Nd isotope and geochemical constraints on the depositional setting of Paleoproterozoic metasedimentary rocks along the margin of the Archean Hearne craton, Saskatchewan, Canada

Abstract

Abstract The Cree Lake Zone in the southwestern part of the Hearne craton, reworked during the Paleoproterozoic Trans-Hudson orogeny, includes the Wollaston Group, a sequence of Paleoproterozoic metasedimentary rocks. As these rocks have been multiply deformed and metamorphosed to high-grade, a geochemical and isotopic study was initiated to see through the overprinting events to better constrain the provenance and evolution of the Wollaston Group. The Wollaston Group consists of thick sedimentary successions representing different depositional and tectonic environments that can be subdivided into two subgroups. The Lower subgroup is represented by deep-water turbidite to shallow-water marine deposits. The Upper subgroup, separated from the Lower one by a regional unconformity, comprises molasse-type assemblages ranging from talus to shallow-marine/lacustrine, with subordinate volcanogenic rocks. Major and trace element data obtained from 31 psammopelitic rocks indicate that the clastic material was derived from sources consisting of both felsic and mafic material, although felsic sources are more important in the samples from the Lower subgroup. Sedimentary tectonic discrimination diagrams demonstrate that most of the Wollaston Group was deposited on an active continental margin rather than a passive margin. The e Nd values for the Wollaston Group psammopelites, calculated at 1.92 Ga, the approximate age of the boundary between the Lower and Upper subgroups, vary from −3.4 to −6.8, which yields an approximate 70:30 mixture between juvenile Paleoproterozoic detritus and Archean detritus from the Hearne craton. A secondary source of detritus may be 1.95–2.0 Ga rocks of the Taltson Orogen, which is exposed to the west of the Archean Rae–Hearne craton. The Wollaston Group records the complete evolution of the western margin of the Trans-Hudson Orogen during the Paleoproterozoic. The rifting of an Archean continent at ca. 2.1 Ga led to the deposition of the passive margin siliciclastic sediments comprising the lowermost, but poorly preserved part of Wollaston Group. The formation of a continental arc (Rottenstone) along the Hearne margin at ca. 1.92 Ga resulted in a change in tectonic environment from a passive margin to a back-arc basin, and detritus in most of the Lower subgroup was then derived from both basement and newly formed magmatic arc. Collision with the juvenile La Ronge arc led to uplift along the Hearne margin which transformed the back-arc basin into a retroarc foreland basin at ca. 1.88 Ga. Sedimentary starvation and shallowing of the basin led to widespread accumulation of carbonate and evaporite deposits in the west, whereas rapidly uplifted orogenic belts in the east provided sources of coarse-grained clastic detritus, forming the Wollaston Upper subgroup. Closure and inversion of the sedimentary basin occurred by ca. 1.86 Ga, coincident with the emplacement of mafic to felsic intrusive rocks in the Cree Lake Zone and the emplacement of the voluminous ca. 1860 Ma Andean-type Wathaman batholith into the continental margin. Closure was manifested by the onset of foreland-directed thrusting of the Wollaston Group.