BRUCE L. REED U.S. Geological Survey, Anchorage, Alaska 99501 MARVIN A. LANPHERE U.S. Geological Survey, Menlo Par\, California 94025
Alaska-Aleutian Range Batholith: Geochronology, Chemistry, and Relation to Circum-Pacific Plutonism
ABSTRACT and Tertiary plutons are associated with ex- trusive rocks, these plutons are largely post- Potassium-argon mineral ages and reconnais- tectonic, and magma was generated beneath sance mapping of approximately 65,000 sq km both stable platform areas as well as beneath in south-central Alaska indicate that the former eugeosynclinal regions in which defor- Mesozoic and Cenozoic plutonic rocks in the mation had essentially ceased. region were emplaced during three discrete in- Magma for the Jurassic plutonic rocks ap- trusive epochs. Most of the plutonic rocks are pears to have been generated along or above an part of the Alaska-Aleutian Range batholith; early Mesozoic subduction zone. The region the remainder appear as outcrops in isolated southeast of the zone is considered to represent plutons northeast of the main batholith. a classic example of continental accretion of Maximum and minimum concordant mineral eugeosynclinal sediments caused by under- ages on coexisting biotite and hornblende are thrusting of oceanic crust beneath an island arc. used to mark the beginning and ending, re- Magma generation for the Cretaceous and spectively, of each intrusive epoch. Tertiary plutons in the northern part of the The oldest intrusive epoch, Early and Mid- batholith does not appear related to the early dle Jurassic, contains plutonic rocks emplaced Mesozoic subduction zone, for it would require between about 176 and 154 m.y. ago. Jurassic that the zone shift inland, or toward the con- plutonism occurred along a magmatic arc at tinent, from its position during the Jurassic and least 1,300 km long, extending from about 480 then shift away from the continent once again km southwest of Becharof Lake northeast to to its present position. the Talkeetna Mountains. Aeromagnetic data suggest that the magmatic arc, which rep- INTRODUCTION resents the roots of the arc portion of an early The area we are studying covers approxi- Mesozoic arc-trench system, also continues mately 65,000 sq km in an arcuate belt of southwest into the Bering Shelf. The associated mountain ranges in south-central Alaska. The trench is thought to be represented by an mountain ranges comprise the southern and imbricated mélange of ophiolite and submarine part of the central Alaska Range and the lava with associated chert and argillite that northern part of the Aleutian Range. These occupies a belt 140 km southeast of the mag- ranges were formed by uplift and consequent matic arc. Clastic sediments more than 4.5 km dissection of a huge block of the Earth's crust thick occupy the 140-km-wide arc-trench gap. in late Cenozoic time. They are composed Late Cretaceous and early Tertiary plutonic chiefly of Mesozoic and Cenozoic granitic rocks rocks, emplaced between about 83 and 58 m.y. and Mesozoic sedimentary and volcanic rocks. ago, are found mainly in the northern part of In the Aleutian Range, these rocks are dis- the batholith and in isolated plutons to the continuously overlain by Cenozoic volcanic and northeast toward Mount McKinley. Middle sedimentary rocks that extend southwestward Tertiary plutonic rocks ranging in age from 38 into the linear chain of volcanoes of the to 26 m.y. occur in two areas within the Aleutian arc. batholith and also in the Mount McKinley The Alaska-Aleutian Range batholith is an area. Although some of the Late Cretaceous informal name applied to the granitic rocks of
Geological Society of America Bulletin, v. 84, p. 2583-2610, 12 figs., August 1973 2583
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this mountain belt (Fig. 1). A geologic map of the Alaska-Aleutian Range batholith at a scale of approximately 1:1,000,000 has been pub- lished recently (Reed and Lanphere, 1972). The generalized geology of the batholith is shown in Figure 2 at somewhat smaller scale. The batholith extends from at least as far southwest as the vicinity of Becharof Lake northeast to about 62° N. lat and is comparable in areal extent to the Sierra Nevada batholith of California. The Alaska-Aleutian Range batholith is not a genetic unit but rather a convenient physiographic grouping of granitic rocks. As will be discussed later, rocks of part of the batholith trend into other areas of south- central Alaska that are underlain by granitic rocks of similar age and origin. Although most of the Alaska-Aleutian Range batholith is very well exposed, the area is re- mote and the cost and difficulty of access have limited detailed geologic investigations. The geology of much of this part of Alaska is there- fore either unknown or poorly known. Previous work on the geology of the southern Alaska Range and Alaska Peninsula was summarized in Figure 1. Map showing distribution of plutonic an earlier paper (Reed and Lanphere, 1969). rocks in parts of the Alaska and Aleutian Ranges. Subsequent to this, a reconnaissance geologic Areas covered in Figures 2 and i are outlined. map covering about 13,000 sq km in the south- unravelling the precise timing of plutonic and ern Alaska Range between Chakachamna Lake associated tectonic activity in a mountain belt and Farewell has been released (Reed and is not a simple matter of obtaining scattered Elliott, 1970). The geology of the Mount age determinations from granitic or meta- McKinley quadrangle, which covers the north- morphic rocks. On the contrary, the signifi- eastern part of the area of this report, has beei cance of individual ages can be judged only by compiled by Reed (1961). Although geologic integrating information of many kinds from control is moderately good in a few areas, for many localities." Despite its limitations, we example, the Iliamna quadrangle and the area feel the study has provided a considerable north of Chakachamna Lake, most of the work body of information about the Alaska-Aleutian has been of a reconnaissance nature, and age Range batholith and is a practical first step dates are widely scattered. Two areas in which toward understanding the age and chemistry of geologic control of the batholith is especially this large province of plutcnic rocks. deficient are those east of Lake Clark and The geochronology of the plutonic rocks northeast of Chakachamna Lake. In this pre- studied by us is based on age measurements on liminary synthesis, we are forced to draw 130 rock samples. Potassium-argon analyses tentative conclusions about the geology of the were made on 115 biotite separates, 69 horn- plutonic rocks of some rather large areas from blende separates, and 4 muscovite separates. very sparse data. It should be emphasized that Age measurements for 33 of these rock samples none of the plutons discussed in this report has (nos. 1 to 33, Fig. 3) have been reported pre- been mapped in detail, and our work thus far viously (Reed and Lanphere, 1969). Age has been limited to blocking out plutonic rock measurements for samples 34 to 130 (Fig. 3 units and sampling them and observing their and Table 1) are new data. Modal analyses as intrusive and structural relations from widely well as chemical analyses by the "rapid scattered locations. We share the views of method" (Shapiro and Brannock, 1962) have Gabrielse and Reesor (1964, p. 97), who, in been made for nearly all o:: the samples dated their study of the geochronology of plutonic isotopically and for 97 samples that were not rocks in the Canadian Cordillera, stated "... dated. The rock samples were collected during
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reconnaissance mapping and are considered rep- by more than the C. V. are considered dis- resentative of the plutonic units in the vicinity. cordant. In general, the C. V. for ages measured Modal analyses were determined by point in this study is slightly more than 4 percent. counting on stained rock slabs (Reed and Lanphere, 1969; Laniz and others, 1964). These GEOLOGIC SETTING AND analyses were used to classify the rocks after STRUCTURE OF THE ALASKA- the system used by Bateman and others (1963). ALEUTIAN RANGE BATHOLITH Where the age relations of intrusive rock The Alaska-Aleutian Range batholith lies on masses seem well established and the masses the northern part of the circum-Pacific have well-defined boundaries and character- orogenic belt and is part of the chain of istic chemical and petrologic properties, an ap- elongate regional granitic batholiths that ex- propriate geographic name is prefixed by the tend south along the Coast Ranges of British dominant lithology of the mass. The term Columbia and into California and Mexico. "sequence" is used where intrusive masses have As is characteristic of this belt, the Alaska and a similar age and similar chemical and petrologic Aleutian Ranges have undergone repeated de- properties but occur either as separate masses formation, batholithic intrusion, orogenic sedi- in the same general area, or as a large mass with mentation, and volcanism during Mesozoic and less well-defined boundaries. The term "rocks" Cenozoic times. The Alaska-Aleutian Range is applied to plutonic masses that are of the batholith trends northeast, essentially parallel same general age but whose chemical properties to the mountain ranges (Fig. 1). The main are not necessarily uniform and whose geo- batholithic mass is about 500 km long and 16 graphic distribution is uncertain. to 80 km wide. The apparent narrowing of the Argon measurements were made by isotope batholith in the Aleutian Range is due to a dilution using equipment and techniques cover of Tertiary volcanic rocks on the west described previously (Dalrymple and Lan- side. At about 62° N. lat, the main batholithic phere, 1969). Potassium was measured by mass terminates rather abruptly. North and flame photometry using a lithium internal northeastward from this point, into the Mount standard. The plus-or-minus value assigned McKinley area, plutonic rocks occur as iso- each age measurement (Table 1) is an estimate lated bodies that range in area from less than 50 of the standard deviation of analytical precision to more than 300 sq km. The presence of these using the method of Cox and Dalrymple small granitic plutons and their similarity in (1967), together with an estimate of accuracy chemistry and age (early Tertiary) to the based on evaluation of the uncertainties in the plutonic rocks of the northern part of the main isotopic composition and in concentration of batholith suggest that the region north and the Ar38 tracer and flame photometer standards. northeast of the main batholith may be under- Complete data for replicate analyses and lain at shallow depths by granitic rocks. sample locations were reported previously The Alaska-Aleutian Range batholith is a (Reed and Lanphere, 1972). composite body composed chiefly of quartz- In order to establish ages of emplacement of bearing rocks that range in composition from plutonic units, we depend heavily on con- gabbro to granite. The granitic rocks occur as cordant ages of coexisting hornblende and discrete masses that generally are in sharp biotite. A decision on whether ages are con- contact with one another. These sharp con- cordant was made using an objective criterion, tacts, as well as wall-rock deformation caused the critical value (C. V.) test (Mclntyre, 1963). by forceful emplacement of plutons, chilled Some workers have defined concordant ages border zones, and inclusions of wall rocks as those that differ by less than some arbitrary along the margins of plutons, indicate that the amount, regardless of the age, but this proce- plutons rose from below as magma. Small dure clearly is unsatisfactory because precision masses of metamorphosed country rock are is a percentage value rather than an absolute scattered throughout the batholith as in- amount. The C. V. is determined by comparing clusions, and three volcanoes that have been analytical precision of mineral ages (excluding active in the Holocene—Mount Iliamna, uncertainties in concentration of the Ar38 Mount Redoubt, and Mount Spurr—cut the tracer and flame photometer standards that batholith. In general, the older plutonic rocks affect accuracy, not precision) at the 95 percent (Jurassic) are more mafic than the Cretaceous confidence level. Ages of minerals that differ and Tertiary plutonic rocks, and, in the part
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Tup Unossigned plutonic rocks TKme Granodiorite of Mount £stelle
Merrill Pass sequence TKy Yenlno sequence Granodiorite of Mount Foraker "-TKsl1 Summit Lake rocks Ts Snowcap sequence Undivided plutonic rocks
TKmk Mc Kinley sequence
TKt Quartz monzonite of Tired Pup Quartz diorite and granodiorite northwest of Cook Inlet Crystal Creek sequence Sedimentary and volcanic rocks of Paleozoic to Tertiary age TKh Hartman sequence
Figure 2. Generalized geologic map of the Alaska- age (SV) define the limits of the Alaska and Aleutian Aleutian Range batholith and related plutonic bodies. Mountain Ranges. Southern portion (left page) and Sedimentary and volcanic rocks of Paleozoic to Tertiary northern portion (right page) are outlined on Figure 1.
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Tup Middle Tertiary TERTIARY Tmp Tf Ts
TKmk
TKt Lower Tertiary and (or) TKc Upper Creta- TKh TKme ceous TKy
—TKsl^ lorn and Ss Hiddtt JURASSIC Jurassic SV
Figure 2. (Continued).
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o* A CRETACEOUS MIDDLE AND TERTIARY EARLY TERTIi,RY CHEMICAL ANALYSES AND AGE DATE Solid symbols denote established age, open symbols denote inferred age, numbers refer to mop numbers given in Tobte /
100 Km
Figure 3. Maps showing locations of samples used tacts correspond to scratch boundaries between units on for age measurements and chemical analyses. Areas of Figure 2. maps are the same as those of Figure 2. Dashed con- of the batholith south of Chakachamna Lake of Early Jurassic age that belong to a complex where sampling has been relatively more de- volcanoplutonic arc extending from southwest tailed, plutonic rocks in the eastern part of the of Becharof Lake into the Talkeetna Moun- batholith appear to be more mafic than those in tains, which lie east of the Alaska-Aleutian the western part. Range batholith (Fig. 1). The Jurassic rocks of In the southern part of the batholith, con- the batholith were emplaced within and tacts tend to be concordant with the enclosing beneath a consanguineous volcanic sequence, country rocks, and the larger plutons are the Talkeetna Formation. Although no fossils elongate in a northeasterly direction, parallel to have been found in the Talkeetna Formation the long axis of the batholith. In the northern west of Cook Inlet, the formation is correlated part of the batholith, contacts tend to be sharp with a similar sequence of marine sedimentary and discordant, and the long axes of individual and volcanic rocks in the Talkeetna Mountains plutons trend in a northerly direction trans- containing a fauna of Sinemurian, Pliens- verse to the regional northeast strike of the bachian, and Toarcian (Early Jurassic) age country rocks. (Grantz and others, 1963a). West of Cook In the area from about 61° N. lat southward, Inlet, this 1,800 to 2,700-m- thick formation is a most of the batholith consists of plutonic rocks marine sequence composed chiefly of andesite
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Figure 3. (continued).
lavas and volcanoclastic strata; graywacke detritus. This trough, interpreted as an arc- sandstone and shale are present in lesser trench gap, is discussed later in the paper. amounts (Detterman and Hartsock, 1966). On the northwest side of the batholith, These rocks are characteristic of orogenic belts from about 61° N. lat northward to a major of continental margins. The prebatholithic fault (shown by the dashed line, Fig. 1), the country rocks are highly deformed locally, batholith is flanked by a thick eugeosynclinal probably due to forceful emplacement of the sequence of dark-colored graywacke sandstone, batholith. Folds and foliation of country rocks slate, and argillite. Fossils have not been found trend northeast and generally conform to the in this sequence, but on the basis of lithologic contact of the plutonic rocks. Contact and similarity and depositional environment, the dynamothermal metamorphism have locally re- rocks are thought to be correlative with the crystallized the rocks to upper greenschist and Kuskokwim Group of Cretaceous age in south- lower almandine-amphibolite facies. Uplift western Alaska (Cady and others, 1955). took place intermittently during this period A major fault (dashed line, Fig. 1) that of volcanism and plutonism, and in Middle trends northeast into the Denali fault appears Jurassic time, an elongate arcuate trough, the to be the oldest fundamental break of the Matanuska geosyncline, received more than Denali fault system in the southern Alaska 4.5 km of first-cycle plutonic and volcanic Range. This fault juxtaposes Mesozoic eugeo-
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K 0 2 Är?äd Calculated age' ip no. Mineral (percent) (10-'° mol/gm) (m.y.)
Biotite 8.70 3.512 27.1 ± 0.6 34 Hornblende 0.370 0.1402 25.4 ± 1.0 35 Hornblende 0.358 0.1384 26.0 ± 0.9 36 Biotite 8.20 3.465 28.4 ± 0.9 Biotite 8.16 3.176 26.2 ± 0.8 37 Hornblende 0.338 0.1256 25.0 ± 1.0 38 Hornblende 0.162 0.4266 171. ±8.6 Biotite 6.40 12.95 132. ± 5.0 39 Hornblende 0.992 2.247 147. t 4.5 Biotite 6.80 15.95 152. ± 4.6 40 Hornblende 0.706 1.670 154. ± 4.6 Biotite 6.45 15.37 155. ± 4.6 41 Muscovite 9.30 22.45 157. ± 4.7 42 Biotite 8.92 12.90 95.4 1 2.8 Biotite 8.84 21.69 159. ± 4.7 43 Hornblende 0.938 2.279 158. ± 4.7 Biotite 7.39 18.33 161. ± 4.8 44 Hornblende 0.632 1.545 159. + 4.8 B1otite 8.44 7.805 61.6 ± 1.8 45 Hornblende 0.984 0.9970 67.4 ± 2.0 Biotite 9.21 8.857 64.0 ± 1.8 46 Hornblende 0.826 0.8333 67.1 ± 2.0 Biotite 8.88 5.088 38.4 ± 1.1 47 Hornblende 0.667 0.4066 40.8 ± 1.3 48 Biotite 8.28 4.187 33.9 ± 1.0 Biotite 6.82 49 4.954 37.6 ± 1.1 Hornblende 0.295 0.1688 38.3 ± 1.9 Biotite 8.70 50 9.253 70.7 ± 2.0 Hornblende 0.852 1.207 93.6 + 2.8 51 Biotite 8.94 8.94 64.9 ± 1.9 Biotite 8.74 52 8.616 65.6 ± 1.9 Hornblende 0.732 0.7684 69.7 ± 2.1 Biotite 9.42 9.684 68.4 ± 2.0 53 Hornblende 1.033 1.129 72.5 ± 2.2 54 Hornblende 0.537 1.173 142. t 4.3 Biotite 8.88 8.508 63.8 ± 1.8 55 hornblende 0.838 0.8308 65.9 ± 2.0 Biotite 9.14 56 8.726 63.5 ± 1.9 Hornblende 0.784 0.8117 68.8 ± 2.0 Biotite 9.18 57 7.898 57.4 ± 1.7 Hornblende 0.731 1.054 95.1 ± 3.8 58 Biotite 8.72 4.754 36.5 ± 1.0 59 Biotite 8.80 4.941 37.6 ± 1.1 Biotite 9.02 4.178 60 31.1 ± 0.9 Hornblende 0.476 0.2508 35.4 ± 1.8 61 Biotite 8.58 4.309 33.7 ± 1.3 62 Biotite 8.75 4.705 36.1 i 1.0 Biotite 9.23 8.113 58.6 ± 1.7 63 Hornblende 0.895 1.064 78.8 ± 2.3 Biotite 6.88 6.477 62.6 ± 1.8 64 Hornblende 0.874 0.8517 64.8 t 1.9 Biotite 9.18 9.613 69.6 ± 2.0 65 Hornblende 0.926 0.9602 68.9 ± 2.1 Biotite 9.54 8.712 66 60.8 ± 1.8 Hornblende 0.694 0.6454 61.9 ± 1.9 Biotite 8.64 8.159 67 62.9 ± 1.8 Hornblende 0.6«; 0.7108 68.6 ± 2.1 68 Biotite 5.42 4.631 57.0 ± 1.7 Biotite 9.01 7.169 69 53.1 ± 1.6 Hornblende 0.569 0.5438 63.6 ± 1.9
uo 10 1 Decay constants for y. : X£ = 0.555 x JO" yr" . 10 l Xg = 4.72 * 20" yr~ . Atomic abundance of JC1*0 = 1.19 x 10~U. Potassium measurements: L. B. Schlocker. Argon Measurements and age calculations: M. A. Lanphzre and J. C. Von Essen. The ± figures are estimates cf analytical precision c.t the 68 percent confidence level.
