Continental tholeiitic mafic rocks of the Paleoproterozoic Hurwitz Group, Central Hearne sub-domain, Nunavut: insight into the evolution of the Hearne sub-continental lithosphere

2003 ◽  
Vol 40 (9) ◽  
pp. 1219-1237 ◽  
Author(s):  
H A Sandeman ◽  
B L Cousens ◽  
C J Hemmingway
Keyword(s):  
Lithospheric Mantle ◽  
Primitive Mantle ◽  
Geochemical Data ◽  
Mafic Rocks ◽  
Isotopic Signatures ◽  
Basaltic Rocks ◽  
Depleted Mantle ◽  
Wide Range ◽  
Continental Lithospheric Mantle ◽  
Hurwitz Group

The Paleoproterozoic Hurwitz Group of the western Churchill Province is an erosional remnant of an areally extensive, predominantly shallow-water intracratonic basin comprised of four major sequences. Sequence 2, forming the central part of the stratigraphy, contains the Ameto Formation, a sequence of pillowed and massive basaltic rocks and associated gabbro sills termed the Happotiyik Member that are interlayered with subordinate deep-water mudstones, siltstones, and diamictites. Whole-rock geochemical data for the mafic rocks reveals a suite of homogeneous tholeiitic basalts with affinities to both continental and volcanic-arc tholeiites. Compatible trace elements and large-ion lithophile elements exhibit scattered behavior, whereas all high field strength elements show a systematic increase with Zr. The rocks are large-ion lithophile and light rare-earth element enriched, and have parallel primitive mantle normalized extended trace element patterns with prominent negative Nb, Ta, and Ti anomalies. εNd(t=2200 Ma) values for the rocks range from 0.0 to +0.8. The data indicate that the parental magmas were derived from a heterogeneous, predominantly depleted mantle source that included a minor metasomatically enriched component. Contamination by Neoarchean, juvenile silicic upper crust during ascent was minimal. We envisage that the rocks of the Happotiyik Member were generated from sub-continental lithospheric mantle that was stabilized immediately after formation of the ca. 2680 Ma, Neoarchean Central Hearne sub-domain. This enrichment occurred via metasomatic infiltration of subduction-derived fluids and melts into the overlying lithosphere. A wide range of Paleoproterozoic intra-continental mafic rocks in the western Churchill Province exhibit comparable geochemical and isotopic signatures that suggest an origin in the lithospheric mantle. These observations imply that the Hearne sub-continental lithospheric mantle has endured since the Neoarchean and likely persists today.

scholarly journals Elemental and Sr–Nd–Pb isotopic geochemistry of Mesozoic mafic intrusions in southern Fujian Province, SE China: implications for lithospheric mantle evolution

2007 ◽  
Vol 144 (6) ◽  
pp. 937-952 ◽  
Author(s):  
JUN-HONG ZHAO ◽  
RUIZHONG HU ◽  
MEI-FU ZHOU ◽  
SHEN LIU
Keyword(s):  
Trace Element ◽  
Mantle Source ◽  
Lithospheric Mantle ◽  
Type A ◽  
Primitive Mantle ◽  
Trace Element Geochemistry ◽  
Spinel Lherzolite ◽  
Fujian Province ◽  
Mafic Rocks ◽  
Se China

