North Mountain basalt from Digby, Nova Scotia: models for a fissure eruption from stratigraphy and petrochemistry

1988 ◽  
Vol 25 (1) ◽  
pp. 74-83 ◽  
Author(s):  
V. S. Papezik ◽  
John D. Greenough ◽  
John A. Colwell ◽  
Terry J. Mallinson
Keyword(s):  
Nova Scotia ◽  
Fissure Eruption ◽  
Chemical Diversity ◽  
Early Mesozoic ◽  
High K ◽  
Granular Matrix ◽  
Crystal Settling ◽  
Lower Crustal ◽  
Crustal Magma ◽  
Zoned Plagioclase

The early Mesozoic, quartz normative, North Mountain basalts in southwestern Nova Scotia (Digby area) form three units: a coarse massive lower flow (~190 m) bearing minor lenses of mafic pegmatite, a middle unit of thin amygdaloidal basaltic flows (~50 m), and an upper flow unit of massive phenocryst-rich basalt (~160 m). The two thick units show phenocrysts of orthopyroxene (bronzite) and (or) pigeonite, augite, and zoned plagioclase in a granular matrix of augite, pigeonite, and plagioclase. Variation diagrams and chondrite-normalized rare-earth-element patterns relate all chemical diversity between and within flows to removal and (or) accumulation of plagioclase and pyroxene phenocrysts (~1:1). High K, Rb, and Ba, appear related to assimilation of continental crust. Constancy of fractionation-independent element ratios and variations in phenocryst content vertically and along the 200 km strike of the basalts suggest (1) crystal settling and accumulation together with assimilation and mixing in a lower crustal magma chamber, (2) rise to upper crustal levels in a central conduit followed by northeast-ward emplacement along a tension-induced dyke system, and (3) extrusion along a fissure in two major and numerous minor pulses that formed the lower, upper, and middle units. Assimilation did not occur as magma moved through the dyke system, for assimilation-related variations in composition do not occur along strike.

Deep seismic reflection data from the Bay of Fundy and Gulf of Maine: tectonic implications for the northern Appalachians

1991 ◽  
Vol 28 (7) ◽  
pp. 1096-1111 ◽  
Author(s):  
C. E. Keen ◽  
W. A. Kay ◽  
D. Keppie ◽  
F. Marillier ◽  
G. Pe-Piper ◽  
...  
Keyword(s):  
Nova Scotia ◽  
Seismic Reflection ◽  
Gulf Of Maine ◽  
Deep Penetration ◽  
United States Geological Survey ◽  
Bay Of Fundy ◽  
Seismic Reflection Data ◽  
The North ◽  
Geological Features ◽  
Lower Crustal

Three deep-penetration seismic reflection profiles were collected off southwest Nova Scotia to determine the crustal structure and geometry beneath the Avalon and Meguma zones of the Appalachian Orogen in Canada. Onshore geological features have been traced seawards using new gravity and magnetic anomaly maps. The seismic data can also be correlated with the previous United States Geological Survey profile in the central Gulf of Maine.Two seismically distinct lower crustal blocks are identified: the Avalon and Sable lower crustal blocks, separated by a major north-dipping reflection zone that cuts the entire crust. The recognition of the Sable block adds a fourth block to the three already identified in the Canadian Appalachians. The Sable block is overlain by the Meguma Zone. The Avalon Zone overlies at least the northern part of the Avalon lower crustal block. Although offshore extension of geological features is not unequivocal, it appears that a north-dipping reflection zone southwest of Nova Scotia marks the site of Devonian thrusting of Avalon Zone over Meguma Zone. In the Bay of Fundy to the north, two south-dipping reflection zones are interpreted as major thrusts, possibly placing Avalon lower crust over a unit with different tectonic affinities. The Fundy Fault is a Carboniferous thrust within the Avalon block along the coast of New Brunswick; this was reactivated during Mesozoic extension as a transtensional fault. Extensional displacement farther southwest was probably accommodated along east-west-trending faults and small rift basins associated with them.

