Neoarchean arc magmatism and Paleoproterozoic high-pressure granulite-facies metamorphism in the southern Motloutse Complex, eastern Botswana: Implications for the western extension of the Limpopo Complex
Neoarchean arc magmatism and Paleoproterozoic high-pressure granulite-facies metamorphism in the southern Motloutse Complex, eastern Botswana: Implications for the western extension of the Limpopo Complex
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DOI：
10.1016/j.precamres.2021.106534
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发表时间：
2022-02
影响因子：
3.8
通讯作者：
T. Basupi;T. Tsunogae;Kazuki Takahashi;Y. Tsutsumi
中科院分区：
文献类型：
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作者：
T. Basupi;T. Tsunogae;Kazuki Takahashi;Y. Tsutsumi
我们提供了第一个记录的高压石榴石斜长角闪岩和镁铁质麻粒岩托管的斜长角闪岩从毛佩地区南部Motloutse复杂在博茨瓦纳东部。岩石的矿物组合为石榴子石+埃丹石+斜长石+钛铁矿+金红石+石英（含石榴子石斜长角闪岩），镁角闪石+斜方辉石+单斜辉石+斜长石+石英（镁铁质麻粒岩），镁角闪石+斜长石+石英+钛铁矿+绿帘石+钛铁矿（寄主斜长角闪岩）。含石榴角闪岩中的石榴子石石榴子石包裹体中含有浅成粒、钛铁矿、石英和斜长石等变质矿物，同时被退变质作用形成的铁浅成粒+斜长石冠状体所覆盖。基性麻粒岩和斜长角闪岩的地球化学特征，结合大离子亲石元素的富集和高场强元素（Nb和Ta）的亏损，表明与俯冲有关的火山弧的亲和性，而含石榴子石的斜长角闪岩则表明与俯冲有关的火山弧或板内玄武岩的亲和性。根据Na_2 O-CaO-FeO-MgO-Al_2O_3-SiO_2-H_2 O-TiO_2-Fe_2O_3（NCFMASHTO）体系相平衡模拟和地质温压法，确定了该斜长角闪岩的峰期变质条件为850-910 °C和10.0- 10.5kbar。从镁铁质麻粒岩（745-825 °C）和角闪岩（610-750 °C）获得的条件略低。含石榴子石斜长角闪岩中的锆石207 Pb/206 Pb年龄为2020.2 ± 4.9Ma，Th/U比值为0.02-0.23，反映了变质作用的强烈程度。而Th/U比值较高（0.44-0.86）的镁铁质麻粒岩中的锆石则记录了2648.8 ± 9.8Ma的岩浆年龄。由于年龄是一致的，与以前报道的年龄英云闪长片麻岩（2645马）从同一地区，我们推断双峰弧岩浆活动在新太古代。研究结果表明，南莫特卢茨杂岩在古元古代（约2.02 Ga）经历了一次沿着单向逆时针P-T演化的高压麻粒岩相变质作用。我们的综合岩石学和年代学研究表明，南部Motloutse复杂几乎相当于拜特桥复杂的碰撞缝合线（林波波复杂的中心地带）之间的太古代Kaapvaal和津巴布韦的火山岩可能向西延伸。CA的发生。2.0来自Motloutse杂岩南部的Ga高压麻粒岩相岩石可能表明在新太古代至古元古代期间多次碰撞的离散地壳单元。
We provide the first record of the occurrence of high-pressure garnet-bearing amphibolite and mafic granulite hosted by amphibolites from the Maope area of the southern Motloutse Complex in eastern Botswana. The mineral assemblages of the rocks are garnet + edenite + plagioclase + ilmenite + rutile + quartz (garnet-bearing amphibolite), magnesio-hornblende + orthopyroxene + clinopyroxene + plagioclase + quartz (mafic granulite), and magnesio-hornblende + plagioclase + quartz + ilmenite + epidote + titanite (host amphibolite). The poikiloblastic garnet in the garnet-bearing amphibolite contains inclusions of edenite, ilmenite, quartz, and plagioclase as prograde minerals, while it is mantled by ferro-edenite + plagioclase coronae formed during retrograde metamorphism. The geochemical signatures of the mafic granulite and amphibolite, combined with the enrichment of large-ion lithophile elements and depletion of high-field strength elements (Nb and Ta), indicate subduction-related volcanic arc affinity, whereas the garnet-bearing amphibolite indicates subduction-related volcanic arc or within-plate basalt affinity. The peak metamorphic condition of the garnet-bearing amphibolite was constrained as 850–910 °C and 10.0–10.5 kbar based on phase equilibrium modeling in the Na2O-CaO-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe2O3(NCFMASHTO) system and geothermobarometry. Slightly lower conditions were obtained from the mafic granulite (745–825 °C) and amphibolite (610–750 °C). Zircon grains from the garnet-bearing amphibolite with lower Th/U ratios of 0.02–0.23 yielded a weighted-mean207Pb/206Pb age of 2020.2 ± 4.9 Ma as the timing of high-grade metamorphism. In contrast, oscillatory-zoned zircons from the mafic granulite with higher Th/U ratios of 0.44–0.86 recorded a magmatic age of 2648.8 ± 9.8 Ma. As the age is consistent with previously reported age of a tonalite gneiss (2645 Ma) from the same region, we infer bimodal arc magmatism during Neoarchean. The results of this study indicate that the southern Motloutse Complex underwent high-pressure granulite-facies metamorphism with post-peak decompression along a single clockwiseP–Tevolution during Paleoproterozoic Era (∼2.02 Ga). Our integrated petrological and geochronological studies suggest that the southern Motloutse Complex is nearly equivalent to the Beit Bridge Complex as a possible western extension of a collisional suture (the Central Zone of the Limpopo Complex) between the Archean Kaapvaal and Zimbabwe Cratons. The occurrence of ca. 2.0 Ga high-pressure granulite-facies rocks from the southern Motloutse Complex might suggest multiple collisions of discrete crustal units during Neoarchean to Paleoproterozoic.