New trace-element, radiogenic isotopic, and geochronologic data from the Troodos ophiolite, considered in concert with the large body of previously published data, give new insight into the tectonic history of this storied ophiolite, as well as demonstrating the variability of suprasubduction-zone ophiolites, and differences between them and commonly used modern analogs. Similar to earlier studies, we fi nd that island-arc tholeiite of the lower pillow lava sequence erupted fi rst, followed by boninite. We further divide boninitic rocks into boninite making up the upper pillow lava sequence, and depleted boninites that we consider late infi ll lavas. We obtained an Ar-Ar age from arc tholeiite of 90.6 ± 1.2 Ma, comparable to U-Pb ages from ophiolite plagiogranites. New biostratigraphic data indicate that most of the basal pelagic sedimentary rocks that conformably overlie the boninitic rocks are ca. 75 Ma. This suggests that voluminous eruption of boninitic rocks persisted until ca. 75 Ma. Limited eruption of boninitic lavas may have continued until 55.5 ± 0.9 Ma, based on the Ar-Ar age we obtained. The duration of arc magmatism at Troodos (at least 16 m.y., with some activity perhaps extending 35 m.y.) without the development of a mature arc edifi ce greatly exceeds that of other well-studied suprasubduction-zone ophiolites. We propose that Troodos was formed over a newly formed subduction zone, similar to many proposed models, but that the extended period of magmatism (boninitic) resulted from a prolonged period of ridge subduction.
Lotic dragonflies and damselflies are expected to be more affected by vicariance than lentic sister species. We demonstrated that severe vicariant speciation acted on lotic Coeliccia in contrast to lentic Copera damselflies, which are both included in the family Platycnemididae. We constructed maximum likelihood and Bayesian inference trees of these Platycnemididae species from the continental islands of Ryukyu (Amami, Okinawa, and Yaeyama islands), Taiwan, and Japan relative to Chinese species using raxmlGUI and BEAST, based on the mitochondrial COI gene (682 bp), COII gene (494 bp), 16SrRNA (478 bp), and the nuclear 28SrRNA gene (807 bp). In BEAUti, we calibrated the splitting age of the MRCA of all the Coeliccia species as 1.55−0.15 million years ago (Ma), a date that corresponds to a geologic constraint: the Okinawa trough and associated straits, including the Yilan basin in Taiwan, began to rift at 1.55 Ma, isolating the Ryukyu-Taiwan islands from the Chinese continent. The vicariance split Coeliccia into the Ryukyu-side clade of Coeliccia ryukyuensis (Coe. r. ryukyuensis in Okinawa and Coe. r. amamii in Amami) and Coeliccia flavicauda (Coe. f. masakii in Yaeyama and Coe. f. flavicauda in southern Taiwan), and the Chinese-side clade of Coeliccia cyanomelas (northern Taiwan and China), separated by the Okinawa trough. These Coeliccia species were further deeply differentiated to form local populations on the major islands and some of the minor islands. The Copera clade constituted a sister of the lotic Coeliccia clade, but genetic differentiation was not recognizable in lentic Copera between China, Taiwan, and Japan. Base substitution rates applying a strict clock model were estimated for COI: 0.0783, COII: 0.0803, 18SrRNA: 0.0186, 28SrRNA: 0.00577, and combined: 0.0408 substitutions/site/myr, and these rates are relatively high.
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