Showing papers on "Metamorphism published in 1968"

Sevier Orogenic Belt in Nevada and Utah

Abstract: In Nevada and Utah, sedimentation in the Cordilleran miogeosyncline began before the appearance of Cambrian fossils and continued without orogenic interruption through the Triassic. During the Jurassic, deformation and regional metamorphism occurred in the western part of the miogeosyncline, and the area of sediment accumulation shifted onto the Colorado Plateau. A major source of clastic material appeared along the eastern margin of the Cordilleran miogeosyncline in Early Cretaceous time; this source supplied the sediments that filled the Cretaceous to Paleocene Rocky Mountain geosyncline. Clasts in the Cretaceous conglomerates show an inverted stratigraphy, reflecting successive exposure of older and older rocks in an evolving orogenic belt along the eastern side of the Cordilleran miogeosyncline. This source area was the Sevier orogenic belt, which had a history of deformation through most of the Cretaceous (Sevier orogeny). Decollement thrusts with displacements of tens of miles are the characteristic structures of the belt, but several large folds are also known. The largest thrusts are overlain unconformably by uppermost Cretaceous conglomerates. Thrusting in the Sevier orogenic belt had virtually ceased by the time the Laramide orogeny began east of the Sevier belt in latest Cretaceous time. Laramide mountains were the result of uplift of great blocks of crystalline basement along nearly vertical, reverse, and steep thrust faults. The Uinta arch, which intersects the Sevier orogenic belt almost at a right angle, is the only one of these basement uplifts closely involved with the deformation of the Cordilleran miogeosyncline. North-south-trending regional normal faulting of post-Oligocene age has broken up the orogenic belt so that it is not immediately recognizable on geologic maps. Arch ranges, intrusive domes, and gravity slides are additional complications of the Tertiary geology, but widespread Tertiary deposits, particularly Oligocene ignimbrites, make a paleogeologic reconstruction possible; thus, the Sevier orogenic belt can be viewed as it existed before the normal faulting.

Volcanism and metamorphism in the Mid-Atlantic Ridge, 22°N latitude

Abstract: The petrology of the mid-Atlantic ridge between 22° and 23°N latitude may be typical of those portions of the ridge characterized by a linear topography parallel to the axis, a well-developed median valley, and an absence of volcanic cones. Submarine basalt lavas dredged at fifteen stations on the crest of the ridge are of three eruptive facies, all derived from essentially identical magmas; (1) pillow lavas, (2) sideromelane-rich tuffs, and (3) massive, mainly holocrystalline basalts. This association is well known from continental exposures of ancient submarine lavas. Chemically, the lavas are oceanic tholeiites and thus support the view that these low-potassium olivine basalts are by far the dominant eruptive on the deep-sea floor. In the 22° area, they are probably the product of voluminous fissure eruptions. The oceanic tholeiite is evidently the counterpart of the continental flood basalts, but it differs compositionally from these, especially in a lower potassium content. As a further characterization of the basalts, seven new analyses of major, minor, and trace elements are presented. Post-cooling hydrothermal metamorphism under some overburden has transformed some of these basalts into greenschists and lower-grade metamorphic rocks. New data indicate that faulting and shearing along the median valley combined with the introduction of hot, probably saline solutions were major agents in the metamorphism.

