogeneous and low radiogenic Pb isotope compositions (206Pb/204Pb = 18.378 to 18.601, 207Pb/204Pb = 15.519 to 15.811 and 208Pb/204Pb = 38.666 to 39.571) that are plotted in the upper crust Pb evolution curve and overlap with that of Devonian to Carboniferous carbonate rocks and Proterozoic basement rocks in the SYG province. Pb isotope compositions suggest derivation of Pb metal from mixed sources. Sulfide minerals have 87Sr/86Sr ratios ranging from 0.7125 to 0.7167, higher than Sinian to Permian sedimentary rocks and Permian Emeishan flood basalts, but lower than basement rocks. Again, Sr isotope compositions are supportive of a mixture origin of Sr. They have an Rb–Sr isotopic age of 191.9 ± 6.9Ma, possibly reflecting the timing of Pb–Zn mineralization. C–O–S–Pb–Sr isotope compositions of the Tianqiao Pb–Zn deposit indicate a mixed origin of ore-forming fluids, which have Pb–Sr isotope homogenized before the mineralization. The Permian flood basalts acted as an impermeable layer for the Pb–Zn mineralization hosted in the Devonian–Carboniferous carbonate rocks.  相似文献   

10.
Numerous Fe-Cu deposits with mineralization styles similar to iron oxide-copper gold (IOCG) deposits form the Kangdian Fe-Cu metallogenic province, southwestern (SW) China. As one of the largest deposits in the region, the ~ 1.0 Ga Lala Fe-Cu deposit is hosted in a Paleoproterozoic volcanic-sedimentary succession named the Hekou Group which is alternately intruded by ~ 1.0 Ga doleritic plutons. This deposit has a paragenetic sequence evolving from Stage I of Na-alteration to Stage II of Fe mineralization, and finally to Stage III of Cu-(Mo, REE) mineralization, coeval with mafic-felsic intra-plate magmatism in the region. This study conducted in-situ Sr isotopic analyses on apatite and carbonate, aiming to resolve the long controversial issue regarding the origin of the Fe and Cu mineralizing fluids in the deposit. Apatite of Stage II has 87Sr/86Sr ratios varying from 0.71380 to 0.72733, much higher than those of synchronous igneous rocks in the region (0.7074 to 0.7091), but similar to the Paleoproterozoic host rocks (0.71368 to 0.71837 at ~ 1.0 Ga). This similarity indicates that radiogenic Sr of the Fe mineralizing fluid was dominantly sourced from the host rocks. Apatite and calcites of Stage III have 87Sr/86Sr ratios (0.75758–0.79293) much higher than apatite of Stage II and the host rocks but similar to the Archean basement rocks (as high as 0.80 at ~ 1.0 Ga) beneath the cover of the Yangtze Block, suggesting that the highly radiogenic Sr isotopic composition of the Cu mineralizing fluid was mainly inherited from the old basement rocks. In combination with previous C-O-S isotopic data indicating a magma-hydrothermal origin, it was suggested that the Fe mineralizing fluid was exsolved from a mafic magma that generated the ~ 1.0 Ga doleritic plutons, and inherited radiogenic Sr from the host rocks during fluid-rock interaction. By contrast, the Cu mineralizing fluid might have been sourced from another pulse of magmatic, Cu-Mo-REE- and CO2-rich fluid which have once interacted with Archean basement rocks prior to mineralization. The source of such a Cu-Mo-REE-rich fluid was not well constrained in current study but was inferred to be exsolved from a hidden felsic magma. We propose that intrusions of the bimodal magmas in Kangdian are responsible for regional hydrothermal circulation which led to Fe-Cu-(Mo, REE) mineralization in the Kangdian province.  相似文献   

