“Roots” of magmatic systems in the large continental igneous provinces

Abstract

Large igneous provinces (LIPs) are known from theArchean and were widely abundant in the Phanerozoic, when they formed large fields of flood basalts(traps) on the continents and in the oceans [10].According to the data available, the evolution of thePhanerozoic LIPs in most cases started from the continental riftogenesis accompanied by eruptions of midalkaline Fe–Ti basalt and picrobasalt, which are replacedby tholeiitic basalt at the second stage [5, 10, 11].The appearance of a LIP results from the rise ofmantle superplumes formed at the liquid core–mantleboundary [3, 10, 14]. However, the concrete state oftheir material and the conditions of its adiabatic melting are not clear. The main source of objective information may be provided by the mantle xenoliths inbasalt and basanite, which are fragments of the material of the front part of the plume above the zone ofadiabatic melting, where the primary magmas areformed. Our study is mainly aimed at concretizationof the ideas on the structure of the front parts of mantleplumes, their material, the level of penetration into thelithosphere, and the conditions of magma generationfrom an example of mantle xenoliths in basalt of theQuaternary TellDanun Volcano, Harrat Ash Samahvolcanic plateau adiabatic, South Syria [8].STRUCTURE OF THE FRONT PART OF THE PLUMEAs is evident from the data available on the structure of Phanerozoic LIPs, melting was localized in thefront parts of secondary plumes, “protuberances” onthe surface of the mantle superplume. Most likely, theirrise resulted from the local accumulation of fluid components providing additional floatage of the material.According to the data of [7], the diffluent plumehead should have a triangular crosssectional shapewith the appearance of a chamber of adiabatic meltingin its upper part (Fig. 1). The intruding newly formedmagma mostly removes fragments of the upper, cooledmarginal part of the plume. These are mantle xenolithsobserved in alkaline (subalkaline) Fe–Ti basalt andbasanite in the modern rift systems [6]. This materialdoes not come from the melting zone, but carriesimportant information on the material of mantleplumes, its phase composition, and components participating in melting. Most populations do not containxenoliths of the lower crustal rocks, which distinguishes these rocks from kimberlite pipes [6] and mayshow that the frontal parts of the local (secondary)mantle plumes reached the base of the sialic uppercrust.MANTLE XENOLITHS IN BASALT AND BASANITE OF SOUTH SYRIAThe populations of mantle xenoliths in basalt arecharacterized by surprising sameness and are dividedinto two major types: (1) the most abundant rocks ofthe “green, or Crdiopside” series and (2) more occasional rocks of the “black, or Al–Tiaugite” series,which often form veins in rocks of the “green” series[6]. The composition of these populations practicallydoes not depend on a concrete region [6], which provides evidence for the uniform nature of melting mantle substrates across the Earth. Most likely, this is thematerial of the mantle thermochemical plumes risingfrom the liquid core–mantle boundary and formed bythe silicate mantle material, as well as by fluid components of the core [1, 3, 14]. We studied these xenolithsin the example of the abovementioned QuaternaryTellDanun Volcano [8].Xenoliths of the “green” series are characterized bythe presence of Crdiopside and a very constant highMg composition of olivine and pyroxenes (Mg# =88–92). They are mostly represented by spinel peridotite with cataclastic, rarely protogranular texturewith the composition varying from predominant spinellherzolite to depleted spinel harzburgite and occasional spinel pyroxenite (mostly websterite). The latterforms veins in the peridotite matrix and is often characterized by the presence of thin kelyphytic rims of