Oxidation of the Kaapvaal lithospheric mantle driven by metasomatism

The oxidation state, reflected in the oxygen fugacity (fO₂), of the subcratonic lithospheric mantle is laterally and vertically heterogeneous. In the garnet stability field, the Kaapvaal lithospheric mantle becomes progressively more reducing with increasing depth from Δlog fO₂ FMQ-2 at 110 km to FMQ-4 at 210 km. Oxidation accompanying metasomatism has obscured this crystal-chemical controlled depth-fO₂ trend in the mantle beneath Kimberley, South Africa. Chondrite normalized REE patterns for garnets, preserve evidence of a range in metasomatic enrichment from mild metasomatism in harzburgites to extensive metasomatism by LREE-enriched fluids and melts with fairly unfractionated LREE/HREE ratios in phlogopite-bearing lherzolites. The metasomatized xenoliths record redox conditions extending up to Δlog fO₂ = FMQ, sufficiently oxidized that magnesite would be the stable host of carbon in the most metasomatized samples. The most oxidized lherzolites, those in or near the carbonate stability field, have the greatest modal abundance of phlogopite and clinopyroxene. Clinopyroxene is modally less abundant or absent in the most reduced peridotite samples. The infiltration of metasomatic fluids/melts into diamondiferous lithospheric mantle beneath the Kaapvaal craton converted reduced, anhydrous harzburgite into variably oxidized phlogopite-bearing lherzolite. Locally, portions of the lithospheric mantle were metasomatized and oxidized to an extent that conversion of diamond into carbonate should have occurred. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Mesozoic lithospheric mantle of the northeastern Siberian craton (evidence from inclusions in kimberlite)

Several thousand clinopyroxene, garnet, and phlogopite inclusions of mantle rocks from Jurassic and Triassic kimberlites in the northeastern Siberian craton have been analyzed and compared with their counterparts from Paleozoic kimberlites, including those rich in diamond. The new and published mineral chemistry data make a basis for an updated classification of kimberlite-hosted clinopyroxenes according to peridotitic and mafic (eclogite and pyroxenite) parageneses. The obtained results place constraints on the stability field of high-Na lherzolitic clinopyroxenes, which affect the coexisting garnet and decrease its Ca contents. As follows from analyses of the mantle minerals from Mesozoic kimberlites, the cratonic lithosphere contained more pyroxenite and eclogite in the Mesozoic than in the Paleozoic. It virtually lacked ultradepleted harzburgite-dunite lithologies and contained scarce eclogitic diamonds. On the other hand, both inclusions in diamond and individual eclogitic minerals from Mesozoic kimberlites differ from eclogitic inclusions in diamond from Triassic sediments in the northeastern Siberian craton. Xenocrystic phlogopites from the D’yanga pipe have ⁴⁰Ar/³⁹Ar ages of 384.6, 432.4, and 563.4 Ma, which record several stages of metasomatic impact on the lithosphere. These phlogopites are younger than most of Paleozoic phlogopites from the central part of the craton (Udachnaya kimberlite). Therefore, hydrous mantle metasomatism acted much later on the craton periphery than in the center. Monomineral clinopyroxene thermobarometry shows that Jurassic kimberlites from the northeastern craton part trapped lithospheric material from different maximum depths (170 km in the D’yanga pipe and mostly < 130 km in other pipes). The inferred thermal thickness of cratonic lithosphere decreased progressively from ~ 260 km in the Devonian-Carboniferous to ~ 225 km in the Triassic and to ~ 200 km in the Jurassic, while the heat flux (Hasterok-Chapman model) was 34.9, 36.7, and 39.0 mW/m², respectively. Dissimilar PT patterns of samples from closely spaced coeval kimberlites suggest different emplacement scenarios, which influenced both the PT variations across the lithosphere and the diamond potential of kimberlites.     