11.
Zircon U–Pb ages, geochemical and Sr–Nd isotopic data are presented for the late Carboniferous Baoligaomiao Formation (BG Fm.) and Delewula Formation (DW Fm.) volcanic rocks, widely distributed in northern Inner Mongolia, in the northern part of the Xing'an–Mongolia Orogenic Belt (XMOB). The BG Fm. rocks mainly consist of basaltic andesites and andesites while the DW Fm. rocks include dacites, trachytes, rhyolites, pyroclastic rocks and minor andesites. New LA-ICPMS zircon U–Pb analyses constrain their eruption to late Carboniferous (317–322 Ma and 300–310 Ma, respectively). The BG Fm. volcanic rocks are characterized by enriched large ion lithophile elements (LILE) and depleted high field strength elements (HFSE), with initial 87Sr/86Sr ratios of 0.70854–0.70869 and negative εNd(t) (− 2.1 to − 2.4) values. They have low La/Ba (0.03–0.05), high La/Nb (2.05–3.70) ratios and variable Ba/Th (59.5–211) ratios. Such features suggest that they are derived from melting of heterogeneous sources including a metasomatized mantle wedge and Precambrian crustal material. The DW Fm. volcanic rocks are more depleted in HFSE with significant Nb, Ta, P, Ti anomalies. They have high initial 87Sr/86Sr ratios (0.72037–0.72234) and strong negative εNd(t) (− 11 to − 11.6) values which indicate those igneous rocks were mainly derived from reworking of the Paleoproterozoic crust. The late Carboniferous volcanic rocks have geochemical characteristics similar to those of the continental arc rocks which indicate the northward subduction of the Paleo Asian Ocean may have continued to the late Carboniferous. The volcanic association of this study together with the early Permian post-collisional magmatic rocks suggests that a tectonic transition from subduction-related continental margin arc volcanism to post-collisional magmatism occurred in the northern XMOB between the late Carboniferous and the early Permian.  相似文献   

12.
Based on the concepts (a) that the stable C and O isotopes combined with the Sr isotope ratios of fracture fills should reflect the source groundwater from which the solid phases precipitated and (b) that U-series disequilibria (USD) enable the calculation of residence time for the U by using Fe oxides as the best candidate, an “isotopic toolbox” was applied to fracture fill from the crystalline basement of the Vienne district. The fracture fills are formed mainly of carbonates, clays and Fe oxides. The isotope data indicate two main generations of carbonate that originated from hydrothermal circulation and equilibrium with present-day groundwaters but the Sr isotope ratios highlight another component with a higher 87Sr/86Sr ratio reflecting the complexity of the water–rock interactions.For the USD, the Fe-hydroxides located at 207 m depth yield an age of 102 ± 5 ka (St. Germain I interglacial stage), whereas those located at 277 m and 300 m yield respective ages of 173 ± 15 ka and 181 ± 10 ka. These corresponding to the transition between the penultimate glacial period (isotopic stage 6) and the end of the preceding interglacial stage (isotopic sub-stage 7a). Investigating water–rock interaction (87Sr/86Sr, 18O, 13C, USD) in the fracture-fill minerals from the crystalline basement has shown that such an approach is relevant to developing an understanding of how the groundwater system has changed over time.  相似文献   

