Experimental constraints on ureilite petrogenesis

This experimental study explores the petrogenesis of ureilites by a partial melting/smelting process. Experiments have been performed over temperature (1150-1280 °C), pressure (5-12.5 MPa), and low oxygen fugacity (graphite-CO gas) conditions appropriate for a hypothetical ureilite parent body ∼200 km in size. Experimental and modeling results indicate that a partial melting/smelting model of ureilite petrogenesis can explain many of the unique characteristics displayed by this meteorite group. Compositional information preserved in the pigeonite-olivine ureilites was used to estimate the composition of melts in equilibrium with the ureilites. The results of 20 experiments saturated with olivine, pyroxene, metal, and liquid with appropriate ureilite compositions are used to calibrate the phase coefficients and pressure-temperature dependence of the smelting reaction. The calibrated coefficients are used to model the behavior of a hypothetical residue that is experiencing fractional smelting. The residue is initially olivine-rich and smelting progressively depletes the olivine content and enriches the pyroxene and metal contents of the residues. The modeled residue composition at 1260 °C best reproduces the trend of ureilite bulk compositions. The model results also indicate that as a ureilite residue undergoes isothermal decompression smelting over a range of temperatures, Ca/Al values and Cr₂O₃ contents are enriched at lower temperatures (below ∼1240 °C) and tend to decrease at higher temperatures. Therefore, fractional smelting can account for the high Ca/Al and Cr₂O₃ wt% values observed in ureilites. We propose that ureilites were generated from an olivine-rich, cpx-bearing residue. Smelting began when the residue was partially melted and contained liquid, olivine, and carbon. These residues experienced varying degrees of fractional smelting to produce the compositional variability observed within the pigeonite-bearing ureilites. Variations in mineral composition, modal proportions, and isotopic signatures are best described by heterogeneous accretion of the ureilite parent body followed by minimal and variable degrees of igneous processing.     

Shallow impact: Isotopic insights into crustal contributions to the Sudbury impact melt sheet

The largest known terrestrial impact melt sheet occurs within the 1850 Ma Sudbury Structure, Ontario. In order to evaluate the relative contributions of different target lithologies to the melt sheet, we have investigated the Pb isotope compositions of feldspar separates from early-formed quartz diorite magmas within Offset Dykes from around the impact structure. The samples define a linear array on plots of age-corrected ²⁰⁶Pb/²⁰⁴Pb versus ²⁰⁷Pb/²⁰⁴Pb. Samples from Offset Dykes hosted by the Huronian Supergroup (South Range) have a range of ²⁰⁶Pb/²⁰⁴Pb₁₈₅₀ from 15.424 to 17.255 and ²⁰⁷Pb/²⁰⁴Pb₁₈₅₀ from 15.390 to 15.801, whilst those hosted by Archean gneisses of the Superior Province (North Range) cluster around ²⁰⁶Pb/²⁰⁴Pb₁₈₅₀ ≈ 14.8 and ²⁰⁶Pb/²⁰⁴Pb₁₈₅₀ ≈ 15.1. These values can be approximated by binary mixing between the two major groups of target lithologies. A mix of 60-70% of Superior Province gneisses with 30-40% of Huronian metasedimentary material closely matches the Pb isotope compositions of North Range Offset Dyke samples, whereas in the South Range the required Huronian component is up to ca. 80%.These mixing proportions are consistent with Sr, Nd and Os isotope and trace element constraints. A third minor component, either locally-exposed Paleoproterozoic mafic rocks or the lower crust is also required. However, the isotopic, trace element and Ni-Cu-platinum group element characteristics of the melt sheet can be accommodated without the involvement of an average lower crustal or meteoritic component.A major contribution of Huronian supracrustal material, which had a pre-impact thickness of up to 12 km, is required to explain the chemical characteristics of the impact melts, which also have a strong upper crustal affinity (e.g. Eu/Sm = 0.22, Rb/Sr = 0.2-0.35). As such, a shallower level of melting is apparent than that predicted by many previous impact models for the Sudbury event. This can be accommodated by considering approach trajectories for the impactor oblique to the Earth’s surface. In addition, the isotopic and trace element variability identified indicates that the melt sheet was heterogeneous at an early stage, and may not have been completely homogenised during crater formation. Our findings have significant implications for the nature of the Sudbury impact event, the evolution of the melt sheet and the crustal sources of metals contained in Sudbury’s world class Ni-Cu-PGE sulphide ores.     