Paleomagnetism of the Siberian traps: New data and a new overall 250 Ma pole for Siberia

The flood basalt province in Siberia is one of the largest in the world but the number of reliable paleomagnetic data on these volcanics is still limited. We studied lava flows and trap-related intrusions from two areas in the north and west of the Siberian platform. A dual-polarity characteristic component was isolated from most samples with the aid of stepwise thermal and alternating field demagnetization. We then compiled all published paleomagnetic data on the Siberian traps that have been obtained according to modern standards; also included are presumably trap-related overprint directions in Paleozoic rocks. Although these overprints and trap results may locally differ, the corresponding mean poles based on remagnetized sediments and volcanics show excellent overall agreement and justify pooling of both data types. Several ways of data grouping were attempted; the trap mean pole proved to be rather insensitive to statistical treatment. Irrespective of the averaging procedure used, the overall mean poles for the Siberian traps (NSP2: 55.1°N, 147.0°E; N = 8, K = 123, A₉₅ = 5.0° or NSP4: 57.2°N, 151.1°E; N = 8, K = 192, A₉₅ = 4.0°) differ slightly, but significantly from the coeval mean poles of Baltica [Torsvik, 2001; Van der Voo, R., and Torsvik, T.H., The quality of the European Permo-Triassic paleopoles and its impact on Pangea reconstructions, in: Timescales of the Paleomagnetic Field, J. E. T. Channell, D.V. Kent, W. Lowrie, and J.G. Meert, eds., AGU Geophys. Monogr., 2004, 135, 29–42]. We consider possible causes for this difference and conclude that it could be explained either by persistent non-dipole terms in the Permo-Triassic geomagnetic field or widespread inclination shallowing in the European data.     

Seismic velocity model of the crust and uppermost mantle around the Mirnyi kimberlite field in Siberia

We present the first detailed seismic velocity models of the crust and uppermost mantle around the Mirnyi kimberlite field in Yakutia, Siberia. We have digitized vintage seismograms that were acquired in 1981 and 1983 by use of Taiga analogue seismographs along two perpendicular seismic profiles. The 370-km long, northwest striking profile I across the kimberlite pipe was covered by 41 seismographs, which recorded seismic signals from 21 chemical shots along the line, including one off-end shot. The perpendicular, 340-km long profile II across profile I ca. 30 km to the south of the Mirnyi kimberlite field was covered by 45 seismographs, which recorded seismic signals from 22 chemical shots, including four off-end shots. Each shot involved detonation of between 1.5 and 6.0 tons of TNT, distributed in individual charges of 100–200 kg in shallow water (< 2 m deep). The data is of high quality with high signal/noise ratio to the farthest offsets. We present the results from two-dimensional ray tracing, forward modelling.Both velocity models show normal cratonic structure of the ca. 45-km-thick crust with only slight undulation of the Moho. However, relatively small seismic velocity is detected to 25-km depth in a ca. 60-km wide zone around the kimberlite pipe, surrounded by elevated velocity (> 6.3 km/s) in the upper crust. The lower crust has a relatively constant velocity of 6.8–6.9 km/s. It appears relatively unaffected by the presence of the kimberlite field. Extremely large P-wave velocity (> 8.7 km/s) of the sub-Moho mantle is interpreted along profile I, except for a 70-km wide zone with a “normal” Pn velocity of 8.1 km/s below the kimberlite. Profile II mainly shows Pn velocities of 8.0–8.2 km/s, with unusually large velocity (> 8.5 km/s) in two, ca. 100-km wide zones, at its southwestern end, one zone being close to the kimberlite field. The nature of these exceptionally large, sub-Moho mantle velocities is not yet understood. The difference in velocity in the two profile directions indicates anisotropy, but the effect of unusual rock composition, e.g. from a high concentration of garnet, cannot be excluded.     

