Ordovician volcanism and mineralization in the Wild Bight group, central Newfoundland: a geological, petrological, geochemical and isotopic study

Swinden, H. S. (1987) Ordovician volcanism and mineralization in the Wild Bight group, central Newfoundland: a geological, petrological, geochemical and isotopic study. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The Wild Bight Group, part of the Newfoundland Dunnage Zone or Central Mobile Belt, is a thick (probably more than 8 km.) sequence of dominantly epiclastic (~75%) and lesser volcanic (~25%) rocks which outcrops in and to the south of central Notre Dame Bay. The base of the group is not exposed. It passes conformably upward into fossiliferous shale with a Caradocian (Middle Ordovician) graptolite fauna. On this basis, it is considered as Early to Middle Ordovician in age. -- Volcanic rocks occur throughout the stratigraphic section of the Wild Bight Group. Eleven separate volcanic sequences are identified and sampled, most of which comprise dominantly or wholly mafic pillow lava. Less commonly, massive basalt or pillow breccia and associated mafic pyroclastic rocks are the dominant lithology. Felsic volcanic rocks occur in five of these sequences and volcanogenic sulphide deposits or prospects in four of them. The entire assemblage is intruded by fine to medium grained mafic sills and, less commonly, dykes, which are interpreted on field and geochemical evidence to be subvolcanic. -- The mafic volcanic rocks exhibit a greenschist facies metamorophic assemblage of chlorite - albite - quartz - epidote ± (actinolite, sphene, magnetite and calcite). The only primary mineral remaining is clinopyroxene. Comparison of secondary mineral chemistry with ancient and modern oceanic rocks and with experimental results are consistent with metamorphism at temperatures in the range 200°C to 280°C and low water/rock ratios. The local presence of secondary amphibole is interpreted to reflect slightly higher temperatures. -- Major and trace element whole rock analyses of volcanic and subvolcanic rocks, as well as clinopyroxene mineral chemistry, reveal a complex geochemical association. Two broad paleotectonic environments can be identified using high field strength elements; volcanic rocks in the lower and middle parts of the group have a clear island arc geochemical signature (negative Ta and Nb and positive Th with respect to La on a chondrite-normalized basis) whereas those in the upper parts of the group generally do not. However, basalts of both affinities occur together in one sequence at the top of the group, suggesting that magmatism of island arc and non-arc affinity overlapped in time. -- Within these two environments, further variations can be recognized. Rocks with an island arc geochemical signature include both LREE-depleted and LREE-enriched island arc tholeiites as well as a group of very incompatible element-depleted tholeiites which are interpreted to represent partial melting of refractory sources. Rocks lacking the island arc geochemical signature range from slightly LREE-enriched basalts to alkali basalts and include a group of basalts that have geochemical characteristics transitional between the two end members. Mafic subvolcanic rocks represent all of these eruptive types. -- Felsic volcanic rocks are low-K, high-SiO₂ rhyolite. They occur only in the central stratigraphic parts of the group and are associated with mafic rocks of island arc affinity. -- Petrogenetic modelling of the Wild Bight Group volcanic rocks, using a comprehensive suite of Nd isotope analyses on selected whole rocks, allows further interpretation of their origin. Rocks of island arc affinity, for the most part, have epsilon Nd in the range -1.2 to +4.8, indicating the involvement of enriched mantle sources in the magmas. Negative Nb and Ta and positive Th anomalies with respect to the LREE indicate that this component was most likely a continental crustal source from the subducting slab. The island arc tholeiites can be modelled as resulting from mixing of this crustal source and normal depleted mantle followed by varying amounts of partial melting. However, the melting history of the depleted tholeiites must be more complex, as suggested by a strong negative correlation between epsilon Nd and both Sm/Nd and atomic Mg/(Mg+Fe) (Mg#). All rocks that lack the island arc geochemical signature have epsilonNd in the range +4 to +7 and are interpreted to have resulted from varying degrees of partial melting of an ocean island basalt-type source, locally mixed with normal depleted mantle. [Please see thesis for remainder of abstract.]

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