Geology, geochemistry and tectonic importance of the Horse Cove complex: a late neoproterozoic igneous complex in the eastern Avalon Zone, Newfoundland

Skipton, Diane R. (2011) Geology, geochemistry and tectonic importance of the Horse Cove complex: a late neoproterozoic igneous complex in the eastern Avalon Zone, Newfoundland. Masters thesis, Memorial University of Newfoundland.

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Abstract

Mafic-to-felsic intrusive rocks of the Horse Cove Complex have been mapped in detail in the area surrounding Bauline on the east coast of Conception Bay in the Avalon Zone, Newfoundland. The Horse Cove Complex is characterized by a swarm of mafic-to- felsic dykes that coincide with the extrapolated trace of the Topsail Fault along the east coast of Conception Bay and are hosted by mafic submarine volcanic rocks and, locally, by diorite. In the Bauline area, the Horse Cove Complex is hosted by granodiorite, which has been dated at 625 ± 1.5 Ma using U/Pb zircon geochronology with the chemical abrasion thermal ionization mass spectrometry (CA-TIMS) method. This granodiorite is correlative to the felsic plutonic rocks of the Holyrood and White Hills intrusive suites, located south of Bauline, within and along the eastern margin of the Holyrood Horst. Based on detailed mapping and petrography in the Bauline area, the Horse Cove Complex has been divided into ten rock units. These rocks units include: feldspar porphyry, gabbro and diorite, which are the host rocks to the dyke swarm, and eight rock units that represent dykes of mafic-to-felsic composition. The Horse Cove Complex has been affected by greenschist-facies metamorphism, resulting in partial replacement of igneous mineral assemblages by greenschist-facies minerals, but igneous minerals and textures are still recognizable. The age of magmatism in the Horse Cove Complex has been bracketed by CA-TIMS U/Pb zircon ages of feldspar porphyry (580.6 ± 2.0 Ma) and an andesitic dyke (578.4 ± 2.3 Ma). Since field relationships indicate that these rock units represent the oldest and youngest datable rocks in the Complex, magmatism occurred over a period of 6.5 Ma or less: from 582.6 to 576.1 Ma, at maximum age limits. This age of magmatism overlaps, within uncertainties, with several magmatic events on the NE A val on Peninsula, including felsic volcanic and fine-grained intrusive rocks along the eastern margin of the Holyrood Horst and near Cape St. Francis. Based on lithogeochemistry, feldspar porphyry and rhyolitic dykes in the Horse Cove Complex appear to represent volcanic arc magmatism and may be co-magmatic. The mafic to intermediate dykes and host rocks in the Horse Cove Complex are comprised of calc-alkaline and tholeiitic rocks that exhibit a range of compositions, from rocks with EM ORB-like geochemistry to rocks that are LREE-enriched and have negative Nb anomalies, con1parable to subduction-related calc-alkaline basalts and andesites. There is an overall progression from rocks with E-MORB-like geochemistry to rocks with arc signatures, but exceptions exist due to the episodic emplacement and complex crosscutting relationships of the dyke swarm. Seven rocks from the Horse Cove Complex, including a rhyolitic dyke and several mafic-to-intermediate rocks, have εNd values (at 580 Ma) ranging from +4.1 to +6.4. Thus, these rocks appear to have depleted mantle sources that have undergone various degrees of mixing or assimilation with older, LREEenriched sources, such as continental crust and/or sediments in a subduction zone. The interpreted paleo-tectonic setting of the Horse Cove Complex is a back-arc basin environment, in which rocks with LREE-enriched mantle sources and subductioncontaminated sources were emplaced side-by-side and closely in time. The Horse Cove Complex may represent the last phase of subduction-related magmatism in the eastern Avalon Zone in Newfoundland prior to deep marine, deltaic and alluvial fan sedimentation during the late Neoproterozoic.

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