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Geologic Setting and Petrology of Apollo 15 Anorthosite (15415)

Wilshire, H. G. and Schaber, G. G. and Silver, L. T. and Phinney, W. C. and Jackson, E. D. (1972) Geologic Setting and Petrology of Apollo 15 Anorthosite (15415). Geological Society of America Bulletin, 83 (4). pp. 1083-1092. ISSN 0016-7606. doi:10.1130/0016-7606(1972)83[1083:gsapoa];2.

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Apollo 15 sample number 15415, popularly called the “Genesis Rock,” is coarse-grained anorthosite composed largely of calcic plagioclase with small amounts of three pyroxene phases. The rock was found as a clast in a piece of friable soil breccia on the lip of Spur crater, a small young crater on the lower slopes of the Apennine Mountains. The mode of occurrence of sample 15415 indicates that it has undergone at least two, and possibly three or more, fragmentation events. These events are reflected in the texture of the rock by shattered and granulated minerals. An earlier thermal metamorphic event is represented by irregular bands of coarsely recrystallized plagioclase and minor pyroxene that cross larger plagioclase grains. Preliminary observations of textural relations of the large plagioclase grains are consistent either with accumulation of plagioclase followed by overgrowth of cumulus grains and post-cumulus crystallization of minor interstitial pyroxene, or with metamorphic recrystallization that eradicated original textures. Any of the events in the complex history of this rock may have affected apparent radiometric ages. Comparative abundance of similar, though smaller, pieces of anorthositic rock in the area and dominance of originally coarse-grained gabbroic-anorthositic clasts in breccia at Spur crater suggest that sample 15415 is the least-deformed member of a suite of similar rocks that were ejected from beneath the regolith at Spur crater.

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Additional Information:© 1972, The Geological Society of America, Inc. Copyright is not claimed on any material prepared by U.S. government employees within the scope of their employment. Manuscript received by the society October 28, 1971. We are greatly indebted to David R. Scott and James B. Irwin for discussions of the field relations of sample 15415. Open exchange of information on the mineralogy and petrology of the sample with David B. Stewart was very beneficial. We are grateful to Ray Sabala for his excellent drafting of the illustrations, to C. G. Utter for his expert photographic work, and K. A. Howard and D. C. Ross, U.S. Geological Survey, for critical review of the manuscript. This work was done under NASA contracts T-4738A and T-65253-G.
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Caltech Division of Geological Sciences2121
Issue or Number:4
Record Number:CaltechAUTHORS:20220729-282315900
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Official Citation:H. G WILSHIRE, G. G SCHABER, L. T SILVER, W. C PHINNEY, E. D JACKSON; Geologic Setting and Petrology of Apollo 15 Anorthosite (15415). GSA Bulletin 1972; 83 (4): 1083–1092. doi:[1083:GSAPOA]2.0.CO;2
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:115948
Deposited By: Tony Diaz
Deposited On:29 Jul 2022 15:51
Last Modified:29 Jul 2022 15:51

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