AbstractCretaceous mafic dykes in Fujian province, SE China provide an opportunity to examine the nature of their mantle source and the secular evolution of the Mesozoic lithospheric mantle beneath SE China. The mafic rocks have SiO2 ranging from 47.42 to 55.40 wt %, Al2O3 from 14.0 wt % to 20.4 wt %, CaO from 4.09 to 11.7 wt % and total alkaline (K2O+Na2O) from 2.15 wt % to 6.59 wt %. Two types are recognized based on their REE and primitive mantle-normalized trace element patterns. Type-A is the dominant Mesozoic mafic rock type in SE China and is characterized by enrichment of light rare earth elements (LREE) ((La/Yb)n = 2.85–19.0) and arc-like trace element geochemistry. Type-P has relatively flat REE patterns ((La/Yb)n = 1.68–3.43) and primitive mantle-like trace element patterns except for enrichment of Rb, Ba and Pb. Type-A samples show EMII signatures on the Sr-Nd isotopic diagram, whereas type-P rocks have high initial 143Nd/144Nd ratios (0.5126–0.5128) relative to the type-A rocks (143Nd/144Nd = 0.5124–0.5127). The type-A rocks have 207Pb/204Pb ranging from 15.47 to 15.67 and 206Pb/204Pb from 18.26 to 18.52. All the type-A rocks show a negative correlation between 143Nd/144Nd and 206Pb/204Pb ratios and a positive relationship between 87Sr/86Sr and 206Pb/204Pb ratios, indicating mixing of a depleted mantle source and an EMII component. Geochemical modelling shows that the parental magmas were formed by 5–15 % partial melting of a spinel lherzolite, and contaminated by less than 1 % melt derived from subducted sediment. The type-P magmas were derived from a mantle source unmodified by subduction components. The wide distribution of type-A dykes in SE China suggests that subduction-modified lithospheric mantle was extensive beneath the Cathaysia Block. Geochemical differences between Mesozoic and Cenozoic mafic rocks indicate that lithospheric thinning beneath SE China occurred in two episodes: firstly by heterogeneous modification by subducted components in early Mesozoic times, and later by chemical–mechanical erosion related to convective upwelling of the asthenosphere during Cenozoic times.

Were Karoo flood basalts derived from a LLSVP-related plume source?

2020 ◽  
Author(s):  
Arto Luttinen ◽  
Jussi Heinonen ◽  
Sanni Turunen ◽  
Richard Carlson ◽  
Mary Horan
Keyword(s):  
Oceanic Crust ◽  
Mantle Plume ◽  
Flood Basalt ◽  
Planetary Science ◽  
Large Igneous Province ◽  
Primitive Mantle ◽  
Geochemical Data ◽  
Flood Basalts ◽  
Depleted Mantle ◽  
Mantle Reservoir

<p>Examination of the least-contaminated rocks of the Jurassic Karoo flood basalt province indicates considerable compositional variability in the mantle source. New and previously published Sr, Nd, and Pb isotopic data are suggestive of two main categories of mantle reservoirs: one coincides with the field of depleted mantle (DM) -affinity oceanic crust and the other has low initial eNd (+3.3 to 0.3) and high <sup>87</sup>Sr/<sup>86</sup>Sr (0.7039 to 0.7057) and Δ8/4 (92 to 138) typical of enriched mantle 1 (EM1) -affinity oceanic crust. Previous studies have proposed the DM type reservoir included domains affected by subduction-related fluids and recycled oceanic components (e.g. Heinonen et al., 2014). The EM1 type reservoir probably also contained subducted crustal components, but the geochemical data are suggestive of an additional primitive mantle (PM) type component (Turunen et al., 2019).</p><p>Importantly, the two reservoirs can be geochemically linked to a recently identified bilateral compositional asymmetry in the volumious Karoo flood basalts (Luttinen, 2018): The DM type  reservoir is the most likely source of Nb-depleted flood basalts in the southeastern Karoo subprovince (Lebombo rifted margin and Antarctica), whereas the EM1-PM type reservoir has been identified as the principal source of the Nb-undepleted flood basalts in the northwestern subprovince (Karoo-Kalahari-Zambezi basins). The boundary between the flood basalt subprovinces and the occurrences of the DM-affinity and EM1-PM-affinity rocks overlie the Jurassic location of the margin of the Jurassic sub-African LLSVP. Magmas derived from the EM1-PM type reservoir were largely emplaced above the deep mantle anomaly, whereas those derived from the DM type reservoir were emplaced outside the footprint of the LLSVP.</p><p>Based on isotopic similarity, the EM1-PM type reservoir of the Karoo province may record the same overall LLSVP material as the Gough component in the zoned Tristan da Cunha plume (e.g. Hoernle et al., 2015). Furthermore, it is possible that the DM type reservoir of the Karoo province, which has been interpreted to represent depleted upper mantle heated by mantle plume, could also represent a plume component and that the bilateral Karoo flood basalt province as a whole could thus register melting of a large zoned plume source associated with the margin of the sub-African LLSVP.</p><p>References</p><p>Heinonen, J.S., Carlson, R.W., Riley, T.R., Luttinen, A.V., Horan, M.F. (2014). Subduction-modified oceanic crust mixed with a depleted mantle reservoir in the sources of the Karoo continental flood basalt province. Earth and Planetary Science Letters 394, 229–241. http://dx.doi.org/10.1016/j.epsl.2014.03.012</p><p>Hoernl, K., Ronde, J., Hauff, F., Garbe-Schönberg, D., Homrighausen, S., Werner, W., Morgan, J.P. (2015).  How and when plume zonation appeared during the 132 Myr evolution of the Tristan Hotspot. Nature Communications 6:7799. doi: 10.1038/ncomms8799</p><p>Luttinen, A.V. (2018). Bilateral geochemical asymmetry in the Karoo large igneous province. Scientific Reports 8:5223. doi:10.1038/s41598-018-23661-3</p><p>Turunen, S.T., Luttinen, A.V., Heinonen, J.S., Jamal, D.L. (2019). Luenha picrites, Central Mozambique – Messengers from a mantle plume source of Karoo continental flood basalts? Lithos 346–347. https://doi.org/10.1016/j.lithos.2019.105152</p>

