Crust–Mantle Xenoliths from the Kharchinsky Volcano (Central Kamchatka Depression): Mineralogy and Petrogenesis

—We present results of a study of plutonic-rock xenoliths from the Kharchinsky Volcano (Central Kamchatka depression). The studied xenolith collection comprises nine samples of peridotites and clinopyroxenites. The peridotites are identified as wehrlites, dunites, and harzburgites composed of olivine, clino- and orthopyroxenes, amphibole, and chromite in varying amounts. The clinopyroxenites consist mostly of clinopyroxene and often contain subordinate olivine, amphibole, hercynite, and magnetite. The xenoliths have interstitial segregations and veins composed of chlorite, plagioclase, K-feldspar, orthopyroxene, barite, fluorapatite, ilmenite, and, more seldom, anhydrite, phlogopite, and some other minerals. The study has revealed that veinlet minerals sometimes replace primary minerals and form pseudomorphs, thus indicating the metasomatic origin of interstitial and vein mineral assemblages. The thermobarometric calculations for minerals have shown that peridotites formed at ~1140 °C and ≤10 kbar in the intermediate chambers at the depths from the spinel stability field to the Moho. Interstitial metasomatic alterations of rocks took place at ~400–850 °C.

Ion microprobe U–Pb zircon geochronology of a late tectonic granitic–gabbroic rock complex within the Hercynian Iberian belt

Ion microprobe (SIMS) dating shows that three plutonic rock bodies representative of the major, ∼15 000 km2, late tectonic (late-D3), plutonic rock series in the Central Iberian Zone of the Hercynian belt in west-central Iberia have indistinguishable zircon crystallization ages. Ledrada biotite granite and Colmenar cordierite-bearing biotite granite show gradual transitions in field appearance, petrography and chemistry and have statistically indistinguishable weighted average ages of 306.8 ± 1.9(2σ) Ma and 306.5 ± 1.5(2σ) Ma, respectively, which indicates that they originated during a single event involving a heterogeneous magma which notably varied in Al-content. The third rock body, Navahermosa meta-gabbronorite, has a weighted average zircon crystallization age of 305.6 ± 1.4(2σ) Ma, statistically indistinguishable (variance analysis, F-test, α = 0.05) from the granites. Zircon crystals in the gabbronoritic rock are anhedral, skeletal, millimetre-sized and partake in main magmatic textures, whereas the zircon grains in the granites are of more common appearance, much smaller, usually euhedral and enclosed in main magmatic crystals. As gestation times of granitic zircon, the time between zircon crystallization and magmatic intrusion, may be up to 5 Myr, the crystallization age of the gabbronoritic zircon may be the best estimate of the time of emplacement of the magmatic complex. Our study indicates co-existence of basic and silicic magmas in the Hercynian crustal section at c. 306 Ma, suggesting common genetic control. The two granitic rocks carry inherited zircon ranging from c. 1300 to 330 Ma, indicating that pre-Hercynian basement rocks of Proterozoic to Palaeozoic age contributed to the granitic magma.