Mantle-crustal nature of early Paleozoic alkaline intrusions in central Sangilen, Tuva (from Nd, Sr, Pb, C, and O isotope data)

The Paleozoic foidolite-foyaite plutons of the Sangilen upland (Bayan-Kol, Dakhu-Nur, Chik, and Kharly ones) might have formed in the Late Cambrian-Early Ordovician ( approximately 490-500 Ma, Sm-Nd and U-Pb); they are the result of the oldest alkaline magmatism in southeastern Tuva. The intrusion was accompanied by the formation of high-temperature (up to approximately 600-900 degrees C) endogenous carbonate rocks containing calcite, alkali pyroxene, Na-Ca amphibole, biotite, fluorapatite, microcline, and nepheline. Silicate and carbonate derivates were produced, most likely, from genetically related heterogeneous sources with epsilon Nd(T) varying from 3.0 to 6.3 and from -0.5 to 6.5, respectively, which might be due to the mixing of the depleted (PREMA) and enriched (EM) mantle materials. Initial ratios (super 207) Pb/ (super 206) Pb nearly equal 0.89 and (super 208) Pb/ (super 206) Pb nearly equal 2.15 in K-feldspar from calcitic rocks are close to those of EM 1. The correlation between the stable-isotope ratios (delta (super 18) O approximately 7.2-19.5, delta (super 13) C from -6.0 to -1.4 ppm) and the high (super 87) Sr/ (super 86) Sr(T) ratio (0.7057-0.7076) indicates a significant crustal contamination of magma in the upper horizons of the lithosphere and a minor impact of a meteoric fluid. The assumed synchronous formation of the studied plutons and other alkaline rock complexes of the Early Paleozoic Large Igneous Province in the west of the Central Asian Orogenic Belt as well as their isotope similarity do not rule out that the intrusion took place in the plume-lithosphere interaction setting.