Quartz arenite-bearing assemblage Directory UMM :Data Elmu:jurnal:P:Precambrian Research:Vol101.Issue2-4.2000:

Fig. 3. Fragmental andesite with 15 – 30 cm fragments of massive andesite with fragments of uniform mineralogic com- position but varying in colour from grey to green. North shore of Lake Verkhnee; hammer 35 cm long. tion pattern with the sills and dikes concentrated on the northeast side of the belt Fig. 2, Table 1. As the goal of the present paper is to character- ize and interpret a quartz arenite – andesite associ- ation, uncommon for Archean greenstone belts, these units are described in more detail below.

3. Quartz arenite-bearing assemblage

This assemblage is comprised of three members: calc-alkaline andesites unit A, quartz arenite horizons unit Q and a unit dominated by felsic volcanic, volcano-sedimentary, reworked volcanic and chemical sedimentary rocks unit F. The boundary between the andesites unit A and the underlying Lower Mafic assemblage is most prob- ably a thrust, indicated by intense carbonatization and silicification near the contact as well as marked up to 1300 stretching of the andesite amygdale elongation along the a-lineation paral- lel to dip Kozhevnikov et al., 1992. The upper boundary of the assemblage is the unconformity at the base of the coarse clastic assemblage and the upper mafic assemblage. 3 . 1 . Andesite unit unit A 3 . 1 . 1 . Field description Massive amygdaloidal, glomeroporphyritic and coarse pyroclastic andesites are distinguished. The andesite unit varies markedly in thickness 100 – 700 m laterally Fig. 2. In the thickest part of the andesite unit, the following succession of rock types is observed from the base upward: 1 ca. 200 m-thick amygdaloidal, partly coarse, frag- mental andesites Fig. 3; 2 massive andesites locally showing some vague indications of internal heterogeneity thickness about 450 m; 3 amyg- daloidal types, seldom with indications of primi- tive pillow textures; thickness on the scale of a few metres to tens of metres; and 4 thick andesite flows with concentrations of plagioclase phe- nocrysts occasionally glomerophenocrysts to- ward the top. The thin glomeroporphyritic andesite horizon is overlain by the quartz arenite unit that can be mapped laterally for several km. However, there are substantial structural differ- belt as well as thin lenticular units among the rocks of the previous assemblage. Some indica- tions of discordance between these rocks and the underlying and overlying assemblages are appar- ent in the map patterns Fig. 2. The dacite-rhyo- lite to rhyolite clasts within these rocks are compositionally uniform and are more felsic than the dacitic matrix. This unit may represent either oligomictic conglomerates or matrix-supported volcaniclastic rocks. The latter interpretation is favoured by their close association with felsic flow top breccia observed at some localities. 2 . 3 . 4 . Upper mafic assemblage The fourth essentially volcanogenic assemblage is represented by a thick pile of pillowed tholeiitic basalts with some thin pyroxenitic komatiite flows and sills in the lower part. This unit rests uncon- formably on the second and third assemblages and is cut by individual undated rhyodacite and granodiorite dykes. 2 . 3 . 5 . Intrusi6e rocks Near the margins of the belt, supracrustal rocks are cut by tonalites in the north and by granites in the south. The interior of the belt is cut by andesite-dacite sills, rhyodacite-rhyolite quartz and quartz-plagioclase porphyry dykes and stocks as well as mafic and ultramafic sills and dykes. The rocks that cut the volcano-sedimentary assemblages show an asymmetric areal distribu- ences between detailed sections A and B Fig. 2. The basic difference lies in the fact that on the southern shore of Lake Verkhneye section B two stratigraphic subunits of andesites A1 and A2 are distinguished Fig. 4. Here, as in section A, glomeroporphyritic andesites are overlain by a cross-bedded quartz arenite unit. Resting on the quartz arenites are two texturally similar andesite horizons, each 6 – 7 m thick. The horizons consist of flow material succeeded upwards by subunits which display vertical grading from pyroclastic breccia to tuff and lapilli tuff. The third horizon, a massive flow is overlain by a ca. 7.0 m-thick quartz arenite bed Fig. 5. Except for this local- ity, no sedimentary rocks have been found in the andesite unit. All the contacts between andesites and quartz arenites are well-defined and occasion- ally tectonized. In section B, A.B. Samsonov exposed a contact between glomeroporphyritic andesites and a lower quartz arenite horizon Fig. 6. In the ca. 50 cm-thick zone near the contact, weathering in the andesite is indicated by rock disintegration and fine regolith-filled cracks. The andesite immedi- ately beneath the quartz arenite contact displays coarse garnet and quartz stringers not found else- where in the unit. These features suggest modifica- tion of original chemical composition near the contact. At two points, the reddish colour of andesite, observed near the contact within a ca. 1.0 m-wide band, is due to groundwater circula- tion in the more porous contact zone. 3 . 1 . 2 . Petrography All varieties of andesites have consistent min- eral compositions. Their distinct crystallization, schistosity, mineral or aggregate lineation and nematogranoblastic structure indicate the meta- morphic nature of the mineral assemblages. The assemblage green hornblende + plagioclase An = Fig. 4. Outcrop map of the andesite-quartz arenite association in the section B area. Fig. 5. Stratigraphic columns for andesite-quartz arenite asso- ciation in sections A and B. Sample numbers appear to the left of the columns. 23 + quartz + brown biotite + chlorite + epi- dote is most common. Magnetite and apatite occur as accessories. Hornblende is clearly oriented along the earlier lineation which is parallel to the dip. The homoaxial replacement of hornblende by colour- less twinned cummingtonite, observed along ca. 10 m-thick subconcordant zones within the andesite unit, suggests a local rise in temperature in these zones. The formation of carbonate ankerite near the lower contact with ferrobasalts is related to the circulation of carbon dioxide solutions in the tec- tonically active contact zone. In a ca. 1.0 m-thick zone near the upper contact with the quartz arenite unit, the garnet poikiloblasts with numerous unde- formed rounded quartz inclusions are flattened in the schistosity plane. Also in this zone, large hornblende poikiloblasts are oriented across earlier lineation along the late subhorizontal lineation which is parallel to the A c -axes of late shear zones. Fine up to 2.0 mm granoblastic quartz veinlets and chlorite rosettes, concordant with schistosity, are observed here. In amygdaloidal andesites, amygdales are filled with milky quartz as well as quartz-plagioclase or quartz – chlorite – carbonate aggregates. Plagioclase plates, up to 1.0 cm in length, that occasionally form stellate aggregates are characteristic of glomeroporphyritic andesites. 