Igneous Petrology Andri's Lecture Note 5p1e27

Created & compiled by Andri SSM Octo-2004 Petrology & Economic Geology Laboratory Dept. of Geology – FIKTM ITB



History of Concepts In the 1960s, geologists were seeking ways to prove or disprove the new idea of moving plates. Exploration of magnetic anomalies at mid-ocean ridges provided strong for seafloor spreading . Geologists studied other ocean features to see how they related to plate tectonics. While visiting Hawaii, Tuzo Wilson, one of the founders of the theory of plate tectonics, noticed some interesting features about ocean islands. On a map of the Pacific basin, he found three linear chains of volcanoes and submarine volcanoes (seamounts).



An interesting pattern emerged. For each chain, the islands become progressively younger to the southeast. The extreme southeast end of each chain is marked by active volcanoes. Wilson proposed that the Hawaiian islands formed successively over a common source of magma called a hot spot. The Island of Hawaii is currently located above the hot spot.


Hot, solid rock rises to the hot spot from greater depths. Due to the lower pressure at the shallower depth, the rock begins to melt, forming magma. The magma rises through the Pacific Plate to supply the active volcanoes. The older islands were once located above the stationary hot spot but were carried away as the Pacific Plate drifted to the northwest .

General Rock Type

Texture - ave. size of minerals Fine grained Extrusive, volcanic

IGNEOUS ROCKS Interlocking homogenous crystalline texture - no preferred orientation to the mineral grains

Medium grained Dikes, sills, etc.

Coarse grained Generally intrusive

Glassy Frothy


General color and/or composition Miscellaneous observations

Rock Name

Felsic - Light colored

Rhyolite

Intermediate

Andesite

Mafic - Dark colored

Basalt


------

Intermediate

Dacite


Diabase


Granite

Intermediate

Diorite


Gabbro

Ultramafic

Peridotite

Dark to black - felsic (DOES NOT follow normal color index)

Obsidian


Pumice


Scor

Table 1: The Organization of Igneous Rocks Felsic

Felsic

Inter-mediate

Inter-mediate

Inter-mediate

Mafic

Ultrabasic

(With quartz)

(Little quartz)

(No quartz)

(No quartz)

(No quartz)

(No quartz)

(No quartz)

Subgroup „

Minerals „

Orthoclase in similar quantities as plagioclase. Also biotite, amphibole, pyroxene, hornblende, augite

More plagioclase than orthoclase. Also biotite, amphibole, pyroxene, augite, olivine, horn-blende, biotite

No feldspar. Few silicates. Pyroxene, olivine.

Course-grained: cooled slowly underground

Pegmatite, Granite

Syenite

Monzonite

Granodiorite

Diorite

Gabbro

Peridotite, Dunite, Pyroxenite

Porphyritic: cooled first below then above ground

Granite Porphyry, Rhyolite Porphyry

Syenite Porphyry

Monzonite Porphyry

Granodiorite Porphyry

Andesite Porphyry

Basalt Porphyry

Fine-Grained: cooled quickly above ground

Rhyolite

Trachyte, Felsite

Latite

Dacite

Andesite

Basalt

Glassy: cooled very quickly above ground

Pumice

Fragmental: made of igneous fragments

Tuff

Origin †

Scoria, Basalt Glass

Obsidian

Volcanic Breccia


More orthoclase than plagioclase. Also muscovite, biotite, amphibole, hornblende


Igneous Rocks. Crystallization of Magma Magma is a hot liquid (made of melted rock), with a abundant gas in solution when it is under pressure in the earth. It may contain crystals of high temperature minerals. It originates in the asthenosphere. Magma is the liquid (with dissolved gases) from which all igneous rocks solidify. The geothermal gradient, about 30 degrees C/km at shallow depths, would produce temperatures above 1000 degrees C at just 33 km below the surface so it must decrease rapidly with depth. At depth pressure raises the melting point so melting only occurs in a narrow zone where the temperature in the earth overcomes the pressure and partial melting occurs. This zone is the asthenosphere. Click the diagram at right. At high temperature in the liquid the ions have so much vibrational energy that bonds cannot form. As the temperature drops the atoms vibrate less and bonding can occur. Crystallization implies that the ions bond together in a regular pattern so that the exterior of the crystal will have a regular geometric form (as we saw in the images of the crystals in the chapter on mineralogy. The rate of cooling affects the size of the crystals There are two types of igneous rocks Plutonic rocks (intrusive) form when the magma cools slowly beneath the surface, while volcanic rocks (extrusive) form when magma reaches the surface and cools rapidly as lava flows or fragmental material. Created & compiled by Andri SSM Octo-2004 Petrology & Economic Geology Laboratory Dept. of Geology – FIKTM ITB


