Dolerite - petrology

2. • Introduction
• Mineralogy
• Textures
• Classification of dolerite
• Dolerite dyke
• Formation of dolerite
• Distribution
• Uses
• Conclusion

3. • A dolerite is the medium-grained equivalent of a
basalt - a basic rock dominated by plagioclase and
pyroxene.
• Dolerites also often include olivine or quartz and
can be alkali basalts, olivine tholeiites or quartz
tholeiites. They can contain a wide range of
accessory minerals including hornblende and
biotite. Dolerites usually have an ophitic texture

4. • Dolerite is typically found as a hypabyssal igneous
rock, typically within dykes. Dykes may also
contain basalt instead of dolerite.
• Diabase is often used as a synonym of dolerite by
american geologists, however, in europe the term
is usually only applied to altered dolerites.

5. Mineralogy: Phenocrysts comprise olivine (olivine diabase) and/or
pyroxene or plagioclase. The groundmass comprises the same
minerals with iron oxide, and sometimes with some quartz,
hornblende or biotite.

6. • Ophitic texture - laths of plagioclase in a coarse
grained matrix of pyroxene crystals, where in the
plagioclase is totally surrounded by pyroxene
grains. This texture is common in diabases and
gabbros.
• Subophitic texture - similar to ophitic texture
where in the plagioclase grains are not completely
enclosed in a matrix of pyroxene grains.

9. • Pyroxene and calcic plagioclase,typically
labradorite ,as essential constituents,with or
without some olivine and a variable amount of
interstitial residum consisting of alkali feldspar
and quartz, often intergrowth .
• The accessory constituents are iron-ore and
apatite with ,in some a little hornblende or
biotite.

10. • Single lime-rich more or less titaniferous
augite as their pyroxene.
• They normally have olivine as a further
essential constituent and not infrequently
have a little interstitial analcite.
• They may be illustrated by those occurring
in Tertiary igneous province of scotland.

11. • Vertical Triassic
diabase cross-
cutting Jura-
Triassic
sedimentary strata
• Note dike offset

12. A view of
the dolerite
dyke cutting
through the
granite.

13. Sam Edwards on
the first ascent of
Slap Dancer (26,
5.12b), a dolerite
pillar in the Organ
Pipes, Tasmania,
Australia

14. Formation ofFormation of
doleritedolerite
Dolerite cools under
basaltic volcanoes, like those
at mid-ocean ridges. It cools
moderately quickly
when magma moves up into
fractures and weak zones below a
volcano. There, it forms dikes
(tabular igneous rock bodies that
cut across pre-existing rock layers
or bodies) or sills(tabular igneous
rock bodies that form parallel to
pre-existing rock layers). The
moderate cooling rate allows small
visible crystals to form in the rock.

15. Distribution
• Tholeiitic dolerite - occur in karroo of
South Africa and those of Tasmania and
Antartica were intruded.
• Alkali dolerites - occur in Scotland
particularly in where they may form large
sills. Midlands of England, as at Rowley
Regis, near Birmingham; in the Clee hills,
Shropshire and Derbyshire.

16. In geological timescale this Karoo dolerite intruded
approximately 83 million years ago during the
Mesozoic era

17. • Dolerite are used in monumental
masonry,building material,concrete
aggregate, paving stone, road stone, road
stones and ornamental stones.
UsesUses

18. ConclusionConclusion
• A dolerite is the medium-grained equivalent of a
basalt - a basic rock dominated by plagioclase and
pyroxene.
• One of its distinguishing character is its ophitic
texture.
• Dolerite is typically found as a hypabyssal igneous
rock, typically within dykes, however, it may also
occur in sills.
• Tholeiitic dolerite and alkali dolerite are the two
types.

19. • Dr.S.R Nockolds, PETROLOGYPETROLOGY FORFOR
STUDENTS,STUDENTS, Pp:120-135.
• Dietrich, R. and Skinner, B., 1979, Rocks, Rocks
and Rock Mineralsand Rock Minerals, pg. 108.
• en.wikipedia.org/wiki/Diabase
• en.wikipedia.org/wiki/Quartz-dolerite