Late Precambrian to Late Devonian mafic magmatism in the Antigonish Highlands of Nova Scotia: multistage melting of a hydrated mantle

1988 ◽  
Vol 25 (4) ◽  
pp. 473-485 ◽  
Author(s):  
J. Brendan Murphy
Keyword(s):  
Nova Scotia ◽  
Trace Element ◽  
Tectonic Setting ◽  
Mantle Metasomatism ◽  
Late Devonian ◽  
Element Abundances ◽  
Late Precambrian ◽  
Metasomatic Fluid ◽  
Metasomatic Event ◽  
Alkalic Basalts

Five suites of alkalic basalt ranging in age from Late Precambrian to Late Devonian are found in the Antigonish Highlands of Nova Scotia. In contrast, on neighbouring Cape Breton Island, alkalic basalts are rare even in suites that are contemporaneous with those in the Antigonish Highlands. Late Precambrian alkalic basalts in the Antigonish Highlands are genetically associated with calc-alkalic rocks and are probably subduction related, whereas the younger suites are continental, rift related, and within plate. Major and compatible trace-element abundances can be explained by crystal fractionation of olivine ± clinopyroxene ± orthopyroxene ± spinel ± garnet. However, incompatible trace-element concentrations are strongly influenced by mantle metasomatism that occurred prior to, or synchronously with, the oldest alkalic rocks. The metasomatic event enriched the mantle in Fe, Ti, P, Zr, and light rare-earth elements. The trace-element composition of the younger suites is similar to that of the oldest alkalic rocks and may have been strongly influenced by the Late Precambrian metasomatic event. The anomalously low Nb/Y ratio (generally less than 1 in all suites) and application of phase-equilibria studies indicate that the metasomatic fluid was probably rich in H2O. This fluid may have been derived from dehydration of the subducting slab in Late Precambrian time, resulting in metasomatism of the overlying mantle wedge in the Late Precambrian. It is proposed that the younger suites obtained their fluids by dehydration of the previously metasomatized mantle associated with the generation of local pull-apart basins. Thus, the metasomatic fluid was exotic with respect to the oldest basalts but indigenous with respect to the younger basalts. In the younger basalts, the indigenous fluid was probably focussed at the site of melting by structural events (i.e., rifting). In situations in which the chemistry of mafic magmas is predetermined by earlier metasomatic events, caution is advised in using trace-element criteria to evaluate the tectonic setting.

Geochemical constraints on the evolution of the early continental crust

1981 ◽  
Vol 301 (1461) ◽  
pp. 423-442 ◽  
Keyword(s):  
Trace Element ◽  
Continental Crust ◽  
Lower Crust ◽  
Plate Tectonics ◽  
Tectonic Setting ◽  
Mantle Plumes ◽  
Element Abundances ◽  
Ocean Island ◽  
Rock Types ◽  
Greenstone Belts

The most important process affecting both major and trace-element concentrations in the mantle and crust is melting producing silicate liquids which then migrate. Another process whose effects are becoming more apparent is the transport of elements by CO 2 - and H 2 O-rich fluids. Due to the relatively small amounts of fluids involved they have but little effect on the major-element abundances but may severely affect minor- and trace-element abundances in their source and the material through which they travel. The Archaean crust was a density filter which reduced the possibility of komatiite or high FeO melts with relative densities greater than about 3.0 from reaching the surface. Those melts retained in the lower crust or at the crust-mantle boundary would have enhanced the possibility of melting in the lower crust. The high FeO melts may have included the Archaean equivalents of alkali basalt whose derivatives may form an important component in the Archaean crust. The occurrence of ultramafic to basic to alkaline magmas in some Archaean greenstone belts is an assemblage most typical of modern ocean-island suites in continental environments. The rock types in the assemblage were modified by conditions of higher heat production during the Archaean and thus greater extents of melting and melting at greater depths. If modern ocean-island suites are associated with mantle plumes, which even now may be an important way to transport heat upward from the deeper mantle, it is suggested that during the Archaean mantle plumes were an important factor in the evolution of the continental crust. It appears that the Archaean continental crust was of comparable thickness to that of the present based on geobarometeric data. If the freeboard concept applied then, this would suggest that plate tectonics was also an active process during the Archaean. If so, it is probably no more realistic to assume that all Archaean greenstone belts had a similar tectonic setting than to assume that all modern occurrences of basic rocks have a common tectonic setting.

