Mantle melting

We present a model to assess the viability of the creation of volcanic eruptions of up to flood-basalt size from a giant impactor striking a relatively thin lithosphere. A 300-km-radius crater in 75-km-thick lithosphere can create 106 km3 of magma from instantaneous in situ decompression of mantle material with a potential temperature of 1300 °C. For a range of lithospheric

Type I and Type II eclogite xenoliths from the Roberts Victor kimberlite (South Africa) show marked differences in terms of microstructures, mineralogy, major- and trace-element compositions and oxygen-isotope compositions. The unequilibrated microstructures of Type I eclogites, their typical accessory assemblages (phologopite, diamond, sulphides, fluid inclusions) and the ubiquitous presence of “melt pockets” in garnets provide strong evidence of metasomatism. Type II eclogites systematically lack such features and are microstructurally more equilibrated. Type I eclogites are more magnesium-rich than most Type II (mean Mg# = 0.56 vs. 0.46), while Type II eclogites are generally more Ca-rich (mean CaO = 9 vs. 12 wt%) and Fe-rich (mean FeO = 10 vs. 12 wt%). Type I eclogites are systematically enriched in LREE, Sr, Ba, alkali elements, HFSE, Th and U compared to the more depleted Type II eclogites. Calculated trace-element patterns of fluids in equilibrium with Type I eclogites are closely similar to those of volatile-rich small-volume mantle melts in the carbonatite–kimberlite spectrum commonly inferred to be responsible for mantle metasomatism. Although oxygen isotopes are often used to argue for a subduction origin of mantle eclogites, correlations between δ18O of garnet and typical metasomatic tracers suggest that the metasomatic process also has shifted the oxygen-isotope compositions of the Type I eclogites toward heavier values. Roberts Victor Type I eclogites thus carry the imprint of a metasomatic process that strongly modified their major-element, trace-element and isotopic compositions, while the more pristine Type II eclogites escaped this modification. Therefore, attempts to constrain the origin of Roberts Victor eclogites should not be based on the much more abundant Type I eclogites, which retain little geochemical memory of their protoliths. The most suitable materials for such investigations may be the less metasomatised, but more rare, Type II eclogites.

