U-PB ISOTOPIC DATING OF TROODOS PLAGIOGRANITE, CYPRUS BY LA-ICP-MS

This study examines the crystallization history of plagiogranite from the western Troodos ophiolite.  Previous geochronologic studies reported evidence of multiple zircon populations from several exposures of plagiogranite in the eastern part of the Troodos massif.

The western Troodos plagiogranite is medium-grained and composed of quartz, Na-rich feldspar, minor K-feldspar, and trace hornblende.  Zircons are 20-100 microns long, range from clear to whitish pink and orange, and appear unzoned in cathodoluminescence images.  Laser Ablation–Inductively Coupled Plasma–Mass Spectrometry (LA-ICP-MS) was used to determine ²⁰⁶Pb/²³⁸U and ²⁰⁷Pb/ ²⁰⁶Pb ratios of 14 zircons.  The weighted average of the individual zircon ²⁰⁶Pb/²³⁸U ages, corrected for common lead, is 90.9 ± 0.5 Ma (2s).  Most grains are only slightly discordant (< 5%) on a Tera-Wassenburg diagram and yield an intercept age of 91.1 ± 1.3 Ma (2s, MSDW=1.3; assuming an error of 1.4% relative, based on the observed variation of analyzed standards).  This result agrees with the previously reported U-Pb zircon age (91.6 ± 1.4 Ma) of plagiogranite exposed in the eastern Troodos massif (Mukasa and Ludden, 1987) and with biostratigraphic studies (Blome and Irwin, 1985) that place the overlying umberiferous strata in the middle to upper Turonian (89.3–92.1 Ma).  Mukasa and Ludden also reported that the eastern plagiogranite contains zircons with a bimodal age distribution (90.3 ± 0.7 and 92.1 ± 0.6 Ma).  Using the age un-mixing function in Isoplot (Ludwig, 2003), we observe a similar bimodal age distribution (90.6 and 91.8 Ma) in the western Troodos plagiogranite, but these ages are statistically similar assuming an error of 1.4% relative.

The results of this study indicate a U-Pb zircon intercept age of 91.1 ± 1.3 Ma for the plagiogranite of the western Troodos massif, in agreement with previous U-Pb zircon results from the eastern Troodos.  This new age supports previous observations of a decreasing trend of ophiolitic plagiogranite ages along the Neo-Tethyan suture zone (e.g. 93-98 Ma at Oman, Tilton et al., 1981; 93-95 Ma at Zagros, Ghatzi et al., 1999; 91-92 Ma at Cyprus, Mukasa and Ludden, 1987 and this study).  These results also indicate that the LA-ICP-MS U-Pb technique can be successfully used to determine crystallization ages of zircons from young (<100 Ma) igneous suites.