Transcript Chapter 15: Continental Flood Basalts

Chapter 15: Continental Flood
Basalts
Large Igneous Provinces (LIPs)
 Oceanic plateaus
 Some rifts
 Continental flood basalts
(CFBs)
Table 15-1. Major Flood Basalt Provinces
Name
Volume
5
3
Age
Locality
CRB
(1.7x10 km )
Miocene
NW US
Keeweenawan
(4x105 km3)
Precambrian
Superior area
Deccan
(106 km3)
Cret.-Eocene
India
Parana
(area > 106 km2 ) early Cret.
Karroo
(2x106 km3?)
Brazil
early Jurassic S. Africa
Figure 15-1. Columbia River Basalts at Hat Point, Snake
River area. Cover of Geol. Soc. Amer Special Paper 239.
Photo courtesy Steve Reidel.
Tectonic Setting of CFBs


Continental hot spots
Continental rifting may be associated with hot
spots
 Successful rifts
 Failed rifts (aulacogens)
Figure 15-2. Flood basalt provinces of Gondwanaland prior to break-up
and separation. After Cox (1978) Nature, 274, 47-49.
Figure 15-3. Relationship of the Etendeka and Paraná plateau provinces to
the Tristan hot spot. After Wilson (1989), Igneous Petrogenesis. Kluwer.
Present setting of the Columbia River Basalt Group in the Northwestern United States.
Winter (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
Clarkston
Table 15-2. Stratigraphy of the Columbia River
Basalt Group.
Magnetic K/Ar
Formation
Member
Polarity* Dates
Lower Monumental
N
6 Ma
Ice Harbor
N,R
8.5
Saddle
Buford
R
Elephant Mountain
R,T
10.5
Mountains Pomona
R
12
Esquatzel
N
Basalt
Weissenfels Ridge
N
Asotin
N
13
Wilbur Creek
N
Umatilla
N
Priest Rapids
R
14.5
Wanapum Roza
T,R
Basalt
Frenchman Springs
N
15.3
Eckler Mountain
N
See Reidel et al .
Grande
N2
15.6
Ronde
Picture
(1989) for
R2
Basalt
Gorge
Grande
N1
Ronde Units
R1
16.5
See Hooper et al .
R1
Imnaha
(1984) for Imnaha
T
Basalt
Units
N0
R0
17.5
Data from Reidel et al . (1989); Hooper and Hawkesworth (1993).
* N = normal, R = reversed, T = transitional
Figure 15-5. Time-averaged extrusion rate of CRBG basalts as a function of time, showing cumulative volume. After
Hooper (1988a) The Columbia River Basalt. In J. D. Macdougall (ed.), Continental Flood Basalts. Kluwer. 1-34.
Imhana first, Grande Ronde second. Flows increase in SiO2, and K with time;
decrease in Al+++, Ca++ as Mg# decreases. Compatible element depletion
consistent with fractional crystallization of Plagioclase plus Orthopyroxene and/or
Olivine.
Figure 15-6. Variation in wt.% of selected major element oxides vs. Mg# for
units of the Columbia River Basalt Group. Winter (2001). An Introduction
to Igneous and Metamorphic Petrology. Prentice Hall. Data from BVTP
(Table 1.2.3.3), Hooper (1988a), Hooper and Hawkesworth (1993).
Imhana 1st, Grande Ronde 2nd, Wanapum 3rd, Saddle Mts. 4th
Note LREE enrichment. Tholeiitic Continental Flood Basalts (CFBs) therefore
show compatible element depletion and incompatible element enrichment, which
distinguishes them from N-MORBS. They are considerably more fractionated
“since separating from their peridotitic mantle source” Winter p. 282.
Most incompatible
on left
Figure 15-7. Condrite-normalized rare earth element patterns of some typical CRBG samples. Winter (2001).
An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Data from Hooper and Hawkesworth
(1993) J. Petrol., 34, 1203-1246.
Late magmas enriched in incompatible LILs such as
K, Rb, Ba and Th compared to N-MORBs
Most incompatible
in center
Figure 15-8. N-MORB-normalized spider diagram for some representative analyses from the CRBG. Winter (2001). An
Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Data from Hooper and Hawkesworth (1993) J. Petrol., 34,
1203-1246. Picture Gorge from Bailey (1989) Geol. Soc. Amer. Special Paper, 239, 67-84.
CFB magmas compared to similar OIBs
Most incompatible
on left
Figure 15-9. OIB-normalized spider diagram for some representative CRBG analyses. Winter (2001). An
Introduction to Igneous and Metamorphic Petrology. Prentice Hall. (data as in Figure 15-8).
The later Saddle
Mountains floods are
closer to DUPAL
(Dupre and Allegre),
suggesting ( to Winter )
a mixture of EMI and
EMII also. I think they
just get most of the
Thorium, and thus
radiogenic Pb 208
Imhana, Grande Ronde,and Picture Gorge plot on
Northern Hemisphere Reference Line, suggesting a
HIMU component with Radiogenic Lead. Pb 206
Figure 15-11. 208Pb/204Pb vs. 206Pb/204Pb for the basalts of the CRBG. Included for reference are EMI, EMII, the
DUPAL group, the MORB array, and the NRHL (northern hemisphere reference line) connecting DM and HIMU
mantle reservoirs from Figure 14-6. Winter (2001). An Introduction to Igneous and Metamorphic Petrology.
Prentice Hall. Data from Hooper (1988a), Carlson et al. (1981), Carlson (1984), McDougall (1976), Brandon et al.
(1993), Hooper and Hawkesworth (1993).
Recall the OIB case, and note
the thinning of the plume with time
Figure 15-13. A model for the origin of the Columbia River Basalt Group From Takahahshi et al. (1998) Earth Planet. Sci. Lett., 162, 63-80.
• Melting
within a heterogeneous plume head (initial stages of the Yellowstone hot spot).
•The plume head contains recycled stringers of recycled oceanic crust that melts before the Peridotite,
yielding a silica-rich basaltic magma equivalent to the main Grande Ronde basalts and leaves a garnetclinopyroxene residue.
•The large plume head stalls and spreads out at the base of the resistant lithosphere and the basaltic magma
ponds (underplates) at the base of the crust, where it melts some crust to create rhyolite.
•Basalt escapes along a northward trending rift system to feed the CRBG.
Figure 15-14. Diagrammatic cross section illustrating possible models for the development of continental flood basalts. DM is
the depleted mantle (MORB source reservoir), and the area below 660 km depth is the less depleted, or enriched OIB source
reservoir. Winter (20010 An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.