Petrogenetic Link between Carbonatite and Silicate Rocks in the Wajilitage Carbonatite Complex: Insights from In Situ Geochemical and Isotope Analysis of Apatite and Calcite

The petrogenetic link between carbonatites and associated silicate rocks remains a longstanding debate in igneous petrology. Some minerals, such as apatite and calcite, which crystallize across diverse lithologies during magmatic differentiation, can record geochemical changes in their crystallizing environments, thereby providing valuable insights into the genesis of these rock suites. The Wajilitage Carbonatite Complex (WCC) in northwest China, part of the Tarim Large Igneous Province, is a typical carbonatite–alkaline complex composed of calcite and dolomite carbonatites, aillikite, nephelinite, and nepheline syenite. In this study, in situ geochemical and C–O isotope analyses of apatite and calcite are utilized to investigate their genetic relationships. The Mg content of apatite (Mgap) serves as an effective recorder of the magmatic evolution of this carbonatite–alkaline complex. Apatite in the carbonatite has higher Mg contents than apatite in the nephelinite and nepheline syenite, precluding an origin via fractional crystallization or liquid immiscibility from these silicate melts. Although the Mg contents of apatite in the carbonatites overlap with those of apatite phenocrysts in aillikite, their distinct trace-element compositions (e.g. La, Sr, and Y) and δ18OV-SMOW values rule out a direct genetic relationship between these rocks. We propose that the carbonatites formed independently by low-degree partial melting of a carbonated mantle source. Subsequent fractional crystallization of calcite and dolomite from this parental magma produced the calcite and dolomite carbonatites, respectively. Trace elements and δ18OV-SMOW values of apatite suggest that nephelinite evolved to nepheline syenite, the latter of which assimilated the aillikite. Differentiation of the carbonatite magma generated distinct styles of rare earth element (REE) mineralization, with light REE (LREE) being enriched in dolomite carbonatite, and both LREE and heavy REE (HREE) being enriched in calcite carbonatite. This study presents an integrated petrogentic model for the WCC, highlighting the utility of apatite geochemistry in unraveling the complex magmatic evolution of carbonatite–alkaline complexes.