Archean Subcontinental Lithospheric Mantle Research Articles

Time-space variations in the East African Rift magmatism: the role of different mantle domains

The East African Rift System (EARS) is the classic example of an active continental rift where extensional tectonics and lithospheric thinning have been closely associated to the generation of large volumes of magmas and represents the environment with the largest range of erupted magma types all over the world. The geochemical signature of erupted magmas testifies the involvement of different mantle domains and depths (i.e., subcontinental lithosphere, asthenosphere and deeper mantle sources). Our aim is to investigate the variable contribution of different mantle domains in the genesis of the EARS magmas through space and time, considering not only the geochemical signature of erupted magmas but also the geochemical message of mantle xenoliths. The main goal is to provide a large-scale view of the common process driving the origin of magmas in the EARS beyond the local peculiarities linked to specific settings. To this aim, we screened an exhaustive geochemical database of basalts and mantle xenoliths from the EARS, and we report original trace element and Sr-Nd isotope data of new samples collected from the Main Ethiopian Rift and Turkana depression. The data were subdivided according to spatial and temporal criteria. From a spatial point of view, the samples were ascribed to five groups, namely: Afar, Ethiopia, Turkana, Eastern Branch, and Western Branch; from a temporal point of view, the magmatic activity of the EARS was subdivided into three main temporal intervals: 45-25 Ma, 25-10 Ma and 10-0 Ma. The geochemical and radiogenic isotope (Sr, Nd, Pb) signature of the selected basalts denotes the variable contributions of a mantle plume, a more depleted asthenospheric mantle (DMM), and different SubContinental Lithospheric Mantle (SCLM) domains, depending on their temporal and spatial distribution. The geochemistry of the selected basalts shows a marked correspondence with the compositional heterogeneity of mantle xenoliths, whose isotopic systematics (Sm-Nd, Re-Os) indicates the formation of the local SCLM in the Archean and during the Pan-African orogeny. Both SCLM domains contributed significantly to magma genesis in the Western Branch (whose signature points towards a contribution of the Pan-African lithosphere) and Eastern Branch (which is also affected by Archean SCLM domains) magmas. The contribution of the SCLM generally increases with time, possibly related to an increase of the geothermal gradient in response to the arrival and flattening of the plume head at the base of the lithosphere and later extension, thinning and shallower melting. Our interpretation supports a pivotal role of the different SCLM domains in magma genesis that is able to fully explain the large compositional heterogeneity of the EARS basalts and represents a reasonable alternative to the putative presence of multiple mantle plumes or a heterogeneous mantle upwelling.

On the Role of Lower Crust and Midlithosphere Discontinuity for Cratonic Lithosphere Delamination and Recycling

AbstractDelamination is an important mechanism of continental lithosphere removal and recycling. It has been invoked as a likely cause for the observed thinning in the eastern North China (North China Craton) and several other cratons, returning thick depleted cratonic roots to the deep mantle. However, how delamination occurred, its lateral extent, and detachment depth of delamination are still debated. Here we test this process by means of 2‐D numerical modeling and find that delamination below lower crustal depths can only initiate near cratonic margins, where thickening‐induced eclogitization can significantly destabilize the lower part of the lithosphere, allowing it to detach and leading to intraplate orogeny. When a weak midlithosphere discontinuity is present, widespread delamination can initiate near cratonic margins and easily propagate to cratonic interiors, yet without obvious intraplate deformation. We illustrate how this delamination mechanism can recycle large volumes of depleted Archean subcontinental lithospheric mantle using numerical models. Our models explain the large‐scale delamination in the North China Craton, where a midlithosphere discontinuity is inferred by geophysical studies and shows that large portions of subcontinental lithospheric mantle can be recycled in the deep mantle potentially affecting mantle heterogeneity.

Elemental and Sr-Nd-Pb isotope geochemistry of the Florianópolis Dyke Swarm (Paraná Magmatic Province): crustal contamination and mantle source constraints

The Florianópolis Dyke Swarm is located in Santa Catarina Island, comprising also the adjacent continental area, and belongs to the Paraná Magmatic Province (PMP). The dyke outcrops in the island are 0.1–70m thick and most of them are coast-parallel (NE-SW trending), with subordinate NW-SE trending. The vast majority of the dykes has SiO2 varying from 50 to 55wt% and relatively high-Ti (TiO2>3wt%) contents and these rocks were divided using the criteria commonly used to distinguish the different magma-types identified in the volcanic rocks from the PMP. The Urubici dykes (Sr>550μg/g) are the most abundant and some of them experienced crustal contamination reaching to 10%, as evidenced by low P2O5/K2O (0.30–0.21), high (Rb/Ba)PM (1.0–2.2), and radiogenic Sr and Pb isotope compositions (87Sr/86Sri up to 0.70716 (back to 125Ma) and 206Pb/204Pbm up to 19.093). The Pitanga (Sr<550μg/g) and the basaltic trachyandesite dykes are less abundant and almost all of them were also substantially affected by at least 15% of crustal assimilation, evidenced by high (Rb/Ba)PM (up to 2.6) and Sr (87Sr/86Sri=0.70737–0.71758) and Pb (206Pb/204Pbm=18.446–19.441) isotope ratios, as well as low P2O5/K2O values (0.30–0.18). The low-Ti (TiO2<2wt%) dykes are scarce and show a large compositional variability (SiO2: 50.4–64.5wt%), with similar geochemical characteristics of the low-Ti volcanic rocks (Gramado-Palmas) from southern PMP, although the most primitive dykes show hybrid characteristics of Ribeira and Esmeralda magmas. The presence of granitic xenoliths with border reactions and dykes with diffuse contacts indicate that crustal contamination probably occurred by assimilation from re-melted the host rocks. Considering only the high-Ti Urubici dykes that were not affected by crustal contamination, the Sr, Nd and Pb isotope mixing modelling indicates the participation of a heterogeneous metasomatized (refertilized) subcontinental lithospheric mantle (SCLM). This mantle source was originated by partial melting of a depleted sublithospheric mantle (DMM – Depleted Mantle MORB), which was hybridized by addition of pyroxenite (<5%) and carbonatite (up to 2%) melts. The isotope mixing modelling also points to a significant participation (up to 50%) of Archean SCLM, not evidenced in the mantle sources of the northern PMP high-Ti Pitanga flows (dominated by Neoproterozoic SCLM).

