Study On The Petrogenesis Of The Aillikites In The Tarim Large Igneous Province

Aillikites are carbonate-rich ultramafic lamprophyres.Although they are volumetrically minor components in some large igneous province?LIP?,these rocks provide important clues to melting and metasomatism in the deep mantle domain during the initial stages of LIPs.This study reports the aillikites exposed in the Wajilitag complex in the Tarim LIP.The Wajilitag carbonate-rich ultramafic lamprophyres,located in the northwestern TLIPs,were previously recognized as kimberlites.These rocks are reclassified as aillikites,based on the occurrence of micro-phenocrysts of amphibole and clinopyroxene,as well as the presence of abundant Ti-rich andradite garnet.Olivines in these aillikites can be divided into two textural types,namely,?i?<5mm microcrysts in matrix??,?,and?a?,which are common in all aillikites;and?ii?0.5-2.5 cm anhedral macrocrysts??b?in matrix,which only occur in the most evolved aillikite dyke and anhedral macrocrysts in dunite and Ol-bearing clinopyroxenite nodules??and??.The microcrysts are phenocrysts,whereas the macrocrysts are probably Fe-rich antecrysts produced from early evolved aillikite melts.Olivine microcrysts in the more primitive aillikite dykes?Dyke 1?display relatively higher forsterite values(Fo_82-89)and normal mantle-like O isotope ratios(average?¹⁸O=4.9±0.22‰),whereas those in the more evolved dykes such as Dyke 2 and 3 exhibit lower Fo_79-87 and lower-than-mantle O isotope compositions(average?¹⁸O=4.72±0.27‰and 4.67±0.11‰,respectively).In addition,the decreasing?¹³C values(?¹³C=-4.5‰to-5.2‰)of carbonate from Dyke 1 through to Dyke 2 and 3,in combination with the indistinguishable Sr-Nd isotopes of these dykes,indicate that the lower?¹⁸O composition of olivine phenocrysts in Dyke 2 and 3 compared to the mantle-like?¹⁸O values of olivine in Dyke 1 probably resulted by carbonate melt/fluid exsolution from compositionally similar parental melts.These exsolved carbonate melts might have contributed to the formation of some of the Western Tarim carbonatites thus supporting a close petrogenetic relationship between aillikites and carbonatites.Finally,the mantle-like?¹⁸O values of olivine phenocrysts in Dyke 1,coupled with the mantle-like?¹⁸O values of cognate clinopyroxene(average?¹⁸O=5.78±0.11‰)and geochemically depleted bulk-rock Sr-Nd isotope compositions((⁸⁷Sr/⁸⁶Sri)=0.703669to 0.704179;?Nd?270?=+1.9 to+3.7)are indicative for a mantle peridotite source.Moreover,the high 100*Mn/Fe ratios?1.5-1.8?at high Ni contents?up to 3418 ppm?,overlapping with OIB olivine phenocrysts,and 100*Ni/Mg values??1?of high-Mg?Fo>85?olivine phenocrysts indicate the presence of phlogopite-and carbonate-rich metasomatic veins within the source.