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1.
In past decades, the formation of dolomite at low temperature has been widely studied in both natural systems and cultured experiments, yet the mechanism(s) involved in the nucleation and precipitation of dolomite remains unresolved. Late Eocene dolomitic deposits from core in the upper Niubao Formation (Lunpola Basin, central Tibetan Plateau, China) are selected as a case study to understand the dolomitization process(es) in the geological record. Dolomite formation in Lunpola Basin can be ascribed to a different mechanism forming the large quantities of replacive dolostones in the geological record; and provides a potential fossil analogue for primary dolomite precipitation at low temperature. This analogue consists of an alternation of laminated dolomitic beds, organic-rich and siliciclastic layers; formed in response to intense evaporation interpreted to take place in a continental shallow lake environment. Mineralogical, textural and stable isotopic evaluations suggest that the dolomite from those dense-clotted laminated beds is a primary precipitate. At the nanoscale, these dolomitic beds are composed of Ca–Mg carbonate globular nanocrystals (diameter 80 to 100 nm) embedded in an organic matrix and attached to clay flakes. Micro-infrared spectroscopy analyses have revealed the presence of aliphatic compounds in the organic matrix. Microscopic and elemental compositional studies suggest that clay surfaces may facilitate the nucleation of dolomite at low temperature in the same way as the organic matrix does. The dolomite laminae show values for δ18OVPDB from −3.2 to −1.76‰ and for δ13CVPDB from −2.62 to −3.78‰. Inferred δ18OSMOW values of the lake water reveal typical evaporitic hydrological conditions. These findings provide a potential link to primary dolomite formation in ancient and modern sedimentary environments; and shed new light on the palaeoenvironmental conditions in central Tibet during the Eocene.  相似文献   

2.
塔里木盆地寒武-奥陶系白云岩储层类型与分布特征   总被引：6，自引：3，他引：3  
塔里木盆地寒武-奥陶系白云岩是台盆区最重要的储层之一,发育4种类型:①潮坪白云岩。以含膏泥晶白云岩为主,石膏溶孔及白云岩砾间孔发育,发育于潮间-潮上坪蒸发环境。白云石表现为MgO-CaO呈线性正相关、低Mg/Ca值及高∑REE值、锶同位素值分布在0.7085～0.7100之间,略高于同期海水值0.7090、阴极发光不发光或暗色光。储层分布主要受沉积相控制,发育于中下寒武统地层;②蒸发台地白云岩。以藻丘及颗粒灰岩选择性白云石化为特征,发育铸模孔、膏溶孔和残留粒间孔,白云石Mg/Ca值变化范围大、δ13C、δ18O值相对偏正、分别大于2‰和-4‰、阴极发光发较亮红光。储层主要发育于台内靠近台缘一侧;③埋藏白云岩。发育细晶、中晶及粗晶白云岩,以晶间孔及晶间溶孔为主,δ18O值偏负在-5‰～-10‰(PDB)之间,87Sr/86Sr值相对较大,为0.7090～0.7110,阴极发光以发暗棕褐色、紫色光为主。埋藏白云岩储层发育主要受成岩相控制,但也表现出与沉积相具有相关性,这是因为物性好的台缘、台内礁滩体及有裂缝沟通构成的开放体系更有利于埋藏白云石化作用发生;④热液白云岩。以受热液改造的结晶白云岩为特征,往往伴生热液矿物,白云石δ18O值异常偏负、一般小于-9‰(PDB)、阴极发光多发明亮红光、稀土元素标准化配分曲线中Eu出现正异常、出现高于地层背景值的异常高温包裹体;主要发育在具有上覆隔挡层的不整合面之下地层及大断裂发育带附近。上述四类白云岩在规模与分布上有不同,但都可预测。埋藏和热液白云岩规模较大,受原始沉积相带和成岩流体来源双重约束。潮坪和蒸发台地白云岩规模可变性较大,可由沉积环境重建,结合成岩相研究预测评价。  相似文献   

3.
Magnesite forms a series of 1‐ to 15‐m‐thick beds within the ≈2·0 Ga (Palaeoproterozoic) Tulomozerskaya Formation, NW Fennoscandian Shield, Russia. Drillcore material together with natural exposures reveal that the 680‐m‐thick formation is composed of a stromatolite–dolomite–‘red bed’ sequence formed in a complex combination of shallow‐marine and non‐marine, evaporitic environments. Dolomite‐collapse breccia, stromatolitic and micritic dolostones and sparry allochemical dolostones are the principal rocks hosting the magnesite beds. All dolomite lithologies are marked by δ13C values from +7·1‰ to +11·6‰ (V‐PDB) and δ18O ranging from 17·4‰ to 26·3‰ (V‐SMOW). Magnesite occurs in different forms: finely laminated micritic; stromatolitic magnesite; and structureless micritic, crystalline and coarsely crystalline magnesite. All varieties exhibit anomalously high δ13C values ranging from +9·0‰ to +11·6‰ and δ18O values of 20·0–25·7‰. Laminated and structureless micritic magnesite forms as a secondary phase replacing dolomite during early diagenesis, and replaced dolomite before the major phase of burial. Crystalline and coarsely crystalline magnesite replacing micritic magnesite formed late in the diagenetic/metamorphic history. Magnesite apparently precipitated from sea water‐derived brine, diluted by meteoric fluids. Magnesitization was accomplished under evaporitic conditions (sabkha to playa lake environment) proposed to be similar to the Coorong or Lake Walyungup coastal playa magnesite. Magnesite and host dolostones formed in evaporative and partly restricted environments; consequently, extremely high δ13C values reflect a combined contribution from both global and local carbon reservoirs. A 13C‐rich global carbon reservoir (δ13C at around +5‰) is related to the perturbation of the carbon cycle at 2·0 Ga, whereas the local enhancement in 13C (up to +12‰) is associated with evaporative and restricted environments with high bioproductivity.  相似文献   