13.
Zijinshan is a large porphyry–epithermal Cu–Au–Mo–Ag ore system located in the Zijinshan mineral field (ZMF) of southwestern Fujian Province, China. Although it is commonly accepted that the early Cretaceous magmatism and the metallogenesis of the mineral field are closely related, the tectonic setting for the ore-forming event(s) has been controversial and regarded as either extensional or subduction-related. New U–Pb zircon geochronology, Sr–Nd–Pb isotopic systematics, and geochemical data presented here from granites and volcanic rocks in the mineral field help to clarify this uncertainty.LA–MC–ICP-MS U–Pb zircon analyses yield weighted mean ages of between ca. 165 and 157 for the monzogranite, ca. 112 Ma for granodiorite, and between ca. 111 and 102 Ma for nine samples of volcanic units in the study area. These dates, integrated with previous geochronological data, indicate that there were two magmatic events in the area during the Middle to Late Jurassic and the Early Cretaceous. Major and trace element geochemistry indicates that these rocks are high-K, calc-alkaline granites, are enriched in LREE and Th, U, Ta, Nd, Sm and Yb, and depleted in Ba, K, Sr, P, Ti and Y. These features are characteristic of volcanic-arc granites or active-continental margin granites. The Middle to Late Jurassic monzogranitic plutons in the region have initial 87Sr/86Sr ratios of 0.7096 to 0.7173, εNdT values of − 10.1 to − 7.6, 206Pb/204Pb isotope ratios of 18.51–18.86, 207Pb/204Pb isotope ratios of 15.64–15.73, and 208Pb/204Pb isotope ratios of 38.76–39.18. The Early Cretaceous granodiorite and volcanic rocks are distinctly different with initial 87Sr/86Sr ratios of 0.7055–0.7116, εNdT values of − 8 to 0.5, 206Pb/204Pb ratios ranging between 18.49 and 19.77, 207Pb/204Pb ratios of 15.63–15.71, and 208Pb/204Pb ratios of 38.71–40.62. These characteristics suggest that the source for the Middle to Late Jurassic monzogranitic plutons is a partially melted Mesoproterozoic substrate, with a minor component from Paleozoic material, whereas the Early Cretaceous granodiorite and volcanic rocks may represent mixing of crustal and mantle-derived melts. It is therefore suggested that the Middle to Late Jurassic monzogranitic plutons, and the Early Cretaceous granodiorite and volcanic rocks in the ZMF are the result of an active continental-margin setting related to the subduction of the Paleo-Pacific Plate beneath the Eurasian continent. Given that the mineralization and the early Cretaceous granodiorite and volcanic rocks in the area are genetically related, the Zijinshan porphyry–epithermal ore system formed in the subduction-related tectonic setting.  相似文献   

14.
《Chemical Geology》2007,236(1-2):112-133
The Cida A-type granitic stock (∼ 4 km2) and Ailanghe I-type granite batholith (∼ 100 km2) in the Pan-Xi (Panzhihua-Xichang) area, SW China, are two important examples of granites formed during an episode of magmatism associated with the Permian Emeishan mantle plume activity. This is a classic setting of plume-related, anorogenic magmatism exhibiting the typical association of mantle-derived mafic and alkaline rocks along with silicic units. SHRIMP zircon U–Pb data reveal that the Cida granitic pluton (261 ± 4 Ma) was emplaced shortly before the Ailanghe granites (251 ± 6 Ma). The Cida granitoids display mineralogical and geochemical characteristics of A-type granites including high FeO⁎/MgO ratios, elevated high-field-strength elements (HFSE) contents and high Ga/Al ratios, which are much higher than those of the Ailanghe granites. All the granitic rocks show significant negative Eu anomalies and demonstrate the characteristic negative anomalies in Ba, Sr, and Ti in the spidergrams. It can be concluded that the Cida granitic rocks are highly fractionated A-type granitoids whereas the Ailanghe granitic rocks belong to highly evolved I-type granites.The Cida granitoids and enclaves have Nd and Sr isotopic initial ratios (εNd(t) = − 0.25 to + 1.35 and (87Sr/86Sr)i = 0.7023 to 0.7053) close to those of the associated mafic intrusions and Emeishan basalts, indicating the involvement of a major mantle plume component. The Ailanghe granites exhibit prominent negative Nb and Ta anomalies and weakly positive Pb anomalies in the spidergram and have nonradiogenic εNd(t) ratios (− 6.34 to − 6.26) and high (87Sr/86Sr)i values (0.7102 to 0.7111), which indicate a significant contribution from crustal material. These observations combined with geochemical modeling suggest that the Cida A-type granitoids were produced by extensive fractional crystallization from basaltic parental magmas. In contrast, the Ailanghe I-type granites most probably originated by partial melting of the mid-upper crustal, metasedimentary–metavolcanic rocks from the Paleo-Mesoproterozoic Huili group and newly underplated basaltic rocks.In the present study, it is proposed that petrogenetic distinctions between A-type and I-type granites may not be as clear-cut as previously supposed, and that many compositional and genetically different granites of the A- and I-types can be produced in the plume-related setting. Their ultimate nature depends more importantly on the type and proportion of mantle and crustal material involved and melting conditions. Significant melt production and possible underplating and/or intrusion into the lower crust, may play an important role in generating the juvenile mafic lower crust (average 20 km) in the central part of the Emeishan mantle plume.  相似文献   

