Granitic Rocks and Geologic History of the Idaho Batholith
Elizabeth M. King, Illinois State University
You are each part of a larger research team investigating the Idaho Batholith.
You are given a geologic map and you note that there are three different main categories
of intrusions in the batholith, mostly based on age.
Regional Geology of the Idaho Batholith
The Idaho batholith is primarily mesozonal granitic rocks of both Cretaceous and
Tertiary age. The southern, and larger, portion of the batholith is defined as the Atlanta
lobe (25,000 km2), whereas the northern portion is defined as the Bitterroot lobe (14,000
km2). The main phases of the Bitterroot lobe are thought to have intruded at depths of
approximately 10 km or greater, based on metamorphic mineral assemblages in the
country rocks and the presences of magmatic muscovite in the granites. Regionally
metamorphosed (up to sillimanite grade) Precambrian Belt Supergroup sediments of the
Salmon River arch separate the two lobes. Belt Supergroup rocks are also exposed north
and east of the Bitterroot lobe and include sequences of metamorphosed siltstone,
claystone, and sandstone. The majority of detrital zircons in the Belt rocks have ages of
1070-1860 Ma, constraining the depositional age to mid-Proterozoic.
Geochronological studies indicate that the Idaho batholith spans a significant
range in ages (generally 100-54 Ma), and many of the mesozonal intrusions contain
inherited zircons that indicate interaction with Proterozoic crust.
The boundary between Precambrian basement on the east side of the boundary
and Triassic/Jurassic accreted terranes on the west has been termed the Salmon River
suture zone.
Your assignment
For your group, you have a suite of rocks that represent a coherent representation of part
of the batholith. There are sample location numbers on the map that match the numbers
on your rocks. Your job is to:
a. Identify the minerals present in your suite of rocks (some are more unique than
others),
b. determine rock names
c. determine if you have I-, S-, or A-type granites
d. based on this determination, what might be a rough estimate of magma source,
what, if any, assimilation occurred, and what a typical tectonic scenario may be
for the formation of these rocks.
e. determine what depth (relatively) in the crust that your rocks were intruded, and
provide evidence for this estimation.
f. Also see (and describe) if your rocks have any distinctive textures to them that
might help you figure out a relative history of the batholith.
At the beginning of lab tomorrow all groups will report their findings. Remember each
suite represents a similar, but slightly different, history so the key will be to relate suite 1,
2, and 3 and not so much the geographic distribution of them.
Based on the three groups findings, what sort of history can you put together for the
batholith, both igneous and tectonic? Write up a 1-2 page history of the batholith based
on the entire class’ findings.
Suite 1 (50-42 Ma)
98IB-19, 98IB-18, 98IB-21, 98IB-57
These plutons have also been the source of moderate size fluorite mine operations.
Suite 2 (75-65 Ma)
98IB-67, 98IB-39, 98IB-10 , 98IB-11
Suite 3 (75-65 Ma)
The zircons have cores of Proterozoic age approximately 1,750-1,800 Ma
98IB-40, 98IB-36, 98IB-12
116°
47°
115°
Lolo
batholith
27
26 25
0.706 line
24
23 21
22
2
4 3
5
7
6
12 Bitterroot
8
9
10
11
TERTIARY
GRANITE
1
TERTIARY
MONZODIORITE
13
14
46°
15
16
17
Elk City
31
30 32 34
33
35
36 3839
37&40 41
42
iver S
uture Zone
on R
Sal
m
44°
Boise
19 18
20
43
Salmon
46 47
48
49
45
99IB1
50
Casto
pluton
113°
Challis
Atlanta Lobe
53
51 52
54
55
60
59
61
58
62
56
57
68
Stanley
63
Sun
Valley
66
Hailey
64
67
0
43°
na
ta
o
on dah
M
I
45°
44
CRETACEOUS
INTRUSIVES
Lobe
0
116°
115°
25
25 kilometers
114°
50 miles