Showing papers on "Petrography published in 1982"

Composition of modern deep-sea sands from arc-related basins

Abstract: SUMMARY: Petrographic and chemical examination of modern deep-sea sands shows a clear distinction between active and passive plate-tectonic settings Among active settings, sands from forearc basins of island arcs and basins from strike-slip continental margins can be distinguished, but those from other arc-related settings overlap considerably in composition In particular, sands from continental margin subduction zones and those from the backarc of island arcs appear to be indistinguishable Chemically, arc-related sands are very close in composition to ancient greywacke sandstones For instance, sodium almost always exceeds potassium Thus it is not necessary for sodium to be added to sands diagenetically to make greywackes, as has been suggested The influence of tectonics on the composition of sandstones has long been a central feature of sandstone petrology The geosynclinal model of global tectonics stimulated much thought on this subject Probably the best-known ideas are those of Krynine (eg 1942) Subsequently such research declined, but the plate model for global tectonics has produced a new wave of interest (eg Crook 1974; Schwab 1975; Potter 1978; Dickinson & Suczek 1979) Much of this work has relied on data from ancient sediments, where diagenetic changes and difficulty in de- ciphering the tectonic setting limit the useful- ness of the conclusions Much more work is needed on recent sediments to establish a base- line against which ancient sediments can be compared We have previously reported (Valloni & Maynard 1981) the results of our work on the petrography of a set of recent deep-sea sands from piston cores In this paper we shall com- pare this petrographic information with bulk chemistry, and attempt to relate these to tecto- nics in arc-related settings

Igneous Rocks of the British Isles

Abstract: A team of geologists explores the major occurrences of igneous rock in Britain and Ireland. Draws on classical accounts and modern assessments in terms of crustal plate movements, geochemistry, and magma genesis. Arranged chronologically, and the changing pattern of magnetism is viewed in the perspective of the evolution of the British Isles. Includes an illustrated petrographic appendix, 24 tables of selected chemical analysis and a compilation of geochronological data.

The Cerchar Abrasivity Index and its relation to rock mineralogy and petrography

Abstract: The Cerchar Abrasivity Index and Its Relation to Rock Mineralogy and Petrography To evaluate the relation between the Cerchar Abrasivity Index (a parameter used in calculating advance rates of full face tunnelling machines) and the petrography of the rocks, measurements were made on minerals and monomineralic rocks. From these data a theoretical abrasivity (quartz equivalence) can be calculated for every rock composition. From the comparison of the theoretical and experimentally determined abrasivity the influence of fabric and other factors besides mineralogical composition were deduced.

Paleomagnetism of the Moenkopi Formation, Colorado Plateau: Basis for long-term model of acquisition of chemical remanent magnetism in red beds

Abstract: Paleomagnetic studies of Moenkopi samples from several sites on the Colorado Plateau, including a complete 427-m section at Sinbad Valley, western Colorado, reveal the following: (1) the presence of both normal and reversed Triassic components in many of the same samples at many localities, (2) the close correlation of the magnetic polarity and intensity of magnetic remanence with the lithologic characteristics in the Sinbad Valley section, (3) a streaked pattern of the remanence directions in any one member in the Sinbad Valley section similar to that expected from polar wander, (4) a lack of progressive change from the base upward, and (5) the common intermediate remanence directions of weak intensity (suggesting the presence of two or more nearly balanced anti-parallel components) throughout the formation. The results of our investigation, combined with previously reported petrographic and paleomagnetic studies of the Moenkopi Formation, have led us to conclude that the principal remanence carried by the formation is chemical remanent magnetism (CRM) acquired diagenetically over a geologically long time interval. From the available data we have formulated the following model to explain how the CRM was acquired through natural processes acting intermittently at geologically reasonable rates. When initially deposited, the Moenkopi sediments consisted primarily of first-cycle detritus containing a few percent of iron-bearing accessory minerals such as biotite, hornblende, magnetite, and ilmenite. At some time after deposition, under favorable subsurface conditions, intrastratal alteration began, involving breakdown of iron-bearing silicate and oxide minerals, partial replacement of some iron-rich silicate grains by hematite, martitization of magnetite, growth of hematite pods and blebs in ilmenite and biotite, and precipitation of crystalline hematite in both primary and secondary voids. The authigenesis spanned at least two polarity events and probably continued over several tens of millions of years. Stratigraphic units with grossly similar lithologies underwent similar diagenetic alterations at rates and to extents that were different from other stratigraphic units having different lithologies. Because of variations in the rate and duration of alteration and variations in the length of normal versus reversed polarity events, any given lithologic unit tended to be dominated by one polarity. The model given above seems to be capable of explaining not only those magnetic and petrographic features found in the Moenkopi Formation but also those of most other red beds.

