Geological Survey

Tastw 3,—Quantitative petrographic summary of chemically analyzed rocks from Umnak and Bogoslof Islands
(Numbers of columns correspond to those in table 2}

Bulletin 1028-L, Table 3

Northeastern Umnak

1A 2 3 4 5 6

8 9 10 11 12 13

14 15 16 17 18 19 20

Mode (volume percent): !
Xenolithic fragments.
Crystals or phenoerys

Composition of less calcic plagioclase crystals
Composition of more calcic plagioclase crystals
Estimated average composition of less calcic plagioclase crystals.
Estimated average composition of more calcic plagioclase crystals
Approximate composition of groundmass plagioclase (+: Ans).
‘Average beta index of augite crystals 1_
‘Average 2V of augite crystals TM

Composition of olivine crystals TM
Estimated average composition of olivine crystals.
Refractive index of groundmass or intersertal glass.

71.575

71, 560

Southwestern Umnak

Bogoslof

22 23

326 27 228 229 30

31 32 33 34 235

Mode (volume percent):
Crystals or phenocrysts >0.2 mm:
uartz...

Less calcic plagioclase
More calcic plagioclase. :
Biotite (and minor chloritic alteration).
‘Amphibole (includes decomposed amphibole)
Clinopyroxene (epidote in column 33)..-..
Orthopyroxene: hypersthene-.

Olivine.

Composition of less calcic plagioclase crys
Composition of more calcic plagioclase crystals
Estimated average composition of less caicie plagic ys
Estimated average composition of more calcic plagioclase ¢1
Approximate composition of groundmass plagioclase (+t An,
mama index of biotite crystals

Beta index of amphibole crystals
Beta index of ferriferous # clinopyroxene erystals
Beta index of calciferous * clinopyroxene crystals TM_
2V of ferriferous clinopyroxene crystals ¥_.

2V of caleiferous clinopyroxene crystals
Composition of hypersthene crystals #__
Average composition of hypersthene crystals.
Composition of olivine crystals #_____
Average composition of olivine erystals_
Refractive index of groundmass or intersertal glass_

tals.

‘aia ll a

Anso-35 2 Anss-o2
Angas Anges Amie-co ( (absent) (
33 a0
Any Ang:
62 An,
s Ani |-

=| Fogz—12
Foro |--
1. 505

1, 543 1. 499 1,514

3
1.9 12
‘x 62.3 K 60.9
n, Dee
(absent) Dyemet Anre-o4
Anas Anas
85 gs
Anssi Ansa

1. 498

1 Determined by point counter described by Chayes (1940).

? Minimum size of erystals about 0.1 mm; modes of intrusive rocks include somewhat smaller
crystals.

3 Visual estimate, Rare hypersthene also present.

«“Ghosts” of olivine, now antigorite, penninite, quartz, and magnetite.

5 Includes palagonitized glass, zeolites, etc.

© Includes 0.6 percent of calcite and 2 percent of chlorite, antigorite, quartz, and zeolites.

1 Includes 0.1 percent of zeolite-filled vesicles.

"Includes felsic groundmass with microveinlets of quartz, zeolite, and magnetite.

* Plagioclase compositions from refractive index data, using curves by Calkins-Hess (Kennedy,
1947, fig. 1) and by Tsuboi (1925, pl. 1). Most determinations within --Ans, limnit, Ans.

1 Based on optical determination of refractive indices; partial analysis for alkalis (Na-K, 1.14 per-
cent) indicates composition of Ann.

Refractive indices determined with sodium light at 25° C.-2° C. Most measurements within
0.00, excepting thoso on harnblende.

1 Data on groundmass sugite.

4 Diopsidic augite (6=1,606) also present; probably xenocrysts.

142V measurements on grains in thin section, using 4-axis universal stage; average of several read-
ings, exeept for No. 9, in which full range of 2V variation of augite is shown.

41 Olivine composition from refractive index data, using curves by Bowen and Schalrer (Kennedy,
1947, fig. 6). Most determinations within +-Fos.

1 Also at least two additional olivines, probably xenocrysts, with overall range of composition from
Fos to Fou.

# Data on unpalagonitized clear glass.

4 Bleached blotite; probably a chlorite.

# Entirely decomposed to opaque oxide-clinopyroxene aggregate,

% Largely decomposed to secondary minerals: opaque oxide rims, clinopyroxene, fibrous green mica
and amphibole, sodic plagioclase, brown glass with apatite needles, and carpholite(?).

3 Determination by refractive indices (a=1.527—8; y=1.538) indicates nearly pure albite.

% A few plagioclase erystals are zoned through entire range, Anse-ss.

% Includes ferriferous augite and ferroaugite following classification of Poldervaart and Hess (1951,
474).

44 Includes diopsidic augite, salite, and ferrosalite following classification of Poldervaart and Hess
(1961, p. 474).
= Orthopyroxene composition from refractive index data, using curves by Hess (Kennedy, 1947,

3).

489292—59 (In pocket)