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1.
In the Deer Lake Basin of western Newfoundland, Canada, analcime has been found within fine-grained, siliciclastic and carbonate, lake and lake-margin sediments of the Carboniferous Rocky Brook Formation. Analcime is the only zeolite observed in these unmetamorphosed and non-volcanogenic rocks. Microscopic analcime occurs as vug-filling, limpid, isotropic to anisotropic crystals. Microprobe analyses show this type of analcime to be lower in silica than most analcime in sedimentary rocks. A second textural type of finer-grained submicroscopic analcime can be detected from X-ray diffraction spectra of Rocky Brook Formation mudstones, where it is associated with phyllosilicates (illite, interstratified chlorite/smectite, smectite, chlorite), other silicates (quartz, feldspar) and carbonates (dolomite, calcite). Results of this study suggest that the analcime in the Rocky Brook Formation formed either by direct lakewater/porewater precipitation or by reaction of these waters with one or more clay mineral types or plagioclase. 相似文献
2.
霍普布鲁克(Hope Brook)金矿床是加拿大第六大金矿床,产于新元古界的阿巴拉契亚山脉阿瓦隆区岩体中,地表出露面积为3000×400m2,向深部逐渐变小。该矿具有复杂的岩脉构造,与浅水体为主的火山碎屑序列的长英质、镁铁质火山碎屑岩密切相关。矿床赋存于大范围的热液蚀变区中,蚀变区内普遍有酸浸现象,并具有大量的泥质蚀变特点。同位素年代学证据表明,成矿时代为早—中寒武世(576±10Ma),是多期次热液作用,同时伴有侵入岩浆以及火山作用产生的凝灰岩共同影响的产物,属于典型的高硫化的浅成低温热液金矿床。 相似文献
3.
十八重溪位于福建省闽侯县南通镇，系省级风景名胜区，其地貌、水文、生物组合奇特。本文从地质角度，着重对十八重溪强碳酸盐化火山岩及其地貌的成因进行分析，并对季节河、珍稀生物的形成条件作初步探讨。 相似文献
4.
ABSTRACT Microstructural and petrological data from the Jumping Brook metamorphic suite, western Cape Breton Highlands, suggest that a single episode of syntectonic prograde metamorphism, followed by uplift, cooling and associated retrogression, affected these rocks during mid-Palaeozoic times. Microstructures indicative of progressive crenulation foliation development can be traced from low-grade (chlorite zone) through high-grade (kyanite zone) rocks, allowing a clear sequence of porphyroblast growth to be established. Metamorphic reactions and P-T calculations suggest metamorphic conditions of 700-750°C at 8-10 kbar were achieved in kyanite zone rocks. Although a complete P-T-t path was not defined, combined petrological and geochronological data can be used to constrain computed P-T-t models. These models suggest that a component of post-metamorphic tectonic exhumation is required to explain the observed times of cooling and uplift. The microstructural and petrological data to not support the interpretation that the high-grade rocks represent pre-existing crystalline basement. Indeed, the metamorphic history, geochronology and computed tectonic models all point to a single, short-lived episode of Silurian-Devonian volcanism, intrusion, convergence, regional metamorphism and uplift, probably resulting from collision tectonics at an irregular continental margin. 相似文献
5.
