New insights into deep carbon recycling and formation of nepheline-bearing alkaline rocks from Sr-Nd-Mg isotope compositions

Magnesium isotopes are widely used to trace recycled carbonates in the mantle source. We recently recognized extremely light Mg isotope values (δ26Mg = −0.50 −0.62‰) nepheline syenites Tarim large igneous province (TLIP), NW China. To evaluate significance of isotopes, we conducted petrological, mineral chemical, zircon U-Pb ages, and geochemical (Sr-Nd-Mg) analyses on syenite understand its petrogenesis. Laser ablation−inductively coupled plasma−mass spectrometry dating yields an age 272.5 ± 1.4 Ma for syenite. Petrographic studies show that nephelinite TLIP display similar assemblages, clinopyroxene Sr compositions bulk-rock Nd (87Sr/86Sr(i) 0.70364−0.70399, εNd(t) +3.51 +4.49 versus 87Sr/86Sr(i) 0.70348−0.70371, +3.28 +3.88 nephelinite, respectively), indicating they possibly co-magmatic. Rhyolite-MELTS modeling shows formed from by fractional crystallization spinel, olivine, clinopyroxene, apatite, biotite. In combination with information previous studies, correlated magnesium “genetic genes” −0.35 −0.55‰) which were produced reaction between peridotite carbonated silicate melt derived eclogite. invoke a three-stage model genesis TLIP. Initially, subduction oceanic crust delivered sedimentary carbonate rocks into deep The melting eclogite reacted ambient form primary nephelinitic magma. Finally, during ascent Our study provides insights implication carbon recycling origin alkaline rocks.