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| | anorthosites (Site not responding. Last check: 2007-11-01) |
 | | A complex Proterozoic terrain in the northern Burro Mountains consists of metamorphic rocks (Bullard Peak and Ash Creek Series, 1550-1570 Ma, U/Pb) that are intruded by granitic and mafic rocks. |
 | | The granitic rocks include 1) Burro Mountain granite (oldest, ~1445 Ma, U/Pb), 2) gneissic granite/granodiorite (~1380 Ma, K/Ar), 3) Jack Creek rapakivi granite (1220.9±3.5, 1198±4.7 Ma, 40Ar/39Ar), 4) Redrock granite (1210±5, 1204.8±12 Ma, 40Ar/39Ar; 1328±58 Ma Sm/Nd isochron), 5) fine-grained alkali-feldspar and biotite granite dikes, 6) rhyodacite/dacite porphyry dikes, and 7) pegmatite dikes (youngest). |
 | | The petrogenesis of rapakivi granites is controversial and questions remain as to the source of the rapakivi magma, the genetic relationships between the granite and penecontemporaneous mafic rocks, the origin of the rapakivi texture, and the tectonic setting of these rocks. |
| geoinfo.nmt.edu /staff/mclemore/projects/anorthosites.html (1125 words) |
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