Mineral Gallery


PHYSICAL CHARACTERISTICS: Color is variable and tends toward pale yellows, browns, grays, and also white, blue, black, reddish, greenish and colorless. Luster is adamantine to waxy. Transparency crystals are transparent to translucent in rough crystals. Crystal System is isometric; 4/m bar 3 2/m Crystal Habits include isometric forms such as cubes and octahedrons, twinning is also seen. Hardness is 10 Specific Gravity is 3.5 (above average) Cleavage is perfect in 4 directions forming octahedrons. Fracture is conchoidal. Streak is white. Associated Minerals are limited to those found in kimberlite rock, an ultramafic igneous rock composed mostly of olivine. Other Characteristics: refractive index is 2.4 ( very high), dispersion is 0.044, fluorescent. Notable Occurrences include South Africa and other localities throughout Africa, India, Brazil, Russia, Australia, and Arkansas. Best Field Indicator is extreme hardness. Created & compiled by Andri SSM Octo-2004 Petrology & Economic Geology Laboratory Dept. of Geology – FIKTM ITB

Texture is the overall size and appearance of the minerals in the rock. The most important factor affecting the texture of the rock is the rate of cooling. Igneous rocks are classified on the basis of texture and mineral composition (1) Aphanitic texture: This fine-grained texture indicates that the rock crystallized rapidly at or near the surface (usually lava flows) of the earth. Mineral composition is often difficult to identify but if the rock is light colored it is probably made of nonferromagnesium minerals and if dark, ferromagnesium minerals. Many aphanitic rocks (basaltic lava) have vesicles which are cavities and small openings from gas bubbles (2) Phaneritic texture: A coarse grained texture indicates that the rock crystallized slowly deep within the earth. These rocks are now exposed at the surface because of uplift and erosion Aphanitic vs phaneric texture indicates very rapid cooling at the surface because no minerals have had a chance to grow. The rock obsidian for example is a glassy rock. Obsidian-2, result from violent ejection from the volcano. Some are made of fine ejected fragments (tuff) while others are large angular blocks. They are all glassy


Igneous Textures

Magmatic Differentiation, Assimilation and Magma Mixing. There is a wide variety of igneous rocks types but only a few basic types of magma, because the asthenosphere and upper mantle have a fairly uniform composition. (1) One process of developing more than one type of rock from a common magma is called magmatic differentation. If early formed crystals sink (crystal settling) to the bottom and the magma crystallizes or is then injected upward into overlying rocks, the resulting rock will have a different compostion from the original magma.

(3) Magma mixing etc. At convergent boundaries rising molten bodies, may overtake one another and mix to form average compostions. How do magmas move toward the surface? (1) By Assimilation, that is, melting the surrounding rocks. (2) By Stoping: the magma forces its way into fractures and large blocks (inclusions) drop into the magma chamber. (3) By Forceful Intrusion: Simply pushing up the surrounding rock.


(2) When a molten body moves up through "country rock" it assimilates rock.(melts and incorporates elements from the surrounding rock). This changes the magma composition.

Naming Igneous Rocks: This is a table of igneous rocks classified on the basis of texture vs.mineral composition. Granitic Basaltic (mafic) Texture Andesitic Ultramafic (low temp. (high temp. minerals) minerals) High in Si and Al K-feldspar and Qtz. Coarsegrained Fine-grained

Intermediate

High in Mg and Fe Pyroxene and NaCa feldspar

Olivine and Pyroxene Mantle rocks

Granite

Diorite

Gabbro

Peridotite

Rhyolite

Andesite

Basalt Created & compiled by Andri SSM Octo-2004 Petrology & Economic Geology Laboratory Dept. of Geology – FIKTM ITB

Amorphous (glass) --fragmental

Pyroclastic = Tuff if < about 4mm and Breccia if > 4 mm.

--massive

Obsidian

--frothy

Pumice makes lava flows and is also the main component of volcanic tuff.

It clear that the rocks on left are rich in SiO2. In contrast, Ultramafic rocks are very rich in Mg and Fe and are "mantle rocks". Typically granites, rhyolites and tuffs have over 70% silica and basalts less than 50 %. Less silicon makes basalts more fluid, e.g. less viscous and affects the eruptive style of the volcano.


Basaltic Rocks Basalt: this dark colored, fine-grained rock (often vesicular) is common in volcanic areas such as the ocean basins and areas of continental tension in which the magma came up from the mantle with little or no contamination. Because of this it is often called a "primary magma". Islands in the oceans such as Hawaii, are make almost entirely of basalt. The ocean basins themselves are made of basalt. Basalt is composed of pyroxene, Ca-feldspar and sometimes olivine but is very fine-grained. When basalt cools it contracts and cracks. These fractures and often make six-sided columns. This is called columnar ting Gabro is the intrusive equivalent of Basalt and is made up of same ferromagnesium minerals as in basalt with some lighter colored Cafeldspar. So we should expect to find gabbro as the intrusive rocks below the basalt in the ocean basins.





