On and Off the North China Craton: Where is the Archaean Keel? (original) (raw)
Related papers
The origin and evolution of Archean lithospheric mantle
Precambrian Research, 2003
The composition of the subcontinental lithospheric mantle (SCLM) varies in a systematic way with the age of the last major tectonothermal event in the overlying crust. This secular evolution in SCLM composition implies quasi-contemporaneous formation (or modification) of the crust and its underlying mantle root, and indicates that crust and mantle in many cases have remained linked through their subsequent history. Archean SCLM is distinctively different from younger mantle; it is highly depleted, commonly is strongly stratified, and contains rock types (especially subcalcic harzburgites) that are essentially absent in younger SCLM. Some, but not all, Archean SCLM also has higher Si/Mg than younger SCLM. Attempts to explain the formation of Archean SCLM by reference to Uniformitarian processes, such as the subduction of oceanic mantle ("lithospheric stacking"), founder on the marked differences in geochemical trends between Archean xenolith suites and Phanerozoic examples of highly depleted mantle, such as abyssal peridotites, island-arc xenolith suites and ophiolites. In Archean xenolith suites, positive correlations between Fe, Cr and Al imply that no Cr-Al phase (i.e. spinel or garnet) was present on the liquidus during the melting. This situation is in direct contrast to the geochemical patterns observed in highly depleted peridotites from modern environments, which are controlled by the presence of spinel during melting. It is more likely that Archean SCLM represents residues and/or cumulates from high-degree melting at significant depths, related to specifically Archean processes involving major mantle overturns or megaplumes. The preservation of island-arc like SCLM at shallow levels in some sections (e.g. Slave Craton, E. Greenland) suggests that this specifically Archean tectonic regime may have coexisted with a shallow regime more similar to modern plate tectonics. Preliminary data from in situ Re-Os dating of sulfide minerals in mantle-derived peridotites suggest that much Archean SCLM may have formed in a small number of such major events >3.0 Ga ago. The survival of Archean crust may have been critically determined by the availability of large plugs of very buoyant SCLM (a "life-raft model" of craton formation). Many Archean SCLM sections have been strongly affected by Proterozoic and Phanerozoic metasomatism, and much of the observed secular evolution in SCLM composition, at least through Proterozoic time, may reflect the progressive modification of relict, buoyant Archean lithosphere.
A refined model for lithosphere evolution beneath the decratonized northeastern North China Craton
Contributions to Mineralogy and Petrology, 2019
The eastern North China Craton (NCC), where an initially diamondiferous deep cratonic mantle root was lost during Paleozoic and Mesozoic time, represents a prime natural laboratory to study the processes and mechanisms of continental lithospheric mantle destruction and replacement, which remain, however, controversial. In this study, detailed petrography, whole-rock and mineral compositions of spinel-facies peridotite xenoliths from Cenozoic basalts in the Huinan area, northeastern NCC, are presented to provide new constraints on the transformation of the subcontinental lithospheric mantle (SCLM). These xenoliths define two groups based on textural observation and mineral modes: Group 1 peridotites show protogranular textures and consist of harzburgites and dunites. They have low Al 2 O 3 contents in whole-rock and orthopyroxene (0.53-1.06 wt.% and 2.10-3.21 wt.%, respectively), high olivine modes (79-96%), whole-rock MgO (44.8-47.9 wt.%) and Mg# (100 Mg/(Mg + Fe T) molar: 90.1-90.7), suggesting that they were derived from moderately refractory SCLM. In contrast, Group 2 xenoliths display porphyroclastic to protogranular textures and consist of lherzolites and harzburgites with rare spinel-pyroxene intergrowths. They have overall higher Al 2 O 3 (1.48-3.23 wt.% and 3.02-4.65 wt.%, respectively) in wholerock and orthopyroxene, lower olivine modes (64-83%), MgO (38.6-44.5 wt.%) and whole-rock Mg# values 87.6-90.1, and they may represent fertile SCLM. Peridotites of both groups have similar equilibration temperatures (i.e., 923-977 °C and 881-1110 °C, respectively), which are not correlated with Mg# in olivines, suggesting that they coexist over a range of depths. However, clinopyroxenes in the Group 1 xenoliths display LREE-enriched and convex-upward REE patterns, whereas those in Group 2 mainly show LREE-depleted and spoon-shaped REE patterns, with minor LREE-enriched and convex-upward ones. In addition, spinel-pyroxene intergrowths indicative of garnet destabilization are ubiquitous in Group 1, consistent with variable Al 2 O 3 over a narrow range of Mg# in some opx and low HREE in some cpx, but rare in Group 2 peridotites. Interaction of the fertile mantle with melts similar to the Cenozoic basalts at high melt-rock ratios eradicated most signatures of their origin in the garnet stability field, whereas the refractory peridotites, which reacted with residual melts or fluids at low melt/fluid-rock ratios, retained evidence for the former presence of garnet. We suggest that, combined, these observations are best reconciled if portions of ancient refractory lithosphere, which were partly delaminated during multiple subduction episodes affecting the eastern NCC, were re-accreted together with fertile mantle during asthenospheric upwelling driven by extension.
Re���Os evidence for replacement of ancient mantle lithosphere beneath the North China craton
2002
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.
