The Quad-Wyoming-Line Creeks area is in the northeastern part of the Beartooth Mountains of Montana. The rocks of the area consist mainly of banded migmatite, granitic gneisses, amphibolite, quartzite, and agmatite; small amounts of biotite schist and biotite gneiss, iron-silicate rocks, ultramafic rocks, mafic dikes, and felsic porphyries are also present. Quartzite outcrops continuously around the major folds, and agmatite is especially widespread in the axial zone of a major anti-form. Two fold sets have been revealed by detailed mapping (scale 1:7200) and statistical analysis of the penetrative subfabric elements. The large later folds, F 2 , are the south-plunging Quad Creek synform and the south-southwest-plunging Wyoming Creek antiform; they are nearly upright and non-isoclinal. A metanorite intrusion is located in the axial zone of the Quad Creek synform. Small F 2 fold style varies from similar in relatively ductile rocks to open, concentric folds in non-ductile rocks. Only one large F 1 , located immediately north of the metanorite pluton, has been mapped. F 1 folds are characteristically similar in style. Refolding of small F 1 folds by small F 2 folds has been observed throughout the map area. Statistical analysis of the compositional layering, So, and the axial planes of small F 1 folds, S 1 , shows that the πS 0 - and πS 1 -axes constructed in the Wyoming Creek antiform are nearly coincident with each other and with south-southwest-plunging maxima of small F 2 fold axes, B 2 . Both B 2 and the axial planes of F 2 folds, S 2 , are dispersed, showing that the later folds are noncylindrical and nonplane. The axes of small F 1 folds, B 1 , are generally dispersed along well defined π-girdles. B 1 and B 2 maxima are generally coincident at some stations in the Wyoming Creek antiform-axial zone, suggesting that B 1 and B 2 are, at least locally, collinear. The long axes of hornblende crystals, L 1 , are also dispersed, but are not parallel to B 1 ; the L 1 -subfabric appears to indicate that L 1 developed during the F 1 folding but plunged somewhat more steeply to the south-southwest to south than Bi and that the F 2 folds are concentric rather than similar in the amphibolite. In the Quad Creek synform (F 2 ), all of the above-mentioned fabric elements diverge around the north side of the metanorite body. The Precambrian deformational history of the present map area, and probably the remainder of the eastern Beartooth Mountains, appears to be comprised of two phases of folding. During the first phase (F 1 ) south-southwest plunging, isoclinal or nearly isoclinal folds formed by passive flow during metamorphism to at least the upper amphibolite facies; these F, folds were not upright and may have been nearly recumbent. L 1 was formed during this phase. Metamorphic differentiation of a sedimentary sequence occurred at this time, but it is not clear whether any new material was added. The second phase of deformation is characterized by upright, non-isoclinal folds which also formed about south-southwest to south plunging axes. Flexural flow was the dominant mechanism in the development of the Quad Creek synform and the Wyoming Creek antiform, but passive flow was important in layers of ductile rocks. Granitization was generally synchronous with the F 2 phase; pegmatite dikes were emplaced later in a passive manner. Quartzite and amphibolite acted as resisters to granitization; their termination in granitic gneisses and migmatites is usually most adequately explained by refolding rather than granitization of the quartzite. It is possible that partial melting occurred in some areas such as the agmatized zones. Time of emplacement of the metanorite body is still questionable, but it clearly predates the F 2 folding; intrusion prior to F 1 is probable because B 1 , B 2 , and L 1 diverge in the vicinity of the body.

