Lithospheric Replacement Research Articles

The lithosphere-asthenosphere boundary and lithosphere replacement as informed by mantle xenoliths and their host lavas

Alkaline lavas hosting mantle xenoliths occur in the hot back arc of the Cordillera of western Canada and Alaska and provide constraints on lithosphere thickness (within 5 to 10 km) and controls on the lithosphere-asthenosphere boundary (LAB). Such information is complementary to geophysical investigations of lithosphere strength and deformation in similar continental regions. Barometry of spinel lherzolite xenoliths and alkaline lavas from the Cordillera show LAB depths from 50 to 80 km, and marked thinning of the lithosphere at the Cordillera-craton transition coincident with the Rocky Mountain Trench-Tintina Fault. Similar barometric data globally for continental intraplate lavas show a relatively common lava equilibration depth of 65 to 80 km, that mostly coincide with LAB depths observed by seismology, and with the spinel- to garnet peridotite transition. The LAB is inferred as accumulated partial melt, modulated in depth by mantle solidus depression caused by varying H2O storage capacity across the garnet-spinel peridotite phase change. This mechanism does not explain cases where LAB depths are < 65 km, which instead require locally hotter asthenosphere. In many regions preserving a shallow LAB, foundering and wholesale replacement of mantle lithosphere is invoked. With the exception of North China, mantle xenoliths sampled from continental margins record cooling timescales that appear far longer than required for foundering and replacement in the past 50 Myr. Either cooling histories cannot be adequately modelled using closure/diffusion arguments or wholesale replacement of mantle lithosphere is a dubious process.

Seismic velocity structure and tectonic evolution of the Continent-ocean transition in the mid-northern South China Sea

Continent-ocean transition (COT) is a key area for studying the lithospheric replacement from the thinned continental to oceanic crust. The extent and nature of the COT in the mid-northern margin of the South China Sea (SCS) have been reported recently after multiple geological and geophysical surveys, including IODP Expeditions 367/368/368X. However, the debate still remains due to limited constraints on the deep crustal structure and lateral variations. A 172 km long wide-angle reflection/refraction profile was conducted. The detailed crustal structure of the COT was mapped based on the forward and inverse modeling. The crust thickness decreased from ∼12 km in the thinned continental domain to ∼7 km in the COT, and then transitioned to ∼5 km in the oceanic domain. A ∼ 3 km thick layer with high velocity of 7.0–7.5 km/s at the bottom of the crust occurred continuously from the thinned continental to COT domains, which is interpreted as syn-rift underplating. Based on the features of crustal structure, sedimentary sequence and gravity anomaly, the landward and oceanward boundaries of the COT were determined which delimit a ∼ 20 km wide COT. The COT documents prominent tectono-magmatic interaction and rapid transition from final rifting to seafloor spreading. Eventually a model was proposed to explain the breakup processes characterized by brittle faulting in the upper crust coevally with ductile flow in the lower crust.

Geochemical constraints on the evolution of the lithospheric mantle beneath central and southern Vietnam

We present comprehensive geochemical and isotopic (Sr-Nd-Hf) datasets for two suites of ultramafic rock in Vietnam, namely xenoliths of spinel lherzolite entrained in late Cenozoic alkali basalts, and Paleozoic ultramafic massifs that occur along the Tam Ky-Phuoc Son suture zone in central and southern Vietnam. The ultramafic massifs are the products of high degrees of melt extraction (up to 40%), and have relatively low equilibrium temperatures of 603 to 778 °C. The lherzolites are residues of relatively low degrees of fractional melting (< 1% up to 20%). The compositions of minerals in the xenoliths reveal that the late Cenozoic lithospheric mantle beneath central and southern Vietnam was hotter (825–1058 °C) than during the Paleozoic. The calculated trace element patterns of metasomatic melts that equilibrated with clinopyroxenes in the LREE-enriched xenoliths show enrichments in Th, U, and LREEs, and depletions in Nb. These data, together with the elevated Ti/Eu ratios of the clinopyroxenes, reflect the role of hydrous silicate melts as the main agents of metasomatism. The ultramafic rocks of the Paleozoic massifs contain spinel with TiO2 contents higher than expected for residual spinel, which suggests the influence of boninitic melt(s). In the lherzolite xenoliths, the clinopyroxenes have MORB-like depleted Sr-Nd-Hf isotopic compositions (87Sr/86Sr = 0.70241–0.70416; eNd = +6.6 to 12.3; eHf = +13.1 to 25.1), suggesting metasomatic melts/fluids from upwelling asthenosphere. We suggest that subduction of the (Paleo-)Pacific Plate and continental collision during the Permian–Triassic played key roles in lithospheric replacement and thinning beneath central and southern Vietnam.

