Mixed Rock Research Articles

Lithologic analysis from multispectral thermal infrared data of the alkalic rock complex at Iron Hill, Colorado

Airborne thermal‐infrared multispectral scanner (TIMS) data of the Iron Hill carbonatite‐alkalic igneous rock complex in south‐central Colorado are analyzed using a new spectral emissivity ratio algorithm and confirmed by field examination using existing 1:24 000‐scale geologic maps and petrographic studies. Color composite images show that the alkalic rocks could be clearly identified and that differences existed among alkalic rocks in several parts of the complex. An unsupervised classification algorithm defines four alkalic rock classes within the complex: biotitic pyroxenite, uncompahgrite, augitic pyroxenite, and fenite + nepheline syenite. Felsic rock classes defined in the surrounding country rock are an extensive class consisting of tuff, granite, and felsite, a less extensive class of granite and felsite, and quartzite. The general composition of the classes can be determined from comparisons of the TIMS spectra with laboratory spectra. Carbonatite rocks are not classified, and we attribute that to the fact that dolomite, the predominant carbonate mineral in the complex, has a spectral feature that falls between TIMS channels 5 and 6. Mineralogical variability in the fenitized granite contributed to the nonuniform pattern of the fenite‐nepheline syenite class. The biotitic pyroxenite, which resulted from alteration of the pyroxenite, is spatially associated and appears to be related to narrow carbonatite dikes and sills. Results from a linear unmixing algorithm suggest that the detected spatial extent of the two mixed felsic rock classes was sensitive to the amount of vegetation cover. These results illustrate that spectral thermal infrared data can be processed to yield compositional information that can be a cost‐effective tool to target mineral exploration, particularly in igneous terranes.

Evolution of burrowing in marine gastropods: observations on rocky-shore species

Trueman & Brown1 have suggested that the burrowing habit observed in representativ es of at least 20 families of marine gastropods2 could most easily have evolved on shores of mixed rock and sand. Gastropods are primitively adapted for locomotion over hard substrata3 but if faced with a habitat in which rock and sand alternate, it would clearly be advantageous for them to develop the ability to crawl over soft substrata as well, if only to get from one patch of rock to another. Evolution of the burrowing habit might follow, together with the ability to extricate themselves from sand if buned The latter would be of particular benefit in intertidal habitats where marked sand movements are in evidence The whelk Bumupena catarrhacta (Gmelm) is essentially a rocky-shore species which nevertheless crawls easily over sand,* employing the same smooth motion observed when it crawls over rock. When disturbed, its pattern of locomotion over sand changes to a discontinuous stepping motion, by which means it can also burrow1. Such stepping is characteristic of burrowing in prosobranchs inhabiting soft substrata. This whelk can also extricate itself if buned, by thrusting vertically upwards. We have pursued this topic further by observing the behaviour on sand of a number of rocky-shore prosobranchs from the coast of the Cape Peninsula, South Ainca, in the field or the laboratory, or both. We find it appropriate to group the rocky-shore species studied into four categories with regard to this behaviour.

Origin and thermal evolution of icy satellites

The paper reviews the problem of formation and evolution of the so-called “regular satellites “ of the giant planets, and it consists of two parts: the first describes the possible origin of the satellites, the second studies their evolution, attempting to stress the relations of the present status of the satellites with their evolutionary history. The formation of regular satellite systems around giant planets is probably related to the formation of the central planet. Some characteristics of regular satellite systems are quite similar, and suggest a common origin in a disk present around the central body. This disk can originate through different mechanisms which we will describe, paying attention to the so-called “accretion disk” model, in which the satellite-forming material is captured. The disk phase links the formation of the primary body with the formation of satellites. The subsequent stages of the disk's evolution can lead first to the formation of intermediate size bodies, and through the collisional evolution of these bodies, to the birth of satellite “embryos” able to gravitationally capture smaller bodies. Given the scenario in which icy satellites may be formed by homogeneous accretion of planetesimals made of a mixtures of ice and silicates, if no melting occurs during accretion, the satellites have a homogeneous ice-rock composition. For the smaller satellites this homogeneous structure should not be substantially modified; only sporadic local events, such as large impacts, can modify the surface structure of the smaller satellites. For the larger satellites, if some degree of melting appears during accretion, a differentiation of the silicate part occurs, the amount of differentiation and hence the core size depending on the fraction of gravitational potential energy retained during the accumulation process. Melting and differentiation soon after the accretion, for the larger satellites, could also depend on the convective evolution in presence of phase transitions and generate an intermediate rock layer, considerably denser than the underlying, still homogeneous core, and unstable to overturning on a geologic time scale. Moreover the liquid water mantle could be a transient feature because the mantle would freeze over several hundred million years. For these large bodies the stable configuration is expected to be one consisting of a silicate core and a mantle of mixed rock and ice.

