All kinds of geological bodies in the survey area have undergone intense multi-stage structural deformation transformation, but due to the different structural evolution history and structural position, the deformation characteristics of each structural layer (geological body) are also quite different, which are discussed separately below.
Structural deformation characteristics of 1. basement metamorphic rock series
It is preliminarily found that the metamorphic rock series in the basement of orogenic belt has undergone at least four stages of structural deformation:
(1) The first stage of structural deformation: a series of strong and weak zones of ductile deformation are often formed, and some felsic veins often run through along the foliation direction due to partial melting at the strongly deformed parts. The regional migmatization in the investigation area may occur during this deformation period.
(2) The second stage of structural deformation: the foliation and felsic veins formed in the early stage of deformation are often used as deformation surfaces, and strong structural metasomatism occurs, forming a composite banded structure.
(3) The third stage of structural deformation: it is extensional and often forms a series of detachment ductile shear zones along the crystalline basement and Yunkai rock group. There are many syntectonic secretory time-related pulses running through the foliation direction of ductile shear zone, which may be closely related to the formation and evolution of Tiantangshan metamorphic core complex.
(4) The fourth stage of tectonic deformation: it is compressive, and the deformation surface is usually extensional foliation and syntectonic secretory pulse, forming shallow columnar folds.
2. Structural deformation characteristics of Paleozoic metamorphic rock series.
Through field geological investigation and comprehensive indoor research, it is preliminarily found that the Paleozoic metamorphic rock series has undergone three cycles, six generations of ductile and brittle-ductile structural deformation sequence transformation.
(1) The first stage of structural deformation: mainly developed in the Lower Paleozoic, with the lithologic layer as the deformation surface, strong bedding shear occurred, forming some bedding concealed folds and small bedding ductile shear zones. The structural characteristics formed by structural deformation in this period are often most developed near the Tiantangshan metamorphic core complex, which may be related to the uplift of Tiantangshan metamorphic core complex.
(2) The second stage of tectonic deformation: it is compressive, often forming a series of closed syncline folds and forming a number of ductile shear zones with dextral shear properties. The structure of this period mainly developed in the Lower Paleozoic, but the Upper Paleozoic was not reformed by it, indicating that it should be the product of Caledonian structural deformation.
(3) The third stage of structural deformation: Upper Paleozoic is the most developed, and a series of small bedding folds with sliding properties are often formed.
(4) The fourth stage of structural deformation: a series of large-scale thrust nappe structures are formed in the area from Beiliu City to the south of Maolin Town, Yulin City, in which the clastic rocks of Lower Paleozoic and Lower Devonian thrust northwest on the carbonate rocks of Upper Paleozoic, forming a series of "structural windows" and forming a large low-angle ductile shear zone at the root of the nappe structure; A large ductile shear zone with a width of several hundred meters to several thousand meters has also been formed in the southeast edge of Darongshan rock mass from the north of Beiliu Anzhen to Shilixu in Rongxian County, which has the property of left-handed thrust. It is preliminarily considered that the above nappe structure and ductile shear zone may be the product of Hercynian-Indosinian orogeny, that is, the Paleo-Tethys (ocean) subducted in the south to the north.
(5) The fifth stage of structural deformation: it is often superimposed on the ductile shear zone formed by the third stage of structural deformation, resulting in two stages of ductile deformation in the ductile shear zone from the north of Min 'an Town to Shilixu in Rong County. In the first stage, the ductile deformed feldspar minerals often undergo strong fine recrystallization, and some particles still undergo recrystallization and regrowth except for the fine recrystallization of the timely minerals, forming rectangular timely zones or polycrystalline quartz zones; In the second stage, the ductile deformed feldspar minerals are mainly broken and refined, and the timely minerals are mainly refined and recrystallized. In addition, it can be seen that the timely strips formed by early ductile deformation are deformed again, forming a lens-like aggregate. During this period, structural deformation also formed some small ductile shear zones in the Darongshan rock mass on the south side of Cenxi City, and its deformation formation mechanism was dextral thrust.
(6) Sixth stage structural deformation: A series of thrust belts (or ductile faults) were often formed in Upper Paleozoic, which led to the inversion of the whole rock series.
3. Mesozoic-Cenozoic structural deformation characteristics
Various brittle faults and folds are mainly developed.
(2) Characteristics of metamorphism
The exploration area is located at the junction of Cathaysian ancient land and South China active belt, and it is the most complicated area of geological structure and metamorphism evolution in Guangxi. Metamorphism is manifested in Proterozoic to Mesozoic, and there are different types and stages of metamorphism and magmatism superimposed transformation. Therefore, metamorphic rocks are widely distributed in the survey area. According to the types and causes of metamorphism, it can be divided into three types: regional metamorphic rocks, contact metamorphic rocks and dynamic metamorphic rocks, of which regional metamorphic rocks are the most developed.
According to the chronological evidence of tectonic-thermal events, combined with the corresponding deformation events and magmatic activity periods, it is preliminarily determined that the survey area has experienced six cycles, 1 1 metamorphic events (Table 2-2).
Table 2-2 Investigation Area Deterioration Event Sequence Table