U–Pb geochronology, Nd–Sm geochemistry, structural setting, and tectonic significance of Late Devonian and Paleogene intrusions in northern Yukon and northeastern Alaska

2019 ◽  
Vol 56 (6) ◽  
pp. 585-606
Author(s):  
Larry S. Lane ◽  
James K. Mortensen
Keyword(s):  
Partial Melting ◽  
Ocean Basin ◽  
Volcanic Edifice ◽  
Late Devonian ◽  
Upper Crust ◽  
Upward Migration ◽  
Middle Crust ◽  
Structural Setting ◽  
Magmatic Underplating ◽  
Tectonic Significance

A suite of six Devonian granites and one syenite were emplaced into the upper crust of northern Yukon between 364.8 ± 2.7 and 371.2 ± 1.4 Ma. The Bear Mountain syenite and related rhyolite porphyry in adjacent Alaska intruded at 52.3 ± 0.4 and 53.5 ± 0.2 Ma, respectively. A felsic volcaniclastic unit and quartz-phyric sill are newly documented adjacent to the Mount Sedgwick granite. The volcaniclastic unit may indicate the presence of a related volcanic edifice. The presence of xenocrystic zircon grains in most of the intrusions suggests initial emplacement of magmas began 10–20 Myr before final emplacement into the upper crust. A Famennian final intrusion age coincides with Late Devonian encroachment of Ellesmerian deformation into the region. Attendant crustal flexure, or evolving foreland structures, may have facilitated upward migration of the magmas. Geochemistry of the intrusions indicates that the Devonian magmatism was largely derived from partial melting of lower and middle crust, implying widespread mafic magmatic underplating in Middle to Late Devonian time. Only Dave Lord syenite retains evidence of an original mantle geochemical signature. Mantle underplating may have played a role in localizing extension, volcanism, and rifting that led to the Late Devonian opening of the Angayucham ocean basin. The Eocene Bear Mountain pluton is inferred to be a northerly example of widespread Cenozoic within-plate magmatism in Alaska.

scholarly journals Thermal conditions during deformation of partially molten crust from TitaniQ geothermometry: rheological implications for the anatectic domain of the Araçuaí belt, eastern Brazil

Solid Earth ◽  
2014 ◽  
Vol 5 (2) ◽  
pp. 1223-1242 ◽  
Author(s):  
G. C. G. Cavalcante ◽  
A. Vauchez ◽  
C. Merlet ◽  
M. Egydio-Silva ◽  
M. H. Bezerra de Holanda ◽  
...  
Keyword(s):  
Partial Melting ◽  
Flow Direction ◽  
Regional Scale ◽  
Thermal Conditions ◽  
Middle Crust ◽  
Low Viscosity ◽  
Lateral Extrusion ◽  
Eastern Brazil ◽  
Gravity Driven ◽  
Gravity Driven Flow

Abstract. During the Neoproterozoic orogeny, the middle crust of the Araçuaí belt underwent widespread partial melting. At the regional scale, this anatectic domain is characterized by a progressive rotation of the flow direction from south to north, suggesting a 3-D deformation of the anatectic middle crust. To better determine whether melt volumes present in the anatectic middle crust of the Araçuaí orogen were large enough to allow a combination of gravity-driven and convergence-driven deformation, we used the titanium-in-quartz (TitaniQ) geothermometer to estimate the crystallization temperatures of quartz grains in the anatectic rocks. When possible, we compared these estimates with thermobarometric estimates from traditional exchange geothermobarometers applied to neighboring migmatitic kinzigites. TitaniQ temperatures range from 750 to 900 °C, suggesting that quartz starts crystallizing at minimum temperatures of ≥ 800 °C. These results, combined with the bulk-rock chemical composition of diatexites, allows the estimation of a minimum of ~ 30% melt and a corresponding viscosity of ~ 109–1010 Pa s. Such a minimum melt content and low viscosity are in agreement with interconnected melt networks observed in the field. Considering that these characteristics are homogeneous over a wide area, this supports the finding that the strength of the middle crust was severely weakened by extensive partial melting, making it prone to gravity-driven flow and lateral extrusion.

