Two populations of mantle xenoliths from the Proterozoic Premier kimberlite show an absence of potassic metasomatism common in Phanerozoic kimberlites. The Premier samples are relatively enriched in Fe and Ti, and contain Fe mica and aluminous amphibole instead of Mg-phlogopite and K-richterite. These features are consistent with a recently identified ρ wave anomaly beneath this part of the Kaapvaal craton ascribed to refertilization of the mantle. Upwelling of sublithospheric mantle to produce the Bushveld Igneous Complex is considered to be the source of silicate melt available for metasomatism. The resultant refertilized Fe-, Ti-, and Al-enriched mantle composition resembles that which is required to form Proterozoic troctolitic magmas. 相似文献
Determination of the age of the mantle part of continental roots is essential to our understanding of the evolution and stability of continents. Dating the rocks that comprise the mantle root beneath the continents has proven difficult because of their high equilibration temperatures and open-system geochemical behaviour. Much progress has been made in the last 20 years that allows us to see how continental roots have evolved in different areas. The first indication of the antiquity of continental roots beneath cratons came from the enriched Nd and Sr isotopic signatures shown by both peridotite xenoliths and inclusions in diamonds, requiring isolation of cratonic roots from the convecting mantle for billions of years. The enriched Nd and Sr isotopic signatures result from mantle metasomatic events post-dating the depletion events that led to the formation and isolation of the peridotite from convecting mantle. These signatures document a history of melt– and fluid–rock interaction within the lithospheric mantle. In some suites of cratonic rocks, such as eclogites, Nd and Pb isotopes have been able to trace probable formation ages. The Re–Os isotope system is well suited to dating lithospheric peridotites because of the compatible nature of Os and its relative immunity to post-crystallisation disturbance compared with highly incompatible element isotope systems. Os isotopic compositions of lithospheric peridotites are overwhelmingly unradiogenic and indicate long-term evolution in low Re/Os environments, probably as melt residues. Peridotite xenoliths from kimberlites can show some disturbed Re/Os systematics but analyses of representative suites show that beneath cratons the oldest Re depletion model ages are Archean and broadly similar to major crust-forming events. Some locations, such as Premier in southern Africa, and Lashaine in Tanzania, indicate more recent addition of lithospheric material to the craton, in the Proterozoic, or later. Of the cratons studies so far (Kaapvaal, Siberia, Wyoming and Tanzania), all indicate Archean formation of their lithospheric mantle roots. Few localities studied show any clear variation of age with depth of derivation, indicating that >150 km of lithosphere may have formed relatively rapidly. In circum-cratonic areas where the crustal basement is Proterozoic in age kimberlite-derived xenoliths give Proterozoic model ages, matching the age of the overlying crust. This behaviour shows how the crust and mantle parts of continental lithospheric roots have remained coupled since formation in these areas, for billions of years, despite continental drift. Orogenic massifs show more systematic behaviour of Re–Os isotopes, where correlations between Os isotopic composition and S or Re content yield initial Os isotopic ratios that define Re depletion model ages for the massifs. Ongoing Sr–Nd–Pb–Hf–Os isotopic studies of massif peridotites and new kimberlite- and basalt-borne xenolith suites from new areas, will soon enable a global understanding of the age of continental roots and their subsequent evolution. 相似文献
Crustal or mantle xenoliths are not common in evolved, tholeiitic flood basalts that cover huge areas of the Precambrian shields. Yet, the occasional occurrences provide the most direct and unequivocal evidence on basement composition. Few xenolith occurrences are known from the Deccan Traps, India, and inferences about the Deccan basement have necessarily depended on geophysical studies and geochemistry of Deccan lavas and intrusions. Here, we report two basalt dykes (Rajmane and Talwade dykes) from the central Deccan Traps that are extremely rich in crustal xenoliths of great lithological variety (gneisses, quartzites, granite mylonite, felsic granulite, carbonate rock, tuff). Because the dykes are parallel and only 4 km apart, and only a few kilometres long, the xenoliths provide clear evidence for high small-scale lithological heterogeneity and strong tectonic deformation in the Precambrian Indian crust beneath. Measured 87Sr/86Sr ratios in the xenoliths range from 0.70935 (carbonate) to 0.78479 (granite mylonite). The Rajmane dyke sampled away from any of the xenoliths shows a present-day 87Sr/86Sr ratio of 0.70465 and initial (at 66 Ma) ratio of 0.70445. The dyke is subalkalic and fairly evolved (Mg No. = 44.1) and broadly similar in its Sr-isotopic and elemental composition to some of the lavas of the Mahabaleshwar Formation. The xenoliths are comparable lithologically and geochemically to basement rocks from the Archaean Dharwar craton forming much of southern India. As several lines of evidence suggest, the Dharwar craton may extend at least 350–400 km north under the Deccan lava cover. This is significant for Precambrian crustal evolution of India besides continental reconstructions. 相似文献
The Homestead kimberlite was emplaced in lower Cretaceous marine shale and siltstone in the Grassrange area of central Montana. The Grassrange area includes aillikite, alnoite, carbonatite, kimberlite, and monchiquite and is situated within the Archean Wyoming craton. The kimberlite contains 25–30 modal% olivine as xenocrysts and phenocrysts in a matrix of phlogopite, monticellite, diopside, serpentine, chlorite, hydrous Ca–Al–Na silicates, perovskite, and spinel. The rock is kimberlite based on mineralogy, the presence of atoll-textured groundmass spinels, and kimberlitic core-rim zoning of groundmass spinels and groundmass phlogopites.
