Geochemistry and in-situ U-Th/Pb Geochronology of the Jambil Meta-Carbonatites, Northern Pakistan: Implications on Petrogenesis and Tectonic Evolution

The putative Jambil meta-carbonatites of Swat, northern Pakistan, occur as discrete intrusions into the Proterozoic Manglaur Formation, which are difficult to be distinguished from nearby calc-silicate marble because both rock types experienced regional metamorphism during Himalayan orogenesis that resulted in similar mosaic textures and mineral assemblages. Carbonatites are often significant repositories of economic mineral resources and, therefore, are important to be distinguished from calc-silicate marble. We present new geochemical and geochronology data to distinguish between the two rock types and interpret the petrogenesis and tectonic evolution of the Jambil meta-carbonatites. Whole rock chemical data from the Jambil meta-carbonatites show characteristically high rare earth element (REE), Sr contents and lack of negative Eu anomaly, consistent with average calcio-carbonatite values worldwide and an igneous origin. More than 0.5 wt.% SrO in the meta-carbonatites and SrO > 0.15 wt.% in constituent rock forming calcite are discriminating signatures of the Jambil meta-carbonatites. Chemically, the Jambil meta-carbonatites are relatively depleted in Rb, Nb, Ta, Ti, Zr and Hf, relatively enriched in Ba, Th, Sr, and have a high LREE/HREE ratio when normalized to primitive mantle. Their carbon and oxygen isotope compositions vary from −3.5‰ to −4.3‰ and from 9.7‰ to 12.3‰, respectively. These geochemical characteristics indicate generation of the carbonatites through small degree of partial melting from a carbonated eclogitic source. In-situ , U/Pb analysis of titanite indicates that the Jambil meta-carbonatites were emplacement at 438 ± 3 Ma. When combined with regional geological observations, we interpret the emplacement of the Jambil meta-carbonatites to have taken place during the Silurian back arc extension within greater Gondwana and mark a transition from a compressional tectonic regime, brought about by collision of microcontinental blocks along the northern margin of Gondwana, to post-orogenic extension in the waning stages of the pre-Himalayan Ordovician orogeny. Finally, in-situ 208 Pb/ 232 Th monazite dates (40.3−27.6 Ma) extracted from the meta-carbonatites are consistent with the Cenozoic metamorphism of the area.