Noye M. Johnson Charles T. Driscoll John S. Eaton Gene E. Likens William H. McDowell 《Geochimica et cosmochimica acta》1981,45(9):1421-1437
Contemporary ‘acid rain’ in the Hubbard Brook ecosystem has induced a series of geochemical responses. Neutralization is accomplished in essentially a 2-step process. Initially, hydrogen ion acidity is neutralized by the dissolution of reactive alumina primarilly found in the soil zone. In the Hubbard Brook area this reactive alumina has solution properties much like natural gibbsite. Aluminum-rich surface waters with a pH of 4.7 5.2 are typical of this neutralization stage. In a second step, both hydrogen ion acidity and aluminum acidity are neutralized by the chemical weathering of primary silicate minerals, i.e. by the alkali and alkaline earths contained in the bedrock and glacial till of the watershed. The chemical weathering reaction is much slower than the alumina dissolution reaction, so that the aluminum acidity stage (pH 4.7 5.2) may persist for substantial periods. Typically, however, in the Hubbard Brook area the aluminum acidity has been neutralized and a pH > 5.2 is obtained before surface waters reach a third-over stream channel. Because of the relatively low pH's throughout the soil zone and in the streamwater, carbonic acid reactions are essentially absent at the present time in the Hubbard Brook system. Water pathlength (or residence time) in the soil zone is the crucial factor in the state of acid rain neutralization, aluminum chemistry and chemical weathering. As measured by the losses of alkali and alkaline earths from the ecosystem, chemical weathering rate in the Hubhard Brook area at the present time is not especially high relative to other areas. 相似文献
6.
The Wentworth plutonic complex, consisting of gabbro and granite, was emplaced in the earliest Carboniferous in the Cobequid shear zone of the northern Appalachians. The plutonic complex is coeval with a 5-km-thick pile of volcanic rocks. Early alkalic A-type granite correlates with thick felsic pyroclastics and minor basalt, which are overlain by 1.5-km-thick basalts that correlate with a large gabbro pluton that is intruded, in turn, by late granites. The basalt and gabbro are Fe-rich tholeiites. The geochemistry of the late granites suggests that they formed by differentiation of a granodioritic magma resulting from assimilation of early granite by the gabbroic magma. The Wentworth plutonic complex lies on the north side of the dextral Rockland Brook fault, near the western tip of wedge-shaped basement block of the Avalon terrane. Field observations of mesoscopic structures and map contacts show that the plutonic bodies at all structural levels are related to transpressive strike–slip faults. Dykes parallel to the mylonitic foliation in the Rockland Brook fault zone and at the contacts between igneous phases suggest that the plutons developed largely through dyke to pluton construction. The plutonism was initiated by dyking related to major faults under transpression that was partitioned into shear zone-bounded blocks, while the sinking of those blocks finally provided the space for mafic magma emplacement. Dyking was active over at least a 10-Ma time period. The overall location of plutonism in the Cobequid shear zone appears related to its position at the intersection of the shear zone bounding the southwestern margin of the Magdalen basin and the E–W transpressional contact of the Avalon and Meguma terranes. Magmatism enabled thermomechanical softening of the crust and the vertical and lateral extrusion of the wedge-shaped basement blocks, whose movement controlled the localisation of the voluminous magmatic activity. 相似文献
7.
Fabrics in the mid-crustal Bronson Hill zone of the southern New England Appalachian orogen record a range of apparent finite strains and conflicting kinematics, but structural relationships indicate coeval development. At the smallest scale of this study, shortening was accommodated in granitic orthogneiss, while transcurrent deformation was partitioned into relatively thin zones of metastratified rocks along the margins. The Monson orthogneiss can be broadly characterized by subvertical to steeply dipping S > L tectonites, subvertical to subhorizontal stretching lineations, closed to isoclinal folds, and dextral/reverse kinematics. The east-bounding Conant Brook shear zone and Greenwich syncline are characterized by steeply dipping mylonitic foliations, a range of lineations, and dextral/reverse kinematic indicators. The west-bounding Mt. Dumplin high strain zone is comprised of steeply dipping mylonites, subhorizontal lineations, and sinistral/normal kinematics. These structures reflect coeval partitioned dextral transpression, vertical extrusion, and north-directed lateral escape of the orthogneiss that was facilitated by bounding conjugate shear zones. Comparison of structural subdomains with transpressional modeling indicates vertical pseudo-monoclinic to inclined triclinic coaxial to simple shear influenced transpression. Compatibility between laterally adjacent subdomains was maintained by meso-/microscale partitioning. Absolute and relative timing constraints show that transpression was sustained from 330 Ma to 300 Ma. 相似文献
8.