Geochemical constraints on the petrogenesis of high-Mg basaltic andesites from the Northern Taiwan Volcanic Zone

The Northern Taiwan Volcanic Zone (NTVZ) is a Late Pliocene–Quaternary volcanic field that occurred as a result of extensional collapse of the northern Taiwan mountain belt. We report here mineral compositions, major and trace element and Sr/Nd isotope data of high-Mg basaltic andesites from the Mienhuayu, a volcanic islet formed at ∼2.6 Ma in the central part of the NTVZ. The rocks are hypocrystalline, showing porphyritic texture with Mg-rich olivine (Fo≈81–80), bronzite (En≈82–79) and plagioclase (An≈66–58) as major phenocryst phases. They have uniform whole-rock compositions, marked by high magnesium (MgO≈5.9–8.1 wt.%, Mg value≈0.6) relative to accompanying silica contents (SiO₂≈52.8–54.5 wt.%). The high-Mg basaltic andesites contain the highest TiO₂(∼1.5 wt.%) and lowest K₂O (∼0.4 wt.%) among the NTVZ volcanic rocks. In the incompatible element variation diagram, these Mienhuayu magmas exhibit mild enrichments in large ion lithophile (LILE) and light rare earth elements (LREE), coupled with an apparent Pb-positive spike. They do not display depletions in high field strength elements (HFSE), a feature observed universally in the other NTVZ volcanics. The high-Mg basaltic andesites have rather unradiogenic Nd (εNd≈+5.1–7.2) but apparently elevated Sr (⁸⁷Sr/⁸⁶Sr≈0.70435–0.70543; leached values) isotope ratios. Their overall geochemical and isotopic characteristics are similar to mid-Miocene (∼13 Ma) high-Mg andesites from the Iriomote-jima, southern Ryukyus, Japan. Despite these magmas have lower LILE and LREE enrichments and Pb positive spike, their “intraplate-type” incompatible element variation patterns are comparable to those of extension-induced Miocene intraplate basalts emplaced in the Taiwan–Fujian region. Therefore, we interpret the Mienhuayu magmas as silica-saturated melts derived from decompression melting of the ascended asthenosphere that had been subtly affected by the adjacent Ryukyu subduction zone processes. This interpretation is consistent with the notion that in the northern Taiwan mountain belt post-orogenic lithospheric extension started in Plio–Pleistocene time.     

REE distribution in some layered migmatites: constraints on their petrogenesis

The REE distributions in mesosomes, neosomes, leucosomes and melanosomes of four layered migmatites have been investigated. In one example (Arvika migmatites) the REE patterns in adjacent paragneisses, the presumed parent rock of the migmatites, were also determined. REE patterns of neosomes and mesosomes of Arvika migmatites are similar to the finegrained layers and coarse-grained layers, respectively, observed in the adjacent paragneiss. This is in agreement with the layer-by-layer paragneiss-migmatite transformation model.

The REE patterns of mesosomes and neosomes indicate that these lithologies may have been closed systems (for REE) during the formation of the migmatites. No indication of metasomatic reactions, melt segregation or injection could be detected. Within the neosomes, leucosomes are depleted and melanosomes enriched in REE contents. This is interpreted to be due to separation and concentration of accessory minerals (monazite, epidote, allanite, zircon, sphene, apatite, garnet) into the melanosomes. The behaviour of accessory minerals during migmatite formation is closely allied to that of biotite, which is also concentrated in the melanosomes.     

