Name Petrology Spring 2006

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Katherine Johns
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1 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. #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

2 #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% #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

3 #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

4 #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 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 An xx Ab yy.

5 #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.

6 Using the appropriate IUGS diagram, name the following rocks:

Name Petrology Spring 2006 Igneous rocks lab Part II Hand samples of igneous rocks Due Tuesday 3/7

Name Petrology Spring 2006 Igneous rocks lab Part II Hand samples of igneous rocks Due Tuesday 3/7 Igneous rocks lab Part II Hand samples of igneous rocks Due Tuesday 3/7 1. Use the color index and density of the rock to establish whether it is felsic, intermediate, mafic, or ultramafic. 2. Determine