Geology topics
Warren J. Nokleberg
Publications and source records attributed to Warren J. Nokleberg.
Summary Circum-North Pacific Tectonostratigraphic Terrane Map
Explore the source record for details and available documents.
Tectonostratigraphic terrane and overlap assemblage map of Alaska
Explore the source record for details and available documents.
Geologic road guide to Kings Canyon and Sequoia national parks, central Sierra Nevada, California
Explore the source record for details and available documents.
Circum-North Pacific tectonostratigraphic terrane map
The companion tectonostratigraphic terrane and overlap assemblage of map the Circum-North Pacific presents a modern description of the major geologic and tectonic units of the region. The map illustrates both the onshore terranes and overlap volcanic assemblages of the region, and the major offshore geologic features. The map is the first collaborative compilation of the geology of the region at a scale of 1:5,000,000 by geologists of the Russian Far East, Japanese, Alaskan, Canadian, and U.S.A. Pacific Northwest. The map is designed to be a source of geologic information for all scientists interested in the region, and is designed to be used for several purposes, including regional tectonic analyses, mineral resource and metallogenic analyses (Nokleberg and others, 1993, 1994a), petroleum analyses, neotectonic analyses, and analyses of seismic hazards and volcanic hazards. This text contains an introduction, tectonic definitions, acknowledgments, descriptions of postaccretion stratified rock units, descriptions and stratigraphic columns for tectonostratigraphic terranes in onshore areas, and references for the companion map (Sheets 1 to 5). This map is the result of extensive geologic mapping and associated tectonic studies in the Russian Far East, Hokkaido Island of Japan, Alaska, the Canadian Cordillera, and the U.S.A. Pacific Northwest in the last few decades. Geologic mapping suggests that most of this region can be interpreted as a collage of fault-bounded tectonostratigraphic terranes that were accreted onto continental margins around the Circum- . orth Pacific mainly during the Mesozoic and Cenozoic (Fujita and ewberry, 1983; 1987; Parfenov, 1984, 1991; Howell, 1985; Watson and Fujita, 1985; Parfenov and Natal'in, 1984; Jones and others. 1987; Monger and Berg, 1987, Fujita and Cook. 1990; Zonenshain and others. 1990; Natal'in, 1991, 1993; Moore and others, 1992; Silberling and others, 1992; Nokleberg and others. 1992, 1993, 1994a; Parfenov and others. 1993; Plaflcer and Berg, 1994; Tabor, 1994). This map is the result of extensive geologic mapping and associated tectonic studies in the Russian Far East, Hokkaido Island of Japan, Alaska, the Canadian Cordillera, and the U.S.A. Pacific Northwest in the last few decades. Geologic mapping suggests that most of this region can be interpreted as a collage of fault-bounded tectonostratigraphic terranes that were accreted onto continental margins around the Circum- . orth Pacific mainly during the Mesozoic and Cenozoic (Fujita and ewberry, 1983; 1987; Parfenov, 1984, 1991; Howell, 1985; Watson and Fujita, 1985; Parfenov and Natal'in, 1984; Jones and others. 1987; Monger and Berg, 1987, Fujita and Cook. 1990; Zonenshain and others. 1990; Natal'in, 1991, 1993; Moore and others, 1992; Silberling and others, 1992; Nokleberg and others. 1992, 1993, 1994a; Parfenov and others. 1993; Plaflcer and Berg, 1994; Tabor, 1994). On the companion map and in the descriptions bel?w· terranes are interpreted according to inferred tectonic environments. These environments are (I) cratonai; (2) passive continental margin; (3) metamorphosed continental margin; (4) continental-margin arc; (5) island arc; (6) oceanic crust, seamount, and ophiolite; (7) accretionary wedge and subduction zone: (8) turbidite basin; and (9) metamorphic for terranes that are too highly-deformed and metamorphosed to de~ermine the original tectonic environment. For terranes with complex geologic histories, the chosen color indicates the tectonic environment most prevalent during this history of the terrane. The tectonic environments inf~rred for igneous rocks are both temporal and genetic. The temporal environments are preaccretion and postaccretion. The genetic environments are subduction-related, rift-related, and collisional (anatectic)- related. In addition to terranes, the map also depicts postaccretion units that include: (I) Cenozoic and Mesozoic overlap assemblages of sedimentary and volcanic rocks that