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1.
Two distinct series of Variscan granitic rocks have been distinguished in the Gravanho-Gouveia area of Portugal, based on field work, variation diagrams for major and trace elements, rare earth patterns and δ18O versus total FeO diagram of rocks, anorthite content of plagioclase, BaO and P2O5 contents of feldspars and AlVI versus Fe2+ diagram for magmatic muscovite. One series consists of a late-orogenic porphyritic biotite > muscovite granite (G1), less evolved beryl-columbite pegmatites and more evolved beryl-columbite pegmatites showing gradational contacts. The other series consists of post-orogenic porphyritic muscovite > biotite granodiorite to granite (G2), slightly porphyritic muscovite > biotite granite (G3) and lepidolite pegmatites. In each series, pegmatites are derived from the parent granite magma by fractional crystallization of quartz, plagioclase, K-feldspar, biotite and ilmenite. Some metasomatic effects occur like muscovite replacing feldspars, chlorite in pegmatites of the first series and a late muscovite in pegmatites of the second series, probably due to hydrothermal fluids. The lepidolite pegmatites contain cassiterite and two generations of rutile. The first magmatic generation consists of homogeneous crystals and the second generation occurs as heterogeneous zoned crystals derived from hydrothermal fluids. The beryl-columbite pegmatites and lepidolite pegmatites also contain the first magmatic generation and the late hydrothermal generation of zoned columbite-group minerals. More evolved beryl-columbite pegmatites were converted into episyenite by intense hydrothermal alteration and regional circulation of fluids in the granitic rocks.  相似文献   

2.
Unusual 18O depletion, with δ18O values as negative as −10‰ to −4‰ relative to VSMOW, was reported in zircons from ultrahigh-pressure eclogite-facies metamorphic rocks in the Dabie-Sulu orogenic belt, China. But it is critical for the negative δ18O zircons to be distinguished between magmatic and metamorphic origins, because the 18O depletion can be acquired by high-T eclogite-facies metamorphism of meteoric-hydrothermally altered low δ18O rocks. While zircon O diffusion kinetics has placed a reasonable constraint on this, zircon trace element compositions can provide a straightforward distinction between the magmatic and metamorphic origins. This paper reports our finding of unusual 18O depletion in zircon from granitic gneiss in the northeastern end of the Sulu orogen. Zircon δ18O values vary from −7.8‰ to −3.1‰ along a profile of 50 m length at Zaobuzhen. They are close to extremely low δ18O values of −9.0‰ to −5.9‰ for metagranite at Qinglongshan and adjacent areas in the southwestern end of the Sulu orogen. CL imaging suggests that the low δ18O zircons at Zaobuzhen are primarily of magmatic origin, but underwent different degrees of metamorphic modification. Zircon U-Pb dating yields middle Neoproterozoic ages of 751 ± 27 to 779 ± 25 Ma for protolith crystallization and Triassic ages of 214 ± 10 to 241 ± 33 Ma for metamorphic resetting. However, no metamorphic modification occurs in zircon REE patterns that only indicate magmatic recrystallization and hydrothermal alteration, respectively. Thus, the negative δ18O zircons are interpreted as crystallizing from negative δ18O magmas due to melting of meteoric-hydrothermally altered negative δ18O rocks in an active rift setting at about 780 Ma. The variation in zircon δ18O values indicates considerable O isotope heterogeneity in its granitic protolith. Zircon Lu-Hf isotope analyses give positive εHf(t) values of 1.6-4.1 and Hf model ages of 1.18-1.30 Ga. This suggests that the granitic protolith was derived from the mid-Neoproterozoic reworking of late Mesoproterozoic juvenile crust. The metagranites at Zaobuzhen and Qinglongshan, about 450 km apart, are two known occurrences of the unusually low δ18O zircons below −6‰ so far reported in the Sulu orogen. They are similar to each other in both protolith and metamorphic ages, so that they share the same nature of both Neoproterozoic protolith and Triassic metamorphism. Therefore, the locally negative δ18O zircons may register centers of low δ18O magmatism during the supercontinental rifting.  相似文献   

