Western Washington University Western CEDAR
WWU Graduate School Collection WWU Graduate and Undergraduate Scholarship
Spring 1980
Geology and Petrology of the Lake Ann Stock and Associated Rocks
Eric William James Western Washington University
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Recommended Citation James, Eric William, "Geology and Petrology of the Lake Ann Stock and Associated Rocks" (1980). WWU Graduate School Collection. 733. https://cedar.wwu.edu/wwuet/733
This Masters Thesis is brought to you for free and open access by the WWU Graduate and Undergraduate Scholarship at Western CEDAR. It has been accepted for inclusion in WWU Graduate School Collection by an authorized administrator of Western CEDAR. For more information, please contact [email protected]. GEOLOGY AND PETROLOGY OF THE LAKE ANN STOCK
AND ASSOCIATED ROCKS
A Thesis Presented to The Faculty of Western Washington University
In Partial Fulfillment Of the Requirements for the Degree
Master of Science
by Eric William James
May, 1980 GEOLOGY AND PETROLOGY OF THE LAKE ANN STOCK
AND ASSOCIATED ROCKS
by Eric William James
Accepted in Partial Completion
of the Requirements for the Degree
Master of Science
Dean, GracmateIracms School
ADVISORY COMMITTEE
Chairperson MASTER'S THESIS
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Date: Table of Contents
Page 1. Abstract 1 2. Introduction 2 3. Methods 6 4. Field, Petrographic and Chemical Data 7 The Lake Ann Stock 7 Dike Rocks 12 Inclusions in the Stock 19 The Swift Creek Volcanics 19 Contact Relations 21 Contact Hetamorphism 22 5. Discussion 25
6. The Lake Ann Stock and Current Theories of Calc-Alkaline Magmatism 41 7. The Lake Ann Stock in Relation to the Magmatic History of the Pacific Northwest 46 8. Summary and Conclusions 49
9. Acknowledgments 51
10. References 52
11. Appendix 1 57 12. Plate 1, Geologic and Sample Location Map Back of the Lake Ann Stock Area Pocket 11
List of Figures
Figure Page 1 Location Map of Late Cenozoic Magma- tism in the Cascade Mountains 3
Regional Geologic Map Centered on the Lake Ann Stock . 4
3 Complex Plagioclase Zoning Pattern 9 4 Axial Angles of Alkali Feldspars in the Lake Ann Stock 11
5 Normative Classification of Samples from the Lake Ann Stock 18
6 Cheveron Folds in Chilliwack Group Sediments 23 7 Overturned Folds and Shearing 24 8 Conditions of Contact Metamorphism 27
9 Interstitial Granophyre in Lake Ann Granodiorite 30
10 Ab-An-0r-H20 System 31 11 A F M Diagram of Lake Ann Stock Samples 35
12 A,B,C Silica Variation Diagrams for the Lake Ann Stock 36
13 Comparison of Silica Variation Diagrams from Four Cascade Suites 39 14 Comparison of Silica Variation Diagrams of Lake Ann Stock and Tuolumne Intrusive Series 40
15 Late Cenozoic Magmatic Episodes 48 Ill
List of Tables
Table Page 1 Modes of Representative Samples of the Lake Ann Stock 8
2,3,4,5, Chemical Analyses and Niggli and 6 Norms of the Lake Ann Stock 13 7 Contact Metamorphic Assemblages 26
8 Characteristics of "S" and ''I” Type Granitoids 42 1
ABSTRACT
The Lake Ann stock is a two pyroxene quartz monzo- diorite to granodiorite epizonal intrusive emplaced into the Shuksan thrust fault in the latest Pliocene. It is similar to other Late Tertiary Cascade intrusives in mineralogy, texture, chemistry, and setting. These simi larities are attributable to near surface emplacement, quick cooling and magmatic arc setting. Epizonal emplacement is indicated by fine, equigranular textures, granophyre, late stage alteration, late miarolitic dikes, plagioclase resorbed by alkali feldspar, and low pressure contact metamorphic assemblages. Temperature of emplace ment was between 600°C and 925°C; pressure was less than 2 kb. Chemistry of individual samples and variation diagrams closely resemble those of Sierra and Cascade arc suites. Petrologic data from the stock are compared to two recently developed theories of magmatic evolution, magma mixing and the ''restite'' model. Evidence for formation of the stock by mixing of basaltic and rhyolitic magmas is equivocal. However, complexly zoned plagioclase crystals, the chemical composition, and the stock's magmatic arc setting suggest that the stock contains a small restite component from an igneous source. The Late Cenozoic history of the Lake Ann area is: 1) eruption of the Hannagan volcanic rocks about 3.4 Ma and emplacement and cooling of the Lake Ann stock about 2.5 Ma; 2) a lull in magmatism follows and erosion unroofs the stock; 3) Swift Creek volcanic rocks erupt and mantle the topography; 4) valleys develop in the Swift Creek volcanics and are filled with flows of Mt. Baker andesite. The timing and westward migration of magmatism fits the regional pattern which may be related to episodic accelerated spreading of the Juan de Fuca Ridge. 2
INTRODUCTION The Lake Ann stock, a quartz monzodiorite to grano- diorite intrusive, lies between Mt. Shuksan and Mt. Baker, Washington, at the heads of Shuksan and Swift Creeks (Fig ures 1 and 2). Wall rocks of the stock are mid-Devonian to Permian Chilliwack Group, composed of basalt and grey- wacke, and the Shuksan Metamorphic Suite composed of the
Darrington phyllite and the Shuksan greenschist. The south trending Shuksan thrust fault, which the stock intrudes, separates the Shuksan and Chilliwack units. The western edge of the Tertiary Chilliwack composite batholith, (not to be confused with the Chilliwack Group) lies five kilo meters to the east. Tuff breccias overlie the Lake Ann stock and are capped by columnar Mt. Baker andesite. The most comprehensive study of the North Cascades has been made by Misch (1952, 1966, 1977), who mapped the major units and structures of the range. Misch (1952) considered the tuff breccias adjacent to the stock in Swift Creek and the Hannagan volcanics to be mutually correlative. On the basis of lithologic similarities and field relationships he considered the Hannagan volcanics to be equivalent to the Oligocene (?) Skagit Volcanics, and the Lake Ann stock probably to be equivalent to the Late Eocene main phase of the Chilliwack composite batholith (Misch, 1966).
Radiometric ages compiled by Engles et al. (1976) conflict with this interpretation. Andesite clasts from 3 4
Figure 2
10 km
Regional geologic map centered on the Lake Ann stock.
