Early Proterozoic granitoid magmatism and crustal evolution in the Makkovik province of Labrador: a geochemical and isotopic study

• Main Author: Kerr, Andrew
• Format: Thesis
• Language: English
• Published: Memorial University of Newfoundland 1989
• Subjects: Makkovik
• Online Access: https://research.library.mun.ca/6795/; https://research.library.mun.ca/6795/1/AndrewKerr.pdf; https://research.library.mun.ca/6795/3/AndrewKerr.pdf

The Lower to Middle Proterozoic Trans-Labrador Granitoid Belt in the Makkovik Structural Province is divided on the basis of field relationships and geochronology into Makkovikian and Labradorian plutonic assemblages, representing approximate time intervals of 1840 - 1720 and 1670 - 1600 Ma respectively. The Makkovikian assemblage includes both syn- and post-tectonic associations, but precise U-Pb zircon ages mostly cluster around 1800 Ma, suggesting a single main intrusive episode that transcended the later deformation associated with the Makkovikian orogeny. The Labradorian assemblage is not associated with local deformation or metamorphism, but is probably a distal effect of the ca. 1650 Ma Labradorian orogeny, prevalent south of the study area. Both assemblages are broadly correlative with volcanic sequences of similar age and geochemical affinity. -- The Makkovikian assemblage is dominated by siliceous, potassic, commonly porphyritic, granites and alkali-feldspar granites, associated with subordinate monzonite to quartz syenite. High-silica granite suites are commonly fluorite-bearing. In geochemical terms, most Makkovikian granitoids are metaluminous to slightly peralkaline, Fe-enriched, and enriched in Zr, Nb, Hf, REE, Zn and fluorine. A comparative analysis suggests that they are transitional in character between Phanerozoic post-orogenic (post-collisional) assemblages and "A-type" or "within-plate" granitoid assemblages. Similarly enigmatic characteristics have been reported from Early Proterozoic granitoid batholiths elsewhere in the world. -- The subordinate, bimodal, Labradorian plutonic assemblage comprises gabbro-diorite-monzonite-syenite suites, derived from mafic parental magmas, and an assortment of siliceous, generally leucocratic, granitoid rocks. Mafic rocks resemble high-K calc-alkaline or shoshonitic basalts, and their associated felsic differentiates are enriched in Rb, Cs, Th, and U, as a consequence of protracted fractionation. Other Labradorian granites (s.s.) are metaluminous to ...