Origin and stabilization of off-craton continental lithosphere recorded by cratonic mantle relics in Pali-Aike (southern Patagonia)

Chemical and petrographic observations on mantle-derived xenoliths and xenocrysts define a fundamental distinction between Archean cratonic mantle and Phanerozoic circumcratonic mantle. Archean xenoliths are more depleted on average, and those from South Africa and Siberia, the best-studied areas, have higher Si/Mg than Phanerozoic xenoliths of similar Mg#; subcalcic harzburgites are wellrepresented in Archean xenolith and xenocryst suites, but rare in younger ones. Analysis of >13,000 garnet xenocrysts from volcanic rocks worldwide shows a correlation of garnet composition with the tectonothermal age of the crust penetrated by the volcanic rocks. Typically, garnet suites from volcanics penetrating Archean crust have higher Cr#, lower Y, Y/Ga and HREE/Sc, and higher Zr/Y than those from volcanic rocks in Proterozoic and Phanerozoic terranes. These differences, and the rarity of subcalcic garnets in mantle beneath Proterozoic and Phanerozoic crust, are consistent with a decrease in mean modal cpx/gnt and (cpx+gnt) from Archean to Proterozoic to Phanerozoic mantle. Temperatures calculated from the Ni content of peridotitic garnets (TNi) can be referred to a local paleogeotherm to map the distribution of garnet types with depth. These maps show that Archean mantle sections are typically strongly stratified, with concentrations of harzburgitic rocks and depleted lherzolites in the middle to lower parts of the sections, and more fertile lherzolites at depth. Proterozoic mantle sections are less depleted and typically less strongly stratified, while the garnet-facies parts of Phanerozoic sections are still less depleted and essentially homogeneous. Inversion of TNi and composition data on garnets allows mapping of the vertical distribution of the XFo of coexisting olivine in mantle sections. Archean sections typically show decreasing XFo with depth; Proterozoic sections are less regular, and many show the lowest XFo in the upper parts of the sections. These differences imply changes in the processes that have produced subcontinental lithospheric mantle (SCLM) at different times in Earth history. In xenolith suites, the Cr content of peridotitic garnet correlates well with the Al content of the host peridotite, and Al is closely related to the contents of other major and minor elements. These correlations allow calculation of the mean composition of a mantle section, given the Cr content of garnet xenocrysts from that section. The calculated mean composition of SCLM beneath Archean, Proterozoic and Phanerozoic terrains shows a secular evolution in all measures of depletion, such as Al, Ca, Mg#, and Fe/Al; Proterozoic SCLM is intermediate in composition between Archean and Phanerozoic SCLM. Cenozoic SCLM, exemplified by garnet peridotites from young extensional areas (China, Siberia, Australia) is only mildly depleted (²10% melt extraction) relative to Primitive Mantle. Similar compositions are found in some exposed spinel lherzolite massifs in extensional settings, such as Zabargad Island. Shallow spinel-facies SCLM beneath some Phanerozoic terrains, especially in Europe (both xenolith suites and exposed massifs), is more depleted and may represent reworked Proterozoic SCLM; this is consistent with some Re- Os depletion ages. Archean xenolith suites show positive correlations between Al, Fe and Cr that are not present in younger mantle samples; these correlations indicate that no Cr-Al phase was present on the liquidus during the melting event that produced Archean mantle. We suggest that most Archean SCLM was derived by high-degree melting at depths ³150 km, and that subduction processes have not been significant in its genesis. Phanerozoic circumcratonic xenolith suites are typically much less depleted than ophiolitic and abyssal peridotites; this suggests that subducted oceanic mantle is not a major component of Phanerozoic SCLM. Instead, the fertile composition of circumcratonic xenolith suites, and comparison with the Zabargad situation, suggests that most existing Phanerozoic SCLM probably has been generated in extensional post-orogenic environments. Density modelling shows that Phanerozoic SCLM, once it has cooled to typical conductive geotherms, will be gravitationally unstable with respect to the asthenosphere. This instability provides a mechanism for removing such lithosphere and replacing it with upwelling fertile material on a short time scale. In contrast, typical sections of Archean and Proterozoic SCLM are gravitationally stable, allowing their long-term preservation. The broad correlation of SCLM composition with crustal age implies quasi-contemporaneous formation of crustal volumes and their underlying SCLM, and crust-mantle coupling over periods of aeons beneath Archean and Proterozoic terranes; it also requires an evolution in fundamental processes involved in the formation of continents and their roots. This evolution may be related to the secular cooling of Earth, and a consequent decrease in the melting temperature and depth of melting in mantle plumes.

