Subalkaline Basaltic Rocks Petrography of basaltic rocks Petrography of basaltic rocks Field relations of basaltic rocks Field relations of basaltic rocks Continental basaltic association Continental basaltic association

Petrography of Basaltic Rocks Fabric Fabric Classification Classification Alteration Alteration

Field Relations - Basaltic Rocks IntrusionsIntrusions Dikes, sills, plugs, necksDikes, sills, plugs, necks ExtrusionsExtrusions Lava flows (pahoehoe, aa aa)Lava flows (pahoehoe, aa aa) Shield volcanoesShield volcanoes Scoria conesScoria cones Tuff ringsTuff rings HyaloclastitesHyaloclastites

Pahoehoe Flows b Very fluid lavas b Smooth surface skin b Ropy textures, surface pleats b Flow moves as growing bubbles or buds b Sometimes a gap at the top of the bud b Shelly pahoehoe

Mount Etna, Italy

Scoria Cones b Simplest and commonest volcanic form b Characterized by three parameters Height, width, crater widthHeight, width, crater width b Standard initial slope of 30 o b Conical shape b Occur in several environments b McGetchin model of cone growth b Erosion is systematic

Sunset Crater, Arizona

Diatremes b Breccia pipes b Kimberlite b Contains diamonds b Ultramafic magmas b Mixture of rocks b Driven by deep CO 2

Tuff Cones b Massive deposits b Thickly bedded b Palagonitized b Bedding up to 30 o b Wet surges Vulcano, Italy

Tuff Rings b Thinly-bedded b Poorly-indurated b Beds less than 12 o b Sandwave beds b Dry surges

Continental Basaltic Association Plateau basaltsPlateau basalts CharacteristicsCharacteristics OriginOrigin Local basalt fieldsLocal basalt fields Basin and Range of USABasin and Range of USA

Examples of Plateau Basalts Columbia River Plateau, USA (T) Columbia River Plateau, USA (T) Deccan, India (K-T) Deccan, India (K-T) Parana, Brazil (J-K) Parana, Brazil (J-K) Keeweenaw, Lake Superior (PreC) Keeweenaw, Lake Superior (PreC) Karoo, South Africa (J) Karoo, South Africa (J) Greenland-Great Britain (K-T) Greenland-Great Britain (K-T)

Plateau Basalt Characteristics Fissure eruptions, associated dike systems Fissure eruptions, associated dike systems Huge volume (>10 5 km 3 ) Huge volume (>10 5 km 3 ) Large discharge rate Large discharge rate May herald the breakup of continents May herald the breakup of continents

Chemical Characteristics Typically more evolved composition than MORBTypically more evolved composition than MORB Higher Si, K, Ti, P, and BaHigher Si, K, Ti, P, and Ba Lower Mg, and NiLower Mg, and Ni Evolved, olivine-poor compositionsEvolved, olivine-poor compositions Suggest some fractionation prior to eruptionSuggest some fractionation prior to eruption

Isotopic Evidence Low initial Sr isotope ratios (<0.704)Low initial Sr isotope ratios (<0.704) Suggest partial melting of upper mantle peridotiteSuggest partial melting of upper mantle peridotite High initial Sr isotope ratios (>0.704)High initial Sr isotope ratios (>0.704) Suggest contamination with crustal materialsSuggest contamination with crustal materials

Origin of Plateau Basalts Low degree of fractionation Low degree of fractionation Low initial Sr isotope ratio Low initial Sr isotope ratio Phase relationships Phase relationships Suggest an origin from a peridotite zone within the asthenosphere at a depth between 60 to 100 km. Suggest an origin from a peridotite zone within the asthenosphere at a depth between 60 to 100 km.

Local Basalt Fields May occur in areas of continental extensionMay occur in areas of continental extension Basin and Range of western USABasin and Range of western USA Characterized by scoria cones and lavasCharacterized by scoria cones and lavas Some surround composite andesitic conesSome surround composite andesitic cones Minor bimodal basalt-rhyolite (pyroclastic) associationMinor bimodal basalt-rhyolite (pyroclastic) association

Basaltic Fields b 10s to 1000s of cones b General elliptical shape b Aspect ratio of 2:1 to 5:1 b 10 to 70 km in length b Areas of extensional tectonics b Elongate perpendicular to tension b Widespread in western USA b Pinacate example

Small Field b North rim of Grand Canyon b Scoria cones aligned along falut planes

Origin of Local Continental Basalt Fields Hot magma from the mantle intrudes rifting crust Hot magma from the mantle intrudes rifting crust Accumulation of basalt at depth melts silicic crust Accumulation of basalt at depth melts silicic crust Silicic melt buoyantly rises to shallow chambers Silicic melt buoyantly rises to shallow chambers Shallow chambers erupt to produce evolved pyroclastic deposits Shallow chambers erupt to produce evolved pyroclastic deposits