Clay mineral diagenesis in Rotliegend aeolian sandstones of the southern North Sea

Abstract: The distribution of diagenetically-formed clay minerals in Rotliegend dune sandstones of the Southern North Sea Basin is closely related to the following factors: (i) the petrography of the sandstones, (ii) the paleoburial depth and tectonic setting of the area, (iii) the thickness of gas-generating Carboniferous strata underlying the Rotliegendes, and (iv) the facies distribution of the overlying Zechstein The diagenetic clay minerals are mainly conversion products of feldspars and, to a lesser extent, of detrital clays and micaceous lithic fragments Sandstones containing dominant drusy illitic and chloritic clay minerals have been buried to depths > 3000 m; if kaolinite is the dominant clay mineral, burial depths were less Sandstones containing feldspars (detrital and authigenic) up to approximately 7% of bulk volume have permeabilities that are about four times higher than sandstones with similar amounts of kaolinite, and as much as 200 times higher than sandstones with similar amounts of illite and/or chlorite

Limitations of Use of Organic Petrographic Techniques for Identification of Petroleum Source Rocks

Abstract: Organic petrographic and organic geochemical studies have been conducted on 58 sediment samples of differing ages and depositional environments. In this study, interpretations of petroleum source-rock potential, based on data from transmitted-light microscopy, show a poor correlation with those derived from chemical data. This lack of correlation arises from the failure to distinguish consistently between hydrogen-poor and hydrogen-rich amorphous organic matter and inability to detect hydrogen-rich components (exinite and resinite) in coal fragments. Interpretations based on data from reflected-light microscopy show a better correlation with those derived from chemical studies, but difficulties still remain. These difficulties arise from inability, except on the basis of luorescence intensity, to distinguish hydrogen-rich from hydrogen-poor organic matter in a finely dispersed state.

Noble metals in the marginal rocks of the Bushveld Complex

Abstract: Two major subdivisions of marginal rocks occur in the eastern Bushveld Complex. A pyroxenitic group borders the lower and lower critical zones, and a gabbroic group borders the upper critical and main zones. Three main facies of the gabbroic group can be recognized on the bases of field relations, petrography, and chemical composition. Five sampling sites were chosen that would best represent the varieties of marginal rocks by study of the field relations and bulk rock compositions of 184 previously analyzed samples. These sites were resampled and the rocks analyzed for major and trace elements and the noble metals (Ir, Rh, Pt, Pd, and Au). The field relations, petrography, and mineral and bulk rock composition of the samples are presented. The range in noble metal abundances is: Ir: 0.2 to 0.6, Rh: 3 to 12, Pt: 30 to 50, Pd: <10 to 20, and Au 10 to 40 ppb. The values are considerably higher than corresponding abundances in most basalts and chilled margins of other layered complexes--especially for Au. All marginal rocks have broadly similar chondrite-normalized noble metal distribution patterns, with enrichment in Rh and Au relative to Pt and Pd.Abundances of noble metals in a liquid from which the UG-2 chromitite layer and the Merensky Reef may have crystallized is calculated from marginal rock compositions, assuming a conical magma chamber. The liquid is composed of a approximately 52/48 percent pyroxenitic/gabbroic mixture and has a composition of Ir: 0.41, Rh: 7.7, Pt: 40, Pd: 16, and Au: 21 ppb. Maximum distribution ratio values (D (super sulfide liquid) (super silicate liquid) ) computed for the mixture, assuming no fractional crystallization, are between 1.8 and 26 times higher in the UG-2 (chromite) environment than in the Merensky Reef (silicate) environment. Distribution ratio values of Ir: 6,600, Rh: 1,100, Pt: 3,900, Pt: 4,000, Au: 480, and Cu: 480 were obtained for the Merensky Reef. With 50 percent fractional crystallization and incompatible behavior of the metals, the distribution ratios attain more realistic values of Ir: 3,300, Rh: 550, Pt: 1,950, Pd: 2,000, Au: 240, and Cu: 240.

