Near-Ultrahigh Pressure Processing of Continental Crust: Miocene Crustal Xenoliths from the Pamir

Xenoliths of subducted crustal origin hosted by Miocene ultrapotassicigneous rocks in the southern Pamir provide important new informationregarding the geological processes accompanying tectonism duringthe Indo-Eurasian collision. Four types have been studied: sanidineeclogites (omphacite, garnet, sanidine, quartz, biotite, kyanite),felsic granulites (garnet, quartz, sanidine and kyanite), basalticeclogites (omphacite and garnet), and a glimmerite (biotite,clinopyroxene and sanidine). Apatite, rutile and carbonate arethe most abundant minor phases. Hydrous phases (biotite andphengite in felsic granulites and basaltic eclogites, amphibolesin mafic and sanidine eclogites) and plagioclase form minorinclusions in garnet or kyanite. Solid-phase thermobarometryreveals recrystallization at mainly ultrahigh temperatures of1000–1100°C and near-ultrahigh pressures of 2·5–2·8GPa. Textures, parageneses and mineral compositions suggestderivation of the xenoliths from subducted basaltic, tonaliticand pelitic crust that experienced high-pressure dehydrationmelting, K-rich metasomatism, and solid-state re-equilibration.The timing of these processes is constrained by zircon agesfrom the xenoliths and ⁴⁰Ar/³⁹Ar ages of the host volcanic rocksto 57–11 Ma. These xenoliths reveal that deeply subductedcrust may undergo extensive dehydration-driven partial melting,density-driven differentiation and disaggregation, and sequestrationwithin the mantle. These processes may also contribute to thealkaline volcanism observed in continent-collision zones. KEY WORDS: xenolith; high-pressure; subduction; Pamir; Tibet     

Fundamental and derived modes of climate variability: concept and application to interannual time-scales

The notion of mode interaction is proposed as a deterministic concept for understanding climatic modes at various time-scales. This concept is based on the distinction between fundamental modes relying on their own physical mechanisms and derived modes that emerge from the interaction of two other modes. The notion is introduced and applied to interannual climate variability. Observational evidence is presented for the tropospheric biennial variability to be the result of the interaction between the annual cycle and a quasi-decadal mode originating in the Atlantic basin. Within the same framework, Pacific interannual variability at time-scales of about 4 and 6 yr is interpreted as the result of interactions between the biennial and quasi-decadal modes of climate variability. We show that the negative feedback of the interannual modes is linked to the annual cycle and the quasi-decadal mode, both originating outside the Pacific basin, whereas the strong amplitudes of interannual modes result from resonance and local positive feedback. It is argued that such a distinction between fundamental and derived modes of variability is important for understanding the underlying physics of climatic modes, with strong implications for climate predictability.