湘东北地区晚燕山期细碧质玄武岩的发现及地质意义

DISCOVERY OF THE LATE YANSHANIAN SPILITIC BASALTS IN NORTHEASTERN HUNAN PROVINCE AND ITS GEOLOGICAL IMPLICATIONS

湘东北地区发育一套晚燕山期（距今85～95Ma）的细碧质玄武岩。该岩石具碱性-拉斑质玄武岩亲和性，矿物成份以钠长石为主，呈斑状和交织结构，气孔-杏仁构造发育，富钠低钾，轻稀土元素富集而重稀土元素亏损，大离子亲石元素（LILE）可变地亏损或富集，高场强元素（HFSE）Th、U、Ti和Hf相对富集而Y、Yb、Zr、P及Nb和Ta表现适度的亏损。Nd同位素成份（εnd（t）=-1．29～0．66）则暗示该岩石并不起源于软流圈地幔。结合中生代华南地区特别是江南造山带大地构造演化，该细碧质玄武岩应产于板内构造环境，起源于一个不太富集的陆下岩石圈地幔低度熔融，可能与晚三叠世扬子地块与华北地块的碰撞导致华南大陆内部因古缝合线或古深断裂活化而诱发的陆内造山事件有关。湘东北晚燕山期细碧质玄武岩是华南燕山期陆内造山运动的后造山阶段于距今90Ma左右开始的岩石圈沿长沙一平江断裂伸展裂陷的产物。郯庐断裂可能向南延伸至湖南境内。

The Late Yanshanian spilitic basahs （ca. 85 -95 Ma） occurred in northeastern Hunan province, whose eruption was controlled by the Changsha-Pingjiang fault, which could be regarded as southward extension of the Tan-Lu fault zone. These rocks had a main mineral assemblage of albite ＋ chlorite ＋ epidote, with the presence of minerals olivine and pyroxene generally as porphyroblasts, typically showing the characteristics of spilites. The geochemistry of the spilitic rocks was characterized by high in Na but low in K, slightly enriched in LREE, Th, U, Ti and Hf but relatively depleted in HREE, Y, Yb, Zr, P, Nb and Ta. However, the contents of LILEs were relatively variable. The Nd isotopic composition （εnd（t）=-0.66～-1.29） revealed that the parental melts could not have been derived from an asthenospheric mantle. Considering the Mesozoic tectonic development of the South China, and particularly regional evolution of the Jiangnan orogen, these rocks might have been generated in a withinplate setting, in which their parental magma was induced by low-degree melting of a slightly enriched, sub-continental lithospheric mantle. We suggested that the Yanshanian intracontinental orogenic event induced by activation of Paleo-surtures and/or Paleo-fauhs in the amalgamated South China due to the Late-Triassic collision between the Yangtze and North China Blocks, probably had played a key role in the generaton of the Mesozoic magmatism in northeastern Hunan province, where the Late Yanshanian spilitic basahs might mark the recommence of lithospheric detachment and extension of the Yanshanian orogenic belt before 90 Ma in South China.