Eocene arc-continent collision and crustal consolidation in Kamchatka, Russian Far East

Abstract

The age and origin of high-grade metamorphic rocks of the Sredinnyi Range, Kamchatka have been the subject of a long and controversial debate. Based on geochronologic data and its association with a collage of accreted oceanic terranes, leading interpretations argue that the Sredinnyi Range metamorphic rocks represent an accreted Precambrian or Mesozoic microcontinent. In this contribution, we present new data that indicate that these metamorphic rocks were formed from the Cretaceous-Paleocene sedimentary margin of northeast Russia when it was overridden during Eocene obduction of the Olyutorsky arc, a far-travelled oceanic island arc. Our data include new mapping and structural observations along the northern and eastern flanks of the Sredinnyi Range, and SHRIMP zircon and monazite U-Th-Pb age data from 15 key samples. These new isotopic data demonstrate that paragneissic units were formed from sediments with depositional ages locally no older than Late Cretaceous to Paleocene. Furthermore, the statistical similarity of zircon U-Pb grain-ages from the Kamchatka Schist with very low-grade turbidite sandstones of the Ukelayat and Khozgon Groups indicate that metasediments of the Sredinnyi Range are upgraded stratigraphic equivalents of northeast Russian marginal strata. SHRIMP U-Pb ages of zircon overgrowths and metamorphic monazite extracted from migmatite and gneiss indicate that peak metamorphism occurred at 52 Ma, which is synchronous with the onset of the Olyutorsky arc-continent collision. Heating and cooling occurred rapidly, at rates approaching 80°C/m.y. Rapid heating is attributed to syn-collisional, subductionrelated magmatism. Thermochronology and structural observations indicate that exhumation was due to a combination of ductile and brittle thinning of the crust. We speculate that this thinning was caused by diapiric ascent of a low-density low-viscosity continental material beneath a dense structural lid of the obducted island arc.

Early Eocene magmatism in the Sredinnyi Range, Kamchatka: Composition and geodynamic aspects

“…We believe that the metamor phic complexes have a thrust-imbricated structure, as was first suggested by Zhegalova (1978Zhegalova ( , 1981 and later confirmed in (Richter, 1995;Bondarenko, 1997;Kir masov et al, 2004). Here we adhere to a scheme (Soloviev, 2008;Shapiro et al, 2008;Hourigan et al, 2009), which is underlain by geological structural data and newly obtained isotopic dates, according to which the Kolpakova Group, granitoids of the Krutogorova Complex that intrude it, and the overlying Kamchatka Group (Shikhtinskaya Formation) and Kheivan For mation compose the autochton (Figs. 1-3) (Shapiro et al, 2008).…”

“…2 2012 EARLY EOCENE MAGMATISM IN THE SREDINNYI RANGE, KAMCHATKA 149 et al, 2008;Hourigan et al, 2009). The granitoids are syncollisional and mark the emplacement of the "newly formed" continental crust in Kamchatka.…”

“…Early Eocene granitoids and migmatites are widespread in fields of metamorphic rocks in the south ern part of the Sredinnyi Range (Hourigan et al, , 2009Soloviev, 2008;Luchitskaya et al, 2008). Along with them, the area hosts coeval intrusions of the norite-cortlandite association, which are accompanied by Ni-Cu sulfide ore mineralization (Konnikov et al, 2006a(Konnikov et al, , 2010Stepanov and Trukhin, 2007;Sidorov, 2009;Trukhin et al, 2008Trukhin et al, , 2009, and hence, many lines of evidence are now available to recognize an Early Eocene phase of magmatic activity in Kamchatka.…”

“…Cutting across the main thrust zone, these granitoids can thus be classified as "sewing" intrusions. The age of the Early Eocene magmatism and anatexis is confirmed by U-Pb zircon and mona zite dates (Table 1) and is assumed to be 52 ± 2 Ma (Hourigan et al, 2009). …”

Reply to comment on “Accordion vs. quantum tectonics: Insights into continental growth processes from the Paleozoic of eastern Gondwana”

“…For this reason, we suggest Taiwan and Oman as two possible analogues to illustrate a range of possibilities in intra-oceanic arc emplacement events. Other examples are well documented from the Urals (Brown et al, 2006), Timor (Audley-Charles, 2011), New Guinea (Cloos et al, 2005) and the Russian Far East (Hourigan et al, 2009). Our point was to make a comparison rather than present any exact match.…”