Genesis Of Rocks Research Articles

Late Cretaceous subduction-related magmatism on the southern edge of Sabzevar basin, NE Iran

Despite detailed geological studies on the Late Cretaceous Sabzevar ophiolites, the non-ophiolitic, subduction-related igneous rocks of NE Iran are still enigmatic. This paper focuses on the Late Cretaceous, non-ophiolitic Sabzevar magmatic rocks, and presents both precise U–Pb ages and a tectonomagmatic scenario for the genesis of these igneous rocks. A thick sequence of Late Cretaceous acidic volcanic rocks associated with sandstones, shales and pelagic limestones dominates the northern parts of the Lut block. This sequence is intruded by a series of shallow mafic to felsic intrusions. New zircon U–Pb results on the intrusive and extrusive rocks show ages of 75–101 Ma. These ages show a pulsed magmatism in NE Iran, which lasted for c. 25 myr. The whole-rock geochemistry of the igneous rocks shows suprasubduction-zone-related geochemical signatures, represented by negative anomalies in high field strength elements and enrichment in large ion lithophile elements. The initial 87 Sr/ 86 Sr ratios and εNd( t ) values of the extrusive rocks range from 0.70411 to 0.70628 and from +5.9 to +7.4 respectively, and for intrusive rocks are in the range of 0.70423–0.70579 and +5.8 to +7.2. High εNd( t ) values for these rocks confirm that their melts were derived from a depleted-mantle source. Both geochemical and isotopic data indicate that the genesis of these rocks was related to the partial melting of mid-ocean ridge basalt-type slab or depleted-mantle wedge sources during the northward movement of the Sabzevar oceanic slab beneath the southern edge of Eurasia (Turan block) in Late Cretaceous time.

Using Radioactive Elements and the Th/U Ratio in Study of the Geochemical Typification of Granitoids and Their Intrusive Character

Abstract —This paper considers the use of radioactive elements for the solution of geochemical problems related to granitoid magmatism. The metallogenic types of granites are recognized on the basis of the contents of radioactive (Th-bearing, rare-metal, U-bearing, gold ore, etc.) elements in them. Special attention is focused on the Th/U ratio, which is higher than 2.5 in primary igneous rocks, such as Li–F granites of the Ongon-Hayrhan deposit (Mongolia). Granitoids with Th/U < 2 (and, the more so, Th/U < 1) cannot be considered igneous. They are either metasomatized or of metasomatic genesis. Petrochemical and geochemical data on these types of rocks should be used for petrological implications with great care. Radiogeochemical indices can and must be used as additional indicators of the genesis of rocks, when it is controversial because of the widely manifested convergence and metasomatism processes.

Material source analysis and element geochemical research about two types of representative bauxite deposits and terra rossa in western Guangxi, southern China

In this study, geologic field observations and geochemical indicators of the types of representative bauxite deposits, terra rossa and underlying carbonate rocks in western Guangxi, southern China, were investigated. Their interrelations, material sources, formation and evolution processes were discussed. The results show an inherited relationship between terra rossa and its underlying limestone. The relevant parameter diagrams of trace and rare earth elements suggest that the two types of bauxite deposits probably possess identical material sources. Both bauxite deposits and terra rossa have close affinities with each other, implying that carbonate rocks similar to the underlying limestone might be a major material source for their formation. However, the eolian component represented by Chinese loess and igneous rock genesis with respect to Emeishan basalt plume may not contribute so much to the material source of bauxite deposits and terra rossa. In the process of leaching and pedogenesis, the mobile elements, such as Ca, Mg and Na, were preferentially dissolved and removed from parent rocks, and the less mobile elements Al, Fe, Ti and Si were accumulated and enriched. During the period of diagenesis and bauxitization, elements K and Si were largely removed from the system. Especially, in transformation from primary bauxite to secondary bauxite deposit, the contents of many “less mobile” elements, such as Fe and Ti, decreased. This finding is possibly due to the strongly acidic environment caused by decomposition of pyrite in primary sedimentation-type bauxite deposit. This study systematically analyzed the inherited relations and evolution processes among carbonate rock, terra rossa, and two types of bauxite deposits, and gave a guide for the bauxite deposit prospecting in western Guangxi.