3.
Microorganisms are ubiquitous in deep subsurface environments, but their role in the global carbon cycle is not well-understood. The natural abundance δ13C and Δ14C values of microbial membrane phospholipid fatty acids (PLFAs) were measured and used to assess the carbon sources of bacteria in sedimentary and granitic groundwaters sampled from three boreholes in the vicinity of the Tono Uranium Mine, Gifu, Japan. Sample storage experiments were performed and drill waters analyzed to characterize potential sources of microbial contamination. The most abundant PLFA structures in all waters sampled were 16:0, 16:1ω7c, cy17:0, and 18:1ω7c. A PLFA biomarker for type II methanotrophs, 18:1ω8c, comprised 3% and 18% of total PLFAs in anoxic sedimentary and granitic waters, respectively, sampled from the KNA-6 borehole. The presence of this biomarker was unexpected given that type II methanotrophs are considered obligate aerobes. However, a bacterium that grows aerobically with CH4 as the sole energy source and which also produces 56% of its total PLFAs as 18:1ω8c was isolated from both waters, providing additional evidence for the presence of type II methanotrophs. The Δ14C values determined for type II methanotroph PLFAs in the sedimentary (−861‰) and granite (−867‰) waters were very similar to the Δ14C values of dissolved inorganic carbon (DIC) in each water (∼−850‰). This suggests that type II methanotrophs ultimately derive all their carbon from inorganic sources, whether directly from DIC and/or from CH4 produced by the reduction of DIC. In contrast, δ13C values of type II PLFAs in the sedimentary (−93‰) and granite (−60‰) waters indicate that these organisms use different carbon assimilation schemes in each environment despite very similar δ13CCH4 values (∼−95‰) for each water. The δ13CPLFA values (−28‰ to −45‰) of non-methanotrophic bacteria in the KNA-6 LTL water do not clearly distinguish between heterotrophic and autotrophic metabolisms, but Δ14CPLFA values indicate that >65% of total bacteria filtered from the KNA-6 LTL water are heterotrophs. Ancient Δ14C values (∼−1000‰) of some PLFAs suggest that many heterotrophs utilize ancient organic matter, perhaps from lignite seams within the sedimentary rocks. The more negative range of δ13CPLFA values determined for the KNA-6 granitic water (−42‰ to −66‰) are likely the result of a microbial ecosystem dominated by chemolithoautotrophy, perhaps fuelled by abiogenic H2. Results of sample storage experiments showed substantial shifts in microbial community composition and δ13CPLFA values (as much as 5‰) during 2-4 days of dark, refrigerated, aseptic storage. However, water samples collected and immediately filtered back in the lab from freshly drilled MSB-2 borehole appeared to maintain the same relative relationships between δ13CPLFA values for sedimentary and granitic host rocks as observed for samples directly filtered under artesian flow from the KNA-6 borehole of the Tono Uranium Mine.  相似文献   

4.
The Sin Quyen-Lung Po district is an important Cu metallogenic province in Vietnam, but there are few temporal and genetic constraints on deposits from this belt. Suoi Thau is one of the representative Cu deposits associated with granitic intrusion. The deposit consists of ore bodies in altered granite or along the contact zone between granite and Proterozoic meta-sedimentary rocks. The Cu-bearing intrusion is sub-alkaline I-type granite. It has a zircon U-Pb age of ~776 Ma, and has subduction-related geochemical signatures. Geochemical analysis reveals that the intrusion may be formed by melting of mafic lower crust in a subduction regime. Three stages of alteration and mineralization are identified in the Suoi Thau deposit, i.e., potassic alteration; silicification and Cu mineralization; and phyllic alteration. Two-phase aqueous fluid inclusions in quartz from silicification stage show wide ranges of homogenization temperatures(140–383℃) and salinities(4.18wt%–19.13wt%). The high temperature and high salinity natures of some inclusions are consistent with a magmatic derivation of the fluids, which is also supported by the H-O-S isotopes. Fluids in quartz have δD values of –41.9‰ to –68.8‰. The fluids in isotopic equilibrium with quartz have δ~(18)O values ranging from 7.9‰ to 9.2‰. These values are just plotted in the compositional field of magmatichydrothermal fluids in the δD_(water) versus δ~(18)O_(water) diagram. Sulfide minerals have relatively uniform δ~(34)S values from 1.84‰ to 3.57‰, which is supportive of a magmatic derivation of sulfur. The fluid inclusions with relatively low temperatures and salinities most probably represent variably cooled magmatic-hydrothermal fluids. The magmatic derivation of fluids and the close spatial relationship between Cu ore bodies and intrusion suggest that the Cu mineralization most likely had a genetic association with granite. The Suoi Thau deposit, together with other deposits in the region, may define a Neoproterozoic subduction-related ore-forming belt.  相似文献   

5.
In the Segura area, Variscan S-type granites, aplite veins and lepidolite-subtype granitic aplite-pegmatite veins intruded the Cambrian schist-metagraywacke complex. The granites are syn D3. Aplite veins also intruded the granites. Two-mica granite and muscovite granite have similar ages of 311.0 ± 0.5 Ma and 312.9 ± 2.0 Ma but are not genetically related, as indicated by their geochemical characteristics and (87Sr/86Sr)311 values. They correspond to distinct pulses of magma derived by partial melting of heterogeneous metapelitic rocks. Major and trace elements suggest fractionation trends for: (a) muscovite granite and aplite veins; (b) two-mica granite and lepidolite-subtype aplite-pegmatite veins, but with a gap in most of these trends. Least square analysis for major elements, and modeling of trace elements, indicate that the aplite veins were derived from the muscovite granite magma by fractional crystallization of quartz, plagioclase, K-feldspar and ilmenite. This is supported by the similar (87Sr/86Sr)311 and δ18O values and the behavior of P2O5 in K-feldspar and albite. The decrease in (87Sr/86Sr)311 and strong increase (1.6‰) in δ18O from two-mica granite to lepidolite-subtype aplite-pegmatite veins, and the behaviors of Ca, Mn and F of hydroxylapatite indicate that these veins are not related to the two-mica granite.  相似文献   

