The significance of Nd isotope mapping in Precambrian orogenic belts: A case study from the Saguenay region of the Quebec Grenville Province

Fifty new Nd isotope analyses are presented for the Saguenay region of the Quebec Grenville Province. Most samples yield juvenile Pinwarian model ages, with an average depleted mantle model age (TDM) of 1.55 Ga, typical of the Quebecia accreted arc terrane. However, ten samples yield older model ages, averaging 1.78 Ga. These samples have an approximately linear geographical distribution, and are interpreted as melts of old basement panels trapped between juvenile Pinwarian crustal blocks during the accretion of the Quebecia terrane as a composite arc belt. These panels are the most tangible remnants of pre-Pinwarian crust in the Saguenay region, and their mapping is an important step in understanding the geometry of Mesoproterozoic crustal accretion on the SE margin of Laurentia. This study illustrates the usefulness of Nd isotope mapping for interpolating crustal formation ages between U-Pb dated samples in deeply exhumed orogenic belts. The data confirm that the Quebecia composite arc belt consists almost entirely of accreted juvenile Pinwarian arc crust, with less than 5% of older Paleoproterozoic crustal slivers. A comparison with the modern analogue of the Japan Arc suggests that normal plate tectonic processes can insert juvenile arc terranes behind rifted slivers of old crust to create the observed geometrical pattern of crustal formation ages.