Evolution of crust vs. mantle contributions to continental arc granitoids within a few Myr: evidence from zircon Hf-O isotopes and high-precision U-Pb dating in the Famatinian Arc, Argentina

2020 ◽  
Author(s):  
Julien Cornet ◽  
Oscar Laurent ◽  
Jörn-Frederik Wotzlaw ◽  
Juan Otamendi ◽  
Olivier Bachmann
Keyword(s):  
Continental Crust ◽  
High Precision ◽  
Lower Crust ◽  
Plate Tectonics ◽  
Complex Function ◽  
Mafic Magma ◽  
Isotopic Signatures ◽  
Mafic Intrusions ◽  
Relative Contribution ◽  
Lower Crustal

<p>The presence of a thick continental crust makes Earth a unique planet in the solar system. During post-Archaean times, with the onset of plate tectonics, processes by which continents form is a complex function of juvenile growth and recycling of pre-existing crust. Indeed, post-Archean mantle-derived magmas commonly intrude pre-existing, felsic continental crust. As a result, the origin of upper crustal granitoids, the most accessible products of planetary differentiation, is either accounted for by the melting of the pre-existing mid- to lower crust or the differentiation of mantle-derived mafic magmas. It is therefore critical to identify the relative contribution of these two different granite-forming processes in a given magmatic province, as well as how this relative contribution evolves over time, to assess crustal growth and/or recycling. To shed some light on this question, we used the combination of oxygen, hafnium and uranium-lead isotopic systems in zircons from granitoids of the Ordovician Famatinian Arc (Argentina) representing a typical crust-forming geotectonic setting. While the lower crustal section of Valle Fertíl, representing the basal level of the Famatinian crust, is already well studied, little is known on the timing and nature of igneous processes that built up the mid- and upper crust. </p><p>From our study, we observe a systematic co-variation of the O and Hf isotopic signatures of zircon in the mid- to upper crustal rocks, from a clearly crustal footprint (granodiorites with zircon δ<sup>18</sup>O of ca. +8 ‰; εHf<sub>t</sub> of ca. –3) to a mantle-like signature (granites and rhyolites: zircon δ<sup>18</sup>O of ca. +5 ‰; εHf<sub>t</sub> of ca. +5). Moreover, the high-precision (ID-TIMS) U-Pb dating obtained from the same zircons seem to record a progressive building of the Ordovician continental crust lasting for ca. 13Myrs from 483 to 470 Myrs ago. The results overlap with published ID-TIMS U-Pb data for the Famatinian lower crust, clustering at 470 Myrs, which confirms that the Famatinian Arc was a transcrustal magmatic system ultimately fed by mantle-derived magmas. In details, the oldest granitoids (483 Myrs) show the strongest crustal Hf-O isotopic fingerprint while the younger ones define a continuous range from this end-member towards the mantle signature. These results could be explained by (i) continuous ingrowth and “self-shielding” of lower crustal mafic intrusions progressively decreasing crustal melting or contamination of ascending mafic magma from a homogenous mantle source; (ii) progressive defertilization of an enriched lithospheric mantle or a strongly slab-enriched mantle wedge. The fact that the earliest (483 Myr-old) granitoids also show a more significant crustal contribution (ASI >1.1, inherited zircon cores) supports the first scenario. In this case, the combination of Hf-O isotopic studies as well as high precision U-Pb dating for the Famatinian arc comply with a progressive building of a magmatic column where a certain amount of time is needed for the system to mature and eventually reach mantle dominated processes in the formation of granites and so, new continental crust.</p>

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>

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.

