Granites and Carbonatites 


I. Granitoids

A. Def: felsic plutonic rocks

-Most occur in areas of thickened continental crust (continental arcs or collision zones)

-Granitic magma => source of heat to melt crust

-Mantle involvement: heat and/or actual magma component

B. Composition

-Main minerals :Plagioclase ; Alkali feldspar;Quartz

-Accessory:Muscovite; Biotite ; Hornblende; Pyroxene


-Complexity may reflect source(s), assimilation, fractional crystallization

-most granitoids plot near ternary eutectic :F.C. or melting of felsic source

C. Classification
-S-type :

-Lower Na, Ca, Sr, 87Sr/86Sr, Fe+3/Fe+2 and higher Cr, Ni, 144Nd/143Nd
-Oxide phase usually ilmenite, mafic is biotite
-S is for sedimentary, or more generally, a melt of weathered crustal material
-Commonly associated with subduction


-Higher Na, Ca, Sr, 87Sr/86Sr, Fe+3/Fe+2 and lower Cr, Ni, 144Nd/143Nd
-Oxide is magnetitc, mafic is hornblende
-I is for igneous => derived from melt with a mantle source


-Highly-evolved products derived directly from mantle-source melts
-Found associated with MORB and OIB, also ophiolites


-Anorogenic - late-stage magmatism after continental collision followed by relaxation
-Or, found in rift zones (early stage of active rifting or within failed rifts)

- tectonic classification of granitoids - some overlap with previous

D. Proterozoic anorogenic magmatic event (AMCG suite)

-AMCG suite:

-Anorthosite - almost pure plagioclase (Origin)
-Mangerite - opx-monzonite
-Charnockite - opx-granite ( High T anhydrous rocks (igneous or high-grade met. origin)May include Fe-rich olivine)
-Granite - actually a range of felsic rocks including quartz monzonite/syenite

-Wilson Cycle:Subduction - collision - supercontinent - rifting

II. Carbonatites

A. Alkaline rocks

-Found in all igneous provinces

-Particularly associated with anorogenic continental areas, especially rift zones

-Only 1% of exposed igneous rocks

B. History of the term "carbonatite"

-Introduced in 1921 - interpreted as igneous:

-Strong objection from Bowen
-Experiments showed carbonate melts could exist at low T and P, but ...
-Field relations showed cross-cutting contacts and chilled margins, but ...

-Igneous origin not accepted until carbonate lavas were observed erupting from Oldoinyo Lengai, Tanzania, in 1958:

-Carbonatites are known
-At ~350 localities
-From Proterozoic to modern
-Term is used loosely to apply to all carbonate igneous rocks, intrusive and extrusive, but there are many more specialized rock names according to texture and composition

C. Associated rocks

-Occur in small ring complexes within alkaline rocks:

-Ijolite (intrusive) = nepheline + aegerine (Na-diopside) - extrusive equivalent is nephelinite

-Nepheline syenite = nepheline + alkali feldspar - extrusive is phonolite

-At OL, interlayered alkaline and carbonititic tuffs and lavas
-rocks often arranged concentrically :

-Progressively poorer in Si from rim toward the core
-Cut by alnoite dikes
-Whole suite usually surrounded by a zone of Na-metasomatism
-Silicic country rock + Na-met = fenite (aegerine syenite)
-Fenitization may extend hundreds of meters

D. Composition

-Mineralogical : 75% carbonate; Cpx, phlogopite, apatite, pyrochlore [(Ca,Na)2(Nb,Ta)2O6(OH,F)], others

-Chemical: low Si, high Na, Ca, K, CO2; enriched in trace elements

-Isotopic: Sr initial values = 0.701-0.704 ( limestone = 0.709); correspond to those of surrounding alkaline rocks => same mantle source

E. Carbonate melts

-May separate as immiscible liquid from kimberlite magma (sill in South Africa)
-Or from ijolitic magma (carbonate globules trapped in apatite, Kenya)
-Or from high-K magma (quenched carbonate droplets in alnoite)

-Carbonatite intrusions may be zoned, from outer: calcite-carb, dolomite-carb, ankerite/siderite-carb (ank, sid) : experiments show this is a F.C. trend for carbonate magma

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