Elsonian magmatism in Labrador: age, characteristics and tectonic setting

1978 ◽  
Vol 15 (3) ◽  
pp. 438-453 ◽  
Author(s):  
R. F. Emslie
Keyword(s):  
Continental Crust ◽  
Country Rock ◽  
Tectonic Setting ◽  
Blocking Temperature ◽  
Average Depth ◽  
Geological Record ◽  
Sequence Of Events ◽  
Magmatic Processes ◽  
Grenvillian Orogeny ◽  
Silicic Magmas

Paleohelikian Elsonian magmatism (~ −1500 to −1400 Ma) in central Labrador occurred in the interval following the Hudsonian Orogeny (~ −1700 Ma) and preceding the Grenvillian Orogeny (after −1200 Ma). Elsonian plutons, dominated by anorthosite and adamellite compositions, have physical and petrogenetic characteristics of anorogenic intrusions and were intruded into stable, continental crust; evidence to support the existence of a 'pre-Elsonian' orogenic event (thus making the intrusions postorogenic) is slim. An average depth of intrusion about 15–20 km implies that the ambient country rock temperatures were at or near the argon blocking temperature for biotite; strong uplift and cooling, during and (or) immediately following Elsonian plutonism, is the probable cause of 'Elsonian' ages in surrounding country rock terrane.The Elsonian plutonic suite has characteristics consistent with derivation from bimodal magmatic processes. Products of basic magmas (relatively older) and of silicic magmas (relatively younger) are well represented but intermediate compositions are much less common and small in volume.Consideration of the geological record succeeding Elsonian magmatism suggests that it was a precursor of a sequence of events that led ultimately to intracontinental rifting or incipient rifting.

Magmatic processes that generate chemically distinct silicic magmas in NW Costa Rica and the evolution of juvenile continental crust in oceanic arcs

2011 ◽  
Vol 163 (2) ◽  
pp. 259-275 ◽  
Author(s):  
Chad D. Deering ◽  
Thomas A. Vogel ◽  
Lina C. Patino ◽  
David W. Szymanski ◽  
Guillermo E. Alvarado
Keyword(s):  
Costa Rica ◽  
Continental Crust ◽  
Magmatic Processes ◽  
Silicic Magmas

Pyroxene megacrysts in Proterozoic anorthosites: Implications for tectonic setting, magma source and magmatic processes at the Moho

2014 ◽  
Vol 389 ◽  
pp. 74-85 ◽  
Author(s):  
G.M. Bybee ◽  
L.D. Ashwal ◽  
S.B. Shirey ◽  
M. Horan ◽  
T. Mock ◽  
...  
Keyword(s):  
Tectonic Setting ◽  
Magma Source ◽  
Magmatic Processes

Changing exhumation potential of (U)HP eclogite through geological time

2020 ◽  
Author(s):  
Richard Palin
Keyword(s):  
Continental Crust ◽  
Plate Tectonics ◽  
Global Network ◽  
Ultrahigh Pressure ◽  
Compositional Variation ◽  
Uhp Metamorphism ◽  
Geological Time ◽  
Geological Record ◽  
Mafic Intrusions ◽  
Archean Crust

<p>Ultrahigh-pressure (UHP) metamorphism is defined by achieving P–T conditions sufficient to transform quartz to coesite (~26–28 kbar at ~500–900 °C), which occurs at ~90-100 km depth within the Earth under lithostatic conditions. Thus, the occurrence of UHP metamorphism is often taken as being a diagnostic indicator of subduction having operated in the geological record, and hence plate tectonics. Yet, the oldest such coesite-bearing rocks belong to the Pan-African belt in northern Mali, and formed at 620 Ma, although there exist multiple lines of evidence to show that a global network of subduction had been operative on Earth for billions of years beforehand. Why, then, are these key geodynamic indicators missing from the majority of the rock record? Here, I show how secular cooling of the Earth's mantle since the Mesoarchean (c. 3.2 Ga) has affected the exhumation potential of UHP (and HP) eclogite through time due to time-dependent compositional variation of both oceanic and continental crust. Petrological modeling of density changes during metamorphism of Archean, Proterozoic, and Phanerozoic composite continental terranes shows that more mafic Archean crust reaches a point-of-no-return during transport into the mantle at shallower depths than less MgO-rich modern-day crust, regardless of whether this occurs via subduction of stagnant lid-like vertical 'drip' tectonics. Thus, while Alpine- and Himalayan-type (U)HP orogenic eclogites represented by metamorphosed mafic intrusions into continental crust may readily have formed during the Precambrian, they would have lacked the buoyancy required for exhumation and preservation in the geological record.</p>

scholarly journals Comment on Bybee et al. (2014): Pyroxene megacrysts in Proterozoic anorthosites: Implications for tectonic setting, magma source and magmatic processes at the Moho

