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Giant Steps Dike
• Orientation: 105 º SE• 8’ – 12’9”• Surrounding Formation: Cape
Elizabeth– Protolith: Interbedded Pelite and
Sandstone– Age: Precambrian/Ordovician
• Dike• Porphyritic Basalt• Minerals within basalt are altered
in composition from their original state
Middle of DikeNa Rich Plagioclase Feldspar, K Feldspar, Apatite, Chlorite, Pyroxene
Pyroxene: 220 x 302 micronsPlagioclase Feldspar: 213 x 350 microns
Edge of Dike (Contact)Na Rich Plagioclase Feldspar, Pyroxene, Titanite, Chlorite, K Feldspar
Pyroxene: 27 x 53 micronsPlagioclase Feldspar:26 x 28 microns
Mineral CompositionNotice that at the same scale, the texture of the minerals is drastically different. The center of the dike is more phaneriticthan the edge of the dike.
Pyroxene, chlorite and plagioclase feldspar
PPL; 10x; from the edge of the dike
Apatite and Chlorite PPL, 10x; from the center of the dike.
Feldspar and pyroxeneXPL
1
10
100
La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Rock/Chondrites REEs-Sun and McD 89
Rare Earth Elements and Geochemistry
•Samples 130 and 131 (Giant Steps)•relatively parallel, with the center of the dike always having more REE than the edge of the dike.•More abundant in lighter REE than heavier REE
•Compared to Andesitic Basalt (Christmas Cove, blue squares), rich in lighter REE and similar concentration of heavier REE
•Compared to Granite (Rte 124, Green Triangles) more stable decrease and much more abundant in all Rare Earth Elements
35 40 45 50 55 60 65 70 750
2
4
6
8
10
12
14
16
Na2O+K2O
SiO2
Picro-basalt
Basalt BasalticandesiteAndesite Dacite
Rhyolite
Trachyte
TrachydaciteTrachy-andesiteBasaltictrachy-andesiteTrachy-basalt
TephriteBasanite
Phono-Tephrite
Tephri-phonolite
Phonolite
Foidite
Basalt Classification
Varying Amounts of Elements
0
10
20
30
40
50
60
SiO[2] Al[2]O[3] Fe[2]O[3] MgO CaO Na[2]O K2O TiO[2]
Elements
Am
ount
s
CB130 (Middle of Dike)
CB131 (Edge of Dike)
CB128 (An Andesitic Basalt)
CB131 (Edge of Dike) - Element Percentages
Al[2]O[3]15%
Fe[2]O[3]14%
MgO6%
CaO9%
Na[2]O3%
K2O2% TiO[2]
3%
SiO[2]48%
Temperature of Formation:
Our Basalt Falls here:
1150º Celsius
<http://mac01.eps.pitt.edu/CEJones/Geology0040/LectureNotes/5igneousrocks.pdf><http://geology.50megs.com/cgi-bin/I/rockmin/chart/ign>
Altered Feldspar
Grungy FeldsparNormal Feldspar
Feldspar abnormality
Chlorite Alteration
Notice the difference in texture, shape
and color between the
normal chlorite and the
feathery chlorite in
Plain and Cross Polarized light.
The bottom slides show the
chlorite replacing anther
mineral.
Plain Polarized Light Cross Polarized Light
Conclusions• The rock surrounding the dike was formed after the Precambrian/Ordovician Age
– Cross Cutting Relationship shows that the Dike was formed after the surrounding rock.• The exact age could be determined through isotopic dating
• Based on analysis of geochemical data, the basalt falls on the ultramafic side of the basalt range.
– Therefore it was formed at around 1150º Celsius
• At some point, most likely at the time of or before solidification of the dike intrusion, there was a change which caused some alterations or abnormalities in mineral and rock composition.
• Some explanations include:– Inclusion of gas bubbles or water in the basaltic magma– Bowen’s Reaction Series affecting crystallization pattern of minerals.