Preservation of ancient and fertile lithospheric mantle beneath the southwestern United States.
TL;DR: It is suggested that depleted mantle is intrinsically less dense than fertile mantle (due to iron having been lost when melt was extracted from the rock), which allows the depleted mantle to form a thicker thermal boundary layer between the deep convecting mantle and the crust, thus reducing tectonic activity at the surface.
Abstract: Stable continental regions, free from tectonic activity, are generally found only within ancient cratons—the centres of continents which formed in the Archaean era, 4.0–2.5 Gyr ago. But in the Cordilleran mountain belt of western North America some younger (middle Proterozoic) regions have remained stable, whereas some older (late Archaean) regions have been tectonically disturbed, suggesting that age alone does not determine lithospheric strength and crustal stability. Here we report rhenium–osmium isotope and mineral compositions of peridotite xenoliths from two regions of the Cordilleran mountain belt. We found that the younger, undeformed Colorado plateau is underlain by lithospheric mantle that is 'depleted' (deficient in minerals extracted by partial melting of the rock), whereas the older (Archaean), yet deformed, southern Basin and Range province is underlain by 'fertile' lithospheric mantle (not depleted by melt extraction). We suggest that the apparent relationship between composition and lithospheric strength, inferred from different degrees of crustal deformation, occurs because depleted mantle is intrinsically less dense than fertile mantle (due to iron having been lost when melt was extracted from the rock). This allows the depleted mantle to form a thicker thermal boundary layer between the deep convecting mantle and the crust, thus reducing tectonic activity at the surface. The inference that not all Archaean crust developed a strong and thick thermal boundary layer leads to the possibility that such ancient crust may have been overlooked because of its intensive reworking or lost from the geological record owing to preferential recycling.
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TL;DR: The Pinto shear zone is one of several Late Cretaceous shear zones within the Mesozoic magmatic arc of the southwest Cordilleran orogen that developed synchronous with continued plate convergence and backarc shortening as discussed by the authors.
35 citations
TL;DR: Spinel lherzolite and harzburgite xenoliths from the Basin and Range and Colorado Plateau Provinces in the southwestern United States (SWUS) show a broad range in incompatible element distributions from depleted to enriched, both within and between sites as mentioned in this paper.
34 citations
TL;DR: Arclogites, or clinopyroxene-, garnet-, amphibolite, amphibole-, and Fe Ti oxide-bearing cumulates and restites (collectively representing residues) to andesitic continental arc magmas, are reviewed in this paper.
34 citations
TL;DR: In this paper, the authors studied deep mafic xenoliths of granulites and gabbroids from the Late Neogene alkali basalt lavas of the Enmelen and Viliga volcanic fields (Russia) and the Imuruk volcanic field in the Seward Peninsula, St. Lawrence Island, and Nunivak Island (Alaska) and concluded that a considerable portion of the lower crust of the continental margins is much younger and was generated by Cretaceous postaccretion magmatic events.
