LINDLINE, J., Geology Dept, Bryn Mawr College, Bryn Mawr PA 19010; MORA-KLEPEIS,
G., CSIRO Minerals Research Laboratories, PO BOX 136, North Ryde, NSW 1670
Australia; CRAWFORD, M.L. and CRAWFORD, W.A., Geology Dept,
Bryn Mawr College, Bryn Mawr PA 19010, mcrawfor@brynmawr.edu
Abstract:
In the central Coast Mountains the Eocene was a time of magmatic quiet
following emplacement of the ~70 to 50 Ma Coast Mountains batholith
(CMB) and before a pulse of Miocene igneous activity. Structural observations
document a change in deformation from dominantly compressional to extensional
in the early Eocene. Sr and Nd isotopic data from igneous rocks suggest
this change in tectonic style was accompanied by a change in the sources
of the magmas intruding the crust that we interpret to reflect crustal
thinning after 50 Ma. Between 70 and 65 Ma orogen-parallel folds and nappes
document convergent tectonism during emplacement of the CMB. After 65 Ma
orogen parallel extension was accompanied by 58-50 Ma bimodal mafic and
felsic plutons. Four samples of CMB plutons (64.5-51.4 Ma) have initial
(Nd values of -4.4 to -1.7 and 87Sr/86Sri of 0.70558 to 0.70664.
These data and similar isotopic ratios reported by others indicate that
the CMB plutons contain a significant component of recycled continental
crustal material isotopically similar to the Yukon-Tanana terrane rocks
that host the plutons. By 50 Ma extension resulted in rapid uplift of the
deep-seated rocks of the batholith. The youngest (52-50 Ma) plutons
probably resulted from decompression melting. As the tectonic setting changed
translation magmatism declined markedly. The next igneous activity produced
bimodal stocks and dikes across the region with (Nd of +1.0 to +4.6 and
87Sr/86Sri of 0.70381 to 0.70559. Most of these are dated at 22-20 Ma,
some of the mafic dikes are as old as 45 Ma. These data suggest that the
source of these magmas is juvenile, a combination of material derived from
the mantle and from melting nonradiogenic rocks resembling those of the
nearby Alexander or Stikine terranes. Quaternary and younger alkali basalt
and related volcanic rocks with 87Sr/86Sri of .70322-.70358 are mantle-derived
with only minor evidence of crustal contamination.
