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.