Sublithospheric Source Research Articles

Petrology of early Tertiary nephelinites off mid-Norway. Additional evidence for an enriched endmember of the ancestral Iceland plume

Plugs of alkaline volcanic rocks (≈56 Ma) penetrating Mesozoic sediments crop out at 63°25′N on the continental shelf off Norway. The rocks are highly silica-undersaturated, comprising both nephelinites and basanite. The nephelinitic rocks are porphyritic, with abundant phenocrysts of Mg-rich olivine and some clinopyroxene. Many olivine crystals are altered along grain margins and fractures. The fine-grained matrix consists of fresh augite, biotite, rhönite, chlorite, oxides, and altered nepheline. Apatite and calcite are accessories. The basanite has much less olivine and abundant matrix plagioclase. The occurrence of rhönite indicates crystallization in the presence of a fluid phase at P<60 MPa and temperatures between 840 and 1200°C. The rocks are characterized by relatively high contents of MgO, CaO, P 2O 5, Ni, Cr, Sr, and incompatible elements. The REE are highly fractionated with LREE up to 500× chondrite. Initial strontium isotope ratios ( 87 Sr/ 86 Sr ) i range from 0.703685 to 0.704025 and ϵ Nd from +3.13 to +3.31 in the nephelinites. In the basanite ( 87 Sr/ 86 Sr ) i is 0.704447 and ϵ Nd+0.01. The nephelinites are enriched in 208 Pb/ 204 Pb . We suggest that the basanite formed from enriched lithospheric mantle and that the nephelinites were low melt fractions of a sublithospheric (asthenospheric) source. This source probably represents an enriched, low-temperature melting end member component of the North Atlantic (Icelandic) plume.

A tectonic perspective of incipient charnockites in East Gondwana

Abstract Examples of spectacular occurrences of charnockite in an arrested stage of formation in crustal segments of East Gondwana (comprising India, Sri Lanka and Antarctica) are evaluated to constrain the broad tectonic framework under which the charnockite formed. Field structures, petrologic features, and information including fluids trapped within inclusions in minerals suggest that the conversion of upper amphibolite facies gneisses to charnockites was brought about by the local desiccation induced by the infiltration of CO2-rich fluids through faults and shears under brittle-quasi-brittle conditions. The stable isotopic composition of these fluids is consistent with a “juvenile”, sublithospheric source. Available geochronologic data indicate multiple events of the incipient charnockite formation at ∼2500, 1000, and 500 Ma in these crustal domains. In each case, incipient charnockite formation post-dates regional metamorphism. Hence, the process of local desiccation is inferred to have operated in cratonized crustal segments. The P-T-t paths, characterized by isothermal decompression, are consistent with incipient charnockite development during the waning stages of collisional mobile belts, when the tectonic regime changed from compressional to extensional. Crustal distension triggers the release of CO2-rich fluids from depth, which are transported to mid-crustal levels through magmatic conduits, a notion supplemented by the occurrence of alkaline intrusives of Pan-African age. Degassing of these magmas could have supplied the CO2 required for stabilizing the anhydrous assemblages in the charnockites.

Growth of the European lithospheric mantle—dependence of upper-mantle peridotite facies and chemical heterogeneity on tectonics and age

In Europe, during the Phanerozoic, collision of microplates caused the juxtaposition of disparate lithospheres of variable age and provenance. The complex prehistory of these plates, together with the present-day tectonic regime, generated considerable topography at the lithosphere-asthenosphere boundary. From north to south across Europe there exists a considerable variation in lithosphere thickness, seismic velocity and heat flow, with concomitant changes in the mantle helium flux, the extent, type and source of Cenozoic volcanism, and the age and origin of the lithospheric mantle protolith. Consideration of Moho depth and lithosphere thickness reveals that the lithospheric mantle should be dominated by garnet-diamond facies mantle beneath stable shield areas (e.g. the Baltic Shield) and young mountain belts (e.g. the Alps and Betics), spinel-garnet facies mantle beneath Variscan Europe and spinel-plagioclase facies mantle in the western Mediterranean, Pannonian Basin and Rhinegraben. However, consideration of mantle xenolith data reveals that garnet peridotites are rare beneath Variscan Europe and that plagioclase peridotites are unreported from the Pannonian Basin and the Rhinegraben. A tectonic dimension to lithosphere thickness, as well as a function that relates to the initial stabilisation age, is illustrated by the presence of thick lithosphere (diamond facies) beneath old tectonically stable areas such as the Baltic Shield, and young tectonically active regions such as the Alps. Thermo-tectonic processes have also produced lithosphere under the Archaean of NW Scotland that is as thin as that under parts of the Alpine Orogen (e.g. in western Mediterranean and the Pannonian Basin). Extreme chemical heterogeneity in the lithospheric mantle can result from both time-integrated effects over several billion years (e.g. in the North Atlantic craton) and mixing over several tens of millions of years along the tectonically active southern margin of Europe (e.g. in the Betics). Volcanic rocks provide a valuable probe of the lithospheric mantle and the asthenosphere, and it is apparent that throughout the Phanerozoic the lithospheric mantle evolved in response to repeated cycles of collision-subduction and intraplate extension. Volcanic rocks erupted during the Caledonian, Variscan and Alpine orogenies were derived from shallow lithospheric reservoirs containing a sedimentary component. In contrast, during syn- or post-orogenic extension volcanic rocks were derived from sub-lithospheric sources or by passive reactivation of young lithospheric mantle.

