An Archean quartz arenite-andesite association in the eastern Baltic Shield, Russia: implications for assemblage types and shield history

Shallow water sedimentary units are generally considered scarce in Archean greenstone belts. We describe an unusual quartz arenite-subaerial andesite association within the Archean Hisovaara greenstone belt, a fragment of the Parandovo-Tikshozero belt within the Karelian craton of the Baltic Shield. The Hisovaara greenstone belt consists of several lithotectonic assemblage s: (1) a komatiite-tholeiite assemblage > 2803 Ma (based on ages of cross-c utting dikes); (2) an andesite-quartz arenite assemblage cut by similar dik es; (3) an assemblage of coarse volcaniclastic rocks, and (4) an upper mafi c assemblage of tholeiitic basalts with minor pyroxene komatiite volcanic r ocks. The andesite-quartz arenite assemblage (100 to ca. 750 m thick) has b asal amygdaloidal fragmental andesites overlain by massive andesites, then amygdaloidal and plagioclase phyric andesites. Unconformably overlying the andesite is a unit of quartz-rich sandstones (6-40 m thick) dominated by qu artz arenite extending several km along strike. At the north end, the quart z arenite succession consists of basal andesite overlain by quartz arenite exhibiting hummocky cross-stratification followed by aluminous coarse metas ediments and sulfidic argilite and an unconformably overlying tholeiite/kom atiite unit. At the south end, the succession is basal andesite, regolith, cross-bedded quartz arenite, weathered andesites, a second quartz arenite, argillite and then subaerial rhyolite. REE and HFSE geochemistry has been o btained on the rocks of the andesite-quartz arenite assemblage. The quartz arenites contain low abundances and chondrite normalized patterns vary from relatively fractionated to flat with most of the variation related to grai n size, with pebbly units having higher abundance and more fractionated pat terns. Combined major and trace element geochemistry indicates that a sodic felsic source with some admixture of mafic material will explain the geoch emistry of the quartz arenites. The andesites display moderately fractionat ed spidergrams with negative anomalies at Ti, Ta and Nb typical of are volc anism. The andesite-quartz arenite assemblage represents accumulation of sh allow water quartz rich sediments in a setting typical of the later stages of are volcanism in which the volcanic edifice is subaerial at the southern end of the assemblage. However, at the north end, our evidence is interpre ted as indicating subaerial andesitic volcanism, subsidence to a shallow ma rine basin which then deepens and rifts. Therefore the Hisovaara andesite-q uartz arenite assemblage provides a linkage in Archean greenstones between assemblages representing continental volcanism and a platform-to-rift setti ng. The presence of an erosional interval in > 2.8 Ga greenstones suggests possible pre-2.7 Ga orogeny in the Baltic shield. The pre-2.7 Ga quartzrich sedimentation is similar in age to platformal assemblages in the pre-2.7 G a North Caribou terrane of the Superior Province, Canada. Crown copyright ( C) 2000 Published by Elsevier Science B.V. All rights reserved.