Nature of mantle heterogeneity in the North Atlantic: evidence from deep sea drilling

1980 ◽  
Vol 297 (1431) ◽  
pp. 179-202 ◽  
Keyword(s):  
Partial Melting ◽  
North Atlantic ◽  
Subduction Zones ◽  
Alkali Basalts ◽  
Upward Migration ◽  
The North ◽  
Subduction Zone Processes ◽  
Wide Range ◽  
Mantle Sources ◽  
The North Atlantic

Studies of dredged and drilled samples from the North Atlantic ocean have revealed that basalts with a wide range of major and trace element compositions have been generated at the Mid-Atlantic Ridge (M.A.R.). Many of the basalts erupted between latitudes 30° and 70° N do not have the geochemical characteristics of normal mid-ocean ridge basalts (m.o.r.b.) depleted in the more-hygromagmatophile (hyg.) elements. Drilling along mantle flow lines transverse to the ridge has shown that different segments of the M.A.R. have produced basalts with a distinct compositional range for tens of millions of years. As more data have become available, the nature and scale of this variation have been established and tighter constraints can now be placed on the petrogenetic processes involved. The rare earth elements are used to test quantitatively the effects of open and closed system fractional crystallization, equilibrium partial melting (including continuous melting), zone refining and mantle mixing processes on basalt chemistry. When evaluated in terms of the more-hyg. elements, the results show that major heterogeneities must exist in the mantle sources feeding the M.A.R. Ratios of many of the more-hyg. elements remain consistent in space and time in basalts erupted at a particular ridge segment, but vary widely between different ridge segments. These ratios are not significantly modified by the processes of basalt generation. The hyg. element relations provide a major constraint on the nature of heterogeneity in the Earth’s mantle and the processes producing it. The mantle sources of anomalous ridge segments can be best explained in terms of variable veining of a hyg. element depleted host by a hyg. element enriched liquid or fluid generated by very small degrees of partial melting. Such incipient melting, as well as subduction zone processes, may be viable mechanisms for changing hyg. element ratios in the mantle source regions on the scale observed. These processes can be integrated into a model for mantle evolution which involves (1) upward migration of incipient melts to provide a hyg. element enriched source for alkali basalts and a hyg. element depleted source for normal m.o.r.b., and (2) extraction of continental crust and recycling of the depleted residue into the mantle at subduction zones. Also, some recycling of continental material into the mantle may be required to explain Pb isotope patterns.

Isotopic ages from the Appalachians and their tectonic significance: Reply to J. G. Dennis

1968 ◽  
Vol 5 (4) ◽  
pp. 961-962
Author(s):  
C. T. Harper
Keyword(s):  
Late Ordovician ◽  
Late Devonian ◽  
Isotopic Data ◽  
Tectonic Significance

The K–Ar isotopic data indicate that two major periods of post-metamorphic uplift and cooling occurred in the Northern Appalachians, the first during Late Ordovician and Silurian times, the second during the Late Devonian Epoch. These recorded events were separated by the Acadian magmatic episode.

scholarly journals Reassessing zircon-monazite thermometry with thermodynamic modelling: insights from the Georgetown igneous complex, NE Australia

2020 ◽  
Vol 175 (12) ◽  
Author(s):  
S. Volante ◽  
W. J. Collins ◽  
E. Blereau ◽  
A. Pourteau ◽  
C. Spencer ◽  
...  
Keyword(s):  
Saturation Temperature ◽  
Thermodynamic Modelling ◽  
Accessory Mineral ◽  
Compositional Variation ◽  
Upper Crust ◽  
Middle Crust ◽  
Deep Crust ◽  
Granite Magmatism ◽  
Zircon Thermometry ◽  
Zircon Saturation