15.
River water composition (major ion and 87Sr/86Sr ratio) was monitored on a monthly basis over a period of three years from a mountainous river (Nethravati River) of southwestern India. The total dissolved solid (TDS) concentration is relatively low (46 mg L−1) with silica being the dominant contributor. The basin is characterised by lower dissolved Sr concentration (avg. 150 nmol L−1), with radiogenic 87Sr/86Sr isotopic ratios (avg. 0.72041 at outlet). The composition of Sr and 87Sr/86Sr and their correlation with silicate derived cations in the river basin reveal that their dominant source is from the radiogenic silicate rock minerals. Their composition in the stream is controlled by a combination of physical and chemical weathering occurring in the basin. The molar ratio of SiO2/Ca and 87Sr/86Sr isotopic ratio show strong seasonal variation in the river water, i.e., low SiO2/Ca ratio with radiogenic isotopes during non-monsoon and higher SiO2/Ca with less radiogenic isotopes during monsoon season. Whereas, the seasonal variation of Rb/Sr ratio in the stream water is not significant suggesting that change in the mineral phase being involved in the weathering reaction could be unlikely for the observed molar SiO2/Ca and 87Sr/86Sr isotope variation in river water. Therefore, the shift in the stream water chemical composition could be attributed to contribution of ground water which is in contact with the bedrock (weathering front) during non-monsoon and weathering of secondary soil minerals in the regolith layer during monsoon. The secondary soil mineral weathering leads to limited silicate cation and enhanced silica fluxes in the Nethravati river basin.  相似文献   

16.
Copper and iron skarn deposits are economically important types of skarn deposits throughout the world, especially in China, but the differences between Cu and Fe skarn deposits are poorly constrained. The Edong ore district in southeastern Hubei Province, Middle–Lower Yangtze River metallogenic belt, China, contains numerous Fe and Cu–Fe skarn deposits. In this contribution, variations in skarn mineralogy, mineralization-related intrusions and sulfur isotope values between these Cu–Fe and Fe skarn deposits are discussed.The garnets and pyroxenes of the Cu–Fe and Fe skarn deposits in the Edong ore district share similar compositions, i.e., dominantly andradite (Ad29–100Gr0–68) and diopside (Di54–100Hd0–38), respectively. This feature indicates that the mineral compositions of skarn silicate mineral assemblages were not the critical controlling factors for variations between the Cu–Fe and Fe skarn deposits. Intrusions associated with skarn Fe deposits in the Edong ore district differ from those Cu–Fe skarn deposits in petrology, geochemistry and Sr–Nd isotope. Intrusions associated with Fe deposits have large variations in their (La/Yb)N ratios (3.84–24.6) and Eu anomalies (δEu = 0.32–1.65), and have relatively low Sr/Y ratios (4.2–44.0) and high Yb contents (1.20–11.8 ppm), as well as radiogenic Sr–Nd isotopes (εNd(t) = − 12.5 to − 9.2) and (87Sr/86Sr)i = 0.7067 to 0.7086. In contrast, intrusions associated with Cu–Fe deposits are characterized by relatively high Sr/Y (35.0–81.3) and (La/Yb)N (15.0–31.6) ratios, low Yb contents (1.00–1.62 ppm) without obvious Eu anomalies (δEu = 0.67–0.97), as well as (87Sr/86Sr)i = 0.7055 to 0.7068 and εNd(t) = − 7.9 to − 3.4. Geochemical evidence indicates a greater contribution from the crust in intrusions associated with Fe skarn deposits than in intrusions associated with Cu–Fe skarn deposits. In the Edong ore district, the sulfides and sulfates in the Cu–Fe skarn deposits have sulfur isotope signatures that differ from those of Fe skarn deposits. The Cu–Fe skarn deposits have a narrow range of δ34S values from − 6.2‰ to + 8.7‰ in sulfides, and + 13.2‰ to + 15.2‰ in anhydrite, while the Fe skarn deposits have a wide range of δ34S values from + 10.3‰ to + 20.0‰ in pyrite and + 18.9‰ to + 30.8‰ in anhydrite. Sulfur isotope data for anhydrite and sedimentary country rocks suggest that the formation of skarns in the Edong district involved the interaction between magmatic fluids and variable amounts of evaporites in host rocks.  相似文献   