Abstract: Despite the exposures of Precambrian and Paleozoic rocks and the accretionary tectonic history of the northern Pacific (northeastern Asia, Alaska, and Kamchatka), it is likely that a considerable portion of the lower crust of the continental margins is much younger and was generated by Cretaceous postaccretion magmatic events. Data on xenoliths suggest that Late Cretaceous and Paleocene mafic intrusions and cumulates of calc-alkaline magmas may become more important with increasing depth. This conclusion is based on the petrological and geochronological investigation of lower-middle crustal xenoliths borne by mantlederived alkali basalt lavas and U-Pb dating of zircon cores from the igneous rocks of the region. We studied deep mafic xenoliths of granulites and gabbroids (accounting for <2% of the general xenolith population) from the Late Neogene alkali basalt lavas of the Enmelen and Viliga volcanic fields (Russia) and the Imuruk volcanic field in the Seward Peninsula, St. Lawrence Island, and Nunivak Island (Alaska). Depleted MORB-like varieties and relatively enriched in radiogenic isotopes and LREE rocks were distinguished among plagioclase-bearing xenoliths. The most representative collection of Enmelen xenoliths was subdivided into three groups: LREE enriched charnockitoids and mafic melts, pyroxene-plagioclase cumulates with a positive Eu anomaly, and LREE depleted garnet gabbroids. Mineral thermobarometry and calculated seismic velocities (P = 5–12 kbar, T = 740–1100°C, and V
p = 7.1 ± 0.3 km/s) suggest that the xenoliths were transported from the lower and middle crust, and the rocks show evidence for their formation through the magmatic fractionation of calc-alkaline magmas and subsequent granulite-facies metamorphism. The U-Pb age of zircon from the xenoliths ranges from the Cretaceous to Paleocene, clustering mainly within 107–56 Ma (147 crystals from 17 samples were dated). The zircon dates were interpreted as reflecting the magmatic and metamorphic stages of the growth and modification of the regional crust. The distribution of the obtained age estimates corresponds to the main magmatic pulses in two largest magmatic belts of the region, Okhotsk-Chukchi and Anadyr-Bristol. The absence of older inherited domains in zircons from both the xenoliths and igneous rocks of the regions is a strong argument in favor of the idea on the injection of juvenile material and underplating of calc-alkaline magmas in the lower crust during that time interval. This conclusion is supported by isotope geochemical data: the Sr, Nd, and Pb isotope ratios of the rocks and xenolith minerals show mantle signatures (87Sr/86Sr = 0.7040–0.70463, 143Nd/144Nd = 0.51252–0.51289, 206Pb/204Pb = 18.32–18.69) corresponding to an OIB source and are in general similar to those of the Cretaceous calc-alkaline basalts and andesites from continental-margin suprasubduction volcanoplutonic belts. Xenoliths from Nunivak Island and Cape Navarin show more depleted (MORB-like) geochemical and isotopic characteristics, which indicates variations in the composition of the lower crust near the southern boundary of the Bering Sea shelf.
34 citations
Cites background from "Preservation of ancient and fertile..."
...This is similar to other regions, such as the Sierra Nevada and the Basin and Range province in the United States, where the injection of mafic mag mas into the base of the crust or mantle underplating predicted on the basis of petrological data is also lack ing or recycled in the mantle during delamination (Ducea and Saleeby, 1996; Lee et al., 2001)....
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...…and the Basin and Range province in the United States, where the injection of mafic mag mas into the base of the crust or mantle underplating predicted on the basis of petrological data is also lack ing or recycled in the mantle during delamination (Ducea and Saleeby, 1996; Lee et al., 2001)....
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TL;DR: In this article, the Yilgarn Craton has been used to study the evolution of the first cratons in the Hadean-Eoarchaean.
Abstract: The formation of stable, evolved (silica-rich) crust was essential in constructing Earth’s first cratons, the ancient nuclei of continents. Eoarchaean (4000–3600 million years ago, Ma) evolved crust occurs on most continents, yet evidence for older, Hadean evolved crust is mostly limited to rare Hadean zircons recycled into younger rocks. Resolving why the preserved volume of evolved crust increased in the Eoarchaean is key to understanding how the first cratons stabilised. Here we report new zircon uranium-lead and hafnium isotope data from the Yilgarn Craton, Australia, which provides an extensive record of Hadean–Eoarchaean evolved magmatism. These data reveal that the first stable, evolved rocks in the Yilgarn Craton formed during an influx of juvenile (recently extracted from the mantle) magmatic source material into the craton. The concurrent shift to juvenile sources and onset of crustal preservation links craton stabilisation to the accumulation of enduring rafts of buoyant, melt-depleted mantle. Why Earth’s crust only started becoming widely preserved in the Eoarchaean, 500 Ma after planetary accretion, is poorly understood. Here, the authors document a shift to juvenile magmatic sources in the early Eoarchaean, linking crustal preservation to the formation of stabilising melt-depleted mantle.
33 citations
References
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TL;DR: In this paper, the authors compared the relative abundances of the refractory elements in carbonaceous, ordinary, and enstatite chondritic meteorites and found that the most consistent composition of the Earth's core is derived from the seismic profile and its interpretation, compared with primitive meteorites, and chemical and petrological models of peridotite-basalt melting relationships.