Types, abundances and distribution of kimberlite indicator minerals in alluvial sediments, Wawa–Kinniwabi Lake area, Northeastern Ontario: implications for the presence of diamond-bearing kimberlite

In response to the discovery of diamonds within modern alluvium in the glaciated area of Wawa, Ontario, Canada, the Ontario Geological Survey undertook a regional program of surficial mapping and modern alluvial sediment sampling to assess the potential of the area for diamond-bearing kimberlite. Five varieties of kimberlite-derived indicator minerals were recovered and the composition of three varieties was evaluated, resulting in the identification of G10 Cr-pyrope garnet, inclusion field chromite and Mg-ilmenite. The distribution of indicator minerals was examined in the context of the glacial and bedrock geology. Glacial dispersal from non-kimberlitic marker units is restricted (commonly less than 200 m) and many kimberlite indicator minerals were recovered from samples collected close to cross-cutting NE–SW and NW–SE faults and a strong NE–SW trend in the bedrock associated with the Kapuskasing Structural Zone. From this, several potential exploration targets for diamond-bearing kimberlite are defined.     

The evolution of refertilized lithospheric mantle beneath the northeastern Siberian craton: Links between mantle metasomatism,thermal state and diamond potential

Although the diamond potential of cratons is linked mainly to thick and depleted Archean lithospheric keels, there are examples of craton-edge locations and circum-cratonic Proterozoic terranes underlain by diamondiferous mantle. Here, we use the results of comprehensive major and trace-element studies of detrital garnets from diamond-rich Late Triassic (Carnian) sedimentary rocks in the northeastern Siberia to constrain the thermal and chemical state of the pre-Triassic mantle and its ability to sustain the diamond storage. The studied detrital mantle-derived garnets are dominated by low- to medium-Cr lherzolitic (～45%) and low-Cr megacrystic (～39%) chemistries, with a significant proportion of eclogitic garnets (～11%), and only subordinate contribution from harzburgitic garnets (～5%) with variable Cr₂O₃ contents (1.2–8.4 wt.%). Low-Cr megacrysts display uniform, “normal” rare-earth element (REE) patterns with no Eu/Eu* anomalies, systematic Zr and Ti enrichment (mainly within 2.5–5), which are evidence of their crystallization from deep metasomatic melts. Lherzolitic (G9) garnets exhibit normal or humped to MREE-depleted sinusoidal REE patterns and elevated Nd/Y (up to 0.33–0.41) and Zr/Y ratios (up to 7.62). Rare low- to high-Cr harzburgitic (G10) garnets have primarily “depleted”, sinusoidal REE-patterns, low Ti, Y and HREE, but vary significantly in Zr-Hf, Ti and MREE-HREE contents, Nd/Y (within 0.1–2.4) and Zr/Y (1.53–19.9) ratios. The observed trends of chemical enrichment from the most depleted, harzburgitic garnets towards lherzolitic (including high-Ti high-Cr G11-type) garnets and megacrysts result from either voluminous high-temperature metasomatism by plume-derived silicate melts or recurrent mobilization of less voluminous kimberlitic or related carbonated mantle melts, rather than the initially primitive, fertile nature of the Proterozoic SCLM. Calculated Ni-in-garnet temperatures (primarily within ～1150–1250 °C) indicate their derivation from at least ～220 km thick Cr-undersaturated lithosphere at the relevant Devonian to Triassic thermal flow of ～45 mW/m² or cooler. We suggest the existence of rare harzburgitic domains in the primarily lherzolitic diamond-facies SCLM beneath the northeastern Siberian craton at least by Triassic, whereas the abundance of eclogitic garnets, predominance of E-type inclusions in placer diamonds and specific morphologies argue for diamondiferous eclogites occurring within a ～50–65 kbar diamond window of the Olenek province by the same time.     