Lunar volcanic glasses and their constraints on mare petrogenesis

Volcanic glasses from the Apollo 11, 14, 15, and 16 landing sites have been analyzed for major elements and Ni by electron microprobe. The 19 varieties of volcanic glass define two distinct chemical arrays that provide new insights into (a) the petrogenesis of mare basalts and (b) the structure of the deep lunar interior. A simple model is proposed whereby mare basaltic liquids may have been derived from two isolated, cumulate systems occurring at depths of ~300 km and ? 400 km. Each system was itself composed of two lithologic components (low-Ti vs high-Ti) that underwent hybridization, assimilation, or mixing to generate the large compositional range of magmas observed within each array. While the distribution of the two components within each system was locally heterogeneous, data indicate that the components themselves were chemically uniform on at least a regional scale. The surface-correlated volatiles associated with the lunar volcanic glasses seem to have come from some other reservoir within the Moon.The simplicity of the chemical relationships observed among the lunar volcanic glasses allow specific-predictions to be made. We believe that they should readily reveal any strengths or weaknesses of this new model.     

Isotopic and trace element constraints on the petrogenesis of lavas from the Mount Adams volcanic field,Washington

Strontium, Nd, Pb, Hf, Os, and O isotope compositions for 30 Quaternary lava flows from the Mount Adams stratovolcano and its basaltic periphery in the Cascade arc, southern Washington, USA indicate a major component from intraplate mantle sources, a relatively small subduction component, and interaction with young mafic crust at depth. Major- and trace-element patterns for Mount Adams lavas are distinct from the rear-arc Simcoe volcanic field and other nearby volcanic centers in the Cascade arc such as Mount St. Helens. Radiogenic isotope (Sr, Nd, Pb, and Hf) compositions do not correlate with geochemical indicators of slab-fluids such as (Sr/P) _n and Ba/Nb. Mass-balance modeling calculations, coupled with trace-element and isotopic data, indicate that although the mantle source for the calc-alkaline Adams basalts has been modified with a fluid derived from subducted sediment, the extent of modification is significantly less than what is documented in the southern Cascades. The isotopic and trace-element compositions of most Mount Adams lavas require the presence of enriched and depleted mantle sources, and based on volume-weighted chemical and isotopic compositions for Mount Adams lavas through time, an intraplate mantle source contributed the major magmatic mass of the system. Generation of basaltic andesites to dacites at Mount Adams occurred by assimilation and fractional crystallization in the lower crust, but wholesale crustal melting did not occur. Most lavas have Tb/Yb ratios that are significantly higher than those of MORB, which is consistent with partial melting of the mantle in the presence of residual garnet. δ ¹⁸O values for olivine phenocrysts in Mount Adams lavas are within the range of typical upper mantle peridotites, precluding involvement of upper crustal sedimentary material or accreted terrane during magma ascent. The restricted Nd and Hf isotope compositions of Mount Adams lavas indicate that these isotope systems are insensitive to crustal interaction in this juvenile arc, in stark contrast to Os isotopes, which are highly sensitive to interaction with young, mafic material in the lower crust.     

Boninite petrogenesis and alteration history: constraints from stable isotope compositions of boninites from Cape Vogel,New Caledonia and Cyprus

¹⁸O values of unaltered olivine and pyroxene phenocrysts in boninites from several areas range from 5.8 to 7.4 and indicate that the source for most boninites is more ¹⁸O-rich than MORBs and other oceanic basalts. The source for oxygen and other major elements is most likely a refractory portion of the mantle having a ¹⁸O value of up to 7.0 to which must be added a small amount of H₂O-rich fluid to induce partial melting. This fluid, which is derived from subducted crust, is the vehicle for LREEs including Nd. The variable, normally low _Nd values typical of boninites do not correlate with the ¹⁸O values.Post eruptive exchange of oxygen in the glass of boninites with that of sea water at low temperatures (<150° C) produces ¹⁸O values of >10 in optically fresh glass. Hydration of the glass has increased the water contents of most boninites from estimated magmatic values of 1–2 wt% to 2–4 wt% and produced D values of < –80, which may be lower than the original magmatic D values. In contrast to most submarine pillow basalts, the magmatic volatile composition of boninite lavas has been extensively modified as a result of post eruptive interaction with seawater.     