O18/O16 Ratios of Coexisting Minerals in Glaucophane-Bearing Metamorphic Rocks

Abstract: Oxygen isotope analyses have been obtained for coexisting minerals in several blue-schist-facies metamorphic rocks from California, Oregon, and New Caledonia. Detailed isotopic studies were made on a continuous exposure of schist in Ward Creek, California, previously described by Coleman and Lee (1962). The oxygen isotope fractionations among coexisting minerals in a variety of rock types, including metasediments and metabasalts, are systematic and larger than those measured in pelitic schists metamorphosed at the grade of biotite zone or higher. Therefore, these Ward Creek rocks (termed Type III) must have formed at lower temperatures than have such pelitic schists. Evidence for significant isotopic equilibration and homogenization is observed in the Ward Creek sequence. Six different metasediments and metavolcanics collected within 25 m of one another show almost identical mineral δ; -values: quartz (15.8 to 16.3), aragonite (13.1 to 13.3), glaucophane (9.8 to 10.0), muscovite (10.9 to 11.3), lawsonite (9.3 to 9.5), and garnet (8.0 to 8.4), given as per mil enrichment in O 18 relative to mean ocean water. These rocks seem to have reached equilibrium at about the same temperature in contact with abundant metamorphic pore fluids. Not all the Ward Creek rocks have completely equilibrated with the postulated metamorphic pore fluids. In particular, the metacherts seem to have been relatively impermeable to the aqueous fluids during metamorphism, as indicated by the large δ -values of quartz in such rocks (17 to 19.2) and by their appreciably higher Fe +3 /Fe +2 ratios. Gradients in O 18 /O 16 and Fe +3 /Fe +2 have been generally “smoothed out” in the rocks during metamorphism, but the process has gone to completion only locally. Cherts and limestones have apparently been lowered in O 18 content by 10 to 15 per mil, and the metabasalts are enriched by 3 to 4 per mil over their unmetamorphosed parent rocks. Using the calibrated quartz-muscovite and quartz—CaCO 3 geothermometers, the measured quartz-muscovite and quartz-aragonite fractionations indicate essentially “concordant” temperatures of formation for the Type III rocks of 270° to 315° C. Inasmuch as aragonite is part of the equilibrium assemblage, these rocks must therefore have been metamorphosed at pressures of at least 6.4 to 7.0 kb. Oxygen isotope fractionations for the mineral pairs aragonite-lawsonite, quartz-muscovite, and quartz-glaucophane progressively decrease from Type II (low-grade) through Type III to Type IV (high-grade tectonic blocks) metabasalts. The higher-grade blueschists from New Caledonia exhibit quartz-muscovite and quartz-glaucophane fractionations similar to the Type IV metabasalts at Ward Creek, indicating temperatures of formation of 400° to 550° C. Thus, glaucophane-bearing metamorphic rocks apparently form over a temperature range of 200° C to 550° C, encompassing the probable temperature range of the entire greenschist and epidote-amphibolite facies. This suggests that glaucophane schists should be separated into at least two metamorphic facies, a lower-grade, lawsonite-aragonite blueschist facies and a higher-grade, epidote-rutile blueschist facies, both representing higher pressures than are attained during ordinary low-rank and middle-rank regional metamorphism.

Isotopic Age of the Nevadan Orogeny and Older Plutonic and Metamorphic Events in the Klamath Mountains, California

Abstract: Several metamorphic and plutonic events have been recognized in the Klamath Mountains utilizing potassium-argon and rubidium-strontium mineral and whole-rock ages. The oldest known metamorphic event in the region produced the Abrams Mica Schist and the co-extensive Salmon Hornblende Schist. Strontium evolution diagrams indicate that the age of primary metamorphism of the Abrams Mica Schist is approximately 380 m.y. (Devonian). The Stuart Fork Formation of Davis and Lipman (1962), the schists of Condrey Mountain, and related schists were produced during a Middle and Late Jurassic metamorphic event. Granitic plutonic rocks are divided into four groups that are different in age and somewhat different in chemical characteristics. The oldest pluton, the Pit River stock, has a minimum age of 246 m.y. (Permian) and is grouped with the Castle Crags pluton. The other three plutonic groups are Middle and Late Jurassic and are characterized by the following ranges in isotopic age: 165 to 167 m.y., 145 to 155 m.y., and 127 to 140 m.y. The principal lode gold deposits in the California part of the Klamath Mountains seem to be related to the youngest group of plutons. If the Nevadan orogeny in this area is restricted to a Middle and Late Jurassic deformational, metamorphic, and plutonic event, the orogeny includes the emplacement of plutons of the three younger groups and the metamorphic development of the Stuart Fork Formation, schists of Condrey Mountain, and related schists.

Age of Precambrian events in the Northeastern Front Range, Colorado

Abstract: Precambrian metasedimentary rocks of the northeastern Front Range have undergone a long and complex geologic history involving multiple periods of metamorphism, deformation, and intrusive activity. The first event following sedimentation involved deformation which produced large east-west isoclinal folds and metamorphism in the greenschist facies and possibly higher. It has been impossible to date this first event by radiometric techniques. The second major event was a period of dominantly medium- to high-grade regional metamorphism accompanied by folding and syntectonic intrusions, such as the Boulder Creek granite. Rb-Sr dating of high-grade gneisses, pegmatites, and Boulder Creek granite places this event at 1700–1800 m.y. ago. Precambrian metasedimentary rocks younger than 1700 m.y. have not been recognized in the northeastern Front Range. A major period of plutonism is dated at 1390–1450 m.y. ago during which time the Sherman and Silver Plume granites, pegmatites, and basalt and andesite dikes were emplaced. The regional heating of the country rock at this time was effective in lowering most of the Rb-Sr and K-Ar mineral ages determined on prebatholithic rocks and probably caused the retrogressive metamorphism which locally affected the gneisses and schists. Post-batholith faulting and cataclasis, possibly at about 1300 m.y., may have been effective in further reducing some mineral ages.

The geochronology of the igneous rocks of Eastern Queensland

Abstract: Isotopic dating of igneous rocks in eastern Queensland south of Bowen has indicated phases of granite emplacement in the Devonian (360 m.y.), Carboniferous (310 and 285 m.y.), Permian (265, 245 and 235 m.y.), Triassic (220 m.y.) and Cretaceous (125 and 110 m.y.). This activity moved generally eastwards with time. The combination of Rb‐Sr and K‐Ar dating has produced a better understanding of the geological history in the complex area of the Connors Arch than would have resulted from the use of either method alone. Also, it was found that, contrary to predictions that the argon retentivity of hornblende is greater than that of biotite during metamorphism, the two minerals had a similar degree of retentivity under burial metamorphism. The relationship between granite intrusion and tectonic phases is examined. The emplacement of Carboniferous granites in the Connors and western Auburn Arches was synchronous with the Kanimbla Movement, while episodes of igneous intrusion in the Late Permian, from Mar...

