The Montana-Yellowstone Geologic Field Guide Database
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Cumulate Xenoliths in the Lodgepole, Enos Mountain and Susie Peak Intrusions: A Guide part of MT Field Guides
The Lodgepole, Enos Mountain and Susie Peak plutons represent multiple intrusions of intermediate magma that were emplaced at depths ranging from near-surface to 2 km during the Late Cretaceous....These intrusions lie, respectively, 8, 9 and 12 km north of the nearest outcrops of the Stillwater Complex. The Lodgepole intrusion is composed of an early, dacitic phase and a later, dioritic phase; the diorite contains abundant xenoliths (maximum 31 cm diameter) in the area north of Clover Basin near its west margin. These xenoliths include foliated mafic amphibolite, gneiss, Paleozoic sedimentary rocks and mafic cumulates. Smaller xenoliths of similar rock types are found in the Enos Mountain and Susie Peak intrusions....Mineral compositions and textures lead to the conclusion that the cumulate xenoliths were brought up from a buried underlying extension of the Stillwater Complex.

Topics: Igneous rocks
Geographic Location: Southwest Montana
Geologic Province: Central Rocky Mountains Foreland Province

Guide to the Picket Pin Mountain Area part of MT Field Guides
This traverse through part of the Banded series provides relatively easy access to the upper half of the exposed part of the Stillwater Complex. The percentage of outcrops is high, and even in areas of soil or tundra cover, the float appears to be sufficiently immobile and concentrated to give an accurate picture of the underlying bedrock geology....The traverse begins in the Middle gabbro zone and terminates where Paleozoic limestone unconformably overlies the Upper anorthosite zone. Above the Middle anorthosite zone, it follows the Picket Pin section described by McCallum and others (1980) and Raedeke (1982b)....An optional traverse through some lithologically complex and intriguing rocks in the Upper anorthosite zone north of the Castle Creek fault is also described. Total traverse time, including the drive from Carter's Camp and return, is 8-10 hours. Elevations range from 9,300 to 10,000 feet.

Topics: Igneous rocks
Geographic Location: Southwest Montana
Geologic Province: Central Rocky Mountains Foreland Province

Guide to the Benbow Area part of MT Field Guides
The Benbow area (named for T.C. Benbow, who first discovered chromite there) is at the east end of the exposed Stillwater Complex....Most of the localities described...are in the Ultramafic series....The features emphasized at these localities include the nature of the cyclic units, pegmatoids associated with chromite seams, evidence for slumping anbd slope instability in the Stillwater magma chamber, lateral persistence of some of the chromite seams, and the unconformity between the Banded series of the Stillwater Complex and overlying Cambrian limestone.

Topics: Structures, Resources, Igneous rocks, Sedimentary rocks
Geographic Location: Southwest Montana
Geologic Province: Central Rocky Mountains Foreland Province

Plutonism at Deep Crustal Levels: The Idaho Batholith, Montana and Idaho part of MT Field Guides
The Idaho batholith field trip traverses a well exposed cross-section of the northern Idaho batholith, briefly examines the broad aspects of this deep-seated granitoid batholith and its regionally metamorphosed country rocks, and considers the role of the synplutonic mafic magmas from the mantle in providing heat for melting of continental crustal rocks to form the more felsic main-phase units of the batholith.

Topics: Igneous rocks, Metamorphic rocks
Geographic Location: Southwest Montana
Geologic Province: Rocky Mountain Fold-Thrust Belt

Field Guide to the Mountain View and West Fork Areas, Stillwater Complex, Montana part of MT Field Guides
The Mountain View area of the Stillwater Complex, which is exposed on the west side of the Stillwater River valley, contains a well exposed, easily accessible section through the Ultramafic series. In this area the ultramafic cumulates have, for the most part, escaped the serpentinization common in other parts of the complex. The Basal series rocks and the lowermost ultramafic cumulates, however, have suffereed extensive alteration. The hornfels and the sill/dike complex are reasonably fresh and well exposed in the Verdigris Creek area, the site of intensive exploration for Cu/Ni sulfides. The Banded series rocks are well exposed along the mine road leading to the abandoned Mouat chromite mine, although only Lower Banded series cumulates are present in this area.

