Petrology and Petrogenesis of the Bokan Granite Complex, Southeastern Alaska

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

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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.

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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. Four varieties of aegirine-bearing granite Protoclastic texture is exhibited by bent or occur within the aegirine granite porphyry offset albite twin planes and by strained Riebeckite Aplite Porphyry unit of Figure 2 (see legend for relative quartz. Aegirine is anhedral, interstitial, ages). They are not distinguished on the and constitutes 7 to 15 volume percent of The transition from aegirine- to rie- generalized map (Fig. 2) due to the small the rock. The quartz content of the aegirine- beckite-bearing rocks was initiated by areal extent of three of the varieties (see bearing granites of the annular zone varies magma roof collapse and emplacement of Thompson and others, 1980, for detailed from 23% to 45%. riebeckite aplite porphyry ring dikes (Fig. geologic map). The most extensive variety is Figure 4A shows diagrammatically the 3B). Separation of an aqueous vapor with

a porphyritic rock with quartz and micro- geometry of the aegirine granite porphyry. C02, F, Na, U, Th, REE, and Be was facili- perthite phenocrysts as much as 8 mm in Although not controlled on the northwest, tated by the pressure release during collapse. diameter. Finer (3-5 mm) quartz, micro- diamond drill holes and mine openings to The riebeckite aplite porphyry ring dikes cline, albite (Ab94), and aegirine form the southeast show the porphyry thinning at contain 50% to 54% round quartz grains (1 the groundmass (Fig. 4B). Sphene, zircon, depth. The bilobate bottom of the aegirine- to 1.5 mm diam) with sparse quartz and as

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much as 10% riebeckite phenocrysts (Fig. 4C). The riebeckite is euhedral with crystal lengths up to 8 mm. Flow orientation of riebeckite is present, and there are bands in which riebeckite is more concentrated than in the normal aplite. Microcline (30 to 35 volume percent) and albite (5 volume percent) make up the rest of the aplitic groundmass. Protoclastic texture is common.

Aegirine Syenite

Aegirine syenite is found at the surface around Bokan Mountain only in the southeastern part of the Complex, and at the present map scale (Fig. 2), the syenite is too small to be shown. It is exposed in mine workings as well as in diamond drill core (Thompson and others, 1980, Pis. 1 through 4). It occurs as (1) cylindrical to tabular masses adjacent to ring dikes (Fig. 3B) or (2) transverse to the ring dikes along faults that were active during magma crystallization. The rock is not vuggy like the episye- nites of the Bois Noirs Granite of the French Massif Central (Cuney, 1978). The aegirine syenite contains primarily albite and aegirine with a grain size range of 2 to 5 mm. As much as 18% very fine grained quartz is dispersed along albite and aegirine grain boundaries. Fluorite, pyrrho- tite, pyrite, monazite, zircon, and sphene constitute the accessory minerals. The lack of quartz phenocrysts as well as the over-all reduction in quartz compared to other rocks of the Bokan Granite Complex serves to distinguish the syenite (Fig. 4D). It occurs in sharp contact with the granitic rocks.

Riebeckite Granite Porphyry

As the collapse of the magma-chamber roof occurred, riebeckite granite porphyry Figure 6. A. Photograph of flow foliation in riebeckite granite porphyry. B. Photograph began to crystallize beneath the aegirine of delicate prismatic riebeckite in riebeckite granite porphyry. The elongate riebeckite granite porphyry cap (Fig. 3B). The riebeck- crystals grew perpendicular to the crystalline rock margin during stagnation of convective ite granite porphyry comprises the majority magma movement. The direction of crystal growth is indicated by the arrow (knife for of the central mass within the Bokan Gran- scale). ite Complex (Fig. 2). Principal minerals include quartz, microperthite, microcline, albite, and riebeckite. Quartz forms pheno- Flow lineation of riebeckite is well devel- margins (Pierson, 1980). Lack of magma crysts as much as 5 mm in diameter but oped, and in places, bands of oriented rie- movement is apparent in a few localities. At ranges down to 0.5 mm. Riebeckite forms beckite, 1 cm to 1 m in width, constitute a such places delicate riebeckite crystals subhedral phenocrysts as much as 4 mm in significant part of the rock (Fig. 6A). Stud- formed perpendicular to the solidified mar- length and constitutes 10% of the rock (Fig. ies of lineation indicate that convective gin (Fig. 6B). 5A). Albite is slightly more abundant (30% magma movement developed with upwel- The riebeckite granite porphyry exhibits to 33%) than the potash feldspars (25% to ling near the center of the complex and with protoclastic texture with offsets and kinking 28%). flow radially out and down along the outer of albite twin planes, and strained quartz.

