Early Mesozoic lamproites and monzonitoids of southeastern Gorny Altai: geochemistry, Sr–Nd isotope composition, and sources of melts

Small intrusions of lamprophyres and lamproites (Chuya complex) and K-monzonitoids (Tarkhata and Terandzhik complexes) are widespread in southeastern Gorny Altai. Geochronological (U-Pb and Ar-Ar) isotope studies show their formation in the Early-Middle Triassic (~ 234-250 Ma). Lamproites have been revealed within two magmatic areas and correspond in geochemical parameters to the classical Mediterranean and Tibet orogenic lamproites. According to isotope data ((87Sr/86Sr)T = 0.70850-0.70891, (143Nd/144Nd)T = 0.512157-0.512196, 206Pb/204Pb = 17.95-18.05) and Th/La and Sm/La values, the Chuya lamproites and lamprophyres melted out from the enriched lithospheric mantle with the participation of DM, EM1, EM2, and SALATHO. The monzonitoid series of the Tarkhata and Terandzhik complexes are similar in petrographic and geochemical compositions but differ significantly in Sr-Nd isotope composition: The Tarkhata monzonitoids are close to the Chuya lamproites, whereas the Terandzhik ones show a higher portion of DM ((87Sr/86Sr)T = 0.70434-0.70497, (143Nd/144Nd)T = 0.512463-0.512487) in their source, which suggests its shallower depth of occurrence and the higher degree of its partial melting as compared with the derivates of the Chuya and Tarkhata complexes. The studied rock associations tentatively formed in the postcollisional setting under the impact of the Siberian superplume

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 ± 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 ºC) paragenesis of apatite, clinopyroxene, ferromonticellite, phlogopite, and magnetite. They are enriched in P2O5 (up to 6.4 wt.%), Sr (up to 3000–4500 ppm; Sr/Ba ~ 5–7), and REE + Y (up to 800 ppm) and show evidence for liquation genesis. The predominant magmatic source (εNd(T) = 5–7) was moderately depleted PREMA, possibly combined with E-MORB and EM. According to the isotopic data ((87Sr/86Sr)T ~ 0.7024–0.7065; δ18O ~ 6.3–15.5‰; δ18C ~ –3.5 to –2.0‰), 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. © 2015, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved