n Alas - Alaska Division of Geological & Geophysical Surveys - State ...

east of the gnelss dome and is separated from it by the generay the case. We presumed that the clear eunorthewt-trending
Shaw Creek fa~llt. The sillirnanite hedral zircons grew during metamorphism, end so we
gneiss on both sides of the fault is petrographically selected those zircons in the +I 50 size fraction and the
similar was probabIy continuous before faulting. The entire population in the -325*400 and -400 size fracsense
and extent of movement dong the Shaw Creek tions of semple 80APr 114 for analysis. In sample
fault ~equired for the palinspastic reconstruction agree 81ADb 35A, greater than 75 percent of the zircons arc
with the 50 km of left-lateral movement postulated by clear and euhedral, with anvx ratio of 1 to 3, whereas
Griscom (1980), primwily on the basis of aeromagnetic less than 25 percent me subhedral and btown. Aldata.
though the yleld of zlrcons in sample 8kADb 35B was
Three Yimpla of sillimanlte gneiss from the very small, most were euhedral and light brown. Algneia
dome were studied, including one sample (BOA Fr though many of the zircons in sample 81ADb 38A are
114) from an area of Typicaln sillimanite gneiss and detrital, in marked contrast to the other sillirnanite
two samples from n locality where partial melting may gneiss samples, a few clear euhedral crystals ore also
have occurred. One of these samplea (81ADb 3SA) is present; these were the only zircons analyzed from
another "typicalM sillimanite gneiss; the other (81~Db this sample. Sample 79AFr 2010, an unfoliated
35B) is a weakly foliated equigranular granitic rock, granite, contains zircons that range in color from dear
possibly 61 leucosome In the slllimanite gneiss. An to light brown; all are euhedral, and the ratio
unfollated crosscutting granite wa5 sampled (7BAPr ranges from 2 to 8. There is no optical evidence for
2010) to provide evidence for a minimum age of the cores or overgrowths.
gneiss. Finally, a sample (BlADb 38A) of what appears
both in hand semple and thIn section to be "typicaiTt
sillimanite gneiss from east of the Shaw Creek fault
Table
was studied for comparison with those from the gneiss
8 .--Major-element data for selected sm les of
sflllmanire gm ~ z!!%!21e &imLj7v,
dome.
Table 8 lists m40relement mdyses for the - Alaska
three samples of silllmanfte gneim. The chemical [All analyses in weight percent by quantltatlve X-ray
compositions of the rocks are compatible with a sedi-
spectroscopy 1
mentary protolith proposed for the gnelss, in that
potassium is greater than sodium, magnesium greater -
than calcium, and normative quartz close to or greater Smplc------------- 8IADb 35A 8LADb 36A 81ADb 38A
thnn the 50percent value considered to indicate a
sedimentary potolith (M-, 1866, p. 249). Normative
quartz values, calculated on the assumption that sfo2-------- ---- --- 69.3 74.0 74.7
two-thirds of the iron is ferric, are: 49 percent ~ 1 ------------ ~ 0 ~ 14.9 9.65 10.5
(sample 81ADb 35~); 59 percent (80~Pr 114); and 52
percent (81ADb 38A). The major-element abundances Total Fe (as FcgOj)- 6.21 3.43 5.49
for a sample of the sillimmite gneiss collected east of go----------------- 2.18 1.20 1.81
the fauit (8lADb 38A) are intermediate between those Ca&---------------- .41 .84 .77
for the two samples from the gneiss dome for all elements
except Na20 and MnO and thus are permissive Naq&--------------- .66 1.02 1.14
evidenoe for these areas having had the same proto- ~~0 ----------------- 3.53 2 -89 3.34
lith. Tfo2-------------- -76 .52 .75
The fntent of this study was to determine the
P~O~---~~-----"- .05
age(s) of metamorphism of the sillirnanite meIss
.07 .05
bodies and to examine the morphology of their zir MnO----------------- .06 ,04 .08
cons. Zircons were extracted from approximately 40 Loss oh Ignltton---- 1.58 .81 .81
kg of rock and processed by routine ion exchange
(modified from Krogh, 1973) for the separation of
uranium and lead isotopes; these elements were Total---------- 99.64 99.47 99.44
analyzed on a 30-em NBS mass spectrometer with digital
control and data processing.
Interpretation of zircon morphology is perticularly
important for this study because of the possible
occurrence of both detrital artd metamorphic popula- Table 9 lists the botopic data plotted in figure
tions. In sample 8OAPr 114, about 25 percent of the 27. The most geolagically reasonable interpretation d
+I50 size fraction are clear euhedral crystals with these data (fig. 27) involves calculating the best-fit
length-to-width WE) ratios of 1 to 3; approximately 50 line through only the upper four data points for the
percent of the crystals are brown in color, subhedral to coarsest size fractions of zircons from all three sillieuhedral,
with partly pitted faces and anl/xratio of 1 rnanite gneiss samples (8OAPr 114, 01ADb 35A, 8IADb
to 5; and the remaining 25 percent are broken and (or) 38A). me discordia through these points has concordia
rounded brown grains. In the finer grain sizes, at least intercepts of 302f156 and 2,383i398 m.y. The three
75 percent of the pains me clear and euhedral, lower data points, from two finer grained zircon fracwhereas
25 percent or less are brown and partly tions from sillimanite gneiss (sample 80AFr 114) and
rounded. Although euhedral zircons that are unequivo- one coarse fraction from the probable leucosome
cally detrital in origin have been found in metasedi- (9Etmple 81ADb 358), were not used in determining tile
mentary mcks (Grauert and others, 19731, this is not discordia. The zircons from these fractions contain