《Applied Geochemistry》2003,18(9):1387-1397
An integrated approach involving the use of ion chromatography–inductively coupled plasma-mass spectrometry (IC–ICP-MS), X-ray absorption spectroscopy (XAS) and sequential extraction procedures has been employed to elucidate the solution and solid phase speciation and partitioning of As in a polluted urban watercourse. Dissolved As concentrations exceeding 130 μg l−1 and comprising entirely inorganic species were determined in the waters of Tinker Brook, a contaminated stream. Upon mixing with a relatively As-free stream, White Ash Brook, both the total concentration of dissolved As and the proportion of As(V) were observed to decrease dramatically below values expected for conservative mixing. This was ascribed to adsorption onto the Fe (oxyhydr)oxides that characterise White Ash Brook on the basis of sequential extraction and direct analysis of the solids via XAS . The shift in oxidation state is speculated to be due to the faster rate of adsorption of As(V) on Fe (oxyhydr)oxides than As(III) in this fast flowing stream system. During periods of reduced supply of anthropogenic As, a small, secondary input of As(III) to White Ash Brook is detectable, delivered by a small ochreous seepage. The Fe (oxyhydr)oxide As-rich deposits surrounding this discharge may also act as a significant source of As upon dissolution during stormflow conditions. 相似文献
9.
阿西金矿地处川甘陕“金三角”成矿集中区,中三叠统为本区内最重要的金矿源层和赋矿层。阿西金矿田的形成经历了一个从沉积－成岩－富集成矿的漫长复杂的由量的积累到质的飞跃过程,是多阶段、多层次、多因素作用有机结合的产物。赋矿岩系特征表明,阿西金矿田的主要成矿环境为半深海斜坡环境。区内岩石类型极为复杂,三大岩类均有产出。主要有细砂岩、杂砂岩、沉凝灰岩、碳酸盐岩、硅质岩;变粒岩、石英岩、角岩、大理岩、夕卡岩、角砾岩;岩浆岩类主要为英安斑岩（玢岩）、闪长岩等。其中最重要的赋矿岩石为石英岩,其主要特征为具块状、不规则条带状构造,呈褐黄色／浅灰色变余层状构造。由石英（75％－80％）、高龄石（10％－15％）、绢云母（2％－3％）、钛铁质（3％－4％）、玉髓（2％－3％）及少量碳质组成。其岩石化学成分与典型硅质岩相比,SiO2偏低,Al2O3总体偏高,出现较强的高岭土化。通过研究认为,该区主要赋矿岩石石英岩的原岩应属正常碎屑沉积岩。 相似文献
10.
About 25 economically significant, Kuroko-type massive sulfide bodies lie in a metamorphosed volcano-sedimentary complex (probable Middle Ordovician) known as Tetagouche Group, in Bathurst area, New Brunswick. Despite unresolved structural complexities, it does appear that they were deposited during a particular phase of volcanic activity and are, therefore, contemporaneous. Most of the sulfide bodies are closely associated with iron-rich rocks representing various facies of iron formation, and together with sulfides it constitutes the “ore horizon‘ which, therefore, is highly magnetic. Aero-magnetic and ground-magnetic techniques are useful to locate the ore horizon but problems are created because of the occurrence of iron-rich rocks with no sulfide, along another horizon in the Tetagouche Group.Petrographic and geochemical characteristics of various types of iron-rich rocks have been studied to see if the iron-rich rocks of the ore horizon can be distinguished from the iron-rich rocks of the other horizon. The iron-rich rocks found in the Tetagouche Group can be classified into five types: (1) cherty magnetitic rocks; (2) iron-rich chloritic rocks; (3) sideritic rocks; (4) basic iron formation; and (5) maroon shale. The basic iron formation, which is quite magnetic, gives a false indication of the ore horizon wherein the presence of any of the first three types of rocks is expected. Moreover, the basic iron formation is generally similar in appearance and mineralogy to some of the cherty magnetitic and chloritic rocks.Regarding major element composition, TiO2, Na2O, Al2O3 and CaO are higher whereas Fe2O3, FeO and MnO are lower in the basic iron formation than in the other iron-rich rocks. These geochemical characteristics can help distinguish the barren rocks of the basic iron formation from those of the ore horizon during the exploration programs. 相似文献
11.
