Continental crust composition and nature of the lower crust: constraints from mantle Nd–Sr isotope correlation

Nature ◽  
1980 ◽  
Vol 286 (5771) ◽  
pp. 342-346 ◽  
Author(s):  
Barry L. Weaver ◽  
John Tarney
Keyword(s):  
Continental Crust ◽  
Lower Crust ◽  
Sr Isotope ◽  
Isotope Correlation

scholarly journals Heat- and melt-fluxed melting of lower continental crust: Insights from two types of subduction-related granitoids in northeastern China and the implications for crustal reworking and growth

Lithosphere ◽  
2019 ◽  
Vol 11 (4) ◽  
pp. 488-506
Author(s):  
Xing-Hua Ma ◽  
Shi-Lei Qiao ◽  
Peng Xiang ◽  
Andrei V. Grebennikov ◽  
Renjie Zhou
Keyword(s):  
Continental Crust ◽  
Continental Margin ◽  
Lower Crust ◽  
Magma Mixing ◽  
Hybrid Origin ◽  
Plate Boundaries ◽  
Mafic Enclaves ◽  
Primary Location ◽  
Crustal Reworking ◽  
Zircon Saturation

AbstractConvergent plate boundaries are the primary location for the formation of continental crust by the intrusion of arc batholiths that contain essentially mantle-derived magmas. This paper presents two types of arc granitoids (enclave-free monzogranites and enclave-bearing granodiorites) in northeastern (NE) China to understand crustal evolution and growth in the eastern Asian continental margin. The monzogranites (189 Ma) show characteristics typical of upper continental crust, with high SiO2 contents and enrichment of K, Rb, and Pb. These monzogranites have low ISr (87Sr/86Sr) ratios (0.70378–0.70413) and positive εNd (t) (+2.2 to +2.3) and εHf (t) (+7.3 to +10.2) values. These features, combined with high zircon saturation temperatures (TZr > 800 °C), suggest that the monzogranites were generated by the heat-fluxed melting of juvenile lower crust. In contrast, the granodiorites (171 Ma) contain abundant coeval mafic enclaves and show relatively low silica contents, low TZr (748–799 °C), and particularly wide variation in εHf (t) (−3.5 to +5.6), implying a hybrid origin involving both mantle- and crust-derived components. Isotopic modeling indicates that mantle material accounts for around 60%–70% of the hybrid magmas by volume. The granodiorites have adakite-like signatures (e.g., Sr/Y > 21 and [La/Yb]N > 15), which may have been primarily caused by a process of magma mixing and hornblende-dominated fractional fractionation, rather than through melting of a subducting slab or thickened lower crust. The two distinct granitoids (monzogranites and granodiorites) represent continental crustal reworking and growth, respectively, related to the subduction of the Paleo-Pacific Plate beneath the eastern Asian continental margin during the Jurassic.

Combined Sm-Nd and Rb-Sr isotope systematics in the Donegal granitoids and their petrogenetic implications

1990 ◽  
Vol 127 (1) ◽  
pp. 75-80 ◽  
Author(s):  
C. S. Dempsey ◽  
A. N. Halliday ◽  
I. G. Meighan
Keyword(s):  
Continental Crust ◽  
Isotope Ratios ◽  
Amphibolite Facies ◽  
Sr Isotope ◽  
Isotopic Data ◽  
Isotopic Compositions ◽  
Isotopic Variations ◽  
Isotope Systematics

AbstractThe metaluminous to peraluminous granitoids of the Donegal batholith, northwest Ireland, were emplaced at c. 400 Ma into greenschist-amphibolite facies metasediments of the Dalradian Supergroup. Sm-Nd and Rb-Sr isotopic data are provided for eleven granitoid samples from six of the plutons and one specimen from the northeast granodiorite pluton of the Newry complex, Co. Down; the Donegal results reveal essentially similar initial Sr isotope ratios (0.7051–0.7068) but highly variable initial eNd values, −1.2 to −8.3 (and −0.5 for Newry). Certain granitoids have distinctive Nd isotopic compositions characteristic of the involvement of old, LREE-enriched continental crust in some cases or young crust and/or mantle-derived magmas in others. The Nd and Sr isotopic variations can be explained by a variety of mixing hypotheses.

