阿拉斯加麥金利系列岩體之鋯石定年與地球化學研究

Abstract

Plutonic rocks in the Central Alaska Range were emplaced during important tectonic events including the end of Late Cretaceous-Paleocene arc magmatism, the terminal phase of terrane accretion, the Kula ridge subduction, regional strike-slip faulting, oroclinal rotation of western Alaska, and Eocene resumption of arc magmatism along the proto-Aleutian arc system. Here we report the first set of LA-ICPMS and SHRIMP zircon U-Pb ages for granitoids from (1) the McKinley Sequence, (2) the Composite Plutons and (3) the Foraker Pluton, central Alaska Range. Two peraluminous granites from the McKinley Sequence yielded 206Pb/238U ages of 62±1 and 60±1 Ma; an A-type granite from the same area gave age of 64±1 Ma. Two granodiorites from the Composite Plutons and a granite from the Foraker Pluton gave ages of 69±1, 67±1 and 37±1 Ma, respectively. The age results indicate that the Composite Plutons and the McKinley Sequence granites are older than previously reported and that the McKinley granites can be divided into two magma suites that consist of peraluminous and A-type compositions occurring at ca. 62-60 and 64 Ma, respectively. Nd and Sr isotope data suggest a temporal change in the magma’s isotopic compositions, with the Foraker granites showing higher and more heterogeneous εNd(T) values that range from about +6 to -2 and the Composite and McKinley Sequence granitoids showing lower and less heterogeneous εNd(T) values from +1 to -3. Besides, zircon Hf isotopic data of the dated samples show that a granite from the Foraker plutons has the highest zircon εHf(T) values from +15 to +10, in contrast to the Composite plutons that show apparently lower zircon εHf(T) values from +4 to -3. Peralumous and A-type granites from the McKinley Sequence have zircon εHf(T) values from +2 to -5 and +8 to +1, respectively. The Composite and McKinley Sequence plutonic rocks are interpreted as products of the ending phase of Late Cretaceous-Paleocene arc magmatism. The peraluminous granites from the McKinley sequence may have been sourced from the remnant mantle wedge, previously responsible for generating the Composite plutons and earlier arc magmas. Besides, generation of the A-type granites can be attributed to the v oroclinal rotation of western Alaska and ridge subduction that started around 64 Ma, a mechanism that may also have resulted in broadly coeval volcanism in the nearby Cantwell basin and northern Talkeetna Mountains. The Foraker pluton are interpreted as the product of renewed magmatism along the proto-Aleutian arc, in response to north or northwestward subduction of the Pacific plate during 45-30 Ma.