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.