The Denali fault of Alaska is considered by the majority of the geologic community to be an active dextral strike-slip feature cutting across the entire width of Alaska for over 1900 km with a prominent northward-convex surface trace. The two major earthquakes in 2002 at the apex of the fault in the central Alaska Range, and the associated dextral offsets in eastern Alaska appear to lend credence to the concept of a statewide dextral strike-slip fault.A different character for the Denali fault is indicated by our various geologic investigations and reconnaissance mapping for the last 30 years in large tracts of central Alaska, including a roughly 240 km segment of the Denali fault. We believe the fault is a strike-slip feature in eastern Alaska, but westward it progressively changes into a dip-slip feature, and is only one of a swarm of subparallel, dominantly dip-slip faults which converge downward and outline a number of upthrust wedge-shaped crustal blocks of regional dimensions. First-motion studies and ground-motion analyses at the epicenters of the two 2002 earthquakes indicate the presence of strong compressional components, including newly developed thrust faults. These are the types of structural features one would expect in a transition zone where a strike-slip fault radically alters its surface course and changes into a number of dip-slip faults. We interpret the changing character of the Denali fault in central Alaska to be the manifestation, in the relatively thin upper plate, of the junction of trench and transform in the lower plate. We further interpret the wedge-shaped fault blocs in central Alaska to be analogous to the structural features described and named composite wedges by Migliorini (1948) from the Apennines of Italy. The inclined field of compressional force required for the development of these features we interpret to have been caused by the upwardly moving and shallowing position of the subducting plate beneath central Alaska within the last approximately 6 million years.Polymetallic mineralizations of primarily Au, Ag, Sb, Sn, and Cu, and placer gold deposits occur throughout the region, but only the Valdez Creek placer gold mine has produced commercially significant amounts of gold. The polymetallic mineralizations occur as vein deposits, breccias pipe, and skarn. At the Denali Prospect, Cu mineralization also includes small stratiform bodies. As most of the region has been investigated by prospectors, mining concerns, and systematically covered by geological mapping and geochemical and geophysical surveys, further discoveries of major ore deposits is deemed possible only after additional detailed geological investigations.

This map presents the results of U.S. Geological Survey (USGS) geologic bedrock mapping studies in the mostly glacier covered Atlin 1:250,000-scale quadrangle, northern southeastern Alaska. These studies are part of a long-term systematic effort by the USGS to provide bedrock geologic and mineral-resource information for all of southeastern Alaska, covering all of the Tongass National Forest (including Wilderness Areas) and Glacier Bay National Park and Preserve. Some contributions to this effort are those concerned with southwesternmost part of the region, the Craig and Dixon Entrance quadrangles (Brew, 1994; 1996) and with the Wrangell-Petersburg area (Brew, 1997a-m; Brew and Grybeck, 1997; Brew and Koch, 1997). As shown on the index map (fig. 1), the study area is almost entirely in the northern Coast Mountains adjacent to British Columbia, Canada. No previous geologic map has been published for the area, although Brew and Ford (1985) included a small part of it in a preliminary compilation of the adjoining Juneau quadrangle; and Brew and others (1991a) showed the geology at 1:500,000 scale. Areas mapped nearby in British Columbia and the United States are also shown on figure 1. All of the map area is in the Coast Mountains Complex as defined by Brew and others (1995a). A comprehensive bibliography is available for this and adjacent areas (Brew, 1997n).