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K 0 Ar1*0 2 rad Calculated age* Map no. Mineral (percent) (10"11 mol/gm) (m.y.)
70 Biotite 8.68 7.241 55.7 ± 1.6 71 Hornblende 0.799 1.157 95.6 ± 3.7 B1otite 8.90 7.761 58.1 + 1.7 72 Hornblende 0.946 0.8179 57.6 ± 1.7 Biotite 8.94 7.148 53.4 ± 2.1 73 Hornblende 0.854 0.7588 59.2 + 2.3 74 Biotite 8.77 6.905 52.6 ± 1.5 75 Biotite 8.50 4.395 34.7 + 1.1 76 Biotite 8.60 4.568 35.6 ± 1.0 77 Biotite 8.88 8.329 62.4 ± 1.8 78 Biotite 8.75 7.491 57.1 ± 1.6 79 Biotite 8.44 7.046 55.6 ± 1.1 80 Biotite S. 30 5.537 39.9 ± 1.1 81 Biotite 8.42 4.269 34.0 t 1.0 62 Biotite 8.98 5.420 40.4 ± 1.1 83 Biotite 8.30 4.458 36.0 ± 1.0 84 Biotite 7.98 4.372 36.7 ± 1.1 Biotite 7.36 6.533 59.1 ± 1.8 85 Hornblende 0.639 0.5740 59.8 ± 1.9 86 Biotite 7.99 7.253 60.5 ± 1.6 Biotite 7.16 6.472 60.2 ± 1.8 87 Hornblende 0.943 0.7851 56.1 ± 1.7 88 Biotite 8.25 6.931 56.0 ± 1.6 89 Hornblende 0.700 1.137 107. ± 3.2 Biotite 6.06 5.139 56.5 i 1.6 SO Hornblende 0.807 0.7107 58.7 i 1.8 91 Biotite 6.51 5.694 58.3 ± 1.7 92 Biotite 3.54 3.134 59.0 ± 2.4 Biotite 6.80 6.189 60.7 i 1.8 93 Hornblende 0.344 0.3237 62.6 ± 2.5 Biotite 8.01 7.528 62.6 ± 1.8 94 Hornblende 0.402 0.3658 60.5 t 1.9 Biotite 7.67 6.732 58.5 i 1.7 95 Hornblende 1.10 0.9664 58.4 ± 2.9 96 Biotite 9.03 9.474 69.7 i 2.0 97 Hornblende 1.061 1.185 74.1 ± 2.2 98 Biotite 8.52 7.374 57.7 ± 1.6 99 Biotite 8.80 5.135 39.1 ± 1.1 100 Biotite 8.44 6.998 55.3 ± 1.6 Biotite 8.88 3.413 25.8 ± 0.8 101 Hornblende 0.906 0.3499 26.0 i 0.9 102 Biotite 8.69 3.247 25.1 i 0.8 103 Biotite 8.24 3.042 24.8 ± 0.7 104 Biotite 8.71 8.114 62.0 i 1.9 105 Biotite 9.20 4.922 35.9 ± 0.7 106 Biotite 8.72 8.541 65.1 ± 1.9 107 Biotite 8.94 9.074 67.4 ± 2.0 108 Biotite 8.70 8.613 65.9 ± 1.9 109 Biotite 8.62 3.672 28.6 ± 0.8 110 Biotite 8.26 3.703 30.1 + 0.9 111 Hornblende 0.972 0.4192 29.0 ± 0.9 112 Biotite 8.25 6.930 56.0 + 1.6 113 Biotite 8.51 7.443 58.3 ± 1.6 114 Hornblende 0.428 0.2449 38.3 1 1.9 115 Biotite 8.54 8.290 65.5 ± 1.9
1 0 10 1 Decay constants for K * : X£ = 0.585 x 10~ yr" . 10 1 Aft = 4.72 x lO' yr" .
Atomic abundance of k1,0 » 1.19 x 10~l>. Potassium measurements : I. B. Schlocker. Argon measurements and age calculations: M. A. Lanphere and J. C. Von Essen. The ± figures are estimates of analytical precision at the 68 percent confidence level.
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TABLE 1 {cjr.tinusd}
M Ar"° . raa Map no. Mineral Calculated age* (percent) (10 "10 mol/gm) (m.y.)
116 Biotite 8.91 7.509 56.2 ± 1.6 117 Biotite 8.80 8.545 64.6 ± 1.8 118 Biotite 8.91 7.371 55.2 ± 1.6 Biotite 8.68 119 7.255 55.7 i 1.6 Muscovite 10.51 8.228 52.3 ± 1.6 120 Biotite 8.28 7.009 56.5 ± 1.6 121 Biotite 8.37 7.048 56.2 ± 1.5 122 Biotite 8.27 6.902 55.7 ± 1.5 123 Biotite 9.34 7.739 55.3 ± 1.6 Biotite 9.14 124 4.464 32.8 ± 1.0 Hornblende 0.626 0.3290 35.2 ± 1.4 Biotite 9.23 125 4.459 32.4 ± 0.9 Hornblende 0.401 0.2115 35.4 ± 1.6 Biotite 8.64 126 4.424 34.3 ± 0.9 Hornblende 0.312 0.1661 35.6 ± 1.4 Biotite 8.99 127 4.620 34.5 ± 1.0 Hornblende 0.308 0.4800 38.5 ± 1.4 Biotite 6.36 128 3.430 35.8 ± 1.1 Hornblende 0.365 0.2116 38.8 i 1.6 129 Biotite 8.87 7.446 56.0 ± 1.6 130 Biotite 8.19 6.892 56.1 ± 1.6
1 0 10 1 Decay constants for K * : X£ = 0.585 x 10~ yr~" . Xg » 4.72 x 10'" yr'1.
Atomic abundance of K1*0 = 1.19 x 10"1*. Potassium measurements.- L. B. Schlocker.
Argon measurements and age calculations. M. A. Lanphere tnd J. C. Von Essen. The ± figures are estimates of analytical precision at the 6b percent confidence level.
synclinal rocks on the south against Paleozoic active during the Middle and Late Jurassic miogeosynclinal rocks to the north (Reed and and also during the Cretaceous period (Burk, Elliott, 1970) and may be an extension of the 1965). A minimum age fo:: movement in the ancient subduction zone in the eastern Alaska Iliamna Lake area is set by undeformed Range described by Richter and Jones (1970). Quaternary lava flows that overlie the fault Later transcurrent movement along this major (Detterman and Hartsock, 1966). There is structural break produced a new break, the no apparent offset of the outcrop pattern of the Farewell segment of the Denali fault system, a middle Tertiary stock that lies athwart the dextral fault which forms an escarpment on the fault east of Nonvianuk Lake (Fig. 2); the north flank of the Alaska Range and extends fault trace in this area, however, is covered, and southwest into the Holitna fault (Grantz, the lack of vertical movement within the past 1966). Eastward along the Denali fault, in the 25 m.y. is not proven. direction of Mount McKinley, Paleozoic rocks occur generally north of the fault and only SUBDIVISIONS OF THE ALASKA- locally are found south of the fault. ALEUTIAN RANGE BATHOLITH The Alaska-Aleutian Range batholith has The Alaska-Aleutian Range batholith and markedly influenced uplift and erosion of the related plutons comprise a suite of granitic mountain range. The highest and most rugged rocks that were intruded into the Earth's crust peaks are carved from parts of the range that over a period of approximately 155 m.y. The contain the greatest proportion of granitic batholith can be subdivided rather naturally rocks. The batholith has also influenced sub- into three major intrusive epochs of Early and sequent deformation of the region because of its Middle Jurassic, Late Cretaceous and early greater mechanical strength. For example, the Tertiary, and middle Tertiary age. This three- Bruin Bay fault, which can be traced for more fold subdivision was suggested previously (Reed than 300 km from Becharof Lake to the area and Lanphere, 1969), and our subsequent of Tuxedni Bay, is a high-angle reverse fault studies have not required any significant that is coincident with the eastern limit oE revision. granitic rocks. This fault appears to have been Each intrusive epoch will be discussed
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separately in following sections. Features that Becharof Lake and Cook Inlet and extend will be discussed include age and distribution northeastward along the southeast side of the of plutonic rocks of a given epoch and their batholith northwest of Cook Inlet. The oldest petrology, contact relations, structure, and concordant pair of ages, 176 m.y. on biotite gross chemical composition. and 172 m.y. on hornblende, was measured on Certain criteria were used in interpreting the granodiorite (sample 1, Fig. 3) from the south- pattern of these ages. In most cases for those westernmost exposure in the batholith. The plutonic rocks which yielded concordant ages oldest age measured on Jurassic plutonic rocks on coexisting minerals, the measured age is is a 179-m.y. potassium-argon date on horn- assumed to closely approximate the time of blende from a hornblende gabbro, a "basic crystallization of the rock from a melt. As forerunner" of the batholith southeast of there is no persuasive evidence for the presence Iliamna Lake (sample 9, Fig. 3). This age in- of excess radiogenic argon in micas or horn- dicates that plutonism was coeval with the blendes from plutonic rocks such as those from accumulation of at least part of the Talkeetna the Alaska-Aleutian Range batholith (see Formation. We interpret the younger limit summary in Dalrymple and Lanphere, 1969), for emplacement of Jurassic plutonic rocks to the measured ages are interpreted to be no be about 155 m.y. based on concordant horn- older than the time of emplacement of the blende and biotite ages of 154 and 152 m.y. granitic melts. Both long cooling history or re- from a quartz diorite on the west side of the heating caused by younger intrusions or meta- batholith south of Iliamna Lake (sample 40, morphism probably would produce discordant Fig. 3). Younger ages have been measured on mineral ages; the older age measured on a pair several samples of plutonic rock (Fig. 3; of coexisting minerals with discordant ages is samples 17 to 20, 28, 42, 50, 54, 57, 63, 71 to interpreted as a minimum age for crystallization 73, and 89) that we infer to be of Jurassic age. of the rock. These samples show marked similarity in chemistry and petrology to undoubted Jurassic Chemical data for the plutonic rocks are not rocks, and generally either have discordant presented in tabular form, but are plotted in ages for coexisting biotite and hornblende, or various types of diagrams that emphasize are in close proximity to Cretaceous or similarities and differences among plutonic