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

Kerr, Andrew (1989) Early Proterozoic granitoid magmatism and crustal evolution in the Makkovik province of Labrador: a geochemical and isotopic study. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

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 peraluminous in composition, and depleted in Zr, Nb, Hf, REE, Zn and fluorine, indicating that they had quite different sources from their Makkovikian counterparts. -- The Makkovikian assemblage displays geographically systematic Nd isotopic variations. In the west, negative εNdCHUR indicates ancient (probably Archean) crustal material in sources, but values are too high to permit derivation entirely from such material. These rocks probably represent mixtures of juvenile, mantle-derived, magma and older polycyclic Archean crust. In the east, positive εNdCHUR indicates juvenile, Proterozoic sources, and gneissic rocks representing possible basement have depleted-mantle Nd model ages of ca. 2100 Ma. Discrete, high-silica, "A-type" granite plutons show an east-west shift in εNdCHUR from +6 to -6, but retain remarkably similar elemental characteristics in both domains. It is concluded that magmas in the east also contained a significant crustal component, but that this component had a short crustal residence period. Nd isotopic data thus define a fundamental crustal boundary between an Archean Craton and a younger (accreted?) Proterozoic crustal province. Makkovikian magmatism accompanied and followed accretion of the younger domain, and is suggested to have resulted from emplacement of anhydrous, hot, mantle-derived mafic magmas into the lower crust, where they crystallized, assimilated and mixed with crustal rocks and melts thereof. -- Labradorian rocks mostly have εNdCHUR of +1 to -2, regardless of composition or location. Olivine-bearing mafic rocks, which cannot represent crustally-derived magmas, have εNdCHUR of ca. +1, significantly below postulated values for concurrent depleted mantle. It is suggested that this crustal component was introduced to depleted mantle via subduction of continent-derived sediment, as suggested for modern arc magmas. However, some Labradorian granites that lack mafic parents could be derived by anatexis of Makkovikian "mixed" crust, or juvenile material underplated during Makkovikian events. Labradorian magmatism is tentatively interpreted as a distal-arc assemblage above a subduction zone that records renewal of tectonic activity following Makkovikian terrane accretion and magmatism. -- The isotopic characteristics of both assemblages underline the importance of the Early Proterozoic as a period of new crustal growth. Makkovikian magmatism reorganized previously generated crust, and added significantly to it, particularly in the "juvenile" eastern domain. Largely ensialic crustal growth of this type may have been more important in a hotter, Proterozoic Earth and, in concert with ensimatic crust generation linked to subduction, may account for the high crustal growth rates deduced for this crucial period in planetary evolution. -- The transitional characteristics of the Makkovikian assemblage may also be a function of greater heat flow. Recent models for "anorogenic" magmatism invoke insulation of the mantle by newly-accreted crust or aggregated megacontinents, resulting in mantle upwelling, plume activity, magmatism and (eventually) rifting. In a hotter Proterozoic Earth, the time-lag between aggregation and mantle upwelling would be shorter, and conditions now associated with "anorogenic" magmatism would prevail widely in post-orogenic environments. It is perhaps significant in this respect that worldwide Lower Proterozoic (1900 - 1700 Ma) orogenic events have recently been interpreted to record the assembly of an early Pangean-type supercontinent. -- KEY WORDS: Labrador; Precambrian Proterozoic; Granitoids; Granite; Batholiths; Petrology Ge chemistry; Comparative; Isotopes; Neodymium; Petrogenesis; Crustal Evolution.

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