Large-scale silicate liquid immiscibility: a possible example from northeastern Brazil

Ultrapotassic, peralkalic silica-saturated plutons (580 Ma) are widespread in the Cachoeirinha-Salgueiro foldbelt, northeastern Brazil. They consist of alkali-feldspar syenites with pyroxenite as co-magmatic inclusions and syn-plutonic or late-stage dikes. Pyroxenite and syenite have the same mineral phases (aegirine-augite, microcline, sphene, apatite, blue amphibole, magnetite), but only in different proportions. Rare inclusions of a “mixed” rock (about 60% syenite+40% pyroxenite in an emulsion-like texture) are also present. Pyroxenes in the three units are all only slightly zoned, silica-saturated and extremely low in Al 2O 3 (0.2–1.4%). Amphiboles are mostly K-rich richterite, characterized by high SiO 2, low Al 2O 3 and TiO 2 contents and low Mg#. The three rock types have similar REE chondrite-normalized patterns, with negative slopes and lack of Eu anomaly, with the total REE in the pyroxenite greater than that of the syenite. Trace element patterns for the mixed rock are intermediate between those for the pyroxenite and syenite. Major element partitioning between pyroxenite and syenite has the same sense as that one observed between immiscible liquids in volcanic lavas and trace element partitioning is similar to the experimentally determined partition of immiscible liquid pairs. The rocks have similar high δ 18O values (avg. w.r.+ 8‰ SMOW, corrected from pyroxene), high initial 87 Sr 86 Sr ratios (about 0.710), and low 143 Nd 144 Nd (avg. 0.51104). Field and geochemical characteristics indicate chemical equilibrium among the three rock types and suggest liquid immiscibility between syenite and pyroxenite, the mixed rock representing the original magma composition.

Water Management for Profit Maximization

Managing irrigation water to replace soil moisture depletion is equivalent to managing water for yield maximization, and does not constitute efficient resource use. Irrigation water allocation models should be based on profit maximization, which will occur at the same point that maximizes water use efficiency. The objectives of this study were to: (i) optimize water allocation based on profit maximization; and (ii) compare curvilinear and linear water production functions for profit maximization. Water production functions were developed for four seasonal harvests of alfalfa (Medicago sativa L.) from data collected over a 6-yr field research project conducted at the University of Nevada, Reno [...]

Paleoecology and Biogeography of Late Cretaceous Molluskan Assemblages Near Loma Prieta, California: ABSTRACT

Deformed, fossilliferous turbidite deposits resembling the Great Valley sequence are present in a fault wedge just east of the San Andreas fault near Loma Prieta Santa Cruz Mountains, California. Near the top of these deposits late Campanian to early Maastrichtian Baculites rex is found along with late Campanian B. inornatus and B. aff B. anceps. Turbidite deposits associated with a conglomerate near the base of the Upper Cretaceous strata contain abundant exogyrid oysters and many rare or uncommon taxa including Amphidonte parasitica, Lyriochlamys traskii, Spondvlus subnodosus, and Hipponix dichotomus, which also imply a late Campanian age. Composition of the oyster-rich assemblage indicates a relatively high-energy, nearshore source area for those turbidite deposits. This source area consisted of mixed sand and rock or shell bed substrates. Fossils found in the overlying Upper Cretaceous turbidites are indicative of a more offshore shelf habitat, suggesting a seaward shift in sediment source area or transgression through time. Molluskan assemblages include North Pacific species having tectonically transported distributions ranging from Baja California to Vancouver Island. Some species in the oyster beds have been previously reported only to the north and some only to the south of Loma Prieta; thus, the unusual faunal composition is apparently more » due to the preservation of a very nearshore assemblage, which is rare in strata of this age in California, rather than to biogeographic constraints. The apparent absence of rudists in this very nearshore assemblage, where they would be expected if present suggests significant displacement of the Upper Cretaceous rudist-bearing rocks found to the west of the San Andreas fault relative to Loma Prieta. « less

Compression effects in rock-ice mixtures: an application to the study of satellites