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TABLE 1. WHOLE-ROCK. OXIDE ANALYSES FOR THE BOK.AN GRANITE COMPLEX

BOKAN GRANITE COMPLEX

Oxides Aegirine granite porphyry Quartz-rich aegirine granite Riebeckite-aplite Riebeckite-granite Lamprophyre porphyry porphyry

n = 44 n = 1 n = 1 n = 14 n = 4 X min max X min max X min max X min max X min max 76.90 53.62 46.81 64.00 Si02 73.67 70.76 78.30 77.80 na na 76.90 na na 73.20 66.90 13.80 19.04 AI2O3 11.02 9.17 14.54 7.40 na na 9.60 10.15 6.70 12.18 16.92 MGO 0.02 0.0 0.15 0.15 na na 0.01 0.07 0.0 0.50 2.29 0.84 3.86 CaO 0.32 0.0 1.15 0.68 na na 0.05 0.42 0.13 1.48 6.89 4.70 9.03 5.70 Na20 5.14 3.28 6.56 4.10 na na 4.50 5.03 4.18 6.10 4.96 3.32 2.60 2.25 3.50 K20 4.08 3.01 5.40 2.70 na na 3.60 3.98 3.30 4.53 Fe*3 4.02 0.73 11.32 4.70 na na 2.90 4.68 2.20 7.70 5.57 1.10 10.09 0.16 0.11 0.34 0.49 0.33 0.71 Ti02 0.17 0.03 0.43 0.15 na na 0.21 0.17 0.23 p2o5 0.04 0.0 0.77 0.02 na na 0.01 0.02 <0.01 0.04 0.05 MnO — - — - na na - - - -- 0.23 0.21 0.24 Agpaitic Ratio 1.171 0.914 1.486 1.306 • na na 1.177 1.275 1.065 2.079 0.668 0.412 0.906

Note: this table from Thompson and others (1980).

Lamprophyre is indicated by fragments included in the The fine-grained riebeckite granite por- fine-grained riebeckite granite porphyry. phyry (Fig. 5B) is distinctive due to a fine Thin, irregular fine-grained or porphy- groundmass peppering of riebeckite through- ritic dikes were intruded into all older rocks Fine-grained Riebeckite Granite out the rock. Albite (45%-48%), microcline with completion of the crystallization of Porphyry (11%-13%), and quartz (30%-32%) com- riebeckite granite porphyry. All the dikes prise the fine groundmass of the rock, and have suffered extensive deuteric alteration A second subsidence of the magma- with resulting assemblages of epidote, chlor- chamber roof was accompanied by em- ite, and carbonate. The porphyritic rocks placement of ring dikes of fine-grained Si02 were probably andesitic in composition as riebeckite granite porphyry (Fig. 3C). Stop- some phenocrysts appear to have been pla- ing accompanied ring-dike emplacement. gioclase. The relative age of the mafic dikes

NaAISi,0B KAISi308 WEIGHT PERCENT

Figure 8. Curves for water-saturated liquids in equilibrium with quartz and alkali feldspar at indicated confining pressures (500, 1,000, 3,000, and 5,000 bars). Isobaric minima are labeled m except at 5,000 bars, where a ternary eutectic e is generated by intersection of the alkali-feldspar solvus with the liquidus surface. Plots of normative Q, Or, and Ab of analyzed granites from the Bokan Granite Al2 Oj Complex concentrate in the stippled area (modified from Tuttle and Figure 7. Binary plot of molecular AI2O3 versus (Na20 + K^O). Bowen, 1958).

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TABLE 1. (Continued)

LIRUEI, NIGERIA'

Andesite Fine-grained Felty aegirine Riebeckite-aegirine Riebeckite-aegirine riebeckite-granite granite aplite granite porphyry n = 4 n = 2 n = 3 n = 3 n = 1