Tertiary plutons whose intrusion presumably rocks of the various intrusive epochs. Data for resulted in argon loss from the Jurassic rocks. some of the oxides of the various plutonic units The southern limit of exposed Jurassic rocks is are presented as SiC>2 (Harker) diagrams and as Becharof Lake, but the batholith (and prob- triangular diagrams on which normative quartz, ably volcanoplutonic arc, although the strongly albite plus anorthite, and orthoclase are plotted. volcanic character of Early Jurassic rocks Modal data are presented on ternary diagrams diminishes southwest of Becharof Lake) must on which the abundances of quartz, K-feldspar, have extended another 480 km to the southwest and plagioclase are plotted. Modal analyses for because coarse Upper Jurassic and Lower each of the plutonic units are also presented as Cretaceous arkosic sediments with abundant mineral abundance diagrams in which quartz, hornblende and biotite as accessory minerals plagioclase, K-feldspar, and mafic minerals indicate that a granite source (not now ex- (biotite and hornblende) are graphically rep- posed) lay northwest of the present axis of the resented. As accessory minerals are grouped Alaska Peninsula (Burk, 1965, p. 44, 73). We with the mafic minerals, the mineral abundance also believe, as discussed later in this report, diagrams do not precisely represent the modal that the belt of positive magnetic anomalies in analyses. The accessory minerals, however, the Bering Shelf (Pratt and others, 1972) re- rarely exceed 1 percent and their inclusion with flects the arcuate continuation of Jurassic the mafic minerals does not materially affect plutonic rocks more than 600 km southwest of the diagrams at the scale presented. Becharof Lake. Thus, Jurassic plutonism must have occurred within a curvilinear belt that Jurassic Plutonism extended from at least 480 km southwest of The oldest recognized period of plutonism Becharof Lake northeast to the Talkeetna in the Alaska-Aleutian Range began in the Mountains (Grantz and others, 1963a; Reed Early Jurassic, about 175 m.y. ago, and con- and Lanphere, 1969, 1973), a distance of more tinued for about 20 m.y. (Fig. 4). Rocks of this than 1,300 km. Tertiary and Quaternary de- age make up most of the batholith between
Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/8/2583/3418107/i0016-7606-84-8-2583.pdf by guest on 26 September 2021 20T—, 103 , | ° n m0 MP MF 102 37 124 s \ Plutonic rocks of 6' 75 48 060 8 ï Ï o. j middle Tertiary age rag Ss8 o|05 125 126 40 59 ¡¡47 °26 83 Quartz diorite and 49 gronodiorite Quartz monzonite and granite 73
A Plutonic rocks 60 of Late Creta- •77 en ceous and early 66 er 50 Tertiary age IFM -H 27 < 65 LU >- 80 SL ME H Y Li. o
1 A CO 42 100 EXPLANATION »89 INFERRED JURASSIC SAMPLE NUMBER MINERAL PAIRS ANALYZED ON SAME SAMPLE 120 PLUTONIC ROCKS (Believed to be affected Solid symbols hornblende-, by younger plulons) open symbols, biotite; M -muscovite
O 140 n P northeast prong ME gronodiorite of Mount Estelle A 20M MP Merrill Pass sequence H Hartman sequence 40 MF gronodiorite of Mount Foraker 1 S «I lu 11 Y Yentna sequence ì° M • gS M 43 WF Windy Fork body TP quartz monzonite of Tired Pup 16 160 — 4 9 K ^ „P N l_ nunrt7 Hinrito nnrt nrnnndinritp QC Crystal Creek sequence 2b I 44 32 5 east of Novianuk^Lake M McKinley sequence I . s 15 S Snowcap sequence 38 21 C undivided plutonic rocks west L SL Summit Lake rocks of Chakachamna Lake 180
Figure 4. Diagrammatic representation of mineral ages for the Jurassic, Late Cretaceous and early Tertiary, and middle Tertiary plutonic rocks shown on Figure 2.
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posits cover the suspected continuation of the A large block of diorite to granodiorite that Jurassic batholith across the Cook Inlet-Susitna extends southward from Chakachamna Lake is lowland, but sample 32 (Fig. 3) proves that considered to be of Jurassic age because it is Jurassic plutonic rocks are iocally present in this composed of rocks that are chemically and area, and aeromagnetic data (Grantz and petrologically similar to the belt of Jurassic others, 1963b) suggest that plutonic rocks rocks to the south. Biotite-hornblende pairs extend at least part way across the lowland. from dated specimens (samples 28, 50, 57, 63, Plutonism was coeval with, and closely fol- 73, Fig. 3) yielded only discordant ages, and lowed, accumulation of the thick lower Jurassic clear evidence of reheating is shown east of the volcanic sequence, and both rock types are Merrill Pass sequence, where differences in intimately associated from Becharof Lake to ages between coexisting biotite and horn- the Talkeetna Mountains. Although plutonism blende are as much as 38 m.y. A maximum age was confined to a narrow northeast-trending of 95 m.y. does not correlate with any plutonic curvilinear belt, the Lower Jurassic eugeo- event presently known in this part of Alaska. synclinal sequence covers a much larger area; One specimen (sample no. 72, Fig. 3) yielded and rocks believed equivalent to it are present concordant ages of about 58 m.y., but this age in the Alaska Range southwest of Mount is interpreted to reflect reheating of Jurassic McKinley (Reed and Eberlein, 1972), an area rocks above the temperature of closure for where Jurassic intrusive rocks have not been argon during emplacement of the early Tertiary found. Crystal Creek sequence. We believe that prior to intrusion of the Late Cretaceous and Jurassic plutonism cannot be subdivided into Tertiary plutonic rocks, these rocks were con- individual pulses, but the relatively long (20 to tinuous with the main mass of Jurassic rocks. A 25 m.y.) time interval defined by mineral ages single K-Ar age of 107 m.y. (sample 89, Fig. 3) suggests that magma generation probably was was measured on a hornblende-pyroxene episodic during this interval. A short time for syenite from the undivided Tertiary plutonic crystallization and unroofing of magma cham- rocks northeast of Chakachamna Lake. This bers is suggested by the presence of granitic apparently is a small body, and we consider detritus in units of early Middle Jurassic age the syenite to be a roof pendant of Jurassic which unconformably overlie the Talkeetna plutonic rocks. Hornblende from this syenite Formation (Detterman and others, 1965). apparently lost some argon during Tertiary Granitic boulders of Jurassic age (Grantz and plutonism. others, 1963a; Detterman and others, 1965) are very common in Upper Jurassic (lower Ox- Samples of Jurassic plutonic rocks from the fordian) conglomerate, indicating that part of southern end of the batholith are chiefly the Jurassic plutonic rocks were intruded and granodiorite (Reed and Lanphere, 1969, p. 32), unroofed within a few million years. The pat- but Burk (1965, p. 73) noted that "potassic tern of concordant mineral ages, however, in- granite" appears to be more abundant than dicates that other parts of the batholith other granitic lithologies in this area. The crystallized over a longer period of time, or presence of "granite" clasts in Upper Jurassic perhaps the rocks now exposed represent deeper and Lower Cretaceous arkosic sandstones and parts of the batholith that cooled more slowly. conglomerate indicates a granitic source con- Jurassic plutonic rocks range in composition taining potassium feldspar. from gabbro to quartz monzonite, but quartz Most Jurassic plutons vary in composition diorite and granodiorite are the most common internally, and wherever intrusive relations and compositions (Fig. 5). Only a few plutons of K-Ar ages have been determined within an quartz monzonite of this age have been intrusive sequence, the older plutons are the recognized. An apparently small body of most mafic and successively younger plutons biotite quartz monzonite (sample 31, Fig. 3) more felsic. Individual plutons range in outcrop intrudes rocks of the Talkeetna Formation on area from a few square kilometers to at least the east side of the batholith south of Chaka- 1,500 sq km. The limits of the larger plutons chamna Lake, and small bodies of quartz have not yet been delineated, but reconnais- monzonite, thought to be late differentiates of sance mapping indicates that they are elongate quartz diorite magma, are present near Mount parallel to the northeast regional strike of the Iliamna and Tuxedni Bay (Juhle, 1955; enclosing rocks. Smaller plutons are irregularly Grantz, 1956). round, and some are concentrically zoned.