Lithospheric mantle beneath the south-eastern Siberian craton: petrology of peridotite xenoliths in basalts from the Tokinsky Stanovik

We provide petrographic, major and trace element data for over 30 spinel peridotite xenoliths from the Tokinsky Stanovik (Tok) volcanic field on the Aldan shield to characterize the lithospheric mantle beneath the south-eastern margin of the Siberian craton, which formed in the Mesoproterozoic. High equilibration temperatures (870–1,010°C) of the xenoliths and the absence of garnet-bearing peridotites indicate a much thinner lithosphere than in the central craton. Most common among the xenoliths are clinopyroxene-poor lherzolites and harzburgites with Al₂O₃ and CaO contents nearly as low as in refractory xenoliths from kimberlite pipes (Mir, Udachnaya) in the central and northern Siberian craton. By contrast, the Tok peridotites have higher FeO, lower Mg-numbers and lower modal orthopyroxene and are apparently formed by shallow partial melting (3 GPa). Nearly all Tok xenoliths yield petrographic and chemical evidence for metasomatism: accessory phlogopite, amphibole, phosphates, feldspar and Ti-rich oxides, very high Na₂O (2–3.1%) in clinopyroxene, LREE enrichments in whole-rocks.Electronic Supplementary Material Supplementary material is available for this article at     

The physicochemical dynamics of carbonatization of the rocks of lithospheric mantle beneath the Siberian Platform

Software modeling flow multireservoir systems was used to study the dynamics of carbonatization of lithospheric mantle with flows of magmatic fluids directed from a chamber in the upper mantle to the permeable zone, which dissects continental lithosphere. It has been shown that the region of physicochemical conditions of carbonatization in depleted mantle rocks corresponds to the narrow range of the compositions of hypothetical fluids. If the total content of the fluid is ~ 4 wt.% and the contents of SiO₂ and Ca are 0.5-0.1 moles, (1) the ratio of the molar fractions of Si to Ca is less than unity; (2) the ratios of molar fractions in the C-H-O system are 1:2:3 or 2:1:2; (3) -8 < log p_O2 < -11; and (4) CO₂ content in the fluid is higher than H₂O content by a factor of 1.5-2, and chlorine significantly dominates over fluorine. If the content of the fluid phase is lower and this phase has a lower major-element content by an order of magnitude, the carbonatization becomes stronger as Ca content decreases.     

辽宁瓦房店金刚石矿田金伯利岩侵位机制分析

辽宁省瓦房店金刚石矿田位于华北陆块辽东新元古代-古生代坳陷带。区内各时代地层均有出露,其中新元古界出露面积最大。区内断裂构造发育,较大的有北北东向的金州断裂,已发现的金伯利岩体基本上分布在该断裂以西。矿田内金刚石矿均为金伯利岩型,已发现100多个金伯利岩体,划分成4条矿带,已提交4个大型原生金刚石矿床和3个近源小型金刚石砂矿床,资源量占全国的一半以上,是我国重要的金刚石矿集区,其中50号金伯利岩管因其出产的金刚石质量优越而在宝石界享有盛誉。但本区的金伯利岩绝大部分是20世纪70年代、80年代发现的,为了更好地开展金刚石勘查工作,对本区金伯利岩的成矿条件和控矿因素进行了研究,金伯利岩体的平面分布位置表明,瓦房店地区的金伯利岩体成群、成带分布,既有岩管也有岩脉,以岩脉为主,岩管约占20%左右,岩体大小不等、形态各异,钻孔控制的岩管、岩脉大多具有向下延伸突然中断的特征,钻孔中见到的金伯利岩显示,很多金伯利岩管底界平直或具有多个水平标高上出现平移错动的现象,典型岩管、岩脉与等轴或近等轴状构造盆地的关系密切。通过对区内金伯利体岩空间分布特征、岩体形态特征进行分析,并探讨了本区金伯利岩的侵位过程和...     

The crater-facies kimberlite system of Tokapal, Bastar District, Chhattisgarh, India

