Intrusive Igneous Rocks, part 3

1. Intrusive Igneous Rocks, part 3 Syenite, Phonolite, Ijolite, Carbonatite, Ultramafic Rocks and Lamprophyre

2. IUGS Intrusive Igneous Rock Chart

3. General Intrusive Rocks Classification

4. Position on IUGS Diagram • Syenites are on the border between the QAP and the APF triangles • The remaining rocks are in the APF triangle • Phonolite is actually an extrusive or hypabyssal equivalent of nepheline syenite • The ultramafic rocks and lamprophyre are very rich in mafic minerals • Consequently they cannot be plotted on the main part of the main IUGS diagram • They plot on Figure2-2c (p. 27, Winter) in a separatetriangle

5. Syenite • Intrusive igneous, plutonic • Q = 0-5 • P/(A+P) is 10-35 • Name is for Syene (near Aswan), Egypt - Pliny the Elder named granite-like rocks from this area for the locality

6. Syenite Mineralogy • Major mineral is feldspar, with greater than 65% alkali feldspar (K-spar or albite) • K-spar is typically orthoclase, microcline or perthite - in dikes sanidine may be present • The ferromagnesian minerals are usually < 20% • Biotite is often present and is usually brown

7. Syenite Mineralogy • Plagioclase, if present, is generally subhedral and is often zoned (normal or oscillatory) • Small amounts of feldspathoids, like nepheline and sodalite, may be present - if more than 5% of these minerals are present, the rock is called nepheline (or sodalite) syenite • Accessory minerals: Titanite, apatite, ilmenite, magnetite, zircon, and monzonite

8. Syenite Photomicrograph • Process: Albite and K-spar are mutually soluble in the crystalline state to a limited extent • Mutual solubility is greater at high temperature • As a high-temperature K-rich alkali feldspar cools past the solvus into the two-feldspar domain, the dissolved albite exsolves to form irregular lamellae of albite within the potassium-feldspar host (perthite) Perthite texture in twinned alkali feldspar (perthite) crystals in Syenite [CN, medium power.]

9. Syenite Hand Specimen Photo • K-feldspar megacrysts • Pyroxene • Location: Brenner Pluton, Yukon • Photo: James Lang

10. Alkali Syenite • Intrusive igneous, plutonic • Q = 0-5 • P/(A+P) is 0-10

11. Alkali Syenite Mineralogy • A syenite rich in sodium, this rock has strongly perthitic K-spars or anorthoclase • Plagioclase is albite to sodic oligoclase. • Mafic minerals include iron-rich biotite, iron or sodic amphiboles including hastingsite, arfvedsonite, or riebeckite, and pyroxene is either aegirine-augite or aegirine. • Accessory feldspathoids may be present

12. Nepheline Syenite • Intrusive igneous, plutonic • P/(A+P) is 10-35 • F = 0-10 • Foid syenite composed of granular aggregates, typically orthoclase, microcline, microperthite, cryptoperthite, or albite. • Other essential felsics: nepheline • Accessory feldspathoids: cancrinite, sodalite, hauyne, or nosean

13. Mafic Minerals in Nepheline Syenite • Mafics are soda-rich • Amphiboles arfvedsonite, or hastingsite • Pyroxenes aegirine-augite, aegirine, or titanaugite

14. Nepheline Syenite Photomicrograph • Large, simple-twinned crystal of perthite (alkali feldspar exhibiting wispy exsolution lamellae) • Equant, colorless crystals of nepheline. The turbid areas of the nepheline crystals are where it has been altered to sericite (fine-grained muscovite). Slide also shows green alkali pyroxene (bottom left, interference color distorted by green body color) and altered biotite [CN, low power]

15. Nephelinite • Nephelinite (alkaline mafic lava) consisting of equant, low-birefringence crystals of nepheline and euhedral crystals of alkali pyroxene, often showing zoning, set in a glassy groundmass • Presence of the feldspathoid nepheline indicates that the magma was silica-undersaturated • CN, medium power

16. Nephelinite • Nephelinite (alkaline mafic lava) consisting of equant, sometimes 6-sided colorless crystals of nepheline and euhedral to subhedral greeny-brown crystals of alkali pyroxene • PP, medium power