On the track of the elusive Sudbury impact: geochemical evidence for a chondrite or comet bolide

Siderophile and lithophile trace element data for 69 samples from the Sudbury impact crater fill (Onaping Formation) and quartz diorite offset dikes help constrain the sources of the established moderately elevated platinum group element signature associated with the impact structure. The siderophile element distribution of the crater fill requires contributions from bulk continental crust, mafic rocks and a chondritic component. A mantle component is absent, but the involvement of mid to lower crust is implied. After considering post‐impact hydrothermal alteration, melt heterogeneity and mafic target admixture, the projectile elemental ratios were determined on a more robust data subset. Chondrite discrimination diagrams of these ratios identify an ordinary or enstatite chondrite as the most probable source of meteoritic material in the Sudbury crater fill. However, the relative and absolute siderophile element distributions within the impact structure as well as bolide size models are congruent with the bolide being a comet that had a chondritic refractory component.     

Postcollisional magmatism: Geochemical constraints on the petrogenesis of Mesozoic granitoids in the Sulu orogen,China

A combined study of zircon U–Pb ages and Lu–Hf isotopes, mineral O isotopes, whole-rock elements and Sr–Nd isotopes was carried out for Mesozoic granitoids from the Shandong Peninsula in east-central China, which tectonically corresponds to the eastern part of the Sulu orogen that formed by the Triassic continental collision between the South and North China Blocks. Four plutons were investigated in this region, with the Linglong and Guojialing plutons from the northwestern part (Jiaobei) and the Kunyushan and Sanfoshan plutons from the southeastern part (Jiaodong). The results show that these granitoids mostly have high Sr, low Yb and Y contents, high (La/Yb)_N and Sr/Y ratios with negligible to positive Eu anomalies (Eu/Eu* = 0.69–1.58), which are similar to common adakites. On the other hand, they have relatively low MgO, Cr, Ni contents and thus low Mg#. Zircon U–Pb dating yields Late Jurassic ages of 141 ± 3 to 157 ± 2 Ma for the Linglong and Kunyushan plutons, but Early Cretaceous ages of 111 ± 2 to 133 ± 3 Ma for the Guojialing and Sanfoshan plutons. Some zircon cores from the Linglong and Kunyushan granitoids have Neoproterozoic U–Pb ages. All the granitoids have variably negative zircon ε_Hf(t) values of ?39.6 to ?5.4, with Mesoproterozoic to Paleoproterozoic Hf model ages of 1515 ± 66 to 2511 ± 97 Ma for the Sanfoshan pluton, but Paleoproterozoic to Paleoarchean Hf model ages of 2125 ± 124 to 3310 ± 96 Ma for the other three plutons. These indicate that the Mesozoic granitoids formed in the postcollisional stage and were derived mainly from partial melting of the subducted South China Block that is characterized by Paleoproterozoic juvenile crust and Neoproterozoic magmatic rocks along its northern edge. However, there are some differences between the Jiaobei and Jiaodong plutons. Compared to the Jiaodong granitoids, the Jiaobei granitoids have very old zircon Hf model ages of 3310 ± 96 Ma suggesting the possible involvement of a Paleoarchean crust that may be derived from the North China Block. Therefore, the continental collision between the two blocks would bring crustal materials from both sides into the subduction zone in the Triassic, yielding subduction-thickened crust as the magma source for the adakite-like granitoids. While lithospheric extension and orogenic collapse are considered a major cause for postcollisional magmatism, anatexis of the subducted mafic crust is proposed as a mechanism for chemical differentiation of the continental crust towards felsic composition.     

吉林东南部晚中生代中酸性火山作用成因的地球化学制约

详细的主、微量元素和Sr-Nd-Pb同位素研究结果显示,延吉地区晚中生代(早白垩世)中酸性火山岩具有岛弧型微量元素特征(富集轻稀土(LREE)、大离子亲石元素(LILE)和亏损高场强元素(HFSE)),和中等放射成因Sr、Pb及类似于硅质地球的Nd同位素组成(~(87)Sr/~(86)Sr(i)=0.70437～0.70525;~(206)Pb/~(204)Pb(i)=18.19～18.37,~(207)Pb/~(204)Pb(i)=15.54～15.57,~(208)Pb/~(204)Pb(i)=38.13～38.22;ε_(Nd)(t)=-2.75～+1.61)。其Sr-Nd-Pb同位素组成特征类似于区域同期基性火山岩和大兴安岭地区同时代火山岩。这些广泛分布于吉林南部的晚中生代中酸性火山岩是岩石圈地幔来源岩浆经分离结晶或地壳物质混染/AFC过程的产物。吉林省东南部晚中生代不同区域火山岩的Sr-Nd-Pb同位素对比结果显示,辽源-延吉与通化地区分属不同构造单元,其中辽源-延吉一带则为中亚造山带的东段,而通化地区为华北克拉通的一部分,夹皮沟-松江断裂可能是华北克拉通东北缘的边界。     

