Sources and geodynamic setting of petrogenesis of the Middle Cambrian Upper Petropavlovka alkaline basic pluton (Kuznetsk Alatau, Siberia)

Early Paleozoic alkaline basic magmatism in the Kuznetsk Alatau is manifested in the Upper Petropavlovka pluton of gabbro, feldspathoid rocks (theralites, mafic foidolites, and nepheline syenites), and Ca-carbonatites. According to Sm-Nd and Rb-Sr isotope data, the pluton formed in the Middle Cambrian (509+ or -10 Ma). The silicate igneous rocks correspond in the contents of silica, alumina, and alkalies to derivates of a K-Na alkaline basic association. The Ca-carbonatites are characterized by a high-temperature (600-900 degrees C) paragenesis of apatite, clinopyroxene, ferromonticellite, phlogopite, and magnetite. They are enriched in P (sub 2) O (sub 5) (up to 6.4 wt.%), Sr (up to 3000-4500 ppm; Sr/Ba approximately 5-7), and REE + Y (up to 800 ppm) and show evidence for liquation genesis. The predominant magmatic source (epsilon Nd(T) = 5-7) was moderately depleted PREMA, possibly combined with E-MORB and EM. According to the isotopic data (( (super 87) Sr/ (super 86) Sr) (sub T) approximately 0.7024-0.7065; delta (super 18) O approximately 6.3-15.5 ppm; delta (super 18) C approximately -3.5 to -2.0 ppm), the fractionation of the melts was accompanied by their crustal contamination. The trace-element composition of the mafic rocks testifies to the participation of a substance similar to the substrata of the parental magmas of MORB, IAB, and OIB in the magma generation. This suggests intrusion in the geodynamic setting of interaction between the active continental margin and an ascending mantle diapir. Most likely, the intrusion led to the mixing of material from different sources, including the components of PREMA, enriched suprasubduction lithospheric mantle (EM), and continental crust. The assumption is made that the complexes of highly alkaline rocks and carbonatites in the western Central Asian Fold Belt are of plume origin and belong to an Early Paleozoic large igneous province.