The Highline Trail Lakes area lies within the Archean basement complex in the Beartooth Mountains of Montana and Wyoming. Eighteen small, pod-shaped, ultramafic bodies associated with granitic gneiss, migmatite, amphibolite, and pegmatite are distributed along a narrow, slightly arcuate zone within the area. The ultramafic rocks are mineralogically complex, containing olivine, ortho-pyroxene, hornblende, anthophyllite, phlogopite, chlorite, serpentine, talc, magnetite, and green spinel in almost every combination in the three dominant rock types: olivine-rich, hornblende-rich, and hornblende-anthophyllite rocks. All textures are crystalloblastic; relict igneous textures were not found. Hornblende-anthophyllite rocks show pronounced lineation of mineral grains and appear to be derived from sheared hornblende-rich rocks. Strongly lineated rocks at the margins of some bodies consist almost entirely of anthophyllite. Closely spaced, highly planar orthopyroxene layers (2 to 3 mm thick) produce rhythmic orthopyroxene-hornblende layering in a few hornblende-rich rocks. These layers, commonly one grain-diameter thick, are coincident with nearly planar surfaces defined by magnetite trails, which appear to represent fractures healed by recrystallization. Locally, less planar layers of identical orthopyroxene traverse the more planar set at high angles. Orthopyroxene-hornblende layering is not associated with any other distinctive fabric element. Later shearing produced a parallel set of serpentinized layers containing fragments of hornblende and orthopyroxene, and bordered by intensely fractured rock. Both types of layering are best explained as resulting from the structural control of metasomatic processes. The bodies are structurally and chemically similar to alpine ultrabasic rocks, and appear to be metamorphic derivatives of a strongly differentiated picrite. The sequence of events following consolidation is as follows: (1) recrystallization under high amphibolite facies conditions to olivine-rich and hornblende-rich rocks; (2) development of orthopyroxene along shear fractures near granulite facies conditions; (3) tectonic breakup of parent body and movement of fragments through plastic country rocks; (4) growth of clinochlore, phlogopite, and anthophyllite at temperatures below about 600°C penecontemporaneously with feldspathization of of country rocks; and (5) serpentinization and steatitization, probably representing several later events. The observed nonequilibrium mineral assemblages resulted from incomplete replacement of earlier assemblages. Most replacements appear to involve metasomatism. Age relationships and geographic distribution suggest that these masses are parts of a disrupted, metamorphosed, satellite intrusion of the Stillwater igneous complex.

The Beartooth Mountains consist of a Precambrian crystalline core uplifted during the Laramide orogeny. This core has been studied by several groups under the direction of Arie Poldervaart to determine the origin and geologic history of the rocks. All published and unpublished work is summarized herein as well as the evolution of ideas of origin of the granitic rocks. Rock types include: microclinerich and microcline-poor granitic gneiss, migmatite, biotite gneiss, para- and orthoamphibolite, ultramafic and mafic igneous rocks, and various quartzites and schists. Facies indicators point to cordierite-amphibolite facies of Abukuma-type metamorphism for relict metasedimentary rocks that have escaped metasomatism. K and Na metasomatism is indicated by: replacement of plagioclase by albite and microcline, alteration of amphibolite to biotite gneiss, mantles of mica around ultramafic pods, progressive variation between quartzite and granite, and growth of orbicular granite. All rocks were deformed by passive folding along north- or northeast-trending axes that plunge from 0° to 40° toward the south. Individual fold axes are traceable for more than 5 miles. Penetrative fabric elements show monoclinic symmetry on a mesoscopic scale. Fabric relations show that folding was contemporaneous with metamorphism. One large folded fold has been found that may indicate a two-phase deformational history. Consistent radiogenic ages indicate metamorphism 2750 m.y. ago. However, a complex history is indicated for similar rocks along the base of the Stillwater Mafic Complex. Radiogenic ages indicate this intrusion was emplaced between 3200 and 3800 m.y. ago, but biotite and plagioclase ages within the complex are 2660 m.y. The intrusive has altered gneisses and amphibolites which are thus older than the complex. It appears that a polymetamorphic history is possible for rocks of the Beartooth Range. Earlier theories postulated an event 2750 m.y. ago that statically metamorphosed and grantized a folded sedimentary series. It is now believed that folding was contemporaneous with this event. Accordant contacts formerly considered indicative of sedimentary layering are explained as being produced during flow of rocks at varying temperatures under high confining pressures in the presence of a fluid phase.