X min max X min max X min max X min max X min max

49.43 49.42 52.00 74.05 73.60 74.50 73.83 70.60 77.50 69.63 67.00 74.10 74.05 na na 19.74 18.20 21.24 11.75 11.60 11.90 7.60 6.20 8.30 14.90 12.50 16.60 11.20 3.70 2.89 4.74 0.03 0.02 0.03 0.02 0.01 0.04 0.02 0.02 0.02 0.40 9.33 8.81 9.95 0.05 0.04 0.05 0.09 0.07 0.12 0.04 0.03 0.05 0.45 3.30 2.50 4.22 5.55 5.50 5.60 5.47 4.40 6.80 6.73 5.70 7.50 4.60 0.96 0.73 1.15 3.95 3.90 4.00 3.37 2.90 3.80 4.23 4.00 4.50 5.20 7.41 3.80 10.25 2.25 1.60 2.90 8.27 7.10 10.10 2.33 2.00 2.70 2.10 0.53 0.19 0.78 0.16 0.14 0.17 0.25 0.21 0.27 0.16 0.10 0.19 0.35 0.19 0.11 0.23 0.02 0.02 0.02 0.01 0.01 0.01 0.01 0.01 0.02 0.06

0.20 0.18 0.21 - - - - - — - — — —

0.327 0.283 0.377 1.141 1.129 1.153 1.664 1.526 1.791 1.056 0.968 1.103 1.13

'Analysis from Liruei Ring Complex obtained from Bowden and Turner (1974).

microcline, quartz, and riebeckite form ring dikes of the Bokan Granite Complex peralkalinity was undoubtedly in response phenocrysts. Some deuteric aegirine is pres- (Figs. 2 and 3D). The rock is a white to light to the early alkali feldspar crystallization ent rimming riebeckite. Protoclastic texture pink aplite porphyry with 1 to 2 cm long prior to aegirine precipitation. The alkali is again well defined in the rock. euhedral riebeckite phenocrysts (Fig. 5D). feldspar (that is, microperthite) exhibits a Small quartz-riebeckite-albite pegmatites Microcline (52%), quartz (23%), and albite gradual increase in Ab component inward and/or quartz veinlets are invariably found (17%) form the aplitic groundmass. Aegi- to the aegirine-riebeckite stability contact in near the outer contact of the fine-grained rine has formed by deuteric alteration of the the Complex. Bailey (1969) demonstrated riebeckite granite porphyry. Most of these margins of riebeckite prisms. Some hema- that acmite crystallizes only from a liquid bodies are weakly radioactive; however, no tite staining is found in most outcrops. that contains excess sodium silicate. The distinct uranium or thorium minerals were aegirine at Bokan is a magmatic product found. GEOCHEMISTRY OF THE BOKAN and does not appear to have replaced iron GRANITE COMPLEX oxide minerals or early riebeckite. Variation Felty Aegirine Granite in Po2 during crystallization of riebeckite- Introduction stable rocks has given rise to irregular aegi- The felty aegirine granite forms an irregu- rine mantling and then continued riebeckite lar, thick ring dike of more than 90° arc in The peralkaline nature of the Bokan precipitation. the northeastern quadrant of the complex Granite Complex is clearly seen in Figure 7. Paragenetically, microperthite is the (Fig. 2). The dike was emplaced in the wan- Agpaitic ratios vary from 0.90 to 2.08. The earliest formed mineral in the Bokan gran- ing stages of magma emplacement (Fig. variation from early aegirine-bearing rocks ites. Some of the microperthite contains 3D). The rock is characterized by as much to later riebeckite-bearing rocks reflects a poikilitic muscovite. Crystallization of al- as 32% fine aegirine needles 2 to 6 mm long decrease through time in peralkalinity and bite and microcline postdates microperthite.

in an aplitic groundmass (Fig. 5C). The in PQ2 (Bailey, 1969) of the Bokan Granite Whitney (1975) demonstrated that alkali groundmass is composed of microperthite magma. The peralkalinity appears to be the feldspar crystallization without plagioclase and albite. Doubly terminated beta quartz result of alumina deficiency rather than in a synthetic granite requires pressures with aegirine inclusions forms sparse phe- excess alkalis (Table 1). greater than 5 kb and greater than 7 wt per- nocrysts. A typical mode consists of quartz, The early work of Bowen and Tuttle cent H2O. The experimental work of Lam- 25.20%; K-feldspar, 26.60%; albite, 15.80%; (1958) noted in experimental studies that bert and others (1969) on the system aegirine, 31.30%; and less than 3% of riebeckite is stable only when water content KAlSi30g-SiC>2-H20 showed that musco- sphene, fluorite, hematite, and chlorite. of the melt is less than 3.9% and tem- vite crystallization reflects pressures exceed- Along the margins of the ring dike, large peratures are below 610 °C. Ernst (1959) ing 5 kbar. Decreasing pressure and water prisms (as much as 8 mm) of riebeckite are demonstrated that increased partial oxygen content are required for plagioclase and present. pressure favors acmite (aegirine) rather than quartz to precipitate simultaneously with riebeckite precipitation. alkali feldspar. The aegirine present in the

