U-PB & O-ISOTOPE INVESTIGATION OF AN EOCENE MYLONITIC ORTHOGNEISS ON SERIFOS: A RECORD OF MELTING OR HP METAMORPHISM IN THE WESTERN CYCLADES?

In order to obtain timing constraints of a crustal-scale mylonitic S-type granite on Serifos, western Cyclades, seven samples were analyzed for zircon U-Pb ages and O-isotope signatures. The unit is characterized as a ductilely deformed mylonitic orthogneiss, with pervasive normal-motion top-to-southwest kinematics. Imaging of the zircon displays internal sector zoning and some zircon contain a thin rim overgrowth. Surface textures of unpolished grains are variable and include pristine crystal faces, crystal faces with wormy appearance, and zircon with notable inclusions (bio, plag, qtz, rutile, xenotime, Fe-oxides). We employed the Cameca ims1270 ion microprobe (UCLA) to perform depth-profiling analysis on unpolished grains from six of the samples in an attempt to resolve the age of the zircon rim overgrowth. One sample was analyzed via conventional polished mount techniques yielding a lower intercept of 37 ± 6 Ma (MSWD: 0.4) based on seven concordant, but distinct, spot analyses. For the unpolished grains, 76 ion-drilling analyses were conducted on the remaining six samples typically with one analysis per grain; 20 of the analyses were obtained by subsequent ion-drilling into the same analytical pit to produce a rim-core profile. U-Pb zircon core ages are Triassic-Jurassic, similar to ages determined on zircon in meta-igneous rocks from structurally coherent sequences elsewhere in the Cyclades. Regression of the youngest subset of 32 ages from concordant or near concordant analyses yields an intercept age of 43.3 ± 1.6 Ma (MSWD: 2.2); a weighted average ²⁰⁶Pb/²³⁸U age is within error at 41.2 ± 3.2 Ma (MSWD: 2.6). In addition to the U-Pb ages, 53 O-isotope analyses were conducted on the same zircon following similar ion-drilling methods (reported as ± 0.5‰). Single spot δ¹⁸O analyses range from 2.2‰ to 14.8‰, with a moderate bimodal population at 4.1‰ and 7.3‰. There is no apparent correlation between age and δ¹⁸O values. White mica ⁴⁰Ar/³⁹Ar cooling ages from adjacent crystalline units are 39-35 Ma, suggesting rapid cooling following zircon neogrowth. Further work is being carried out to determine if these data are a record of the widespread Cycladic high-pressure metamorphism or are registering partial melting and intrusion during an exhumation-deformation event.