Nature of three episodes of Paleoproterozoic magmatism (2180 Ma, 2115 Ma and 1890 Ma) in the Liaoji belt, North China with implications for tectonic evolution

Three episodes of Paleoproterozoic magmatism with distinctively different nature in the Liaoji belt of the Eastern North China Craton are discussed here: the 2200–2140Ma Liaoji A-type granites, the 2160–2110Ma Haicheng mafic sills and the ∼1890Ma Qingchengzi I-type granites. The Mafeng monzogranitic pluton, representative of the Liaoji A-type granites, gives a SIMS U-Pb zircon age of 2181±6Ma (n=20, MSWD=4.3). The Qingchengzi plagiogranitic pluton, representative of the Qingchengzi I-type granites, gives a SIMS U-Pb zircon age of 1891±10Ma (n=8, MSWD=1.8). The Mafeng monzogranites are high in Fe, Ti, K but low in Mg, Al and Ca. They have high abundance of total REEs (ΣREE=213–346ppm). They are relatively depleted in feldspar-compatible elements (e.g., Eu and Sr) and HFSEs (e.g., Nb, Ta, P and Ti). They have high 10000×Ga/Al ratios of 3.19–3.61 and Zr+Nb+Ce+Y concentrations of 472–656ppm, which are typical for A-type granite. The Qingchengzi plagiogranites have relatively high Al, Ca, Na but low Fe, Ti and K contents. They have low abundance of total REEs (ΣREE=17.6–21.6ppm). They are enriched in LILEs (e.g., Ba, K and Sr) but depleted in HFSEs (e.g., Nb, Ta, P, Ti and Y). These features, combined with extremely high Sr/Y ratios (327–413), are comparable with those of typical modern adakites. The Liaoji A-type granites show a source same to that of the Haicheng mafic sills and they originated from the Archean subcontinental lithospheric mantle plus with contamination from the Archean granites. However, the Qingchengzi I-type granites are interpreted to originate from subducted oceanic crust with significant contributions from sediments. The Liaoji A-type granites and the coeval bimodal volcanism in the Liaohe Group as well as the Haicheng mafic sills may represent episodic magmatism which was related to a protracted intra-continental rifting caused by lithospheric extension. The ca. 1890Ma Qingchengzi I-type granites and coexisting S-type granites as well as the coeval regional metamorphism are favored to represent an active continental magmatism linked to Paleoproterozoic subduction.

Formation and evolution of the Paleoproterozoic meta-mafic and associated supracrustal rocks from the Lushan Taihua Complex, southern North China Craton: Insights from zircon U-Pb geochronology and whole-rock geochemistry

The Upper Taihua Complex in the Lushan area is one of the best preserved Paleoproterozoic metamorphic basements exposed in the southern North China Craton, which is mainly composed of Khondalite-dominated supracrustal rocks and associated meta-mafic intrusions (amphibolites) and granitic plutons. Detrital zircon U-Pb ages of four paragneisses from the supracrustal rocks can be divided into two groups: 2.39–2.10Ga and 1.97–1.83Ga. Concordant 207Pb/206Pb ages from the older group show typical magmatic features with a distinct peak around 2.31–2.23Ga, whereas the metamorphic zircons define a remarkable age peak at ∼1.93Ga. Magmatic zircons from the amphibolite sample have a weighted mean 207Pb/206Pb age of 2128±29Ma, representing the crystallization age of the protolith. Therefore, the depositional age of the supracrustal rocks in the Upper Taihua Complex can be well constrained at ∼2.23–2.13Ga. Geochemically, the amphibolites are classified as subalkaline high-Fe tholeiitic basalts in compositions and characterized by slightly enrichment in LREE, flat HREE and weakly negative Eu anomalies, with depleted high field strength elements (e.g., Nb, Ta, and Ti), inconsistent with typical rocks derived from the asthenospheric mantle. The high FeOT (>13.59wt%), CaO (>6.67wt%), low Al2O3 (<13.09wt%), Th (<3.09ppm), and lack of correlation between εNd (t) values and crustal contamination sensitive elements suggest that crustal assimilation was limited during magma evolution. The varied εNd (t) values (−2.99 to +0.44) and Nd model ages (2.65–2.98Ga) suggest that they were likely originated from Archean subcontinental lithospheric mantle (SCLM) which have been refertilized by metasomatic processes. These geochronological and geochemical signatures indicate the meta-mafic rocks and associated supracrustal rocks of the Upper Taihua Complex in the Lushan area were most likely formed in an intra-continental rift setting, implying the extensional tectonic scenario rather than subduction-related regime occurred between ∼2.2 and 2.0Ga at least in the southern margin of the NCC. Metamorphic ages suggest that the Taihua Complex in the Lushan area was involved in subduction-collision processes from 1.97Ga to 1.83Ga.

