INCLUSIONS OF PHLOGOPITE AND PHLOGOPITE HYDRATES IN CHROMITE FROM THEHONGGULELENG OPHIOLITE IN XINJIANG, NORTHWEST CHINA

Two types of chromite deposits occur in the Hongguleleng ophiolite in Xinjiang, northwest China. One is located in the mantle sequence, the other occurs in the transition zone between the mantle sequence and la yered cumulates. Abundant primary silicate inclusions such as phlogopi te, pargasite, clinopyroxene, orthopyroxene, and olivine are found in the segregated chromite, but silicate inclusions occur only rarely in accessory chromite of the ultramafic rocks from the transition zone an d cumulates. These silicate phases are considered to have been entrapp ed as discrete and rare composite inclusions during magmatic precipita tion of chromite rather than formed by postmagmatic entrapment. Phlogo pite is the most abundant mineral found as inclusions in chromite in t he Hongguleleng ophiolite. There are two types of substitutions for K in the phlogopite inclusions: (1) Na substitutes for K, and phlogopite shows a continuous range from almost pure sodium phlogopite to phlogo pite; (2) Ca partially substitutes for K, resulting in the formation o f Ca-bearing phlogopite. It is proposed that the alkalic aqueous liqui d (melt) responsible for the formation of the phlogopite inclusions wa s derived from the mixture of the K-rich aqueous liquid related to the subduction of the oceanic slab and the Na-rich aqueous liquid from th e primary magma of the ophiolite. Two types of phlogopite hydrates, hy drate I and possibly a new hydrate (hydrate H), occur as inclusions in the chromite and result from later hydrothermal processes. The fractu res from brittle deformation provided passage for meteoric water to en ter and react with the phlogopite inclusions. Compared with those in t he transition zone, the inclusions of phlogopite and phlogopite hydrat es in the mantle sequence are characterized by (1) smaller grain size and greater abundance, (2) undulatory extinction, (3) higher Si, Cr, N i, and Ca, and (4) lower Ti and Al. These differences are possibly due to (I) P, T, and composition of chromite-precipitating magma, (2) sub solidus reequilibration with the host chromite, and (3) postmagmatic d eformation and hydrothermal processes.