6.
In the Eastern Dharwar craton, among the many shear zone-hosted lode gold deposits, those at Ramagiri and Penakacherla are located near the western margin of the craton. Mineralized quartz (± sulfide ± carbonate) veins are hosted by the schistose (metavolcanic and carbonaceous metasedimentary) rocks, in close spatial association with granitoids having quartz and quartzofeldspathic veins representing hydrothermal activities associated with them. Mineralized quartz veins from the ore zones (in Ramagiri and Penakacherla regions) and quartz (or pegmatitic) veins from the surrounding granitic terrane were chosen for δ18O analysis. Samples from the schistose and granitic domains show δ18Oquartz values in the range of 10.4–14.9 and 9.3–10.9‰ respectively. The ore-zone fluids from the Ramagiri and Penakacherla regions give δ18O values of 7.9 ± 1.5 and 5.1 ± 0.8‰, calculated at pressure-corrected temperatures obtained from fluid inclusion microthermometry. The late-magmatic fluid is relatively 18O-poor with δ18O values estimated at 4.5 ± 0.7‰ and the value is closer to what is obtained for the ore zones. Based on the δ18O values reported and a possible magmatic contribution to ore fluid deciphered from fluid inclusion characteristics, a genetic relationship between granitic magmatism and gold mineralization is surmised. The observed increase in the 18O/16O ratio from the magmatic fluid to ore fluid in the shear zone is attributed to interaction of the magmatic fluid with host metasediments, that agrees well with the variation in the CO2/CH4 ratio of carbonic component in such fluids.  相似文献   

7.
The Yurungkash and Karakash rivers, also known respectively as the White Jade and Black Jade rivers, located in Hetian, Xinjiang Province, Northwest China, are the two main sources in China of white, green, and black placer nephrite, with a long history (~ 5000 years) of exploration and mining. The twenty-nine placer nephrite samples collected from both rivers and analyzed in the present study possess fine-grained and compact microstructures. The mineral assemblages in the samples provide clues to the metamorphic/metasomatic processes that formed the nephrite, which was the result of one of the following reactions: dolomitic marble → tremolite, or dolomitic marble → diopside → tremolite. White nephrite from the Yurungkash River and green nephrite from the Karakash River are predominantly tremolite. Based on electron probe microanalytical data, backscattered electron images, and Raman spectra, two kinds of black nephrite from the Karakash River are identified: one dominated by actinolite aggregates, and another consisting of tremolite aggregates with graphite crystals up to 2 mm in length. Compared with black nephrite, white and green nephrites contain fewer mineral inclusions and have lower FeO and MnO contents. All the amphiboles in the nephrites have very low contents of Cr2O3 (0.00–0.07 wt.%) and NiO (0.00–0.05 wt.%) relative to serpentinite-related nephrite (0.07–0.43 wt.% Cr2O3, 0.08–0.36 wt.% NiO). Most of the nephrite samples have low total rare earth element (ΣREE) contents, ranging from 12.22 to 49.40 ppm. In two nephrite samples, relatively high ΣREE concentrations (161 and 190 ppm) are related to the presence of REE-bearing minerals. Whole-rock REE chondrite-normalized patterns of all samples are characterized by strong negative Eu anomalies (0.16–0.48), moderate light-REE enrichment (La/NdN = 1.8–5.0), and nearly flat heavy-REE distributions (Gd/YbN = 0.3–1.7). Nephrite samples from both river locations have δ18O and δD isotope compositions ranging from 1.1‰ to 5.6‰ and − 55.7‰ to − 72.4‰, respectively. These values are closer to those recorded in dolomite-related nephrites than those in serpentinite-related deposits. Importantly, δ18O and δD values correspond to fluid isotope compositions of δ18O = 1.6‰ to 6.1‰ (330 °C), 1.8‰ to 6.3‰ (350 °C), and 2.5‰ to 7.0‰ (430 °C), and δD = − 34.9‰ to − 52.5‰ (350 °C to 650 °C). These values are close to or within the field of magmatic water. Geochemical and petrographic characteristics point to a dolomite-related metamorphic/metasomatic origin for nephrite at both locations. The placer nephrite is likely to have been derived from primary nephrite deposits in the Kunlun Mountains around Hetian, based on the geological occurrence of the deposits.  相似文献   

8.
We have developed a quantitative model of CO2 and H2O isotopic mixing between magmatic and hydrothermal gases for the fumarolic emissions of the La Fossa crater (Vulcano Island, Italy). On the basis of isotope balance equations, the model takes into account the isotope equilibrium between H2O and CO2 and extends the recent model of chemical and energy two-end-member mixing by Nuccio et al. (1999). As a result, the H2O and CO2 content and the δD, δ18O, and δ13C isotope compositions for both magmatic and hydrothermal end-members have been assessed. Low contributions of meteoric steam, added at a shallow depth, have been also recognized and quantified in the fumaroles throughout the period from 1988 to 1998. Nonequilibrium oxygen isotope exchange also seems to be occurring between ascending gases and wall rocks along some fumarolic conduits.The δ13CCO2 of the magmatic gases varies around −3 to 1‰ vs. Peedee belemnite (PDB), following a perfect synchronism with the variations of the CO2 concentration in the magmatic gases. This suggests a process of isotope fractionation because of vapor exsolution caused by magma depressurization. The hydrogen isotopes in the magmatic gases (−1 to −‰ vs. standard mean ocean water [SMOW]), as well as the above δ13CCO2 value, are coherent with a convergent tectonic setting of magma generation, where the local mantle is widely contaminated by fluids released from the subducted slab. Magma contamination in the crust probably amplifies this effect.The computed isotope composition of carbon and hydrogen in the hydrothermal vapors has been used to calculate the δD and δ13C of the entire hydrothermal system, including mixed H2O-CO2 vapor, liquid water, and dissolved carbon. We have computed values of about 10‰ vs. SMOW for water and −2 to −6.5‰ vs. PDB for CO2. On these grounds, we think that Mediterranean marine water (δDH2O ≈ 10‰) feeds the hydrothermal system. It infiltrates at depth throughout the local rocks, reaching oxygen isotope equilibrium at high temperatures. Interaction processes between magmatic gases and the evolving seawater also seem to occur, causing the dissolution of isotopically fractionated aqueous CO2 and providing the source for hydrothermal carbon. These results have important implications concerning fluid circulation beneath Vulcano and address the more convenient routine of geochemical surveillance.  相似文献   