are deposited across two or more terranes that formed generally after accretion of most terranes in the region; (2) Cenozoic and Mesozoic basinal deposits that occur within a terrane or on the craton; (3) plutonic rocks. The postaccretion igneous units are identified by age-lithologic abbreviations and by name. These overlap assemblages and basinal deposits formed mainly during sedimentation and magmatism that occurred after accretion of terranes to each other or to a continental margin. Overlap assemblages provide minimum ages on the timing of accretion of terranes. Some Cenozoic and Mesozoic overlap assemblages and basinal deposits, as well as fragments of terranes, are extensively offset by movement along postaccretion faults. In addition, in onshore areas, the map depicts major preaccretion plutonic rocks that are limited to individual terranes. and in offshore areas. the map depicts major oceanic plates,-ocean floor magnetic lineations. oceanic spreading ridges, and seamounts. The map consists of five sheets. Sheets I and 2 depict, at a scale of I :5.000.000. the tectonostratigraphic terranes. preaccretion plutonic rocks, and postaccretion Cenozoic and Mesozoic overlap sedimentary, volcanic. and plutonic assemblages, and basinal deposits for the Circum- orth Pacific including the Russian Far East, northern Hokkaido Island of Japan, Alaska. the Canadian Cordillera, part of the U.S.A. Pacific Northwest. and adjacent offshore areas. Sheet 3 provides the list of map units for Sheets I and 2. Sheet 4 is a index map showing generalized onshore terranes and overlap assemblages for onshore parts of the Circum-North Pacific at a scale of I: I 0,000,000. Sheet 4 is a guide to the more complicated onshore features depicted on Sheets I and 2. Sheet 5 is an index map showing the major geographic regions for the Circum-North Pacific. Significant differences exist between the representation of onshore and offshore geology on Sheets I and 2. These are: (I) compared to the onshore part of the map, the offshore part is depicted in a more schematic fashion because of more limited data and because the offshore terranes and early Cenozoic and older overlap assemblages generally are obscured by extensive late Cenozoic sedimentary cover that is not shown unless thicker than two kilometers; (2) marginal contacts of offshore Cenozoic and Cretaceous sedimentary basins do not match contacts of onshore Cenozoic and Cretaceous sedimentary units because offshore basins are limited to those regions with sediment thicknesses greater than two kilometers; (3) stratigraphic columns, included at the end of this explanation. are provided only for onshore terranes because the geology of offshore terranes is generally less well-known; and (4) for simplicity, the major onshore Cenozoic sedimentary basins are generally not defined and described separately because the onshore part of the map is designed to emphasize terranes and overlap volcanic assemblages that are crucial for both for tectonic and metallogenic analyses published elsewhere (Nokleberg and others, 1993, 1994a). Several key geologic sources were used in the compilation of the map. For Alaska. the basic outcrop pattern for the map is from Beikman (1980), Gehrels and Berg (1992, 1994). Barker and others ( 1994). Brew (1994), and Moli-Stalcup and others ( 1994b). The distribution of terranes is from Jones and others (1987) and Monger and Berg (1987), with modification by Grantz and other (1991 ). Worall (199 1 ), okleberg and others (1993, 1994a), the cited references, and the Alaskan co-authors of this report. For the Canadian Cordillera. the basic outcrop pattern is from Monger and Berg ( 1987), Wheeler and other (1988). and Wheeler and McFeeley ( 1991) with modifications by the Canadian authors. For the northern part of the Russian Far East. the basic outcrop pattern is from So unov (1985) with modifications by the Russian authors. For the outhern part of the Russian Far East, the basic outcrop pattern is from Krasny (1991) and Bazhanov and Oleinik ( 1986) with modification by the Russian authors. The Russian Far East part of the map is the first attempt to define and delineate terranes in that region. In their compilation. the Russian authors utilized the methodology of U.S.A. and Canadian geologists. Because this map is the first attempt to display the terranes. Cenozoic and Mesozoic overlap assemblages. basinal deposit , and plutonic belts of the Russian Far East. the Russian author will appreciate constructive sugge tions for improving the map.
