Geology of Mesoproterozoic anorthosite intrusions in the vicinity of Nain, Labrador

Voordouw, Ronald Jaap (2006) Geology of Mesoproterozoic anorthosite intrusions in the vicinity of Nain, Labrador. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

This thesis investigates aspects of the 'anorthosite problem' through geological mapping and petrographic study of Proterozoic anorthositic rocks in the Nain batholith (NB), Labrador. The aspects of this problem to be investigated include the tectonomagmatic setting of this batholith, and the controls on ascent, emplacement and crystallization of the anorthositic rocks. Secondary emphasis is placed on the mineral and whole rock chemical evolution of these anorthositic rocks over time. Brief descriptions of Mesoproterozoic gabbronoritic, Fe-rich gabbroic, Fe-rich dioritic rocks and granitoid rocks that are spatially associated with anorthositic rocks are also presented, as well as descriptions of Paleoproterozoic and Archean country rock. -- The main findings of the thesis are based on 1:20 000 scale geological mapping over a 400 km² area that was previously covered only at reconnaissance scale. This area comprises four belts of Paleoproterozoic and/or Archean country rock and three Mesoproterozoic 'centered intrusive complexes'. These intrusive complexes are here referred to as 'centered' because each consists of a 'core' that is enveloped by a 'margin'. The core is sub-horizontally oriented and consists of anorthositic rocks, whereas the margin is sub-vertically oriented and consists of anorthositic, gabbronoritic +/- monzonitic, troctolitic and Fe-rich gabbroic rocks. The centered intrusive complexes in the study area are referred to as Tikkoatokak, Sophie and Kikkertavak. -- The four belts of country rock are here referred to as the Anaktalik Brook, Kangilialuk Lake, Nain Hill and Kauk Harbor country rock belts. Each of these belts contains gneiss and granulite that is of Paleoproterozoic +/- Archean age and structures that strike parallel to Paleoproterozoic structures of the Tomgat orogen. Compositional layering in the Nain Hill country rock belt shows an arcuate strike that is interpreted as a Paleoproterozoic structure that was rotated into a Mesoproterozoic east-west shear zone. This Mesoproterozoic shear zone was active during emplacement of the Nain batholith. This interpretation contrasts with previous work that suggested the arcuate strike was formed by a pre-Mesoproterozoic episode of folding or by the intrusion of Mesoproterozoic anorthositic rocks. -- The margins of all intrusive complexes in the study area were deformed in subvertical, north-south striking, shear zones and/or sub-vertical, east-west striking, shear zones. Field relations and compiled U-Pb ages indicate that this deformation was synchronous with emplacement of the Nain batholith (1365-1270 Ma). Map compilation is used to show that these shear zones continue north of the study area. Field relations of strongly and weakly deformed plutonic rocks in these shear zones indicate that Mesoproterozoic deformation on individual shear zones was episodic, which is consistent with new and compiled U-Pb ages that show north-south deformation occurred between 1351 and 1313 Ma and east-west deformation occurred between 1340 and 1311 Ma. -- Mesoproterozoic plutonic rocks in the study area were deformed in Mesoproterozoic north-south and east-west shear zones. The north-south striking shear zones lie adjacent to and strike parallel to Paleoproterozoic structures in the country rock, and it is suggested that the shear zones developed on re-activated Paleoproterozoic structures. The east-west striking shear zones strike parallel to the orientation of Mesoproterozoic faults in the Gardar Province of western Greenland, which comprised part of Mesoproterozoic Laurentia with the rocks in the study area. In addition to previous work on the tectono-magmatic setting of the Nain batholith, the findings indicate that emplacement of this batholith was contemporaneous with: (a) development of north-south shear zones, (b) development of east-west shear zones, (c) lithospheric thinning, graben development, and (d) back-arc extension. These findings are in contrasts to a mantle plume-related setting that has been proposed for petrogenesis of the Nain batholith. -- The margins of Mesoproterozoic intrusive complexes in the study area are: (a) composite, (b) bound by shear zones, (c) elongate parallel to shear zones, (d) are structurally controlled at shear zone intersections, and (e) contain xenoliths of strongly deformed plutonic rocks. These features are all characteristic of shear zone assisted melt transport and emplacement. These findings contrast with the widely held belief that the emplacement of anorthositic magmas is diapiric, and suggests a liquid-state rheology for anorthositic magmas during ascent and emplacement. -- The Tikkoatokak centered intrusive complex contains the oldest plutonic rocks in the study area. This intrusion shows a sub-rectangular and bell-jar form that suggests emplacement into a ductile crust that was controlled by sub-vertical and north-south