Name _______________________ Petrology — Spring 2006 Igneous rocks lab — Part I Due Tuesday 3/7 Igneous rock classification and textures For each of the rocks below, describe the texture, determine whether the rock is plutonic or volcanic, and describe its color index. #1. Dunite: Precambrian, from Jackson county, North Carolina. Note the high birefringence and lack of cleavage of the olivine. The olivine grains form a mosaic and meet at angles of about 120°. The resultant texture is called mosaic or granoblastic, and characterizes high grade metamorphic rocks. Some slides contain traces of serpentine ( a, c, d, e), talc ( d, e ), or magnesian chlorite ( c, d ) that formed by alteration of olivine. There appears to be a trace of calcite ( of secondary origin ) in at least one slide, and of a mineral that resembles apatite (unlikely in a dunite) in slide a. The opaque mineral clearly is a member of the spinel family, and may contain considerable chromium. There is a trace of phlogopite (?) in d. Plutonic or Volcanic Felsic — Intermediate — Mafic — Ultramafic Texture: Color index: #9. Gabbronorite: Red Mountain - Del Puerto Complex, Diablo Range, California. Hypersthene — 15%, faintly pleochroic in flesh pink and pale green; exsolution of augite Augite — 15%, colorless to very pale green Amphibole — 5%, pale yellowish green; weakly pleochroic. Mantles and replaces (?) both pyroxenes. A calcium-bearing amphibole of the actinolite or hornblende family. Plagioclase — 65% #9-f: a conspicuous shear is filled with fibrous amphibole Plutonic or Volcanic Felsic — Intermediate — Mafic — Ultramafic Texture: Color index:Name _______________________ Petrology — Spring 2006 #14. Alkali olivine basalt: Tertiary (~55 Ma) basalt from the San Francisco volcanic field, Flagstaff, Arizona. This texture is subophitic to intersertal. Olivine — ~8% as microphenocrysts and ~8% in the groundmass. The olivine is readily identified because it is slightly altered to reddish iddingsite Pyroxene — 30%, encloses laths of plagioclase in subophitic texture Plagioclase — 40% Glass and opaque minerals — 15% Plutonic or Volcanic Felsic — Intermediate — Mafic — Ultramafic Texture: Color index: #19. Diorite: Guadaloupe Igneous Complex, west of Mariposa, California. Plagioclase — 60% Augite — ~10% Hypersthene — ~15%, distinguished from augite only by its lower birefringence, a very weak pleochroism, and its negative sign. The relatively low 2V and relatively (for an orthopyroxene) high birefringence suggest that it is rich in iron, but the weak pleochroism suggests that it is not very rich in iron. Hornblende — 15%, pleochroic in yellow-green-brown Apatite — x Plutonic or Volcanic Felsic — Intermediate — Mafic — Ultramafic Texture: Color index:Name _______________________ Petrology — Spring 2006 #21. Pyroxene andesite: Boggs Mountain, south of clear Lake, California. The pyroxenes are not easy to distinguish because the hypersthene is almost imperceptibly pleochroic. But in this rock extinction angles are useful in distinguishing them because the phenocrysts are euhedral. And thus we can observe rational crystal faces against which to measure extinction angles. Recall that a clinopyroxene will present an apparent extinction angle that will range from zero up to its true extinction angle, depending upon its orientation. The groundmass (~60%) appears to be composed of the same minerals together with magnetite. There is some glass in the groundmass of some of the slides. In #21 a both red and black glass are present. Some of the sections are weathered around a hairline crack that bisects them. This texture is hyalopilitic. Plagioclase — 20%, zoned Hypersthene — 10% Augite — 10%, occur as phenocrysts Plutonic or Volcanic Felsic — Intermediate — Mafic — Ultramafic Texture: Color index:Name _______________________ Petrology — Spring 2006 #27. Sherman "granite": Precambrian, of the Laramie Range, Wyoming. Sodic plagioclase — 15-20% Microcline — 48-58%, megacrysts Quartz — 15-20%, undulose extinction; Boehm lamellae Biotite — 5%, in #27b epidote is included in the Biotite Arfvedsonite (?, and amphibole) — 5%, pleochroic in blue-green, green, and brown. Very low 2V, negative sign. It is difficult to determine the interference figure of this mineral. Titanite (aka, sphene) — 1%, weakly pleochroic Zircon — x Apatite — x Muscovite — 1%, present only as a minor alteration product of feldspar Chlorite — x, secondary after biotite. No chlorite is present in slide #27a. Calcite — trace amount in slide #27a Plutonic or Volcanic Felsic — Intermediate — Mafic — Ultramafic Texture: Color index: Determine the proportion of alkali feldspars versus plagioclase in this rock and name the rock according to the IUGS system. Is this rock a true granite? IUGS classification: Estimate the plagioclase composition using the Michel Levy method. Write the composition in terms of the percentage anorthite and albite according to the convention AnxxAbyy.Name _______________________ Petrology — Spring 2006 #30. Rhyolite porphyry: Chaffee County, Colorado. Phenocrysts — 20%: Oligoclase — 5% Sanidine — 10%, Carlsbad twins; very low 2V, negative sign Quartz — 5% Oxyhornblende (?) — very rare Oxybiotite — trace in slide 30c. Groundmass — 80%: Alkali feldspar and quartz: low relief. The groundmass probably is devitrified glass. Notice that mafic minerals are very rare in this rock. Garnet as small xenocrysts in 30e, and in filled vesicles in 30c. There may be trace amounts of topaz. Plutonic or Volcanic Felsic — Intermediate — Mafic — Ultramafic Texture: Color index:Name _______________________ Petrology — Spring 2006 Using the appropriate IUGS diagram, name the following rocks: 1 2 3 4 5 6 7 8 9 10 11 12 13 14