IGNEOUS PROVINCES IN WESTERN UNITED STATES 



571 



crystal settling, crystal zoning, and filter pressing. The late differentiates 

 of the olivine basalt magma were much more alkalic than those of the 

 augite magma because of the initial difference in the composition of the 

 parent magmas (Lovering and Goddard, 1950). 



The eastern slope olivine basalt differentiate series is similar to that of 

 the San Juan volcanic field except one is an intrusive succession and the 

 other mostly an extrusive. Relative volumes are unknown, but at least, 

 both are postulated to have come from an olivine basalt parent. Some 

 assimilation may have occurred in the San Juan magma reservoirs but 

 Lovering and Goddard, if the writer correctly understands, do not pre- 

 sume assimilation for the olivine basalt series of the east slopes of the 

 Front Range However, the augite diorite parent magma may have been 

 generated entirely by fusion of a crystalline basement rock. 



Both magma subprovinces of the Front Range developed across the 

 ancestral Colorado Range of Pennsylvanian age (see Chapter 25). This 

 general area all through Paleozoic time had been dominantly positive 

 and had received only a very thin veneer of sediments on the crystalline 

 basement, which was broadly exposed by erosion as the Colorado Range 

 was uplifted. The range was gradually buried during the Mesozoic, and 

 Cretaceous beds were deposited on the Precambrian over wide areas of 

 the old range and constituted in these places the only sedimentary rock 

 at the time of Laramide orogeny. Again in Laramide times uplift was 

 prominent but large-scale overthrusting occurred, especially in the west- 

 ern part of the old Colorado Range, now the Vasquez Mountains, the 

 Williams Range, and the Gore Range, and the uplift, thrusting, and in- 

 trusive sequence are closely related in time. 



Yellowstone Subprovince. The Yellowstone subprovince will here in- 

 clude the Absaroka, the Crazy Mountains, the Livingston and Adel Moun- 

 tain fields as well as the Yellowstone Park field. The rocks of this province 

 are generally calc-alkalic in mild contrast to the alkalic rocks of central 

 Montana, previously described and also to the andesites and quartz 

 latites of the Elkhorn Mountains volcanics and the Hogan formation. See 

 Figs. 36.1 and 36.3. Actually the differences are slight and boundaries 

 separating the three provinces are difficult to draw, principally because 

 two of the volcanic centers have episodes of alkalic rock eruption sepa- 



rated by episodes of calc-alkalic rock eruptions. Superposed volcanic se- 

 quences are subprovinces in Larsen's nomenclature (1940). 



Yellowstone Field. The eruptive rocks of Yellowstone Park range from 

 basalt to rhyolite, with the basalts containing calcic plagioclase, augite, 

 hypersthene, and olivine. The Absaroka Range has trachydolerites and 

 orthoclase gabbros (alkalic) as its mafic rocks, and where the age rela- 

 tions have been determined the older effusives are generally calc-alkalic 

 and the younger alkalic. The Absaroka field may therefore be placed in 

 either the Yellowstone calc-alkalic province or the central Montana 

 alkalic province. 



Crazy Mountains Field. The igneous rocks of the Crazy Mountains 

 consist of an older calc-alkalic series of two stocks and associated dikes, 

 sills, and laccoliths, and a younger alkalic series, found chieflv in the 

 northern part of the mountains, and occurring as sills, laccoliths, and 

 dikes. The alkalic bodies are richer in soda than anv of the other groups 

 of central Montana, and have been determined as granite porphyry, 

 syenite, nepheline syenite, shonkinite, and lamprophyre. The older and 

 more calcic stocks are chiefly diorite with minor amounts of granodiorite, 

 gabbro, and picrite (Larsen, 1940). 



Livingston Field. The Livingston formation is a series of pyroclastic 

 rocks several thousand feet thick which crop out on the north side of the 

 Beartooth Mountains. They grade laterally into the Claggett, Judith River, 

 Bearpaw, and Lennep formations and hence represent a center of vol- 

 canism that was active during most of the Montana epoch of the Upper 

 Cretaceous. Pyroxene andesite breccias are by far the most abundant, and 

 occur both above and below hornblende andesite breccias (Vhay, 1939). 



Adel Mountain. A fairly large volcanic field in the southern end of the 

 Foothill belt of the Canadian and Montana Rockies, west of the Highwood 

 Mountains and on the northern end of the Big Belt Mountains may be 

 divided into two parts. Its southeastern part, the Adel Mountain, has been 

 studied by Lyons (1944). He finds that the volcanic rocks consist of po- 

 tash-rich basalts which were erupted on Cretaceous sediments. The 

 trachybasalt breccias, flows, and agglomerates are 3200 feet thick and have 

 been intruded by many chonoliths, sills, and dikes ranging from gabbro 

 to quartz monzonites. The chemical and mineralogical analyses relate 



