Description: The magmatic activity (0–16 Ma) in Iceland is linked to a deep mantle plume that has been active for the past 62 My. Icelandic and northeast Atlantic basalts contain variable proportions of two enriched components, interpreted as recycled oceanic crust supplied by the plume, and subcontinental lithospheric mantle derived from...

Description: Subduction earthquakes release vast amounts of energy to crust and mantle lithosphere. The products of such drastic events are rarely observed in the field because they are mostly lost by subduction. We present new observations of deformation products formed by a few very large and numerous small intermediate-depth Alpine subduction earthquakes that are preserved along the exhumed gabbro–mantle peridotite contact of the Piemont-Liguria oceanic basin in Corsica. The abrupt release of energy resulted in shear heating events that completely melted both gabbro and peridotite. The large volumes of melt that were generated can be studied in the fault and injection vein breccia complex along the fault zone. The energy required for wholesale melting of a large volume of peridotite along the fault combined with previous estimates of stress drops show that very large earthquakes took place along the Moho of the subducting plate. Because these fault rocks formed by intraplate seismicity, we suggest, by analogy with present-day subduction, that they represent a proxy for the lower seismogenic zone.

Description: The lower Main Zone (LMZ) in the Northern Limb of the Bushveld Complex as intersected by the 1563·02 m deep Moordkopje (MO-1) drill hole shows very little large-scale differentiation as exemplified by parameters such as the An% of plagioclase, the Mg# of pyroxene and the modified differentiation index over a vertical interval in excess of 1·3 km. These features, coupled with the late entry of pigeonite (now inverted to orthopyroxene), are suggestive of the repeated intrusion of magma into the magma chamber of the Northern Limb during the early stages of the development of the Main Zone. Importantly, the lower Main Zone (LMZ) exhibits significant Sr isotopic disequilibrium between coexisting plagioclase (Sr i ~0·708) and orthopyroxene (Sr i up to ~0·711) in certain samples, a feature that has not previously been described for the Main Zone. Many of the features of the lower Main Zone (e.g. bulk compositions of LMZ cumulates unsuitable for the crystallization of plagioclase + two pyroxenes; poorly developed layering; non-cotectic proportions of plagioclase and pyroxene; decoupling of the differentiation trends of plagioclase and pyroxene over short vertical intervals) in the Northern Limb may be explained if it was intruded not as repeated influxes of aphyric magma, but instead by the repeated intrusion of crystal mushes from a deeper, sub-compartmentalized, crustal staging chamber. The model proposed is an alternative to previous models suggesting the loss of extrusive components from the Bushveld Complex, which were developed to explain compositional paradoxes within the Bushveld Complex.

Description: The anorthositic members of the Mealy Mountains Intrusive Suite (MMIS; Labrador, Canada) are host to 0·5–5 m diameter pegmatitic, pod-like segregations, originally described as graphic granite pods. U–Pb zircon geochronology confirms that the pods are coeval with the 1650–1630 Ma emplacement age range for the MMIS, yielding ages of 1654 ± 8 to 1628 ± 3·5 Ma. Petrographic and geochemical analysis of five pods from anorthositic rocks of the MMIS reveals that the pods have a diverse compositional range from monzodiorite to granite, varying from Fe-rich and Si-poor, to Fe-poor and Si-rich compositions. Fe-rich, Si-poor pods in the MMIS and other massifs (e.g. Laramie Anorthosite Complex) tend to be hosted by olivine-bearing anorthosites, whereas Si-rich, Fe-poor pods are hosted by pyroxene-bearing anorthosites. Each pod shows a range of graphic, myrmekitic and symplectitic textures, along with distinctive mineral assemblages (e.g. apatite and zircon) and highly enriched trace-element compositions. Evolved mineral assemblages, high concentrations of Fe, Ti and P (and in some cases SiO 2 ), and 10–1000 x chondrite enrichment in light rare earth elements, U, Th and Rb indicate that many of the pods are highly fractionated. The array of textural intergrowths provides clues about the final stages of crystallization in the pods and, by extension, the anorthosites. Macroscopic quartz–K-feldspar graphic intergrowths indicate high-viscosity, fluid-bearing and significantly undercooled magmatic conditions, whereas microscopic myrmekitic (plagioclase–quartz) and symplectitic (plagioclase–orthopyroxene) intergrowths on primary grain boundaries indicate replacement of phases in the presence of reactive fluids. In assessing the nature of these pegmatitic pods based on field, petrographic and geochemical evidence, we conclude that they represent the fluid-bearing, late-stage crystallization products of a residual liquid in the massif anorthosite system. The Fe and Si compositional variations observed in these late-stage pods can be linked to a fundamental olivine–pyroxene dichotomy observed in most Proterozoic anorthosite massifs, suggesting that pulses of magma experience variable contamination (in amount and/or composition) leading to varying differentiation paths. A range of lithologies (monzonites, monzonorites, ferrodiorites and jotunites) observed in similar pod-like structures, as well as dykes and plutons, has been observed in other Proterozoic anorthosite massifs and all have, at one time or another, been interpreted as the residual liquids of anorthosite crystallization. Our observation of in situ pods with similar compositions to all of the aforementioned lithologies, and displaying textures indicative of late-stage crystallization, supports the notion that all of these associated lithologies can be interpreted as comagmatic with, but variably contaminated and isolated residual liquids of, anorthosite crystallization. However, using isotopic evidence we cannot support the notion that the far larger granitic plutons associated with Proterozoic anorthosites are also residual liquids of anorthositic magma fractionation.