3 . 1 . 3 . Geochemistry The Hisovaara andesites belong to the tholeiitic magma clan using the scheme of Jensen 1976. With respect to major element geochemistry, the Hisovaara andesites are tholeiitic andesites similar to those of the Blake River Group in the Abitibi greenstone belt Xie, 1996. Based on the field appearance of the unit in general and the low LOI values, we expected to see little evidence for alter- ation. Two pairs of andesite samples 94-PCT-002 004 and 94-PCT-022 023 were taken, with one of each pair from within a few cm of the quartz arenite and the other member of the pair from 1 – 2 m beneath the contact. When major element data for the andesites are compared, we observe with increasing proximity to the con- tact: addition of Fe + 3 , K, and P, loss of Mg and Na and variable behaviour of Si, Fe + 2 , Al, Na and Ca. Spidergrams of trace element geochemistry Fig. 7 display a fractionated pattern with nega- Fig. 6. The basal contact of the quartz arenite with the underlying andesite at location B. The lighter coloured quartz arenite lies above the lighter about 7.5 cm long and the weathered andesite beneath the lighter. tive anomalies for TiO 2 , Ta and Nb typical of island arc volcanism. The negative Hf anomalies in some samples are due to incomplete sample dissolution verified by comparing Zr values ob- tained by XRF with those obtained by ICP-MS and corroborated by complete dissolution of simi- lar samples using the closed beaker technique Jenner, 1996. REE data for these sample pairs are plotted in Fig. 8a and b. The LREE are quite variable for a suite of samples obtained within a stratigraphic section of a mesoscopically similar rock type a few metres in thickness. In basaltic rocks within a coherent unit, LREE variation is conventionally ascribed to fractionation of clinopyroxene 9 pla- gioclase. No conventional igneous fractionation process will yield sample-to-sample variation in Ce anomalies or crossing REE patterns. Comparison of the samples immediately under- lying the quartz arenite vs. those somewhat re- moved from the contact reveals: marked LREE depletion, a negative Ce anomaly, some depletion of the HREE and loss of Rb and Cs Figs. 7 and 8a, b. A sample of the regolith Fig. 8c shows negative Ce and Eu anomalies and extreme deple- tion of the HREE. Precambrian weathering of basaltic rocks Kimberley and Grandstaff, 1986 results in Na depletion, variable behaviour of the heavier alkalies K,Rb, Cs and the alkaline earths Ca, Mg. Fe shows an upward decrease into the paleosol but this pattern can be disturbed by downward percolation of Fe-bearing ground- water. These authors also report depletion of the LREE in weathering of the Kinojevis basalts in the Abitibi subprovince. In general the behaviour of REE in weathering of mafic rocks seems some- what variable Braun et al., 1990; Marsh, 1991; Price et al., 1991. Leaching experiments indicate that incipient alteration tends to release REE with behaviour controlled by groundwater parameters flux, Eh, pH and the secondary minerals pro- duced during alteration Price et al., 1991. The proximity of samples 94-PCT-004 and 94-PCT- 022 to the quartz arenite contact and the presence of mineralogical changes near the contact and the similarity to the above studies of Precambrian and younger basaltic weathering suggest that the chemical changes we observe may be related to weathering of andesite. If the alteration were due to recent weathering, this cannot explain the min- Fig. 7. Spidergram extended trace element diagram using the normalizing factors of Wood et al. 1986 for the Hisovaara andesites. eralogical differences in samples proximal to the quartz arenites, reddening of the fresh surface near the quartz arenite or the more intense alter- ation being in samples taken closest to the quartz arenites. Therefore weathering took place prior to quartz arenite deposition. 3 . 2 . Quartz-arenite unit Q 3 . 2 . 1 . Field description Quartz arenites associated with andesites were traced over several kilometres on the northern flank of the Hisovaara structure and were studied in detail at some localities Fig. 2. Quartz arenite unit Q is subdivided into subunits Q1 and Q2. Lower subunit Q1, in contact with the andesite unit, was revealed at all the points shown on the map Fig. 2. Upper subunit Q2 was found only on the northern shore of Lake Verkhneye in section B. Considerable variations in diverse tex- tures and primary structures, apparent both later- ally and vertically, are characteristic of unit Q. For example, subunit Q1 varies in thickness from 6 – 8 m points C and D, Fig. 2 to 10 – 12 points B, E, 913 and even 40 m in section A. Other differences are apparent in comparing sections A and B. In section A, white fine- to medium- grained thin-laminated quartz arenites rest with a sharp direct contact on glomeroporphyritic andes- ites Fig. 9. Hummocky cross-bedding with char- acteristic low angles between cross-bedded units and erosion surfaces is observed in some out- crops, depressions being filled with micaceous argillic material Fig. 10. Quartz clasts are pre- dominantly sand sized, except sample 94-PCT-012 that includes small B 1.0 cm quartz pebbles. The fairly high degree of rounding of fine quartz pebble material reflects the textural maturity of the rocks. The prevalent white colour of the rocks with a very small admixture of stained minerals indicates the high mineralogical maturity of these quartz arenites. One-centimetre-thick feldspar-rich laminae are preserved locally. Primary stratifica- tion is retained, despite a pressure-solution cleav- age which cross-cuts bedding at a medium angle. The rocks are recrystallized strongly enough to form metamorphic quartz veins, with the degree of recrystallization increasing toward the upper contact of subunit Q1. Fig. 8. A Chondrite normalized REE profile for andesites 94-PCT-002 and 94-PCT-004 taken from beneath the quartz arenite at location A. B Chondrite normalized REE profile for andesites 94-PCT-022 and 