9.
The Rb-Sr isochron age of igneous ankerite-calcite and siderite carbonatites in central Tuva is estimated at 118 ± 9 Ma. The following ranges of initial values of O, C, Sr, and sulfide and S isotopic compositions were established: δ18Ocarb = +(8.8?14.7)‰, δ13Ccarb = ?(3.6?4.9)‰, δ18Oquartz = +(11.6?13.7)‰, δ34Spyrite = +(0.3?1.1)‰, and (87Sr/86Sr)i =0.7042?0.7048 for ankerite-calcite carbonatite and δ18Osid = +(9.2?12.4)‰, δ13Csid = ?(3.9?5.9)‰, δ18Oquartz = +(11.2?11.4)‰, δ34Spyrite = ?(4.4–1.8)‰, δ34Ssulfate = +(8.6?14.5)‰, and (87Sr/86Sr)i = 0.7042?0.7045 for siderite carbonatite. The obtained isotopic characteristics indicate that both varieties of carbonatites are cognate and their mantle source is comparable with the sources of Late Mesozoic carbonatites in the western Transbaikal region and Mongolia. The revealed heterogeneity of isotopic compositions of carbonatites is caused by their contamination with country rocks, replacement with hydrothermal celestine, and supergene alteration.  相似文献   

10.
In the eastern, external part of the Grenvillian Belt in SW Sweden, five formations of granitic rocks were found in the basement of the Dalslandian Supracrustal Group. The granitic rocks have been strongly recrystallized but have preserved most of their granitic texture in the process. Most magmatic crystals have been pseudomorphed by metamorphic minerals: quartz, albite, chlorite, biotite, white mica, epidote, titanite, hematite, pyrite and carbonate. Two of the formations have subsequently been affected by a cataclastic deformation and at present consist of mylogneisses. 18O whole-rock values for the granitic rocks vary from +3.0 to +11.1. Quartz-apatite, quartz-zircon and quartz-titanite pairs show 18O/16O fractionations corresponding to equilibrium temperatures of 550–700° C, which are believed to reflect in the main continued closed-system isotopic exchange at high temperatures following solidification. In contrast highly positive 18O/16O fractionations for quartz-K-feldspar, quartz-biotite, quartz-chlorite and quartz-sericite pairs in some granitic samples indicate that these rocks have exchanged oxygen with heated, meteoric, H2O dominated fluids. Other granitic samples, however, show virtually undisturbed magmatic 18O/16O fractionation values for the same mineral pairs, even though these rocks are equally altered.It is concluded that all granitic rock units recrystallized under greenschist facies conditions during the infiltration of fluids under the influence of hydrothermal convection systems set up by the intrusion of the granitic plutons. The fluids probably had a range of 18O values from ca. -14 to ca. +10, indicating the mixing of distinct fluid reservoirs, one of meteoric origin and the others of magmatic and/or metamorphic origin. The temperature of alteration is estimated at 450–500° C.Estimation of pre-alteration 18O whole rock values for the five granitic units suggests that three units should be assigned a dominantly S-type origin, where as the other two units may partly or wholly have an I-type origin.  相似文献   