Distribution, facies, ages, and proposed tectonic associations of regionally metamorphosed rocks in east- and south-central Alaska
Most of the exposed bedrock in east- and south-central Alaska has been regionally metamorphosed and deformed during Mesozoic and early Cenozoic time. All the regionally metamorphosed rocks are assigned to metamorphic-facies units on the basis of their temperature and pressure conditions and metamorphic age. North of the McKinley and Denali faults, the crystalline rocks of the Yukon- Tanana upland and central Alaska Range compose a sequence of dynamothermally metamorphosed Paleozoic and older(?) metasedimentary rocks and metamorphosed products of a Devonian and Mississippian continental-margin magmatic arc. This sequence was extensively intruded by postmetamorphic mid-Cretaceous and younger granitoids. Many metamorphic-unit boundaries in the Yukon-Tanana upland are low-angle faults that juxtapose units of differing metamorphic grade, which indicates that metamorphism predated final emplacement of the fault-bounded units. In some places, the relation of metamorphic grade across a fault is best explained by contractional faulting; in other places, it is suggestive of extensional faulting. Near the United States-Canadian border in the central Yukon- Tanana upland, metamorphism, plutonism, and thrusting occurred during a latest Triassic and Early Jurassic event that presumably resulted from the accretion of a terrane that had affinities to the Stikinia terrane onto the continental margin of North America. Elsewhere in the Yukon-Tanana upland, metamorphic rocks give predominantly late Early Cretaceous isotopic ages. These ages are interpreted to date either the timing of a subsequent Early Cretaceous episode of crustal thickening and metamorphism or, assuming that these other areas were also originally heated during the latest Triassic to Early Jurassic and remained buried, the timing of their uplift and cooling. This uplift and cooling may have resulted from extension. South of the McKinley and Denali faults and north of the Border Ranges fault system, medium-grade metamorphism across much of the southern Peninsular and Wrangellia terranes was early to synkinematic with the intrusion of tonalitic and granodioritic plutons of primarily Early and Middle Jurassic age in the Peninsular terrane and Late Jurassic age in the Wrangellia terrane. Areas metamorphosed during the Jurassic episode that crop out near the Border Ranges fault system were subsequently retrograded and deformed in Cretaceous and early Tertiary time during accretion of younger units to the south. North of the Jurassic metamorphic and plutonic complex, low-grade metamorphism affected the rest of the Wrangellia terrane sometime during Jurassic and (or) Cretaceous time. North of the Wrangellia terrane and immediately south of the McKinley and Denali faults, flyschoid rocks, which were deposited within a basin that separated the Wrangellia terrane from the western margin of North America, form a northeastward-tapering wedge. Within the western half of the wedge, flysch and structurally interleaved tectonic fragments were highly deformed and weakly metamorphosed; much of the metamorphism and deformation probably occurred sometime during mid- to Late Cretaceous time. In the eastern half of the wedge, flyschoid rocks form an intermediate-pressure Barrovian sequence (Maclaren metamorphic belt). Metamorphism of the Maclaren metamorphic belt was synkinematic with the Late Cretaceous to earliest Tertiary intrusion of foliated plutons of intermediate composition. Isotopic data suggest metamorphism extended into the early Tertiary and was accompanied by rapid uplift and cooling. Low- to medium-grade metamorphism throughout the wedge was probably associated with the accretion of the outboard Wrangellia terrane, as has been proposed for the Maclaren metamorphic belt. South of the Border Ranges fault system lie variably metamorphosed sequences of oceanic rocks that comprise the successively accreted Chugach, Yakutat, Ghost Rocks, and Prince William terranes. The Chugach terrane consists of three successively accreted sequences of differing metamorphic histories. Metamorphism in all the sequences was associated with north-directed underthrusting beneath either the combined Peninsular-Wrangellia terrane or the older and inner parts of the Chugach terrane. These sequences, from innermost to outermost are: (1) intermediate- to highpressure, transitional greenschist- to blueschist-facies metabasalt and metasedimentary rocks that were metamorphosed during the Early and Middle Jurassic; (2) prehnite-pumpellyite-facies