Sub-lithospheric mantle sources for overlapping southern African Large Igneous Provinces

2021 ◽  
Author(s):  
L.D. Ashwal
Keyword(s):  
Lithospheric Mantle ◽  
Isotopic Data ◽  
Isotopic Signatures ◽  
Large Igneous Provinces ◽  
Igneous Provinces ◽  
Depleted Mantle ◽  
Heating Event ◽  
Mantle Sources ◽  
Short Time ◽  
Melt Composition

Abstract At least four spatially overlapping Large Igneous Provinces, each of which generated ∼1 x 106 km3 or more of basaltic magmas over short time intervals (<5 m.y.), were emplaced onto and into the Kaapvaal Craton between 2.7 and 0.18 Ga: Ventersdorp (2 720 Ma, ∼0.7 x 106 km3), Bushveld (2 056 Ma, ∼1.5 x 106 km3), Umkondo (1 105 Ma, ∼2 x 106 km3) and Karoo (182 Ma, ∼3 x 106 km3). Each of these has been suggested to have been derived from melting of sub-continental lithospheric mantle (SCLM) sources, but this is precluded because: (1) each widespread heating event sufficient to generate 1 to 2 x 106 km3 of basalt from the Kaapvaal SCLM (volume = 122 to 152 x 106 km3) would increase residual Mg# by 0.5 to 2 units, depending on degree of melting, and source and melt composition, causing significant depletion in already-depleted mantle, (2) repeated refertilization of the Kaapvaal SCLM would necessarily increase its bulk density, compromising its long-term buoyancy and stability, and (3) raising SCLM temperatures to the peridotite solidus would also have repeatedly destroyed lithospheric diamonds by heating and oxidation, which clearly did not happen. It is far more likely, therefore, that the Kaapvaal LIPs were generated from sub-lithospheric sources, and that their diverse geochemical and isotopic signatures represent variable assimilation of continental crustal components. Combined Sr and Nd isotopic data (n = 641) for the vast volumetric majority of Karoo low-Ti tholeiitic magmatic products can be successfully modelled as an AFC mixing array between a plume-derived parental basalt, with <10% of a granitic component derived from 1.1 Ga Namaqua-Natal crust. Archaean crustal materials are far too evolved (εNd ∼ -35) to represent viable contaminants. However, a very minor volume of geographically-restricted (and over-analysed) Karoo magmas, including picrites, nephelinites, meimechites and other unusual rocks may represent low-degree melting products of small, ancient, enriched domains in the Kaapvaal SCLM, generated locally during the ascent of large-volume, plume-derived melts. The SCLM-derived rocks comprise the well-known high-Ti (>2 to 3 wt.% TiO2) magma group, have εNd, 182 values between +10.5 and -20.9, and are characteristically enriched in Sr (up to 1 500 ppm), suggesting a possible connection to kimberlite, lamproite and carbonatite magmatism. These arguments may apply to continental LIPs in general, although at present, there are insufficient combined Sr + Nd isotopic data with which to robustly assess the genesis of other southern African LIPs, including Ventersdorp (n = 0), Bushveld (n = 55) and Umkondo (n = 18).

