North American Feldspathoidal Rocks in Space and Time
Feldspathoidal rocks are generally considered to be postorogenic and concentrated in the stable portions of continents. These erroneous ideas have biased petrogenetic hypotheses. Of the more than 120 feldspathoidal rock occurrences in North America, only one-third are in the shield and stable craton; the remainder are within Phanerozoic orogenic belts. Approximately 20 percent of all occurrences are pretectonic or syntectonic. In several regions (usually considered petrographic provinces) superficially identical feldspathoidal rocks were emplaced over spans of several hundred million years. Time correlation based upon petrologic similarity and geographic proximity is as risky for these igneous rocks as it is for sedimentary rocks. Hypotheses stating that feldspathoidal rocks are generated during “crustal tension” and rift faulting seem inapplicable to North America.