A Discussion on the evolution of the Precambrian crust - Geochemistry of granulite facies rocks and problems of their origin

1973 ◽  
Vol 273 (1235) ◽  
pp. 429-442 ◽  
Keyword(s):  
High Pressure ◽  
Continental Crust ◽  
Major Element ◽  
Granulite Facies ◽  
Sr Isotope ◽  
Average Surface ◽  
Magmatic Rocks ◽  
Mineral Associations ◽  
High Pressure Granulite ◽  
Critical Mineral

Occurrences of granulite facies rocks are widespread in continental regions where they mostly are parts of stable shield areas. Granulite facies terrains are classified as low-, medium- or high-pressure terrains on the basis of critical mineral associations. Special interest is attached to the medium- and highpressure terrains, as they are representative of the deepest crustal levels available for study in any areal extent on the surface, and may give information about the composition of the lower continental crust. Granulite facies terrains are mainly composed of metamorphic and metasomatic rocks, but magmatic rocks with primary igneous textures interpreted as formed by crystallization of magmas under granulite facies conditions are frequent in some areas. Examples of such rocks are anorthosites, gabbros and mangerites. The low-pressure—high-temperature granulite facies rocks are chemically indistinguishable from the amphibolite facies gneisses with which they characteristically occur. It is therefore important to make a distinction between these and the higher pressure types. The medium- to high-pressure granulite facies terrains are characterized by a less ‘acidic’ average major element compositions, and significant depletions in Rb, Cs, Th and U compared with average surface shield compositions. Available data also indicate low initial Sr isotope ratios, even in the gneissic types. In the author’s opinion the important problem associated with granulite facies rocks is not that of their origin, but rather of their importance as constituents of the continental crust, and how they attained their present chemistry.

The Athabasca Granulite Terrane and Evidence for Dynamic Behavior of Lower Continental Crust

2018 ◽  
Vol 46 (1) ◽  
pp. 353-386 ◽  
Author(s):  
Gregory Dumond ◽  
Michael L. Williams ◽  
Sean P. Regan
Keyword(s):  
Continental Crust ◽  
Lower Crust ◽  
Tectonic Evolution ◽  
Paradigm Shift ◽  
Lower Continental Crust ◽  
Deep Crust ◽  
And Behavior ◽  
Lower Crustal ◽  
New Evidence ◽  
Lower Crustal Xenolith

Deeply exhumed granulite terranes have long been considered nonrepresentative of lower continental crust largely because their bulk compositions do not match the lower crustal xenolith record. A paradigm shift in our understanding of deep crust has since occurred with new evidence for a more felsic and compositionally heterogeneous lower crust than previously recognized. The >20,000-km2Athabasca granulite terrane locally provides a >700-Myr-old window into this type of lower crust, prior to being exhumed and uplifted to the surface between 1.9 and 1.7 Ga. We review over 20 years of research on this terrane with an emphasis on what these findings may tell us about the origin and behavior of lower continental crust, in general, in addition to placing constraints on the tectonic evolution of the western Canadian Shield between 2.6 and 1.7 Ga. The results reveal a dynamic lower continental crust that evolved compositionally and rheologically with time.

scholarly journals The earthquake cycle in the dry lower continental crust: insights from two deeply exhumed terranes (Musgrave Ranges, Australia and Lofoten, Norway)

2021 ◽  
Vol 379 (2193) ◽  
pp. 20190416 ◽  
Author(s):  
Luca Menegon ◽  
Lucy Campbell ◽  
Neil Mancktelow ◽  
Alfredo Camacho ◽  
Sebastian Wex ◽  
...  
Keyword(s):  
Continental Crust ◽  
Lower Crust ◽  
Shear Zones ◽  
Wall Rock ◽  
Earthquake Cycle ◽  
Geological Time ◽  
Lower Continental Crust ◽  
Seismic Slip ◽  
Crustal Earthquakes ◽  
Deep Source

This paper discusses the results of field-based geological investigations of exhumed rocks exposed in the Musgrave Ranges (Central Australia) and in Nusfjord (Lofoten, Norway) that preserve evidence for lower continental crustal earthquakes with focal depths of approximately 25–40 km. These studies have established that deformation of the dry lower continental crust is characterized by a cyclic interplay between viscous creep (mylonitization) and brittle, seismic slip associated with the formation of pseudotachylytes (a solidified melt produced during seismic slip along a fault in silicate rocks). Seismic slip triggers rheological weakening and a transition to viscous creep, which may be already active during the immediate post-seismic deformation along faults initially characterized by frictional melting and wall-rock damage. The cyclical interplay between seismic slip and viscous creep implies transient oscillations in stress and strain rate, which are preserved in the shear zone microstructure. In both localities, the spatial distribution of pseudotachylytes is consistent with a local (deep) source for the transient high stresses required to generate earthquakes in the lower crust. This deep source is the result of localized stress amplification in dry and strong materials generated at the contacts with ductile shear zones, producing multiple generations of pseudotachylyte over geological time. This implies that both the short- and the long-term rheological evolution of the dry lower crust typical of continental interiors is controlled by earthquake cycle deformation. This article is part of a discussion meeting issue ‘Understanding earthquakes using the geological record’.

Reflection seismic surveys to site the Drilling the Ivrea Verbano zonE (DIVE) proposed drill-holes, Val Sesia and Val d’Ossola, Italy.

