摘要
在鄂尔多斯盆地动力学和热力作用讨论基础上,以紫金山岩体为代表,进行了鄂尔多斯盆地东部岩浆活动的深部地质背景、岩石学、地球化学、同位素年代学和构造演化等研究。认为盆地热力作用是盆地动力学研究的弱点和难点,盆地热力作用具明显非均一性,热力盆地特征复杂、类型多样,盆地热力构造可分为五类。针对盆地热力作用研究的薄弱性和其与深部圈层活动的复杂性及特殊性,提出了未来研究应注意的要点。中生代鄂尔多斯盆地为克拉通内改造叠合型盆地,盆地的形成演化和区域岩石圈深部具有明显多阶段或多世代沉降更替、演变迁移特点。紫金山岩体具有富碱、较富铁、贫镁、钙,SiO2不饱和,属碱性-过碱性系列;稀土元素含量高,无负铕异常;碱性岩微量元素构造环境判别属于大陆板内拉张区;岩浆活动有多期性;锶(87Sr/86Sr)、钕(143Nd/144Nd)同位素比值落入EMⅠ型地幔,岩浆物质来源较深;单颗粒锆石SHRIMP测年数据表明岩浆活动主期在125~132Ma间,为早白垩世。侏罗纪-白垩纪盆地东部及周缘存在着丰富的深部岩浆活动-热力作用,并发生了岩浆侵入或喷发活动。中生代深部背景是处于吕梁软流圈上涌区,在吕梁软流圈上涌柱软块区与西侧厚硬块区相间的上涌热浮物质聚集处,岩石圈破裂,造成盆地东部出现软流圈热浮物质底侵和岩石圈局部减薄,底侵物质再熔融上侵,形成了紫金山幔源岩浆侵入和火山喷发构成的碱性复式岩体,代表鄂尔多斯盆地与华北陆块在早白垩世经受过一期重要的热力构造事件,它是该盆地东部吕梁软流圈上涌、盆地东翼大面积抬升隆起、断隆带翘倾的表现,此与盆地构造动力体制转换期及岩浆活动-热事件对应。中生代是该盆地油气煤铀等多种能源矿产形成的重要时期。构造过程是:古生代属于大鄂尔多斯盆地,与华北地区一同升降;中生代早期(T-J)为连续沉积区,晚期(J3-K1)发生较强烈的构造转换,岩石圈减薄,地壳拉张伸展变形,伴随岩浆侵入和火山喷发,形成紫金山复式岩体;新生代为整体抬升剥蚀期。
In the study of tectonic kinetics of basins,thermal process and thermal structure are the weakness and difficulty. In recent years,according to the influencing depth of magma-thermal action,there are five types of thermal tectonic units:(ancient) geothermal anomaly gathered fault zone,volcanic hydrothermal active zone(volcanic eruption of shallow layer-hydrothermal action-hypabyssal porphyry intrusion),thermal dome,intermediate-deep intrusion (including intercrustal thermal anomaly plume) and mantle thermal plume. The Ordos basin is not only one of the basin in which oilfields were found earliest,it is also the second biggest petroliferous basin. It is located in the middle-west of the north China block,which is in the binding domain of east and west tectonic regions influenced by multistage repeated extensions and compressions,and it is a composite craton basin formed by overlapping of multi-stage and multi-system structures on the basis of the original Ordos basin,belongs to interior intracraton basin. In the eastern Ordos basin,more attention was paid in the past to study earthquake and magma activity on the basin margin such as the Fen (river) -Wei (river) rift in the southeast margin part of the basin. In the eastern part of the basin,west part of the Shanxi flexural fold belt is a large monocline stratum,which is sloping from east to west. There are a series of open and flat meridional,latitudinal extending sub-folds and a nearly east-westward fracture named the Wubu Fracture (F1) which developed well. In the east of study area,there is a fracture named the Lishi Fracture (F2) in south and north direction. Most folds and fractures belong to semi concealed style. Along with the study of tectonic background,mineralization and petroleum accumulation in the basin,we have recognized that there exists point type of thermal process from deep sourced magma in the eastern basin and its periphery. By field investigation,sample analysis,SHRIMP age determination and implication for magma source,and according to the characteristics of geology and geochemistry,the stage and sequence of magmatic activity and latest SHRIMP chronology were discussed,which led to a suggestion that there would have been a type of middle-deep sourced magma intruding in the eastern part of the basin,China. The main period of a magma thermal process by SHRIMP chronology is early Cretaceous,and then the magma activity sequence can be determined. The acquisition of geochemical and dating data has an important significance to analyze the tectonic evolution and its background of the study area.Based on the discussion of the dynamic process and thermodynamic effect of the Ordos basin,as represented by the Zijinshan complex,the magma activity in the eastern part of the Ordos basin with the respect of deep geological background,petrology,geochemistry,isotopic dating and tectonic evolution is studied. The research holds the view that the thermodynamic process is weakness and difficulty in the light of study on tectonic kinetics of basins. The thermodynamic process of the basin is evidently inhomogenous and the characteristics of the thermal basin are complicated and diverse. The basin thermal structure can be summarized into 5 categories. Aiming at weak researches of thermodynamic process and complication and specialty with deep lithosphere activity,the main points needing attention for future research are put forward. In the Mesozoic,the Ordos basin belonged to intracratonic reformed superposition basin. The development and evolution of the basin as well as the deep of regional lithosphere have the obvious characteristics of multi-stage or multi-generation settlement alteration,evolution and migration. The Zijinshan complex is rich in alkali,relatively rich in iron,poor in magnesium and calcium and unsaturated in SiO2,belonging to an alkaline-peralkaline series. The complex has high content of rare earth elements,without negative Eu abnormal. Tectonic setting discrimination of trace elements of the alkaline rock belongs to an extension area of the continental intraplate. The magmatic intrusion has many stages. The rate of 87Sr/86Sr and 143Nd/144Nd are fallen into the range of EMⅠlithospheric mantle,which shows that the igneous material comes from a deeper source. The SHRIMP chronology of single-grain zircon shows that the main stage of magmatic activity is in 125~132Ma,is equal to early Cretaceous. In the Jurassic-Cretaceous,a large amount of deep magma activity and thermodynamic process took place in the eastern basin and its periphery,along with magma intrusion and eruption. The deep background of the region in the Mesozoic was the upwelling area of Lliang asthenosphere. The lithosphere broke in the spot where the interphase between the Lliang asthenosphere