4.
Late Cambrian to Early Ordovician sedimentary rocks in the western Tarim Basin, Northwest China, are composed of shallow-marine platform carbonates. The Keping Uplift is located in the northwest region of this basin. On the basis of petrographic and geochemical features, four matrix replacement dolomites and one type of cement dolomite are identified. Matrix replacement dolomites include (1) micritic dolomites (MD1); (2) fine–coarse euhedral floating dolomites (MD2); (3) fine–coarse euhedral dolomites (MD3); and (4) medium–very coarse anhedral mosaic dolomites (MD4). Dolomite cement occurs in minor amounts as coarse saddle dolomite cement (CD1) that mostly fills vugs and fractures in the matrix dolomites. These matrix dolomites have δ18O values of ?9.7‰ to ?3.0‰ VPDB (Vienna Pee Dee Belemnite); δ13C values of ?0.8‰ to 3.5‰ VPDB; 87Sr/86Sr ratios of 0.708516 to 0.709643; Sr concentrations of 50 to 257 ppm; Fe contents of 425 to 16878 ppm; and Mn contents of 28 to 144 ppm. Petrographic and geochemical data suggest that the matrix replacement dolomites were likely formed by normal and evaporative seawater in early stages prior to chemical compaction at shallow burial depths. Compared with matrix dolomites, dolomite cement yields lower δ18O values (?12.9‰ to ?9.1‰ VPDB); slightly lower δ13C values (?1.6‰–0.6‰ VPDB); higher 87Sr/86Sr ratios (0.709165–0.709764); and high homogenization temperature (Th) values (98°C–225°C) and salinities (6 wt%–24 wt% NaCl equivalent). Limited data from dolomite cement shows a low Sr concentration (58.6 ppm) and high Fe and Mn contents (1233 and 1250 ppm, respectively). These data imply that the dolomite cement precipitated from higher temperature hydrothermal salinity fluids. These fluids could be related to widespread igneous activities in the Tarim Basin occurring during Permian time when the host dolostones were deeply buried. Faults likely acted as important conduits that channeled dolomitizing fluids from the underlying strata into the basal carbonates, leading to intense dolomitization. Therefore, dolomitization, in the Keping Uplift area is likely related to evaporated seawater via seepage reflux in addition to burial processes and hydrothermal fluids.  相似文献   

5.
Post-collisional K-rich volcanic rocks (KVRs) can provide an opportunity to constrain the architecture of the lithosphere and the mechanisms of plateau uplift. However, their petrogenesis and geodynamic setting remain in dispute. Lithium concentrations and isotopic compositions of 87 potassic, ultrapotassic and Mg-rich potassic volcanic rocks (PVRs, UPVs, and MPRs, respectively) in SW Tibet, along with new Pb–Sr–Nd isotope data and whole-rock analyses, are used to constrain their mantle source and genesis. These rocks are characterized by very similar δ7Li values: PVRs vary from −4.9‰ to +3.2‰, UPVs from −3.9‰ to +1.7‰, and MPRs from −1.2‰ to +3.5‰. They can be classified into two groups: Group I (19 out of 87 samples) with heavier δ7Li values (+1.0‰ to +3.5‰) similar to those reported for mid-ocean-ridge and ocean-island basalts (MORBs and OIBs, respectively), and Group II (68 out of 87 samples) with lighter values (−4.9‰ to +1.0‰) similar to those of Indian lower crust. These variable isotopic compositions may record the isotopic signature of the early-middle Miocene subcontinental lithospheric mantle (SCLM). This paper demonstrates the existence of isotopically light mantle domains beneath the Lhasa terrane, which were ascribed to the interaction with fluids/melts derived from the subducted Indian lower crust. The modeling curves of Indian lower crust with a metasomatized mantle composition fully account for compositional variations in the PVRs, UPVs, and MPRs. They were generated by the partial melting of SCLM, which was metasomatized by fluids/melts derived from the subducted Indian lower crust (ca. 4–14%, ca. 4–10%, and ca. 6–10% for the PVRs, UPVs, and MPRs, respectively). The Li isotopic data indicate that the Indian lower crust was subducted beneath the central Lhasa subterrane, and this sheds new light on the formation of the Tibet Plateau.  相似文献   