94-PCT-023 taken from beneath the quartz arenite at location B. C Regolith. Fig. 9. Unconformity between massive andesite bottom of photo and overlying quartz arenite. Lighter colour of andesite adjacent to the quartz arenite can be seen. Lens cap, 52 mm in diameter. tion A. Here, a 10 – 20 cm-thick fine pebble quartz conglomerate horizon lies at its base and has a sharp contact with weathered glomeroporphyritic andesite. White, moderately rounded, flattened quartz pebbles, up to 3.0 cm in size along the long axis, are closely packed and supported by sand- sized mica and quartz matrix. These rocks gradu- ally pass upwards to fine pebble arenites. Poorly rounded, often angular pebbles measuring 1.0 – 1.5 cm are represented solely by white quartz. The grey quartz arenite matrix consists of unequally rounded commonly angular quartz grains, 1.0 – 2.0 mm in size, with the addition of biotite, which suggests the low textural and mineralogical matu- rity of the rock. In this unit, which has an ex- posed thickness of ca. 7.0 m, trough bedding, most distinct in its lower half, is obvious. The upper contact between subunit Q1 and the andes- ites of subunit A2 is not visible because it is covered by Quaternary deposits. In section B, both subunits are represented, with subunit Q1 markedly differing in some char- acteristics from its stratigraphic analogue in sec- Fig. 10. Map of quartz arenite exposures with hummocky cross-bedding subunit Q1 in section A. The second quartz-rich rock horizon subunit Q2 is up to 7.0 m thick. It rests with a sharp contact on the weathered andesites of subunit A2. Subunit Q2 differs in some macroscopic features from subunit Q1. It is characterized by: 1. The yellowish, locally rusty colour of the rocks caused by the presence of finely dispersed al- tered iron sulphides. 2. Thin, locally deformed parallel lamination. 3. A dominant sand size and smaller dimensions of quartz grains and scarce thin 10 – 15 cm horizons containing fine B 1.0 cm quartz pebbles sample 94-PCT-015. 4. The presence of ca. 30 cm-thick horizons with very fine-grained B 0.05 mm quartz sample 94-PCT-021. 5. The occurrence of thin 10 – 15 cm muscovite- enriched horizons sample 94-PCT-019 re- sponsible for the graded nature of individual beds of this subunit. Resting directly on subunit Q2 are 2.0 – 3.0 m of carbonaceous rocks overlain by felsic rocks that represent rhyolitic ash flows. 3 . 2 . 1 . 1 . Petrography. Highly siliceous rocks are divided based upon microscopy into several types that differ in the nature of clastic material, the degree of quartz recrystallization, and the rela- tionships between quartz and other silicates etc. Quartz arenites, the most abundant rock type of the unit, are bedded rocks in which quartz-rich beds alternate with beds that contain quartz and other silicates. Ninety to ninety-five per cent of the rock consists of quartz grains varying from 0.2 to 2 – 3 mm in diameter. Intense recrystalliza- tion gives rise to the less common sutured polygo- nal boundaries of grains in quartz monocrystals and deformation that continues until lenticular aggregates are formed. It is, therefore, impossible to estimate the degree of roundness of fine clastic quartz. Two types of plagioclase are observed in the quartz arenites: a fine, poorly rounded clas- tic grains are commonly filled with grey dust-like opaque material along cleavage cracks, which is presumably due to weathering; b newly-formed plagioclase grains occur together with garnet and amphibole in the form of fine chains that fill interstices between quartz grains. Trace minerals occur as newly-formed grains or chains of mica, amphibole, garnet, kyanite, staurolite and chlorite aggregates. Zircon, sphene and opaque ore miner- als occur as detrital accessories. Pebbly quartz arenites are second in abundance in the section. Unlike the quartz arenites de- scribed above, they contain polycrystalline quartz interpreted as vein quartz strongly elongated along the A c axis which is parallel to the mineral lineation and dip of the rocks. In cross section, normal to lineation, various shapes of quartz pebbles subrounded to angular, but generally poorly rounded that vary in size from 0.2 × 1 – 1 × 2 cm over 5 cm along lineation in these cross-sections are easily observed in sawed slabs. Individual quartz pebbles constitute thin centime- tre-scale laminae in quartz arenites that provide the matrix for coarser quartz. Trace minerals and accessories are similar to those described above, but their quantities vary markedly. For example, pebbly quartz arenites from section B contain more biotite, and at station 913 zircon is present in large amounts about 30 grains in one thin section. In subunit Q2, two more rock types were seen in section B. In sample 94-PCT-021, fine equigranular quartz grain size B 0.05 mm in which individual coarser 1 – 2 mm deformed de- trital grains are observed. The quartz arenites pass upwards into thin-laminated muscovitic quartz arenites. This subunit includes chemical sediments and tuffaceous material. Several populations of zircon are found in the quartz-rich sedimentary rocks of the quartz arenite-andesite association at Hisovaara. The transparent long-prismatic grains are probably metamorphic based upon similarity to metamor- phic zircons cf. D.W. Davis, pers comm 1996 Rounded, transparent and dark metamict grains are prominent among detrital zircons. Of great interest are scarce poorly rounded, broken zircon prisms that may indicate a fairly proximal source of zircon. It is also important that both macro- scopic and petrographic observations point to the absence of lithic fragments in the quartz arenite unit. Vein quartz pebbles and clastic zircons are the most distinct indications of detritus whose textural maturity was presumably variable. A neg- Table 2 Mean chemical compositions of quartz-arenites from Hisovaara and other Archean regions a 1 2 3 4 5 6 7 8 94.5 92.13 SiO 2 95.97 94.4 87.02 88.65 92.11 93.45 0.05 0.06 0.02 0.1 0.04 0.26 TiO 2 0.07 0.04 2.21 Al 2 O 3 2.44 4.34 2.24 4.47 7.71 4.58 4.07 1.4 1.07 Fe 2 O 3 0.79 1.58 1.95 0.7 1.09 0.55 0.02 0.01 0.01 0.08 0.02 MnO 0.01 0.02 0.42 MgO 0.35 0.31 0.2 2.47 0.44 0.88 0.27 0.26 0.26 0.07 2.98 CaO 0.1 0.43 0.09 0.14 0.46 0.98 0.04 0.07 0.76 0.36 Na 2 O 0.24 0.17 0.15 K 2 O 0.42 0.83 0.65 0.83 1.76 1.01 1.24 0.05 0.02 B 0.01 P 2 O 5 0.02 0.02 0.01 0.01 0.03 38.8 21.2 42.8 19.5 42.7 11.5 SiO 2 Al 2 3 20.1 23 0.2 K 2 ONa 2 O 0.9 0.8 16.2 11.9 4.9 4.21 7.3 5.3 4.2 56 63.9 21.4 19.1 23.9 Al 2 O 3 Na 2 O 2.9 63 62 71 n.d. 52 74 CIA 73 69 4 9 N 1 10 22 3 8 49 a Recalculated to 100 on a volatile-free basis. 