Gender, genius and rock and roll in‘Roy Orbison and Friends: A Black and White Night’

AbstractTo a considerable extent, the mythology of rock and roll rebellion is predicated upon a similarly mythologised male sexual potency that Simon Frith and Angeld McRobbie have characterised as ‘aggressive, dominating, and boastful, … [constantly seeking] to remind the audience of [its] prowess, [its] control’ (Frith and McRobbie 1990, p. 319). In this article, I look to Roy Orbison – a musician who was a key figure in the genesis of rock and roll, but who nevertheless subverts this phallocentric meta-narrative. Focusing on the 1987 concert film,Roy Orbison and Friends: A Black and White Night, I argue that Orbison's staid performance style, unusual voice and unconventional songwriting as evidenced (and amplified) by that film trouble the purportedly monolithic rock and roll masculinity, and the concomitant mythology of rebellion. At the same time, however, I propose that even as normative masculinity appears to be destabilised, a close reading of the film reveals that the performance situates Orbison within a different masculinist discourse: the 19th-century Romantic discourse of masculine genius that continues to inflect 21st century notions about artists and art music. Thus, inBlack and White Night, normative and non-normative masculinities are thoroughly imbricated, each simultaneously destabilising and reaffirming the other.

ELECTRONIC MICROSCOPE INVESTIGATION OF THE UPPER JURRASSIC MICRITE OF THE CARPATHIAN FOREDEEP IN CONTEXT OF THEIR GENESIS

On the base of the electronic microscope investigation of the Upper Jurrassic micrite of the Carpathian Foredeep two types of ”replica surfaces” were revealed, such as: block and agregatic smoothly block. The relicts of gelly–like threads of Cyanophyta algaes, which show the biochemogenic genesis of rocks, formed in conditions of the tidal shallows of ancient shelf of the Carpathian – Podolian continental margin of Tethys ocean in the high salinity waters were detected in the latter type at for the first time.

Magmatic epidote from gabbro

Epidote is traditionally considered a mineral indicating metamorphic and metasomatic genesis of rocks. Recently, the magmatic nature of epidote of many granitoid intrusions has been proved. We present the first data on the magmatic origin of epidote, which is one of the rock-forming minerals of anorthite fassaite and biotite-amphibole gabbros. These rocks form block inclusions in the serpentinite melange plate localized in gneisses of the Proterozoic Selyankino strata of the Il’meny Mountains complex in the South Urals. The holocrystalline structure and cenotypal appearance of the epidote gabbros, their gabbro, porphyritic, and ophitic textures, the induction surfaces of synchronous growth between the hosted minerals, the mineral crystal faceting, and high-temperature paragenesis evidence the magmatic origin of the rocks.

