Zircon from banded iron formation as a sensitive indicator of its polychronous background: a case study on the Kostomuksha greenstone belt, Karelian Craton, Fennoscandian Shield

Zircon is not a common accessory mineral of banded iron formation (BIF), but it occurs in all BIF samples from the Kostomuksha Greenstone Belt (KGB) in the Karelian Craton of the Fennoscandian Shield. The Kostomuksha BIF is of Algoma type and is part of three variably old rock associations: BIF-1 is part of a sedimentary sequence in Mesoarchean (2.87–2.84 Ga) basaltic komatiites with dacites, BIF-2 is located in Meso-Neoarchean (2.8–2.79 Ga) felsic volcanics, and BIF-3 is part of a Neoarchean (2.76–2.74 Ga) greywacke unit with felsic volcanics. Analysis of zircon has revealed grains with cores and several generations of rims. The U-Th-Pb systems of zircon indicate that its isotopic age varies considerably from 2.98 to 1.89 Ga, forming up to 4 age clusters in each sample. Most samples contain single inherited grains. The bulk of zircon in BIF is younger than the host rocks and is interpreted as metamorphic. Most of metamorphic zircons display an oscillatory zoning, and their Th/U ratio varies from 0.01 to 4.7, i.e. their origin is hard to interpret reliably, based on the above characteristics. Paleoproterozoic (1.89–1.85 Ga) zircons from BIF-1 and 3, occurring as rims and individual grains, are related to local tectono-thermal processes simultaneous with the formation of the Svecofennian orogen and were described for the central Karelian Craton for the first time. Thus, both inherited and metamorphic zircons have been revealed in Meso-Neoarchean BIF metamorphosed repeatedly in the Neoarchean-Paleoproterozoic under up to amphibolite-facies conditions. All metamorphic generations of zircons from BIF are correlated with the tectono-thermal events in the Karelian Craton, including the effect of the Svecofennian orogeny.

锆石（zircon）并非条带状铁建造（banded iron formation, BIF）的常见副矿物，但在芬诺斯坎底亚地盾（Fennoscandian Shield）卡累利阿克拉通（Karelian Craton）的科斯托穆克什绿岩带（Kostomuksha Greenstone Belt, KGB）的所有条带状铁建造样品中均有产出。科斯托穆克什条带状铁建造属于阿尔戈马型（Algoma type），隶属于三套年代各异的岩石组合：BIF-1赋存于中太古代（2.87~2.84 Ga）玄武质科马提岩与英安岩构成的沉积序列中；BIF-2产于中-新太古代（2.8~2.79 Ga）长英质火山岩中；BIF-3则属于新太古代（2.76~2.74 Ga）长英质火山岩杂砂岩单元的一部分。对锆石的分析显示，其颗粒均具有核部及多世代边部。锆石的铀-钍-铅（U-Th-Pb）体系表明，其同位素年龄跨度极大，介于2.98 Ga至1.89 Ga之间，单个样品中最多可形成4个年龄簇。多数样品含有单颗继承型锆石颗粒。条带状铁建造中绝大多数锆石的形成时代晚于赋存围岩，被认为是变质成因的。多数变质锆石呈现振荡环带结构，其钍铀（Th/U）比值介于0.01至4.7之间，仅基于上述特征难以可靠判定其成因。采自BIF-1和BIF-3的古元古代（1.89~1.85 Ga）锆石，以边部或独立颗粒形式产出，与斯维科芬造山带（Svecofennian orogen）形成同期的局部构造热事件相关，这也是卡累利阿克拉通中部首次报道此类锆石。综上，在新太古代-古元古代期间经历角闪岩相变质作用的中-新太古代条带状铁建造中，已同时发现继承型与变质成因锆石。条带状铁建造中所有世代的变质锆石均与卡累利阿克拉通内的构造热事件相关，包括斯维科芬造山运动（Svecofennian orogeny）的影响。