11.
The role of fluids during Archaean intra-crustal magmatism has been investigated via integrated SHRIMP U–Pb, δ18O and LA-MC-ICPMS 176Hf isotopic zircon analysis. Six rock samples studied are all from the Nuuk region (southern West Greenland) including two ~3.69 Ga granitic and trondhjemitic gneisses, a 3.64 Ga granitic augen gneiss, a 2.82 Ga granodioritic Ikkattoq gneiss, a migmatite with late Neoarchaean neosome and a homogeneous granite of the 2.56 Ga Qôrqut Granite Complex (QGC). All zircon grains were thoroughly imaged to facilitate analysis of magmatic growth domains. Within the zircon analysed, there is no evidence for metamictization. Initial εHf zircon values (n = 63) are largely sub-chondritic, indicating the granitic host magmas were generated by the remelting of older, un-radiogenic crustal components. Zircon from some granite samples displays more than one 207Pb/206Pb age, and correlated with 176Hf/177Hf compositions can trace multiple phases of remelting or recrystallization during the Archaean. Model ages calculated using Lu/Hf arrays for each sample indicate that the crustal parental rocks to the granites, granodiorites and trondhjemites segregated from a chondrite-like reservoir at an earlier time during the Archaean, corresponding to known formation periods of more primitive tonalite–trondhjemite–granodiorite (TTG) gneisses. Zircon from the ~3.69 Ga granite, the migmatite and QGC granite contains Eoarchaean cores with chondritic 176Hf/177Hf and mantle-like δ18O compositions. The age and geochemical signatures from these inherited components are identical to those of surrounding tonalitic gneisses, further suggesting genesis of these granites by remelting of broadly tonalitic protoliths. Zircon oxygen isotopic compositions (n = 62) over nine age populations (six igneous and three inherited) have weighted mean or mean δ18O values ranging from 5.8 ± 0.6 to 3.7 ± 0.5‰. The 3.64 Ga granitic augen gneiss sample displays the highest δ18O with a mildly supra-mantle composition of 5.8 ± 0.6‰. Inherited Eoarchaean TTG-derived zircon shows mantle-like values. Igneous zircon from all other samples, spanning more than a billion years of Archaean time, record low δ18O sub-mantle compositions. These are the first low δ18O signatures reported from Archaean zircon and represent low δ18O magmas formed by the remelting and metamorphism of older crustal rocks following high-temperature hydrothermal alteration by meteoric water. Meteoric fluid ingress coupled with crustal extension, associated high heat flow and intra-crustal melting are a viable mechanism for the production of the low δ18O granites, granodiorites and trondhjemites reported here. Both high and low δ18O magmas may have been generated in extensional environments and are distinct in composition from Phanerozoic I-type granitic plutonic systems, which are typified by increasing δ18O during intra-crustal reworking. This suggests that Archaean magmatic processes studied here were subtly different from those operating on the modern Earth and involved extensional tectonic regimes and the predominance of remelting of hydrothermally altered crystalline basement.  相似文献   

12.
Evaluation of the extent of volatile element recycling in convergent margin volcanism requires delineating likely source(s) of magmatic volatiles through stable isotopic characterization of sulfur, hydrogen and oxygen in erupted tephra with appropriate assessment of modification by degassing. The climactic eruption of Mt. Mazama ejected approximately 50 km3 of rhyodacitic magma into the atmosphere and resulted in formation of a 10-km diameter caldera now occupied by Crater Lake, Oregon (lat. 43°N, long. 122°W). Isotopic compositions of whole-rocks, matrix glasses and minerals from Mt. Mazama climactic, pre-climactic and postcaldera tephra were determined to identify the likely source(s) of H2O and S. Integration of stable isotopic data with petrologic data from melt inclusions has allowed for estimation of pre-eruptive dissolved volatile concentrations and placed constraints on the extent, conditions and style of degassing.Sulfur isotope analyses of climactic rhyodacitic whole rocks yield δ34S values of 2.8-14.8‰ with corresponding matrix glass values of 2.4-13.2‰. δ34S tends to increase with stratigraphic height through climactic eruptive units, consistent with open-system degassing. Dissolved sulfur concentrations in melt inclusions (MIs) from pre-climactic and climactic rhyodacitic pumices varies from 80 to 330 ppm, with highest concentrations in inclusions with 4.8-5.2 wt% H2O (by FTIR). Up to 50% of the initial S may have been lost through pre-eruptive degassing at depths of 4-5 km. Ion microprobe analyses of pyrrhotite in climactic rhyodacitic tephra and andesitic scoria indicate a range in δ34S from −0.4‰ to 5.8‰ and from −0.1‰ to 3.5‰, respectively. Initial δ34S values of rhyodacitic and andesitic magmas were likely near the mantle value of 0‰. Hydrogen isotope (δD) and total H2O analyses of rhyodacitic obsidian (and vitrophyre) from the climactic fall deposit yielded values οf −103 to −53‰ and 0.23-1.74 wt%, respectively. Values of δD and wt% H2O of obsidian decrease towards the top of the fall deposit. Samples with depleted δD, and mantle δ18O values, have elevated δ34S values consistent with open-system degassing. These results imply that more mantle-derived sulfur is degassed to the Earth’s atmosphere/hydrosphere through convergent margin volcanism than previously attributed. Magmatic degassing can modify initial isotopic compositions of sulfur by >14‰ (to δ34S values of 14‰ or more here) and hydrogen isotopic compositions by 90‰ (to δD values of −127‰ in this case).  相似文献   

13.
The Cangyuan Pb-Zn-Ag polymetallic deposit is located in the Baoshan Block, southern Sanjiang Orogen. The orebodies are hosted in low-grade metamorphic rocks and skarn in contact with Cenozoic granitic rocks. Studies on fluid inclusions (FIs) of the deposit indicate that the ore-forming fluids are CO2-bearing, NaCl-H2O. The initial fluids evolved from high temperatures (462–498 °C) and high salinities (54.5–58.4 wt% NaCl equiv) during the skarn stage into mesothermal (260–397 °C) and low salinities (1.2–9.5 wt% NaCl equiv) during the sulfide stage. The oxygen and hydrogen isotopic compositions (δ18OH2O: 2.7–8.8‰; δD: −82 to −120‰) suggest that the ore-forming fluids are mixture of magmatic fluids and meteoric water. Sulfur isotopic compositions of the sulfides yield δ34S values of −2.3 to 3.2‰; lead isotopic compositions of ore sulfides are similar to those of granitic rocks, indicating that the sulfur and ore-metals are derived from the granitic magma. We propose that the Cangyuan Pb-Zn-Ag deposit formed from magmatic hydrothermal fluids. These Cenozoic deposits situated in the west of Lanping-Changdu Basin share many similarities with the Cangyuan in isotopic compositions, including the Laochang, Lanuoma and Jinman deposits. This reveals that the Cenozoic granites could have contributed to Pb-Zn-Cu mineralization in the Sanjiang region despite the abundance of Cenozoic Pb-Zn deposits in the region, such as the Jingding Pb-Zn deposit, that is thought to be of basin brine origin.  相似文献   

