Origin Of Pseudotachylite From The Eastern Dabieshan Orogenic Belt And Its Tectonic Implications

Pseudotachylites are dark aphanitic vein rocks showing intrusive relations and sharp boundaries with the wall rocks and containing clasts or crystals of the host rocks. They can be formed in different geological settings. Some pseudotachylites have been described as products of impact events. More studies have related pseudotachylites to palaeoseismic events, where they have been interpreted as a response to rapid faulting with large stress drops and as the fossil remnant of paleoseismic events. It provides direct observation of the characteristics and behavior of the products along an exhumed fault. Being one of the particular tectonites, the pseudotachylite has long attracted geologists for almost one century. but owing to the invariably fine-grained to microscopic nature of the rock's matrix, there is still controversy over the origin and the formation mechanism of the pseudotachylite so far. Two main mechanisms of formation have been proposed, one by frictional melting, the other by cataclasis. Although the most workers now prefer a frictional melt origin, the number of crushing-originated (ultra-comminution) pseudotachylites has recently been increasing in the world. Also, the fault-generated pseudotachylite can occur in varying depth from 1.5 km to about 15 km or 60 km or more deeper along the palaeoseismic zone, which can be as an indicator for the exhumation rate of the fault zone.Recent field survey in the eastern Dabieshan has revealed extensive occurrences of pseudotachylite. They occur in different metamorphic facices from ultrahigh pressure metamorphic facies rocks to epimetamorphic greenschist facies and the granitic gneisses, but they tend to generally develop along the NE-SW trending fracture zones or shear zones which parallel the Tan-Lu fault zone and cut all the pre-Cretaceous geologic bodies, such as granitic plutons and metamorphic sequence, et al.. Being similar to most of the modes of occurrence in the literature, the pseudotachylites from the Dabieshan orogenic belt also occur as simple dikes (fault veins) and complex networks of veins. and they have sharp contacts and intrusive relations with the host rock. Macroscopically, they exhibit maroon aphanitic groundmass textures with varying concentrations of wall rock fragments, and some pseudotachylites develop the mylonitic structure.This paper made a minute description to the field features of the pseudotachylites from 5 localities in the eastern Dabieshan. On the basis of the charateristics of microstructures (including the optical microscope, the SEM, and powder X-ray diffraction) and geochemistry and chronology exhibited by the pseudotachylites and their host rocks from 5 localities of the eastern Dabieshan, this paper discussed the origin and the formation depth of the pseudotachylites and the uplift rate of the Dabieshan orogenic belt from the late Cretaceous period constrained by the depth and by dating the age of the pseudotachylite and the relative rocks.The main conclusions are as follows:(1) Pseudotachylites developed in the eastern Dabieshan orogenic belt are a kind of fault-rock that developed during 80-90Ma along the NE-SW fracture zones or shear zones, which suggests that there occurred extensively high-speed movement along the Tan-Lu strike-slip fault zone during the late Cretaceous and the earlier Tertiary period. It is the product of the exhumation of the late orogenic belt, other than that of the subduction.(2)According to the characteristics of deformation, there occurred two kinds of pseudotachylites in the Dabieshan, one having been coexisting with the mylonites and behaving ductile deformation itself and the other by mainly cataclasis-ultracataclasis with no ductile deformation, which suggests that there maybe some slightly differences in the formation depth between the two kinds of pseudotachylites.(3) The veins show the characteristic features of progressive fracturing of the wall rocks interspersed with many rock fragments. There exists a gradation in cataclasis from the host rock to the pseudotachylte, which strongly indicates that antecedent cataclasis or pre-existing tectonic soften belt was a precursor to the formation of pseudotachylite. The pseudotachylites were formed mainly by ultracataclasis, but the ultramicro-cryptocrystalline texture and the similar vitreous composition of the matrix itself and the abundant corrosion borders along the margins of some porphyroid crystals of feldspar and plagioclase and pyrite suggest that there might have been the presence of melt during the pseudotachylite generation.(4) Bulk and REE analyses of 17 pseudotachylytes and their respective host rocks indicate that the pseudotachylites from the Dabieshan orogenic belt have approximately the same composition and similar REE patterns as their host rocks, suggesting that the pseudotachylites have been formed from the rocks in which they occur. But the amount of Fe₂O₃ and SiO₂ increases in most of the pseudotachylites to varying degree with decreases of the whole REE amount in all pseudotachylites.(5) The pseudotachylites from the Dabieshan orogenic belt were formed at 80-90Ma. The bulk K-Ar ages of the pseudotachylites and related host rocks from different localities in the Dabieshan orogenic belt range in 66.67-115.56 Ma, which is consistent with the main ages of the NE trending faults, Among them, the mylonitized adamellite from Loc.1 has an age of 93.44Ma, whereas the pseudotachylite generated after this kind of host rock has an age of 92.51±1.36Ma. The age sameness between the pseudotachylite and its host rock suggests that the pseudotachylite was formed just following the mylonitization and the formation depth of the pseudotachylite should lie in the transition zone between the ductile and brittle tectonic zone, i.e., about 12 km. The laser-probe ⁴⁰Ar/³⁹Ar ages of those newly-formed phengite formed simoutaniously or followed closely with the pseudotachylite.are among 79±1 l～87.9±3.2Ma, the average age is about 80.4Ma;(6) According to the P-T conditions of phengite occurred in the matrix of pseudotachylite, we estimate that the pseudotachylites were formed 9-12 km depth at 80 Ma and they were exhumed by an average of vertical rate of 0.125 mm/yr since then. The value implies the very slow isostatic uplift of the Dabieshan orogenic belt since the late Cretaceous period.(7) Moreover, in order to further understand the formation mechanism of the psudotachylite, the high-speed frictional experiment on the granitic rocks was carried out in this paper, which suggests that the process of the pseudotachylite formation is very complex. It depends on the relations among a series of factors such as the velocity-shear stress-displacement-lasting time-the mechanical properties of minerals et al.