Sr-Nd isotopic constraints on the petrogenesis of the Central Bohemian Pluton,Czech Republic

The Sr-Nd isotopic data for selected granitoids of the Central Bohemian Pluton show a broad negative correlation with the total range of (⁸⁷Sr/⁸⁶Sr)₃₃₀ = 0.7051–0.7129 and _Nd ³³⁰ = +0.2 to –8.9. The older intrusions have more depleted Sr-Nd compositions and calc-alkaline geochemistry (Sázava suite), whereas the younger intrusions shift towards K-rich calc-alkaline (Blatná suite) and shoshonitic rocks (íany and ertovo bemeno suites) with more evolved isotopic signatures. The distribution of the data is interpreted as reflecting a diversity of sources and processes, rather than a single progressive crustal contamination trend. The Sázava suite could have originated by partial melting of metabasites, or of a mantle source with an isotopic composition close to bulk earth, or by hybridization of crustally-derived tonalitic and mantle-derived magmas. Variation within the Blatná suite is modelled by mixing between a moderately enriched [(⁸⁷Sr/⁸⁶Sr)₃₃₀ 0.708, _Nd ³³⁰ –3] mantle component with either an isotopically evolved metasedimentary component, or with more evolved magmas of the suite. The íany suite was most probably produced by partial melting of peraluminous lithologies, possibly of the adjacent Moldanubian unit. The ertovo bemeno suite evolved from strongly enriched mantle-derived magmas [(⁸⁷Sr/⁸⁶Sr)₃₃₀0.7128, _Nd ³³⁰ –7], either through closed-system fractional crystallization or interaction with magma corresponding to leucogranites of the Central Bohemian Pluton.     

Geochronological and geochemical constraints on the petrogenesis of alkaline ultramafic dykes from southwest Guizhou Province,SW China

Geochronological, geochemical and whole-rock Sr–Nd isotopic analyses have been completed on a suite of alkaline ultramafic dykes from southwest (SW) Guizhou Province, China with the aim of characterising their petrogenesis. The Baiceng ultramafic dykes have a LA-ICP-MS zircon ²⁰⁶Pb/²³⁸U age of 88.1 ± 1.1 Ma (n = 8), whereas two phlogopites studied by ⁴⁰Ar/³⁹Ar dating methods give emplacement ages of 85.25 ± 0.57 Ma and 87.51 ± 0.45 Ma for ultramafic dykes from Yinhe and Lurong, respectively. In terms of composition, these Late Mesozoic ultramafic dykes belong to the alkaline magma series due to their high K₂O (3.31–5.04 wt.%) contents. The dykes are characterised by enrichment of light rare earth element (LREE) and large-ion lithosphile elements (LILEs) (Rb and Ba), negative anomalies in high field strength elements (HFSEs), such as, Nb, Ta and Ti relative to primitive mantle, low initial ⁸⁷Sr/⁸⁶Sr ratios (0.7060–0.7063) and positive ε_Nd(t) values (0.3–0.4). Such features suggest derivation from low degree (< 1%) partial melting of depleted asthenospheric mantle (garnet-lherzolite), and contamination to various degrees (～ 10%) by interaction with upper crustal materials.     

The role of mantle-hybridization and crustal contamination in the petrogenesis of lithospheric mantle-derived alkaline rocks: constraints from Os and Hf isotopes

