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STRUCTURAL GEOLOGY OF NORTH AMERICA 



in general more than one chamber was tapped during an eruption (Waters, 

 1955). 



Although the existence of an orogenic root is questionable, Waters 

 suggests: 



. . . [This] part of the Rocky Mountain root was undergoing renewed granitiza- 

 tion and anatexis, and that the volcanic rocks were fed from growing pods 

 filled with mixtures of magma and migma. Pluto's genetic traits can actually 

 be seen in the volcanic rocks! But it is not a root of argillites and graywackes 

 that was undergoing partial melting as in the Cascades. Instead the richness 

 in potash, and the abundance of biotite and hornblende in process of resorption 

 point to a mountain root in a much later stage of metamorphic development — 

 one in which the principal rocks were mica schists, amphibolites, and granodio- 

 rite intrusives. 



Spanish Peaks Field. The igneous rocks of the Spanish Peaks area 

 (Knopf, 1936) consist of two central stocks of which the older is a mass 

 of granite porphyry. It is cut by the later pyroxene syenodiorite. A strik- 

 ing system of radial dikes (Chapter 25) evidently emanated from the 

 stocks, and they range from highly silicic to mafic varieties. The order of 

 intrusion is: 1, granite porphyry stock and granite porphyry dikes; 2, 

 granodiorite porphyry stock and biotite porphyry dikes; 3, pyroxene 

 syenodiorite stock; 4, microsyenodiorite; 5, teschenite, camptonite, and 

 shonkinite, and trachydolerite; 6, augite syenodoirite porphyry; 7, campto- 

 nite and biotite lamprophyres. Their origin is discussed by Waters ( 1955) 

 as follows: 



The order of intrusion in the stocks is the order of decreasing silica content, 

 the reverse of the normal plutonic order. Noteworthy, too, is the great variety 

 and abundance of the lamprophyres. Another interesting fact is that some 

 of the lamprophyres are of the kind commonly considered related to calc- 

 alkalic masses, whereas others are of the kind believed to be genetically 

 related to alkalic rocks. 



Without added evidence from mineral paragenesis and inclusions it would 

 be presumptuous to suggest that the Spanish Peak rocks may be igneous 

 offshoots from a zone of biotite-rich metamorphic rocks that were undergoing 

 partial fusion. Nevertheless, such a hypothesis, in contrast to derivation from a 

 parental basalt magma, better fits the reversal in the "normal" sequence of 

 intrusion. Also the rising temperature during anatexis, resulting ultimately in 

 partial fusion of hornblende and biotite, can account for the formation of the 

 varied suite of lamprophyres and can explain their heteromorphism (Waters, 

 1955). 



Chico Field. In northeastern New Mexico adjacent to the Spanish 

 Peaks field basalt flows cover over 700 square miles. They are here col- 

 lectively called the Chico field. There were three periods of basalt ex- 

 trusion separated by active stream erosion, and all are believed to be of 

 Quaternary age although it is possible that the oldest is Pliocene. The 

 basalt extrusions are mostly fissure-type eruptions, but some necks are 

 noted. The extrusion loci have not been tied to post-Eocene structure. 

 The volcanics occur on the southeast flank of the Raton basin of Creta- 

 ceaus and Tertiary age. Olivine basalts predominate in all three periods. 

 The intermediate flows have the greatest variation and include olivine 

 basalt, olivine-free basalt, olivine basalt with quartz inclusions, felspathoid 

 basalts (tephrite?, olivine absent), and basanites (olivine present). 

 Dacites, andesites, soda trachytes, and phonolites in minor amounts are 

 also noted. All these rocks of the area probably originated from one 

 magma whose original composition approximated olivine basalt. The suite 

 is sodic alkalic (Collins, 1949; Stobbe, 1949). 



The Chico field basalts are grouped in the same province with the San 

 Juan andesites and rhyolites because they are adjacent and have had the 

 same parentage, namely an olivine basalt magma. It should be noted that 

 the Chico basalts are distinctly in a nonorogenic region. 



Front Range Igneous Rocks. The transverse porphyry or mineral belt 

 of Laramide intrusions and related rocks of the Front Range of Colorado 

 has been reviewed in Chapter 25. The succession of igneous rocks and 

 their chemical and mineralogical composition suggest that those of the 

 western slopes were derived from an augite diorite magma, and those on 

 the eastern slopes from an olivine basalt magma. The augite diorite 

 magma differentiated into a series ranging from porphyritic diorite 

 through porphyritic quartz monzonite to granite porphyry in compara- 

 tively shallow hearts. The olivine basalt magma gave rise to the differ- 

 entiate series: diorite, monzonite, quartz monzonite, granite, alaskite, 

 lead-silver ores; alkalic syenite, bostonite, pyrite gold ores; and biotite 

 monzonite, biotite latite, latitic intrusion breccia, gold telluride ores, 

 and tungsten ores. This was accomplished by withdrawal of portions of 

 the changing residuum of the slowly solidifying magma into shallow 

 reservoirs, and further differentiation by the subtractive processes of 