Paleoproterozoic magmatism across the Archean-Proterozoic boundary in central Fennoscandia: Geochronology, geochemistry and isotopic data (Sm–Nd, Lu–Hf, O)

The central Fennoscandia is characterized by the Archean-Proterozoic (AP) boundary and the Central Finland Granitoid Complex (CFGC), a roundish area of approximately 40,000km2 surrounded by supracrustal belts. Deep seismic reflection profile FIRE 3A runs across these units, and we have re-interpreted the profile and crustal evolution along the profile using 1.92–1.85Ga plutonic rocks as lithospheric probes. The surface part of the profile has been divided into five subareas: Archean continent (AC) in the east, AP, CFGC, boundary zone (BZ) and the Bothnian Belt (BB) in the west. There are 12 key samples from which zircons were studied for inclusions and analyzed (core-rim) by ion probe for U–Pb dating and oxygen isotopes, followed by analyzes for Lu–Hf by LA–MC–ICP–MS.The AC plutonic rocks (1.87–1.85Ga) form a bimodal suite, where the proposed mantle source for the mafic rocks is 2.1–2.0Ga metasomatized lower part of the Archean subcontinental lithospheric mantle (SCLM) and the source for the felsic melts is related plume-derived underplated mafic material in the lower crust. Variable degrees of contamination of the Archean lower crust have produced “subduction-like” Nb–Ta anomalies in spidergrams and negative εNd (T) values in the mafic-intermediate rocks. The felsic AC granitoids originate from a low degree melting of eclogitic or garnet-bearing amphibolites with titanite±rutile partly prevailing in the residue (Nb–Ta fractionation) followed by variable degree of assimilation/melting of the Archean lower crust. The AP plutonic rocks (ca. 1.88Ga) can be divided into I-type and A-type granitoids (AP/A), where the latter follow the sediment assimilation trend in ASI diagram, have high δ18O values (up to 8‰) in zircons and exhibit negative Ba anomalies (Rb–Ba–Th in spidergram), as found in sedimentary rocks. A mixing/assimilation of enriched mantle-derived melts with melts from already migmatized sedimentary rocks±amphibolites is proposed. The CFGC is characterized by both I-type and A-type (CFGC/A) intermediate and felsic granitoids. The I-type granitoids are divided into two groups at ≥1885Ma and ≤1882Ma, where the latter overlap in age with the CFGC/A granitoids. Both I-type CFGC and CFGC/A granitoids are interpreted to have formed from mixing of Paleoproterozoic SCLM-derived melts with crustal melts from hydrous and dry intermediate-felsic igneous sources, respectively. The geochemistry, dominantly δ18O values below 6.5‰ in zircons and TDM (2.11–2.42Ga) of the CFGC granitoids favor the occurrence of older crust (ca. 2.1–2.0Ga) in their genesis. The BZ granitoids are similar in age but more juvenile with TDM ages between 1.94Ga and 2.16Ga. The 1.92Ga granodiorite in the BB is correlated with juvenile gneissic tonalites and granodiorites found from the AP boundary.We suggest that the present high-velocity lower crust under the CFGC is composed of melt-extracted granulites (crustal source age≥2.0Ga) and mafic cumulates which both formed during 1.90–1.88Ga arc magmatism. The ≤1.88Ga stage represents the end of compression/transpression and is followed by 1.87–1.86Ga buckling, forming the Bothnian Oroclines.

Geochemistry of metamafic dykes from the Quanji massif: Petrogenesis and further evidence for oceanic subduction, Late Paleoproterozoic, NW China

A suite of ~1.84–1.92 Ga metamafic dykes within the paragneiss suite (khondalite) of the Quanji massif in NW China, has been chosen in this study for further understanding the tectonic evolution and possible links to the global Columbia supercontinent. Occurrence and field relations suggest that they were formed coevally with a previous studied ~1.83–1.85 Ga metamafic dyke swarms. Whole-rock major and trace elemental geochemistry suggests precursor magma of the amphibolites being generated from a volcanic arc-related tectonic setting rather than a back-arc environment where the metamafic dyke swarms were emplaced. The metamafic dykes show enrichment of LREE and strongly negative anomalies for Ta-Nb, Zr-Hf and Ti, have high SiO2 (49.3 wt.%–52.5 wt.%) but low MgO (6.40 wt.%–7.76 wt.%) contents and Mg# (Mg#=[100×(MgO/40.3)]/[MgO/40.3+FeO/71.8]) values (45.7–52.1), suggesting evolved precursor magma. The high values of La/Ta (22.2–42.8) and La/Nb (1.71–2.47), mildly negative eNd(t) values (-2.51–0.15), with depleted mantle model ages (T DM) of 2.45–2.84 Ga, suggest that their precursor magmas were possibly derived from a subduction-related fluid metasomatized Archean sub-continental lithospheric mantle. This study provides further evidence for oceanic plate subduction prevailing before or around ~1.85 Ga, which confirms a prolonged subduction-accretion-collision history in the NW China which is possibly linked to the assembly of the Columbia supercontinent.

