Home – Home – Jornals – Russian Geology and Geophysics 2002 number 5

Within the Late Precambrian crystalline complexes of the Arzybei and Shumikha blocks two contrasting geochemical types of tonalite-trondhjemite granitoids have been distinguished - high- and low-alumina, similar in petrogeochemistry to continental and oceanic plagiogranitoids (according to 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 tonalities and trondhjemites testifies to contrasting PT -conditions of formation of their parental melts in subductional settings. The high-alumina varieties resulted from high-pressure melting of metabasic rocks of the subducting oceanic plate, 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 development of island arc or during the back-arc spreading. The above association can be considered typical of Precambrian oceanic suprasubductional settings.