Excess ⁴⁰Ar-free phengite in ultrahigh-pressure metamorphic rocks from the Lago di Cignana area, Western Alps

详细信息    查看全文

• 作者：Chitaro Gouzu ; Tetsumaru Itaya ; Hironobu Hyodo ; Takaaki Matsuda
• 关键词：UHPM rocks ; Lago di Cignana ; Western Alps ; Ar/Ar age ; Excess ⁴⁰Ar
• 刊名：Lithos
• 出版年：2006
• 出版时间：December 2006
• 年：2006
• 卷：92
• 期：3-4
• 页码：418-430
• 全文大小：2072 K

文摘

Ar/Ar analyses of phengites and paragonites from the ultrahigh-pressure metamorphic rocks (zoisite–clinozoisite schist, garnet–phengite schist and piemontite schist) in the Lago di Cignana area, Western Alps were carried out with a laser probe step-heating method using single crystals and a spot dating method on thin sections. Eight phengite and two paragonite crystals give the plateau ages of 37–42 Ma with 96–100 % of ³⁹Ar released. Each rock type also contains mica crystals showing discordant age spectra with age fractions (20–35 Ma) significantly younger than the plateau ages. Phengite inclusions in garnet give ages of 43.2 ± 1.1 Ma and 44.4 ± 1.5 Ma, which are significantly older than the spot age (36.4 ± 1.4 Ma) from the matrix phengites, and the plateau ages from the step-heating analyses. Inclusion ages (43 and 44 Ma) are consistent with a zircon SHRIMP age (44 ± 1 Ma) in this area. These results suggest that the oceanic materials that underwent a simple subduction related UHPM, form excess ⁴⁰Ar-free phengite and that the peak metamorphism is ca. 44 Ma or little older. We suggest that matrix phengites experienced a retrogression reaction changing their chemistry contemporaneously with deformation related to the exhumation of rocks releasing significant radiogenic ⁴⁰Ar from the crystals. This has lead to the apparent ages of the matrix phengites that are significantly younger than the inclusion age.