Big insights from tiny peridotites: Evidence for persistence of Precambrian lithosphere beneath the eastern North China Craton

Previous studies have shown that the eastern North China Craton (NCC) lost its ancient lithospheric mantle root during the Phanerozoic. The temporal sequence, spatial extent, and cause of the lithospheric thinning, however, continue to be debated. Here we report olivine compositions, whole-rock Re–Os isotopic systematics, and platinum-group element abundances of small (<2cm in maximum dimension) mantle peridotite xenoliths from two basalt localities from the eastern NCC, Wudi (Cenozoic) and Fuxin (Cretaceous). These locations lie far (~150–200km) from the Tan–Lu fault, which has been linked to lithospheric replacement in the eastern NCC. Peridotites at both locations have fertile to moderately refractory compositions (Fo<91.5), while highly refractory (Fo>92) lithospheric mantle is largely absent. Osmium isotopic data suggest the Wudi peridotites experienced melt depletion primarily during the Paleoproterozoic (~1.8Ga), although an Archean Os model age for one xenolith indicates incorporation of a minor component of Archean lithospheric mantle. These data suggest that a previously unrecognized Paleoproterozoic orogenic event removed and replaced the original Archean lithospheric mantle beneath the sedimentary basin at the southern edge of the Bohai Sea. By contrast, the Fuxin peridotites, entrained in Cretaceous basalts that crop out along the northern edge of the eastern NCC, document the coexistence of both ancient (≥2.3Ga) and modern lithospheric mantle components. Here, the original Late Archean–Early Paleoproterozoic lithospheric mantle was, at least partially, removed and replaced prior to 100Ma. Combined with literature data, our results show that removal of the original Archean lithosphere occurred within Proterozoic collisional orogens, and that replacement of Precambrian lithosphere during the Mesozoic may have been spatially associated with the collisional boundaries and the strike–slip Tan–Lu fault, as well as the onset of Paleo-Pacific plate subduction.

Heat-flow data in the Four Corners area suggest Neogene crustal warming resulting from partial lithosphere replacement in the Colorado Plateau interior, southwest USA

The uplift of the Colorado Plateau is of great geologic interest, and low-noise terrestrial heat-flow data can provide boundary conditions that should be met by proposed models of uplift. The structural continuity of the plateau interior with respect to the neighboring Basin and Range and Southern Rocky Mountains implies a distinctive Neogene lithosphere-crustal evolution. The Four Corners area represents a unique location in the Colorado Plateau interior, and likely the world, where both heat-flow data with little variability, and mineralogy studies relating to pre-Neogene lithosphere temperatures, are available. Together these studies allow one to calculate data-derived heat-flow values at two different times, ca. 25 Ma and the present. Using a first-order model for temperature-depth profiles, these two heat-flow values imply ∼100 km of Neogene lithosphere-asthenosphere boundary (LAB) shallowing in the study area. If crustal radiogenic heat remains constant, the calculated Neogene increase in heat flow of ∼17 mW m –2 will be produced below the Moho. Several mechanisms are considered that might cause the LAB to shallow and the near-surface heat flow to increase. Derived Neogene lithosphere thinning of ∼100 km leads one to consider lithosphere displacement of the same order. If the presented finite-difference model approximates Neogene thermal phenomena in the Four Corners, then partial lithosphere replacement of the upper-mantle lithosphere appears to be the most suitable cause of the heat-flow rise in the area, as well as the near-uniform heat flow over large regions of the plateau interior identified by low-noise heat-flow data.

From enriched to depleted mantle: Evidence from Cretaceous lamprophyres and Paleogene basaltic rocks in eastern and central Guangxi Province, western Cathaysia block of South China