New directions in Wilson Cycle concepts: Supercontinent and Tectonic Rock Cycles

Modern earth science pedagogy is increasingly based on an integrated systems framework, where all of the major earth systems, including lithospheric cycles, are interlinked and dependent on each other through feedback loops. Most secondary school and introductory college-level geology courses present the concepts of plate tectonics and rock classifi cations. However, many instructional approaches fail to integrate these topics within an earth systems viewpoint, where supercontinent cycles are viewed in both spatial and temporal dimensions, and the classifi cation of rock types is intrinsically dependent on the tectonic, as well as the depositional environment in which they were formed. This contribution presents new tectonic animations and images that allow students to investigate supercontinent cycles (e.g., the assembly and breakup of Rodinia, and the Paleozoic interactions of Laurentia, Gondwana, and Baltica) and integrated Wilson Cycle and Tectonic Rock Cycles that equate rock genesis with tectonic and environmental settings. Central to these visualizations is the concept that processes of rock genesis have evolved, and will continue to evolve, through geologic time. We discuss the conceptual and historical background for each of these visualizations, and follow this with detailed descriptions of, and educational uses for, the images and animations. To be of optimal instructional utility, the animations and images consider the visual domain as a primary, rather than secondary instructional tool. As such, they are designed to function optimally in an inquiry-driven educational setting. They take into account the complex cognitive interactions between visual and verbal representations in learning environments by providing rapid interchange between these two domains. Where factual information is of interest, e.g., when introducing a topic or relaying important background information, the typical verbal primacy is observed. But in instances where spatial and temporal relationships are of interest, such as in the Rodinia and Pangaea supercontinent cycles, and the “No Rock is Accidental” tectonic rock cycle, the visualizations assume the primary role, with text-based annotations or verbal discussion as secondary. We conclude with discussions on the importance of inquiry-based educational approaches and effective ways of evaluating educational visualizations. We suggest that to best utilize the available media (digital and paper based), the level of cognitive engagement of the learning task should be closely tied to a taxonomy of visualizations that encompass detailed, integrated representations and animations. Inquiry-based interfaces, such as we present in this paper, promote more mindful articulations of the desired learning tasks and an increased retention of the subject material outside the bounds of the classroom. Teaching a systems-based understanding of the Earth and the concepts of evolving tectonic and rock cycles provides students with holistic foundations from which they can better evaluate, and make decisions about, their living environment.

Neoarchean trans-subprovince sediment transport in southwestern Superior Province: sedimentological, geochemical, and geochronological evidence

At ~2700 Ma volcanism spawned by northward-directed subduction under Wabigoon subprovince, Canadian Shield, fed sediment into an adjacent fore-arc basin – trench system. Volcanically active island arcs to the south (Wawa subprovince) were transported northward during this interval and at ~2690 Ma collided with the accretionary complex (Quetico subprovince). Metamorphosed sedimentary units intercalated with the ocean floor and island-arc volcanic rocks of northern Wawa subprovince provided the opportunity to amass data useful in extending the paleogeographic interpretation of the area. This study examines the McKellar Harbour Formation, a several kilometre thick assemblage of graded metasandstone beds outcropping to the north of Lake Superior. Its sedimentology, geochemistry, and detrital U–Pb zircon geochronology were investigated in conjunction with the same attributes of possible correlative units in the region. Similarities between the McKellar Harbour Formation and sedimentary rocks composing the fore-arc basin and Quetico trench to the north indicate that by 2696 Ma the trench had been overwhelmed by sediment, to the point where turbidity currents were overflowing onto the abyssal ocean plain, building a prograding submarine ramp–fan that was to become the McKellar Harbour Formation. The turbiditic ramp–fan assemblage comprising most of this extensive depositional system crosses much of southern Superior Province, highlighting the interrelated genesis of rocks formed during amalgamation of the individual subprovinces.

Géochimie du volcanisme birimien (Protérozoïque inférieur) de l'Unité de Haute-Comoé. N-E de la Côte-d'Ivoire: Premiers résultats

The Lower Proterozoic (Birimian) Haute-Comoé Unit (West African Craton of Côte-d'Ivoire) consists of a metavolcanic and metasedimentary suite. The metavolcanic formation deals with a main T-MORB type tholeiitic serie with intercalations of little-extended calc-alkaline volcanites chiefly composed of andesitic to dacitic rocks. Local hydrothermal alteration is responsible of the genesis of rocks with spilitic affinities. The volcanism setting would be anorogenic, in a tectonic context of frifting in an ensialic environment, trough a strongly slimmed crust, after Kröner's pattern for Precambrian formations.

The Precambrian metavolcano-sedimentary sequence east of Ife and Ilesha/SW Nigeria. A Nigerian ‘greenstone belt’?