The behaviour of a porous material consisting of mixed rock and ice grains is studied theoretically. Two aspects are considered: (1) self-compression of a rock and ice mixture forming a celestial body, and (2) the heat conduction coefficient of the mixture. The work is aimed at an application to the study of the internal structure of medium-sized icy satellites. A compaction time-scale for the porous structure of the latter is calculated. It is suggested that Mimas could be porous down to its center even today, provided it accreted in a cold environment. Mimas thus appears to be the most suitable object to which our model could be applied. For a realistic porosity-pressure relation the equation governing pressure distribution within a non-rotating spherical body is solved numerically for Mimas. It is shown that the presence of voids may significantly lower the moment of inertia. Our results also suggest that porosity is sufficient to explain the actual shape of Mimas. The heat conduction coefficient of a porous rock-ice mixture is calculated by means of a Monte Carlo method based on the random resistor concept. It is suggested that the results so obtained could be useful in a study of thermal evolution. Throughout the paper the need for an experimental determination of the various material parameters used in this work is emphasized.

Geochemistry of the J-M (Howland) Reef of the Stillwater Complex, Minneapolis Adit area; I, Sulfide chemistry and sulfide-olivine equilibrium

Platinum mineralization in the Stillwater Complex occurs within a complex sequence of olivine-bearing, plagioclase-rich cumulates about 400 m above the base of the Banded zone. The reef consists of a 2-m-thick layer of disseminated sulfides extending over the entire strike length of the complex. In the area of this study the host rock to the reef sulfides shows a lateral transition over a strike length of 700 m from a heterogeneous olivine-plagioclase cumulate (mixed rock) to a plagioclase-olivine cumulate containing a high proportion of postcumulus bronzite, with olivine preserved only as rare resorbed cores on bronzite grains.Platinum-group elements within the reef correlate strongly with abundances of sulfides, which show convincing textural evidence for having originated as immiscible sulfide liquid. Strong correlations are also observed between all six platinum-group elements, Ni, and Cu. Despite the gradation in host-rock mineralogy, the sulfide fraction of the reef displays a constant bulk composition over 700 m of strike length, containing 9.3 percent Ni, 6.9 percent Cu, and approximately 0.7 percent total platinum-group elements (principally Pd). These platinum-group element concentrations are two to three orders of magnitude higher than those found in typical magmatic sulfide deposits and are attributed to equilibration of sulfide liquid with a very large volume of silicate melt.Olivines from the reef show a range of compositions from Fo 71 to Fo 85 , and from 2,400 to 4,500 ppm Ni. A positive correlation is observed between NiO and FeO in olivines from sulfide-bearing samples, indicating Ni-Fe exchange between olivine and sulfides on a local scale, such that Ni contents of olivines were buffered by that of the sulfide phase. A value of 9.70 + or - 1.49 is obtained for the Ni-Fe distribution coefficient between sulfide and olivine, in agreement with values obtained at magmatic temperatures by Boctor (1982).These observations may be interpreted in the framework of the plume model of Campbell et al. (1983). A pulse of replenishing magma enters the Stillwater chamber as a buoyant plume and subsequently spreads to form a layer of hybrid melt. Sulfide liquation and olivine crystallization occur within the plume and the hybrid layer in response to mixing and subsequent cooling. Sulfide liquid attains high platinum-group element concentrations during turbulent ascent and spreading of the plume. Crystallization of olivine from the hybrid layer gives rise to olivine-sulfide boulders, which settle to the floor of the chamber. Reaction between the boulders and the evolved liquid at the bottom of the chamber causes olivine to become more iron rich. Subsequent reequilibration between olivine and sulfide grains gives rise to the observed Ni-Fe distribution.

Magma mixing in alkaline series: an example from Sancy volcano (Mont-Dore, Massif Central, France)

