Morphology versus U-Pb systematics in zircon: a high-resolution isotopic study of a zircon population from a Variscan dike in the Central Alps

Details Morphology versus U-Pb systematics in zircon: a high-resolution isotopic study of a zircon population from a Variscan dike in the Central Alps Morphology versus U-Pb systematics in zircon: a high-resolution isotopic study of a zircon population from a Variscan dike in the Central Alps

Jul 1986

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1986e%26psl..78..339b&link_type=abstract

Earth and Planetary Science Letters, Volume 78, Issue 4, p. 339-354.

Mathematics

Logic

17

Scientific paper

U-Pb isotopic measurements on individual zircon crystals combined with morphological analyses permit the identification of three distinct components within the zircon population of the Saedelhorn diorite, a Variscan dike from the western Gotthard (Central Alps, Switzerland): (i) 94% of the grains in the zircon population are elongate crystals with pronounced skeletal morphology indicative of rapid growth from a supercooled melt. (ii) 5% of the population consist of turbid, mostly subhedral zircons frequently showing D-type morphology (classification according to Pupin and Turco [1]) and elevated uranium contents compared to the skeletal variety. Single-crystal and multi-grain U-Pb isotopic data of group (i) and (ii) zircons define an intrusion age of 293 +5/-4 m.y. for the dike. (iii) Rare, transparent zircon crystals (<1% of the zircon population) yield apparent U-Pb ages in the range of 370-490 m.y. and display morphological and isotopic characteristics closely resembling those of a Caledonian orthogneiss intruded by the dike. This implies presence of assimilated wall-rock components in the macroscopically homogeneous dike sample.
A comparison of the data obtained by conventional analysis of multi-grain zircon fractions and those obtained by grain-by-grain analysis demonstrates that age resolution is considerably improved by single-crystal U-Pb dating. Furthermore, quantitative identification of zircon components assimilated by the ascending magma along its path to the present level of exposure is feasible by the latter technique. Since it is likely that such zircon grains are common to a broad variety of magma types, valuable information on age and composition of crustal layers not accessible to direct observation may readily become available by application of precise micro-analytical techniques.
Low initial 143Nd/144Nd (ɛNd = -2.7) at the time of intrusion of the Saedelhorn dike requires the magma to be derived partially or totally from a crustal source. For this crustal precursor, a model age of 1050 m.y. (TDM) is obtained, indicating that Proterozoic crust was involved in the petrogenesis of the Variscan intrusives of the Gotthard area.

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