Crustal recycling patterns during the evolution of the Neoproterozoic Arabian-Nubian Shield (ANS) were defined using the oxygen isotope ratio of zircon [δ18O(Zrn)]. Evidence for early (~ 870–740 Ma) crustal recycling in the northernmost ANS (southern Israel and Sinai, Egypt) is given by laser fluorination analysis of bulk zircon separates, which yield higher than mantle δ18O(Zrn) values of several island arc complex (IAC) orthogneisses (6.9 to 8.2‰) and also from the average δ18O(Zrn) value of 6.4‰ determined for detrital zircons (~ 870–780 Ma) from the Elat-schist; the latter representing the oldest known rock sources in the region. These results indicate prolonged availability of surface-derived rocks for burial or subduction, melting, and assimilation at the very early stages of island arc formation in the ANS. Other IAC intrusions of ~ 800 Ma show mantle-like δ18O(Zrn) values, implying that not all magmas involved supracrustal contribution. Much younger (650–625 Ma) deformed syn-collisional calc-alkaline (CA1) intrusions are characterized by δ18O(Zrn) values of 5.0 to 7.9‰ indicating continued recycling of the felsic crust.The main sample set of this study comprises rocks from the mostly granitic, post-collisional calc-alkaline (CA2: ~ 635–590 Ma) and alkaline (AL: ~ 608–580 Ma) magmatic suites. Despite having distinct geochemical characteristics and petrogenetic paths and spans of magmatic activity, the two suites are indistinguishable by their average δ18O(Zrn) values of 5.7 and 5.8‰ pointing to the dominance of mantle-like δ18O sources in their formation. Nonetheless, grouping the two suites together reveals geographical zoning in δ18O(Zrn) where a large southeastern region of δ18O(Zrn) = 4.5 to 5.9‰ is separated from a northwestern belt with δ18O(Zrn) = 6 to 8‰ by a ‘6‰ line’.It is thus suggested that all CA2 and AL magmas of the northernmost ANS were derived from mantle-like δ18O reservoirs in the mafic lower-crust and the lithospheric-mantle, respectively. However, while in the northwestern belt these magmas intruded a thick crustal section and assimilated ~ 15–35%, high-δ18O IAC+CA1 material, magmas in the southeastern region intruded a thinner crust and little or no contamination occurred. The proposed NW–SE variance in crustal thickness during the late Neoproterozoic fits well with the geometry of the fan shaped rifting model proposed by Stern [Stern, R.J., 1985. The Najd Fault System, Saudi Arabia and Egypt: a late Precambrian rift related transform system. Tectonics 4, 497–511.] for this region. Deep parts of the lithosphere were beginning to rift at ~ 630 Ma, allowing the asthenospheric mantle to rise and transfer heat to the lithosphere. This resulted in vast melting of the mafic lower-crust to produce the batholithic CA2 magmas. Later (~ 610 Ma) percolation of lithospheric-mantle melts (possibly along deep seated lithospheric-scale faults) introduced AL magmas to shallow levels of the crust. Intrusion of CA2 and AL mantle-like δ18O parent magmas into the thinned southeastern crust did not involve assimilation of older crust whereas similar intrusion into the thicker northwestern crust resulted in mild assimilation of high-δ18O pre-635 Ma crust.An important implication from our results is that petrogenesis of some high-δ18O AL magmas of the northernmost ANS involved assimilation of supracrustal material. Felsic intrusions of the AL suite were previously described as A-type granites derived solely from mantle melts with no crustal components. Our results contribute to the “A-type petrogenesis debate” by showing that their formation can involve recycling of crustal material.