A search for splitSKSwaveforms in the North Atlantic

SUMMARY To test a prediction of the anisotropic properties of the upper mantle beneath the central North Atlantic, we measure splitting of the waveforms of SKS phases recorded at WWSSN stations in the Azores (PDA), and on Bermuda (BEC). Because the analysis of individual, analogue records does not provide sufficient resolution, we have developed a new technique for representing the confidence limits that permits a straightforward estimation of the average splitting beneath a station from a group of events. If events from a variety of different azimuths are recorded, the average splitting parameters can be tightly constrained. We found five records for PDA and eight for BEC that have good signal-to-noise ratio and a prominent SKS phase. Only one of the phases at PDA shows evidence of splitting; the average anisotropy must be small if it exists at all. At BEC, the average anisotropy is significant with a fast direction between NW-SE and N-S and a time delay between the two split components of about 1 s. Both of these observations are significantly smaller than predicted by a model of the upper mantle anisotropy based on SS-S differential traveltimes. The absence of systematic anisotropy at PDA may be attributable to a complex flow pattern in the upper mantle at this site near the Eurasian-African plate boundary in the vicinity of an active hotspot. At BEC, the observed direction is consistent with the prediction and both the direction and the magnitude of the average anisotropy are indicative of a sublithospheric source. To check the reliability of these results from analogue records, we have performed a similar analysis for station KIP on Oahu where digital records are available for comparison. The results from digital and analogue records are similar. At KIP, the splitting is more variable than expected for uniform anisotropy, with the fast directions for individual records ranging from about N26E to about N83E and with variable time delays. Similar variability is observed at all three stations. The average anisotropy at KIP is well-constrained, however, with a time delay of about 0.6 s and fast direction N75E. The fast direction coincides with the fossil sea-floor spreading direction beneath the Hawaiian islands, suggesting that the dominant source of anisotropy beneath KIP is lithospheric.

The Mount Gharib A-type granite, Nubian Shield: petrogenesis and role of metasomatism at the source

The Mount Gharib peralkaline A-type complex (476±2 Ma), located in the Nubian Shield of Egypt, contains sodic-calcic to sodic amphiboles, accessory astrophyllite, zircon, fluorite, apatite, allanite, aenigmatite, elpidite(?) and ilmenite. This “within plate” hypersolvus suite is enriched in large-ion lithophile (LIL) and high field-strength (HFS) elements, and characterized by a fractionated REE pattern (Ce/Yb=49) and a significant negative Eu anomaly. A fine-grained acicular-amphibole-bearing roof facies shows further enrichment in the LIL and HFS elements. The suite was emplaced in a Pan-African granodiorite-adamellite host, which it locally metasomatized. The affected rocks contain hydrothermal albite, end-member arfvedsonite, astrophyllite, and levels of the LIL and HFS elements intermediate between those in the peralkaline granite and the roof facies. Trace element and isotopic modeling of this A-type granite, with its high initial 87Sr/86Sr value (0.7110), documents an active role of the lithosphere in magma generation. Lithospheric extension, expressed by regional dyke-swarms, was caused by cooling, fracturing and relaxation of the thin, newly formed Pan-African crust. Localized partial melting took place in an open system, possibly as a result of an influx of alkali-rich fluid derived from a sublithospheric source. Metasomatic reactions similar to those observed in the metasomatized wallrocks are considered to have played an important role just prior to the onset of anatexis and generation of the A-type melt.