Sm-Nd mineral ages of pegmatite veins and their host rocks from Swat area, Chilas complex, northern Pakistan

Pegmatite veins and their host rocks from the Chilas complex, N. Pakistan are dated using the Sm-Nd chronometer. The dated pegmatite veins are mainly composed of plagioclase and hornblende. The granulite-facies host rocks consist of plagioclase, clinopyroxene, orthopyroxene and hornblende. Sm-Nd mineral isochrons of the coarse-grained pegmatite veins give ages of 107 and 94 Ma for the two dated veins, respectively. In contrast, the host rocks in the vicinity of the sampled veins give ages between 69 and 72 Ma for the first sampled vein and between 60 and 76 Ma for the second sampled vein. The fact that the vein ages are apparently older than their host rocks might be explained in different ways. (1) The apparent ages may be the consequence of isotopic mixing and bear no geochronological meaning. (2) If the apparent ages are real, then there are three possible interpretations that are discussed here: (i) the vein ages may indicate that the magmatic age of the Chilas complex could have been > 107 Ma, which is much older than the previously reported magmatic age of 84 Ma; (ii) vein ages may date the time span of granulite-facies metamorphism in the Chilas complex which approximately coincides with the period of the Kohistan-Asia collision (102-85 Ma); and/or: (iii) the vein ages show the effects of the variation of closure temperature with grain size as coarser minerals have higher closure temperatures and vice versa, which consequently affects the Sm-Nd mineral age. The third interpretation gains particular significance, as there is a close correlation in apparent grain size with measured absolute age in both sets of samples. This interpretation is therefore discussed in some detail.