Origin of Archean anorthosites: Evidence from the Bad Vermilion Lake anorthosite complex,Ontario

The Bad Vermilion Lake anorthosite complex (2,700 m.y.) is exposed over an area of about 100 km² near Rainy Lake, Ontario. As is typical of other Archean anorthosites, it is composed of coarse (1–30 cm across), equidimensional, euhedral to subhedral, calcic (An₈₀) plagioclase, in a finer grained mafic matrix. The amount of mafic matrix in individual samples ranges from none to about 70% by volume. The complex has been variably metamorphosed to greenschist facies. Zoisite, chlorite, and hornblende are abundant, but primary plagioclase is preserved in many places. The anorthosite complex is associated with gabbro and with mafic to felsic metavolcanic rocks, and is cut by tonalite plutons and by mafic dikes. Some gabbros contain local concentrations of Fe-Ti oxides and/or apatite, but no chromite. The mafic groundmass of the anorthositic rocks is similar in major and trace element chemistry, including rare earth elements, to the associated basaltic metavolcanics, suggesting that the anorthositic complex may have accumulated from a subvolcanic magma chamber which fed mafic lavas to the surface during its crystallization. Mafic flows and dikes chemically similar to the mafic metavolcanics contain plagioclase megacrysts akin to those of the anorthositic rocks, and thus may represent a link between the anorthosite complex and associated mafic lavas. Elongate pretectonic tonalite intrusions were comagmatic with the felsic metavolcanics, but not with the anorthosites or metabasalts. These silicic rocks may represent low-pressure partial melts of the mafic rocks. There is no direct or indirect evidence for significant volumes of ultramafic material at the present exposure level of the complex. An estimate of the bulk composition of all rocks presumed to be comagmatic with the anorthosites, including gabbros and mafic metavolcanics, is an aluminous basalt with about 20 wt.% Al₂O₃. This composition has REE abundances unlike those of typical Archean high-Al basalts and probably does not represent that of a primary or evolved melt. The possibility must be considered, therefore, that a substantial fraction of material comagmatic with the anorthosites has been separated from the complex, either by magmatic or tectonic processes.     

Isotopic (O, Sr, Nd) and trace element geochemistry of the Laouni layered intrusions (Pan-African belt, Hoggar, Algeria): evidence for post-collisional continental tholeiitic magmas variably contaminated by continental crust

The three layered intrusions studied in the Laouni area have been emplaced within syn-kinematic Pan-African granites and older metamorphic rocks. They have crystallized at the end of the regional high-temperature metamorphism, but are free from metamorphic recrystallization, revealing a post-collisional character. The cumulate piles can be interpreted in terms of two magmatic liquid lines of descent: one is tholeiitic and marked by plagioclase–olivine–clinopyroxene cumulates (troctolites or olivine bearing gabbros), while the other is calc-alkaline and produced orthopyroxene–plagioclase rich cumulates (norites). One intrusion (WL (West Laouni)-troctolitic massif), shows a Lower Banded Zone where olivine-chromite orthocumulates are interlayered with orthopyroxene-rich and olivine–plagioclase–clinopyroxene cumulates, whereas the Upper Massive Zone consists mainly of troctolitic and gabbroic cumulates. The other two massifs are more homogeneous: the WL-noritic massif has a calc-alkaline differentiation trend whereas the EL (East Laouni)–troctolitic massif has a tholeiitic one. Separated pyroxene and plagioclase display similar incompatible trace element patterns, regardless of the cumulate type. Calculated liquids in equilibrium with the two pyroxenes for both noritic and troctolitic cumulates are characterized by negative Nb, Ta, Zr and Hf anomalies and light REE enrichment inherited from the parental magmas. Troctolitic cumulates have mantle-derived δ¹⁸O (+5 to +6‰), initial ⁸⁷Sr/⁸⁶Sr (Sr_i=0.7030 to 0.7054), _Nd (+5 to −1) values whereas noritic cumulates are variably enriched in δ¹⁸O (+7 to +9‰), show negative _Nd (−7 to −12) and slightly higher Sr_i (0.7040–0.7065). Based on field, isotopic ratios are interpreted as resulting from a depleted mantle source (Sr_i=0.7030; _Nd=+5.1; δ¹⁸O=+5.1‰) having experience short term incompatible element enrichment and variable crustal contamination. The mantle magma was slightly contaminated by an Archaean lower crust in troctolitic cumulates, more strongly and with an additional contamination by an Eburnian upper crust in noritic cumulates. Lower crust input is recorded mainly by Sr and Nd isotopes and upper crust input by O isotopes. This is probably due to the different water/rock ratios of these two crust types. Assimilation of low amounts (<10%) of quartz-bearing felsic rocks, coming from both lower and upper crust, can explain the rise of SiO₂ activity, the enrichment in ¹⁸O and ⁸⁷Sr and the lowering of _Nd in the noritic cumulates compared to troctolitic ones. The geodynamic model proposed to account for the Laouni tholeiitic magmatism involves a late Pan-African asthenospheric rise due to a rapid lithospheric thinning associated with functioning of shear zones, which allowed tholeiitic magmas to reach high crustal levels while experiencing decreasing degrees of crustal contamination with time.     