Introduction Methods and Limitations Previous Studies of PGE in the Volcanic Rocks Description of the Rock Units Copper, Nickel, and Chromium Concentrations Platinum-Group-Element (PGE) Concentrations Comparisons with Alaskan-Type Intrusions Summary Acknowledgements References Page 3 6 9 11 12 15 17 22 24 26 27 1 LIST OF TABLES (following references) Table 1. Lower limits of determination and uncertainties for elements analyzed for by different methods and reported in Tables 2 and 3. Table 2, Summary of Cu, Ni, and Cr determinations and ratios for samples of rndk metavolcanic rocks of the Gravina belt and Wrangellia terrane of southeastern Alaska. Table 3. Summary of platinum-group element concentrations and ratios for samples of rnafic metavolcanic rocks of the Gravina belt and Wrangellia terrane of southeastern Alaska. LIST OF FIGURES (following tables) Figure 1. Index map of southeastern Alaska, showing areas of Gravina belt rocks (JU, PE, KE) and of Wrangellia terrane rocks. Figure 2. Variations of Pd and Pt with Cu concentrations in rnetavolcanic units of this study. Figure 3. Variations of Pd and Pt with MgO concentrations in metavolcanic units of this study. Figure 4. Histograms showing distribution of percentages of samples in 2 ppb intervals of Pd and Pt contents in metavolcanic rocks of the Gravina belt of the Juneau area and the upper and the lower parts of the Wrangellia tenane. Figure 5. Chondrite-normalized PGE diagram comparing metavolcanic rocks of southeastern Alaska. Figure 6. Variation between W(Pt+Pd) and Cu/(Cu+Ni) in metavolcanic units of this study compared with Alaskan-type intrusions and other mafic igneous rocks of Loney and Himelberg (1 992), SUMMARY INFORMATION ABOUT PLATINUM-GROUP ELEMENTS, COPPER, NICKEL, AND CHROMIUM IN GREENSTONES OF THE WRANGELLIA TERRANE AND GRAVINA BELT, SOUTHEASTERN ALASKA By Arthur B. Ford and David A. Brew ABSTRACT Platinum-group elements (PGE), Cu, Ni, and Cr concentrations have been determined for 134 samples of greenstone and other metavolcanic rocks of the upper Paleozoic and Triassic Wrangellia terrane and of the Upper Jurassic and Cretaceous terrane-overlap assemblage of the Gravina belt in the western part of the Coast Mountains between the Chilkat Peninsula and northern Admiralty Island and the results are summarized here. The elemental concentrations and ratios of the Gravina belt rocks arePlatinum-group elements (PGE), Cu, Ni, and Cr concentrations have been determined for 134 samples of greenstone and other metavolcanic rocks of the upper Paleozoic and Triassic Wrangellia terrane and of the Upper Jurassic and Cretaceous terrane-overlap assemblage of the Gravina belt in the western part of the Coast Mountains between the Chilkat Peninsula and northern Admiralty Island and the results are summarized here. The elemental concentrations and ratios of the Gravina belt rocks are compared with corresponding data from the Alaskan-type mafic-ultramafic intrusions to test previously published hypotheses regarding their parent magmas. Finally, we speculate about the magmatic and other processes that led to the relative concentrations of the above-listed elements in these rock units. a This study is limited by the sampling and analytical methods used, by the elements selected, and by the lack of data pertinent to all of the processes that may have affected the original magmas; nevertheless it accomplishes three specific purposes: (1) it summarizes an original and comprehensive set of PGE, Cu, Ni, and Cr data for these units for the first time; (2) it compares summarized Douglas Island Volcanics data with those of the Alaskan-type mafic-ultrarnafic complexes, and suggests a new preliminary hypothesis regarding the parent magmas of the latter; and (3) it speculates about the magmatic and other processes that led to some regular, albeit loose, grouping of the data for the different units. This last purpose is controversial because it is especially fraught with uncertainties. The rocks analyzed from the Wrangellia terrane include greenstone of Late Triassic age on Chilkat Peninsula near Haines and the Gastineau Volcanics of late Paleozoic to Late Triassic age near Juneau. The PGE-Ni-Cr concentrations and concentration ratios for these rocks supplement previous trace-element studies and may serve to further differentiate these rocks from those of Cretaceous age. The previous trace-element studies showed that the Wrangellia tholeiitic basalt metavolcanic units and the Cretaceous-age Douglas Island Volcanics alkalic basalt units erupted in different tectonic settings and have different elemental concentrations and ratios. Our new studies, like the previous ones, are of potential value to mineral explorationists and others attempting