6.
Zhaxikang is one large Sb–Pb–Zn–Ag deposit located in the North Himalaya of southern Tibet. To date, the genesis of this deposit still remains controversial. Here, we present new pyrite Fe and sphalerite Zn isotopic data for the first three stages of mineralization, Fe–Zn isotopic data for Mn–Fe carbonate that formed during the first two stages of mineralization, and Zn isotopic data for the slate wall rocks of the Jurassic Ridang Formation to discuss the genesis of the Zhaxikang deposit. The overall δ56Fe and δ66Zn values range from −0.80‰ to 0.43‰ and from −0.03‰ to 0.38‰, respectively. The δ56Fe values of Mn–Fe carbonates are lighter than those of associated pyrite in six mineral pairs, indicating that the iron carbonates are preferentially enriched in light Fe isotopes relative to pyrite. The sphalerite has lighter δ66Zn values than associated Mn–Fe carbonates in three mineral pairs.The δ56Fe values of pyrite that formed during the first three stages of mineralization gradually increase from stage 1 (−0.33‰ to −0.09‰) through stage 2 (−0.30‰ to 0.19‰) to stage 3 (0.16‰–0.43‰). In comparison, the sphalerite that formed during these stages has δ66Zn values that gradually decrease from stage 1 (0.16‰–0.35‰) through stage 2 (0.09‰–0.23‰) to stage 3 (−0.03‰ to 0.22‰). These data, in conjunction with the observations of hand specimens and thin sections, suggest that the deposit was overprinted by a second pulse of mineralization. This overprint would account for these Fe–Zn isotopic variations as well as the kinetic Rayleigh fractionation that occurred during mineralization. The temporally increasing δ56Fe and decreasing δ66Zn values recorded in the deposit are also coincident with an increase in alteration, again supporting the existence of two pulses of mineralization. The δ56Fe values of the first pulse of ore-forming fluid were calculated using theoretical equations, yielding values of −0.54‰ to −0.34‰ that overlap with those of submarine hydrothermal solutions (−1‰ to 0‰). However, the δ56Fe values of the stage 3 pyrite are heavier than those of typical submarine hydrothermal solutions, which suggests that the second pulse of mineralization was probably derived from a magmatic hydrothermal fluid. In addition, the second pulse of ore-forming fluid has brought some Fe and taken away parts of Zn, which results the lighter δ66Zn values of sphalerite and heavier δ56Fe values of pyrite from the second pulse of mineralization. Overall, the Zhaxikang deposit records two pulses of mineralization, and the overprint by the second pulse of mineralization causes the lighter δ66Zn values and heavier δ56Fe values of modified samples.  相似文献   

7.
The study focuses on the formation of lacustrine dolomite in late Miocene lakes, located at the East Mediterranean margins (Northern Israel). These lakes deposited the sediments of the Bira (Tortonian) and Gesher (Messinian) formations that comprise sequences of dolostone and limestone. Dolostones are bedded, consist of small‐sized (<7 μm), Ca‐rich (52 to 56 mol %) crystals with relatively low ordering degrees, and present evidence for replacement of CaCO3 components. Limestones are comprised of a wackestone to mudstone matrix, freshwater macrofossils and intraclasts (mainly in the Bira Formation). Sodium concentrations and isotope compositions differ between limestones and dolostones: Na = ~100 to 150 ppm; ~1000 to 2000 ppm; δ18O = ?3·8 to ?1·6‰; ?2·0 to +4·3‰; δ13C = ?9·0 to ?3·4‰; ?7·8 to 0‰ (VPDB), respectively. These results indicate a climate‐related sedimentation during the Tortonian and early Messinian. Wet conditions and positive freshwater inflow into the carbonate lake led to calcite precipitation due to intense phytoplankton blooms (limestone formation). Dry conditions and enhanced evaporation led to precipitation of evaporitic CaCO3 in a terminal lake, which caused an increased Mg/Ca ratio in the residual waters and penecontemporaneous dolomitization (dolostone formation). The alternating lithofacies pattern reveals eleven short‐term wet–dry climate‐cycles during the Tortonian and early Messinian. A shift in the environmental conditions under which dolomite formed is indicated by a temporal decrease in δ18O of dolostones and Na content of dolomite crystals. These variations point to decreasing evaporation degrees and/or an increased mixing with meteoric waters towards the late Messinian. A temporal decrease in δ13C of dolostones and limestones and appearance of microbial structures in close association with dolomite suggest that microbial activity had an important role in allowing dolomite formation during the Messinian. Microbial mediation was apparently the main process that enabled local growth of dolomite under wet conditions during the latest Messinian.  相似文献   

8.
通过对塔里木盆地中、北部地区寒武系—奥陶系碳酸盐岩的研究,发现鞍形白云石胶结物发育比较普遍,常见于孔洞或裂缝之中,乳白色,晶体粗大,晶面弯曲或呈阶梯状,镜下波状消光,晶体内部常见微裂缝,常与热液矿物共生。本文对28个鞍形白云石样品进行了碳、氧、锶同位素测试,结果显示鞍形白云石的δ~(13)C和δ~(18)O值分别介于-2.446‰～0.686‰和-9.101‰～-5.117‰之间,~(87)Sr/~(86)Sr值介于0.708 6～0.710 2之间;流体包裹体测温分析表明,鞍形白云石中气—液两相包裹体的均一温度(T_h)介于121～159.5℃之间,但集中分布在135～145℃之间;根据最后冰融点温度(T_m)求得的白云岩化流体盐度介于21.3%～23.1%之间。这些数据表明,该类型白云石形成于热卤水(盐度是海水的5～8倍)之中。塔里木盆地鞍形白云石与世界范围内其它盆地的鞍形白云石的碳、氧同位素特征基本相似,但其~(87)Sr/~(86)Sr值相对偏低。导致这一现象的原因可能是鞍形白云石形成于来自深部的岩浆热液流体之中,这些流体伴随岩浆侵位或通过切穿基底的深大断裂及其与之相连的次级断裂系统从深部直接进入碳酸盐岩地层中,未经过碎屑岩输导层的长时间运移,所以导致其中形成的鞍形白云石~(87)Sr/~(86)Sr值偏低。  相似文献   