1–4 Quartz arenites from Hisovaara: quartz arenites, subunits Q1 1 and Q2 2; pebbly quartz arenites, subunits Q1 3 and Q2 4. 5–6 Quartz arenites, Keewaywin formation 5 and Keeyask Lake Formation 6, Sandy Lake greenstone belt Superior Province, Canada Cortis, 1991. 7 Quartz arenites, Pongola supergroup, S. Africa Wronkiewicz, Condie, 1989. 8 Quartz arenites, Yavanahalli belt, S. India Argast and Donnelly, 1982. ligible amount of plagioclase and the absence of lithic fragments suggest that in a quartz-feldspar- lithic fragment sandstone provenance, the compo- sitions of the quartz-rich metasediments of the quartz arenite-andesite assemblage at Hisovaara lie on the ‘quartz-feldspar line’ near the quartz apex i.e. in field 1, suggestive of a cratonic provenance Dickinson, 1985. 3 . 2 . 1 . 2 . Geochemistry. The major and rare earth element geochemistry of quartz arenites and asso- ciated rocks from the Hisovaara greenstone belt is shown in Tables 2 – 4. Both field and microscopic characteristics are used to define three groups: quartz arenites Q1 and Q2 and pebbly quartz arenites Q1. Pebbly quartz arenite sample 94- PCT-015 and mica quartz arenite sample 94PCT-019 from subunit Q2 were analysed sepa- rately. In the above groups, some major compo- nents such as SiO 2 , Al 2 O 3 , FeO, Fe 2 O 3 , CaO, Na 2 O and K 2 O vary over a wide range. Variations in TiO 2 and MgO content are less appreciable. We compare the average compositions of the Hiso- vaara quartz arenites with similar rocks from other Archean regions Table 2 but they have some distinctive characteristics. For example, Hisovaara quartz arenites contain more SiO 2 , CaO and Na 2 O and less Al 2 O 3 and K 2 O. Their SiO 2 Al 2 O 3 ratio is higher and K 2 ONa 2 O ratio is lower. The high CaO content of the quartz arenites in the Keewaywin Formation is obviously due to superimposed carbonatization Cortis, 1991. The CIA value CIA = [Al 2 O 3 Al 2 O 3 + CaO + Na 2 O + K 2 O] × 100; Nesbitt and Young, 1982 estimated for this group is not given be- cause in the calculations CaO represents Ca in a silicate form McLennan et al., 1990. Many sam- ples collected in subunit Q1 show ultralow B 0.2 K 2 ONa 2 O ratios Fig. 11. The above character- istics of the rocks described, emphasized by the strong predominance of Na 2 O over K 2 O, indicate that the matrix of Hisovaara arenites, in which undecomposed plagioclase grains play a major role and the pelite component is less significant and chemically immature. The only exception is mica quartz arenite sample 94-PCT-019 which is abnormally poor in SiO 2 and abnormally rich in Al 2 O 3 and K 2 O. The chemical indices of alteration CIA; Nes- bitt and Young, 1982, calculated for the above rock groups are much lower than those for quartz arenites from other Archean regions. This indi- Fig. 11. SiO 2 Al 2 O 3 -K 2 ONa 2 O plot for quartz arenites. cates the lower chemical maturity of quartz arenites at Hisovaara. The lowest mean CIA value of 52 was determined for the quartz arenites of subunit Q1. A fairly unusual combination of low chemical maturity and very high SiO 2 content, observed in the rocks discussed, is largely respon- sible for their trace element geochemistry. Analysis of REE content and REE distribution patterns has revealed some distinctive features of sedimentary rocks at Hisovaara. First of all, it should be noted that their SREE values are much lower than those of Pongola quartzose sandstones Fig. 12. Extremely small quantities of rare earths SREE=4.46–15.20 ppm were determined for the quartz arenites of subunit Q1. The pebbly arenites of both subunits and the quartz arenites of subunit Q2 contain REE in much greater quan- tities. The REE content of mica quartz arenite in sample 94-PCT-019 is abnormally high SREE= 96.76 ppm. This supports the conclusion that REE dominantly form part of micas, i.e. the argillic matrix of quartz arenites Wronkiewicz and Condie, 1989. All rock samples show a nega- tive Eu anomaly Eu Eu = Eu n Sm n ,Gd n 12 value of 0.67 Taylor, 1979. About 80 of Archean sedimentary rocks have Eu ] 0.85 McLennan et al., 1984, 1990. There are marked differences in REE distribu- tion pattern between the groups studied. This primarily applies to the quartz arenites of sub- units Q1 and Q2. In subunit Q1, two types of samples are distinguished. Type 1 samples 94- PCT-005 and 011 is characterized by slightly fractionated REE distribution La N Yb N = 2.78 – 4.32, flat HREE distribution Gd N Yb N = 0.9 – Fig. 12. Chondrite-normalized generalized REE diagrams for quartz-rich sedimentary rocks at Hisovaara and Pongola Wronkiewicz and Condie, 1989. Hisovaara rocks are markedly depleted in REE. Both groups are similar to typical post-Archean REE distribution pattern with a negative Eu anomaly. Fig. 13. Chondrite normalized REE profiles for Hisovaara quartz arenites. A Unit Q1 type 1; B unit Q1, type 2; C unit Q2 fractionated REE patterns; and D pebbly quartz arenites. 1.02, a higher EuEu value 0.75 and a mini- mum SREE value 4.46–6.41 ppm Fig. 13A. Type 2 samples 94-PCT-007, 008 and 010 is characterized by a higher La N Yb N ratio 8.82 – 12.15, more fractionated SREE distribution Gd N Yb N = 1.74 – 2.0, lower EuEu values 0.59 – 0.74 and higher SREE values 10.23– 15.20 ppm Fig. 13B. Subunit Q2 shows even more diverse REE characteristics. For example, sample 94-PCT-016 is fairly similar in REE frac- tionation pattern La N Yb N = 3.91, Gd N Yb N = 0.92 to samples of type 1 from subunit Q1. However, its higher REE value is observed to- gether with a far more intense negative Eu anomaly EuEu = 0.59 Fig. 9A. Samples 018 and 020 occupy an intermediate position in terms of REE characteristics between types 1 and 2 in subunit Q1 La N Yb N = 8.29 – 8.51, Gd N Yb N = 1.20 – 1.49, EuEu = 0.70 – 0.73 Fig. 13B. Sam- ples 94-PCT-019 and 021 show maximum REE La N Yb N = 24.57 and 25.43 and SREE Gd N Yb N = 3.08 and 2.76 fractionation, but they dif- fer markedly in EuEu ratio 0.82 and 0.59, respectively Fig. 13C. In both subunits, pebbly quartz arenites generally have more persistent REE and SREE fractionation patterns, but their EuEu values vary substantially 0.53 – 0.80 Fig. 13D. Sample 94-PCCT-017 from altered felsic tuff differs greatly in REE distribution from the rocks described. It has a slightly fractionated La N Yb N = 2.46 REE distribution, marked SREE enrichment Gd N Yb N = 0.76 and a pro- nounced positive Eu anomaly EuEu = 2.00 Fig. 13C. In the Gd N Yb N – EuEu