Much of Eastern China underwent significant lithospheric thinning during Phanerozoic time. Compared to the wealth of detailed research on the destruction of the North China Craton since the Mesozoic, the lithospheric evolution of South China (including the Yangtze Craton and the Cathaysia block) is still unclear. In particular, studies of the western part of the Cathaysia block, which connects the Yangtze Craton in the west to the eastern Cathaysia block in the east, are sparse. The study of the mantle-derived magmatic rocks from the western Cathaysia block can provide a better understanding of the continental evolution of South China, and of greater East Asia. Elemental and SrNd isotopic compositions of the Cretaceous (89Ma) lamprophyres and Paleogene (51–28Ma) basaltic rocks in eastern and central Guangxi Province, western Cathaysia block, were analyzed to reveal the nature of their mantle sources. The Cretaceous lamprophyres are ultrapotassic, strongly enriched in LILE–LREE and depleted in HFSEs (e.g., Nb, Ta, Ti), and have high (87Sr/86Sr)i and negative εNd (t) values. They probably were derived from low-degree partial melting of an EM2-type garnet-facies (>80km) subcontinental lithospheric mantle (SCLM), followed by the fractionation of olivine and clinopyroxene. The modeled SCLM, mainly a phlogopite-bearing harzburgite, represents a refractory mantle that was metasomatically enriched by subduction, prior to the Pacific subduction during Mesozoic. The Paleogene basaltic rocks are enriched in alkalis, LILEs and LREE but positive NbTa anomalies, and are similar to alkali oceanic-island basalts. These basaltic rocks have DM-type SrNd isotopic signatures, with low (87Sr/86Sr)i and high εNd (t). They probably were derived from the low-degree partial melting of fertile (asthenospheric) mantle in spinel- to spinel-garnet-facies lherzolite (<80km). The differences imply that the source region was transformed from EM2 in Cretaceous time to DM-mantle in Paleogene time, and that lithospheric replacement occurred over Cretaceous–Paleogene time. It was driven by the two-sided dynamics around the Cathaysia block, with Pacific subduction in the east and the Eurasian–Indian plate collision in the west. The complex subduction dynamics led to complex asthenospheric convection, a regional lithospheric extension beneath the Cathaysia block, and the removal or modification of the ancient SCLM.

Contributions of the lower crust to Mesozoic mantle-derived mafic rocks from the North China Craton: implications for lithospheric thinning

Abstract The lithospheric mantle underneath the North China Craton changed completely from the Palaeozoic to the Cenozoic. This study reviews geochemical data from Mesozoic mantle-derived mafic rocks from the North China Craton to investigate the role of mafic lower continental crust in lithosphere replacement. Samples from the North China Craton have typical ‘continental’ geochemical signatures, including depletion of high field strength elements, enrichment of large ion lithophile elements and Pb, unradiogenic Pb isotopes, and enriched Sr–Nd isotopic ratios. Positive correlation between initial 87 Sr/ 86 Sr and 206 Pb/ 204 Pb, low Ce/Pb and Nb/U, high Ba/Nb and La/Nb, and unradiogenic Pb isotopes of Mesozoic mafic rocks cannot simply be explained by derivation from a lithospheric mantle enriched by ancient (Archaean or Mesoproterozoic) fluid or melt metasomatism. Instead, they more probably result from a lithospheric mantle or upwelling asthenosphere underneath the North China Craton that was modified by the lower continental crust in the Mesozoic. Because oceanic plate subduction zones surrounded the North China Craton during the late Palaeozoic, the lithospheric mantle underneath the North China Craton was weakened by fluids derived from subducted slabs, and thus shortened and thickened by continent–continent collisions of the North China Block with the South China Block and the Siberian plate. Metamorphic reactions occurred in the mafic lower continental crust beneath the North China Craton, creating garnet-bearing assemblages (eclogite and garnet pyroxenite) with densities of up to 3.8 g cm −3 , which led to negative buoyancy in the over-thickened lithosphere. The unstable lithosphere was delaminated and subsided into the uppermost mantle. The delaminated lower crust partially melted, producing SiO 2 -rich melts that metasomatized surrounding asthenospheric mantle, which upwelled and replaced the volume formerly occupied by the delaminated lithospheric mantle, resulting in the ‘continental’ geochemical signatures widely observed in Mesozoic mantle-derived mafic rocks from the North China Craton. The ‘continental’ geochemical signatures of Mesozoic mantle-derived mafic rocks suggest that lithospheric delamination could have occurred by the time of volcanic eruption in the northern margin of the North China Craton in the mid-Jurassic and later in the southern margin and Dabie–Sulu Orogen in the early Cretaceous.