9.
Pervasive dolomites occur preferentially in the stromatoporoid biostromal (or reefal) facies in the basal Devonian (Givetian) carbonate rocks in the Guilin area, South China. The amount of dolomites, however, decreases sharply in the overlying Frasnian carbonate rocks. Dolostones are dominated by replacement dolomites with minor dolomite cements. Replacement dolomites include: (1) fine to medium, planar‐e floating dolomite rhombs (Rd1); (2) medium to coarse, planar‐s patchy/mosaic dolomites (Rd2); and (3) medium to very coarse non‐planar anhedral mosaic dolomites (Rd3). They post‐date early submarine cements and overlap with stylolites. Two types of dolomite cements were identified: planar coarse euhedral dolomite cements (Cd1) and non‐planar (saddle) dolomite cements (Cd2); they post‐date replacement dolomites and predate late‐stage calcite cements that line mouldic vugs and fractures. The replacement dolomites have δ18O values from ?13·7 to ?9·7‰ VPDB, δ13C values from ?2·7 to + 1·5‰ VPDB and 87Sr/86Sr ratios from 0·7082 to 0·7114. Fluid inclusion data of Rd3 dolomites yield homogenization temperatures (Th) of 136–149 °C and salinities of 7·2–11·2 wt% NaCl equivalent. These data suggest that the replacive dolomitization could have occurred from slightly modified sea water and/or saline basinal fluids at relatively high temperatures, probably related to hydrothermal activities during the latest Givetian–middle Fammenian and Early Carboniferous times. Compared with replacement dolomites, Cd2 cements yield lower δ18O values (?14·2 to ?9·3‰ VPDB), lower δ13C values (?3·0 to ?0·7‰ VPDB), higher 87Sr/86Sr ratios (≈ 0·7100) and higher Th values (171–209 °C), which correspond to trapping temperatures (Tr) between 260 and 300 °C after pressure corrections. These data suggest that the dolomite cements precipitated from higher temperature hydrothermal fluids, derived from underlying siliciclastic deposits, and were associated with more intense hydrothermal events during Permian–Early Triassic time, when the host dolostones were deeply buried. The petrographic similarities between some replacement dolomites and Cd2 dolomite cements and the partial overlap in 87Sr/86Sr and δ18O values suggest neomorphism of early formed replacement dolomites that were exposed to later dolomitizing fluids. However, the dolomitization was finally stopped through invasion of meteoric water as a result of basin uplift induced by the Indosinian Orogeny from the early Middle Triassic, as indicated by the decrease in salinities in the dolomite cements in veins (5·1–0·4 wt% NaCl equivalent). Calcite cements generally yield the lowest δ18O values (?18·5 to ?14·3‰ VPDB), variable δ13C values (?11·3 to ?1·2‰ VPDB) and high Th values (145–170 °C) and low salinities (0–0·2 wt% NaCl equivalent), indicating an origin of high‐temperature, dilute fluids recharged by meteoric water in the course of basin uplift during the Indosinian Orogeny. Faults were probably important conduits that channelled dolomitizing fluids from the deeply buried siliciclastic sediments into the basal carbonates, leading to intense dolomitization (i.e. Rd3, Cd1 and Cd2).  相似文献   

10.
与热液有关白云岩以晶体粗大为特征,较易形成物性良好的油气储集体。塔中地区寒武系-下奥陶统发育大量深层白云岩,以粉-细晶白云岩为主,局部可见中-粗晶白云岩。该中-粗晶白云岩流体包裹体均一温度经压力校正主要集中在173~199℃,比该层位最大埋藏温度(约160℃)高10~40℃,故认为其形成与热液活动有关。镜下观察两种类型的白云岩存在渐变过渡现象和碳氧同位素值的部分重叠,说明中-粗晶白云岩可能由粉-细晶白云岩在热液作用下重结晶而来。进一步结合其流体包裹体盐度(为正常海水盐度的3~5倍)和富87Sr、贫13C的特征认为,其成岩流体可能为岩浆热液经过下寒武统富87Sr、贫13C泥页岩质烃源岩的改造,并向上运移与中寒武统膏岩层孔隙间浓缩海水混合而形成的混合性流体。前者为其提供热量,后者可增加其盐度,遍布区内的深大断裂体系为其提供了运移通道。  相似文献   