Topics: Resources, Igneous rocks
Geographic Location: Southwest Montana
Geologic Province: Central Rocky Mountains Foreland Province

Field Guide to an Archean Transect, Eastern Beartooth Mountains, Montana-Wyoming part of MT Field Guides
The eastern Beartooth Mountains of Montana and Wyoming contain a record of crustal evolution that spans almost 1000 Ma and culminates in a major episode of crustal growth 2700-2800 Ma. The earlier record is sparse and complex as a result of extensive magmatism and intense metamorphism associated with Late Archean activity. In general, however, it appears that continental material was present in this area by roughly 3600 Ma, and that a stable continental shelf accumulated quartzites, iron-formation, and lesser amounts of pelitic to psammitic units interspersed with small volumes of mafic to silicic volcanic rocks. This cycle of accumulation was apparently terminated by an episode of granulite facies metamorphism 3300-3400 Ma, perhaps as a result of continent-continent collision....About 2800-2900 Ma, a second major cycle of crustal growth began that bears some resemblance to those associated with modern continent-ocean subduction zones....The first igneous rocks produced during this cycle were andesitic or dioritic rocks, both coarse and fine grained, that were subsequently metamorphosed to amphibolite facies....This interval is restricted by the presence of a granodioritic series (Long Lake granodiorite) that was intruded late in the kinematic cycle... (2779 Ma)...and marks the lower limit for the last major episode of regional metamorphism. The last and most volumetrically important rock produced during the cycle was the Long Lake granite. This unit composes approximately 80-90% of the eastern portion of the range and engulfs all older rock types. It appears to have been intruded about 2740 Ma....This excursion will attempt to view the evidence of these two major cycles in four main stops: (1) Hellroaring Plateau, (2) Lower Quad Creek, (3) Upper Quad Creek, and (4) Long Lake. Late Archean mafic dikes (at Beartooth Lake) will also be examined.

Topics: Metamorphic rocks, Igneous rocks
Geographic Location: Southwest Montana
Geologic Province: Central Rocky Mountains Foreland Province

Field Guide; Belt Butte and Tiger Butte part of MT Field Guides
This field trip extends east from Great Falls across rolling glacial plains to the summit of Belt Butte, where its formation and the collapse structure adjacent to it will be discussed. In addition, the Tiger Butte laccolithic intrusion, contact metamorphic effects, associated dikes, and structure resulting from the intrusion will be examined.

Topics: Fossils, Hazards, Resources, Surficial geology, Igneous rocks, Sedimentary rocks, Metamorphic rocks
Geographic Location: North-Central Montana
Geologic Province: Montana Plains

Field Guide; North Moccasin and Judith Mountains part of MT Field Guides
The emphasis of this field trip is to examine gold mineralization hosted in karsts breccias at the Kendall mining district and the relationship between intrusives, Paleozoic sedimentary rocks and mineralization in the Judith Mountains.

Topics: Surficial geology, Hydrology, Sedimentary rocks
Geographic Location: North-Central Montana
Geologic Province: Montana Plains

Field Guide; Little Rocky Mountains part of MT Field Guides
This fieldtrip examines deformed Paleozoic sedimentary rocks on the margins of the Little Rocky Mountains, Tertiary intrusive rocks (porphyrys, magmatic-hydrothermal breccias, and dikes), and associated mineralization.

Topics: Fossils, Resources, Structures, Igneous rocks, Sedimentary rocks, Metamorphic rocks
Geographic Location: North-Central Montana
Geologic Province: Montana Plains

Field Guide; Little Rocky Mountains part of MT Field Guides
This fieldtrip examines deformed Paleozoic sedimentary rocks on the margins of the Little Rocky Mountains, Tertiary intrusive rocks (porphyrys, magmatic-hydrothermal breccias, and dikes), and associated mineralization.

Topics: Metamorphic rocks, Sedimentary rocks, Igneous rocks
Geographic Location: North-Central Montana
Geologic Province: Montana Plains