Gabbro - petrology

2.  INTRODUCTION
 MINERALOGY
 TEXTURES
 (IUGS CLASSIFICATION) STRECKEINSEN
CLASSIFICATION OF GABBRO(1967)
 THEORIES BASED ON ORIGIN
 WORLD DISTRIBUTION
 ECONOMIC IMPORTANCE
 USES OF GABBRO
 CONCLUSIONS
 REFERENCES

3.  Gabbro refers to a large group of dark, coarse-grained,
intrusive mafic igneous rocks plutonic equivalent to
basalt.
 It was named by the German geologist Christian
Leopold von Buch after a town in the Italian Tuscany
region.
 These occurs as lopolith, batholith,stock and large
layered bodies.

4.  Gabbro is dense, greenish or dark-colored and contains pyroxene,
plagioclase, amphibole, and olivine (olivine gabbro when olivine
is present in a large amount).
 The pyroxene is mostly clinopyroxene; small amounts of
orthopyroxene may be present. If the amount of orthopyroxene is
substantially greater than the amount of clinopyroxene, the rock is
then a norite.
 Quartz gabbros are also known to occur and are probably
derived from magma that was over-saturated with silica.
 Gabbros contain minor amounts, typically a few percent, of iron-
titanium oxides such as magnetite, ilmenite.

5.  Plagioclase (labrodorite ,bidunite ,or rarely anorthite)
 Clinopyroxene (augite or diapside)
 Orthopyroxene (hypesthene &rarely enstetite)
 Olivine
 Hornblende
 Biotite (as altered product)
 Quartz(quartz gabbros)
 Orthoclase
Apatite, titaniferrous magnetite, illmnite, pyrite, pyrrhotite
are the accsessary minerals.

6. The rock gabbro shows the following textures:
 Cumulus:- Here they are formed by the accumulation of crystals
that settled out from a magma by the action of gravity they are in
liquid form.It shows euhedral grain size.
 Ophitic texture:- Is one where random plagioclase laths are
enclosed by pyroxene or olivine. If plagioclase is larger and
encloses the ferromagnesian minerals, then the texture
is subophitic and the laths typically impinge on one another to form
sharp angles.
 Poikilitic texture:- It refers to small, typically euhedral crystals
(chadacrysts), that are enclosed (included) within a much larger
mineral of different composition. Unlike the porphyritic texture.

7.  Orbicular texture:-here the rock are charecterised by oval to
circular patches usually consists of cores of plagioclase enclosed
with alternating rings of pyroxenes or hornblende.
 Symplectitic intergrowth:- It consists of myrmekite like bulbous
outgrowth of plagioclase , occasionally twinned in normal
manner,riddled with vermicular inclusions of orthopyroxene.
 Corona texture:-It consists of zone of minerals, usually with
radial arrangement around another mineral.

8. Gabbro
Photomicrograph of a thin section of
gabbro.
Orbicular texture

9. Cumulate texture Ophitic texture
Poikilitic texture

10. Gabbro
Norite
(IUGS Classification) Streckeinsen
Classification Of Gabbro(1967)

12.  Possible explanation involves multiple injection of batches of
identical magma, which subsequently differentiate to produce
each cycle.
 Similarities between bedding and grain fabric of clastic
sedimentary bodies with the rhythmic layering and cumulus
fabric of stratiform intrusions suggest similar process of
sedimentation has been considered to involve simultaneous
gravity induced crystal settling.
 Convection was responsible for the rhythmic layering, igneous
layering cross bedding. Here the magma near the roof would
begin to crystallize first become denser gravitationally unstable
would like to sink along the walls of intrusion and spread
horizantal along the floor.

13.  At a number of times in the earth history particularly during
the Precambrian, conditions varied as continental rifting and
meteoritic impact led to the development of enormous
volumes of tholeitic magma which on slowly cooling
differentiated into remarkable suites of cumulates.

14. The form of a typical LMI

15. Large or particularly well-studied LMIs exposed in
continents (many in flood basalt provinces)
Some Principal Layered Mafic Intrusions
Name Age Location Area (km
2
)
Bushveld Precambrian S. Africa 66,000
Dufek Jurassic Antarctica 50,000
Duluth Precambrian Minnesota, USA 4,700
Stillwater Precambrian Montana, USA 4,400
Muskox Precambrian NW Terr. Canada 3,500
Great Dike Precambrian Zimbabwe 3,300
Kiglapait Precambrian Labrador 560
Skaergård Eocene East Greenland 100
WORLD DISTRIBUTION
Layered Mafic Intrusions

16. Bush weld igneous
complex - South Africa
The Stillwater Complex, Montana
Great dyke – Zimbabwe

17.  Gabbro layered intrusions though few in number contain the
worlds main reserves of chromium and platinum.The so called
chromite horizon in bushweld &stillwater more specially layers are
found with olivine rich peridotites, orthopyroxenites.
 It also contains larger number of titano magnetite layers Fe, Ti,
vanadium ,Tin. It also yielded nickel, copper, cobalt, sulfur, gold,
silver, tellurium, pyrrhotite, chalcopyrite.
 Gabbro sometimes contains economic amounts of some
relatively rare metals. Gabbros containing significant
amounts of the mineral ilmenite are mined for their titanium
content.

18.  Gabbros are popularly known as “Black granites”.
 Brightly polish gabbro is used to make cemetery markers,
kitchen counter tops, floor tiles, facing stone and other
dimension stone products.
 It is also used to make a number of rough-cut products such
as curbing, ashlars, paving stones and other products.
 Crushed gabbro is used as a base material in construction
projects, as a crushed stone for road construction.

19.  It is a highly desirable rock that stands up to
weathering.
 Most of the intrusions are lopolith, some are
elliptical or lobate in plan such as Bushweld
complex of South Africa.
 It contains economic amount of rare minerals.
 It is of great economic importance. These rocks get
altered to chlorite, serpentine, talc, & clay minerals.

20.  Donald W.Hyndman-1972-Petrology of
igneous and metamorphic rocks-Pp-105-
113,222-232.
 Myron G.Best-1976-Igneous and
metamorphic petrology-CBS publishers and
distributors Delhi-Pp:-167-16
 http://www.ucl.ac.uk/es/impact/geology/lon
don/glossary/rocks/igneous/gabbro
 http://www4.nau.edu/meteorite/Meteorite/B
ook-Textures.html
 http://geology.com/rocks/gabbro.shtml