Geochemical constraints on the tectonic setting of the mafic rocks of the Bathurst Camp, Appalachian Orogen

1990 ◽  
Vol 27 (9) ◽  
pp. 1182-1193 ◽  
Author(s):  
A. Dogan Paktunc
Keyword(s):  
Trace Element ◽  
Gulf Of California ◽  
Tectonic Setting ◽  
Source Region ◽  
Ocean Basin ◽  
Mafic Rocks ◽  
Middle Ordovician ◽  
Element Abundances ◽  
Fine Grained ◽  
Back Arc

Abundant mafic rocks comprising basalts and gabbros occur in the Bathurst Camp, a complexly deformed Ordovician terrane in northeastern New Brunswick. The mafic rocks form a consanguineous suite of aphyric lavas, subvolcanic sills, and (or) dikes. Gabbros and basalts have somewhat similar major-element compositions but differ in terms of their trace-element contents. Medium-grained gabbros display tholeiitic compositions, whereas basalts and fine-grained gabbros have alkalic affinities. In general, trace-element abundances indicate an enriched source region for the Bathurst mafic rocks. Trace-element characteristics of the tholeiitic group point to a transitional setting going from back-arc to ocean basin, whereas the alkalic group has geochemical characteristics in common with within-plate basalts. Mixing between magmas of these contrasting settings could explain some of the trace-element characteristics of both groups. The back-arc-basin setting appears to be ensialic and is characterized by the absence of an underlying subducted slab during the formation of the basin. The tectonic reason for rifting in such a case could be the strike separation along a series of en echelon faults similar to those of the Gulf of California. Calc-alkaline characteristics of the upper mantle underlying the basin seem to have been inherited from southeasterly subduction of the proto-Atlantic Ocean in Early to Middle Ordovician times.

Structural setting and age of the Partridge Island block, southern New Brunswick, Canada: a link to the Cobequid Highlands of northern mainland Nova Scotia

2014 ◽  
Vol 51 (1) ◽  
pp. 1-24 ◽  
Author(s):  
Adrian F. Park ◽  
Robert L. Treat ◽  
Sandra M. Barr ◽  
Chris E. White ◽  
Brent V. Miller ◽  
...  
Keyword(s):  
Nova Scotia ◽  
New Brunswick ◽  
Tectonic Setting ◽  
Sedimentary Rocks ◽  
Alkali Feldspar ◽  
Early Carboniferous ◽  
Late Devonian ◽  
Island Formation ◽  
Lake Formation ◽  
Saint John