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Contacts with country rock are generally 100 sharp and concordant, although intrusive rocks M locally cut across the bedding in the wall rocks. 50 Structural features of the wall rocks appear to
be the result of forceable emplacement of the Mafic and accessory Plogioclose granitic rocks, and no pre-existing structural minerols Quartz features in the country rocks have been K-Feldspor and perthite U7, 100 recognized. Primary foliation is locally well n developed near the contacts of the plutons. The mm so ; foliation parallels the contact and may extend w into the pluton for as much as 2 km. Contact migmatites are locally present. These features, when compared with those of younger plutons, indicate that, at the present level of exposure, many of the Jurassic plutonic rocks may be syntectonic, more forcefully intruded, and per- haps emplaced at greater depths than were the Cretaceous or Tertiary plutonic rocks. Some of the Jurassic plutonic rocks, however, must have crystallized at relatively shallow depths because the abundance of pyroclastic material pre- Plogioclose K-Feldspor served in the Lower Jurassic Talkeetna Forma- tion implies that some of the plutons vented to feed volcanic eruptions. The lack of direct evidence indicating shallow emplacement of Jurassic plutons is probably due to the fact that these are the oldest plutonic rocks in the batholith, and erosion has consequently cut deeper into their roots and removed their up- per, eruptive parts. Late Cretaceous and Early Tertiary Plutonism k, Concordant ages on coexisting biotite and Range batholith that began in Late Cretaceous £ time, about 74 m.y. ago (samples 22, 97, Fig. 3), and ended in early Tertiary time, about 58 m.y. ago (Fig. 4). The tight group of ages for 45 55 65 75 the McKinley sequence of plutons, which gen- , WEIGHT PERCENT erally contain only biotite, suggests, however, Figure 5. Mineral abundance, modal, and norma- that at least in this area, plutonism continued tive diagrams, 2nd plots ofK^O against SiC>2 for Jurassic until about 55 m.y. ago. The Knutson Bay plutonic rocks. intrusive north of Iliamna Lake (Reed and Lanphere, 1969) yielded slightly older mineral clear break in plutonism curing the interval ages (sample 22, Fig. 3), and the affinities of between 74 m.y. and 55 m.y. (Fig. 4). Mineral this pluton are not clear. Coexisting horn- pairs in six samples of Summit Lake rocks blende and biotite (sample 23, Fig. 3) from yielded discordant ages. In all six samples, the this pluton yielded ages of 83.4 and 80.7 m.y., hornblende age is older than the biotite age, respectively. Mineral ages on Late Cretaceous and this may reflect the cooling history of the and early Tertiary plutonic rocks south of Summit Lake rocks. An age of 40.4 m.y. was Chakachamna Lake, herein informally called measured on biotite from a gabbro (sample 82, the Summit Lake rocks, and isolated plutons Fig. 3), but this locality is close to a 35-m.y.-old west and northwest of Chakachamna Lake pluton, and we believe that the gabbro was (samples 27, 77, 82, 96, and 97, Fig. 3) show no affected by intrusion of the younger pluton. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/8/2583/3418107/i0016-7606-84-8-2583.pdf by guest on 26 September 2021 ALASKA-ALEUTIAN RANGE BATHOLITH 2585 The chemistry of the gabbro differs markedly from the chemistry of the adjacent Tertiary M VA granites. A contact between the Summit Lake and §tt 50 ki Jurassic rocks was not recognized during a. reconnaissance mapping, and plutonic rocks of Late Cretaceous and early Tertiary age may /jMofic ond occessory well occur along the northwest side of the A mmerols Plogiodose I K-Feldspor and Quortz Jurassic plutonic belt from the Iliamna Lake I perthite area to Chakachamna Lake and northeast to the 'M'WtZMt '//j'M SS6S0SSSSm Cook Inlet-Susitna lowland (sample 33, Fig. w OSS wn .. . -1 3) and southwestern Talkeetna Mountains 5 (Reed and Lanphere, 1969). The Summit Lake — 50 S (Fig. 2) comprise the Late Cretaceous and 55 60 65 70 early Tertiary quartz diorite and granodiorite. SiOj .WEIGHT PERCENT These rocks range in composition from quartz Figure 6. Mineral abundance, modal, and norma- diorite to quartz monzonite with a Si02 con- tive diagrams, and plots of K2O against Si02 for Late tent between 56 and 68 percent, averaging Cretaceous and early Tertiary Summit Lake rocks. about 65 percent (Fig. 7). On silica variation diagrams and normative plots, they do not Alk-F-M triangular diagrams (Fig. 8). The two form as tight a group as the early Tertiary rock categories also differ in age (Fig. 4). In- quartz monzonite and granite, but they are trusion of quartz diorite and granodiorite distinct and do not overlap the fields of quartz began about 67 m.y. ago and ended about 62 monzonite and granite when plotted on m.y. ago, whereas the oldest quartz monzonite normative quartz-plagioclase-orthoclase and and granite rocks are about 60 m.y. old. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/8/2583/3418107/i0016-7606-84-8-2583.pdf by guest on 26 September 2021 2584 REED AND LANPHERE Granodiorite of Mt Estelle Hartman Yentna South body North body sequence sequence lOOr HWg. 100 k. W/AW/jY/ß S $ Uj (o 50 El1 i i 0 50 1 I il • • •1 1 1I 1 1 HI ••I • • n 1• s Mafic and accessory • minerals Flagioclase K-Feldspar and Cuartz perthile 50/ Ab + An 50 Or Ab + An • North body • Hartman sequence o South body ° Yentna sequence H Z 6 6 - UJ O z SiO , WEIGHT PERCENT Si02, WEIGHT PERCENT Figure 7. Mineral abundance, modal, and norma- Cretaceous and early Tertiary quartz diorite and tive diagrams, and plots of K2O against Si02 for Late granodiorite. The two plutons which constitute the bearing quartz-sulfide veins s.re present (Reed granodiorite of Mount Estelle, which are and Elliott, 1970). Hornblende ages of about thought to be offset by a fault, are silicic 55 and 66 m.y. (Fig. 3, samples 106, 108) were granodiorites. The north pluton is slightly measured on rocks from the north pluton, finer grained (1 to 2.5 mm) than the south whereas concordant biotite and hornblende pluton (2 to 4 mm). The north pluton contains mineral pairs from the south pluton (Fig. 3, tourmaline as an accessory mineral, and gold- samples 93 and 94) indicate that it was em- Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/8/2583/3418107/i0016-7606-84-8-2583.pdf by guest on 26 September 2021 ALASKA-ALEUTIAN RANGE BATHOLITH 2585 Mount Estelle plutons. Farther west, the granodiorite of Mount Estelle is intruded by rocks of the Crystal Creek sequence. All of the Late Cretaceous and early Tertiary quartz monzonite and granite units have similar mineral compositions (Fig. 9). Biotite, the chief mafic mineral, rarely exceeds 10 volume percent. Hornblende is present as an accessory mineral only in the Crystal Creek sequence. Ratios of quartz to plagioclase and K-feldspar to plagioclase are approximately the f F (FiOtlF^O^UiOl 50^ same for the three felsic units. The chemical compositions of the three units are also similar. Their Si02 content averages about 74 percent, the alkali content about 8 percent. Normative plots and plots of K2O against Si02 (Fig. 9) show a remarkably tight grouping, particularly when the size of individual bodies and their broad geographic distribution is considered. • .. ¿Ik (Kfl'Naflt 50 w {MqO! These plutonic rocks were also emplaced within + Ouartz monjonile of Tired Pup ° Yenln(> sequence a relatively narrow time interval of about 5 * Crystal Creek sequence GfonodioMe of Mounl Estelle m.y. The total spread of potassium-argon ages a Mc Kmley sequence • north body on the quartz monzonite of Tired Pup is 2.4 • Horfmon sequence o south body m.y., and the total spread of biotite ages from Figure 8. Normative and Alk-F-M diagrams of Late the McKinley sequence is only 1.3 m.y. The Cretaceous and early Tertiary quartz diorite and Crystal Creek sequence and the undivided granodiorite, and quartz monzonite and granite. plutonic rocks to the east are slightly older and have a slightly greater age range; potassium- argon ages on these rocks range from 55.7 to placed about 62 m.y. ago. A single biotite age 60.5 m.y. Sample 74 (Fig. 3, Table 1) yields a of 62 m.y. (Fig. 3, sample 104) from the Hart- single biotite age of 52.6 m.y. This sample lies man sequence suggests that this north-trending close to the middle Tertiary Merrill Pass se- group of small stocks was emplaced about the quence, and we believe the slightly younger age same time as the southern Mount Estelle reflects argon loss caused by emplacement of pluton. Biotite ages from the mafic stocks of the younger body. the Yentna sequence (Fig. 3, samples 107, 115, 117) range from 64.4 to 67.4 m.y., indicating The ages for the quartz monzonite and that this sequence is about the same age as the granite, and for the quartz diorite and granodi- northern Mount Estelle pluton. The Yentna orite in the northern part of the Alaska- sequence locally displays primary lineation of Aleutian Range batholith do not overlap but hornblende. Structures of the flow stage are not do fall within a relatively narrow period of commonly present in the other quartz diorite time. This permits one to speculate on the and granodiorite bodies. possibility that these rocks have been derived The quartz monzonite of Tired Pup, the from the same parental source materials with Crystal Creek sequence, and the McKinley the quartz monzonite and granite being dif- sequence comprise the early Tertiary quartz ferentiates of the more mafic rocks. However, monzonite and granite. The undivided plutonic the compositional fields of the two suites do not rocks east of the Crystal Creek sequence in the overlap (Fig. 8), and the trends of the two northeast part of the batholith (Fig. 2) are suites on a K20/Si02 plot (Fig. 10) do not ap- chiefly quartz monzonite and granite and have pear to project to a similar parental magma been included with the Crystal Creek sequence composition. This evidence suggests that each on the diagrams of Figures 8 and 9. The un- group represents a different comagmatic suite. divided Tertiary plutonic rocks, however, in- Two plutons of the McKinley sequence are clude areas of granodiorite and quartz diorite different in their mineral and trace-element which are cut by the more felsic rocks. No content. The mass southwest of Mount Mc- mineral ages are available from these mafic Kinley (Fig. 3, samples 118 and 119) is a two- rocks, but they may be equivalent in age to the mica granite. The small pluton north of the Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/8/2583/3418107/i0016-7606-84-8-2583.pdf by guest on 26 September 2021 McKinley sequence Quartz monzonite of Tired Pup Crystal Creek sequence L_ 1 ' ' Quartz \50 I I // • \ Plogioclose 50 K-Feldspar 0 50/ / \ / \ \ / v Ab-"An Or Ab»An ^ 6 - ^ 6 - S fc ! £ • ci ^ 5 < < h ^ • * V i * « ** | 4 |4 • • cT Ci" • CT • 1 1 1 l 80 70 80 70 80 70 Si0? , weight percent SiOj, weight percent Si02, weight percent Figure 9. Mineral abundance, modal, and normative diagrams and plots of K20 against Si02 for Late Cretaceous and early Tertiary quartz monzonite and granite. