Chronology of early Archaean granite-greenstone evolution in the Barberton Mountain Land, South Africa, based on precise dating by single zircon evaporation.
Chronology of early Archaean granite-greenstone evolution in the Barberton Mountain Land, South Africa, based on precise dating by single zircon evaporation.
复制标题
南非巴伯顿山区早期太古代花岗岩-绿岩演化的年代学，基于单锆石蒸发的精确测年。
DOI：
10.1016/0012-821x(91)90148-b
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发表时间：
1991
影响因子：
5.3
通讯作者：
D. Lowe
中科院分区：
文献类型：
--
作者：
A. Krüner;G. Byerly;D. Lowe
我们报告的精确207 Pb 206 Pb单锆石蒸发年龄的低品位长英质变火山岩内的翁韦尔瓦赫特和无花果树群的巴伯顿绿岩带（BGB），南非，并从花岗岩类岩体接壤的带。翁韦尔瓦赫特群上部Hooggenoeg组英安质凝灰岩的年龄为3445 ± 3 Ma ~ 3416 ± 5 Ma，含有以3504 ± 4 Ma的锆石捕虏晶为代表的较老地壳成分。英安质凝灰岩和团块中自形锆石的年龄在3259 ± 5 Ma和3225 ± 3 Ma之间，反映了它们的结晶时间。一个令人惊讶的复杂捕虏晶群记录的年龄为3323 ± 4 Ma至3522 ± 4 Ma。我们怀疑这些捕虏晶是在长英质熔体流到地表的过程中，从不同的来源继承而来的，如绿岩和花岗岩类岩石，这些岩石沿着BGB南部和西部边缘以英云闪长岩-奥长花岗岩岩体的形式暴露出来，以及早于任何暴露的绿岩或侵入岩的单元。几个花岗岩类沿着南部边缘的带与年龄在3490和3440马之间的锆石人口，与或略早于翁韦尔瓦赫特长英质火山作用，而Kaap谷岩体沿着西北边缘的带是与无花果树英安质火山作用。这些结果强调了绿岩长英质火山岩单元与周围深成岩套之间的同源岩浆关系。一些火山岩和深成岩单元含有比任何出露岩石更古老的锆石捕虏晶。这些表明存在更古老的单元，可能是地层上更低和更古老的部分绿岩序列本身，更古老的花岗岩类侵入岩，或机构的更古老的，无关的地壳物质。我们的数据表明，翁韦尔瓦赫特和无花果树长英质单元具有明显不同的年龄，因此不代表一个单一的，构造重复的单位，如其他人提出的。与加拿大的晚白垩世Abitibi绿岩带不同，该带形成于约30 Ma，BGB中的暴露岩石形成于至少220 Ma的时期。在这项研究中遇到的复杂的锆石人口意味着，传统的多颗粒锆石定年可能无法准确地确定在古绿岩长英质火山活动的时间。非洲南部卡普瓦尔克拉通的BGB和西澳大利亚皮尔巴拉地块的绿岩在岩石类型、构造演化和年龄上惊人的相似性表明，这两种地形可能是早白垩世时期一个更大地壳单元的一部分。
We report precise207Pb206Pbsingle zircon evaporation ages for low-grade felsic metavolcanic rocks within the Onverwacht and Fig Tree Groups of the Barberton Greenstone Belt (BGB), South Africa, and from granitoid plutons bordering the belt. Dacitic tuffs of the Hooggenoeg Formation in the upper part of the Onverwacht Group yield ages between3445 ± 3and3416 ± 5Ma and contain older crustal components represented by a3504 ± 4Ma old zircon xenocryst. Fig Tree dacitic tuffs and agglomerates have euhedral zircons between3259 ± 5and3225 ± 3Ma in age which we interpret to reflect the time of crystallization. A surprisingly complex xenocryst population in one sample documents ages from3323 ± 4to3522 ± 4Ma. We suspect that these xenocrysts were inherited, during the passage of the felsic melts to the surface, from various sources such as greenstones and granitoid rocks now exposed in the form of tonalite-trondhjemite plutons along the southern and western margins of the BGB, and units predating any of the exposed greenstone or intrusive rocks. Several of the granitoids along the southern margin of the belt have zircon populations with ages between 3490 and 3440 Ma, coeval with or slightly older than Onverwacht felsic volcanism, while the Kaap Valley pluton along the northwestern margin of the belt is coeval with Fig Tree dacitic volcanism. These results emphasize the comagmatic relationships between greenstone felsic volcanic units and the surrounding plutonic suites. Some of the volcanic and plutonic units contain zircon xenocrysts older than any exposed rocks. These indicate the existence of still older units, possibly stratigraphically lower and older portions of the greenstone sequence itself, older granitoid intrusive rocks, or bodies of older, unrelated crustal material. Our data show that the Onverwacht and Fig Tree felsic units have distinctly different ages and therefore do not represent a single, tectonically repeated unit as proposed by others. Unlike the late Archaean Abitibi greenstone belt in Canada, which formed over about 30 Ma, exposed rocks in the BGB formed over a period of at least 220 Ma. The complex zircon populations encountered in this study imply that conventional multigrain zircon dating may not accurately identify the time of felsic volcanic activity in ancient greenstones. A surprising similarity in rock types, tectonic evolution, and ages of the BGB in the Kaapvaal craton of southern Africa and greenstones in the Pilbara Block of Western Australia suggests that these two terrains may have been part of a larger crustal unit in early Archaean times.