Showing papers on "Petrography published in 1983"

Melt origin of fault-generated pseudotachylytes demonstrated by textures

Abstract: A petrographic and electron-microprobe study of fault-generated pseudotachylytes from the Outer Hebrides Thrust Zone, Scotland, demonstrates that the textures have resulted from the primary crystallization of a clast-laden melt rather than the devitrification of a glass, or by crushing and cataclasis. The observations are consistent with these pseudotachylytes having formed by frictional fusion followed by rapid quenching—implying faulting at shallow crustal depths.

Submarine metamorphism of gabbros from the Mid-Cayman Rise: Petrographic and mineralogic constraints on hydrothermal processes at slow-spreading ridges

Abstract: The coarse-grained, submersible-collected rocks from the Mid-Cayman Rise in our study span a vertical section of about 700 m and range from amphibolite to gabbro. Petrographic and mineralogic studies showed that: (1) some deformation is present in all rocks; (2) the distribution of deformation is not uniform within a specimen; and (3) alteration has taken place preferentially in the deformed zones. The abundance of amphibole decreases with sample depth and argues that seawater flux into the oceanic crust decreased with depth. The compositional changes in rocks indicate that partially exchanged seawater was the metamorphic fluid that supplied Na, K, and H2O to and removed Ca from the rock. In all, about 15% of the original rock has been transformed to amphibolite in the 700 m vertical section. Our study indicates that deformation of oceanic crust is necessary for providing pathways for seawater penetration which in turn is necessary for the submarine metamorphism to occur. If deformation continues to take place during the spreading of seafloor, alteration may also continue to occur along newly formed fractures and cracks.

Petrography and Diagenesis of Silcrete from the Kalahari Basin And Cape Coastal Zone, Southern Africa

Abstract: Silcrete is widely distributed in southern Africa and occurs both in genetic association with deeply weathered bedrock (weathering profile type) and as a significant component of the Cenozoic Kalahari Beds where it has formed through silicification of a variety of continental deposits (nonweathering profile type) Silcrete fabrics are classified into four major types: GS (grain-supported--skeletal quartz grains in contact), F (floating--skeletal grains `floating' in matrix), M (matrix skeletal grains < 5% of volume), and C (conglomeratic--detrital component containing pebbles) F and M fabric types are subdivided on the presence/absence of glaebules Replacement by silica of a precursor matrix can account for the development of all four fabric types, but at least some GS and C fabr c silcretes have formed by passive silica precipitation in packing voids Glaebules and colloform features predominantly composed of concentrations of anatase and iron oxide are common in weathering profile silcretes, but their precise origin is uncertain Authigenic silica is present as megaquartz, microquartz, length-slow and length-fast chalcedony (predominantly in void-fills), cryptocrystalline silica and opal-CT depending on the silica and ionic concentrations of pore waters, pH, and host sediment characteristics Diagnostic petrographic features, including glaebules, colloform structures, and chalcedonic vugh-fills, may be used in conjunction with geochemical data and sedimentary associations to interpret silcretes in the stratigraphic record with respect to weathering profile types (formed in humid, low pH environments) or nonweathering profile types (formed in high-pH ? semiarid to arid environments)

Origin of Archean anorthosites: Evidence from the Bad Vermilion Lake anorthosite complex, Ontario

Abstract: Studies of the petrology and geochemistry of the anorthosite complex at Bad Vermillion Lake, Canada, based on 400 samples collected in summer, 1979, are presented. Petrographic, microprobe, X-ray-fluorescence, and instrumental-neutron-activation analyses were performed. Major and trace-element abundances of the anorthositic rocks and surrounding mafic and felsic rocks are reported in tables, chondrite-normalized rare-earth-element patterns are shown, and the anorthositic, intrusive, and metavolcanic formations are characterized in detail. The anothrositic plagioclases are found to have a coarse porphyritic texture and calcic composition (80 normative mol percent An) similar to those of other Archean anorthosite complexes. Chemical similarities indicate that the gabbro and mafic to felsic metavolcanic formations associated with the anorthosite complex may be comagmatic with it, while the absence of ultramafic material and the bulk composition of the comagmatic basalt (about 20 wt percent Al2O3) suggest that much of the original comagmatic material has been separated.