Metamorphic evidence for an inverted crustal section, with constraints on the Main Karakorum Thrust, Baltistan, northern Pakistan 总被引：1，自引：0，他引：1
Abstract The Shyok Suture Zone separates rocks in the Asian plate from rocks in the Kohistan-Ladakh island arc. In Baltistan, this suture has been reactivated by the late 'break-back'Main Karakorum Thrust (MKT). The P-T histories of metamorphic rocks both north and south of the MKT have been determined in an effort to place constraints on the tectonic history of this zone. The terranes north and south of the MKT have different, unrelated metamorphic histories. Rocks from the Kohistan-Ladakh island arc south of the MKT have undergone a static low- P (2–4 kbar, c. 500° C) thermal metamorphism. The P-T paths and metamorphic textures of these rocks are consistent with metamorphism due to emplacement of plutonic rocks into the island arc. This metamorphism pre-dates folding and deformation of these rocks. Rocks in the Karakorum Metamorphic Complex, north of the MKT, have experienced a complex deformational and metamorphic history. Prograde metamorphic isograds have been deformed by subsequent south-verging folding and by gneiss dome emplacement. However, decompression metamorphic reactions occurred during nappe emplacement. Higher pressure rocks are associated with higher level nappes, creating an inverted pressure metamorphic sequence (8–9-kbar rocks over 5–6-kbar rocks). There is little variation in temperature with structural level (550–625° C). These two different terranes have been juxtaposed after metamorphism by the late south-directed MKT. 相似文献
12.
The travertine dams of Slade Brook, Gloucestershire: their formation and conservation 总被引：1，自引：0，他引：1
Travertine dams along the Slade Brook, Gloucestershire, occur over a 700-m length of stream, making this the largest active site of its kind so far recorded in the British Isles. The dams have considerable conservation value owing to their unusual geomorphology and influence on water flow (ponding and cascades) and their specialized flora and fauna. 相似文献
13.
Theodore J. Armbrustmacher Carl E. Hedge 《Contributions to Mineralogy and Petrology》1982,79(4):424-435
Concentrations of Rb, Sr, and REE (rare earth elements), and Sr-isotopic ratios in rocks of the Cambrian alkaline complexes in the Wet Mountains area, Colorado, show that rocks formed as end-products of a variety of magmas generated from different source materials. The complexes generally contain a bimodal suite of cumulus mafic-ultramafic rocks and younger leucocratic rocks that include nepheline syenite and hornblende-biotite syenite in the McClure Mountain Complex, nepheline syenite pegmatite in the Gem Park Complex, and quartz syenite in the complex at Democrat Creek. The nepheline syenite and hornblende-biotite syenite at McClure Mountain (535±5m.y.) are older than the syenitic rocks at Democrat Creek (511±8m.y.). REE concentrations indicate that the nepheline syenite at McClure Mountain cannot be derived from the hornblende-biotite syenite, which it intrudes, or from the associated mafic-ultramafic rocks. REE also indicate that mafic-ultramafic rocks at McClure Mountain have a source distinct from that of the mafic-ultramafic rocks at Democrat Creek.In the McClure Mountain Complex, initial87Sr/86Sr ratios for mafic-ultramafic rocks (0.7046±0.0002) are similar to those of hornblende-biotite syenite (0.7045±0.0002), suggesting a similar magmatic source, whereas ratios for carbonatites (0.7038±0.0002) are similar to those of nepheline syenite (0.7038±0.0002). At Democrat Creek, initial ratios of syenitic rocks (0.7032±0.0002) and mafic-ultramafic rocks (0.7028±0.0002) are different from those of corresponding rocks at McClure Mountain. 相似文献
14.
