RECORD FOR MESOZOIC OROGENY AND BACK-ARC BASIN FORMATION AND MAGMATISM IN WESTERN STIKINIA; EVIDENCE FROM STRATA AT JOHN PEAKS, EASTERN ISKUT RIVER AREA, NORTHWESTERN B.C
Detailed field mapping, structural measurements, petrography, geochemistry and U-Pb geochronology help interpret the nature, age, and volcanogenic evolution of these strata. Stratigraphy in the area is divisible into six members. The basal member (124 m thick) is a granitoid-bearing, cobble conglomerate overlying an angular unconformity on Upper Triassic Stuhini Group; granitoid fragments suggest stripping of the Late Triassic volcanic arc to its plutonic roots in a Triassic-Jurassic mountain-building episode.
Overlying the basal member is a subfeldspathic volcaniclastic sandstone, siltstone and shale sequence (75 m) correlative with Pleinsbachian strata to the north. Overlying the sedimentary strata is a thin andesitic crystal tuff (15 m), and a feldspathic, locally calcareous, sandstone (212 m) which hosts common dacite sills. This succession is capped by a thick sequence of felsic lapilli tuff, tuff breccia, and volcanic conglomerate (483 m), including a fiamme-bearing, rhyolitic tuff that yielded a U-Pb age of 174.7 ± 1.6 Ma, coeval with footwall rhyolite at EC. Geochemistry of volcanic facies and apparently co-magmatic sills within the lower five members show a volcanic arc affinity.
An uppermost unit comprises pillowed, locally plagioclase-phyric, basalt flows interbedded with siliceous siltstone and volcanic mudstone and siltstone (>475m; 'pajama beds'). These tholeiitic flow rocks have an E-MORB affinity distinct from the underlying succession, and host several sub-economic VMS-style mineral occurrences similar to EC.
The strata at John Peaks provide evidence for Triassic-Jurassic mountain building and for the subsequent, bimodal magmatism of the Middle Jurassic trans-tensional back-arc basin which extends for more than 200 km N-S in the region.