1- Lake Ann stock 2- Swift Creek volcanics 3- Hannagan volcanics 4- Mt. Baker volcanics 5- Shuksan Metamorphic Suite 6- Chilliwack Group 7- Chilliwack composite batholith (From Hunting, aj_. , 1961) 5 the Hannagan volcanics at the type area yield K/Ar dates on hornblende of 3.3± 1.0 and 3.6± 1.0 Ma. Dates on biotite separates from samples of hornfels at the edge of the Lake
Ann stock are 2.7± 0.3 and 2.5± 0.1 Ma. The similarities in age suggest that the Hannagan volcanics and Lake Ann stock may be cogenetic. Coombs (1939) has described the most recent rocks in the area, andesite flows related to Mt. Baker. These rocks crop out as topographically inverted intercanyon flows which form Table Mountain and Kulshan Ridge. A K/Ar age of
400,000± 100,000 years b.p. on a whole rock sample from
Table Mountain has been published by Easterbrook and Rahm
(1970). The economic geology of the study area has been re viewed by Moen (1969) who describes two prospects and a patented claim located on the periphery of the stock. No mining has been done in the area since the turn of the century. Staatz et al. (1972) investigated the economic potential of the eastern edge of the area as part of the geologic survey of the North Cascades National Park. The eastern-most edge of the stock, which lies in the park, was mapped during this investigation. Previous work in the Cascades (Fuller, 1925; Fiske
et al., 1963; Hopson ^ ^. , 1965; Tabor and Crowder,
1969; Erikson, 1969; Wise, 1969; Richards and McTaggart, 1976) has revealed many similarities among the Late ,
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FIELD, PETROGRAPHIC, AND CHEMICAL DATA
The Lake Ann Stock The Lake Ann stock is composed of dark to medivim grey equigranular quartz monzodiorite to granite. The grain size is about one millimeter throughout the stock. In thin section the rock is hypidiomorphic-granular. Minerals ob served include plagioclase, quartz, alkali feldspar, bio- tite, augite, hypersthene, magnetite, hornblende, apatite, and zircon. Representative modes are listed in Table 1.
Plagioclase in the stock is divided into two groups, complexly zoned and simply zoned. Simply zoned crystals, which comprise more than 90 percent of the plagioclase, are lath shaped to equant and exhibit normal zoning. Cores are An 59 to 45 grading outward to An 17 or in a few cases An 5 rims. Complexly zoned crystals are larger, up to 5 mm in length, and commonly occur in groups. These crystals exhibit a combination of normal and oscillatory zoning. Some have patchy zoning and exhibit evidence for several episodes of resorption. A typical zoning and resorption pattern is shown in Figure 3. Many of the complexly zoned crystals are broken. 8
Table 1
Modes of Representative Samples of the Lake Ann Stock
10.14 Sample Number 10.14.6A 10.30.5 10.15.1A 9.18.1A
Quartz 15.2% 12.9% 12.0% 11.6%
Alkali feldspar 29.5% 11.0% 19.8% 12.5%
PIagioclase 40.9% 54.9% 49.8% 58.3%
Biotite 2.3% 9.9% 1 .0% 0.4%
Clinopyroxene 6.1% 5.1% 10.7% 8.2%
Orthopyroxene 3.2% 3.7% 4.5% 6.4%
Hornblende 0.5% 0.0% 0.0% 0.0%
Opaques 2.3% 2.3% 2.2% 2.6%
Apatite T 0.3% 0.1% T
One thousand points counted per sample. Amphibole other
than primary hornblende was reassigned to pyroxene.
Quartz and alkali feldspar percentages in sample 10.14.6A
may be inaccurate due to fine grain granophyric inter
growths . ’i
9
Figure 3
0.5 mm
Complex plagioclase zoning pattern 10 Alkali feldspar in the stock ranges from fine, patchy microperthite to apparently homogeneous orthoclase. Optic angles range from 15 to 43 degrees. Figure 4 shows the distribution of 19 measurements of 2V. Interstitial, anhedral alkali feldspar occurs in all samples. Euhedral crystals are scarce and limited to the most felsic grano- diorites. In these samples the interstitial alkali feld spar corrodes the rims of plagioclase crystals and is granophyrically intergrown with quartz. Alkali feldspar is dusted with fine red inclusions of hematite, particularly in the more felsic samples. Samples from the stock contain two pyroxenes, augite and hypersthene. Hypersthene compositions range from En 75 to En 50. Augite is diopsidic. Hypersthene is only slightly pleochroic. In most samples it is altered to orthoamphibole, probably cummingtonite. Augite is generally less altered than hypersthene but is also subject to uralitization. In a few sections pyroxenes are rimmed by biotite and magnetite. Quartz appears to have crystallized in two generations. The first exhibits a subhedral to anhedral form. The second and most common is interstitial and granophyric. Other minerals include biotite, magnetite, hornblende and accessories. Brown biotite occurs as isolated subhedral crystals and replacing pyroxene. Magnetite is associated with much of the biotite. Primary hornblende is scarce, occurring only in felsic granodiorites. Accessories include Figure 11 [I- m
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1 5 16 Cr 37 37 32 47 40 30 Co 29 15 33 16 22 25 La 25 23 24 28 26 20 Ni 12 11 15 18 18 18 Sc 11 18 15 18 16 14 Sn 10 10 10 10 10 10 Sr 467 568 479 497 616 556 V 73 130 no 143 124 122 Y 25 24 26 28 23 23 Zn 54 70 51 42 5 6 48 Zr 105 119 91 1 32 113 71 Gd 20 22 20 22 21 22 Pb 10 10 10 10 10 10 Analyst: Scott Morgan 14 Table 3 Chemical Analyses and Niqqli Norms of the Lake Ann Stock 10.30.5 Samp 1el0.14.7A 10.14 10.14.2A 1 0.1 5 .1A 9.14.lA 7.30.2A S i O2 64.31 60.89 60.88 60.22 61 .52 60.40 AI2O3 15.75 16.19 16.01 16.72 1,6.47 16.75 T 5.01 6.43 6.27 6.31 5.63 6.38 MgO 1.79 2.70 2.73 3.14 2.72 3.04 CaO 3.79 5.30 5.02 5.42 4.60 5.09 0 CM 3.75 3.93 3.68 4.21 3.89 4.58 K2O 3.21 2.29 2.63 2.16 2.58 2.08 Ti02 0.80 0.98 0.99 1 .05 0.89 0.96 P2O5 0.22 0.30 0.23 0.24 0.19 0.24 MnO 0.092 0.113 0.094 0.125 0.094 