Joseph M. Pyle 《Lithos》2006,88(1-4):201-232
Analysis of monazite-bearing lithologies from the Precambrian Honey Brook Upland (HBU) and overlying metasedimentary Paleozoic Chester Valley Sequence (CVS) (SE PA, USA) reveals overprinting of primary major and accessory phase parageneses by texturally and compositionally disparate secondary accessory phase parageneses. Two-pyroxene temperatures of 915–945 °C for reconstituted pyroxene reflect emplacement temperatures of felsic plutonic rocks (opdalite, charnockite) prior to Mesoproterozoic metamorphism. Monazite in metavolcanic felsic gneiss yields three age domains at 1009 ± 4 Ma (2 s.e.), 965 ± 6, and 876 ± 10 Ma. The first two domains record metamorphism of the HBU after anorthosite intrusion; peak monazite–xenotime temperatures for the monazite core domain are 700 °C, and high Th/U values in the second (overgrowth) age domain likely reflect a second high-T monazite growth episode. Formation of cummingtonite coronas on orthopyroxene in opdalite constrains maximum 1010 Ma metamorphic temperatures in the “granulite-facies” terrane to 730–740 °C. Evidence of increased Cl fluid activity in the 965 Ma metamorphism includes higher Cl content of matrix apatite relative to garnet-included apatite (metavolcanics), and Cl-bearing K-hornblende succeeding cummingtonite in coronal overgrowths (opdalite). Extreme monazite Th/U values (75–250) in the rim domain suggest growth during low-T hydrothermal alteration. In the opdalite, secondary singe-grain monazite and monazite + xenotime metasomites in apatite yield ages of 714 ± 24 and 586 ± 88 Ma, temperatures of 325–425 °C, and are interpreted to reflect thermal disturbances associated with late Proterozoic plutonic and volcanic activity in the Upland. This thermal disturbance may be recorded by Rb–Sr age of 567 Ma for biotite from a HBU gneiss. Monazite age domains in metaquartzite (378 ± 28, 272 ± 44 Ma) suggest that low-grade metamorphism (260–320 °C, Mnz–Xno thermometry) of the CVS is not a result of Taconian orogenesis. 相似文献
15.
Quartz‐rich veins in metapelitic schists of the Sanandaj‐Sirjan belt, Hamadan region, Iran, commonly contain two Al2SiO5 polymorphs, and, more rarely, three coexisting Al2SiO5 polymorphs. In most andalusite and sillimanite schists, the types of polymorphs in veins correlate with Al2SiO5 polymorph(s) in the host rocks, although vein polymorphs are texturally and compositionally distinct from those in adjacent host rocks; e.g. vein andalusite is enriched in Fe2O3 relative to host rock andalusite. Low‐grade rocks contain andalusite + quartz veins, medium‐grade rocks contain andalusite + sillimanite + quartz ± plagioclase veins, and high‐grade rocks contain sillimanite + quartz + plagioclase veins/leucosomes. Although most andalusite and sillimanite‐bearing veins occur in host rocks that also contain Al2SiO5, kyanite‐quartz veins crosscut rocks that lack Al2SiO5 (e.g. staurolite schist, granite). A quartz vein containing andalusite + kyanite + sillimanite + staurolite + muscovite occurs in andalusite–sillimanite host rocks. Textural relationships in this vein indicate the crystallization sequence andalusite to kyanite to sillimanite. This crystallization sequence conflicts with the observation that kyanite‐quartz veins post‐date andalusite–sillimanite veins and at least one intrusive phase of a granite that produced a low‐pressure–high‐temperature contact aureole; these relationships imply a sequence of andalusite to sillimanite to kyanite. Varying crystallization sequences for rocks in a largely coherent metamorphic belt can be explained by P–T paths of different rocks passing near (slightly above, slightly below) the Al2SiO5 triple point, and by overprinting of multiple metamorphic events in a terrane that evolved from a continental arc to a collisional orogen. 相似文献
16.