to establish the presence or absence of Triassic metavolcanic units; this question is important because the Triassic rocks locally contain volcanogenic-massive-sulfide deposits . Rocks analyzed from the Gravina belt principally belong to the Lower Cretaceous Douglas Island Volcanics exposed near Juneau. These latter rocks previously have been proposed to be the extrusive equivalent of magmas that formed Lower Cretaceous (1 18-100 Ma) Alaskan-type mafic and ultramafic intrusions, but our data do not support this interpretation. Instead, our study suggests that mafic dikes and plugs emplaced into the Douglas Island Volcanics either during a late stage of M1 (120 Ma) low-grade regional metamorphism or possibly during the same time as the M2 contact metamorphism that marks the aureoles of the Alaskan-type intrusions, are the parents of those intrusions. a According to our data, the metavolcanic rocks of both the Wrangellia terrane and the Gravina assemblage are characterized by dominance of Pd over Pt as shown by mean Pd/Pt ratios of 2.30 and 1.61, respectively, for the lower and upper parts of the Wrangellia terrane, and of 1.82 for the Gravina belt. Average Pt/(Pt+Pd) ratios are about the same (0.3-0.4) for all the metavolcanic units; they are significantly lower than values ( 0 5 1.0) for the Pt-dominant Alaskan-type intrusions. The mean total PGE concentrations of samples of the Douglas Island Volcanics and equivalents [16 parts per billion (ppb)] and of associated dike rocks (10 ppb), however, are much lower than for Alaskan-type intrusions (generally 75100 ppb). Dikes in the Douglas Island Volcanics are Pt dominant, with mean Pd/Pt of 0.74 and Pt/(Pt+Pd) of 0.58; these values are similar to those in some of the Alaskan-type bodies, suggesting that the late dikes may represent their parental magmas, instead of the parents of the extrusive rocks of the Douglas Island Volcanics. All of these analytical values and calculated ratios are subject to significant uncertainty. The Cu, Ni, and Cr variations are complex; overall, the average Cu concentration of the units shows a general increase with increasing Pd but no systematic variation with Pt. In volcanic rocks of @ the Gravina belt of the Juneau area, Cu shows positive covariation with Pd and lacks systematic covariation with Pt . In contrast, in rocks of the Wrangellia terrane, Cu has a general overall increase with both increasing Pd and Pt. Neither Ni or Cr show covariation with Pt and Pd in all units. It should be noted that, in comparison with the Gravina assemblage rocks from the Juneau area, inferences regarding the Cu, Ni, and Cr concentrations of the metavolcanic rocks of both the Wrangellia terrane and the Gravina assemblage in the Ketchikan and Petersburg areas are limited by the small sample population for those areas. INTRODUCTION This report summarizes platinum-group element (PGE), Cu, Ni, and Cr concentrations in 134 samples of upper Paleozoic and Mesozoic volcanic rocks from southeastern Alaska. The PGE's analyzed are Pt, Pd, Rh, Ru, and Ir. This study is part of a long-term study of the volcanogenic history of southeastern Alaska (Brew, 1968; Ford and Brew, 1978, 1987, 1988, 1993). The complete data set is in the files of co-author A.B. Ford. The primary purpose of the report is to summarize the new data on the PGE, Cu, Ni, and Cr content of the volcanic rocks. The secondary purpose is to compare the Douglas Island Volcanics PGE data with those of the Alaskan-type complexes; this topic is of interest because of uncertainty about the parent magma of the Alaskan-type complexes and because the Douglas Island Volcanics have been suggested to be the extrusive equivalent of the Alaskan-type complexes. The third purpose is to speculate on the processes that led to the concentrations now present in the different geologic units; this purpose is similar to that of Ely and others (1998) in their study of Ontong Java Plateau basalts. The fourth purpose is to explore the possibility that PGE and other data may be used to differentiate the Wrangellia metavolcanic rocks, which locally contain volcanogenic massive sulfide deposits, from the Gravina belt metavolcanics, which generally do not. Samples are principally from (1) the Wrangellia terrane of northern southeastern Alaska; it consists of two parts: (a) an upper Paleozoic and Triassic(?) and Upper Triassic lower and upper parts of the Gastineau Volcanics of Ford and Brew (1993) and (b) the Upper Triassic metabasalt of the Chillcat Peninsula and northern Admiralty Island; and (2) the Douglas Island Volcanics of Early Cretaceous age (Lathram and others, 1965). The Wrangellia rocks were assigned to the Taku terrane by Berg and others (1978), but have since been recognized on paleontological and chemical evidence to belong to Wrangellia (Ford and Brew, 1993). The Douglas Island Volcanics are part of the Upper Jurassic and Lower Cretaceous Gravina terrane-overlap assemblage that forms a discontinuous belt extending throughout southeastern Alaska (Berg and others, 1972; Brew and Ford, 1985; Brew and Karl, 1988a, b; McClelland and others, 1991; Cohen, 1992; Cohen and Lundberg, 1994). The Gravina belt is used in this paper as defined in Br