14.
A combined in situ SIMS and LA-(MC)-ICPMS study of U-Pb ages, trace elements, O and Lu-Hf isotopes was conducted for zircon from eclogite-facies metamorphic rocks in the Sulu orogen. The two microbeam techniques sampled various depths of zircon domains, revealing different element and isotope relationships between residual magmatic cores and new metamorphic rims and thus the geochemical architecture of metamorphic zircons which otherwise cannot be recognized by the single microbeam technique. This enables discrimination of metamorphic growth from different subtypes of metamorphic recrystallization. Magmatic cores with U-Pb ages of 769 ± 9 Ma have positive δ18O values of 0.1-10.1‰, high Th/U and 176Lu/177Hf ratios, high REE contents, and steep MREE-HREE patterns with negative Eu anomalies. They are interpreted as crystallizing from positive δ18O magmas during protolith emplacement. In contrast, newly grown domains have concordant U-Pb ages of 204 ± 4 to 252 ± 7 Ma and show negative δ18O values of −10.0‰ to −2.2‰, low Th/U and 176Lu/177Hf ratios, low REE contents, and flat HREE patterns with weak to no Eu anomalies. They are interpreted as growing from negative δ18O fluids that were produced by metamorphic dehydration of high-T glacial-hydrothermally altered rocks during continental subduction-zone metamorphism. Differences in δ18O between different domains within single grains vary from 0.8‰ to 12.5‰, suggesting different degrees of O isotope exchange between the positive δ18O magmatic core and the negative δ18O metamorphic fluid during the metamorphism. The magmatic zircons underwent three subtypes of metamorphic recrystallization, depending on their accessibility to negative δ18O fluids. The zircons recrystallized in solid-state maintained positive δ18O values, and REE and Lu-Hf isotopes of protolith zircon, but their U-Pb ages are lowered. The zircons recrystallized through dissolution exhibit negative δ18O values similar to the metamorphic growths, almost completely reset U-Pb ages, and partially reset REE systems. The zircons recrystallized through replacement show variably negative δ18O values, and partially reset REE, and U-Pb and Lu-Hf isotopic systems. Therefore, this study places robust constraints on the origin of metamorphic zircons in eclogite-facies rocks and provides a methodological framework for linking the different types of metamorphic zircons to petrological processes during continental collision.  相似文献   

15.
Abstract: Sulfur isotope data (δ34S) of sulfides of more than 6700 samples from 157 ore deposits associated with Early and Late Yanshanian granitic and volcanic activities in South China are reviewed and summarized. Averaged δ34S values of individual deposits vary from ‐9. 3 to +20. 6%, and show a normal distribution pattern with the average of +2%. About 88 % of the ore deposits have values within the range, ?2.5 ? +13.6‰, of associated Yanshanian granitoids. There is a temporal‐spatial variation of δ34S values of the ore deposits. However, no clear zonal distribution parallel to geotectonic NNE lineaments was observed. Spatial distribution of ore sulfide δ34S values in most of the NE part of the whole studied area coincides with that of Yanshanian granitoids and volcanic rocks. A downward tendency of the average values in time is: +3. 0% (n=7, J1) → +1. 6% (n=29, J2) → +1. 7% (n=68, J3) → +1. 8% (n=37, K1) → ?1. 5% (n=16, K2). There is an “island” of high and variable δ34S values (0? +16.5‰) occurring within a generally low trough zone (?8 ? 0%) of N‐S about 800 km and E‐W 100 to 300 km, bounded by 110°E ? 116°E longitudes and 22°N ? 31°N latitudes. The island occurs at the junction of three tectonic units and a NE‐trending crustal matching line implying a variety of magmatism occurred at the junction. The low trough zone coincides with a low ferric/ferrous ratio zone of Early Yanshanian granitoids, indicating their genetic relationship. Different genetic types of ore deposits show different histogram patterns suggesting different relationships to magmatic rocks and host strata. Granite/greisen/pegmatite type deposits are most closely associated with granitoids, with average ore sul‐fide δ34S values for individual ore deposits ranging between ‐2. 0 and +4. 1%, and an average of +0. 5% (n = 15) close to type meteoric value of 0%. Porphyry‐type deposits have also narrow range of ?2.2 ? + 4.9‰, with an average value of +1. 1% (n = 18). Skarn‐type dominated ore deposits have a nearly normal distribution pattern with an average of +1. 6% (n = 62), ranging from ‐5. 3 to +11. 5%. Volcano‐subvolcanic ore deposits range between ‐3. 1 and +5. 9% with an average of +2. 3% (n = 19). Other types of hydrothermal ore deposits have averaged δ34S values of individual ones from ‐9. 3 to +20. 6%, with average value of +1. 3% (n=43). Vertical and horizontal zonations of δ34S values of ore deposits around their associated granitoid plutons are observed in several localities. Such zonations may be caused by interaction between magma and/or magmatic fluids and host sedimentary rocks, as well as the evolution of physico‐chemical conditions of ore‐forming fluids. Spatial distribution of ore sulfur isotope compositions is also clearly controlled by tectonics and deep faults. Ore sulfur isotope composition is sometimes strongly affected by host sedimentary rocks, especially by evaporite sulfur with much higher δ34S value and partly by biogenic sulfur with low δ34S value. The δ34S values of Yanshanian granitoids are from ‐2. 5 to +13. 6% for both rock samples and pyrite/pyrrhotite separates from granitic rocks, with similar spatial distribution pattern to those of associated ore deposits. The ore deposits associated with ilmenite‐series granitoids have δ34S values ranging between ‐7. 5 and +10. 4% with an average of +1. 0%, while the ore deposits associated with magnetite‐series granitoids ranging between ?8.0 ? +11.5‰ with an average of +1. 1%. δ34S values of ore deposits tend to converge to +3% as the Fe2O3/FeO ratio of associated granitoids increases from 0. 45 to 8. 7.  相似文献   