Sphene: uranium-lead ages.

Abstract: Uranium-lead ages were measured on 14 samples of sphene from rocks aged from 1000 to 2750 x 10 6 years. All samples gave concordant or nearly concordant ages, the maximum difference between the Pb 206 -U 238 and Pb 207 -Pb 206 ages being 10 percent. Sphene has more concordant ages than has the coexisting zircon in each of seven rocks in which they were compared. Sphene sometimes has greater ages than does coexisting biotite, although in two metamorphic rocks, in which metamorphism was sufficiently intense to cause redistribution of radiogenic strontium-87 between various mineral phases, sphene dates the time of metamorphism rather than of original crystallization of the rocks.

Strontium isotope variation and whole‐rock isochron studies, Grenville Province of Ontario

Abstract: Rb-Sr whole-rock isochrons were obtained for five granitic bodies and two metasedimentary horizons distributed over a distance of 100 miles in a region of typical Grenville metasediments in southeastern Ontario. Ages (in millions of years: m.y.) of plutonic rocks obtained and their geological significance are as follows: 1285 ± 41 m.y., a time of emplacement for a major pretectonic sill that was deformed on a region scale (the Blue Mountain nepheline syenite). 1103 ± 39 m.y., a maximum value for the time of intense metasomatism that accompanied the formation of the gneisses in the Burleigh paragneiss dome. 1096 ± 50 m.y., a time of emplacement for a major syenite mass that was deformed during regional deformation (the Gananoque syenite). 1035 ± 60 m.y., a minimum time for the formation of a major sill-like granitic body (the Kaladar gneissic granite). 1016 ± 39 m.y., a time of emplacement for a major posttectonic granite (the Westport pluton). Projected initial Sr87/Sr86 ratios for these rocks range from 0.704 to 0.7055, whereas measured values for gabbros and marbles are 0.703 and 0.705, respectively (with the exception of the Kaladar granite for which a less precise value of 0.711 was determined). The indicated age of 1230 ± 72 and 1056 ± 33 m.y. for the Morton sillimanite paragneiss and the metasediments adjacent to the Burleigh paragneiss dome may indicate the time of metamorphism of these rocks. The low initial strontium ratios (0.707 and 0.705) for these metasediments indicate a very short pre-Grenville history. Two granitic bodies in the Grenville province (1 and 9 miles south of the Grenville front) in the North Bay area gave somewhat inconsistent isochrons that approach the age of the Superior province to the northwest (2000–2700 m.y.). Isochron data for metasedimentary rocks in this geological setting are inconsistent. Their high initial strontium ratio (greater than 0.726 in some cases) is an indication of the pre-Grenville history of these rocks. It is therefore concluded that the rocks adjacent to the Grenville front in the areas studied are the metamorphic equivalents of the rocks to the northwest. The rocks studied in southeastern Ontario were deposited or emplaced between 1300 and 1000 m.y. ago and were not derived from an ancient sialic basement such as that found north of the Grenville front. We refer to this time of plutonism and metamorphism in this area as the ‘Grenville orogeny’ but acknowledge that a revision of ‘Grenville’ nomenclature is needed.

The chemical evolution of the Canadian Shield

Abstract: Archean surface crystalline rocks of the Canadian Shield differ chemically from those of Proterozoic age. The younger rocks are higher in K2O, TiO2, U, and Th and lower in Na2O, Cr, Ni, and possibly MgO and CaO. This may be a secondary result of vertical chemical zoning of the crust as a result of metamorphism accompanied by anatectic melting. The erosion of a zoned crust would result in the enrichment of elements such as K, Na, Si, U, and Th in younger sedimentary basins. The crystalline crust subsequently evolved from the sediments of these basins would be enriched in these elements relative to older rocks exposed as a result of erosion. The secular chemical evolution of the shield may also reflect a progressive, though irregular, shift in the composition of new sial being added to the crust. Evidence for this progressive change is found in the increase in the abundance of potassium of successive dike swarms that were feeders of new sial to the upper part of the crust.

A synthesis of folding and metamorphism in the Mt Lofty Ranges, South Australia

Abstract: The Upper Precambrian and Lower Palaeozoic Rocks in the Mt Lofty Ranges, South Australia, have been subjected to at least three phases of folding. The first involved the formation of inclined folds and less common reclined folds. These structures are overprinted by usually upright, moderately tight, second and third generation folds which may show a well developed axial plane crenulation cleavage. The metamorphism commenced prior to the appearance of penetrative structures and continued in many areas until after the third phase of deformation. It appears to have had its greatest effect during the static period following the first phase of folding. Mineral assemblages of the pelitic rocks indicate that the metamorphism is of the low pressure‐intermediate type and that there are at least four progressive zones of metamorphism, namely, chlorite, biotite, andalusite‐staurolite, and sillimanite. Cordierite occurs in the sillimanite zone and kyanite is sporadically distributed in the andalusite‐staurol...

