New geochronological and geochemical constraints on petrogenesis and tectonic setting of the Loe-Shilman carbonatite complex, Northwest Pakistan

Abstract

The Loe-Shilman carbonatite complex in northwest Pakistan comprises carbonatite and ultrapotassic silicate rocks. The age and environment of emplacement of these rocks have long been controversial, and therefore, impeded our understanding on the regional tectonic setting. New whole-rock geochemistry, Sr-Nd isotopic and zircon petrochronological data provide age and petrogenetic information on the carbonatite and ultrapotassic silicate rocks of the Loe-Shilman complex. Zircon U/Pb data yield 206Pb/238U weighted mean ages of 90.6 ± 1.0 Ma (n = 33) for the carbonatite and 89.3 ± 1.0 Ma (n = 30) for the ultrapotassic rocks, which indicates that the two lithologies were emplaced simultaneously during a single tectonic/thermal event. The Nd isotope data of the ultrapotassic rocks is homogenous (εNd(i) = −2.5 to −2.9) and overlap εNd(i) of the carbonatite (−2.6 to −2.8), indicating derivation from a common, enriched mantle source. Despite this temporal overlap, whole-rock geochemical data, however, indicates that generation of both rock types by fractional crystallization or liquid immiscibility is unlikely and that the carbonatite and ultrapotassic rocks are actually products of two independent melts generated by partial melting of an enriched sub-continental lithospheric mantle (SCLM) source at the same time. Trace element data further indicate that the ultrapotassic rocks are products of fractional crystallization of a melt produced by a low degree partial melting of an enriched mantle source, within the garnet stability field. Moreover, the ultrapotassic rocks and carbonatites show association with anorogenic and within-plate plume type tectonic settings, coeval with juxtaposition of the northwestern margin of the Indian plate over the Réunion hotspot mantle-plume during Late Cretaceous time.