First Discovery of a Nonophiolite-Type Spinel Lherzolite Xenolith in the Back-Arc Basin of the Mariana Island Arc System
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NE GEOLOGY
First Discovery of a Nonophiolite-Type Spinel Lherzolite Xenolith in the Back-Arc Basin of the Mariana Island Arc System A. V. Koloskova, *, V. A. Rashidova, **, and V. V. Ananyeva, *** a
Institute of Volcanology and Seismology, Far Eastern Branch, Russian Academy of Sciences, Petropavlovsk-Kamchatsky, Russia *e-mail: kolosav@kscnet **e-mail: [email protected] ***e-mail: [email protected] Received January 12, 2019; revised May 16, 2019; accepted December 16, 2019
Abstract—The first sampling of mantle horizons underlying the ophiolite peridotite complex of the Mariana back-arc basin was carried out based on a spinel lherzolite xenolith in basanites from the submarine Esmeralda Volcano. The paper shows significant differences between the bulk, petrological, geochemical, and mineralogical compositions of this xenolith and similar characteristics of ophiolite peridotites dredged from the Central Trough of the north part of the Mariana Trench, on the west wall of one of the fault zones of the Santa Rosa Bank Fault (southern part of the trough), and ultramafic rocks dredged from the Conical and Torishima seamounts in the fore-arc part of the Mariana and Idzu-Bonin Island Arc Systems. Using different geothermometers and geobarometers, we revealed the PT conditions for the formation of Esmeralda xenolith’s mineral paragenesis. The optimal temperature is 950–1050°C and the pressure is 13–15 kbar. These parameters significantly exceed those suggested for the compared ophiolite peridotites. Keywords: lherzolite xenolith, back-arc basin, ophiolite peridotites, Mariana Island Arc System, petrological, geochemical, and mineralogical compositions, geothermometers and geobarometers DOI: 10.1134/S0001437020040128
INTRODUCTION Reported finds of peridotites in back-arc basins are limited to data on the Mariana Trench [17, 28, 32, 43, 44], the Parece Vela basin [31], and the Sea of Japan [15, 30]. In addition, there are known finds of peridotite xenoliths, primarily spinel lherzolites, in alkaline basalts on islands and submarine volcanoes in the shelf zone of some marginal marine basins: the South China Sea [3, 4, 9] and the Bering Sea region on the islands of Pribylov and Nunivak [19, 21, 37]. However, only a few studies present results of their detailed petrological study [3, 28, 33]. There are also descriptions of rare finds of spinel lherzolites in the ophiolite complexes of Mamonia, southwestern Cyprus [1], and Troodos in Cyprus [12]. Here, the lherzolites are closely associated with the prevailing harzburgites and, according to [1, 12], were the result of fractional melting of a mantle source close in composition to a MORB source. In this paper, we present the results of a study on a xenolith sample in basanite, which we will call Esmeralda, recovered during dredging of the active submarine Esmeralda Volcano in the southern part of the Mariana back-arc basin on cruise 4 of the R/V Vulkanolog [2] (Fig. 1).
Esmeralda Volcano is the central edifice of a submeridional submarine volcanic massif. The two other volcanic str
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