During the Variscan orogenic cycle, central Europe was intruded by numerous granitoid plutons. Typological and age relationships show that the characteristics of the granitoid magmatism changed during the course of the Variscan orogeny. Five genetic groups of granitoids may be distinguished: Late Devonian to early Carboniferous “Cordilleras” I-type granitoids (ca. 370-340 Ma): These early Variscan granitoids are mainly tonalites and granodiorites. They often have hornblende and occur in association with diorites and gabbros. They form plutonic massifs in the Saxothuringian unit, in Central Bohemia and the intra-Alpine Variscides. In terms of existing models, they can be interpreted as volcanic arc granites, being related to the subduction of early Variscan oceans. Models involving mantle sources and AFC may be feasible. Early Carboniferous, deformed S-type granite/migmatite associations (ca. 340 Ma): These occur in the footwall of a thick thrust in Southern Bohemia (Gföhl nappe) and seem to represent a phase of water-present, syn-collisional crustal melting related to nappe stacking. Late Visean and early Namurian S-type and high-K, I-type granitoids (ca. 340-310 Ma): These granitoids are mainly granitic in composition and particularly abundant along the central axis of the orogen (Moldanubian unit). This zone experienced a high heat flow at this time, probably as a consequence of post-collisional extension and magmatic underplating. Most of group 3 granitoids formed through high-T fluid-absent melting in the lower crust. Enriched mantle melts interacted with some crustal magmas on a local scale to form durbachites. Partial melting events in the middle crust produced a number of high-T/low-P, S- and I-type diatexites and some S-type granite magmas. Post-collisional, epizonal I-type granodiorites and tonalites (ca. 310-290 Ma): These plutons can be found throughout the Central European Variscides. However, most of them occur in the Alps (near the southern flank of the orogen). Such late I-type plutons could be related to renewed subduction along the southern fold belt flank, and/or to extensional decompression melting near the crust/mantle boundary. Post-collisional mantle or slab melting may have occurred in connection with remnant subduction zones below the orogen undergoing thermal relaxation and dehydration. Late Carboniferous to Permian leucogranites (ca. 300-250 Ma): Many of these rocks are similar to sub-alkaline A-type granites. Potential sources for this final stage of plutonism could have been melt-depleted lower crust or lithospheric mantle. Im Verlauf der variszischen Orogenese intrudierten im mitteleuropäischen Raum große Massen von Granitoiden. Eine Bewertung geochronologischer and granittypologischer Daten zeigt, daß sich die Magmencharakteristik mit der Zeit verändert hat. Fünf Hauptgruppen von Granitoiden können unterschieden werden: I-Typ Granitoide des sädten Devon and fruhen Karbon (ca. 370-340 Ma): Es handelt sich dabei durchwegs um I-Typ Tonalite and Granodiorite, welche häufig Hornblende fühen. Typisch für these Plutone ist die Präsenz gabbroischer oder dioritischer Endglieder. Eine Magmenentstehung aus Mantelquellen mit Modifikation durch AFC und eine genetische Verbindung zu frühvariszischen Subduktionszonen ist denkbar. Syntektonische S-Typ Granite and Migmatite (ca. 340 Ma): Große Massen solcher Granitoide treten im Deckenstapel der südlichen Böhmischen Masse auf. Sie repräsentieren wassergesättigte, syn-kollisionale Krustenschmelzen, die sich in der Nähe von tektonischen Überschiebungsbahnen gebildet haben. S-Typ and kalireiche L-Typ Granitoide des spdten Vise and fruhen Namur (ca. 340-310 Ma): Diese Plutone haben in der Regel granitische Zusammensetzung und intrudierten vornehmlich in der moldanubischen Zentralzone des Orogens. Die dortige kontinentale Kruste war zu dieser Zeit einem extrem hohen Wärmefluß ausgesetzt, der vermutlich durch postkollisionale Extension mit rascher Krustenhebung und magmatischem „underplating” verursacht wurde. Die meisten dieser Granite bildeten sich durch Dehydratationsschmelzen der Unterkruste aus Paragneisen und eventuell auch intermediären kaliumreichen Orthogneisen. Einige wenige Plutone zeigen Interaktionen mit mafischen Magmen, die aus einem angereicherten Lithosphärenmantel stammen (Durbachite). Schmelzprozesse in der mittleren Kruste führten weiträumig zur Bildung von Migmatiten mit grßgen Anteilen an S-Typ and I-Typ Diatexiten. Postkollisionale, epizonale I-Typ Granodiorite and Tonalite (ca. 310-290 Ma): Die Hauptverbreitung dieser Plutone liegt in den Alpen. Eine genetische Verbindung zu einer spätvariszischen Subduktionszone am Variszikums-Siidrand erscheint möglich. Andererseits könnte auch die bloße Reaktivierung and Dehydratation von alten (frühvariszischen) Subduktionszonen unter dem Orogen die Produktion entsprechender I-Typ Magmen bewirkt haben, ebenso wie ein postkollisionales Druckentlastungsschmelzen von I-Typ Quellen im Bereich der Krusten-Mantel Grenze ohne Subduktionzusammenhang. Leukogranite des sädten Karbon and Perm (ca. 300-250 Ma): Viele dieser Plutone zeigen Eigenschaften von A-Typ Graniten. Die entsprechenden Magmen sind vermutlich durch Schmelzprozesse in einer restitischen Unterkruste oder im lithosphärischen Mantel entstanden.

