Petrology and Petrogenesis of the Bokan Granite Complex, Southeastern Alaska
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Petrology and petrogenesis of the Bokan Granite Complex, southeastern Alaska TOMMY B. THOMPSON ) JOHN R. PIERSON > Department of Earth Resources, Colorado State University, Fort Collins, Colorado 80523 THOMAS LYTTLE ) ABSTRACT rite, and biotite quartz monzonite extending Granite Complex at the Universite des over a much larger area (MacKevett, 1963). Sciences et Techniques du Languedoc (Ber- The Bokan Granite Complex is a peralka- nard Collot, 1981, personal commun.). line ring-dike complex emplaced into marine PREVIOUS WORK shales, volcanic flows and tuffs, and plu- REGIONAL SETTING tonic rocks. Mineralogically and chemi- Published accounts of regional geology cally, the Complex is composed of I-type from shoreline reconnaissance by Budding- The stratigraphic section in the Bokan granites. Aegirine- and riebeckite-bearing ton and Chapin (1929) provide the earliest Mountain area consists of deep marine sed- granite aplites, porphyries, and pegmatites attempts to establish regional stratigraphy iments, volcanic flows and tuffs, and upper comprise twelve distinct intrusive episodes. and tectonic setting in southeastern Alaska. reefal limestone formations that developed The aegirine-bearing rocks occur in an outer MacKevett (1963) first mapped the Bokan in a eugeosynclinal setting with island-arc annular zone formed during early crystal- Mountain area after uranium was discov- volcanic sources. The formations exhibit lization. Subsequent rocks are riebeckite- ered there in 1955. Lanphere and others rapid facies changes and, when considered bearing, due to devolatilization of the (1964) published radiometric dates on min- in conjunction with the volcanic systems, magma chamber during a collapse-ring- eral separates from several intrusive phases. reflect a belt of continued tectonic ad- dike emplacement event. Early crystalliza- Churkin and Eberlein (1977) made detailed justment. tion of alkali feldspar occurred during mag- studies of stratigraphic sequences in south- Rhyolitic volcanism is thought by Chur- ma ascension from a lower crustal-upper eastern Alaska and interpreted the regional kin and Eberlein (1977) to have been gener- mantle source. At shallow depths, subsolvus setting in a plate-tectonic sense. Hudson ated along a westward-dipping subduction crystallization allowed microcline and albite (1979) defined five major plutonic belts zone. Older volcanic arcs to the west con- to dominate. Local bodies of aegirine syen- formed in southeastern Alaska during verged on the shallow ocean basins to the ite were formed during the early collapse- Mesozoic plate convergence. Staatz (1978) east. Alkaline volcanism developed during ring-dike emplacement, in response to the provided detailed mineralogical and geo- subduction in response to vertical uplift magma devolatilization. The riebeckite gran- chemical data on the I and L vein system on rather than to tension. Pitcher (1979) char- ites reflect lower PQ2 and possibly declining the east-central margin of the Bokan Gran- acterized such volcanic-plutonic systems as peralkalinity. ite Complex. The present authors com- including I-type granitoid and basic bath- The granitic rocks at Bokan all exhibit pleted detailed field and laboratory studies oliths feeding volcanic vents. The mixed NajO contents greater than K2O. Litho- of the Bokan Granite Complex as part of a basaltic, andesitic, and sodic rhyolites seen phile elements are concentrated in all the contract with Bendix Field Engineering on Prince of Wales and surrounding islands rocks, especially in zones where hydrother- Corporation (BFEC Subcontract No. 78- lend credence to such interpretation. 245-E) during 1978-1979. This paper sum- mal albite and chlorite formed. Rb/Sr Magmatic activity occurred throughout marizes the petrology-petrogenesis of the ratios increase in progressively younger much of southeastern Alaska during Meso- Bokan Granite Complex, part of a major rocks in the Complex. Agpaitic ratios vary zoic time (Hudson, 1979). MacKevett alkaline province in southeastern Alaska from 0.92 to 2.08 for the granitic rocks. (1963) and Lanphere and others (1964) only poorly defined to date. Two M.S. reported Triassic-Jurassic ages for the theses (Pierson, 1980; Lyttle, in progress) LOCATION Bokan Granite. have extended the data base beyond the contract report (Thompson and others, Country Rock The Bokan Granite Complex is located in 1980). One additional M.S. thesis is in pro- southeastern Alaska, approximately 60 km The Bokan Granite Complex intrudes gress on the I and L vein system at the Uni- south-southwest of Ketchikan (Fig. 1), near metasedimentary, metavolcanic, and plu- versity of Alaska (Dave Gaard, 1981, the southern end of Prince of Wales Island. tonic rocks. Along the western and southern personal commun.). A Ph.D. dissertation is The Complex extends over an area of 28.5 margins of the complex, graphitic slates are also currently in progress on the Bokan km2, with diorite, quartz diorite, granodio- the country rock, but elsewhere metavol- Geological Society of America Bulletin, v. 93, p. 898-908, 10 figs., 2 tables, September 1982. 