melange that was metamorphosed sometime during the Jurassic and Cretaceous; and (3) low-pressure prehnite-pumpellyite- or greenschist- facies flysch and metavolcanic rocks that were initially metamorphosed during latest Cretaceous to early Tertiary time and, in the eastern Chugach Mountains, were subsequently overprinted by low-pressure amphibolite-facies metamorphism that accompanied widespread intrusion during Eocene time. A similar low-pressure-facies series also developed within melange and flysch of the Yakutat terrane; these rocks are also intruded by Eocene plutons and are correlated with similar rocks of the Chugach terrane. Seaward of the Chugach terrane are the strongly deformed but weakly metamorphosed (prehnite-pumpellyite-facies) deep-sea metasedimentary rocks and oceanic metavolcanic rocks of the Ghost Rocks and Prince William terranes. Metamorphism and deformation occurred during underthrusting of these terranes beneath the Chugach terrane in early Tertiary time and predated, perhaps by very little, intrusion by early Tertiary granitoids.
Geologic map of the Tehipite Dome quadrangle, Fresno County, California
Explore the source record for details and available documents.
Map, tables, and summary of fossil and isotopic age data, Mount Hayes Quadrangle, eastern Alaska range, Alaska
This report describes, summarizes, and interprets all known bedrock fossil and isotopic age studies for the Mount Hayes quadrangle, eastern Alaska Range, Alaska. The accompanying map shows the location of all known bedrock fossil and isotopic sample localities in the quadrangle on a generalized geologic base map. These fossil and isotopic age data are obtained from new studies, unpublished data of the U.S. Geological Survey, contributed unpublished data, and published data. This report is one result of a five-year mineral resource assessment of the quadrangle that was done during the summers of 1978 through 1982, with additional topical studiesin 1985 and 1986. This report is one part of a folio on the geological, geochemical, geophysical, and mineral-resource assessment studies of the quadrangle prepared as part of the Alaskan Mineral Resource Assessment Program (AMRAP) of the U.S. Geological Survey.
Chapter 14: Middle Cretaceous silicic metavolcanic rocks in the Kings Canyon area, central Sierra Nevada, California
Metamorphosed silicic volcanic and hypabyssal rocks of middle Cretaceous (110 to 100 Ma) age occur in two roof pendants in the Kings Canyon area of the central Sierra Nevada. The metavolcanic remnants are similar in age to or are only slightly older than the voluminous enclosing batholithic rocks. Thus, high to surface levels of the batholith are implied for this region. This is interesting considering that deep-level (∼25 km) batholithic rocks of the same age as the metavolcanic rocks occur at the southern end of the range. Apparent structural continuity between these two regions suggests that the southern half of the range offers an oblique section through young (˜100 Ma) sialic crust. The middle Cretaceous ages of the two volcanic sequences are indicated by U/Pb zircon and Rb/Sr bulk-rock isochron data. The two isotopic systems agree very closely with one another. Some of the U/Pb systems within the Boyden Cave pendant are discordant due to the inheritance or entrainment of Proterozoic zircon. This is a common phenomenon in volcanic or plutonic rocks erupted or emplaced within the Kings sequence metamorphic framework, a belt of distinct pendants with abundant continent-derived sedimentary protoliths. In conjunction with other petrochemical parameters, lavas and magmas of this framework domain are shown to be contaminated with sedimentary admixtures. The contaminated domain of the batholith reflects the bounds of the Kings sequence framework, which along its eastern margin probably represents a major pre-batholith to early batholith tectonic break. The middle Cretaceous metavolcanic sequences were apparently built on two distinctly different early Mesozoic substrates separated by a major tectonic break. In the Boyden Cave pendant, the substrate may be represented by the shallow to deep-marine Kings sequence; to the east in the Oak Creek pendant, the substrate consists of a thick silicic ignimbrite sequence. In both areas the middle Cretaceous rocks and adjacent sequences share intense ductile deformation fabrics. Earlier views that considered these fabrics as an expression of Jurassic orogenic deformation are in error. Structural and age relations indicate that the fabrics developed between 105 and 100 Ma and during the medial phases of Cretaceous composite batholith growth.
