Magmatism and Tectonics

Basaltic Lava Fountain and Flows – (Hawaii)

Andesitic Explosive Eruption – (Mt. St. Helens)

Magma Types Based on Chemistry gabbro/basaltdiorite/andesitegranite/rhyolite 50% SiO 2 60% SiO 2 70% SiO 2

Physical Properties of Different Magma Types

Extrusive and Intrusive Igneous Rocks Granite Basalt

IUGS classification of phaneritic igneous rocks with more than 10% (quartz + feldspar + feldspathoids). Quartz-rich Granitoid Alkali Fs. Quartz Syenite Quartz Syenite Quartz Monzonite Quartz Monzodiorite Syenite Monzonite Monzodiorite (Foid)-bearing Syenite (Foid)-bearing Monzonite (Foid)-bearing Monzodiorite 90 Alkali Fs. Syenite (Foid)-bearing Alkali Fs.Syenite 10 (Foid) Monzosyenite (Foid) Syenite (Foid) Monzodiorite (Foid) Gabbro Qtz. Diorite/ Qtz. Gabbro 5 10 Diorite/Gabbro/ Anorthosite (Foid)-bearing Diorite/Gabbro 60 (Foid)olites Quartzolite Granite Grano- diorite Tonalite Alkali Feldspar Granite Q A P F 60 Classification of Igneous Rocks

Quartz-rich Granitoid Alkali Fs. Quartz Syenite Quartz Syenite Quartz Monzonite Quartz Monzodiorite Syenite Monzonite Monzodiorite Alkali Fs. Syenite Qtz. Diorite/ Qtz. Gabbro 5 Diorite/Gabbro/ Anorthosite Quartzolite Granite Grano- diorite Tonalite Alkali Feldspar Granite Q A P QAP diagram

IUGS classification of the phaneritic igneous rocks.OlivineClinopyroxene Orthopyroxene Lherzolite Harzburgite Wehrlite Websterite Orthopyroxenite Clinopyroxenite Olivine Websterite Peridotites Pyroxenites Dunite (c) Gabbroic Rocks Ultramafic Rocks

Major elements: principle components of rocks mineralogy expressed as wt% of oxide; usually greater than 0.1% –SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K2O P2O5 H2O Trace elements: everything else in ppm or ppb; not principle components in minerals Isotopes: –Special isotopes of certain elements (ie. Sr, Nd, Pb, O) that are particularly important for understanding igneous processes Bulk (or Whole Rock) Chemistry of Igneous Rocks

Table 8-3. Chemical analyses of some representative igneous rocks PeridotiteBasaltAndesiteRhyolitePhonolite SiO TiO Al2O Fe2O FeO MnO MgO CaO Na2O K2O H2O Total Major Elements

Early on it was recognized that some major element chemical parameters were very useful in regard to distinguishing magmatic groups (classification) and for showing important petrogenetic trends –Total Alkalis (Na 2 O + K 2 O) vs (SiO 2 ) –AFM triangular diagram (Na 2 O + K 2 O, FeO, MgO) –Harker diagrams (Oxides vs. SiO 2 )

Alkali vs. Silica diagram for Hawaiian volcanics

Classification of Volcanic Rocks Alkaline Subalkaline

AFM diagram can further subdivide the subalkaline magma series into a tholeiitic and a calc-alkaline rocks A = Na2O + K2O F = FeO + Fe2O3 M = MgO Red circles = tholeiitic rocks from Iceland, the Mid-Atlantic Ridge, the Columbia River Basalts, and Hawaii Pink circles = calc-alkaline rocks of the Cascade Range, Washington

A world-wide survey suggests that there may be some important differences between the three series