The Kamchatka arc represents a 700 km long convergent margin in the NW Pacific Ocean, which contains >240 volcanic centers. Avachinsky volcano (AV, 53.255șN, 158.83șE) represents one of the 34 currently active volcanoes in the Eastern Volcanic Front arc segment of Kamchatka. AV is a stratovolcano (2751 m) composed of a Holocene active volcanic cone nested within a Late Pleistocene - Early Holocene caldera. The volcano is located ~110 km above the Benioff zone, 187.5 km from the trench, and in continental crust of average (35 km) thickness. Compositionally, AV is comprised primarily of medium-K, calc-alkaline basalt-andesite, but with subordinate diamond-bearing, high-Mg (14-20% MgO) picrites that host mantle xenoliths. High Mg# (76-82), Ni (380 ppm), and Cr (1460 ppm) content in the picrites indicates a primary mantle melt derivation. This study focuses upon the mineralogy and mineral chemistry of the mantle xenoliths and host volcanics of AV. Point count data from the mantle xenoliths indicate a heterogeneous lithologic character and includes: dunite, harzburgite, olivine (ol) clinopyroxenite, ol websterite, and websterite. Mantle derived fabrics (porphyroclastic and protoclastic) are present in the xenoliths due to variable deformation under mantle conditions. Mineral defect structures, such as undulose ol and kink-bands in pyroxene (px), are ubiquitous in the AV xenoliths. The dominant mantle xenolith mineral assemblage is olivine (ol), clinopyroxene (cpx), orthopyroxene )opx), Cr-spinel (sp), +/- hornblende (hbl). Mantle ol compositions are Fo90+/-2 with the lower Fo content coinciding with the hbl-bearing/metasomatized samples. Xenolith cpx is typically diopside (Wo46+/-1En48+/-1Fs6+/-2) with some Cr-rich (~1.0% Cr2O3) varieties. Enstatite to ferroan enstatite compositions dominate the mantle opx (Wo1En87+/-3Fs12+/-3). Metasomatic hbl is found replacing px and ol in the xenoliths either in patches or as veinlets and ranges from magnesio-hbl to edenitic or pargasitic hbl in composition. Host picrite phenocryst mineral assemblage includes ol (Fo83+/-6), cpx (Cr-diopside and Mg-rich augite), and Cr-sp. Quantitative P-T estimates have been made on the mantle xenoliths and host picrite using cpx-ol-sp and cpx-opx thermometry and single px thermobarometry. Mantle xenoliths record T = 950-1050șC and P = 27-30 kb (90-105 km); whereas the picrites yield comparable T-P of 1000-1100șC and 27 kb. We suggest that the AV mantle xenolith - host picrite relationship represents fragments of source mantle wedge materials that are entrained within a parental arc magma, providing us with invaluable information on the most primitive stages of arc volcanic development and a rare glimpse into the sub-arc mantle wedge.
From 1996 Abstracts with Programs. The Geological Society of America.
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