17.
《Applied Geochemistry》2005,20(4):749-766
A synthesis of Sr isotope data from shallow and deep groundwaters, and brines from the Fennoscandian and Canadian Shields is presented. A salinity gradient is evident in the water with concentrations varying from approximately 1–75 g L−1 below 1500 m depth in the Fennoscandian Shield and from 10 up to 300 g L−1 below 650 m depth in the Canadian Shield. Strontium isotope ratios were measured to assess the origin of the salinity and evaluate the degree of water–rock interaction in the systems. In both shields, the Sr concentrations are enriched relative to Cl, defining a positive trend parallel to the seawater dilution line and indicative of Sr addition through weathering processes. The depth distribution for Sr concentration increases strongly with increasing depth in both shields although the variation in Sr-isotope composition does not mirror that of Sr concentrations. Strontium-isotope compositions are presented for surface waters, and groundwaters in several sites in the Fennoscandian and Canadian Shields. Numerous mixing lines can be drawn reflecting water–rock interaction. A series of calculated lines links the surface end-members (surface water and shallow groundwater) and the deep brines; these mixing lines define a range of 87Sr/86Sr ratios for the deep brines in different selected sites. All sites show a specific 87Sr/86Sr signature and the occurrence of large 87Sr/86Sr variations is site specific in both shields. In Canadian Shield brines, the Sr isotope ratios clearly highlight large water rock interaction that increases the 87Sr/86Sr ratio from water that could have been of marine origin. In contrast to the Canadian Shield, groundwater does not occur in closed pockets in the Fennoscandian, and the well-constrained 87Sr/86Sr signatures in deep brines should correspond to a large, well-mixed and homogeneous water reservoir, whose Sr isotope signature results from water–rock interaction.  相似文献   