Major rock units of the central and eastern Beartooth Mountains are granitic gneiss, amphibolite, and biotite schist. Migmatitic interlayering and gradational sequences are common. In the western Beartooth Mountains, biotite schist and quartzite are dominant units and granitic gneiss is relatively minor. A summary of available data shows that amphibolite has a composition similar to tholeiitic basalt, and biotite schist is probably a metamorphosed pelitic rock. Granitic gneiss is similar in composition to magmatic granitic rocks. Summary data are evaluated by use of four basically different models for the origin of granitic gneiss: sedimentary-volcanic depositional, magmatic, anatectic, and metasomatic models. Combinations of processes are likely in nature; therefore, combined models such as the magmatic-metasomatic are also considered. Depositional and anatectic models are inadequate to explain field relationships and composition of major rock units. The magmatic model is plausible only if magma injection can take place over hundreds of square miles and vertical distance of thousands of feet without significantly disturbing the homogeneity of fabric and conformable nature of contacts. Rounded zircons in granitic gneiss are evidence against the magmatic model. If gradational sequences are explained by reaction between magma and country rock, there is a problem of disposal of elements, mainly aluminum, that are shown not to be needed in the reaction. The metasomatic model explains many of the observed relationships, but little evidence is available on source of material and transport mechanism. Some experimental evidence suggests that metasomatism under conditions of upper amphibolite facies may produce granitic rocks similar in composition to magmatic or anatectic granites. No single-process model can explain the observed relationships in the Beartooth Mountains. The author is most convinced by a theory of metasomatism as the dominant process, but the evidence can also be interpreted to favor other processes, particularly a combination of synkinematic magmatism and metasomatism.

Alkalic dolerites form many sheets intrusive into the Cretaceous formations widely developed in the Nemuro Peninsula. Sometimes the effects of differentiation in situ and pillow structures are observable in these bodies, and the following three groups are distinguished in the field: (1) differentiated dolerite-monzonite sheets, (2) undifferentiated dolerite sheets, and (3) pillow lava flows. The first group is characterized by the association of rock types ranging from picritic dolerite through dolerite and monzonite to syenite, as the result of gravitative separation of olivine and augite within the sheets. The second group is composed of dolerite only, and sometimes shows remarkable pillow structures in the central parts of the sheets. The third group probably represents offshoots from pillowed sheets of the second group. Several rock types and their minerals were studied in detail, and some were chemically analyzed. The suite forms an “alkali series” with alkali-lime index of 49.0. Predominance of K 2 0 and H 2 0 are characteristic features that differ from alkalic rocks in other parts of Japan and the surrounding areas. The parent magma is inferred to be potash-rich olivine basalt, probably derived from mica-peridotite in the upper mantle under the continent. At first, differentiation occurred at a deep level in the upper mantle, and alkali dolerite magma in a slightly advanced stage, ascended toward a magma reservoir at a higher level in the crust. As the Nemuro Formation was being deposited in the deep sea, the dolerite magma began its activity, intruded the well-stratified unconsolidated sediments, and formed pillow structures in the central portions of sheets. The high-temperature melts were in contact with water derived from wet sediments, but still retained their mobility. Columnar joints were formed in the more quickly chilled upper and lower portions where no movement was possible. No differentiation could occur in these thinner sheets. After deposition of the Nemuro Formation was completed, more primitive parent magma from the deeper reservoir came up and intruded the already consolidated and dewatered sediments as thick sheets, generally more than 100 m in thickness. Since virtually no water was available in this case, pillow structures were not developed, and slower cooling favored differentiation in situ, resulting in the formation of several rock types within the sheets.

