IGNEOUS PROVINCES IN WESTERN UNITED STATES 



569 



olivine basalt, the laccolith is an hauyne basanite, and some of the sills 

 in one place are nepheline-hauyne basalt. The basalts were intruded con- 

 siderably later than the microgranites. 



It is evident, in review, that the west Texas and northeastern Mexico 



alkalic province contains differentiates similar to the Spanish Peaks field 



of Colorado, the Rattlesnake Hills field of central Wyoming, and some of 



the igneous groups of central Montana. Fairly stable crustal conditions 



I obtained in most all places, an olivine basalt was the parent magma, but 



I probably some assimilation of alkalic country rock occurred, and in 



) places a mixing of magmas in different states of differentiation seems to 



I be necessary to explain the unusual types. 



- Calc-Alkalic Subprovinces 



San Juan-Front Range Subprovince. The San Juan-Front Range will 

 here include the igneous rocks of the San Juan Mountains, and the Front 

 Range as well as the Spanish Peaks, Chico, and Raton basin fields ( see 

 map, Fig. 36.1). All the rocks of this large area have a notable calc- 

 i alkalic composition, range from basalt to rhyolite, and show a great vari- 

 ation from one flow to another. 



San Juan Field. The great bulk of the San Juan Mountains volcanic 

 field, about 100 miles in diameter, is made up of andesitic and rhyolitic 

 rocks in about equal amounts. Rasalts transitional to andesites are sub- 

 ordinate. 



In the following stratigraphic sequence (Larson and Cross, 1956) the 

 Miocene volcanics of the Potosi series are by far the most extensive and 

 aggregate between 5000 and 6000 cubic miles in total bulk. 



Quaternary andesite: one small body. 

 Erosion to mountain topography. 

 Pliocene ( ?) andesite, andesite-basalt, and rhyolite. 

 Erosion to peneplain. 

 Miocene latite-andesite. 

 Erosion to mountain topography. 



Miocene (Potosi series) andesites, quartz latites, rhyolites, and sub- 

 ordinate andesitic basalts; several internal erosion intervals separating 



conformable sequences of lavas in which dominantly quartz-latite 

 lavas and tuffs are succeeded upward by dominant andesites. 



Erosion to mountain topography. 



Upper Cretaceous to Eocene andesite (dominant), latite, and rhyolite; 

 all occur locally and several internal erosion intervals can be rec- 

 ognized. 



The volcanics lie partly on the northeast flank of a dome some 50 miles 

 in diameter. They spread principally across the central part of the Un- 

 compahgre Range of the Ancestral Rockies (Chapter 15). This range 

 rose in Pennsylvanian time and was gradually buried during succeeding 

 Permian, Triassic, and Jurassic time. In large parts of the range and the 

 area upon which the volcanics accumulated not more than 3000 feet of 

 strata existed, chiefly Cretaceous, prior to the Laramide doming. The 

 area was characterized by doming on the west. To the east compres- 

 sional deformation occurred in South Park and the Front Range ( Chapter 

 25). At the time of Miocene volcanism large areas had been stripped of 

 any sedimentary veneer, and the volcanics accumulated directly on the 

 Precambrian crystalline rocks. The volcanics cannot be directly related 

 therefore, to a basin of sedimentation, to a broad Laramide uplift, or to 

 a belt of strong Laramide orogeny. As for the ancestral Uncompahgre 

 uplift it would seem that its roots would long since have disappeared by 

 isostatic adjustment before Tertiary volcanism occurred. This andesite 

 assemblage is therefore somewhat of an anomaly but must not be neg- 

 lected in shaping a theory of the origin of andesitic magmas in the oro- 

 genic belts. 



Serial derivation from basic magma by fractional crystallization was 

 the dominant process, but also prominent was the thorough mixing of 

 magmas from the same common parentage but at different stages of dif- 

 ferentiation. Some assimilation of country rock may also have occurred 

 (Larsen and Cross, 1956). 



The evidence of mixing of magmas, contamination by foreign material. 

 resorption of hornblende and biotite, and great variation in composition from 

 one flow to another characterizes the San Juan volcanic pile . . . the evidence 

 demonstrates that magmas of chemically related but quite dissimilar composi- 

 tions, were generated locally within spongv subterranean chambers, and that 