Geoscience Data Integration: Insights into Mapping Lithospheric Architecture

SUMMARY In order to develop a 4D understanding of the architecture of the entire lithosphere, it is necessary to embrace integration of multi-disciplinary, multi-scale data in a GIS environment. An holistic understanding has evolved whereby geologic, geochemical and geophysical signals are consistent with a subcontinental lithospheric mantle (SCLM) dominated by a mosaic of domains of Archean ancestry, variably overprinted by subsequent tectonothermal events. Pristine Archean SCLM is mostly highly depleted (high Mg#), low density, high velocity and highly resistive, and preserves intact Archean crust. There is a first order relationship between changes to these signals and the degree of tectonothermal overprint (by melts, fluids). Continental crust is comprised largely of reconstituted Archean components, variably diluted by juvenile addition, symptomatic of the various overprinting events. These events impart crustal fabrics and patterns dictated by SCLM architecture, influenced by the free surface and crust-mantle decoupling.

Episodic refertilization and metasomatism of Archean mantle: evidence from an orogenic peridotite in North Qaidam (NE Tibet, China)

Episodic refertilization and metasomatism of depleted subcontinental lithospheric mantle (SCLM) is rarely revealed within a single orogenic peridotite. We have investigated the Shenglikou peridotite massif in the North Qaidam Orogen (NE Tibet, China) to understand its origin and secular re-enrichment history and possible links with global tectonism, using multiple chemical and isotopic techniques. The massif contains two main groups of rocks, one rich in olivine (Ol) and the other rich in pyroxene (Pyro). In situ Re–Os isotope analyses on sulfides and previous reported whole-rock Os data show the Shenglikou peridotite massif has an Archean origin. The Ol-rich group has Mg# up to 0.923, Al2O3 contents down to 0.55 wt%, low FeO contents, whole-rock HREE ~0.01–0.1 times chondritic values and high spinel Cr# (up to 0.71). Modeling suggests that the Ol-rich peridotites are residues after ~30–40 % melt extraction at ~1600–1500 °C and ≤5 GPa. Mineral compositions, oxygen-isotope compositions (δ18OV-SMOW = 5.7 ± 0.1 ‰) and attainment of Hf-isotope internal equilibration at ~1.5–1.4 Ga with initial ratios similar to that of depleted convective mantle indicate that the Ol-rich group was refertilized by silicate melts derived from the depleted asthenosphere at ~1.5–1.4 Ga, corresponding to one phase in the breakup of the Columbia supercontinent. On the other hand, chemical compositions and Hf–O isotopic signatures of the pyroxene-rich rocks suggest a second refertilization superimposed on the Archean lithospheric mantle at ~700 Ma, coeval with the global extension that affected Rodinia. Both groups show metasomatism by fluids, sourced from subducting slabs during the early Paleozoic convergence between the Qaidam and Qilian blocks. This study shows that the episodic refertilization and metasomatism of depleted Archean SCLM, temporally linked to supercontinental cycles, can be revealed in a single orogenic peridotite, and suggests that continental extension and convergence play an important role in the destruction of cratonic keels.

Re–Os isotope systematics of Archean chromitites from the Chimalpahad Anorthosite Complex, south-east India: Implications for mantle extraction processes

The Chimalpahad Anorthosite Complex, which contains layers of chromitites, is located within the Archean Peninsular Gneissic Complex in the SE Indian shield. Here we present Re–Os isotope systematics of the chromitites and associated websterites and anorthosites in tracing the petrogenetic history and heterogeneity of the source mantle. The average initial 187Os/188Os compositions of chromitites are 0.1045 with negative γOs values of −17.67, whereas the websterites and anorthosites have average initial 187Os/188Os compositions of 14.86 and 11.79 with positive γOs values of +467 and +215, respectively. Mantle extraction ages (TMA) of the chromites indicate depletion of primitive mantle beginning as early as ~3.5Ga. The subchondritic signature of Os in the chromitite is comparable with those of the chromites from ultramafic intrusions in the Zimbabwe Craton interpreted to reflect sequential extraction from evolving Archean subcontinental lithospheric mantle. The younger (TMA) ages of 2.77–1.41Ga of the websterites and anorthosites indicate Proterozoic melt depletion or complex isotopic systematics resulting from overprinting of tectonic events. The geochemical data and Re–Os isotope systematics presented in this study suggest that the Chimalpahad Complex formed from protracted evolution of ultramafic–mafic magmas in a deep-seated chamber of a magmatic arc along the SE margin of the Indian plate.