Picritic magma — residual dunite relationships in garnet peridotite at Kalskaret near Tafjord, South Norway

Abstract: Field and textural relationships have indicated the tectonic emplacement of the Norwegian garnet peridotites as relatively cold intrusions into their present environment. Mineralogical data demonstrate considerable heterogeneity. Olivines and orthopyroxenes in garnet rich peridotites are significantly more ferriferous than those in garnet free peridotites. Mineralogical features indicate that the mineral assemblages have been equilibrated at subsolidus temperatures. However, the hypothesis that these garnet peridotites have resulted from the eclogite facies metamorphism in deep levels of the crust of other peridotite mineral facies assemblages is considered and rejected.

The Origin of Crossed-Girdle Orientations of Optic Axes in Deformed Quartzites

Abstract: In the narrow metamorphic aureole of the Papoose Flat granitic pluton, Inyo Mountains, California, progressive flattening of stratified sedimentary rocks is observed; it is accompanied by recrystallization and development of foliation, lineation, and preferred orientation of minerals. The field and petrographic evidence indicate that these phenomena occurred concomitantly during a single episode of deformation and metamorphism associated with the emplacement of the pluton into previously unmetamorphosed rocks. The type of strain in the rocks is reflected in distortion of fossils and well-developed boudins. Preferred orientation in recrystallized quartzites is shown to be related to this episode, and the purpose of the paper is to correlate the preferred orientations with the strain and stress insofar as they are specified by the structures noted. The quartz axes are typically oriented in two nearly perpendicular planes that are symmetrically inclined to the foliation and intersect in the foliation perpend...

Mantled Gneiss Domes in the Albion Range, Southern Idaho

Abstract: The Albion Range, in southern Idaho east of the Antler orogenic belt and west of the Sevier orogenic belt, exposes a northeast-trending chain of four mantled gneiss domes. The Green Creek Complex (Precambrian [2.5 b.y.] gneiss and metasediments), which forms the cores of the domes, is unconformably overlain by the Paleozoic Dove Creek Group, consisting of sparsely fossiliferous metasediments approximately 22,000 feet thick. The basal quartzite and overlying schists, marbles, and thin quartzites are of probable Cambrian age. A massive quartzite and overlying schist and limestone may be Cambrian or Ordovician in age. Ordovician quartzite more than 7000 feet thick forms the middle part of the metasedimentary section, and this is overlain by massive pure dolomite, calcareous black schist, and quartzite of Ordovician through Mississippian(?) age. Devonian strata are thin or absent. At the top of the metasedimentary section are limestone, schist, and quartzite of the Pennsylvanian Oquirrh(?) Formation. Younger Paleozoic sandstone, limestone, and chert, and Triassic(?) shale occur in fault contact overlying the metasediments. Cenozoic sedimentary and volcanic rocks unconformably overlie the deformed older rocks. Metamorphic grade (greenschist and amphibolite facies) increases northwestward and downward in the area. Staurolite and kyanite occur in Cambrian rocks in the northern half of the range; cordierite, andalusite, and sillimanite occur in Precambrian rocks in the southern half of the range, but these may be due to Tertiary contact metamorphism. Deformation during Mesozoic metamorphism produced two fabric systems; locally a Precambrian metamorphic fabric survived the younger metamorphism. Early bedding foliation and northeast-trending, northwest-vergent folds and lineations predated rise of the domes. Northwest-trending folds and lineations with northeast and southwest vergence were produced during and after doming; over much of the western half of the range these structures destroyed all older fabrics. During the Oligocene (30 m.y.) a postkinematic adamellite stock—the Almo Pluton—and related granitic dikes were emplaced. Following emplacement of the pluton, uplift of the range accelerated to an average rate of .5 mm per year, bringing the gneiss domes to the surface from a depth of at least 10 km. The Albion Range is part of a belt of rocks along the western part of the Cordilleran miogeosyncline which was affected by high-grade regional metamorphism during the middle of the Mesozoic. The metamorphic rocks are part of the Cordilleran infrastructure, which can be traced from California into southern British Columbia and beyond.