10,830 citations
"Preservation of ancient and fertile..." refers background in this paper
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TL;DR: In this article, the Wilson cycle is used to balance the tectosphere by depleting the continental upper mantle in a basalt-like component, which stabilizes the old continental nuclei against convective disruption.
Abstract: Beneath the old continental nuclei are thick root zones which translate coherently during plate motions. These zones are apparently stabilised against convective disruption by the depletion of the continental upper mantle in a basalt-like component. Construction of this delicately balanced tectosphere is accomplished by the dynamic and magmatic processes of the Wilson cycle.
770 citations
"Preservation of ancient and fertile..." refers background in this paper
...This allows the depleted mantle to form a thicker thermal boundary laye...
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TL;DR: Isotope analyses of Os, Sr, Nd, and Pb elements were caried out on twelve peridotite xenoliths from the Jagersfontein, Letseng-la-terae, Thaba Patsoa, Mothae, and Premier kimberlites of southern Africa, to investigate the timing and the nature of melt extraction from the continental lithosphere and its relation to the continent formation and stabilization.
688 citations
"Preservation of ancient and fertile..." refers background in this paper
...Assuming that partial melting leads to stabilization of the lithospheric mantle, the Re–Os isotope systematics of peridotite xenoliths (samples of the lithospheric mantle) can be used to date this time of stabilization; this is because partial melting fractionates Re/Os (Re is moderately depleted and Os is sequestered in the residu...
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TL;DR: In this article, three age provinces have been delineated, each generally northeast-southwest trending, having decreasing crystallization ages and increasing initial e nd values with increasing distance southeastward from the Archean craton.
Abstract: Initial Nd isotopic ratios of crystalline rocks from an area of ∼ 1.5 × 10 6 km 2 of the western United States have been determined in order to map Precambrian age province boundaries and thus document the growth and modification of the North American continent in the Proterozoic. The use of three representative rock suites of different ages— Mesozoic and Tertiary peraluminous granitic rocks, middle Proterozoic (ca. 1.4 Ga) “an-orogenic” granitic rocks, and lower Proterozoic (ca. 1.7 Ga) igneous and metamorphic rocks—allows the ages of the provinces to be distinguished on the basis of different Nd isotopic evolution paths rather than solely on the basis of model ages. Three age provinces have been delineated, each generally northeast-southwest trending, having decreasing crystallization ages and increasing initial e Nd values with increasing distance southeastward from the Archean craton. Province 1 is composed of crustal rocks of central Utah and northeastern Nevada, which are characterized by average values of e Nd (1.7 Ga) ≈ 0 and T DM ≈ 2.0–2.3 Ga. Province 2 covers Colorado, southern Utah, and northwestern Arizona and has e Nd (1.7 Ga) ≈ +3 and T DM ≈ 1.8–2.0 Ga. Province 3, which comprises the basement rocks of New Mexico and southern Arizona, has e Nd (1.7 Ga) ≈ +5 and T DM ≈ 1.7–1.8 Ga. An additional region of province 1-type isotopic characteristics, herein named “Mojavia,” is found in eastern California and western Nevada. Crust formation in each province involved a large component of mantle-derived material plus a moderate amount (∼20%) of pre-existing crust. As the new crust was built outward from the Archean nucleus, however, contributions of Archean material to the newly forming crust were more effectively screened, so that the most distal province (3) is derived almost entirely from Proterozoic mantle. The province boundaries are subparallel to the crystallization age trends determined by other workers. An exception to this is the Mojavia region of province 1, which crosscuts and truncates the other provinces in the region of the lower Colorado River. This region appears to be displaced relative to other areas of the North American basement that have similar isotopic characteristics. This suggests the presence of a previously unrecognized large-scale, left-lateral, north-south–trending basement offset of Proterozoic age in the vicinity of the California-Arizona border.
518 citations
"Preservation of ancient and fertile..." refers background in this paper
...But in the Cordilleran mountain belt of western North America some younger (middle Proterozoic) regions have remained stabl...
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