The first finding of A(Adak)-type magnesian andesites on the Kamchatka Peninsula

Spessartites and vogesites close in chemical composition to magnesian andes ites have been found in the complex of Late Miocene-Early Pliocene dikes on the Kamchatka Peninsula. Amphiboles and clinopyroxenes in these rocks have high magnesium contents: in most of them K-mg varies within 0.72-0.78 and 0.82-0.88, respectively. The rocks underwent significant low-temperature hy drothermal metamorphism, which gave rise to secondary minerals (albite, chl orite, K-feldspar), high LOI values, and elevated Na2O contents. Neverthele ss, the geochemistry of these rocks remained the same, and in REE compositi on (first of all La/Yb, K/La, Rb/La, and La/Cs ratios) they are close to A- type magnesian andesites, whose prototype is lavas of Adak Island (Central Aleutians). Geochemical data point to the absence of genetic relationship b etween the Kamchatka magnesian andesites and ordinary calc-alkaline diorite -porphyrite-microdiorites whose dikes are found within a single dike field.