Tectonic Implications of the State Line Kimberlites
There is a general
worldwide
correlation between the emplacement of kimberlites, and periods of
relative orogenic and magmatic inactivity, although some have argued
for a plate tectonic origin. As is the case in many of the worlds
cratonic regions,
North America appears to have experienced multiple episodes of
diatreme emplacement (Lester et al., 2001). However there is
somewhat of
a dearth of reliable ages for the emplacement of many of these
formations. A date of approximately 380 Ma has been suggested for
the Sloan kimberlite (Carlson et al., 2004). Analysis of U-Pb
systematics in perovskite grains from the Iron Mountain and Chicken
Park kimberlites yields weighted average ages of 410.0 +/-6.5 Ma and
613.8 +/-8.7 Ma respectively. The Estes Park diatreme has been
given a Rb - Sr phlogopite date of 386 +/-9 Ma, and George Creek has
been dated by the same method at about 600 Ma (Heaman et al.,
2003). 40Ar/39Ar dating of phlogopite
from
the Chicken Park formation has given an age of 620 - 640 Ma, and the
Green Mountain diatreme has been dated using Sm - Nd at 572 +/-49 Ma
(Lester et al., 2001). Some have proposed a correlation between
the Cambrian - Eocambrian period of kimberlitic magmatism and rifting
of the Iapetus ocean (Heaman et al., 2003).
There is some debate as to
the nature and geometry of the suture between the Archean Wyoming
Craton and the Proterozoic Yavapai province to the south.
Studies of xenoliths from many of these diatremes can offer a snapshot
of the lithosphere at the time of eruption; for example, U-Pb zircon
dates from lower-crustal xenotilths of the Sloan kimberlite mostly fall
in the
range of 1640 - 1750 Ma (Carlson et al., 2004). It should be
noted that there are three ages that must be considered for these
formations; the age of eruption recorded by the kimberlitic
magma, the ages of xenoliths from various layers that the diatreme has
sampled, and the age of the diamonds and their inclusions.
Diamondiferous formations are almost invariably associated with Archean
cratons. This has led
some to propose that Archean lithosphere extends south beneath
the Proterozoic rocks on a dipping interface, whereas others have
proposed that the interface is of opposite polarity, placing
Paleozoic lithosphere underneath the Archean craton. The latter
hypothesis is based on seismic imaging that indicates a
northward-dipping structure (Carlson et al., 2004). An
alternative explanation has been put forth, calling for structural
complexity beneath the State Line district with intermixed regimes of Archean and Proterozoic material (Schulze et al., 2008). An
apparant dichotomy exists between the geochemical signatures of
diamonds from the northern and southern State Line diatremes, with the
northern group having predominantly peridotitic inclusions in diamonds
with "normal" ð13C
values and high N contents, and the southern group having eclogitic
inclusions in diamonds with anomalously low values of ð13C.
Worldwide there is a general correlation between P-suite inclusions and
Archean encapsulation dates. The northern group includes the
Kelsey Lake and Schaffer kimberlites, and the Sloan and George Creek
formations belong to the southern suite (Schulze et al., 2008).
This is certainly a generalization, however, as significant isotopic
variation is seen in the zonation within single diamonds from
locations such as George Creek. It has been suggested that this
variation may reflect two distinct periods of diamond growth
(Fitzsimmons et al., 1999).
The figure
at right (modified from Lester et al., 2001) shows the relative
positions of the State Line and Iron Mountain districts, as well as the
Estes Park and Green Mountain diatremes.
The table at
left gives dates of emplacement for various kimberlites across western North
America. Modified from Heaman et al., 2003.
The figure below
(modified from Lester and Farmer 1998) shows one possible
configuration for the Archean-Proterozoic suture zone beneath the State
Line district. It is important to note that this is the
interpreted disposition of the interface at the time of sampling by the
kimberlites. It has been proposed that the lithospheric root
beneath the region has subsequently been removed. This hypothesis
is supported by an apparant change in heat flow from the cold geotherm
suggested by Proterozoic xenoliths and the heat flow measured in the
area today (Carlson et al., 2004).