The origin and evolution of Archean lithospheric mantle

Peridotitic sulphide inclusions in diamonds from the central Slave craton constrain the age and origin of their subcontinental lithospheric mantle (SCLM) sources. These sulphides align with either a ca. 3.5 Ga (shallow SCLM) or a ca. 3.3 Ga isochron (deep SCLM) on a Re–Os ischron diagram, with variably enriched initial 187Os/188Os. Since some Archaean to recent plume-derived melts carry a subducted crust (eclogite) signature and some cratonic SCLM may have been generated in plumes by extraction of komatiitic liquids, we explain these data by subduction of evolved lithospheric material (shallow SCLM) and melting in a hybrid mantle plume that contains domains of recycled eclogite (deep SCLM), respectively. In upwelling hybrid mantle, eclogite-derived melts react with olivine in surrounding peridotites to form aluminous orthopyroxene, convert peridotite to pyroxenite and confer their crustal isotope signatures. We suggest that it is subsequent to orthopyroxene enrichment of peridotite in an upwelling plume that partial melting of this Al- and Si- enriched source generated komatiites and complementary ultradepleted cratonic mantle residues. Although subduction is needed to explain some cratonic features, melting of a hybrid plume source satisfies several key observations: (1) suprachondritic initial 187Os/188Os in subsets of lithospheric mantle samples and in some coeval Archaean komatiites; (2) variable enrichment of cratonic mantle by high-temperature aluminous orthopyroxene; (3) high Mg# combined with high orthopyroxene content in cratonic mantle due to higher melt productivity of an Al- and Si-richer source; (4) variable orthopyroxene enrichment possibly linked to varying mantle potential temperatures (Tp), plume buoyancy and resultant eclogite load and/or variable availability of subducted material in the source; and (5) absence of younger analogues due to a secular decrease in Tp. Most importantly, this model also alleviates a mass balance problem, because it predicts a hybrid mantle source with variably higher SiO2 and Al2O3 than primitive mantle, and, contrary to a primitive mantle source, is able to reconcile compositions of komatiites and complementary cratonic mantle residues.

