1.  Introduction

•  Petrology is the study of rocks at various scales (outcrop, hand specimen, thin section).  Two main branches of petrology.
• petrography-  description/classification by mineralogy and texture.
• petrogenesis-  origin/history of rocks based on field relations, experiments, and chemical analysis.
 
• This course cover igneous and metamorphic petrology only.
• Igneous rocks crystallize from magma.
• Metamorphic rocks are igneous, sedimentary rocks that have changed mineralogically and texturally due to heat (T), pressure (P), directed stresses, and/or chemical reactive fluids (H₂O, CO₂).

• Importance:  most of the crust and mantle of the Earth (and the inner planets of our solar system) are made up of igneous and metamorphic rocks.

2.  Igneous Rock Structures

-  Important for petrogenesis and basic field interpretations.
-  Two main types: (1) extrusive (volcanic) and (2) intrusive (plutonic).
 

 Extrusive Structures

• Lava flows -   tabular or lobate masses of solidified magma, mostly basaltic (relatively low SiO₂ content).
• Subaerial (erupted on land) flows are three main types:

1) pahoehoe-  smooth, ropy flows of low viscosity magma that contains dissolved gases.
2) aa-  rough, irregular fragments that form flows; higher viscosity magma than pahoehoe mainly due to degassing.  3) blocky- high viscosity flows made up of large blocks (larger than aa) with smooth surfaces.
 
• Flow units-  Lava flows from single eruptions.  Can be composed pahoehoe, aa, and blocky flows depending on viscosity (SiO2 and gas content). Usually 10-30 m thick, but can be up 100- 200 m thick.  Multiple flow units can form thick piles up to 1-2 km and cover 10,000-100,000 km².  These large piles of basaltic lavas are called plateau basalts (or flood basalt provinces); examples include the Deccan (India) and Siberian Traps, the Columbia River basalts (E. Washington, and the Parana basalts in Brazil.  NJ examples include the three Watchung basalts of the Triassic/Jurassic Newark Basin (e.g. Orange Mtn Basalt, Preakness Basalt, and the Hook Mtn Basalt).
 
• Pillow lavas-  Subaqueous basaltic lavas (erupted underwater) that form ellipsoidal or pillow-shaped bodies (10 cm - 6 m in diameter).  The pillows have fine-grained interiors with glassy (quenched) crusts.  Mostly form at mid-ocean ridges (MOR) and are the common type of lava found in oceanic crust.  Important for interpreting ancient tectonic environments from the rock record.  Rare in non-marine environment (but, NJ has some examples in Newark Basin).
 
• Pyroclastic material -  any volcanic material that is ejected into the air (ejecta).  Compositions can range from basaltic to rhyolitic, but higher viscosity magmas (andesitic/rhyolitic w/ higher SiO₂ contents) are usually more commonly erupted as ejecta.  Accumulations of ejecta are called pyroclastic rocks or tephra.  Classified according to size.
• blocks and bombs-   (>32 mm diam.), blocks are solid when ejected, bombs are liquid.
• lapilli-  (4-32 mm), rock fragments formed from ejected droplets of magma.
• ash-  (<4 mm), usually glass, but sometimes contain mineral fragments; consolidated ash (welded) is called tuff.
 
• Nuee Ardente ("glowing cloud") -  In between ejecta and flow; hot (700°-1000°C), dense, mixture of incadescent rock fragments, ash, and gases that sweeps down the flanks of stratovolcanoes.  Can be either solid, liquid, or a mixture when finally comes to rest.  Form ignimbrites (or ash flow sheets) which are of two types: (1) ash flow tuff = deposit of mainly solid particles and is unconsolidated or (2) welded ash flow tuff = enough internal residual heat to weld particles together and is thus consolidated.  Usually 1-100 m thick; can be single or multiple cooling units.  Some areas on Earth have thick deposits (>1 km) and cover large areas (10,000-100,000 km²).  Examples include the western US (Cenozoic) and southern South America (Mesozoic).
 
• Types of Volcanoes
• Shield volcanoes -  broad, flat cones; 10's km in diameter with slopes of 2-10°. Represent huge volumes of magma and can be quite high (>3000 m above sea level, for example in Hawaii).  Caldera structures are common at summit.  Calderas are circular depressions caused by collapse of empty magma chambers at depth.  Usually erupt passively and compositions are dominantly low viscosity basaltic magmas.  Most common at oceanic intraplate hotspots (ie. Hawaii).
 
• Composite or stratovolcanoes -  steep-sided cones of alternating lava flows and pyroclastic material.  Usually the tallest volcanoes (above sea level; up to ~6000 m in the Andes).  Generally erupt explosively and compositions are more often relatively high viscosity andesitic magmas.  Only found associated with subduction at convergent plate boundaries (ie. Cascades of NW USA; Andes of South America).
 