Platinum-group elements (PGE), Cu, Ni, and Cr concentrations have been determined for 134 samples of greenstone and other metavolcanic rocks of the upper Paleozoic and Triassic Wrangellia terrane and of the Upper Jurassic and Cretaceous terrane-overlap assemblage of the Gravina belt in the western part of the Coast Mountains between the Chilkat Peninsula and northern Admiralty Island and the results are summarized here.The elemental concentrations and ratios of the Gravina belt rocks are compared with corresponding data from the Alaskan-type mafic-ultramafic intrusions to test previously published hypotheses regarding their parent magmas.Finally, we speculate about the magmatic and other processes that led to the relative concentrations of the above-listed elements in these rock units.This study is limited by the sampling and analytical methods used, by the elements selected, and by the lack of data pertinent to all of the processes that may have affected the original magmas; nevertheless it accomplishes three specific purposes: (1) it summarizes an original and comprehensive set of PGE, Cu, Ni, and Cr data for these units for the first time; (2) it compares summarized Douglas Island Volcanics data with those of the Alaskan-type mafic-ultramafic complexes, and suggests a new preliminary hypothesis regarding the parent magmas of the latter; and (3) it speculates about the magmatic and other processes that led to some regular, albeit loose, grouping of the data for the different units.This last purpose is controversial because it is especially fraught with uncertainties.The rocks analyzed from the Wrangellia terrane include greenstone of Late Triassic age on Chilkat Peninsula near Haines and the Gastineau Volcanics of late Paleozoic to Late Triassic age near Juneau.The PGE-Ni-Cr concentrations and concentration ratios for these rocks supplement previous trace-element studies and may serve to further differentiate these rocks from those of Cretaceous age.The previous trace-element studies showed that the Wrangellia tholeiitic basalt metavolcanic units and the Cretaceous-age Douglas Island Volcanics alkalic basalt units erupted in different tectonic settings and have different elemental concentrations and ratios.Our new studies, like the previous ones, are of potential value to mineral explorationists and others attempting to establish the presence or absence of Triassic metavolcanic units; this question is important because the Triassic rocks locally contain volcanogenic-massive-sulfide deposits .Rocks analyzed from the Gravina belt principally belong to the Lower Cretaceous Douglas Island Volcanics exposed near Juneau.These latter rocks previously have been proposed to be the extrusive equivalent of magmas that formed Lower Cretaceous (118-100 Ma) Alaskan-type mafic and ultramafic intrusions, but our data do not support this interpretation.Instead, our study suggests that mafic dikes and plugs emplaced into the Douglas Island Volcanics either during a late stage of Ml (-120 Ma) low-grade regional metamorphism or possibly during the same time as the M2 contact metamorphism that marks the aureoles of the Alaskan-type intrusions, are the parents of those intrusions.According to our data, the metavolcanic rocks of both the Wrangellia terrane and the Gravina assemblage are characterized by dominance of Pd over Pt as shown by mean Pd/Pt ratios of 2.30 and 1.61, respectively, for the lower and upper parts of the Wrangellia terrane, and of 1.82 for the Gravina belt.Average Pt/(Pt+Pd) ratios are about the same (0.3-0.4) for all the metavolcanic units; they are significantly lower than values (0.5-1.0) for the Pt-dominant Alaskan-type intrusions.The mean total PGE concentrations of samples of the Douglas Island Volcanics and equivalents [16 parts per billion (ppb)] and of associated dike rocks (10 ppb), however, are much lower than for Alaskan-type intrusions (generally 75-100 ppb).Dikes in the Douglas Island Volcanics are Pt dominant, with mean Pd/Pt of 0.74 and Pt/(Pt+Pd) of 0.58; these values are similar to those in some of the Alaskan-type bodies, suggesting that the late dikes may represent their parental magmas, instead of the parents of the extrusive rocks of the Douglas Island Volcanics.All of these analytical values and