Abstract

Two minor intrusions within the Hawke River terrane in the Grenville Province, eastern Labrador, have upper intercept U–Pb ages as follows: (i) graphic for a megacrystic intermediate dyke and (ii) 1622 ± 3 Ma for a pegmatite dyke. The upper intercepts date time of emplacement and the lower intercepts (508 and 320 Ma, respectively) record the timing of early Paleozoic events in the region. A third sample, a sillimanite-bearing pelitic gneiss from the Paradise metasedimentary gneiss belt in the Hawke River terrane, yielded a range of ages between 1647 and 1627 Ma from seven single zircon analyses. The zircons in the metasedimentary gneiss are interpreted as detrital because of their variation in morphology, the range of ages obtained, and their extreme variation in U content. A detrital origin implies that the sedimentary protolith must have been deposited after 1627 Ma, and is therefore not the same age as morphologically similar metasedimentary gneiss that occurs as enclaves within pre-1670 Ma migmatized quartz diorite. The timing of the post-1627 Ma high-grade metamorphic event that subsequently affected the gneiss is not known. It is unlikely to have been Grenvillian, as other evidence denies the likelihood that the Hawke River terrane was affected by more than moderate Grenvillian metamorphism. Using the emplacement ages of the minor intrusions, coupled with previous U–Pb dating and unequivocal field relationships, the following history is proposed. An early (northward subduction?) event, which possibly should be defined as being pre-Labradorian, occurred at ca. 1710 Ma and included distal magmatism in the Makkovik Province. There were two short-lived, closely related calc-alkaline plutonic events at 1677 and 1670 Ma, then mafic dyke injection and migmatization. These events are interpreted to reflect the times of rapid island-arc formation (over a southward-dipping subduction zone) from a mantle reservoir having a chondritic isotopic signature. After local granitoid emplacement at 1663 Ma (succeeded by further mafic dyke injection and then megacrystic dyke injection at 1660 Ma?), there was a widespread, major felsic magmatic event at ca. 1650 Ma. This was followed by another phase of mafic dyke injection and amphibolite facies metamorphism, which ended by ca. 1646 Ma. These events are interpreted to record the time of accretion of the island arcs to proto-Laurentia. A final Labradorian felsic magmatic event occurred between 1632 and 1622 Ma and is considered to be related to postcollisional anatectic plutonism and crustal thickening. This crustal thickening resulted in erosion that led to the deposition and burial of sediments in the Paradise metasedimentary belt.

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