Origins of ultramafic rocks in the Sulu Ultrahigh-pressure Terrane, Eastern China

Ultramafic rocks in the Sulu belt are associated with high-pressure (HP) and ultra-high-pressure (UHP) rocks, a metamorphic product of the northern margin of the Yangtze Craton (YZC). These ultramafic rocks are important because they provide information relevant to the nature and evolution of the Mesozoic collisional belt. We selected ultramafic rocks from Yangkou Bay (YKB), Suoluoshu (SLS) and Hujialin (HJL) in the central region of Sulu belt. The ultramafic rocks in YKB and SLS are hydrated to form serpentinites and they contain low concentrations of moderately incompatible elements (Al, Ti, and V), high contents of Ir-group platinum-group elements (IPGE; Ir, Os, and Ru; 12.8–21.7ppb in total), and high ratios (1.2–5.5) of IPGE to Pd-group PGE (PPGE) in bulk rocks. Spinel contains high Cr (Cr#=atomic ratio of Cr/[Cr+Al], 0.57–0.79). The data suggest that they likely represent hydrated forearc mantle peridotites underlying the margin of the North China Craton (NCC). Dunite samples from HJL are not fully hydrated with loss on ignition (LOI) values ranging from 6.6 to 13.2wt.%, and contain olivine grains with high forsterite components (Fo=100*Mg/[Mg+Fe], 91.7–92.4) and NiO contents (0.36–0.41wt.%). Spinel grains show high Cr# (0.68–0.76). The bulk rock contains high IPGE (5.0–22.7ppb total) and show high ratios (up to 8.8) of IPGE to PPGE. The data suggest that they are also residual mantle peridotites after high degrees of influx partial melting in the subduction setting. The geochemical features of our serpentinite and dunite samples are different from peridotites of young Cenozoic subcontinental lithospheric mantle (SCLM) that have been brought to the surface as xenoliths of young volcanic rocks. Instead, our samples are similar but even more refractory than peridotites of relict Archean SCLM below the NCC that were enclosed as xenoliths in Paleozoic–Mesozoic igneous rocks. Thus, the studied ultramafic rocks likely represent the relic of old refractory lithospheric mantle peridotites that underwent partial melting in the margin of the NCC during the subduction of oceanic lithosphere before the collision with the YZC. The peridotites in the forearc mantle were dragged into the subduction channel during the Mesozoic subduction of the YZC below the NCC, and exhumed together with the buoyant granitic metamorphic rocks in the Sulu belt.

Coupling, decoupling and metasomatism: Evolution of crust–mantle relationships beneath NW Spitsbergen

The Bockfjord area of NW Spitsbergen (Norwegian Arctic) exposes a long history of crustal evolution, culminating in the Caledonian (400–500Ma) orogeny; prior to the opening of the N. Atlantic Ocean, this area was part of the Laurentian (Greenland) side of the orogen. The N-striking Breibogen–Bockfjorden (BB) fault marks the western margin of a large graben filled with Devonian redbeds. West of the fault the basement consists of gneisses, schists and granites of the Hekla Hoek formation, inferred to represent a Caldeonian thrust sheet. U–Pb and Hf-isotope data for detrital zircons from this area show that the Hekla Hoek protoliths formed at ca 1.8Ga, but were heavily reworked ca 800–1000Ma ago, and again during the Caledonian orogeny.Quaternary alkali-basalt volcanism has provided abundant xenoliths of mantle and crustal rocks from both sides of the BB fault. Lower-crustal xenoliths are mainly mafic to intermediate granulites. Whole-rock Sr-, Nd- and Hf-isotope data for the granulites from both sides of the BB fault show significant disequilibrium, implying the removal of melts late in the evolution of the lower crust. Most zircons from eight xenoliths have Neoarchean/Paleoproterozoic and Paleozoic U–Pb ages; several also contain zircons with ages and/or Hf model ages >3.2Ga.The peridotite xenoliths are dominantly spinel lherzolites, metasomatized to varying extents. Xenoliths from basalts east of the BB fault commonly contain metasomatic amphibole, phlogopite and apatite; peridotites from west of the fault rarely display these phases. West of the fault, there is no clear correlation between cpx REE patterns and whole-rock Al contents; east of the fault there is a clear negative correlation between LREE enrichment and whole-rock Al2O3. In-situ Re–Os isotope analysis of sulfides in the peridotites shows another dichotomy. Xenoliths from west of the fault contain sulfides with Re depletion (TRD) model ages extending back to 3.3Ga, with major populations at 2.4–2.6Ga, 1.6–1.8Ga and 1.2–1.3Ga; the Caledonian orogeny is weakly represented. East of the BB fault the peaks in the TRD spectrum are at 900–1100Ma and 400–500Ma; only a few grains have TMA>2.5Ga.The data demonstrate a major disjunct, on both sides of the BB fault, between the Archean lower crust and a Proterozoic–Paleozoic upper crust; this suggests that the original Archean upper (and middle?) crust was detached and replaced by thrust sheets of younger material during the Caledonian orogeny. The striking differences in the sub-continental lithospheric mantle (SCLM) on either side of the BB fault suggest that major transcurrent movement has juxtaposed lithospheric sections that evolved independently. West of the fault, the presence of Archean lower crust overlying Archean SCLM suggests coupling of the crust and mantle for ≥3Ga. East of the BB, similar Archean lower crust overlies an apparently younger, more fertile SCLM. We suggest that pervasive melt-related metasomatism refertilized the SCLM east of the BB fault, largely obscuring its Archean origin. Typical Archean SCLM is 150–250km thick, whereas the Bockfjord volcanoes carry no samples from deeper than 80km. The detached lower part of the Archean SCLM may now lie beneath the Gakkel Ridge to the NW, as proposed by Goldstein et al. (2008) on the basis of basalt geochemistry. This proposition is supported by the mean TRD of the Sverrefjell sulfides (1740±718Ma), which is similar to the oldest whole-rock TRD values for peridotites from the Gakkel Ridge (1.8–2.2Ga; Liu et al., 2010).

