New SHRIMP U-Pb, Sm-Nd, and Rb-Sr Data on the Campanian Age and Genesis of Gneissic Plagiogranites of the Kol and Krutogorova Massifs (Sredinnyi Kamchatka Uplift)

摘要

The southern Sredinnyi (Median) Range of the Kamchatka Peninsula hosts the large (250 x 40 km2) Sredinnyi Kamchatka uplift composed of Upper Mesozoic volcanogenic-terrigenous sequences metamorphosed to the greenschist and high-temperature amphibolite facies during the early Eocene. The uplift is characterized by the presence of many granitoid intrusions among metamorphic rocks, which form bodies variable in shapes, structural positions, inner structures, and ages. Gneissic biotite plagiogranites (gneissic granites) constituting the large Kol (KLM, ＞350 km~2), Icha (100 km~2), Pravaya Kolpakova (75 km~2), Krutogorova (KRM, 65 km~2), Poperechnyi (25 km~2), and smaller massifs, which were subdivided in two, the Krutogorova and Kol massifs, are traditionally thought to represent the oldest granitoid mag-matism phase. At the same time, interpretations of the relationships between granitoids and overlying metamorphic sequences are highly controversial: they are considered as intrusive stratigraphically unconform-able with the basal conglomerate member at the base of terrigenous or tectonic sequences. The differences in the structural positions of these massifs, compositions, and textural-structural features of gneissic granites are responsible for many different interpretations of their genesis and age (Proterozoic to Mesozoic), which were based on the views of the researchers on the evolution of the Sredinnyi Kamchatka uplift as a whole. The wide spectrum of Mesozoic isoch-ron K-Ar and Rb-Sr dates obtained subsequently for these rocks confirmed their belonging to two complexes: Early Cretaceous Krutogorova and Late Cretaceous Kol. According to the recent SHRIMP U-Pb investigations of zircons from several massifs of the Krutogorova Complex, its formation age is estimated to be 76-83 Ma (Campanian). The age of gneissic granites of the Kol Complex still remains unknown.