scholarly journals Petrogenesis and Geological Implications of the Oligocene Mingze monzodiorites, Southern Lhasa

Minerals ◽  
2020 ◽  
Vol 10 (4) ◽  
pp. 301
Author(s):  
Kailiang Zhang ◽  
Zeming Shi ◽  
Rong Liao ◽  
Feilin Zhu
Keyword(s):  
Tibetan Plateau ◽  
Field Strength ◽  
Lithospheric Mantle ◽  
High Field ◽  
Geochemical Data ◽  
High Field Strength ◽  
Mafic Rocks ◽  
Mafic Enclaves ◽  
Continental Plate ◽  
Geological Implications

The Mingze Cu-Mo deposit is located in the southern margin of the Lhasa block of the Himalayan Tibetan Plateau. Here, we report the geochronological and geochemical data from Mingze monzodiorites, which hosts the Mingze deposit. Zircon dating indicates that the Mingze monzodiorites were emplaced at ca. 31 Ma (i.e., the Oligocene). The monzodiorites have variable SiO2 and MgO contents, strongly negative high field-strength element (HFSE, such as Ta, Nb, Zr and Hf) anomalies on the normalized trace element diagram and show uniform (87Sr/86Sr)i (0.7066–0.7076), εNd(t) (−2.50 to −4.04) and εHf(t) (+1.50 to +7.50). Their geochemical compositions are different from coeval (40–30 Ma) adakite-like rocks but comparable to coeval mafic enclaves and gabbros. We propose that Mingze monzodiorites were derived from partial melting of the lithospheric mantle, which previously metasomatized by the subducted Indian continental plate that probably subducted into the overlying mantle. The concurrency of the genetically related mafic enclaves and associated intermediate to mafic rocks implies the heterogeneity of the Lhasa lower crust.

scholarly journals Gabbroides of Peterman island (West Antarctica). First information on geochemistry

2019 ◽  
Vol 65 (4) ◽  
pp. 449-461
Author(s):  
G. V. Artemenko ◽  
V. I. Ganotskiy
Keyword(s):  
Antarctic Peninsula ◽  
High Strength ◽  
Primitive Mantle ◽  
Geochemical Data ◽  
Magma Source ◽  
Magma Chambers ◽  
Small Bodies ◽  
Wide Range ◽  
The Antarctic ◽  
Crustal Magma