11.
《Comptes Rendus Geoscience》2007,339(3-4):223-239
Neoproterozoic glacial and post-glacial sediments from the Volta Basin (West Africa) form a stratigraphic ‘Triad’ of tillites, carbonates and cherts. The carbonates that cap the tillites were studied in detail at Bwipe (Ghana), in the western part of the basin. They are made of finely-laminated dolostones with well-preserved sedimentary features, suggesting that dolomite formation was penecontemporaneous of deposition in a warm, arid peritidal environment, with microbial activity. Rare-earth element distribution display seawater-like patterns. Redox-sensitive trace elements indicate suboxic conditions. High Ba/Al ratios can be related to high organic productivity. δ13C values are nearly constant at −3.7‰ and δ18OPDB are about −6‰. 87Sr/86Sr ratios range between 0.7061 and 0.7073. The δ13C signatures are nearly similar to those of coeval cap dolostones from the northern part of the Basin. The dolostones change upwards to limestones with secondary textures, as well as more negative δ13C and higher 87Sr/86Sr ratio. Therefore, only the dolostones witness the post-glacial conditions in seawater. It is proposed that, due to a bloom of microbial productivity following ice thaw, organic matter likely accumulated at the water–sediment interface and was consumed by sulphate-reducing bacteria, leading to the precipitation of δ13C-negative dolomite. This microbially mediated model is supported by present-day field evidence from hypersaline lagoons in Brazil and by previously published culture experiments. It is consistent with the sedimentological and geochemical data from the Ghana cap dolostones and can be applied to other Neoproterozoic cap dolostones worldwide.  相似文献   

12.
Dolomites from the upper calcareous-siliceous member of the Miocene Monterey Formation exposed west of Santa Barbara, California, were analysed for geochemical, isotopic and crystallographic variation. The data clearly document the progressive recrystallization of dolomite during burial diagenesis in marine pore fluids. Recrystallization is recognized by the following compositional and crystallographic variations. Dolomites have decreasing δ18O and δ13C compositions, decreasing Sr contents and increasing Mg contents with increasing burial depths and temperatures from east to west in the study area. δ18O values vary from 5·3‰ in the east to − 5·5‰ PDB in the west and are interpreted to reflect the greater extent and higher temperature of dolomite recrystallization in the west. δ13C values correlate with δ18O and decrease from 13·6‰ in the east to − 8·7‰ PDB in the west. Sr concentrations correlate positively with δ18O values and decrease from a mean of 750 ppm in the east to a mean of 250 ppm in the west. Mol% MgCO3 values inversely correlate with δ18O values and increase from a minimum of 41·0 in the east to a maximum of 51·4 in the west. Rietveld refinements of powder X-ray diffraction data indicate that the more recrystallized dolomites have more contracted unit cells and increased cation ordering. The fraction of the Ca sites in the dolomites that are occupied by Ca atoms increases slightly with the approach to stoichiometry. The fraction of the Mg sites occupied by Mg atoms strongly correlates with mol% MgCO3. Even in early diagenetic, non-stoichiometric dolomites, there is little substitution of Mg in Ca sites. During recrystallization, the amount of Mg substituting for Ca in Ca sites decreases even further. Most of the disorder in the least recrystallized, non-stoichiometric dolomites is related to substitution of excess Ca on Mg sites.  相似文献   

13.
Located in the western Yangtze Block, the Qingshan Pb–Zn deposit, part of the Sichuan–Yunnan–Guizhou Pb–Zn metallogenic province, contains 0.3 million tonnes of 9.86 wt.% Pb and 22.27 wt.% Zn. Ore bodies are hosted in Carboniferous and Permian carbonate rocks, structurally controlled by the Weining–Shuicheng anticline and its intraformational faults. Ores composed of sphalerite, galena, pyrite, dolomite, and calcite occur as massive, brecciated, veinlets, and disseminations in dolomitic limestones.

The C–O isotope compositions of hydrothermal calcite and S–Pb–Sr isotope compositions of Qingshan sulphide minerals were analysed in order to trace the sources of reduced sulphur and metals for the Pb–Zn deposit. δ13CPDB and δ18OSMOW values of calcite range from –5.0‰ to –3.4‰ and +18.9‰ 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 had a mixed origin of mantle, marine carbonate rocks, and sedimentary organic matter. δ34S values of sulphide minerals range from +10.7‰ to +19.6‰, similar to Devonian-to-Permian seawater sulphate (+20‰ to +35‰) and evaporite rocks (+23‰ to +28‰) in Carboniferous-to-Permian strata, suggesting that the reduced sulphur in hydrothermal fluids was derived from host-strata evaporites. Ores and sulphide minerals have homogeneous and low radiogenic Pb isotope compositions (206Pb/204Pb = 18.561 to 18.768, 207Pb/204Pb = 15.701 to 15.920, and 208Pb/204Pb = 38.831 to 39.641) that plot in the upper crust Pb evolution curve, and are similar to those of Devonian-to-Permian carbonate rocks. Pb isotope compositions suggest derivation of Pb metal from the host rocks. 87Sr/86Sr ratios of sphalerite range from 0.7107 to 0.7136 and (87Sr/86Sr)200Ma ratios range from 0.7099 to 0.7126, higher than Sinian-to-Permian sedimentary rocks and Permian Emeishan flood basalts, but lower than Proterozoic basement rocks. This indicates that the ore strontium has a mixture source of the older basement rocks and the younger cover sequence. C–O–S–Pb–Sr isotope compositions of the Qingshan Pb–Zn deposit indicate a mixed origin of the ore-forming fluids and metals.  相似文献   