0.115 Total 98.31% 99.13% 98.59% 100.09% 98.57% 99.65% +L0F 100.00% 100.52% 98.82% 100.29% 99.25% 100.74% Qz 17.74 12.93 13.41 10.09 13.55 12.12 Or 19.43 •13.8 15.93 12.83 15.56 12.50 Ab 34.5 35.89 33.88 38.01 35.66 35.53 An 17.07 20.21 19.89 20.46 20.28 21 .98 Mt 1 .91 2.4 2.35 2.51 2.10 2.37 11 1 .14 1 .39 1 .41 1 .47 1 .27 1 .36 Di 1 .35 5.28 4.52 5.26 2.42 2.97 Hy 6.86 8.1 8.61 9.35 9.16 10.18 PI ag An 33 An36 An37 An35 An36 An38 Semi-quantitative Trace Elements in ppm B 10 10 10 10 10 10 Ba 867 610 644 594 766 650 Be 2.0 1 .7 1 .8 1 .8 1 .9 2.0 Co 11 12 16 17 13 16 Cr 29 38 36 38 23 37 Cu 24 22 26 25 15 22 La 23 22 26 25 21 23 Ni 10 13 16 17 12 18 Sc 12 16 16 18 15 16 Sn 10 10 10 10 10 11 Sr 376 401 459 490 457 501 V 30 no 123 142 109 128 Y 27 27 26 28 27 28 Zn 54 73 60 59 46 81 Zr 179 127 97 120 94 43 Ga 19 19 21 22 18 20 Pb 10 10 10 10 .10 10 Analyst: Scott Morgan 15 Table 4 Sample9.13.2A 9.14.3A 9.13.3B 7.24.48 9.13.7A 9.14.2A SfO^ 61.57 62.41 60.19 61 .29 65.68 63.01 AI2O3 16.11 16.17 16.44 16.74 16.62 15.50 T 5.54 5.69 6.32 6.01 4.31 5.69 MgO 2.44 2.47 2.66 2.61 1.57 2.25 CaO 4.36 4.44 4.88 5.00 4.05 4.30 Na^O 3.95 4.02 4.27 4.37 4.52 4.04 K2O 2.63 2.52 2.45 1 .95 2.11 2.89 Ti02 0.76 0.87 1 .06 0.08 0.53 0.92 0.18 0.19 0.25 0.20 0.16 0.21 MnO 0.081 0.102 0.106 0.118 '0.078 0,096 Total 97.61% 98.88% 98.63% 99.06% 99.65% 98.95% + LOF 100.71% 99.95% 99.86% 100.47% 100.97% 100.77% Qz 14.10 14.59 10.62 12.34 18.08 14.83 Or 16.02 15.17 14.77 11 .68 12.56 17.42 Ab 36.57 36.78 39.11 39.80 40.90 37.01 An 19.04 18.99 18.83 20.57 18.98 15.94 Mt 2.09 2.12 2.37 2.23 1 .59 2.12 11 1 .09 1 .24 1 .51 1.13 0.74 1 .31 Di 2.62 2.77 4.7 3.68 1 .02 4.66 Hy 8.47 8.34 8.1 8.57 6.12 6.7 Plag An34 An34 An32 An34 An32 An30 Semi -quantitative Trace Elements In PPM B 10 10 10 10 10 10 Ba 799 735 715 621 754 865 Be 1 .9 2.2 2.0 1 .9 1 .8 2.5 Co 13 1 5 14 14 8 14 Cr 37 34 29 27 20 29 Cu 16 26 17 22 12 19 La 20 22 25 21 20 28 Ni 17 21 13 13 6 12 Sc 14 15 16 15 10 15 Sn 11 14 10 11 10 13 Sr 567 496 481 574 903 473 V 113 107 117 115 66 115 Y 24 30 31 26 19 33 Zn 57 46 50 51 43 42 Zr 15 68 237 33 39 1 54 Gd 21 21 1 1 19 20 21 Pb 17 10 10 10 10 10 16 Table 5 Chemical Analyses and Niqqll Norms of the Lake Ann Stock Samp 1e 8.1 .3A 9.18.3B 9.26.3C 7.3C.7A Si02 63.12 59.43 68.35 58.58 AI2O3 15.89 17.81 14.38 16.90 FejOjT 5,57 6.12 3.90 4.35 MgO 2.26 2.60 1 .22 3.34 CaO 4.12 5.34 2.44 5.96 Ma20 4.10 3.90 3.41 4.21 K2O 2.80 2.02 4.19 1 .34 Ti02 0.84 0.87 0.58 1 .09 '"2^5 0.18 0.19 0.10 0.25 MnO 0.093 0.097 0.064 0.135 Total 98.97% 98.37% 98.62% 96.66% +L0F 100.92% 101.67% 100.94% 99.38% Qz 14.83 11 .63 23.35 9.69 Or 16.84 12.21 25.42 11.22 Ab 37.47 35.83 31 .45 39.02 An 16.99 25.72 11.87 22.49 Mt 2.07 2.29 1 .47 1 .64 11 1.19 1 .24 0.33 1 .57 Di 3.06 1.12 0.45 6.43 Hy 7.54 9.86 5,16 7.93 Plag An31 An42 An27 An37 Semi -quantitative Trace Elemen B 10 10 10 10 Ba 696 746 881 669 Be 2.1 1.7 2.4 1 .8 Co 13 13 5 19 Cr 29 26 21 30 Cu 24 25 7 18 La 25 20 20 23 Ni 16 11 7 14 Sc 14 13 10 19 Sn 10 10 10 14 Sr 414 568 233 553 V 112 92 49 151 Y 30 27 30 28 Zn 61 72 24 63 Zr 171 100 163 1 09 Gd T9 22 18 22 Pb 10 10 10 10 Analyst: Scott Morgan Chemical Analyses of the Lake Ann Stock and Volcanics of Swift Creek 00 o CO ro f r c r ro LO c CO CO <: r r OO r ro oo o ro 00 O ro o CO 00 o CO o. o> — — — — — — • • • • • • • • • • •r* ro r r o VO ro VO ro VO CM VO ro VO LO 'd- ro CO VO ro VO ro LO VD o o VO CVJ ro V£> LD o i£> CO o — — CM 1 c r-^ r fmm VO ■ p- o fmmm CO CO ro VO VO CM ro LO VO CM o o ro CO LO o CO CO — “ — CM ro o p P" f Lu 1 LO LO ro cr> LO VO CTl LO r-» r o> lO ro o VO 00 ro o — “ o “ — ro CSJ CM O CM ro LO o ro LO VO CM «3- 00 VO P-* ro CM ro 00 o 00 LO CM 0 p p O ro p CM r CM ro ro ro VD ^ <_3 ro o O LO O Ch ro ro VO o LO O o^ 00 fO — “ — — 2 4 Z 'd' ^ p p o ro f o^ ro ro CM ro cn ro CM ro o ro o o cn o ro o> fC — “ CO 4 “ p VD CM p» o o CM CM LO VO CTk CM o lO LO p. CM LO VO ro VO — CM •r- r h- o CM CO VO CM VO •d' o o 00 ro CM o LO o o •d" o crt ro o — CM s; r VO o O p>- o ro O O O o 00 O CM O CT> o VO O O ro o O o o cr\ o O O — c r CM p* LO r «a- ro «d CM ro ro ro LO t p** 00 r«^ ^ LO VO CT^ CO o CM LO o VO 00 4-^ a» — — — o fO 17 “ > V4- + o to to c o to o c o> o pi* CM CM VO CM CM r * f r CM VO CM CM O cr> o o ro o-o Cn ro p CM ro ro VD CM CO VO CM o CO “ — — N — — • • • • • • CM p r ro O CO -K ro CT> o CT> — — ■ p- p O 00 P CM ro LO CM 00 rt ^ p ^ p-» VO o LO CM LD 00 o o LO ro CM VO CM o — u “ — “ r- f CM r CM ^ f f VO ro ro VO ro VD LO 00 ro LO < JO ro ro VD ^ ro VO ro r 1 1 CM 1 f CM c f LO r p- ^ ro o o crv LO VO 00 VO CM VD CM IT) VO CO ^ VD CO CTi O VO — — — — — — c — r r CM CM r lO o CO e CM CM VO LO ro CO ro CTi O LO O CM VO o — “ — ; ^ ^ » ro "cP p f 'S' pw f o ro 'J- CO ro r* VO VD O 0\ MH o a. 00 O o o LO VO — — — 0 — 0 “ CM f 1 LO LO »r r CM f CO LO ro VD CTi 00 CM O — 0 0 — — — “ «d- 1 VO ro ^ CO w CM p p ro «d o ro LO CTi o o Ch O CTi CM o> — >1 — — • • “ • < CM < p < o. c <0 p Determinations by atomic absorbtion spectrophotometry at Western Washington University. Iron apportioned 0.35=Fe203/(Fe203+ FeO) for norm calculations. Analysts: Eric James, Bruce Christenson, and Jack Cruver Figure 18 o s- 'V 65% '/ EVIZSPYQIXVMGEPP]QMRSV%ALIHMOIWEVIPIWWXLER'&GQ [MHIERHGERVEVIP]FIXVEGIHQSVIXLER'&QFIJSVIXLI] TMRGLSYX%1X]TMGEPTMROHMOIMWGSQTSWIHSJ++TIVGIRX QMGVSTIVXLMXI#)+TIVGIRXUYEVX^#-TIVGIRXGSVVSHIH TPEKMSGPEWI#ERH)TIVGIRXSVPIWWEPXIVIHEQTLMFSPIERH FMSXMXI%ALIUYEVX^ERHQMGVSTIVXLMXIJSVQKVERSTL]VIMR WSQIWEQTPIW#IWTIGMEPP]EXHMOIQEVKMRW%ALIGLIQMGEP GSQTSWMXMSRWSJX[STMROHMOIW%@EQTPIW-%)'%*2ERH '&%'*%-2#ETTIEVMRAEFPI,%;MEVSPMXMGGEZMXMIWEVIGSQQSR MRXLIWIHMOIWFYXHSRSXSGGYVMRXLIWYVVSYRHMRKVSGOW% 9RGPYWMSRWMRXLI@XSGO 9RGPYWMSRWSFWIVZIHMRXLIWXSGOVERKIJVSQ(QQGPSXW SJQEJMGQMRIVEPWERHGSQTPI\P]^SRIHTPEKMSGPEWIXSE(&Q FPSGOSJ3LMPPM[EGO7VSYTWIHMQIRXW%1FSYX/&TIVGIRXSJ XLIMRGPYWMSRWETTIEVXSFISJMKRISYWTEVIRXEKI%9RGPY$ WMSRWEVIMREPPWXEKIWSJEWWMQMPEXMSR%ALI]VERKIJVSQ WLEVT$IHKIHGLYROWXSHMJJYWIFPSFWSJQEJMGQMRIVEPW%ALI QMRIVEPSK]SJXLIMKRISYWMRGPYWMSRWMWWMQMPEVXSXLIWXSGO I\GITXXLEXXLIVIMWEPEGOSJEPOEPMJIPHWTEVERHQEJMG QMRIVEPWEVIQSVIRYQIVSYW%@IHMQIRXEV]\IRSPMXLWEVI MHIRXMGEPXSXLILMKLIWXKVEHIWIHMQIRXEV]LSVRJIPWEWWS$ GMEXIH[MXLXLIWXSGO% ALI@[MJX3VIIOCSPGERMGW :MKLXXERT]VSGPEWXMGVSGOW#MRJSVQEPP]VIJIVVIHXS EWXLI@[MJX3VIIOZSPGERMGW#EVII\TSWIHMRGPMJJWEXXLI LIEHSJ@[MJX3VIIOERHMXWYTTIVXVMFYXEVMIW%ALIWIVSGOW (& PMIRSRGSRJSVQEFP]EKEMRWXXLIWXSGO%?SYRHIHGSFFPIW HIVMZIHJVSQXLIWXSGOSGGYVMRXLIZSPGERMGVSGOWRIEV XLIGSRXEGX% @IZIVEPTMTIWSJXYJJMHIRXMGEPXSXLIZSPGERMGWGYX XLIWXSGOSRXLIVMHKIIEWXSJTIEO+.,/#;X%1RR#!