PROTEROZOIC CRUSTAL EVOLUTION IN LAURENTIA AND EAST ANTARCTICA AS SAMPLED BY GRANITE MAGMAS

The Proterozoic framework of southern Laurentia is well known, but the origin and assembly of lithotectonic belts, and their relation to other cratons in Rodinia remain controversial. More enigmatic is the composition, age and origin of Archean-Proterozoic crust in East Antarctica because of nearly complete ice cover. Geologic and isotopic correlations between East Antarctica and southwest Laurentia indicate a Rodinia connection as early as 1.7-1.4 Ga, but direct comparison of pre-rift crustal signatures is difficult. Hf- and O-isotope compositions of dated zircons in Cordilleran- and A-type granites provide geochemical clues to the origin and evolution of their crustal hosts. Precise Hf-isotope compositions of ~1.4 Ga Laurentian A-type granites from the southwest U.S. to the upper mid-continent are broadly similar, yet εHf(i) values of -2 to +7 and depleted-mantle model ages discriminate the granites by age of crust that they intrude. The isotopic compositions of these late- to post-orogenic Mesoproterozoic granites are controlled mainly by melting of heterogeneous lower crust that evolved between 2.0-1.6 Ga, with minor to absent mantle contributions. Melting of lower crustal sources eliminates petrogenetic models for the A-type granites by fractionation from mantle-derived parental magmas. Early Paleozoic to Mesozoic (500-200 Ma) orogenic granitoids forming the root of the modern Transantarctic Mountains were emplaced within formerly rifted East Antarctic crust. New Hf-isotope values from dated zircons show significant variation in εHf(i) of -14 to +5, reflecting spatially heterogeneous basement evolution. The Cordilleran-type granites have average crustal residence ages that range from 1.0 to ≥2.0 Ga, including discrete inherited components reflecting primary igneous crust of ~1.0 and ~1.65 Ga. Zircon δ¹⁸O values range from +5 to +12‰, consistent with crustal melting; variation along the TAM reflects variable assimilation and melt interaction with crust showing Grenville and older signatures. The isotopic and age patterns given by granites indicate that Proterozoic crustal provinces terminating along the southwestern rift margin of Laurentia, including Mesoproterozoic orogens, can be traced into central East Antarctica, thereby refining and delimiting the paleogeography of Rodinia.