Geochemical Indication of Sinian Bedded Siliceous Rocks in the Hunan-Guizhou-Guangxi Area and Their Environmental Significance

: In the Hunan-Guizhou-Guangxi area there have developed very thick bedded siliceous rocks of the late Sinian. The rocks have a fairly pure composition, with an average content of siliceous minerals exceeding 95%. They are relatively rich in Fe and Mn, and poor in Al, Ti and Mg. The Fe/Ti, (Fe+Mn)/Ti, Al/(Al+Fe+Mn) and U/Th ratios and the Al-Fe-Mn and Fe-Mn-(Ni+Co+Cu)×10 triangle diagrams all show that they are hydrothermal sedimentary siliceous rocks. In the rocks the total amount of REEs is low, the δCe shows an obvious negative anomaly and the δEu a weak anomaly, and LREE>HREE, all indicating that they are products of hydrothermal processes. The δ30Si and δ18O values, as well as the formation temperature of the rocks all clearly show that the silica forming the rocks comes from hot water. Besides, analyses of the depositional environment of the rocks using the MnO/TiO2 ratio and the δCe and δ30Si values yield the same conclusion that they are formed in environments from continental marginal slopes of a bathyal environment to oceanic basins of a deep-sea environment.

Geochemical Characteristics of Wuyang Siliceous Rocks in the Southern Margin of North China Craton and its Constraint on the Formation Environment of BIF of Tieshanmiao Formation

Precambrian banded iron formation(BIF)is one of the most important mineral resources in China,mostly abundant in the North China Craton(NCC)with relatively less common in South China.Since the BIF and siliceous rocks both originated from chemical deposition,the syngenetic BIF and Siliceous rocks can help evaluate their environment of formation.We examine here the mineralogy and geochemistry of siliceous rocks associated with the Tieshanmiao Formation BIF,aiming to decipher the conditions of formation of both BIF and Siliceous rocks in the Wuyang area in the NCC.Analysis of the geochemical characteristics of whole rock shows that the Si O2 content of the siliceous rock ranges from 90.11%to 94.85%and is relatively high overall.Trace element contents of Ba and U are also high,the Ba/Sr ratio ranges from 3.89 to 25.28 and the U/Th ratio ranges from 0.09 to 0.20.Finally,theΣREE value of rare earth elements ranges from 57.03 ppm to 152.59 ppm,and these indexes all indicate that siliceous rock resulted from hydrothermal deposition.Plots of Al2 O3-Si O2,Si O2/(K2 O+Na2 O)-Mn O2/Ti O2 and Mn-10×(Cu+Co+Ni)-Fe in discrimination diagrams also verify this interpretation.However,both the Mg O content,ranging from 0.16 to 0.32,and the Fe/Ti ratio,ranging from 2.50 to 9.72,suggest that terrigenous material was added during the depositional process.Major and trace element parameters of siliceous rock,such as the Al/(A1+Fe+Mn)ratio(from 0.81 to 0.93),Mn O/Ti O2(from 0.00 to 0.17),Al/(Al+Fe)(from 0.82 to 0.93),Sc/Th ratio(from 0.21 to 0.50),U/Th(from 0.09 to 0.20),(La/Yb)N(from 0.83 to 3.04),and the(La/Ce)N(from 0.01 to 0.02)all imply that the siliceous rock formed in a continental margin.In addition,the Sr/Ba ratio from 0.08 to 0.26,theδCe value from 0.31 to 0.90,and theδEu value from 0.14 to 0.58,all indicate that the siliceous rock was formed at a relatively deeper water depth and under weak hydrodynamic conditions.Siliceous rock and BIF formed in the same geological setting,with the Si O2/(K2 O+Na2 O)ratio of siliceous rock ranging from 28.61 to 47.43,the Si O2/Al2 O3 ratio from 16.53 to 32.37,and the Si O2/Mg O ratio from 287.28 to 592.81,which are all in agreement with chemical deposition associated with volcanic eruptions.The Al2 O3/Ti O2 ratio from 37.82 to 50.30 indicates that the magma source of siliceous rock was of slightly intermediate composition.During the Late Archean in the Wuyang area,the high concentration and high purity Si O2 quickly precipitated from hydrothermal fluids to finally result in the accumulation of siliceous rock in a marginal sea,while the input corresponding to iron formation components was deposited to form iron formation layers,and limestone was only the product formed during the deposition intervals of siliceous rock and iron formations.In this study,the synsedimentary siliceous rocks of BIF act as a new way to provide direct evidence to understand the formation environment of BIF due to its high geochemical stability.