Geochemistry and petrogenesis of the Fiskenaesset anorthosite complex,southern West Greenland: Nature of the parent magma

The petrogenesis of the Fiskenaesset anorthosite body has been investigated using major and trace element data for a large range of rock types from each zone of the complex. The chemistry of these ultramafic to anorthositic cumulates is interpreted in terms of crystal fractionation of a parental, trace element impoverished, tholeiitic magma, involving crystallisation of the cumulus phases olivine, orthopyroxene, clinopyroxene and (dominant) plagioclase feldspar. Amphibole appears not to have been a significant cumulus phase at any stage of crystallisation of the body, the abundant amphibole found in the rocks of the complex being produced by primary intercumulus crystallisation, supplemented by secondary metamorphic recrystallisation. Similarly, magnetite is unlikely to have been a significant early cumulus phase, although, together with chromite, it crystallised as a cumulus phase at high stratigraphic levels in the complex. The metamorphism appears to be largely isochemical, although sub-solidus metamorphic re-equilibration of the REE can be demonstrated on a grain-size scale.The spatial and temporal association between the anorthosite complex and the bordering metavolcanic amphibolites is matched by a strong similarity between the observed trace element chemistry of the amphibolites and the trace element chemistry of calculated successive liquids for the complex. This is taken to suggest a genetic relationship between the evolution of the anorthosite complex and enclosing amphibolites. The presence of trace element impoverished amphibolites (which are not cumulates) with trace element abundances comparable to those of the suggested parental liquid to the anorthosite complex, is used to derive a major element composition for the primary Fiskenasset magma. This composition approximates a moderately aluminous tholeiitic basalt, which may have been generated by hydrous fusion of previously depleted mantle. This primary magma underwent crystal fractionation under low pressure conditions, allowing the development of extensive plagioclase cumulates.The Fiskenaesset anorthosite, and similar bodies, cannot represent a cumulate residue complementary to the enclosing voluminous tonalitic gneisses, which have a calc-alkaline chemistry controlled by high pressure crystal liquid fractionation. Rather, the association between the cumulate layered complex and bordering supracrustal sequence may imply an ancient ocean crust analogue for the development of this component of Archaean high-grade terrains. It is suggested that slices of such Archaean ocean floor may be emplaced laterally into the base of the continental crust during subduction of oceanic lithosphere at Cordilleran type continental margins.     

Andesites as Magmatic Liquids or Liquid-crystal Mixtures; Insights from Egmont and Ruapehu Volcanoes, New Zealand Research Article

Late Quaternary andesitic magmas in New Zealand contain complexly zoned antecrysts and glomerocrysts that are not in equilibrium with either the host whole rock compositions or siliceous groundmass glass and glass inclusions. Glass inclusions represent partial melts of mafic to gabbroic cumulates in the lower crust that mix with restite crystals, or cumulates from earlier magma batches. Assimilation of partial melts of mid-crustal rocks, represented by glass in crustal xenoliths, contributes a crustal component to the andesites. Magmas at Egmont are stored at about the brittle/ductile transition at about 10 km depth and variability in the composition of erupted material is a function of the composition of the recharging magma, and which parts of the storage system are tapped during the eruption. At Taranaki recharge occurs on a c. 1400 year cycle while interactions within the storage give rise to shorter period events. A similar process on a less well constrained timescale operates at Ruapehu. Andesites are therefore complex mixtures of fractionated mantle basalts, siliceous partial melts of both the lower crust and underplated cumulates, restite and cumulate crystals. Further modification occurs by interaction with partial melts of lower to middle crustal basement as geotherms increase with time.     

