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

north of Twelvemile Summit (fig. 321, Q a more alumi- assemblages: (I) cryp tocrystallhe black marble, (2)
nous quarte~l@~~+muscovite+Pe-~~orite~~o~ clinohumiteYbrsterIte+dolomite+calcIte+Ma-*NoritoIdf
garnet sahist. These assemblages indicate Ite+MgserpantCne marble, (3) green acdno~te+calgarne
t e e mediumpressure faciesseries metamor- eite+epidote+diopside+quartz+pl~gi0~109e+sphene eels
phism; they are steble from about 400'~ at 0.1 to 0.4 with sllky white trernolite layers, and (4) massive red-
GPs (limited by the breakdown of pycophyllite) to brown grossularite+green hedenbarg~te+quartz~dclte
550 C at 0.7 GPa (using the petrngenetic grid of fels.
Labotka, 1881, big. 11). An increase in metamorphic
grade within this range of temperatures is seen toward
the east, as indicated by increasingly Mg rich chlorite
and Ca rich plagioclase in the less aluminow assemblage
(fig. 32).
North of the lineament, the pelitlc rocks are [Ydutlength-dl%pers(ve analyses an a CMZCA mobel *81 clacVon alcroroh.
ustnq Tram-r-Uartbrn model IN-1310 buurmatlon. ccnvarrpd to wight percant
composed of qunrtz+pIag;Ioclase+m uscovite+bio- according ta the Bencc-Albce whniaye VIU nlbee-RW correction Ixurs.
AccelOrdtlnQ voltape 15 kY. sample current 12 nA on brass- spot stre. 16 tr
tite*Fe-chlorite. Garnet is rare and wcurs only in a for plsgt~lbsc IVLG~ rnd ~ 'm for garnet lu01 And pymrcn; (PVII, n.d.,
few samples as resorbed grains or as inclusions in pla-
not dchmined. SEc figure J2 far mlt local lrfcs. Pn, FOR. a d d\G
nonml(zM to total cations of 4. 5, an 6. res IIVcly; A) and Al
gioclase. The porphyroblas tic plagioclase resembles rrslgnrd from asrued stolchl-: feq' anl Fmj5 .%timated frp. afrrenl
charge b ~ dncsl l
that of pelitic rocks south of the lineament. Two generations
of biotite are present. The first occurs as
alined inclusions in plagioclase that define a weak
folintion, which were apparently formed during
regional metamorphism. The second k randomly Hlneral------ GAR PIX PLC
oriented biotite that crosses the early foliation and
gives the rock a hornfebic texture. These
observatiorts suggest that contact metamorphism was
superimposed on a preexisting garnet-grade epidoteamphibollte-facies
regional metamorphism. Contact
effects do not extend south of the lineament.
Petrographic examination indicates that the plagloclase
porphyroblasts are of albite In the pelitic
schist south of the lineament near Twelvemile Summit,
but farther east they are zoned and have albite cores
( ~ n ~ , and ~ ! oligoclase rims (An24,27E. In contrast,
prelirn~nacy electron microprobe data (table 12) on
zoned phgioclase froin near the pluton Indicate that
the cores are An23, and the rims are Anl. -More
data are needed to ieterrnine whether this zoning is
continuous and, if not, to assess whether the cores and
rims represent polymetamorphism or equilibrium
mineral growth across the peristerite gap, which would
confine the metamorphic temperature to between
approximately 400' and 550' (Smit3 1872, fig. 9).
Metamorphic temperatures above 600 could not have
been reached on either side of the Uneament or else
the pelitic rocks would have undergone partial melting.
South of the lineament, mafic schist contains
amphibole+plagioclase+quartz+M~hlorite+biotite+epidote+garnet+sphene.
Petrographic data suggest that
the amphibole has an actlnoUte core and a hornblende
rim and that the plagloclase porphyroblasts have cores
of albite rimmed with oligoclase. Coexistence of
hornblende with oUgoclase is consistent with the garnet-Jlgoclase
qede of the intercalated pelitic rocks
(Laird, 1980, fig. 2).
Mafic rocks north of the Uneament are mineralogically
and texturally distinct. The assernbhge is ollgocW+quartz+actinolite+biotite+epidotephene
Fe- AssembIege 2 may be described by the
chlorite. Garnet is rare and occurs as resorbed grains equilibrium lor8 terite+dolorn ite+ A O=clinohumite+calor
a8 inclusions in plagioclase. Actinolite needles form eite+Co2, which constrains the rnaarnorphie temperasmall
radial clusters; random orientations of white ture to a minimum of about 450'~ (Rice, 1980).
mica, blotite, and tourmaline imply contact metarnor- Absence of evidence of partial melting of the inter*
phism. Coexistence of actinolite with oligoclase l&ed pelitic rocks sets the upper temperature limit at
rather than albite also indicates low-pressure facies 600'~. This range of metamorphic temperatures is
series metamorphism (compare Laird, 1980, figs. 2, 5). consistent with asembhges 3 and 4 and with the pres-
Calc-silicate rocks, which crop out only north of ence of calcitequartz rather than wouastonite
the Uneament, include four thin-bedded mineral twinkler, 3979, figs. 9-5, 9-6, 10-2). Low values of