The Denali fault system and Alaska Range of Alaska: Evidence for underplated Mesozoic flysch from magnetotelluric surveys
Regional magnetotelluric surveys recently completed across the central and eastern Alaska Range of Alaska provide evidence for large volumes of conductive rocks beneath the core of the range. These conductive rocks may represent a formerly extensive, but now collapsed, Mesozoic flysch basin formed on the leading edge of the Talkeetna superterrane (amalgamated Wrangellia, Peninsular, and Alexander terranes). The docking of the Talkeetna superterrane caused large-scale oblique thrusting, folding, and metamorphism in the flysch basin, and formation of a megasuture along which the Cenozoic strike-slip Denali fault system developed. The deep magnetotelluric soundings and seismic reflection data suggest the possibility that the highly conductive rocks were tectonically emplaced beneath the thin crystalline sheet constituting the southern Yukon-Tanana terrane over a broad region of the Alaska Range. The conductive rocks are locally correlated with surface outcrops of Mesozoic black shales that are part of Upper Jurassic and Cretaceous flysch but may be composed of Paleozoic carbonaceous shales as well. In either case, their extremely low resistivities make them a valuable marker horizon for tectonic studies. The conductive rocks are interpreted to extend to depths of greater than 20 km and were mapped north and northeast of the Denali fault for more than 50 km. The magnetotelluric surveys represent the first large-scale surveys done in Alaska, but the structures mapped are similar to those observed in large, compressed flysch basins in the eastern Alps and Carpathian Mountains of Europe. The results of these surveys bear on several key tectonic questions, including development of the ancestral Denali fault, and collapse and possible underplating of an extensive Mesozoic flysch system and associated igneous arc.
Lead isotopic fingerprinting of tectono-stratigraphic terranes, east-central Alaska
Common lead isotopic compositions have been determined on feldspars from meta-igneous rocks from nine tectono-stratigraphic terranes or subterranes in east-central Alaska. Most of the terranes have distinct and well-defined signatures in terms of isotopic composition; thus, most can be distinguished on conventional lead isotopic diagrams. Lead isotopic ratios provide evidence for (1) possible sources for the igneous rocks, (2) time of metamorphism, (3) correlation of terrane fragments, and (4) delineation of juxtaposed terranes. Determination of lead isotopic ratios from igneous rocks can be useful in characterizing tectono-stratigraphic terranes (as to mantle or crustal origin) and in correlation, particularly where terrane relationships are enigmatic.
Significant metalliferous lode deposits and placer districts of Alaska
Explore the source record for details and available documents.