scholarly journals Neoproterozoic magmatic evolution of the southern Ouaddaï Massif (Chad)

2020 ◽  
Vol 191 ◽  
pp. 34 ◽  
Author(s):  
Félix Djerossem ◽  
Julien Berger ◽  
Olivier Vanderhaeghe ◽  
Moussa Isseini ◽  
Jérôme Ganne ◽  
...  
Keyword(s):  
Geothermal Gradient ◽  
Isotopic Signature ◽  
Biotite Granite ◽  
Granite Sample ◽  
Metasedimentary Rocks ◽  
Rock Types ◽  
Pan African ◽  
High K ◽  
Back Arc ◽  
Lower Crustal

This paper presents new petrological, geochemical, isotopic (Nd) and geochronological data on magmatic rocks from the poorly known southern Ouaddaï massif, located at the southern edge of the so-called Saharan metacraton. This area is made of greenschist to amphibolite facies metasediments intruded by large pre- to syn-tectonic batholiths of leucogranites and an association of monzonite, granodiorite and biotite granite forming a late tectonic high-K calc-alkaline suite. U-Pb zircon dating yields ages of 635 ± 3 Ma and 613 ± 8 Ma on a peraluminous biotite-leucogranite (containing numerous inherited Archean and Paleoproterozoic zircon cores) and a muscovite-leucogranite, respectively. Geochemical fingerprints are very similar to some evolved Himalayan leucogranites suggesting their parental magmas were formed after muscovite and biotite dehydration melting of metasedimentary rocks. A biotite-granite sample belonging to the late tectonic high-K to shoshonitic suite contains zircon rims that yield an age of 540 ± 5 Ma with concordant inherited cores crystallized around 1050 Ma. Given the high-Mg# (59) andesitic composition of the intermediate pyroxene-monzonite, the very similar trace-element signature between the different rock types and the unradiogenic isotopic signature for Nd, the late-kinematic high-K to shoshonitic rocks formed after melting of the enriched mantle and further differentiation in the crust. These data indicate that the southern Ouaddaï was part of the Pan-African belt. It is proposed that it represents a continental back-arc basin characterized by a high-geothermal gradient during Early Ediacaran leading to anatexis of middle to lower crustal levels. After tectonic inversion during the main Pan-African phase, late kinematic high-K to shoshonitic plutons emplaced during the final post-collisional stage.

scholarly journals Experimental evidence for decompression melting of metasomatized mantle beneath Colima Graben, Mexico

2020 ◽  
Vol 175 (11) ◽  
Author(s):  
Eduardo Becerra-Torres ◽  
Elena Melekhova ◽  
Jon D. Blundy ◽  
Richard A. Brooker
Keyword(s):  
Experimental Approach ◽  
Mantle Wedge ◽  
Thermal Structure ◽  
Liquidus Surface ◽  
Chemical Diversity ◽  
Phase Assemblage ◽  
Cinder Cones ◽  
Piston Cylinder ◽  
High K ◽  
Metasomatized Mantle

Abstract Primitive subduction zone magmas provide information about the composition and thermal structure of the underlying mantle wedge. In the Colima Graben, Mexico, primitive lavas erupted from cinder cones range from high magnesium calc-alkaline basalts to high-K trachybasalts. This chemical diversity suggests that the sub-arc mantle wedge from which they derive is heterogeneous. To explore the conditions of magma generation in the wedge beneath Colima we used an inverse experimental approach to constrain multiple saturation points on the liquidus surface of a primitive high-K basanite (COM-1). Equilibrium piston-cylinder experiments were carried out between 1.0 and 2.4 GPa under hydrous (1.8–3.8 wt% H2O) and oxidizing (ƒO2 = − 0.5 to 4.3 log units relative to NNO) conditions. COM-1 + 3.8 wt% H2O is shown to be multiply-saturated with a phlogopite-bearing spinel pyroxenite assemblage (cpx + opx + phl + sp) close to its liquidus at 1.9–2.4 GPa and 1300 ºC. Experimental mapping of the liquidus surface reveals a multiple saturation point (MSP) where a lherzolitic phase assemblage of ol + cpx + opx + sp + phl coexist. The topology of the MSP indicates a peritectic of the form cpx + opx + phl + sp = liquid + ol. Four bracketing experiments define the MSP of COM-1 as 1300 ± 10 °C, 1.7 ± 0.1 GPa, ∆NNO = 3.4 ± 0.5 log units, for melt containing 3.6 ± 0.4 wt% H2O. The MSP olivine is too forsterite-rich (Fo92-94) to be in equilibrium with mantle lherzolite, but matches phenocryst core compositions in the natural basanite. Thus, experimental results indicate that COM-1 was produced by incongruent melting of an olivine-free, phlogopite-pyroxenite source that itself is the result of metasomatism of mantle wedge by slab-derived fluids. These conditions provide a valuable constraint on the thermal structure and chemical composition of the mantle wedge beneath Colima.