Riebeckite-Aegirine Aplite The decrease Po2 is supported by ferric: early-formed rocks of the Bokan Granite ferrous ratios of > 15 for aegirine and <3.5 Complex crystallized late as small intersti- Riebeckite-aegirine aplite forms, in the for riebeckite separates from the Bokan tial aggregates between quartz and feld- north-central part, the last in the series of Granite Complex rocks. The decrease in spars. The aegirine clearly reflects strong

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oxidizing conditions (Bailey, 1969), and the Major-Element Geochemistry rite, and tourmaline. Concentrations of stage II (Fig. 3B) collapse and ring-dike unique trace elements are associated with emplacement allowed for liberation of O2 Whole-rock chemical analyses of 76 spec- the altered rock. (Details of wall-rock alter- plus other volatiles. The crystallization of imens from the ring-dike complex are ation and hydrothermal uranium-thorium riebeckite in subsequent rocks of the com- reported (Table 1). Except for the minor deposits will be discussed in a paper now in

plex indicates a lower Po2 and/or a lower lamprophyre and andesite dikes, the rocks preparation by Thompson.) degree of peralkalinity (Bailey, 1969). exhibit remarkably consistent compositions, Normative calculations for the Bokan The crystallization of small tabular to with Na20 exceeding K2O in all rocks. Con- rocks lend support to the chemical homo- cylindrical masses of aegirine syenite in trasted with the Nigerian granitic ring dike geneity of the rocks with the exceptions of association with granitic rocks is closely at Liruei, the significant differences are in the aegirine syenite and the mafic dikes related to the stage II collapse-ring-dike Na20, K20, and MgO (Table 1). The larger (Fig. 9). The aegirine syenite is albitized, emplacement (Fig. 3B). Crystallization of MgO content at Liruei reflects a higher and whole-rock chemical comparisons are anhydrous minerals, magma emplacement modal riebeckite compared to the Bokan not possible; the syenite does have signifi-

to a shallow crustal level (Whitney, 1975), suite. The total alkali concentrations at cantly lower silica (58.5%-70.4% Si02, with and an apparent initially high-volatile mag- Bokan are similar to those reported (Rogers a mean value of 66.1% for 44 samples). ma indicate that the magma volatile content and Greenberg, 1981) for alkali granites; Normative quartz, orthoclase, albite, and increased to the saturation point in the api- however, the Bokan suite is different in that acmite characterize all of the Bokan granitic cal and marginal portions of the magma Na20>K.20. Also, the alumina values at rocks. chamber. Description of the crystallization Bokan are significantly lower than those of A differentiation trend is shown in Figure sequence is facilitated utilizing the Or-Ab- typical alkali granites (Rogers and Green- 10, which illustrates a linear trend for K2O- berg, 1981). SÌO2 ternary system (Fig. 8). Relative pres- (CaO + Na20) versus Al203-(Na20-K.20- sure increases require depressing the cotectic Hydrothermal alteration is present in the CaO/7), with a distinct gap between the boundary toward the Ab-Or join; however, aegirine granite rocks at Bokan. It is syenitic and granitic rocks. The gap clearly the collapse-ring-dike emplacement allowed manifested by albitization with complete signifies a major event during magma for volatile loss to occur and a shift of the replacement of all potash feldspar by albite. crystallization—the devolatilizing of the cotectic away from the Ab-Or join. Local In these rocks, K.2O values are on the order magma during stage II collapse-ring-dike masses of magma were left in the field of of 0.0X percent, but Na20 values increase emplacement (Fig. 3B). alkali-feldspar-only crystallization, yielding to 8.5% to 13%. Additionally, aegirine is re- The Bokan Granite Complex exhibits the syenitic bodies with crystallization. placed by high-iron chlorite, calcite, fluo- characteristics typical of I-type granites: (1) no biotite and <0.5% muscovite are present; (2) Na20 values >3.3% with

K20 < Na20; (3) molecular Al203/alkalis + CaO < 1.3; (4) rocks are oxidized with high Q ferric iron content (Table 1); (5) no alumi- nosilicate, garnet, or cordierite are present; (6) whole-rock isochrons give regular and linear point sets (Thompson and others, 1980); and (7) primary sphene occurs as an accessory.