Petrography and geochemistry of some jordanian oil shales from north jordan

Abstract: Twenty-two "oil shale" samples from north Jordan have been investigated. They are in fact all bituminous limestones, of ages ranging from Uppermost Cretaceous to Paleocene. Major elements, organic matter and trace elements, were determined, together with aspects of mineralogy and petrography. Although macrofossils are rare, trace fossils were encountered. These rocks are postulated to be normal limestones deposited in an oxygenated shallow marine environment. The H2S/O2 interface coincided or lay just below the water/sediment interface. The organic matter is indigenous and not related to migrating oil.

Burial Cementation—Is it Important? A Case Study, Stuart City Trend, South Central Texas

Abstract: The Stuart City trend is a shelf-edge buildup of Lower Cretaceous bioclastic and reefal carbonates that is currently buried to depths of between 3,300 and 5,000 m (11,000 and 16,000 ft). Compaction and cementation have generally reduced rock porosities to less than 9%. Sediments were cemented in the marine environment by finely crystalline bladed, isopachous cement and volumetrically important (14 volume %) coarse to very coarsely crystalline, fibrous to bladed, isopachous, Mg-calcite cement. These cements have been neomorphically altered End_Page 536------------------------------ to low Mg-calcites forming an unusual radiaxial texture observed in the coarse to very coarsely crystalline, bladed, calcite cement. Evidence of their marine origin consists of a relative 1 mole % Mg++ memory, a marine-like isotopic character (^dgr18O ^approx -2.5 and ^dgr13C ^approx +2.0), and early relative timing of precipitation. Diagenetic alteration of these carbonate sediments by the interaction with meteoric water in lenses that formed within topographic highs along the shelf margin changed the initial marine chemical, isotopic, and textural character of the sediments. Secondary porosity formation, mineral stabilization, aggrading neomorphism, and equant spar calcite cementation are the important products of meteoric diagenesis. The equant spar calcite cements make up approximately 16% by volume of the limestones studied. They are iron and manganese poor. The majority have a ^dgr13C composition which falls in the range of modern marine carbonates, i.e., 0.9 to +3.5^pmil. The ^dgr18O compositions range from -1.3 to -6.6^pmil relative to the PDB standard. Oxygen stable isotopic and petrographic data suggest that over 50% of the equant spar calcite cements were formed in a near-surface meteoric environment. A large percentage of the remaining equant spar calcite cements formed at shallow burial depths in a water-limited system where mass transfer was dominated by diffusional processes. Thermally induced ^dgr18O depletion of the equant spar calcites, indicating significant fluid flow, was of minimal importance. Pyrobitumen pore fillings and inclusions in the outer 1.0 mm-thick rims of the very coarsely crystalline, equant spar calcite cements indicate that only minor amounts of cementation have occurred since the introduction of hydrocarbons. Deep burial diagenesis (i.e., post-hydrocarbon migration) consisted of the precipitation of minor amounts of galena, fluorite, and Sr++-rich equant spar calcites. These diagenetic products can be directly related to the present-day formation water. The chemical, isotopic, and textural characteristics of the Stuart City trend limestones contain the imprints of their initial marine composition and shallow diagenetic alterations in a hydrodynamic system. Burial diagenesis has not significantly altered these limestones. Fault and fracture control on the movement of formation waters in this system determine the location and intensity of late stage diagenetic events. End_of_Article - Last_Page 537------------

Porosity Preservation in the Upper Smackover (Jurassic) Carbonate Grainstone, Walker Creek Field, Arkansas: Response of Paleophreatic Lenses to Burial Processes: REPLY

