赞比亚东北部班韦乌卢地块泛非期以来构造演化的裂变径迹约束

本文利用锆石和磷灰石裂变径迹方法探讨了班韦乌卢地块泛非期以来的主要构造演化时间.为研究班韦乌卢地块的构造活动,本文从班韦乌卢地块中部卡帕图地区的沉积盖层姆波罗科索群的姆巴拉组、萨马组和卡布韦卢马组中采集多件新鲜的岩石样品,并获得5件锆石和2件磷灰石样品裂变径迹分析结果.5件锆石裂变径迹年龄介于575±35～380±22 Ma之间,其峰值年龄组为600～572 Ma和420～390 Ma.2件磷灰石裂变径迹年龄介于59±6～27±4 Ma之间,其峰值年龄组为99～80 Ma和19～11 Ma.它们的峰值年龄组分别对应的主要时代为新元古代的埃迪卡拉纪、早泥盆世、晚白垩世和中新世.对比区域上已有的年龄数据可知,南部非洲地区新元古代的埃迪卡拉纪、早泥盆世和晚白垩世的构造活动是一个区域性的构造事件,仅中新世(19～11 Ma)的构造活动在区域上的表现尚不明确.综合前人成果资料,自泛非构造运动以来,班韦乌卢地块至少经历600～572 Ma、488～441 Ma、420～390 Ma、375～293 Ma、99～80 Ma及19～11 Ma六个构造事件.班韦乌卢地块泛非期以来的构造演化研究程度很低,此次研究成果可为今后在该地区的深入研究提供重要参考.     

赞比亚东北部陇都地区首次发现中元古代辉长岩:哥伦比亚超大陆裂解在班韦乌卢地块的响应

赞比亚东北部陇都地区位于班韦乌卢地块中东部,区内首次发现中元古代基性岩浆侵入事件,对于了解班韦乌卢地块中元古代构造演化历史及哥伦比亚超大陆的重建具有重要意义.针对研究区的辉长岩进行了系统的岩相学、LA-MC-ICP-MS锆石U-Pb年代学、岩石地球化学和锆石Hf同位素研究.结果表明,辉长岩锆石中17粒具有他形板柱状,内...     

The LATEA metacraton (Central Hoggar, Tuareg shield, Algeria): behaviour of an old passive margin during the Pan-African orogeny

Historically, the Tuareg shield is divided into three parts bordered by mega-shear zones with the centre, the Central Polycyclic Hoggar, characterized by Archaean and Palaeoproterozoic lithologies. Nearly 10 years ago, the Tuareg shield was shown to be composed of 23 displaced terranes [Geology 22 (1994) 641] whose relationships were deciphered in Aïr to the SE [Precambr. Res. 67 (1994) 59]. The Polycyclic Central Hoggar terranes were characterized by the presence of well preserved Archaean/Palaeoproterozoic and Neoproterozoic lithologies.We show here that the terranes from Central Hoggar (Laouni, Azrou-n-Fad, Tefedest, Egéré-Aleksod) belonged to a single old passive margin, to which we gave the acronym name LATEA, which behaved as a craton during the Mesoproterozoic and the Early-Middle Neoproterozoic but was partly destabilized and dissected during the Late Neoproterozoic as a consequence of its involvement as a passive margin in the Pan-African orogen.An early Pan-African phase consisted of thrust sheets including garnet-bearing lithologies (eclogite, amphibolite, gneiss) that can be mapped and correlated in three LATEA terranes. In the Tin Begane area, P–T–t paths have been established from>15 kbar––790 °C (eclogite) to 4 kbar––500 °C (greenschist retrogression) through 12 kbar––830 °C (garnet amphibolite) and 8 kbar––700 °C (garnet gneiss), corresponding to the retrograde path of a Franciscan-type loop. Sm–Nd geochronology on minerals and laser ablation ICP-MS on garnet show the mobility of REE, particularly LREE, during the retrograde greenschist facies that affects, although slightly, some of these rocks. The amphibolite-facies metamorphism has been dated at 685 ± 19 Ma and the greenschist facies at 522 ± 27 Ma. During the thrust phase, the Archaean–Palaeoproterozoic basement was only locally affected by the Pan-African tectonics. LATEA behaved as a craton. Other juvenile terranes were also thrust early onto LATEA: the Iskel island arc at ≈850 Ma to the west of LATEA, the Serouenout terrane in the 700–620 Ma age range to the east. No subduction-related magmas have intruded LATEA during this epoch, which behaved as a passive margin.During the main Pan-African phase (625–580 Ma), LATEA was dissected by mega-shear zones that induced several hundreds km of relative displacement and allowed the emplacement of high-K calc-alkaline batholiths. Smaller movements continued till 525 Ma, accompanied by the emplacement of subcircular plutons with alkaline affinity. Here is dated the Ounane granodiorite (624 ± 15 Ma; ⁸⁷Sr/⁸⁶Sr_i=0.70839 ± 0.00016; 6WR, MSWD=0.87) and the Tisselliline granite (552 ± 15 Ma; ⁸⁷Sr/⁸⁶Sr_i=0.7074 ± 0.0001; 5WR, MSWD=1.4). Nd isotopes indicate a preponderant Palaeoproterozoic crustal source for these two plutons: _Nd=−14 to −21 at 624 Ma and T_DM=1650–2320 Ma for Ounane and _Nd=−13 to −15 at 555 Ma and T_DM=1550–1720 Ma for Tisselliline. Our model links these intrusions to a linear lithospheric delamination along mega-shear zones, allowing the hot asthenosphere to rise, melt by adiabatic pressure release and inducing the melting of the Palaeoproterozoic and Archaean lower crust.The LATEA cratonic microcontinent remained however sufficiently rigid to preserve Archaean and Palaeoproterozoic lithologies as well as Middle Neoproterozoic oceanic thrust sheets. This corresponds to the notion of metacraton [J. African Earth Sci. 34 (2002) 119], i.e. a craton that has been remobilized during an orogenic event but is still recognizable dominantly through its rheological, geochronological, isotopic and sometimes petrological characteristics.     

