UNITED STATE DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY Chemical analyses and rock series classification ~f 102 tonalite samples for the Jurassic magmatic arc, Alaska-Aleutian Range batholirh, Alaska Bruce L. Reed, Marvin A. Lanphere, and Alfred T. Miesch Open-File Report 83-94 This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards and stratigraphic nomenclature
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U N I T E D STATE DEPARTMENT OF THE I N T E R I O R GEOLOGICAL SURVEY
Chemical analyses and rock series classification ~f 102 tonalite samples for the Jurassic magmatic arc,
Alaska-Aleutian Range batholirh, Alaska
Bruce L. Reed, Marvin A. Lanphere, and Alfred T. Miesch
Open-File Report
83-94
This report is preliminary and has not been reviewed for conformity with
U.S. Geological Survey editorial standards and stratigraphic nomenclature
Chemical ana lyses and rock s e r i e s c l a s s i f i c a t i o n of 102 t o n a l i t e samples f o r the J u r a s s i c magmatic a rc ,
I l laska-Aleutian Range b a t h o l i t h , Alaska
Bruce L. Reed, Marvin A. Lanphere, and Alfred T. Miesch
The purpose of t h i s r e p o r t i s t o make a v a i l a b l e major oxide chemi- c a l ana lyses and t h e corresponding CIPW norms of 102 t o n a l i t e samples from t h e J u r a s s i c magmatic a r c i n t h e Alaska-Aleutian Range b a t h o l i t h . Discussion of chemical v a r i a t i o n s and t h e p o l a r i t y of t h e J u r a s s i c a r c i s given i n Reed, Miesch, and Lanphere (1983).
More t h a n 80 percen t of t h e exposed p l u t o n i c rocks i n t h i s p a r t of the J u r a s s i c a r c ( f i g s . 1 and 2 ) comprise a composi t ional ly continuous c a l c - a l k a l i n e s u i t e t h a t ranges from hornb lend i te through hornblende gabbro, hornb lende-b io t i t e d i o r i t e , hornb lende-b io t i t e q u a r t z d i o r i t e , t o n a l i t e , t o r a r e g r a n o d i o r i t e and q u a r t z mozonite. T o n a l i t e and q u a r t z d i o r i t e a r e the dominant rock t y p e s , and t h i s s u i t e of p l u t o n i c rocks i s in formal ly r e f e r r e d t o as t o n a l i t e . I s o l a t e d bodies of high-A1203- type t rondhjemi te on t h e nor thwest s i d e of the b e l t and q u a r t z monzonite and g r a n o d i o r i t e p lu tons p resen t l o c a l l y on t h e s o u t h e a s t margin of t h e b e l t a r e not included i n this s tudy ,
The average and ranges of 64 modal ana lyses made on s t a i n e d s l a b s a r e given i n t a b l e 1. Forty-four of t h e s e samples con ta in less than 1 volume percen t K-feldspar, and on a Q-A-P diagram most of t h e samples f a l l i n t h e q u a r t z d i o r i t e o r t o n a l i t e f i e l d c l o s e t o t h e quar tz-plagio- c l a s e j o i n ( f i g . 3 A ) . Examination of t h i n s e c t i o n s of t h e remaining samples shows them t o c o n t a i n g e n e r a l l y less than 3 volume percen t K- f e l d s p a r . Ferromagnesian minera l s comprise between 15 and 60 percen t of each rock ( f i g . 3B) and c o n s i s t c h i e f l y of green hornblende, brown bio- t i t e , and, r a re ly , pyroxene.
Chemical ana lyses and t h e corresponding CIPW norms f o r t h e 102 samples a r e given i n t a b l e 2 . A s t a t i s t i c a l summary of t h e s e d a t a and a few simple r a t i o s a r e given i n t a b l e 3.
On a t e r n a r y normative f e l d s p a r diagram ( f i g . 4 A ) , a l l 102 samples f a l l i n t h e t o n a l i t e f i e l d . A l l but one are c l a s s i f i e d as c a l c a l k a l i n e on an A-F-M p l o t ( f i g . 4B).
Figures SA, 5B, and 6 show t h a t , f o r t h i s s u i t e of p l u t o n i c r o c k s , use of commonly used x-y rock c l a s s i f i c a t i o n diagrams may r e s u l t i n samples being placed i n d i f f e r e n t rock s e r i e s . The grouping of samples on Si02-versus-Na20+K20 p l o t (fig. SA) i s no t d i s t i n c t , and about half t h e samples l i e i n t h e t h o l e i i t i c f i e l d . The t o n a l i t e samples have a high a l k a l i - l i m e index (2 6 6 , fig. 5 A ) , and t h e i r K20/Na20 r a t i o s a r e low, ranging from 0.08 i n t h e m a f i c rocks t o 0.73 i n t h e more s i l i c i c rocks (table 3 ) . The FeO*/MgO r a t i o s r e l a t i v e t o Si02 have a uni- formly low r a t e of i n c r e a s e ( f i g . 6 ) , range between 1.41 t o 4.75, and
References Cited
Barker, F., 1979, Trondhjemite: definition, environment and hypothesis of origin, p, 1-12, Barker, F., ed., Trondhjemltes, dacites and related rocks, Amsterdam, Elsevier Scientific Publishing Company.