18.
The Gaoligong belt is located in the southeastern margin of the Tibetan plateau, and is bound by the Tengchong and Baoshan blocks. This paper presents new data from zircon geochronology, geochemistry, and whole-rock Sr–Nd–Pb–Hf isotopes to evaluate the tectonic evolution of the Gaoligong belt. The major rock types analysed in the present study are granitic gneiss, granodiorite, and granite. They are metaluminous to peraluminous and belong to high-K, calc-alkaline series. Laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) analyses of zircons from nine granitic rocks yielded crystallization ages of 495–487 Ma, 121 Ma, 89 Ma, and 70–63 Ma. The granitoids can be subdivided into the following four groups. (1) Early Paleozoic granitic gneisses with high εNd(t) and εHf(t) values of − 1.06 to − 3.45 and − 1.16 to 2.09, and model ages of 1.16 Ga to 1.33 Ga and 1.47 Ga to 1.63 Ga, respectively. Their variable 87Sr/86Sr and Pb values resemble the characteristics of the Early Paleozoic Pinghe granite in the Baoshan block. Our data suggest that the rocks were derived from the break-off of the Proto-Tethyan oceanic slab between the outboard continent and the Baoshan block, which induced the partial melting of Mesoproterozoic pelitic sources mixed with depleted mantle materials. (2) Early Cretaceous granodiorites with low εNd(t) and εHf(t) values of − 8.92 and − 4.91 with Nd and Hf model ages of 1.41 Ga and 1.49 Ga, respectively. These rocks have high initial 87Sr/86Sr (0.711992) and lower crustal Pb values, suggesting that they were derived from Mesoproterozoic amphibolites with tholeiitic signature, leaving behind granulite residue at the lower crust. (3) Early Late Cretaceous granites with low εNd(t) and εHf(t) values of − 9.58 and − 4.61 with Nd and Hf model ages of 1.43 Ga and 1.57 Ga, respectively. These rocks have high initial 87Sr/86Sr (0.713045) and lower crustal Pb isotopic values. These rocks were generated from the partial melting of Mesoproterozoic metapelitic sources resulting from the delamination of thickened lithosphere, following the closure of the Bangong–Nujiang Ocean and collision of the Lhasa–Qiangtang blocks. (4) Late Cretaceous to Paleogene granitic gneisses with low εNd(t) and εHf(t) values of − 4.41 to − 10 and − 5.95 to − 8.71, Nd model ages ranging from 1.08 Ga to 1.43 Ga, and Hf model ages from 1.53 Ga to 1.67 Ga, respectively. These rocks show high initial 87Sr/86Sr (0.713201 and 714662) and lower crustal Pb values. The data suggest that these rocks are likely related to the eastward subduction of the Neo-Tethyan Oceanic slab, which induced partial melting of Mesoproterozoic lower crustal metagreywacke. The results presented in this study from the Gaoligong belt offer important insights on the evolution of the Proto-Tethyan, Bangong–Nujiang, and Neo-Tethyan oceans in the southeastern margin of the Tibetan Plateau.  相似文献   

19.
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   

20.
This paper presents geochemical, Sr–Nd isotopic, and U–Pb zircon geochronological data on the Alvand plutonic complex in Sanandaj–Sirjan zone (SSZ), Western Iran. The gabbroic rocks show a trend of a calc-alkaline magma suite and are characterized by low initial 87Sr/86Sr ratios (0.7023–0.7037) and positive εNd(t) values (2.9–3.3), which suggest derivation from a moderately depleted mantle source. Geochemical features of the granites illustrate a high-K calc-alkaline magma series, whereas the leucocratic granitoids form part of a low-K series. Granites have intermediate 87Sr/86Sr ratios (0.707–0.719) and negative εNd(t) values (−1.0 to −3.4), while leucocratic granitoids have higher initial 87Sr/86Sr ratio (0.713–0.714) and more negative εNd(t) values (−3.5 to −4.5). Potential basement source lithologies for the granites are Proterozoic granites and orthogneisses, and those for the leucocratic granites are plagioclase-rich sources such as meta-arkoses or tonalites. The U–Pb dating results demonstrate that all granitoids were exclusively emplaced during the Jurassic instead of being Cretaceous or younger in age as suggested previously. The pluton was assembled incrementally over c. 10 Ma. Gabbros formed at 166.5 ± 1.8 Ma, granites between 163.9 ± 0.9 Ma and 161.7 ± 0.6 Ma, and leucocratic granitoids between 154.4 ± 1.3 and 153.3 ± 2.7 Ma. Granites and leucocratic granitoids show some A-type affinity. It is concluded that the Alvand plutonic complex was generated in a continental-arc-related extensional regime during subduction of Neo-Tethyan oceanic crust beneath the SSZ. The U/Pb zircon age data, recently corroborated by similar results in the central and southern SSZ, indicate that Jurassic granitoids are more areally extensive in this belt than previously thought.  相似文献   

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