Abstract: A section of the ARCO Bodcaw No. 1 core from the Upper Smackover (Jurassic) of southern Arkansas was studied in detail in an effort to understand the control of porosity. Petrographic, isotopic, atomic absorption, and cathodoluminescence data on whole rocks and components were generated on 24 samples from a 115-foot length of core. The core is composed almost entirely of accretionary and cryptocrystalline grains and cement. Porosity is unrelated to grain type, size. or sorting on a sample-to-sample basis. The porous intervals are interpreted to have been stablized to calcite under fresh-water phreatic conditions. The porous intervals approach "fresh water" values of -5.0 per thousand delta 18 O, 1.5 mol percent Mg, and 300 ppm Sr and are further characterized by luminescent oolites and relatively slight solution compaction effects. The nonporous intervals represent relict vadose and subphreatic environments which stabilized after burial. The nonporous intervals approach "burial" values of -2.8 per thousand delta 18 O, 2.5 mol percent Mg, and 600 ppm Sr and are further characterized by nonluminescent grains and intense solution compaction. Data indicate that oolites were initially composed of a metastable mineralogy, cryptocrystalline grains were composed of a more stable mineralogy, and algal grains were composed of a mixture of mineralogies. Oolites in the vadose and subphreatic environments were not fully stabilized under fresh water conditions, but vadose grains carried with them a light delta 13 C soil gas signal after burial. These grains were fully stabilized under post-fresh-water, burial conditions. Observed porosity distribution is readily explained in terms of solution compaction and cementation. Grains with metastable mineralogy in relict vadose and subphreatic intervals were heavily solution-compacted at relatively shallow depths before being converted to stable calcite. Intense solution compaction destroyed pore space in these intervals. Dolomite cementation seems linked with the compaction process. The stablized mineralogy of the phreatic lens, together with associated meteoric cements, resisted solution compaction and thereby preserved pore space.

Fluid inclusion petrography of SW England granites and its potential in mineral exploration

Abstract: Fluid inclusions in granite quartz from SW England provide a record of the complex and protracted hydrothermal history of this important metallogenic province. Regional variations in terms of the different types of inclusions can be correlated on an inter-pluton scale with both the texture of the host granite and the extent to which it is mineralized. On an intra-granite scale those areas where mineralization is particularly pronounced show a higher overall inclusion abundance than areas where little or no mineralization is known to occur. The types of fluid inclusion most commonly related to Sn-W-Cu mineralization are halite-free, moderate temperature inclusions. Inclusions containing visible CO2 at room temperature are restricted to two localities in SW England. Both of these contain stockwork/vein-swarm tungsten mineralization. These regional ‘fluid inclusion anomalies’ show that fluid inclusion petrography using a simple petrographic microscope has potential application in the field of mineral exploration.

Textural and Rb-Sr isotopic evidence for late Paleozoic mylonitization within the Honey Hill fault zone, southeastern Connecticut

Abstract: A petrographic and Rb-Sr isotopic study of rocks within and near the Honey Hill fault zone places important constraints on its history of movement. Rb-Sr apparent ages for micas and plagioclase from these rocks have been reset and range from Permian to Triassic, considerably younger than the minimum stratigraphic age (Ordovician) of the rocks studied or of Acadian (Devonian) regional metamorphism. Permian Rb-Sr ages of dynamically recrystallized muscovite date the development of mylonite fabric. An older age is precluded by the excellent preservation of unrecovered quartz, which indicates that these rocks did not experience temperatures high enough to anneal quartz or thermally reset Rb-Sr isotopic systems in muscovite since the time of mylonitization. Metamorphic mineral assemblages and mineral apparent ages in rocks north of the fault zone indicate recrystallization under similar upper greenschist-lower amphibolite grade conditions during Permian to Triassic time. Collectively these results indicate that the Honey Hill fault zone was active during the Late Paleozoic and that ductile deformation and metamorphism associated with the Alleghanian orogeny extend well into southern Connecticut. An Alleghanian age for mylonitization within the Honey Hill fault zone suggests it should be considered as a possible site for the major Late Paleozoic strike-slip displacementsmore » inferred from paleomagnetic studies for parts of coastal New England and maritime Canada.« less

A petrographic study of phyllic alteration superimposed on potassic alteration; the Sibert porphyry deposit (Rhone, France)

Abstract: Zoned wallrock alteration surrounds each phyllic vein. Three zones distinguished: (1) a nearest wallrock zone where the preexisting minerals (except quartz) are wholly transformed into muscovite; (2) an intermediate wallrock zone where the alteration intensity decreases (30% of the rock is replaced by white micas); and (3) a farthest wallrock zone where a small proportion of preexisting biotites and plagioclases recrystallizes (less than 10% of the primary minerals recrystallize).--Modified journal abstract.