The metavolcano-sedimentary sequence east of Ife and Ilesha southwest Nigeria has previously been classified as a proterozoic mobile belt. Field observations indicate, however, that this sequence has a typical, for Archaean, granite-greenstone rock succession. Geochemical data could support this opinion. In spite of the lack of age determinations, it is possible to place these rock successions in the early Archaean. This new classification of the metavolcano-sedimentary sequence of the Ife-IIesha area is, therefore, of far-reaching economic importance. The known mineralizations of gold, cassiterite, niobite, tantalite and tungsten in this area and its surroundings call for further prospection, not only in southwest Nigeria, but also in the other mobile belt region of Nigeria. One important result is the fact that geochemical methods are a tool for the stratigraphic classification and rock genesis in areas with strong lateritic weathering or metamorphic overprint.

Geochemical modelling of basalts from DSDP Leg 65, East Pacific Rise, Gulf of California

Major and rare earth element (REE) data for basalts from Holes 483, 483B, and 485A of DSDP Leg 65, East Pacific Rise, mouth of the Gulf of California, support a simple fractional crystallization model for the genesis of rocks from this suite. The petrography and mineral chemistry (presented in detail elsewhere) provide no evidence for magma mixing, but rather a simple multistage cooling process. Based on its lowest TiO 2 content (0.88%), FeO ∗ MgO ratio (0.95 with total Fe as FeO), and Mg# (100 Mg Mg + Fe ″ = 70 ), sample 483-17-2-(78–83) has been selected as the most primitive primary magma of the samples analyzed. This is supported by the REE data which show this sample has the lowest total REE content, a La Sm cn (chondrite-normalized) = 0.36, and Eu Sm cn = 1.05. Because other samples analyzed have higher SiO 2, lower Mg#, and a negative Eu anomaly ( Eu Sm cn as low as 0.89), they are most likely derivative magmas. Wright-Doherty and trace element modelling support fractional crystallization of 14.1% plagioclase (An 88), 6.7% olivine (Fo 86), and 4.7% clinopyroxene (Wo 41En 49Fs 10) from 483-17-2-(78–83) to form the least differentiated sample with Mg# = 63. The La Sm cn of this derivative magma is almost identical to the parent magma (0.35 to 0.36), but the other samples have higher La Sm cn (0.45 to 0.51), more total REE, and lower Mg# (60 to 56). Both Wright-Doherty and trace element modelling indicate that the primary magma chosen cannot produce these more evolved samples. For the major elements, the TiO 2 and P 2O 5 are too low in the calculated versus the observed (1.38 to 1.90; 0.11 to 0.17, respectively, for example). Rayleigh fractionation calculates a lower La Sm cn and requires about 60% crystal removal versus 40% for the Wright-Doherty. These more evolved samples must be derived from a parent magma different from the one selected here and, unfortunately, not sampled in this study. A magma formed by a smaller degree of partial melting with slightly more residual clinopyroxene left in the mantle than for sample 483-17-2-(78–83) is required.

Removal of atmospheric argon contamination and the use and misuse of the K–Ar isochron method: Discussion

Hayatsu and Carmichael(1977) have shown that they can obtain reproducible analyses for the amount of tightly bonded 36Ar in rock samples but they have not proven that the observed 36Ar is initial in the sense that it was acquired at the time of crystallization of the rocks that they studied. They are not alone in their interpretation that tightly bonded 36Ar is of initial origin (Brown et al. 1974; Mellor and Mussett 1975; Fitch et al. 1976). The observation of reproducible 36Ar content in rocks is not new it is familiar to anyone who has run large numbers of replicate Ar analyses. Much of the observed 36Ar in the experiments described could have been acquired at any time between initial crystallization and removal to the dating laboratory and my own experience leads me to believe that post-crystallization contamination is the normal case. Considerable amounts of tightly bonded 36Ar are acquired by rocks in the field by gradual adsorption and trapping of argon from interstitial natural fluids. In rare circumstances the 40Ar/36Ar atio in the interstitial fluid may not be atmospheric but in general I think it is. Those who associate the tightly bonded 36Ar with igneous rock genesis and use 40Ar/36Ar 40K/36Ar isochrons to calculate ages are operating on false premises and creating numbers of doubtful significance. I have observed tightly bonded 36Ar in hundreds of dated rocks. The quantities vary considerably from specimen to specimen and most significantly they are consistently small in young and very fresh volcanic rocks (Fig. 1). Unless one has studied rocks such as modern Hawaiian basalts, Holocene basalt from the Snake River Plain. or Pleistocene volcanics from Antarctica one has never seen fresh I rocks. This point needs emphasis. Rocks often described as fresh, with well preserved igneous textures, usually contain detectable alteration. This is often dismissed as deuteric, and insignificant, but it is invariably present. I have never been able to collect an older (greater than a few million years in temperate climates) basalt that was completely fresh by the criteria calibrated on the young volcanic suites mentioned, in spite of deep quarries, generous application of sledge hammer, or trimming with a diamond saw. I am convinced it is this incipient and secularly increasing alteration that is the rock component that traps 36Ar from interstitial 1 I I I I