calculated ratios are subject to significant uncertainty.The Cu, Ni, and Cr variations are complex; overall, the average Cu concentration of the units shows a general increase with increasing Pd but no systematic variation with Pt.In volcanic rocks of the Gravina belt of the Juneau area, Cu shows positive covariation with Pd and lacks systematic covariation with Pt.In contrast, in rocks of the Wrangellia terrane, Cu has a general overall increase with both increasing Pd and Pt.Neither Ni or Cr show covariation with Pt and Pd in all units.It should be noted that, in comparison with the Gravina assemblage rocks from the Juneau area, inferences regarding the Cu, Ni, and Cr concentrations of the metavolcanic rocks of both the Wrangellia terrane and the Gravina assemblage in the Ketchikan and Petersburg areas are limited by the small sample population for those areas.^ * ^Analysis by fire-assay and emission-spectrographic method of Hafty and Riley (1968); lower limits of determination from Hafty and Riley (1968, p. Ill); lower limits of determination in parentheses from Baedecker (1987, p. 3); analytical uncertainty judged to be like that of Meier and others (1996, p. 163-164).Pt,Pd,Rh,Ru,Ir

The Dufek intrusion is a mafic, differentiated stratiform igneous complex exposed in the northern Pensacola Mountains.Due to its similar age and geochemistry, it is considered to be part of the Ferrar Dolerites that are the hypabyssal phases of the Jurassic tholeiitic Ferrar group of Antarctica.The intrusion is composed principally of cumulate gabbros with lesser amounts of anorthositic, pyroxenitic, and magnetite cumulates that are capped conformably by a thick unit of granophyre of granodioritic composition.The Dufek intrusion was sampled from its exposed parts in two ranges in the Pensacola Mountains.The stratigraphically lower section is called the Dufek Massif and the upper one is called the Forrestal Range section.This report summarizes Rb and Sr abundances and 87Sr/86Sr isotopic values for 89 whole-rock and 52 mineral (plagioclase, pyroxene, apatite) samples, and d 180 values for 89 whole-rock and 54 mineral (plagioclase, pyroxene, magnetite) samples from the Dufek intrusion.The calculated initial 87Sr/86Sr values for mafic whole-rock specimens generally increase upward within a range between 0.7083 and 0.7121 and indicate the Dufek intrusion crystallized from a mantle derived tholeiitic magma that was variably contaminated with crustal melts.Along with reversals in trends in mineral chemistry, a major reversal in the general upward increase of initial 87Sr/86Sr probably reflects an introduction of new magma in the Forrestal Range section about 1000 meters below the top of the intrusion.The initial 87Sr/86Sr of the capping granophyre of 0.7128, and values from 0.7154 to 0.7360 for six cross-cutting aplite dikes throughout the intrusion indicate all of these siliceous rocks are crystallized crustal melts.In several specimens initial 87Sr/86Sr values of pyroxene, and/or plagioclase and apatite are different and reflect a lack of isotopic equilibrium between cumulate and noncumulus phases in these rocks.Whole-rock d 180 values from the Dufek Massif section are within the range (+5.0 to+6.9 per mil) reported for other layered gabbroic intrusions, whereas d 180 values for rocks from the lower part of the Forrestal Range section are generally lower (+3.0 to+6.0 per mil) than those of the Dufek Massif.In the rocks from about 1000 meters below, to the top of the intrusion, d 180 ranges from 0.0 to +5.0 per mil.The low d 180 values along with elevated initial 87Sr/86Sr indicates rocks of the Forrestal Range section were derived from magma variably contaminated with 180 depleted, metamorphosed sedimentary rocks.Reversals of A 180 plagioclasepyroxene in some rocks suggest that some low d 180 values resulted from subsolidus 180 depletion by interaction with a meteoric waterhydrothermal system.Regressions of Rb-Sr whole-rock, mineral data for sixteen specimens yield ages that range from 148 Ma to 224 Ma.Many regressions have high MSWD due to the isotopic disequilibrium between cumulate and noncumulate phases in some rocks and ages derived from these regressions indicate only that these rocks are Mesozoic.Regressions with MSWD <1 in other rocks are interpreted to indicate the age of an event.An age of 184 ± 4 Ma is calculated from a precise Rb-Sr whole-rock isochron (MSWD=0.27)for six specimens from a 100 meter section of the capping granophyre.This age, along with a similar published U-Pb zircon age for the granophyre and incremental heating 40Ar/39Ar isochron ages for two plagioclase separates from gabbros indicate the Dufek intrusion is greater than 180 Ma; older than the main mass of Ferrar dolerites.