Three magmatic units (Grande Cascade pyroclastic deposits, Grande Cascade lava flow, Durbise nuee ardente deposits) from the Quaternary volcano Sancy (Mont-Dore area, Auvergne, France) show textural evidences of magma mixing between a silica undersaturated basic magma (alkali basalt and hawaiite) and an acid magma (quartz-bearing trachyte). Three kinds of mixed rock types are described: basic inclusions within an acid host, « emulsified rocks » showing infracentimetric basic globules disseminated within an acid groundmass, and « banded rocks » in which elongated acid and basic zones alternate. The chemical compositions of mixed rocks plot systematically onto linear trends in petrographic diagrams. Microprobe analyses of the groundmass show similar linear variations between basic and acid end-members. The mineralogical associations of these mixed rocks are highly complex and present many disequilibrium features. Olivine is stable in the basic component and becomes rimmed by orthopyroxene in the acid one. Zoning patterns of feldspars are complex. Clinopyroxene, kaersutite and phlogopite phenocrysts have increasing component Mg contents from core to rim both in the basic and the acid. Titanomagnetite and hemoilmenite phenocrysts were equilibrated at 900-800° C under high oxygen fugacities. Mixing results primarily from a mechanical disintegration of partly liquid basic inclusions within their acid host, and also from a mechanical transfer of phenocrysts from one component to the other, in which they often remain surrounded by a coating of their original groundmass. Chemical data on the groundmass indicates that some « true » hybridization between coexisting acid and basic liquids may also have occurred. The extent of mixing is controlled by the type of emplacement. For pyroclastic deposits a chemical gap exists between basic inclusions and their acid hosts; in contrast, mechanical mixing was enhanced during the emplacement of the viscous Grande Cascade lava flow, and complete transitions occur between basic and acid components. The two end-members are genetically associated, the latter deriving from the former by crystal fractionation. Mixing appears as a late-stage phenomenon in the petrogenetic history of the Mont-Dore series; in the case of the Grande Cascade lava flow, its extent is primarily dependent on emplacement modalities.

Tectonic outlier of Great Valley sequence in Franciscan terrain, Diablo Range, California

Research Article| June 01, 1979 Tectonic outlier of Great Valley sequence in Franciscan terrain, Diablo Range, California JANET M. BAUDER; JANET M. BAUDER 1Department of Geology, Stanford University, Stanford, California 943505 Search for other works by this author on: GSW Google Scholar J. G. LIOU J. G. LIOU 1Department of Geology, Stanford University, Stanford, California 943505 Search for other works by this author on: GSW Google Scholar Author and Article Information JANET M. BAUDER 1Department of Geology, Stanford University, Stanford, California 943505 J. G. LIOU 1Department of Geology, Stanford University, Stanford, California 943505 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1979) 90 (6): 561–568. https://doi.org/10.1130/0016-7606(1979)90<561:TOOGVS>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation JANET M. BAUDER, J. G. LIOU; Tectonic outlier of Great Valley sequence in Franciscan terrain, Diablo Range, California. GSA Bulletin 1979;; 90 (6): 561–568. doi: https://doi.org/10.1130/0016-7606(1979)90<561:TOOGVS>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract A dismembered ophiolite, tectonically overlain by Great Valley strata, rests upon Franciscan rocks southeast of Cedar Mountain in the northern Diablo Range. The klippe of sedimentary rocks is the first reported outlier of the Great Valley sequence in the interior of the range. The dismembered ophiolite is composed of serpentinized ultramafic rocks, hornblende gabbro, diorite, and plagiogranite, and it is partially altered to greenschist facies assemblages containing chlorite + epidote + albite + actinolite.The Franciscan Complex northeast of the ophiolite is characterized by lithologic heterogeneity, and it includes mélanges comprised of mixed rock fragments of varied metamorphic grade incorporated in a pervasively sheared shale matrix. The coherent Franciscan terrain southwest of the ophiolite may belong to a structural unit of Late Jurassic age which overlies younger Franciscan rocks in other parts of the Diablo Range. All Franciscan samples examined petrographically contain blueschist facies minerals such as lawsonite, pumpellyite, sodic amphibole, and jadeitic pyroxene.The Great Valley strata consist of interbedded sandstone and shale. Arkoses of the Great Valley klippe are generally calcareous and contain well-preserved detrital potassium feldspar and biotite. Locally, the sandstone contains megafossils of probable Valanginian and Cenomanian age, including Linearia multicostata (Gabb), Pterotrigonia oregana (Packard), Turritella hearni Anderson, Trigonia sp., Panope sp., and small ammonite scraps similar to Thurmanniceras (?) sp. Incipient low-grade metamorphism is indicated by alteration of calcic plagioclase to albite + calcite + white mica and by partial chloritization of biotite. The lithology and the age, structural, and metamorphic relationships indicate that the Great Valley outlier and its underlying dismembered ophiolite are erosional remnants of the Coast Range thrust which once extended across the Diablo Range. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Depositional Systems of Cisco Group: Their Relation to Reservoir Distribution and Petroleum Production on Eastern Shelf, Midland Basin: ABSTRACT

