Megacrysts of “bubbled” kaersutite in the Neogene-Quaternary of Western Syria: evidence of crystallization in a boiled melt/fluid?

<p>The territory of Syria is a classic area of intraplate Neogene-Quaternary plateau basaltic magmatism (Ponikarov et al., 1969; Sharkov, 2000; Lustrino, Sharkov, 2006; Trifonov et al., 2011, etc.). These basalts belong to the Afro-Arabian large igneous province (LIP) (Ernst, 2014), whose origin, according to geophysical data, is related to the ascent of a mantle thermochemical plume that originated at the liquid iron core-silicate mantle boundary of (Hansen et al., 2012).</p><p>The basalt plateaus of Syria have a similar structure and are formed by numerous basaltic flows, as well as scoria and pyroclastic cones, often containing mantle xenoliths. Approximately 80% of them are represented by green spinel lherzolites and harzburgites, and subordinate amount (~20 %) of xenoliths belong to black series (hornblendite, hornblende clinopyroxenites, clinopyroxenites, phlogopitites, etc., as well as megacrysts of kaersutite, clinopyroxene, ilmenite, sanidine, etc.). Some of the kaersutite megacrysts have unusual “bubbled” structure, containing oval cavities up to 3-4 mm in diameter. We believe that these xenoliths are fragments of the upper cooled margin of the mantle plume above the adiabatic melting zone (Sharkov et al., 2017). Thus, they probe substance of mantle plume and bear important information about the processes within its interior.</p><p>As previously shown (Sharkov et al., 2017), the black series rocks were formed from a melt/fluid released fluid during the incongruent ("secondary") melting of the mantle plume head at the final stage of the magmatic system evolution. The crystallization of this fluid-supersaturated melt could be accompanied by its retrograde boiling, which led to the appearance of "bubbled" crystals.</p><p> </p>

Sedimentary and volcanic record of the nascent Izu-Bonin-Mariana arc from IODP Site U1438

The oldest known, intact sedimentary record of a nascent intraoceanic arc was recovered in a ∼100-m-thick unit (IV) above ca. 49 Ma basaltic basement at International Ocean Discovery Program Site U1438 in the Amami Sankaku Basin. During deposition of Unit IV the site was located ∼250 km from the plate edge, where Izu-Bonin-Mariana subduction initiated at 52 Ma. Basement basalts are overlain by a mudstone-dominated subunit (IVC) with a thin basal layer of dark brown metalliferous mudstone followed by mudstone with sparse, graded laminae of amphibole- and biotite-bearing tuffaceous sandstone and siltstone. Amphibole and zircon ages from these laminae suggest that the intermediate subduction-related magmatism that sourced them initiated at ca. 47 Ma soon after basement formation. Overlying volcaniclastic, sandy, gravity-flow deposits (subunit IVB) have a different provenance; shallow water fauna and tachylitic glass fragments indicate a source volcanic edifice that rose above the carbonate compensation depth and may have been emergent. Basaltic andesite intervals in upper subunit IVB have textures suggesting emplacement as intrusions into unconsolidated sediment on a volcanic center with geochemical and petrological characteristics of mafic, differentiated island arc magmatism. Distinctive Hf-Nd isotope characteristics similar to the least-radiogenic Izu-Bonin-Mariana boninites support a relatively old age for the basaltic andesites similar to detrital amphibole dated at 47 Ma. The absence of boninites at that time may have resulted from the position of Site U1438 at a greater distance from the plate edge. The upper interval of mudstone with tuffaceous beds (subunit IVA) progresses upsection into Unit III, part of a wedge of sediment fed by growing arc-axis volcanoes to the east. At Site U1438, in what was to become a reararc position, the succession of early extensional basaltic magmatism associated with spontaneous subduction initiation is followed by a rapid transition into potentially widespread subduction-related magmatism and sedimentation prior to the onset of focused magmatism and major arc building.

Silicic volcanism in the Scottish Lower Carboniferous; lavas, intrusions and ignimbrites of the Garleton Hills Volcanic Formation, SE Scotland

Extensional tectonics and incipient rifting on the north side of the Iapetus suture were associated with eruption of (mainly) mildly alkaline olivine basalts. Initially in the Tournaisian (Southern Uplands Terrane), magmatic activity migrated northwards producing the Garleton Hills Volcanic Formation (GHVF) across an anomalous sector of the Southern Uplands. The latter was followed by resumption of volcanism in the Midland Valley Terrane, yielding the Arthur's Seat Volcanic Formation. Later larger-scale activity generated the Clyde Plateau Volcanic Formation (CPVF) and the Kintyre lavas on the Grampian Highlands Terrane. Comparable volcanic successions occur in Limerick, Ireland. This short-lived (c. 30 myr) phase was unique in the magmatic history of the Phanerozoic of the British Isles in which mildly alkaline basaltic magmatism locally led to trachytic differentiates. The Bangly Member of the GHVF represents the largest area occupied by such silicic rocks. The most widespread lavas and intrusions are silica-saturated/oversaturated trachytes for which new whole-rock and isotopic data are presented. Previously unrecognized ignimbrites are described. Sparse data from the fiamme suggest that the magma responsible for the repetitive ignimbrite eruptions was a highly fluid rhyolite. The Bangly Member probably represents the remains of a central-type volcano, the details of which are enigmatic.