Subduction geodynamics in Archean and formation of diamond-bearing lithospheric keels and early continental crust of cratons

The diamond-bearing mantle keels underlying Archean cratons are a unique phenomenon of Early Precambrian geology. The common stable assemblage of the Archean TTG early continental crust and underlying subcontinental lithospheric mantle clearly shows their coupled tectogenesis, which was not repeated in younger geological epochs. One of the least studied aspects of this phenomenon is concerned with the eclogitic xenoliths carried up by kimberlite pipes together with mantle-derived nodules. The eclogitic xenoliths reveal evidence for their subduction-related origin, but the Archean crustal counterparts of such xenoliths remained unknown for a long time, and the question of their crustal source and relationships to the formation of early continental crust remained open. The Archean crustal eclogites recently found in the Belomorian Belt of the Baltic Shield are compared in this paper with eclogitic xenoliths from kimberlites in the context of the formation of both Archean subcontinental lithospheric mantle (SCLM) and early continental crust. The crustal eclogites from the Belomorian Belt are identical in mineral and chemical compositions to the eclogite nodules (group B), including their diamond-bearing varieties. The eclogite protoliths are comparable in composition with the primary melts of the Meso- and Neoarchean oceanic crust, which was formed at a potential temperature of the upper mantle which exceeded its present-day temperature by 150–250 K. The reconstructed pathways of the Archean oceanic crust plunging in the upper mantle suggest that the Archean mantle was hotter than in the modern convergence settings. The proposed geodynamic model assumes coupled formation of the Archean diamond-bearing SCLM and growth of early continental crust as a phenomenon related to the specific geodynamics of that time controlled by a higher terrestrial heat flow.

Archean lithospheric mantle beneath Arkansas: Continental growth by microcontinent accretion

The Cretaceous Prairie Creek lamproites of southern Arkansas intrude Proterozoic crust near the boundary between the 1.5–1.3 Ga Granite-Rhyolite Province and the 1.3–1.0 Ga Grenville orogen. They carry xenocrysts and rare xenoliths derived from the subcontinental lithospheric mantle (SCLM) and the deep crust. U-Pb age dating of groundmass perovskite in the Prairie Creek lamproites gives a poorly constrained Cretaceous age. U-Pb dating and in situ Sr and Nd isotope data show that perovskite micronodules in the Twin Knobs #2 lamprophyre are ca. 600 Ma old, and may represent samples of rift-related alkalic magmas derived from a juvenile mantle. A lithologic section constructed from the mantle-derived xenocrysts shows a moderately depleted SCLM that has experienced a high degree of melt-related metasomatism, especially in the depth range 150 to 140 km. In situ Re-Os analysis of sulfide grains in the xenoliths yields model ages ranging up to 3.4 Ga, with major peaks at 1.4–1.5 Ga and 200–300 Ma. Early Paleoproterozoic model ages appear to reflect mixing between residual Archean high-Os sulfides and later low-Os sulfide melts. These data suggest that the SCLM beneath the Prairie Creek area formed in Archean time, and has been progressively refertilized by a series of magmatic events, which appear to correlate in time with events in the overlying crust. The Archean SCLM sampled by the lamproites may represent the mantle root of the Sabine microcontinent, which lies mainly to the south of the lamproite field and is recognizable in seismic tomography as a feature with higher shear-wave velocity (Vs) (100–175 km depth). Seismic tomography also shows several blocks with high Vs beneath the Grenville province to the east, which may represent other microcontinental blocks. These findings suggest that the growth of individual continents is significantly affected by the accretion of older microcontinental blocks, and that the extent of early continental crust therefore may be greater than generally estimated.

Zircons in the Shenglikou ultrahigh-pressure garnet peridotite massif and its country rocks from the North Qaidam terrane (western China): Meso-Neoproterozoic crust–mantle coupling and early Paleozoic convergent plate-margin processes

