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

Abstract

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 δ13C (−6.5 to −1.9 ‰), δ18O (9.2-23 %„), δD (−58 to −41 %„), and δ34S (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.