The central Karakoram can be divided into three main tectonic units from north to south: a northern Karakoram terrane, the Karakoram batholith, and the Karakoram metamorphic complex. In the Baltoro Glacier region the Karakoram magmatism includes intrusive suites that predate and postdate the India-Eurasia collision. The oldest subduction-related phases include Jurassic hornblendite to biotite monzogranite of the Hushe complex, and Cretaceous (ca. 82 to 75 Ma) hornblende-biotite metagranitoids of the Muztagh Tower unit, all of which were deformed during the India-Kohistan-Karakoram collision. Volumetrically dominant is a postcollisional granite, the Baltoro Plutonic Unit (BPU), which consists of biotite monzogranites to two-mica ± garnet leucogranites and pegmatite-aplites of mildly peraluminous affinity. The BPU represents the youngest magmatic phase of the composite Karakoram batholith with a U-Pb age of 21 ± 0.5 Ma and K-Ar mica cooling ages ranging from 11.7 to 5.25 Ma.
The Masherbrum migmatite complex (MMC), one of a series of such complexes along the southern margin of the batholith, immediately predates the BPU. Leucocratic dikes that cross-cut the MMC yield an Rb-Sr age of 14.1 ± 2.1 Ma and K-Ar ages of 17 to 10 Ma. The BPU is interpreted as a crustal melt ultimately derived from deep crustal levels and may not be related to leucogranite generation associated with the migmatite terrain to the south. Petrogenesis of the BPU is fundamentally different from that of the High Himalayan granites and may involve a degree of selective mantle contamination.
Four major metamorphic-deformation phases can be distinguished in the central Karakoram. The earliest, M1, is represented by low-pressure andalusite-staurolite–bearing assemblages that are spatially associated with igneous components of the Hushe complex of Jurassic age. The dominant thermal event was a widespread Barrovian-type metamorphism (M2), which was syntectonic to the main deformation and overprinted M1 assemblages. M2-related structures are cut by the 37.0 ± 0.8 Ma Mango Gusar two-mica granite pluton. M2 kyanite-garnet-plagioclase-quartz-muscovite-biotite-staurolite assemblages indicate minimum pressure temperature (P-T) conditions of 550°C and 5.5 kbar (550 MPa).
Thermal effects related to intrusion of the BPU constitute M3. Along its northern margin at Mitre Peak, the assemblage andalusite-cordierite-chlorite-biotite-muscovite-quartz-plagioclase indicates a maximum pressure of 3.75 kbar (375 MPa, ≃ 12.5-km depth). Along its southern margin at Paiyu, the presence of granitic melt pods with sillimanite, muscovite, plagioclase, and quartz indicates a minimum pressure of ca. 3.5 kbar (350 MPa) and a temperature 75° higher than local M2 assemblages. The replacement of kyanite by sillimanite and the appearance of granitic melt pods approaching the BPU along the Baltoro Glacier transect, may be an M3 overprinting of M2. M4 (<5 Ma) is a syntectonic retrogressive metamorphism along the hanging wall of the Main Karakoram Thrust—a breakback thrust responsible for the recent uplift of the Karakoram.
Structural culminations of midcrustal rocks occur in the K2 and Broad Peak areas within Carboniferous-Lower Cretaceous sediments of the Gasherbrum Range. Metamorphism of the K2 gneiss (dominantly biotite-hornblende-K-feldspar orthogneiss) occurred during middle to Late Cretaceous time. Pegmatite dikes dated as 70 to 58 Ma (K-Ar-mica) cut the gneisses.
