U-PB GEOCHRONOLOGICAL CONSTRAINTS ON THE GEOLOGICAL EVOLUTION OF THE PINWARE TERRANE AND ADJACENT AREAS, GRENVILLE PROVINCE, SOUTHEAST LABRADOR, CANADA

U-Pb geochronological data show that the Pinware terrane and adjacent areas in the Grenville Province in southeast Labrador experienced thre e orogenic events: Labradorian, Pinwarian and Grenvillian. Labradorian (1710-1600 Ma) rocks in the Pinware terrane, previously only known fr om a dated felsic volcanic enclave in a younger granite, are now recog nized to be widespread and include both supracrustal units and granito id intrusions. One quartzite was deposited between similar to 1805 and 1500 Ma, the ages of the youngest detrital zircon and subsequent meta morphism, respectively, and includes 1878 and 2720 Ma detrital zircon suggesting derivation of source material from pre-Labradorian Laurenti a. A volcaniclastic(?) unit is interpreted to have an age of 1637 +/- 8 Ma. Two Labradorian quartz monzonite intrusions yielded ages of 1650 (-9)(+18) Ma and 1649 +/- 7 Ma. Coupled with previous information, the se data indicate coeval plutonism, volcanism and sedimentation taking place at the same time as formation of the Trans-Labrador batholith fa rther north. The Pinwarian Orogeny is now extended to include events b etween 1510 and 1450 Ma, Granitoid rocks of the Upper Paradise River p luton, a large AMCG suite in the Mealy Mountains terrane, has been dat ed at 1495 +/- 7 and 1501 +/- 9 Ma, making it the only suite of this t ype and age known in North America. Migmatitic quartz monzonite, dated at 1450(-21)(+15) Ma, provides the first proof of high-grade metamorp hism in the Pinware terrane, and, in conjunction with recent geochrono logical data from other parts of the eastern Grenville Province, justi fy upgrading the Pinwarian to orogenic status, Grenvillian metamorphis m throughout the Pinware terrane occurred between 1050 and 985 Ma, as indicated mainly on the basis of zircon lower intercepts, but includin g some monazite data and constraints imposed by dated younger rocks, T his range of ages contrasts with the time span for Grenvillian metamor phism in the Lake Melville terrane, previously dated to between 1080 a nd 1000 Ma, and with which a newly determined age of 1047 +/- 2 Ma fro m a granodiorite dyke at the Mealy Mountains-Lake Melville terrane bou ndary conforms. In the Pinware terrane, near the end of Grenvillian or ogenesis, widespread alkalic magmatic activity occurred between 990 an d 980 Ma. Units investigated include an aegerine-bearing alkali-feldsp ar syenite emplaced at 991 +/- 5 Ma, a clinopyroxene-fayalite alkali-f eldspar syenite having a probable age of 985 Ma, and alkalic mafic dyk es emplaced at 985 +/- 6 Ma. Similar (although amygdaloidal) alkalic m afic dykes in the Lake Melville terrane may be slightly younger, havin g an age of 974 +/- 6 Ma. The short-lived alkalic activity was followe d by late-tectonic magmatism in the Pinware terrane at 983 +/- 3 Ma, w hich heralded post-tectonic granitoid plutons between 974 and 956 Ma. Cooling and uplift in the Pinware terrane is documented by titanite ag es between 972 +/- 5 Ma and 939 +/- 5 Ma. It is inferred that cooling occurred 30 million years sooner in the southeast Pinware terrane than near its northwest margin.