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 Hydrated Peridotite – Basaltic Melt Interaction Part I: Planetary Felsic Crust Formation at Shallow Depth

2021 ◽  
Vol 9 ◽  
Author(s):  
Anastassia Y. Borisova ◽  
Nail R. Zagrtdenov ◽  
Michael J. Toplis ◽  
Wendy A. Bohrson ◽  
Anne Nédélec ◽  
...  
Keyword(s):  
Continental Crust ◽  
Plate Tectonics ◽  
Mafic Magma ◽  
Aqueous Fluid ◽  
Early Earth ◽  
Magma Ocean ◽  
Crust Formation ◽  
Basaltic Melt ◽  
Low Pressures ◽  
Direct Melting

Current theories suggest that the first continental crust on Earth, and possibly on other terrestrial planets, may have been produced early in their history by direct melting of hydrated peridotite. However, the conditions, mechanisms and necessary ingredients for this crustal formation remain elusive. To fill this gap, we conducted time-series experiments to investigate the reaction of serpentinite with variable proportions (from 0 to 87 wt%) of basaltic melt at temperatures of 1,250–1,300°C and pressures of 0.2–1.0 GPa (corresponding to lithostatic depths of ∼5–30 km). The experiments at 0.2 GPa reveal the formation of forsterite-rich olivine (Fo90–94) and chromite coexisting with silica-rich liquids (57–71 wt% SiO2). These melts share geochemical similarities with tonalite-trondhjemite-granodiorite rocks (TTG) identified in modern terrestrial oceanic mantle settings. By contrast, liquids formed at pressures of 1.0 GPa are poorer in silica (∼50 wt% SiO2). Our results suggest a new mechanism for the formation of the embryonic continental crust via aqueous fluid-assisted partial melting of peridotite at relatively low pressures (∼0.2 GPa). We hypothesize that such a mechanism of felsic crust formation may have been widespread on the early Earth and, possibly on Mars as well, before the onset of modern plate tectonics and just after solidification of the first ultramafic-mafic magma ocean and alteration of this primitive protocrust by seawater at depths of less than 10 km.

Petrogenesis of Archaean granites in the Barberton region of South Africa as a guide to early crustal evolution

2021 ◽  
Vol 124 (1) ◽  
pp. 111-140
Author(s):  
L.J. Robb ◽  
F.M. Meyer ◽  
C.J. Hawkesworth ◽  
N.J. Gardiner
Keyword(s):  
South Africa ◽  
Continental Crust ◽  
Lower Crust ◽  
Plate Tectonics ◽  
Sedimentary Basins ◽  
Crustal Thickening ◽  
Gravity Inversion ◽  
Crustal Evolution ◽  
Broad Variety ◽  
Parallel Arrays

ABSTRACT The Barberton region of South Africa is characterized by a broad variety of granite types that range in age from ca. 3.5 Ga to 2.7 Ga and reflect the processes involved in the formation of Archaean continental crust on the Kaapvaal Craton. These granites are subdivided into three groups, as follows: A tonalite-trondhjemite-granodiorite (TTG) suite diapirically emplaced at 3 450 Ma and 3 250 Ma into pre-existing metamorphosed greenstone belt material. TTG melts were derived from melting amphibolite in the lower crust, with individual plutons being emplaced at various crustal levels. The dome-and-keel geometry that characterizes the TTG-greenstone dominated crust at this time is inconsistent with a plate tectonic domain and reworking was likely controlled by gravity inversion or ‘sagduction’; Regionally extensive potassic batholiths (the GMS suite) were emplaced at 3 110 Ma during a period of crustal thickening and melting of a TTG-dominated lower crust. Subsequent to emplacement of the voluminous GMS granites, the thickened continental crust had stabilized sufficiently for large sedimentary basins to form; Late granite plutons were emplaced along two distinct linear and sub-parallel arrays close to what might have been the edge of a Kaapvaal continent at 2 800 to 2 700 Ma. They are subdivided into high-Ca and low-Ca granites that resemble the I- and S-type granites of younger orogenic episodes. The high-Ca granites are consistent with derivation from older granitoids in the lower crust, whereas the low-Ca granites may have been derived by melting metasedimentary precursors in the lower-mid crust. Granites with similar characteristics are associated with a subduction zone in younger terranes, although the recognition of such a feature at Barberton remains unclear. The petrogenesis of granites in the Barberton region between 3.5 Ga and 2.7 Ga provides a record of the processes of Archaean crustal evolution and contributes to discussions related to the onset of plate tectonics.

