Igneous Petrology Part 3

Based on:

Composition
Fabric and Texture
Field Relation

How do we classify igneous rocks?

Modal Composition

Most straightforward approach to determining rock mineralogy.

Normative Mineralogy

is an indirect scheme using data derived from chem analysis of a rock sample

CIPW Norm Classification

The first norm classification

Cross
Iddings
Pirsson
Washington

CIPW Norm Classification means __________

In terms of Geology, the term BASIC and ACIDIC is different from chemistry because they pertain to the silica content of a rock.

Chemical Composition: Normative Mineralogy

The percent by weight of silica content is under what classification of ignx rx?

Chemical Composition: Modal Composition

The percentage of dark color- colored or light- colored minerals is under what classification of ignx rx?

Dark-colored Minerals

are generally enriched with Fe and Mg and referred as ferromagnesian or mafic minerals

Light-colored Felsic Minerals

are depleted with ferromagnesian elements and rich in Si,O2,K,Na

Andesite

is an INTERMIDIATE rock

Basalt

is a BASIC rock

Rhyolite

is an ACIDIC rock

Ultramafic

Dark or greenish rocks that contain olivine, pyroxene, and amphibole

Mafic

Dark colored rocks that contain Olivine, Pyroxene, Amphibole and Biotite

Intermediate

Grayish to Salt and Pepper rocks that contain Plagioclase, amphibole, Biotite and Quartz

Felsic

Light colored or red rocks that are rich in Potassium Feldspar, Biotite, Muscovite and Quartz

RECITE WEIGHT AND COLOR PERCENTAGES OF ROCKS

SiO2 Oversaturated

QUARTZ+feldspar or magnesium orthopyroxene

SiO2 Saturated

Feldspar or Magnesium orthopyroxene ONLY

SiO2 Undersaturated

forsterite olivine+nepheline+leucite etc

Shand Classification
Ellis Classification

2 types of classification based on Percentage Dark Colored Minerals by COLOR INDEX of MAFIC MINERALS

Pumice

Vesicular light colored rock

Scoria

Vesicular dark colored rock

Peridotite

very dark colored ultramafic rock depleted in SiO2

Basalt and Gabbro

are dark colored mafic and SiO2 poor rocks

Andesite and Diorite

Are gray colored intermediate to salt pepper rocks

Dacite and Granodiorite

are light-colored felsic rocks containing APPROXIMATELY 2/3 SiO2

Rhyolite and Granite

are light-colored felsic rocks containing MORE THAN 2/3 SiO2

Fabric

Encompasses the non-compositional properties of rocks which comprises texture and large scale textures.

Texture

Also called MICROSTRUCTURE and is based on the proportions of glass relative to mineral grains

Structures

Are larger size of features that are commonly seen in an outcrop. Examples include bedding in a pyroclastic flow and pillows in a submarine lava flow.

Aphanitic

a texture that is very fined grained as a result of rapid cooling at surface

Phaneritic

a texture that is course grained due to magma cooling at depth

Porphyritic

a texture that has very large crystals (phenocrysts) are embedded to (groundmass) smaller crystals

Fine grained

1mm-3mm

Medium grained

3mm-1cm

Course grained

1cm-3cm

Glassy

contain variable proportion of glass
When a molten rock is quenched quickly at it was ejected from the atmosphere

Pyroclastic

Produce by fragmenting processes which creates broken fragments of volcanic rocks.

Vitrophyre

Is a pyroclastic rock that contains scattered phenocrysts in a glassy matrix.

Holocrystalline
Hypocrystalline
Holohyaline

3 degree of crystallinity

Holocrystalline

Wholly crystalline texture
(pure crystals)

Hypocrystalline

Partially crystalline and partially glass texture (half crystals half glass)

Holohyaline

Wholly Glassy texture
(pure glass)

Euhedral

Bounded by crystal faces

Euhedral

also called idiomorphic

Subhedral

intermediate stage of development

incompletely bounded by crystal faces

Hypidiomorphic

There is a mix of euhedral, subhedral and anhedral grains.

Anhedral

also called xenomorphic

Anhedral

crystal faces are absent

Obsidian

product of quenching

Massive and high silica glass appearing in hand sample with ZERO crystallinity

Devitrification

weathering process in glassy/pyroclastic materials

Palagonite

Important alteration product of devitrification

Spherulitic texture

these are fibrous alkali feldspar and SiO2 polymorph and a product of devitrification

Perlitic texture

develops by hydration of obsidian

Pitchstone

waxy luster dark color with 6-16% of the water has been absorbed. example of perlitic tecture

Pegmatitic Texture

a texture characterized by large crystals more than 30 mm in diameter. It also display early form euhedral and later formed subhedral

Aplitic Texture

in contrast with pegmatitic, this refers to extremely fined grained minerals

Vesicular Texture

contain spherical to ellipsoidal void spaces called vesicles

Amygdaloidal

Gas cavities are filled with secondary minerals

vesicles are filled with secondary minerals

Miarolitic

Gas cavities are filled with secondary minerals

vesicles are filled with primary minerals

Intrusive

these are Plutonic and Phaneritic in Texture rocks

Extrusive

these are Volcanic and Aphanitic in Texture rocks

Hypabyssal Rock

igneous rocks that form at intermediate depth and not clearly distinct if volcanic or plutonic rx

Stock

A small discordant pluton (less than 100km2)

Batholith

A discordant pluton with more than 100km2

Dike

A discordant tabular body that cross-cuts a bedding (vertical)

Sill

a concordant tabular body (lateral/horizontal)

Discordant

it cuts the bedding and it goes along with the bedding planes

Concordant

it does not cut the bed but it goes along with the bedding planes

Laccolith

an intrusive igneous body with a blister shaped sill

Lopolith

a saucer shaped concordant bodies

Bysmalith

a cylindrical shaped discordant body

Phacolith

a concordant lens-shaped pluton that typically occupies the crest of an anticline or trough of a syncline