Relations of zoned pegmatites to other pegmatites, granite, and metamorphic rocks in the southern Black Hills, South Dakota

Journal Article · · American Mineralogist; (USA)

OSTI ID:6285778

Norton, J J ^[1]; Redden, J A ^[2]

---

1. Geological Survey, Denver, CO (USA)
2. Geological Survey, Denver, CO (USA) South Dakota School of Mines and Technology, Rapid City (USA)

The pegmatite field and the Harney Peak Granite of the southern Black Hills, South Dakota, form an igneous system that progresses from slightly biotitic muscovite granite through layered pegmatitic granite, with alternating sodic and potassic rocks, to simple plagioclase-quartz-perthite pegmatites, and on to zoned pegmatites. Most of the country rocks are Lower Proterozoic mica schists that were isoclinally folded and metamorphosed to biotite and almandine grades before granitic activity began. At 1,700 Ga, intrusion of the Harney Peak Granite created a large dome in these rocks, a thermal aureole with a staurolite, a first sillimanite isograd, and a small area of metamorphism above the second sillimanite isograd. The estimated pressure is 3.7 kbar. The zoned pegmatites have a strong tendency to occur in clusters, and the types of pegmatites are different in different clusters. A less obvious tendency is a regional zonation in which rare-mineral pegmatites become more abundant and muscovite pegmatites less abundant toward the outskirts of the region. The composition of the granite indicates that its magma originated by partial melting of metasedimentary mica schists similar to those at the present surface. The pegmatitic nature of most of the granite probably reflects exsolution of an aqueous phase. The resulting processes continued into the simple pegmatites and zoned pegmatites, which are 4% and 0.5% of the exposed granitic rocks. For rare-element pegmatites, residual concentration in granitic magma can account for their modest contents of Be, Sn, Ta, Nb, and Cs, but Li presents complications because a large share of the Li in the exposed parts of the system is in Li-rich zoned pegmatites.

• OSTI ID: 6285778
• Journal Information: American Mineralogist; (USA), Journal Name: American Mineralogist; (USA) Vol. 75:5-6; ISSN AMMIA; ISSN 0003-004X
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580000* -- Geosciences
ALKALI METALS
ALKALINE EARTH METALS
ANORTHOSITES
BERYLLIUM
BIOTITE
CESIUM
CHALCOGENIDES
CHEMISTRY
CRYSTALLIZATION
ELEMENTS
FEDERAL REGION VIII
FELDSPARS
GABBROS
GEOCHEMISTRY
GEOLOGIC AGES
GEOLOGIC DEPOSITS
GEOLOGIC HISTORY
GEOLOGY
GRANITES
IGNEOUS ROCKS
LITHIUM
MAGMA
METALS
METAMORPHIC ROCKS
METAMORPHISM
MICA
MINERALS
MUSCOVITE
NIOBIUM
NORTH AMERICA
ORIGIN
OXIDE MINERALS
OXIDES
OXYGEN COMPOUNDS
PEGMATITES
PHASE TRANSFORMATIONS
PLUTONIC ROCKS
PRECAMBRIAN ERA
QUARTZ
RESERVOIR TEMPERATURE
ROCKS
SILICATE MINERALS
SILICON COMPOUNDS
SILICON OXIDES
SOUTH DAKOTA
TANTALUM
TIN
TRANSITION ELEMENTS
USA