Geochemistry of upper Permian siliceous rocks from the Lower Yangtze region, southeastern China:implications for the origin of chert and Permian ocean chemistry

The Permian Chert Event is of great significance to understanding the geological evolution of the entire Permian; however,the origin of widespread chert formation is debated. We report new geochemical data from deep-marine siliceous rocks of the upper Permian Da-long Formation, Lower Yangtze region, southeastern China. Their geochemical results show that these thin-bedded siliceous rocks have a clear biologic origin, with rare to no evidence of hydrothermal influence. The values of Al/(Al + Fe + Mn) and Eu/Eu~* are 0.60-0.84(mean = 0.72) and 0.45-1.08(mean = 0.77), respectively, and Mn/Ti ratios are relatively low(mean = 0.72). The correlations of LaN/CeN, LaN/YbN, and Fe203/Ti02 with Al_2 O_3/(Al_2 O_3 + Fe_2 O_3), along with the Ce anomaly, indicate that the Da-long siliceous rocks were deposited at a transitional zone between a continental margin and the open ocean; i.e., relatively close to terrestrial sediment input and far from hydrothermal activity. The accumulation of chert is related to its unique paleogeographic location in an equatorial setting with many submarine paleo-highlands.Intense upwelling and frequent local volcanism are the main factors that promoted the development of siliceous rocks in the studied area. Ocean acidification triggered by large-scale volcanism(Large Igneous Province) during the late Permian led to extensive silica precipitation and preservation.

Germanium/silica ratio and trace element composition of Early Cambrian siliceous rocks in Keping:implications for the siliceous rocks'formation and paleoenvironment interpretations

This study used the germanium/silica(Ge/Si)ratios,together with rare earth elements and other trace elements to infer the siliceous source and sedimentary environment of the siliceous rocks located at the bottom of Yuertusi Formation in Northwestern Tarim Basin,Keping,China.Previous studies have shown that this siliceous rock stratum formed at the edge of the carbonate platform on the continental shelf.Researchers suggest that these siliceous rocks were formed by hydrothermal activity,but some still draw different conclusions.Understanding the silicon source and depositional environment of these siliceous rocks would help us learn the processes of environmental changes and the causes of biological explosions during this period.The value of germanium/silica ratios of these siliceous rocks is from 0.15 to 0.37 pmol/mol and much lower than above 10|imol/mol values in siliceous rocks that are known formed by hydrothermal activity.All samples are rich in HREE,which differ from hydrothermal siliceous rocks that are rich in LREE.Most samples lack hydrothermal related elements.All these features show that the source of these siliceous rocks'siliceous is not hydrothermal flu ids.The samples'Ce/Ce*range from 0.88 to 1,and Th/U ratios range from 0.01 to 0.36.These features suggest these siliceous rocks were formed in an anoxic environment.Considering all the evidence,we conclude that the siliceous rock stratum at the bottom of the Lower Cambrian Yuertusi Formation in northwest Tarim Basin.Keping,was formed in anoxic seawater at the edge of the carbonate platform on the continental shelf.Its silicon source is seawater instead of hydrothermal fluid.

Petrogenesis of siliceous high-Mg series:Evidence from Early Paleoproterozoic mafic volcanic rocks of the Vodlozero Domain,Fennoscandian Shield

Compositional peculiarities of the siliceous high-Mg series(SHMS)rocks formed at the Archean-Paleoproterozoic boundary as a function of plume activity are discussed using example of Early Paleoproterozoic mafic volcanic rocks of the Vodlozero Domain,Fennoscandian Shield.These rocks are characterized by wide variations in Mg#(33-67)and Cr contents(25-1123 ppm),LREE enrichment,and weakly negative_(εNd)(from-0.7 to-2.9).The high Gd/Yb ratio in the primitive high-Mg rocks of the Vodlozero Domain suggests their generation from a garnet-bearing source.At the same time,their negative _(εNd)in combination with LREE enrichment points to the crustal contamination.A new model was proposed to explain the remarkable global-scale similarity of SHMS.Such rocks can be generated by the contamination of a high-degree(30%)partial melt derived from a depleted mantle.The lower crustal sanukitoid-type rocks can be considered as a universal crustal contaminant.Modeling showed that such mixing can provide the observed narrow_(εNd)variations in Early Paleoproterozoic volcanics.The Neoarchean sanukitoid suites,which are widespread on all cratons,presumably composed the lower crust at the beginning of the Paleoproterozoic.Therefore,this mechanism can be considered universal for the genesis of the SHMS rocks.The high-to low-Cr rock series can be produced by the fractionation of the mafic melt coupled with an insignificant crustal assimilation of felsic end members of the sanukitoid suite of the Vodlozero Domain en route to the surface,as suggested by the positive correlation of_(εNd)with Cr and Mg#,negative correlation with Th,and slight decrease of_(εNd)in the more evolved varieties.