Andesitic Rocks. (intermediate)


Andesite is the common kind of volcanic rock associated withe the subduction zones in the circum-Pacific volcanic chains both on the continents and in the island arcs.The common minerals are plagioclase feldspars (light and rectangular) and amphiboles.(elongate and dark) Diorite is the plutonic equivalent of andesite. It looks like granite but does not have quartz and has more plagioclase feldspar and dark minerals

Current Seismicity for Australia - Indonesia Plate boundaries in yellow. Open circles: Earthquake Activity in the last 30 days; not clickable.


Updated as of Sun Nov 2 23:12:20 UTC 2003.



BASIC DEFINITIONS


Minerals Minerals are: Naturally occurring Inorganic Solids Minerals have a definite chemical composition Minerals have an orderly internal crystal structure Minerals are the building blocks of rocks. Each mineral has different physical and chemical properties which allow it to be identified. Physical properties you will use to identify the minerals include color, hardness, luster, cleavage, magnetism, reaction to acid, etc. Rocks An aggregate of one or more minerals. Rocks are the building blocks of the Earth's crust. The Earth's continental crust is dominated by granite, and the oceanic crust is dominated by basalt. Both of these are igneous rocks. There are three basic categories of rocks: Igneous (or crystallized from hot lava or magma) - ex. granite, basalt Sedimentary (or fragments laid down by water or wind) - ex. sandstone, shale, limestone Metamorphic (or rocks changed by heat and or pressure) - ex. gneiss, schist, slalte, marble Physical Properties of Minerals Color - The color of the mineral as it appears in reflected light to the naked eye. Luster - The character of the light reflected from the mineral. A mineral may have a metallic luster (in other words, you would call it a metal), or a non-metallic luster. Non-metallic lusters may be described in more detail as:

ALPHABETICAL MOHS TABLE OF HARDNESS Alabaster

2

Garnet

7-7.5

Amber

2-2.5

Glass

5-6

Andalusite

7-7.5

Hematite

Apatite

5

Azurite

3.5-4

Brazilianite Calcite

5.5 3

5.5-6.5

Rhodochrosite

1.5-4.5

Rhodonite

5.5-6.5

Rutile

6-6.5

Iolite

7-7.5

Serpentine

2-4

Jadeite

6.5-7

Soapstone

1-1.5

Jet

2.5-4

Sodalite

5-6

5-6

Sphene

5-5.5

Lazulite

Chalcedony

6.5-7

Labradorite

6

Spinel

8

Chrysoberyl

8.5

Lapis-Lazuli

5-6

Topaz

8

Coral

3.5-4

Malachite

3.5-4

Tourmaline

7-7.5

9

Marcasite

6-6.5

Turquoise

5-6

8.75

Moldavite

5.5

Variscite

4-5

Diamond

10

Nephrite

6-6.5

Zircon (low)

6.5

Diopside

5-6

Obsidian

5-5.5

Zircon (med.)

7.5

Dioptase

5

Opal

5-6.5

Zoisite

6-7

Emerald

7.5-8

Pearl

2.5-4.5

.

Epidote

6-7

Peridot

6.5-7

.

Fluorite

4

Quartz

7

.

Corundum (Sapphire / Ruby) Cubic Zirconia

. Created & compiled by Andri. SSM Octo-2004 Petrology & Economic Geology Laboratory Dept. of Geology – FIKTM ITB.

•IGNEOUS COMPOSITIONAL NAMES AND MAGMA TYPES

SiO2 (wt. %)

<45

45 -52

52 - 57

57 - 63

63 - 68

>68

intermediate

intermediate to acidic or silicic

acidic or silicic

Compositional or Chemical Equivalent

Ultrabasic

basic

basic to intermediate

Magma Type

ultramafic

mafic

mafic to intermediate

intermediate

intermediate to felsic

felsic

Extrusive Rock Name

komatiite

basalt

basaltic andesite

andesite

dacite

rhyolite

Intrusive Rock Name

peridotite

gabbro

diorite

diorite or quartz diorite

granodiorite

granite

Mafic Mineral Content Water Content

Mg/Fe

Ca/Na or Ca/K


Liquidus Temperature

Classification of igneous rocks Igneous Rocks have a two-dimensional classification scheme based on chemistry, grain size and texture.