Discovery of diamondiferous kimberlites in the Mainpur Kimberlite Field, Raipur District, Chhattisgarh in central India, encouraged investigation of similar bodies in other parts of the Bastar craton. The earlier known Tokapal ultramafic intrusive body, located beyond the 19-km milestone in Tokapal village along the Jagdalpur–Geedam road, was reinterpreted as crater-facies kimberlite. Its stratigraphic position in the Meso-Neoproterozoic intracratonic sedimentary Indravati basin makes it one of the oldest preserved crater-facies kimberlite systems. Ground and limited subsurface data (dug-, tube-wells and exploratory boreholes) have outlined an extensive surface area (>550 ha) of the kimberlite. The morphological and surface color features of this body on enhanced satellite images suggest that there is a central feeder surrounded by a collar and wide pyroclastic apron. Exploration drilling indicates that the central zone probably corresponds to a vent overlain by resedimented volcaniclastic (epiclastic) rocks that are surrounded by a 2-km-wide spread of pyroclastic rocks (lapilli tuff, tuff/ash beds and volcaniclastic breccia). Drill-holes also reveal that kimberlitic lapilli tuffs and tuffs are sandwiched between the Kanger and Jagdalpur Formations and also form sills within the sedimentary sequence of the Indravati basin. The lapilli tuffs are commonly well stratified and display slumping. Base surges and lava flows occur in the southern part of the Tokapal system. The geochemistry and petrology of the rock correspond to average Group I kimberlite with a moderate degree of contamination. However, the exposed rock is intensely weathered and altered with strong leaching of mobile elements (Ba, Rb, Sr). Layers of vesicular fine-grained glassy material represent kimberlitic lava flows. Tuffs containing juvenile lapilli with pseudomorphed olivine macrocrysts are set in a talc–serpentine–carbonate matrix with locally abundant spinel and sphene. Garnet has not been observed, and phlogopite is very rare. Very limited microdiamond testing (two 18-kg samples) proved negative; however, the composition of chromite grains indicate crystallization in the diamond stability field.     

Depletion and enrichment processes in the lithospheric mantle beneath the Kola Peninsula (Russia): Evidence from spinel lherzolite and wehrlite xenoliths

A suite of spinel lherzolite and wehrlite xenoliths from a Devonian kimberlite dyke near Kandalaksha, Kola Peninsula, Russia, has been studied to determine the nature of the lithospheric mantle beneath the northern Baltic Shield. Olivine modal estimates and Fo content in the spinel lherzolite xenoliths reveal that the lithosphere beneath the Archaean–Proterozoic crust has some similarities to Phanerozoic lithospheric mantle elsewhere. Modal metasomatism is indicated by the presence of Ti-rich and Ti-poor phlogopite, pargasite, apatite and picroilmenite in the xenoliths. Wehrlite xenoliths are considered to represent localised high-pressure cumulates from mafic–ultramafic melts trapped within the mantle as veins or lenses. Equilibration temperatures range from 775 to 969 °C for the spinel lherzolite xenoliths and from 817 to 904 °C for the wehrlites.

Laser ablation ICP-MS data for incompatible trace elements in primary clinopyroxenes and metasomatic amphiboles from the spinel lherzolites show moderate levels of LREE enrichment. Replacement clinopyroxenes in the wehrlites are less enriched in LREE but richer in TiO₂. Fractional melt modelling for Y and Yb concentrations in clinopyroxenes from the spinel lherzolites indicates 7–8% partial melting of a primitive source. Such a volume of partial melt could be related to the 2.4–2.5 Ga intrusion of basaltic magmas (now metamorphosed to garnet granulites) in the lower crust of the northern Baltic Shield. The lithosphere beneath the Kola Peninsula has undergone several episodes of metasomatism. Both the spinel lherzolites and wehrlites were subjected to an incomplete carbonatitic metasomatic event, probably related to an early carbonatitic phase associated with the 360–380 Ma Devonian alkaline magmatism. This resulted in crystallisation of secondary clinopyroxene rims at the expense of primary orthopyroxenes, with development of secondary forsteritic olivine and apatite. Two separate metasomatic events resulted in the crystallisation of the Ti–Fe-rich amphibole, phlogopite and ilmenite in the wehrlites and the low Ti–Fe amphibole and phlogopite in the spinel lherzolites. Alternatively, a single metasomatic event with a chemically evolving melt may have produced the significant compositional differences seen in the amphibole and phlogopite between the spinel lherzolites and wehrlites. The calculated REE pattern of a melt in equilibrium with clinopyroxenes from a cpx-rich pocket is identical to that of the kimberlite host, indicating a close petrological relationship.     