Lithological and Si–O–S isotope geochemistry:constraints on the origin and genetic environment of the selenium(Se)-rich siliceous rocks in Enshi,Hubei Province,China

Se-rich black rock series of the Middle and Late Permian system is widely distributed in Enshi Prefecture with an exposed area of 850 km~2,among which the unique Yutangba black rock series independent selenium deposit with industrial mining significance in the world is produced.However,the source and metallogenic mechanisms of Se are still controversial.In general,terrestrial weathering and submarine hydrothermal processes are the main source end members of Si and Se,and the related siliceous rocks record the deposition process of Si and Se from different sources.The study of lithofacies and paleogeography shows that western Hubei belongs to the near eastwest turn of the Yangzi platform in the Middle and Late Permian and becomes an inter-platform basin with nearly north-south direction.Therefore,the comparative study of the Yutangba deposit and the selenium-rich black rock series in the northern Shadi with high selenium content is expected to reveal the provenance evolution of the two sections in space,and further restrict the Se mineralization mechanism in the Enshi basin.From the element geochemistry study,the black rock series in two study areas may have formed in a transitional position of either the continental margin or continental slope,in the process of sedimentary,more terrigenous clastic materials entered.They are rich in lithophile elements V and Cr.δU>1.0,U/Th and V/(V+Ni)ratio indicate that the Se-rich strata of black rock series in the Enshi areas occurred in an anoxic reducing environment and formed in an environment between the ocean basin and the continental margin.From Si-O isotope geochemistry,the original Si source of the study area is thought to relate to a volcanic eruption,which leads to the enrichment of Si in the seawater.The determined values of S isotope in the black rock series of the two study areas both show the characteristics related to organic reduction/biogenic.

Residual stress measurement and analysis of siliceous slate-containing quartz veins

Engineering geological disasters such as rockburst have always been a critical factor affecting the safety of coal mine production.Thus,residual stress is considered a feasible method to explain these geomechanical phenomena.In this study,electron backscatter diffraction(EBSD)and optical microscopy were used to characterize the rock microcosm.A measuring area that met the requirements of X-ray diffraction(XRD)residual stress measurement was determined to account for the mechanism of rock residual stress.Then,the residual stress of a siliceous slate-containing quartz vein was measured and calculated using the sin^(2) ϕ method equipped with an X-ray diffractometer.Analysis of microscopic test results showed homogeneous areas with small particles within the millimeter range,meeting the requirements of XRD stress measurement statistics.Quartz was determined as the calibration mineral for slate samples containing quartz veins.The diffraction patterns of the(324)crystal plane were obtained under different ϕ and φ.The deviation direction of the diffraction peaks was consistent,indicating that the sample tested had residual stress.In addition,the principal residual stress within the quartz vein measured by XRD was compressive,ranging from 10 to 33 MPa.The maximum principal stress was parallel to the vein trend,whereas the minimum principal stress was perpendicular to the vein trend.Furthermore,the content of the low-angle boundary and twin boundary in the quartz veins was relatively high,which enhances the resistance of the rock mass to deformation and promotes the easy formation of strain concentrations,thereby resulting in residual stress.The proposed method for measuring residual stress can serve as a reference for subsequent observation and related research on residual stress in different types of rocks.

Fluid inclusion, siliceous rock geochemistry of Shewushan lateritic gold deposit, Hubei Province, eastern China: Implication for the genesis of primary orebody

The Shewushan gold deposit is located 16 km southwest of Jiayu County, Hubei Province, eastern China, which is the largest lateritic gold deposit in Asia, consisting of a series of mineralized faults containing gold grades of 1.0–19.5 g/t set within a larger, lower-grade(0.2–1.0 g/t) zone. According to the fluid inclusions study, the homogenization temperature ranges from 70–350℃, and concentrates between 140–220℃. The laser Rama results show that the CO2 and CH4 exist in mineralized fluid. In addition, the major and trace element and REE geochemical data show that the genesis of the siliceous rocks is hydrothermal genesis, formed by mineralized fluid bearing SiO2 transmitted along faults to the surface, and replace the carbonate rocks to begin deposition. Primary gold mineralization is attributed to hydrothermal activity that followed the main period of tectonic deformation of the Indosinian orogeny, which caused the regional detachment regime in southeast Hubei. The reversed fold and the fault system formed the fluid migration channel in the Shewushan area. EPMA results show that Au exists in arsenopyrite(850×10-6–1550×10-6 Au) and pyrite(470×10-6–1340×10-6 Au). Therefore, based on the above results and combined with the field observation, we suggest that the genesis of primary orebody in Shewushan deposit is carlin type, while not weathered residual type.