The North Qaidam terrane lies between the Qaidam and Qilian blocks of western China, and contains the Shenglikou ultrahigh-pressure (UHP) garnet peridotite massif. Simultaneously analyzed U–Pb ages, trace element patterns and Hf-isotope compositions of zircons from the peridotite massif and its country rocks (migmatite and gneiss) reveal the complex evolution of the terrane. Zircons in peridotite and pyroxenite from the massif document metamorphism at ca. 429 ± 2 Ma (2 σ, n = 30, MSWD = 1.03). The peridotitic and pyroxenitic zircons have similar trace-element patterns and ranges in ɛ Hf( t) (+3.5 to −7.0), suggesting that these zircons crystallized from a metamorphic fluid, sourced mainly from the surrounding garnet-bearing ultramafic rocks. The protolith of the peridotite massif is interpreted as a fragment of the metasomatized Archean subcontinental lithospheric mantle (SCLM) from the Qilian block. Hf depleted-mantle model ages ( T DM) mainly cluster in the interval 1.1–0.9 Ga, implying that the protolith of the massif probably underwent mantle metasomatism in Meso-Neoproterozoic time. Zircons from the migmatite yield concordant 206Pb/ 238U ages, with a major peak at 468 ± 4 Ma (2 σ, n = 7, MSWD = 2.2) and smaller populations of 1003 ± 6 Ma, 951 ± 8 Ma, 738 ± 4 Ma and 577 ± 14 Ma. Due to the systematically lower Th/U ratios and trace element concentrations in the early Paleozoic zircons, compared to the Proterozoic grains, we suggest the early Paleozoic (∼468 Ma) zircons were formed by metamorphic recrystallization of their Proterozoic precursors. Their variable Hf isotopic compositions, i.e., ɛ Hf( t) from +14.2 to −15.9, T DM from 1.7 Ga to 0.6 Ga and T crust (average crustal model ages) from 2.4 Ga to 0.6 Ga, imply complex origins, including the introduction of juvenile components during events at ∼1.1–0.9 Ga. Zircons from the gneiss show a major concordant population with 206Pb/ 238U ages of 430 ± 5 Ma (2 σ, n = 10, MSWD = 2.3) and minor populations with ages of 806 ± 13 Ma and 613 ± 24 Ma, indicating that metamorphic recrystallization of the protolith occurred in the early Paleozoic (∼430 Ma). The original protolith of the gneiss was ancient (ca. T crust = 2.8–1.8 Ga), with some Meso-Neoproterozoic (ca. 1.1 Ga) juvenile crustal components. These data together with published results suggest that the North Qaidam terrane is a collisional orogen with Archean crustal components, and the Qaidam and Qilian blocks underwent crust–mantle coupling events in Meso-Neoproterozoic time (∼1.1–0.9 Ga) and convergent plate-margin processes including the UHP metamorphism in early Paleozoic (∼468–430 Ma), i.e., the beginning of continental subduction at ∼468 Ma, the subsequent continental deep subduction and final continent–continent collision at ∼430 Ma.

High-precision U–Pb age and geochemistry of the mineralized (Ni–Cu–Co) Suwar intrusion, YemenThis article is one of a series of papers published in this Special Issue on the theme of Geochronology in honour of Tom Krogh.

High-precision U–Pb isotope dilution – thermal ionization mass spectrometry (ID–TIMS) geochronology on chemically abraded zircon grains from a noritic gabbro of the Ni-bearing Suwar mafic–ultramafic layered complex, northwestern Yemen, gives a mean 206Pb/238U age of 638.46 ± 0.73 Ma (2σ; MSWD = 1.4). At Wadi Qutabah, ∼30 km to the north, a similar mafic sample has an identical age of 638.58 ± 0.51 Ma (2σ; MSWD = 0.32), which supports the possibility of there being a single, large intrusive complex with an estimated areal extent of ∼250 km2. This is supported by geochemical data of samples from each locality, which are postkinematic, gabbroic rocks that contain variable amounts of cumulus olivine, plagioclase, orthopyroxene, and ilmenite with intercumulus augite, hornblende, and Ni-sulphides. Straight rare Earth element (REE) patterns, Ba/La ∼30, Rb/Ba ∼0.04, and negative primitive-mantle-normalized P anomalies resemble EM1 (Enriched Mantle 1) of oceanic island basalts and Archean subcontinental mantle lithosphere. The mineralogy and magmatic/tectonic discrimination diagrams suggest within-plate continental tholeiitic and noritic cumulates typical of a rift setting for both intrusions. The complex intrudes retrograded, amphibolite-facies paragneiss of the Pan-African Afif lithotectonic terrane, and is generally undeformed and unaltered, and, therefore, unaffected by the Pan-African orogeny. Emplacement of the 639 Ma complex occurred during a tensional tectonic regime on the Arabian Peninsula and marks the time of proto-Iapetan rifting. The estimated size of the intrusion, its noritic composition, and Archean subcontinental lithospheric mantle signature and its position in thin Proterozoic lithosphere abutting Archean cratonic rocks, give it the key characteristics of known, mostly Proterozoic, intrusions that host world-class Ni–Cu–Co ore deposits.

Coupled evolution of Archean continental crust and subcontinental lithospheric mantle

The observations that Archean continental crust and the subcontinental lithospheric mantle (SCLM) have different compositions from their Phanerozoic counterparts, that komatiite extraction models for the origin of the Archean SCLM do not work, and that non-arc Archean basalts are not necessarily formed in a plume setting are used to challenge the mantle plume model for the formation of the Archean SCLM. Petrological modeling suggests that, instead, the SCLM formed at a hot ocean ridge giving rise to dense, Fe-rich basaltic ocean crust and highly depleted thick oceanic lithosphere. Typically this lithosphere would subduct, but where slab melting and tonalite-trondhjemite-granodiorite (TTG) production took place, the SCLM coupled to felsic crust would be sufficiently buoyant to be conserved. Thus Archean SCLM is transposed normal Archean oceanic lithosphere created at a hot ridge.