2014 ◽  
Vol 401 ◽  
pp. 378-380 ◽  
Author(s):  
Jacqueline Vander Auwera ◽  
Bernard Charlier ◽  
Jean Clair Duchesne ◽  
Bernard Bingen ◽  
John Longhi ◽  
...  
Keyword(s):  
Tectonic Setting ◽  
Magma Source ◽  
Magmatic Processes

A Discussion on global tectonics in Proterozoic times - The Grenville Province in Helikian times: a possible model of evolution

1976 ◽  
Vol 280 (1298) ◽  
pp. 499-515 ◽  
Keyword(s):  
Continental Crust ◽  
Granulite Facies ◽  
Continental Collision ◽  
Canadian Shield ◽  
Grenville Province ◽  
East Central ◽  
Metasedimentary Rocks ◽  
Grenvillian Orogeny ◽  
Triangular Area ◽  
Global Tectonics

It is suggested that the Helikian (1650-1000 million years (Ma) ago) evolution of the Grenville Province in the Canadian Shield was marked by three events: emplacement of anorthosites around 1450-1500 Ma ago, rifting associated with opening of a proto-Atlantic ocean between 1200 and 1300 Ma ago, and continental collision responsible for the Grenvillian ‘orogeny’ about 1100-1000 Ma ago. Emplacement of rocks of the anorthosite suite (anorthosites and adamellites or mangerites) into continental crust was accompanied by formation of aureoles in the granulite facies. The Grenville Group was deposited in the southern part of the Province between 1300 and 1200 Ma ago and comprises marbles, clastic metasedimentary rocks and volcanics. It occupies a roughly triangular area limited on the northwest by the Bancroft—Renfrew lineament and on the southeast by the Chibougamau—Gatineau lineament. It is thought to have been accumulated in an aulacogen that would have developed along a fracture zone separating two basement blocks. The Grenvillian thermotectonic event may represent a Tibetan continental collision in the sense of Burke & Dewey. The suture zone would now be hidden under the Appalachians. Collision would cause reactivation of continental crust and renewed movement on pre-existing lineaments. The east—central part of the Grenville Province appears to have been more intensively reactivated than the western part.

Mafic and ultrapotassic rocks from the Canyon domain (central Grenville Province): geochemistry and tectonic implications

2012 ◽  
Vol 49 (2) ◽  
pp. 412-433 ◽  
Author(s):  
Carolina Valverde Cardenas ◽  
Aphrodite Indares ◽  
George Jenner
Keyword(s):  
Tectonic Setting ◽  
Source Region ◽  
Crustal Contamination ◽  
Granulite Facies ◽  
Grenville Province ◽  
Mafic Rocks ◽  
Isotopic Signatures ◽  
Late Evolution ◽  
Grenvillian Orogeny ◽  
Laurentian Margin

The Canyon domain and the Banded complex in the Manicouagan area of the Grenville Province preserve a record of magmatic activity from ∼1.4 to 1 Ga. This study focuses on 1.4–1.2 Ga mafic rocks and 1 Ga ultrapotassic dykes. Geochemistry and Sm–Nd isotopic signatures were used to constrain the origin of these rocks and evaluate the changing role of the mantle with time and tectonic setting from the late evolution of the Laurentian margin to the Grenvillian orogeny, in the Manicouagan area. The mafic rocks include layers inferred to represent flows, homogeneous bodies in mafic migmatite, and deformed dykes, all of which were recrystallized under granulite-facies conditions during the Grenvillian orogeny. In spite of the complexities inherent in these deformed and metamorphosed mafic rocks, we were able to recognize suites with distinctive geochemical and isotopic signatures. Integration of this data along with available ages is consistent with a 1.4 Ga continental arc cut by 1.2 Ga non-arc basalts derived from depleted asthenospheric mantle, with varied degrees of crustal contamination and inferred to represent magmatism in an extensional environment. The 1 Ga ultrapotassic dykes postdate the Grenvillian metamorphism. They are extremely enriched in incompatible elements, have negative Nb anomalies, relatively unradiogenic Sr-isotopic compositions (initial 87Sr/86Sr ~ 0.7040) and εNd –3 to –15. Some dykes have compositional characteristics consistent with derivation from the mantle, ruling out crustal contamination as a major process in their petrogenesis. The most likely source region for the ultrapotassic dykes is a metasomatized subcontinental lithospheric mantle, with thermal input from the asthenosphere in association with post-orogenic delamination.