Many thin sills and some dikes younger than almost all the associated normal Karroo dolerites occur in a narrow belt that extends from Durban along coastal Natal into Zululand where it flanks Stormberg (Lebombo) volcanic rocks on the west. Characteristic features of these intrusions in the field are glassy and variolitic margins, abundant minute amygdales, and conspicuous xenoliths of typical Natal Precambrian granite and quartzo-feldspathic gneiss and schist. They contain more silica and potash, less iron and lime, and much less magnesia than average Karroo dolerite. Petrographically they are all very similar. The holocrystalline rock has plagioclase ranging from some labradorite to much andesine-albite associated with prismatic clinopyroxene, titanomagnetite plates, much fine quartz-feldspar micrographic intergrowth, and xenocrysts mainly of quartz fragments. The chemical composition spans the range from rhyodacite to trachyandesite. The Effingham rock type is considered to have been produced from the melting and subsequent assimilation of sialic rocks by a slightly differentiated, volatile, and iron-enriched magma at depth. Heat energy supplied by a thermal convective process contributed to the formation of the new magma which was injected in the later phases of Karroo magmatism, during downwarping of what is now coastal region, and probably just prior to continental disruption.

Detailed study of the Palisades Sill has revealed new information on its mineralogy, and, in particular, on the relationships between pyroxenes and olivine in the late fractionation stages, beyond the so-called two-pyroxene field in the pyroxene quadrilateral. In addition, the chemical investigation identified the sill as a multiple intrusion containing two main magma phases. The geochemical study on the distribution of elements, and their behavior with fractionation in tholeiitic magma, indicated the importance of fractional crystallization in the differentiation process. The degree of differentiation shown by the sill was largely controlled by the rate at which the intrusion cooled, for this determined the contribution to differentiation by gravity settling in the early stages of crystallization and the extent of fractional crystallization possible throughout cooling. Differentiation was toward iron-, silica-, and alkali-enrichment, and its range and trend are similar to those in other tholeiite provinces elsewhere.

Mafic and ultramafic nodules of Hawaii are classified into (1) a lherzolite series, (2) a wehrlite series comprising dunite, wehrlite, and pyroxenite, (3) an eclogite series comprising pyroxenite and eclogite, and (4) a group of gabbro. The lherzolite series appears to constitute the major part of the upper mantle. It ranges in Mg0/∑Fe0 from 6.0 to 2.3. With the decrease of this ratio, Al 2 0 3 , total Fe0, Ca0, Na 2 0 + K 2 0, Ti0 2 , and Mn0 increase, whereas Mg0 and Cr 2 0 3 decrease. This variation may have been produced by crystal accumulation in some mafic basaltic magma in a remote past. It is a little difficult, if not impossible, to explain the whole variation as due to different degrees of subtraction of basaltic magmas erupted from the Hawaiian volcanoes. But the lower Mg0/∑Fe0 members of the lherzolite series may be a potential source of various basaltic magmas, provided the amount of partial melting is relatively small. The hypothetical primordial mantle material “pyrolite” is very close in composition to some members of the lherzolite series. The series can be distinguished from the wehrlite series by the compositions of clinopyroxenes. The latter series was probably formed as crystal accumulates at comparatively shallow depths, possibly representing the lower portions of solidified magma reservoirs of the Hawaiian volcanoes. The eclogite series is distinct from the lherzolite series in its trend of compositional variation. With the decrease of Mg0/∑Fe0, A1 2 0 3 , total Fe0, Na 2 0 + K 2 0, Ti0 2 , and Mn0 increase, whereas Si0 2 , Mg0, Ca0, and Cr 2 0 3 decrease. The main sequence of crystallization is (1) pyroxenite, (2) eclogite with clinopyroxene-garnet intergrowth, (3) eclogite, and (4) olivine eclogite. These rocks are supposed to constitute a local pocket or pockets within the mass of the lherzolite series, probably at depths between 40 and 60 km from the surface, where some mafic magma was trapped. It is suggested that the fractional crystallization of the magma which produced the eclogite series gave rise to a magma having a composition close to some of the Hawaiian oceanites. The gabbro nodules are fragments of the crust, some of them being possibly the upper portions of the mass of the wehrlite series.

