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 assemblages: (1) a komatiite-tholeiite assemblage>2803 Ma (based on ages of cross-cutting dikes); (2) an andesite-quartz arenite assemblage cut by similar dikes; (3) an assemblage of coarse volcaniclastic rocks, and (4) an upper mafic assemblage of tholeiitic basalts with minor pyroxene komatiite volcanic rocks. The andesite-quartz arenite assemblage (100 to ca. 750 m thick) has basal 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 quartz arenite extending several km along strike. At the north end, the quartz arenite succession consists of basal andesite overlain by quartz arenite exhibiting hummocky cross-stratification followed by aluminous coarse metasediments and sulfidic argillite and an unconformably overlying tholeiite/komatiite 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 obtained 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 grain size, with pebbly units having higher abundance and more fractionated patterns. Combined major and trace element geochemistry indicates that a sodic felsic source with some admixture of mafic material will explain the geochemistry of the quartz arenites. The andesites display moderately fractionated spidergrams with negative anomalies at Ti, Ta and Nb typical of arc volcanism. The andesite-quartz arenite assemblage represents accumulation of shallow water quartz rich sediments in a setting typical of the later stages of arc volcanism in which the volcanic edifice is subaerial at the southern end of the assemblage. However, at the north end, our evidence is interpreted as indicating subaerial andesitic volcanism, subsidence to a shallow marine basin which then deepens and rifts. Therefore the Hisovaara andesite-quartz arenite assemblage provides a linkage in Archean greenstones between assemblages representing continental volcanism and a platform-to-rift setting. 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 Ga North Caribou terrane of the Superior Province, Canada.

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 assemblages: (1) a komatiite-tholeiite assemblage > 2803 Ma (based on ages of cross-cutting dikes); (2) an andesite-quartz arenite assemblage cut by similar dikes; (3) an assemblage of coarse volcaniclastic rocks, and (4) an upper mafic assemblage of tholeiitic basalts with minor pyroxene komatiite volcanic rocks. The andesite-quartz arenite assemblage (100 to ca. 750 m thick) has basal 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 quartz arenite extending several km along strike. At the north end, the quartz arenite succession consists of basal andesite overlain by quartz arenite exhibiting hummocky cross-stratification followed by aluminous coarse metasediments and sulfidic argilite and an unconformably overlying tholeiite/komatiite 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 obtained 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 grain size, with pebbly units having higher abundance and more fractionated patterns. Combined major and trace element geochemistry indicates that a sodic felsic source with some admixture of mafic material will explain the geochemistry of the quartz arenites. The andesites display moderately fractionated spidergrams with negative anomalies at Ti, Ta and Nb typical of are volcanism. The andesite-quartz arenite assemblage represents accumulation of shallow 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 interpreted as indicating subaerial andesitic volcanism, subsidence to a shallow marine basin which then deepens and rifts. Therefore the Hisovaara andesite-quartz arenite assemblage provides a linkage in Archean greenstones between assemblages representing continental volcanism and a platform-to-rift setting. 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 Ga North Caribou terrane of the Superior Province, Canada. Crown copyright (C) 2000 Published by Elsevier Science B.V. All rights reserved.