14.
HAIRUO Qing 《Sedimentology》1998,45(2):433-446
The petrography and geochemistry of fine- and medium-crystalline dolomites of the Middle Devonian Presqu’ile barrier at Pine Point (Western Canada Sedimentary Basin) are different from those of previously published coarse-crystalline and saddle dolomites that are associated with late-stage hydrothermal fluids. Fine-crystalline dolomite consists of subhedral to euhedral crystals, ranging from 5 to 25 μm (mean 8 μm). The dolomite interbedded with evaporitic anhydrites that occur in the back-barrier facies in the Elk Point Basin. Fine-crystalline dolomite has δ18Ο values between ?1·6 to –3·8‰ PDB and 87Sr/86Sr ratios from 0·7079–0·7081, consistent with derivation from Middle Devonian seawater. Its Sr concentrations (55–225 p.p.m., mean 105 p.p.m.) follow a similar trend to modern Little Bahama seawater dolomites. Its rare earth element (REE) patterns are similar to those of the limestone precursors. These data suggest that this fine-crystalline dolomite formed from Middle Devonian seawater at or just below the sea floor. Medium-crystalline dolomite in the Presqu’ile barrier is composed of anhedral to subhedral crystals (150–250 μm, mean 200 μm), some of which have clear rims toward the pore centres. This dolomite occurs mostly in the southern lower part of the barrier. Medium-crystalline dolomite has δ18O values between ?3·7 to ?9·4‰ PDB (mean ?5·9‰ PDB) and 87Sr/86Sr ratios from 0·7081–0·7087 (mean 0·7084); Sr concentrations from 30 to 79 p.p.m. (mean 50 p.p.m.) and Mn content from 50 to 253 p.p.m. (mean 161 p.p.m.); and negative Ce anomalies compared with those of marine limestones. The medium-crystalline dolomite may have formed either (1) during shallow burial at slightly elevated temperatures (35–40 °C) from fluids derived from burial compaction, or, more likely (2) soon after deposition of the precursor sediments by Middle Devonian seawater derived from the Elk Point Basin. These results indicate that dolomitization in the Middle Devonian Presqu’ile barrier occurred in at least two stages during evolution of the Western Canada Sedimentary Basin. The geochemistry of earlier formed dolomites may have been modified if the earlier formed dolomites were porous and permeable and water/rock ratios were large during neomorphism.  相似文献   

15.
The North Lhasa terrane in Tibet is generally interpreted to be paleotectonically unrelated to the East African Orogen (EAO) and is instead thought to have derived from northeastern India or northwestern Australia. In this study, we present petrogenetic and geochronological results pertaining to the analysis of gabbros (ca. 652 Ma), diorites (ca. 658 and 646 Ma), and tonalites (ca. 652 Ma) from the North Lhasa terrane. The gabbros are calc-alkaline and exhibit arc-like geochemical features. Low positive zircon εHf(t) values (+1.0 to +3.8), high zircon δ18O (6.25‰ to 7.94‰), and low negative whole-rock εNd(t) values (−3.5 to −1.4) indicate that the gabbros were derived from the lithospheric mantle, with geochemical modification by a subduction component. The diorite suite is characterized by a wide range of whole-rock chemistries (e.g., SiO2 = 51.33–61.98 wt%) and Hf–O–Sr isotopic compositions (εHf(t) = −10.8 to −0.1; δ18O = 5.17‰ to 7.11‰; ISr = 0.706 to 0.710), and negative whole-rock εNd(t) values (−7.0 to −4.7). These diorites are geochemically similar to OIB and are interpreted to be products of the partial melting of a relatively deep mantle source (>85 km) prior to extensive modification by continental crustal material. The tonalites are adakitic and have moderate Mg# values (47–54), low compatible element abundances, positive zircon εHf(t) values (+3.4 to +6.2), high ISr values (0.714 to 0.715), and small negative whole-rock εNd(t) values (−1.6 to −0.4). These tonalites most likely formed by the melting of thickened Mesoproterozoic continental crust. The generation of these ca. 650 Ma magmatic rocks was related to slab break-off in a collision zone. By integrating the findings of previous studies with the data of the present study, we suggest that the North Lhasa terrane was most likely located in the northern segment of the EAO in paleotectonic reconstructions of the Gondwana supercontinent.  相似文献   