chemistry: The key to chemical classification in igneous rocks is the amount of Silica (SiO2) in the magma. (Of course people who study this make a much bigger deal out of it! If magmas don't have much silica, their minerals are dominated by magnesium and iron (Fe) - hence the term MAFIC (MA- from the magnesium and FIC from the Fe), or even ULTRAMAFIC for the really silica poor varieties. Silica rich magmas have a mineral named FELDSPAR in them (see book) and are called FELSIC as a result. You will also see the words "acidic" and "basic" used for felsic and mafic respectively and you should be aware that this has nothing to do with pH! One can often tell about how much silica is in a rock just by its color. The more silica, the lighter the color.

grain size: The main control of grain size is how fast the rock cooled from the molten state. Slow cooling allows bigger crystals to form, and fast cooling makes smaller crystals and even glass (no crystals). So the second dimension of igneous rock classification is whether the rock was formed by cooling on the surface as an extrusive rock. or in the crust as an intusive rock. Magma can either be erupted (extruded) as ash to make pyroclastic rock or as lava to make volcanic rocks.

Texture Igneous textures are classified by the presence or absence of crystals, the size of the crystals, and the size and density of vesicles (holes). Check out this page for a nice summary of IGNEOUS TEXTURES

Extrusive rocks: Pyroclastic rocks are classified by grain size from BOMBS (>64mm) to ash (<2mm). Lapilli are pea-like grains often in a finer matrix. Here is a nice picture I found to illustrate the classification scheme of pyroclastics:

Volcanic Rocks: Volcanic rocks are mainly classified by the amount of silica. There are four main categories with increasing silica: basalt, andesite, dacite and rhyolite.



Igneous Rocks:


Rocks that form from magma: mixture of liquid, mineral crystals and gas (mostly water vapor); 95% of earth's crust is igneous or metamorphosed igneous rock usually divided into two broad genetic groups: Intrusive (plutonic) rocks that form from magma cooling beneath the earth's surface

Extrusive (volcanic): rocks that form from magma cooling at the earth's surface; but how can you tell where an igneous rock originally formed? Texture :is a proxy for cooling rate; coarse-grained (phaneritic) cooled slowly all coarse-grained igneous rocks are intrusive Fine-grained (aphanitic) cooled quickly includes all volcanic rocks AND some intrusive rocks (for example: those that formed when magma squeezed into other rocks along fracture planes) a mixture of coarse and fine crystals can reflect two stages of cooling and sometimes cooling can be so rapid that gas bubbles are trapped (vesicles, pumice) or that mineral crystals don't even have time to form (volcanic glass)

Igneous Rock Types formed by crystallization of molten rocks called magma Classified based on:

A. Texture- primarily crystal size. Intrusive - coarse-grained, slow cooling at depth; "plutonic rocks" (plutons, batholiths, stocks, dikes, sills). Extrusive - fine-grained, rapid cooling at or near surface; "volcanic rocks" (lavas, pyroclastic rocks) B. Chemical and mineral composition (SiO2 varies from 45 to 70 wt%) Felsic- mostly quartz, K- and Na-feldspars, muscovite; high SiO2; lightcolored; low-T (~700-800°C), high-viscosity. Granite, Rhyolite. Mafic- mostly Fe-Mg rich olivine and pyroxene, Ca-rich feldspars; low SiO2; dark-colored; high-T (~1100-1200°C), low-viscosity. Basalt, Gabbro. Intermediate- Na-rich feldspars, amphibole, biotite, minor quartz. Granodiorite, Dacite, Diorite, Andesite. Ultramafic - Mg-rich olivine and pyroxenes. Peridotite, Komatiite


-

2. Magma formation and differentiation Magma forms when rocks melting T° is exceeded; melting T° depends on rock composition and conditions of T and P. Rocks are multicomponent systems, thus they melt over a range of T°'s. Minerals that melt at the lowest T melt first and produce a partial melt. Partial melts can have very different composition than a completely melting rock (ie., basalt from the mantle). Partial melts rise and coalesce to form magma chambers. Increasing P causes melting T to increase. Explains why most of crust and mantle are solid. Partial melting may be induced by lowering P rapidly (decompression melting). Composition: Rocks with minerals that crystallize at low-T also melt at low T. Water lowers the melting T significantly. Diversity of igneous rocks was first explained by Magmatic Differentiation (Bowen, 1928). Process by which a uniform parent magma evolves into daughter magmas of varied composition. Occurs via fractional Crystallization. (gravity settling or by compaction/deformation). Bowen's Reaction Series. Modern theories are more complex but incorporate Bowen's theories. Important concepts are: (1) partial melting of various source rock compositions (2) dynamic magma chamber processes