Volcanic greywackes from the upper Devonian Baldwin formation, Tamworth-Barraba district, New South Wales

Abstract: The Baldwin Formation consists of interbedded greywackes and mudstones deposited in the New England Eugeosyncline during Late Devonian times. The greywackes consist essentially of andesitic detritus and they may be termed volcanic greywackes. They contain very little detrital quartz or sedimentary rock fragments. Chemical analyses of ten greywackes are presented. These show that the greywackes have an andesitic composition, differing only in their relatively high sodium contents. The high sodium values may be due to reaction between the detritus and sea water. Burial metamorphism of the greywackes has produced minerals characteristic of the prehnite‐pumpellyite metagreywacke facies. Secondary minerals include albite, chlorite, calcite, prehnite, pumpellyite and epidote.

Precambrian Geochronology of the Medicine Bow Mountains, Southeastern Wyoming

Abstract: The Medicine Bow Mountains of Wyoming are located in a transitional zone between the 2.5 b.y. old Superior Geochronologic Province to the north and the 1.3 to 1.7 b.y. old Central U.S. Geochronologic Province to the south. The mountains are crossed by a major northeast-trending shear zone, northwest of which is a quartzofeldspathic-gneiss complex that is intruded by the Baggot Rocks granite, by metagabbro, and by pegmatite. The complex is overlain nonconformably by more than 35,000 feet of miogeosynclinal metasedimentary rocks. Rb-Sr whole-rock isochrons indicate that the quartzofelds-pathic gneiss is 2410 ± 50 m.y. old, and the Baggot Rocks granite is 2340 ± 50 m.y. old. The metasedimentary rocks are at least 1550 ± 50 m.y. old, as indicated by a biotite Rb-Sr date, and are younger than 2410 ± 50 m.y. Whole-rock isochrons of 1550 ± 425 m.y. and 1650 ± 60 m.y. for two formations in the metasedimentary sequence may represent times of metamorphism but define at least minimum dates for sedimentation. Pegmatite intruding the gneiss complex is at least 1600 m.y. old; an anomalous initial Sr 87 /Sr 86 (0.745) permits the interpretation that the primary age is closer to 2400 m.y. Gneiss southeast of the shear zone is more calcic and mafic than that to the northwest, and it is intruded by metagabbro, pegmatite, foliated granite, and massive granite thought to be related to the Sherman Granite. The gneiss is unsuitable for direct dating because of low Rb/Sr ratios. Pegmatite yields mineral isochrons ranging from 1455 ± 40 to 1570 ± 40 m.y., and the Sherman-type granite yields a whole-rock isochron of 1335 ± 30 m.y. Analyzed whole-rock samples of foliated granite do not form a single isochron but show no evidence of being older than about 1715 m.y. No evidence has been found to indicate an age of 2.5 b.y. for rocks southeast of the major shear zone in the Medicine Bow Mountains, nor has any been found in nearby areas southeast of the projections of the shear zone. Rubidium-strontium mineral isochrons and individual mineral dates indicate one or more episodes of metamorphism in the Medicine Bow area between 1600 and 1455 ± 40 m.y. ago. We suggest that miogeosynclinal sedimentary rocks in the northern Medicine Bow Mountains, roughly correlative with Animikie and Huronian strata of Minnesota, Michigan, and Ontario, were deposited on the southern edge of a craton consisting of rocks mainly ≥ 2400 m.y. Gneiss of the southern Medicine Bow Mountains and gneiss and graywacke of the Front Range may be the more metamorphosed, eugeosynclinal counterparts of these sedimentary rocks which have been metamorphosed and transformed during the 1600 to 1455 m.y. old, or older, orogeny.

Age Relations and Provenance of the Dalradian Series of Scotland

Abstract: Rb-Sr and K-Ar isotopic measurements from Dalradian metasediments and associated intrusions of the Caledonian belt of Scotland indicate a magmatic and metamorphic history extending over 130 m.y. Magmatic events took place approximately 530, 440, and 400 m.y. ago. The mid-Cambrian activity included the formation of minor granitic bodies (previously referred to as the Older Granites), as well as nonfoliated granites and pegmatites. Microcline granites and pegmatites were emplaced 440 m.y. ago during Ordovician times. The last phase of Caledonian magmatic activity took the form of forcefully and passively emplaced postorogenic granites 400 m.y. ago. Two regional thermal events at 440 to 460 m.y. and 400 to 420 m.y. ago are recorded. The former event is discussed in terms of either a widespread retrograde metamorphism (M 3 ), or uplift and cooling of a high-grade, static, pre-Arenig (M 2 ) event. The 400 to 420 m.y. event was restricted to the westerly margins of the present Dalradian outcrop and is thought to be associated with a younger (M 4 ) metamorphism. Whole rock analyses of metasediments from widely differing structural and stratigraphic levels show that the majority of sediments had a relatively uniform Sr 87 /Sr 86 ratio of about 0.719 at time of deposition. A whole rock isochron of 670 ± 30 m.y. is interpreted as either an approximate or maximum depositional age for the bulk of the Dalradian sedimentary rocks. Unusually high initial Sr 87 /Sr 86 ratios in most of the granites reflect remobilization of crustal material enriched in radiogenic Sr. A close grouping of initial Sr 87 /Sr 86 ratios around 0.717 from most of the granites, regardless of age, is interpreted as indicating a common source region. High initial Sr 87 /Sr 86 ratios of approximately 0.730 from two granitic masses may reflect remobilization of the Dalradian sedimentary rocks during regional metamorphic activity. Although the 400 m.y. passively and forcefully emplaced granites cannot be distinguished chronologically from one another, distinct differences in their initial Sr 87 /Sr 86 ratios of 0.717 and 0.707, respectively, may indicate distinct petrogenetic differences. Because of the variable distribution of radiogenic Sr between the Dalradian sediments and lower crustal levels, the initial Sr 87 /Sr 86 ratios of the magmatic intrusions can be used as an indicator of their relative depth of formation. A working hypothesis is outlined for the region including a series of magmatic and metamorphic events 530, 440, and 400 m.y. ago.