16.
The Dapingzhang volcanogenic Cu–Pb–Zn sulfide deposit is located in the Lancangjiang tectonic zone within the Sanjiang region, Yunnan province of southwestern China. The deposit occurs within a felsic volcanic dome belonging to a mid-Silurian volcanic belt stretching for more than 100 km from Dapingzhang to Sandashan. The mineralized volcanic rocks are predominantly keratophyre and quartz keratophyre with subordinate spilite. The Dapingzhang deposit is characterized by well-developed vertical zonation with stockwork ores in the bottom, disseminated sulfide ores in the middle, and massive sulfide ores in the top, overlain by a thin layer of chemical sedimentary exhalative rocks (chert and barite). The Re–Os age of the pyrites from the deposit is 417 ± 23 Ma, indistinguishable from the age of the associated felsic volcanic rocks. The associated felsic volcanic rocks are characterized by negative Nb–Ta anomalies and positive εNd(t) values (+ 4.4–+6.5), similar to the coeval calc-alkaline volcanic rocks in the region. This observation supports the interpretation that the felsic volcanic rocks associated with the Dapingzhang deposit are the derivatives of arc basaltic magma by extensive fractional crystallization. The δ34S values of the sulfides from the deposit vary from − 1.24 to + 4.32‰, indicating a predominantly magmatic source for the sulfur. The sulfides are also characterized by homogeneous and relatively low radiogenic Pb isotope compositions (206Pb/204Pb = 18.310–18.656, 207Pb/204Pb = 15.489–15.643 and 208Pb/204Pb = 37.811–38.662), similar to the Pb isotopic compositions of the associated volcanic rocks. The Pb isotopic data indicate that mantle-derived Pb is more prevalent than crust-derived Pb in the deposit. The S–Pb isotopic data indicate that the important ore-forming materials were mainly derived from the associated volcanic rocks. The δ13CPDB and δ18OSMOW values of the associated hydrothermal calcite crystals vary from − 2.3‰ to + 0.27‰ and from + 14.6 to + 24.4‰, respectively. These values are between the mantle and marine carbonate values. The narrow range of the δ13CPDB values for the calcite indicates that carbon-bearing species in the hydrothermal fluids were primarily derived from marine carbonates. The δ18O values for the hydrothermal fluids, calculated from the measured values for quartz, are between − 2.1‰ and + 3.5‰. The corresponding δD values for the fluids range from − 59‰ to − 84‰. The O–H isotopic data indicate mixing between magmatic fluids and seawater in the ore-forming hydrothermal system. Similar to a typical volcanogenic massive sulfide (VMS) deposit, the ore-forming fluids contained both magmatic fluids and heated seawater; the ore metals and regents were derived from the underlying magma as well as felsic country rocks.  相似文献   

17.
《Resource Geology》2018,68(3):303-325
The Lujing uranium deposit, located in the southeastern part of the Nanling metallogenic province, is one of the representative granite‐related hydrothermal uranium deposits in South China. Basic geology, geochemistry, and geochronology of the deposit have been extensively studied. However, there is still a chronic lack of systematic research on the genesis and metallogenic process of the deposit. Thus, we recently carried out an electron microprobe and stable isotopic analysis. The main research results and progresses are as follows: Uranium minerals in this deposit include coffinite, pitchblende, and uranothorite, and small amounts of uranium exist in accessory minerals in the form of isomorphism. Coffinite, which occurs predominantly as the pseudomorphs after pitchblende, also occurs as a primary mineral and is locally formed from the remobilization of uranium from adjacent uranium‐bearing minerals. The mineralizing fluid was originally composed of a magmatic fluid generated by late Yanshanian magmatism. The high As content of pyrite in ores may reflect the addition of meteoric water, or the formation water (or both), to the magmatic hydrothermal system. The δ34S values vary from −14.4‰ to 13.9‰ (mean δ34S = −3.9‰), showing a range that is similar to nearby Cambrian metamorphic strata and Indosinian granites, indicating that these host rocks represent the source of sulfur; however, the possibility of a mantle source cannot be completely ruled out. According to our new isotopic data and recent Pb isotopic data, we conclude that the uranium in ores was derived by leaching dominantly from the uranium‐rich host rocks, especially the Cambrian metamorphic strata. The δ13CPDB values (−8.75‰ to 1.40‰; mean δ13CPDB = −5.41‰) and δ18OSMOW values (5.45–18.62‰; mean δ18O = 13.02‰) of reddish calcite from the ore‐forming stage suggest that the CO2 in the mineralizing fluids was derived predominantly from the mantle, with a small component contributed by marine carbonates. Based on these new data and previous research results, this paper proposes that uranium metallogenesis in the Lujing deposit is closely associated with mafic magmatism resulting from crustal extension during the Cretaceous to Paleogene in South China.  相似文献   

18.
New major and trace elemental, Sr–Nd–Pb isotope, and zircon U–Pb geochronological and Hf–O isotope data of post-collisional potassic and ultrapotassic volcanic rocks (PVRs and UPVs, respectively) along with geochemical data of PVRs, UPVs, and Mg-rich potassic rocks (MPRs) in the literature are used to constrain their mantle source and genesis. The PVRs, UPVs, and MPRs share similar geochemical features but with some discrepancies, suggesting that they were derived from subcontinental lithospheric mantle (SCLM) with isotopic heterogeneity resulting from the varying contributions of subducted Indian lower crust into the mantle source (ca. 6–20%, ca. 8–30%, and ca. 9–30%, respectively). The zircon Hf–O isotopic compositions of these rocks can be classified into two groups, including Group I rocks with high δ18O (6.7–11.3‰), low εHf(t) (− 17.0 to − 12.0), and old Hf crustal model ages (1.87–2.19 Ga) that indicate an ancient SCLM source, and Group II rocks with δ18O values of 6.8–10.7‰, εHf(t) values of − 11.8 to − 6.3, and younger Hf crustal model ages (1.50–1.86 Ga). The negative correlation defined by δ18O and εHf(t) of Group II samples suggests a two-component mixing between mantle- and crust-derived melts, in which the latter would be the subducted Indian lower crust as indicated by the similar negative εHf(t) values between Group II samples (− 11.8 to − 6.3) and the High Himalayan gneiss (− 14.2 to + 0.3). Thus we propose two enrichment events to account for the Hf–O isotopic compositions of the PVRs and UPVs/MPRs: the first involves the enrichment of the overlying SCLM that was metasomatized by fluids derived from dehydration of the subducted Indian lower crust, and the second invokes the enrichment of the overlying SCLM metasomatized by melts of the already dehydrated different proportions of the Indian lower crust. We argue that break-off of the northwards subducted Indian Plate in the early Miocene caused the asthenospheric upwelling under the Indian plate through slab window, resulting in varying degrees of partial melting of the overlying metasomatized heterogeneous SCLM to produce the primitive magmas of the PVRs, UPVs, and MPRs in an extensional setting. These observations and interpretations imply that the Indian lower crust was subducted beneath the Lhasa terrane in the Early–Middle Miocene.  相似文献   