Geomagnetic induction anomaly near Kootenay Lake—a strike–slip feature in the lower crust?

1970 ◽  
Vol 7 (6) ◽  
pp. 1568-1579 ◽  
Author(s):  
J. J. Lajoie ◽  
B. Caner
Keyword(s):  
Surface Expression ◽  
Strike Slip ◽  
Conductive Layer ◽  
Geomagnetic Induction ◽  
Western Cordillera ◽  
Early Mesozoic ◽  
Conductivity Structure ◽  
Kootenay Lake ◽  
Lower Crustal ◽  
East West

A geomagnetic induction anomaly in southeastern British Columbia has been investigated in detail with a 20-station network. The results indicate a sharp (near vertical) discontinuity in deep electrical conductivity structure, trending roughly east–west and located at the south end of Kootenay Lake. It is interpreted as evidence for sinistral strike–slip which intersected the edge of the main conductive layer which underlies most of the western Cordillera.Geological evidence indicates that such a feature (which has no known surface expression) must predate the late Paleozoic or early Mesozoic (200–250 m.y.). The persistence of sharp discontinuities over such long periods would confirm the compositional, rather than thermal, nature of the lower crustal conductive layer under the western Cordillera.

Neoproterozoic evolution of Western Ethiopia: igneous geochemistry, isotope systematics and U–Pb ages

2003 ◽  
Vol 140 (4) ◽  
pp. 373-395 ◽  
Author(s):  
T. GRENNE ◽  
R. B. PEDERSEN ◽  
T. BJERKGÅRD ◽  
A. BRAATHEN ◽  
M. G. SELASSIE ◽  
...  
Keyword(s):  
Trace Elements ◽  
Crustal Material ◽  
East African ◽  
Plate Convergence ◽  
High K ◽  
East African Orogen ◽  
Incompatible Trace Elements ◽  
Back Arc ◽  
Lower Crustal ◽  
Western Ethiopia

New geochemical, isotopic and age data from igneous rocks complement earlier models of a long-lived and complex accretionary history for East African Orogen lithologies north of the Blue Nile in western Ethiopia, but throw doubt on the paradigm that ultramafic complexes of the region represent ophiolites and suture zones. Early magmatism is represented by a metavolcanic sequence dominated by pyroclastic deposits of predominantly basaltic andesite composition, which give a Rb–Sr whole-rock errorchron of 873±82 Ma. Steep REE patterns and strong enrichments of highly incompatible trace elements are similar to Andean-type, high-K to medium-K calc-alkaline rocks; εNd values between 4.0 and 6.8 reflect a young, thin continental edge. Interlayered basaltic flows are transitional to MORB and compare with mafic rocks formed in extensional, back-arc or inter-arc regimes. The data point to the significance of continental margin magmatism already at the earliest stages of plate convergence, in contrast with previous models for the East African Orogen. The metavolcanites overlap compositionally with the Kilaj intrusive complex dated at 866±20 Ma (U–Pb zircon) and a related suite of dykes that intrude thick carbonate-psammite sequences of supposedly pre-arc, continental shelf origin. Ultramafic complexes are akin to the Kilaj intrusion and the sediment-hosted dykes, and probably represent solitary intrusions formed in response to arc extension. Synkinematic composite plutons give crystallization ages of 699±2 Ma (Duksi, U–Pb zircon) and 651±5 Ma (Dogi, U–Pb titanite) and testify to a prolonged period of major (D1) contractional deformation during continental collision and closure of the ‘Mozambique Ocean’. The plutons are characterized by moderately peraluminous granodiorites and granites with εNd values of 1.0–2.0. They were coeval with shoshonitic, latitic, trachytic and rare trachybasaltic intrusions with very strong enrichments of highly incompatible trace elements and εNd of 0.4–8.0. The mafic end-member is ascribed to partial melting of enriched sub-continental mantle that carried a subduction component inherited from pre-collision subduction. Contemporaneous granodiorite and granite formation was related to crustal underplating of the mafic magmas and consequent melting of lower crustal material derived from the previously accreted, juvenile arc terranes of the East African Orogen.