Westward Thrust Faulting in the South-Central Klamath Mountains, California

Abstract: Structural studies in the Cecilville quadrangle of the Klamath Mountains, northwestern California, indicate a tectonic imbrication of three of the four Klamath geologic subprovinces. The structural sequence is interpreted as follows (bottom to top): (l) rocks of the western Paleozoic and Triassic subprovince; (2) the Siskiyou thrust zone; (3) the Siskiyou thrust plate composed of rocks of the central metamorphic subprovince; (4) the Trinity thrust zone and the sheetlike Trinity ultramafic pluton present along it; and (5) the Trinity thrust plate composed of rocks of the eastern Klamath subprovince. Minimum westward displacements of the Siskiyou and Trinity thrust plates are estimated at 15 to 20 miles and 20 miles, respectively, based on the positions of fensters and klippen. All rocks previously assigned to the Stuart Fork Formation are now recognized as parts of the western Paleozoic and Triassic subprovince, some exposed within fensters in the Siskiyou thrust plate. The relationship of the Trinity ultramafic pluton to the Trinity thrust zone has not been resolved. The peridotitic pluton may have been injected along the thrust contact, presumably during Mesozoic:(?) thrusting. An alternative is that the pluton was intruded into the Grouse Ridge Formation of the central metamorphic subprovince during Paleozoic regional metamorphism, and that later thrust faulting was localized across it. Regional relationships favor the first alternative; whereas, field relationships of the pluton with rocks of the Grouse Ridge Formation favor the second. The problem can be resolved if there has been more than one episode of ultramafic emplacement in the south-central Klamath region. Regional metamorphism of moderate grade of the central metamorphic subprovince has been isotopically dated by Lanphere and Irwin (1967) as Early to Middle Devonian. Thrust faulting of probable Jurassic age was accompanied in some areas by low-grade regional metamorphism, responsible for partial retrogressive metamorphism of central subprovince rocks. It is unlikely that any Paleozoic formation now exposed in the Klamath Mountains can be considered as the parental unit for the Salmon Hornblende Schist of the central metamorphic subprovince, and the same may be true for the overlying Grouse Ridge Formation. The two formations possibly represent basaltic oceanic crust (sima) and Ordovician or pre-Ordovician eugeosynclinal rocks deposited on it. The presence of westward-directed thrust plates in the Klamath Mountains is in keeping with recent discoveries of westward thrusting elsewhere in Cordilleran eugeosynclinal areas. Apparent westward migration in northern California of tectonic episodes characterized by thrust faulting and accompanying injection of mantle peridotites along the faults can best be explained by periodic underthrusting of the continental margin by an active oceanic block.

Geochronology in the Lake Superior region

Abstract: During the past 10 years many radiometric ages have been determined on minerals and rocks in the Lake Superior region. The oldest known rocks are the granitic gneisses in the Minnesota River Valley, which have been dated at 3300 to 3550 m.y. ago. Both K–Ar and Rb–Sr methods have been applied to samples from a number of the metasedimentary formations in the region. The ages, however, appear to be the time of folding or of metamorphism rather than of deposition for which only limits or ranges can be given from the ages for associated igneous and metamorphic rocks.Although considerable progress has been made, significant uncertainties remain in the decay constants and in the analytical measurements. More serious problems, however, are geologic ones, such as the effects of metamorphism and of weathering on the parent–daughter nuclide ratios. Both analytical and geological considerations must enter into any proposal for a time classification of the Precambrian.A three-fold division of the Precambrian with time...