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Aegirine granite porphyry Riebeckite granite porphyry Aegirine syenite Riebeckite oplite Quartz monzonite Figure 10. Differentiation trends for the Bokan Granite Complex. Andesite Lamprophyre _

are significantly lower than those reported for the Nigerian Complex. No correlation CO exists (R2 = 0.013) between Zr contents and agpaitic ratios for the Bokan suite. Mean values for both La and Y are consid- ° o^ir o erably lower than the values from Liruei O • o (Table 2), but the La/Y ratio of 1.4 falls with the LREE/HREE ratios of 1.3 to 4.5 for peralkaline granites within the Nigerian Complex. Rb/Sr ratios increase in progres- A sively younger rocks within the Bokan Granite Complex; however, intensely albit- i ized rocks exhibit significantly lower Rb values due to potash feldspar replacement (Thompson and others, 1980).

K20- ( Ca0 + Na20 ) PETROGENESIS OF THE TRACE-ELEMENT eluding lithophiles such as Zr, Nb, Rb, BOKAN GRANITE COMPLEX GEOCHEMISTRY REE, uranium, and thorium. Bowden and Turner (1974) suggested that Zr concentra- The granitic magma of the Bokan Granite The Bokan Granite Complex rocks con- tion might be used as an index of peralkalin- Complex is an I-type granite, requiring tain the characteristic trace elements of ity based on studies of peralkaline granite derivation from a partial melt of upper- alkaline/peralkaline systems (Table 2), in- complexes; however, the Bokan Zr contents mantle rocks. The pressure conditions neces-

TABLE 2. SELECTED TRACE-ELEMENT DATA FOR THE BOKAN GRANITE COMPLEX

Unalbitized

Bokan Mountain Liruei, Nigeria Trace Aegirine granite Riebeckite granite Andesite Lamprophyre Aegirine granite elements n = 28 n = 1 n = 1 n = 2 n = 1 X min max X min max X min max X min max X min max U 16 5 30 19 na na ±1 na na 6 5 8 na na Th ±50 ±50 350 ±50 ±50 ±50 ±50 ±50 Be 4 2 10 10 1 13 5 20 Ga 12 5 20 10 10 8 5 10 55 La 125 ±20 200 200 ±20 30 ±20 50 185 Li 62 5 163 107 19 158 144 172 575 Mo 8 ±2 100 ±2 2 ±2 ±2 ±2 Nb 38 20 100 50 10 15 1500 PPm Rb 351 174 439 311 73 251 210 293 1400 Sn 10 ±10 20 10 ±10 ±10 10 150 Sr 22 4 118 9 770 440 440 440 V ±10 ±10 ±10 + 10 100 25 20 30 Y 89 ±10 300 200 ±10 28 5 50 535 Zr 327 100 1000 700 20 13 10 15 2160 (Fe+3 2.21 0.51 2.93 2.10 2.66 1.71 0.77 2.66 (Fe+2 0.56 0.12 1.94 5.05 3.58 2.53 2.18 2.88 Fe*3/Fe*2 7.017 0.263 21.595 4.153 0.743 0.744 0.268 0.744 U/Th 0.534 0.058 1.175 0.739 na na na na Rb/Sr 29 3 95 3*7 0.09 0.57 0.48 0.67 K/Rb 96 66 120 115 83 95 91 99 25

Note: this table after Thompson and others (1980).