Closed-system geochemical recycling of crustal materials in alpine-type peridotite

We examined aluminous mafic rock (with or without corundum or sapphirine) alternating with peridotite from the Ronda peridotite massif, southern Spain. On the basis of petrographic characteristics, these mafic rocks show a decompression history from high pressure (P > 1.5 GPa), but on the basis of their geochemical characteristics, they are crystal accumulates of plagioclase, clinopyroxene, and olivine formed within the lower crust (P < 1 GPa). A complex evolution history, including higher-pressure recrystallization after initial formation as cumulate gabbros at lower-pressure conditions, is proposed. The aluminous mafic rocks and their peridotite hosts are inferred to be recycled crustal materials now observed as centimeter-scale layered components in alpine-type peridotite. The rocks retained their original cumulate compositions; that is, their compositions were not affected by melting and metasomatic modifications during subduction, intense deformation within the upper mantle, and upwelling to the surface.     

Gabbroic xenoliths in tuff-breccia pipes from the Hyblean Plateau: insights into the nature and composition of the lower crust underneath South-eastern Sicily, Italy

Summary Crust-derived xenoliths hosted by Miocene basaltic diatremes in the Hyblean Plateau (south-eastern Sicily, Italy) provide new information regarding the nature of a portion of the central Mediterranean lower crust. These xenoliths can be divided into three groups: gabbros (plagioclase + clinopyroxene + Fe–Ti oxides ± apatite ± amphibole ± Fe-rich green spinel), diorites (An-poor plagioclase, clinopyroxene ± Fe–Ti oxides ± orthopyroxene) and mafic granulites (plagioclase + clinopyroxene + green spinel ± orthopyroxene ± Fe–Ti oxides). Gabbros form the main subject of this paper. They represent cumulates whose igneous texture has been locally obliterated by metamorphic recrystallization and shearing. They were permeated by Fe–Ti-rich melts related to tholeiitic-type fractional crystallisation. Incompatible element ratios (Zr/Nb = 5–26; Y/Nb = 1.4–11) indicate that these cumulate gabbros derived from MORB liquids. Late-stage and hydrothermal fluids caused diverse, sometimes important, metasomatic trasformations. Petrographic and geochemical comparison with gabbroids from well-known geodynamic settings show that the Hyblean lower crustal xenoliths were probably formed in an oceanic or oceanic-continent transition environment.     

Rare earth element geochemistry and strontium isotopic composition of a massif-type anorthositic-charnockitic body: the Hidra Massif (Rogaland,SW Norway)

The Hidra Massif (Rogaland Complex, SW Norway) mainly consists of plagioclase cumulates (anorthosites and leuconorites), which grade progressively into a fine-grained (200 μm). locally porphyritic, jotunitic rock towards the contact with the granulite facies gneisses. The massif is cross-cut by thin (10 cm up to 1 m) charnockitic dykes.The petrographical and geochemical evolution of the Hidra Massif can be explained by fractional crystallization of a jotunitic parental magma. Major and trace element constraints indicate that mafic phases are underabundant in the exposed levels of the massif, most likely as a result of plagioclase flotation in the early stages of solidification. Partitioning into the cumulate minerals (mainly plagioclase and orthopyroxene) governs the trace element contents of the leuconoritic adcumulates. However, the trace element geochemistry of the apparently early formed anorthositic orthocumulates largely depends upon the amount of a trapped intercumulus liquid. On the basis of trace element abundances (high REE, Rb, Th, U; negative Eu anomalies) the silicic charnockitic dykes can be considered as the residual liquid of the anorthositic fractionation trend. The higher initial ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios (0.7086 ± 0.0006 vs 0.7055 ± 0.0004 for the plagioclase cumulates and jotunites) point to contamination of the charnockitic liquids by surrounding gneissic material.     

Anorthosite formation by plagioclase flotation in ferrobasalt and implications for the lunar crust

