Geochemistry and mineralogy of sediments from the Ventersdorp and Transvaal Supergroups, South Africa: Cratonic evolution during the early Proterozoic
Geochemistry and mineralogy of sediments from the Ventersdorp and Transvaal Supergroups, South Africa: Cratonic evolution during the early Proterozoic
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DOI：
10.1016/0016-7037(90)90323-d
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发表时间：
1990-02
影响因子：
5
通讯作者：
D. Wronkiewicz;K. Condie
中科院分区：
文献类型：
--
作者：
D. Wronkiewicz;K. Condie
为了监测南部非洲Kaapvaal克拉通早元古代的演化，对来自文特斯多普(~2.7Ga)和德兰士瓦(~2.6-2.1Ga)超群的约100个泥灰岩和12个石英岩样品进行了分析。从最古老到最年轻的泥质岩分别来自文茨多普-博塔维尔(BOT)、德兰斯瓦尔-塞拉蒂(SEL)、黑礁(BR)、Timeball Hill(TH)、Strubenkop(STR)和Silverton(Silverton)地层。与BOT-SEL-BR相比，TH-STR-SIL-SIL中的泥页岩富含重稀土、大分子稀土元素、Zr、Hf、Nb和Ta，亏损K2O、MgO、Ni和Cr，具有较低的Cr/Zr、Sc/Th、K2O/Na2O和K/Rb比值。与SEL-BR相比，BOT-TH-STR-SIL球粒岩轻稀土含量和La/Yb比值较高，Eu/Eu∗比值较低(0.61~0.66)。与北美页岩复合体(NASC)相比，THSTR-Silil球粒岩富集轻稀土、Th、U、Ta、Nb、Sc、Cs，La/Yb比值较高，K2O和MgO亏损。与NASC相比，BOT-SEL-BR榴辉岩富K2O、MgO、Cr和Ni，K2O/Na2O、Sc/Th和Eu/Eu∗比值较高，Th、U、重稀土和高场强元素亏损。该源区比BOT-SEL-BR源区演化程度更高，表明2.2Ga时是原始(富镁铁质)上地壳向演化(富长英质)上地壳的转变。这一转变遵循了穆迪-庞戈拉(3.3-3.0Ga)和威特沃特斯兰德(~2.8Ga)的早期原始到演化趋势。这些数据表明，在3.3-2.1Ga之间，Kaapvaal克拉通发生了几次上大陆地壳的变化旋回。
Approximately 100 pelite and 12 quartzite samples from the Ventersdorp (~2.7 Ga) and Transvaal Supergroups (~2.6–2.1 Ga) have been analyzed to monitor the early Proterozoic evolution of the Kaapvaal Craton, southern Africa. From oldest to youngest, pelites were sampled from the Ventersdorp-Bothaville (BOT), Transvaal-Selati (SEL), Black Reef (BR), Timeball Hill (TH), Strubenkop (STR), and Silverton (SIL) Formations. Paleocurrent measurements in Transvaal quartzites indicate sources lying predominantly to the north and east.Relative to the BOT-SEL-BR, pelites from the TH-STR-SIL are enriched in heavy-REE, LILE, Zr, Hf, Nb, and Ta, depleted in K2O, MgO, Ni, and Cr, and have lower Cr/Zr, Sc/Th, K2O/Na2O, and K/ Rb ratios. Compared to SEL-BR, BOT-TH-STR-SIL pelites have higher light-REE contents and La/Yb ratios, and lower Eu/Eu∗ ratios (0.61–0.66). Relative to NASC (North American Shale Composite), THSTR-SIL pelites are enriched in light-REE, Th, U, Ta, Nb, Sc, Cs, have higher La/Yb ratios, and are depleted in K2O and MgO. BOT-SEL-BR pelites are enriched in K2O, MgO, Cr, and Ni, have higher K2O/Na2O, Sc/Th, and Eu/Eu∗ ratios, and are depleted in Th, U, heavy-REE, and High Field Strength Elements (HFSE) relative to NASC.Compositions of TH-STR-SIL pelites suggest a provenance similar to average Phanerozoic uppercontinental crust. This source is more evolved than that of BOT-SEL-BR pelites, indicating a transformation from primitive (mafic-rich) to evolved (felsic-rich) upper-crust at 2.2 Ga. This transition follows earlier primitive to evolved trends in Moodies-Pongola (3.3-3.0 Ga) and Witwatersrand (~2.8 Ga) successions. These data suggest that several cycles of changing upper-continental crust occurred in the Kaapvaal craton between 3.3-2.1 Ga.