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/8/2583/3418107/i0016-7606-84-8-2583.pdf by guest on 26 September 2021 ALASKA-ALEUTIAN RANGE BATHOLITH 2585 Denali fault (Fig. 3, sample 129) can be classified as a "tin granite" on the basis of its trace-element content. Semiquantitative spec- • • trographic analyses of five samples of biotite v+ granite from this pluton gave 7 to 30 ppm Sn, +**>< 2 to 20 ppm Be, 15 to 30 ppm Nb, 100 to 500 XX « X ppm Y, and 200 to 500 ppm Li. Biotite from CDO oo° sample 129 contains 100 ppm Sn, 300 ppm Nb, SD 200 ppm Y, and 700 ppm Li. * .4 1 + Other isolated stocks of early Tertiary age «PI occur on the west side (Fig. 3, sample 25) and north of the main batholith (Fig. 3, samples 112 and 113). As only single biotite ages were determined for these bodies, they are mapped as undivided Cretaceous and Tertiary plutonic Si Op WEIGHT PERCENT rocks on Figure 2. + Quartz monzonite of Tired Pup QUARTZ MONZONITE o Crystal Creek sequence AND GRANITE Middle Tertiary Plutonism x Mc Kinley sequence Plutonic rocks of middle Tertiary age are • Yetna sequence QUARTZ DIORITE • Hortman sequence presently known in three areas: the northern AND GRANODIORITE part of the Alaska-Aleutian Range batholith, Granodiorite of Mount Estelle in the Mount McKinley area, and on the Figure 10. KjO plotted against SÍO2 for the Late Alaska Peninsula (Fig. 2). Although individual Cretaceous and early Tertiary quartz diorite and plutons have rather uniform chemical and granodiorite and quartz monzonite and granite in the modal compositions (Fig. 11), there is no northern part of the Alaska-Aleutian Range batholith. characteristic chemistry for this age group as a between 37.6 and 40.8 m.y. Rocks north of the whole. The rocks range in composition from dashed contact (hereafter referred to as quartz diorite to granite. The largest area of "northern rocks") rarely contain hornblende middle Tertiary plutonic rocks is the Merrill and yield biotite ages between 33.7 and 36.7 Pass sequence, a 150-km-long elongate body of m.y. The position of the dashed contact be- quartz monzonite and granite on the north- tween the northern and southern rocks of the western side of the main batholith that yields Merrill Pass sequence is inferred from widely mineral ages ranging from 33.7 to 40.8 m.y. scattered sample localities. (Fig. 4). Modes of the Merrill Pass sequence Separate intrusive masses in the "northern are scattered across the quartz monzonite and rocks" were not observed, and we believe that granite fields, but norms show a relatively tight this part of the body represents a single in- group (Fig. 11). Some of the rocks are quite trusion. Biotite from the northern end of the fine grained, and they commonly contain "northern rocks" yields ages that are in gen- granophyric and perthitic intergrowths that eral 2 to 3 m.y. older than biotite from the make point counting difficult and tend to southern end of the northern rocks (Fig. 2; produce scatter in the modes. The most mafic Table 1). As this is the youngest pluton of ap- rock (Fig. 3, sample 60) is a hornblende- preciable size in this part of the Alaska Range, biotite quartz diorite collected adjacent to a the difference in mineral ages cannot be at- septum of metavolcanic rock (too small to tributed to reheating by younger plutons. Nor show at the scale of Fig. 2) and may reflect do the measured ages correlate with the alti- wall-rock contamination of a more felsic tudes of sample localities. There is, however, magma. geologic evidence indicating that the northern Two pulses of magma generation within the part of the "northern rocks" represents a Merrill Pass sequence (Figs. 4 and 11) are sug- higher level in the magma chamber than the gested by slight differences in mineralogy, southern part. The average grain size is 3 to 5 chemistry, and age of the rocks. Rocks south of mm in the southern part, and the grain size the dashed contact (Fig. 2) in the southern part decreases northward to 0.5 to 1 mm. Miarolitic of the body ("southern rocks" on Fig. 11) have cavities are abundant in the northern part of lower K2O contents, generally contain horn- the northern rocks, and there is a general north- blende as an accessory mineral, and yield ages ward increase of roof pendants. The roof Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/8/2583/3418107/i0016-7606-84-8-2583.pdf by guest on 26 September 2021 Merrill Poss sequence GronodioritE of "Northeast Mount Foroker prong" 100 100 I- ¡j M z m éjttpMI UJ ÌÌ o 50 50 j.— — Quartz diorite and granodiorite east Southerp rocks' I00r 100 o o cr £C UJ UJ Q. Q. Mafic and accessory minerals K-Feldspar and perthite Plagioclase K-Feldspar Plagioclase K- Feldspar Ab + An Ab + An Granodiorite of ^ Mount Foraker 6 H Southern rocks + Northeast prong Z o UJ 5 A Quartz diorite and z 5 O granodiorite a. UJ UJ o a. 4 o Windy Fork body ^U J 4^ 1- • Snowcap sequence + X 3 0 I UJ o s 2 a" • a 0" 01 I * 0 60 70 80 60 70 80 Si02, WEIGHT PERCENT Si02, WEIGHT PERCENT Figure 11. Mineral abundance, modal, and normative diagrams and plots of K2O against SiOj for middle Tertiary plutonic rocks. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/8/2583/3418107/i0016-7606-84-8-2583.pdf by guest on 26 September 2021 ALASKA-ALEUTIAN RANGE BATHOLITH 2585 pendants do not appear to be foundered Biotite granite in the northeast prong of the blocks, but rather parts of an irregular but once quartz monzonite of Tired Pup (Fig. 2) yields continuous roof that has been largely removed concordant biotite-hornblende ages of about by erosion. Reconnaissance mapping suggests 26 m.y.—or about 30 m.y. younger than the that the northern part of the Merrill Pass main part of the Tired Pup body. This is the sequence vented explosively, erupting lava and youngest plutonic event presently known in pyroclastic rocks. This initial phase appears to this part of the Alaska Range. There is little have been followed by cauldron subsidence and difference megascopically between biotite rise of magma through its own volcanic ejecta. granite in the northeast prong and biotite The genetic association with volcanic rocks granite of the Tired Pup pluton, and no con- that are cut by the pluton is further evidence tacts were seen during reconnaissance mapping that at the level of exposure, the northern part of the pluton. The contact shown on Figure 2 of the body was emplaced at a relatively shal- is simply drawn between sample locations. low depth and consequently cooled more Biotite is greenish brown in rocks from the rapidly than the southern part. This could re- northeast prong and reddish brown in the sult in older mineral ages for the northern part. Tired Pup rocks (R. L. Elliot, 1971, oral commun.), and rocks in the northeast prong are A single biotite age of 34 m.y. (Fig. 3, slightly enriched in SiC>2 (Figs. 9 and 11). An sample 81) from one of several small bodies of aeromagnetic survey of the Tired Pup pluton granodiorite, herein informally called the shows that the northeast prong has more Snowcap sequence, suggests that these rocks magnetic anomalies and that the anomalies are belong to the same period of magma emplace- greater in magnitude than in the main part of ment as the Merrill Pass sequence. the Tired Pup body (Reed and Anderson, In the Mount McKinley area, the grano- 1969). This was attributed to a higher con- diorite of Mount Foraker, which is truncated centration of ferromagnesian minerals in the on its north side by the Denali fault, yields northeast prong (Reed and Anderson, 1969, p. concordant biotite-hornblende ages of about 5), although no significant difference in fer- 35 m.y. and some slightly discordant ages romagnesian mineral content is apparent in the with hornblende as old as about 39 m.y. samples collected. Biotite ages of about 39 Biotite-hornblende-granodiorite and quartz m.y. in other parts of the Tired Pup pluton diorite are common in the Mount Foraker (samples 80 and 99, Fig. 3) are believed to body, which is chemically and modally quite result from reheating of Tired Pup rocks during distinct from the McKinley sequence, which is intrusion of the younger rocks of the northeast chiefly biotite granite (Figs. 9 and 11). The prong. elongate pluton north of the fault (northeast of the granodiorite of Mount Foraker), called Two pulses of middle Tertiary plutonism are the McGonagall batholith, is reported to be a present on the Alaska Peninsula east of quartz diorite (Reed, 1961, p. 16). The Mc- Nonvianuk Lake. Concordant biotite-horn- Gonagall batholith, shown as unassigned mid- blende ages on sample 8 (Fig. 3, Table 1) dle Tertiary plutonic rocks on Figure 2, is indicate that the earlier pulse took place about truncated on the south by the Denali fault and 36 m.y. ago, and biotite and hornblende ages on was once part of the Mount Foraker body that six samples (Fig. 3, samples 6, 7, and 34 through has subsequently undergone about 38 km 37) indicate the younger pulse occurred 26 to of right-lateral displacement along the fault 28 m.y. ago. The biotite ages of three samples (Reed and Lanphere, unpub. data). (samples 6, 34, and 37, Fig. 3) are