Tonalite-trondhjemite complexes of subduction-related settings (by the example of late Riphean plagiogranitoids of the southwestern margin of the Siberian Platform)

Within the Late Precambrian crystalline complexes of the Arzybei and Shumikha blocks two contrasting geochemical types of granitoids of tonalite-trondhjemite series have been distinguished - high- and low-alumina, similar in petrogeochemistry to "continental" and "oceanic" plagiogranitoids, respectively (according to the classification by J. Art). The geologic position, composition, and petrogenesis of the granitoids evidence their formation under strongly different PT-conditions related to the development of island-arc systems. The available data show that such associations of tonalites and trondhjemites of two types are also present in other Precambrian subductional complexes. The coexistence of high- and low-alumina tonalites and trondhjemites testifies to contrasting PT-conditions of formation of their parental melts in subduction settings. The high-alumina varieties resulted from high-pressure melting of metabasic rocks of the subducting oceanic slab, whereas the low-alumina ones are the products of fractional crystallization of basaltic melt or low-pressure melting of metabasic rocks at the initial stage of evolution of island arcs or during the back-arc spreading. The above association can be considered indicative for Precambrian oceanic suprasubduction settings.