16.
Zircons from metamorphosed granites exposed near Qinglongshan have δ18OVSMOW values of −7 to 0‰ in both grain rims and cores. The concordant 238U/206Pb ages of zircon cores are 684 to 754 Ma with rims at 221 Ma. Discordant 238U/206Pb ages range from 242 to 632 Ma. Results demonstrate a Neoproterozoic age for the origin of the Qinglongshan oxygen and hydrogen isotope anomaly. The low δ18O values were imprinted on the rocks by a hydrothermal system charged with meteoric water from a cold climate. Groundwater circulation was driven by heat from cooling granitic magma. The geologic age of the hydrothermal system correlates with that of the Nantuo tillite in the Sinian strata of the South China block, suggesting that Qinglongshan’s cold climate may be a manifestation of Neoproterozoic “snowball Earth.”  相似文献   

17.
Tourmalinization associated with peraluminous granitic intrusions in metapelitic host-rocks has been widely recorded in the Iberian Peninsula, given the importance of tourmaline as a tracer of granite magma evolution and potential indicator of Sn-W mineralizations. In the Penamacor-Monsanto granite pluton (Central Eastern Portugal, Central Iberian Zone), tourmaline occurs: (1) as accessory phase in two-mica granitic rocks, muscovite-granites and aplites, (2) in quartz (±mica)-tourmaline rocks (tourmalinites) in several exocontact locations, and (3) as a rare detrital phase in contact zone hornfels and metapelitic host-rocks. Electron microprobe and stable isotope (δ18O, δD, δ11B) data provide clear distinctions between tourmaline populations from these different settings: (a) schorl–oxyschorl tourmalines from granitic rocks have variable foititic component (X□ = 17–57 %) and Mg/(Mg + Fe) ratios (0.19–0.50 in two-mica granitic rocks, and 0.05–0.19 in the more differentiated muscovite-granite and aplites); granitic tourmalines have constant δ18O values (12.1 ± 0.1 ‰), with wider-ranging δD (?78.2 ± 4.7 ‰) and δ11B (?10.7 to ?9.0 ‰) values; (b) vein/breccia oxyschorl [Mg/(Mg + Fe) = 0.31–0.44] results from late, B- and Fe-enriched magma-derived fluids and is characterized by δ18O = 12.4 ‰, δD = ?29.5 ‰, and δ11B = ?9.3 ‰, while replacement tourmalines have more dravitic compositions [Mg/(Mg + Fe) = 0.26–0.64], close to that of detrital tourmaline in the surrounding metapelitic rocks, and yield relatively constant δ18O values (13.1–13.3 ‰), though wider-ranging δD (?58.5 to ?36.5 ‰) and δ11B (?10.2 to ?8.8 ‰) values; and (c) detrital tourmaline in contact rocks and regional host metasediments is mainly dravite [Mg/(Mg + Fe) = 0.35–0.78] and oxydravite [Mg/(Mg + Fe) = 0.51–0.58], respectively. Boron contents of the granitic rocks are low (<650 ppm) compared to the minimum B contents normally required for tourmaline saturation in granitic melts, implying loss of B and other volatiles to the surrounding host-rocks during the late-magmatic stages. This process was responsible for tourmalinization at the exocontact of the Penamacor-Monsanto pluton, either as direct tourmaline precipitation in cavities and fractures crossing the pluton margin (vein/breccia tourmalinites), or as replacement of mafic minerals (chlorite or biotite) in the host-rocks (replacement tourmalinites) along the exocontact of the granite. Thermometry based on 18O equilibrium fractionation between tourmaline and fluid indicates that a late, B-enriched magmatic aqueous fluid (av. δ18O ~12.1 ‰, at ~600 °C) precipitated the vein/breccia tourmaline (δ18O ~12.4 ‰) at ~500–550 °C, and later interacted with the cooler surrounding host-rocks to produce tourmaline at lower temperatures (400–450 °C), and an average δ18O ~13.2 ‰, closer to the values for the host-rock. Although B-metasomatism associated with some granitic plutons in the Iberian Peninsula seems to be relatively confined in space, extending integrated studies such as this to a larger number of granitic plutons may afford us a better understanding of Variscan magmatism and related mineralizations.  相似文献   