Volcanism 1. VOLCANIC ROCKS - Lavas - magma that extrudes relatively quietly onto surface. A. Basaltic- high T° (~1100°C), dark, low viscosity, far traveling (10's of km). Flood basalts- immense plateaus, fluid-lava, on flat terrain, 100's m thick. Pahoehoe - smooth, ropey flows, dissolved gases, forms tubes. Aa - blocky, jagged flows, degassed lava. Pillow lavas - underwater cooling, spheriodal blocks, like toothpaste. B. rhyolitic - low T° (800-1000°C). light, high viscosity, local, forms domes.Pyroclastic Deposits - vapor P release explosively ejects magma into air. Pyroclasts- material ejected, classed by size. Ash to house-size; tuffs; breccias. Pyroclastic Flow - hot mix of ash, gas, and dust that travels near surface, flows down flanks of volcanoes up to 200 km/hr; most dangerous volcanic hazard.


VOLCANIC LANDFORMS and eruptive styles - Shield Volcanoes - very large, convex, broad, "shield-shaped" cone, thousands of very fluid-lavas (mostly basalt), typical at hotspots; Mauna Loa, Kilauea. - Cinder Cones - small, concave cones, made of layers of cinders, commonly basaltic - Composite Volcanoes - large, concave, steep-sided, alternating lavas flows and pyroclastic deposits, andesitic, erupt explosively, subduction zones; Mt. St. Helens. - Volcanic Domes - small, steep-sided domes, viscous rhyolitic magma, usually plugs the vent of composite cones after explosive eruption, periodically collapse or explode. - Calderas - large collapse structures; emptying of large, shallow magma chamber during violent eruptions. Occur on shield and composite volcanoes. Very dangerous. Yellowstone; Long Valley - Phreatic Eruption - magma in with water (ground, sea, lake, ice); can be very explosive; Krakatoa 1883. - Fissure Eruptions - large volumes of lava from linear cracks; shield volcanoes; mid-ocean ridges; Iceland; flood basalts. - Lahars (mudflows) - warm mix of wet volcanic debris; moves rapidly in stream valleys; melting glacial ice or rain on recent pyroclastic deposits. Dangerous Created & compiled by Andri SSM Octo-2004 Petrology & Economic Geology Laboratory Dept. of Geology – FIKTM ITB

Origin


"fire-formed rocks" Crystallize from molten material: •Magma - below the Earth's surface •Lava - erupts onto the Earth's surface through a volcano or crack (fissure) Lava cools more quickly because it is on the surface.

Cooling Rates Cooling rates influence the texture if the igneous rock: •Quick cooling = fine grains •Slow cooling = coarse grains Aphanitic - fine grain size (< 1 mm); Phaneritic - coarse grain size; visible result of quick cooling grains (1-10 mm); result of slow cooling

Rhyolite Granite

Porphyritic-


Mixture of grain sizes caused by mixed cooling history; slow cooling first, followed by a period of somewhat faster cooling. for the textural components: Phenocrysts - the large crystals; Groundmass or matrix - the finer crystals surrounding the large crystals. The groundmass may be either aphanitic or phaneritic. Types of porphyritic textures: Porphyritic-aphanitic & Porphyritic-phaneritic

Origin: mixed grain sizes and hence cooling rates, imply upward movement of magma from a deeper (hotter) location of extremely slow cooling, to either: a much shallower (cooler) location with fast cooling (porphyritic- aphanitic), or a somewhat shallower (slightly cooler) location with continued fairly slow cooling (porphyritic-phaneritic).





Hydrothermal cycles at spreading zone that mainly associated with Mid Oceanic Ridge

Primary magmas Primary magma-any chemically unchanged melt derived

from a partial melting of its (mantle) source rock

Primitive magma-ambiguous but means unmodified Parental-magmas that give rise to other derivative

magmas.

Interested in knowing if magmas are co-genetic

Primary Melt reflect process-fractional vs equilibrium,

depths (init, cease), source, degree of melting Modified by -fractional crystallization -magma mixing -assimilation (crust, lithoshere) -liquid immiscibility


Vessiculation:

depressurization leads to gas becoming less soluble and formation of bubbles-depends on amount of gas, composition of melt, and pressure Once bubbles form, magma is less dense, rises more, loses more gas...

If viscosity is low, bubbles coalesce and escape if slow flow, bubbles leave non-violently. if fast flow, magma gets thrown out with a burp-bombs or scoria.