The Lesser Caucasus occurs in the hinterland of the Arabia–Eurasia collision zone in the broad Alpine–Himalayan orogenic belt and includes Cenozoic plutonic and volcanic sequences that provide important clues for collision-driven continental magmatism and mantle dynamics. Two main magmatic episodes (Eocene and late Miocene–Quaternary) formed the volcanic landscape and the igneous assemblages in the Lesser Caucasus of Azerbaijan. (1) The Eocene sequence consists of trachybasalt and basaltic trachyandesite with subordinate tephrite-basanite, basaltic andesite, and trachyandesite, showing shoshonitic and mildly alkaline compositions. The Miocene–Quaternary magmatic episode is represented by (2a) an early phase of upper Miocene–lower Pliocene andesite, trachyandesite, trachydacite, dacite and rhyolite lavas, and by (2b) a late phase of upper Pliocene–Quaternary trachybasalt, basaltic trachyandesite, basaltic andesite, trachyandesite, trachyte, and rhyolite flows. The rocks of the early phase have high-K calc-alkaline compositions, whereas those of the late phase show high-K shoshonitic compositions, defining an alkaline trend and a K2O-enriched melt source. All three volcanic associations show variant troughs in Nb, Ta, Hf, and Zr, strong enrichment in Rb, Ba, Th, La, and depletion in Ti, Yb, Y relative to mid-ocean ridge basalt N-(MORB) in their multi-element patterns. The enrichment of incompatible elements and K suggests derivation from a metasomatized mantle source, whereas the troughs in Nb and Ta indicate a subduction influence in the mantle melt sources. Mantle-derived magmas were modified by AFC/FC processes for all three volcanic sequences. These geochemical features are similar to those of coeval volcanic associations in the peri-Arabian region, and indicate the existence of subduction-metasomatized lithospheric mantle beneath the Lesser Caucasus during the Cenozoic. Partial melting of this subduction-modified subcontinental lithospheric mantle in the peri-Arabian region was triggered initially by slab breakoff following discrete continental collision events in the early Eocene. The heat source for the later Miocene–Quaternary volcanism in the entire peri-Arabian region was provided by asthenospheric upwelling, which itself was caused by delamination of the mantle lithosphere following the final Arabia–Eurasia collision at ∼13 Ma. Increased alkalinity of successively younger units in the Plio-Quaternary volcanic associations resulted from the input of enriched asthenospheric melt during the last stages of post-collisional magmatism. Active, crustal-scale and orogen-parallel, transtensional fault systems in the peri-Arabian region facilitated the formation of fissure eruptions and stratovolcanoes in the latest Cenozoic.

The abundances of 30 trace elements, including the volatile chalcophile/siderophile elements As, Cd, Ga, In and Sn were determined by laser ablation ICP-MS in minerals of 19 anhydrous and 5 hydrous spinel peridotite xenoliths from three continents. The majority of samples were fertile lherzolites with more than 5% clinopyroxene; several samples have major element compositions close to estimates of the

Unusually magnesian (Mg# ∼76) basalts have been sampled from a small submarine volcano situated on the Mariana arc magmatic front. Total alkalis range from 1.7 to 1.94%, Al2O3 from 9.09 to 10.3% and CaO from 13.9 to 14.09%. These lavas can be classified based on mineralogy as picrite and ankaramite. Olivine-hosted melt inclusions (MIs) have median MgO contents of 17.17–17.86 wt%, 0.35–0.5% TiO2, 42–50% SiO2 and 1.66–3.43% total alkalis, which suggest that the parental magmas were primitive mantle melts. Trace element concentrations for both MIs and lavas are arc-like, although more depleted than most arc lavas. Chlorine (182–334 ppm) and H2O contents (0.11–0.64 wt%) in the MIs are consistent with the estimated median oxygen fugacities (log ΔFMQ of + 1.53–1.66) which lie at the low end of the range estimates for arc basalts and picrites (ΔFMQ =  + 1 to + 3). Isotopic compositions of Sr, Nd, Hf and Pb are similar to those of other Mariana arc lavas and indicate derivation from an Indian Ocean mantle domain. The averaged magmatic temperature estimate from several geothermometers was 1,367°C at 1–1.5 GPa. We propose that high-Mg magmagenesis in this region results from the adiabatic decompression melting of relatively anhydrous but metasomatized mantle wedge. This melting is attributed to enhanced upwelling related to unusual tectonics on the over-riding plate related to a tear or other discontinuity on the subducted slab.