Lateral crustal flow along discrete flat-lying lower crustal shear zones during Archean continent construction

2020 ◽  
Author(s):  
Graham Hill ◽  
Eric Roots ◽  
Ben Frieman ◽  
Jim Craven ◽  
Richard Smith ◽  
...  
Keyword(s):  
Gravitational Collapse ◽  
Lower Crust ◽  
Plate Tectonics ◽  
Shear Zones ◽  
Transitional Period ◽  
Superior Province ◽  
Near Surface ◽  
Low Resistivity ◽  
Crustal Shear Zones ◽  
Lower Crustal

<p>The nature of lithospheric evolution and style of the driving ‘tectonic’ processes occurring during Archean continent construction remain enigmatic. A significantly hotter thermal regime characterised the early Earth and was pervasive for much of the Archean. This resulted in construction of continents that were significantly weaker and unable to support the thick crustal sequences and topographies common to modern orogens. Gravitational collapse of these early continents may have occurred when deeper material became less dense by heating or partial melting and created a density contrast beyond the crustal competence and/or due to post-orogenic relaxation. Such a collapse could result in large scale horizontal spreading within the middle to lower crust and the development of lateral crustal flow along flat-lying shear zones producing fluid-deposited graphitic and metallic sulphide films at these depths, which, if preserved would produce broad scale quasi-horizontal mid-lower crustal low resistivity anomalies. Here we show 3D magnetotelluric resistivity models of the Archean Superior Province of Canada that reveal these types of anomalies that could represent lateral crustal flow in the middle to lower crust. Further, the magnetotelluric model shows narrow sub-vertical zones of low resistivity extending from the mid crust to the near surface, interpreted to represent remnant fluid pathways that potentially formed prior to gravitational collapse. These sub-vertical low resistivity features correlate spatially with crustal-scale deformation zones that potentially are host to hydrothermal ore deposits and abundant metasomatic mineral assemblages. The well preserved record of primary crustal amalgamation within the Superior Province of Canada with both features expected of autochthonous vertical ‘drip’ tectonics (sub-vertical fluid pathways) and allochthonous horizontal plate tectonics (flat-lying lower crustal shear zones) regimes, suggests a potential transitional period of tectonic evolution might have characterised the region during the late Archean.</p>

scholarly journals Titanium isotopes as a tracer for the plume or island arc affinity of felsic rocks

2019 ◽  
Vol 116 (4) ◽  
pp. 1132-1135 ◽  
Author(s):  
Zhengbin Deng ◽  
Marc Chaussidon ◽  
Paul Savage ◽  
François Robert ◽  
Raphaël Pik ◽  
...  
Keyword(s):  
Continental Crust ◽  
Plate Tectonics ◽  
Indirect Evidence ◽  
Island Arc ◽  
Island Arcs ◽  
Calc Alkaline ◽  
Magma Differentiation ◽  
Isotopic Signatures ◽  
Crustal Rocks ◽  
Felsic Rocks

Indirect evidence for the presence of a felsic continental crust, such as the elevated 49Ti/47Ti ratios in Archean shales, has been used to argue for ongoing subduction at that time and therefore plate tectonics. However, rocks of intermediate to felsic compositions can be produced in both plume and island arc settings. The fact that Ti behaves differently during magma differentiation in these two geological settings might result in contrasting isotopic signatures. Here, we demonstrate that, at a given SiO2 content, evolved plume rocks (tholeiitic) are more isotopically fractionated in Ti than differentiated island arc rocks (mainly calc-alkaline). We also show that the erosion of crustal rocks from whether plumes (mafic in average) or island arcs (intermediate in average) can all produce sediments having quite constant 49Ti/47Ti ratios being 0.1–0.3 per mille heavier than that of the mantle. This suggests that Ti isotopes are not a direct tracer for the SiO2 contents of crustal rocks. Ti isotopes in crustal sediments are still a potential proxy to identify the geodynamical settings for the formation of the crust but only if combined with additional SiO2 information.

