STRUCTURAL AND THERMAL EVOLUTION OF THE KARAKORAM CRUST

Prior to the Eocene (c. 50 Ma) collision of the Indian and Asian plates, the southern margin of Asia along the Karakoram plate was an Andean-type margin dominated by tonalitic-granodioritic magmatism of Jurassic-Lower Cretaceous age (Hushe gneiss, Muztagh Tower gneiss and K2 orthogneiss) and associated low pressure andalusite, staurolite and garnet grade metamorphism (Ml). Following India-Asia collision, crustal shortening, thickening and regional Barrovian metamorphism (M2) occurred between 50-37 Ma. Thermobarometry of kyanite-grade metapelites indicate burial to depths of around 30-35 km. Simultaneous solution of the garnet-biotite geothermometer with the garnet-muscovite-biotite-plagioclase and garnet-AI 2SiO 5-quartz-plagioclase geobarometers indicates peak M2 P-T conditions of 696 f 20 °C at 8.6 ± 0.7 kbar (860 MPa). Temperatures may have exceeded 700°C in sillimanite-grade metapelites to produce in situ partial melting and leucogranitic melt pods. Peak M2 metamorphism occurred prior to 37±0.8 Ma, the crystallization age of the Mango Gusar two-mica granite pluton which cross-cuts syn-metamorphic deformation fabrics. Post-M2 thermal relaxation followed from 37-25 Ma, after which localized high heat concentrations at the baseo f the thickened crust caused widespread crustal melting and intrusiono f the Baltoro granite batholith at 25-21 Ma. A high temperature-low pressure thermal aureole (M3) along the northern contact is synchronous with the 21 ± 0.5 Ma zircon age of the Baltoro granite. Andalusite hornfels along the northern contact of the batholith (Mitre thermal aureole) indicates maximum pressures of 3.75 kbar (375 MPa). A 75 °C increase of temperature in kyanite-sillimanite grade gneisses approaching the southern granite contact of the Baltoro granite is interpreted as the thermal upwarping of pre-37 Ma Barrovian metamorphic M2 isograds around the 21 Ma contact aureole M3 isotherms

AGE OF CRYSTALLIZATION AND COOLING OF THE K2 GNEISS IN THE BALTORO KARAKORAM

The mountains of K2 (8611 m) and Broad Peak (8047 m) in the Baltoro Karakoram (northern Pakistan) are composed of plagioclase-homblende and biotite-hornblende-K-feldspar orthogneisses and amphibolite-facies para gneisses, intruded by garnetbiotite-muscovite-tourmaline leucogranitic veins. A U-Pb zircon age of 115-120 Ma was obtained on an orthogneiss from the south face of K2. 40Ar- 39Ar analysis on hornblende yields a plateau age of 90.6 ± 1.8 Ma, consistent with a mid-Cretaceous phase of magmatism, concomitant with early subduction-related components of the Karakoram batholith (Muztagh Tower unit, Hunza plutonic unit). We interpret the K2 gneiss as representing a culmination of midcrustal rocks along a discontinuous but wide zone north of the Karakoram batholith

Late Cenozoic volcanism and rates of active faulting in eastern Iran

We present new 40Ar/39Ar ages of samples of volcanic rock exposed along the remote margins of the Dasht-e Lut desert in eastern Iran. Close spatial relationships between the volcanic rocks and the trace of active strike-slip faults allow us to determine the slip rates of two major faults, averaged since eruption of the volcanics. Our study shows that the Nayband fault at the western margin of the Dasht-e Lut has a slip rate of ∼1.4 ± 0.5 mm yr−1 averaged over 2.25 Ma. The East Neh fault, one of several active strike-slip faults within the Sistan Suture Zone at the eastern margin of the Dasht-e Lut, has a minimum slip rate of ∼1.2 mm yr−1 averaged over ∼1.7 Ma. The rates of slip on major active faults in eastern Iran are largely unknown, and the slip rates our data provide, though limited, are a significant increase on what is known of the faulting within this remote and relatively inaccessible desert region. We also present analyses of the major and trace element concentrations within the volcanic rocks. The chemistry of the volcanic rocks is typical of intracontinental melts with an overall signature similar to that of ocean island basalts. Inversion of rare earth element distributions suggests some melting has occurred at depths of ∼80 km, indicating the presence of a relatively thin lithosphere beneath eastern Iran, in agreement with recently published maps of lithospheric thickness derived from shear wave velocities