Late syn- to post-collisional magmatism in Madagascar: the genesis of the Ambalavao and Maevarano Suites

The East African Orogen involves a collage of Proterozoic microcontinents and arc terranes that became wedged between older cratonic blocks during the assembly of Gondwana. The Ediacaran–Cambrian Ambalavao and Maevarano Suites in Madagascar were emplaced during the waning orogenic stages and consist of weakly deformed to undeformed plutonic rocks and dykes of mainly porphyritic granite but also gabbro, diorite and charnockite. U-Pb geochronological data date emplacement of the Ambalavao Suite to between ca. 580 Ma and 540 Ma and the Maevarano Suite to between ca. 537 Ma and 522 Ma. Major and trace element concentrations are consistent with emplacement in a syn- to post-collisional tectonic setting as A-type (anorogenic) suites. Oxygen (δ¹⁸O of 5.27‰–7.45‰) and hafnium (εHf(t) of –27.8 to –12.3) isotopic data from plutons in the Itremo and Antananarivo Domains are consistent with incorporation of an ancient crustal source. More primitive δ¹⁸O (5.27‰–5.32‰) and εHf(t) (+0.0 to +0.2) isotopic values recorded in samples collected from the Ikalamavony Domain demonstrate the isotopic variation of basement sources present in the Malagasy crust. The Hf isotopic composition of Malagasy zircon are unlike more juvenile Ediacaran–Cambrian zircon sources elsewhere in the East African Orogen and, as such, Madagascar represents a distinct and identifiable detrital zircon source region in Phanerozoic sedimentary provenance studies. Taken together, these data indicate that high-T crustal anatexis, crustal assimilation and interaction of crustal material with mantle-derived melts were the processes operating during magma emplacement. This magmatism was coeval with polyphase deformation throughout Madagascar during the amalgamation of Gondwana and magmatism is interpreted to reflect lithospheric delamination of an extensive orogenic plateau.Donnelly B. Archibald, Alan S. Collins, John D. Foden, Justin L. Payne, Peter Holden, Théodore Razakamanan

Single Photons on Pseudo-Demand from Stored Parametric Down-Conversion

We describe the results of a parametric down-conversion experiment in which the detection of one photon of a pair causes the other photon to be switched into a storage loop. The stored photon can then be switched out of the loop at a later time chosen by the user, providing a single photon for potential use in a variety of quantum information processing applications. Although the stored single photon is only available at periodic time intervals, those times can be chosen to match the cycle time of a quantum computer by using pulsed down-conversion. The potential use of the storage loop as a photonic quantum memory device is also discussed.Comment: 8 pages, 7 Figs., RevTe

The geochemistry and crustal origin of the Archaean acid intrusive rocks of the Agnew Dome, Lawlers, western Australia

Acid intrusive rocks of the Agnew Dome intrude a supracrustal sequence, part of the Lawlers-Mt. White greenstone belt. The acid intrusives form a complex batholith with multiple intrusions. Three distinct groups of granitoids have been recognised, both in terms of style of intrusion and on petrological grounds. The oldest intrusion was that of the Lawlers tonalite (granitoid group I) which includes a variety of rock-types from quartz diorite through tonalite to granodiorite. These are characterised by high Na/K, high concentrations of LREE, Ba and Sr, very high La/Yb, low HREE and low Y. Intrusion of the Lawlers tonalite pre-dates the first deformation of the intruded greenstones.A second granitoid suite (group II) occupies the dominant area of the batholith and comprises a diverse group ranging from granodiorite through adamellite to trondhjemite. These have lower Na/K and La/Yb ratios and a higher SiO2 content than the tonalites. Their intrusion pre-dates the main folding of the greenstones (D2).The third granitoid group (group III) comprises highly leucocratitic, true granites of near minimum-melt composition, K- and Rb-rich, but with very low concentrations of P, Zr, Ti, Sr and Ba. They have flat REE patterns with Eu-anomalies. These are clearly the latest intrusive phase and form a network of intersecting dykes, sheets and small plugs.On geochemical grounds the three granitoid suites of Lawlers are not related. The tonalites are likely to have been derived from the fusion of mafic source rocks, while some features of the leucogranites suggest source material must include a K-rich granite component. These features suggest that at least, in this part of the Yilgarn Block, the crust which pre-dates both the 2.65 Ga Lawlers tonalite and the Mt. White-Lawlers greenstone belt must have been of mixed character with mafic and sialic rocks very like the presently exposed Yilgarn Archaean suite