The Xitieshan deposit (~ 64 Mt at 4.86% Zn, 4.16% Pb, 58 g/t Ag, and 0.68 g/t Au) is hosted by the Middle to Late Ordovician Tanjianshan Group of the North Qaidam tectonic metallogenic belt, NW China. This belt is characterized by island arc volcanic, ultra-high pressure (UHP) metamorphic and ophiolitic rocks. The Tanjianshan Group constitutes a succession of metamorphosed bimodal volcanic and sedimentary rocks, which are interpreted to have formed on the margin of a back-arc ocean basin between the Qaidam block and the Qilian block.Four stratigraphic units are identified within the Ordovician Tanjianshan Group. From northeast to southwest they are: 1) unit a, or the lower volcanic-sedimentary rocks, comprising bimodal volcanic rocks (unit a-1) and sedimentary rocks (unit a-2) ranging from carbonates to black carbonaceous schist; 2) unit b, or intermediate-mafic volcaniclastic rocks, characterized by intermediate to mafic volcaniclastic rocks intercalated with lamellar carbonaceous schist and minor marble lenses; 3) unit c, a purplish red sandy conglomerate that unconformably overlies unit b, representing the product of the foreland basin sedimentation during the Early Silurian; 4) unit d, or mafic volcanic rocks, from base to up, comprising the lower mafic volcaniclastic rocks (unit d-1), middle clastic sedimentary rocks (unit d-2), upper mafic volcaniclastic rocks (unit d-3), and uppermost mafic volcanic rocks (unit d-4). Unit a-2 hosts most of the massive sulfides whereas unit b contains subordinate amounts.The massive stratiform lenses constitute most of the Xitieshan deposit with significant amount of semi-massive and irregularly-shaped sulfides and minor amounts in stringer veins. Pyrite, galena and sphalerite are the dominant sulfide minerals, with subordinate pyrrhotite and chalcopyrite. Quartz is a dominant gangue mineral. Sericite, quartz, chlorite, and carbonate alteration of host rocks accompanies the mineralization.U-Pb zircon geochronology yields three ages of 454 Ma, 452 Ma and 451 Ma for the footwall felsic volcanic rocks in unit a-1, sedimentary host rocks in unit a-2 and hanging-wall unit b, respectively. The Xitieshan deposit is considered to be coeval with the sedimentation of unit a-2 and unit b of the Tanjianshan Group. The Xitieshan deposit has been intensely deformed during two phases (main ductile shear and minor ductile-brittle deformation). The main ductile shear deformation controls the general strike of the ore zones, whereas minor deformation controls the internal geometry of the ore bodies. 40Ar-39Ar age of muscovite from mylonitized granitic gneisses in the ductile shear zone is ~ 399 Ma, which is interpreted to date the Xitieshan ductile shear zone, suggesting that Early Devonian metamorphism and deformation post-dated the Tanjianshan Group.The Xitieshan deposit has many features similar to that of the Bathurst district of Canada, the Iberian Pyrite Belt of Spain, the Wolverine volcanogenic massive sulfide deposit in Canada. Based on its tectonic setting, host-rock types, local geologic setting, metal grades, geochronology, temperatures and salinities of mineralizing fluid and source of sulfur, the Xitieshan deposit has features similar to sedimentary exhalative (SEDEX) and VMS deposits and is similar to volcanic and sediment-hosted massive sulfide (VSHMS) deposits. 相似文献
17.