Petrogenesis of the Higashi-Akaishi Ultramafic Body: Implications for Lower Crustal Foundering and Mantle Wedge Processes

2020 ◽  
Author(s):  
Meghan R Guild ◽  
Christy B Till ◽  
Tomoyuki Mizukami ◽  
Simon Wallis
Keyword(s):  
Continental Crust ◽  
Sierra Nevada ◽  
Lower Crust ◽  
Mantle Wedge ◽  
Metamorphic Belt ◽  
Subduction Channel ◽  
Compositional Evolution ◽  
Garnet Pyroxenite ◽  
Lower Crustal ◽  
Crustal Foundering

Abstract Recycling of ultramafic lower crustal cumulates via delamination or foundering is often invoked as a mechanism to return mafic material to the mantle during continental crust formation. These recycled pieces of the lower crust are rarely sampled but are preserved in several locations including the Kohistan and Talkeetna arc sections, Sierra Nevada and Colorado Plateau pyroxenite xenoliths and, as discussed here for the first time, the exhumed Higashi-Akaishi (HA) ultramafic body in Japan. The HA is located in the Besshi region of the Sanbagawa metamorphic belt in southwestern Japan and is dominantly composed of dunite with lesser garnet pyroxenite and harzburgite lenses. Although the petrogenetic history of the HA body is still debated, our new bulk major and trace element compositions, radiogenic isotope data, as well as petrologic and field observations, are consistent with a lower crustal cumulate origin for the HA dunite and pyroxenite, with a later slab-derived fluid overprint. Clinopyroxene and olivine in the foliated HA dunite have compositions consistent with ultramafic cumulates with high Mg#s (Mg# clinopyroxene = 0·94, Mg# olivine = 0·88), high NiO in olivine (∼0·26 wt %) and low-Al clinopyroxene. In addition, the bulk major element chemistry of the HA dunite and garnet pyroxenite follow systematic behavior in Mg# vs SiO2 wt %, similar to those observed in other lower crustal cumulate lithologies and corresponding intrusive lithologies, pointing to different liquid lines of descent for the corresponding melts. Our new thermobarometric estimates (peak pressure–temperature at 2·6 GPa, 713ºC) are consistent with a hot slab surface subduction path, rather than the lower crustal temperatures recorded in arc sections (Kohistan & Talkeetna: 1 GPa, 800ºC). A pervasive slab-fluid influence is also indicated in the HA lithologies by LREE and Ce enrichments and strong Nb and Zr depletions. The trace elements and the pressure–temperature estimates, as well as the thermodynamic modeling results necessitate removal of the HA body from the lower crust and incorporation into cooler portions of a mantle wedge. At lower crustal conditions, the bulk density of the HA lithologies is greater than the background mantle, indicating the feasibility of lower crustal foundering into a mantle wedge where the HA was incorporated in the subduction channel to reach its peak conditions. Hydration of the HA body while in the subduction channel likely provided the change in density necessary to facilitate its rapid exhumation to the surface. Thus, the HA cumulate likely represents a piece of the subduction system that is rarely preserved, as well as a key component in the compositional evolution of the continental crust.

Isotopic abundances relevant to the identification of magma sources

1984 ◽  
Vol 310 (1514) ◽  
pp. 591-603 ◽  
Keyword(s):  
Isotopic Composition ◽  
Continental Crust ◽  
Oceanic Crust ◽  
Rare Gas ◽  
Isotopic Signatures ◽  
Isotope Tracers ◽  
Isotope Tracer ◽  
Relative Contribution ◽  
Magma Sources ◽  
Isotopic Abundances

The behaviour of natural radiogenic isotope tracers in the Earth that have lithophile and atmophile geochemical affinity is reviewed. The isotope tracer signature of oceanic and continental crust may in favourable circumstances be sufficiently distinct from that of the mantle to render a contribution from these sources resolvable within the isotopic composition of the magma. Components derived from the sedimentary and altered basaltic portion of oceanic crust are recognized in some island arc magmas from their Sr, Nd and Pb isotopic signatures. The rare-gas isotope tracers (He, Ar, Xe in particular) are not readily recycled into the mantle and thus provide the basis of an approach that is complementary to that based on the lithophile tracers. In particular, a small mantle-derived helium component may be readily recognized in the presence of a predominant radiogenic component generated in the continents. The importance of assessing the mass balance of these interactions rather than merely a qualitative recognition is emphasized. The question of the relative contribution of continental-oceanic crust and mantle to magma sources is an essential part of the problem of generation and evolution of continental crust. An approach to this problem through consideration of the isotopic composition of sediments is briefly discussed.

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