Литосфера (Jan 2016)
Xenoliths in the alkali basalts of Makhtesh Ramon of Desert Negev (Israel) as indicators of mantle metasomatosis and magma generation
Abstract
Xenoliths in the Early Cretaceous alkali basalts of Makhtesh Ramon basin (southern Israel) are represented by essentially olivine rocks: dunite (usually including clinopyroxen) - 5% of the total amount, lherzolite - 21%, wehrlite - 28%, clinopyroxenite - 34%, gabbro - 12 %. According #Mg = Mg/(Mg + Fe), xenolith rocks form several discrete groups corresponding to the following values #Mg: >0.85 (dunite, lherzolite, some wehrlites), 0.85-0.75 (wehrlite, olivine clinopyroxenite), 0.75-0.65 (olivine clinopyroxenite, clinopyroxenite), 0.60-0.45 (gabbro). Primary mantle rocks are represented by lherzolite, other xenoliths are the products of metasomatism, which preceded and accompanied magma generation. The main minerals of ultramafic xenoliths- olivine slightly enriched by CaO, clinopyroxene with varying content of TiO2 (1-4%), Al2O3 (2-12%), Na2O (0.5-2%) and #Mg = 0.92-0.59, spinelids: chromite (Cr2O3 = 20-38%), Al spinel and titanomagnetite (TiO2 = 10-21%, Cr2O3 = 0.3-8%, Al2O3 = = 1.5-13%, MgO = 2-7%). Rich inTiO2, Al2O3 and Na2O clinopyroxene together with plagioclase, anorthoclase, kaersutite, rhenite, ilmenite, “orthopyroxene” and “feldspar” glass веlong to late ultramafic paragenesic associated with the process of partial melting. Orthopyroxene in ultramafic rocks is unstable and usually is replaced by minerals of the late paragenesis. Gabbroic xenoliths consist of low-titanium and low-aluminiferous clinopyroxene (#Mg = 0.66-0.56), orthopyroxene (#Mg ≈ 0.5), plagioklase An45-55, often with rims of anorthoclase, titanomagnetite of the same composition as in the ultramafic rocks, ilmenite. Xenoliths have the signs of partial melting and metasomatic transformation preсеding to melting. As a result the orthopyroxene from lherzolite is replaced by clinopyroxene. This leads to the width development of wehrlites and olivine clinopyroxenites. During metasomatosis the content of Mg, Cr and Ni falls while Ti, Fe, Al, Ca grows, as well as the content of large ion lithophile an high-strength elements providing increase of fertility of basalt magmatic source. Composition of the produced melt is close to basanite. The glass, cementing crystallization products, preserved in xenoliths, has a composition close to the orthopyroxene-feldspar mixtures. The mineral phases in such glass presented by clinopyroxene, kaersutite, rhenite, plagioclase, anorthoclase, nepheline, titanomagnetite and ilmenite.