Gondwana Research, 2013
High-Mg# peridotite xenoliths in the Cenozoic Hebi basalts from the North China Craton have refractory mineral compositions (Fo > 91.5) and highly heterogeneous Sr-Nd isotopic compositions ( 87 Sr/ 86 Sr = 0.7031-0.7048, 143 Nd/ 144 Nd = 0.5130-0.5118) ranging from MORB-like to EM1-type mantle, which are similar to those of peridotites from Archean cratons. Thus, the high-Mg# peridotites may represent relics of the ancient lithospheric mantle. Published Re-Os isotopic data for Cenozoic basalt-borne xenoliths show T RD ages of 3.0-1.5 Ga for the peridotites from Hebi (the center of the craton), 2.2-0 Ga for those from Hannuoba and Jining (north margin of the craton), and 2.6-0 Ga for those from Fanshi and Yangyuan (midway between the center and north margin of the craton). In situ Re-Os data of sulfides in Hannuoba peridotites suggest that wholerock Re-Os model ages represent mixtures of multiple generations of sulfides with varying Os isotopic compositions. These observations indicate that initial lithospheric mantle beneath the Central Zone of the North China Craton formed during the Archean and was refertilized by multiple melt additions after its formation. The refertilization became more intensive from the interior to the margin of the craton, leading to the high heterogeneity of the lithospheric mantle: more ancient and refractory peridotites with highly variable Sr-Nd isotopic compositions in the interior, and more young and fertile peridotites with depleted Sr-Nd isotopic composition in the margin. Our data, coupled with published petrological and geochemical data of peridotites from the Central Zone of the North China Craton, suggest that the lithospheric mantle beneath this region is highly heterogeneous, likely produced by refertilization of Archean mantle via multiple additions of melts/ fluids, which were closely related to the Paleoproterozoic collision between the Eastern and the Western Blocks and subsequent circum-craton subduction events.
Lithos, 2011
Despite the relatively long-standing availability of numerical approaches for estimating palaeogeotherms using peridotite xenolith Pressure-Temperature (P-T) data, the practise of fitting xenolith P-T arrays to simple models of lithospheric heat generation, in a non-quantitative manner, remains widespread. The lack of quantification in both the magnitude and uncertainty of heat flow and lithosphere thickness estimates leads to difficulty in evaluating proposed models for lithosphere evolution on a local and regional scale. Here, we explore the advantages of using a numerical approach to palaeogeotherm fitting, in terms of the ability to make objective comparisons of the effect that differing thermobarometer combinations and varying states of mineral and textural equilibrium have on the shape of the palaeogeotherm, and the resulting estimates of lithospheric thickness and heat flow. We also make quantitative comparisons between lithospheric mantle properties estimated using peridotite xenoliths versus single mineral xenocrysts. Using two reference peridotite xenolith databases from Bultfontein (S. Africa) and Somerset Island (Canada) we show that the same lithospheric mantle properties are predicted using harzburgite versus lherzolite thermobarometry methods. Filtering mineral data for the effects of inter-mineral disequilibrium does not produce significantly different palaeogeotherms but does increase the quality of fit of the palaeogeotherm to the P-T data, allowing more confidence to be placed in comparisons between locations. Palaeogeotherms calculated using xenocryst data, screened for peridotitic affinities, show misfits that are 2-3 times greater than those obtained using xenoliths. Lithospheric properties calculated from the Somerset Island xenocrystbased geotherm yield results that are within error of the xenolith estimate. A mutually consistent and quantitative palaeogeotherm fitting approach is used to evaluate existing hypotheses for the evolution of the southern African lithosphere. We find very similar estimates for the heat flow and thickness of the lithosphere between SW Namibia (off-craton) and Bultfontein (on-craton). This supports suggestions of a cratonic thermal regime and equivalent lithospheric thickness across that region of southern Africa at the time of kimberlite sampling, with concurrent local thermal disturbance evident in Namibia. Complimentary, novel, seismically-obtained geotherm estimates show that the lithosphere in Namibia is now significantly thinner than the estimate at 70 Ma obtained from xenolith thermobarometry.
Petrology, geochemistry and Re-Os isotopes of peridotite xenoliths from Yantai (Shandong Province) are reported in this paper with aims of constraining the age and evolution of the lithospheric mantle beneath eastern North China Craton. The Yantai xenoliths contain predominant harzburgites and subordinate lherzolites. Although their highly incompatible element compositions have been modified by metasomatism, the heavy rare earth element (REE) and Y contents in the Yantai peridotites are primarily governed by partial melting, which started in garnet stability field then continued in spinel stability field after breakdown of garnet to two pyroxenes and spinel. Such a polybaric melting produced a residual mantle in which degree of depletion decreases with depth. Os isotopic ratios of the most refractory peridotites (Al 2 O 3 b 1.2 wt.%) range from 0.117 to 0.126, yielding T RD model ages between 0.5 and 1.7 Ga. This suggests co-existence of Phanerozoic and Proterozoic mantle beneath Shandong Province. Alternatively, the whole lithospheric mantle beneath Yantai was likely formed during the Phanerozoic, given the resemblance of their Os isotopic ratios with those of abyssal peridotites. The latter interpretation is consistent with the fact that all the studied samples plot along the oceanic trend in a plot of forsterite content in olivine versus olivine mode. It also gains further support from the contrasting ε Nd between the late Mesozoic lithospheric mantle and Cenozoic mantle beneath the region. The data presented in this study therefore argue for a complete replacement of the cratonic mantle by upwelling asthenosphere.