An unusually large, elongate Jurassic pluton of trondhjemite, about 120- by 10--15 km in dimensions, intruded Jurassic plutonic and metamorphic rocks of the Peninsular terrane in the central Talkeetna Mountains of south-central Alaska. Muscovite and biotite yield minimum ages of 150--145 Ma. The N40[degree]E-trending body is concordant with regional structures. It is the youngest member of a subduction-related Jurassic plutonic suite in the Peninsular terrane that, along with Wrangellia, was accreted to the North American continent in the middle Cretaceous. Rocks, commonly sheared, are medium to coarse grained and leucocratic (CI = 3--9). Biotite is the chief mafic mineral. Minor muscovite and garnet are common and green hornblende rare. Samples (n = 27) from the body's entire length have an average Mg[number sign] of 45 and an SiO[sub 2] continuum of 67--74% (avg. 70.7%). High Al[sub 2]O[sub 3] (14.4--17.9%, avg. 16.5%) is typical of continental trondhjemite. Averages for Zr (109 ppm) and Nb (3.5 ppm) and the ratios K/Rb (491) and Zr/Nb (34) are typical of orogenic igneous rocks of subduction origin. Four samples analyzed have low ([sup 87]Sr/[sup 86]Sr)[sub i] (avg. 0.7036). Very low Rb/Sr (avg. 0.027) is similar to Idaho batholith trondhjemites. REE patterns with low to moderatemore » LREE and HREE with flat patterns and low contents suggest residual garnet or hornblende during partial melting or fractionation. The pluton appears homogeneous in outcrop. However, some geographic variations in chemistry, as in SiO[sub 2] contents and especially in Eu/Eu[sup *], suggest existence of perhaps three regionally separate plumbing systems, or chambers in which different processes such as plagioclase accumulation or hornblende fractionation were active.« less

The western metamorphic belt is part of the Coast plutonic-metamorphic complex of western Canada and southeastern Alaska that developed as a result of tectonic overlap and/or compressional thickening of crustal rocks during collision of the Alexander terrane and Gravina assemblage on the west against the Yukon Prong and Stikine terranes to the east. Sub-greenschist to lower greenschist facies metabasalts exposed along the west end of the western metamorphic belt near Juneau, Alaska record the earliest metamorphic event (M1). These low-grade rocks are gradational with younger, higher-grade assemblages that define an inverted metamorphic gradient (metamorphic event M5). The most common metamorphic mineral assemblages are chlorite-epidote-actinolite with or without pumpellyite and stilpnomelane. There is no systematic distribution of metamorphic mineral assemblages in the study area, and all assemblages are in the pumpellyite-actinolite facies near the transition to the lower greenschist facies. Different low-variance assemblages can be attributed to minor differences in pressure, temperature, or X[sub CO[sub 2]]. Mineral chemistry and phase equilibria suggest that thermal peak metamorphism of pumpellyite-bearing assemblages occurred at about 325 C and 2 to 4.5 kbar. The geologic setting, the pumpellyite-actinolite to lower greenschist facies mineral assemblages, and the deduced P and T of peak metamorphism are all more » compatible with burial metamorphism of the Douglas Island Volcanics at a depth of 15 to 20 km. Preservation of the low-grade metamorphic mineral assemblages during collisional crustal thickening and during establishment of the inverted metamorphic gradient is attributed to instantaneous thickening displacing the pre-thickening isotherms to substantially greater depths and rapid uplift after establishment of the inverted gradient. « less

