Lithosphere-asthenosphere mixing in a transform-dominated late Paleozoic backarc basin: Implications for northern Cordilleran crustal growth and assembly

Abstract

The Slide Mountain terrane is part of a North American Cordillera-long backarc basinal assemblage that developed between the ensialic arc terranes (Yukon-Tanana and affi liated pericratonic terranes) and the North American craton in the middle to late Paleozoic. The Slide Mountain basin started to open in the Late Devonian, and spreading continued through the late Paleozoic in an oblique (transform-dominated) manner such that the pericratonic terranes were translated into southerly latitudes. The basin closed, also in an oblique manner, by the Early Triassic, resulting in the reaccretion of the Yukon-Tanana terrane to the northwestern Laurentian margin. Both the opening and closing likely involved hundreds to possibly thousands of kilometers of intra-ocean and/or intra-arc strike-slip displacement, sinistral during the ocean's Late Devonian to mid-Permian opening and dextral during its Late Permian closing.In southeastern Yukon, Canada, the Early Permian Slide Mountain terrane is dominated by mafi c and ultramafi c volcanic and plutonic rocks of the Campbell Range Formation. These rocks are narrowly distributed, for over 300 km, on either side of the Jules Creek-Vangorda fault, a fault that separates Slide Mountain terrane from Yukon-Tanana terrane. The Campbell Range basaltic volcanic and high-level intrusive rocks have geochemical and isotopic signatures that vary systematically across the Jules Creek-Vangorda fault: ocean-island basalt (OIB) and enriched mid-ocean ridge basalt (E-MORB) suites with lower εNd t occur exclusively south of the fault, whereas north of the fault they have normal midocean ridge basalt (N-MORB) and backarc basin basalt (BABB) signatures with higher εNd t values. The εNd t values are inversely correlated with Nb/Th pm and Nb/La pm , suggesting that the lower εNd t values present in the E-MORB and OIB are mantle source features of these basalts and not due to continental crustal contamination. Isotopic and multi-element mixing calculations illustrate that the OIB-like basalts were derived primarily from enriched continental lithospheric mantle, whereas the N-MORB and BABB suites were sourced primarily from the upwelling backarc asthenospheric mantle; E-MORBs represent mixtures of depleted asthenospheric and enriched lithospheric mantle. The geochemical and isotopic variations in the Campbell Range Formation across the Jules Creek-Vangorda fault is attributed to formation in different parts of an extending continental-backarc basin and then their subsequent juxtaposition by continued displacement along the fault.Despite the juvenile isotopic signatures present in the Slide Mountain terrane, they occur as thin klippe atop rocks of recycled continental crustal affi nity, suggesting that they were likely only minor contributors to Cordilleran crustal growth.

Geologic Setting of Eclogite-facies Assemblages in the St. Cyr Klippe, Yukon–Tanana Terrane, Yukon, Canada

“…The Late Devonian to Early Mississippian Finlayson assemblage, the oldest arc assemblage, is characterized by mafic to felsic metavolcanic and metaplutonic rocks of arc and back-arc affinities Piercey et al 2006Piercey et al , 2012. The assemblage also includes ultramafic rocks, carbonaceous pelite, quartzite, volcaniclastic rocks and minor marble (Mortensen 1992;Colpron et al 2006a;Murphy et al 2006).…”

“…The remnants of this basin are preserved today in a discontinuous belt along the eastern edge of the Yukon-Tanana terrane and called the Slide Mountain terrane (Wheeler and McFeely 1991;Mortensen 1992;Pigage 2004;Colpron et al 2006a;Piercey et al 2012). In southeastern Yukon, the Slide Mountain terrane is found as fault-bounded slivers of oceanic lithosphere preserved between the Yukon-Tanana terrane and the underlying North American margin.…”

“…1;Mortensen 1992;Nelson 1993;Nelson et al 2006;Colpron et al 2006a;Piercey et al, 2006Piercey et al, , 2012. This marginal basin was originally termed the Anvil Ocean by Tempelman-Kluit (1979).…”

“…In southeastern Yukon, the Slide Mountain terrane is found as fault-bounded slivers of oceanic lithosphere preserved between the Yukon-Tanana terrane and the underlying North American margin. Representative assemblages of the Slide Mountain terrane include unmetamorphosed to greenschist-facies basalt, gabbro, leucogabbro, and variably serpentinized ultramafite, as well as deep-water sedimentary sequences such as chert and argillite (TempelmanKluit 1979;Colpron 2006;Colpron et al 2006a;Murphy et al 2006;Nelson et al 2006;Piercey et al 2012). Other sedimentary rocks found within the Slide Mountain terrane include siltstone, limestone, epiclastic sandstone and conglomerate, and phyllitic chert.…”

“…Three Late Devonian to Late Permian, unconformity-bounded, geochronologically and geochemically distinct volcanic and volcaniclastic assemblages are built upon pre-Early Devonian metasedimentary basement known as the Snowcap assemblage (Fig. 1;Mortensen 1992;Piercey et al 2002Piercey et al , 2006Piercey et al , 2012Colpron et al 2006a;Piercey and Colpron 2009). The Snowcap assemblage is a heterogeneous mix of psammitic, pelitic and calc-silicate schist, quartzite, marble and amphibolite (Colpron et al 2006a(Colpron et al , 2006bPiercey and Colpron 2009).…”