Emplacement history of the Pearly Gates anorthosite pluton and spatially related Tessiarsuyungoakh intrusion, and metamorphic petrology of the adjacent Tasiuyak paragneiss, northern Labrador

Tettelaar, Tanya Anne (2004) Emplacement history of the Pearly Gates anorthosite pluton and spatially related Tessiarsuyungoakh intrusion, and metamorphic petrology of the adjacent Tasiuyak paragneiss, northern Labrador. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Mesoproterozoic Nain Plutonic Suite consists of anorthositic and granitic intrusions, which were emplaced along an older suture zone marked by the Torngat Orogen in northern Labrador. This study concerns the western margin of the Nain Plutonic Suite where the Pearly Gates Anorthosite pluton and spatially related Tessiarsuyungoakh intrusion, and the Makhavinekh Lake pluton, were intruded into the Paleoproterozoic Tasiuyak paragneiss of the Southeastern Churchill Province. Detailed geological mapping, petrography and U-Pb Thermal Ionization Mass Spectrometry dating of zircon and baddeleyite of the Pearly Gates Anorthosite pluton and Tessiarsuyungoakh intrusion were used to ascertain the mode of emplacement and crystallization ages of the major intrusive units. In addition, studies of the Paleoproterozoic regional metamorphism and Mesoproterozoic contact metamorphism of the Tasiuyak paragneiss were performed using the NaKFMASH petrogenetic grid to determine P-T conditions. The results of this thesis project indicate the following geological history of the study area. -- The Tasiuyak paragneiss contains evidence of the Paleoproterozoic regional metamorphic mineral sub-assemblage garnet + biotite + sillimanite. Detailed petrography of this sub-assemblage indicated that the prograde and retrograde history of this unit occurred within the sillimanite stability field and peak P-T conditions were constrained in the range of 7.2 to 10.2 kbar and 800° to 830°C. Prior to ca. 1363 Ma, reactivation of structures in the paragneiss provided fractures into which monzonitic and monzodioritic magmas intruded as sheets. The 1363 ± 3 Ma monzonite and 1360 ± 4 Ma monzodiorite formed the composite Tessiarsuyungoakh intrusion. This intrusion produced a small (< 20 m) contact aureole in the Tasiuyak paragneiss. Incipient development of the contact metamorphic sub-assemblage cordierite + orthopyroxene + spinel replaced the regional sub-assemblage of garnet + biotite + sillimanite. This suggests that the Tessiarsuyungoakh intrusion was a small body with a short-lived emplacement and crystallization history. The P-T estimate of this contact aureole is between 3 and 4 kbar and 775° and 800°C. -- Prismatic zircons included in plagioclase crystals from the Pearly Gates Anorthosite pluton yielded a maximum concordant crystallization age of 1370 ± 5 Ma. These crystals are interpreted as having formed at depth at ca. 1370 Ma, prior to emplacement at a mid-crustal level. The earliest evidence of anorthosite emplacement is a relatively undeformed anorthosite body, herein called the Fraser Canyon Anorthosite, which intruded into the Tessiarsuyungoakh intrusion at 1355 ± 1.3 Ma. It is unknown if the Fraser Canyon Anorthosite is a part of the Pearly Gates Anorthosite pluton, however, both have similar characteristics such as an inner zone of massive anorthosite and an outer zone of layers of anorthosite and norite. The outer zone is interpreted as having crystallized relatively fast and the inner zone as having cooled relatively slowly. During crystallization of the outer zone, syn-emplacement deformation occurred along the contact between the Tessiarsuyungoakh intrusion and Pearly Gates Anorthosite pluton. Anhedral zircon fragments from the Pearly Gates Anorthosite pluton yielded an age of 1335 +7/-3 Ma, of which the uppermost error limit of 1342 Ma is interpreted as approximating the final crystallization age of the pluton. Norite bodies that intruded the Pearly Gates Anorthosite pluton at ca. 1340 Ma are undeformed and indicate that the anorthosite had completely crystallized and deformation had ceased by this time. -- At ca. 1322 Ma, the Makhavinekh Lake pluton intruded into the Tasiuyak paragneiss and produced a contact aureole 4 km wide with cordierite + orthopyroxene + spinel replacing regional garnet + biotite + sillimanite. The temperature gradient along this aureole ranged from 675°C at the 4 km limit of the contact aureole, to at least 850°C at the contact with the pluton, and isobaric conditions were between 3 to 4 kbar.

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