Archean Volcanism and Geochemistry, Washeibamaga-Thundercloud Lakes Area, Wabigoon Subprovince - Superior Province, Northwest Ontario

Abstract

<p>The Washeibamaga - Thundercloud Lakes area in the Wabigoon Greenstone Belt, southeast of Dryden, Ontario, was studied and mapped. Petrographic examination of the various rock types was carried out and geochemical whole rock and trace element data was obtained using X.R.F. methods.</p> <p>The area can be subdivided into three units: 1) The lower volcanic sequence of metabasalts (lower greenschist facies) is preserved as a steeply-dipping, north-facing homoclinal volcanic pile six kilometres thick. The basalts show trace element (Y, Nb, Zr, Ni, Ba, Rb, Sr), geochemical similarities to modern ocean-floor tholeiitic basalts.</p> <p>2) The Thundercloud Quartz-Porphyry intrudes the lower sequence and is believed to represent a vent-plug filling a late-stage felsic volcano. Accompanying explosive volcanism produced a three-kilometre thick sequence of coarse pyroclastic and epiclastic rocks and tuffs, hereafter called the middle sequence. Associated dacitic and auto-brecciated rhyolitic flows have calc-alkaline affinities and are chemically distant from both volcanic sequences. They appear not be a differentiated product but to have originated as a separate magma.</p> <p>3) The upper volcanic sequence of metabasalts is composed of tightly folded, massive to pillowed flows. The contact with the underlying epiclastic and pyroclastic rocks is at an angle of thirty degrees, implying either profound angular unconformity or fault contact. The upper sequence is chemically distinct from the lower sequence. Trace element abundances (K, Rb, Sr, Ba), suggest similarities with modern island-are tholeiites or back-arc-basin basalts.</p> <p>The data will be used to compare the thesis area with other Archean settings and with modern analogues. In doing this a tectonic interpretation of the Washeibamaga - Thundercloud Lakes area will be presented.</p>