Geochemical Studies Of Copper Related Adakitic Rocks And The Magmatic Rocks Of Post-mineralization In The Middle And Lower Yangtze Belt

Late Mesozoic magmatic activities are an important geological event that controlled the present landform of the eastern China and strecth about 2000 km long from the south to the north. The pollymetallic deposits are generally igneous related and concentrated in the east part of Yangtze block of the Eastern China. The Middle and Lower Yangtze River belt (MLYRB) is one of the most important metallogenic belts in China that has about 200 deposits. As research continues, the different geochemistry characters between the west and the eastern part of the MLYRB had been discovered and produced more and more scientific problems to be solved.Copper, gold and iron are the major ore-forming elements in the MLYRB with just small amount of lead, zinc, molybdenum, phosphorus and Uranium enrichment as associated elements. Previous studies have confirmed that copper-gold deposits were formed generally earlier than iron deposits. Spatially, copper-gold deposits are often distributed in the uplifts with a thick crustal section, such as the Edong, Jiurui, Anqing and Tongling areas, while iron deposits are often distributed in the rift-controlled basins, such as the Jinniu, Luzong, Ningwu basins. Interesting, the copper-gold related igneous rocks in the east of the Yangtze block generally show adakitic characters. As the research focusing on adakitic rocks continues, the MLYRB has also become one of the most important hot areas of adakitic rocks in the world. In addition, in the post-mineralization era, there are amount of ore-barren A-type grantioids and sparsely distributed mafic rocks of the MLYRB.This study has reported and summarized geochronological, petrological, and geochemical data of copper-related adakitic rocks in the MLYRB from west to east and is trying to discuss their petrogenesis and tectonic settings. The objective of this dissertation is to explore the relationship between adakitic rocks in the continent interior and the subducted Paleo-pacific plate during Late Mesozic. In addition, we also analysised the coeval maifc rocks and the Huayuangong A-type granitoids as the typical rock association in the Lower Yangtze River belt forming in the post mineralization to supply petrological and tectonic constraint of the central-eastern China during the early Cretaceous.Three major research results of this dissertation are acquired as followings:.1. Paleo-Pacific subduction in the interior of eastern China:Evidence from adakitic rocks in the Edong-Jiurui districtThe Edong-Jiurui district is located more than 1000 km away from the current Pacific subduction zone. It is part of the famous middle and lower Yangtze River Cu-Fe-Au belt in central-easitern China. Cu-mineralization of this area is spatially and temporally associated with Late Mesozoic magmatic rocks. These rocks exhibit geochemical features of adakites but their origin is not yet fully well understood. To explore the relationship between Cu-mineralization and Mesozoic magmatism, we report geochemical, Sr-Nd-Pb isotopic and zircon U-Pb age data from adakitic rocks in the Edong-Jiurui area. Zircon U-Pb ages point to a protracted period of magmatic activity ranging from 151 to 139 Ma. This time span coincides with the Cu-mineralization (146-137 Ma) in the middle and lower Yangtze River belt. Adakitic features of the rocks are displayed by high contents of SiO2, Al2O3, Na2O and Sr, enrichments of light REEs and LILEs and depletion in HREEs, positive Sr-and negative Nb-, Ta-and Ti-anomalies and high Sr/Y and La/Yb ratios. We favor a model of melt segregation from a plagioclase-free and garnet-bearing residue. Compared to non-Cu-bearing Mesozoic adakitic rocks in the Dabie terrane, adakitic rocks in the Edong-Jiurui area have higher initial εNd values (-3.4 to-6.3), Pb isotopic ratios and Th-contents and lower Pb/Ce values. Altogether, these features indicate that the melts were probably derived from subducted ocean mixed with marine sediment.2. Age and composition of Cu-Au related rocks from the Lower Yangtze River belt:Contraints on paleo-Pacific slab roll-back beneath eastern ChinaCu-Au mineralization in the Yangtze River belt is closely related to the formation of adakitic rocks. The origin of these rocks is still controversially