New Evidence for Genesis of the Zoige Carbonate-Siliceous-Pelitic Rock Type Uranium Deposit in Southern Qinling:Discovery and Significance of the 64 Ma Intrusions

The carbonaceous-siliceous-argillitic rock type uranium deposit in the Zoige area is located in the northeastern margin of the Tibetan Plateau, and has gained much attention of many geologists and ore deposit experts due to its scale, high grade and abundant associated ores. Because of the insufficient reliable dating of intrusive rocks, the relationship between mineralization and the magmatic activities is still unknown. In order to study this key scientific issue and the ore-forming processes of the Zoige uranium ore field, the LA-ICP-MS zircon U-Pb dating of magmatic rocks was obtained：64.08±0.59 Ma for the granite-prophyry and ~200 Ma for the dolerite. U-Pb dating results of uraninite from the Zoige uranium ore field are mainly concentrated on ~90 Ma and ~60 Ma. According to LA-ICP-MS U-Pb zircon dating, the ages for the dolerite, porphyry granite and granodiorite are 200 Ma, 64.08 Ma approximately and 226.5-200.88 Ma, respectively. This indicates that the mineralization has close relationship with activities of the intermediate-acidic magma. The ages of the granite porphyry are consistent with those uraninite U-Pb dating results achieved by previous studies, which reflects the magmatic and ore-forming event during the later Yanshanian. Based on the data from previous researches, the ore bodies in the Zoige uranium ore field can be divided into two categories：the single uranium type and the uranium with polymetal mineralization type. The former formed at late Cretaceous（about 90 Ma）, while the latter, closely related to the granite porphyry, formed at early Paleogene（about 60 Ma）. And apart from ore forming elemental uranium, the latter is often associated with polymetallic elements, such as molybdenum, nickel, zinc, etc.

Early Triassic Silicic Volcanics of South China:Petrogenesis and Constraints on the Geodynamic Evolution of the Paleo-Tethys Ocean Region

The only occurrence of Lower Triassic silicic volcanic rocks within the South China Block is in the Qinzhou Bay area of Guangxi Province.LA-ICP-MS zircon U-Pb dating reveals that volcanic rocks of the Beisi and Banba formations formed between 248.8±1.6 and 246.5±1.3 Ma,coeval with peraluminous granites of the Qinzhou Bay Granitic Complex.The studied rhyolites and dacites are characterized by high SiO_(2),K_(2)O,and Al_(2)O_(3),and low MgO,CaO,and P_(2)O_(5) contents and are classified as high-K calc-alkaline S-type rocks,with A/CNK=0.98-1.19.The volcanic rocks are depleted in high field strength elements,e.g.,Nb,Ta,Ti,and P,and enriched in large ion lithophile elements,e.g.,Rb,K,Sr,and Ba.Although the analyzed volcanic rocks have extremely enriched zircon Hf isotopic compositions(ε_(Hf)(t)=-29.1 to-6.9),source discrimination indicators and high calculated Ti-in-zircon temperatures(798-835℃)reveal that magma derived from enriched lithospheric mantle not only provided a heat source for anatectic melting of the metasedimentary protoliths but was also an endmember component of the S-type silicic magma.The studied early Triassic volcanics are inferred to have formed immediately before closure of the Paleo-Tethys Ocean in this region,as the associated subduction would have generated an extensional setting in which the mantle-derived upwelling and volcanic activity occurred.