《Chemie der Erde / Geochemistry》2021,81(1):125687
Sanandaj-Sirjan Zone (SaSZ) is one of the most dynamic structural zones of Iran, which is divided into three main parts: Northern, Central and Southern. The northern SaSZ has been affected by deformation due to fault activities near the Zagros suture zone, and mylonitic structures have overprinted these rocks and was affected by three episodes of magma injection during the Permian-Carboniferous, Early Cretaceous and Cenozoic. In this study, the rock units investigated that have been considered Precambrian-Paleozoic basement on geological maps. This paper considers zircon U-Pb dating, whole-rock chemistry and Sr-Nd isotope ratios of Cretaceous magmatic rocks in the N-SaSZ to develop a new geodynamic model for the evolution of these magmatic rocks. The new zircon U-Pb ages obtained in this study show that the magmatic rocks crystallized at 115–107 Ma in the Early Cretaceous (Aptian-Albian) and are much younger than the supposed ages presented on geological maps. This complex classified into two main groups of basic-intermediate and acidic rocks based on SiO2 contents. The whole-rock chemistry of the basaltic and andesitic rocks, which are interbedded with marine shallow-water sedimentary deposits, shows their typical calc-alkaline affinity and subordinate tholeiitic series on an active margin. The positive εNd(t) of approximately +4 for some undifferentiated basalts with negative Ti and Nb anomalies shows the relation of these rocks to calc-alkaline magmatism and was generated by the partial melting of subcontinental lithospheric mantle (SCLM). Granitoid rocks with some affinity to the peraluminous group with a negative εNd(t) value (-3.2) mainly and negative Ti and Nb anomalies plot in an active margin tectonic setting. Simultaneous mafic calc-alkaline volcanism and the generation of granitic intrusions in the Early Cretaceous could have occurred on an active margin. Due to the absence of Jurassic arc related magmatic rocks in northern SaSZ and presence of Cretaceous calc alkaline magmatic activity, which are not observed in the central SaSZ, support the idea that the subduction of the Neotethys beneath the northern SaSZ started in the Early Cretaceous. 相似文献
18.
The importance of oxygen isotope geochemistry in studies of terrestrial and extra-terrestrial silicate rocks was recognized nearly sixtyeight years ago soon after the discovery of O18 and O17. As early as 1934, the significance of oxygen isotope variations in rocks and minerals was stressed by Russian geochemists who also pioneered the discipline of silicate oxygen isotope geochemistry. It is now known that processes involving isotopic interaction between rock and water, magmatic differentiation, and metamorphic recrystallization fractionate oxygen isotopes in the lithosphere. δO18 (the conventional notation for reporting O18/O16 ratios in rocks and minerals) is highest in sedimentary rocks (17 to 35 ‰) and lowest in igneous rocks (4 to 12‰). Metamorphic rocks have intermediate values. δO18 in mafic minerals (1 to 8‰) is lower than in felsic minerals (8 to 16‰). In igneous and metamorphic rocks, quartz is most enriched in O18 (10 to 16‰) and magnetite the least (1 to 2‰). An important application of O18/O16 techniques is in geothermometry, where these are capable of elucidating several petrological processes. 相似文献
19.