discussed. In this study, we report zircon U-Pb ages, major-trace element and Pb-Sr-Nd isotopes of Cu-Au related adakitic rocks from the Anqing and Tongling areas of the lower Yangtze River belt (LYRB). Geochemically, the rocks show similarities to adakites derived from slab melting. Statistical evaluation of existing zircon U-Pb data from the LYRB reveals a protracted magmatic activity from 148 Ma to 106 Ma with peak activity at-139 Ma. Cu-Au related magmatism tends to decrease in age from the Gan-Hang belt in the southwest to the LYRB in the northeast. This trend is consistent with a roll-back process of the paleo-Pacific plate. The Cu-Au related adakitic rocks in the Gan-Hang belt and the middle and lower Yangtze River belt show systematic variation in geochemical and Pb-Sr-Nd isotopic compositions, pointing to a component change of magma source with time. The rocks have features in common with Cenozoic slab-derived adakites, but have experienced different levels of melt peridotite interaction and complex crustal material infusion process. Model calculation shows that interfusion with crustal material increased with decreasing crystallization age. Combined with a decline trend of the estimated copper reserves with the decreasing crystallization age, it is concluded that the amount of slab melts decreased during the course of the paleo-Pacific subduction process.3. Petrology and integrated geochemical study of the Huayuangong metaluminous A-type granitoids and coeval mafic rocks in the Lower Yangtze River belt, eastern China:Implications for petrogenesis and tectonic settingThe Lower Yangtze River belt (LYRB) in eastern China is characterized by intense magmatic activity during Late Mesozoic and product abundant polymetallic ore deposits. Huayuangong (HYG) granitoids are ore-barren and locate in the south part of the region. Zircon U-Pb dating results of three samples suggest that the HYG granitoids are formed contemporarily at-126 Ma. The age is coeval with formation ages of the mafic rocks (131-125 Ma) in the LYRB. The HYG pluton covers an area of about 110 km2 and has metaluminous A-type affinity. According to the Nb/Y versus Zr/TiO2 discrimination diagram, samples of mafic rocks from the Anqing and Tongling districts are classified into the fields of Andesite and Andesite or Basalt series. These mafic rocks exhibit similar trace elemental characteristics to those of continental arc andesite (CAA) with depletion in high field strength elements (HFSE; e.g., Nb, Ta, Zr, Hf and Ti) and enrichment of large ion lithophile elements (LILE; e.g., Rb, Th, U and Pb). The fertile Sr-Nd isotopes and linearly correlation with Pb/Ce and 207Pb/204Pb(t) ratios of these with-plate mafic rocks suggest the mantle source in the LYRB might have been metasomatized by fluids and/or melts from subducted sediment.The HYG A-type rocks exhibit similar whole-rock Sr-Nd isotopes with the coeval mafic rocks. This suggests the A-type magma may also origin from a fertile mantle source metasomatized by recycled sediments. The distinct elements variations in Hark diagrams preclude a likeness that the HYG A-type magma was evolved directly from the mafic melt by fractional crystallization. The distinction in whole rocks Pb isotopes implies the A-type and mafic magmas may originate from heterogeneous mantle sources and belong to different fractionating magma series. The HYG A-type magma show slightly variation of zircon Hf isotopes with εHf(t) values from-4.8 to-11.1. In the Yb/Ta versus Y/Nb and Ce/Nb versus Y/Nb diagrams, both of the A-type and mafic rocks show a large range of Y/Nb ratios and plot in the field between IAB and OIB. This suggests the A-type and mafic magmas in LYRB may be produced by mixing between asthenosphere and enriched lithospheric mantle. According to tectonic discrimination diagrams, both of the coeval A-type and mafic rocks are classified as with-plate tectonic settings. In contrast to the early stage adakitic rocks (148-130 Ma) showing VAG character, the rock association from early stage formed subduction related adakitic rocks to the latest stage magmatic activities of with-plate rocks of post mineralization in the LYRB are corresponding to roll-back processes of the paleo-Pacific plate beneath the Eurasia plate with subsequently upwelling of asthenosphere and lithospheric thinning during the Late Mesozoic.