>PEXI '"%ALIZSPGERMGWEVISZIVPEMRF]MRXIVGER]SRJPS[WSJ ;X%2EOIVERHIWMXI%6IIHIVHMOIWJSVXLIWIJPS[WGYXXLI WXSGOERHXLI@[MJX3VIIOZSPGERMGW%ALIXLMGORIWWSJXLI @[MJX3VIIOZSPGERMGWMWEXPIEWX,&&Q#FYXXLIFEWISJ XLIHITSWMXLEWRSXFIIRSFWIVZIHERHQYGLQE]LEZIFIIR VIQSZIHF]IVSWMSR%7SSHI\TSWYVIWSJXLIWIVSGOWPMI WSYXLSJXLIIEWXIRHSJAEFPI;SYRXEMR% ALIZSPGERMGWEVIHEGMXIXYJJWERHXYJJFVIGGMEW% ALIFVIGGMEWEVIGSQTSWIHSJTYQMGIERHJIPWMGXYJJJVEKQIRXW &%+XS+&GQMRHMEQIXIVMREQEXVM\SJKVI]$XEREWLERH GV]WXEPW%@SQISJXLIPEVKIVGPEWXWLEZIGSQTSWMXMSREPP] KVEHIHFIHHMRKSJEWLERHLSVRFPIRHIGV]WXEPW%9RLERH WTIGMQIRXLIGPEWXWEVIERKYPEVERHWLS[RSIZMHIRGISJ JPEXXIRMRK%ALIHITSWMXMWQEWWMZI[MXLRSSFZMSYWFIHHMRK% 9RXLMRWIGXMSRXLIQMRIVEPSK]SFWIVZIHMWSWGMPPEXSV] ^SRIHTPEKMSGPEWI#IYLIHVEPLSVRFPIRHIERHFMSXMXI#WGEVGI WERMHMRIERHUYEVX^%ALIKVSYRHQEWWSJXLIZSPGERMGWMW KPEWWERHHIZMXVMJ]MRKKPEWW%1PXLSYKLTLIRSGV]WXWEVI GPIEVERHYREPXIVIH#QSWXEVIWLEXXIVIHSVFVSOIR% @TIGXEGYPEVKPSQIVSTSVTL]VMXMG^SRIHTPEKMSGPEWIGV]WXEPW SGGYVMRXLITYQMGI%?SYRHIHI\SXMGGPEWXW#MRGPYHMRK 21 basalt, granitics, and phyllite, are numerous at the contact with the stock but comprise less than 1 percent of other outcrops. Contact Relations The contacts between the Lake Ann stock, the Chilliwack Group and the Shuksan Suite are intrusive and generally steep. The southern edge of the stock dips 60 degrees to the south; elsewhere the contact is vertical. The stock truncates the Cretaceous Shuksan thrust (Misch, 1966), which is a complex zone of tectonically intercalated Barrington phyllite and Chilliwack Group sediments and greenstones. Contacts of the stock exhibit evidence for several mechanisms of emplacement. Contacts are brecciated, stoped, diked, migmatitic and sharp. Wall rock is undis turbed in some outcrops but very deformed in others. Stoped blocks of Chilliwack Group rocks are locally numerous within 20 m of the contact but become scarce toward the interior of the stock. Chilliwack basalts make up the preponderance of the stoped blocks. The best examples of intrusive breccias and stoping are along the base of the southwest side of Shuksan Arm and southeast of peak 5869 ("Mt. Ann”). Dikes injected in the Chilliwack Group are numerous on Shuksan Arm, especially in the Shuksan thrust and in the hydro- thermally altered "gossan” zone. Some contacts with the Barrington phyllite and the rhythmically bedded shale and 22 greywacke of the Chilliwack Group are migmatitic. Quartz veins and granitic material separated by biotite-rich layers form contorted folds. Outcrops of the relatively incompetent Chilliwack sed iments and the Barrington phyllite are commonly deformed. There are two styles of folding, chevron-style, with wave lengths of about 25 cm (Figure 6), and 10 cm overturned folds verging to the southwest (Figure 7). Contact Metamorphism The contact aureole around the Lake Ann Stock reaches the upper cordierite-K-feldspar hornfels facies of Winkler (1976). The protolith for the hornfels includes basalt, interbedded shale and greywacke, and phyllite. Hornfelsic textures overprint previous regional metamorphic and cata- clastic textures. Polymetamorphism, similar bulk composi tions, and locally the introduction of fluids from the stock have made the Chilliwack sediments and Barrington phyllite virtually indistinguishable. The lateral extent of the contact aureole is difficult to ascertain because contacts are exposed on steep ground and the nearby Chilliwack com posite batholith may have added its own thermal aureole to the area. Assemblages in the aureole are divided on the basis of protolith. Phyllite and Chilliwack Group sediments contain quartz and potassium feldspar or sericite. Meta morphosed Chilliwack basalt generally lacks these minerals 23 Figure 6 Cheveron folds in Chilliwack Group sediments Overturned folds and shearing related to Shuksan thrusting. Hammer handle is in axial plane trending N60W 45NE. Folds are overturned toward the SW. 25 and contains more plagioclase and pyroxene. Complete assemblages of the samples are listed in Table 7. The coexistence of quartz and orthopyroxene and the lack of textures indicating anatexis indicate that the con ditions of contact metamorphism lie between 600° and 925°C and less than about 2 kb (Figure 8). There are several possible inaccuracies in these estimates. The orthopyroxene in these rocks is about En 60, while the experimental data (Chernosky and Autio, 1979) are for pure enstatite. The addition of iron shifts the curve toward low temperature an unknown, but probably small, amount (Chernosky and Autio, 1979). The assiamption that water pressure equals total pressure in the contact rocks is probably good. The proto- liths contain hydrous phases that would yield water on decomposition. However, any deviation toward drier condi tions would increase the stability range shown in Figure 8. The pressure estimates are in good accord with evidence for shallow emplacement in the stock