Peterman Island is located in the archipelago of the Wilhelm Islands on the west coast of the Antarctic Peninsula (Graham Land). It is composed of gabbroids and granitoids of the Andean complex, which formed almost 100 million years later than the volcanic group of the Antarctic Peninsula. To clarify their genesis and geodynamic conditions of formation, gabbroids of the Andean complex are of particular interest, since the petrological models of their formation are well developed. Gabbroid intrusions comprise small bodies that are widespread along the Antarctic Peninsula. Among them stand out olivine gabbros, normal gabbros, norites and hornblende gabbros. Also are found small bodies of melanogabbro-pegmatites and intramagmatic dykes, that are associated with the manifestations of ore mineralization of magnetite, ilmenite and sulfides. For this reason, they are of interest for both the minerals search and for solving the question of their genesis. To this end, we performed geochemical studies of Peterman Island gabbroids. Gabbroids of Peterman Island are represented by amphibolized medium-grained gabbro with hypidiomorphic texture. Among them, xenoliths of thinly stratified gabbroids 3 × 8 m in size were found, which are characteristic of stratified intrusions, for example, Stillwater, Bushveld, etc. Gabbroids of Peterman Island have low content of silica and potassium and according to the petrochemical characteristics correspond to peridotite gabbro. They have low contents of Cr, Ni, V and high strength lithophilic Y and Nb elements. Gabbroids have been crystallized from basic magma, differentiated in the intermediate crustal magma chambers. Positive anomalies of Sr, Eu, and Ti in the multielement diagrams and positive anomalies of europium Eu/Eu* suggest the accumulation of plagioclase and apparently, ilmenite in the magmatic chamber. The primary magma source for gabbroids was probably the primitive mantle (PM). Gabbroids are contaminated with crustal matter. This contamination is probably due to their regressive metamorphism, caused by the introduction of later intrusions of Andean complex granitoid. Finely layered xenolithic gabbroids do not differ from other homogeneous gabbros of Peterman Island in terms of chemical composition.This xenolith most likely represents a part (fragment) of the wall of the magma chamber in which the differentiation of the initial main magma took place. According to the obtained geochemical data, a wide range of compositions of the Andean complex gabbroids formed as a result of crystallization differentiation of magma melted from rocks of the composition of the primitive mantle (PM) in crustal magma chambers, which also resulted in the accumulation of ore elements — V, Co, and Cu in the residual magmatic melts.

scholarly journals Old subcontinental mantle zircon below Oahu

2021 ◽  
Vol 2 (1) ◽  
Author(s):  
John D. Greenough ◽  
Sandra L. Kamo ◽  
Donald W. Davis ◽  
Kyle Larson ◽  
Zhen Zhang ◽  
...  
Keyword(s):  
Mass Spectrometry ◽  
Lithospheric Mantle ◽  
Inductively Coupled Plasma ◽  
Prolonged Storage ◽  
Ionization Mass ◽  
Isotopic Signatures ◽  
Inductively Coupled ◽  
Mantle Evolution ◽  
Plasma Mass Spectrometry ◽  
Continental Lithospheric Mantle

AbstractOur understanding of mantle evolution suffers from a lack of age data for when the mantle geochemical variants (mantle components) developed. Traditionally, the components are ascribed to subduction of ocean floor over Earth history, but their isotopic signatures require prolonged storage to evolve. Here we report U-Pb age results for mantle-derived zircon from pyroxenite xenoliths in Oahu, Hawaii, using laser ablation inductively coupled plasma mass spectrometry and isotope dilution - thermal ionization mass spectrometry. The zircon grains have 14 million-year-old rims, Cretaceous cores, and Proterozoic Lu-Hf model ages which are difficult to reconcile with transport of the pyroxenites in the Hawaiian mantle plume because the ages would have been reset by high temperatures. We suggest the zircons may have been preserved in sub-continental lithospheric mantle. They possibly reached Oahu by asthenospheric transport after subduction at Papua New Guinea or may represent fragments of sub-continental lithospheric mantle stranded during Pangean breakup.

From the Finero phlogopite peridotite to the shoshonitic magmatism of the Dolomites: unveiling the evolution of the Sub-Continental Lithospheric Mantle beneath the Southern Alps (Northern Italy)

2021 ◽  
Author(s):  
Federico Casetta ◽  
Massimo Coltorti ◽  
Ryan B. Ickert ◽  
Darren F Mark ◽  
Pier Paolo Giacomoni ◽  
...  
Keyword(s):  
Partial Melting ◽  
Lithospheric Mantle ◽  
Planetary Science ◽  
Southern Alps ◽  
Late Triassic ◽  
Spinel Lherzolite ◽  
Depleted Mantle ◽  
Significant Enrichment ◽  
Continental Lithospheric Mantle ◽  
Ultramafic Cumulates