Determining CO_2 consumption from elemental change in soil profiles developed on carbonate and silicate rocks

To further understand the roles of carbonate and silicate rocks in regulating the atmosphere/soil CO_2level,the flux of CO_2 consumed by the chemical weathering of silicate and carbonate rocks was determined from the elemental change in soil profiles.Results showed that the chemical weathering of carbonate rocks mainly occurred at the rock-regolith interface,and that the further weathering of the residua soil on the carbonate rocks was similar to that of the granite profile.Chemical weathering of the silicate rocks occurred through the whole profiles.Therefore,CO_2 consumed per volume by the silicate profiles[M_(sr)(CO_2)]and the residues on carbonate rocks[M_(cr)(CO2)]were calculated based on the elemental weathering gradients.CO_2 consumed by carbonate protolith[M_(cp)(CO_2)]was calculated from the elemental change at the rock-regolith interface.The M_(sr)(CO_2) were about tens to thousands orders of magnitude greater than M_(cr)(CO_2).Even so,this demonstrated that the residues on carbonate rocks could be a sink of CO2_ on long-term scales.The M_(cp)(CO_2) was about four times larger than M_(sr)(CO_2),which demonstrated that carbonate rocks played a more important role in regulating the CO_2 level than the silicate rocks did during the pedogenic process of the profiles.

Geochemical Characterization of Intermediate to Silicic Rocks in the Global Ophiolite Record

Ophiolites consist predominantly of mafic-ultramafic rocks but also contain in subordinate amounts intermediate to silicic intrusive–extrusive rocks in them.These rocks,although not in large volumes in comparison to mafic–ultramafic units,reveal significant clues about melt evolution,magmatic accretion processes,and partial melting of pre-existing oceanic crust during ophiolite evolution.They also yield ample accessory minerals,such as zircon and monazite,which are widely used to date the timing of oceanic crust generation.We present a global synthesis of the occurrence of such leucocratic,intrusive and volcanic rocks from 150 Phanerozoic to Archean ophiolites,and evaluate models for their genesis during the development of oceanic crust in different tectonic environments.In the ophiolite complexes we have investigated,intrusive and extrusive rocks show a wide range in composition.Intermediate rocks（with Si O2 between 52and 63 wt.%）include andesite and diorite,whereas the silicic rocks（with Si O2 more than 63 wt.%）include dacite,trondhjemite,tonalite and granite.For subduction-relatedandsubduction-unrelated MORB-type ophiolites in the Phanerozoic orogenic belts,the most commonly reported intermediate to silicic intrusive rocks,independent of their original tectonic setting,are trondhjemite,plagiogranite and tonalite,and less commonly quartz-diorite and diorite.These rock types have been documented from 80 percent of the 104ophiolites included in this study.Intermediate to silicic lavas and/or pyroclastic rocks（basaltic andesite,andesite,dacite and rhyolite）are less abundant,and have been reported from only 35 percent of the 104 ophiolites.Trachyte occurs in some of the Rift/Continental Margintype,subduction-unrelated ophiolitesEvolvedrocksinsubduction-unrelated,Rift/Continental Margin ophiolites are predominantly basaltic andesite and andesite,whereas MOR type（mid-ocean ridge）ophiolites exhibit nearly equal proportions of basaltic andesite/andesite and rhyodacite and Plume/MOR type ophiolites are characterized by rhyolites.Intermediate to silicic volcanic uints in the Backarc sub-group of subduction-related ophiolites are characterized by similar amounts of basaltic andesite/andesite and rhyodacite,whereas in the Backarc to Forearc,Forearc,and Volcanic Arc sub-groups they are mainly basaltic andesite/andesite.Intermediate to silicic rocks in Rift/Continental Margin and Plume/MOR type ophiolites are generally LREE-enriched,whereas those in the MOR type vary from LREE-depleted to LREE-enriched.The Backarc and Backarc to Forearc types are similar to the MOR type;silicic rocks of the Forearc and Volcanic Arc types are generally LREE-enriched.The main process in the formation of the majority of the intermediate to silicic rocks in both subduction-unrelated and subduction-related ophiolites is partial melting of basaltic and/or gabbroic rocks beneath the spreading centers,whereas a minor volume in subduction-related ophiolites are adakites that were produced from partial melting of a subducting slab.Silcic to intermediate rocks in Plume/MOR type ophiolites are generated by fractional crystallization of basaltic melt.The incompatible,non-conservative elements,such as Ba and Th,are weakly to strongly enriched in subduction-related ophiolites as a result of shallow to deep enrichment associated with subduction zone processes.The field occurrence and the geochemical character of leucocratic rocks in ophiolites show considerable variations,providing additional constraints on the petrogenesis of ophiolites in different tectonic settings.Fractional crystallization appears to have been an important process in the formation of the intermediate to silicic rocks in Rift/CM and P/MOR ophiolites in the subduction-unrelated class,as well as VA ophiolites in the subduction-related class.In these types of ophiolites magma chambers might have operated as closed-systems for a sufficient amount of time for fractionation and compositional zoning of the magma to take place（Fig.1C1）.Fractional crystallization was of subordinate importance during the igneous construction of the MOR,BA,FA,and BA-FA ophiolites.Instead,partial melting of hydrothermally altered crust was the most common process resulting in the majority of the intermediate to felsic rocks of the Rift/CM（estimated to 60%）,MOR（estimated to 95%）,and the BA,FA and BA-FA（estimated to 75%）ophiolites,as well as a high proportion of the VA ophiolites（;0%）(Fig.1C2).Slab-generated melts that result in the production of intermediate to silicic rocks of adakitic character represent a relatively small fraction（;5%in Phanerozoic ophiolites）of subduction-related ophiolites（Fig.1C3）.Some adakites are very low in incompatible elements（e.g.,La）,suggesting that these melts were generated from highly depleted MORB.Precambrian ophiolites contain a higher percentage of adakites（ca.35%）,indicating that physical conditions were more favorable for slab melting during the Precambrian than in the Phanerozoic.This phenomenon may have resulted from higher geothermal gradient along relatively shallow-dippingsubductionzonesduringthe Precambrian.