Geologic setting, petrology, and geochemistry of stratiform sphalerite-galena-barite deposits, Red Dog Creek and Drenchwater Creek areas, northwestern Brooks Range, Alaska
Similar stratiform sphalerite-galena-barite deposits occur in the Red Dog Creek area, De Long Mountains quadrangle, and in the Drenchwater Creek area, Howard Pass quadrangle, northwestern Brooks Range, Alaska. The deposits, located approximately 180 km apart, are hosted by Mississippian and Pennsylvanian strata of the Kagvik structural sequence of late Paleozoic and early Mesozoic age. This sequence is situated in the lowermost structural plate of a series of predominantly east-striking, shallow-dipping thrust plates that characterize the regional structure. The deposits occur in shale, tuff, tuffaceous sandstone, altered chert, and quartz exhalite. These units are in places interlayered with or adjacent to bedded or nodular barite or submarine keratophyre flows. Chert and shale commonly contain abundant highly altered, fine-grained feldspar, pumice lapilli, and mafic volcanic rock fragments.The main types of occurrences at Red Dog Creek are: (1) disseminated and stratiform sulfides thinly bedded in organic-rich Mississippian and Pennsylvanian shale and chert, (2) massive sulfide veins and breccia fillings in silicified Mississippian and Pennsylvanian shale, and (3) stratiform sulfide-bearing lenses subdivided into (3a) quartz-exhalite with up to several percent sulfides and barite, (3b) massive sulfide-quartz lenses with up to several percent barite, and (3c) barite-quartz lenses with up to several percent sulfides.At Drenchwater Creek sulfides and barite occur as: (1) disseminations in shale, chert, tuff, and tuffaceous sandstone; (2) disseminations to aggregates in quartz exhalite; and (3) occurrences in veins crosscutting cleavages in brecciated chert and shale.The stratiform sulfides occur in zones up to several thousand meters long and several tens of meters thick. Preliminary grades at Red Dog Creek are 17.1 percent zinc, 5.0 percent lead, and 2.4 oz silver per ton contained in at least 85 million tons.The range of sulfide sulfur isotope values at Red Dog Creek is -16.6 per mil to +3.6 per mil. Sphalerite averages 0.8 per mil. Pyrite is isotopically lighter than coexisting sphalerite in quartz exhalite and massive sulfide lenses. This isotopic relationship is compatible with paragenetic relationships which show pyrite crystallized both earlier and later than other sulfides. Galena is isotopically lighter than coexisting sphalerite. Three groups of geologically and isotopically distinct barites exist including isotopic values appropriate for derivation from late Paleozoic seawater sulfate.Early in the depositional sequence isotopically light pyrite formed from organically reduced seawater sulfate. Sphalerite, galena, late pyrite, and some barite precipitated from two sulfur-bearing solutions that mixed at the mineralization site. Isotopically light barite associated with sulfides precipitated from oxidized H 2 S without seawater sulfate contribution. Lead isotope data from Red Dog and Drenchwater Creeks are compatible with generation of lead in a Phanerozoic mature island-arc setting, a typical orogene, or possibly a back-arc type intracratonic basin.The stratiform sphalerite-galena-barite deposits probably formed in an incipient island-arc environment near a continental margin, containing magmas of keratophyric and andesitic composition, where metal-laden hydrothermal fluids discharged into a deep (?) water, low-energy environment. Later intense deformation, associated with plate docking and thrusting, disrupted and partly remobilized and stratiform deposits.
