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Geologic units containing alkalic intrusive rock
Alkalic intrusive rock
A plutonic rock that contains more sodium and/or potassium than is required to form feldspar with the available silica.
- Igneous rocks (Phanerozoic | Mesozoic | Cretaceous)
- Includes undifferentiated altered sedimentary rocks of Paleozoic age at Granite Mountain, Pulaski County, and Mufreesboro, Pike County. Acid to intermediate - Nepheline syenite complexes at Magnet Cove and Potash Sulfur Springs, stocks at Granite Mtn., explosion breccias, and dikes and sills (principally trachyte,tinguaite, phonolite, some ranging from syenite to diabase.) Basic to ultrabasic - Lamproite pipes (Murfreesboro, Pike County) and dikes and sills (principally lamprophyres of fourchite, ouachitite, and monchiquite)
- Tertiary coarse-grained intrusive rocks (Phanerozoic | Cenozoic | Tertiary)
- Tertiary coarse-grained rocks: Some of these rocks have the composition of quartz diorite, monzonite, and similar rocks but most are alkalic syenite, leucite-rich rocks, and other alkalic varieties. Some of the rocks that are associated areally with the Cretaceous volcanic rocks may be Cretaceous in age. Includes the larger intrusive masses in the eastern part of western Montana and the western part of eastern Montana, most of which have domed upper surfaces. Many of these masses have been regarded as laccoliths, but generally without adequate proof. The Boulder batholith and related stocks are mapped separately.
- Alkalic intrusive rocks (Oligocene and Eocene) (Eocene to Oligocene)
- Sills, dikes, stocks, and irregular intrusions of porphyritic or aphanitic camptonite, shonkinite, and. nepheline syenite or phonolite. Potassium-argon ages of 32 to 35 Ma obtained on camptonite and nepheline syenite (Snavely and others, 1976c; Fiebelkorn and others, 1983)
- Alkalic extrusive and intrusive igneous rocks (Phanerozoic | Cenozoic | Tertiary | Eocene)
- ALKALIC INTRUSIVE AND EXTRUSIVE IGNEOUS ROCKS (AGE ABOUT 44 Ma)--Light- to greenish-gray porphyry.
- Alkalic extrusive and intrusive igneous rocks (Phanerozoic | Cenozoic | Quaternary)
- ALKALIC EXTRUSIVE AND INTRUSIVE IGNEOUS ROCKS (AGE 1.25 Ma)--Leucite- and nepheline-rich flows, scoria, and necks.