Barker, F., and Arth, J. G., 1976, Generation of trondhjemiric-tonalitic liquids and Archean bimodal trondjhemite-basalt suites: Geology, v. 4, p. 596-600.
Ewart, A , , 1976, Mineralogy and chemistry of modern orogenic lavas-- same statistics and implications: Earth and Planentary Science Let- ters, v. 31, p. 417-432.
Gill J.B., 1981, Orogenic andesites and plate tectonics, v. 16 of Kyllie, P.J., Ed. in Chief, Minerals and Rocks: New York, springe7 Verlaag, 390 p .
Miyashiro, A., 1974, Volcanic rock series in island arcs and active continental margins: American Journal of Science, v. 274, p. 321-355.
OIConnor, J. T., 1965, A classification for quartz-rich igneous rocks based on feldspar ratios: U.S. Geological Survey Professional Paper 525B, p. B79-B84.
Peacock, M. A. , 1931, Classification of igneous rocks: Journal of Geology, v. 39, p. 54-67.
Reed, B. L., Miesch, A. T., and Lanphere, M. A . , 1983, Plutonic rocks of Jurassic age in the Alaska-Aleutian Range batholith: chemical varia- tions and polarity: Geological Society of America Bulletin, in press.
F i g u r e 1.--Map showing Alaska-Aleutian Range b a t h o l i t h and Jurassic magmatic arc (Jurassic p l u t o n i c rocks)
Q (quartz)
4 / C'; \
Granodior i te
Quartz d ior i te
Quar tz monzodiorlte
rnaf ic minerals
plagioclase + alkal i feldspar
F i g u r e 3.--Modal d iag rams of 6 4 plutonic rocks f rom the Jurassic magmatic arc. ( A ) , Q-A-P; (B) Quartzlnafic minerals-total feldspar.
F (total Fe as FeO)
Figure 4A.--Normative p l o t s of An-Ab-Or for Jurassic i ona l i t e samples. Rock c lass i f icat ion af te r 0' Connor (1965) GS
modified 5y Barker (1979).
4B.--A-M-F diagram showing trend of Jurassic tonalitc samples.
Alkaline field
Calc -alkaline
Tholeiitic field
\
Tholeiitic field
N O R M A T I V E PLAGIOCLASE COMPOSITION
Figure 5A.--Plot of Si02 versus K2O+Na20 with least-squares E i t l i n e (dashed) for Jurassic t o n a l i t e samples. Sol id l i n e i s least-squares f i t for CaO. The a lkal i - l ine index (weight percent of S i 0 2 ) , where the K2QtNa20 l i n e intersects the CaO l i n e , for the Jurass ic s u i t e of rocks is about 65, def- ining the rocks as c a l c i c (Peacock, 1931). Alkaline, calc-alkal ine, and . h o l e i i t i c f i e l d s from Barker and Arth (1976). See t ex t for explanation 5f sample numbers and symbols.
5 B . --Plot of normative plagioclase composition versus weight percent A l 2 0 3 . Dashed l i n e separating calc-alkaline and t h o l e i i t i c fields from Irving and Baragar (1971) .
Thole i i t ic f i e l d /
f i e l d
0 0 45 5 0 5 5 60 65 70
SiO, ( w t %)
Figure 6.--Plot of SiO2 versus FeO*/MgO. Dashed line t h a t separates a l k a l i n e and tholeiitic fields from G i l l (1981) , following Miyashi to (1974). See text for explanation of sample numbers and symbols.
Table 1. Averages and ranges of modal analyses (volume percent) of 64 samples collectively referred to as tonalite from the Jurassic magmatic arc in the Alaska-Aleutian Range batholith.
Table 2.--Chemical analyses, norms, differentiation indices and location of 102 plutonic rocks collectively referred to as tonalite from the Jurassic magmatic arc in the Alaska-Aleutian Range batholith.
[Rapid chemical analyses of rocks by Lowell Artis, S. D. Botts, Gilson Chloe, P. L. D. Elmore, J. L. Glen, James Kelsey, Herbert Kirschenbaum and Hezekiah Smith. X-ray spectroscopic analyses and partial chemical analyses are indicated by a 7 symbol, and analyses were made by J. W. Barker, E. L. Brandt, E. E. Engleman, G. Manson, H. G. Neiman, F. Newrnan and J. N. Taggar t . ]