The role of CO2 in alkali rock genesis

The role of CO₂ in alkali rock genesis

Trace elements in the Singhbhum granite, India

A study of the trace elements in the Singhbhum granite from its northern borders (Saraikela) and the central mass (Chaibasa) reveals that there are differences not only in the limit of concentration but also in their behaviour. It is found that while correlating with the petrographic and also the major elements behaviour, the trace elements always do not mark an increase or decrease during the various stages of the evolution of the Singhbhum granite. The behaviour of the trace-elements as revealed from the previous studies relating to both igneous and metamorphic rocks, is significant in working out the physico-chemical factors involved in the genesis of rocks. In the magmatic rocks, the trace elements obviously follow a trend and go in accordance withGoldschmidt's principles of camouflage, capture and admission. In the present study such uniformities are not found. The present study reveals firstly, the different nature of chemical gradients formed by these elements during different geological environments; secondly, the mutual relationships and the ratio of certain traceelements in the different associations, indicating the original heterogeneity of these rocks and their formation under different geological as well as physico-chemical conditions. It has been concluded (Roonwal, 1968) that the rocks studied here as representing parts of Singhbhum granite do not form one granite series as referred to byRead. In fact they represent rocks formed from material having originally different lithological composition.

Classification and origin of the igneous rocks

Present day igneous petrology requires three systems of rock classification: (1) a quantitative mineralogical and/or chemical system that is suitable for systematic petrography, (2) a genetic system that is designed for use in petrogenetic discussions, and (3) a flexible system of petrography that can be employed in mapping igneous rocks in limited areas of particular interest. Current ideas on igneous rock genesis are discussed; and it is concluded that while some rocks may be classified as allochthonous igneous bodies, the most common igneous rocks evolve from (1) simple primary magmas, (2) compound primary magmas, (3) secondary magmas that differentiate at high-level, (4) secondary cumulate magmas, (5) syntectic magmas, or (6) magmas generated by tectonic activity.

Uranium-Thorium-Lead Isotope Relations in Lunar Materials

The lead isotopic compositions and uranium, thorium, and lead concentrations have been measured on six samples of material from the Sea of Tranquillity. The leads are moderately to very radiogenic; the initial lead concentrations are very low; the uranium and thorium levels are 0.26 to 0.88 and 0.87 to 3.35 parts per million, respectively. The Th/U ratios cluster about a 3.6 value. Apparent ages calculated for four rocks are 4.1 to 4.2 x 10(9) years. Dust and breccia yield apparent ages of 4.60 to 4.63 x 10(9) years. The uranium-lead ages are concordant, or nearly so, in all cases. The lunar surface is an ancient region with an extended record of events in the early history of the solar system. discrepancy between the rock ages and dust ages poses a fundamental qusestion about rock genesis on the moon.

