Geochemistry and geochronology of the Troodos ophiolite: An SSZ ophiolite generated by subduction initiation and an extended episode of ridge subduction?

Abstract

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.

Geochemistry of Vein Calcites Hosted in the Troodos Pillow Lavas and Their Implications for the Timing and Physicochemical Environment of Fracturing, Fluid Circulation, and Vein Mineral Growth

“…Considering the onefold crack and sealing characteristic of completely sealed syntaxial veins, both, fracturing and simultaneous precipitation may be dated (Ramsay, 1980;Roberts & Walker, 2016). If intersection ages approximate the timing of the earliest host pillow lava formation (≥90 Ma; Mukasa & Ludden, 1987;Osozawa et al, 2012), fracturing independent of the growth mechanism may be dated as well, because veining postdates pillow lava crystallization. Low Rb concentrations of calcite (93% of all laser ablation spot analyses <1 ppm Rb or even below detection limit, remaining spots mostly <4 ppm) do not require the calculation of initial 87 Sr/ 86 Sr ratios.…”

Microtextures and fluid inclusions from vein minerals hosted in the Pillow Lavas of the Troodos supra-subduction zone

“…Pillow lava extrusion spans a wide time range from 91 to 75 Ma with some limited depleted boninite eruptions until 56 Ma along the southwestern flank of the Troodos ophiolite (Osozawa et al, 2012). It is therefore supposed that veins and vesicles have formed within this temporal framework.…”

“…75 Ma although ca. 56 Ma old depleted boninite eruptions provide a larger time range (Osozawa et al, 2012). The lack of volcanoclastic material indicates that mature arc volcanism was absent .…”

From ocean depths to mountain tops: Uplift of the Troodos ophiolite (Cyprus) constrained by low‐temperature thermochronology and geomorphic analysis

“…The Late Cretaceous average zircon (U‐Th)/He ages and the combined zircon (U‐Th)/He isochron age of 92.6 ± 1.4 Ma obtained for the Marathasa Valley samples are similar within error to the U‐Pb zircon crystallization age of 91.6 ± 1.4 Ma obtained for two plagiogranite samples [ Mukasa and Ludden , ] and Ar‐Ar step heating age of 90.6 ± 1.2 Ma obtained for fresh volcanic glass from the pillow basalts [ Osozawa et al ., ]. Given the difference in closure temperatures for the U‐Pb and (U‐Th)/He systems in zircon (800–1000°C and 140–200°C, respectively) [ Cherniak and Watson , ; Guenthner et al ., ; Reiners et al ., ], this similarity indicates early cooling of the plagiogranite plutons following their emplacement in Late Cretaceous time.…”

“…Our data from the Marathasa Valley indicate that deep crustal levels in the vicinity of the Troodos spreading axis were already cold several million years after the ophiolite formation and thus highlights that subduction, rollback related extension, and magmatism were short lived (≤10 Myr). The late Cretaceous cooling envisaged here for the Troodos oceanic crust conflicts with limited previous K‐Ar and Ar‐Ar data, which were interpreted to suggest that voluminous magmatism in Troodos persisted until circa 75 Ma or even until circa 55 Ma, possibly in incipient volcanic arc setting [ Delaloye et al ., ; Osozawa et al ., ]. Widespread post ~90 Ma magmatism should have reset the (U‐Th)/He system in zircon, due to its relatively low closure temperature (~140–190°C) [ Guenthner et al ., ; Reiners et al ., ], but such a reset was not observed in most of the samples.…”