Alkaline Rocks 354b59

ALKALINE ROCKS AND CARBONITITE S

Alkaline rocks Excess alkalis in feldspathoids, sodic

pyroxene/amph In this deficient of SiO2 so no quartz <1% exposed igneous rocks are alkaline

Despite their relatively small volume worldwide,

alkaline rocks for most of the hundreds of rocktype names in the geologic literature because of unusually great variation in chemical, mineralogical, and modal composition. Regrettably, there is no consensus among petrologists as to the precise definition of alkaline rocks, and their classification continues to be challenging. Alkaline rocks have a relative excess of alkalies over silica (Figure 2.16) but the exact ratio of these constituents has not been established. Most are silica-undersaturated and contain normative nepheline and real feldspathoids (nepheline, leucite). Alkaline rocks commonly include one or more of analcime, alkali feldspar, alkali-rich amphiboles; Na-TiAlrich clinopyroxenes, biotite-phlogopite solid solutions;olivine; and no orthopyroxene or quartz.

However, some very rare rocks known as lamproites

contain significant modal proportions of leucite yet are quartz normative by virtue of their very low concentration of Al2O3. Alumina deficient peralkaline rocks are also sometimes considered to be alkaline, even though theyn may be silica-oversaturated. Because Na and K are relatively abundant in alkaline rocks, a twofold subdivision into sodic and potassic series is used.  More common subalkaline rocks are usually silicasaturated or silica-oversaturated and lack normative nepheline. Real minerals include combinations of feldspars, hornblende, augite clinopyroxene, orthopyroxene, biotite, quartz

Table 19-1

. Nomenclature

of some alkaline igneous rocks

( mostly volcanic/hypabyssal) Basanite feldspathoid-bearing basalt. Usually contains nepheline, but may have leucite + olivine Tephrite olivine-free basanite Leucitite a volcanic rock that contains leucite + clinopyroxene  olivine. It typically lacks feldspar Nephelinite a volcanic rock that contains nepheline + clinopyroxene  olivine. It typically lacks feldspar. Fig. 14-2 Urtite plutonic nepheline-pyroxene (aegirine-augite) rock with over 70% nepheline and no feldspar Ijolite plutonic nepheline-pyroxene rock with 30-70%

Melilitite a predominantly melilite - clinopyroxene volcanic (if > 10% olivine they are called olivine melilitites) Shoshonite K-rich basalt with K-feldspar ± leucite Phonolitefelsic alkaline volcanic with alkali feldspar + nepheline. See Fig. 14-2. (plutonic = nepheline syenite) Comendite peralkaline rhyolite with molar (Na2O+K2O)/Al2O3 slightly > 1. May contain Napyroxene or amphibole Pantellerite peralkaline rhyolite with molar (Na2O+K2O)/Al2O3 = 1.6 - 1.8. Contains Na-pyroxene or amphibole

Lamproite a group of peralkaline, volatile-rich, ultrapotassic, volcanic to hypabyssal rocks. The mineralogy is variable, but most contain phenocrysts of olivine + phlogopite ± leucite ± K-richterite ± clinopyroxene ± sanidine. Lamprophyre a diverse group of dark, porphyritic, mafic to ultramafic hypabyssal (or occasionally volcanic), commonly highly potassic (K>Al) rocks. They are normally rich in alkalis, volatiles, Sr, Ba and Ti, with biotite-phlogopite and/or amphibole phenocrysts. They typically occur as shallow dikes, sills, plugs, or stocks.

Kimberlite a complex group of hybrid volatile-rich (dominantly CO2), potassic, ultramafic rocks with a finegrained matrix and macrocrysts of olivine and several of the following: ilmenite, garnet, diopside, phlogopite, enstatite, chromite. Xenocryst s and xenoliths are also common Group I kimberlite is typically CO2-rich and less potassic than Group 2 kimberlite Group II kimberlite (orangeite) is typically H2O-rich and has a mica-rich matrix (also with calcite, diopside, apatite)

Carbonatite an igneous rock composed principally of carbonate (most commonly calcite, ankerite, and/or dolomite), and often with any of clinopyroxene alkalic amphibole, biotite, apatite, and magnetite. The Ca-Mg-rich carbonatites are technically not alkaline, but are commonly associated with, and thus included with, the alkaline rocks. • Chemistry of alkaline rocks The analytical table here  gives 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.