itself. DISCUSSION The spatial, temporal, textural and chemical character istics of the Lake Ann stock can be attributed to two major factors. The first is its shallow level of emplacement. The texture and mineralogy of the stock are a consequence of epizonal low pressure and quick cooling. The second and most important factor is the magmatic arc environment in 26 Table 7 Contact Metamorthic Assemblages Basalt Protoliths PI ag K-feld Qtz Biot Opx Cpx Hbd Mag Sericite Other 9.16.2B X X T X X 9.16.3A X T X X X Olivine 9.16.3B X X X X T X 9.16.3D X X X X X 9.18.3A X X X X X X Oamph 9.26.2A X X X X X 9.26.2C' X X X X X X 9.26.2D X X X X X 9.26.2E X X X 9.26.2F X X X X Phyl1ite and Sedimentary protoli ths 7.29.3A X X X X X 7.19.3B X X X X X X 7.30.IB X X X X X 7.10.8A X X X X T? X Sarnet 7.31.3A X X X X X Garnet 8.5.IB X X X X X X 9.13.3A X X X X X 9.13.3C X X T X X 9.13.3F X X X X X 9.13.5A X X X X X X X T = trace 27 Figure 8 The approximate conditions of metamorphism lie in the ruled area. A= anthophy11ite , En- enstatite, Q= quartz. A= En+ Q+ H2O from Chernosky and Autio (1979), Shale melting curve from Wyllie and Tuttle (1961). 28 which the Lake Ann stock was formed. These two factors have combined throughout the Cascades to form a string of Tertiary epizonal plutons with surprisingly similar characteristics. The textural effects of rapid cooling at low pressure appear in samples from throughout the stock. The stock is relatively fine grained (1 mm) and equigranular. This is generally accepted as evidence of quick cooling (although this is not necessarily so; see Swanson, 1977). Granophyric texture in the stock is strong evidence for rapid cooling. Buddington (1959) states, "Granophyre, in general, occurs extensively in the epizone.” Experimental studies show that the development of granophyre is probably due to substantial undercooling of a liquid-quartz-feldspar mixture. Under these conditions quartz crystallizes in a dendritic habit. Growth of the dendrite arms takes up silica, enriching the area between the arms in alkali feldspar component. The final product is a fine-scale intergrowth of hypersolvus alkali feldspar and quartz (Fenn, personal communication; Barker, 1970). In conjunction with the granophyric mesostases, minute flakes of hematite and alteration in alkali feldspar and pyroxene are also evidence for rapid crystallization. Rapid crystallization of anhydrous phases would lead to the generation and trapping of hot aqueous fluids capable of altering pyroxene to amphibole and causing turbidity in the feldspars (Barker, 1970). The pink dikes in the stock represent accumulations of this aqueous fluid. 29 Granophyre and late alteration are common in Tertiary Cascade plutons. Granophyre is well developed in the Tatoosh and Cloudy Pass plutons. Figure 35 of Fiske et al. (1963) and Figures 6, 7, and 9 of Tabor and Crowder (1969) show granophyre in the interstitial areas of the felsic phases of each pluton. Fiske e_t aT. (1963) found granophyre in areas that were inferred to be below volcanic vents. Nearly identical textures in a granodiorite of the Lake Ann stock are shown in Figure 9. Miarolitic cavities are considered good evidence for shallow emplacement. These cavities are formed when a magma becomes saturated in volatile components, primarily water and carbon dioxide. The Tatoosh, Cloudy Pass and Snoqualmie plutons all have miarolitic vugs (Fuller, 1925; Fiske ^ al., 1963; Tabor and Crowder, 1969). Only the late pink dikes of the Lake Ann stock have miarolitic cavities. This implies that there were insufficient volatiles in the magma to reach saturation until the latest stages of crystallization. Replacement and embayment of plagioclase by alkali feldspar have been reported in the Tatoosh and Cloudy Pass Batholith (Fiske et al., 1963; Tabor and Crowder, 1969). This texture can be explained in two ways. Intrusion of the stock would lower pressure in the magma, enlarging the one feldspar field illustrated in Figure 10. Plagioclase crystallized earlier would be out of equilibrium with the melt and would be replaced by alkali feldspar. Similar 1 mm Interstitial granophyre in Lake Ann granodiorite, sample 10. 14. 7A. 31 Figure 10 An Ab-An-0r-H20 system showing the expansion of the one feldspar field with reduction of pressure. The boundry curve E-F changes from a coprecipi tation curve to a resorption curve along its length. Diagram after Stull (1978). 32 explanations have been advanced for patchy zoning (Vance, 1965) and for rapakivi texture (Stoss, 1978). There are no flow or cataclastic textures in the Lake Ann stock that indicate upward movement late in the crystallization history. The second, and preferred, explanation is that resorption is the result of normal crystallization in the system An-Ab-0r-Q-H20. According to Tuttle and Bowen (1958, pp 133- 134), the last section of the boundary curve E-F in Figure 10 is a resorption curve rather than a coprecipitation curve at pressures less than 3.6 kb. The same situation occurs in the quartz normative system. Resorption in the Lake Ann stock indicates that the final crystallization of the magma took place at pressures less than 3.6 kb. Lastly, the mineralogy of the stock is indicative of quick cooling and is typical of Tertiary Cascade plutons. Pyroxene granodiorites occur in the Cloudy Pass Batholith, the Snoqualimie Batholith, and in the Tatoosh pluton (Grant, 1969; Tabor and Crowder, 1969; Erikson, 1969; Fiske ^ al., 1963). Tabor and Crowder (1969) state, "The pyroxene is thus preserved in border rocks or small stocks which may have undergone rapid cooling before hydrous minerals of Bowen's reaction series could form." Pyroxenes are also indicative of a high temperature, low PH2O magma. The chemical, spatial and temporal characteristics of the stock were controlled by its setting in a continental magmatic arc. As with most arc intrusive rock, the Lake Ann stock is associated with volcanic rocks. 