Литосфера (Jun 2015)
New data on the nature of substrate Late Paleozoic granites of the Southern Urals
Abstract
Isotopic Sm-Nd and Rb-Sr systems of gneisses forming dome structures and enclosing Late Paleozoic granites in Kochkar anticlinorium of the Southern Urals were studied. Rb-Sr isochron age of gneisses - 354.9 ± 4.5 Ma - coincides with the age of the granite migmatites of Varlamovsk migmatite-pluton and age of granitic rocks widespread in the Urals gabbro-tonalite-granodiorite-granite (GTGG) massifs which are the products of hornblende gabbro water anatexis. Isotopic parameters (87Sr/86Sr)i, (εSr)i, (143Nd/144Nd)i as far as geochemical features of GTGG granitoids and gneisses were proved to be similar. Closeness of the age, isotopic characteristics and composition of the tonalite-granite-gneisses forming dome structures enclosing Late Paleozoic granite massifs, on the one hand, and the rocks of GTGG massifs, on the other hand, suggests that the gneisses are metamorphosed by Devonian granitoids. The high water content of the eutectic composition magma of Late Paleozoic granites ( PH2O = 0.7-0.8 Ptot at a value of Ptot = 4-5 kbar) prevents its vertical movement. The maximum possible ascent of granitic magma to the values of PH2O = Ptot under which it should crystallize corresponds to pressure differential not more than 0.5-1.0 kbar or in absolute numbers - 1-3 km. These values do not exceed the thickness of orthogneiss domes and do not contradict the assumption that the granite rocks occur among the possible substrate. These data testify that the main substrate Late Paleozoic (Permian) granites in the eastern part of the Southern Urals can be the rocks of Late Devonian GTGG complexes metamorphosed to amphibolite facies. This conclusion explains the isotopic parameters of granites, in particular, the low content of radiogenic strontium and increased value of143Nd/144Nd, corresponding to a positive value εNd. The rocks of Mesoproterozoic basement also participated in the granite formation, but played a minor role.