The Partridge Island block is a newly identified tectonic element in the Saint John area of southern New Brunswick, located south of and in faulted contact with Proterozoic and Cambrian rocks of the Ganderian Brookville and Avalonian Caledonia terranes. It includes the Lorneville Group and Tiner Point complex. The Lorneville Group consists of interbedded volcanic and sedimentary rocks, subdivided into the Taylors Island Formation west of Saint John Harbour and West Beach Formation east of Saint John Harbour. A sample from thin rhyolite layers interbedded with basaltic flows of the Taylors Island Formation at Sheldon Point yielded a Late Devonian – Early Carboniferous U–Pb (zircon) age of 358.9 +6/–5 Ma. Petrological similarities indicate that all of the basaltic rocks of the Taylors Island and West Beach formations are of similar age and formed in a continental within-plate tectonic setting. West of Saint John Harbour, basaltic and sedimentary rocks of the Taylors Island Formation are increasingly deformed and mylonitic to the south, and in part tectonically interlayered with mylonitic granitoid rocks and minor metasedimentary rocks of the Tiner Point complex. Based on magnetic signatures, the deformed rocks of the Tiner Point complex can be traced through Partridge Island to the eastern side of Saint John Harbour, where together with the West Beach Formation, they occupy a thrust sheet above a redbed sequence of the mid-Carboniferous Balls Lake Formation. The Tiner Point complex includes leucotonalite and aegirine-bearing alkali-feldspar granite with A-type chemical affinity and Early Carboniferous U–Pb (zircon) ages of 353.6 ± 5.7 and 346.4 ± 0.7 Ma, respectively. Based on similarities in age, petrological characteristics, alteration, iron oxide – copper – gold (IOCG)-type mineralization, and deformation style, the Partridge Island block is correlated with Late Devonian – Early Carboniferous volcanic–sedimentary–plutonic rocks of the Cobequid Highlands in northern mainland Nova Scotia. Deformation was likely a result of dextral transpression along the Cobequid–Chedabucto fault zone during juxtaposition of the Meguma terrane.

Melt geometry, movement and crystallization, in relation to mantle dykes, veins and metasomatism

1993 ◽  
Vol 342 (1663) ◽  
pp. 1-21 ◽  
Keyword(s):  
Trace Element ◽  
Fractional Crystallization ◽  
Three Dimensional ◽  
Mantle Peridotite ◽  
Mantle Metasomatism ◽  
Mantle Xenoliths ◽  
Chemical Components ◽  
Combined Processes ◽  
Element Abundances ◽  
Natural Rock

Consideration of theoretical, experimental and natural rock data show that basic-ultrabasic melt will disperse along mineral grain edges in olivine-rich mantle rock and thereby form a connected three-dimensional network throughout the rock even when present in only small (less than 1%) volumes. The viscosity of such melts will also allow small (less than 1-5%) volumes to move on appropriate geological timescales as a result of gravity-driven compaction. These features mean that small volume basic-ultrabasic melts are capable of infiltrating and metasomatizing mantle peridotites. Modally metasomatized mantle xenoliths are commonly closely associated with an array of dyke-like and vein injection phenomena. Textural, structural and modal characteristics of a wide array of mantle dykes, veins and metasomatic rocks suggest that such rocks have certain features in common with cumulates, and might usefully be distinguished as dyke cumulates and metasomatic infill cumulates . They represent partial crystal precipitates from melt flowing along channelways or pervasively through peridotite, and their bulk rock compositions provide poor guides to actual mantle melt compositions. The crystallization of the minerals in dykes/veins/ metasomites causes differentiation of the melt by crystal fractionation processes, but at the same time the melt may maintain equilibrium with host rock phases (e.g. olivine) and chromatographic column or percolation effects will control the range of transport of different chemical components by the melt. These combined processes are referred to as percolative fractional crystallization . Data on the actual trace element compositions of melt in equilibrium with the minerals of mantle dykes/veins/metasomites are calculated from trace element analyses of the minerals by using partition coefficients. For a wide variety of metasomatic suites, the calculated melt compositions show a progression of trace element abundances from ones similar to primitive asthenospheric OIB-like compositions towards more incompatible element enriched compositions. Thus they support the hypothesis that fractional crystallization and percolative fractional crystallization processes operating upon initial primitive asthenospheric melts may yield melt compositions matching those necessary for wide varieties of mantle metasomatism. The differentiation of the melts and evolution of the metasomatic rocks proceed together. No evidence for the involvement of volatile-rich fluids distinct from melts has been found. The trace element compositions of many kimberlitic and lamproitic melts may also arise by processes of percolative fractional crystallization of initially primitive melts with oIB-like trace element compositions, as a result of flow through mantle peridotite.

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