33 The Swift Creek volcanics and Mt. Baker andesites are in contact with the stock and the Hannagan volcanic rocks crop out 8 km to the east. Close spatial relationships of plutonic and volcanic rocks have been demonstrated in several Cascade suites: the Cloudy Pass Batholith (Carter, 1969; Tabor and Crowder, 1969); the Snoqualmie Batholith (Fuller, 1925; Erikson, 1969); near Mt. St. Helens (Wise, 1969); and the Tatoosh volcanic-plutonic complex (Fiske et al., 1963). However, a direct cogenetic link between vol canic and plutonic rocks has not been proven, and in the case of the Tatoosh complex, recent work shows that the plutonics postdate the volcanics by 5 million years (Mattinson, 1977). The Lake Ann stock is clearly older than either of the adjoining Cenozoic volcanic units. The Swift Creek volcan ics are apparently flat lying and lie nonconformably against a steep erosion surface developed on the stock. The steep ness of the contact, the considerabi^e thickness of the volcanic rocks and the Late Pliocene age of the stock indi cate that the Swift Creek volcanics may have filled a glacial valley. Subsequently, valleys developed in the easily eroded tuffs, and these valleys were in turn filled by Mt. Baker andesite flows. The nearest exposed volcanic rocks that might be related to the Lake Ann stock are the Hannagan volcanics. The Hannagan volcanic rocks are dated at 3.3± 1.0 and 3.6± 1.0 Ma 34 and the Lake Ann stock at 2.5 ± 0.1 and 2.7 ± 0.3 Ma (Engles et , 1976) . The Hannagan volcanics and the Lake Ann stock are probably part of the same magmatic episode. The chemistry of the stock is typical of calc-alkaline intrusives. Alkalies versus iron versus magnesivim tri angular diagrams show trends similar to other orogenic suites (Figure 11). Variation diagrams of percentage Si02 against percentage major oxide show little scatter and are quite linear (Figures 12 A,B,C). Other Cascade suites have very similar trends (Figure 13), except that the Lake Ann rocks are slightly richer in potassium and have slightly less calcium and aluminum. This is reflected in mineral content; the stock is richer in alkali feldspar and the other suites contain more anorthite. The trend of the Tuolumne Intrusive Series (Bateman and Chappell, 1979) is also similar (Figure 14). It appears that the chemical composition of the stock is a function of its magmatic arc origin in general, and is not due solely to its Cascade magmatic arc parentage. 35 Figure 11 Na20 Lake Ann stock ------Cloudy Pass batholith ------Chilliwack batholith ------Snoqualmie Batholith ...... K i ij u r e 12 o2 CM O o o oo ^ (5^ Q O c ^ Q d un o O O o 0) CM o 36 o Q un O O o o en o CM O F ig u r e12 37 P e r c e S n iO t F i q u r e 12 38 I’e r c e nS1 tO2 39 Figure 13 40 41 THE LAKE ANN STOCK AND CURRENT THEORIES OF CALC-ALKALINE MAGMATISM Recent work in the Sierra Nevada Batholith of Calif ornia and the New England Batholith of Australia has been instrumental in the development of new petrogenetic theories that augment the classical ideas of primary melts and frac tional crystallization. Piwinskii (1967) and Presnall and Bateman (1973) concluded that some Sierra Nevada magmas contain residual materials carried from the lower crust by a rising partial melt. They argue that the ubiquitous mafic inclusions and the cores of complexly zoned and mottled plagioclase crystals are residual materials that were en trained with the melt. This idea has been termed the "restite model" (Chappell and White, 1974). The restite model has been developed further by Chap pell and VJhite (1974) and Chappell (1978). They divide granitoid rocks into two classes based on the source materials inferred from the chemistry and residual materials (restite) in the rock. "S"-type granitoids are formed by partial melting of a sedimentary source, "I"-types from an igneous source. Table 8 summarizes the characteristics of "S"- and "I"-type granitoids. In general the calc-alkaline coastal batholiths of North America are "I"-types. A diffuse belt of "S"-type granitoids lies to the landward side of the "I"-types (Miller, 1979). 42 Table 8 Characteristics of "S" and "I" Type Granitoids I-Types S-Types Relatively high in sodium, Relativly low in sodium, less than Na^jO normally greater than 3.2% in rocks with 5% K2O, decreasing 3.2% in felsic rocks, de to less than 2.2% in rocks with creasing to 2.2% in mafic 2% K2O rocks Mol Al203/(Na20+K20+Ca0) Mol Al20o/(Na20+K20+Ca0) greater less than 1.1 than l.r C.I.P.W. normative diopside Greater than 1% C.I.P.W normative or less than 1% normative corundum corundum Broad spectrum of compo Relatively restricted in compo sitions from felsic to mafic sition to felsic types Regular interelement Irregular variation diagrams, lots variations within plutons; of scatter near linear variation diagrams Initial Sr^^/Sr^^ 0.704-0.706 Initial Sr^^/Sr^^ greater than 0.708 Hornblende bearing inclusions Metasedimentary inclusions Hornblende, sphene, apatite, Muscovite, monazite, aluminosilicates, in mafics garnet, cordiorite, and apatite as descrete crystals Late in intrusive sequence Early in composite intrusive sequence Massive or primary foliation May have metamorphic foliation From Chappell and White (1974) 43 Another theory of the genesis of intermediate grani toids and andesites is that they are formed by the mixing of basaltic and rhyolitic magmas. This idea has a long history (Carmichael £t al., 1974, p 67) and has been cited as the means of differentiation in specific suites (Larson et al., 1938; Wilcox, 1944; Macdonald and Katsura, 1965). Anderson (1976) and Eichelberger (1978) argue that magma mixing is widespread and important in the genesis of most calc-alkaline rocks. The evidence for mixing observed by Anderson (1976) and Eichelberger (1978) include crystals of plagioclase, quartz or forsteritic olivine not in equilibrium with the bulk rock composition; coexistence of very sodic and very calcic plagioclase; embayed quartz; sodic plagioclase cores mantled by calcic rims; inclusions of rhyolitic glass in mafic inclusions; inclusions of basaltic glass in felsic inclusions; vesicular basaltic xenoliths with coarse-grained cores and fine-grained rims; and isotopic disequilibrium between groundmass and phenocrysts. Evidence is best pre served in volcanic rocks since slow cooling obliterates most evidence of magma mixing. Andersont (1976) states that mixing may be subvolcanic and must involve large batches of magma in order to form large amounts of homogeneous intermediate magmas. He also observes that vapor-saturated systems appear to favor mixing. Eichelberger (1978 and personal communication) has developed 44 a magma mixing model for the basalt-andesite-dacite-rhyolite association. Basaltic magmas from the mantle emplaced in the lower crust cause partial melting which forms rhyolite bodies. Upward moving basaltic magma mixes with the slower moving rhyolitic magma or mixes in high level chambers to form andesites and dacites. Mixing is facilitated by thermal convection and by quenching of the basalt at the basalt-rhyolite interface. Quenching causes the exsolution of volatiles which vesiculate the basalt and lower its density. Vesiculated pieces of quenched basalt glass rise through the rhyolite where they are assimilated. Compres- sional tectonics appear to favor mixing yielding andesites and dacites. Extensional tectonics impede mixing and result in the eruption of bimodal rhyolite-basalt suites. Application of these ideas to the Lake Ann stock is difficult. The assimilation of wall rock poses difficulties for comparisons with restite theory and magma mixing. Evidence is obscured by equilibration in a plutonic environ ment. Also, restite theory, magma mixing, and fractional crystallization are not mutually exclusive and may operate together to multiply their individual complexities. The origin of the xenoliths in the stock is important to both restite theory and magma mixing. Many of the xenoliths are clearly stoped blocks. Ignoring the zone within 20 m of the contact, the proportion of sedimentary to igneous inclusions is constant and roughly equal to 45 their amount of outcrop at the contact with the stock. The sedimentary inclusions are derived from the Chilliwack wall rocks and therefore it is reasonable to assume that the associated mafic igneous inclusions are too. Mineral- ogically the inclusions are the same as the contact meta morphosed Chilliwack Group rocks. Therefore, few, if any, of the macroscopic igneous inclusions are restite, and it seems unlikely that the inclusions represent the mafic part of a basalt-rhyolite mixture. Petrologic evidence indicates that there may be a small restite component in the Lake Ann stock. The complexly zoned plagioclase crystals appear identical to those cited by Presnail and Bateman (1973) , Chappell (1978), and Bateman and Nokelberg (1979) as restite crystals. However, Wiebe (1968) attributes similar crystals to changes in pressure as the magma rises. The occurrence of these crystals in groups suggests that they are the remains of digested restite inclusions. The stock has many of the properties of an "I”-type granitoid (Table 8), and its location in an arc environment also is in agreement with an igneous source for the magma. The high temperature mineralogy of the stock, the apparent lack of macroscopic restite, and the scarcity of complexly zoned plagioclase suggest that it contains only a small restite component. There is no clear evidence for magma mixing, but obliteration of evidence by slow cooling and reequilibration 46 makes interpretation difficult. The Si02 variation diagrams may be construed to imply magma mixing. Exceptionally linear trends could be caused by mixing different proportions of rhyolitic and basaltic melt. However, Chappell and White (1974) note that most ''I''-type granitoids have variation diagrams with linear trends. The occurrence of a few plagio- clase crystals with sodic cores and oscillatory zoning could represent felsic magma, but these could equally well be restite or the result of pressure changes in an anhydrous system (Vance, 1965; Wiebe, 1968). If magma mixing did play a part in the evolution of the Lake Ann stock, the evidence has been destroyed. THE LAKE ANN STOCK IN RELATION TO THE MAGMATIC HISTORY OF THE PACIFIC NORTHWEST Reconstruction of the Cenozoic magmatic history of the Pacific Northwest has moved forward rapidly in the last several years. These advances allow the Lake Ann stock to be seen in an historic perspective and in a plate tectonic framework. Models have been published by Hammond (1979) , McBirney (1978), Armstrong (1978) and Vance (1977). Per tinent observations on the Late Tertiary history of Western British Columbia have been made by Bevier et al. (1979). To one degree or another, these models link the magmatic history to plate movements along North America's western edge. 47 Major Cenozoic magmatism in the northwest began with the Challis volcanic episode (Armstrong, 1979) about 55 Ma. Volcanic activity formed a northwest-southeast trending arc in British Columbia and Washington. Diorites of this episode are exposed in the Chilliwack Batholith south of the Skagit River (Engles et al., 1976). Activity