Paleoarchean U–Pb (SIMS SHRIMP-II) age of mafic granulites of the bug complex (Ukrainian shield)

Mafic granulites (metamorphosed tholeite-komatiite volcanic rocks) represent the inclusions in gneissenderbite of the Bug complex (southwest of the Ukrainian shield). The studied mafic inclusion has two zircon generations of magmatic origin that yield, respectively, SHRIMP zircon U–Pb ages of 3628±58 Ma and 2845±65 Ma. The first age is the oldest date the earliest stage of basic magmatism on the Ukrainian shield. The age of the second zircon is interpreted as a result of partial melting, synchronous with the metamorphism in the area.

U–Pb zircon ages in supracrustal and plutonic rocks; North Spirit Lake area, Northwestern Ontario

The North Spirit Lake greenstone belt in the Sachigo Subprovince of the Superior Province comprises parts of three sequences of volcanic and sedimentary rocks; the main (youngest) is separated from the older sequences by an unconformity to disconformity. The belt is bounded by large granitoid batholiths and was metamorphosed under greenschist to, locally, hornblende–cordierite facies. U–Pb zircon dating was performed on volcanic, sedimentary, and plutonic rocks in order to establish an absolute chronology for the evolution of the area.A tuff breccia in the lowermost supracrustal sequence is dated at 3023 ± 2 Ma. Zircons from a tuff in the middle sequence show complex U–Pb relationships; although they do not allow a precise age determination, the data suggest that the tuff formed sometime between 2950 and 2800 Ma ago. Zircons from a quartz arenite, also in the middle sequence, yield a simple data pattern and define an age of [Formula: see text], suggesting derivation of the sediment from a uniform source of this age. A zircon analysis from a tonalitic clast in a conglomerate at the base of the upper sequence yields a minimum age of 2975 Ma. The clast may have had the same origin as the zircons in the quartz arenite. Another tonalitic clast from the same conglomerate yields a slightly older age of 3001 ± 3 Ma. No remnant of these tonalites can be recognized in the field, suggesting that they have been largely removed during subsequent erosional processes.Two subvolcanic intrusions from the upper sequence yield zircon ages of 2743 ± 2 and 2731 ± 2 Ma, respectively. A crystal tuff, also in the upper sequence, contains two generations of zircons: newly formed magmatic zircons, which date the extrusion of the tuff at 2735 ± 10 Ma, and older grains with a minimum age of 2862 Ma, which represent inherited zircons. This tuff was thus generated at least in part by anatectic melting of >2862 Ma crust.A quartz diorite from MacDowell Lake in the adjacent Berens River Subprovince yields an age of 2744 ± 2 Ma. A mafic inclusion in the quartz diorite contains amoeboidal, strongly fractured zircons, which point to an intercept age of about 2727 Ma. This age could either reflect a metamorphic event or represent a geologically meaningless mixed age.