赞比亚班韦乌卢变质克拉通东北部姆巴拉组碎屑锆石U-Pb年龄和Hf同位素特征

班韦乌卢变质克拉通活动时间和地壳增长问题一直是地质学家关注的焦点。本文通过对班韦乌卢地块中卡帕图地区姆波罗科索群姆巴拉组石英砂岩进行了碎屑锆石U-Pb年代学和Hf同位素研究,结合变质克拉通已有成果获得认识如下：(1)姆巴拉组的形成时间可能介于(1833±22 Ma)～(1712±22 Ma)之间,属于古元古代。(2)姆巴拉组具有丰富的物质来源,其中2728～2602 Ma(峰值为2650 Ma)的物质可能来源于坦桑尼亚克拉通中的花岗岩类、粗面安山岩及流纹岩等岩石。2246～1833 Ma(峰值为1880 Ma)的物质主要来源于班韦乌卢地块中花岗岩类、石英闪长岩及火山岩类等岩石。(3)班韦乌卢变质克拉通的活动时间包括中太古代、新太古代、古元古代和中元古代四个时期,其中古元古代(1870 Ma)为活动的高峰期,涉及范围广泛,可能与哥伦比亚超大陆的演化密切相关。(4)班韦乌卢变质克拉通沉积岩的源区地壳增生除来自古老地壳的再循环物质外,还存在新生地壳的物质。同时,地壳在古太古代-古元古代均实现了增长,其中凯诺兰超大陆聚合时期(2550 Ma)增长最快。在古元古代之前的地壳主增长期与坦桑尼亚克拉通及全球大陆地壳主增长期基本一致。     

赞比亚北部卡帕图地区古元古代花岗岩成因:岩石地球化学、锆石年代学及Hf同位素约束

赞比亚北部地区广泛发育古元古代花岗岩,是研究班韦乌卢地块构造岩浆作用特征的天然实验室.本文对赞比亚北部卡帕图地区大面积出露的花岗岩体开展较系统的岩相学、岩石地球化学、锆石U-Pb年代学和Hf同位素研究.结果显示,卡帕图地区主体花岗岩形成于2012±11～1970±20Ma,晚期浅肉红色似斑状正长花岗岩侵位年龄为1959±16Ma,均为古元古代岩浆活动,大量太古宙捕获锆石信息显示班韦乌卢地块可能存在太古宙地壳组分残留.地球化学数据表明,卡帕图花岗岩具有高硅(SiO2=67.93％～72.80％)、高铝(13.85％～14.91％)、富钾(Na2O/K2O=0.29～0.89)、低镁(0.38％～1.93％)、低磷(0.10％～0.23％)的特点,铝饱和指数A/CNK在1.04～1.19之间,CIPW刚玉标准分子指数大于1％(1.01％～2.51％);稀土元素分配模式整体右倾,其中主体花岗岩稀土含量较高(ΣREE=191.79×10-6～294.25×10-6),Eu负异常(δEu=0.26～0.53),晚期浅肉红色似斑状正长花岗岩稀土含量相对较低(ΣREE=48.58×10-6～54.00×10-6),Eu弱正异常(δEu=1.04～1.37);微量元素中,卡帕图花岗岩富集Rb、K等大离子亲石元素,亏损Ba、Sr和高场强元素Nb、Ta等,并具有较低的Cr(3.56×10-6～65.6×10-6)、Co(1.99×10-6～12.5×10-6)和Ni(2.16×10-6～33.7×10-6)含量;以上特征表明研究区古元古代花岗岩属于S型花岗岩.Hf同位素结果表明,卡帕图花岗岩的εHf(t)值和二阶段模式年龄tDM2值分别介于-15.9～+4.7和2289～3596Ma,远大于其锆石的结晶年龄,指示岩浆源区成分的强不均一性,岩浆源区系太古宙-古元古代陆壳部分熔融产物,存在不同程度的地幔物质混入.此外,主体岩石较高的CaO/Na2O和低的Rb/Sr比值,表明主体花岗岩原岩是由地壳砂质岩部分熔融而来;晚期浅肉红色似斑状花岗岩低的CaO/Na2O和高的Rb/Sr比值,指示泥质岩源区熔融.综合区域和全球构造演化历史,本次研究认为卡帕图花岗岩形成于坦桑克拉通与班韦乌卢地块同碰撞的构造环境,该期岩浆活动可能是Columbia超大陆的碰撞造山事件在班韦乌卢地块的地质响应.     