The Cordilleran orogen in southeastern Alaska includes 24 distinct magmatic belts, ranging in age from Cambrian to Holocene, that are defined by map relations, lithology, age, and chemical composition. The youngest magmatic features are Quaternary-age pre- and post-glacial volcanic rocks that occur in three major fields in the region, as well as in isolated locations. Cenozoic magmatic features consist of four major and three minor belts. The major Tkope-Portland Peninsula belt of Oligocene age includes both volcanic and plutonic rocks. The major calcalkalic Coast Mountains belt of early and middle Eocene age is the single largest magmatic feature of the region. Early Tertiary and latest Cretaceous magmatism is represented by the major calcalkalic great tonalite sill belt, a remarkable long and narrow feature along the west side of the Coast Mountains. Cretaceous and Jurassic intrusive rocks occur in five major belts and two minor belts in the region and Paleozoic intrusive rocks occur in four major and two minor belts. The three major plutonic-metamorphic complexes (PMC), from east to west, are: the Coast PMC in the Coast Mountains; the Glacier Bay-Chichag of plutonic complex (Chugach MC) in the northern outer islands. The Coast PMC records dynamothermal and regional contact metamorphicmore » events related to regional plutonism within several juxtaposed terranes; its lengthy and complicated history is related to the Late Cretaceous collision of the Alexander and Wrangellia terranes and the Gravina overlap assemblage to the west against the Yukon prong and Stikine terrane to the east. The relatively simple Glacier Bay PC history is recorded as the roots of a Late Jurassic through late Early Cretaceous island arc that probably developed during the early stages of the above tectonic event. The complicated Chugach MC history developed during and after the Late Cretaceous collision of the Chugach terrane with the Wrangellia and Alexander terranes.« less

Magnetic susceptibility (MS) measurements on 194 granitic rock samples from a transect across the Coast plutonic-metamorphic complex near Juneau help define and characterize individual plutons and plutonic units that form 3 major plutonic belts and 6 major subbelts of the region. Plutons along the western margin (Admiralty-Revillagigedo belt) form two subbelts with distinctive MS. A group of small 95-Ma tonalitic plutons have very low MS ( 3,000) and magnetite content, and an eastern subbelt of foliated granodioritic plutons of variable but generally high MS. Three small deformed tonalitic sills located east of Juneau are spatially part of the great tonalite sill belt but exhibit very low MS similar to those tonalites of the Admiralty-Revillagigedo belt. On a regional scale the MS shows no correlation with total iron (FeO) except within suites or subbelts, but it does show a positive correlationmore » with the oxidation state (molecular ratio Fe[sup 3])/(Fe[sup 2]+Fe[sup 3]). The differences in MS between the major plutonic belts appear to be related to differences in the ages of the belts and the host terranes in to which they were emplaced. The differences between subbelts and within subbelts is less clear, but is probably due to variable oxidation states and FeO content caused by differentiation trends and contamination, and possibly related to size of plutons, and the amount of tectonic stress and deformation, and grade of metamorphism during synkinematic emplacement.« less

Petrographic, chemical, and age data are presented for six plutonic sills of Late Cretaceous to early Tertiary age located east of Juneau, Alaska.Five of the sills-the Mount Juneau, Carlson Creek, Lemon Creek Glacier, Mendenhall Glacier, and Taku Cabin plutons-are mostly tonalite but range from quartz diorite to granodiorite; their mean color indexes range from 19 to 32.The more silicic Annex Lakes pluton is mostly granodiorite but has significant quartz monzodiorite and tonalite and a mean color index of 14.Although the six plutons are calc-alkaline and generally similar in average chemical and modal composition, they vary in form, mineralogy, texture, and petrographic and chemical characteristics.The Mount Juneau and Annex Lakes plutons exhibit the greatest internal differences, whereas the Taku Cabin and Mendenhall Glacier plutons are the most homogeneous.Plots on silica-oxide variation and AFM diagrams exhibit coherent trends with some scatter.