The Twin Lakes intrusion is composed mainly of coarse-grained porphyritic granodiorite, and is zoned from a felsic core to a slightly more mafic border. Steeply dipping mineral layers, typically a few inches to 5 feet thick and several tens of feet long, occur in discontinuous marginal zones as wide as 5000 feet. Four main types of layers are defined by increased abundances of orthoclase, quartz, plagioclase, and mafic minerals. The characteristic minerals of each type of layer differ markedly in size (orthoclase, average length about 10 cm; quartz, average diameter about 1 cm; plagioclase, average length .45 mm; and mafic minerals, average length .15 mm). Textural evidence from fine-grained granodiorite porphyry and deformed mafic layers indicates that the magma contained 50 to 60 volume percent suspended crystals during emplacement. Structures in the mafic layers such as size and concentration grading normal to the plane of layering, wedge layering, and cross layering superficially resemble sedimentary structures. Inspection of these structures, however, reveals a number of features that are difficult to explain by a process of sedimentation, but which are consistent with a flow sorting process accompanied by deformation. The layering probably formed by size sorting of the suspended crystals in marginal zones of the intrusion by essentially vertical shear flow during emplacement.

Criteria for the recognition of different types of pyroclastic and related rocks, especially breccias, in the vent or cone-complex facies of volcanic provinces are reviewed. These criteria include structural features, character of the fragments, and the composition and texture of the groundmass of the volcanic rocks. Autobrecciation of lava flows produces monolithologic autoclastic volcanic breccias with angular, lithic, unsorted fragments, usually with a central zone or lens of nonbrecciated material. Underground brecciation of previously consolidated material and subsequent extrusion as breccia flows deposit thick, unstratified, unsorted, heterolithologic, nonglassy, nonvesicular, chaotic volcanic breccias. Pyroclastic flows (nuée ardentes) are responsible for coarse blocky breccias, pumice breccias, and ash-flow tuffs, all of which are unsorted, unbedded, and monolithologic. Their fragments contain much glassy vesicular material, especially pumice fragments of all sizes. The crumbling of domes and spines produces a type of glowing avalanche deposit of limited lateral extent and with a larger proportion of dense, angular fragments to vesicular fragments. The deposits of volcanic mudflows or lahars are unsorted, unbedded, heterolithologic, chaotic tuff breccias of great lateral extent. Many pyroclastic rocks are formed by the explosive ejection of tephra with sub-aerial deposition. Vulcanian eruptions produce poorly stratified, unsorted, hetero-lithologic, chaotic tuff breccias as a part of volcanic cone structure. These breccias contain angular, accessory, and accidental fragments and subrounded juvenile materials. Strombolian and lava fountain eruptions form the common cinder and spatter cones of the world. These deposits are sorted, have graded and mantle bedding, and are monolithologic. Their fragments are subrounded, vesicular, cindery, and contain breadcrust and other bombs; the resulting breccias include the true agglomerates. Phreatic eruptions with deposition on land produce breccias similar to vulcanian-eruption deposits if no new magma is present. With new magma these eruptions are phreatomagmatic and build saucer-shaped tuff cones of graded, mantle-bedded, essential, glassy granules, usually palagonitized. Subaqueous breccia flow deposits may be formed from subaerial eruptions in which deposition only is subaqueous or both eruption and deposition may be sub-aqueous. Terrestrial eruptions with subaqueous deposition result in chaotic tuff breccias with poor stratification, low initial dips, and no sorting in coarse-grained beds, but with interbedded stratified lenses of tuffs or volcanic sandstones which show the minor structures of turbidity-current deposits. Phreatic submarine eruptions produce breccias with very similar structural relations but with a much larger amount of glassy granular material, usually palagonitized. Brecciation of lava flows under water produces pillow breccias or hyaloclastites. These glassy pyroclastics produce a group of chaotic, unsorted, unstratified, pillow breccias with angular, glassy, palagonitized material in their groundmass. Underwater slumping and reworking by currents of the finer material from such growing extrusive piles deposit stratified, sorted tuffs and tuff breccias intermixed with marine sediments and fossils. Submarine eruptions of rapidly vesiculating magmas result in quick quenching of materials in a submarine eruptive column which settle back to the ocean floor and are spread laterally by turbidity-current slides to build partially sorted, graded-bedded layers of lapilli tuffs with pumice fragments, crystal fragments, and shattered glass shards.