<p>The Mid-Triassic emplacement of shoshonitic magmas at the NE margin of the Adria plate in concomitance with extensional/transtensional tectonics is one of the most intriguing and peculiar aspects typifying the geodynamic evolution of the Western Tethyan realm. Although often hypothesized, the link between this magmatic event and the metasomatised Southern Alps Sub-Continental Lithospheric Mantle (SCLM) has never been constrained.</p><p>Geochemical and petrological analyses of lavas, dykes and ultramafic cumulates belonging to the shoshonitic magmatism of the Dolomites, coupled with pre-existing data on peridotite massifs (i.e. Finero, Balmuccia, Baldissero), were used to reconstruct the evolution of the Southern Alps SCLM between Carboniferous and Triassic. According to our model, a metasomatised amphibole + phlogopite-bearing spinel lherzolite, similar to the Finero phlogopite peridotite and likely generated by interaction between a depleted mantle and slab-derived components during the Variscan subduction, was able to produce magmas with orogenic-like affinity during Mid-Triassic. In this context, partial melting degrees of ca. 5-7% were required for producing primitive SiO<sub>2</sub>-saturated to -undersaturated melts with shoshonitic affinity (<sup>87</sup>Sr/<sup>86</sup>Sr<sub>i</sub> = 0.7032-0.7058; <sup>143</sup>Nd/<sup>144</sup>Nd<sub>i</sub> = 0.51219-0.51235; Mg #~ 70; ~1.1 wt% H<sub>2</sub>O). As testified by the H<sub>2</sub>O content in mineral phases from the Finero phlogopite peridotite (Tommasi et al., 2017), the modelled Mid-Triassic fertile lithospheric mantle could have been able to preserve a significant enrichment and volatile content (600-800 ppm H<sub>2</sub>O) for more than 50 Ma, i.e. since the Variscan subduction-related metasomatism. During the Mid-Triassic partial melting event, the modelled Finero-like mantle exhausted the subduction-related signature inherited during the Variscan subduction. Around 20 Ma later, the same lithosphere portion was affected by an asthenospheric upwelling event related to the Late Triassic-Early Jurassic opening of the Alpine Tethys (Casetta et al., 2019).</p><p>Casetta, F., Ickert, R.B., Mark, D.F., Bonadiman, C., Giacomoni, P.P., Ntaflos, T., Coltorti, M., 2019. The alkaline lamprophyres of the Dolomitic Area (Southern Alps, Italy): markers of the Late Triassic change from orogenic-like to anorogenic magmatism. Journal of Petrology 60(6), 1263-1298.</p><p>Tommasi, A., Langone, A., Padrón-Navarta, J.A., Zanetti, A., Vauchez, A., 2017. Hydrous melts weaken the mantle, crystallization of pargasite and phlogopite does not: Insights from a petrostructural study of the Finero peridotites, Southern Alps. Earth and Planetary Science Letters 477, 59-72.</p>

scholarly journals Ancient helium and tungsten isotopic signatures preserved in mantle domains least modified by crustal recycling

2020 ◽  
Vol 117 (49) ◽  
pp. 30993-31001
Author(s):  
Matthew G. Jackson ◽  
Janne Blichert-Toft ◽  
Saemundur A. Halldórsson ◽  
Andrea Mundl-Petermeier ◽  
Michael Bizimis ◽  
...  
Keyword(s):  
Isotopic Signature ◽  
Geochemical Data ◽  
Recycled Materials ◽  
Isotopic Signatures ◽  
Crustal Recycling ◽  
Ocean Island Basalts ◽  
Depleted Mantle ◽  
History Of ◽  
Earth’S Interior ◽  
Earth's Interior

