Olivine diabase (Lafayette Bluff Sill, Proterozoic; Lafayette Bluff Tunnel, northeastern Minnesota, USA) 5 : 無料・フリー素材/写真
Olivine diabase (Lafayette Bluff Sill, Proterozoic; Lafayette Bluff Tunnel, northeastern Minnesota, USA) 5 / James St. John
| ライセンス | クリエイティブ・コモンズ 表示 2.1 |
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| 説明 | Olivine diabase in the Precambrian of Minnesota, USA.Lafayette Bluff is located along Lake Superior's northern shore in northeastern Minnesota, USA. The rocks are principally olivine diabase, which is a mafic, crystalline-texture, intrusive igneous rock. Typical diabase is composed of plagioclase feldspar and pyroxene - these particular rocks also have a significant olivine component. The diabase intrusion here is a sill - a tabular intrusion that more or less parallels the country rocks. The sill has been folded into a plunging anticline-syncline pair, possibly due to subsidence of underlying rocks into the Lafayette Bluff Sill's source magma chamber (see Pope, 1976).Lafayette Bluff Sill rocks are amygdaloidal porphyritic olivine diabases. Vesicular olivine diabase is also present. Published analyses indicate that the olivine diabase is quartz tholeiitic in composition, with 46.9% silica. Its average mineralogy is the same as the nearby Silver Creek Cliff Sill - 66% plagioclase feldspar, 16% augite pyroxene, 12% olivine, 2% orthopyroxene, 2% opaque oxide minerals (ilmenite + magnetite), 2% altered interstitial material, plus trace apatite, quartz, and K-feldspar (see Pope, 1976).Here's a description of the Lafayette Bluff Sill from Boerboom et al. (2003): "Irregularly, discordant, sheet- to dike-shaped body composed of soft, dark greenish-black, amygdaloidal and porphyritic diabase, with local differentiated pods of coarse-grained prismatic ferrmonzodiorite. Typically deeply weathered to a brown crumbly grus bearing round corestones of fresh diabase. . . . The diabase locally contains abundant anorthosite xenoliths . . . as well as scattered xenoliths of metamorphosed basalt."and"Diabasic olivine gabbro - Dark green, medium- to coarse-grained, typically ophitic, commonly porphyritic and amygdaloidal. Contains 55 to 80 percent generally fresh, fine- to coarse-grained plagioclase . . . 10 to 25 percent fresh, subpoikilitic to poikilitic augite; 5 to 12 percent variably altered olivine; 2 to 3 percent opaque oxides; trace amounts of pigeonite, inverted pigeonite, and orthopyroxene; and minor granophyric mesostasis. Amygdules and diktytaxitic cavities are filled with fibrous zeolite mienrals. Readily recognized by its distinctive color, softness, variably amygdaloidal and porphyritic character, and weathered nature. . . . Relatively abundant amygdules indicate that the magma may have been unusually enriched in volatiles . . . Contains scattered xenoliths of coarse-grained anorthosite up to 10 feet (3 meters) in diameter and metamorphosed basalt.”Geologic unit & age: Lafayette Bluff Sill, Midcontinent Rift Intrusive Supersuite, ProterozoicLocality: eastern end of Lafayette Bluff Tunnel - roadcut along Rt. 61, northeast of the town of Two Harbors, near the northern shore of western Lake Superior, northeastern Minnesota, USA (47° 05’ 47.97" North latitude, 91° 33’ 01.76” West longitude)-----------------Some references on Lafayette Bluff Sill geology:Pope (1976) - Petrology and structure of the late Precambrian Silver Creek Cliff and Lafayette Bluff mafic intrusions, Lake County, Minnesota. Proceedings, Twenty Second Annual Institute on Lake Superior Geology, May 3-7, 1976, Saint Paul, Minnesota, p. 49.Miller et al. (2001) - Geologic map of the Duluth Complex and related rocks, northeastern Minnesota. Minnesota Geological Survey Miscellaneous Map Series Map M-119.Boerboom et al. (2003) - Bedrock geology of the Castle Danger Quadrangle, Lake County, Minnesota. Minnesota Geological Survey Miscellaneous Map Series Map M-140. |
| 撮影日 | 2015-06-09 09:19:41 |
| 撮影者 | James St. John |
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