Preliminary CHIME dating of granites from the Nkambe area, northwestern Cameroon, Africa
Preliminary CHIME dating of granites from the Nkambe area, northwestern Cameroon, Africa
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非洲喀麦隆西北部 Nkambe 地区花岗岩的初步 CHIME 测年
DOI：
10.18999/joueps.46.57
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发表时间：
1999
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影响因子：
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通讯作者：
M. Adachi
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作者：
S. Tetsopgang;Kazuhiro Suzuki;M. Adachi
引言化学钍-铀-总铅等时线法（CHIME）已被用于对喀麦隆西北部同构造叶状和构造后无叶状花岗岩进行测年。片状黑云母花岗岩样品 I10 中的铜榴石和锆石的 CHIME 年龄分别为 532±35 Ma 和 523±45 Ma，片状二云母花岗岩样品 N9 中的独居石为 530±9 Ma，无片状二云母花岗岩样品 I4 中的独居石年龄为 510±25 Ma。来自样品 N9 和 I4 的几个独居石颗粒分别产生 436±13 和 420±16 Ma CHIME 年龄。 532-510Ma年龄被解释为花岗岩侵位时期，436-420Ma年龄被解释为热液蚀变时期。 CHIME 年龄加上叶状花岗岩和混合岩片麻岩之间的一致关系表明大约有 2000 年。喀马隆西北部 530 Ma 变质-深成事件，而不是花岗岩侵入的热压印。泛非造山运动在非洲广大地区都有记录（Kroner，1977），但各个移动区域的时间和性质仍不清楚。争论的要点是，众多的寒武纪（约 530 Ma）年龄是否记录了具有完整的沉积-变质-深成阶段的单一旋回，或者是通过花岗岩侵入而在较古老的岩石上留下的简单热印记。 Tingey (1991)将东冈瓦纳500 Ma事件视为无形变的热事件。然而，最近的地质年代学研究表明，在南极洲东部（521-553Ma，Shiraishi等，1994；529±14541±15Ma，Asami等，1996，1997）和印度南部（527±10Ma，Bindu等，1998）广泛存在寒武纪麻粒岩相副片麻岩。和马达加斯加（527±15 534±10 Ma，Ito 等，1997）。由于副片麻岩含有 622-797 Ma 碎屑独居石 (Asami et al., 1997; Ito et al., 1997) 和 640-800 Ma 碎屑锆石 (Shiraishi et al., 1994; Bindu et al., 1996)，因此它们并不代表改造后的较旧基底，而是由单一的 620 Ma 后沉积物新形成的。与东冈瓦纳碰撞有关的变质作用（Shiraishi 等，1994；Asami 等，1997；Ito 等，1997）。另一方面，西冈瓦纳大陆被认为是由西非克拉通、刚果克拉通和泛非-巴西利亚诺造山运动期间的元古代移动带碰撞形成的(Castaing et al., 1993; Trompette, 1994)。泛非-巴西造山运动期间变质作用和深成作用的时间对于正确理解冈瓦纳超大陆的合并历史非常重要。喀麦隆，位于.. 58 S. Tetsopgang 等人的北部。刚果克拉通（图1：如Kennedy，1964；Rocci，1965；Clifford，1970）在泛非-巴西造山运动研究中占有重要地位。图 1. 显示喀麦隆西北部 Nkambe 地区位置的地图。 Lasserre 和 Soba (1976) 以及 Lasserre 等人报告了刚果克拉通 Ntem 复合体的位置和辐射年龄。 （1981）也显示了。插图显示了非洲克拉通的分布（肯尼迪之后，1964 年）。 59 喀麦隆花岗岩的初步 CHIME 测年 地质轮廓和样品描述 Lasserre (1966) 将喀麦隆的基岩分为喀麦隆南端的 Ntem 杂岩和该领土其他地区的中非流动区。 Ntem 杂岩体构成刚果克拉通的北部，主要由霞岩片麻岩和花岗岩组成。对埃博洛瓦花岗岩的研究得出了 2.5 和 3.2 Ga Pb-a 锆石年龄（Cahen 和 Snelling，1966）、2566±100 Ma K-Ar 角闪石年龄（Lasserre，1969）和 3010±50 Ma Rb-Sr 全岩等时线年龄（Lasserre 和 Bessoles，1976）。其他花岗岩的铷-锶黑云母年龄为 1800-2350 Ma（Cahen 和 Snelling，1966；Lasserre，1969）。中非移动带由片岩、片麻岩、混合岩和花岗岩的混合物组成。移动带中的花岗岩被分为两类：具有明显叶理和/或糜棱岩结构的“无边界”花岗岩和没有叶理的“边界”花岗岩（Bessoles 和 Trompette，1980）。此后，我们将无外接花岗岩和外接花岗岩分别称为片状花岗岩和无片状花岗岩。有叶状花岗岩可能是在泛非造山运动期间形成的，而无叶状花岗岩则可能是在造山运动后期形成的。已经报告了来自 Groutchoumi（约 525 Ma，Lasserre 等人，1981）、Poli（520±20 Ma，Lasserre 和 Soba，1976）、Linte（521±19 Ma，Lasserre 等人，1981）和 Lom（498±5 Ma，Lasserre 和 Soba）的花岗岩的 Rb-Sr 年龄数据。荞麦面，1976）。然而，我们并不确定这些年龄到底是片状花岗岩的侵入时间，还是片状花岗岩的侵入叠印时间。喀麦隆西北部的 Nkambe 地区（图 1）下方主要是花岗岩。这些花岗岩与中非流动带其他地区的花岗岩一样，可分为叶状和未叶状类型。作者目前正在进行 CHIME 测年，以了解花岗岩的就位年龄。本文报告了恩卡姆贝地区片状和未片状花岗岩代表性样品的 CHIME 年龄数据。 Nkambe 地区的纬度范围为北纬 6 25' 至 6 53'，经度范围为东经 10 22' 至 11 00'（图 1）。该地区主要由高原组成，东北部有深邃的河谷流入东加河，东南部有高耸的火山山脉。 Nkambe地区地质如图2所示。基岩包括岩浆片麻岩、深熔花岗岩、角闪岩和花岗岩，部分覆盖新生代玄武岩、安山岩和碱性流纹岩(Peronne，1969)。混合岩片麻岩走向NNE-SSW方向。 Nkambe 地区的花岗岩由片状和未片状花岗岩组成。虽然图中没有区分这些类型，但前一类型包括图 2 中混合岩片麻岩域中的黑云花岗岩、深熔花岗岩和小型卵石岩。由于片状花岗岩在边缘分级进入混合岩片麻岩，并随混合岩片麻岩一起变形，因此可以将其视为同构造侵入体。未剥落的花岗岩以直径约 500-1000 m 的小块侵入混合岩片麻岩中。两个片状花岗岩样品（样品 I10 和 N9）和一个未片状花岗岩样品（样品 I4）用于本次 CHIME 测年。样品'