Isograds and mineral assemblages in the eastern axial zone, Montagne Noire (France): implications for temperature gradients and P–T history

Abstract: Detailed petrography across a metapelitic sequence in the eastern axial zone of the Montagne Noire, France, is the basis for a sequence of isograds marking the first appearance of biotite–cordierit...

Organic Petrography and Extractable Hydrocarbons of Sediments from the Gulf of California, Deep Sea Drilling Project Leg 64

Abstract: We determined the quantity, type, and maturity of organic matter in Pleistocene sediment from the Gulf of California. We analyzed extractable hydrocarbons by capillary column chromatography and combined gas chromatography/mass spectrometry (GC/MS). We investigated kerogens by Rock-Eval pyrolysis and microscopy. Organic carbon is mostly between 1 and 2°7o in samples from the mouth of the Gulf (Holes 474 and 474A) and in the Guaymas Basin (Holes 478, 481, and 481 A) and between 2 and 4% at the Guaymas Basin slope (Hole 479). We determined total extracts of about 10 mg/g Corg for all samples. The organic matter seems to be mostly terrigenous in Holes 474,474A, 478,481, and 481 A, as indicated by the extractable hydrocarbons and the macefal analysis. But we believe that the large amounts of steroid and triterpenoid hydrocarbons in some samples indicate marine and bacterial biomass, which probably is finely disseminated and cannot readily be identified under the microscope. Rederived, inert organic matter is common in these holes. In Hole 479, the terrigenous contribution is less pronounced, and algal liptinites exceed terrigenous liptinites. But in all samples, perylene is the predominant aromatic hydrocarbon. Optical and chemical investigations show that the organic matter in the Gulf of California sediment is immature, despite the high heat flow and local basaltic intrusions. We could not establish a regular maturity trend with increasing depth.

Metamorphism of the pelitic rocks of the Snyder Group in the contact aureole of the Kiglapait layered intrusion, Labrador: effects of buffering partial pressures of water

Abstract: The petrography and mineral chemistries of the Aphebian Snyder Group pelitic rocks in the contact aureole of the Kiglapait layered intrusion, Labrador reveal a rapid increase in metamorphic grade o...

Jurassic volcanic rocks of the northern North Sea

Abstract: The lavas and minor intrusive rocks from the Jurassic volcanic province of the Forties-Piper area of the North Sea can be subdivided into three distinct series on the basis of petrography, whole-rock chemistry and mineral chemistry These series, which probably evolved from one or more alkali olivine basalt parent magmas are: ( i ) an ankaramite-basaltic hawaiite-hawaiite series, ( ii ) an alkali olivine basalt-hawaiite series, and ( iii ) an intrusive hawaiite-intrusive mugearite series Variations within series are due mainly to differential distribution of mafic phenocryst phases but liquid immiscibility and magmatic fractionation are also believed to have contributed The Middle Jurassic volcanism in the North Sea area is thought to be related to the tensional tectonic regime which existed in NW Europe during the Jurassic or to be due to a region of high heat flow extending northeastwards from the contemporary spreading axis between Iberia and Newfoundland and possibly to a combination of the two

Stratigraphy and petrography of some metasediments and associated intrusive rocks from central Fiordland, New Zealand

