The S-type Ladybird leucogranite suite of southeastern British Columbia:Geochemical and isotopic evidence for a genetic link with migmatite formation in the North American basement gneisses of the Monashee complex

摘要

The 62-52 Ma Ladybird granite (LBG) suite is a peraluminous,leucocratic,S-type,quartz monzonitic to granitic suite which occurs as batholiths,stocks,dikes,sills,and pegmatite veins predominantly in the high-grade rocks of the Shuswap complex,in southeastern British Columbia.The emplacement of the LBG was synchronous with the production of abundant migmatites within Thor-Odin dome of the Monashee complex,an exposure of North American basement,exhumed from depths of ca.26-33 km by Eocene extensional faults.The LBG and the leucosome in migmatites from Thor-Odin dome have similar major and trace element patterns,and are both characterized by zircons which have inherited Precambrian cores.Whole rock Nd isotope compositions show a range of values for the LBG with epsilon Nd_((55 Ma)) values from-5.0 to-17.2.The epsilon Nd_((55 Ma)) for the leucosome samples range from-9.5 to-23.6,overlapping with those of the granitic suite.These data support the interpretation of a genetic link between formation of the LBG suite and melting of North American basement rocks,such as those exposed in the core of Thor-Odin dome.The leucosome samples have lower high field strength element (HFSE) concentrations and positive Eu anomalies,whereas the LBG samples have higher HFSE concentrations and negative Eu anomalies.The similar trace element characteristics suggest that the leucosome from the migmatites and the LBG are related,whereby most of the leucosome samples are cumulates and the LBG samples represent evolved or residual melts.The initial ~(87)Sr/~(86)Sr isotope values for both the LBG and leucosome samples have a large range.However,the initial Sr isotopic ratios for the LBG suite are lower than those of the leucosome samples,with ~(87_Sr/~(86)Sr_((55 Ma)) ranging from 0.70603 to 0.73688 and 0.74256 to 0.76593,respectively.This isotopic discrepancy suggests either:a) isotopic disequilibrium during partial melting in the mid-to lower crust where the leucosome formed,b) the distribution of Sr during partial melting was controlled by different melt-producing reactions,and/or c) isotopic heterogeneity in the source rocks.At least part of the LBG suite likely formed via melting of North American basement rocks that were dominantly of sedimentary origin.Melting of the Proterozoic supracrustal metasedimentary rocks overlying North American basement may also have contributed to the formation of the different phases of the suite found at the regional scale.However,the abundant leucosomes in the basement rocks of Thor-Odin dome may mark the paths along which anatectic melt migrated in the structurally overlying Ladybird granites of the South Fosthall pluton.