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The Třebíč Pluton (TP) comprises plutonic rocks of the 'durbachite series', i.e., ultrapotassic biotite–amphibole quartz melasyenites–melagranites with conspicuous K-feldspar phenocrysts and abundant mafic enclaves. The TP with its satellite bodies intruded the eastern Moldanubian Zone (orogenic root of the Variscan Bohemian Massif) at c. 338 Ma. Overall, the entire TP durbachite series (SiO2 = 52–67 wt%) remains highly magnesian, reduced, and (except when strongly contaminated) metaluminous. The durbachitic rocks are markedly enriched in lithophile elements (esp. Cs, Rb, Th, U, K and Pb) and LREE; in contrast, the contents of high-field strength elements (Nb, Ta, and Ti) and Sr are much lower, resulting in conspicuous negative anomalies in Primitive mantle-normalized multielement plots. The proposed petrogenetic model requires re-enrichment of the depleted, harzburgitic lithospheric mantle by deeply subducted crustal material of the Saxothuringian lower plate, directly and/or via (U)HP melt/fluid. This process produced phlogopite-rich metasomes (phlogopite harzburgites and/or glimmerites). The metasomes re-melted soon thereafter by a short thermal pulse caused by slab break-off or decompression melting of rising diapirs of the relatively light metasomatized mantle, yielding primary ultrapotassic melts (87Sr/86Sr337 > 0.712 and ε Nd 337 ≤ 7.6). Upon their passage through the Moldanubian upper plate, ultrapotassic primary magmas mixed with felsic anatectic melts (87Sr/86Sr337 < 0.710; ε Nd 337 > – 6.5). K-feldspar-driven fractional crystallization ± accumulation represented only a late modifying influence. The occurrence of such ultrapotassic enriched/crustally contaminated mantle-derived magmatism complicates any conventional mass-balance assessments of relative roles of crustal recycling vs. growth in hot collisional orogens, e.g., Variscides or Himalayas. [ABSTRACT FROM AUTHOR]

Deep crustal felsic xenoliths from classic Scottish Midland Valley localities, carried to the surface by Permo-Carboniferous magmatism, are shown for the first time to include metaigneous varieties with dioritic and tonalitic protoliths. Four hypotheses regarding their origin have been evaluated: (1) Precambrian basement; (2) Permo-Carboniferous underplating; (3) 'Newer Granite' magmatism; (4) Ordovician arc magmatism. U–Pb zircon dating results rule out the Precambrian basement and Permo-Carboniferous underplating hypotheses, but establish that the meta-igneous xenoliths represent both 'Newer Granite' and Ordovician (to possibly Silurian) arc magmatism. The metadiorite xenoliths are shown to have protolith ages of c. 415 Ma with εHf t zircon values ranging from +0·1 to +11·1. These are interpreted to represent unexposed 'Newer Granite' plutons, based on age, mineralogical, isotopic and geochemical data. This shows that Devonian 'Newer Granite' magmatism had a greater impact on the Midland Valley and Southern Uplands crust than previously realized. Clinopyroxene–plagioclase–quartz barometry on the metadiorites from the east and west of the Midland Valley yielded a similar pressure range of c. 5–10 kbar, and a metadiorite from the east yielded a minimum two-feldspar temperature estimate of c. 793–816°C. These results indicate that the metadiorites once resided in the middle–lower crust. In contrast, two metatonalite xenoliths have a Late Ordovician protolith age (c. 453 Ma), with zircon εHf t values of +7·8 to +9·0. These are interpreted as samples of a buried Late Ordovician magmatic arc situated within the Midland Valley. Inherited zircons with similar Late Ordovician ages and εHf t =453 values (+1·6 to +10·8) are present in the metadiorites, suggesting that the Devonian 'Newer Granites' intruded within or through this Late Ordovician Midland Valley arc. A younger protolith age of c. 430 Ma from one of the metatonalites suggests that arc activity continued until Silurian times. This validates the long-standing 'arc collision' hypothesis for the development of the Caledonian Orogen. Based on U–Pb zircon dating, the metatonalite and metadiorite xenoliths have both experienced metamorphism between c. 400 and c. 391 Ma, probably linked to the Acadian Orogeny. An older phase of metamorphism at c. 411 Ma was possibly triggered by the combined effects of heating owing to the emplacement of the 'Newer Granite' plutons and the overthrusting of the Southern Uplands terrane onto the southern margin of the Midland Valley terrane. [ABSTRACT FROM AUTHOR]