Abstract: Some 400 km2 of high grade metasediments and intrusive rocks from central Fiordland are described in detail and petrographic descriptions given for some of the more important lithologies. Doubtful Sound and Western Manapouri Provinces are differentiated on the lithological content of each region and confirm earlier subdivisions proposed by Turner. The boundary between the provinces is tectonic, and complicated by the fact that Doubtful Sound Province is locally thrust faulted over the Western Manapouri Province. Doubtful Sound Province is subdivided into 9 formations. Deep Cove Gneiss is lithologically diverse and predominantly comprises hornblende and biotite gneisses interstratified with lesser amounts of marble and calc-gneiss. Townley Calcsilicate and Stella Psammite are characterised by thin-bedded sequences of calc-gneiss and psammite and differ mainly in respect to the relative proportions of these predominant rock types. Mackenzie Schist consists largely of pelitic and semipelitic schists...

Petrologic variations in Apollo 16 surface soils

Abstract: Source rock, maturation history and intrasite variation data are derived for the Apollo 16 regolith by comparing modal analyses of 15 surface soils with rake and rock sample data. Triangular source rock component plots show that Apollo 16 soils have similar source rocks that are well homogenized throughout the site. The site can be divided into three soil petrographic provinces. Central site soils are mature, well homogenized, and enriched in glass. They are probably the most typical Cayley Plains materials present. North Ray soils are immature to submature, containing North Ray ejecta. South Ray soils are mature, but contain small amounts of fresh impact melts and plagioclase, due perhaps to the breakdown of blocky South Ray ejecta. The different compositions and physical properties of North and South Ray ejecta support the hypothesis that the latter event excavated Cayley material, while the former excavated Descartes materials.

A rock magnetic study of the lower massive sandstone, Moenkopi Formation (Triassic), Gray Mountain Area, Arizona

Abstract: This paper describes the results of rock magnetic and petrographic studies of the Lower Massive sandstone of the Moenkopi Formation, north central Arizona, which heretofore has been considered to be a reliable magnetostratigraphic unit. The rock-magnetic studies included stepwise thermal demagnetization of samples of intraformational mudstone clasts and a deformed shale lens, both of which occur within the lower part of the sandstone, and stepwise thermal and selective destructive demagnetization (SDD) of samples of the sandstone containing black sand laminae composed largely of opaque grains. In samples both from the claystone clasts and deformed shale lens two secondary components, one of modern normal polarity and one of Triassic normal polarity, were unambiguously identified, and a third CRM component of Triassic reversed polarity was tentatively identified. There is no evidence in samples of either the claystone clasts or deformed shale lens of a remanence acquired prior to deposition or to deformation, respectively. SDD, by means of which the small-scale homogeneity of remanence within samples of the Lower Massive sandstone containing black sand laminae could be assessed, shows that each sample primarily contains a heterogeneous three-dimensional mixture of Triassic normal and reversed components. The Triassic normal direction that is characteristic of the Lower Massive sandstone is a vector resultant of almost balanced proportions of the two nearly antiparallel components, averaging about 55 to 60% normal and 45 to 40% reversed. The local variability in both direction and intensity of remanence within and between individual black sand laminae confirms that acquisition of the remanence took place over a geologic time interval that spanned at least two and probably several polarity intervals and that, on a small scale, the remanence was sporadically acquired more or less randomly, probably in response to local conditions in the interstitial physicochemical environment that affected the growth of authigenic hematite. No primary component of remanence acquired at or soon after deposition could be identified. The remanence in the black sand-bearing Lower Massive sandstone is largely carried by the black sand grains which, as determined from reflected-light petrographic studies, are primarily martite (67%) and altered ilmenite grains (20%) composed of complex intergrowths of hematite and iron-titanium minerals. These data strongly suggest that these grains, deposited initially as unaltered to only moderately altered magnetite and ilmenite, subsequently underwent oxidation during which they acquired their remanence.