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sary for muscovite and alkali feldspar ACKNOWLEDGMENTS Martin, R. F., and Bonin, B., 1976, Water and crystallization without plagioclase or quartz magma genesis: The association hypersolvus (Whitney, 1975; Lambert and others, 1969) This paper was reviewed by Marvin Lan- granite-subsolvus granite: The Canadian Mineralogist, v. 14, pt. 3, p. 228-237. support crystallization of both under rela- phere and Russell Jacoby. We are indebted Pierson, J. R., 1980, Petrology and pedogenesis tively dry conditions at depths of greater to both reviewers for their suggested correc- of the Bokan granite complex, Alaska [M.S. than 15 km. As the melt moved into shal- tions and clarifications. thesis]: Fort Collins, Colorado, Colorado lower depths, subsolvus crystallization pre- State University, 86 p. REFERENCES CITED vailed with the crystallization of two Pitcher, W. S., 1979, The environmental control feldspars and quartz. As the melt stoped to of granitic emplacement in orogenic belts: Bailey, D. K., 1969, The stability of acmite in the Geological Society of America Abstracts within 2 to 4 km of the surface, water and presence of H20: American Journal of with Programs, v. 11, no. 7, p. 496. volatiles became concentrated in the upper Science, Schairer vol. 267-A, p. 1-16. Rogers, J.J.W., and Greenberg, J. K„ 1981, parts of the magma chamber (Whitney, Bowden, P., and Turner, D., 1974, Peralkaline Trace elements in continental-margin mag- 1975), where residual sodium combined and associate ring complexes in the Nigeria- matism: Part III, Alkali granites and their with oxidized iron to form aegirine. In addi- Niger Province, West Africa, in Sorensen, relationship to cratonization: Summary: H., ed., The alkaline rocks: New York, John Geological Society of American Bulletin, tion to water, C03=, H2S, F~, and lithophile Wiley & Sons. Part I, v. 92, p. 6-9. and chalcophile elements were concentrated Buddington, A. F„ and Chapin, T., 1929, Geol- Staatz, H. H., 1978, I and L uranium and in the upper parts of the magma chamber. ogy and mineral deposits of southeastern thorium vein system, Bokan Mountain, The system behaved similarly to the Alaska: U.S. Geological Survey Bulletin 800, southeastern Alaska: Economic Geology, hypersolvus-subsolvus granites described by 398 p. v. 73, p. 512-523. Churkin, M„ Jr., and Eberlein, G. D„ 1977, Thompson, T. B., Lyttle, T., and Pierson, J. R., Martin and Bonin (1976), with generation Ancient borderland terranes of the North 1980, Genesis of the Bokan Mountain, of hydrothermal fluids from subsolvus gran- American Cordillera: Correlation and micro- Alaska, uranium thorium deposits: U.S. ite. With the inception of stage II collapse- plate tectonics: Geological Society of Amer- Department of Energy Report (Bendix Field ring-dike emplacement, the volatiles and ica Bulletin, v. 88, no. 6, p. 769-786. Engineering Corporation Subcontract 78- associated lithophile and chalcophile ele- Cuney, M., 1978, Geologic environment, miner- 245-E), Grand Junction, Colorado, 232 p. alogy and fluid inclusions of the Bois Noirs- ments were released into fractures where Tuttle, D. F., and Bowen, N. L„ 1958, Origin of Limouzat uranium vein, Forez, France: Eco- granite in the light of experimental studies in cooling, lower pressures, and reducing nomic Geology, v. 73, no. 8, p. 1567-1610. the system NaAlSi308-KAlSi308-Si02-H20: conditions caused precipitation of oxides, Ernst, W. G., 1959, Alkali amphiboles, in Report Geological Society of America Memoir 74, carbonates, fluorides, and sulfides. Con- of the Director of the Geophysical Labora- 153 p. temporaneously, small bodies of syenite tory: Carnegie Institute Washington Paper Whitney, J. A., 1975, Vapor generation in a No. 1300, p. 121-126. quartz monzonite magma: A synthetic model were crystallized. An Andinotype (Pitcher, Hudson, T., 1979, Mesozoic plutonic belts of with application to porphyry copper depos- 1979), plate-edge relationship appears to southern Alaska: Geology, v. 7, no. 5, p. its: Economic Geology, v. 70, no. 2, have been the structural setting for the 230-234. p. 346-358. Bokan system. Volcanic rocks were proba- Lambert, 1. B., Robertson, J. K., and Wyl- bly generated from the collapse-ring-dike lie, P. J., 1969, Melting reactions in the system KAlSi 0 -Si0 -H 0 to 18.5 kilobars: events. 3 8 2 2 American Journal of Science, v. 267, p. 609-626. Subsequent magma crystallization reflects Lanphere, M. A., MacKevett, E. M., Jr., and a lower degree of magma oxidation and/or Stern, T. W., 1964, Potassium-argon and peralkalinity. Local pressure variations al- lead-alpha ages of plutonic rocks, Bokan lowed for riebeckite-aegirine aggregates to Mountain, Alaska: Science, v. 145, no. 2622, MANUSCRIPT RECEIVED BY THE SOCIETY form in the riebeckite granites. Local mag- p. 705-707. MacKevett, E. M„ 1963, Geology and ore depos- MARCH 13, 1981 ma repressuring is apparent as indicated by its of the Bokan Mountain uranium-thorium REVISED MANUSCRIPT RECEIVED emplacement of the felty-aegirine granite area, southeastern Alaska: U.S. Geological SEPTEMBER 14, 1981 ring dike (Figs. 2 and 3D) of stage IV. Survey Bulletin 1154, 125 p. MANUSCRIPT ACCEPTED SEPTEMBER 28, 1981

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