The Sept Iles layered intrusion (Quebec, Canada) is dominated by a basal Layered Series made up of troctolites and gabbros, and by anorthosites occurring (1) at the roof of the magma chamber (100-500 m-thick) and (2) as cm- to m-size blocks in gabbros of the Layered Series. Anorthosite rocks are made up of plagioclase, with minor clinopyroxene, olivine and Fe-Ti oxide minerals. Plagioclase displays a very restricted range of compositions for major elements (An₆₈-An₆₀), trace elements (Sr: 1023-1071 ppm; Ba: 132-172 ppm) and Sr isotopic ratios (⁸⁷Sr/⁸⁶Sr_i: 0.70356-0.70379). This compositional range is identical to that observed in troctolites, the most primitive cumulates of the Layered Series, whereas plagioclase in layered gabbros is more evolved (An₆₀-An₃₈). The origin of Sept Iles anorthosites has been investigated by calculating the density of plagioclase and that of the evolving melts. The density of the FeO-rich tholeiitic basalt parent magma first increased from 2.70 to 2.75 g/cm³ during early fractionation of troctolites and then decreased continuously to 2.16 g/cm³ with fractionation of Fe-Ti oxide-bearing gabbros. Plagioclase (An₆₉-An₆₀) was initially positively buoyant and partly accumulated at the top of the magma chamber to form the roof anorthosite. With further differentiation, plagioclase (<An₆₀) became negatively buoyant and anorthosite stopped forming. Blocks of anorthosite (autoliths) even fell downward to the basal cumulate pile. The presence of positively buoyant plagioclase in basal troctolites is explained by the low efficiency of plagioclase flotation due to crystallization at the floor and/or minor plagioclase nucleation within the main magma body. Dense mafic minerals of the roof anorthosite are shown to have crystallized from the interstitial liquid.The processes related to floating and sinking of plagioclase in a large and shallow layered intrusion serve as a proxy to refine the crystallization model of the lunar magma ocean and explain the vertically stratified structure of the lunar crust, with (gabbro-)noritic rocks at the base and anorthositic rocks at the top. We propose that the lunar crust mainly crystallized bottom-up. This basal crystallization formed a mafic lower crust that might have a geochemical signature similar to the magnesian-suite without KREEP contamination, while flotation of some plagioclase grains produced ferroan anorthosites in the upper crust.     

Pyroxenite xenoliths and clinopyroxene megacrysts from the Cenozoic Jbel Saghro Volcanic Field (Anti-Atlas,Morocco): Petrography,mineral chemistry and equilibration conditions

A suite of mafic pyroxenite xenoliths and clinopyroxene megacrysts was brought to the surface by Cenozoic nephelinites of the Jbel Saghro Volcanic Field (Anti-Atlas, Morocco). The large population of samples was subdivided into five groups: (i) clinopyroxenites sensu stricto; (ii) olivine clinopyroxenites; (iii) mica-bearing clinopyroxenites; (iv) kaersutite-bearing clinopyroxenites; (v) clinopyroxene megacrysts. These xenoliths display a cumulate texture (adcumulate, heteradcumulate with poikilitic clinopyroxene including olivine). The clinopyroxenes have the composition of augite and show an appreciable variation of MgO (7.02–14.80 wt.%), TiO₂ (0.58–5.76 wt.%) and Al₂O₃ (2.81–12.38 wt.%) contents in grains. The clinopyroxenes are characterized by convex upward chondrite-normalized REE patterns, they display very similar trace element compositions with low contents of incompatible elements such as Rb (0−0.9 ppm), Ba (0.1–8.3 ppm), Th (0.1−0.3 ppm), U (0.01−0.04 ppm) and Nb (1.3–3.2 ppm). REE contents of the calculated melts in equilibrium with the clinopyroxene megacrysts and clinopyroxene from pyroxenite xenoliths are similar to those of the nephelinites exposed in Jbel Saghro. Crystallization temperatures of pyroxenite xenoliths and clinopyroxene megacrysts range from 950 °C to 1150 °C. Clinopyroxene barometry yielded pressure of crystallization ranging from 0.4 to 0.8 GPa for pyroxenite xenoliths and 0.3 to 0.7 GPa for clinopyroxene megacrysts. This pressure range is in agreement with pyroxenite xenoliths and clinopyroxene megacrysts being crystallized from their parental melts at the lower and upper crust.     

The crustal tongue melting model and the origin of massive anorthosites

Recent detailed field studies in several anorthosite complexes have shown that anorthosites are frequently associated with weakness zones in the crust which may have favoured their emplacement at mid-crust levels. Recent experimental data have shown that the parent magma compositions of various anorthosite massifs lie on thermal highs in the relevant phase diagrams at 10–13 kbar, indicating that these magmas cannot be derived by fractionation of peridotitic mantle melts but by melting of gabbronoritic sources in the lower crust at 40–50 km depths. In the Sveconorwegian Province terne boundaries have been traced in deep seismic profiles to Moho offsets or to tongues of lower crustal material underthrust to depths higher than 40 km. In Southern Norway, we suggest that a lithospheric-scale weakness zone (the Feda transition zone?) has channelled the Rogaland anorthosites through linear delamination, asthenospheric uprise and melting of a mafic lower crustal tongue.     