K-Ar dating, geochemical and Sr-Nd-Pb isotopic systematics of Paleocene mafic rocks in Central Jiangxi, SE China: Evidence for lithosphere replacement

Geochemical and isotopic studies were carried out on Paleocene mafic rocks in the Taihe Basin, central Jiangxi, Southeast China, in order better to understand their magma sources and tectonic implications. K-Ar dating results show that these mafic rocks intruded during the Paleocene (50‐65 Ma). These Paleocene mafic rocks are porphyritic diabase and have similar geochemical features, such as strong enrichment of large ion lithophile element (LILE, e.g., Rb, Ba, Sr), slight enrichment of light rare earth element (LREE) and high field strength element (HFSE, e.g., Nb, Ta, Ti, P). The mafic rocks are also characterized by high rare earth element (REE) and minor Eu anomalies (δEu = 0.88‐1.2) and have uniform initial 87 Sr/ 86 Sr (0.7041‐0.7064) and Pb isotopic composition ( 206 Pb/ 204 Pb = 18.338‐18.677) as well as a relatively wide range of initial Nd isotopic ratios (eNd(T)) varying from +0.8 to +6.2. These geochemical characteristics are different from those of subduction-related basalts, but similar to oceanic island basalts (OIB). Geochemical and isotopic evidence suggests that mafic rocks in the Taihe Basin were not significantly affected by crustal contamination and originated from asthenospheric mantle with a minor involvement of EM 2 mantle component. It is noted that Cretaceous basaltic rocks in Southeast China show enriched geochemical and isotopic compositions and were derived from a highly heterogeneous lithospheric mantle with minor asthenosphere components. On the other hand, Late Cenozoic basalts in Southeast China, which display OIB-like geochemical characteristics, were probably generated by melting of depleted asthenospheric mantle (DMM or MORB) with an involvement of EM 2 lithospheric mantle and a paucity of EM 1 lithospheric mantle. The Paleocene mafic rocks of the Taihe Basin exhibit transitional geochemical features between Cretaceous and Late Cenozoic basaltic magmatism, indicating that the involvement of asthenospheric mantle beneath Southeast China increased with time from Late Mesozoic to Cenozoic. It is suggested that the lithosphere replacement beneath Southeast China was associated with lithosphere extension and thinning, with decompression melting of the upwelling asthenosphere having taken place during the Paleocene period. Lithosphere replacement might be related to subduction roll-back or/and steepening of Paleo-Pacific plate due to injection of asthenospheric material into the mantle wedge beneath Southeast China.

Late Mesozoic-Eocene Mantle Replacement beneath the Eastern North China Craton: Evidence from the Paleozoic and Cenozoic Peridotite Xenoliths

Xenolith-bearing Paleozoic kimberlites and Cenozoic basalts from the eastern part of the North China craton provide unusual insights into intraplate processes and Phanerozoic lithospheric evolution. Paleozoic peridotite xenoliths represent samples of ancient cratonic mantle; P-T estimates show that a thick (˜230 km), cold (ca 40 mW/m2) lithosphere existed beneath the craton during mid- Ordovician time. However, xenoliths from Tertiary basalts sample a thin (> 90 km), hot (mean geotherm ca 80 mW/m2), compositionally heterogeneous lithosphere beneath the same area in Cenozoic time. Fertile, spinel-facies mantle makes up much of the Cenozoic lithosphere beneath the eastern North China craton, especially in regions along the translithospheric Tanlu fault. However, refractory spinel-facies xenoliths are found locally along the north-south gravity lineament in areas far away from the Tanlu fault. These refractory xenoliths are interpreted as derived from shallow relics of the cratonic mantle embedded in more fertile Cenozoic lithosphere. The increasing incidence of fine-grained, sheared microstructures in xenoliths from the north-south gravity lineament progressively toward the Tanlu fault suggests that the translithospheric fault system played an important role in the Mesozoic-Cenozoic replacement of pre-existing lithospheric mantle by more fertile material. Modification of cratonic mantle beneath the eastern North China craton involved irregular replacement of old lithosphere by cooling products of weakly depleted asthenosphere welling up along major shear systems. This lithosphere replacement was accompanied by an elevated geotherm and a shallower asthenosphere-lithosphere boundary.

Petrology and geochemistry of spinel peridotite xenoliths from Hannuoba and Qixia, North China craton

We report mineralogical and chemical compositions of spinel peridotite xenoliths from two Tertiary alkali basalt localities on the Archean North China craton (Hannuoba, located in the central orogenic block, and Qixia, in the eastern block). The two peridotite suites have major element compositions that are indistinguishable from each other and reflect variable degrees (0–25%) of melt extraction from a primitive mantle source. Their compositions are markedly different from typical cratonic lithosphere, consistent with previous suggestions for removal of the Archean mantle lithosphere beneath this craton. Our previously published Os isotopic results for these samples [Earth Planet. Sci. Lett. 198 (2002) 307] show that lithosphere replacement occurred in the Paleoproterozoic beneath Hannuoba, but in the Phanerozoic beneath Qixia. Thus, we see no evidence for a compositional distinction between Proterozoic and Phanerozoic continental lithospheric mantle. The Hannuoba xenoliths equilibrated over a more extensive temperature (hence depth) interval than the Qixia xenoliths. Neither suite shows a correlation between equilibration temperature and major element composition, indicating that the lithosphere is not chemically stratified in either area. Trace element and Sr and Nd isotopic compositions of the Hannuoba xenoliths reflect recent metasomatic overprinting that is not related to the Tertiary magmatism in this area.