The Cisco Group is a mixed terrigenous clastic and carbonate rock stratigraphic unit deposited on the Eastern shelf, a late Paleozoic constructional platform developed on the margin of the sediment-starved Midland basin. Detailed facies mapping of the Harpersville format, a boundary-defined unit within the Cisco Group, outlines 3 depositional systems that are differentiated by gross lithologic composition and position relative to the equivalent shelf edges. They are the (1) Cisco fluvial-deltaic system, (2) Sylvester shelf-edge bank system, and (3) Sweetwater slope system. The Cisco fluvial-deltaic system is composed of dip-fed fluvial and deltaic facies and associated strike-fed interdeltaic embayment facies. The Sylvester shelf-edge bank system consists of an offlapping series of elongated, prismatic limestone banks that lie along the shelf margin. The Sweetwater slope system is composed of numerous slope wedges, or fans, which include shelf-margin, slope-trough, and distal-slope sandstone facies. The eastern shelf prograded into the Midland basin by local upbuilding through fluvial, deltaic, and shelf-edge bank deposition contemporaneous with outbuilding by slope-fan deposition. Oil pools are found in all 3 depositional systems. Productive facies include fluvial, distributary channel, and distributary-mouth bar sandstones of the fluvial-deltaic system and distal-slope and shelf-margin sandstones of the slope system. Production is concentrated in areas where 2 broad, subparallel, structurally-related NE-SW trends intersect the mapped fluvial-deltaic lobes. The complex, lenticular geometry of these thin deltaic sandstones affords maximum opportunity for development of stratigraphic and combination traps. End_of_Article - Last_Page 619------------

Pleistocene Sea-Level Fluctuations off Southern California and Their Relation to Continental Slope Sedimentation: ABSTRACT

Fluctuations of sea level as much as 700 ft during the late Pleistocene have modified the continental margin of southern California and exercised a dominant control on the offshore sediment-distribution patterns. Narrow terraces and low sea cliffs were cut into bedrock during lowered sea stands. They are associated with anomalous occurrences of sand-algal nodules, concentrates of shallow-water shells, phosphorite, and rounded cobbles of mixed rock type. The rock outcrops exposed in the low sea cliffs provide a window that permits sampling and the mapping of bedrock End_Page 457------------------------------ structures from submersibles in an area normally covered with prograding slope deposits. The concentration of relict shallow-water fossils in deep terrace zones and the rough topography can be confused with nonexistent structural features if not recognized during sampling programs. End_of_Article - Last_Page 458------------

The Lakeview Mountains Pluton, Southern California Batholith Part I: Petrology and Structure

The Lakeview Mountains pluton, a part of the southern California batholith, is a steep-walled body with a tear-shaped ground plan, exposed discontinuously over an area of 100 to 130 km 2 . It is located at an abrupt local deflection of the regional structural grain and is concordant with planar structures in the enclosing rocks. These are gabbro, granitic rocks, mixed rock composed of granitic and metamorphic rocks, and regionally metamorphosed rocks of the cordierite-amphibolite facies. Most of the adjacent granitic rocks are older than the Lakeview Mountains pluton, although some may be of the same age. The pluton is almost entirely coarse-grained hornblende-biotite quartz diorite that lacks potassium feldspar. Schlieren are ubiquitous, and small lenticular inclusions, common throughout the pluton, parallel the schlieren. The schlieren, which make the rock extremely inhomogeneous on a small scale, geometrically fall into three orientation groups. One group, the most pronounced, is concordant to the outline of the pluton and planar structures in the adjacent rocks; the other two, discordant with respect to the shape of the pluton, strike northeast and northwest. Some outcrops exhibit schlieren intersecting at a large angle, and mesoscopic fabric analysis indicates that mineral orientation of much of the massive-appearing rocks is complex on the scale of an outcrop. Granitic pegmatite dikes, masses of hypersthene gabbro, and both mafic and leucocratic quartz diorite are concentrated in the geometrically and structurally deduced center of the pluton. It is concluded that the concordant schlieren resulted from outward growth of the pluton and the discordant sets in response to regional forces. The concordance between the outline of the pluton and its wall rocks and the abrupt deflection of the regional structural grain resulted from the outward growth of the pluton. The pegmatite represents the fugitive constituents of the pluton-forming magma, and the sites of pegmatite concentration in the wider part of the tear-shaped body mark the last portions of the pluton to crystallize.