Diagenetic Sequence Related to Structural History and Petroleum Accumulation: Spindle Field, Colorado

Abstract: Two Upper Cretaceous marine sandstones have produced 36 million bbl of oil and 164 bcf of gas at Spindle field in the Denver basin of Colorado. The sandstones are offshore bars enclosed in finer grained rocks of the middle Pierre Shale. They were deposited in response to western interior sea-level fluctuations that affected water depths, energy levels, and sediment distribution across structurally controlled sea-floor topography during several intervals of Cretaceous sedimentation. Some of the structures that influenced Pierre deposition are believed to have controlled later emplacement of oil and gas. Preliminary petrographic analysis of the sandstones indicates that four phases of diagenesis have occurred. Early diagenetic events of compaction and formation of authigenic chlorite rims, silica overgrowths, and calcite cement within the sandstones (phases I and II) largely destroyed initially good primary porosity and permeability in these well-sorted sandstones. These early events approximately coincide with the energy-of-formation (^Dgr G°f) related paragenetic sequences in fine to medium-grained, well-sorted marine rocks as reported by other workers. However, later diagenetic events (phases III and IV) clearly record separate phases with no sequential relationship to earlier events. Later diagenetic events of calcite solution to form secondary porosity and partial occlusion of the secondary porosity by authigenic kaolinite and minor illite reflect a change in pore-fluid chemistry believed to be related to vertical migration of acidic fluids from below. Acid water, resulting from dissociation of the CO2 gas produced during degradation of organic matter in the source rocks, is considered a likely solvent for the calcite. Secondary porosity has created the reservoir at Spindle field; elsewhere, thick, originally well-sorted sandstones are tight. Both the acidic fluids and the oil and gas are postulated to have migrated vertically, under pressure related to petroleum generation, into the reservoir along tensional faults and fractures related to a paleostru tural element at depth. Spindle field hydrocarbons represent a combination of stratigraphic and diagenetic trapping. Diagenetic sequences record aspects of the total burial history of a rock that may be combined with other geologic information to give a better understanding of the total burial history. Predictability of timing and mechanism of hydrocarbon emplacement may be enhanced by relating diagenetic sequences to the structural features and history of an area. The diagenetic sequence observed in the sandstones at Spindle field is recognized as a response to structural influence on fluid migration.

Modal petrology of six soils from Apollo 16 Double Drive Tube Core 64002

Abstract: Petrographic data form six size fractions for six samples of Apollo 16 drive tube section 64002 show source rocks similar to those of core 60009. Analysis of modal data from the 64002 core show that the upper three and lowest core soils are mature and have similar maturation histories, while the two middle soils are submature and have histories that are similar to each other but unlike those from the aforementioned soils. In all of these soils, mixing has dominated over reworking, and appears to involve two mature soils distinguished by differing source rocks and an immature, plagioclase-rich soil which is correlated with larger clasts of chalky, friable breccia. These breccias and the plagioclase-rich soil are tentatively associated with the Descartes Formation.

Petrography and Geochemistry of Basement Rocks from Five Leg 60 Sites

Abstract: . Bulk chemistry and trace elements data were measured in 72 samples, selected from 5 basement sections, which have been recovered by Leg 60 drilling (Sites 453, 454, 456, 458, and 459). According to analytical results a metagàbbro-metabasalt breccia, deposited about 5 Ma at the westernmost flank of the Mariana Trough (Site 453), was derived from an island arc source. Basalts from the Mariana Trough (Sites 454 and 456) are similar in many respects to midoceanic ridge basalts (MORB). Yet rocks of unusual geochemistry, reflecting the possible influence of arc volcanism, were found among the pillow lavas at the easternmost trough (Site 456). The acoustic basement in the Mariana fore-arc region was formed by submarine eruptions of arc tholeiites (Sites 458 and 459) and peculiar high-MgO andesites related to the boninite suite.

