Petrogenesis of the Early Paleozoic Mafic Rock in Fangxian Area on the Southern Margin of Wudang Mountain and Its Tectonic Significance
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摘要:
武当山南缘青峰—襄广断裂是分隔南秦岭和扬子板块的重要界线, 亦是勉略带东延的重要纽带.通过开展房县门古和五台地区路线地质调查和室内研究, 查明门古地区发育的原定新元古代耀岭河群火山岩为晚奥陶世变玄武岩块体((444.9±2.5) Ma), 并与原划震旦系—寒武纪的灰岩、硅质岩块体包裹于砂质片岩中, 显示出构造混杂岩带特征; 五台地区侵入志留系梅子垭组的辉绿岩为早志留世岩浆活动产物((432.4±3.3) Ma), 岩石地球化学特征显示两地区样品均具有高TiO2、富集高场强元素Nb、Ta、Zr等特征, 不相容元素比值及稀土配分曲线和微量元素蛛网图等均表现出OIB亲缘性, 两地区样品岩浆均起源于较深部的软流圈地幔, 母岩浆为高压环境下石榴石二辉橄榄岩低度部分熔融形成, 并发生了分离结晶作用, 且受到地壳轻微混染; 区域地质特征和构造判别图解共同指示二者非地幔柱成因, 其形成环境与大陆裂谷无关,而是形成于大洋板内环境, 这为进一步理解南秦岭显生宙构造演化提供了证据.
Abstract:The Qingfeng–Xiangguang fault on the southern margin of Wudang Mountain is a dominant boundary for the South Qinling and Yangtze plates and is also an important link between the Mianlue zone and its eastward extension. Detailed field route investigation and systematic research reveal that the formerly assigned Neoproterozoic Yaolinghe Group volcanic rocks outcropped in Mengu area should be meta-basalt block of Late Ordovician ((444.9 ± 2.5) Ma), and it consists of a distinct mélange feature with the formerly assigned Sinian to Cambrian limestone and siliceous blocks and sandy schist matrix; zircon U-Pb dating of diabase which intruded into Meiziya Formation of Wutai area indicates an Early Silurian magmatic age ((432.4±3.3) Ma). Whole rock geochemistry of these samples indicates that both Mengu meta-basalts and Wutai diabase are typically characterized by high concentrations of TiO2, Nb, Ta, and Zr. The ratio of incompatible elements, rare earth element distribution patterns and trace element spider diagrams all display the feature of affinity of Ocean-island basalt (OIB). In addition, samples from the two areas appear to have all originated from the asthenospheric mantle and the magma was mainly formed by the high-pressure and low degree of partial melting of garnet lherzolite and underwent varying degrees of fractional crystallization and slight crustal contamination. Regional geological and discrimination diagrams both indicate that these samples do not represent a plume or continental rift, but exhibit a setting of oceanic intraplate. These results provide the pivotal evidence for accurate investigation of the Phanerozoic tectonic evolution of South Qinling Orogeny.
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Key words:
- mafic rocks /
- petrogenesis /
- Early Paleozoic /
- Wudang Mountain /
- South Qinling
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