Age and geochemistry of the granitoid from the Lunte area,Northeastern Zambia: Implications for magmatism of the Columbia supercontinent

The Paleoproterozoic tectonic evolution of the Bangweulu Block has long been controversial. Paleoproterozoic granites consisting of the basement complex of the Bangweulu Block are widely exposed in northeastern Zambia, and they are the critical media for studying the tectonic evolution of the Bangweulu Block. This study systematically investigated the petrography, zircon U-Pb chronology, and petrogeochemistry of the granitoid extensively exposed in the Lunte area, northeastern Zambia. The results show that the granitoid in the area formed during 2051±13–2009±20 Ma as a result of Paleoproterozoic magmatic events. Geochemical data show that the granites in the area mainly include syenogranites and monzogranites of high-K calc-alkaline series and are characterized by high SiO₂ content (72.68% –73.78%) and K₂O/Na₂O ratio (1.82–2.29). The presence of garnets, the high aluminum saturation index (A/CNK is 1.13–1.21), and the 1.27%–1.95% of corundum molecules jointly indicate that granites in the Lunte area are S-type granites. Rare earth elements in all samples show a rightward inclination and noticeably negative Eu-anomalies (δEu = 0.16–0.40) and are relatively rich in light rare earth elements. Furthermore, the granites are rich in large ion lithophile elements such as Rb, Th, U, and K and are depleted in Ba, Sr, and high field strength elements such as Ta and Nb. In addition, they bear low contents of Cr (6.31×10⁻⁶–10.8×10⁻⁶), Ni (2.87×10⁻⁶–4.76×10⁻⁶), and Co (2.62×10⁻⁶–3.96×10⁻⁶). These data lead to the conclusion that the source rocks are meta-sedimentary rocks. Combining the above results and the study of regional tectonic evolution, the authors suggest that granitoid in the Lunte area were formed in a tectonic environment corresponding to the collision between the Tanzania Craton and the Bangweulu Block. The magmatic activities in this period may be related to the assembly of the Columbia supercontinent.©2021 China Geology Editorial Office.     

班韦卢地块和伊鲁米德带区域地质及构造演化特征

"变质克拉通"作用为稳定克拉通的被动边缘受到陆内碰撞作用的影响而发生变质作用和岩浆作用。班韦卢地块位于赞比亚东北部,被认为是坦桑尼亚克拉通或刚果克拉通的变质克拉通边缘,伊鲁米德带位于班韦卢地块的东南部,为其元古宙变质克拉通边缘。本文通过对两者的基底、地层和岩浆岩特征的系统整理研究,发现元古宙变质基底从北向南随变质作用的增强,由片岩类渐变为片麻岩类,这可能是由于两者构造层位的不同和遭受变质克拉通作用的期次不同共同造成或其中之一造成的。穆瓦超群大规模沉积作用年龄为1800Ma左右,姆波洛科索群沉积碎屑主要来源于盆地南部的变质基底,锆石年龄显示含少量太古宙地壳组分。多期次的岩浆岩具有相似的地球化学性质,其来源可能为太古宙地壳的局部熔融作用。根据地层和岩浆岩特征,可以将构造演化分为3个阶段:班韦卢地块和伊鲁米德带"变质克拉通"阶段(≥1880Ma)、稳定沉积阶段(1880Ma~1050Ma)和伊鲁米德带"变质克拉通"阶段(≤1050Ma),不同阶段地幔岩浆加入量较少,但可作为热源导致太古宙地壳多次熔融而形成元古宙地质体。     

班韦乌卢地块中部变质表壳岩碎屑锆石U-Pb年代学、Hf同位素研究及其构造意义

通过LA-MC-ICP-MS碎屑锆石微区U-Pb定年和Hf同位素分析,对班韦乌卢地块中部地区基底变质表壳岩进行了同位素定年及物源示踪研究.结果显示,两件样品的碎屑锆石峰值年龄分布表现出较好的一致性,总体呈现出2490～2059 Ma、2044～1954 Ma、1942～1887 Ma、1865～1817 Ma以及164...     

