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

(Mg3.09 pe2+ 0.69 pe 0.~1~0.01~~0.98)~~7.~4~~0.66)-
3+
greater1 (3) their AMP is less aluminous and sodlc and
022P0.11 OH)^,^^. Blue-green pleochroic rims me
is caldc rather than sodic-calcic; and (4) calcite, plagioclase,
and sphene accur in some samples. .Dffferdistinguished
by enrichment in pe2+, pe3+, and Ca aces 1 through 3 suggest metamorphism at lower 2
(table 13), add compositions range from magnesiohorn- and P for the Fairbanks eclogites, which Is consistent
blende to tschermakitic hornblende.
withThe estimates of 540' to 590'~ and 0.55 to 0.75
GAR becomes slightly less calcic toward the GPa by Swainbank and Forbes (1975).
grain margins (fig. 34). The least calelc composition,
represented by the formula ~ e ~ ~ ~ ~ ~ g ~ ~ ~ ~ ~ n ~ . ~ ~
~o.63"l.98~e~to6"2.95~*.050~2~ * lcom edge
of a grain at its contact with a blue-green pleochroie
AMP rim. The E'e3'/(~1 VI +Fe
3+
Ti+Cr) rntlo in clinozoisite
(CZO) is small and ranges from 3.0 to 4.5 atom
percent. White mica is muscovite showing considerable
phengite substitution. The K/(K+Na+Ca) ratio
ranges from 81.6 to 86.0 atom percent, and the
ca/(K+~a+Ca) ratio is no more than 0.9 percent.
On the basis of GAR composition (fig. 341, thb
eclwte falls within group C (eclogltes from alpinetype
orogenic terranes) of Coleman and others (1965)
but is near the "contactn with group B (eclogite from
gneiss and migmattte terranes). Coexisting GAR and
omphecite compositions are also comparable to thase
at the group Clgroup B boundary (compare fig. 34 with
Coleman and others, 1985, fig. 11). On the basis of
omphacite composition, the ecloglte Is simUar to eclogites
from highpressure fietamorphic terranes,
whereas the distribution of Fe /Mg between GAR and
PYX suggests amphibolite- or granulite-factes terranes
(Miyashiro, 1973, figs. 124, 12-51.
The occurrence of biotite+Peldspar+quartzCgarnet+whlte
miea gneiss (sample 79AWr93, fig. 33)
associated with eclogite-appearing rubble suggests
group B affinities. Further petrographic and electronmicroprobe
studies of the mineralogic variations in the
eelogitic rocks represented and in the associated
peUtic and felsic rocks are planned to test this
hypothesis.
Figure 35 ilIustrates our best estimate of the
pressure() and temperature (3 of metamorphism on
the basis of the experimental date presently available.
The omphacite +quartz assembGe constrains P
to above the jdq5 Isopleth for the reaction
alq&eqadelte+quartz. T is constrained by the
Pe /Mg distribution between GARand PYX but must
be less than the high T estimates from the EUis and
Green (1979) calibration because the eclogite appears
to be intercalated with pelitic sahist, which would
have partially melted if 2 were above that of the
reaction muscovite+K-feldsparfalbf te+quartz+vapor =
liquid-just above 600'~ for these pressures (Thompson
and Algor, 1977, fig. 8). Using the muscoviteparegonite
solvus of Eugger and others (1972) gives a
minimum T of about 530 C, but the magnitude of the
effect of phengite substitution on this solvus Is
unknown.
The only eclogites previously described from the
Yukon-Tanana Upland are group C eclogites (Swainbank
and Porbes, 1975) in the Fairbanks district, about
39 km southwest of the eclogite locality near Twin
Buttes (fig. 33). These Pairmks district ecl
distinct from the sample studied in that:
GAR is almandine richer but pyroq$ poorer; (2) their
PYX Is jadeite poorer, and ED=(Pe /Wg)GAR/PY is
Figwe 35.-Estimated pressure (9 and temperature (T)
(within box) of metamorphism for sample 79~~rlO&
(fig. 33). 52 stability for reaction albite=jsdeite+
quartz b from Holland (1980). jdqO and jdq isopleths,
repraentkng minimum and maximum jadel?e contents
observed In omphacite, calculated from model of
Ho)#nd (1979, 1980). Lines of constant ED=
Fe /~g)=- py representing minimum and
maximum v8des %served, extrapolsted from Raheim
and Green (1974) (dotted Unes) and Ellis and Green
(1979) (dashed lines).
Whole-rouk analyses or the Pairbanks district ec-
logites indicate sedimentarg protoliths. However, the
distinction in mineral assemblage (difference 4) md
the field relations suggest that an Igneous protolith Is
more probable for the eclogite nem Twin Buttes.
Swainbank and Porbes (1975) suggested that the
Fairbanks district eclogite terrane Is a tectonic wln-
dow. Occurrence of brecciated rocks about 1J krn
south of the eclogite and associated pelitlc schist and
quartzite near Twin Buttes suggests a possible tectonlc
contact with the pelltic, mafic, and calc schist end
quartzite to the south. Fairly good preservation in the
sample from near Twin Buttes except for formation of
cryptocrgstaLLIne material around the PYX and blue-
green rims on the AMP (possibly associated with Ter-
tiary plutonlsm) suggests that thL eclogite was ex-
humed quickly. More detailed mapplng and petrologic
study are required to test this hypothesis, to determine
the relation of this eclogite and associated rocla to
adjacent metamorphic terranes, including that of the
Fairbanks district, and to further assess its geologic
implications.