The North China Craton (NCC) has been thinned from >200 km to in its eastern part. The ancient subcontinental lithospheric mantle (SCLM) has been replaced by the juvenile SCLM in the Meoszoic. During this period, the NCC was destructed as indicated by extensive magmatism in the Early Cretaceous. While there is a consensus on the thinning and destruction of cratonic lithosphere in North China, it has been hotly debated about the mechanism of cartonic destruction. This study attempts to provide a resolution to current debates in the view of Mesozoic mafic magmatism in North China. We made a compilation of geochemical data available for Mesozoic mafic igneous rocks in the NCC. The results indicate that these mafic igneous rocks can be categorized into two series, manifesting a dramatic change in the nature of mantle sources at ~121 Ma. Mafic igneous rocks emplaced at this age start to show both oceanic island basalts (OIB)-like trace element distribution patterns and depleted to weakly enriched Sr-Nd isotope compositions. In contrast, mafic igneous rocks emplaced before and after this age exhibit both island arc basalts (IAB)-like trace element distribution patterns and enriched Sr-Nd isotope compositions. This difference indicates a geochemical mutation in the SCLM of North China at ~121 Ma. Although mafic magmatism also took place in the Late Triassic, it was related to exhumation of the deeply subducted South China continental crust because the subduction of Paleo-Pacific slab was not operated at that time. Paleo-Pacific slab started to subduct beneath the eastern margin of Eruasian continent since the Jurrasic. The subducting slab and its overlying SCLM wedge were coupled in the Jurassic, and slab dehydration resulted in hydration and weakening of the cratonic mantle. The mantle sources of ancient IAB-like mafic igneous rocks are a kind of ultramafic metasomatites that were generated by reaction of the cratonic mantle wedge peridotite not only with aqueous solutions derived from dehydration of the subducting Paleo-Pacific oceanic crust in the Jurassic but also with hydrous melts derived from partial melting of the subducting South China continental crust in the Triassic. On the other hand, the mantle sources of juvenile OIB-like mafic igneous rocks are also a kind of ultramafic metasomatites that were generated by reaction of the asthenospheric mantle underneath the North China lithosphere with hydrous felsic melts derived from partial melting of the subducting Paleo-Pacific oceanic crust. The subducting Paleo-Pacific slab became rollback at ~144 Ma. Afterwards the SCLM base was heated by laterally filled asthenospheric mantle, leading to thinning of the hydrated and weakened cratonic mantle. There was extensive bimodal magmatism at 130 to 120 Ma, marking intensive destruction of the cratonic lithosphere. Not only the ultramafic metasomatites in the lower part of the cratonic mantle wedge underwent partial melting to produce mafic igneous rocks showing negative e Nd( t ) values, depletion in Nb and Ta but enrichment in Pb, but also the lower continent crust overlying the cratonic mantle wedge was heated for extensive felsic magmatism. At the same time, the rollback slab surface was heated by the laterally filled asthenospheric mantle, resulting in partial melting of the previously dehydrated rocks beyond rutile stability on the slab surface. This produce still hydrous felsic melts, which metasomatized the overlying asthenospheric mantle peridotite to generate the ultramafic metasomatites that show positive e Nd( t ) values, no depletion or even enrichment in Nb and Ta but depletion in Pb. Partial melting of such metasomatites started at ~121 Ma, giving rise to the mafic igneous rocks with juvenile OIB-like geochemical signatures. In this context, the age of ~121 Ma may terminate replacement of the ancient SCLM by the juvenile SCLM in North China. Paleo-Pacific slab was not subducted to the mantle transition zone in the Mesozoic as revealed by modern seismic tomography, and it was subducted at a low angle since the Jurassic, like the subduction of Nazca Plate beneath American continent. This flat subduction would not only chemically metasomatize the cratonic mantle but also physically erode the cratonic mantle. Therefore, the interaction between Paleo-Pacific slab and the cratonic mantle is the first-order geodynamic mechanism for the thinning and destruction of cratonic lithosphere in North China.

If the subcontinental lithospheric mantle (SCLM) formed through the repeated underthrusting of oceanic slabs, peridotitic SCLM should resemble oceanic peridotites, and mafic rocks (eclogites, s.l.) should be distributed throughout the SCLM. However, cratonic peridotites (both exposed massifs and xenoliths) differ markedly from oceanic and ophiolitic peridotites in their Fe-Cr-Al relationships and abundances of trace elements (Li and B) diagnostic of subduction. "Typical" cratonic peridotites have experienced extensive metasomatism; modelling of their refractory protoliths indicates high-degree melting at high P, perhaps a uniquely Archean process. Cratonic eclogites are strongly concentrated at the base of the depleted SCLM or at major layer boundaries, and are accompanied by intense meltrelated metasomatism in adjacent peridotites. This distribution, and the preservation of exsolution microstructures, suggest an origin by the ponding and cooling of magmas at a compositional/rheological boundary. Compositionally, cratonic eclogites are similar to Phanerozoic garnet pyroxenites that originated as cumulates of high-Al pyroxenes and as reaction zones between melts and peridotite wall rocks. Eu anomalies in peridotitic garnets from the lithospheric mantle are unlikely to reflect plagioclase fractionation, but may be redox-related metasomatic signatures; such anomalies in eclogitic minerals or whole rocks are thus not prima facie evidence of low-P origin. Mg-isotope fractionation in high-T mantle xenoliths indicates that stableisotope variations (including O and C) in cratonic eclogite suites may not be evidence of ocean-floor processes. Covariations between C and O isotope ratios suggest that high-T redoxrelated Rayleigh fractionation, and mixing processes involving carbonatitic melts, can explain the ranges of ! 18 O and ! 13 C in eclogite suites. There is thus little compelling evidence that any rocks in the cratonic SCLM represent unambiguous samples of subducted oceanic plates.