If viscosity is high, bubbles can’t coalesce they get bigger and eventually cause disruption of magma, explosive eruption Other possible driving forces tectonic forces-squeezing like toothpaste? diapirs-blobs rising



How do Rocks Melt? Magma, Lava and their products

The Geotherm-Why is the Interior Hot? •Sources of Heat •Adiabatic Compression •Primordial Heat •Radioactivity •Sunlight

How does Earth get rid of Heat? •Radiation •Conduction •Convection •Plate Tectonics Created & compiled by Andri SSM Octo-2004 Petrology & Economic Geology Laboratory Dept. of Geology – FIKTM ITB

Temperature of The Earth The Geotherm


Why is the Earth's Interior Hot? Sources of Heat •Pressure-Adiabatic Compression •Primordial Heat-Left over from the formation •Decay of Radioactive Isotopes- (K, Th, U) •Sunlight-Only penetrates a few inches


How to get rid of Heat?


•Convection-(mass) •Radiation-(light) •Conduction-(atomic vibrations) •Most efficient for Earth's Interior-leads to Plate Tectonics

When do Rocks Melt? •When Geotherm Crosses Melting Curve •All Materials have a Melting Point •The Melting Point changes with Pressure & Composition

(Melting Curve)



Causes of Melting •Adiabatic Decompression or Pressure Release Melting

Addition of Water



What Happens after Melting?

Where does melting happen?

•Transport •Eruption •Ponding •Solidification

What Solidifies First? Bowen's Reaction Series •One Magma-All sorts of rocks •Discontinuous Series-Fe, Mg rich Silicates •Continuous Series-Ca, Na, K, rich Silicates


PLUTONIC=COOL BELOW THE SURFACE; COOL SLOWLY -> LARGE CRYSTALS VOLCANIC=COOL ABOVE THE SURFACE; COOL FAST -> SMALL CRYSTALS & GLASS


PLUTONIC VOLCANIC

~50%

<--SILICA CONTENT-->

as high as 72%

LOW

<--LAVA VISCOSITY-->

HIGH

DARK/MAFIC

COLOR

LIGHT/FELSIC

GABBRO

DIORITE

GRANITE

BASALT

ANDESITE

RHYOLITE

•Glassy - instantaneous cooling •Obsidian = volcanic glass •Aphanitic - fine grain size (< 1 mm); result of quick cooling • Basalt •Rhyolite •Andesite

Obsidian

Rhyolite


Igneous textures:

PHANERIC •Granite •Diorite •Gabbro •Pegmatitic - very large crystals (many over 2 cm)


•Phaneritic - coarse grain size; visible grains (1-10 mm); result of slow cooling •Granite pegmatite or pegmatitic granite

Porphyritic- Mixture of grain sizes caused by mixed cooling history; slow cooling first, followed by a period of somewhat faster cooling. • for the textural components: •Phenocrysts - the large crystals •Groundmass or matrix - the finer crystals surrounding the large crystals. The groundmass may be either aphanitic or phaneritic. •Types of porphyritic textures: •Porphyritic-aphanitic •Porphyritic-phaneritic •Origin: mixed grain sizes and hence cooling rates, imply upward movement of magma from a deeper (hotter) location of extremely slow cooling, to either: •a much shallower (cooler) location with fast cooling (porphyritic- aphanitic), or •a somewhat shallower (slightly cooler) location with continued fairly slow cooling (porphyritic-phaneritic). •Rock name = porphyry •Granite porphyry or porphyritic granite (porphyritihaneritic) - phenocrysts usually potassium feldspar •Andesite porphyry or porphyritic andesite (porphyritic-aphanitic) - phenocrysts usually hornblende (amphibole)


Low viscosity basaltic lava flow from an active volcano on one of the Hawaiian Islands. Created & compiled by Andri SSM Octo-2004 Petrology & Economic Geology Laboratory Dept. of Geology – FIKTM ITB


A cinder cone is a small volcano (high viscosity magma), between 100 and 400 meters tall, made up of exploded rock blasted out of a central vent at a high velocity

Mount St. Helens eruption on May 18, 1980


The above image is a post-eruption computer rendering of Mount St. Helens from a U.S.Geological Survey digital elevation model (DEM). The lateral eruption removed 2.8 cubic kilometers of rock and sediment from from the volcano and lowered its height by 400 meters. Detectable amounts of ash were spread over 50,000 square kilometers of area surrounding the volcano. The large crater created by the explosive eruption is about 600 meters deep and can be seen in the center of the image above.

The most explosive type of volcano is the CALDERA. The cataclysmic explosion of these volcanoes leaves a huge circular depression at the Earth's surface. This depression is usually less than 40 kilometers in diameter. These volcanoes form when "wet" GRANITIC MAGMA quickly rises to the surface of the Earth. When it gets to within a few kilometers of the surface the top of the magma cools to form a dome. Beneath this dome the gaseous water in the magma creates extreme pressures because of expansion. When the pressure becomes too great the dome and magma are sent into the Earth's atmosphere in a tremendous explosion. On the island of KRAKATOA , a caldera type volcano exploded in 1883 ejecting 75 cubic kilometers of material in the air and left a depression in the ground some 7 kilometers in diameter.