Genesis of intermediate igneous rocks at the end of the Sveconorwegian (Grenvillian) orogeny (S Norway) and their contribution to intracrustal differentiation

Abundant ferroan, metaluminous granitoids (970–950 Ma) emplaced at the end of the Sveconorwegian collisional orogeny (1130–900 Ma) are dominated by intermediate to silicic compositions with rare mafic facies. Both 73% fractional crystallization of an amphibole-bearing gabbroic cumulate substracted from the parent mafic composition and 30% non-modal batch melting of an amphibolitic source equivalent in composition to the mafic facies produce a monzodioritic liquid with appropriate trace element composition. A better fit is obtained for the partial melting process. Both processes could have occurred simultaneously to produce mafic cumulates and restites. As there is no evidence for large volumes of dense mafic rocks in the Sveconorwegian upper crust, these dense mafic rocks were probably produced in the lower crust. Formation of these granitoids, thus, contributed to the vertical stratification of the Proterozoic continental crust and also to the transfer of water from the lower crust to the surface.     

Aluminous websterite and granulite xenoliths from the Chyulu Hills volcanic field, Kenya: gabbro–troctolitic cumulates subjected to lithospheric foundering

Whole rock major and trace element abundances in aluminous garnet–spinel websterite, sapphirine-bearing Mg–Al granulite and hibonite-bearing Ca–Al granulite xenoliths from the Chyulu Hills volcanic field, Kenya, suggest that the samples represent a meta-igneous suite linked by fractionation. The incompatible major element contents increase from the websterites to the Mg–Al granulites and further to the Ca–Al granulites. High bulk rock Mg#s and very low concentrations of most incompatible trace elements indicate that the rocks are cumulates rather than crystallized melts. Elevated Ni abundances, impoverishment in Cr and HFSE and high contents of normative plagioclase and olivine in the granulites indicate that their protoliths were similar to troctolite. The textures and metamorphic reaction paths recorded in the granulites suggest igneous emplacement in the crust and cooling from igneous to ambient crustal temperatures accompanied or followed by compression. For the websterite xenoliths, there is an apparent contradiction between the results of P–T calculations that suggest high P and T of crystallization of early generation pyroxenes and elevated P–T conditions during final equilibration (1.4–2.2 GPa/740–980°C) on the one hand and the positive Eu anomaly that suggests shallow-level plagioclase accumulation on the other hand. This contradiction can be reconciled by a model of compression of a plagioclase-bearing (gabbroic) protolith to mantle depths where it recrystallized to an ultramafic assemblage, which requires foundering of dense lower crustal material into the mantle.     

Erupted cumulate fragments in rhyolites from Lipari (Aeolian Islands)

Over the last ~267 ky, the island of Lipari has erupted magmas ranging in compositions from basaltic andesites to rhyolites, with a notable compositional gap in the dacite field. Bulk geochemical and isotopic compositions of the volcanic succession, in conjunction with major and trace elemental compositions of minerals, indicate that the rhyolites were dominantly generated via crystal fractionation processes, with subordinate assimilation. Radiogenic (Sr, Nd, and Pb) and stable (O) isotopes independently suggest ≤30 % of crustal contamination with the majority of it occurring in mafic compositions, likely relatively deep in the system. Within the rhyolites, crystal-rich, K₂O-rich enclaves are common. In contrast to previous interpretations, we suggest that these enclaves represent partial melting, remobilization and eruption of cumulate fragments left-over from rhyolite melt extraction. Cumulate melting and remobilization is supported by the presence of (1) resorbed, low-temperature minerals (biotite and sanidine), providing the potassic signature to these clasts, (2) reacted Fo-rich olivine, marking the presence of mafic recharge, (3) An_38–21 plagioclase, filling the gap in feldspar composition between the andesites and the rhyolites and (4) strong enrichment in Sr and Ba in plagioclase and sanidine, suggesting crystallization from a locally enriched melt. Based on Sr-melt partitioning, the high-Sr plagioclase would require ~2300 ppm Sr in the melt, a value far in excess of Sr contents in Lipari and Vulcano magmas (50–1532 ppm) but consistent with melting of a feldspar-rich cumulate. Due to the presence of similar crystal-rich enclaves within the rhyolites from Vulcano, we propose that the eruption of remobilized cumulates associated with high-SiO₂ rhyolites may be a common process at the Aeolian volcanoes, as already attested for a variety of volcanic systems around the world.     