Age and Sr-Nd-Hf isotopes of the sub-continental lithospheric mantle beneath the Cameroon Volcanic Line: Constraints from the Nyos mantle xenoliths

The compositional and mineralogical distinction between mantle lithosphere underlying old continental cratons and that beneath orogenic belts and ocean basins is well established (Boyd, 1989). The origin of many orogenic and oceanic peridotites as residues of partial melting at pressures less than 3 GPa in the spinel stability field is generally accepted. At such pressures, residues will be Ol-rich. In contrast, the highly depleted character, yet relatively Olpoor (and Opx-rich) mineralogy recognized in xenoliths from many cratons require that such lithosphere is not a simple residue. Hypotheses for the formation of cratonic lithosphere were reviewed by Kelemen et al. (1998) who provided compelling evidence that most of the mantle lithosphere beneath cratons originates by partial melting in the spinel stability field, and that cratonic garnet peridotites with higher equilibration pressures have been tectonically transported to such depths. In this way, ‘on-craton’ lithosphere could be considered oceanic peridotite that has underplated to form Archean continents with the deep lithospheric keels. Nonetheless, of the hypotheses reviewed by Kelemen et al. (1998), none have convincingly tied the petrological origin of cratonic mantle lithosphere to a specific tectonic setting. It has been speculated that Opx-enrichment in cratonic mantle resulting from melt/rock reaction would proceed in a subduction zone, whereas the same feature would occur from high degrees of melting and related cumulate enrichment in the root of a mantle plume. Distinct chemical or mineralogical lines of evidence to link the mantle samples to these tectonic environments is so far lacking. When interpreted in the light of a growing experimental data base for V partitioning (Canil, 1997; 1999) the systematics for V in cratonic and oceanic mantle help to further distinguish the conditions and the tectonic setting during melt depletion to form these two contrasting types of lithosphere. Examination of a data base of 200 spinel- and garnet peridotite analyses of on- and off-craton mantle xenoliths, and orogenic massifs from various settings show that the V abundances in mantle lithosphere lies along two distinct coherent trends (see Figure) when plotted against a depletion index such as wt% MgO. For a given degree of depletion, the V abundances of all cratonic peridotites, whether spinel- or garnet-bearing, are at lower levels than those of oceanic peridotites. The covariation of V with depletion for oceanic peridotites is explained (and modelled) as partial melting in the spinel stability field. Spinel has a DV that is ~ 10 times higher than any other peridotite mineral at a given oxygen fugacity (fO2), and as a residual phase during partial melting at low/moderate pressures, and fO2’s near that of MORB generation (NNO- 2 or -3), will dominate the budget for V. On the other hand, the spectrum of cratonic mantle compositions in the Figure requires either of the following: (1) Oceanic and cratonic peridotites represent residues from two completely different primitive mantle (PUM) sources; the source for cratonic mantle having a lower overall V abundance relative to PUM. (2) Unlike oceanic peridotites, cratonic peridotites are residues of melting at high pressures in the garnet stability field. Because DV Gt is less than DV Sp, less V is retained in the residue for a given degree of depletion. Quite possibly, neither garnet nor spinel are residual in the original melt depletion events related to cratonic peridotites, and both pyroxene and/or olivine, having low DV are responsible for the smaller amount of V in the residues. (3) Cratonic mantle originates by depletion in the Sp stability field at low pressure where Sp is a residual phase, but at much higher fO2 than oceanic peridotites related to MORB generation. The DV for Sp is very fO2 sensitive and varies almost an order of magnitude over the range of terrestrial fO2’s. The lower DV Sp during melting at higher fO2 (near NNO) would result in less V being retained in the residue. Hypothesis (3) would constrain the formation of cratonic peridotites to a region of melt generation in the mantle of relatively high FO2, most likely related to subduction. The distinctly lower Ti/V ratios recognized in many arc-related tholeiites (Shervais, 1982) are a complimentary signature related to this process. The above hypotheses will be quantitatively described and explored using appropriate peridotite melting models and partition coefficients. If hypothesis (3) is accepted then partial melting in the Archean was more oxidized than hitherto realized, some form of subduction was operative, and the mantle lithosphere beneath Archean cratons is intimately related to the subduction process.