Origin and tectonic evolution of the Maclaren and Wrangellia terranes, eastern Alaska Range, Alaska
Major portions of the eastern Alaska Range, south of the Denali fault, in the McCarthy, Nabesna, Mount Hayes, and eastern Healy quadrangles, consist predominantly of the Maclaren and Wrangellia tectono-stratigraphic terranes. The Maclaren terrane consists of the Maclaren Glacier metamorphic belt and the regionally deformed and metamorphosed East Susitna batholith. The Maclaren Glacier metamorphic belt is composed of argillite, metagraywacke, and sparse andesite flows that are progressively regionally metamorphosed from lower greenschist facies to middle amphibolite facies near the East Susitna batholith. The East Susitna batholith is composed of gabbro, quartz diorite, granodiorite, and sparse quartz monzonite. Isotopic ages are as old as a K-Ar hornblende age of 87.5 m.y., possibly reset, and a U-Pb zircon age of 70 m.y. The batholith is intensely deformed and regionally metamorphosed under conditions of the middle amphibolite facies. The Wrangellia terrane is divided into two subterranes: (1) the Slana River subterrane, composed of late Paleozoic andesite to dacite flows, tuff, limestone, and argillite, unconformably overlying massive basalt flows of the Triassic Nikolai Greenstone, Late Triassic limestone, and younger Mesozoic flysch; and (2) the Tangle subterrane, a deeper-water equivalent of the Slana River subterrane, composed of late Paleozoic and Early Triassic aquagene tuff, chert, minor andesite tuff and flows, limestone, unconformably overlying pillow basalt and massive basalt flows of the Triassic Nikolai Greenstone, and Late Triassic limestone. Both subterranes are intruded by locally extensive gabbro and diabase dikes and by cumulate mafic and ultramafic sills. Less extensive terranes (two) are the Clearwater terrane, a sequence of intensely deformed chlorite schist, muscovite schist, marble, and greenstone of Late Triassic age; and an unnamed terrane of ultramafic and associated rocks of presumable Paleozoic or Mesozoic age. Each terrane or subterrane generally has (1) a distinctive time-stratigraphic sequence reflecting a unique geologic history; (2) a missing provenance for bedded sedimentary or volcanic rocks; and (3) bounding thrust or strike-slip faults, interpreted as accretionary sutures. The Maclaren and Wrangellia terranes are juxtaposed along the Broxson Gulch thrust, which consists of an imbricate series of north-dipping thrust faults. Paralleling the Broxson Gulch thrust, a few kilometres to the south, is the north-dipping Eureka Creek thrust, along which are juxtaposed the Slana River and Tangle subterranes. The Maclaren terrane is correlated with the Kluane Schist and the Ruby Range batholith in the southern Yukon Territory, which represent the northward extension of the Taku and Tracy Arm terranes. If correct, this correlation defines a minimum displacement of the Maclaren terrane along the Denali fault of ∼400 km. The Maclaren terrane is interpreted to have formed in a synorogenic Andean-type arc setting on the west margin of Mesozoic North America in the middle to late Mesozoic and early Cenozoic. The Wrangellia terrane is interpreted to have initially formed in an island-arc setting during the late Paleozoic. Subsequently in the Late Triassic, the Wrangellia terrane underwent rifting near the paleoequator, with formation of the Nikolai Greenstone and associated mafic and ultra-mafic igneous rocks. In the middle and late Mesozoic, Wrangellia migrated toward, and was accreted during, the middle Cretaceous to the Maclaren terrane along the Broxson Gulch thrust. Subsequent dispersion of both the Maclaren and Wrangellia terranes along the Denali fault and the Broxson Gulch thrust commenced during the early Tertiary and continues through the present.
Geologic setting, petrology, and geochemistry of zoned tungsten-bearing skarns at the Strawberry Mine, central Sierra Nevada, California
The Strawberry mine, 90 km northeast of Fresno, California, occurs on the margin of a small roof pendant of Early Jurassic metasedimentary rocks and middle Cretaceous metaigneous rocks. Middle Cretaceous granitic intrusions surround and intrude the roof pendant. Adjacent to one granodiorite intrusion, several subvertical marble layers are replaced by scheelite-bearing skarns. The skarns form spears that project as much as 300 m upward and into the marble layers. Between the marble and granodiorite, the skarns comprise a metasomatic column and are subdivided into four zones: (1) wollastonite skarn, adjacent to and replacing marble, and containing wollastonite, grossularite, idocrase, and diopside; (2) garnet