RECOGNITION OF THE TIME AND LEVEL OF EMPLACEMENT OF THE SILLAI PATTI CARBONATITE COMPLEX, MALAKAND DIVISION, NORTHWESTERN PAKISTAN: <i>constraints from fission-track dating</i>
The Sillai Patti carbonatite complex represents the second largest carbonatite body of the Peshawar Plain Alkaline Igneous Province of northern Pakistan. It is situated about 20 km west of Malakand, near Sillai Patti Village. Here, the carbonatite occurs along a fault in the form of a sheet striking in the NNE-SSW direction and dipping southward. The carbonatite body is about 12 km long and 2-20 m thick, predominantly intruded along the faulted contact of metasediments and granite gneiss but locally, within the metasediments. A fission-track age of 29.40 ± 1.47 Ma was obtained for the Sillai Patti carbonatite complex. Close resemblance of the fission-track apatite age of this study with the fission-track as well as other high-temperature radiometric ages from the same and the neighboring carbonatite complexes of the alkaline belt of northern Pakistan suggests emplacement of the Sillai Patti carbonatite complex at higher crustal level and subsequent extremely fast cooling to near ambient temperatures (<60 ºC) required for the complete retention of fission tracks in apatite. The age data also point out that the fission-track age of 29.40 ± 1.47 Ma of this study is the age of intrusion of the carbonatitic magma of the Sillai Patti carbonatite complex to shallow, near-surface level. Comparison of the uplift-induced denudation rates of the region with the world data clearly reflects the presence of a postcollisonal extensional environment in the region south of the Main Mantle Thrust during Oligocene time. This strongly negates the idea of the earlier workers of emplacement of the carbonatite complexes of the Loe-Shilman and Sillai Patti areas along thrust faults during the Oligocene.