898 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/93/9/898/3444739/i0016-7606-93-9-898.pdf by guest on 02 October 2021 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/93/9/898/3444739/i0016-7606-93-9-898.pdf by guest on 02 October 2021 900 THOMPSON AND OTHERS canic and plutonic rocks are in contact with ring-dike emplacement through time (Fig. feldspar (Fig. 4A). Quartz-rich feldspar the Complex. By far the most common of 2). The ring dikes extend through an arc of aplite with average grain size less than 1 mm the latter group is a biotite quartz monzo- as much as 100 degrees. Rock types are dis- surrounds the pegmatite. The aplite also nite mass. In the east-central complex con- cussed in order from oldest to youngest. contains riebeckite prisms up to 6 mm in tact zone, the quartz monzonite has been Discussions include descriptive and chemi- length. There is no preferred crystal orienta- intruded by biotite-bearing aplite with cal data and distribution and style of tion, suggesting crystallization in a motion- quartz veins and pegmatites. emplacement of each rock type. less magma. The following discussion will focus on the Bokan Granite Complex emplacement-petro- Border Zone Pegmatite-Aplite Aegirine Granite Porphyry genesis. The border zone pegmatite-aplite is a dis- Aegirine granite porphyry is present as a BOKAN GRANITE COMPLEX continuously exposed rock unit (Figs. 2 and nearly continuous annular zone around the 3) along the outer contact of the Bokan outer part of the Bokan Granite Complex Introduction Granite Complex. It attains a maximum (Figs. 2 and 3). Maximum thickness of thickness of 13 m and gives way abruptly on aegirine-bearing rock is 180 m, and the The Bokan Granite Complex is a ring-dike its inner contact to aegirine granite por- inner contact of the porphyry occurs where system (Fig. 2) composed of twelve distinct phyry. The border zone contains pegmatitic a transition zone up to 15 m thick from intrusive rock types. The Complex exhibits a clots of coarsely crystalline (5 cm) aegirine aegirine- to riebeckite-bearing granite is progressively smaller diameter of collapse- and riebeckite prisms with quartz and alkali present. The inner contact of the porphyry ROCK UNI TS Riebeckite-aegirine aplite --Tìofg Felty-aegirine granite Fine-grained riebeckite "Rfrg granite porphyry Lamprophyre « a. EE E <3 "Rrg Riebeckite granite porphyry "Ras Aegirine syenite M Riebeckite aplite porphyry Fine-grained aegirine granite Aegirine granite porphyry Quartz-rich aegirine granite Aegirine aplitic granite 2 miles Border zone pegmatite-aplite 0 I 2 km Country rock Figure 2. Generalized geologic map of the Bokan Granite Complex, southeastern Alaska. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/93/9/898/3444739/i0016-7606-93-9-898.pdf by guest on 02 October 2021 A. B. Southeast Northwest Southeast Northwest Riebeckite Aplite Border Figure 3. Diagrammatic cross sections illustrating emplacement history of the Bokan Granite Complex. See text for discussion of A, B, C, and D. c. a. Granite Northwest •Northwest Southeast Southeast aegirine Aplite Present surface Figure 4. A. Border-zone pegmatite-aplite with riebeckite (r)-aegirine (a) crystal clusters in an aplitic quartz, riebeckite, and feldspar matrix. B. Photograph of three varieties of aegirine granite: fine grained (b-1), aegirine granite porphyry (3-56), and quartz-rich aegirine granite (b-6). q = quartz, a = aegirine. C. Photograph of riebeckite aplite porphyry with flow-oriented riebeckite. D. Photograph of aegirine syenite (Ku.-1-41) and aegirine granite porphyry (3-56). Note the absence of quartz (q) phenocrysts in the syenite. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/93/9/898/3444739/i0016-7606-93-9-898.pdf by guest on 02 October 2021 902 THOMPSON AND OTHERS Figure 5. A. Photograph of riebeckite granite porphyry (3-68) and aegirine granite porphyry (bK-78-19). Paragenetically early riebeckite (r) and late aegirine (a) are shown in respective samples, q = quartz. B. Photograph of riebeckite granite porphyry (b-8) contrasted with fine-grained riebeckite granite porphyry (b-5). Note the quartz (q) and riebeckite (r) phenocrysts in the riebeckite granite porphyry and the "pepper-like" nature of the fine riebeckite and the microcline (m) phenocrysts in the fine-grained riebeckite granite porphyry. C. Photograph of the aegirine granite porphyry (bK-78-19) and felty aegirine granite (a-7). The fine needle-like habit in the felty aegirine granite is compared to the dark interstitial aggregates of aegirine in the aegirine granite porphyry. D. Photograph of riebeckite-aegirine aplite (b-3). Note the euhedral dark prisms of riebeckite. is shown in Figure 2, where riebeckite is the monazite, muscovite, and fluorite form the bearing rocks is exposed only in diamond only alkali ferromagnesian mineral present accessory minerals. The muscovite occurs drill core. No preferred crystal orientation toward the center of the complex. only as inclusions within microperthite. was seen in the aegirine granite porphyries.