skarn, containing andradite-grossularite, diopside-hedenbergite, and scheelite; (3) pyroxene skarn containing hedenbergite-diopside, plagioclase, magnetite, and scheelite; and (4) hornblende skarn, adjacent to granodiorite, and containing hornblende, plagioclase, microcline, magnetite, and scheelite. Important features of the skarn zonation are: sharp contacts between skarn zones; relative constancy of whole-rock, mineral, and modal compositions within each skarn zone; major changes in whole-rock, mineral, and modal compositions across contacts between skarn zones; and systematic replacement between skarn zones, with any given skarn zone replaced by the next lower and adjacent skarn zone toward the granodiorite. Volume remained approximately constant in the replacement of marble by various skarn zones. Marble and skarn exhibit uniformly low porosity. Data on mineral paragenesis and mineral composition indicate that in the replacement of any skarn the most calcic mineral is consumed and the residual calc-silicate minerals in the replacing skarn increase in abundance and are richer in total Fe and MnO. This mineral paragenesis can be related to systematic variations in whole-rock composition. From the marble to the granodiorite, skarns are progressively depleted in CaO and CO 2 , and progressively enriched in SiO 2 , total Fe, MgO, MnO, Al 2 O 3 , Na 2 O, K 2 O, and WO 3 . The skarns probably formed when an aqueous fluid containing a solute fraction relatively depleted in Ca and CO 2 , and relatively enriched in Si, Fe, Mg, Mn, Al, Na, K, and W, was expelled from a granitic magma into country rock. The tendency toward local equilibrium between fluid and marble resulted in formation of skarns with systematic stepwise changes in mineral and rock composition at contacts between skarns. Achievement of local equilibrium occurred mainly by infiltration metasomatism with a possible minor component of diffusion metasomatism.
Comments and replies on ‘Collision-deformed Paleozoic continental margin, western Brooks Range, Alaska’: Reply
No abstract available.
Paleozoic and Mesozoic deformations in the central Sierra Nevada, California
Analysis of structural and stratigraphic data indicates that several periods of regional deformation, consisting of combined folding, faulting, cataclasis, and regional metamorphism, occurred throughout the central Sierra Nevada during Paleozoic and Mesozoic time. The oldest regional deformation occurred alono northward trends during the Devonian and Mississippian periods in most roof pendants containing lower Paleozoic metasedimentary rocks at the center and along the crest of the range. This deformation is expressed in some roof pendants by an angular unconformity separating older thrice-deformed from younger twice-deformed Paleozoic metasedimentary rocks. The first Mesozoic deformation, which consisted of uplift and erosion and was accompanied by the onset of Andean-type volcanism during the Permian and Triassic, is expressed by an angular unconformity in several roof pendants from the Saddlebag Lake to the Mount Morrison areas. This unconformity is defined by Permian and Triassic andesitic to rhyolitic metavolcanic rocks unconformably overlying more intensely deformed Pennsylvanian, Permian(?), and older metasedimentary rocks. A later regional deformation occurred during the Triassic along N. 20?_30? W. trends in Permian and Triassic metavolcanic rocks of the Saddlebag Lake and Mount Dana roof pendants, in upper Paleozoic rocks of the Pine Creek roof pendant, and in the Calaveras Formation of the western metamorphic belt; the roof pendants are crosscut by Upper Triassic granitic rocks of the Lee Vining intrusive epoch. A still later period of Early and Middle Jurassic regional deformation occurred along N. 30?-60? E. trends in upper Paleozoic rocks of the Calaveras Formation of the western metamorphic belt. A further period of deformation was the Late Jurassic Nevadan orogeny, which occurred along N. 20?_40? W. trends in Upper Jurassic rocks of the western metamorphic belt that are crosscut by Upper Jurassic granitic rocks of the Yosemite intrusive epoch. Structures of similar age occur in intensely deformed oceanic-lithospheric and syntectonic plutonic rocks of the lower Kings River area, in Jurassic metavolcanic rocks of the Ritter Range roof pendant, and in Triassic metasedimentary rocks of the Mineral King roof pendant. The final Mesozoic deformation occurred along N. 50?-80? W. trends in both high-country roof pendants and the lower Kings River area; structures of this generation are crosscut by relatively undeformed Upper Cretaceous granitic rocks of the Cathedral Range intrusive epoch.