Rare high-3He/4He signatures in ocean island basalts (OIB) erupted at volcanic hotspots derive from deep-seated domains preserved in Earth’s interior. Only high-3He/4He OIB exhibit anomalous182W—an isotopic signature inherited during the earliest history of Earth—supporting an ancient origin of high3He/4He. However, it is not understood why some OIB host anomalous182W while others do not. We provide geochemical data for the highest-3He/4He lavas from Iceland (up to 42.9 times atmospheric) with anomalous182W and examine how Sr-Nd-Hf-Pb isotopic variations—useful for tracing subducted, recycled crust—relate to high3He/4He and anomalous182W. These data, together with data on global OIB, show that the highest-3He/4He and the largest-magnitude182W anomalies are found only in geochemically depleted mantle domains—with high143Nd/144Nd and low206Pb/204Pb—lacking strong signatures of recycled materials. In contrast, OIB with the strongest signatures associated with recycled materials have low3He/4He and lack anomalous182W. These observations provide important clues regarding the survival of the ancient He and W signatures in Earth’s mantle. We show that high-3He/4He mantle domains with anomalous182W have low W and4He concentrations compared to recycled materials and are therefore highly susceptible to being overprinted with low3He/4He and normal (not anomalous)182W characteristic of subducted crust. Thus, high3He/4He and anomalous182W are preserved exclusively in mantle domains least modified by recycled crust. This model places the long-term preservation of ancient high3He/4He and anomalous182W in the geodynamic context of crustal subduction and recycling and informs on survival of other early-formed heterogeneities in Earth’s interior.

Magmatic evolution of the southern Coast Belt: constraints from Nd–Sr isotopic systematics and geochronology of the southern Coast Plutonic Complex

1995 ◽  
Vol 32 (10) ◽  
pp. 1681-1698 ◽  
Author(s):  
R. M. Friedman ◽  
J. B. Mahoney ◽  
Y. Cui
Keyword(s):  
Middle Jurassic ◽  
Igneous Rocks ◽  
Geochemical Data ◽  
Crustal Rock ◽  
Southern Coast ◽  
Magmatic Arc ◽  
Field Evidence ◽  
Depleted Mantle ◽  
Zircon Crystallization ◽  
Wide Range

Igneous rocks of the southern Coast Belt (SCB) and adjacent Insular Belt developed within a Jurassic–Quaternary magmatic arc built across accreted juvenile-arc and oceanic terranes. SCB plutons are mostly of intermediate composition, with I-type characteristics and major element, trace element, and rare earth element geochemistry consistent with genesis in a subduction-related magmatic arc. Ubiquitous xenoliths and migmatitic zones at pluton–county rock contacts indicate that assimilation of crustal rock was an important magmatic process. U–Pb zircon crystallization ages for SCB and Insular Belt igneous rocks indicate an overall eastward migration of the magmatic axis from Middle Jurassic through Late Cretaceous time. Although absent in most rocks, traces of old inherited zircon are present in several Middle Jurassic–Upper Cretaceous plutons in the southeastern Coast Belt. The primitive character and restricted range of Nd–Sr isotopic data for Middle Jurassic to Quaternary igneous rocks of the SCB (εNd = +2.4 to +8.0; Sri = 0.7030 − 0.7042) indicate they were generated in an isotopically juvenile magmatic arc. The distribution of isotopic values along the mantle array and the wide range of fSm/Nd values suggest magma was derived from depleted mantle within a mantle wedge, with little or no contribution from old, isotopically evolved continental material. Although field evidence suggests that assimilation of juvenile crust was an important process during magma ascent, isotopic and geochemical data do not permit discrimination between direct mantle derivation of magmas followed by fractionation and crustal assimilation, and wholesale melting of mafic arc-derived lower crust.

