Olivine and Cr-spinel as indicators of the petrogenesis and partial melting conditions of the high-MgO ultramafic volcanic rocks from NW Ad Dhala Province—Yemen

The high-MgO ultramafic volcanic rocks in the NW Ad Dhala province are classified as meimechite according to the IUGS classification scheme.This province represents the southeastern outcrops of the Yemen Volcanic Group(YVG),which constitutes part of the AfroArabian continental large igneous province(LIP) and located within the boundary of the Afar mantle plume.In this study,we present the chemical compositions of olivine and Cr-spinel in meimechite rocks from Bagah Village in NW Ad Dhala province,aiming to characterize the genesis and partial melting conditions and to estimate the crystallization temperatures of these high-MgO rocks.Olivine crystals are characterized by high forsterite,ranges from Mg-rich core(up to Fo_(89.69)) to relatively Fe-rich rim(down to Fo_(78.57)),high CaO,MgO and MnO whereas Cr-spinel crystals have high TiO_(2) and Cr# values ranging from 0.49 to 0.63 which indicate that they are crystallized from primary magma and are typical of volcanic olivine and Crspinel that formed in intraplate tectonic setting.Olivine and Cr-spinel compositional data and primary melt composition(MgO ~ 23 wt.%) are compatible with the derivation of studied meimechite rocks from peridotite mantle source by small degrees of partial melting under conditions of high temperature and pressure at great depths,mostly within the garnet stability field.Also,these data provide a compelling indicator for the important role of upwelling Afar mantle plume in the genesis of these high-MgO ultramafic volcanic rocks.Finally,based on the different olivine-liquid equilibrium methods and Al-in-olivine thermometer approach the estimated crystallization temperature ranges from 1450 to 1490℃,and mantle potential temperature(Tp) ranges from 1617 to 1677℃,at high pressure(3-4.8 GPa).These high temperatures substantiate the existence of the Afar thermal mantle plume and its important role in the genesis of the studied meimechite rocks.

Chemical Characteristics of Cr-spinel of Chromites in Kuda Ophiolite of Western Kunlun and Its Geological Implications

The Kuda ophiolite belongs to the early Paleozoic ophiolite belt in Western Kunlun Mountains and is composed of metamorphic peridotites,cumulate peridotites,mafic volcanic rocks and quartzites(Li et al.,

Opaque Mineralogy as a Tracer of Magmatic History of Volcanic Rocks: an Example from the Neogene-Quaternary Harrat Rahat Intercontinental Volcanic Field, North Western Saudi Arabia

The Neogene-Quaternary Harrat Rahat volcanic field is part of the major intercontinental Harrat fields in western Saudi Arabia. It comprises lava flows of olivine basalt and hawaiite, in addition to mugearite, benmorite, and trachyte that occur mainly as domes, tuff cones and lava flows. Based on opaque mineralogy and mineral chemistry, the Harrat Rahat volcanic varieties are distinguished into Group I （olivine basalt and hawaiite） and Group II （mugearite, benmorite and trachyte）. The maximum forsterite content （-85） is encountered in zoned forsteritic olivine of Group I, whereas olivine of Group II is characterized by intermediate （Fo=50）, fayalitic （Fo=25） and pure fayalite in the mugearite, benmorite and trachyte, respectively. The more evolved varieties of Group II contain minerals that show enrichment of Fe2＋, Mn2~ and Na~ that indicates normal fractional crystallization. The common occurrence of indicates that pS＋ becomes saturated in this coarse apatite with titanomagnetite in the benmorite rock variety and drops again in trachyte. Cr-spinel is recorded in Group I varieties only and the Cr# （0.5） suggests lherzolite as a possible restite of the Harrat Rahat volcanics. The plots of Cr# vs. the forsterite content （Fo） suggest two distinct trends, which are typical of mixing of two basaltic magmas of different sources and different degrees of partial melting. The bimodality of Harrat Rahat Cr-spinel suggests possible derivation from recycled MORB slab in the mantle as indicated by the presence of high-Al spinel. It is believed that the subcontinental lithospheric mantle was modified by pervious subduction process and played the leading role in the genesis of the Harrat Rahat intraplate volcanics. The trachytes of the Harrat Rahat volcanic field were formed most probably by melting of a lower crust at the mantle-crust boundary. The increase in fo2 causes a decrease in Cr203, and A1203, and a strong increase in the proportion of Fe3＋ and Mg# of spinel crystallizing from the basaltic melt at T -1200~C. The olivine-pyroxene and olivine-spinel geothermometers yielded equilibrium temperature in the range of 935-1025~C, whereas the range of 〈500-850~C from single-pyroxene thermometry indicates either post crystallization re- equilibrium of the clinopyroxene, or the mineral is xenocrystic and re-equilibrated in a cooling basaltic magma.