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Paleomagnetic and Geochemical Analysis of the Summit Creek
Volcanics South-Central Cascades, WA
Abstract
The Basalt of Summit Creek is an enigmatic basalt unit
that is about 55 million years (Ma) in age, located south of
Mount Rainier. These basalts formed in a transitional time
between the formation of the Coastal Range Basalts and
the present Cascade arc. Through geochemical and
paleomagnetic techniques, samples were collected and
analyzed. Paleomagnetic samples showed nearly complete
post eruptive demagnetization. Geochemical samples
show arc type rocks lower in section and OIB at upper
sites.
Goals and Purpose
(1) To determine the tectonic setting in which these rocks
formed
(2) To better understand their relationship to the Crescent
Basalts
(3) To establish the extent to which these basalts have
undergone rotation and/or northward translation.
Figure 1. Geologic
map of the Summit
Creek area and
vicinity. Areas of
study are indicated
by yellow in the
areas labeled Evb.
Two locations were
sampled, Bear Creek
located in the
southeastern region
and US-12 located
where Evb
approaches US-12.
Elli McKinley & Professor Mike Valentine
University of Puget Sound
Summer 2011
[email protected]
Tectonic Setting
100.00 Rock/MORB
Magnetic Direction Results
Table 1. Magnetic directions for sites EM01-EM02, and EM1108EM1111 that have been corrected for bedding tilt.
EM11-09-1
1.00
Sample
No.
Site
US-12
EM01
Bear Creek
Dec.
--
Inc.
--
N
R
--
α95
--
--
k
--
EM02
20.82
58.25
4
3.98
7.41
154.8
EM1108
46.05
-14.85
5
3.99 44.01
3.98
EM1111
53.15
16.07
7
6.08 25.62
6.51
Paleomagnetic Results
The samples show inconsistent directions and many of them are
weakly magnetized making it difficult to infer the translation and
rotation of crust. Samples from site EM01 were weakly magnetized
through alternating field demagnetization. No data was recorded
because of the inconsistencies. Other samples lost their
magnetization in early stages of alternating field demagnetization.
Figure 3. Map showing a
region south of Mount
Rainier (from
Googlemaps). Samples
were collected along US12 and two sample
locations south of US-12,
near
the
confluence
of
Area of the Basalts
Bear Creek and the N.
of Summit Creek
Fork Tieton River (Table
1).
400
EM11-05-4
EM11-07-1
Zr (wt. %)
350
0.10
Sr
300
EM11-05-2
EM11-09-1
200
150
50
0
40
45
50
55
60
65
SiO2 (wt. %)
70
75
80
Figure 5. Variation diagram showing discrepancy in concentration of
Zr and SiO2 in four samples from the Bear Creek location analyzed
with the ICP-MS.
Rb Ba Th Ta Nb Ce
P
Zr Hf Sm Ti
Y
Yb
Conclusions
Paleomagnetic samples using thermal demagnetization
became weakly magnetized after early stages of thermal
demagnetization. This suggests that there has been
alteration in minerals with a low Curie temperature.
Inconsistent directions show similarity to Crescent Basalts as
they are also variable in declination and inclination.
Trace element analysis using the ICP-OES/MS shows that
there are arc-like rocks lower in the section, sample EM1105-4 (Figure 4). Sample EM11-05-4 also shows high amounts
of Zr (ppm).
The tectonic setting appears to change from arc like-rocks to
rocks more characteristic of a hot spot, based on trace
element data.
(4)
250
K
Figure 4. Spidergram showing trace element data from samples
collected along FR 1000 at Bear Creek. All samples except EM1105-4 show Ocean Island hotspot character. EM11-05-4 shows
arc-like fingerprint with a depletion in Ta and Nb and an
enrichment in Rb and Ba.
(2)
(3)
100
Figure 2a. Shows a broken rock core used for paleomagnetic
analysis with the spinner magnetometer.
Figure 2b. Taking sample location notes used for bedding tilt
corrections and orienting core along US-12.
EM11-07-1
Corrected for tilt:
Zirconium Concentrations
a.
EM11-05-4
10.00
(1)
b.
EM11-05-2
Future Work
Make polished thin sections to be analyzed with light
microscopy techniques
Date high Zr flows using zircon crystals
Sampling along the ridge in the Summit Creek region as well
as sampling of other outcrops of Eocene basalts
Rerun samples for P concentrations
Acknowledgements
I would like to thank:
1) The University of Puget Sound for funding my Summer
Research Grant,
2) Professor Mike Valentine for guiding me through this
process as well as sharing his knowledge of
paleomagnetism,
3) Professor Jeff Tepper for is assistance in identifying rocks,
fusing and ICP analysis.