S. B. Lobach-Zhuchenko V. V. Balagansky Sh. K. Baltybaev G. V. Artemenko E. S. Bogomolov A. V. Yurchenko L. M. Stepanyuk V. V. Sukach 《Lithology and Mineral Resources》2014,49(5):381-397
Paleoarchean granulite-facies metasedimentary rocks (quartzites, garnet quartzites, garnet-pyroxene gneisses, pyroxene-magnetite and magnetite quartzites) attributed to the Dniester-Bug Group of the Ukrainian Shield were studied. On the basis of geochemical data, including REE, the primary composition of these rocks was reconstructed as association of Fe-rich sandstones and sublitharenites, Fe-shales, and BIFs. This sedimentary association is similar to the rocks of other ancient greenstone belts and ascribed to the Algama-type iron formation. The sum of Al2O3, CaO, Na2O, and TiO2, high Zr contents (>100 ppm in quartzites), and the presence of detrital zircon grains of different ages are consistent with the terrigenous nature of sedimentary rocks. The Sm/Nd, Ti/Zr, Sc/Zr, and Ni/Zr ratios indicate the predominance of granitoid rocks in the source areas. The elevated Cr contents suggest that, in addition to granitoids, the source area contained ultramafic rocks. Geochemical characteristics, such as Fe/Mn ratio, low REE contents, and variations of REE versus the sum of Ni, Co, and Cu testify that sedimentation occurred under shallow-water conditions on the continent or its slope, similarly as the formation of ancient (3.5–3.2 Ga) basalt-komatiitic series intercalated with sedimentary rocks in the Pilbara Craton. The age of supracrustal rocks of the Dniester-Bug Group was constrained within the time interval of 3.4–3.2 Ga on the basis of U-Pb zircon dating and determination of Nd isotope composition. The DM model age of quartzites varies from 3.37 to 3.5 Ga. Sedimentary rocks together with volcanic rocks represent the oldest supracrustal association of the East European Platform. 相似文献
20.
Marcelo Augusto de Oliveira Roberto Dall’Agnol Fernando Jacques Althoff Albano Antonio da Silva Leite 《Journal of South American Earth Sciences》2009,27(2-3):146-160
The Archean sanukitoid Rio Maria Granodiorite yielded zircon ages of ～2.87 Ga and is exposed in large domains of the Rio Maria Granite-Greenstone Terrane, southeastern Amazonian craton. It is intrusive in the greenstone belts of the Andorinhas Supergroup, in the Arco Verde Tonalite and Caracol Tonalitic Complex (older TTGs). Archean potassic leucogranites, younger TTGs and the Paleoproterozoic granites of Jamon Suite are intrusive in the Rio Maria Granodiorite.The more abundant rocks of the Rio Maria Granodiorite have granodioritic composition and display medium to coarse even-grained textures. These rocks show generally a gray color with greenish shades due to strongly saussuritized plagioclase, and weak WNW-ESE striking foliation. The significant geochemical contrasts between the occurrences of Rio Maria Granodiorite in different areas suggest that this unit corresponds in fact to a granodioritic suite of rocks derived from similar but distinct magmas. Mingling processes involving the Rio Maria Granodiorite and similar mafic to intermediate magmas are able to explain the constant occurrence of mafic enclaves in the granodiorite.The associated intermediate rocks occur mainly near Bannach, where mostly quartz diorite and quartz monzodiorite are exposed. The dominant rocks are mesocratic, dark-green rocks, with fine to coarse even-grained texture. The Rio Maria Granodiorite and associated intermediate rocks show similar textural and mineralogical aspects. They follow the calc-alkaline series trend in some diagrams. However, they have high-Mg#, Cr, and Ni conjugate with high contents of large ion lithophile elements (LILEs), typical of sanukitoids series. The patterns of rare earth elements of different rocks are similar, with pronounced enrichment in light rare earth elements (LREEs) and strong to moderate fractionation of heavy rare earth elements (HREEs).Field aspects and petrographic and geochemical characteristics denote that the granodiorites and intermediate rocks have sanukitoid affinity. However, geochemical data suggest that the intermediate rocks and the granodiorites are not related by a fractional crystallization process. It is concluded that the intermediate rocks derived from similar sources to the granodiorites, but probably result from a higher degree of melting, being both cogenetic, but not comagmatic rocks.Mineralogical aspects associated with experimental evidence suggest that the Rio Maria Granodiorite magma was relatively water-enriched (>4 wt.%), explaining the presence of hornblende at the liquidus and the absence of clinopyroxene and orthopyroxene in the studied rocks. The occurrence of well-preserved magmatic epidote crystals, admitting that the Rio Maria Granodiorite was emplaced at shallow crustal levels, points to a rapid ascent of the Rio Maria Granodiorite magma. 相似文献