The petrology and geochemistry of the english peak intrusive suite, Klamath mountains, California

The English Peak intrusive suite is a calcic to calc-alkaline late middle Jurassic composite pluton located in the central Klamath Mountains of northern California. The suite consists of the Uncles Creek, Heiney Bar, and English Peak plutons. This study was designed to sample the intrusive suite, to determine the mineralogy and rock types of the plutons, to measure the geochemical variation of the plutonic units in the suite, and to establish which magmatic processes were responsible for the chemical variation.

Geochemical analysis revealed that the English Peak intrusive suite was emplaced in at least eight pulses of magma at temperatures in the range 1000 to 1150^ C and a depth of approximately 11 km. The plutonic imits were intruded in the order: Uncles Creek pluton; Heiney Bar pluton; English Peak pluton early stage; English Peak pluton intermediate stage; and English Peak pluton late stage, which is composed of three compositional groups. Trace element compositions indicate the magmas were derived from subduction-modified mantle or subduction derived crust. Initial Sr/ Sr ratios of 0.7038 to 0.7045 are compatible with mantle-derived magmas; however, limited 5^^0 values for quartz separates of approximately +13.5 ^/oo require a high 5^^0 crustal component in the magmas. Water content of the magmas was variable. In the English Peak pluton, with the exception of the extreme southeastern margin, the magmas in the early stage were apparently drier than the in late stage. Geochemical modeling of the late stage of the English Peak pluton suggest an origin by fi'actional crystallization of a parental magma of intermediate composition accompanied by injection and mixing of mafic magmas.