Geochemistry and models of mantle circulation

1989 ◽  
Vol 328 (1599) ◽  
pp. 425-439 ◽  
Keyword(s):  
Mass Balance ◽  
Continental Crust ◽  
Oceanic Crust ◽  
Large Scale ◽  
Oceanic Islands ◽  
Primitive Mantle ◽  
Geochemical Data ◽  
Depleted Mantle ◽  
Subducted Oceanic Crust ◽  
Subcontinental Lithosphere

Geochemical data help to constrain the sizes of identifiable reservoirs within the framework of models of layered or whole-mantle circulation, and they identify the sources of the circulating heterogeneities as mainly crustal and/or lithospheric, but they do not decisively distinguish between different types of circulation. The mass balance between crust, depleted mantle and undepleted mantle based on 143 Nd/ 144 Nd, Nb/U and Ce/Pb, and the concentrations of very highly incompatible elements Ba, Rb, Th, U, and K, shows that ca. 25- 70% (by mass) of depleted mantle balances the trace element and isotopic abundances of the continental crust. This mass balance reflects the actual proportions of mantle reservoirs only if there are no additional unidentified reservoirs. Evidence on the nature and ages of different source reservoirs comes from the geochemical fingerprints of basalts extruded at mid-ocean ridges and oceanic islands. Consideration of Nd and He isotopes alone indicates that ocean island basalts (oibs) may be derived from a relatively undepleted portion of the mantle. This has in the past provided a geochemical rationale for a two-layer model consisting of an upper depleted and a lower undepleted (‘primitive’) mantle layer. However, Pb-isotopic ratios, and Nb/U and Ce/Pb concentration ratios demonstrate that most or all oib source reservoirs are definitely not primitive. Models consistent with this evidence postulate recycling of oceanic crust and lithosphere or subcontinental lithosphere. Recycling is a natural consequence of mantle convection. This cannot be said for some other models such as those requiring large-scale vertical metasomatism beneath oib source regions. Unlike other trace elements, Nb, Ta, and Pb discriminate sharply between continental and oceanic crust-forming processes. Because of this, the primitive mantle value of Nb/U = 30 (Ce/Pb = 9) has been fractionated into a continental crustal Nb/U = 12 (Ce/Pb = 4) and a residual-mantle (morb (mid-ocean ridge basalt) plus oib source) Nb/U = 47 (Ce/Pb = 25). These residual mantle values are uniform within about 20% and are not fractionated during formation of oceanic crust. By using these concentrations ratios as tracers, it can be shown that the possible contribution of recycled continental crust to oib sources is limited to a few percent. Therefore, recycling must be dominated by oceanic crust and lithosphere, or by subcontinental lithosphere. Oceanic crust normally bears a thin layer of pelagic sediment at the time of subduction, and this is consistent with oib sources that are dominated by subducted oceanic crust with variable but always small additions of continental material. Primordial 3 He, 36 Ar, and excess 129 Xe, in oceanic basalts demonstrate that the mantle has been neither completely outgassed nor homogenized, but they do not constrain the degree of mixing or the size of reservoirs. Also, helium does not correlate well with other isotopic data and may have migrated into the basalt source from other regions. The high 3 He/ 4 He ratios found in some oibs suggest that, even though the basalts are not derived from primordial mantle, their sources may be located close to a reservoir rich in primordial gases. This leads to models in which the oib sources are in a boundary layer within the mantle. The primordial helium migrates into this layer from below. The interpretation of the rare-gas data is still quite controversial. It is often argued that the upper mantle is a well-homogenized reservoir, but the data indicate heterogeneities on scales ranging from 10° to 10 6 m. The 206 Pb/ 204 Pb ratios in the oceanic m antle range from 17 to 21, which is similar to the range in most continental rocks. The degree of mixing cannot be directly inferred from these data unless the size and composition of the heterogeneities and the time of their introduction into the system are known. The relative uniformity of Nb/U and Ce/Pb ratios in the otherwise heterogeneous morb and oib sources indicates that this reservoir was indeed homogenized after the separation of the continental crust, and that the observed isotopic and chem ical heterogeneities were introduced subsequently. Overall, the results are consistent with, but do not prove, a layered mantle where the upper layer contains both morb and oib sources, and the lower, primitive mantle is not sampled by present-day volcanism. Alternative models such as those involving a chemically graded mantle have not been sufficiently explored.

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