ALHA 81005: Moon, mars, petrography, and Giordano Bruno

Abstract: Meteorite ALHA 81005, a glassy regolith breccia, is beyond any reasonable doubt of lunar origin, according to its petrographic, chemical, and isotopic characteristics. The sample was shocked at most to 15 GPa during ejection and supports the possibility of some meteorites being of Martian origin. Most lithic fragments are feldspathic granulitic impactites, cataclastic anorthosites, and impact melts, all typical lunar lithologies. Mineral fragments are derived from plutonic troctolitic and noritic rocks, varied mare basalt-like extrusive rocks, and the identified lithic fragment types. Neither petrographically nor chemically are there significant components in ALHA 81005 which were previously unknown. A mare basalt component restricts the origin of the sample: with remotely-sensed data, which eliminates most frontside, as well as farside groundtrack regions, it makes the probability of ejection from Giordano Bruno, near the Moon's NE limb, far greater than from any other lunar crater.

Further Data on the Occurrence of Pectolite in Kimberlite

Abstract: Pectolite has been identified, using petro- graphic, chemical, and X-ray methods, in kimberlites from the De Beers and Dutoitspan Mines, Kimberley, South Africa and the Letseng-La-Terai Mine, Lesotho. It occurs as radiating aggregates of fibrous or acicular crystals not only in the groundmass of certain kimberlites but also in altered xenoliths included within kimberlite. The occurrence of Na-rich minerals in kimberlite is unusual. It is suggested that it can occur as an apparently primary mineral resulting from the incorporation and assimilation of foreign Na-bearing material prior to final consolida- tion, and/or as a secondary mineral after metasomatic introduction of Na-bearing fluids. THIS study presents petrographic, chemical, and X-ray diffraction data for a mineral which occurs as a groundmass constituent of kimberlite and in altered xenoliths included in kimberlite. It is con- cluded that this mineral is pectolite, the same mineral as that discussed by Kruger (1980, 1982). Four samples from the De Beers and Dutoitspan Mines, Kimberley, South Africa and the Letseng- La-Terai Mine, Lesotho are included in this study. Sample 173/33/K3/73 is considered to be either similar to or the same sample as that examined by Kruger (1980, 1982) which was provided by the Geology Department De Beers Consolidated Mines Ltd., Kimberley. In addition to the localities noted above, probable pectolite has been observed by the authors in several other kimberlites.

Geochemistry and tectonic affinities of a Proterozoic bimodal igneous suite, west Texas

Abstract: Diabasic sills from the 1.27-b.y.-old Carrizo Mountain Group of west Texas have immobile element abundances similar to those of modern mid-ocean ridge basalts (MORB). The presence of thick terrigenous sedimentary rocks and peraluminous rhyolitic ash-flow tuffs within the Carrizo Mountain Group preclude the diabases forming in a typical MORB environment. Field, petrographic, and geochemical evidence suggests that the Carrizo Mountain Group was deposited in an incipient continental margin back-arc basin that may have extended northeastward into the Texas panhandle region. The overlying Allamoore Formation consists of sedimentary rocks and basalt accumulated within a mature marginal basin behind the tholeiite-dominated island arc postulated for the Llano area.

Pre-Keweenawan anorthosite inclusions in the Keweenawan Beaver Bay and Duluth Complexes, northeastern Minnesota

