Age, lithology and structural evolution of the c. 3.53 Ga Theespruit Formation in the Tjakastad area, southwestern Barberton Greenstone Belt, South Africa, with implications for Archaean tectonics
Age, lithology and structural evolution of the c. 3.53 Ga Theespruit Formation in the Tjakastad area, southwestern Barberton Greenstone Belt, South Africa, with implications for Archaean tectonics
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c的年龄、岩性和构造演化。
DOI：
10.1016/j.chemgeo.2008.11.006
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发表时间：
2009
期刊：
影响因子：
3.9
通讯作者：
J. Connelly
中科院分区：
文献类型：
--
作者：
M. V. Kranendonk;A. Kröner;E. Hegner;J. Connelly
结合巴伯顿绿岩带西南部Tjakastad附近Theespruit组岩石的野外和岩相学重新评估，以及新的锆石U-Pb年龄和全岩Sm-Nd同位素结果，揭示了该地区与以前的逆冲-增生模型的几个重要不一致之处。发现以前解释为“逆冲断层”的断层是伸展性的，“基底正片麻岩逆冲岩片”是变形不大的石英-长石斑岩或剪切的长英质火山-沉积岩，“沉积杂岩”是长英质团块。这些意见，结合一致的面向方向的层理，识别不同的地层变化的走向，和U-Pb锆石日期从13个长英质变火山岩样品，都表明Theespruit组是一个中等应变，连贯的地层序列沉积在c。3530 Ma，受c. 3230 Ma.之前从角闪岩相Theespruit组长英质片岩中分析的3453±6 Ma锆石群被解释为变质而非碎屑成因，并由长英质深成岩的热引起，该深成岩将Theespruit组缝合到上覆的Komati组，约为2000年。3450 Ma，比推测的逆冲推覆事件早220 Ma。
A field and petrographic re-assessment of rocks from the Theespruit Formation near Tjakastad, southwestern Barberton Greenstone Belt, is combined with new zircon U–Pb ages and whole-rock Sm–Nd isotopic results to reveal several important inconsistencies with a previous thrust-accretion model for this area. It was found that faults previously interpreted as ‘thrusts’ are extensional, ‘thrust slices of basement orthogneiss’ are little deformed quartz-feldspar porphyries or sheared felsic volcaniclastic sedimentary rocks, and ‘sedimentary diamictites’ are felsic agglomerates. These observations, combined with consistent facing directions of bedding, the recognition of distinct stratigraphic variations across strike, and U–Pb zircon dates from 13 felsic metavolcanic samples, all suggest that the Theespruit Formation is a moderately strained, coherent stratigraphic succession deposited at c. 3530 Ma and affected by extensional shear deformation at c. 3230 Ma. A 3453±6 Ma population of zircons analysed previously from an amphibolite-facies Theespruit Formation felsic schist is interpreted as being of metamorphic, rather than detrital, origin, and arising from heat associated with felsic plutonic rocks that stitch the Theespruit Formation to the overlying Komati Formation at ca. 3450 Ma, 220 Ma earlier than the proposed thrusting event.