Comparative characteristics of jadeite and associated rocks from Polar Ural and Prebalkhash region

New additional data on petrography and mineralogy of jadeite and associated rocks from Polar Urals and Prebalkhash region are given. Some questions of the jadeite rock genesis in hyperbasites are also considered. In Pay-Yer massif, Polar Urals, the jadeite rocks are associated with gabbro-pyroxenite complex, garnet-bearing gabbro-amphibolite, metamorphic schists and glaucophane rocks. Jadeitite and albitite form a chain of lens-shaped bodies in hyperbasites and represent metasomatic metagabbro rocks, altered under the conditions of high pressure and high soda potential. Secondary processes of diaphthoresis are strongly revealed. In Kantegir hyperbasite massif, northern Prebalkhash region, jadeitites form drop – like bodies together with numerous inclusions of unaltered granitoids and other metasomatic rocks. Jadeitite and hornblende are the earliest (high temperature) metasomatic rocks. Chemical, optical and X-ray data of jadeitite and accessory and other minerals of jadeite rocks are given. The authors c...

Discussion

Professor B. G. K ing said that he welcomed this investigation, which was of importance in understanding the genesis of rocks containing pyroxenes of the aegirine–augite series. Rocks containing such pyroxenes in association with nepheline or nepheline and alkali feldspar were found in most alkaline complexes, including those of eastern Africa. Field and petrographical study suggested that the variations in the composition of the alkali pyroxenes were likely to be mainly controlled by or dependent on temperature of crystallization. No adequate data on such pyroxenes existed and it was for this reason that a programme of analysis of pyroxenes and of their containing rocks was undertaken under the speaker’s direction. A notable result of the investigation was the discovery of the great effect on equilibrium of relatively small amounts of diopside in the aegirine–augite series. This might explain the prevalence in rocks of pyroxenes on the one hand ranging from diopside to those containing moderate proportions of diopside and on the other hand of nearly pure aegirine. Diopsidic aegirines were rather uncommon, suggesting that they were stable over a very limited range of temperature/compositional conditions. Dr D. K. B ailey said that the results, and their representation, raised several interesting questions: (1) It was important to emphasize that the information on the various composition-planes was in the form of a projection, and expressions such as ‘piercing-point’ (which implied, among other things, a defined liquid composition) might be misunderstood. It would be particularly interesting, for instance, to know the liquid composition

Interpretation of Stratification: ABSTRACT

Ancient environments may be interpreted, at least in part, from primary structures in sedimentary rocks. Because such structures are largely developed during deposition, they provide information on the processes involved and on the general geologic and climatic setting. Unfortunately, many sedimentary structures are poorly understood; numerous data are required from the observation of modern sediments and of controlled experiments before positive conclusions can be drawn concerning rock genesis. Principal types of stratification and cross stratification include (1) flat or horizontal bedding, (2) low-angle, simple or planar cross-stratification, (3) tabular-planar cross-stratification of intermediate angle, (4) high-angle, wedge-planar cross-stratification, (5) ripple lamination, (6) graded bedding, and other less common varieties. Some of these structures are typical of more than one environment; other structures are represented by two or more varieties in a single environment. Information on natural combinations or associations of these structures is especially valuable for paleoecological interpretations. Environments of deposition that have been studied, in greater or lesser degree, with respect to the characteristic type or types of stratification are dunes, river channels, river floodplains, alluvial fans, delta cones, tidal flats, foreshore beaches, backshore beaches, and barrier bars. Very little information is yet available concerning primary structures representative of the various offshore marine environments. End_of_Article - Last_Page 537------------

GEOCHEMISTRY OF ISOTOPES

The isotopic composition of the earth's crust is never static, due to various radioactive processes. This is especially noticeable on geologic time scales. The isotopic exchange reactions, particularly those involving lighter atoms, are intensive in the outer shell of the earth, notably in the biosphere. The biospheric exchange reactions are primarily responsible for local enrichments and anomalous absences of various isotopes, especially those of oxygen, carbon, sulfur, and hydrogen. Isotope geochemistry is a tool which will help solve problems of past major and minor geologic events in the genesis of rocks and minerals. It is hoped that isotope geochemistry will introduce two new parameters into geologic investigation of time and temperature of natural processes. --G. E. Denegar.