Abstract: Large numbers of calcic anorthosite inclusions varying in size from centimetres to several hundred metres occur in the Beaver River gabbro unit of the Beaver Bay Complex exposed along the North Shore of Lake Superior. Lesser numbers of similar inclusions occur in the Duluth Complex. Both host rocks crystallized at about 1.13 b.y. B.P. and are of Keweenawan age. The inclusions have been divided into four groups, based on outcrop and petrographic data, as follows: (1) Tectonite inclusions that were sheared and recrystallized. The tectonites consist of plagioclase (An 76–78 ), minor to accessory olivine (Fo 78 ), and accessory magnetite-ilmenite. (2) Igneous inclusions that have igneous structures and textures. Plagioclase ranges in composition from An 54 to An 80 . Principal mafics are augite (Wo 40 En 45 Fs 15 ) and orthopyroxene (Wo 2 En 80 Fs 18 ), but more iron-rich pyroxenes (Wo 38 En 40 Fs 22 ), including pigeonite (Wo 10 En 55 Fs 35 ), occur as much smaller grains in most igneous inclusions and are comparable to pyroxenes in host rocks. Textural evidence and mineral chemistry suggest that the Fe-rich pyroxenes, as well as the more albitic plagioclase, represent contamination. (3) Intermediate inclusions whose textures are intermediate between the tectonites and igneous rocks. Some of these appear to have been incompletely recrystallized, and some were modified to more igneous-appearing rocks by reaction with host rocks. (4) Cataclastic and brecciated inclusions having brittle deformation that is not apparent in the host rocks. Representatives of each group were analyzed for rare-earth elements (REE) and Sm-Nd, Rb-Sr isotopes. The chemical data correlate with the textural groups. For example, tectonites have low REE abundances with Nd about one times chondrites, whereas those in the igneous-textured group have higher REE abundances with Nd up to as much as seven times chondrites. Isotopic data show that the isotopic systems of the inclusions were disturbed and that contamination was introduced. Tectonites have 87 Sr/ 86 Sr and 143 Nd/ 144 Nd ratios that are less than those of the Beaver River gabbro when recalculated to 1.13 b.y. Igneous-textured inclusions have 143 Nd/ 144 Nd ratios that are lower than tectonite values at 1.13 b.y., but 87 Sr/ 86 Sr ratios vary widely, with some being greater than ratios in the host Beaver River gabbro. None of the inclusions has an isotope ratio that would allow a co-magmatic relationship with either the Duluth Complex or the Beaver River gabbro. Sm-Nd data from an incompletely equilibrated inclusion suggest recrystallization between 1.14 b.y. B.P. and 1.9 b.y. B.P., perhaps as a result of the 1.8 b.y. B.P. Penokean event. Calculations of a crystallization age for the inclusions are speculative but suggest crystallization at or before 1.9 b.y. B.P.

The magma ocean from the Fra Mauro shoreline - An overview of the Apollo 14 crust

Abstract: Petrographic and mineralogical studies of coarse grained, polymineralic, texturally monominct igneous clasts from the Apollo 14 breccias show troctolite and troctolitic anorthosite to be the most abundant rock types. The second most abundant group consists of plagioclase cumulates with more evolved mineral compositions than the Mg-rich trend anorthosite cumulates. Coarse grained ilmenite gabbros and mineralogically evolved monzonoritic and granitic clasts are widespread in occurrence, but not abundant. The present data provide further support for widespread regional heterogeneities within the early lunar crust.

In situ trace element microanalysis

Abstract: We define in situ trace element microanalysis as the measurement of concentrations of trace elements (< 1000 ppm) in individual mineral phases in polished sections, analogous to electron microprobe analysis for major elements. It is our opinion that such measurements are important and can provide a new dimension to petrology and geochemistry. This article is both a review of work already done and a summary of potential advances in the future. The advantages of in situ analyses, as opposed to methods involving physical and/or chemical mineral separations, are that the trace element data can be interpreted in a petrographic context and that ambiguities associated with the purity of the mineral separates can be avoided. With data on individual grains, comparisons of intergrain and intragrain (zoning) variations between major and trace elements can be made, and the importance of inclusions can be assessed, within the spatial resolution of the microanalysis technique.

Diagenetic processes and the mineralization in the triassic of Central England

Abstract: Continental red beds are the host rocks of a characteristic style of mineralization which suggests a genetic link between red bed formation and ore formation. Samples of unmineralized and mineralized Triassic sediments from Central England have been studied sedimentologically, petrographically and geochemically with the aim of clarifying this link which may provide valuable guides for exploration. On the basis of sedimentological and petrographic observations it is suggested that these red beds were formed as a result of diagenetic alteration whereby detrital silicates and oxides are progressively dissolved and the iron released is deposited as hematite. A model for mineralization is proposed involving the release of trace metals from detrital minerals during diagenesis, their retention in saline interstitial solutions, migration to suitable sites of precipitation and deposition by reaction with trapped hydrocarbons or reduced sulphur.

Identification of weathering and hydrothermal alteration in acidic rocks : petrography and mineralogy of clay minerals

Abstract: A major problem in ore prospecting on granitic terrains is to make a distinction between clay mineral assemblages due to weathering and those due to hydrothermal alteration. The most outstanding features of these two types of transformation are : ; 1. Potassic beidellite is typical of hydrothermal alteration. ; 2. Low iron content mineral assemblages in the presence of large amounts of kao-linite is typical of hydrothermal alteration. ; 3. Argillan veins and pore coatings are typical of weathered rocks. ; The control of microprobe analyses by X-ray diffraction on small quantities of matter is very useful for the identification of clay assemblages.

