Multi-isotope study of composite dykes from the northernmost Arabian-Nubian Shield: implications for the origin of A-type granites

Yaron Katzir, Boris Litvinovsky, Ada Zanvilevich, Bor-ming Jahn, John W. Valley, Moshe Eyal, Michael J. Spicuzza

Research output: Chapter in Book/Report/Conference proceedingConference contribution

Abstract

Late Neoproterozoic bimodal dyke suites are abundant in the Arabian-Nubian Shield. In southern Israel the bimodal suite includes dominant alkaline quartz porphyry dykes, rare mafic dykes, and numerous composite dykes with felsic interiors and mafic margins. Composite dykes with either abrupt or gradational contacts between the felsic and mafic rocks bear field, petrographic and chemical evidence for coexistence and mixing of basaltic and A-type rhyolitic magmas. Mixing and formation of hybrid intermediate magmas commenced at depth and continued during emplacement of the dykes. Oxygen isotope ratios of alkali feldspar in quartz porphyry (13 to 15ppm) and of plagioclase in trachydolerite (10 to 11ppm) are much higher than their initial magmatic ratios predicted by equilibrium with unaltered quartz (8 to 9ppm) and clinopyroxene (5.8pppm). The elevation of delta (super 18) O in alkali feldspar and plagioclase, and extensive turbidization and sericitization call for post-magmatic low-temperature water-rock interaction. Hydrous alteration of alkali-feldspar, the major carrier of Rb and Sr in the quartz-porphyry, also accounts for the highly variable and unusually high apparent values of I(Sr) of 0.71253 to 0.73648. Unlike I(Sr) the initial (super 143) Nd/ (super 144) Nd ratios, expressed by epsilon Nd(T) values, are probably unaltered and show small variation in mafic and felsic rocks within a narrow range from +1.4 to +3.3. The Nd isotope signature suggests either a common mantle source for the mafic and silicic magmas or a juvenile crust source for the felsic rocks. However, oxygen isotope ratios of zircon in quartz porphyry [delta (super 18) O (Zrn) = 6.5 to 7.2ppm] reveal significant upper crustal contribution to the rhyolite magma, suggesting that mafic and A-type silicic magmas, though contemporaneous, are not co-genetic. Comparison of (super 18) O/ (super 16) O ratios in zircon allows to distinguish two groups of A-type granites in the region: those with mantle-derived source, delta (super 18) O(Zrn) ranging from 5.5 to 5.8ppm (Timna and Katharina granitoids) and those with major contribution of the modified juvenile upper crust component, delta (super 18) O(Zrn) varying from 6.5 to 7.2ppm (Elat quartz porphyry dykes and the Yehoshafat alkaline granite). This suggests that A-type silicic magmas in the northern ANS evolved by variable amounts of assimilation and magma mixing.
Original languageEnglish
Title of host publication Congres Geologique International, Resumes
Volume33
EditionAbstract 1344377
StatePublished - 1 Jan 2008

Keywords

  • Igneous and metamorphic petrology
  • Isotope geochemistry
  • A-type granites
  • Africa
  • alkaline earth metals
  • Arabian Peninsula
  • Arabian Shield
  • Asia
  • assimilation
  • bimodal composition
  • dikes
  • granites
  • igneous rocks
  • intrusions
  • isotope ratios
  • isotopes
  • magmas
  • mantle
  • metals
  • mixing
  • Nd-144/Nd-143
  • neodymium
  • nesosilicates
  • Nubian Shield
  • O-18/O-16
  • orthosilicates
  • oxygen
  • plutonic rocks
  • quartz porphyry
  • rare earths
  • rhyolites
  • silicates
  • Sr-87/Sr-86
  • stable isotopes
  • strontium
  • volcanic rocks
  • water-rock interaction
  • zircon
  • zircon group

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