The Rum Eastern Layered Intrusion (ELI) is the product of part of an ~60 Ma open-system magma chamber. The 16 coupled peridotite-troctolite ± gabbro macro-rhythmic units it contains represent crystallization of multiple batches of basaltic and picritic magma. Within the ELI, Unit 10 has been considered the type example of batch fractionation of magma on Rum for more than 50 years, successively producing peridotite, troctolite and olivine gabbro. Detailed field observations and logs of the Unit 10 peridotite cumulate are presented here, together with mineralogical and textural analyses of Cr-spinel seams and their peridotite host-rocks. Numerous harrisite layers are commonly associated with diffuse, laterally discontinuous, platinum-group element (PGE) enriched Cr-spinel seams. Multiple millimetre- to centimetre-thick Cr-spinel seams occur at the bases and tops and within harrisite layers. These relationships are inconsistent with simple batch fractionation of magma. Critically, the harrisite layers also exhibit centimetre- to metre-scale, upward oriented apophyses that point to injection of magma into the overlying cumulate, indicating an intrusive origin for the harrisite. Quantitative textural and chemical analysis suggests that the Cr-spinel seams formed via in situ crystallization within the crystal mush together with intrusive peridotites from an assimilation reaction between the replenishing magma and peridotitic crystal mush. Intrusive magma replenishment in Unit 10 caused significant compositional disequilibrium between the crystallizing phases in response to the postcumulus migration of reactive liquid, resulting in chemical zoning of intercumulus plagioclase crystals. We propose that the Unit 10 peridotite is intrusive and that repeated small-volume magma replenishments are responsible for incremental construction of a large proportion of the peridotite body, similar to recent interpretations of parts of Unit 12 and Unit 14. Moreover, it is suggested that some or all of the injections of magma occurred into the crystal mush, rather than at the magma chamber floor. This new model of intra-mush Cr-spinel formation may have significant economic implications for PGE enrichment in other layered intrusions, such as the peridotite-hosted chromitites of the Stillwater Complex Ultramafic Series (Montana, USA). It is also worth noting that thin platiniferous chromitite seams considered to have formed in situ occur below the Merensky Reef of the Bushveld Complex (South Africa). [ABSTRACT FROM AUTHOR]

The authors regret to have inadvertently supplied an erroneous version of the data table displaying recalculated titanite analyses. [ABSTRACT FROM AUTHOR]

Abstract: Thin (~2 mm) Cr-spinel seams are present at the bases of several of the coupled peridotite–troctolite units that comprise the Rum Eastern Layered Intrusion. In some cases, ‘subsidiary’ Cr-spinel seams have also developed at between 6 and 12 cm below the unit boundaries. The subsidiary seams are thinner than the normal seams (approx. 1 mm), discontinuous, and occur within and at the base of a thin (~10 cm) layer of anorthosite, which is sandwiched between peridotite and troctolite. The troctolite contains cumulus olivine, but in the Cr-spinel seam and anorthosite, olivine is intercumulus only. Cr-spinel is scarce in the troctolite, but common in the anorthosite, where it has a different composition (more Fe-and Cr-rich) and crystal size distribution (CSD) profile to the Cr-spinel in the subsidiary seam, suggesting that it represents a different crystal population. A model involving downward infiltration of hot picrite and resulting troctolitic cumulate assimilation is developed here to explain the subsidiary seams. This is based on petrographic observation, quantitative textural measurement and mineral chemical analyses. It is suggested that as the picritic magma was emplaced, downward percolation of this melt occurred into a troctolite mush. The anorthosite represents a layer of almost completely melted troctolite, formed by growth of high-anorthite zones from the contaminated picrite onto residual cumulus plagioclase. Assimilation of large amounts of the troctolite cumulate mush forced the contaminated picrite onto the olivine-spinel cotectic, leading to Cr-spinel crystallisation. Subsequently, the Cr-spinel crystals in the anorthosite have reacted with intercumulus melt over a wider temperature interval and have gained a more Fe3+-rich composition than the subsidiary seam Mg- and Al-rich Cr-spinels. It is suggested that the separation of a small fraction of immiscible sulphide liquid in the Cr-spinel seams is the result of locally changing SiO2 and oxygen fugacity at the crystal–melt interface, during the peritectic reaction of some of the Cr-spinel to produce olivine and plagioclase. [Copyright &y& Elsevier]