U-Th-Pb systematics of morphologically characterized zircon and allanite: a high-resolution isotopic study of the Alpine Rensen pluton (northern Italy)

Details U-Th-Pb systematics of morphologically characterized zircon and allanite: a high-resolution isotopic study of the Alpine Rensen pluton (northern Italy) U-Th-Pb systematics of morphologically characterized zircon and allanite: a high-resolution isotopic study of the Alpine Rensen pluton (northern Italy)

Nov 1989

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1989e%26psl..95..235b&link_type=abstract

Earth and Planetary Science Letters, Volume 95, Issue 3-4, p. 235-254.

Computer Science

23

Scientific paper

The Rensen pluton is a typical member of the Tertiary calc-alkaline plutonic suite structurally connected to the Periadriatic lineament. It intrudes the Austroalpine polymetamorphic basement of the Eastern Alps southwest of the Tauern Window (northern Italy).
Zircons and allanites from tonalitic, quartz-dioritic and monzogranitic lithologies were studied systematically on the basis of refined grain-by-grain microanalytical techniques. High-resolution isotope analyses show that morphology and U-Pb systematics of zircon are highly correlated with magma evolution. Zircons from the main tonalitic part of the intrusion display a gradual increase of U content from early-stage S-type morphology to late-stage G1-type morphology.
Most of the zircon crystals include premagmatic seeds derived from old crustal sources: unevolved S-type zircons contain inherited components of late Proterozoic/early Paleozoic apparent age, whereas late-stage G1-type zircons show inheritance of early Proterozoic apparent age. Both types of inherited components are present in zircons from a fine-grained tonalitic domain located at the center of the pluton, which shows restitic structures. The crustal affinity of this rock is corroborated by its relatively low ɛNd value (-6.0) as compared to values obtained for the main tonalite (-3.9) and the quartz diorite (-3.5).
The accuracy of Cainozoic zircon ages is affected by initial radioactive disequilibrium in the U-Pb decay chains. Particularly uncertain is the fractionation behavior of protactinium, which could be enriched in zircon due to the high stability of the Pa5+ oxidation state. Because the Th-Pb system is devoid of long-lived intermediate radioactive nuclides, the best approximation to the emplacement age of the Rensen pluton is given by allanite Th-Pb ages of 31.70 +/- 0.25 Ma (95% c.l.) for the main tonalite and by a slightly younger age of 31.09 +/- 0.32 Ma for quartz diorite.

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