Late Paleozoic to early Mesozoic rocks in the Sierra Nevada and Klamath Mou
ntains, western United States, preserve a record of lateral growth of conti
nental crust by incorporation of ophiolites, volcanic arcs, and associated
sedimentary rocks. Deciphering the timing of arc construction and metamorph
ism is critical for elucidating tectonic and thermal evolution during this
type of crustal growth, but is difficult in complex accretionary terranes.
In this study, we address the timing of arc construction and regional metam
orphism in the Slate Creek Complex, a volcano-plutonic terrane in the centr
al part of the northern Sierra Nevada.
New Ar-40-Ar-39 ages from relict volcanic hornblende demonstrate that the y
oungest volcanic unit in the Slate Creek Complex is ca. 170 Ma, at least 30
m.y, younger than previous estimates of ca. 200 Ma for metaplutonic rocks
in the lower part of the complex. Consequently, the Slate Creek Complex is
polygenetic, with two distinct episodes of arc magmatism. The distribution
of mineral assemblages and textures indicates that greenschist and epidote-
amphibolite facies metamorphism is younger than 170 Ma and is associated wi
th crosscutting plutons that yield cooling ages of 150 Ma and younger. Amph
iboles from foliated rocks in the Slate Creek Complex yield plateau ages of
156.1 +/- 0.6 and 152.0 +/- 0.7 Ma that date cooling subsequent to or dyna
mic recrystallization during the metamorphic event.
Age and lithologic similarities suggest that the lower parts of the Slate C
reek Complex correlate with plutons (ca. 200 Ma) and the volcanic cover seq
uence of the Rattlesnake Creek terrane in the Klamath Mountains. The upperm
ost volcanic unit correlates with the ca. 170 Ma Western Hayfork terrane th
at structurally overlies the Rattlesnake Creek terrane. Metamorphism in the
Slate Creek Complex correlates broadly with a similar late metamorphic eve
nt in the Klamath Mountains.