Contributions of basaltic underplating to crustal growth in island arc and extensional tectonic settings in the Chinese Tianshan Orogenic Belt,NW China

A mafic–ultramafic intrusive belt comprising Silurian arc gabbroic rocks and Early Permian mafic–ultramafic intrusions was recently identified in the western part of the East Tianshan, NW China. This paper discusses the petrogenesis of the mafic–ultramafic rocks in this belt and intends to understand Phanerozoic crust growth through basaltic magmatism occurring in an island arc and intraplate extensional tectonic setting in the Chinese Tianshan Orogenic Belt (CTOB). The Silurian gabbroic rocks comprise troctolite, olivine gabbro, and leucogabbro enclosed by Early Permian diorites. SHRIMP II U-Pb zircon dating yields a 427 ± 7.3 Ma age for the Silurian gabbroic rocks and a 280.9 ± 3.1 Ma age for the surrounding diorite. These gabbroic rocks are direct products of mantle basaltic magmas generated by flux melting of the hydrous mantle wedge over subduction zone during Silurian subduction in the CTOB. The arc signature of the basaltic magmas receives support from incompatible trace elements in olivine gabbro and leucogabbro, which display enrichment in large ion lithophile elements and prominent depletion in Nb and Ta with higher U/Th and lower Ce/Pb and Nb/Ta ratios than MORBs and OIBs. The hydrous nature of the arc magmas are corroborated by the Silurian gabbroic rocks with a cumulate texture comprising hornblende cumulates and extremely calcic plagioclase (An up to 99 mol%). Troctolite is a hybrid rock, and its formation is related to the reaction of the hydrous basaltic magmas with a former arc olivine-diallage matrix which suggests multiple arc basaltic magmatism in the Early Paleozoic. The Early Permian mafic–ultramafic intrusions in this belt comprise ultramafic rocks and evolved hornblende gabbro resulting from differentiation of a basaltic magma underplated in an intraplate extensional tectonic setting, and this model would apply to coeval mafic–ultramafic intrusions in the CTOB. Presence of Silurian gabbroic rocks as well as pervasively distributed arc felsic plutons in the CTOB suggest active crust-mantle magmatism in the Silurian, which has contributed to crustal growth by (1) serving as heat sources that remelted former arc crust to generate arc plutons, (2) addition of a mantle component to the arc plutons by magma mixing, and (3) transport of mantle materials to form new lower or middle crust. Mafic–ultramafic intrusions and their spatiotemporal A-type granites during Early Permian to Triassic intraplate extension are intrusive counterparts of the contemporaneous bimodal volcanic rocks in the CTOB. Basaltic underplating in this temporal interval contributed to crustal growth in a vertical form, including adding mantle materials to lower or middle crust by intracrustal differentiation and remelting Early-Paleozoic formed arc crust in the CTOB.     

Enigmatic cathodoluminescent objects in the Dhofar 025 lunar meteorite: Origin and sources

Dhofar 025 is a lunar highland breccia consisting mainly of anorthositic, with less common noritic, gabbronoritic, and troctolitic material. Rare fragments of low-Ti basalts are present as well, but no KREEP (component enriched in incompatible elements) was found in the meteorite. The cathodoluminescence study of this meteorite showed that its impact–melt matrix contains unusual cathodoluminescent (CL) objects of feldspathic composition, which frequently contain microlites of Fe-Mg spinel (pleonaste). They were presumably formed by impact mixing and melting of olivine and plagioclase with subsequent rapid quenching of the impact melts. Such mixing could happen either during assimilation of anorthosites by picritic/troctolitic magmas or during impact melting of troctolites. The enrichment of CL objects of Dhofar 025 in incompatible trace elements suggests that the mafic component of the impact mixture may be related to the high-magnesium suite rocks, which are frequently enriched in KREEP component. The depletion of CL objects in alkalis indicates their possible relation with residual glasses formed by evaporation. However, the presence of FeO in most objects points to the insignificant extent of evaporation. Thus, evaporation cannot explain the enrichment of the CL objects in Al₂O₃ and other refractory components, although this process definitely took place in their formation. Their similarity to the lunar pink spinel anorthosites, whose existence was predicted from orbital data, serves as an argument in support of the possible formation of the latters by impact mixing.     