The Anfeg post-collisional Pan-African high-K calc-alkaline batholith (Central Hoggar, Algeria), result of the LATEA microcontinent metacratonization

The Anfeg batholith (or composite laccolith) occupies a large surface (2000 km²) at the northern tip of the Laouni terrane, just south of Tamanrasset in Hoggar. It is granodioritic to granitic in composition and comprises abundant enclaves that are either mafic microgranular enclaves (MME) or gneissic xenoliths. It intruded an Eburnian (≈2 Ga) high-grade basement belonging to the LATEA metacraton at approximately 608 Ma (recalculated from the U–Pb dating of [Tectonics 5 (1986) 955]) and cooled at approximately 4 kbar, with a temperature of about 750 °C. This emplacement occurred mainly along subhorizontal thrust planes related to Pan-African subvertical mega-shear zones close to the attachment zone of a strike-slip partitioned transpression system. Although affected by some LILE mobility, the Anfeg batholith can be ascribed to a high-K calc-alkaline suite but characterized by low heavy REE contents and high LREE/HREE ratios. The MME belong to the Anfeg magmatic trend while some xenoliths belong to Neoproterozoic island arc rocks.The Anfeg batholith defines a Nd–Sr isotopic initial ratios trend (_Nd/(⁸⁷Sr/⁸⁶Sr)_i from −2.8/0.7068 to −11.8/0.7111) pointing to a mixing between a depleted mantle and an old Rb-depleted granulitic lower crust. Both sources have been identified within LATEA and elsewhere in the Tuareg shield (_Nd/⁸⁷Sr/⁸⁶Sr)_i of +6.2/0.7028 for the depleted mantle, −22/0.708 for the old lower crust.The model proposed relates the above geochemical features to a lithospheric delamination along the subvertical mega-shear zones that dissected the rigid LATEA former passive margin without major crustal thickening (metacratonization) during the general northward tectonic escape of the Tuareg terranes, a consequence of the collision with the West African craton. This delamination allowed the uprise of the asthenosphere. In turn, this induced the melting of the asthenosphere by adiabatic pressure release and of the old felsic and mafic lower crust due to the high heat flow. A gradient in the mantle/crust ratio within the source of the Pan-African magmatism is observed in LATEA from the northeast (Egéré-Aleksod terrane) where rare plutons are rooted within the Archaean/Eburnian basement to the southwest (Laouni terrane) where abundant batholiths, including Anfeg, have a mixed signature. Some mantle melts with only slight crustal contamination (Laouni troctolitic layered intrusions) are even present. This suggests that the southern boundary of LATEA microcontinent is not far south of the Tuareg shield.     

赞比亚东北部卡萨马群形成环境:碎屑锆石U-Pb年龄与Hf同位素的限定

赞比亚东北部班韦乌卢地块受到其东北部的古元古代乌本迪构造带、西北部的中元古代基巴拉构造带、东南部的中元古代伊鲁米德构造带及西南部泛非期卢弗里安构造带的影响。班韦乌卢地块主要由基底和沉积盖层两部分组成。其中,基底主要由片岩带、安山质-流纹质变质火山岩、花岗岩类岩基及其他侵入体组成。沉积盖层从老至新依次为姆波罗科索群、卡萨马群、加丹加超群及新生代的河流相和湖泊相沉积。本文对卡萨马地区的卡萨马群地质特征、碎屑锆石U-Pb-Hf同位素及地球化学特征进行了系统研究,获得卡萨马群中紫红色粉砂岩的32颗碎屑锆石U-Pb年龄多集中在(1849±23)～(1993±21)Ma,卡萨马群的形成时代可能晚于(1434±14)Ma,属于中元古代,其中22颗锆石(176Hf/177Hf)i为0.281049～0.281618,εHf(t)为-12.8～-1.6,二阶段模式年龄变化范围为2406～3487 Ma。对卡萨马群的砂岩进行了稀土元素分析和微量元素分析,结果显示卡萨马群砂岩δEu负异常明显,投点主要落在沉积岩区;微量元素显示砂岩形成于被动大陆边缘环境。