Ore petrography of low-grade siliceous manganese ores from the Bonai-Keonjhar belt, Orissa, India: The influence of mineral-fabric on their beneficiation

Low grade siliceous manganese ores from the iron ore group of the Bonai-Keonjhar belt, Orissa, India are found mostly in shear zones. The ore characteristics of siliceous manganese ore samples from three differ- ent mines, viz. the Shankar (Barbil OMC lease hold area), the Sone-Patuli (Patmunda, OMM lease hold area), and the Musaghar (Roida, OMDC lease hold area), were studied. These siliceous manganese ores are of three types, respectively: (i) spongy-granular; (ii) massive-mosaic; and (iii) hard-mylonitized. The spongy-gran- ular type contains granular, saccharoidal quartz and the major manganese mineral present is pyrolusite. The second type contains well crystallized quartz and cryptomelane, while the third has cherty, fine grained quartz (mylonite) along with romanechite. All three ores were subjected to physical beneficiation under similar conditions. Both gravity and magnetic separation techniques were employed. The mineral-fabric of the ores has been correlated to the extent of their beneficiation using these physical techniques. Of these three ores only the spongy-granular type responded well to upgrading. The feed with 22% Mn content could be upgraded to 44% with a 28% yield and a 49% recovery. The good response to beneficiation of the spongy- granular sample could be due to the large euhedral crystals of pyrolusite and the friable nature of the sac- charoidal quartz. This study reveals the influence of mineral-fabric on beneficiation of low grade ore, siliceous Mn ore in particular.

塔里木盆地西北缘埃迪卡拉纪—寒武纪转折期黑色岩系中硅质岩成因及其环境指示意义

埃迪卡拉纪—寒武纪转折期是地质历史上的关键期,全球范围内普遍发育一套黑色岩系,包括硅质岩、泥岩和白云岩。在这套黑色岩系中下部,发育了不同厚度的硅质岩,尤其是在塔里木盆地西北缘,黑白燧石条带韵律性互层且硅质岩分布变化大,其成因对寒武系玉尔吐斯组优质烃源岩形成环境具有重要指示意义。本文以塔西北地区新柯地1井这套黑色岩系中硅质岩为研究对象,运用岩芯观察、镜下薄片鉴定、主微量元素和稀土元素分析,研究硅质岩与有机质的成因及其二者之间的共生机理,硅质岩较高的Ti、V、Y、Ba等元素含量、较低的δ^(30)Si值(1.2‰~0.7‰)、明显的Ce负异常说明其硅质流体为热液来源,但是硅质岩不是热液脉体侵入形成,而是富含硅质的流体与冰冷的海水混合沉积而成,硅质岩沉积受温度、pH值、有机质等因素共同控制,有机质的存在可以促进硅质沉淀。同时系统地研究了黑白燧石条带韵律性互层的成因,白色层由纯的微晶石英组成,而黑色层由碎屑碳质颗粒、石英颗粒和碳质纹层组成,认为是微生物席活动的结果,硅质岩中黑白燧石层受冰期和间冰期控制。硅质岩中较高的Al_(2)O_(3)/(Al_(2)O_(3)+Fe_(2)O_(3))比值和较低的Lan/Cen比值代表受陆源影响的大陆边缘沉积环境,结合古气候、海平面变化等信息,认为塔西北地区寒武系玉尔吐斯组硅质岩以及硅质页岩沉积于局限海湾或潟湖环境,为正确认识硅质页岩的形成环境和烃源岩的评价预测提供理论指导。