• Lavas domes -  small, steep-sided structures of high viscosity rhyolitic magma; ususally form as plug domes in the vents (craters) or calderas of larger stratovolcanoes (ie. Mt St Helens), but can form independent structures (ie. Mono Craters, Long Valley, California).  Usually associated with convergent plate magmatism.
 
• Cinder cones -  small, steep-sided cones composed entirely of lapilli or cinder-seized tephra most commonly of basaltic composition.  Can form in a variety of different tectonic settings.

 
 

Intrusive Structures

Pluton is a general term for intrusive igneous rock, typically has "granitic" texture (granular).  Usually moderate to large size with outcrop area of ~10-100 km².  Intrusion is a general term for smaller body usually with shallower depth of emplacement than a pluton.

 

• Batholiths -  the largest bodies of plutonic rock (>100 km²); they are usually lenticular or tabular in shape (in 3-D) and most often "granitic" in composition.  Composed of multiple, smaller bodies called stocks (<10 km²).  Depth classification of batholiths.
• Epizonal (shallow) -  evidence of forceful injection, chilled border, thin contact metamorphic zone, discordant contact relations.

• Mesozonal (intermediate) -  wide zone of contact metamorphism, discordant or concordant contacts, internal flow structures.
 
• Catazonal (deep) -  concordant contacts, gneissic banding near contact, wide zone of migmatite (melted country rock).
 
• Lopoliths -  large, dish-shaped or lenticular, layered intrusions; usually mafic to ultramafic in composition.  Diameters can be large (10-100 km) and several km thick.  Classic examples include the Skaergaard Complex in East Greenland, the Bushveld Complex in South Africa, and the Stillwater Complex in Montana.
 
• Sills -  concordant sheets, emplaced parallel to bedding or foliation in the country rock.  Usually shallow depths of emplacement and dominantly of basaltic composition.  Can be quite thick 300-400 m thick and outcrop over large distances (>100 km).  The thickest sills show evidence for magma differentiation (e.g. Palisades Sill, NJ).
 
• Laccoliths -  concordant, mushroom-shaped bodies with diameters of 1-10 km, maximum thickness ~1000 m.  Shallow emplacement, tend to form domes in overlying strata.  Usually composed of slightly more viscous and higher SiO₂ content magmas than basaltic magmas of sills.
 
• Dikes -  discordant, thin tabular intrusions; thickness usually <10 m, some larger dikes are >100 m thickness, some can be traced for 10's km along strike.  Often form radiating swarms (dike swarms) that radiate from volcanic necks (eroded throat of volcano).  Also form ring dikes or cone sheets which are cylindrical or inward dipping dike structures due to brittle fracturing above large magma chambers at depth; usually associated with caldera structures.
 
• Small-scale Features (meter-scale or less)
• veins -  crudely tabular bodies that fill fractures.
• apophyses -  short, irregular dikes that extend from larger plutons.
• xenoliths -  accidental inclusions of foreign rock, usually surrounding country rock.
• autoliths -  inclusions of rock that genetically related to the pluton or intrusion.

3.  Igneous Rock Textures

Igneous rocks contain crystals, glass, and/or combination with rock fragments.  Texture is the geometric relationship between crystals, glass, and rockfragments.  This includes grain size, shape, orientation, and grain boundary relationships.  Mostly controlledby cooling rate and history.

• holocrystalline - totally crystalline
• holohyaline -  totally glass (glassy)
• hypocrystalline -  combination of crystalline and glass

• aphanitic -  very fine-grained; individual crystals are not seen easily with the naked eye.
• phaneritic -  crystals can be seen with eye;  fine-grained (<1 mm), medium-grained (1-5 mm), coarse-grained (>0.5-3 cm).
• pegmatitic - very coarse-grained (>5 cm).
• porphyritic texture -  phaneritic crystals set in a finer-grained matrix.

• phenocrysts - crytsals in a porphyritic volcanics rock;  phyric rock has phenos, aphryric rock does not.
• groundmass -  the crystals and/or glass that makes up the fine-grained matrix that phenocrysts are set in.
• euhedral -  crystal shapes that are bounded by well-developed faces, perfect crystal shapes.
• subhedral -  crystals that are partially bound by crystal faces.
• anhedal -  crystals with no crystal faces; sometimes look corroded.
• granular texture -  the classic "granitic" texture; interlocking grains of roughly uniform size.
• graphic texture -  mostly in pegmatitic granites; very systematic intergrowth of quartz and feldspar.
• poikilitic texture -  large grains that totally enclose smaller, early formed minerals.
• ophitic texture - large clinopyroxene graines that enclose plagioclase.

subophitic texture -  simultaneous crystallization of clinopyroxene and plagioclase; enclose one another.
 
 

-  Primary:  minerals that crystallize directly from magma.
-  Secondary:  form at temperatures below any magma; hydration, oxidation by late fluids or weathering.
-  Accessory:  primary or secondary minerals that are in relatively low abundance (<5% of the rock).
 

