Petrogenesis of the Late Cretaceous Sillai Patti carbonatites, NW Pakistan: Insights from C-O-Sr-Nd isotopes and titanite U[sbnd]Pb dating.

The Sillai Patti Carbonatite Complex constitutes the second largest carbonatite complex in Pakistan. However, our current understanding of its origin and geodynamic settings remains unclear. An integrated investigation, involving mineral composition, titanite U Pb dating, whole-rock geochemistry and C O isotopes as well as in situ Sr Nd isotope study of the mineral calcite and titanite from this carbonatite complex was conducted. The carbonatite, mainly consisting of calcite, apatite, amphibole and biotite, occurs as a sheet-like intrusion along an NNE-SSW-striking and southward dipping thrust plane. Titanite U Pb dating results reveal that the Sillai Patti carbonatites were formed at about 83 ± 7 Ma. Chemically, the Sillai Patti carbonatites are characterized by variably high SiO 2 (4.56–24.02 wt%), P 2 O 5 (3.32–6.09 wt%) and total REEs contents (1989–3309 ppm) and are enriched in alkalis (Na 2 O > K 2 O), Nb (29–397 ppm), Zr, Ba, Sr and LREEs. The elevated contents of REEs and Nb and the ubiquitous REE-Nb mineral hosts (e.g., fluorapatite (up to 1.8 wt% LREE 2 O 3), titanite (up to 7360 ppm REE and 10,050 ppm Nb), calcite (350–760 ppm REE), allanite, pyrochlore, zircon and monazite) in the Sillai Patti carbonatites suggest that they have considerable potential of REEs and Nb mineralization. δ13C PDB and δ18O SMOW values for the Sillai Patti carbonatites vary from −6.7 ‰ to −6.9 ‰ and 8.3 ‰ to 10.4 ‰, respectively, which are in good agreement with primary mantle-derived carbonatite values. The Sillai Patti carbonatites show slightly enriched Sr and Nd isotopic compositions ((87Sr/86Sr) i = 0.70472–0.70496; εNd(t) = −2.6 to −0.1), indicating derivation from an enriched sub-continental lithospheric mantle source. We propose that the Sillai Patti carbonatites were formed in an intra-continental rift setting. Along with other analogous Late Cretaceous carbonatites and alkaline silicate rocks in the northwestern margin of the Indian plate, they likely recorded an early period of alkaline magmatism related to the Réunion hotspot mantle plume. • Titanite U-Pb dating yielded ca. 83 Ma for the Sillai Patti carbonatites. • The carbonatites are products of partial melts of an enriched SCLM source. • The Sillai Patti carbonatites occurred during intra-continental rift setting. • Chronicle Réunion hotspot is invoked to contribute to the source of magmatism. [ABSTRACT FROM AUTHOR]