INTRODUCTION The chemical Th-U-total Pb isochron method (CHIME) has been adopted for dating the syntectonic foliated and post-tectonic unfoliated granites in northwestern Cameroon. The CHIME ages are 532±35 Ma for allanite and 523±45 Ma for zircon from Sample I10 of foliated biotite granite, 530±9 Ma for monazite from Sample N9 of foliated two-mica granite, and 510±25 Ma for monazite from Sample I4 of unfoliated two-mica granite. Several monazite grains from Sample N9 and I4 yield 436±13 and 420±16 Ma CHIME ages, respectively. The 532-510 Ma ages are interpreted as the time of granite emplacement and the 436-420 Ma ages as the time of hydrothermal alteration. The CHIME ages coupled with the concordant relation between the foliated granites and migmatitic gneiss suggest a ca. 530 Ma metamorphic-plutonic episode in northwestern Camaroon rather than a thermal overprint by granite intrusion. The Pan-African orogeny was recorded in a wide area of Africa (Kroner, 1977), but the timing and nature of individual mobile zones have remained unclear. The main point of argument is whether the numerous Cambrian (ca. 530 Ma) ages record a single cycle with integral sedimentary-metamorphic-plutonic episodes or a simple thermal imprint an older rocks through granite intrusion. Tingey (1991) regarded the 500 Ma event in East Gondwana as a thermal event free from deformation. Recent geochronogical studies, however, have disclosed a widespread occurence of Cambrian granulite-facies paragneisses in East Antarctica (521-553 Ma, Shiraishi et al., 1994; 529±14 541±15 Ma, Asami et al., 1996, 1997), South India (527±10 Ma, Bindu et al., 1998) and Madagascar (527±15 534±10 Ma, Ito et al., 1997). Since the paragneisses contain 622-797 Ma detrital monazites (Asami et al., 1997; Ito et al., 1997) and 640-800 Ma detrital zircons (Shiraishi et al., 1994, Bindu et al.,1996), they do not represent reworked older basements, but formed newly from post-620 Ma sediments by the single metamorphism linked to the collision of East Gondwana (Shiraishi et al., 1994; Asami et al., 1997; Ito et al., 1997). West Gondwana, on the other hand, has been considered to have formed by the collision of the Western Africa craton, Congo craton and a Proterozoic mobile zone during the Pan-African-Brasilliano orogeny (Castaing et al., 1993; Trompette, 1994). The timing of the metamorphism and plutonism during the Pan-African-Brasilliano orogeny is important for proper understanding of the amalgamation history of the Gondwana supercontinent. Cameroon, located to the north of .. 58 S. Tetsopgang et al. the Congo craton (Fig. 1: e.g. Kennedy, 1964; Rocci, 1965; Clifford, 1970), occupies a key position for studying of the Pan-African-Brasiliano orogeny. Fig. 1. Map showing the location of the Nkambe area in northwestern Cameroon. The location of the Ntem Complex of the Congo craton and radiometric ages reported by Lasserre and Soba (1976) and Lasserre et al. (1981) are also shown. Inset map shows the distribution of craton in Africa (after Kennedy, 1964). 