GEOLOGICAL SETTING AND DEPOSITIONAL ENVIRONMENT OF THE STIRLING GROUPOF SOUTHEASTERN CAPE-BRETON-ISLAND, NOVA-SCOTIA

The Stirling Group consists of ca. 680 Ma volcanic and volcaniclastic rocks, interpreted to have been deposited in and around a trough flank ed by felsic volcanic centres The volcaniclastic rocks are dominated b y lapilli tuff and tuffaceous litharenite sequences. The lapilli tuffs show some evidence of subaqueous reworking but most may have originat ed as air-fall deposits. Sedimentary structures in the litharenites su ggest subaqueous mass transport into the trough and deposition below w ave base. Pyritic siltstone-chert-carbonate laminites, which are local ly associated with the litharenites along the southeastern side of the trough adjacent to felsic volcanic centres, probably represent a mixt ure of fine-grained distal turbidites and chemical precipitates from s ubaqueous volcanic exhalations. The volcanic rocks of the Stirling Gro up do not show convincing evidence for subaqueous deposition: the fels ic rocks appear to form subvolcanic intrusions and domes flanked by fe lsic crystal-rich lapilli tuff and rare welded(?) tuff; the mafic volc anic rocks are not pillowed but arc closely associated with breccias w hich may have formed partly by quench fragmentation. The overall facie s assemblage and its association with VMS Zn-Pb-Cu-Ag-Au mineralizatio n are compatible with generation in an intra-arc basin. As the Stirlin g Group is significantly older than the volcanic belts which flank it, it may represent a tectonically displaced remnant of a pre-existing a rc.