Volcanic activities in this area can be divided into early Variscan (D2-3), middle and late Variscan (C 1-P2), late Indosinian (T3) and early Yanshan (J2). The types of volcanic rocks are rift type volcanic rocks (D2-3, C 1-P2) and shoulder arc type volcanic rocks (T3, J2).
1. Middle-late Devonian volcanic rocks
There are two layers of basalt in limestone of natural group. This is mentioned in the description of rock strata, and there is no other information.
Early Carboniferous-Late Permian volcanic rocks
The metamorphic volcanic rocks in the Kagong Group are 275 meters thick and are mainly composed of olivine tholeiite, almond basalt and dense basalt. There are also metamorphic basalt greenschists produced in the late Carboniferous, metamorphic basalts sandwiched in the lower part of Dongba Formation and basalts occurring in Salon Formation. Silicate analysis shows that the content of SiO2 is less than 45% from late Carboniferous to early Permian, which indicates the existence of picrite. Early Carboniferous-Early Permian basalt types include low-potassium tholeiite, tholeiite, calc-alkali series basalt and alkaline series basalt. In the late Permian, it was mainly alkaline basalt. W (K2O) ranges from 0.04% to 0.18%, mostly less than 0.4% or more than 1%. Generally speaking, w(K)/w(Rb)(0.09) and W have the characteristics of high MgO, low TiO2 _ 2, low P2O5 and low K2O. The distribution pattern of rare earth elements is slowly to the right, and the total amount of rare earth elements is 42.58× 10-6, without europium anomaly. These characteristics are very similar to those of mid-Atlantic ridge basalt. The contents of K2O, TiO2 _ 2 and P2O5 in the alkaline basalt of Sharon Formation are all high, which may belong to a tectonic environment similar to an ocean island or a closed rift valley. In a word, this stage should belong to rift volcanic rocks, which were formed at the end of ocean island or rift closure.
Third, late Triassic volcanic rocks
Produced in Zhuka Group, in which volcanic rocks are 8268 meters thick. The main lithology is rhyolite, dacite, peridotite-bearing dacite, rhyolite, rhyolite tuff and sodalite dacite. , is a series of calc-alkaline volcanic rocks. W (SiO _ 2) is 73% ~ 63%, and W (K2O) > 3.0%, most of which belong to high potassium series, and W (K2O) > W (Na2O). Volcanic rocks are composed of four cycles, and the overall change trend is that the content of SiO2 _ 2 decreases gradually, and the content of porphyry and microcrystal changes from low to high, which belongs to aluminum supersaturation type. It is reflected as orogenic environment on lgτ-lgδ diagram. Late Triassic volcanic rocks in this area are also located in the east of Nujiang expansion zone. During this period, the expansion of the Nujiang River to the rift-ocean basin environment will inevitably form a strong compression on the western plate, forming intermediate-acid volcanic rocks, so the late Triassic volcanic rocks should belong to the shoulder arc volcanic rock combination.
Four. Volcanic rocks of the Middle Jurassic
Produced in Yanshiping Group, there are dacite volcanic breccia, dacite tuff, dacite, andesite and rhyolite near Along in the north of Dingqing, with a cumulative thickness of about 200 m, belonging to calc-alkaline volcanic rock combination and central volcanic eruption type. Yanshiping Group is also located in the eastern part of Nujiang rift-ocean basin environment. With the simultaneous expansion, it is impossible to subduct and collide with the surrounding plates at the same time, but it can only cause strong compression on the south and north blocks, forming remelting-synmelting of crustal materials and forming an open volcanic rock combination. Therefore, the volcanic rocks in some areas of Yanshiping Group also belong to shoulder arc volcanic rocks.
In a word, the tectonic environment of the Dingqing-Tangji volcanic belt is dominated by continental rift volcanic rocks, which lasted for a relatively long time (not including Devonian because there is no data), and lasted about 1 17 Ma from the early Carboniferous to the late Permian. Judging from volcanic rocks, the Lancang River junction zone should still exist. Whether there is an ocean basin environment of ophiolite in Yunnan and northern Tibet at the same time needs further study. However, remote sensing interpretation confirmed the existence of ophiolite in Beikegong Group in Dingqing County. Ophiolite exists in the same stratum in southern Qinghai, indicating that the Lancang River junction zone was a rift-ocean basin tectonic environment from Carboniferous to Permian. In the Late Triassic and Jurassic, this area belonged to the shoulder arc environment.