diagram McLennan and Taylor, 1991, Hisovaara quartz-rich sand- stones plot outside the Archean sedimentary rock field because they have a stronger negative Eu anomaly. Partial overlap with the Archean sedi- mentary rock field is observed in the low Gd N Yb N range Fig. 14A. Th and U content varies between and within the rock groups differentiated Fig. 14. In sub- unit Q1, quartz arenites contain less U average U content is 1.02 ppm and especially Th average Th content is 3.62 ppm than pebbly arenites U = 1.57 ppm, Th = 8.95 ppm. One exception is sample 94-PCT-013 with an abnormally small amount of U 0.49 ppm and, correspondingly, an abnormally high 15.1 ThU ratio. The quartz arenites of subunit Q2 are markedly richer in both elements U = 1.58 ppm and Th = 7.28 ppm than the quartz arenites of subunit Q1. ThU values vary from 2.7 to 15.1, extremely high values being due to either extremely low U content sample 013 or abnormally high Th content sample 94- PCT-021. Generally speaking, in Th versus ThU coordinates McLennan and Taylor, 1991 the Hisovaara quartzose sandstones plot completely within the Archean sedimentary rock field. Th and U distribution seems to be largely controlled by the distribution of heavy minerals, primarily zircon, indicated by the fact that anomalous quantities of some trace elements Zr = 933 ppm; Th = 338 ppm; Y = 57 ppm and Pb = 72 ppm that can only be explained by the accumulation of relatively abundant zircon as found in sample 913 Kozhevnikov, 1992. 3 . 2 . 1 . 3 . Interpretation. The above geological, pet- rographic and geochemical data on the quartz arenite unit can be discussed from two aspects. One aspect is related to the source of both quartz and the other detrital components which consti- tute the unit. The other aspect is the reconstruc- tion of the depositional environment and depositional mechanism of these rocks that are the oldest sedimentary rocks in the Hisovaara greenstone belt. With respect to the provenance of the quartz arenites, identification of class types is the most informative data set. Hisovaara rocks contain no lithic fragments that directly indicate possible sources. Therefore, data on the geochemistry of immobile elements such as REE, major element chemistry, abundance of quartz pebbles and some textural characteristics of the rocks discussed are critical. Vein quartz, normally present in addition to other types of rock fragments, has been reported from practically all Archean quartz-rich sediments Eriksson, 1980; Srinivasan and Ojakangas, 1986; Bhattacharyya et al., 1988; Wronkiewicz and Condie, 1989; Cortis, 1991, etc.. Several rock types can be proposed as hypothetical sources. Fig. 14. Gd N Yb N -EuEu A and Th-ThU B plot for quartz-rich rocks. Same symbols as in Fig. 8. Negative Eu anomalies are markedly higher in Hisovaara rocks than in Archean and a large part of Proterozoic rocks. In the Th-Th U coordinates, they are, in fact, completely delineated by Archean rocks. In Proterozoic rocks, ThU ratio is generally lower. Archean and Proterozoic rock fields are given after McLennan and Taylor, 1991; Fig. 7c, d and Fig. 8c, d. Geochemical, mineralogical and other data place a limit on some sources that are probable in principle. It should be noted that when using REE geochemistry to constrain sources of the clastic material in the quartz arenites, we assume that: 1. Quartz-rich clastic rocks adequately reflect source area geochemistry. This follows from flat REE distribution patterns in quartz sands normalized in terms of the REE content of associated muds in both present-day passive margin environments Biscay, Ganges and ac- tive continental margin settings such as back- arc basins Japan and continental arcs Java McLennan et al., 1990. 2. The low SREE content of Hisovaara rocks is presumably due to the diluting effect of quartz. This phenomenon was used to explain small quantities of SREE in Archean quartz arenites McLennan et al., 1984; Wronkiewicz and Condie, 1989 and the commonly ob- served low REE content of modern sands in comparison to that of associated muds, in which LaYb, LaSm and GdYb ratios being either unchanged or slightly disturbed McLennan et al., 1990. 3. The role of vein quartz in the REE distribu- tion pattern, i.e. La N Yb N , La N Sm N , Gd N Yb N and EuEu values, is presently impossible to assess properly because there are no data on these parameters for the vein quartz of Hisovaara rocks. The relevant evi- dence, in the literature, is scanty Siddaiah et al., 1994. Therefore, such an assessment is of limited value. 4. Low CIA, low K 2 ONa 2 O and Al 2 O 3 Na 2 O and high SiO 2 Al 2 O 3 values observed in the Hisovaara quartz arenites indicate that the initial geochemical parameters of the felsic source areas are retained better here than in similar rocks from other regions. The chemical-exhalative mechanism for devel- opment of a high SiO 2 concentration in a hypo- thetical source of clastic quartz cannot really be applied to Hisovaara rocks because of their dis- tinct negative Eu anomaly which is sharply posi- tive in chemically precipitated rocks Bavinton and Taylor, 1980; Siddaiah et al., 1994. An ad- mixture of chemically precipitated material is pos- sible, in principle, in sample 94-PCT-017 which has slightly fractionated REE distribution and a strong positive Eu anomaly. Weathering of quartz-rich amygdaloidal andes- ites could provide a source of quartz. It could accumulate in some settings, e.g. a nearshore beach zone. However, the possible mechanism for such complete andesite decomposition, needed to explain the complete absence of lithic fragments, remains obscure. Furthermore, weakly positive Eu anomalies in andesites do not favour this option. Quartz-rich metasomatic rocks could be a source of quartz, as observed, for example, in some Proterozoic rocks in Karelia Kozhevnikov and Golubev, 1995. Fragments of metasomatic rocks, fuchsite schists, tourmaline quartz arenites and other rocks are found in some Archean belts Luukkonen, 1988; Cortis, 1991; Kozhevnikov, 1992. Metasomatic quartz rocks usually contain no plagioclase, and the rocks consist of a quartz- mica association, Ca and Na being completely removed. The presence of plagioclase in the detri- tal material of quartz arenites at Hisovaara and their low CIA value does not seem to favour a metasomatic source. The REE distribution pat- tern of most of the samples analysed indicates that the major constituent of the source was rep- resented by felsic rocks, which is reflected in LREE enrichment, fractionated