ALKALINE ROCKS AND CARBONITITES Alkaline rocksExcess alkalis in feldspathoids, sodic pyroxene/amphIn this defcient ofSiO2so no quartz!" exposed i#neous rocks are alkaline$espite their relati%ely small %olume &orld&ide, alkaline rocks account for most of the hundreds of rocktype names in the #eolo#ic literature 'ecause of unusually #reat %ariation in chemical, mineralo#ical, and modal composition( )e#retta'ly, there is no consensus amon# petrolo#ists as to the precise defnition of alkaline rocks, and their classifcation continues to 'e challen#in#(Alkaline rocks ha%e a relati%e excess of alkalies o%er silica *+i#ure 2(!,- 'ut the exact ratio of these constituents has not 'een esta'lished(.ost are silica/undersaturated and contain normati%e nepheline and real feldspathoids *nepheline, leucite-( Alkaline rocks commonly include one or more of analcime, alkali feldspar, alkali/rich amphi'oles0 1a/2i/Alrich clinopyroxenes, 'iotite/phlo#opite solid solutions0oli%ine0 and no orthopyroxene or quartz( 3o&e%er,some %ery rare rocks kno&n as lamproites contain si#nifcant modal proportions of leucite yet are quartz normati%e 'y %irtue of their %ery lo& concentration of Al2O4( Alumina defcient peralkaline rocks are also sometimes considered to 'e alkaline, e%en thou#h theyn may 'e silica/o%ersaturated( 5ecause 1a and 6 are relati%ely a'undant in alkaline rocks, a t&ofold su'di%ision into sodic and potassic series is used( .ore common subalkaline rocks are usually silicasaturated or silica/o%ersaturated and lack normati%e nepheline( )eal minerals include com'inations of feldspars, horn'lende, au#ite clinopyroxene, orthopyroxene, 'iotite, quartz in more silica/rich rocks, and oli%ine in less silica/rich rocks( Table 19-1. Nomenclature of some alkaline igneous rocks

( mostly volcanic/hypabyssal)Basanite felspathoi-bearing basalt. !sually contains nepheline" but may have leucite # olivine Tephrite olivine-free basanite $eucitite a volcanic rock that contains leucite # clinopyro%ene olivine. &t typically lacks felspar Nephelinite a volcanic rock that contains nepheline # clinopyro%ene olivine. &t typically lacks felspar. 'ig. 1(-) !rtite plutonic nepheline-pyro%ene (aegirine-augite) rock *ith over +,- nepheline an no felspar &.olite plutonic nepheline-pyro%ene rock *ith /,-+,- nepheline0elilitite a preominantly melilite - clinopyro%ene volcanic (if 1 1,- olivine they are calle olivine melilitites) 2hoshonite 3-rich basalt *ith 3-felspar 4 leucite 5honolitefelsic alkaline volcanic *ith alkali felspar # nepheline. 2ee 'ig. 1(-). (plutonic 6 nepheline syenite) 7omenite peralkaline rhyolite *ith molar (Na)8#3)8)/9l)8/ slightly 1 1. 0ay contain Na-pyro%ene or amphibole 5antellerite peralkaline rhyolite *ith molar (Na)8#3)8)/9l)8/ 6 1.: - 1.;. 7ontains Na-pyro%ene or amphibole $amproite a group of peralkaline" volatile-rich" ultrapotassic" volcanic to hypabyssal rocks. The mineralogy is variable" but most contain phenocrysts of olivine # phlogopite 4 leucite 4 3-richterite 4 clinopyro%ene 4 saniine. $amprophyre a iverse group of ark" porphyritic" mafic to ultramafic hypabyssal (or occasionally volcanic)" commonly highly potassic (319l) rocks. They are normally rich in alkalis" volatiles" 2r" Ba an Ti" *ith biotite-phlogopite an/or amphibole phenocrysts. They typically occur as shallo* ikes" sills" plugs" or stocks.3imberlite a comple% group of hybri volatile-rich (ominantly 78))" potassic" ultramafic rocks *ith a fine-graine matri% an macrocrysts of olivine an several of the follo*ing< ilmenite" garnet" iopsie" phlogopite" enstatite" chromite. =enocrysts an %enoliths are also common >roup & kimberliteis typically 78)-rich an less potassic than >roup ) kimberlite >roup && kimberlite (orangeite)is typically ?)8-rich an has a mica-rich matri% (also *ith calcite" iopsie" apatite) 7arbonatite an igneous rock compose principally of carbonate (most commonly calcite" ankerite" an/or olomite)" an often *ith any of clinopyro%ene alkalic amphibole" biotite" apatite" an magnetite. The 7a-0g-rich carbonatites are technically not alkaline" but are commonly associate *ith" an thus inclue *ith" the alkaline rocks. Chemistry of alkaline rocksThe analytical table heregives ussample compositions for rocks belonging to both series. We can use it to calculate norms, to compare the geochemistry of both series, and/or to link with the mineral contents.TexturesAsweremember,phasediagramsareapotenttooltointerpret thetexturesofigneousrocks.wevebeenusingsofarmostly diagramsformafic(basaltic)systems;hereisadiagram appropriate for more leucocratic rocks were white minerals form the dominant components.7ommon occurrence7ontinental riftsIntraplate settin#s &ith no clear tectonic controlEnd of %olcanic acti%ity *3a&aii-)ift associated)hine #ra'en, 5aikal )ift, Oslo )ift, East African )ift8plift, extension, 4 km deep #ra'ens9ikely plume causin# uplift and ma#ma sourceAlso in rift: car'onatitescar'onatites;