Proterozoic southward accretion and Grenvillian orogenesis in the interior Grenville Province in eastern Labrador: Evidence from U-Pb geochronological investigations

摘要

This study is a fascinating example, from the eastern Grenville Province, of unraveling the triple challenges inherent in the geochronological investigation of any multi-orogenic region, namely inheritance, em placement and subsequent metamorphism. The existence of hitherto unknown pre-Labradorian crust within allochthonous Grenvillian terranes is demonstrated; formerly unrecognized spatial relations between Labradorian and Pinwarian crust are outlined; and a new perception of mid-Grenvillian magmatic and metamorphic events is presented. The pre-Labradorian crust has an age range of 1800-177OMa and consists of calc-alkaline plutonic rocks intruding earlier supracrustal assemblages containing Archean and early Paleoproterozoic inherited zircons. Evidence is provided from dated enclaves and inheritance in younger rocks that similar-aged pre-Labradorian crust once existed farther south. The geochronologically defined geographic extent of early Labradorian crust having a 1680-1655 Ma age range is expanded. This crust is intruded by the mid-Labradorian Mealy Mountains Intrusive Suite, for which new results confirm its 1650-1630 Ma time of emplacement. Pinwarian orthogneiss, dated between 1513 and 1496 Ma and previously unknown in the region, is now recognized to be common. Sufficient data exist to define, for the first time, a boundary between Labradorian and Pinwarian orthogneiss (cryptic in the field), establishing the presence of Pinwarian-aged crust to the southwest, lacking Labradorian inheritance except close to the boundary. A belt of layered mafic, anorthositic and monzogranitic intrusions is situated close to the boundary. Current data favour a Pinwarian age for emplacement of these rocks. Mid-Grenvillian plutonism is demonstrated by 1043 Ma ages from two K-feldspar megacrystic granodiorite bodies. They are situated on either flank of a crustal-scale, northeast-trending, northeast-closing fold. The fold, which is 40 km wide and at least 160-km long and exposes Pinwarian orthogneiss in its core, post-dates emplacement of the 1043 Ma intrusions. The fold probably formed at the peak of Grenvillian tectonism in the region, between 1030 and 1015 Ma, and certainly before intrusion of weakly deformed granitoid rocks at 992 Ma. Final activity included the emplacement of late- to post-Grenvillian granitoid plutons between 964 and 951 Ma. A pluton having a 951 Ma age is the youngest known in the eastern Grenville Province. Cooling and stabilization was completed by ca. 940 Ma. Broader implications of the results, in conjunction with existing data, are that: (i) the northeasternmost part of the eastern Grenville Province was only peripherally affected by Grenvillian orogenesis, (ii) an east-northeast offshore extrapolation of the Grenville front as a major tectonic feature does not exist, (iii) high-grade metamorphic mineral assemblages in the Groswater Bay and Hawke River terranes are pre-Grenvillian, rather than an eastward expression of a Grenvillian high-pressure belt, and (iv) the north shore of the eastern St. Lawrence estuary represents across-orogen exposure, rather than orogen-parallel as previously supposed.