striking shear zones. The northwestern part of the Kikkertavak centered intrusive complex contains the youngest plutonic rocks in the study area. This intrusion shows a sub-rectangular and convex down form that suggests emplacement into a dilating brittle crust that was controlled by steeply dipping, east-west striking, shear zones. The Sophie centered intrusive complex is intermediate in age with respect to the Tikkoatokak and NW Kikkertavak: centered intrusive complexes and contains plutonic rocks that were deformed in both north-south and east-west shear zones. These findings suggest that over the time span that covers emplacement of the Tikkoatokak: (> 13.33 Ma), Sophie and NW Kikkertavak: (between 1318 and 1311 Ma) centered intrusive complexes, the rheology of the crust changed from ductile to brittle and the structural controls on emplacement changed from north-south to east-west shear zones. These findings contrast with previous studies that suggested crustal uplift occurred in the waning stages (<1290 Ma) of emplacement of the Nain batholith. -- Anorthositic rocks in the study area show granular and/or poikilitic textures and microstructures that indicate high-T deformation and/or recrystallization. In addition, anorthositic rocks show relatively high variation in whole rock Mg/(Mg+Fe) in comparison to the range in normative plagioclase compositions, and are cut by ferrodiorite dikes that show sharp to gradational contacts. These characteristics are used to suggest that the crystallization of anorthositic magmas involved the formation of crystal frameworks and the compaction of these frameworks. Compaction may have been related to tectonic activity on the same shear zones that controlled the ascent and emplacement of magmas into intrusive complexes. It is suggested that this compaction also formed mottled rocks, primary layering and foliated rocks. In addition, compaction may also have produced planar aggregates of pyroxene crystals, calcic rims on plagioclase crystals and elevated wt% Al₂O₃ in the orthopyroxenes of anorthositic rocks. These findings indicate that the textures and structures in anorthositic rocks were produced in magmas that exhibited solid-state rheology and contrasts with previous work that suggests liquid-state processes (convection, crystal sedimentation) formed textures and structures during the crystallization of anorthositic magmas. -- Anorthositic rocks in the Tikkoatokak centered intrusive complex contain high abundances of Na-rich plagioclase, wt% K₂O, ppm Ba and ppm Sr in comparison to anorthositic rocks in the Sophie and Kikkertavak centered intrusive complexes. In contrast, the Sophie and Kikkertavak centered intrusive complexes contain high wt% CaO and Mg/(Fe+Mg). All anorthositic and gabbronoritic rocks in the Tikkoatokak centered intrusion lack olivine, whereas minor abundances of anorthositic and/or gabbronoritic rocks in the Sophie and NW Kikkertavak centered intrusive complexes contain olivine. These findings indicate that relatively Na-, K-, Ba- and Sr-rich anorthositic rocks were emplaced prior to Ca-rich, high Mg/(Fe+Mg) +/-olivine-bearing anorthositic rocks. These characteristics are used to suggest that anorthositic rocks with a greater crustal component were emplaced prior to anorthositic rocks with a lower crustal component. -- Mineral and whole rock chemistry of anorthositic rocks from this study were compared to previous mineral chemistry, whole rock chemistry and geochronology studies. The mineral and whole rock chemistry characteristics of NB noritic anorthosite and NB troctolitic anorthosite are summarized, and follow a sub-division presented by Xue and Morse (1993). The result of this compilation is used to divide emplacement of the Nain batholith into four episodes. Episode 1 (1365 - 1330 Ma) involved the emplacement of NB noritic anorthosite and episode 2 (1330 - 1310 Ma) involved the emplacement of NB troctolitic anorthosite +/- NB noritic anorthosite. Episode 3 (1310 - 1290) involved the emplacement of troctolitic rocks in the northeastern part of the Nain batholith and emplacement of anorthositic and granitic rocks in the southern part of the Nain batholith. Episode 4 (1290 - 1270 Ma) involved the emplacement of peralkaline volcanic rocks and basaltic dikes. -- This study shows that anorthositic rocks in a Proterozoic batholith were generated and emplaced during a localized period of tectonic activity. This activity was accommodated on sub-vertically oriented and north-south, or east-west, striking shear zones, which also controlled the transport and emplacement of the magmas. The magmas that formed anorthosite were likely 'leucogabbroic' (65-90% plagioclase, 10-35% pyroxene + olivine + Fe-Ti oxide) in composition, and the crystallization of anorthosite from such magmas involved compaction and the removal of interstitial liquid. These findings suggest a positive correlation between tectonic activity and the petrogenesis of Proterozoic batholiths with anorthositic rocks, and it is hoped that this study will inspire further research of this correlation.

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