Origin of carbonatites of the Matcha alkaline pluton from Turkestan-Alai ridge, Kyrgyz Southern Tien Shan

Postorogenic alkaline intrusions in the Turkestan-Alai segment of the Southern Tien Shan coexist with dikes and veins of carbonatites dated at ~220 Ma. They are primarily composed of calcite and dolomite (60–85 %), as well as sodic amphibole, phlogopite, clinopyroxene, microcline, albite, apatite, and magnetite, with accessory niobate, ilmenite, Nb-rutile, titanite, zircon, baddeleyite, monazite-(Ce), barite, and sulfides. The rocks share mineralogical and geochemical similarity with carbonatites that originated by liquid immiscibility at high temperatures above 500°C. Silicate and salt-carbonate melts are derived from sources with mainly negative bulk εND(t) ~ from −11 to 0 and high initial 87Sr/86Sr ratios (~ 0.7061-0.7095) which may be due to mixing of PREMA and EM-type mantle material. Pb isotopic ratios in accessory pyrrhotite (206Pb/204Pb = 18.38; 207Pb/204Pb = 15.64; 208Pb/204Pb = 38.41) exhibit an EM 2 trend. The intrusions bear signatures of significant crustal contamination as a result of magma genesis by syntexis and hybridism. Concordant isotope composition changes of δ 13C (−6.5 to −1.9 ‰), δ 18O (9.2-23 %"), δD (−58 to −41 %"), and δ 34S (12.6-12.8 ‰) in minerals and rocks indicate inputs of crustal material at the stage of melting and effect of hot fluids released during dehydration of metamorphosed oceanic basalts or sediments. The observed HFSE patterns of the oldest alkaline gabbro may be due to interaction of the primary mafic magma with IAB-type material. The isotope similarity of alkaline rocks with spatially proximal basalts of the Tarim large igneous province does not contradict the evolution of the Turkestan-Alai Triassic magmatism as the «last echo» of the Tarim mantle plume

Геохимическое сходство кембрийского щелочного и субщелочного магматизма (Кузнецкий Алатау, Сибирь): синтез новых данных

This is a synthesis of recent geochemical evidence from Cambrian granitic, gabbro-monzonitic, and few small gabbro-foidolitic intrusions in the Early Paleozoic Kuznetsk Alatau orogen (western Central Asian Orogenic Belt, Siberia). Their origin is attributed to late- and post-orogenic regional magmatism which produced different rock types sharing basic similarity in isotope systematics and trace-element compositions. The εNd(t) values of ~2 to 8.7 in main igneous lithologies record the extent of interaction between depleted and enriched mantle. Concurrent increase of initial 87Sr/86Sr (0.7039–0.7058) ratios and δ18О values (6.5–13 ‰) in rocks and minerals, as well as 207Pb enrichment (207Pb/204Pb(t) = 15.5–15.7), indicate crustal contamination of mantle melts. Trace-element patterns in the rocks correspond to a mixed IAB + OIB source. The alkaline and subalkaline rocks of the Kuznetsk Alatau area share trace element and isotopic similarity and apparently emplaced almost synchronously late during a regional-scale tectonic event. The synthesized data show that Cambrian magmatism in the area involved mixed mantle and continental crust melt components and acted in a complex tectonic setting of the former active margin of the Paleo-Asian Ocean impacted by a mantle plume

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

Parental sources of high-alumina alkaline melts: Nd, Sr, Pb, and O isotopic evidence from the Devonian Kiya-Shaltyr gabbro-urtite intrusion, South Siberia

The isotope geochemistry (Nd(t) 4.8–5.4, 206Pb/204Pbin 18.05–18.36, 207Pb/204Pbin 15.53–15.57, 208Pb/204Pbin 37.59–37.83, 87Sr/86Sr(t) 0.7048–0.7057, 18OSMOW 8–10.5‰) and trace element compositio of the Kiya–Shaltyr gabbro–urtite pluton allow us to suggest a heterogeneous source and complex geodynamic settings of the Devonian alkali magmatism in the Kuznetsk Alatau. It is assumed that its evolution took place under conditions of partial mingling of matter of the depleted (PREMA) and enriched (EM) mantle with crustal contamination of the evolving melt. Such an interaction could have been a result of superposition of a mantle plume and an active margin (IOB and IAB components). In fold belts this led to the formation of hybrid high-alumina foidoite magmas

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

Composition and origin of rare-metal (Nb–Ta, REE) and sulfide mineralization in magnesiocarbonatites from the Yenisei Ridge, Central Siberia

The Penchenga Neoproterozoic linear fenite-carbonatite complex in the Yenisei Ridge consists of several magnesiocarbonatite sheet-like bodies which lie nearly concordantly with the metamorphic country rocks and are surrounded with alkaline metasomatic fenite aureoles. Their mineralogy includes primary high-temperature phases (ferrodolomite, calcite, phlogopite, sodic amphibole of the eckermannite – magnesio-arfvedsonite series, fluorapatite, magnetite, and ilmenite), late-magmatic accessories (fluorocalciopyrochlore, pyrrhotite, pyrite, Cu-Pb-Zn-Ag sulfide and hessite, monazite-(Ce) and REE-carbonates of synchysite, cordylite, cebaite, and ancylite), as well as secondary phases (Sr-Ba pyrochlore, ferrocolumbite, fersmite, and Fe and Sr carbonates) of hydrothermal and supergene origin that replace primary magmatic minerals. According to their main features of mineralogy and chemistry, the carbonatites correspond to magnesian derivates of a high-pressure alkali-dolomite melt that forms by melting of carbonated peridotite, as obtained in experiments. The Nd, Sr, and Pb isotope compositions of the Penchenga rocks and minerals (εNd ≈ +6; εSr ≈ −19; ∼0.81 207Pb/206Pb; ∼1.99 208Pb/206Pb) suggest a mantle PREMA + EM 2 source of carbonatite magma and mineralization (including Nb and REE). The presence of carbonates with relatively high δ18O and pyrrhotite enriched in δ34S and 207Pb may be due to continental crust inputs to the source of metals. As estimated approximately from the composition of the pyrrhotite-sphalerite assemblage, sulfide minerals crystallize at a depth of ∼30 km in the lower crust, under a pressure of ∼8 kbar. Degassing of the juvenile alkali-dolomite melt and its interaction with meteoric waters leads to fenitization and formation of late magmatic CO2–H2O fluids. These “carbohydrothermal” fluids form from salt melts, and increase in H2O upon cooling from 600 to 200 °C. At lower temperatures of 480–240 °C, the O–H isotope systems of magmatic minerals undergo inversion and ensuing closure. REE-bearing mineral phases precipitate from REE-carrier fluids