Structural and metamorphic evolution of the Nanga Parbat syntaxis, Pakistan Himalayas, on the Indus gorge transect: The importance of early events

Abstract

AbstractField relationships are as important as radiometric age data for unravelling the history of polycyclic metamorphic regions. Field and petrographic data were used to investigate such a region, the Nanga Parbat syntaxis ‐ a major N‐trending transverse structure in the Pakistan Himalayas. Continental rocks of the Indian plate are exposed in the core of this structure due to uplift relative to the surrounding and structurally higher amphibolites of the overthrust Kohistan Island Arc. In the Indus gorge, the Indian plate gneisses are migmatitic, highly deformed and cut by Himalayan granite sheets. Field relationships show that the migmatization pre‐dates the emplacement of a suite of basic intrusive sheets (now amphibolites) which are themselves unlikely to be Himalayan; these amphibolite sheets are cut by the Himalayan granite sheets. There is therefore no genetic link between the granites and the migmatization: the latter might be Precambrian. During the pre‐Himalayan evolution, an eastern group of metapelitic migmatites underwent biotite dehydration melting at granulite facies conditions, leading to the development of assemblages with garnet, orthoclase and kyanite. Melt was not completely lost and back‐reacted during cooling to give biotite‐kyanite coronas around garnets. A western group of orthogneisses is characterized by foliated muscovite and biotite; orthoclase is never found with either garnet or aluminium silicate. In this second group, partial melting and small‐scale segregation gave rise to the migmatitic appearance, but melt was not lost and back‐reacted fully to regenerate the micas.The migmatites and basic sheets were strongly deformed at amphibolite facies during Himalayan collision, producing LS tectonites, mylonitic in places. This strong fabric was then tightly folded on large scale. The NNE trending fold is slightly oblique to the earlier N‐S stretching lineation; this has been reoriented on the steep limbs, but its plunge remains shallow. No new fabric is seen associated with this folding. On the western margin, steeply plunging lineations are associated with shear bands in gneisses, indicating relative uplift of the syntaxis. In contrast, on the steep eastern margin, relative uplift of the syntaxis appears to have been due to rotation without the development of new fabrics. There is no evidence for further melting during this Himalayan evolution, so the granite sheets cannot have been derived locally. This study highlights the importance of distinguishing pre‐Himalayan and Himalayan events in this polymetamorphic region, and the usefulness of field relationships in addition to geochronological constraints in such a task.