是岩浆侵入还是构造侵位?——湖北两郧随枣镁铁岩带野外调查纪实

Is Magmatic Intrusion or Exotic Block Emplacement?——Field Survey and Records of Yunxi—Suizhou Mafic Magmatic Belt in Hubei Province

湖北省武当山—随州—枣阳一线存在一呈北西西走向的镁铁质岩浆岩带,长约390km,宽40～100km。其中超镁铁质岩14处,超镁铁质、镁铁质杂岩21处,镁铁质岩250处,出露面积689km2。自上世纪开展1∶20万区调工作迄今,已出版的成果资料一致认为,这些岩体是镁铁质岩浆沿深大断裂侵入而成。笔者等于2005年随机选择了丹江口和随州境内两条地质调查路线,在实地详细观察了35个大小不一的岩体,证实该岩浆岩带的岩体都不是镁铁质岩浆就地侵入形成,而是众多外来的镁铁质岩块与武当山群和随县群浅变质浊积岩共同构成的混杂岩带。这一论断的具体根据为:邻接岩块的围岩普遍不存在热变质;围岩的纹理、层理及韵律层理均有完好保留;岩块边部并无冷凝边及结晶变细现象;岩块内部的相带及条带状构造与岩块的外形无关,并可常见造岩矿物之结晶条带被接触界面斜截;岩块中穿插有不规则状石英脉体及斜长花岗岩脉,但二者从不侵入围岩,这表明,岩块形成时代早于围岩;一些大、中型岩体都不是单一侵入体,而是许多中、小型岩块的集合体;在岩块内部从来未见到围岩捕虏体。从岩块的岩石化学分析结果看出,调查区内的镁铁质岩均属拉斑玄武岩系列,相当于蛇绿岩套中的堆积杂岩,是古洋脊或边缘海盆扩张脊的产物。据此可以断定,此混杂岩带所在的武当山—大悟山地体,具有增生楔的属性。

The Yunxi—zaoyang—Suizhou mafic magmatic belt occurs in a NWW trend with length of about 390 km and width of 40～100 km in the northern Hubei Province. Among which there are 14 ultramafic rocks, 21 ultramafic—mafic complex rocks, and 250 mafic rocks, with total outcrop areas of 689 km^2. From the second half of the 20th century to the present, the previous investigators unanimously affirmed that all these mafic and ultramafic rocks been originated in magma intrusions along the abyssal fault. The authors selected both the Danjiangkou and the suizhou geological routes at random, and investigated in detail to 35 mafic rock exposures in this years. Based on this research, the authors recognized that the mafic and ultramafic rocks among the whole magmatic belt are not that the liquid magma intruded on the spot, but that a numerous exotic block of mafic rocks emplaced in the turbidite of Wudangshan Group or Suixian Group. In actual fact, there is not so much a zone of mafic magmatite as a large-scale m lange belt. The argument of this paper are as follows： （1） The thermal metamorphism is lacking in general for adjoining rocks close the mafic block. The characteristic lamination, stratification and cyclothem of turbidite among the host rock near the exotic blocks still are clearly discernible; （2） The chilled border is absent in common in the margin of exotic blocks; （3） The cumulate bonded structure and facies plane in the gabbroite not depends fundamentally on the shape of mafic blocks, and it frequently cut off by the cintact plane; （4） The gabbro and diabase inside of mafic blocks was injected occasionally by the irregular quartz vein or the plagioclase-granite dike, which of them never intrude into the turbidite round the exotic blocks. There are indications that the times of ultramafic and mafic rocks be earlier than the host rock; （5） Some mafic intrusions with a large or middle scale in this belt generally are not unitary intrusive bodies, but a aggregation of numerous exotic blocks with different scale; （6） The capture of host rocks in interior of mafic blocks is never seen in this investigation. The ＂captures＂ which was defined by the predecessors are mostly a turbidite in between the different mafic blocks as the ground mass of m lange; （7） Petrochemical studies of the mafic rocks in the Yunxi—Suizhou belt demonstrate that the said rocks are the tholeiitic magma series, analoguous to the cumulate complex of ophiolite suite.Therefore we may conclude that the Wudangshan—Dawushan region herein lies the above said m lange belt is an accretionary wedge in the southwestern margin of Tongboshan—Dabieshan terrane.