Coastal Complex Research Articles

New U-Pb zircon ages of plagiogranites from the Coastal Complex ophiolite and Twillingate batholith, Newfoundland: Evidence for the oldest and overlapping silicic magmatism in the nascent Cambrian peri-Laurentia forearc and arc terranes

The Coastal Complex (CC) and Bay of Islands Complex (BOIC) represent two supra-subduction zone (SSZ) forearc ophiolitic assemblages of different ages within the Taconic Humber Arm Allochthon of the Western Newfoundland Appalachians. To further constrain age relationships between the CC and BOIC and earliest initiation of Iapetus subduction, we acquired LA-ICP-MS U-Pb zircon ages for eleven plagiogranites sampled along strike of the CC belt. These ages combined with three prior legacy ages establish an age range of 514.3–502.7 Ma for the six CC plagiogranite plutons. The oldest precise ca. 514.3 Ma CC pluton age represents the oldest Taconic arc-related plutonic activity yet documented in Western Newfoundland and provides a minimum age for the intruded CC ophiolite basement and initiation of westerly vergent subduction within the peri-Laurentian Iapetus Ocean. This confirms the Early Cambrian CC mafic–ultramafic basement is significantly older than the juxtaposed SSZ BOIC, which formed by later rifting of the CC forearc along younger forearc spreading center in the Late Cambrian by ca. 488.3 Ma. After subduction initiation a prolonged early period CC forearc silicic magmatism lasted at least ∼ 11.6 Ma until ca. 502.7 Ma, a range consistent with age spans of magmatism in modern juvenile forearcs. We also reinvestigated the Central Newfoundland Twillingate Granite batholith thought to be one of the earliest major felsic batholiths formed within the adjacent juvenile arc massif of the Notre Dame Subzone, part of the same peri-Laurentian arc. We determined a precise 3-sample composite age of 504.3 ± 1.8 Ma that signals the formation of earliest Notre Dame arc edifice developed by Middle Cambrian. It overlaps the end-stage CC forearc plagiogranite magmatism and places a minimum mid-Cambrian age constraint on intruded ophiolitic arc tholeiitic and boninitic pillow lavas of the Sleepy Cove and correlated Lushs Bight volcanics, suggesting basement continuity with the CC.

Geochemical and geological evidence bearing on the origin of the Bay of Islands and Coastal Complex ophiolites of western Newfoundland

Suggestions that ophiolitic rocks of the Bay of Islands Complex (BOIC) and Coastal Complex (CC) of southwestern Newfoundland were formed at a seafloor spreading center are supported by: 1. (1) whole-rock trace and major element data from diabases and basalts, 2. (2) spinel mineral chemistry from ultramafic rocks, 3. (3) the internal geology of the ophiolite terranes, and 4. (4) regional geologic and tectonic constraints in the western Newfoundland Appalachians. Diabase and basalt samples from the BOIC and the northern CC massifs are tholeiitic and geochemically indistinguishable from present-day mid-ocean ridge basalts (MORBs). Many diabases from the Lewis Hills massif (the southernmost part of the CC) are also identical to MORBs, but there is also an unusual suite of diabases from the Lewis Hills that is unlike any other suite yet reported from a variety of tectonic settings including island arcs, ocean islands or spreading centers. This suite is interpreted to have been produced by unusual magma generation processes in the deep structural levels of an oceanic ridge-ridge transform fault. Approximately 90% of spinels from ultramafic rocks in the BOIC and CC have Cr/(Cr + Al) ratios that are less that 0.60, comparable to those values reported from ultramafic rocks of the present-day ocean basins. These geochemical constraints, in combination with previously-described geological relationships within the ophiolite terranes and regional tectonic relationships in the western Newfoundland Appalachians, suggest that the BOIC and CC ophiolites formed within a main ocean basin (i.e., the early Paleozoic Iapetus Ocean) along a seafloor spreading center and ridge-ridge transform fault, respectively. There is little evidence to suggest formation in either an island arc or back-arc basin tectonic setting. As such, the BOIC and CC ophiolites are probably well suited for detailed petrologic, structural and geophysical studies to determine the processes involved in the generation and evolution of contemporary oceanic crust and upper mantle.

Reconstruction of the geometry of accretion during formation of the Bay of Islands Ophiolite Complex

Two types of highly allochthonous ophiolitic terrains crop out adjacent to one another in the Bay of Islands region of southwestern Newfoundland, Canada. The first, called the Coastal Complex (CC), is exposed as a long, linear belt along the west coast. It consists of a highly deformed ophiolitic assemblage interpreted as oceanic crust and upper mantle with a previous transform fault tectonic history. The second, called the Bay of Islands Complex (BOIC), crops out to the east and consists of a series of four relatively undeformed ophiolitic massifs interpreted as oceanic crust and upper mantle with a more normal seafloor spreading tectonic history. An autochthonous relationship can be demonstrated between the two types of ophiolitic terrains in the southernmost of these massifs, indicating that each evolved along a single ridge‐transform system. The internal structure of both the BOIC and the CC is consistent with the above interpretations. The NE‐SW structural trends in the CC transform are oriented approximately parallel to the sea‐floor spreading direction inferred from the average orientation of the sheeted diabase dikes within the BOIC. One‐way chilling statistics from the sheeted diabase dike unit, growth directions of dendritic crystals along inclined comb layers in the plutonic section, and the sense of shear along the CC transform all indicate that the BOIC was accreted on the NE side of a NW‐SE trending spreading center (present geographic reference frame). Juxtaposition of the BOIC with the NE‐SW trending CC occurred at a ridge‐transform intersection. The transform exhibited a left‐lateral offset. Magma chamber profiles have been identified in the layered gabbroic sections of the BOIC by using the patterns outlined by the attitudes of fine‐scale igneous layering. These patterns depict arcuate structures which appear to define roughly a cross‐sectional view of a systematically oriented half‐cylinder; the long axis of which is oriented parallel with the plane of the sheeted dikes and major lithologic unit contacts. Thus we believe that a large cylindrical‐shaped crustal level subaxial magma chamber extended in a direction parallel with the ridge axial‐plane at the time of formation of the BOlC. The CC marks the abrupt truncation of this long, linear magma chamber. Petrologic constraints demonstrate that the ultramafic and transition zone cumulates which form the base of the plutonic section, crystallized at a range of high to intermediate pressures possibly along a long narrow conduit extending to 30‐km depth beneath the crustal level magma chamber. After formation, these cumulates were deformed at high temperatures in the zone of mantle upwelling and divergence, then tectonically transported to crustal or near crustal levels to form the basement upon which the remainder of the ophiolite stratigraphy was accreted. A geometric and tectonic model is presented for the accretion process responsible for formation of the Bay of Islands Ophiolite.