Re–Os evidence for replacement of ancient mantle lithosphere beneath the North China craton

Re–Os data for peridotite xenoliths carried in Paleozoic kimberlites and Tertiary alkali basalts confirm previous suggestions that the refractory and chemically buoyant lithospheric keel present beneath the eastern block of the North China craton (and sampled by Paleozoic kimberlites) is indeed Archean in age and was replaced by more fertile lithospheric mantle sometime after the Paleozoic. Moreover, lithospheric mantle beneath the central portion of the craton (west of the major gravity lineament) formed during the last major Precambrian orogeny, around 1900 Ma ago. This age is significantly younger than the overlying crust (2700 Ma), suggesting that the original Archean lithosphere was replaced in the Proterozoic. The timing of lithospheric replacement in the eastern block of the North China Craton is constrained only to the Phanerozoic by the Re–Os results. Circumstantial geologic evidence suggests this new lithosphere is Jurassic or Cretaceous in age and formed after collision of the Yangtze and North China cratons in the Triassic, an event that was also responsible for the subduction and uplift of ultrahigh-pressure metamorphic rocks. Collectively, these data suggest that lithosphere replacement occurred in response to two continent collisional events widely separated in time (∼1900 and ∼220 Ma). Coupled with observations from other Archean cratons we suggest that wholesale replacement of lithospheric mantle (±lower crust) may require large-scale continental collision.

The density structure of subcontinental lithosphere through time

This study uses information on composition, thermal state and petrological thickness to calculate the densities of different types of subcontinental lithospheric mantle (SCLM). Data from mantle-derived peridotite xenoliths and garnet–xenocryst suites document a secular evolution in the composition of SCLM: the mean composition of newly formed SCLM has become progressively less depleted, in terms of Al, Ca, mg# and Fe/Al, from Archean, through Proterozoic to Phanerozoic time. Thermobarometric analyses of xenolith and xenocryst suites worldwide show that the mean lithospheric palaeogeotherms rise from low values (corresponding to surface heat flows of 35–40 mW/m2) beneath Archean terranes, to higher values (>50 mW/m2) beneath regions with Phanerozoic crust. The typical thickness of the lithosphere (defined as a chemical boundary layer), ranges from about 250 to 180 km, 180–150 km and 140–60 km for Archean, Proterozoic and Phanerozoic terranes respectively. The depth of this lithosphere–asthenosphere boundary corresponds to a temperature of 1250–1300°C. Using the estimated compositions, average mineral compositions and experimental data on the densities of mineral end-members (tables 1 and 2), we calculate mean densities at 20°C for Primitive Mantle (3.39 Mg m−3) and for SCLM of Archean (3.31±.016 Mg m−3), Proterozoic (3.35±0.02 Mg m−3) and Phanerozoic (3.36±0.02 Mg m−3) age. Curves of density and cumulative density versus depth, which take into account variations in geotherm with tectonothermal age, have been constructed for each age type of lithospheric section to assess the buoyancy of these columns relative to the asthenosphere, modelled as a Primitive Mantle composition. The density curves show that Archean SCLM is significantly buoyant relative to the asthenosphere at depths greater than about 60 km. Proterozoic sections deeper than about 100 km thick also are significantly buoyant. The buoyancy of Archean and Proterozoic SCLM sections, combined with their refractory composition, leads to high viscosities and explains the longevity and stability of old SCLM. Replacement of Archean lithosphere, as beneath the present-day eastern Sino–Korean craton, probably involves mechanical dispersal by rifting, accompanied by the rise of hot, fertile asthenospheric material. Fertile Phanerozoic lithosphere is buoyant when the geotherm is sufficiently high, as in many Cenozoic volcanic provinces. However, as the geothermal gradient relaxes toward a stable conductive profile, Phanerozoic SCLM sections thinner than about 100 km become denser than the asthenosphere, and hence gravitationally unstable. This could help to induce delamination of the SCLM and upwelling of asthenospheric material, beginning a new cycle. The tectonic consequences of such lithosphere replacement would include uplift and magmatism, and basin formation during subsequent thermal relaxation.