Xenocrystic Cores Research Articles

U-Pb Studies of Zircon Cores and Overgrowths, and Monazite: Implications for Age and Petrogenesis of the Northeastern Idaho Batholith

U-Pb isotopic studies of zircons, many containing xenocrystic cores with euhedral overgrowths, and monazite from igneous rocks and metasedimentary inclusions of the northeastern Idaho batholith yield linear arrays on concordia diagrams. We interpret these as mixing lines between an old component (cores) and a young component (overgrowths and zircons without cores). The lower intercept of such arrays with concordia may yield the minimum age of the rocks if the overgrowths and zircons without cores are discordant, or the crystallization age if they are concordant. Monazites yield apparently concordant ages either equal or less than the lower intercept zircon ages. The samples studied yield lower intercept ages ranging from $73.5 \pm 6 m.y.$ (foliated quartz diorite) to $46.5 \pm 1 m.y.$ (feldspar megacryst granite); ages obtained are consistent with crosscutting relations observed in the field. Upper intercepts yield ages of 1700 to 2349 m.y. These are interpreted to indicate the mean age of xenocrystic zir...

Two hundred and twenty million years of Archean evolution: a zircon U–Pb age stratigraphic study of the Uchi–Confederation Lakes greenstone belt, northwestern Ontario

Three metavolcanic units in the Uchi–Confederation Lakes greenstone belt have been dated by the zircon U–Pb method. Ages obtained reveal a time interval of [Formula: see text] separating the cycle I crystal tuff (2958.6 ± 1.7 Ma) from the cycle III rhyolite ([Formula: see text]). A cycle II rhyolite about 2794 Ma old lies between the other two units. Uranium-rich xenocrystic cores occur in the cycle I zircons and are indirect evidence of preexisting sialic material.A metamorphosed granodiorite, intrusive into the cycle III rhyolite, is about the same age as the rhyolite. A post-metamorphic quartz monzonite pluton is dated at 2729.6 ± 1.3 Ma. A period of dynamothermal metamorphism in this portion of the Superior Province indirectly dated at [Formula: see text] reflects the Kenoran orogeny.

Some Minor Intrusions of Glen Shee, Perthshire

SUMMARYSome additional data are given regarding the “newer” intrusions of an area of which the main petrological features have been described elsewhere. The intrusions are:— (1) Aplites, pegmatites, quartz-veins, granodiorite porphyries, and biotite-granodiorites in or around the Newer Diorite. (2) A “porphyry” contaminated by cordieritic hornfels and having “clouded ” plagioclase “phenocrysts ”. Evidence is given that the “phenocrysts” have xenocrystic cores. (3) Quartz-felsites with cordierite, andalusite, and garnet, and granodiorite-porphyries, lying between the Newer Diorite and its roof of Ben Lawers Schist. The granodiorite-porphyries post-date the Diorite but the age of the quartz-felsites is uncertain. Contamination, and mechanical and chemical alteration, of the quartz-felsites are recorded. (4) Newer quartz-felspar-porphyries and lamprophyres, not obviously associated with the Newer Diorite.