19.
The Cretaceous-Paleogene granites of the Eastern Sikhote Alin volcanic belt (ESAVB) and Late Cretaceous granitoids of the Tatibin Series (Central Sikhote Alin) are subdivided into three groups according to their oxygen isotope composition: group I with δ18O from +5.5 to +6.5‰, group II with δ18O from +7.6 to +10.2‰, and group III with less than +4.5‰. Group I rocks are similar in oxygen isotope composition to that of oceanic basalts and can be derived by melting of basaltic crust. Group II (rocks of the Tatibin Series) have higher δ18O, which suggests that their parental melts were contaminated by sedimentary material. The low 18O composition of group III rocks can be explained by their derivation from 18O-depleted rocks or by subsolidus isotopic exchange with low-18O fluid or meteoric waters. The relatively low δ18O and 87Sr/86Sr in the granitoids of Primorye suggest their derivation from rocks with a short-lived crustal history and can result from the following: (1) melting of sedimentary rocks enriched in young volcanic material that was accumulated in the trench along the transform continental margin (granites of the Tatibin Series) and (2) melting of a mixture of abyssal sediments, ocean floor basalts, and upper mantle in the lithospheric plate that subsided beneath the continent in the subduction zone (granites of the ESAVB).  相似文献   

20.
Magnesium isotopic compositions, along with new Sr–Nd–Pb isotopic data and elemental analyses, are reported for 12 Miocene tourmaline-bearing leucogranites, 15 Eocene two-mica granites and 40 metamorphic rocks to investigate magnesium isotopic behaviors during metamorphic processes and associated magmatism and constrain the tectonic-magmatic-metamorphic evolution of the Himalayan orogeny. The gneisses, granulites and amphibolites represent samples of the Indian lower crust and display large range in δ26Mg from −0.44‰ to −0.09‰ in mafic granulites, −0.44‰ to −0.10‰ in amphibolites, and −0.70‰ to −0.03‰ in granitic gneisses. The average Mg isotopic compositions of the granitic gneisses (−0.19 ± 0.34‰), mafic granulites (−0.22 ± 0.17‰) and amphibolites (−0.25 ± 0.24‰) are similar, indicating the limited Mg isotope fractionation during prograde metamorphism from granitic gneisses to mafic granulites and retrograde metamorphism from mafic granulites to amphibolites. The Eocene two-mica granites and Miocene leucogranites are characterized by large variations in elemental and Sr–Nd–Pb isotopic compositions. The leucogranites and two-mica granites have their corresponding (87Sr/86Sr)i varying from 0.7282 to 0.7860 and 0.7163 to 0.7191, (143Nd/144Nd)i from 0.511888 to 0.512040 and 0.511953 to 0.512076, 207Pb/204Pb from 15.7215 to 15.7891 and 15.7031 to 15.7317, 208Pb/204Pb from 38.8521 to 39.5286 and 39.2710 to 39.4035, and 206Pb/204Pb from 18.4748 to 19.0139 and 18.7834 to 18.9339. However, they have similar Mg isotopic compositions (−0.21‰ to +0.06‰ versus −0.24‰ to +0.09‰), which did not originate from fractional crystallization nor source heterogeneity. Based on hornblende/biotite/muscovite dehydration melting reaction and Mg isotopic variations in two-mica granites and leucogranites with the proceeding metamorphism, along with elemental discrimination diagrams, Eocene two-mica granites and Miocene leucogranites could be related to hornblende dehydration melting and muscovite dehydration melting, respectively. Mg isotopic compositions of Eocene two-mica granites become heavier compared to the source because of residues of isotopically light garnet in the source; while those of Miocene leucogranites become lighter because of entrainment of isotopically light garnet from the source region. Thus, a new model for crustal anatexis and Himalayan orogenesis was proposed based on the Mg isotope fractionation in the leucogranites and metamorphic rocks. This model emphasizes a successive process from Indian continental subduction to rapid exhumation of the Higher Himalayan Crystalline Series (HHCS). The former underwent high-temperature (HT) and high-pressure (HP) granulite-facies prograde metamorphism, which resulted in the hornblende dehydration melting and the formation of Eocene two-mica granites; while the latter experienced amphibolite-facies retrogression and decompression, which resulted in the muscovite dehydration melting and the formation of Miocene leucogranites.  相似文献   

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