Petrology

• They have high alkali concentration Alkalic Rocks relative to Si • Ne appears in the norm • Contain feldspathoids, alkali , Best, Chapter 6 alkali and many unusual minerals • High concentrations of incompatible trace elements (Zr, Nb, Rb, Ti, P, etc.)

Classification Oceanic Alkalic Rocks

• Common volcanic series • Tholeiitic to alkaline series – ---- – Galapagos • Other volcanic types – – , kimberlite, lamprophyres • Alkaline association • Common coarse-grained types – Tristan da Chuna – Syenite, , theralite, – Tahiti

Oceanic Rocks Continental Alkaline Rocks

• East African zone • Carbonatite- • High-potassic series • Kimberlite • Mantle Continental Rocks East African Rift Zone

• Continental rift system • Magmatism for the past 70 my •Ethiopia– transitional • Kenya – basalt, nephelinite, Carbonatite • Tanzania – Oldoinyo Lengai carbonatite, nephelinite • Uganda & Kenya – , trachyte, phonolite • Malawi & Mozambique - Carbonatite

Carbonatite-Nephelinite

• Commonly occur in rift zones • Activity begins with silicate and ends with carbonatite • Alkaline are common (aegerine) • High-T alteration of host rock yields nepheline, k-spar, Na-, Na- pyroxenes, biotite and carbonate

Highly Potassic Series Kimberlite

•K2O/Na2O > 3 • Primary source for diamonds • Leucite is a major • Contain other high-P crystals • Typical minerals are leucite, • Occur on stable continental platforms phlogopite, diopside • Potassic ultrabasic rock • Typical rock is wyomingite • Occur in diatremes • Minerals: , enstatite, Cr-diopside, phlogopite, , Mg-ilmenite Ultramafic Xenoliths

• A key to the composition of the mantle • Generally olivine-rich lherzolite • Metasomatism of inclusions obvious • Early depletion by partial melting of basalts

• Later enriched by CO2 fluids • Enriched in incompatible elements: P, Ti, LIL (K, Rb, Cs, Ba, Pb, Th, U)

Peralkaline Association

•Na2O + K2O > Al2O3 • Contain aegerine and riebeckite • Same environment as alkaline rocks • Derived from fractionation of marginally saturated rocks

Origin of Alkaline

• Residual highly-differentiated melt? • Small degree partial melts? • Partial melt of metasomatized mantle? • Separation of immiscible liquids?