Geology and petrology of a deep crustal zone from the Famatinian paleo-arc, Sierras de Valle Fértil and La Huerta, San Juan, Argentina

The ranges of the Sierras Valle Fértil-La Huerta expose natural cross sections through a paleo-arc crust that formed in the Late Cambrian - Early Ordovician Famatinian magmatic arc, northwestern Argentina. Thick mafic sequences of amphibole gabbronorites to orthopyroxene-amphibole-biotite diorites form the lower levels of the exposed paleo-arc section. This mafic unit includes lens-shaped bodies of olivine-bearing cumulate rocks and tabular-shaped sill/dike intrusions of fine-grained chilled amphibole gabbro. The mafic magmas were emplaced into regional metasedimentary sequences at lower crustal levels, corresponding to pressure from 5 to 7 kbar. Gabbronorites likely representing the parental magmas that fluxed into the exposed paleo-arc crust differ from primitive magmatic arc rocks in having somewhat lower Mg-number (ca. 0.60) and compatible (Cr and Ni) trace element contents, and slightly higher Al₂O₃ contents. This difference is taken to indicate that a pyroxene-rich olivine-bearing assemblage with a bulk high Mg/Fe ratio and low Al₂O₃ content crystallized from mantle-derived melts before mafic magmas reached the crustal levels currently exhumed. However, some gabbronorites have incompatible trace element signatures typical of primitive mafic arc magmatism. Igneous rocks to some extent more evolved than those of the mafic unit make up a tonalite-dominated intermediate unit. The intermediate unit consists of a heterogeneous suite that ranges from orthopyroxene-bearing amphibole-rich diorites to biotite-rich amphibole-poor tonalites. Within the intermediate unit, chilled mafic rocks are found as a network of dikes, whereas metasedimentary migmatites appear interlayered as m-wide septa and km-long strips. The tonalite-dominated intermediate unit passes into a granodiorite batholith through a transitional zone that is up to 2-km wide. The boundary zone separating the tonalite-dominated and granodiorite-dominated units is characterized by mingling of tonalitic and leucogranitic magmas, which together appear multiply-intruded by mafic sill/dike bodies. Within the tonalite- and granodiorite-dominated units, the less evolved mafic rocks occur as: (1) bodies tens of meters long, (2) chilled dikes and sills, and (3) microgranular inclusions (enclaves), supporting the inference that mafic magmatism was the main source for generating a vast volume of intermediate and silicic igneous rocks. Mass balance calculations and trace element systematics are combined to demonstrate that tonalites and granodiorites formed by concurrent closed-system fractional crystallization and open-system incorporation of paragneissic migmatites and/or anatectic leucogranites into the evolving igneous sequence. This study argues that the sequence of igneous rocks from Valle Fértil-La Huerta was formed as the result of complementary petrogenetic processes that operated concurrently at different levels of the Famatinian arc crust.     

韩国Hadong-Sanchung地区斜长岩Sr-Nd同位素初步研究——成因和前寒武纪构造意义

斜长岩呈长条带出露于朝鲜半岛南部,侵入到年代约为2.0Ga的Yeongnam前寒武纪基底岩石中,虽然岩石类型简单(斜长岩和辉长岩质斜长岩),但可以同世界已知块状类型斜长岩相对比。这些斜长岩具有几个重要的差别,例如呈层状构造,镁铁相成分是角闪石而不是辉石,并且不具斜方辉石巨晶。应用Rb-Sr和Sm-Nd同位素系统研究这些岩石的年龄和成因,测定出一种页理化辉长岩质斜长岩矿物的Sm-Nd等时线年龄为1678±90Ma,推断其为侵位年龄,因为中生代绿岩相变质期间这些岩石的Sm-Nd同位素体系呈封闭状态。这一年龄和过去曾报道的元古宙块状斜长岩的年龄范围(1.1~1.7Ga)相吻合。认为斜长岩成因可以用所谓元古宙斜长岩事件来解释。斜长岩的岩浆活动对朝鲜半岛南部前寒武纪基底岩石的构造历史有重要意义。全岩εNd(t)值范围-1.6~-5.2,而87Sr/86Sr初始值变化于0.704~0.706之间,据此可解释地幔成因的斜长岩岩浆是在其结晶作用期间吸收了地壳物质的结果。然而不能排除是下地壳源的可能性。     

Lunar Magma Ocean crystallization revisited: Bulk composition, early cumulate mineralogy, and the source regions of the highlands Mg-suite

