Greenstone belt-hosted Mesoarchaean Mbarga BIF prospect, NW Congo Craton (southern Cameroon): Petrography, geochemistry, Sr-Nd isotopes, zircon u-pb geochronology, petrogenetic, and tectonic implications

IF 3.2 2区地球科学 Q2 GEOSCIENCES, MULTIDISCIPLINARY

Precambrian Research Pub Date : 2024-11-23 DOI:10.1016/j.precamres.2024.107631

George Lemewihbwen Ngiamte , Roland Maas , Eleanor C.R. Green , Olugbenga Akindeji Okunlola , Cheo Emmanuel Suh , Alan Greig , Ashleigh van Smeerdijk Hood , Nowel Yinkfu Njamnsi , Cyril Samndong Tufoin

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Abstract

Banded Iron Formations (BIFs) interbedded with schists characterize the Mbarga prospect in the Ntem Complex at the northwest edge of the Congo Craton. This study presents new whole-rock geochemical, Sr-Nd, and zircon U-Pb isotopic data for the BIFs and schists to constrain the timing and geodynamic setting of the deposit. The abundances of SiO₂ (52.81 to 79.14 wt%) and Na₂O+K₂O (4.24 to 8.54 wt%) in the schists indicate andesitic, dacitic, to rhyolitic protoliths. Trace element signatures, such as high Ba and depleted Nb-Ta concentrations, suggest a volcanic arc affinity. A well-defined U-Pb zircon age of 2890 ± 4 Ma implies a Mesoarchaean protolith age, while an imprecise Rb-Sr whole-rock age of ca. 2.65 Ga is consistent with known tectonothermal events (∼2.75 and 2.65 Ga) in the Ntem Complex. Initial ε_Nd(2.89) values of + 0.8 to + 2.0 for the schists indicate an unevolved, mantle-like source for the protoliths. The BIFs show partial to extensive alterations of magnetite to hematite-martite and are of the Algoma type. They are characterized by high Fe₂O₃ (∼54.06 wt%) and SiO₂ (∼45.40 wt%) but low Al₂O₃ (∼0.14 wt%), TiO₂ (∼0.1 wt%), Zr (∼4.92 ppm), Th (∼0.11 ppm), and REE-Y contents. Rare earth patterns marked by LREE depletion, positive Eu anomalies (∼2), mild Ce depletion (Ce/Ce* 0.67 to 1.16), and super-chondritic Y/Ho ratios (∼34) suggest formation under anoxic to suboxic Archaean marine conditions, possibly involving mixing of Archaean seawater with minor (0.1–1 %) contributions from medium- to high-T hydrothermal fluids. Sparse 2951 ± 24 Ma zircons, presumably of detrital origin, establish a depositional link to the associated schists, redefining the age of BIF deposition within the Ntem Complex to ca. 2.95–2.89 Ga. However, whole-rock Sm-Nd isotope data for five BIF samples define a scattered array with an imprecise slope equivalent to an age near 1004 ± 78 Ma, which may reflect a previously unrecognized recrystallization event in the BIFs. The initial ε_Nd of this array (−11.1 ± 2.0) suggests a crustal source. The mineralogical, geochemical, and isotopic datasets reconcile the Mbarga BIF prospect with arc magmatism in the Late Archaean, suggesting their formation in a back-arc basin setting.

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刚果克拉通西北部（喀麦隆南部）绿岩带中古生代 Mbarga BIF 勘探区：岩石学、地球化学、Sr-Nd 同位素、锆石 u-pb 地质年代、岩石成因和构造影响

刚果克拉通西北边缘恩特姆复合体中的姆巴加探矿区的特点是带状铁地层（BIFs）与片岩互层。本研究提供了有关带状铁岩层和片岩的新的全岩地球化学、Sr-Nd和锆石U-Pb同位素数据，以确定该矿床的形成时间和地球动力环境。片岩中二氧化硅（52.81 至 79.14 wt%）和 Na2O+K2O （4.24 至 8.54 wt%）的丰度表明其原岩为安山岩、闪长岩和流纹岩。微量元素特征，如高钡和贫铌钽浓度，表明与火山弧有亲缘关系。明确的U-Pb锆石年龄为2890 ± 4 Ma，这意味着原岩年龄为中始新世时期，而不精确的Rb-Sr全岩年龄约为2.65 Ga，这与Ntem岩群已知的构造热事件（∼2.75 Ga和2.65 Ga）相一致。片岩的初始εNd(2.89)值为＋0.8至＋2.0，表明原岩来源于未演化的地幔。BIFs显示了部分到广泛的磁铁矿到赤铁矿的蚀变，属于阿尔戈马类型。它们的特征是Fe2O3（∼54.06 wt%）和SiO2（∼45.40 wt%）含量高，但Al2O3（∼0.14 wt%）、TiO2（∼0.1 wt%）、Zr（∼4.92 ppm）、Th（∼0.11 ppm）和REE-Y含量低。稀土模式以LREE贫化、Eu正异常（∼2）、轻度Ce贫化（Ce/Ce* 0.67至1.16）和超软玉Y/Ho比率（∼34）为特征，表明是在缺氧至亚缺氧的太古宙海洋条件下形成的，可能涉及太古宙海水与少量（0.1-1%）中高T热液的混合。稀疏的 2951 ± 24 Ma 锆石（可能来自碎屑岩）建立了与相关片岩的沉积联系，将恩特姆复合体内的 BIF 沉积年龄重新定义为约 2.95-2.89 Ga。然而，五个BIF样品的全岩Sm-Nd同位素数据确定了一个分散的阵列，其斜率不精确，相当于1004 ± 78 Ma附近的年龄，这可能反映了BIF中以前未认识到的再结晶事件。该阵列的初始εNd（-11.1 ± 2.0）表明其来源于地壳。矿物学、地球化学和同位素数据集使巴尔加 BIF 远景与晚太古宙的弧岩浆活动相吻合，表明它们是在弧后盆地环境中形成的。

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来源期刊

Precambrian Research 地学-地球科学综合

CiteScore

7.20

自引率

28.90%

发文量

325

审稿时长

12 months

期刊介绍： Precambrian Research publishes studies on all aspects of the early stages of the composition, structure and evolution of the Earth and its planetary neighbours. With a focus on process-oriented and comparative studies, it covers, but is not restricted to, subjects such as: (1) Chemical, biological, biochemical and cosmochemical evolution; the origin of life; the evolution of the oceans and atmosphere; the early fossil record; palaeobiology; (2) Geochronology and isotope and elemental geochemistry; (3) Precambrian mineral deposits; (4) Geophysical aspects of the early Earth and Precambrian terrains; (5) Nature, formation and evolution of the Precambrian lithosphere and mantle including magmatic, depositional, metamorphic and tectonic processes. In addition, the editors particularly welcome integrated process-oriented studies that involve a combination of the above fields and comparative studies that demonstrate the effect of Precambrian evolution on Phanerozoic earth system processes. Regional and localised studies of Precambrian phenomena are considered appropriate only when the detail and quality allow illustration of a wider process, or when significant gaps in basic knowledge of a particular area can be filled.