Open-File Report 80-78 1980This r e p o r t i s p r e l i m i n a r y and has n o t been edited o r reviewed f o r conformity with Geological Survey standards and n o m e n c l a t u r e .NOW with Amax Exploration, Inc . , Denver, Colorado. ABSTRACT A v a i l a b l e i n f o r m a t i o n on t h e map d i s t r i b u t i o n , composition, and a g e s ofi n t r u s i v e r o c k s i n s o u t h e a s t e r n Alaska h a s been compiled and t h e r e s u l t s i n t e rp r e t e d t o i n d i c a t e t h e presence of s i x major and f i v e minor b e l t s . 2About 30 p e r c e n t of t h e 175,000 km of s o u t h e a s t e r n Alaska i s u n d e r l a i n by i n t r u s i v e igneous rocks. From west t o e a s t , t h e major b e l t s a r e : t h e F a i rweather-Baranof b e l t of e a r l y t o mid-Tertiary g r a n o d i o r i t e ; t h e Muir-Chichagof b e l t of mid-Cretaceous t o n a l i t e and g r a n o d i o r i t e ; t h e Admiralty-Revillagigedo p o r p h y r i t i c g r a n o d i o r i t e , q u a r t z d i o r i t e , and d i o r i t e b e l t of p r o b a b l e Cretaceous age; t h e Klukwan-Duke b e l t of c o n c e n t r i c a l l y zoned o r Alaskan-type ultramafic-mafic p l u t o n s of mid-Cretaceous age which i s mainly w i t h i n t h e Admiralty-Revillagigedo b e l t ; t h e Coast P l u t o n i c Complex t o n a l i t e s i l l of unknown, b u t perhaps mid-Cretaceous, age; and the Coast P l u t o n i c Complex b e l t I of e a r l y t o mid-Tertiary g r a n o d i o r i t e and q u a r t z monzonite.The minor b e l t s a r e d i s t r i b u t e d a s f o l l o w s : l a y e r e d gabbro complexes of i n f e r r e d mid-Tertiary a g e l i e w i t h i n and a r e probably p a r t of t h e FairweatherBaranof b e l t ; t h e Chilkat-Chichagof b e l t of J u r a s s i c g r a n o d i o r i t e and t o n a l i t e l i e s w i t h i n t h e Muir-Chichagof b e l t ; t h e S i t k o h Bay a l k a l i n e , Keadrick Bay pyrox e n i t e t o q u a r t z monzonite, and Annette and Cape Fox t r o n d h j e m i t e complexes, a l l i n t e r p r e t e d t o be of Ordovician ( ? ) a g e , form t h e c r u d e s o u t h e r n Southeast Alaska b e l t w i t h i n the Muir-Chichagof b e l t ; t h e Kuiu-Etolin v o l c a n i c -p l u t o n i c b e l t of mid-Tertiary a g e extends from t h e Muir-Chichagof b e l t eastward i n t o t h e Admiralty-R e v i l l a g i g e d o belt; and t h e B e h Canal b e l t of mid-t o l a t e T e r t i a r y g r a n i t e s l i e s w i t h i n and next t o t h e Coast P l u t o n i c Complex b e l t 11. I n a d d i t i o n , s c a t t e r e d maficu l t r a m a f i c b o d i e s occur w i t h i n t h e Fairweather-Baranof, Muir-Chichagof, and Coast P l u t o n i c Complex b e l t s . P a l i n s p a s t i c r e c o n s t r u c t i o n of 200-km r i g h t -l a t e r a l movement on t h e Chatham S t r a i t f a u l t does n o t s i g n i f i c a n t l y change t h e p a t t e r n of t h e major b e l t s , b u t does b r i n g mid-Tertiary v o l c a n i c -p l u t o n i c and Ord...

The Coast Range megalineament is a prominent, nearly continuous topographic and structural feature that extends southeastward about 550 km (330 mi) from its junction with the Chatham Strait – Lynn Canal fault at Point Sherman to Tongass Passage near the mouth of Pearse Canal where it leaves southeastern Alaska. It probably extends still further southeastward into British Columbia along Work Channel and Chatham Sound – Grenville Channel.The megalineament is a zone a few hundred metres to 10 km (6 mi) wide in which closely spaced joints, foliation, compositional layering, and small faults define the megalineament trend. The zone usually coincides with topographic depressions apparently caused by selective fluvial and glacial erosion of the less resistant rocks of the zone.Studies in the Juneau, Endicott Arm, and Behm Canal areas indicate that the megalineament (1) is locally the site of lateral and (or) vertical separations of no greater than several kilometres; (2) does not mark a major structural or metamorphic discontinuity in the near-surface rocks; (3) may be located near a pre-metamorphic and pre-intrusive discontinuity; (4) is consistently associated with and parallel to steep gradients in both the gravity and aeromagnetic fields; and (5) probably is the surface expression of the western contact, at depth, of the intrusive rocks and gneisses of the Coast Range batholithic complex with the schists to the southwest.