Description: Earth’s residual geoid is dominated by a degree-2 mode, with elevated regions above large low shear-wave velocity provinces on the core–mantle boundary beneath Africa and the Pacific. The edges of these deep mantle bodies, when projected radially to the Earth’s surface, correlate with the reconstructed positions of large igneous provinces...

Description: Trachytes are typically interpreted in terms of extreme fractional crystallization from basaltic magmas. Data from Mauritius suggest otherwise. Here, intrusive, nepheline-bearing trachytes are associated with Older Series basalts (9·0–4·7 Ma), as confirmed by a U–Pb zircon age of 6·8 Ma. Trachyte mineralogy is dominated by low-Ca alkali feldspar with variable Na/(Na + K), lesser high-Na nepheline, aegirine-rich clinopyroxene, titanomagnetite and accessory zircon and apatite. A few samples contain xenocrysts of anorthoclase, Al- and Ti-rich clinopyroxene and kaersutite; these phases show prominent reaction rims that approach typical trachyte mineral compositions. Trachyte major elements cluster at ~63 wt % SiO 2 and Na 2 O + K 2 O ~12 wt %, but reconstructed pre-alteration compositions suggest that they were originally mainly phonolites, and form a prominent Daly Gap when plotted with the basalts. Incompatible trace elements are enriched in all trachytes, except for Ba, Sr and Eu, which show prominent negative anomalies. Rare earth element patterns have variable abundances, prominent negative Eu anomalies and shapes that differ markedly from those of the basalts. Initial Nd values cluster at + 4·03 ± 0·15 ( n = 13), near the lower end of the range for basalts ( Nd = +3·70 to + 5·75), but the initial 87 Sr/ 86 Sr is highly variable (I Sr = 0·70408–0·71034) compared with the relatively constant I Sr of 0·70411 ± 19 for the basalts. Fractional crystallization models, using the PELE and MELTS algorithms, starting with a primitive, nepheline-normative Mauritian basalt parent ( P = 1 kbar, f O 2 = QFM – 3, where QFM is quartz–fayalite–magnetite buffer) fail, because when plagioclase joins olivine in the crystallizing assemblage, successive liquids become depleted in Al 2 O 3 , do not produce nepheline, and do not approach phonolitic or trachytic compositions. Similar results are obtained using more alkaline parent liquids including basanite; however, such compositions are not observed anywhere amongst the Older Series basalts. Plutonic xenoliths from Mauritius do not fill the Daly Gap as in some other occurrences (e.g. Hawaii, Pantelleria, Azores). Fractional crystallization was not the operative process that produced Mauritian trachytes. Likewise, liquid immiscibility is excluded because the compositions do not fall at the ends of known miscibility gaps. What remains as plausible is some type of partial melting process, although the source cannot be Precambrian continental crust, as suggested to exist under Mauritius, because such material should not yield nepheline-bearing melts, and would not account for the Sr–Nd isotopic compositions. Small amounts of contamination with such continental crust, however, can account for the Sr–Nd isotopic compositions of the trachytes. Partial melting of basaltic volcanic rocks or extant gabbroic bodies, either from the oceanic crust or from Réunion plume-related magmas, should yield quartz-saturated melts different from the critically undersaturated Mauritian trachytes. A remaining possibility is that the trachytes represent direct, small-degree partial melts of fertile, perhaps metasomatized mantle; we suggest that the xenocryst assemblage present in some samples might represent fragments of this source. A mantle source is supported by the presence of trachytic and phonolitic glasses in many mantle xenoliths, and experimental results show that low-degree alkaline melts can be produced from mantle peridotites even under anhydrous conditions. If some feldspar is left behind as a residual phase, this would account for the negative Ba, Sr and Eu anomalies observed in Mauritian trachytes. These considerations may also apply to other trachyte and phonolite occurrences worldwide.