Garnet xenocrysts are mainly Cr-pyropes, of which 2–12% are G10 compositions, crustal almandines are rare and eclogitic garnets are absent. Spinel xenocrysts have MgO and Cr2O3 contents ranging into the diamond inclusion field. Mg-ilmenite xenocrysts contain 7–11 wt.% MgO and 0.8–1.9 wt.% Cr2O3, with (Fe+3/Fetot) from 0.17–0.31. Olivine is the only obvious megacryst mineral present. One microdiamond was recovered from caustic fusion of a 45-kg sample.
Upper-mantle xenoliths up to 70 cm size are abundant and are some of the largest known garnet peridotite xenoliths in North America. The xenolith suite is dominated by dunites, and harzburgites containing garnet and/or spinel. Granulites are rare and eclogites are absent. Among 153 xenoliths, 7% are lherzolites, 61% are harzburgites, 31% are dunites, and 1% are orthopyroxenites. Three of 30 peridotite xenoliths that were analysed are low-Ca garnet–spinel harzburgites containing G10 garnets. Xenolith textures are mainly coarse granular, and only 5% are porphyroclastic.
Xenolith modal mineralogy and mineral compositions indicate ancient major-element depletion as observed in other Wyoming craton xenolith assemblages, followed by younger enrichment events evidenced by tectonized or undeformed veins of orthopyroxenite, clinopyroxenite, websterite, and the presence of phlogopite-bearing veins and disseminated phlogopite. Phlogopite-bearing veins may represent kimberlite-related addition and/or earlier K-metasomatism.
Xenolith thermobarometry using published two-pyroxene and Al-in-opx methods suggest that garnet–spinel peridotites are derived from 1180 to 1390 °C and 3.6 to 4.7 GPa, close to the diamond–graphite boundary and above a 38 mW/m2 shield geotherm. Low-Ca garnet–spinel harzburgites with G10 garnets fall in about the same T and P range. Most spinel peridotites with assumed 2.0 GPa pressure are in the same T range, possibly indicating heating of the shallow mantle. Four of 79 Cr diopside xenocrysts have P–T estimates in the diamond stability field using published single-pyroxene P–T calculation methods. 相似文献
The world's oldest diamond deposits occur in 2.67 Ga dikes and heterolithic breccias emplaced into greenstone belts of the Wawa and Abitibi Subprovinces, southern Superior Province, Canada. Thousands of white to yellow microdiamonds and macrodiamonds to 5 mm in width have been recovered by non-contaminating fusion techniques. The host rocks exhibit petrographic and compositional features that are characteristic of post-Archean minettes and spessartites of the calc-alkaline or shoshonitic lamprophyre clan. Based on chemical trends and petrographic evidence, host rocks that contain more than 16 wt.% MgO represent lamprophyre magmas that entrained cumulate olivine, probably at the base of the crust. Breccia bodies that are tens of metres wide at the two localities are somewhat atypical of late Archean lamprophyre occurrences in the Superior Province and owe their size to optimum conditions for magma ascent that were required to preserve the diamonds. Abundant altered ultramafic xenoliths occur in the host rocks. The majority of xenoliths studied (10 of 14) display uniform major element compositions similar to websterite cumulate suites derived from crystal fractionation processes at the base of post-Archean volcanic arcs. The xenoliths display highly variable trace element abundances that are characteristic of cryptic metasomatism associated with the flux of an oxidised fluid above a subduction zone.
The tectonic setting of the deposits and the nature of the host rocks indicate that the diamonds may be derived from the asthenospheric wedge and subducted slab at shallow depths (100 to 160 km) rather than the deep keels of Archean cratons associated with traditional diamond deposit types. Models of low-temperature Phanerozoic diamond formation in active subduction zones, or rapid uplift and emplacement of peridotite massif occurrences, can be adapted to the Archean deposits. The stability field of diamonds in most Phanerozoic subduction scenarios, however, may be too deep to be accessed by the lamprophyric magmas. In contrast, shallow subduction, as invoked for the distinctive occurrence of adakitic (slab-melt) type rocks in the southern Superior Province, could generate two different diamond stability windows at sufficiently shallow depths to account for their presence in lamprophyric magmas.
The multiple requirements imposed on Archean tectonic models by occurrences of diamonds in hydrous shoshonitic rock types (spessartite and minette lamprophyres), along with distinctively metasomatised xenoliths, strongly favour plate tectonic subduction models of orogeny. Evidence of slightly earlier mantle plumes, such as 2.7 Ga komatiites, only strengthens the need for a subduction-driven low-temperature thermal anomaly in the Archean mantle prior to lamprophyric magmatism. 相似文献
Lherzolite xenoliths with calcite-rich microgranular secondary aggregates (0.1–1 mm) have been sampled in a Messinian breccia pipe from the northeastern part of the Languedoc volcanic province (South France). Their study shows that the carbonate crystallized at low pressure from a silico-carbonated melt resulting from partial melting of diopside and spinel at depth. This melting has been induced by injection, shortly before the eruption, of CO2 and H2O-rich fluids, stored probably within the upper lithospheric mantle and reset in motion by the magma ascension. These fluids would derive from decarbonation of levels of deeper lithospheric mantle previously metasomatized by carbonatitic melts. To cite this article: J.-M. Dautria et al., C. R. Geoscience 338 (2006).相似文献