A geochronologic investigation of the anorthosite complex, Adirondack mountains, New York

Abstract: Cogenetic uranium-lead isotope systems in zircons have been analyzed for five rock samples from the anorthositic domes of the Adirondack highlands and compared with data from a cogenetic suite in a charnockitic gneiss from the Ticonderoga dome in the eastern Adirondacks. Textural and structural arguments applied to the zircons and rocks from the anorthosite suite indicate that some of the zircon are metamorphic and reveal that conditions producing granulite facies metamorphism exited in different parts of the anorthosite complex at discretely different times during the interval from 1020 to 1100 m.y. ago. The Ticonderoga gneiss sample yields an interpreted zircon age of 1130 ± 10 m.y. Zircon characteristics argue this to be the age of magmatic crystallization of the host rock. Extensive studies have confirmed this age as the prevailing age in many large bodies of charnockite (or syenite) gneiss which are peripheral to the anorthosite domes. Petrological arguments of persistent spatial association and compositional gradation between the anort11osite and a suite ranging from norite to mangerite, syenite, quartz syenite and granite, imply that all of these rock are comagmatic; this leads to the conclusion that the age of the anorthosite itself is 1130 m.y. No evidence has been found for any older ages in any of the Adirondack orthogneisses to support the hypothesis of a pre-"Grenville" basement. The mounting evidence for almost precise age contemporaneity between the Adirondack anorthosite-syenite complex and the Duluth anorthositic gabbro-granophyre complex should provoke a close comparative petrologic and structural scrutiny of these two great stratiform masses.

Deformation and Metamorphism in the Lukmanier Region, Central Switzerland

Abstract: The Lukmanier region in the central-southern Swiss Alps contains parts of three important structural and stratigraphic units: the Gotthard Massif in the north, the parautochthonous Mesozoic cover of this massif, and the Lucomagno Massif, a frontal part of the Lower Pennine nappe complex to the south. Two successive phases of Alpine folding have been recognized in the Mesozoic cover, both of which occurred after the main northward horizontal transport of the Lower Pennine nappes. The earlier phase, called Phase B, produced three large-scale fold zones in the cover, with a penetrative axial-plane schistosity that strikes east-west and dips steeply north; fold hinge lines are markedly curved in the plane of the schistosity due to differential strain effects. Deformed fossils and ooids indicate the principal axes of Phase B strain and show that the steeply plunging, micaceous mineral lineation, which is found on most schistosity surfaces, is parallel to Phase B maximum strain extension. Regional Phase B folds were modified to various extents by Phase B movements on tectonic slides that lie parallel to the axial-plane schistosity and form the east-west contacts of both massifs with the Mesozoic rocks. The subsequent phase of deformation, called Phase V, produced no large-scale structures, but corrugated bedding and Phase B schistosity and mineral lineation mainly in the southern half of Lukmanier. Fold axial planes strike southeast and dip gently northeast, and fold axes plunge gently southeast. Complex systems of joints and rare, local strike-slip faults developed after Phase V, probably during postorogenic regional uplift. Effects of both phases of Alpine folding are found in the Gotthard and Lucomagno Massifs. Phase B produced the schistosity that strikes east-west and dips steeply north in both massifs, and the shapes of augen, together with deformed schlieren in the Hercynian granodiorite in the Gotthard Massif, indicate that the orientations of Phase B strain axes are similar to those in the Mesozoic cover, with the mineral lineation on the schistosity in both Massifs being parallel to Phase B maximum strain extension. Very intense Phase B deformation in the Gotthard Massif was restricted mainly to mylonitization in narrow east-west zones, and such movements in one zone of ancient paragneisses appear to have controlled the position of the Scopi synclinal zone, the most northerly large-scale Phase B fold zone in the Mesozoic cover. Phase V deformation had little effect in the Gotthard Massif, but Phase V folds are found everywhere in the Lucomagno Massif: their orientation is similar to Phase V folds in the Mesozoic cover. Complex systems of joints in both Massifs probably developed at about the same time as similar joints in the Mesozoic cover. Alpine metamorphism, generally of lower almandine-amphibolite facies, produced a diversity of porphyroblasts in the Mesozoic cover, and they show a variety of textural relations with phases of deformation. These relations change across the area and are due to migration of phases of deformation from south to north relative to stationary metamorphic conditions. Porphyroblast textures also show that shape and predeformational orientation may be important factors controlling amounts of porphyroblast rotation. Regional considerations of Alpine events in Lukmanier and adjacent areas lead to the conclusion that the main northward horizontal transport of the Lower Pennine nappes preceded Phase B deformation in Lukmanier, and in turn Phase B preceded and initiated, by uplift of the Gotthard Massif, the main northward horizontal transport of the Ultrahelvetic and Helvetic nappes.

Chronology of Precambrian events in the Front Range, Colorado

Abstract: The Precambrian rocks of the Front Range have undergone a long and complex history involving multiple periods of metamorphism, deformation, and intrusive activity. Before 1750 m.y. ago sedimentatio...

Guatemela: preliminary zircon ages from central cordillera.

Abstract: Concordia resolution of uranium-lead analyses of zircons from rocks of the Guatemalan Cordillera indicates a period of plutonism, and perhaps metamorphism, in late Paleozoic times (345 +/- 20 million years). Gneisses of the Chuacus Series yield an age of 1075 +/- 25 million years which may be either the age of a source terrain for the original sediments, or the age of principal metamorphism. Zircons from a Paleozoic granite intruding the gneisses appear to contain inherited or contaminating material acquired during the process of magma generation or emplacement, or both.