17. Larvikite • Intrusive igneous, plutonic • Pearl gray alkali syenite, sometimes opalescent, which grades into monzonite • Typically coarse textured • The name is for the locality, Larvik, Norway

18. Larvikite Mineralogy • Prominent crystals are rhombs of very finely twinned sodic-plagioclase and alkali feldspar • Mafics are diopsidic augite and titanaugite • Accessory apatite, ilmenite, titaniferous magnetite, nepheline, olivine, or biotite • Often used as a decorative stone because of a beautiful opalescence when polished

19. Larvikite Polished Section Photo • Polished surface on coarse-grained larvikite rock, a type of syenite from Norway - widely marketed under the trade name 'Blue Pearl' • Composed mainly of large crystals of alkali feldspar (K + Na), with lesser amounts of dark minerals • The large crystal in middle of photo is about 2 centimeters long, and shows a hint of concentric growth layers • The rock is particularly attractive because the feldspar cleavage surfaces (visible as bright areas in photo) show as pearly pale blue iridescent flashes • Location: Larvik district, southeast coast of Norway

20. Ijolite • Intrusive igneous, plutonic to hypabyssal • F = 60-100 • M = 30-70 • Sodium > potassium • The name is for the locality parish Ijo, Finland

21. Ijolite Mineralogy • A feldspathoid-rich rock, containing essential nepheline (50-70%) • Essential: pyroxene, generally aegirine • Ijolites, carbonatites, and syenites are often associated, and are often rich in alkali elements • The silicate and carbonatite minerals have similar trace and minor element chemistries, including high values of Ba, Sr, Ce, La, Zr, P, and Cl • Frequent host rocks for rare-earth and apatite minerals

22. Carbonatite • Intrusive igneous, hypabyssal • Igneous rock in which the carbonate minerals are primary • Apatite and pyrochlore are often considered essential • Accessory: Siderite, calcite, dolomite, or ankerite • Less common accessory minerals: Mg-rich magnetite, phlogopite, and pyrite

23. Carbonatite Occurrence • Carbonatites occur as plugs with surface areas of up to 8 km2. • They are enclosed in mafic alkaline rings. • Mafic rocks include biotite pyroxenite, ijolite, nephelinite (35% nepheline, 65% mafic on average), and jacupirangite (rocks consisting of pure magnetite, magnetite with accessory pyroxene, pyroxene with accessory magnetite, or pyroxene and nepheline with biotite and olivine).

24. Carbonatite Origin • The similarity between silicate and carbonate phase trace and minor elements has convinced most petrologists that the carbonatites must originate together with the silicates, possibly from the same source, and not from limestones, as early workers had suggested

25. Extrusive Rock Classification

26. Phonolite • Igneous extrusive, hypabyssal? • The rock is usually a lava, but may occur in shallow dikes • Name, from the Gr. phone, meaning sound • Supposedly the rock rings when struck with a hammer • Other names are clinkstone and echodolite

27. Phonolite Mineralogy • An aphanitic rock composed of alkali feldspar, often anorthoclase or sanidine, nepheline, and mafics • Other feldspathoids may replace nepheline

28. Ultramafic Rocks • Igneous intrusive, plutonic • M  90 • Examples: dunite, peridotite, and pyroxenite

29. Ultramafic Classification Diagram

30. Mineralogy of Ultramafic Rocks • Ultramafic rocks are composed of minerals near the top of Bowen’s Reaction series • These minerals, especially iron-rich varieties, are prone to weathering. • Serpentine minerals are common weathering products of olivine or pyroxene • Serpentine may be accompanied by talc, another weathering product, or by magnesite, formed by the precipitation of MgCO3 from sea-water after the release of Mg ion

31. Mineralogical Changes in Ultramafic Rocks • Magnetite is a very common accessory mineral in felsic to mafic rocks • In ultramafic rocks, almost all of the iron is associated with various mafic silicates minerals, so there is no iron oxide phase present • Opaque accessory mineral is often chromite • If enough chromite is present, the rock may become an ore of chromium • In hand specimen, chromite is mostly easily seen in fresh specimens of dunite

32. Mineralogical Changes in Ultramafic Rocks • Titanium is often present, and titanoaugites or other titanium rich minerals may be present

