No abstract

Subduction zones are often characterized by the presence of strong trench-parallel seismic anisotropy and large delay times. Hydrous minerals, owing to their large elastic anisotropy and strong lattice preferred orientations (LPOs), are often invoked to explain these observations. However, the elasticity and the LPO of chloritoid, which is one of such hydrous phases relevant in subduction zone settings, are poorly understood. In this study, we measured the LPO of polycrystalline chloritoid in natural rock samples, obtained the LPO-induced seismic anisotropy, and evaluated the thermodynamic stability field of chloritoid in subduction zones. The LPO of chloritoid aggregates displayed a strong alignment of the [001] axes subnormal to the rock foliation, with a girdle distribution of the [100] axes and the (010) poles subparallel to the foliation. New elasticity data of single-crystal chloritoid showed a strong elastic anisotropy of chloritoid with 47% for S-waves (VS) and 22% for P-waves (VP), respectively. The combination of the LPO and the elastic anisotropy of the chloritoid aggregates produced a strong S-wave anisotropy with a maximum AVS of 18% and a P-wave anisotropy with an AVP of 10%. The role of chloritoid LPO in seismic anisotropy was evaluated in natural rock samples and a hypothetical blueschist. Our results indicate that the strong LPO of chloritoid along the subduction interface and in subducting slabs can influence the trench-parallel seismic anisotropy in subduction zones with “cold” geotherms.

We report the first finding of Ediacaran mafic magmatism in northern Victoria Land of the Ross orogen, Antarctica, based on ca. 600-590 Ma magmatic zircon cores in Cambrian eclogites. The mafic magmatism could be either linked to ca. 615-590 Ma incipient convergent margin magmatism in central segment of the Ross orogen, or ca. 600-580 Ma continental rift-related volcanism widespread in eastern Australia. The latter is preferred based on the trace-element compositions distinctive from those of arc-related basalts and the depleted mantle-like Hf isotopic compositions of zircon.Our results suggest dual rifting episodes during both the Ediacaran and Cryogenian (ca. 670-650 Ma) in the East Antarctic margin, correlative with those in eastern Australia. A spatial distribution of coeval rifting and subduction along the Ediacaran margin of East Antarctica is readily accounted for by rift inheritance; the upper-and lower-plate geometry resulting from detachment and transform faulting.

Megacrystic phengite-bearing quartz veins associated with retrogressed eclogites occur along terrane boundary zone in the northern Ross orogen, Antarctica. The vein zircons crystallized from metamorphic fluid under high-pressure (P) conditions rather than captured as xenocrysts from the host eclogites, on the basis of: (1) weak oscillatory-and fir tree-zoned cathodoluminescence textures and extremely low Th/U ratios of zircon, (2) highly steep patterns of heavy rare earth elements and weak negative to positive Eu anomaly of zircons, suggesting their growth not in equilibrium with garnet and plagioclase, respectively; and (3) the high celadonite contents of phengite megacrysts (up to 3.33 a.p.f.u. Si for O = 11). Zircon U-Pb ages dated the high-P quartz vein formation at 501 ± 7 Ma (2σ), which overlaps with time of peak metamorphism of the eclogites at c. 500-498 Ma. The zircon crystallization age and marginal breakdown of phengite megacrysts into biotite-oligoclase symplectites suggest that the high-P quartz vein formation was at near-peak to early-exhumation stages of the eclogite facies metamorphism. The similarity in depths of the vein formation and major dehydration of subducting metamafic rocks further suggests that this fluid activity would be one of possible agents responsible for arc-related magmatism in northern Victoria Land of the Ross orogen.