Geochemical and O-isotope constraints on the evolution of lithospheric mantle in the Ross Sea rift area (Antarctica)

Peridotite xenoliths found in Cenozoic alkali basalts of northern Victoria Land, Antarctica, vary from fertile spinel-lherzolite to harzburgite. They often contain glass-bearing pockets formed after primary pyroxenes and spinel. Few samples are composite and consist of depleted spinel lherzolite crosscut by amphibole veins and/or lherzolite in contact with poikilitic wehrlite. Peridotite xenoliths are characterized by negative Al₂O₃–Mg# and TiO₂–Mg# covariations of clino- and orthopyroxenes, low to intermediate HREE concentrations in clinopyroxene, negative Cr–Al trend in spinel, suggesting variable degrees of partial melting. Metasomatic overprint is evidenced by trace element enrichment in clinopyroxene and sporadic increase of Ti–Fe_tot. Preferential Nb, Zr, Sr enrichments in clinopyroxene associated with high Ti–Fe_tot contents constrain the metasomatic agent to be an alkaline basic melt. In composite xenoliths, clinopyroxene REE contents increase next to the veins suggesting metasomatic diffusion of incompatible element. Oxygen isotope data indicate disequilibrium conditions among clinopyroxene, olivine and orthopyroxene. The highest δ¹⁸O values are observed in minerals of the amphibole-bearing xenolith. The δ¹⁸O_cpx correlations with clinopyroxene modal abundance and geochemical parameters (e.g. Mg# and Cr#) suggest a possible influence of partial melting on oxygen isotope composition. Thermobarometric estimates define a geotherm of 80°C/GPa for the refractory lithosphere of NVL, in a pressure range between 1 and 2.5 GPa. Clinopyroxene microlites of melt pockets provide P–T data close to the anhydrous peridotite solidus and confirm that they originated from heating and decompression during transport in the host magma. All these geothermometric data constrain the mantle potential temperature to values of 1250–1350°C, consistent with the occurrence of mantle decompressional melting in a transtensive tectonic regime for the Ross Sea region.     

An exotic Cretaceous kimberlite linked to metasomatized lithospheric mantle beneath the southwestern margin of the Francisco Craton,Brazil

We present major and trace element compositions of mineral concentrates comprising garnet xenocrysts,ilmenite,phlogopite,spinel,zircon,and uncommon minerals (ti...     

Recycling process and proto-kimberlite melt metasomatism in the lithosphere-asthenosphere boundary beneath the Amazonian Craton recorded by garnet xenocrysts and mantle xenoliths from the Carolina kimberlite