HREE distribu- tion and a negative Eu anomaly. Their sodic nature, which is retained even during partial weathering, suggests that they could be tonalite- type granitoids or felsic volcanics of a sodic series. Judging by the presence of at least two types of detrital zircon in the samples analysed, the felsic source is assumed to be complex. Some problems that arise when the ‘quartz budget’ in quartz-rich sediments is estimated using a granitoid destruc- tion mechanism Pettijohn et al., 1972 can be overcome by assuming that hypabbysal subvol- canic bodies, typically containing abundant vein quartz, were destroyed. To assess the possible sources of the material, which comprises subunit Q, calculations were made using the REE content of the rocks. The amount of REE in the tonalites, rhyolites and komatiites of the lower mafic assemblage was employed as end members. Komatiites were in- cluded in calculations for several reasons. Firstly, it has been found earlier that the Cr content of subunit Q1 varies over a very broad range from 42 to 581 ppm Kozhevnikov and Travina, 1993. Secondly, thin maximum 15 cm Cr-enriched metasandstone horizons that contain finely dis- persed metamorphic fuchsite were observed in quartz arenites at several levels above quartz arenite subunit Q, which testifies to the presence and weathering of an ultramafic source Kozhevnikov, 1992. Thirdly, flat HREE distribu- tion and slight LREE enrichment at low La N Yb N and a strong negative Eu anomaly require the use of an additional component for interpretation. This component must have characteristics most similar to those of the komatiites in the lower mafic association. Table 5 shows the chondrite-normalized REE content of the rocks that hypothetically form the end members of possible sources tonalites, komati- ites and rhyolites, for a series of quartz-rich samples from subunits Q1 and Q2 and in estimated mixtures diluted with quartz containing negligible REEs. Estimated normalized REE values are sim- ilar to those observed in the Hisovaara quartz arenites. Their distribution curves are similar, too Fig. 15. This suggests that such REE distribution, e.g. those observed in samples from unit Q1, could be indicated by the deposition of the products of destruction of a bimodal source with various ratios of felsic to ultramafic components. Tonalite de- struction products were deposited dominantly within individual thin horizons sample 94-PCT- 007. In the course of subunit Q2 formation, the role of a felsic source increased steadily, the degree of quartz dilution being possibly lower. Intense weathering of tonalites and ultramafic rocks must have followed the uplift and deep erosion of the roots of the greenstone belts domi- nated by the rocks of the mafic assemblage, includ- ing komatiites and hypabbyssal tonalitic plutons saturated with vein quartz. In this case, conditions favourable for the concentration of the most resis- tant rock destruction products, e.g. quartz, heavy minerals and partly plagioclase, must have been formed locally. A river channel in a deeply eroded mountain system or in any other small, seasonally drained basin is a possible environment. It seems to be the type of setting in which clastic e.g. pebbly quartz material could be abraded for a short time and the multiple intense scour of deposits, accom- panied by the formation of quartz sands and coarse gravel, could occur. The proximity of sources of clastic material and its very rapid transport without long abrasion is indicated by: 1. The textural immaturity of pebble-sized clasts. 2. The occurrence of unrounded, euhedral zircon fragments. Some characteristics of the quartz arenite accumu- lation field can be reconstructed by summarizing the above evidence. Judging by the bedding pat- terns in quartz-rich and associated rocks, they were deposited in a marine setting. In section A quartz arenites, hummocky cross bedding could be formed in a shelf zone affected by contour currents and storm waves at a depth up to 90 m, i.e. above the storm wave base Duke, 1985. Other possible environments for the occurrence of hummocky cross stratification are known [flash-flood braided delta Hjellbakk, 1993, eolian systems Langford 1989, and antidunes in a fluvial channel Rust and Gibling 1990]. The angularity of quartz pebbles is presumably due to limited transport distances. Furthermore, the transition to overlying sulfidic argillites, could represent basin deepening e.g. Hoffman, 1987 or development of lagoonal condi- tions. This transition could be rapid enough for the burial of texturally immature quartz-rich rocks. The trough cross-bedding, observed in the quartz- rich sandstones of section B, could form in small channels near the shoreline Mueller and Dimroth, 1985. Association of these rocks with subaerial andesites and rhyolites favours such a setting. The two andesite units Fig. 5 differ in texture, but are identical geochemically, thus indicating similar magma-generating conditions. This points to a short time interval between the two episodes of andesite volcanism, the succession of processes being: andesite volcanism — a non-depositional interval — the formation of a thin weathered crust-rapid transport and deposition of quartz rich sedimentary rocks. Addition of tuffaceous material to subunit Q2 indicates that sedimentation associ- ated with the completion of andesite volcanism continued as well as rhyolite volcanism. P .C . Thurston , V .N . Kozhe 6 niko 6 Precambrian Research 101 2000 313 – 340 a 6 7 8 9 10 11 12 13 14 15 16 17 18 1 2 3 4 5 94-PCT- 94-PCT- 92-8 92-12 92-14 94-PCT- 92-10 94-PCT- 92-16 094- 92-35 94-PCT- 92-37 92-38 94-PCT- 94-PCT- 92-36 94-PCT- 003 012 012 008 003 011 010 PCT-05 007 93.80 94.40 96.08 96.48 SiO 2 96.98 91.92 87.52 88.42 91.80 91.94 92.06 92.64 93.48 94.30 92.00 93.00 93.24 03.50 0.08 0.02 0.02 0.02 B 0.01 0.01 0.01 0.10 0.08 0.04 0.06 0.07 0.06 0.03 0.04 TiO 2 0.06 0.06 0.02 2.35 2.35 1.70 1.84 1.18 6.72 6.15 4.20 4.45 3.28 4.46 2.62 3.15 2.62 2.75 2.31 Al 2 3 2.36 2.03 0.16 1.29 1.16 1.28 0.04 0.16 0.10 1.14 t 0.26 0.63 t 0.63 t 0.10 153 t 0.44 0.44 149 1.13 0.72 Fe 2 O 3 0.57 1.00 0.58 0.29 1.22 0.57 0.72 – 0.93 – – 0.72 – 0.86 0.50 1.00 0.43 1.15 FeO 0.00 0.99 0.01 0.01 0.01 0.02 0.01 B 0.01 0.02 0.01 0.01 MnO 0.02 0.01 0.01 0.02 0.00 0.02 0.10 0.35 0.20 0.20 0.15 0.45 0.30 0.60 0.20 MgO B 0.05 0.75 0.30 0.30 0.60 0.20 0.80 0.65 0.40 0.25 0.63 0.42 0.07 0.07 0.07 0.42 0.07 0.52 0.42 0.07 0.07 0.98 0.50 0.07 0.07 CaO 0.30 0.49 1.12 169 1.25 0.85 0.35 0.63 0.56 0.12 0.36 0.03 