soft block and western mass hard block,in which the thermal upwelling material aggregated. As the result,there occurred intrusion of thermal material of asthenosphere and partly lithospheric thinning and then the underplating materials melted and intruded upward,formed the alkali composite body,which is composed of the Zijinshan mantle-source magma intrusion and volcano eruption. The complex represents that in early Cretaceous Ordos basin and the North China plate experienced a stage of significant thermal structural events,which is the representative of the Lliang asthenosphere upwelling in eastern basin,large-scale uplift elevation and tilting of a fault-uplift zone in the east limb of the basin. These features are consistent with the transition of tectonic and dynamic regime and magma activity-thermal event. The Mesozoic is the significant period in which multi-energy resource mineral deposits (oil-gas-coal-uranium,etc) of the basin were formed. It was comparatively steady in the Paleozoic and Triassic inside the North China block and such state of steady changed obviously at Yanshanian Movement in middle-late Mesozoic. Magmatic activity was relatively strong in the period of the Jurassic-Cretaceous,and accordingly many kinds of magmas intruded through faults inside the basin and in its east margin. This stage of deep magmatic activity from late Jurassic to early Cretaceous represented that there experienced one significant and extensive tectonic thermal event in the Ordos basin associated with the North China block in middle-late Mesozoic. The tectonic thermal event experienced tectonic inversion in early Cretaceous(135~125Ma),in the middle-later stage of basin evolution,and the time of initial upwelling of the basin is correspond to that of tectonic inversion and coincides with the tectonic thermal event in North China.The Zijinshan area is located in the steeply contact belt between the east of the Ordos lump and Lliang soft block. In the deep,upwelling plume of asthnosphere (soft block) and thickening area of lithosphere (hard block) was in interval location. In the point of hot upwelling materials assembled,the lithosphere broke,the hot liquid upwelled and basal eroded the lower crust along the fracture,then liquation,formed a set of alkaline and alkali complex,so that formed the Zijinshan mantle-derived complex.The research shows that from late Jurassic to early Cretaceous,there happened a stage of strong tectonic events in the North China block in which the Ordos basin was located. Therefore,magma intrusion or volcanic eruption activity happened in the Mesozoic in the east part of the basin and the area of the Taihang Mountains,led to form the Zijinshan complex. Inside the basin,there existed point deep-sourced intrusive rocks in the range of cover. All these show that the setting of the regional thermal process was special at that time,and that there existed definite setting conditions of the regional thermal process in the depth of the basin. The thermal process of this stage gave rise to upwelling of the Lliang block dome,tilting of fault-uplift zone and large-scale uplift elevation of the east limb of the basin.The sequence of tectonic evolution of magmatic-thermal activity in the study area is listed as follows:Firstly,in the Early Paleozoic,the area along with the North China block was uplifted in mass which indicated by absence of late Ordovician-early Carboniferous sediment. Secondly,the deep magma intruded and monzonite and aegirine-augite syenite formed in the study area. In the further east of the basin around the Taihang Mountains,there existed a magmatic intrusion process in the depth. According to SHRIMP chronology we find that the magmatic zircons were well developed in 343Ma and 287~293Ma,which implies that a series of magma thermal events happened at that time. Thirdly,from the Triassic to Jurassic,east part of the basin and around the Lliang Mountains was in the section of continuously sedimentary sequence. Finally,the Yanshanian tectonic activity was very intense in the basin and its neighborhood with violent deformation in the strata,and intense structural activity mainly occurred in latter Jurassic. In Cretaceous period,tectonic stress and crust deformation pattern in North China had been greatly changed:the main structure activity is the tension of the lithosphere with weak compression,and the lithosphere became thinning and thinning violently,while the crust endured extensional deformation. Then the rift basin and rift valley developed well,along with widespread magmatism and volcanism:141~154Ma,125~132Ma. In the study area,there formed trachyte porphyry,trachyandesite,trachytic volcanic breccia and phonolitic volcanic breccia,partly malignite and nepheline syenite and so on. The magmatic pluton formed in the period is the primary type in the surface and shallow well in the basin margin nowadays and also is the concentrating section of numerous dating data. From Himalayan period late-recent period,this area was in the state of uplift in mass and denudation at all times.The main tectonic processes are:In Paleozoic Era,the basin originated from the great Ordos basin,ascending and descending along with the North China region accordingly. In early Mesozoic(T-J),the basin was in the section of continuously sedimentary sequence. During late Mesozoic (J3-K1),drastic tectonic transformation occurred,lithosphere became thinner,mantle stretched and deformed,accompanied by magma intrusion and volcano eruption,then Zijinshan complex was formed. In Neozoic Era,the whole basin experienced uplift and erosion.