waned toward the end of the Eocene as volcanism moved westward into the Cascades. This period coincides with the bend in the Emperor-Hawaii chain and a possible slowing in convergence rates (Vance, 1977). Apparent magmatic quiescence from 41 to 35 Ma (Vance, 1977) was followed by Oligocene volcanism and plutonism similar to what is seen in the Cascades in the Quaternary (Armstrong, 1978). Oligocene volcanics became more silicic with time. The spatial distribution was unusual; alkaline rocks were erupted in the Coast Range with less alkaline rocks inland (McBirney, 1978). A lull in magmatism in the Early Miocene was followed by the exceptionally voluminous Columbian volcanic episode. Eruption of the Columbia River Basalts and a thick coeval sequence of andesites in the Cascades was the most important magmatic event in the Cenozoic. Three pulses of Late Miocene to Recent magmatism have been noted (Figure 15). Armstrong (1978) places them at 3 to 10, 5 to 7, and 0 to 2 Ma. McBirney (1978) places them at 9 to 10 (Andean), 3 to 6 (Fijian) and 0 to 2 Late Cenozoic Magmatic Episodes a o c >> 48 (McBirneyj 1978) (Armstrong, 197® (Bevier, et al, 1979) i 49 (Cascadian). The 2 to 3 Ma lull is associated with a shift in magmatism to the present arc axis (Cascade or Garibaldi belt) from a position just to the east (Pemberton belt) (Bevier et al., 1979). Changes in subduction geometry caused by reorientation of the Explorer Plate about 3 Ma may be responsible for the movement of the magmatic arc (Bevier et al., 1979). Alternatively, the westward step may be linked to changes in spreading and subduction rates. Circum-Pacific volcanic episodes have been linked to spread ing rates (Noble et , 1974), and increased spreading rates of the Juan de Fuca Ridge (Rea and Scheidegger, 1979) may coincide with the Fijian episode and the Cascadian episode. The cooling dates on the Lake Ann stock, 2.7 ±0.3 and 2.5 ± 0.1 Ma, and the dates on the Hannagan volcanics, 3.3 +1.0 and 3.6 ± 1.0 Ma, coupled with their position east of the present Cascade volcanic arc, indicate that they are a southern extension of the Pemberton volcanic belt of Bevier et (1979) and are part of the Fijian volcanic episode. SUMMARY AND CONCLUSIONS The Lake Ann stock is one of the many plutons of the Late Tertiary Cascade magmatic arc. Granophyre, fine equigranular grain size, miarolitic vugs, resorption of plagioclase by alkali feldspar and high temperature miner- ology are common to these plutons as a result of rapid low 50 pressure crystallization. Estimates of temperatures and pressures in the contact aureole of the Lake Ann stock are between 600°and 925°C and less than about 2 kb. Resorption textures indicate that the pressure in the magma was less than about 3.6 kb. The chemistry, location and time of emplacement of the Lake Ann stock and the Swift Creek volcanics are related to their ensialic arc setting. Application of current theories of arc magma petrogenesis, i.e., resite theory and magma mixing, is problematic. It appears that there is a small component of ''I"-type restite in the stock. The stock's chemical composition is similar to other "I"-type granitoids. There is no unequivocal evidence for magma mixing. The magmatic history of the Lake Ann area fits the pattern of magmatic activity observed in British Columbia by Bevier et al. (1979) and may be related to the pattern of episodic accelerated spreading rates on the Juan de Fuca Ridge (Rea and Scheidegger, 1979). The magmatic history of the Lake Ann area is: 1) eruption of the Hannagan volcanics (3.4 Ma) and emplacement of the Lake Ann stock (2.5 Ma) at the southern end of the Pemberton volcanic belt during the waning stages of the Fijian volcanic episode, 2) magmatic quiescence and (glacial?) erosion to expose the stock; then from the Garibaldi (Cascade) volcanic belt during the Quaternary Cascadian volcanic episode, 3) eruption of the Swift Creek volcanics and 4) flows of Mt. Baker andesite. 51 ACKNOWLEDGMENTS I thank Dr. R. S. Babcock for his encouragement and valuable suggestions in the field and laboratory. Dr. E. H. Brown made important suggestions and provided me with summer emplo3rment. I am very grateful to Dr. D. R. Pevear for his good humor, careful reading of the draft manuscript, and for the provision of office space for two long years. A discussion with Dr. P. Misch helped to clarify several poorly understood field relationships. I thank S. Morgan for his accurate chemical analyses and suggestions. D. B. James, S. K. Cruver and S. Ciacco assisted greatly in the field. I thank D. B. James and L. K. James for their typing. Base maps were kindly provided by Mt. Baker, National Forest Technical Center, U.S. Forest Service. 52 REFERENCES CITED Anderson, A.T., 1976, Magma Mixing; Petrologic process and volcanological tool: J. Vole. Geotherm. Res., v. 1, p. 3-33. Armstrong, R.L., 1978, Cenozoic igneous history of the U.S. Cordillera from lat 42 to 49 N, ^ Smith, R.B., and Eaton, G.P., eds., Cenozoic tectonics and regional geophysics of the western Cordillera: Geol. Soc. Amer ica Mem. 152, p. 265-282. Barker, D.S., 1970, Compositions of granophyre, myrmekite and graphic granite: Geol. Soc. 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Wyllie, P.J., and Tuttle, O.F., 1961, Hydrothermal melting of shales: Geological Magazine, v. 98, p. 56-66. 57 Appendix 1 Relative error and detection limits for ten major and five minor oxides at the X-Ray Spectroscopy Labs, USGS, Menlo Park, California, May 1979 Element % Relative Error Detection Limits (PPM) Si02 0.36 125 A1 2O3 0.98 200 0.74 27 MgO 2.39 205 CaO 1 .09 28 0 CM 1 .68 700 K2O 1 .58 10 Ti02 1 .74 23 P2O3 4.55 50 MnO 2.41 10 NiO 5.8 10 5.0 15 BaO 6.3 10 ZrO 4.2 20 SrO 2.8 20