Search results for "Porphyritic"
showing 5 items of 5 documents
Elements of the Archean thermal history and apparent polar wander of the eastern Kaapvaal Craton, Swaziland, from single grain dating and paleomagnet…
1989
Noble-gas signature of magmas from a heterogeneous mantle wedge: The case of Stromboli volcano (Aeolian Islands, Italy)
2014
Abstract We report on the variation of the elemental (He, Ne, and Ar) and isotopic (He and Ar) compositions of olivine- and clinopyroxene-hosted fluid inclusions hosted in lavas, pyroclastics, and cumulate xenoliths from the last 60 ka of the eruptive history of Stromboli volcano, Italy. Samples belong to (i) the present-day activity as represented by shoshonitic (SHO) basalts, i.e., pumices with low porphyritic (LP) content and high porphyritic (HP) scoriae; (ii) the subalkaline versus alkaline products erupted at Stromboli during its subaerial history among the extreme magmatic series, i.e., calc-alkaline (CA) and potassic (KS); and (iii) the only known ultramafic cumulates (San Bartolo w…
Mimicking shear zones: An example from Wadi Filk, Jordan
2017
Abstract Ductile shear zones can develop in at least two ways: (1) a nucleus can grow laterally by free propagation into undeformed host rock, like most faults or joints; (2) the zone may nucleate and grow on or in a planar discontinuity and mimick its orientation. Most small-scale ductile shear zones are mimicking zones, but large-scale ductile shear zones could be free-propagating. The Wadi Filk mylonite zone in Jordan is a two km long, ten meter wide mylonite zone flanked by ultramylonite zones, developed in undeformed Neoproterozoic porphyritic monzogranite. Since mineral and major element composition of mylonite and monzogranite are identical, the structure seems to have formed by free…
Evolution of the Mayo Kebbi region as revealed by zircon dating: An early (ca. 740Ma) Pan-African magmatic arc in southwestern Chad
2006
Abstract The Mayo Kebbi region in SW Chad is part of the NNE-SSW trending Neoproterozoic Central African Fold Belt (CAFB) and is made up of three calc-alkaline granitoid suites emplaced into a metavolcanic–metasedimentary sequence. The first suite is represented by mafic to intermediate rocks (gabbro-diorite and metadiorite) emplaced between 737 and 723 Ma during early Pan-African convergence. The second consists of the Mayo Kebbi batholith and includes tonalites, trondhjemites and granodiorites, emplaced during several magmatic pulses between 665 and 640 Ma. The third suite includes porphyritic granodiorite and hypersthene monzodiorite dated at ca. 570 Ma. The Mayo Kebbi domain extends sou…
Structural relations and PbPb zircon ages for the Makuti gneisses: evidence for a crustal-scale Pan-African shear zone in the Zambezi Belt, northwes…
1999
Abstract The Makuti Group of northwest Zimbabwe is composed of mafic and intermediate biotite-rich gneisses interlayered with quartzofeldspathic gneisses of granitic composition, and minor sedimentary units. The gneisses have experienced a multi-staged metamorphic history, including an early high temperature-high pressure event and subsequent reworking at upper- to mid-amphibolite-facies conditions. They are positioned along the strongly deformed, southern margin of the east-west trending Zambezi Belt, and have been correlated with supracrustal gneiss units along the northern margin of the Zimbabwe Craton. The Makuti Group is characterised by an intensely developed gneissic layering and com…