Delamination/thinning of sub-continental lithospheric mantle under Eastern China: The role of water and multiple subduction

We present a new model to explain one of the biggest tectonic problems of Earth Sciences today, namely, how and why did the Archean sub-continental lithospheric mantle under the Eastern Block of the North China Craton (NCC) delaminate or thin so drastically in the Cretaceous? The eastern NCC is surrounded by several sutures: to the north the south-dipping Solonker (Permo-Triassic formation age) and Mongol-Okhotsk suture (Jurassic formation age), to the south the north-dipping Dabie Shan and Song Ma sutures (Permo-Triassic formation age), and to the east the Pacific oceanic plate (200-100 Ma). Water was carried down under the eastern NCC by the hydrated Pacific plate for at least 100 Ma, and by oceanic plates subducted along the Solonker, Dabie Shan and Song Ma sutures for at least 250 Ma from the early Paleozoic to the Permo-Triassic, and by the Mongol-Okhotsk suture for at least 200 Ma until the Jurassic. Tomographic images show that the Pacific plate has ponded along the top of the mantle transition zone under the eastern NCC for over ca. 2000 km from the Japan trench. An addition of 0.2 weight percent H 2 O lowered the solidus temperature of hydrous mantle peridotite by 150 °C, which led to extensive melting in the hydrous mantle transition zone, and to the rise of hydrous plumes into the overlying crust-mantle. By the time of formation of the Permo- Triassic sutures, the hydration caused by subduction of four oceanic plates had caused major garnetization of the Archean crustal root of the eastern NCC, and post-collisional thrusting in the Jurassic led to major crust/mantle thickening and this triggered collapse of the hydro-weakened garnet-enriched crustal root. During the extensional Cretaceous period, abundant mafic, adakitic and granitic intrusions and extensive gold mineralization were emplaced and metamorphic core complexes and sedimentary and foreland basins formed in and around the eastern NCC. Part of the root was chemically transformed and replaced by upwelling fertile asthenospheric material, which fed the extrusion of extensive alkaliflood basalts in the Cenozoic.

Melting of enriched Archean subcontinental lithospheric mantle: Evidence from the ca. 1760 Ma volcanic rocks of the Xiong’er Group, southern margin of the North China Craton

The Xiong’er Group is an important geologic unit in the southern margin of the North China Craton. It is dominated by the volcanic rocks, dated at 1763 ± 15 Ma, that have SiO 2 contents ranging from 52.10 wt% to 73.51 wt%. These volcanic rocks are sub-alkaline and can be classified into three subgroups: basaltic andesites, andesites and rhyolites. They unexceptionally show enrichment of light rare earth elements (LREE) and share similar trace element patterns. Depletions in Nb, Ta, Sr, P and Ti relative to the adjacent elements are evident for all the samples. The volcanic rocks are evolved with low MgO contents (0.29–5.88 wt%) and accordingly low Mg # values of 11–53. The Nd isotopes are enriched and show a weak variation with ɛ Nd( t) = −7.12 to −9.63. Zircon Hf isotopes are also enriched with ɛ Hf( t) = −12.02 ± 0.45. The volcanic rocks of the Xiong’er Group are interpreted to represent fractional crystallization of a common mantle source. The volcanic rocks might have been generated by high-degree partial melting of a lithospheric mantle that was originally modified by the oceanic subduction in the Late Archean. This brings a correlation with the subduction-modified lithospheric mantle in an extensional setting during breakup of the Columbia supercontinent in the late Paleoproterozoic, rather than in an arc setting. The elevated SiO 2 contents and evolved radiogenic isotope features indicate the possible incorporation into their source of lower crustal materials that have similar Nd isotopic characteristics to the subcontinental lithospheric mantle. The existence of extensive Xiong’er volcanic rocks (60,000 km 2) indicates an early large-scale subduction-related metasomatism in the area and probably suggest a flat subduction model for the plate-margin magmatism in the Late Archean.

Evolution of the Lüliangshan garnet peridotites in the North Qaidam UHP belt, Northern Tibetan Plateau: Constraints from Re–Os isotopes

Garnet peridotite, interlayered with lesser amounts of dunite and olivine websterite and interwoven with garnet pyroxenite dikes, makes up a 0.5 km 2 massif near Lüliangshan in the North Qaidam ultrahigh-pressure (UHP) metamorphic belt, Northern Tibetan Plateau. Previous studies have suggested that the garnet peridotites represent Alaskan-style magmatic cumulates or oceanic serpentinites that were subducted to depths of > 200 km. Re–Os isotopic data provide new constraints on the origin and formation of the peridotites. Most dunites (Mg# 92–> 95) have high Os contents (2.90–4.25 ppb) and low 187Os/ 188Os (minimum 0.1108), while garnet pyroxenites have high 187Os/ 188Os (up to 0.2847) and low Os contents (0.03–1 ppb); garnet lherzolites lie between these extremes. The Archean model ages (TRD = 2.6 Ga, TMA = 2.8 Ga) of the most refractory dunites represent minimum estimates for the age of the original melt depletion. In an Os– 187Os/ 188Os plot, almost all garnet peridotites fall along two mixing lines: one is defined by mixing between the most refractory dunite and a garnet-rich pyroxenite, and the other by mixing between the dunite and a garnet-poor pyroxenite. The extremely depleted nature of the dunites, the high Ca and Al contents of the lherzolites, the variable enrichment in incompatible elements, and the Re–Os isotopic data suggest that the Lüliangshan garnet peridotites represent a fragment of subcontinental mantle, in which ancient residues after melt extraction (e.g. dunite) were refertilised by mafic melts similar to the garnet pyroxenites. Modelling of the garnet peridotite data suggests that at least some of the metasomatism is Paleoproterozoic in age; Re also may have been introduced into some of the garnet peridotites during the Paleozoic UHP Qaidam orogeny. The Lüliangshan peridotites are interpreted as refertilised Archean subcontinental lithospheric mantle, probably derived from beneath the North China Craton and entrained in the surrounding gneisses during their subduction into the mantle; the garnet peridotite assemblages may have formed during the original refertilisation event, or during the subduction.