older than A small body of granite (Windy Fork body the hornblende ages, and even though these on Fig. 11) north of the quartz monzonite of ages are discordant, the differences between Tired Pup yields biotite and hornblende ages mineral pairs are not significantly greater than ranging from 28.6 to 30.1 m.y. (samples 109— the precision of the individual mineral ages. 111, Fig. 3). This granite has anomalously high The middle Tertiary age of these rocks con- contents of Be, Sn, and Nb. Eudialite, a firms the age suggested by Burk (1965, p. 112), zirconium and rare-earth-bearing silicate, but petrographic examination indicates that usually found in undersaturated rocks, is an magnetite is not an abundant accessory mineral abundant mineral in late-stage quartz-amphi- as Burk suggested was characteristic for mid- bole-K-feldspar pegmatite veins that cut the Tertiary plutons of the Alaska Peninsula. granite. Rocks of both ages include quartz diorite and Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/8/2583/3418107/i0016-7606-84-8-2583.pdf by guest on 26 September 2021 2584 REED AND LANPHERE granodiorite, and they have similar modal and eral pair, and the youngest age for a single min- chemical compositions (Fig. 11). There is no eral determination (Fig. 12). That is, if the apparent offset in the outcrop pattern of the beginning of an intrusive epoch is defined as the younger middle Tertiary stock by the Bruin oldest age for a single mineral determination or Bay fault; sample 37, which was collected tiie oldest age of a discordant mineral pair, and within 60 m of the fault trace, shows no effects the end of an intrusive epoch is defined as the of shearing and yields concordant biotite and hornblende ages of 26.2 and 25.0 m.y., re- 20 spectively. oo Only five samples of middle Tertiary MIDDLE plutonic rocks yielded concordant ages using TERTIARY °o the critical value test. Eight other samples o yielded slightly discordant ages. The largest 40 difference between biotite and hornblende ages is 4.3 m.y.; and for six of these samples, the hornblende age is older. The middle Tertiary plutons are the youngest intrusive rocks in the 60 QOn region. The difference in mineral ages may be Oo related to the cooling history of the plutons. SUMMARY OF INTRUSIVE EPOCHS K-C A large number of radiometric age measure- 80 ( LATE ments are needed in order to establish the CRETACEOUS ° evolution in time of composite batholiths, and AND EARLY these age measurements must be evaluated TERTIARY carefully in light of other geologic data, in- cluding the possibility of reheating of older ^100 rocks by younger plutons. This complex prob- e lem of timing of plutonic events is examined in greater detail elsewhere for the granitic UJ batholiths of circum-Pacific North America ^ 120 (Lanphere and Reed, in prep.). Only a brief summary of intrusive epochs in the Alaska- Aleutian Range batholith will be presented in this paper. The three intrusive epochs of the Alaska- 140 Aleutian Range batholith are defined on the basis of concordant potassium-argon ages for o coexisting minerals. Concordant ages are those r EARLY that meet the critical value test described 160 AND earlier in this paper. Maximum and minimum o concordant ages are taken to mark the be- MIDDLE o ginning and ending, respectively, of each epoch JURASSIC (Fig. 12). Although potassium-argon ages have been measured on 55 mineral pairs, only 27 of 180 these samples have ages that are concordant cn the basis of the critical value test. The three intrusive epochs defined by concordant min- eral ages are Early and Middle Jurassic (154 to 176 m.y.), Late Cretaceous and early Tertiary 200 (58 to 83 m.y.), and middle Tertiary (26 *;o Figure 12. Plutonic episodes in the Alaska-Aleutian 38 m.y.). The maximum and minimum Range batholith as interpreted from concordant K-Ar concordant ages of mineral pairs of the ii- ages on coexisting hornblende and biotite. O, oldest trusive epochs are, however, in close agreement age of a concordant mineral pair;...., oldest age for a single mineral determination or oldest age of a dis- with the oldest age of a single mineral deter- cordant mineral pair; , youngest age for a single mination, the oldest age of a discordant min- mineral determination. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/8/2583/3418107/i0016-7606-84-8-2583.pdf by guest on 26 September 2021 ALASKA-ALEUTIAN RANGE BATHOLITH 2585 youngest age of a single mineral from a rock Lanphere, in prep.). In the Alaska-Aleutian that does not seem to have been reheated, then Range batholith, Si02 decreases and AI2O3 in- the range of intrusive epochs is: Early and creases toward the Pacific margin, whereas Middle Jurassic (154 to 179 m.y.), Late Cre- there are no significant changes in the Si02 and taceous and early Tertiary (55 to 83 m.y.), and AI2O3 content across the Sierra Nevada middle Tertiary (25 to 41 m.y.). batholith (Bateman and Dodge, 1970). Trends Jurassic samples which have discordant ages for Fe203, FeO, CaO, MgO, Na20, H20, and are readily apparent on Figure 4, but the fact MnO in the two batholiths are quite similar. that many middle Tertiary samples have dis- The increase in potash away from the Pacific cordant mineral ages is not so obvious. This is margin is perhaps the most significant chemical because a difference in age of approximately 1.2 trend across circum-Pacific batholiths. A sys- m.y. between coexisting minerals from a tematic increase in potash content across vol- plutonic rock 30 m.y. old makes the ages dis- canic arcs led Dickinson (1970b) to suggest that cordant when the critical value test is applied. batholiths of the circum-Pacific region orig- Although the greatest difference in age between inated beneath volcanic chains and represent coexisting minerals in the middle Tertiary in- the eroded roots of arcs. There also is direct trusive epoch is 4.3 m.y., only 5 of 13 samples correlation of potash content of magmas to the meet the critical value test. For the Merrill depth of the inclined seismic zone beneath Pass sequence and the granodiorite of Mount volcanic arcs in the circum-Pacific region Foraker, the difference between the biotite (Dickinson, 1970a, 1972). The direction of ages and the hornblende ages, the latter being variation in potash, lime, and magnesia content slightly older, may reflect the cooling history of across the Alaska-Aleutian Range batholith is the plutons and the higher blocking tempera- similar to that across volcanic arcs (Hatherton, ture for argon diffusion from hornblende. 1969) which further suggests that an analogy Inspection of Figure 12 shows that plutonic exists between the batholith and volcanic arcs. rocks in the Alaska-Aleutian Range batholith The geologic evidence in Alaska indicates that were emplaced during discrete intrusive epochs the elongate belt of Jurassic plutonic rocks was and that the Mesozoic and Cenozoic plutonism once a part of a volcanoplutonic arc, and the was episodic but not periodic. This is in contrast prevalence of volcaniclastic strata in the to the periodic nature of magma generation eugeosynclinal sequences of Early Jurassic age proposed by Evernden and Kistler (1970) for suggests that the batholiths locally vented to the batholiths in the Sierra Nevada and south- feed surface eruptions. There is little doubt that ern California where the majority of con- some of the interior middle Tertiary plutons, cordant potassium-argon mineral ages fall be- for example, the Merrill Pass sequence, were tween 155 and 80 m.y. This suggests to us that emplaced at shallow depths and locally in- the timing and duration of intrusive episodes in truded their own volcanic ejecta. Plate tectonic the circum-Pacific batholiths of North America models offer a way to derive magma from are not broadly contemporaneous (Lanphere partial melting of the mantle or of descending and Reed, in prep.). crustal slabs, or both. However, as Bateman and Dodge (1970) have pointed out and as TECTONIC OVERVIEW recognized by Dickinson (1970a, p. 843), it is difficult to imagine a paleoseismic zone per- Chemical trends across batholiths in the sisting for more than 100 m.y. (about 140 m.y. circum-Pacific region have been noted by for the Alaska Range) with magma generation many workers (Buddington, 1927; Lindgren, taking place intermittently and at various 1915; Moore, 1959; Moore and others, 1963; depths along this zone. Hutchison, 1972). Recently Bateman and Dodge (1970) convincingly demonstrated that If batholiths represent the eroded deeper in the central part of the Sierra Nevada parts of volcanic chains, the best way to docu- batholith, K2O decreases systematically toward ment meaningful similarities in chemical trends the Pacific margin. In British Columbia, the across batholiths and volcanic chains is to plutons near the Pacific Ocean have trondhje- sample across a wide portion of an elongate belt mitic characteristics, and the inland plutons of plutonic rock of the same age. Although have calc-alkaline characteristics (Hutchison, chemical trends have been established for the 1972). Chemical data also show a systematic batholith south of Chakachamna Lake (Reed decrease in K20 across the Alaska-Aleutian and Lanphere, unpub. data), sample density is Range batholith toward the Pacific (Reed and not yet sufficient to demonstrate a true varia- Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/8/2583/3418107/i0016-7606-84-8-2583.pdf by guest on 26 September 2021 2584 REED AND LANPHERE tion for the Jurassic rocks. The best area tc taceous age 480 km southwest of Becharof Lake determine chemical variations across the belt of (Burk, 1965). rocks would be east of Lake Clark where the The trench associated with the Jurassic Jurassic belt is the widest (Fig. 2). Geologic magmatic arc is thought to be represented 140 mapping and sampling in this area have been km to the southeast by the early Mesozoic completed, and analytical work is in progress. pillow lavas and cherts, ultramafic bodies, and We believe the Jurassic plutonic rocks of the pervasively sheared slate and graywacke with Alaska-Aleutian Range batholith and associated