Here we present new data on the major and trace element compositions of silicate and oxide minerals from mantle xenoliths brought to the surface by the Carolina kimberlite, Pimenta Bueno Kimberlitic Field, which is located on the southwestern border of the Amazonian Craton. We also present Sr-Nd isotopic data of garnet xenocrysts and whole-rocks from the Carolina kimberlite. Mantle xenoliths are mainly clinopyroxenites and garnetites. Some of the clinopyroxenites were classified as GPP–PP–PKP (garnet-phlogopite peridotite, phlogopite-peridotite, phlogopite-K-richterite peridotite) suites, and two clinopyroxenites (eclogites) and two garnetites are relicts of an ancient subducted slab. Temperature and pressure estimates yield 855–1102 °C and 3.6–7.0 GPa, respectively. Clinopyroxenes are enriched in light rare earth elements (LREE) (La_N/Yb_N = 5–62; Ce_N/Sm_N = 1–3; where N = primitive mantle normalized values), they have high Ca/Al ratios (10–410), low to medium Ti/Eu ratios (742–2840), and low Zr/Hf ratios (13–26), which suggest they were formed by metasomatic reactions with CO₂-rich silicate melts. Phlogopite with high TiO₂ (>2.0 wt.%), Al₂O₃ (>12.0 wt.%), and FeO_t (5.0–13.0 wt.%) resemble those found in the groundmass of kimberlites, lamproites and lamprophyres. Conversely, phlogopite with low TiO₂ (<1.0 wt.%) and lower Al₂O₃ (<12.0 wt.%) are similar to those present in GPP-PP-PKP, and in MARID (mica-amphibole-rutile-ilmenite-diopside) and PIC (phlogopite-ilmenite-clinopyorxene) xenoliths. The GPP-PP-PKP suite of xenoliths, together with the clinopyroxene and phlogopite major and trace element signatures suggests that an intense proto-kimberlite melt metasomatism occurred in the deep cratonic lithosphere beneath the Amazonian Craton. The Sr-Nd isotopic ratios of pyrope xenocrysts (G3, G9 and G11) from the Carolina kimberlite are characterized by high ¹⁴³Nd/¹⁴⁴Nd (0.51287–0.51371) and εNd (+4.55 to +20.85) accompanied with enriched ⁸⁷Sr/⁸⁶Sr (0.70405–0.71098). These results suggest interaction with a proto-kimberlite melt compositionally similar with worldwide kimberlites. Based on Sr-Nd whole-rock compositions, the Carolina kimberlite has affinity with Group 1 kimberlites. The Sm-Nd isochron age calculated with selected eclogitic garnets yielded an age of 291.9 ± 5.4 Ma (2 σ), which represents the cooling age after the proto-kimberlite melt metasomatism. Therefore, we propose that the lithospheric mantle beneath the Amazonian Craton records the Paleozoic subduction with the attachment of an eclogitic slab into the cratonic mantle (garnetites and eclogites); with a later metasomatic event caused by proto-kimberlite melts shortly before the Carolina kimberlite erupted.     

山东蒙阴坡里地区金伯利岩带形成时代——来自辉绿岩锆石U-Pb定年数据的证据

金伯利岩是化学成分、矿物组成和结构多变的混杂岩,极易发生蚀变,因此对金伯利岩全岩及各种矿物进行测年的方法很难确定金伯利岩的侵位年龄,且数据结果差别很大。通过分析蒙阴坡里金伯利岩带与辉绿岩的侵入关系,以及辉绿岩锆石U-Pb测年,结合辉绿岩与上覆灰岩的接触关系及金刚石砂矿储集层与已知金刚石原生矿的关系,确定辉绿岩脉的侵位时代为中生代燕山晚期,证实坡里金伯利岩带形成时代为中生代燕山晚期而非加里东期。     

Geology of the Gahcho Kué kimberlite pipes, NWT, Canada: root to diatreme magmatic transition zones

The Cambrian Gahcho Kué kimberlite cluster includes four main pipes that have been emplaced into the Archaean basement granitoids of the Slave Craton. Each of the steep-sided pipes were formed by the intrusion of several distinct phases of kimberlite in which the textures vary from hypabyssal kimberlite (HK) to diatreme-facies tuffisitic kimberlite breccia (TKB). The TKB displays many diagnostic features including abundant unaltered country rock xenoliths, pelletal lapilli, serpentinised olivines and a matrix composed of microlitic phlogopite and serpentine without carbonate. The HK contains common fresh olivine set in a groundmass composed of monticellite, phlogopite, perovskite, serpentine and carbonate. A number of separate phases of kimberlite display a magmatic textural gradation from TKB to HK, which is characterised by a decrease in the proportion of pelletal lapilli and country rock xenoliths and an increase in groundmass crystallinity, proportion of fresh olivine and the degree of xenolith digestion.

The pipe shapes and infills of the Gahcho Kué kimberlites are similar to those of the classic South African pipes, particularly those of the Kimberley area. Similar intrusive magmatic emplacement processes are proposed in which the diatreme-zone results from the degassing, after breakthrough, of the intruding magma column. The transition zones represent ‘frozen’ degassing fronts. The style of emplacement of the Gahcho Kué kimberlites is very different from that of many other pipes in Canada such as at Lac de Gras, Fort à la Corne or Attawapiskat.