Petrology and petrogenesis of lunar breccia 12013

Abstract: Sample 12013 is extremely rich in incompatible elements (K, P, U, Th, Pb, Ba, Rb, Zr, Nb, and rare earths) relative to other lunar samples, and it contains abundant granitic material composed of silica minerals and K-feldspar The present investigation provides detailed petrologic and mineralogic descriptions of the rock and a discussion of its genesis and history The sample is made up of two lithologies, including a mottled gray and white lithology, and a black lithology A detailed petrology of the gray breccia is presented, taking into account lithic clasts, mineral clasts, deformation textures in clasts, the matrix, and felsite A detailed petrology of the black breccia is also provided Attention is given to a model for the formation of the breccia, an evaluation of the possible genetic relationships between the various components, and some constraints on the nature of the source area

Petrography and Mineral Compositions of Gabbros Recovered in Deep Sea Drilling Project Hole 453 on the Western Side of the Mariana Trough

Abstract: The compositions, mineralogies, and textures of gabbros recovered in polymict breccias in Hole 453 indicate that they are the cumulus assemblages of calc-alkalic crystal fractional on that occurred beneath the West Mariana Ridge. They are among a class of gabbros known only from other calc-alkalic associations (e.g., the Lesser Antilles and the Peninsular Ranges batholith of Southern California) and differ from gabbros of stratiform complexes, ophiolites, and the ocean crust. Particularly abundant in the Hole 453 breccias are olivine-bearing gabbros with extremely calcic Plagioclase (An94_97) but with fairly iron-rich olivines (Fo76_77). Other gabbros contain biotite and amphibole and occur in breccias with fairly high-grade greenschist facies (amphibole-chlorite-stilpnomelane) metabasalts. One unusual gabbro has experienced almost complete subsolidus recrystallization to an assemblage of aluminous magnesio-hornblende, anorthite, and green hercynitic spinel. This reaction, the extremely calcic Plagioclase, the occurrence of biotite and amphibole, and the association with greenschist facies metamorphic rocks suggest that crystallization of the gabbros occurred at elevated P(H2O). Comparisons with other calc-alkalic gabbro suites suggest pressures in excess of 4 kbar (about 12 km depth). The gabbros were exposed by the early stages of opening of the Mariana Trough and imply that considerable uplift may have attended rifting. They were also subjected to hydrothermal alteration after breccia formation, resulting in formation of chlorite, epidote, actinolite, and prehnite. Temperatures of at least 200°C—and probably 350°C—were reached, and most likely could not have been attained without extrusion or intrusion of magmas nearby, even though no such rocks were cored.

Geochemistry and Petrography of Organic Matter in Northwest African Continental Margin Sediments: Quantity, Provenance, Depositional Environment and Temperature History

Abstract: This report synthesises the results of organic geochemical and organic petrographic analyses of sediments fron deep sea drilling sites on the Northwest African continental nargin. The investigation of the quantity, type, and composition of the organic matter and of the depositional environment leads to the recognition of a number of organofacies. Based on the organofacies, the sediments can be classified according to their differing hydrocarbon potentials, although the organic matter in all the sediments encountered has been determined to be iirmature, the onset of thermal hydrocarbon generation having not been reached at present burial depths.

Archean and early proterozoic complexes of santa catarina, parana and sao paulo states, south-southeastern brazil : an outline of their geological evolution

Abstract: The Luis Alves, Serra Negra and ltatins complexes are Arehean to early Proterozoic metamorphic units in the Brazilian States of Santa Catarina, Parana and Sao Paulo, south of the 24°S parallel. This paper summarizes their geological evolution based on petrographic, chemical, and mainly geochronological data. The three complexes consist of metamorphic rocks of the granulite facies, dominantly: norites, enderbites. charnoenderbites, ultramafites of granulite facies, kinzigites, quartzites, biotite-hornblende gneisses, meta-quartz diorites, metadiorites, metagabbros, serpentinites and talcites. Petrographic and chemical data support an igneous origin for most of these rocks. The radiometric age determinations made bythe Rb-Sr and the K-Ar methods on different materiais gave results related, for the most part, to the Pre-Jequie (about 3,100 m.y.), Jequie (2,600 ± 200 m.y.) and Transamazonico (2,000 ± 200 m.y.) events. Geochronological data suggest that the three complexes were formed predominantly by crustal accretion-differentiation processes.