The Rhön area as part of the Central European Volcanic Province (CEVP) hosts an unusual suite of Tertiary 24-Ma old hornblende-bearing alkaline basalts that provide insights into melting and fractionation processes within the lithospheric mantle. These chemically primitive to slightly evolved and isotopically (Sr, Nd, Pb) depleted basalts have slightly lower Hf isotopic compositions than respective other CEVP basalts and Os isotope compositions more radiogenic than commonly observed for continental intraplate alkaline basalts. These highly radiogenic initial ¹⁸⁷Os/¹⁸⁸Os ratios (0.268–0.892) together with their respective Sr–Nd–Pb isotopic compositions are unlikely to result from crustal contamination alone, although a lack of Os data for lower crustal rocks from the area and limited data for CEVP basalts or mantle xenoliths preclude a detailed evaluation. Similarly, melting of the same metasomatized subcontinental lithospheric mantle as inferred for other CEVP basalts alone is also unlikely, based on only moderately radiogenic Os isotope compositions obtained for upper mantle xenoliths from elsewhere in the province. Another explanation for the combined Nd, Sr and Os isotope data is that the lavas gained their highly radiogenic Os isotope composition through a mantle “hybridization”, metasomatism process. This model involves a mafic lithospheric component, such as an intrusion of a sublithospheric primary alkaline melt or a melt derived from subducted oceanic material, sometime in the past into the lithospheric mantle where it metasomatized the ambient mantle. Later at 24 Ma, thermal perturbations during rifting forced the isotopically evolved parts of the mantle together with the peridotitic ambient mantle to melt. This yielded a package of melts with highly correlated Re/Os ratios and radiogenic Os isotope compositions. Subsequent movement through the crust may have further altered the Os isotope composition although this effect is probably minor for the majority of the samples based on radiogenic Nd and unradiogenic Sr isotope composition of the lavas. If the radiogenic Os isotope composition can be explained by a mantle-hybridization and metasomatism model, the isotopic compositions of the hornblende basalts can be satisfied by ca. 5–25% addition of the mafic lithospheric component to an asthenospheric alkaline magma. Although a lack of isotope data for all required endmembers make this model somewhat speculative, the results show that the Re–Os isotope system in continental basalts is able to distinguish between crustal contamination and derivation of continental alkaline lavas from isotopically evolved peridotitic lithosphere that was contaminated by mafic material in the past and later remelted during rifting. The Hf isotopic compositions are slightly less radiogenic than in other alkaline basalts from the province and indicate the derivation of the lavas from low Lu–Hf parts of the lithospheric mantle. The new Os and Hf isotope data constrain a new light of the nature of such metasomatizing agents, at least for these particular rocks, which represent within the particular volcanic complex the first product of the volcanism.     

Polyorogenic reworking of ore‐controlling shear zones at the South Range of the Sudbury impact structure: A telltale story from in situ U–Pb titanite geochronology

The post‐impact orogenic evolution of the world class Ni–Cu–PGE Sudbury mining camp in Ontario remains poorly understood. New temporal constraints from ore‐controlling, epidote–amphibolite facies shear zones in the heavily mineralised Creighton Mine (Sudbury, South Range) illuminate the complex orogenic history of the Sudbury structure. In situ U–Pb dating of shear‐hosted titanite grains by LA‐ICP‐MS reveals new evidence for shear zone reworking during the Yavapai (ca. 1.77–1.7 Ga), Mazatzalian–Labradorian (1.7–1.6 Ga) and Chieflakian–Pinwarian (1.5–1.4 Ga) accretionary events. The new age data show that the effects of the Penokean orogeny (1.9–1.8 Ga) on the structural architecture of the Sudbury structure have been overestimated. At a regional scale, the new titanite age populations corroborate that the Southern Province of the Canadian Shield documents the same tectonothermal episodes that are recorded along orogenic strike within the accretionary provinces of the Southwestern United States.     