Diamonds ascend to Earth's surface in rare molten rock, or magma, that originates at great depths. Carrying diamonds and other samples from Earth's mantle, this magma rises and erupts in small but violent volcanoes. Just beneath such volcanoes is a carrot-shaped "pipe" filled with volcanic rock, mantle fragments, and some embedded diamonds. The rock is called kimberlite after the city of Kimberley, South Africa, where the pipes were first discovered in the 1870s. Another rock that provides diamonds is lamproite.

The volcano that carries diamond to the surface emanates from deep cracks and fissures called dikes. It develops its carrot shape near the surface, when gases separate from the magma, perhaps accompanied by the boiling of ground water, and a violent supersonic eruption follows. The volcanic cone formed above the kimberlite pipe is very small in comparison with volcanoes like Mount St. Helens, but the magma originates at depths at least 3 times as great. These deep roots enable kimberlite to tap the source of diamonds. Magmas are the elevators that bring diamonds to Earth's surface.



While minor diamond discoveries were made among alluvial gold in New South Wales starting in 1851, a discovery in 1979 on the Kimberley Plateau of Western Australia enabled the country to be the world's most prolific diamond producer. Based on ancient bedrock, diamond exploration began in 1972, with a kimberlite pipe discovery coming in 1976 in the Ellendale area. In 1979, a large lamproite pipe was found and named the Argyle mine; by 1992 over 200 million carats had been mined there. Only 5% of the production is gem quality. A unique feature of the Argyle mine, though, is a small but consistent supply of valuable pink to red or purple diamonds

The Argyle mine on the Kimberley plateau of Western Australia. Australian Production Total: 428 million carats Annual: 3540 million carats

Most diamonds consist of primeval carbon from Earth's mantle, but those from eclogites probably contain carbon recycled from the ocean crust by plate tectonics -- the carbon of microorganisms. How do we know? Carbon atoms occur in three different masses, or isotopes. Unlike high-temperature processes in deep Earth, lowtemperature, biological processes, such as photosynthesis, are sensitive to the differences in mass, and actively sort different carbon

isotopes.

Thus, the ratios of carbon isotopes in organic materials -- plants, animals, and shells -- vary, and also differ from those in the carbon dioxide of the atmosphere and the oceans. Geochemists "read" the carbon isotopes in samples to interpret nature's record. Virtually all carbon atoms, the ones in a diamond or a tree or you, came from the stars. Particularly at Earth's surface the proportions of 12C and 13C (the carbon isotopes of mass 12 and 13) get redistributed. Expressed as simple numbers in 13C notation -- in which larger numbers mean more 13C -organic carbon has large negative values, average Earth has a mildly negative value, and the carbon in shells is near zero Created & compiled by Andri SSM Octo-2004 Petrology & Economic Geology Laboratory Dept. of Geology – FIKTM ITB

The narrow range of 13C values for harzburgitic diamonds in the histogram on the top resembles the range of average Earth, indicating that the mantle is the likely carbon source. The large range for eclogites suggests mixing of organic carbon (the strongly negative numbers), mantle carbon (mildly negative numbers), and shell-like carbon (values near zero). These data recycling of once-living carbon from Earth's surface deep into the mantle to form diamond. Created & compiled by Andri SSM Octo-2004 Petrology & Economic Geology Laboratory Dept. of Geology – FIKTM ITB

When ocean floor slides into the mantle, the basaltic rock becomes eclogite, and organic carbon in sediments may become diamond Created & compiled by Andri SSM Octo-2004 Petrology & Economic Geology Laboratory Dept. of Geology – FIKTM ITB



Forms of Intrusive Plutons Differentiated Sills Layered Complexes Differentiated Sills Always sill-like in shape and generally hypabyassal Very sharp s with host rocks marked by a thin chill zone Systematic varaitions in chemical and mineralogic composition Modal qtz+kspar increase upward. Plag and pyroxene decrease Fe/Mg increase upward in the pyroxene Plag systematically becomes more albitic Elements Si, Fe, Na, K increase upward; Ca, Mg decrease Grain size of the final differentiate is often pegmatitic Basal rocks are often ultramafic Upper chilled margin closely resembles lower chilled margin in composition (i.e. mafic) Created & compiled by Andri SSM Octo-2004 Petrology & Economic Geology Laboratory Dept. of Geology – FIKTM ITB


Origin Differentiation (Crystal- Liquid Fractionation) Early formed differentiates sink gravitationally giving rise to lower layer of ultramafics. Chilled zone of mafics is due to attachment of crystals to the roof of the magma chamber. It should be smaller than the basal zone and generally it is. Sill then changes compoistion upward due to the differentiation process.