全球硅酸盐岩化学风化—二氧化碳消耗定量模型研究进展

硅酸盐岩通过与二氧化碳的化学反应,去除大气二氧化碳并将其封存在风化产物或海洋碳酸盐岩中,是影响全球碳循环以及气候变化的要素之一。定量计算全球硅酸盐岩通过风化作用消耗的二氧化碳总量是了解地球现今与过去气候变化的关键。作者系统调研了5个硅酸盐岩化学风化—二氧化碳消耗定量模型的数据来源、研究方法、计算公式以及各模型的主要影响因素,并且以最新的Celine模型所计算得出的二氧化碳消耗量为参考标准,对比了各模型的优缺点与适用范围。现有模型估计全球硅酸盐岩化学风化的二氧化碳消耗量为69~169 Tg/yr,其中各模型的主要参数包括气候(温度、径流)与岩性,次要参数包括构造隆升、火山与植物作用等。在未来探索硅酸盐岩化学风化所消耗二氧化碳的定量计算中,应考虑更多控制作用的影响以及各因素之间的相互联系。此外,利用大数据分析方法将这些定量模型推广应用于深时地球古气候重建可能是未来的研究趋势。

北苏鲁威海地区共生伟晶岩脉-钙硅酸盐岩的形成机制和地质意义

威海位于苏鲁造山带最北端,出露片麻岩和榴辉岩等多类超高压岩石,是研究大陆俯冲带变质、深熔和熔体效应的理想靶区。基于详细野外观察,本文对该区的一套钙硅酸盐岩、伟晶岩脉和寄主片麻岩进行了岩相学、地球化学和锆石年代学等研究,旨在深入探究超高压变质岩的多期熔融及与之伴随的地质过程。阴极发光显示,三类岩石中的锆石整体具有核-边结构,继承核年龄为859±20Ma~272±6Ma,与苏鲁变质岩内大多残余岩浆锆石的年龄一致。新生锆石共记录了约237±4Ma、224±7Ma~218±2Ma和177±5Ma~166±3Ma等三组年龄。结合不同微区的稀土元素特征,本文认为,237±4Ma记录了峰期变质时代,而224±7Ma~218±2Ma和177±5Ma~166±3Ma年龄的锆石呈现典型的深熔锆石特征,分别代表俯冲陆壳折返至高压榴辉岩相和加厚地壳初始伸展阶段的部分熔融年代。伟晶岩与片麻岩中继承锆石的Hf同位素组成不同,表明形成伟晶岩的熔体并非寄主岩石直接熔融而来。此外,钙硅酸盐岩及其内部的单斜辉石具有极低的Cr、Ni等相容元素含量和负Eu异常特征,且均显示大离子亲石元素富集、高场强元素亏损的特点。并且,钙硅酸盐岩与苏鲁超高压片麻岩的Sr-Nd同位素组成接近,其内部继承/新生锆石与伟晶岩中相应锆石微区的Hf同位素组成一致。综合分析上述数据并结合前人成果,本文推测,钙硅酸盐岩为深俯冲陆壳折返阶段长英质熔体与片麻岩内部的大理岩团块发生熔岩反应所致。综上,本研究进一步约束了俯冲陆壳在折返及造山垮塌阶段两期熔融的时间及熔体来源等信息,并为大陆俯冲带深部的熔体效应和物质循环提供了参考。

滴定硅酸盐岩石与土壤中氧化亚铁的对比研究

【研究目的】由于硅酸盐岩石与土壤的成分不同,开展硅酸盐岩石与土壤中氧化亚铁的对比研究有利于更加准确地滴定氧化亚铁的含量。【研究方法】本文对重铬酸钾滴定法滴定硅酸盐岩石与土壤中氧化亚铁进行了对比,重点比较了溶样时间对氧化亚铁的影响,实验使用硫酸(1+1)和氢氟酸溶样,多余的氟用硼酸络合,以二苯胺磺酸钠为指示剂,重铬酸钾标准溶液滴定氧化亚铁。【研究结果】分析结果表明,溶样时间对硅酸盐岩石和土壤中氧化亚铁影响较大,硅酸盐岩石和土壤样品分别在溶液完全沸腾0~2 min和8~10 min时滴定为宜;方法经国家一级标准物质分析验证,结果与参考值相符,硅酸盐岩石和土壤的精密度RSD(n=10)分别为1.94%、1.78%。【结论】因此,在实际操作中应根据样品的类型选择合适的溶样时间,能够更加准确地确定氧化亚铁的含量。

