Lithology, geochemistry and geochronology of the Aillik Group and foliated granitic intrusions: implications on the formaton and early evolution of the Aillik domain, Makkovik province, Labrador

LaFlamme, Crystal (2011) Lithology, geochemistry and geochronology of the Aillik Group and foliated granitic intrusions: implications on the formaton and early evolution of the Aillik domain, Makkovik province, Labrador. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Makkovik Province of eastern Labrador is part of an accretionary orogenic belt that formed during the Paleoproterozoic Makkovikian orogeny. The Aillik domain represents one of three domains that make up the Makkovik Province and is composed of the Aillik Group, a package of Paleoproterozoic bi-modal volcano-sedimentary rocks, and abundant variably deformed Paleoproterozoic intrusive suites. The Aillik Group has experienced several phases of deformation and has been metamorphosed to lower amphibolite facies during the Makkovikian orogeny. Two areas, Middle Head and Pomaidluk Point, are the focus of this project and are used as case studies to assess and examine the Aillik Group with respect to the objectives as outlined below. Middle Head is dominated by arkosic sandstone, felsic tuff, rhyolite and basalt; whereas, Pomiadluk Point is composed primarily of felsic tuff and polymictic conglomerate with lesser preserved rhyolite and basalt. This study consists of detailed bedrock mapping in conjunction with: insitu SHRIMP U-Pb zircon geochronology, insitu LA-MC-ICPMS Hf isotopic geochemistry, major and trace element geochemistry, and whole rock Nd isotope geochemistry. These methods are used to: 1) constrain the timing of volcanism within the Aillik Group, 2) determine the source of magmatism, 3) resolve the overall tectonic setting in which the Aillik Group was deposited, and 4) briefly investigate the subsequent evolution of the Aillik domain. -- U-Pb SHRIMP zircon geochronology on felsic tuff samples yields magmatic ages that range from ca. 1852 at Middle Head to ca. 1854 - 1862 Ma at Pomiadluk Point. These U-Pb ages indicate that sections of the Aillik Group occurring 27 km from one another were deposited contemporaneously, and that felsic volcanism continued to as late as ca. 1852 Ma. A foliated Paleoproterozoic granite from Middle Head yields an age of 1805 ±4 Ma, which further constrains the timing of deformation within the Aillik Group as continuing past its emplacement. One population of inherited zircons occurs between 1880 and 1920 Ma and is interpreted to be xenocrystic in nature. -- Initial εHfs in zircon from a ca. 1852 Ma felsic tuff and ca. 1805 Ma granite at Middle Head range uniformly from -1.6 to -5.0 with felsic crustal extraction ages of ca. 2.4 to 2.6 Ga. In contrast, two felsic tuff samples at Pomiadluk Point with magmatic ages of ca. 1854 and ca. 1861 Ma have initial εHfs values in zircon that range from -4.8 to -11.9 in 18 of 20 grains analyzed, giving (felsic) crust formation ages of 2.6 to 3.0 Ga. A third sample from Pomiadluk Point, a ca. 1862 Ma foliated felsic tuff that outcrops between two conglomerate beds, contains magmatic zircons with initial εHfs that range from +2.1 to -1.6, and crust formation ages of 2.3 to 2.5 Ga. None of the felsic volcanic rocks analyzed from the Aillik Group show Hf-isotope evidence of derivation from the North Atlantic Craton as the model ages are too young. Additionally, none of the samples demonstrate their formation on a truly juvenile, 1.9-2.0 Ga crust with short residence times (<100 Ma), as might be expected for an intra-oceanic island arc origin. Inherited zircon grains (1880 to 1920 Ma) demonstrate a similar Hf isotopic signature and depleted mantle model ages as the magmatic grains. -- Trace and REE geochemistry demonstrate that felsic volcanic rocks of the Aillik Group as well as temporally distinct deformed granitic intrusions are 'A-type' in nature. Felsic volcanic rocks and a deformed monzogranite demonstrate a range in Nd isotopic signatures (εNd(T)= -1.1 to -5.0), which reflects partial melting of a heterogeneous felsic crust. Based on geochemical signatures, mafic volcanic rocks can be classified into two groups. Group A basalts have geochemical signatures that demonstrate flat rare-earth element pattern, consistent with melting of a depleted mantle source, and are composed of primary plagioclase and clinopyroxene and metamorphic amphibole and biotite. Group B basalts and mafic tuff are chemically more evolved, and composed of primary plagioclase and metamorphic amphibole and magnetite. The two different REE patterns seen in mafic volcanic rocks are interpreted to reflect a variable amount of crustal contamination. Furthermore, Group A basalts demonstrate systematically more elevated εNd(T) signatures (+2.8 to +4.3) than Group B basalts and mafic tuff (-3.5 to +2.2). Based on mixing models, mafic magmas of the Aillik Group are determined to have formed by mixing of the depleted mantle with a small to moderately significant amount (5 to 35%) of the felsic volcanic rocks of the Aillik Group. -- The combination of early clastic sedimentation, bimodal volcanism and geochemical signatures of felsic and mafic melts suggests that the ca. 1883 to 1852 Ma Aillik Group formed in a continental back-arc setting, forming from a crust that had an age range of at least 700 Ma, including both Paleoproterozic and Late Archean components. The similar Hf isotopic signatures of the Cross Lake granite, the felsic volcanic rocks of the Aillik Group, and the xenocrysts in the felsic volcanic rocks of the Aillik Group confirms that the Aillik domain was generated from the same basement over 115 million years.

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