MAJOR-ELEMENT CHEMISTRY OF PLUTONIC ROCK SUITES FROM COMPRESSIONAL AND EXTENSIONAL PLATE BOUNDARIES

Chemical criteria have been developed to distinguish plutonic rock suites from compressional and extensional plate margins by comparing type examples of Mesozoic and Cenozoic plutonic suites generated at these plate boundaries. Compressional plutonic rock suites are characterized by intermediate unimodal frequency distributions of differentiation index and normative plagioclase, calc/alkali indexes in the range of 60-64, and distinctive patterns on AFM diagrams. Extensional plutonic rock suites are characterized by bimodal distribution of differentiation index and normative plagioclase, calc/alkali indexes in the range of 50-56 and the presence of peralkaline rocks. These criteria are useful in determining tectonic settings of plutonic rock suites of unknown environment. No single criterion should be used to distinguish tectonic setting. The distinguishing chemical features of compressional plutonic rock suites may directly be related to melting and dehydration of the subducted ocean crust. Other processes that may be important are: lowering of the solidus of the overlying peridotitic mantle wedge resulting in partial melting; fractionation of both primary melts; extensive reaction of these melts with the continental crust. The characteristics of extensional plutonic rock suites may directly be related to melting of anhydrous peridotitic mantle; small amounts of melting of continental crust and a lack of mixing of the two magmas. Basic rocks from extensional suites may be generated by smaller amounts of melting at greater depths than those from compressional suites resulting in some with alkaline affinities. © 1979.