The response of mineral chronometers to metamorphism and deformation in orogenic belts

Abstract

Mineral chronometers, especially accessory minerals using the U-Pb decay system, can reveal important information regarding the environmental conditions and duration of metamorphic-deformation events during the re-working of older rocks. Minerals such as zircon can newly grow during amphibolite facies or granulite facies events, providing direct ages of metamorphism. Pre-existing minerals like monazite, allanite, and titanite can preserve a component of their original age in spite of upper amphibolite facies re-working and very thorough recrystallisation of the rock fabric during mylonite development. The degree of incomplete Pb loss can be used to deduce, at least semi-quantitatively, the temperature and duration of the subsequent event. In well-studied examples, the relative retentivity of Pb is highly predictable, and this helps place strong constraints on relative closure temperatures, even when laboratory experimental data are lacking or inconclusive. A number of examples are presented from a wide variety of geological environments to illustrate the response of U-Pb isotope systematics within accessory minerals superimposed deformation, metamorphism and/or mineral growth.2

Allanite behaviour during incipient melting in the southern Central Alps

“…The robustness of allanite is even more significant in samples from Bellinzona and Val d'Arbedo where there is good evidence of prolonged high temperatures from zircon dating and Ti-in-zircon thermometry ). This observation is consistent with, and improves on other empirical estimates of Pb closure temperature in allanite from high-grade gneisses ($650°C, Heaman and Parrish, 1991;Parrish, 2001) and calc-alkaline igneous rocks (700-800°C, von Blanckenburg et al, 1992;Oberli et al, 2004).…”

“…Heaman and Parrish (1991) dated allanite from high grade gneisses by ID-TIMS and found that when metamorphosed to second sillimanite zone, protolith allanite still preserved a component of its original age. Parrish (2001) concluded based on U-Pb isotope systematics that the closure temperature of allanite is likely $650°C. Isotopic inheritance in the form of inherited cores has not been reported for allanite.…”

40Ar/39Ar dating of the Jiali and Gaoligong shear zones: Implications for crustal deformation around the Eastern Himalayan Syntaxis

“…Age data include 40 Ar/ 39 Ar dates obtained for amphibole, biotite, and muscovite separates this study), U-Pb zircon dates Lee et al, 2003;Song et al, 2007;Liang et al, 2008), and fission track dates for zircon and apatite (Chung, 2006). Closure temperatures are taken from Parrish (2001).…”

U-Th-Pb and 230Th/238U disequilibrium isotope systematics: Precise accessory mineral chronology and melt evolution tracing in the Alpine Bergell intrusion

“…U-Pb age data on zircon, allanite and titanite extracted from rocks associated with the metamorphic core zone of southeast British Columbia have shown the retentivity of radiogenic Pb in allanite to exceed that of titanite at upper amphibolite facies conditions (Heaman and Parrish, 1991). A closure temperature of 650 Ϯ 25°C was derived from these observations (Heaman and Parrish, 1991;Spear and Parrish, 1996;Parrish, 2001). As will be demonstrated by the isotopic arguments following below, such a low closure temperature appears to be at variance with our results.…”