Assimilation of Country Rock - Not thought to be important because the s between host and pluton are too sharp and inclusions are lacking.

Mixing - Also not important as this would invalidate the systematic chemical variations


Layered Intrusive Complexes Characteristics Extremely large - Bushveld (65,000 km2, Great Dyke 5000 km2 Usually have the shape of an inverted funnel Layering is very promiment and always discordant with the walls of the funnel Often linear to elliptical in shape Possess both rhythmic (cyclic) layering Also prosses cryptic mineral and chemical variations Graded bedding is common Locally slump structures and cross bedding has been noted

Origin Problem has always been explaining rhythmic and cryptic layering and graded bedding. Rhythmic layering thought to be the result of repeated reinjection of new batches of magma, but cryptic layering invalidates this. Graded bedding implies gravity settling. Cryptic layering suggests fractionation is the dominant process. Local cross beds and slump structures indicate the magma chamber was not undergoing convective motion.

static, but rather


Chemical analyses can be obtained, and a chemical classification, such as the LeBas et al., IUGS chemical classification of volcanic rocks (based on total alkalies [Na2O + K2O] vs. SiO2 diagram shown above)

SiO2 (Silica) Content > 66 wt. % - Acid 52-66 wt% - Intermediate 45-52 wt% - Basic < 45 wt % - Ultrabasic


This terminology is based on the onetime idea that rocks with a high % SiO2 were precipitated from waters with a high concentration of hyrdosilicic acid H4SiO4. Although we now know this is not true, the acid/base terminology is well entrenched in the literature.

Silica Saturation If a magma is oversaturated with respect to Silica then a silica mineral, such as quartz, cristobalite, tridymite, or coesite, should precipitate from the magma, and be present in the rock. On the other hand, if a magma is undersaturated with respect to silica, then a silica mineral should not precipitate from the magma, and thus should not be present in the rock. The silica saturation concept can thus be used to divide rocks in silica undersaturated, silica saturated, and silica oversaturated rocks. The first and last of these are most easily seen. Silica Undersaturated Rocks - In these rocks we should find minerals that, in general, do not occur with quartz. Such minerals are:

Sodalite - 3NaAlSiO4.NaCl

Nosean - 6NaAlSiO4.Na2SO4

Haüyne - 6NaAlSiO4.(Na2,Ca)SO4

Perovskite - CaTiO3

Melanite - Ca2Fe+3Si3O12

Melilite - (Ca,Na)2(Mg,Fe+2,Al,Si)3O7 Thus, if we find any of these minerals in a rock, with an exception that we'll see in a moment, then we can expect the rock to be silica undersaturated.


Forsteritic Olivine - Mg2SiO4

If we calculate a CIPW Norm (we'll see how to do this in lab) the normative minerals that occur in silica undersaturated rocks are nepheline and/or leucite. Silica Oversaturated Rocks. These rocks can be identified as possibly any rock that does not contain one of the minerals in the above list. •

If we calculate a CIPW Norm, silica oversaturated rocks will contain normative quartz. Silica Saturated Rocks. These are rocks that contain just enough silica that quartz does not appear, and just enough silica that one of the silica undersaturated minerals does not appear. In the CIPW norm, these rocks contain olivine, or hypersthene + olivine, but no quartz, no nepheline, and no leucite. To get an idea about what silica saturation means, let's look at a simple silicate system - the system Mg2SiO4 - SiO2 •

Note how compositions between Fo and En will end their crystallization with only Fo olivine and enstatite. These are SiO2-undersaturated. compositions. All compositions between En and SiO2 will end their crystallization with quartz and enstatite. These are SiO2 – oversaturated compositions. Note also that this can cause some confusion in volcanic rocks that do not complete their crystallization due to rapid cooling on the surface. Let's imagine first a composition in the silica-undersaturated field. Cooling to anywhere on the liquidus will result in the crystallization of Fo-rich olivine. . If this liquid containing olivine is erupted and the rest of the liquid quenches to a glass, then this will produce a rock with phenocrysts of olivine in a glassy groundmass. Created & compiled by Andri SSM Octo-2004 Petrology & Economic Geology Laboratory Dept. of Geology – FIKTM ITB

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Olivine-Leucite Basalt (Basanite) of Komba Volcano – Flores Sea Created & compiled by Andri SSM Octo-2004 Petrology & Economic Geology Laboratory Dept. of Geology – FIKTM ITB

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Olivine-Leucite Basalt (Basanite) of Komba Volcano – Flores Sea Created & compiled by Andri SSM Octo-2004 Petrology & Economic Geology Laboratory Dept. of Geology – FIKTM ITB

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