18.
In this paper we consider the mechanisms by which the mineralogy and composition of the margins of the Notch Peak granitic stock, Utah, were affected by calcareous argillite country rocks. The contact zone of the granite relative to the unaffected granite is enriched in: K2O from about 4 to 10 wt.%, Rb from 250 to 510 ppm, Sr from 150 to 790 ppm and Ba from 250 to 2160 ppm. Locally, some of the intrusive rocks at the contact are nearly devoid of quartz and can be classified as syenites. The initial 87Sr/86Sr ratios range from 0.7069 in the unaffected rocks to 0.7100 in the potassium-enriched samples, approaching the values of the calc-silicate country rocks of about 0.7110.Calculations show that the characteristics of the contact zone near the top of the stock are the result of a number of sequential processes. CO2-rich fluids derived from the heated calcareous argillites apparently caused a shift in the phase boundaries in the magma, enhancing accumulation of K-feldspar and high-Ca augite at the expense of other phases. The accumulation resulted in the high Ba and Sr concentrations in some samples. However, the high K2O and Rb concentrations and magmatic 18O values indicate infiltration of magmatic fluid emanating from the solidifying lower portions of the pluton subsequent to solidification of the cap. The minimum fluid-rock ratios of 4.6 by mass, calculated on the basis of K2O and Rb concentration shifts, indicate that a substantial amount of the fluid was channeled through this contact zone. The desilication of the rocks forming the syenitic samples at the contact apparently occurred when a chemical potential gradient of silica between the granite and wall-rocks was established as quartz was consumed in the wall-rocks during calc-silicate reactions. The infiltrating magmatic fluid probably acted as a medium for transport of silica across the contact and perhaps exchange of Sr between the country rocks and the intrusion where up to 30% of strontium in the granitic and syenitic samples from the contact zone was derived from the calc-silicates. The syenitic rocks cannot be explained by desilication reactions involving assimilation of the calc-silicates by the granite magma. The results of this study show that fluids interacting with the country rocks need to be considered to explain the effects of country rocks on the composition of the margins of granitic intrusions.  相似文献   

19.
Well-preserved aragonitic land snail shells (Vallonia) from late Pleistocene Eolian sediment in the Folsom archaeological site in New Mexico exhibit an overall decrease of δ18OPDB from maximum values of +2.7‰ (more positive than modern) to younger samples with lower average values of about −3.6‰ (within the modern range). The age of the samples (approximately 10,500 14C yr B.P.) suggests that the decrease in δ18O may manifest climatic changes associated with the Younger Dryas. Some combination of increased relative humidity and cooler temperatures with decreased δ18O of precipitation during the times of snail activity can explain the decrease in shell δ18O. A well-known Paleoindian bison kill occurred at the Folsom site during this inferred environmental transition.Average δ13C values of the aragonite shells of the fossil Vallonia range from −7.3 to −6.0‰ among different archaeological levels and are not as negative as modern values. This suggests that the proportion of C4 vegetation at the Folsom site approximately 10,500 14C yr B.P. was greater than at present; a result which is consistent with other evidence for higher proportions of C4 plants in the region at that time.  相似文献   

20.
Oxygen isotope analysis and U-Pb dating were carried out on zircons from granite, granitic gneiss and eclogite in the Dabie-Sulu orogenic belt of east-central China. The results show a wide variation in zircon δ18O values from −10.9 to 8.5‰. Most of the values are lower than normal mantle zircon and almost half have prominently negative values. The low δ18O zircons have protolith ages of 700 to 800 Ma and metamorphic ages of 205 to 250 Ma, respectively. Fluid availability within the metaigneous rocks dictates the extent of metamorphic recrystallization and overgrowth. The igneous zircons have preserved their magmatic zoning and middle Neoproterozoic U-Pb age during the Triassic metamorphism, indicating low fluid availability. Widespread low δ18O values are identified in the magmatic zircons of middle Neoproterozoic age, within an outcrop area of over 20,000 km2 along the northern edge of the South China Block. The low δ18O zircons record the presence of large volumes of low δ18O igneous rocks that were derived from remelting of meteoric-hydrothermally altered rocks at some time between 700 and 800 Ma.The U-Pb ages for metaigneous protoliths and granites are correlated not only with the timing of rifting accompanying the breakup of Rodinia, but also with contemporary glacial deposits in the South China Block at paleolatitudes of 30 to 40°N. Melting of glacial ice and snow is suggested, instead of the direct involvement of meteoric water, to produce the low δ18O fluid with oxygen isotopic signatures like the cold-climate meteoric water. The rifting created conditions favorable to anatexis of meteoric-hydrothermally altered rocks. Glaciated regions supplied copious water for the water-rock interaction during magma emplacement along rifting zones. Both rifting and glaciation favored the generation of the low δ18O magmas in the region. The low δ18O zircons are thus interpreted to have crystallized from the low δ18O magmas of middle Neoproterozoic age, and onset of the generation of low δ18O magmas in the northern margin of the South China Block is estimated to occur at about 758 ± 15 Ma. The large-scale remelting of hydrothermally altered crust not only results from repetitive emplacement of mafic magmas along the rifting zones with protracted episodes of water-rock interaction, but also involves rift systems that rapidly introduce large volumes of fluid through confined pathways and traps in a short space of time in response to tectonic triggers. Occurrence of the large-scale 18O depletion during the middle Neoproterozoic may be a manifestation of the cold paleoclimate related to the snowball Earth event.  相似文献   

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