scholarly journals An Overview on the Classification and Tectonic Setting of Neoproterozoic Granites of the Nubian Shield, Eastern Desert, Egypt

Geochemistry ◽  
2021 ◽  
Author(s):  
Gaafar A. El Bahariya
Keyword(s):  
Continental Crust ◽  
Tectonic Setting ◽  
Alkali Feldspar ◽  
Eastern Desert ◽  
Granitic Rocks ◽  
Rock Types ◽  
Nubian Shield ◽  
Magma Sources ◽  
Tectonic Settings ◽  
Ocean Ridge

Granites constitute the main rock components of the Earth’s continental crust, which suggested to be formed in variable geodynamics environments. The different types of granitic rocks, their compositional characteristics, tectonic settings and magma sources are outlined. Mineralogical classification of granites includes four rock types: tonalites, granodiorites, granite (monzogranite and syenogranites) and alkali-feldspar granites. Alphabetical classification subdivided granites into: I-type, S-type, A-type and M-type granites. Moreover, formation of granitic magmas requires distinctive geodynamic settings such as: volcanic arc granite (Cordilleran); collision-related granites (leucogranites); intra-plate and ocean ridge granites. The Eastern Desert of Egypt (ED) forms the northern part of Nubian Shield. Both older and younger granites are widely exposed in the ED. Old granites (OG) comprise tonalites and granodiorites of syn- to late-orogenic granitoid assemblages. They are calcalkaline, I-type, metaluminous and display island arc tectonic setting. Younger granites (YG) on the other hand, include granites, alkali-feldspar granites and minor granodiorites. They are of I- and A-type granites and of post-orogenic to anorogenic tectonic settings. The majority of the YG are alkaline, A-type granite and of within-plate tectonic setting (WPG). The A-type granites are subdivided into: A2-type postorogenic granites and A1-type anorogenic granites. Granite magma genesis involves: (a) fractional crystallization of mafic mantle-derived magmas; (b) anatexis or assimilation of old, upper crustal rocks (c) re - melting of juvenile mafic mantle – derived rocks underplating the continental crust. Generally, older I-type granitoids were interpreted to result from melting of mafic crust and dated at approximately 760–650 Ma, whereas younger granites suggested to be formed as a result of partial melting of a juvenile Neoproterozoic mantle source. Moreover, they formed from anatectic melts of various crustal sources that emplaced between 600 and 475 Ma.

scholarly journals Mafic Archean continental crust prohibited exhumation of orogenic UHP eclogite

2021 ◽  
Author(s):  
Richard Palin ◽  
James Moore ◽  
Zeming Zhang ◽  
Guangyu Huang
Keyword(s):  
Continental Crust ◽  
Plate Tectonics ◽  
Ultrahigh Pressure ◽  
Global Ocean ◽  
Secular Change ◽  
Early Earth ◽  
Low Density ◽  
Geological Record ◽  
Point Of No Return ◽  
Negatively Buoyant

Abstract The absence of ultrahigh pressure (UHP) orogenic eclogite in the geological record older than c. 0.6 Ga is problematic for evidence of subduction having begun on Earth during the Archean (4.0–2.5 Ga). Many eclogites in Phanerozoic and Proterozoic terranes occur as mafic boudins encased within low-density felsic crust, which provides positive buoyancy during subduction; however, recent geochemical proxy analysis shows that Archean continental crust was more mafic than previously thought. Here, we show via petrological modelling that secular change in the composition of upper continental crust (UCC) would make Archean continental terranes negatively buoyant in the mantle before reaching UHP conditions. Subducted or delaminated Archean continental crust passes a point of no return during metamorphism in the mantle prior to the stabilization of coesite, while Proterozoic and Phanerozoic terranes remain positively buoyant at these depths. UHP orogenic eclogite may thus readily have formed on the Archean Earth, but could not have been exhumed, weakening arguments for a Neoproterozoic onset of subduction and plate tectonics. Further, isostatic balance calculations for more mafic Archean continents indicate that the early Earth was covered by a global ocean over 1 kilometre deep.

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