CRUST-FORMING PROCESSES IN THE GEOLOGIC DEVELOPMENT OF THE TUVA-MONGOLIA MASSIF: SM-ND ISOTOPIC AND GEOCHEMICAL DATA FOR GRANITOIDS

Abstract

The compositions of igneous rocks from the Tuva-Mongolia massif were studied. These rocks were formed during the precollisional, syncollisional, and postcollisional stages of massif formation. The compositions of strongly metamorphosed precollisional complexes are correlated with the rocks of island arcs and rift zones of passive margins. Rocks with intraplate chemical signatures (peralkaline granitoids, syenites, and layered gabbro-monzodiorites) were distinguished in the group of syncollisional and postcollisional granitoids. Normal syncollisional and postcollisional granitoids are chemically similar to the crustal rocks that are modeled by the composition of metaterrigenous rocks of the supracrustal association. This is in agreement with the similar εNdCT) values of granitoids and metaterrigenous rocks, from 0.7 to -6.5 and from -2 to -7.6, respectively. Variations in incompatible element contents in the granitoids are related to different degrees of source melting. The rocks with intraplate characteristics bear common geochemical features. It was shown that an increase in incompatible element contents in the sequence gabbro-monzonite was caused by fractionation processes with the participation of plagioclase. The subsequent evolution up to leucogranite and peralkaline granite compositions was accompanied by crustal contamination. The Nd isotopic systematics of the rocks of this group (εNd(T) from +3.5 to +4.6) are similar to the compositions of OIB-type basalts from the Agardagh-Erzin ophiolite zone (εNd(T) = +2.4). Paleogeodynamic reconstructions suggest that the structure of the Tuva-Mongolia massif developed ∼500-490 Ma ago as a result of the collision of fragments of the Riphean and Caledonian (island-arc) lithosphere with an oceanic island similar to Iceland. The lithosphere of the newly formed accretion block overlay a mantle hot spot, which triggered intraplate magmatic activity. In addition, anatectic processes occurred under the influence of the plume source and produced syncollisional and postcollisional granitoids.