Magmatism and tectonic settings

Abstract

Mutual relationships of mobile lithospheric plates define two distinct tectonic environments each characterized by diagnostic magmatism. A compressional environment occurs at loci of plate convergence; quartz‐ and hypersthene‐normative intermediate, acid and basic members of the calc‐alkaline orogenic suite are emplaced above a dipping seismic zone. A tensional environment occurs mainly at loci of plate divergence, areas characterized by networks of deep fractures, offset by transform faults. Products of nonorogenic magmatism typically consist of abundant tholeiitic or alkali olivine basalt, possibly accompanied by rhyolite, trachyte, or phonolite (or plutonic equivalents); intermediate compositions are volumetrically insignificant. On a differentiation index versus frequency plot, orogenic suites define one maximum, nonorogenic suites two maximums. Orogenic rocks show no iron enrichment; nonorogenic suites do. Recognition of older igneous associations as orogenic or nonorogenic by employing chemical and mineralogical criteria can lead to inferences about tectonic environments during emplacement. Such criteria may provide a unique insight into regional tectonic evolution very difficult to deduce from structural investigations alone.

Chapter 9: Recycling of continental crust in Miocene volcanic rocksfrom the Mojave block, southern California

“…Elevated Ti02 is often associated with "nonorogenic" basalts (e.g., Chayes, 1964;Martin and Piwinskii, 1972). However, Glazner and Richardson (1983) showed that Ti02 cannot be used to discriminate syn-and post-subduction basalts in western North America.…”

Geochemistry and evolution of the early Proterozoic, post-Penokean rhyolites, granites, and related rocks of south-central Wisconsin, U.S.A.

“…In general, rock suites emplaced during an anorogenic event are higher in Si02 and P2O5, higher in alkalies (especially K), low in CaO, AI2O3 and MgO, and show definite iron enrichment when compared to orogenic rock associations (Martin and Piwinskii, 1972). Also, in anorogenic suites few rocks have Si02 contents that are less than 65% (Rogers and Greenberg, 1981).…”

Oliverian domes, related plutonic rocks, and mantling Ammonoosuc Volcanics of the Bronson Hill Anticlinorium, New England Appalachians

“…In this context, it is worth noting the well-established association of a bimodal (and, typically, alkalic) plutonic-volcanic suite with anorogenic (extensional) environments; orogenic (compressional) environments, on the other hand, are associated with calcalkaline suites showing a spectrum of compositions (Martin and Piwinskii, 1972). This bimodality, for example, constitutes the major basis for interpreting the Silurian-Devonian Maine coastal volcanic belt as having formed under conditions of crustal extension (Gates and Moench, 1981).…”