An outline of the geology of the central eastern alps

Abstract: The Eastern Alps extend about 500 kilometres in an east-west direction; at the Upper Rhine they pass westwards into the Western Alps, and at Vienna eastwards into the Carpathians, although this latter continuation is buried by the Mio-Pliocene Vienna Basin. Upon an Hercynian basement of metamorphic and sedimentary rocks, late Palaeozoic to earliest Tertiary sediments were deposited in a series of east-west trending facies-belts. Each belt has a complex depositional history, but in the northern belts there is generally a southward transition from thin, shallow-water deposits to thicker, deep-water deposits (i.e. north to south: Helvetic facies; Flysch facies; Schieferhulle, with local ophiolites). In contrast, however, the next adjacent belts to the south have thin shelf-deposits (Lower East Alpine facies, etc.), and pass southwards into very thick carbonates and reef complexes (Calc-Alpine facies). Rates of subsidence and sediment accumulation varied considerably between the different facies belts with time. In the mid-Cretaceous, thrusting and possibly strike-slip faulting (‘pre-Gosau movements’) occurred in the south within the Calc-Alpine rocks; these rocks were reburied within a short time by an unconformable cover of late-Cretaceous clastics (Gosau Beds). In the latest Cretaceous or early Tertiary, however, a series of regional thrust-sheets developed, transporting the more southerly facies-belts relatively northwards on top of each other, and on top of the more northerly belts. Thus, a pile of extensive but relatively thin superjacent sheets was formed within which the Mesozoic section may locally be repeated as many as four times. Present-day relationships are complicated by subsequent normal faulting. In early Tertiary times, after metamorphism of its lower parts, this pile was arched about an east-west axis, and coarse elastic debris was supplied to an Oligo-Miocene Molasse basin to the north. Erosion of this arched pile brought about thepresent-day regional outcrop pattern of east-west trending rock-units, modified locally in the axial zone by exposure of the lowest structural units in tectonic windows. The largest window, the Tauernfenster, exposes regionally metamorphosed (par-)autochthonous Pennine argillites (Schieferhulle) locally bearing kyanite and glaucophane, and resting upon Hercynian gneisses.

Radiometric age determinations, Bighorn Mountains, Wyoming

Abstract: A foliated granitic complex in the north evolved contemporaneously with gneiss in the south. Small ultramafic bodies in both areas were emplaced before and after metamorphism. All are cut by younger dolerite dikes. K-Ar dates in the gneiss and granitics range from 2.31 to 2.78 b.y.; quartz monzonite is 2.78 b.y., and quartz diorite ranges from 2.79 to 3.14 b.y. Other miscellaneous dates are reported. The dates are minimum only, due to argon loss. The major metamorphism-metasomatism event at approximately 3 b.y. is supported by U-Pb dates on zircons and monazite.

Pyroxenes in the blueschist facies of California

Abstract: In the Fransciscan Formation pure end-member jadeite is restricted to metasomatized leucocratic masses within serpentinite. Jadeitic pyroxene coexists with quartz in metagraywackes, and omphacite or acmitic pyroxene are present in metabasalts; metacherts contain acmitic pyroxene. Chemical, optical, and X-ray parameters of pyroxenes from representative rocks show strong variations related to rock composition. Experimentally, jadeite and related pyroxenes are favored by high pressures. In silica-deficient serpentinites, pure jadeite forms, whereas in other rock types, under the same metamorphism, where excess silica is present, only jadeite pyroxene and omphacite are stable.

The Broken Hill ore body, Australia

Abstract: The paper reviews recent works on the Broken Hill ore body to see what data is available on the origin and history of the ore. A high grade metamorphism is recognizable (the Willyama Metamorphism) and this was followed by a number of retrograde metamorphic events. The characteristics of these events is described and this is followed by an analysis of the hypothesis that there is a stratigraphic control of Broken Hill type mineralization. It is concluded that any control that does exist is of a regional nature and that the evidence is equally as good for a restriction of mineralization to areas of high grade metamorphism. Many of the isotopic and geochronological results for Broken Hill are not diagnostic as far as ore genesis is concerned. They do present an internally consistent set of data that records three major events, the Willyama Metamorphism and introduction of the Broken Hill type mineralization at 1700 m. y., the intrusion of the Mundi Mundi Granite and cross cutting pegmatites at 1560 m. y., and retrograde metamorphism and introduction of the Thackaringa Mineralization at or prior to 500 m.y. The sulphides have been deformed in the retrograde schist zones and the ore therefore predates this event. But there is no evidence at the moment that unambiguously establishes that the sulphides were present prior to the Willyama Metamorphism. If it was present prior to or during the Willyama Metamorphism and there is no period of deformation prior to the first recognizable folds at Broken Hill, then the sulphide mass was initially grossly discordant with bedding.