UPb geochronological constraints on the geological evolution of the Pinware terrane and adjacent areas, Grenville Province, southeast Labrador, Canada

Abstract

UPb geochronological data show that the Pinware terrane and adjacent areas in the Grenville Province in southeast Labrador experienced three orogenic events: Labradorian, Pinwarian and Grenvillian. Labradorian (1710-1600 Ma) rocks in the Pinware terrane, previously only known from a dated felsic volcanic enclave in a younger granite, are now recognized to be widespread and include both supracrustal units and granitoid intrusions. One quartzite was deposited between ∼1805 and 1500 Ma, the ages of the youngest detrital zircon and subsequent metamorphism, respectively, and includes 1878 and 2720 Ma detrital zircon suggesting derivation of source material from pre-Labradorian Laurentia. A volcaniclastic(?) unit is interpreted to have an age of 1637 ± 8 Ma. Two Labradorian quartz monzonite intrusions yielded ages of 1650 +18 −9 Ma and 1649 ± 7 Ma. Coupled with previous information, these data indicate coeval plutonism, volcanism and sedimentation taking place at the same time as formation of the Trans-Labrador batholith farther north. The Pinwarian Orogeny is now extended to include events between 1510 and 1450 Ma. Granitoid rocks of the Upper Paradise River pluton, a large AMCG suite in the Mealy Mountains terrane, has been dated at 1495 ± 7 and 1501 ± 9 Ma, making it the only suite of this type and age known in North America. Migmatitic quartz monzonite, dated at 1450 +15 −21 Ma, provides the first proof of high-grade metamorphism in the Pinware terrane, and, in conjunction with recent geochronological data from other parts of the eastern Grenville Province, justify upgrading the Pinwarian to orogenic status. Grenvillian metamorphism throughout the Pinware terrane occurred between 1050 and 985 Ma, as indicated mainly on the basis of zircon lower intercepts, but including some monazite data and constraints imposed by dated younger rocks. This range of ages contrasts with the time span for Grenvillian metamorphism in the Lake Melville terrane, previously dated to between 1080 and 1000 Ma, and with which a newly determined age of 1047 ± 2 Ma from a granodiorite dyke at the Mealy Mountains-Lake Melville terrane boundary conforms. In the Pinware terrane, near the end of Grenvillian orogenesis, widespread alkalic magmatic activity occurred between 990 and 980 Ma. Units investigated include an aegerine-bearing alkali-feldspar syenite emplaced at 991 ± 5 Ma, a clinopyroxene-fayalite alkali-feldspar syenite having a probable age of 985 Ma, and alkalic mafic dykes emplaced at 985 ± 6 Ma. Similar (although amygdaloidal) alkalic mafic dykes in the Lake Melville terrane may be slightly younger, having an age of 974 ± 6 Ma. The short-lived alkalic activity was followed by late-tectonic magmatism in the Pinware terrane at 983 ± 3 Ma, which heralded post-tectonic granitoid plutons between 974 and 956 Ma. Cooling and uplift in the Pinware terrane is documented by titanite ages 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.