AbstractAccessory mineral thermometry and thermodynamic modelling are fundamental tools for constraining petrogenetic models of granite magmatism. U–Pb geochronology on zircon and monazite from S-type granites emplaced within a semi-continuous, whole-crust section in the Georgetown Inlier (GTI), NE Australia, indicates synchronous crystallisation at 1550 Ma. Zircon saturation temperature (Tzr) and titanium-in-zircon thermometry (T(Ti–zr)) estimate magma temperatures of ~ 795 ± 41 °C (Tzr) and ~ 845 ± 46 °C (T(Ti-zr)) in the deep crust, ~ 735 ± 30 °C (Tzr) and ~ 785 ± 30 °C (T(Ti-zr)) in the middle crust, and ~ 796 ± 45 °C (Tzr) and ~ 850 ± 40 °C (T(Ti-zr)) in the upper crust. The differing averages reflect ambient temperature conditions (Tzr) within the magma chamber, whereas the higher T(Ti-zr) values represent peak conditions of hotter melt injections. Assuming thermal equilibrium through the crust and adiabatic ascent, shallower magmas contained 4 wt% H2O, whereas deeper melts contained 7 wt% H2O. Using these H2O contents, monazite saturation temperature (Tmz) estimates agree with Tzr values. Thermodynamic modelling indicates that plagioclase, garnet and biotite were restitic phases, and that compositional variation in the GTI suites resulted from entrainment of these minerals in silicic (74–76 wt% SiO2) melts. At inferred emplacement P–T conditions of 5 kbar and 730 °C, additional H2O is required to produce sufficient melt with compositions similar to the GTI granites. Drier and hotter magmas required additional heat to raise adiabatically to upper-crustal levels. S-type granites are low-T mushes of melt and residual phases that stall and equilibrate in the middle crust, suggesting that discussions on the unreliability of zircon-based thermometers should be modulated.

Interaction among upper crustal, lower crustal, and mantle materials in the Port Mouton pluton, Meguma Lithotectonic Zone, southwest Nova Scotia

2000 ◽  
Vol 37 (4) ◽  
pp. 579-600 ◽  
Author(s):  
D Barrie Clarke ◽  
Raymond Fallon ◽  
Larry M Heaman
Keyword(s):  
Nova Scotia ◽  
Lower Crust ◽  
Early Stage ◽  
Late Devonian ◽  
Upper Crust ◽  
Middle Stage ◽  
Two Component ◽  
Granitoid Rocks ◽  
Mafic Magmas ◽  
Lithotectonic Zone

The Port Mouton pluton is unique among the Late Devonian peraluminous granitoid bodies in the Meguma Lithotectonic Zone of southwestern Nova Scotia in its lithological heterogeneity, extensive physical and chemical interaction with the country rocks, clear evidence for mingling and mixing with mafic magmas, and highly abundant pegmatites. New U–Pb age determinations on monazite establish an intrusion age of 373 ± 1 Ma, similar to the ages of other Meguma Lithotectonic Zone granitoid plutons and mafic intrusions. Field relations, petrology, and geochemistry define three stages of intrusion of the Port Mouton pluton: (i) early stage, discontinuously exposed around the outer margin of the pluton, dominated by coarse-grained tonalite-granodiorite, and with Rb/Sr < 0.55, Eu/Eu* > 0.40, and GdN/LuN < 2; (ii) middle stage, occupying the interior of the pluton, dominated by medium-grained granodiorite-monzogranite, and with Rb/Sr > 0.55, Eu/Eu* < 0.40, and GdN/LuN > 2; and (iii) late stage, consisting of abundant minor sheets throughout the pluton, dominated by fine-grained tonalite, granodiorite, and leucogranite that are similar to rocks of the early and middle stages. The Port Mouton pluton shows a wider range of 87Sr/86Sri (0.7036-0.7154), and a wider range and generally higher εNdi (–3.72 to +2.12), than other granitoid rocks in the Meguma Lithotectonic Zone, potentially reflecting a complex, partially equilibrated, interaction among mantle, lower crust, and upper crust. Field, petrological, and chemical evidence for the involvement of mantle-derived magmas and melting of upper crust permit modelling of the Port Mouton pluton granitoid compositions by three simultaneous mixing equations. These mixing model results suggest that the early stage granitoid rocks can form from simple three-component mixing relationships when the bulk distribution coefficients between residuum and melt for Sr and Nd range from 1.05 to 1.18, or two-component mixing combined with fractionation of material like the known felsic lower crust. The middle stage granitoid rocks only yield solutions involving two-component mixing and fractionation of material unlike the known felsic lower crust. We conclude that the Late Devonian mafic magmas played a major role in the formation of granitoid magmas in the Meguma Lithotectonic Zone by supplying heat and material to cause partial fusion of the Avalon lower crust.

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