• Textures

As  we  ,  phase  diagrams  are  a  potent  tool  to  interpret  the  textures  of  igneous  rocks.  we’ve  been  using  so  far  mostly  diagrams  for  mafic  (basaltic)  systems;  here  is  a  diagram  appropriate for more leucocratic rocks were white minerals form  the dominant components.

Common occurrence Continental rifts Intraplate settings

with no clear tectonic control End of volcanic activity (Hawaii)

Rift associated Rhine graben, Baikal

Rift, Oslo Rift, East African Rift Uplift, extension, 3 km deep grabens Likely plume caulift and magma source Also in rift: carbonatites

carbonatites >50% carbonate

minerals Igneous carbonate rocks 1/2 in Africa, also in Arkansas & Ontario Need stable continental craton

Mineralogy Table 19-4. Some Minerals in Carbonatites. Carbonates Calcite Dolomite Ankerite Siderite Strontanite Bastnäsite (Ce,La)FCO3) * Nyerereite ((Na,K) 2Ca(CO3)2) * Gregoryite ((Na,K) 2CO3) Silicates Pyroxene Aegirine-augite Diopside Augite Olivine Monticellite Alkali amphibole Allanite Andradite Phlogopite Zircon Source: Heinrich (1966), Hogarth (1989)

Sulfides Pyrrhotite Pyrite Galena Sphalerite Oxides-Hydroxides Magnetite Pyrochlore Perovskite Hematite Ilmenite Rutile Baddeleyite Pyrolusite Halides Fluorite Phosphates Apatite Monazite

* only in natrocarbonatite

Carbonatite source Mantle source (isotopes) Direct melt of hydrous carbonated mantle How does mantle get CO2 in it? Deep primordial mantle Subducted limestones/altered ocean crust

natrocarbonatite Tanzania Ol

Doinyo Lengai volcano On east African rift Sodium carbonate lava Very low viscosity Rich in CO2

QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture.

natrocarbonatite Tanzania Ol Doinyo

Lengai volcano Sodium carbonate lava Very low viscosity Rich in CO2 Only volcano like in on Earth Similar to flows on Venus?

QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture.

Black flows <24

hours Then turn gray, then powdery white QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture.

QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture.

Long exposure

photography shows red flows Eruption T: 500°C Basalt ~ 1000°C

QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture.

QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture.

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Active spatter

cone CO2 makes lava foam like sodas

QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture.

Other alkaline rocks on cratons Lamprophyres: porphyritic dikes, basaltic but

crazy chemistry/mineralogy. Commonly with plag and nepheline Lamporites: similar, but no plag and nepheline. Found in Leucite Hills of Wyoming. Ultrapotassic Kimberlites

Kimberlites Emplaced as

explosive breccia from deep in mantle Hard to tell true magma composition So much contamination

Kimberlites Famous for

diamonds only on Archean crust Most occur in South Africa Diamonds are older than host rock (900-3300 Ma vs. 90-1600 Ma)

Diamonds form and

preserved at base of craton (120-200 km thick) Picked up by younger kimberlite and lamproite magmas Inclusions in diamond found in harzburgite found in kimberlites hosted in Archean country rock

Anorthosites Plutonic rocks

>90% plag No known volcanic equivalent Light colored highlands of moon Archean and Proterozoic

Fiskenaesset anorthosites

This is where all

the Eu ends up!

How do we

concentrate so much plag from a mantle melt?

Model for origin a. Mantle-derived magma underplates the crust as

it becomes density equilibrated.

b. Crystallization of mafic phases (which

sink), and partial melting of the crust above the ponded magma. The melt becomes enriched in Al and Fe/Mg

c. Plagioclase forms when the melt is

sufficiently enriched. Plagioclase rises to the top of the chamber whereas mafics sink.

d. Plagioclase accumulations become less

dense than the crust above and rise as crystal mush plutons.

e. Plagioclase plutons coalesce to form massif

anorthosite, whereas granitoid crustal melts rise to shallow levels as well. Mafic cumulates remain at depth or detach and sink into the mantle .

Anorthosite on the moon Highlands: anorthosite of 4.4 Ga Maria: basalt and younger Several km thick layer of magma formed at

surface Magma ocean crystallizes, plag floats Occurs just after formation based on age