The Wild Bight Group, Newfoundland Appalachians : a composite Early to Middle-Ordovician ensimatic arc and continental margin arc-arc rift basin

MacLachlan, Kate (1998) The Wild Bight Group, Newfoundland Appalachians : a composite Early to Middle-Ordovician ensimatic arc and continental margin arc-arc rift basin. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The Wild Bight Group (WBG) and South Lake Igneous Complex (SLIC) represent a peri-Gondwanan Ordovician accreted oceanic terrane of the central Newfoundland Appalachians. The Wild Bight Group is a sequence of volcanic and volcaniclastic rocks, and the South Lake igneous complex is comprised of layered gabbro and sheeted dykes intruded by hornblende diorite and tonalite plutons. -- This study involves detailed mapping (~ 1:12,500) in the South Lake igneous complex and adjacent rocks of the eastern Wild Bight Group, combined with petrography, geochemical, and Sm-Nd isotopic studies, and U-Pb geochronology. It shows that there are two temporally and genetically distinct sequences of volcanic, volcaniclastic and sedimentary rocks in the WBG, separated by up to 10 Ma. The older sequence is genetically related to plutonic rocks of the South Lake igneous complex, and these older rock packages have been structurally interleaved with the younger sequence during subsequent deformation. The younger sequence has also been structurally imbricated and does not represent a simple conformable sequence in its present configuration. -- Magmatic rocks related to the older sequence of the WBG formed predominantly between 486 +/-4 and 489 +/-3 Ma (Tremadoc to early Arenig), and range in composition from normal island arc tholeiitic basalt (IATs), to incompatible element-depleted low-Ti, high-Mg IATs and boninites, and high-Si, low-K rhyolite/tonalite. They are interpreted to represent the initiation and stabilization of a primitive ensimatic oceanic arc. Sm-Nd isotope systematics in the depleted IATs and boninites, show an apparent decoupling of isotopic and geochemical characteristics, which require a complex source or melt generation process for these rocks. The geochemical, isotopic and field relationships in the older volcanic sequence and SLIC provide some insight into tectonomagmatic processes in volcanic arcs in general. -- The younger sequence of the Wild Bight Group comprises two volcanic successions which are stratigraphically separated. The lower one has a calc-alkalic geochemical affinity, and its age is confined to 472 +/-3 Ma (late Arenig), by felsic tuffs within the succession. The upper volcanic succession has enriched tholeiitic to alkalic within-plate geochemical signatures, and geochemically related gabbro sills are dated at 470 +/-5 and 471 +/-4 Ma. Geochemical and isotopic characteristics suggest that these two volcanic sequences were produced from a similar mantle source. Sm-Nd isotope systematics indicate that the calc-alkaline rocks were contaminated by an old continentally-derived component. The younger sequence of the Wild Bight Group is interpreted to represent a volcanic arc that formed on or immediately adjacent to thinned continental crust of the Gondwanan margin, and was subsequently rifted. -- Blocks in melanges and debris flows within the younger succession have geochemical signatures which indicate that they were derived from both the older and younger volcanic sequences of the WBG. This suggests that the older arc formed part of the substrate on which the younger sequence developed. Intrabasinal uplifts exposed the older substrate which then provided a source of detritus for the younger basin. The occurrence of a Precambrian detrital zircon in a volcaniclastic unit of the younger sequence suggests that there was an even older component to the uplifted substrate.The uplifts are interpreted to result from horsts and grabens formed in an extensional back arc and subsequent arc rift setting. In most places the contact between the older and younger sequences is structurally modified, although locally a stratigraphic contact may be preserved. -- The tectonomagmatic evolution of the WBG and SLIC has four main stages: firstly, formation of a primitive ensimatic arc outboard of the Gondwanan margin, by westward (present coordinates) subduction; secondly arc/continental margin "collision", which involved attempted subduction of the thinned continental margin, and resulted in the termination of westward subduction; thirdly, initiation of a second subduction zone outboard of the continental margin, but with the opposite polarity, such that the subsequent arc developed on the composite margin; and finally, arc rifting which formed an extensive marginal basin. -- The Wild Bight Group has traditionally been correlated with the Exploits Group to the east. These groups are interpreted to represent different parts of the same late Arenig to Llanvirn, arc/back-arc/arc-rift system, and were probably also contiguous during generation of the older ensimatic arc. The eastern WBG is interpreted to represent the remnant arc following arc rifting, and the western WBG is interpreted to represent the main rift basin. The Exploits Group probably formed in the pre-rift back arc basin in close proximity to the continental margin. -- There are lithological, structural and stratigraphic differences between ophiolites and volcanic rock packages in the Exploits Subzone interpreted to be correlative with the older sequence of the WBG, and late Arenig to Llanvirn volcanic/volcaniclastic and epiclastic sequences which are interpreted to be correlative with the younger sequence of the WBG. These differences are interpreted to be related to the presence of a promontory on the Gondwanan margin, as suggested by previous workers. The Exploits Subzone can be divided into two Ordovician tectonic zones, one in the east and south which underwent an important compressional event during the middle to late Arenig (Penobscot Orogeny), but only minor late Arenig to Llanvirn/Llandeilo extension, and the other in the north and west with the opposite characteristics. The boundaries of these zones are likely transitional, but roughly coincide with Silurian structures such as Noel Pauls Line and Dog Bay Line, which have been used to subdivide the Dunnage zone in the past.

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