Oceanic Subalkaline Basaltic Association Two types of basaltic provincesTwo types of basaltic provinces Intraplate volcanoes (hot spots)Intraplate volcanoes (hot spots) Spreading plate boundaries (ocean ridges)Spreading plate boundaries (ocean ridges) IcelandIceland Oceanic riftsOceanic rifts Mid-Atlantic riseMid-Atlantic rise East-Pacific riseEast-Pacific rise

Iceland Subaerial outcropping of the Mid-Atlantic RidgeSubaerial outcropping of the Mid-Atlantic Ridge No continental sial is presentNo continental sial is present Mostly contains quartz tholeiiticMostly contains quartz tholeiitic Minor alkali basaltsMinor alkali basalts A few eruptive centersA few eruptive centers Fe-rich andesite, dacite, & rhyoliteFe-rich andesite, dacite, & rhyolite Produced by olivine fractionationProduced by olivine fractionation Origin from rising mantle plume?Origin from rising mantle plume?

Icelandic Shields b Moderate size b Extremely symmetrical b Small size >800 m high b Uniform slope ~ 8 o b Tube-fed pahoehoe lavas

Mauna Kea, Hawaii Skaldbreidur, Iceland

Subglacial Volcanoes b Pillow lavas b Pillow breccias b Hyaloclasstites b Dikes b Flat top with lava

Sub-glacial b Sequence of intrusion b Final form is a table mountain

Oceanic Rifts Their lavas comprise 70% of the earth’s surface Their lavas comprise 70% of the earth’s surface Sea floor spreading is the mechanism of their origin Sea floor spreading is the mechanism of their origin

Oceanic Lithosphere Layer 1Layer 1 0 to 1 km thick, sediment0 to 1 km thick, sediment Layer 2Layer 2 1 to 3 km thick, basalt flows, pillows breccia, dikes1 to 3 km thick, basalt flows, pillows breccia, dikes Layer 3Layer 3 4 to 8 km thick, fractured mafic intrusions 4 to 8 km thick, fractured mafic intrusions Below layer 3 is is subcrustal peridotiteBelow layer 3 is is subcrustal peridotite

Ocean Floor Basalts MORBMORB Reference composition to other basalt typesReference composition to other basalt types See book Table 5-5 for chemical characteristicsSee book Table 5-5 for chemical characteristics Low K 2 O content & large-ion lithophile elementsLow K 2 O content & large-ion lithophile elements Originate in the mantleOriginate in the mantle Partial melts within the asthenospherePartial melts within the asthenosphere Olivine tholeiitic composition (Ol and Hy in norm)Olivine tholeiitic composition (Ol and Hy in norm)

Ocean Floor Lavas Evidence of disequilibriumEvidence of disequilibrium Corroded phenocrysts of Mg olivine and Ca plagioclaseCorroded phenocrysts of Mg olivine and Ca plagioclase Chemically evolved groundmassChemically evolved groundmass Anomalous melt inclusionsAnomalous melt inclusions Uniform composition of lavasUniform composition of lavas Suggest recurrent mixing in shallow chambers under riftsSuggest recurrent mixing in shallow chambers under rifts

Depleted Magma Source Several lines of evidenceSeveral lines of evidence Extremely low concentrations of incompatible elementsExtremely low concentrations of incompatible elements Rb/Sr ratio too low to yield Sr isotopic ratio (~0.703)Rb/Sr ratio too low to yield Sr isotopic ratio (~0.703)

Models for Ocean Floor Lavas Thin lid modelThin lid model Primitive lavas fed from center of chamberPrimitive lavas fed from center of chamber More fractionated materials from marginsMore fractionated materials from margins Evolving systemEvolving system Several small chambers at different stages of fractionationSeveral small chambers at different stages of fractionation Strong role of crystal fractionationStrong role of crystal fractionation Supported by presence of mafic cumulate horizonsSupported by presence of mafic cumulate horizons

Ophiolites Alpine ultramafic bodiesAlpine ultramafic bodies Hartzburgitic typeHartzburgitic type Mainly hartzburgite and duniteMainly hartzburgite and dunite Minor dikes & veins of other typesMinor dikes & veins of other types Can not be the source of basaltic magmas by meltingCan not be the source of basaltic magmas by melting Lherzolitic typeLherzolitic type Mainly lherzolite, minor pyroxeniteMainly lherzolite, minor pyroxenite May yield basaltic magmas by partial meltingMay yield basaltic magmas by partial melting

Alpine Ultramafic Association Steinmann trinitySteinmann trinity Ultramafic rocksUltramafic rocks Pillow basalts (spilitic = metasomatized basalt)Pillow basalts (spilitic = metasomatized basalt) Chert (with argillite and limestone)Chert (with argillite and limestone) Origin by obductionOrigin by obduction Ocean floor thrust onto continental crust during mountain buildingOcean floor thrust onto continental crust during mountain building

Ophiolite Sequence Refractor residue of upper mantle hartzburgiteRefractor residue of upper mantle hartzburgite Deformed and drained of low-melting point materialsDeformed and drained of low-melting point materials Overlying fossil magma chambersOverlying fossil magma chambers Capping of fractionated basaltic lavas and dikesCapping of fractionated basaltic lavas and dikes Sheeted dike complexesSheeted dike complexes