| The Northern Daba Mountain, as an important component of the South Qinling orogen, has experienced a variety of tectonic evolutionary stages, for instance, subduction-accretion orogenesis, collisional orogenesis, and intracontinental orogenesis, and different geologists hold different perspectives on the tectonic attributes of the mountains. The volcani-sedimentary sequence and volcanic rock assemblages have tectonic speciality, and they are an effective measure for identifying patterns of ancient plate tectonics and tracing the tectonic evolution history. With the basaltic volcanic-sedimentary rocks of Taohekou Formation and the trachytic volcanic-sedimentary rocks of Banjiuguan Formation in the hinterland of the northern Daba Mountains as the research objects, a research is conducted on the assemblage type and the sequence characteristics of the volcanic-sedimentary rocks; moreover, in combination with petrology, geochemistry, isotopic geochronology, researches were carried out to explore the tectonic background of the cause and formation of the volcanic rocks, meanwhile, the geodynamic processes are also investigated. In addition, the deposit genesis and metallogenic epochs are also conducted on the Xiaozhen copper deposit occurring in Taohekou Formation.According to the field investigation and the actual and revisional surveys of the geological profile, this paper classifies the volcano facies and reconstructs the palaeo-volcanic mechanisms of the volcanic-sedimentary rocks of Taohekou and Banjiuguan Formations. The research suggests that many volcano vents, such as Zhaxi, Bijiashan, possibly exist in Taohekou Formation. And the establishment of the ideal volcano-sedimentary sequence indicates that Zhaxi area has the features typical environment characteristics of seamounts; besides, the trachytic volcanic rocks of Banjiuguan Formation occur in both the north and the south of Hongchunba Fault. Judging from the field symbiotic rock, rock structures and the structure characteristics, they are products of two different magmatic activities, among which the palaeo-volcanic mechanisms in Langao area is located in the vicinity of the Siji River.Based on petrology, geochemistry, isotope dating and fossils, the studies show that the basaltic volcanic-sedimentary rocks of Taohekou Formation erupted in early Devonian (408Ma) of Late Paleozoic and mainly composed of the picrites, alkaline basalts, tephrites, basanites and nephelinites which is an alkaline basalt series. The eastern of Linhe to Bijiashan area develop the potassic type while the western of Xiaozhen to Zhaxi area develop sodic type, they are comagmatic but have experienced fractional crystallizations of different degrees; they are all originated from the asthenosphere mantle. Early stage of the peridotite at the bottom of the ancient oceanic lithosphere and a small amount of terrigenous sediment will subduct to the asthenosphere, which provided the material source, especially when the mantle metasomatism took place in the source region. The trachytic volcanic-sedimentary rocks in Langao area belong to the potassic-alkali series as formed in middle Silurian (432Ma)of Early Paleozoic, while the trachytic volcanic-sedimentary rocks in Ziyang area are of the shoshonite series and they formed in early Devonian (-410Ma) of Late Paleozoic. They all underwent the strong fractional crystallization, but experienced the crystallization differentiation of different minerals; partial melting is the main process of their magmatic formations, but they are two different kinds of parental magmas; the trachytic rocks in Langao area originated from the independent mantle source,also, the source area has mantle metasomatism, while the trachytic rocks in Ziyang area originated from the partial melting of the lower crust, which is caused by the under plating of the femic magma.Through researches into the volcano-sedimentary sequences, volcanic rock assemblages as well as geochemistry, it can be indicated that the basaltic volcanic rocks of Taohekou Formation and the trachytic rocks in Langao Area are respectively formed in the intraplate environment of back-arc and seamount of back-arc basin setting, while the trachytic rocks in Ziyang Area were developed in the subduction-related orogenic environment, which shows that southward subduction had ever happened in the Northern Daba Mountain.Moreover, based on mineralogy, deposit geochemistry and metallogenic geochronology, the researches indicate that the Xiaozhen copper deposit, formed in154Ma, is a hydrothermaldeposit after the magmatic stage with middle or low temperature, and the ore-bearing albitites are taken as the magma genesis. Therefore, there exist such geological events as diagenesis and mineralization in Yanshanian in the Northern Daba Mountain. |
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