四川阿布郎当超镁铁质侵入体成岩机制的地球化学约束

阿布郎当超镁铁质侵入体位于扬子地台西缘,康滇地轴中段,安宁河深大断裂之西侧。该岩体呈似同心环状相带分布,基性程度很高,岩体的中心为含长橄榄岩,向外依次过渡为含长辉橄岩、辉橄岩及斜长辉橄岩,边缘带斜长橄辉岩。在岩体的边缘带附近存在明显的Cu-Ni-PGE矿化。随着近年来矿产价格的走高和国家对地质普查工作的力度加大,阿布郎当岩体又重新引起人们的关注。该岩体在地质勘探方面已经积累了丰富的资料,但在地球化学方面的研究还很薄弱。本文对阿布郎当超镁铁岩体进行了系统的主要造岩矿物成分、主量元素、微量元素及铂族元素含量的分析,讨论了该岩体的原始岩浆和地幔部分熔融程度,并对成岩过程进行了探讨。研究认为,阿布郎当超镁铁质岩属拉斑玄武岩系列,是峨眉山大火成岩省构造-岩浆活动产物,成岩原始岩浆为苦橄质岩浆,由类似于洋岛玄武岩岩浆源区成分的地幔经18%左右的部分熔融形成。在岩浆上升过程中,最先结晶的镁铁矿物由于岩浆的流动而集中于岩浆管道的中央,于是形成各种岩石的环带分布。当岩浆上升侵入阿布郎当岩浆房以后,发生了以橄榄石为代表的镁铁矿物的堆积,与此同时,岩浆在岩浆房内继续进行着结晶分异并且还可能与围岩发生了混染,导致岩体边缘相附近出现了硫化物的熔离。在后期的地壳运动中,阿布朗当岩浆房露出地表,即今天所见到的阿不朗当超镁铁质岩体。     

The Sudbury Structure (Ontario,Canada): a tectonically deformed multi-ring impact basin

The occurrence of shock metamorphic features substantiates an impact origin for the 1.85 Ga old Sudbury Structure, but this has not been universally accepted. Recent improvements in knowledge of large-scale impact processes, combined with new petrographic, geochemical, geophysical (LITHOPROBE) and structural data, allow the Sudbury Structure to be interpreted as a multi-ring impact structure. The structure consists of the following lithologies: Sudbury Breccia —dike breccias occurring up to 80 km from the Sudbury Igneous Complex (SIC); Footwall rocks and Footwall Breccia — brecciated, shocked crater floor materials, in part thermally metamorphosed by the overlying SIC; Sublayer and Offset Dikes, Main Mass of the SIC and Basal Member of the Onaping Formation (OF) — geochemically heterogeneous coherent impact melt complex ranging from inclusion-rich basal unit through a dominantly inclusion-free to a capping inclusion-rich impact melt rock; Grey Member of OF — melt-rich impact breccia (suevite); Green Member of OF — thin layer of fall back ejecta; Black Member of OF — reworked and redeposited breccia material; Onwatin and Chelmsford Formations — post-impact sediments. Observational and analytical data support an integrated step-by-step impact model for the genesis of these units. Analysis of the present spatial distribution of various impact-related lithologies and shock metamorphic effects result in an estimated original rim-to-rim diameter of the final crater of 200 or even 280 km for the Sudbury Structure, prior to tectonic thrusting and deformation during the Penokean orogeny.     

内蒙古哈沙图北超镁铁杂岩体成因的地球化学制约

哈沙图北超镁铁杂岩体位于华北克拉通西北缘，具小规模多岩相的特征，主要由蛇纹石化橄榄岩、橄榄二辉岩、角闪辉石岩、异剥辉石岩及石英闪长玢岩等组成的铁质超镁铁杂岩。岩体由两大系列岩石组成：第一系列岩石结晶粗大，具平坦型稀土元素分配模式，亏损Nb、Ta、Ti等高场强元素（HFSE）和Rb、Cs、Ba等大离子亲石元素（LILE），富集U和Rb,Zr,Hf及HREE等元素含量变化较大，为大陆地壳拉张环境下富集型岩石圈地幔较高程度熔融形成的岩浆经小规模分异后分期侵入原地结晶形成的，在每期岩浆结晶初期存在一定程度的堆晶作用。第二系列岩石呈致密太，磁铁矿含量较高，岩石风化表面呈黄褐色，亏损Rb、Cs、K、Ti，富集Nb、Ta、Pb、LREE，∑REE高，为初始洋壳下富集地幔较低程度熔融形成的岩浆结晶的产物。Nd同位素组成变化较小（^143Nd/^144Nd=0.5122-0.5124)，两个系列岩石中分配系数相等的微量元素比值分别分布在两个区域，岩石地球化学特征表明，该岩体为一长寿命的岩浆通道中不同源区、不同构造环境中不同期岩浆的堆积物，岩浆源区为EM1型富集地幔；表明华北克拉通西北缘在古元古代以来存在长期的陆壳增生事件，经历了拉张减薄、初始洋壳演化阶段。