Rejuvenation of Rb-Sr mica ages during shearing on the northwestern margin of the Nanga Parbat-Haramosh massif

Abstract

The shear zone bordering the western margin of the Nanga Parbat-Haramosh massif (NPHM) separates terranes with distinct but well-documented cooling histories. This study was undertaken in order to examine isotopic behaviour and if possible to date deformation or cooling within the contact zone itself. Rb-Sr muscovite and biotite ages and petrographic descriptions are presented for a range of metamorphic lithologies and granite sheets which have been affected to varying degrees by greenschist-facies extensional crenulation. This deformation has been kinematically linked to the differential uplift of the NPHM. Ages for metamorphic lithologies and granite sheets are similar and are considered together. Determined muscovite ages mostly lie in the range 9–22 Ma, whilst the majority of biotite ages lie in the range 2–11 Ma. Mica ages from the shear zone are significantly more variable than ages determined for unsheared lithologies from outside the shear zone. Even closely spaced samples yield variable mica ages, and in some cases there is a significant difference between muscovite and biotite ages determined for the same sample. Muscovite ages yield a bimodal distribution, with groups of ages in the range 17–21 Ma and 8–12 Ma. The older group is thought to reflect the original, purely thermostatic cooling of lithologies through the muscovite closure temperature, prior to formation of the NPHM structure. In contrast, the younger group of muscovite ages reflects variable degrees of isotopic resetting during shearing at temperatures below the muscovite closure temperature, associated with the relative uplift of the NPHM. The majority of biotite ages also reflect partial to complete resetting during shearing. However, biotite samples from structurally competent amphibolite horizons preserve older (>16 Ma) ages, indicating that there was locally a strong lithological or fluid control on the degree of resetting. The flanks of the NPHM cooled through the muscovite closure temperature significantly earlier than the deeper structural levels presently exposed within the core of the NPHM. Greenschist-grade deformation was occurring within the shear zone at the same time that deeper levels of Indian crust were passing through the muscovite closure temperature, suggesting that the shearing reflects the relatively passive extensional movement of rocks sliding off the flank of the actively growing Nanga Parbat structure. The results highlight the isotopic complexities of long-lived shear zones present in orogenic belts, but show that useful radiometric age information may be obtained if the regional tectonic history is well understood and if the samples for dating come from well-constrained structural settings.