A sillimanite gneiss dome in the Yukon crystalline terrane, east-central Alaska; petrography and garnet-biotite geothermometry

Abstract: A topographically high, nearly circular area of sillimanite gneiss crops out over approximately 600 km2 in the Big Delta quadrangle, Yukon crystalline terrane, east-central Alaska. Foliation is generally horizontal or subhorizontal in the center of the gneiss body and dips outward in all directions. Felsic dikes, some of which are pegmatitic, occur in the central area of gneiss. Quartzite and marble are locally infolded in the gneiss, particularly near its margins. The gneiss is partly bordered on the north and east by pelitic schist interlayered with lesser amounts of quartzite, marble, and amphibole schist. The ages of the protoliths of these rocks are believed to be Paleozoic or possibly Precambrian; major metamorphism probably took place between Mississippian and Middle Triassic time. Two isograds within the pelitic schist are concentric to the gneiss body and indicate an increase in metamorphic grade toward the gneiss. The outermost isograd is defined by the appearance of staurolite with biotite, at the expense of chlorite and muscovite. Upgrade of this isograd, several combinations of the aluminum silicate polymorphs (andalusite, kyanite, and sillimanite) +garnet+staurolite are present in quartz-biotite-muscovite schist. Textural relations suggest that these three polymorphs all formed during the same prograde metamorphic event in which pressuretemperature conditions were near the triple point. The innermost isograd, defined by the disappearance of staurolite, closely coincides with the schist-gneiss contact. Increase in metamorphic grade in the gneiss (quartz+biotite+sillimanite+orthoclase+plagioclase± muscovite± garnet and locally quartz+biotite+sillimanite+cordierite+orthoclase +plagioclase +muscovite) is indicated by an increase in modal orthoclase and decrease in muscovite toward the center of the body, consistent with the breakdown of muscovite+quartz. Migmatitic textures, although rare, may be evidence of local partial melting of the gneiss. The relative distribution of Fe and Mg between garnet and biotite indicates equilibration at about 535° to 600°±30°C for pelitic schist north of the gneiss body and 655° to 705°±30°C for sillimanite gneiss. Petrographic data from poorly exposed rocks south and west of the gneiss also suggest an increase in pressure and (or) temperature conditions toward the gneiss body; such an increase is indicated by an isograd which separates andalusite-sillimanite-bearing schist and gneiss to the southwest from sillimanite gneiss to the northeast. Evidence that the sillimanite gneiss is a dome consists of: (1) the topographic expression and radial drainage pattern, (2) the outward dip of foliation, (3) an increase in metamorphic grade toward the center of the body, and (4) an increase in garnet-biotite temperatures inward. Minor intrusion of synmetamorphic dikes or small granitic bodies is suggested by the presence of equigranular, aluminum-silicate-bearing rocks at several localities in the proposed dome which contain textural features indicative of both igneous and metamorphic processes. The staurolite-out and andalusite-sillimanite isograds define the northern and eastern and the southwestern margins, respectively, of the proposed gneiss dome. INTRODUCTION Sillimanite gneiss crops out in a 600 km2 area in the central part of the Big Delta quadrangle, eastcentral Alaska, near the northwestern margin of the Yukon crystalline terrane (fig. 1). On an early geologic map of the Yukon-Iknana region (Mertie, 1937), the gneiss and most of the surrounding schists were included in a single geologic unit, the Birch Creek Schist (of former usage). The general geology of the area is shown at a scale of 1:63,360 on a reconnaissance geologic map by Foster and others (1977a) and at a scale of 1:250,000 on a reconnaissance geologic map (Weber and others, 1978). The possibility that the area of sillimanite gneiss might be a gneiss dome was reported initially by Foster and others (1977b). In this paper we present new petrographic and petrologic data on the sillimanite gneiss and surrounding pelitic schist, and we interpret these data in terms of the metamorphic history of these rocks. In particular, we address the hypothesis that the large, topographically high area of sillimanite gneiss is a gneiss dome. This is the first gneiss dome in the Yukon crystalline terrane to be reported in the literature, but reconnaissance mapping in the Alaskan part of the crystalline terrane (Weber and others, 1978) has identified other areas of gneiss which appear roughly circular in plan view and which also may prove to be gneiss domes. This particular body of sillimanite gneiss, flanked by pelitic schist, was chosen for study because of the abundance of mineral assemblages containing the pressure-temperature-sensitive aluminum silicates and the relatively good exposure of the gneiss. The distribution of aluminum silicates in thin sections of E2 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY pelitic schist and sillimanite gneiss enables the estimation of metamorphic pressure and temperature conditions, and garnet-biotite pairs in these same rocks provide an additional means of determining metamorphic temperatures. Because kyanite, andalusite, and sillimanite occur together in single thin sections of pelitic schist from four different localities, we also have attempted to determine whether or not these Al2SiOs polymorphs crystallized during a single metamorphic episode and thus whether their occurrence indicates triple-point conditions in the schist. Study of the sillimanite gneiss dome can provide a basis for subsequent comparisons with other such structures in Alaska and the Canadian Cordillera. This paper is an outgrowth of geologic mapping of the Big Delta quadrangle for the U.S. Geological Survey's Alaska Mineral Resource Assessment Program (Foster and others, 1979). Most of the f ieldwork, all of which was of a reconnaissance nature, was done in 1976 and 1977; the laboratory work was accomplished mainly in 1979 and 1980. ACKNOWLEDGMENTS We wish especially to recognize the many contributions of Florence R. Weber to the mapping of the Big Delta quadrangle and her initial recognition of sillimanite gneiss in the area. We are also indebted to Charles R. Bacon for microprobe analyses, aid in interpretation of the data, and support throughout the study. Discussions of metamorphic petrology and petrography with Jo Laird and Anna Hietanen have been particularly valuable. GEOLOGIC SETTING The sillimanite gneiss occurs in the Yukon crystalline terrane (Tempelman-Kluit, 1976), a geologically complex metamorphic and igneous terrane that is bounded by two major right-lateral strike-slip fault systems, the Tintina to the north and the Denali to the south (fig. 1). Tempelman-Kluit (1976) has proposed that much of this terrane is allochthonous, and

