Crustal And Uppermost Mantle Velocity Structure Andits Relationship With The Formation Of Ore Districts In The Middle-lower Yangtze River Region

The Middle-Lower Yangtze River region includes the southeastern North China Craton, the Dabie-Sulu orogenic belt, the central eastern Yangtze Craton and most parts of the Cathaysia Block. In the center of the Middle-Lower Yangtze River region, there is an important polymetallic metallogenic belt and mineral resource base, where more than 200 kinds of polymetallic deposits have been found. In the narrow arc-like metallogenic belt, also called the Middle-Lower Yangtze River Metallogenic Belt, there are seven big ore-concentrated districts. Why does such a narrow zone contain so rich mineral resources? What is the deep dynamics? How does the deep process control the derivation, the evolution and the intrusion of magma for mineralization? Therefore, geophysical constraints on the crustal and upper mantle structure are important for us to understand the tectonic and magmatic processes responsible for the ore formation. Due to low seismic activity in the Middle-Lower Yangtze River region and sparse seismic station coverage in the past, only a few surface wave tomographic studies have been carried out in this area.In this study, we conduct ambient noise tomography and teleseismic two-plane-wave tomography to probe the crustal and uppermost mantle structures in the Middle-Lower Yangtze River region. The data used include 14-months(from July 2012 to August 2013) continuous ambient noise data and 219 teleseismic earthquakes recorded at 138 broadband seismic stations from Chinese provincial networks and 19 temporary seismic stations deployed by China University of Geosciences(Beijing). First, we apply ambient noise tomography to the collected ambient noise data to generate Rayleigh wave group and phase velocity maps at 5-42 s periods and two-plane-wave tomography method to earthquake data to generate intermediate-to-long period phase velocity maps at 20-143 s periods. We then combine the short-to-intermediate period velocity maps from ambient noise and the intermediate-to-long period phase velocity maps from earthquake data to generate broadband phase velocity maps at periods from 5 to 143 s. By inverting these 5-143 s Rayleigh wave velocity maps, we construct a 3-D shear wave velocity model from the surface to ~250 km depth in the Middle-Lower Yangtze River region. The 3-D model shows that in the upper crust, the basin regions, including the JiangHan, HeHuai, SuBei, HeFei and NanYang basins, are all featured with low velocities, and the mountain regions with high velocities. In the uppermost mantle, a low-velocity zone at ~100-200 km depth is observed beneath the Middle-Lower Yangtze River Metallogenic Belt. Moreover, our tomographic results show that the NingWu and NingZhen ore districts are clearly characterized with the strongest low velocity anomaly in the uppermost mantle at ~70-200 km depth. The depth extent of the low-velocity zone becomes shallower and the amplitude of low velocity anomaly becomes larger from the southwest JiuRui ore district to northeast NingWu ore districts. The change pattern of the low-velocity zone in the depth extent and the amplitude is consistent with the fact that peak ages of magmatic events along the Middle-Lower Yangtze River Metallogenic Belt progressively become younger and younger from 148 Ma in the southwest to 125 Ma in the northeast. The observed low-velocity zone may represent the cooling warm upper mantle rocks resulting from partial melting of paleo-Pacific plate or of an enriched mantle source induced by the westward subduction of the paleo-Pacific plate in the past. The upwelling of the mantle-derived magmas may result in the formation of these granitic rocks and coeval ores deposits along the Middle-Lower Yangtze River Metallogenic Belt.