0.52 1.50 0.87 0.55 0.87 0.06 0.85 0.30 1.00 Na 2 O 0.05 0.31 0.06 0.10 0.21 2.23 0.19 1.15 0.33 K 2 O 0.91 0.04 0.43 0.38 0.90 0.04 0.62 0.07 0.03 0.08 0.10 0.05 0.02 0.07 0.20 0.10 0.10 0.15 0.05 0.04 0.08 0.07 0.15 0.95 H 2 O 0.04 0.08 0.10 0.14 0.25 0.16 0.23 0.17 0.19 0.11 0.76 011 0.58 0.18 0.28 0.56 0.21 0.35 0.27 0.28 0.20 L.O.1 P 2 O 5 n.d. n.d. n.d. B 0.01 B 0.01 B 0.01 0.05 B 0.01 0.03 0.03 B 0.01 0.03 B 0.01 0.01 n.d. n.d. 0.24 B 0.01 100.03 99.95 100.19 100.16 100.03 99.80 99.95 99.77 99.71 99.77 99.89 99.62 99.84 99.76 99.68 99.76 Total 100.08 99.78 53 37 47 58 58 61 68 64 54 58 55 64 58 57 69 36 56 50 CIA SiO 2 28.51 38.95 39.91 40.17 56.52 52.43 82.19 13.02 14.38 21.86 20.66 20.64 2941 35.68 34.29 45.32 35.50 40.36 Al 2 O 3 0.02 0.06 0.89 0.10 0.18 1.75 6.19 0.06 2.21 2.07 0.22 0.07 105 0.78 0.44 15.0 0.05 0.03 K 2 O Na 2 O 1.94 2.76 6.71 2.70 3.28 9.82 18.67 Al 2 O 2.03 1.89 8.08 3.00 5.13 8.73 3.01 45.83 2.39 8.73 2.31 Na 2 O 24 25 26 27 28 29 30 31 21 32 20 33 34 35 36 37 22 23 19 94-PCT- 94-PCT- 94-PCT- 94-PCT- 94-PCT- 94-PCT- 94-PCT- 913 94-PCT- 94-PCT- 576-4 578-2 577-2 92.39 94-PCT- 94-PCT- 831-1 94-PCT- 94-PCT- 016 021 019 24 017 018 004 002 020 014 022 023 021 95.44 77.76 78.30 66.00 57.88 57.98 56.72 57.10 56.40 57.00 56.02 60.42 37.75 SiO 2 67.00 94.36 21.10 93.48 95.20 96.20 0.02 0.12 0.35 0.72 0.77 0.60 0.84 1.32 1.30 0.02 1.04 TiO 2 1.42 0.64 0.31 0.35 0.04 0.08 0.07 0.05 2.23 13.02 12.38 17.74 13.39 13.25 15.73 14.48 14.88 14.20 14.41 14.27 Al 2 O 3 6.24 2.09 16.40 3.11 2.84 2.07 1.70 0.16 0.52 0.88 2.04 1.65 3.56 11.60 t 2.99 2.82 0.44 2.92 0.84 3.44 2.02 1.90 1.10 0.12 Fe 2 O 3 1.55 0.68 0.50 0.50 0.57 1.15 1.01 1.94 7.64 6.36 – 8.38 8.98 8.62 9.05 7.54 4.57 2.15 1.01 0.50 1.15 FeO 0.01 0.01 0.02 0.04 0.12 0.17 0.14 0.13 0.11 MnO 0.15 0.01 0.12 0.11 0.27 0.05 0.02 0.03 0.01 0.01 0.20 0.61 0.40 1.00 4.33 4.44 2.11 3.02 3.98 0.25 4.03 0.10 2.62 3.33 26.40 1.43 MgO 0.40 0.36 0.40 0.07 0.07 0.07 0.42 0.70 2.10 6.03 6.20 6.16 5.68 6.10 4.28 6.00 4.51 5.74 3.27 0.42 0.42 0.14 CaO 0.05 0.15 0.04 0.32 1.51 2.42 5.17 5.98 5.47 4.33 3.45 5.15 4.01 5.17 0.08 5.88 Na 2 O 1.22 0.07 0.41 0.65 4.00 2.75 2.91 0.08 0.26 0.27 0.52 0.33 0.42 0.40 0.76 0.51 0.23 0.02 1.22 K 2 O 0.59 0.38 0.29 0.05 0.07 0.11 0.03 0.10 0.23 0.09 0.66 0.15 0.15 0.33 0.19 0.22 0.06 0.29 0.14 0.07 0.03 0.02 H 2 O 0.28 0.59 0.20 1.79 1.39 2.23 2.56 0.71 0.35 1.18 1.47 1.49 1.45 1.37 11.51 0.83 0.40 0.50 0.10 L.O.1 B 0.01 0.01 0.06 0.16 0.17 0.22 0.24 0.24 0.22 B 0.01 0.18 P 2 O 5 0.26 0.17 n.d. 0.15 B 0.01 n.d. 0.20 0.01 99.70 99.77 99.85 99.52 99.94 99.74 99.78 99.52 100.37 99.65 99.53 99.87 100.08 99.96 Total 100.23 99.98 99.87 99.89 100.09 71 70 73 50 68 61 CIA 66 46.00 45.15 56.59 42.80 5.97 6.32 3.72 29.61 32.92 SiO 2 Al 2 O 3 2.80 16.25 K 2 O 12.5 10.86 1.82 1.20 0.24 0.93 11.80 Na 2 O 11.33 55.75 40.69 8.20 7.33 6.93 41.4 Al 2 O 3 29.86 2.55 Na 2 O a n.d. Not detected. t-Fe recalculated to Fe 2 O 3 . See Figs. 2–5 for locality locations. 1–10 Quartz arenites from the subunit Q1. 11–19 Pebbly quartz arenite from the subunit Q1. 20–23 Quartz arenites from the subunit Q2. 24 Pebbly quartz arenite from the subunit Q2. 25 Mica quartz arenite from the subunit Q2. 26 Altered rhyolite. 27 Altered luff-sandstone. 28–29 Amygdaloidal 28 and homogeneous 29 andesite fom the lower part of the andesite sequence subunit Al. 30–32 Glomeroporphyritic andesites from the upper part of the subunit Al. Unaltered 30, 31 and altered 32 near contact with quartz-rich subunit Q1. 33–35 Andesites from subunit A2; homogeneous unaltered 33 and altered 34 near contact with quartz-rich subunit Q2, coarse pyroclastic 35 from dark fragment. 36 Peridotitic komatlite 2. 37 Tonalite. P .C . Thurston , V .N . Kozhe 6 niko 6 Precambrian Research 101 2000 313 – 340 333 Table 4 Trace element geochemistry ppm of Hisovaara rocks 6 7 8 9 10 11 1 2 3 4 5 94-PCT-011 94-PCT-005 94-PCT-012 94-PCT-013 94-PCT-003 94-PCT-001 94-PCT-021 94-PCT-020 94-PCT-010 94-PCT-008 94-PCT-007 7.53 5.19 4.31 8.17 7.16 6.14 1.96 La 0.70 2.23 3.06 1.12 2.59 1.67 15.48 10.46 9.50 13.95 13.52 11.21 6.59 5.04 Ce 4.35 0.52 0.22 1.84 1.19 1.06 1.93 1.72 128 0.78 Pr 0.57 0.33 6.67 4.16 3.73 6.74 6.08 4.72 0.90 1.95 1.96 Nd 1.25 2.94 0.22 0.27 1.17 0.67 0.68 1.12 0.93 0.85 0.58 0.35 0.40 Sm 0.21 0.11 0.16 0.16 0.15 0.19 Eu 0.06 0.12 0.06 0.08 0.05 0.91 0.52 0.55 0.89 0.65 0.74 0.19 0.22 Gd 0.33 0.28 0.42 0.02 0.03 0.13 0.06 0.07 0.11 0.08 0.11 0.05 0.03 0.04 Tb 0.61 0.26 0.34 0.56 0.33 Dy 0.58 0.18 0.24 0.17 0.22 0.13 0.12 0.05 0.06 0.11 0.06 0.11 0.03 0.04 Ho 0.03 0.05 0.03 0.33 0.14 0.18 0.32 0.17 0.37 Er 0.11 0.14 0.10 0.13 0.10 0.04 0.02 0.02 0.04 0.02 0.06 0.02 Tm 0.02 0.01 0.02 0.02 0.17 0.17 0.34 0.18 0.25 0.39 0.19 0.50 0.17 013 0.15 Yb 0.06 0.04 0.04 0.06 0.03 0.08 0.04 0.03 Lu 0.03 0.02 0.03 0.46 6.41 35.44 23.05 20.95 34.55 31.09 26.94 15.20 10.87 10.23 SREE 2.78 4.32 14.95 19.47 11.64 14.11 25.43 8.29 12.15 11.58 8.82 La N Yb N 4.05 4.88 4.00 4.60 4.85 4.55 2.00 3.32 3.09 4.01 La N Sm N 2.61 1.78 1.02 0.90 2.16 2.34 1.78 1.84 2.76 1.20 2.00 1.74 Gd N Yb N 0.62 0.57 0.80 0.49 0.59 0.73 EuEu 0.75 0.74 0.59 0.67 0.75 9.33 7.39 8.46 10.62 14.59 6.35 3.32 4.38 Th 2.84 21.38 5.18 1.12 1.24 1.78 0.49 1.78 2.22 1.95 1.64 1.29 0.72 0.71 U 3.0 3.5 5.2 15.1 4.8 4.8 7.5 3.9 4.0 3.3 4.0 ThU 17 18 19 20 21 22 15 23 16 24 14 13 12 94-PCT-017 568-2 577-2 94-PCT-004 94-PCT-002 94-PCT-023 94-PCT-022 94-PCT-018 94-PCT-014 94-PCT-016 94-PCT-015 94-PCT-019 94-PCT-024 6.89 24.64 4.09 4.24 9.73 14.63 28.83 14.45 La 3.56 21.11 5.79 3.30 3.28 690 58.82 145 46 50.80 12.42 9.81 23.35 49.79 39.43 9.50 716 12.66 Ce 43.54 1.93 6.14 1.71 1.78 3.00 4.59 6.70 4.40 0.82 1.58 Pr 5.04 1.46 0.84 23.63 2.96 706 25.70 857 8.71 12.70 19.68 18.49 8.07 3.14 5.26 18.02 Nd 0.57 3.45 1.38 4.88 1.71 3.24 3.19 4.35 4.35 3.09 0.70 0.89 Sm 2.99 0.93 1.26 0.43 1.22 1.18 1.37 0.83 1.46 0.13 1.03 0.69 Eu 0.12 0.13 2.54 0.48 1.47 4.57 2.11 3.95 3.85 4.45 4.72 3.41 0.65 0.63 2.21 Gd 0.30 n.d. n.d. 0.64 0.62 0.62 Tb 0.72 0.06 0.53 0.11 0.08 0.30 0.31 2.05 3.78 1.89 3.67 3.65 3.35 1.38 3.91 0.35 2.99 Dy 1.30 0.36 0.70 0.23 0.07 0.47 n.d. n.d. 0.70 0.70 0.64 0.75 0.59 0.15 0.06 0.22 Ho Er 1.50 0.20 1.99 1.03 1.89 1.91 1.76 2.04 1.63 0.46 0.18 0.58 0.68 0.22 n.d. n.d. 0.26 0.25 0.24 0.08 0.28 0.03 0.23 Tm 0.08 0.02 0.08 0.65 0.26 1.57 2.14 0.90 1.66 1.67 1.51 1.80 1.54 0.57 0.20 0.58 Yb 0.24 0.28 0.17 0.24 0.24 0.21 Lu 0.24 0.04 0.22 0.09 0.03 0.10 0.10 41.47 124.19 35.03 42.01 66.04 107.19 128.23 97.04 16.19 37.97 SREE 96.76 27.75 18.08 29.94 8.51 2.96 7.75 3.09 1.72 