2020 ◽  
Author(s):  
Andrew Greenwood ◽  
Ludovic Baron ◽  
Yu Liu ◽  
György Hetényi ◽  
Klaus Holliger ◽  
...  
Keyword(s):  
Continental Crust ◽  
Transition Zone ◽  
Lower Crust ◽  
Seismic Reflection ◽  
Spatial Scales ◽  
Seismic Reflection Data ◽  
Seismic Surveys ◽  
Crustal Rocks ◽  
Drill Holes ◽  
Lower Crustal

<p>The Ivrea-Verbano Zone in the Italian Alps represents one of the most complete and best-studied cross-sections of the continental crust. Here, geological and geophysical observations indicate the presence of the Moho transition zone at shallow depth, possibly as shallow as 3 km in the location of Balmuccia in Val Sesia. Correspondingly, the Ivrea-Verbano Zone is a primary target for assembling data on the deep continental crust as well as for testing several hypotheses regarding its formation and evolution.</p><p>            Within the context of a project submitted to the International Continental Scientific Drilling Program (ICDP), the Drilling the Ivrea-Verbano zonE (DIVE) team proposes to establish three drill holes across pertinent structures within the Ivrea-Verbano Zone. Two of the planned drill holes, each with a length of ~1000 m, are within Val d’Ossola and target the Pre-Permian lower and upper section of the lower crust. The third proposed drill hole, with a length of ~4000 m, is targeting the lower most crust of the Permian magmatic system of the Ivrea-Verbano Zone in the Val Sesia, close to the Insubric Line. Combined, the three drill holes will compose a complete section of the lower crust and the Moho transition zone, and will reveal the associated structural and composition characteristics at different scales.</p><p>To bridge across the range of spatial scales and to support the drilling proposal, we have carried out active seismic surveys using an EnviroVibe source in the Val d’Ossola. These surveys combined 2D transects (in-line) with the simultaneous collection of short cross-lines, and spatially varied source points, to collect sparse 3D data with a preferential CMP coverage across strike. This survey geometry was largely controlled by environmental considerations and access for the vibrator. Accordingly, 2D profiles, both in-line and cross-line, have been processed using crooked-line geometries, which include CMPs from the 3D infill.</p><p>The very high acoustic impedance contrast of the Quaternary valley infill sediments with respect to the predominant metapelitic and gabbroic lower crustal rocks, as well as the highly attenuative nature of the sediments, were both beneficial and problematic. The former enables mapping of the valley structure, while the latter largely prevents the detection of low-amplitude reflections from within the underlying lower crustal rocks.</p><p>Here, we present the latest results of these seismic reflection surveys and discuss the observations with respect to the prevailing structure and the planning of the drilling operations. Beyond the specific objectives pursued in this study, our results have important implications with regard to the acquisition and processing of high-resolution seismic reflection data in crystalline terranes and their capacity for resolving complex, steeply dipping structures.</p>

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.

scholarly journals A New Magma Type in the Continental Collision Zone. The Case of Capraia Island (Tuscany, Italy)

Geosciences ◽  
2021 ◽  
Vol 11 (2) ◽  
pp. 104 ◽  
Author(s):  
Alba Patrizia Santo
Keyword(s):  
Continental Crust ◽  
Lower Crust ◽  
Distinct Type ◽  
Continental Collision ◽  
Time Interval ◽  
Lower Continental Crust ◽  
Mafic Enclaves ◽  
Magma Type ◽  
Rock Types ◽  
Collision Zone

The Tuscany Magmatic Province consists of a Miocene to Pleistocene association of a wide variety of rock types, including peraluminous crustal anatectic granites and rhyolites, calcalkaline and shoshonitic suites and ultrapotassic lamproites. In addition to the magma types already recognised, the occurrence of a new, distinct magma type at Capraia and Elba islands and in mafic enclaves in the San Vincenzo rhyolites has been suggested by recent studies. This particular type of magma, represented by intermediate to acidic calcalkaline rocks showing high Sr, Ba, and LREE, is restricted to the northwestern sector of the province and to a time interval of about 8 to 4.5 Ma. New data obtained on rocks from Capraia Island have allowed for the verification of the occurrence of this new magma type, the exploration of its origin and a discussion of its possible geodynamic significance. The high-Sr-Ba andesite-dacite rocks occurring in the Laghetto area at Capraia display a composition that is intermediate between adakitic and calcalkaline rocks. It is suggested that they represent a distinct type of magma that originated at mantle pressure by melting of the lower continental crust, followed by mixing with other Capraia magmas. The geodynamic model that best explains the composition of the studied rocks is the thickening of the continental crust during continental collision, followed by extension that favoured melting of the lower crust.

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