59 Preliminary CHIME dating of granites from Cameroon GEOLOGICAL OUTLINE AND SAMPLE DESCRIPTION Lasserre (1966) separated the basement rocks in Cameroon into the Ntem Complex in the southern end of Cameroon and the Central-Africa mobile zone in the rest of the territory. The Ntem Complex constitutes the northern part of the Congo craton, and consists predominantly of charnockitic gneiss and granites. Studies on the Ebolowa Granite have yielded 2.5 and 3.2 Ga Pb-a zircon ages (Cahen and Snelling, 1966), a 2566±100 Ma K-Ar hornblende age (Lasserre, 1969) and a 3010±50 Ma Rb-Sr wholerock isochron age (Lasserre and Bessoles, 1976). Other granitoids have Rb-Sr biotite ages of 1800-2350 Ma (Cahen and Snelling, 1966; Lasserre, 1969). The Central-Africa mobile zone is composed of a mixture of schist, gneiss, migmatite and granite. Granitic rocks in the mobile zone have been classified into two groups, "uncircumscribed" granites with a marked foliation and/or mylonitic texture and "circumscribed" granites with no foliation (Bessoles and Trompette, 1980). Hereafter, we designate the uncircumscribed and circumscribed granite as the foliated and unfoliated granites, respectively. The foliated granite was emplaced possibly during the Pan-African orogeny and the unfoliated one on an later anorogenic stage. Rb-Sr age data have been reported for granitoids from Groutchoumi (ca. 525 Ma, Lasserre et al., 1981), Poli (520±20 Ma, Lasserre and Soba, 1976), Linte (521±19 Ma, Lasserre et al., 1981), and Lom (498±5 Ma, Lasserre and Soba, 1976). We, however, are not confident whether these ages date the intrusion time of the foliated granite or the overprint time by the intrusion of the unfoliated granite. The Nkambe area in northwestern Cameroon (Fig. 1) is underlain by predominantly granitic rocks. These granitoids, like those in other area of the Central-African mobile zone, can be classified into the foliated and unfoliated types. CHIME dating by the authors is currently in progress, to know the emplacement ages of the granitoids. This paper reports CHIME age data for representative samples of the foliated and unfoliated granites from the Nkambe area. The Nkambe area ranges in latitude from 6 25' to 6 53' N and in longitude from 10 22' to 11 00'E (Fig. 1). The area consists mainly of plateaus with deep river valleys flowing in to the Donga River in the northeast, and high volcanic mountains in the southeast. The geology of the Nkambe area is shown in Fig. 2. The basement rocks include magmatitic gneiss, anatectic granite, amphibolite and granite, which are overlain in part by Cenozoic basalt, andesite and alkali rhyolite (Peronne, 1969). The migmatitic gneiss trends NNE-SSW direction. Granitic rocks in the Nkambe area consist of the foliated and unfoliated granites. Although these types are not distinguished in the map, the former type includes biotite granite, anatectic granite and small phacoliths in the migmatitic gneiss domains in Fig. 2. Since the foliated granites grade into the migmatitic gneiss at the margins and are deformed along with the migmatitic gneiss, they can be regarded as syntectonic intrusives. The unfoliated granites intrude into the migmatitic gneiss as small stocks of about 500-1000 m in diameter. Two samples (Sample I10 and N9) of the foliated granite and one sample (Sample I4) of the unfoliated granite were used for the present CHIME dating. The sample '
Suzuki, K. 和 Adachi, M.：“日本东北部南北上地体中坪之泽准片麻岩的前寒武纪起源和志留纪变质作用，通过独居石、锆石和磷钇矿的 Th-U-总 Pb 化学等时线年龄揭示。”
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