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An inverted metamorphic gradient is preserved in the western metamorphic belt near Juneau, Alaska. 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 and Stikine terranes. Detailed mapping of pelitic single-mineral isograds, systematic changes in mineral assemblages, and silicate geothermometry indicate that thermal peak metamorphic conditions increase structurally upward over a distance of about 8 km. Peak temperatures of metamorphism increase progressively from about 530 "C for the garnet zone to about 705 "C for the upper kyanite-biotite zone. Silicate geobarometry suggests that the thermal peak metamorphism occurred under pressures of 9-11 kbar. The metamorphic isograds are in general parallel to the tonalite sill that is regionally continuous along the east side of the western metamorphic belt, although truncation of the isograds north of Juneau indicates that the sill intrusion continued after the isograds were established. Our preferred interpretation of the cause of the inverted gradient is that it formed during compression of a thickened wedge of relatively wet and cool rocks in response to heat flow associated with the formation and emplacement of the tonalite sill magma. Garnet rim compositions and widespread growth of chlorite suggest partial re-equilibration of the schists under pressures of 5-6 kbar during uplift in response to final emplacement and crystallization of the tonalite sill. The combined results of this study with previous studies elsewhere in the western metamorphic belt indicate that high-T/high-P metamorphism associated with the collision of the Alexander and Stikine terranes was a long-lived event, extending from about 98 Ma to about 67 Ma.

We thank A.B. Till and J.A. Dumoulin for providing unpublished data on the Seward and York terranes on the Seward Peninsula, and J.H. Dover for providing unpublished data and interpretations for east-central Alaska. We also thank J.P. Calzia and N.J. Silberling for their constructive and thoughtful reviews, C.J. Nokleberg for computer drafting of the stratigraphic columns, and J.A. Nokleberg for coloring of the map. DESCRIPTION OF STRATIFIED MAP UNITS POST-ACCRETION SEDIMENTARY ROCKS Qs Sedimentary rocks (Quaternary) Chiefly Quaternary continental sedimentary rocks and lesser volcanic rocks, and unconsolidated silt, sand, and gravel. Depositionally overlie older overlap assemblages and terranes. Locally folded and faulted Ts Sedimentary rocks (Tertiary) Chiefly Tertiary continental sedimentary rocks and lesser volcanic rocks. Depositionally overlie older overlap assemblages and terranes. Locally folded and faulted Czs Sedimentary rocks (Cenozoic) Chiefly Tertiary continental sedimentary rocks and lesser volcanic rocks, and Quaternary (including Holocene) sedimentary rocks and unconsolidated silt, sand, and gravel. Depositionally overlie older overlap assemblages and terranes. Locally intensely folded and faulted Ks Sedimentary rocks (Cretaceous) Chiefly deep marine shale anft minor conglomerate deposited by turbidity currents. Local coarse-grained sandstone, and fine-grained sedimentary rocks deposited in deepmarine conditions to shallow-marine to nonmarine conditions. Local interlayered volcanic rocks chiefly rhyolite, dacite, and andesite tuff, flows, and breccia. Depositionally overly older Mesozoic overlap assemblages, .and terranes. Locally intensely folded and faulted. In southwestern Alaska, consists chiefly of Upper Cretaceous Kuskokwim Group (Cady and others, 1955; Box and Elder, 1992) that is mainly shale and fine-to medium-grained sandstone, and lesser siltstone and conglomerated that formed as deep-water marine turbidites with abundant, immature, lithic-rich components. Kuskokwim Group forms major overlap assemblage on Dillinger, Goodnews, Nixon Fork, Ruby, Tikchik, and Togiak terranes. In west-central Alaska in the Yukon-Koyukuk basin unit consists mainly of Lower Cretaceous, deep-marine turbidite deposits and Upper Cretaceous, shallow marine and non-marine deltaic sedimentary rocks that overlie Angayucham, Nixon Fork, and Ruby terranes (Nilsen, 1989) GN Gravina-Nutzotin overlap assemblage (mid-Cretaceous to Late Jurassic) Chiefly argillite, graywacke, and conglomerate, with lesser andesitic and basaltic volcanic, and volcaniclastic rocks of the Chisana Formation, Douglas Island Volcanics, and similar unnamed volcanic units. Sedimentary rocks range from deep marine turbidites to shallow-water and nonmarine deposits. In eastern-southern Alaska, coarse clastic rocks in Nutzotin part of assemblage derived locally from the underlying Wrangellia terrane and from unknown metamorphic source terranes. In southeastern Alaska, coarse clastic rocks in Gravina part of assemblage derived from stratigraphically un...

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U–Pb geochronologic studies demonstrate that steeply dipping, sheetlike tonalitic plutons along the western margin of the northern Coast Mountains batholith were emplaced between ~83 and ~57 (perhaps ~55) Ma. Less elongate tonalitic–granodioritic bodies in central portions of the batholith yield ages of 59–58 Ma, coeval with younger phases of the tonalitic sheets. Large granite–granodiorite bodies in central and eastern portions of the batholith were emplaced at 51–48 Ma. Trends in ages suggest that the tonalitic bodies generally become younger southeastward and that, at the latitude of Juneau, plutonism migrated northeastward across the batholith at ~0.9 km/Ma. Variations in the age, shape, location, and degree of fabric development among the various plutons indicate that Late Cretaceous – Paleocene tonalitic bodies were emplaced into a steeply dipping, dip-slip shear zone that was active along the western margin of the batholith. Postkinematic Eocene plutons were emplaced at shallow crustal levels. Inherited zircon components in these plutons range in age from mid-Paleozoic to Early Proterozoic and are coeval with detrital zircons in adjacent metasedimentary rocks. These old zircons, combined with evolved Nd isotopic signatures for most plutons, record assimilation of continental crustal or supracrustal rocks during the generation and (or) ascent of the plutons.