The subcontinental lithospheric mantle (SCLM) plays an essential role in tectonic and metallogenetic processes affecting the continental lithosphere. Kimberlitic, kamafugitic and alkaline-carbonatitic (KKAC) magmas are probes of the SCLM; they may carry mantle xenoliths, xenocrysts and ore-forming elements. Over 700 KKAC intrusions are currently identified on the south-western margin of the São Francisco Craton (SWSFC), within the NW-SE lineament of 125° Azimuth (AZ125). However, the ages of the KKAC rocks and the nature of the SCLM are not well established. Moreover, the diamond content in these rocks is very low, while large volumes and individual gems, commonly >100ct, occur in secondary sources. A re-evaluation of the history of the KKAC magmas combines geochronology with mineralogical/chemical characterization of geochronometers. A critical literature review and new ages demonstrate that inherited xenocrysts and primary minerals occur in the same pipes. The compositions of resistate minerals carry clues on their parental magmas, and microstructural/chemical/isotopic features distinguish maximum vs intrusion ages, refining the main KKAC magmatism at 88-76 Ma. Geochemical data from garnet and spinel xenocrysts in the KKAC magmas reveal a SCLM with typical cratonic model geotherms (37.5-42.5 mW/m2 ) and a lithosphere 110-175 km thick; the mean depth for the Base of the Depleted Lithosphere (BDL) is ca 140 km. Ten unidimensional SCLM sections reveal that fertile lherzolites affected by varying degrees of melt-related metasomatism dominate the SCLM; a few depleted SCLM volumes remain both on- and off-craton. A newly recognized traceelement pattern, the Tecton-lherzolite trend, reflects physical mixing between asthenospheric and lithospheric materials in extensional and/or compressional regimes. The perovskite-based oxygen fugacity of KKAC magmas shows relatively low values, where perovskites with FMQ -2 or below probably represent shallow cumulates from deep-seated magmas. A new oxybarometer based on V/Sc in pyrope garnets (V/Scgnt) was calibrated and key trends of fO2 distribution in cratonic, reworked, and Tecton SCLM were defined. Below the SW-SFC, the V/Scgnt trend reflects a SCLM significantly more reduced than cratonic mantle. The chemical tomography is used for interpretation of seismic and MT data. Lithosphere-scale melt/fluid channels controlled the emplacement of magmas that sampled a relatively thin SCLM with large-scale short-range variability in degrees of depletion/fertility. Local differences in lithosphere thickness might explain the large variety of KKAC magmas in the area. The timing of KKAC magmatism in the AZ125, and perhaps in the Lucapa corridor (Angola), may represent a far-field response to the South Atlantic opening. The Archean SCLM below the SW-SFC was progressively modified by several tectonothermal events linked to intense metasomatism and refertilisation, representing craton-margin lithospheric erosion and asthenosphere-SCLM mixing during continental collision and post-rifting continental magmatism. Types I, II and CLIPPIR diamonds found in secondary sources are survivors of those processes, sampled by numerous low-grade, but potentially high-value, KKAC pipes. These findings imply new exploratory paradigms for diamond exploration and perhaps exploration for magmatic ores in the SW-SFC.

Petrogenesis of Cenozoic, alkalic volcanic lineages at Mount Morning, West Antarctica and their entrained lithospheric mantle xenoliths: Lithospheric versus asthenospheric mantle sources

Thinning and destruction of the lithospheric mantle root beneath the North China Craton: A review

