The externides of Wopmay Orogen, Point Lake and Kikerk Lake map areas, District of Mackenzie

Some results of recent field work are briefly discussed as they pertain to the following topics: (1) north-south stratigraphic continuity of the Precambrian continental-terrace wedge, (2) stromatolite elongation, paleowind direction and global polarity during deposition of the Rocknest dolomite shelf, (3) evidence for primary aragonitic mineralogy of the Rocknest Formation, (4) attempted quantitative paleobathymetry of the upper continental slope, (5) eastward migration of foredeep flysch, (6) nature of basement involvement in Asiak Fold-Thrust Belt, (7) relation of thrusting to the foredeep molasse, (8) mysterious basement-involved cross folding of regional extent, (9) normal faults associated with late transcurrent faulting , and (10) the first reported minor leadzinc vein mineralization in Rocknest dolomite. Future field work is outlined

The externides of Wopmay Orogen, Takijuq Lake and Kikerk Lake map areas, District of Mackenzie

Some r esults of the final summer's fieldwork on this project re outlined as they pertain to the following topics: (l) sedimentary structures and paleoenvironment of the Odjick elastic shelf; (2) paleogeographic zonation of the Rocknest shelf-edge reef complex; (3) nature and origin of Rocknest shelf cycles; (4) correlation of the Rocknest dolomite eastward across Rockinghorse Arch into Kilohigok Basin and implications of a revised correlation of formations in the Epworth and Goulburn groups; (5) regional variation in low grade metamorphism and its relation to deformation events; (6) basement-involved folding without thrusting in the Tree River belt; and (7) development of "shingle" structures (crudely hexagonal crustal blocks) by progressive conjugate transcurrent faulting

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