Metallic Sulfide Deposits in Gulf Coast Salt Dome Cap Rocks

Abstract: Sixty-five core tests at Hockley salt dome, Harris County, Texas, indicate an annular zone of iron, zinc, lead, and silver sulfides around the perimeter of the cap rock in cumulative concentrations from trace amounts to over 50% total sulfides. Barite has been identified in concentrations exceeding 60%. The textures within and the geometry of the mineralized zone suggest rapid precipitation in a highly reducing, tectonically active environment. Petrographic, isotopic, trace element, hydrocarbon and biological information supports origin of the deposit through a complex evolutionary system involving halokinesis; cap rock development and diagenesis; and the generation, migration and accumulation of hydrocarbons and associated brines. Existing information indicates that at least 13 other Gulf Coast salt domes host metallic sulfide deposits containing zinc and/or lead as a result of similar processes.

Petrographic and Metallogenic Outlines of the “La Balma-M. Capio” Ultramafic-Mafic Body (Ivrea-Verbano Basic Complex, NW Italian Alps)

Abstract: The La Balma-M. Capio ultramafic-mafic body, in the Ivrea-Verbano complex (Western Alps, Italy) is a sill-shaped layered intrusion, concordantly emplaced into high-grade metamorphic sediments (Kinzigitic Series). It mainly consists of a thick layer of amphibole-peridotite overlain by thinner pyroxenite and amphibole-gabbro.

Petrology of the Apollo 11 Highland Component

Abstract: New data for 38 highland fragments hand-picked from Apollo 11 coarse fines are reported. Petrographic, mineralogic, and bulk chemical data show that: (1) the Apollo 11 highland component is non-KREEPy and like that of Apollo 16; (2) poikilitic rocks, granulitic breccias, and the anorthosite-norite-troctolite suite are the most abundant rock types, followed by polymict breccias and glasses; (3) both the ferroan anorthosite and Mg-rich plutonic suites are represented in the Apollo 11 highland component; (4) except for one sample, the intermediate-K Fra Mauro and high-K Fra Mauro groups are not represented. The data and observations are consistent with local derivation of the highland material from beneath relatively thin basalt flows and addition to the regolith via vertical mixing.