Isolated, Paired, and Ring Silicates

• Olivine -  (Mg,Fe)₂SiO₄ solid solution.  Mg-rich olivines are common in ultramafic and mafic rocks, particularly peridotites, gabbros, and basalts.  Fe-rich olivine are much rarer, but can occur in highly fractionated "granitic" rocks.
• Garnets, zircon, topaz, sphene, monazite, titanite, allanite, epidote (mostly secondary) are all accessory minerals usually in more fractionated or SiO₂-rich magmas.
• Beryl and tourmaline can be significant minerals in very fractionated, SiO₂- and H₂O-rich pegmatites (very coarse-grained granitic rocks).

 

Chain Silicates

• Pyroxenes (single chain; XYSi₂O₆); most common pyroxenes in igneos rocks are:
•  Ca-rich clinopyroxenes: augite [(Ca,Na)(Mg,Fe,Al)(SiAl)₂O₆], and hedenbergite (CaFeSi₂O₆).  Augite is a very common mineral in mafic volcanic and plutonic rocks.  hedebergite and Fe-rich augite are common in late stage differentiated mafic magmas and some Fe-rich granites.
• Ca-poor orthopyroxenes: Orthopyroxenes [(Mg,Fe)SiO_3] and pigeonite [Ca_0.25(Mg,Fe)_1.75Si₂O₆] are common in ultramafic and mafic plutonic rocks.  Some basalts have phenocrysts of Opx.   
• Na-rich pyroxenes;  aegirine (NaFeSi₂O₆), mostly in alkali-rich plutonic rocks.
• Amphiboles (double chains; WX₂Y₅Z₈O₂₂(OH,F)₂)
• most common is hornblende [(Ca,Na)_2-3(Mg,Fe,Al)_5-6(Si,Al)₂Si₆O₂₂(OH)₂]) in intermediate and silicic rocks (both plutonic and volcanic)
• riebeckite and arfvedsonite [Na₂(Fe²⁺, Fe³⁺)₅Si₈O₂₂(OH)₂] are common in alkali-rich volcanic and plutonic rocks and late-stage differentiated rocks.
 

Mica & Sheet Silicates

• Primary minerals include biotite [K(Mg,Fe)₃AlSi₃O₁₀(OH)₂], muscovite [KAl₃Si₃O₁₀(OH)₂], and phlogopite [KMg₃AlSi₃O₁₀(OH)₂].  Biotite is common is common in all types of igneous rocks; muscovite is common in silicic plutonic rocks; and phologopite is commonly found in ultramafic rocks.
• Secondary minerals include many clay minerals such as chlorite, sericite, iddingsite.

• Framework Silicates  (most abundant minerals in igneous rocks)
• Quartz (SiO₂) usually forms anhedral grains intermediate to felsic rocks, common phenocryst in rhyolites and major component of granites.
• Feldspars:
• Alkali feldspars -  complete solid solution between albite (NaAlSi₃O₈) and orthorclase (microcline, sanidine; KAlSi₃O₈).  Very common in intermediate to felsic rocks; ususally form euhedral crystals in both volcanic and plutonic rocks.
• Plagioclase feldspars - complete solid solution between albite (NaAlSi₃O₈) and anorthite (CaAl₂Si₂O₈).  Most common crustal mineral, found in all compositional types of igneous rocks; Ca-rich plag in mafic rocks, Na-rich in intermediate and felsic rocks.
 
• Others silicates
• Mainly the feldspathoids like nepheline (NaAlSiO₄), leucite (KAlSi₂O₆), and sodalite (Na₃Al₃Si₃O₁₂ lNaCl).  Common constituent in akali-rich igenous rocks.  Zeolites (natrolite, chabazite, etc.) are common secondary minerals that fill cavitites in mafic voclanics.

• Oxides, Sulfides, Phosphates, and Carbonates
• Oxides-  spinel (e.g. chromite (Fe,Mg)CrO₄), magnetite (Fe₃O₄), and ilmentite (FeTiO₃).  Fe-oxides display solid solution between magnetite, ilmentite, and hematite (Fe₂O₃).  Useful for constraining oxidation state of magma and crystallization T°; common in all types of igneous rocks
• Sulfides- pyrite (FeS₂), chalcopyrite (CuFeS₂); comes from magmatic sulfur in mostly mafic and ultramfic magmas.
• Phosphates- apatite [Ca₅(PO₄)₃(OH,F,Cl)]; common in late stage fractionated magmas, and felsic plutonic rocks.
• Carbonates- calcite (CaCO₃) usually secondary, but is a primary mineral in carbonatites (igneous rocks that form from CaO- and CO₂-rich magmas) along with dolomite [CaMg(CO₃)₂] and ankerite [CaFe(CO₃)₂].