33. Pyroxenite • Intrusive igneous, plutonic • ol/(ol + cpx +opx) < 40% • M  90

34. Pyroxenite Mineralogy • Some pyroxenites contain more iron and are predominantly composed of opx - either hypersthene or bronzite • Bronzite is named for the noticeable bronze reflection (Schiller luster) that may be seen in hand specimen. • If clinopyroxene is present it is often the variety diallage which shows well-developed parting parallel to {100} in the augite-diopside composition range. • Ti-bearing augite or aegirine is also possible

35. Pyroxenite Mineralogy • Accessory minerals include: • Hornblende, generally brown, may be present • Chromite - Crystals may be diamond or square shaped - crystals in hand specimen are small, seldom showing octahedral habit

36. Clinopyroxenite • This rock is dominated by the clinopyroxene augite • Location: West Point, GA

37. Bronzitite • Intrusive igneous, plutonic • A type of pyroxenite composed almost exclusively of bronzite • Accessory minerals may include olivine, picotite (chromian spinel), chromite, hornblende. • Secondary serpentine may be present • Also called bronzite

38. Bronzitite Photomicrograph • Almost all of the grains in this photomicrograph are hypersthene • This rock also contains some interstitial plagioclase, better seen in plane light. Location: Stillwater Complex, Montana

39. Hypersthene Photomicrograph • Orthopyroxenes are noted for having low, first-order interference colors • Also note the cleavages that intersect at about 90 degrees. • Another identifying characteristic of orthopyroxene is its parallel extinction.

40. Peridotite • Intrusive igneous, plutonic • ol/(ol + cpx +opx) > 40% • M  90% • The mineralogy is similar to the pyroxenites, except that a mica, phlogopite, is often present • The rock is feldspar free • Name - from the French name for olivine, péridot

41. Dunite • Dunite is olivine peridotite • Ol > 90%

42. Dunite Photomicrograph • Almost all of the grains in this rock are olivine - note the high order interference colors of olivine • Secondary calcite occurs as veinlets through the sample

43. Harzburgite • Intrusive igneous, plutonic • Harzburgite is a type of peridotite in which the major minerals are orthopyroxene (generally near enstatite) and Mg-rich olivine. • M  90% • ol/(ol+opx+cpx) = 40-90 • cpx/(ol+opx+cpx) < 5%.

44. Harzburgite • Harzburgite is believed to represent depleted mantle material after the removal of a basaltic component • Name - for the locality, Harzburg, Germany

45. Chromitite • Intrusive igneous, plutonic • Igneous rock composed of 95%+ of chromite • Similar rocks with larger percentages of ferromagnesian minerals may be named for the mineral, e.g. olivine chromitite • Rock is denser than most other ultramafic rocks

46. Hornblendite • Intrusive igneous, plutonic • M  90% • hb/(hb+px+ol) > 90%

47. Kimberlite • Potassium-rich peridotites which usually occur as “pipes” which originate in the mantle • Usually have an abundant volatile content, especially CO2 and H2O • Found in old (>2500 my), stable continental areas but are themselves much younger, often Mesozoic in age • Kimberlite magmas intrude along deep fractures, sometimes extensions of ocean fracture zones • Name - for the locality Kimberly, South Africa

48. Kimberlite Xenoliths • Kimberlites are most interesting scientifically because they carry a rich variety of xenoliths and xenocrysts • Many are these xenoliths are believed to be part of the mantle or deep crust • They are pieces that fell into the kimberlite magma during its passage to the surface • Xenoliths include garnet peridotites, lherzolites, and eclogites, which are believed to be samples of the mantle lithosphere and even the upper asthenosphere

49. Kimberlite Mineralogical Alteration • Kimberlites often show signs of alteration • Olivine is transformed to serpentine, usually antigorite • Phlogopite is transformed to chlorite, or vermiculite, with accompanying magnetite and/or calcite

50. Kimberlite breccia • Kimberlites may also occur as breccia • This probably represents turbulent emplacement, where the magma is mixed with many different kinds of xenoliths • Xenoliths from depth are rounded, probably as the result of wear during the ascent, and the upper xenoliths are more angular