scholarly journals From long-lived batholith construction to giant porphyry copper deposit formation: petrological and zircon chemical evolution of the Quellaveco District, Southern Peru

2021 ◽  
Vol 176 (2) ◽  
Author(s):  
Chetan L. Nathwani ◽  
Adam T. Simmons ◽  
Simon J. E. Large ◽  
Jamie J. Wilkinson ◽  
Yannick Buret ◽  
...  
Keyword(s):  
High Pressure ◽  
Porphyry Copper ◽  
Marked Change ◽  
Crustal Thickening ◽  
Deposit Formation ◽  
Nazca Plate ◽  
Southern Peru ◽  
Porphyry Cu ◽  
Lower Crustal ◽  
Crustal Magma

AbstractPorphyry Cu ore deposits are a rare product of arc magmatism that often form spatiotemporal clusters in magmatic arcs. The petrogenetic evolution of igneous rocks that cover the temporal window prior to and during porphyry Cu deposit formation may provide critical insights into magmatic processes that are key in generating these systems. This study documents the magmatic evolution of the Palaeocene–Eocene Yarabamba Batholith, Southern Peru, that was incrementally assembled between ~ 67 and ~ 59 Ma and hosts three, nearly contemporaneous, giant porphyry Cu–Mo deposits that formed at 57–54 Ma (Quellaveco, Toquepala and Cuajone). Whole-rock geochemistry, U–Pb geochronology and zircon trace element chemistry are reported from Yarabamba rocks that span the duration of plutonic activity, and from six porphyry intrusions at Quellaveco that bracket mineralisation. A change in whole-rock chemistry in Yarabamba intrusive rocks to high Sr/Y, high La/Yb and high Eu/Eu* is observed at ~ 60 Ma which is broadly coincident with a change in vector of the converging Nazca plate and the onset of regional compression and crustal thickening during the first stage of the Incaic orogeny. The geochemical changes are interpreted to reflect a deepening of the locus of lower crustal magma evolution in which amphibole ± garnet are stabilised as early and abundant fractionating phases and plagioclase is suppressed. Zircons in these rocks show a marked change towards higher Eu/Eu* (> 0.3) and lower Ti (< 9 ppm) compositions after ~ 60 Ma. Numerical modelling of melt Eu systematics and zircon-melt partitioning indicates that the time series of zircon Eu/Eu* in these rocks can be explained by a transition from shallower, plagioclase-dominated fractionation to high-pressure amphibole-dominated fractionation at deep crustal levels from ~ 60 Ma. Our modelling suggests that any redox effects on zircon Eu/Eu* are subordinate compared to changes in melt composition controlled by the fractionating mineral assemblage. We suggest that growth and intermittent recharge of the lower crustal magma reservoir from ~ 60 Ma produced a significant volume of hydrous and metallogenically fertile residual melt which ascended to the upper crust and eventually generated the three giant porphyry Cu–Mo deposits at Quellaveco, Toquepala and Cuajone from ~ 57 Ma. Our study highlights the importance of high-pressure magma differentiation fostered by strongly compressive tectonic regimes in generating world-class porphyry Cu deposits.

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