Serpentinite beds within the Lalm-Sel area in the Caledonides of central Norway occur as a more or less clearly developed “serpentine conglomerate,” a term used in the literature. This rock is unique because in one locality it contains a rich fauna, determined to an age around the boundary Lower to Middle Ordovician. The serpentinite occurs as beds in a strongly folded eugeosynclinal series of greenschists and phyllite. The observations and some evidence from recent literature are used in discussing the origin of the “serpentine conglomerate:” tectonic mélange, lava, metasomatic origin, true sedimentary origin, or pyroclastic origin. The latter is slightly favored, mostly from negative evidence: the “conglomerate” must be a sedimentary formation, but a pyroclastic origin seems less impossible than a truly sedimentary one.

The banded Lewisian gneisses of Mingulay, with minor amphibolites, show the effects of at least five phases of folding and related deformation. The formation of the banding in the gneisses is associated with the earliest recognized phase of deformation during which the main metamorphic reconstitution took place, with the strong linear (L 1 ) and planar (S 1 ) fabric elements related to tight isoclinal folds (F 1 ). The largest folds (F 2 ), which exercise strong control on the gross attitude of the banding, were formed during a second phase of deformation. They are asymmetrical in form with secondary axial planar foliation (S 2 ) and their axial planes controlled the uprise of locally derived quartzo-feldspathic pegmatitic material. Pseudotachylite formation preceded the three subsequent phases of deformation in which the folds (F 3 , F 4 , F 5 ) were formed. These folds are generally open with little or no related mineral reconstitution. Interference structures resulting from the superimposition of these sets of open folds on the asymmetrical folds are common and account for the variation in attitudes of the lithological layering. The most common and largest pegmatitic veins, derived from deeper crustal levels, are situated in the hinge zones of the fourth fold set. Major- and trace-element proportions are consistent with a derivation of the amphibolites from basic igneous rocks. Metamorphic segregation resulted in the formation of hornblendite pods and balls and associated quartzo-feldspathic material. The gneisses are generally comparable to granodiorite in chemical composition and in parts show gradations to more potassium-rich quartzo-feldspathic veins of pegmatitic aspect which are largely concordant. Many of the discordant pegmatites have abundant potassium feldspar. The genesis of the different types of quartzo-feldspathic rock is related to the varying roles of extraction and redeposition of the most soluble substance under conditions of heterogeneous pressure, partial melting, and potassium metasomatism.

The nature and sequence of development of the elements of fabric which characterize the metasediments of the Manhattan Formation occurring on Manhattan Island have been investigated. Three phases of deformation have been separated, and the time relations and geometrical form of the three sets of folds and associated planar and linear structures elucidated on the basis of field observations and structural analysis. Regional metamorphism with the development of the dominant foliation, axial plane to isoclinal folds, was associated with the earliest recognized phase of deformation, termed the Riverside Fold Phase . The most prominent and commonly developed folds were formed during a second phase, the Central Park Fold Phase . Both the banding and dominant foliation were deformed, and the trend of the axes of these folds generally corresponds with the trend of the Appalachian mountain belt. In a third phase of deformation, the Park South Fold Phase , open symmetrical folds were formed. Over most of the Island the effects on pre-existing structures were of a minor nature, but in localized areas deformation was pronounced. The considerable range shown by the published age dates for the metamorphism of the Manhattan Formation is attributed to the varying effects of the three phases on the potassium/argon and rubidium/strontium isotopic proportions.