high-K Calc-alkaline Character Research Articles

The geochemical character of late Cadomian extensional magmatism in Jersey, Channel Islands

Abstract An episode of magmatism, which may represent the last igneous phase attributable to the Cadomian orogenic cycle, is found in southern Jersey, Channel Islands. A zone, some 3 km wide, of intensive dyke injection, the Jersey Main Dyke Swarm, cuts across post-orogenic calc-alkaline plutons, though showing a close temporal link with their emplacement. The nature of the dyke swarm magmatism is dominantly but not exclusively basic. A clear acid—basic bimodality is present with very little intermediate material. Although subjected to varying degrees of alteration under low-grade metamorphic conditions, the primary nature of the rocks is often preserved. All those having a basaltic composition are olivine-normative, but show a high-K calc-alkaline character. Fairly high LREE enrichments are present. MORB-normalized spidergrams confirm that the magmatism was of a mature calc-alkaline type, similar to that generated at active continental margins such as the Andes. The presence of the swarm indicates a period of late post-orogenic crustal extension sub-parallel to the NE-SW strike of the proposed Cadomian active continental margin in the Armorican Massif. It could represent injection of the last remnants of magma from an already decayed subduction zone system into a newly cratonized environment.

O +, O +2, O +3 and O +4 ions in Ar-O 2 sputtering discharges : comments on “Oxidation mechanism in rf CO 2 plasma”, Vacuum, 36, 85 (1986) : Carolyn Rubin Aita and Michel E Marhic, Vacuum, 38, 1988, 37–38

The appinite–granite suite commonly occurs within post-subduction regimes during the final stage of an orogeny and thus holds the key to monitoring critical geodynamic and crustal evolutionary processes. The present zircon U–Pb dating and geochemical study documents the Late Permian appinite–granite complex from northwestern Liaoning, North China Craton. The hornblende-rich appinitic rocks have an SiO2 range from 41.8 to 53.4%, moderate to high alkali contents, enrichment in large ion lithophile elements and depletion in high field strength elements, with initial 87Sr/86Sr ratios of 0.7066 to 0.7128, εNd(t) of − 8.3 to − 13.3 and zircon εHf(t) from − 10.1 to − 18.7. These features suggest that their generation most likely involves metasomatism of mantle peridotites by sediment-derived liquids related to subduction and a later partial melting. The associated monzogranites range in SiO2 from 72.6 to 75.0% and exhibit a high-K calc-alkaline character, with high Sr–Ba abundances and elevated Sr/Y and La/Yb ratios. Such potassic adakitic signatures, plus their evolved isotopic compositions (87Sr/86Sri = 0.7063 to 0.7066, εNd(t) = − 15.6 to − 16.3, zircon εHf(t) = − 16.5 to − 18.8), are consistent with partial melting of mixed protoliths including newly underplated enriched mafic lower crust and minor old lower crustal materials. Such a mafic and felsic magma association not only attests to a heterogeneously enriched sub-continental lithospheric mantle beneath the northern North China Craton, but also indicates reworking within a post-orogenic transcurrent regime of lithospheric delamination. The temporal coincidence of the present appinite–granite complex with regional transcurrent fault activity leads to the further suggestion that such mafic–felsic magmatic suites could provide spatial markers for monitoring important post-orogenic structural and geodynamic processes.