3.93 6.54 5.42 1.56 3.91 19.55 24.57 La N Yb N 3.14 3.18 1.50 0.82 1.92 2.12 2.09 0.72 La N Sm N 3.62 2.97 4.10- 4.44 5.26 3.16 0.76 1.72 1.91 1.92 1.86 2.38 3.08 2.12 0.92 1.79 2.55 Gd N 1.49 Yb N 0.70 0.86 2.00 0.82 0.69 1.04 1.03 0.95 0.99 0.97 0.59 0.53 EuEu 0.82 4.41 7.46 4.91 2.97 3.02 3.53 3.17 Th 5.22 2.21 5.97 6.30 9.82 6.21 0.88 1.01 0.92 0.37 0.53 0.59 1.18 0.81 2.69 0.75 U 0.81 1.90 1.52 ThU 5.0 2.7 7.4 5.3 8.03 5.7 6.0 3.9 7.0 3.1 4.1 3.7 5.3 SREE without Tb, Dy, Ho and Tm; breccia Fig. 13. P .C . Thurston , V .N . Kozhe 6 niko 6 Precambrian Research 101 2000 313 – 340 Table 5 Chondrite-normalized REE contents in komatiite, tonalite, rhyolite and in quartz-rich rocks from Hisovaara and calculated compositions of mixtures that result with various dilutions by quartz a 6 7 1 8 9 10 11 12 13 14 15 16 17 18 19 2 3 4 5 La 12.51 2.17 12.55 8.01 7.97 17.62 17.64 30.78 31.14 23.67 24.66 29.27 28.82 86.30 89.92 49.43 117.9 4.58 4.60 10.33 10.04 6.82 6.58 14.89 14.11 24.27 24.92 19.85 3.86 19.52 Ce 21.19 22.65 68.25 70.63 4.06 92.21 40.80 2.48 Pr 809 2.18 7.86 5.39 5.56 11.00 11.06 19.09 19.77 15.15 15.12 17.84 17.29 52.28 53.88 34.47 69.50 3.42 3.28 6.20 5.87 4.14 4.24 7.87 8.26 14.08 14.59 11.10 2.61 11.02 2.64 12.83 12.49 38.03 38.93 Nd 2.98 26.32 49.87 3.77 3.13 2.60 2.56 4.42 4.41 7.60 7.62 5.78 Sm 5.40 3.77 604 5.84 19.42 18.19 15.91 22.40 1.75 1.76 2.07 1.92 1.38 1.58 2.76 2.70 3.62 4.78 2.24 1.03 3.41 Eu 2.59 3.71 11.90 11.56 1.03 14.31 9.83 1.72 2.06 1.90 1.62 1.48 2.69 2.68 4.45 4.33 3.08 3.04 3.18 3.26 10.82 10.16 Gd 3.87 9.20 12.43 1.08 1.17 0.94 1.11 0.87 1.02 1.34 1.56 2.40 2.49 1.42 0.84 1.58 3.07 1.30 1.40 5.12 4.35 Dy 0.71 5.43 6.34 0.66 2.71 0.84 0.87 0.78 0.76 0.84 1.22 1.99 1.87 1.08 1.18 1.02 1.17 3.49 3.64 4.70 4.10 0.66 Er 0.82 3.57 1.03 0.99 0.91 0.90 1.51 1.39 2.06 2.08 1.21 1.26 1.15 1.17 3.51 3.68 5.63 3.94 103 Yb 1.18 1.13 1.18 1.02 1.57 1.59 2.36 2.26 1.18 0.92 1.33 Lu 1.18 1.22 3.94 3.80 1.18 3.94 6.30 4.72 12.15 11.11 LaYb 8.82 0.61 8.85 11.64 12.69 14.95 14.99 19.56 19.57 25.43 24.63 24.57 24.43 8.82 29.92 4.32 5.61 3.32 4.01 3.09 3.11 4.00 4.00 4.05 4.08 4.10 2.61 4.57 2.61 4.85 4.93 4.44 4.94 LaSm 0.58 3.11 5.26 2.00 1.68 1.78 1.64 1.78 1.93 2.16 2.08 2.55 2.41 2.76 2.79 3.08 GdYb 2.76 1.08 1.64 3.15 0.02 1.43 0.74 0.79 0.67 0.81 0.80 0.78 0.62 0.83 0.53 0.84 0.59 0.85 0.82 0.72 0.85 EuEu 0.45 0.81 0.86 0.75 a 1 Komatiite s.576-4; 2 tonalite s.831-1; 3 rhyolite 94-PCT-.024; 4 quartz arenite 94-PCT-.011; 5 mixture of komatiite: tonalite = 1; 1, diluted 5.6 times by quartz; 6 quartz arenite 94-PCT-010; 7 mixture of komatiite: tonalite: rhyolite : 1:1:1 diluted 4.4 times by quartz; 8 quartz arenite 94-PCT-007; 9 contents in tonalite diluted 6.2 times by quartz; 10 pebbly quartz arenite 94-PCT-003; 11 mixture of komatiite:tonalite:rhyolite : 1:1:1 diluted 3.1 times by quartz; 12 pebbly quartz arenite 94-PCT-012; 13 mixture of tonalite:rhyolite c 3.2 diluted 2.4 times by quartz; 14 pebbly quartz arenite 94-PCT-015; 15 mixture of tonalite:rhyolite = 2:3. diluted 3.7 times by quartz; 16 quartz arenite 94-PCT-021; 17 mixture of tonalite:rhyolite = 1.4 diluted 3.6 times by quartz, 18-mica quartz arenite 94-PCT-019; 19 mixture of tonalite:rhyolite = 1:4, diluted 1.1 times by quartz. Fig. 15. Chondrite-normalized REE diagrams for tonalite, komatiite, rhyolite, some quartz-rich rock samples and rated mixtures at various degrees of quartz dilution. The normalized values given in Table 3 were used. Sample numbers in this figure are those used in Table 3. Fairly aggressive acid rains, related to fumarolic activity between volcanic paroxysms, could well be responsible for the development of weathering on andesites. In summary, the Hisovaara quartz arenites are associated with intermediate to felsic volcanics providing an example of a combination of some features characteristic of platformal settings with those typical of environments that display an active continental margin- or island-arc type of calcareous-alkaline volcanism. It is a rare case in Archean greenstone belts. 3 . 3 . Felsic fragmental rocks unit F 3 . 3 . 1 . Field description This unit is composed of felsic pyroclastic rocks and subordinate reworked equivalents Kozhevnikov, 1992. In this section we provide the field evidence for interpretation of the deposi- tional environment of the unit. In the vicinity of location B, the sequence consists of metre scale units of tuff breccia and lapilli tuff with occa- sional tuff intercalations. Primary textures and structures indicate the presence of both pumice and lithic fragments. Each unit is defined by distinct size ranges of clasts, texture of pumice, a distinct phenocryst content and a regular succes- sion of size and density grading. Within individual units as defined above, reverse grading of pumice, normal grading of lithic fragments, sporadic ex- amples of basal ground surge beds and thinly laminated fine ash tuff beds occur which indicate the units represent ignimbrite deposition Sparks et al., 1973. Evidence for welding con- sists of the presence of branching fumarolic struc- tures cf. Thurston, 1980 and the presence of flattened silicified pumice fragments concentrated toward the middle of the stratigraphic unit. The silicified pumice ignores depositional unit boundaries and is thus interpreted as evidence of vapour phase recrystallization Ross and Smith, 1961 indicative of subaerial eruption and deposi- tion. At location A, in a few exposures, the quartz arenite is overlain by a few metres of a fragmental aluminous metaconglomerate with granitoid and possible metavolcanic clasts. This is overlain by about a 100 m thickness of thin graded beds of sulfidic argillite and carbonate bearing silty sandstones. 3 . 3 . 2 . Petrography The rhyolites at location B consist of varying proportions of quartz, plagioclase and minor potassium feldspar with accessory biotite and sericite. Primary textures are completely obliter- ated by metamorphic recrystallization but gross grain size variation seen mesoscopically is present in thin section. Silicified pumice fragments contain irregular polycrystalline plates of quartz whereas the matrix contains finer rained quartz, feldspar, biotite and sericite. 3 . 3 . 3 . Geochemistry The sole analysis of the rhyolitic unit done for the present study shows the unit to be a high silica rhyolite with potassium dominant over sodium Table 3. In trace element terms, the unit displays a fractionated REE pattern similar to the FI rhyolites of Lesher et al. 1986.

4. Summary and conclusions