Apatite in the Dufek intrusion of Antarctica is F-rich and exhibits systematic variations in contents of Cl, F, Fe, Sr, and rare-earth elements (REEs). Four types of apatite are distinguished on the basis of morphology, grain size, and textural relations: 1) cumulus, 2) early postcumulus (euhedral forms), 3) late postcumulus (anhedral forms), and 4) noncumulus primocryst. Cumulus apatite is relatively rich in F, Fe, and Sr, and poor in Cl and LREEs, whereas postcumulus apatite commonly contains more Cl and LREEs, and less F than cumulus apatite. Apatite primocrysts of the capping granophyre are depleted in Cl and enriched in REEs. Cl, F, Fe, and REEs are partitioned between the two types of postcumulus apatite, with euhedral, early-postcumulus grains typically containing less F, more Cl and Fe, and having higher Ce/Y ratios than anhedral, late-postcumulus grains. Large subhedral apatite sporadically occurs in anorthosite and leucogabbro below the main cumulus-apatite horizon, and shows textural and chemical evidence of being recycled cumulus apatite, deposited by current activity. Whole-rock P contents follow patterns generally comparable to those described for other layered mafic intrusions, but are unusually high in cumulates of the Forrestal Range section. Iron-rich biotite occurs in thin (5-15-mu-m) infilled cavities parallel to the c axes of some well-formed cumulus and early-postcumulus apatite crystals.

The Sitkoh Bay alkalic plutonic suite consists of six elongate small plutons totaling about 10 km wide by 45 km long on Chichagof Island near Tenakee Springs, southeastern Alaska. The suite, of Silurian or older age, contains a wide variety of rock types, ranging from nepheline syenite, syenite and trondhjemite to quartz monzonite and granite. This report provides majorand minor-element chemical data, including rare-earth elements (REE), for four bodies of the suite: the Kook Lake, Basket Lake, Point Hayes, and Tenakee Springs plutons. Rocks analyzed are samples typical of units mostly obtained from 1960 reconnaissance mapping. This part of Chichagof Island in the early Paleozoic was probably in or near a continental-margin volcanic arc based on the nature of country rocks of the alkalic suite, but the rocks of the suite lack fingerprints of such arc-related magmatism (for example, strong Nb depletion). The magmatism may be related to a rapid switch from subduction or collision to transcurrent tectonic activity, such as recorded for alkalic rocks from other areas. The analytical data show no evidence of significant REE or other element concentrations as are economic in other bodies of this type, but lack of mineral-resource directed sampling precludes data interpretation in terms of economic significance.

The western Cordillera of North America extends for over 6000 km from the tip of Baja California to the Alaska Range. It includes a wide variety of metamorphic and plutonic terrains, but none is more spectacular scenically or geologically than the Coast plutonic-metamorphic complex (Brew & Ford 1984) of western Canada and south-eastern Alaska. This report briefly describes the evolution of the western part of the complex, integrating information from the deformational, plutonic and metamorphic events. Most of the original studies are reported by the authors in U.S. Geological Survey Circular numbers 733, 751, 823-B, 868, 939, 945, 967 and 978, and are not cited specifically here. This summary does not contain either a comprehensive bibliography or a comparison of the metamorphic histories of south-eastern Alaska with the adjacent parts of British Columbia. The Coast plutonic-metamorphic complex is here divided into three major elements: the western metamorphic, the central granitic and the eastern metamorphic zones (Fig. 1). The western metamorphic belt is extremely long (900 km), and narrow (7–25 km). It consists of regional dynamothermally and regional thermally metamorphosed rocks with mineral assemblages ranging from prehnite-pumpellyite to upper amphibolite facies, scattered mesozonal to epizonal granitic bodies, and a few concentrically zoned mafic-ultramafic masses. The metamorphic grade and the amount of deformation increase from south-west to north-east, culminating at, or slightly to the north-east of, the ‘great tonalite sill’: a remarkable 700-km-long, 3- to 25-km-wide vertical to northeast-dipping belt of mostly syntectonic plutons of approximately the same age, composition and structural