Skarn zone of the Doshinkubo ore deposits of the Chichibu mine, Saitama Prefecture

The writer surveys the Doshinkubo ore deposits of the Chichibu mine, which belongs to the magnetite deposits of the pyrometasomatic type consisting of pyrite, pyrrhotite, chalcopyrite, and some silicate skarn minerals. At the contact between the quartz-diorite and limestone, the following zones of mineral association were recognized by megascopic and microscopic observations. Zone 1: quartz-diorite, which is composed of quartz, plagioclase (An45 Ab55_??_An55 Ab45), brown hornblende, biotite and blue actinolite. Zone 2: mixed rock zone (width; 20_??_30cm.), which is composed of quartz, plagioclase, brown hornblende, diopside, garnet, chlorite, and calcite. Zone 3: vesuvianite-xanthophyllite zone (width; about 2m), which is composed of vesuvianite, diopside, wollastonite, garnet, and xanthophyllite. Zone 4: garnet-chlorite-epidote zone (width; about 10m.), which is composed of andradite, pistacite, and chlorite. Zone 5: magnetite ore body (width; about 30m.). Zone 6: hedenbergite-ilvite zone (width; about lm.). Zone 7: limestome. The occurrence of the Zones 2 and 3 are closely related, with an intrusive body of the quartz diorite, and those of the Zones 4, 5, and 6 controlled by the formation of the ore body. From the observation mentioned above, the writer suggests that the Zones 2 and 3 were formed by the magmatic “reaction” between the quartz-diorite magma and limestone, but the Zones 4, 5, and 6 may be the products of the mineralization at the later stage.

Application of photoelectric densitometry to the assessment of respirable dust samples

A technique has been developed for rapid assessment of the samples of respirable mine dusts obtained with the long-running thermal precipitator (NCB/MRE dust sampler type 101). The size segregation of particles in these dust samples enables the optical density of a thin strip across the deposit to be converted to the number of particles in that strip, the relationship being a function of mean particle size. The addition of a specially shaped weighting mask in the light beam enables a single measurement of the optical density of the whole deposit to be converted to the total number of particles, independently of the size distribution of the dust. A densitometer has been devised and laboratory and field trials have established appropriate conversion factors from optical density to number of particles. The loss of light caused by the sample is measured over a small angle in order to reduce the difference between coal and the more transparent rock particles. For routine mine samples, which consist in general of mixed coal and rock, use of mean conversion factors is recommended. Operation of the apparatus is simple and rapid, with accuracy and consistency comparable with microscope counting, and it is believed that a high proportion of the routine dust samples can be evaluated in this way. Further advantages of the technique are that it can be adapted, in conjunction with incineration, to the compositional analysis of dust samples and, by using appropriate conversion factors or weighting masks, to the measurement of the mass of dust in the samples.

Igneous rocks of the Highwood Mountains, Montana: Part V. Contact Metamorphism

Very near the contacts of the stocks the sediments have been replaced by sanidine and diopside, through magmatic reaction. An irregular zone of indurated sediments, produced largely by hydrothermal agents, extends outward from the stocks for as much as half a mile. Locally, more intense hydrothermal metamorphism has formed orthoclase and fibrous amphibole with some diopside and locally garnet, phlogopite, spinel, and cordierite. Along the dikes metamorphism extends out from the contacts for a distance of a few inches up to 5 feet. Thermal metamorphism produced recrystallization of the fine interstitial material of the sandstone. Aegirite, calcite, hematite, and pyrite were deposited by hydrothermal solutions. Magmatic reaction produced diopside. Reaction between sandstone and the magma of the headed dike caused a progressive replacement of plagioclase and quartz by potash feldspar. Granitic inclusions in the biotite phonolite dikes have a marginal shell in which plagioclase has been replaced by orthoclase. Surrounding each inclusion is a zone of mixed rock which contains quartz grains much embayed and surrounded by 0.1 mm. prisms of pyroxene.

On Rocks; Their Chemical and Mineral Composition, and Physical Characteristics

All rocks are necessarily composed of minerals,—that is, of certain “substances which, wherever found, present respectively nearly the same forms and physical characters, and are generally composed of nearly the same chemical constituents.” A rock may consist of one single mineral, in which case it is called a simple rock; or it may be made up of an aggregation of several different minerals, when it is called a mixed rock. Crystalline limestone, which consists exclusively of one mineral, calcite, may be given as a familiar example of a simple rock, and granite, made up of an intimate aggregation of three distinct minerals, felspar, quartz, and mica, of a mixed rock.