The old lithospheric mantle beneath the North China Craton (NCC, Fig. 1a) was extensively thinned during the Phanerozoic, especially in the Mesozoic and Cenozoic, resulting in the loss of more than 100 km of the rigid lithosphere (Menzies et al., 1993; Fan et al., 2000). This inference comes from the studies on the Ordovician diamondiferous kimberlites (Fig. 1b), Mesozoic lamprophyre-basalts and Cenozoic basalts, and their deep-seated xenoliths (e.g. Lu et al., 1995; Griffin et al., 1998; Menzies & Xu, 1998; Zhang et al., 2002). This remarkable evolution of the subcontinental lithosphere mantle, which has had profound effects on the tectonics and magmatism of this region, has attracted considerable attention (e.g. Guo et al., 2003; Deng et al., 2004; Gao et al., 2004; Rudnick et al., 2004; Xu et al., 2004; Ying et al., 2004; Zhang et al., 2004a, 2005, 2008; Wu et al., 2005; Tang et al., 2006, 2007, 2008, 2011; Zhao et al., 2010). However, the cause of such a dramatic change, from a Paleozoic cold and thick (up to 200 km) cratonic mantle (Griffin et al., 1992; Menzies et al., 1993) to a Cenozoic hot and thin (< 80 km) “oceanic-type” lithospheric mantle, is still controversial. Based on the Mesozoic basalt development, Menzies and Xu (1998) argued that thermal and chemical erosion of the lithosphere was perhaps triggered by circum-craton subduction and subsequent passive continental extension. This suggestion was first supported by the geochemical studies on the Mesozoic basalts and high-Mg# basaltic andesites on the NCC (Zhang et al., 2002, 2003). A partial replacement model was proposed, having a subcontinental lithospheric mantle in this region composed of old lithosphere in the uppermost part and newly created lithosphere in the lower part (Fan et al., 2000; Xu, 2001; Zheng et al., 2001). The clearly zoned mantle xenocrysts found in Mesozoic Fangcheng basalts (Zhang et al. 2004b) provide the evidence for such a replacement of lithospheric mantle from high-Mg peridotites to low-Mg peridotites through peridotite-melt reactions (Zhang, 2005). Another different model was also proposed that ancient lithospheric mantle was totally replaced by juvenile material in the Late Mesozoic (Gao et al., 2002; Wu et al., 2003). On the basis of Os isotopic evidence from mantle xenoliths enclosed in Cenozoic basalts, Gao et al. (2002) suggested that two times replacement existed in the NCC. They attributed the replacement of the old lithospheric mantle beneath the Hannuoba region to the collision of the Eastern

The geological record of base metal sulfides in the cratonic mantle: A microscale 187Os/188Os study of peridotite xenoliths from Somerset Island, Rae Craton (Canada)

Three novel statistical approaches (Cluster Analysis by Regressive Partitioning [CARP], Patient Rule Induction Method [PRIM], and ModeMap) have been used to define compositional populations within a large database (n &gt; 13,000) of Cr‐pyrope garnets from the subcontinental lithospheric mantle (SCLM). The variables used are the major oxides and proton‐microprobe data for Zn, Ga, Sr, Y, and Zr. Because the rules defining these populations (classes) are expressed in simple compositional variables, they are easily applied to new samples and other databases. The classes defined by the three methods show strong similarities and correlations, suggesting that they are statistically meaningful. The geological significance of the classes has been tested by classifying garnets from 184 mantle‐derived peridotite xenoliths and from a smaller database (n &gt; 5400) of garnets analyzed for &gt;20 trace elements by laser ablation microprobe–inductively coupled plasma‐mass spectrometry (LAM–ICPMS). The relative abundances of these classes in the lithospheric mantle vary widely across different tectonic settings, and some classes are absent or very rare in either Archean or Phanerozoic SCLM. Their distribution with depth also varies widely within individual lithospheric sections and between different sections of similar tectonothermal age. These garnet classes therefore are a useful tool for mapping the geology of the SCLM. Archean SCLM sections show high degrees of depletion and varying degrees of metasomatism, and they are commonly strongly layered. Several Proterozoic SCLM sections show a concentration of more depleted material near their base, grading upward into more fertile lherzolites. The distribution of garnet classes reflecting low‐T phlogopite‐related metasomatism and high‐T melt‐related metasomatism suggests that many of these Proterozoic SCLM sections consist of strongly metasomatized Archean SCLM. The garnet‐facies SCLM beneath Phanerozoic terrains is only mildly depleted relative to Primitive Upper Mantle (PUM) compositions. These data emphasize the secular evolution of SCLM composition defined earlier [Griffin et al., 1998, 1999a] and suggest that at least part of this evolutionary trend reflects reworking and refertilization of SCLM formed in the Archean time.

The evolution of lithospheric mantle beneath the Kalahari Craton and its margins

Helium isotopes in lithospheric mantle: evidence from tertiary basalts of the western USA