Crystallization of the Lunar Magma Ocean (LMO) has been numerically modeled and its products inferred from sample observations, but it has never been fully tested experimentally. This study is a reexamination of the LMO hypothesis by means of the first experimental simulation of lunar differentiation. Two end-member bulk Moon compositions are considered: one enriched in refractory lithophile elements relative to Earth and one with no such enrichment. A “two-stage” model of magma ocean crystallization based on geophysical constraints is simulated and features early crystal suspension and equilibrium crystallization followed by fractional crystallization of the residual magma ocean. An initially entirely molten Moon is assumed. Part 1 of this study, presented here, focuses on stage 1 of this model and considers the early cumulates formed by equilibrium crystallization, differences in mantle mineralogy resulting from different bulk Moon compositions, and implications for the source regions of the highlands Mg-suite.Refractory element enriched bulk Moon compositions produce a deep mantle that contains garnet and trace Cr-spinel in addition to low-Ca pyroxene and olivine. In contrast, compositions without refractory element enrichment produce a deep dunitic mantle with low-Ca pyroxene but without an aluminous phase. The differences in bulk composition are magnified in the residual melt; the residual LMO from the refractory element enriched composition will likely produce plagioclase and ilmenite earlier and in greater quantities. Both compositions produce Mg-rich early cumulate piles that extend from the core-mantle boundary to ∼355 km depth, if 50% equilibrium crystallization and whole Moon melting are assumed. These early LMO cumulates provide good fits for the source regions for a component of the high-Mg^∗, Ni- and Co-poor parental magmas of the Mg-suite cumulates, if certain conditions are called upon. The olivine in early LMO cumulates produced by either bulk Moon composition is far too rich in Cr to be reasonable for the source regions of the Mg-suite, meaning either core formation in the presence of S and/or C must be invoked to deplete the LMO and the crystallizing olivine in Cr, or that current estimates of the bulk lunar Cr content are too high. We infer that melts meeting the criteria of the Mg-suite parents could be produced from early LMO cumulates by solid state KREEP and plagioclase hybridization near the base of the crust and subsequent partial melting. Additionally, we propose a revised model for Mg-suite petrogenesis.     

The composition and formation of Miocene tholeiites in the Central European Cenozoic plume volcanism (CECV)

Tholeiites accompanying a majority of alkali basalts are restricted to the highly productive central part of the CECV plume activity in Vogelsberg and Hessian Depression. They mainly occur as quartz tholeiites which according to experiments of partial melting and material balances are products of olivine tholeiitic primary melts. The differentiation from olivine to quartz tholeiitic melts took place in lower crustal magma chambers where olivine tholeiitic melt intruded due to a density comparable with that of the country rocks. The fractionation due to separation of olivine and some clinopyroxene caused contamination of tholeiite magmas by tonalitic partial melts from the wall rocks of the magma chambers. The latter process is indicated by relatively high Rb, K and Pb and low Nb concentrations and by Nd, Sr and Pb isotopes. Contaminating crustal melts, which roughly attained a proportion of 10%, contained very low ¹⁴³Nd/¹⁴⁴Nd ratios from a Nd/Sm fractionation as old as 2.6 Ga. This is the first evidence from mafic rocks of this high age in the lower crust beneath Central Europe. Modelling with incompatible elements allows to recognize olivine tholeiites as products of about 1% partial melting of plume rocks consisting of 35% primitive and 65% depleted mantle materials. The production of tholeiites other than alkali basalts is restricted to the highest plume activity and the largest fraction of MORB type source rocks. Received: 10 December 1999 / Accepted: 23 June 2000     

Granulite xenoliths from western Saudi Arabia: the lower crust of the late Precambrian Arbian-Nubian Shield

Mafic and intermediate granulite xenoliths, collected from Cenozoic alkali basalts, provide samples of the lower crust in western Saudi Arabia. The xenoliths are metaigneous two-pyroxene and garnet granulites. Mineral and whole rock compositions are inconsistent with origin from Red Sea rift-related basalts, and are compatible with origin from island arc calc-alkaline and low-potassium tholeiitic basalts. Most of the samples are either cumulates from mafic magmas or are restites remaining after partial melting of intermediate rocks and extraction of a felsic liquid. Initial⁸⁷Sr/⁸⁶Sr ratios are less than 0.7032, except for two samples at 0.7049. The Sm-Nd data yield T_DM model ages of 0.64 to 1.02 Ga, similar to typical Arabian-Nubian Shield upper continental crust. The isotopic data indicate that the granulites formed from mantle-derived magmas with little or no contamination by older continent crust. Calculated temperatures and pressures of last reequilibration of the xenoliths show that they are derived from the lower crust. Calculated depths of origin and calculated seismic velocities for the xenoliths are in excellent agreement with the crustal structure model of Gettings et al. (1986) based on geophysical data from western Saudi Arabia. Estimation of mean lower crustal composition, using the granulite xenoliths and the Gettings et al. (1986) crustal model, suggests a remarkably homogeneous mafic lower crust, and an andesite or basaltic andesite bulk composition for Pan-African juvenile continental crust.