interbedded greenstone and limestone on the Early Jurassic volcaniclastic strata represent southeast side of Cook Inlet near Seldovia the roots of an ancient magmatic arc of an (Fig. 2) (Martin, 1915). The structural and early Mesozoic arc-trench system. The mag- stratigraphic relations of these rocks are un- matic arc extended from the Talkeetna Moun- certain, but we infer that these rocks rep- tains down the Alaska Peninsula. West of resent an imbricated mélange of ophiolite, sub- Becharof Lake, the arc axis lies northwest of the marine lava with associated chert, and argillite present Alaska Peninsula; this axis trends which has been underthrust by a descending southwest directly into a broad arcuate belt cf oceanic plate. A similar melange is exposed positive magnetic anomalies on the Bering southwest of Seldovia along the northwestern Shelf 600 km southwest of Becharof Lake side of Kodiak Island (Capps, 1937). At described by Pratt and others (1972) who sug- Seldovia, these rocks an; in apparent fault gested that this belt represents a continuation contact with an interbedded sequence of of the Gemuk series exposed near Cape marbles, cherts, and schists which have been Newenham. This series contains a number of recrystallized to mineral assemblages of the intrusive bodies of granite, quartz diorite, greenschist-to-blueschist facies. The meta- diorite, and gabbro and, according to Pratt ana morphic rocks yield K-Ar mineral ages of about others (1972, p. 4998), "similar intrusive rocks 190 m.y. (Forbes and Larphere, 1973). may account for the high frequency of mag- netic anomalies observed offshore." These in- Middle and Upper Jurassic clastic sediments trusive bodies have been assigned a Tertiary more than 4.5 km thick occupy the 140-km- age (Hoare and Coonrad, 1961), but no wide gap (the Matanuska geosyncline of isotopic ages are available, and the age of these Payne, 1955) between the ancient arc and intrusive rocks is not well established. We sug- trench. In this part of the Cook Inlet area, gest as an alternate interpretation that the bel: the elongate trough-filling lacks complex com- of positive magnetic anomalies on the Bering pressional structural features and volcanic Shelf reflects the southwest continuation of the intercalations and is interpreted to be deposi- generally mafic Jurassic plutonic belt of the tion within an arc-trench gap (Dickinson, 1971, Alaska Peninsula. West of Cook Inlet, Jurassic 1972). This clastic sequence has been found plutonic rocks have a high magnetic intensity in the subsurface in the Cook Inlet basin (Grantz and others, 1963b); south of Iliamna (Kirschner and Lyon, 1972) and extends north- Lake, Jurassic diorite and gabbro plutons east and eastward to the Wrangell Mountains produce very high positive magnetic anomalies where it has been named the Matanuska- (D. Reno, 1972, oral commun.). In the vicinity Wrangell terrane (Berg and others, 1972). of Becharof Lake, aeromagnetic maps show Where this clastic seque ace is exposed along positive magnetic anomalies over the southern- the western side of Cook Inlet, composition and most outcrops of Jurassic plutonic rocks of the sedimentary structures indicate a volcanic and Alaska-Aleutian Range batholith (Andreasen plutonic source to the northwest (Detterman and others, 1963a, 1963b). Plutonic rocks south and others, 1965). of Becharof Lake are covered by surficial The thick sequence of late Mesozoic and deposits, but the southwest continuation of the early Cenozoic graywacke and argillite ex- belt of Jurassic plutonic rocks is indicated by a posed southwest from the Chugach Mountains 12-km-wide ridge of positive magnetic anom- through Kodiak Island and along the Bering alies that extends for 45 km to the southwest Shelf edge may, in part, represent a younger edge of the aeromagnetic maps. Furthermore, arc-trench assemblage accreted to the early pre-Late Jurassic granitic rocks (not now ex- Mesozoic continent (Moore, 1972). The still posed) must have been present northwest of the younger modern volcanic arc-trench system of Alaska Peninsula to serve as a source of arkosic the eastern Aleutians, which is considered the sediments of Upper Jurassic and Lower Cre- late Cenozoic analog of the earlier systems, lies subparallel to but south of both older arc- Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/8/2583/3418107/i0016-7606-84-8-2583.pdf by guest on 26 September 2021 ALASKA-ALEUTIAN RANGE BATHOLITH 2585 trench systems. Thus it appears that this part area. An anatectic model for generation of of Alaska represents a classic example of con- the Summit Lake rocks also seems reasonable. tinental accretion caused by repeated under- These rocks are chemically similar to, and were thrusting of oceanic crust beneath arcs. Under- intruded through, the Jurassic plutonic rocks. thrusting was initiated in the early Mesozoic, The Jurassic plutonic rocks and (or) associated and the 1964 Alaskan earthquake is evidence andesitic volcanic rocks may well have been the that it is continuing today (Plafker, 1972). source of, or at least influenced, the chemistry The timing of middle Tertiary plutonism of subsequent plutonism in this region. Re- east of Nonvianuk Lake in the Alaska Penin- melting of the Jurassic rocks would produce an sula may be related to northward underthrust- average composition similar to that of the ing of the Kula plate and subsequent destruc- Summit Lake rocks. As pointed out by Gilluly tion of the Kula ridge (Grow and Atwater, (1971), it is not necessary, and in many cases it 1970, p. 3720, Fig. 2c; Atwater, 1970). More may be impossible, to attribute all magmatic information on the temporal and spatial occurrences to plate tectonic models. Although distribution and chemistry of Tertiary plutons some of the Late Cretaceous and Tertiary and associated volcanic rocks of the Shumagin- plutons are definitely associated with extrusive Kodiak Shelf-Alaska Peninsula area (Burk, rocks, these plutons are largely post-tectonic, 1965, 1966) is necessary to substantiate a model and magma was generated beneath diverse involving relative motions of plates in the geologic environments that included stable northeast Pacific. platform areas as well as regions of former eugeosynclines in which deformation had es- The current andesitic volcanism and seismic sentially ceased. activity of the eastern Aleutian chain and southern Alaska Peninsula logically can be Isotope-tracer studies on these younger extended to the attractive concept of magma plutonic rocks, as well as on the Jurassic rocks, generation for the Jurassic plutonic rocks along may permit us to learn whether the three or above a subduction zone associated with an intrusive suites of the Alaska-Aleutian Range elongate arc-trench system. The association of batholith evolved through crustal anatexis, dominantly andesitic volcanic rocks and gen- differentiation of primitive mantle and (or) erally mafic Jurassic plutonic rocks further sug- descending oceanic slabs, or contamination of gests that the Mesozoic subduction zone may the latter two primary materials by crustal have involved oceanic against oceanic plates, rocks. whereas the Cenozoic system—at least in the eastern Aleutians and Alaska Peninsula—in- ACKNOWLEDGMENTS volved oceanic material thrust beneath sialic The critical comments and suggestions of crust. It seems unlikely, however, that magma P. C. Bateman, Arthur Grantz, and F.W.C. which produced the Late Cretaceous and Dodge on an earlier draft of this paper have Tertiary plutonic rocks farther inland in the materially improved the report. None of these northern part of the batholith was generated friends, however, necessarily subscribe to the along the same subduction zone. This would thesis presented. require that the zone shift inland, or toward the The co-operation of many geologists from oil continent, from its position during the Jurassic and mining companies working in the region and then shift away from the continent once has been invaluable, particularly in providing again to its present position. Furthermore, if samples. J. J. McDougall of Falconbridge these Late Cretaceous and Tertiary plutons are Nickle Mines, Ltd., C. E. Kirschner and related to arcs, then the younger arcs are sub- C. A. Lyon of the Standard Oil Company of parallel and only partly coincident with the California, Tak Matsumoto of St. Eugene Jurassic arc system. Higher K20/Si02 ratios Mining Company, L. F. Fay of the Atlantic of the rocks in the northwestern part of the Richfield Company, and Herbert Mann of the batholith (Reed and Lanphere, unpub. data) Shell Oil Company have been especially help- are consistent with the hypothesis of magma ful in providing rock samples from and informa- generation occurring along or above an in- tion on some areas that we have not been able clined subduction zone, but the ratios could to examine. Ming Ko provided assistance in equally well reflect anatexis of a thick sialic the various computer programs; R. L. Elliott, layer consisting of geosynclinal sedimentary R. J. May, and M. B. Estlund assisted with material such as represented by the great thick- field studies; Dennis Sorg and M. B. Estlund ness of Cretaceous eugeosynclinal rocks in this assisted with sample preparation and modal Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/8/2583/3418107/i0016-7606-84-8-2583.pdf by guest on 26 September 2021 2584 REED AND LANPHERE analyses; and J. C. Engels, J. D. Luetscher, Alaska: U.S. Geol. Surrey Prof. Paper 512, L. B. Schlocker, J. B. Von Essen, and H. C. 78 p. Whitehead assisted with the age measurements. Detterman, R. L., Reed, B. L., and Lanphere, Rapid rock analyses were made by P. Elmore, M. A., 1965, Jurassic plutonism in the Cook Inlet region, Alaska, in Geological Survey re- G. Chloe, J. Glenn, L. Artis, S. Botts, H. search 1965: U.S. Geol. Survey Prof. Paper Smith, and J. Kelsey. 525-D, p. D16-D21. Dickinson, W. R., 1970a, Relations of andesites, REFERENCES CITED granites, and denvativ: sandstones to arc- Andreasen, G. E., Dempsey, W. J., Vargo, J. L., trench tectonics: Rev. 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