碱熔-标准物质法-电感耦合等离子体原子发射光谱(ICP-AES)法测定硅酸盐岩中10种主量元素

硅酸盐岩元素的准确测定是其地球化学分析研究的基础,其主量元素含量通常可以采用电感耦合等离子体原子发射光谱(ICP-AES)法测定,但其测定方法的系统性研究相对缺乏,尤其是样品前处理和基体干扰的有效消除两方面。前处理过程中,考察不同熔剂用量对硅酸盐岩样品的分解能力,发现当熔剂与样品比例达到6∶1后,熔珠为纯色透明,经稀硝酸提取后溶液澄清,确定了硅酸盐岩前处理时熔剂与样品的最佳配比。测定过程中,通过考察基体匹配法和标准物质法两种基体干扰消除方法对测定结果的影响,发现当采用与岩性一致或者接近的标准物质绘制校准工作曲线时,基体干扰消除效果更好,更适用于测定硅酸盐岩10种主量元素含量。据此,建立了硅酸盐岩经偏硼酸锂熔融,稀硝酸振荡提取处理,以标准物质法绘制校准工作曲线,采用ICP-AES法同时测定SiO_(2)、Fe_(2)O_(3)、Al_(2)O_(3)、CaO、K_(2)O、MgO、Na_(2)O、TiO_(2)、P_(2)O_(5)、MnO 10种成分含量的方法。对岩石标准物质GBW07107进行分析测定,方法的相对标准偏差(RSD)为0.17%~0.75%,方法检出限为0.001%~0.016%,满足硅酸盐岩样品元素定量分析的要求,而且操作简单快速,环境污染小,适用于大批量样品分析。

桂西南碳硅泥岩型铀矿赋矿地层沉积特征

桂西南地区是我国华南重要的碳硅泥岩型铀矿床产出区,区内产铀地层沉积建造与铀成矿作用的关系,对该区勘查找矿具有重要意义。在综合前人资料的基础上,通过野外地质调查、剖面对比分析该区产铀地层特征并阐明桂西南地区不同地段铀成矿主含矿层的沉积环境,进而总结地层沉积建造与铀成矿作用的关系。研究表明,该区铀矿化层位具有多层性,但不同地段铀成矿对层位有一定的专属性,且在空间展布上有一定的规律性;不同地段铀矿成矿主含矿层的沉积环境有滨海局限台地相、浅海陆棚碎屑岩相、浅海浅水台地相、浅海较深水台盆相沉积;且不同沉积环境下地层沉积建造对铀成矿作用具有明显的控制作用。

Relationships between Basic and Silicic Magmatism in Continental Rift Settings:A Petrogeochemical Study of Carboniferous Post-collisional Rift Silicic Volcanics in Tianshan,NW China

Petrogeochemical data are reported for silicic volcanic rocks from the Tianshan Carboniferous rift, with the aim of discussing the petrogenesis of silicic magmas. Incompatible element vs. incompatible element diagrams display smooth positive trends for the Tianshan Carboniferous rift-related volcanic rocks; the isotope ratios of the silicic lavas [^87Sr/^86S（t）=0.699880.70532; eNd（t）=4.76-8.00; ^206pb/^204pb（t）=17.435-18.017; ^207Pb/^204Pb（t）=15.438-15.509; ^208Pb/^204Pb（t） = 37.075-37.723] encompass those of the basic lavas. These data suggest a genetic link between rhyolites and basalts, but are not definitive in establishing whether silicic rocks are related to basalts through fractional crystallization or partial melting. Geochemical modeling of incompatible vs. compatible elements excludes the possibility that silicic melts are generated by the melting of basaltic rocks, and indicates a derivation by fractional crystallization plus moderate assimilation of wall rocks （AFC） starting from intermediate rocks to silicic rocks. Continuous AFC from basalt to rhyolite, with small rates of crustal assimilation, best explains the geochemical data. The presence or absence of bimodal volcanism （the ＂Daly Gap＂） might be related to cooling rates of magma chambers. In central and eastern Tianshan, the crust was thinner and the cooling rates of the magma chamber within the crust were greater. These conditions resulted in a rapid fall in temperature within the magma reservoir and caused a narrow temperature interval over which intermediate melts formed, effectively reducing the volume of the intermediate melts.