Potassium - Argon Ages of Mainly Intrusive Rocks in the Saint Elias Mountains, Yukon and British Columbia

Abstract

One hundred and two new K-Ar age determinations, largely from granitoid rocks, are reported from the Saint Elias Mountains of southwest Yukon, northwest British Columbia, and closely adjacent parts of Alaska. Six new K-Ar dates are also presented for rocks in the northern part of the Coast Plutonic Camplex. New and previously published K-Ar age data from the Saint Elias Mountains aid in defining at least six plutonic episodes in this region: latest Pennsylvanian to Early Permian (270-290 Ma); Late Jurassic to earliest Cretaceous (130-160 Ma); late Early Cretaceous (106-117 Ma); Eocene (41-52 Ma); Oligocene (23-33 Ma); and mid- to Late-Miocene (6-16 Ma) time. Ali can be correlated with plutons or plutonic belts previously outlined in adjoining Alaska. ln the Saint Elias Mountains the plutonic episodes, except for the fast two, are apparently confined to certain terranes, whereas equivalent plutons or plutonic belts in Alaska mostly are not. These six distinct episodes of mainly calc-alkaline plutonism in structurally camplex Saint Elias Mountains and in contiguous Alaska, may be, but are not necessarily, related to periods of plate convergence and subduction.

Geologic studies in Alaska by the U.S. Geological Survey, 1990

“…The spread in K-Ar dates in this region probably reflects partial resetting of mineral dates during the Cretaceous event (described above) and (or) an Eocene thermal event that affected the accretionary prism to the south (Hudson and Plafker, 1982). The geochemistry of the Late Jurassic plutons shows that they are calc-alkaline (Hudson, 1983;Dodds and Campbell, 1988;Plafker and others, 1989). The plutons were intruded during the beginning of a major regional orogeny (MacKevett, 1978) and are interpreted as the roots of an island arc (Hudson, 1983;Plafker and others, 1989).…”

“…Concordant biotite and hornblende K-Ar dates from single samples range from approximately 160 to 130 Ma, with most of the hornblende K-Ar dates falling between 140 and 160 Ma. The plutonic rocks are weakly to strongly foliated, and they typically have concordant contacts with the greenschist to amphibolite facies country rock (MacKevett, 1978;Winkler and others, 1981;Dodds and Campbell, 1988).…”

“…The plutons are typically elongate parallel to the regional fabric and have concordant contacts (Brew and Morrell, 1983;Hudson, 1983). Hudson (1983) refers to these plutons as the Nutzotin-Chichagof belt; this group of plutons includes the Muir-Chichagof belt of Brew and Morrell (1983) and the Kluane Ranges plutonic group of Dodds and Campbell (1988). K-Ar dates from these plutons range from 120 to 103 Ma; concordant biotite-hornblende pairs from samples along the entire length of the belt are tightly clustered between 115 and 107 Ma (MacKevett and others, 1974;Richter and others, 1975;Dodds and Campbell, 1988).…”

“…Kinematic indicators, including bending of foliation in ductile shear zones, S-C asymmetries, "tails" on porphyroclasts, and slickenlines, show that the major phase of movement along the high-angle shear zones and faults was dextral simple shear (Pavlis and others, 1989;Roeske and others, 1990a, b). To refine the timing of this deformation event, we have undertaken an 40 ArP 9 Ar study of rocks in southeast Alaska, based on similar ages and compositions for the whole group (Loney and others, 1967;Dodds and Campbell, 1988;Karl and others, 1988). Hudson (1979) named the belt the Tonsina-Chichagof belt; the segment of the belt that intrudes the combined Wrangellia-Alexander terrane in Canada is referred to as the Saint Elias plutonic group (Dodds and Campbell, 1988).…”

“…To refine the timing of this deformation event, we have undertaken an 40 ArP 9 Ar study of rocks in southeast Alaska, based on similar ages and compositions for the whole group (Loney and others, 1967;Dodds and Campbell, 1988;Karl and others, 1988). Hudson (1979) named the belt the Tonsina-Chichagof belt; the segment of the belt that intrudes the combined Wrangellia-Alexander terrane in Canada is referred to as the Saint Elias plutonic group (Dodds and Campbell, 1988). Concordant biotite and hornblende K-Ar dates from single samples range from approximately 160 to 130 Ma, with most of the hornblende K-Ar dates falling between 140 and 160 Ma.…”