| Low Yangtze River Block is one of the most important metallogenic belt in China, its deposit in the form of ore zone along the Yangtze River was set EW-NE arc distribution, within the development zone of copper (gold), iron, molybdenum, lead, zinc, silver, tungsten, etc., have proven more than200deposits. All types of deposits in the Yangtze River region have related to Yanshanian intrusive rocks, which is recognized as adakite with the adakitic characteristics like high Sr low Y, etc.,This thesis study on three adakitic Mesozoic intrusive rocks (Chuxian, Machang, Shangyaopu rocks) nearby Chuzhou of eastern China region with systemic geochemical analyzes, including field observation and sampling, mineral flakes microscope, biotite EPMA, the whole rock major and trace elements composition analyzes, LA-ICP-MS zircon U-Pb dating and Hf isotope composition analysis to investigate the petrogenesis and geological significance.The lithology results show that Chuxian, Machang, Shangyaopu rock is monzonite-quartz monzonite, mainly high-K calc-alkaline series. Machang rock is more complex, according to the SiO2content can be divided into two groups:high Si (MC-H) and low Si (MC-L). Most samples have high Na low K (K/Na<1), high Mg#(average59) features. Three rock are showing significant Ba, Sr positive anomaly and Nb, Ta, Ti, Th negative anomalies. Chondrite-normalized REE sample curves are right type, light rare earth elements (LREE) enriched and heavy rare earth elements (HREE) strongly depleted, with no obvious negative Eu anomaly-positive Eu anomalies (σEu=0.96-1.04), geochemical characteristics same as typical adakites. Zircon U-Pb geochronology shows the result of age of three rock as:Chuxian,121.8±1.9Ma (2se, MSWD=1.7) and124.0±1.4Ma (2se, MSWD=1.2), Machang,123.1±2.0Ma (2se, MSWD=1.3), and Shangyaopu,126.6±1.8Ma (2se, MSWD=0.2) and123.4±1.9Ma (2se, MSWD=1.17), and coincide with the massive magmatism in eastern China at the late Early Cretaceous.176Hf/177Hf ratio of Chuxian rock samples09CX01is0.282171, εHf(t) is-21.26, two-stage Hf model ages is2352Ma;176Hf/177Hf ratio of09CX10is0.282183,εHf(t) is-20.84, TDM2is2325Ma.176Hf/177Hf ratio of Machang rock sample09MC02is0.281978, εHf(t) is-28.06, TDM2is2777Ma.176Hf/177Hf ratio of Shangyaopu rock sample09SYP06is0.282186, εHf(t) is-20.74, TDM2is2314Ma.176Hf/177Hf ratio of09SYP12is0.282199, εHf (t) is-20.27, TDM2is2289Ma. Studies of diagenetic temperature indicate that Zr saturation temperatures were: Chuxian717-735℃, with average of727℃, Machang729-761℃, with average of749℃; Shangyaopu710-744℃, with average of728℃. Zircon Ti temperatures were:Chuxian679℃, Machang695℃, Shangyaopu705℃; biotite Ti temperature result:Chuxian691℃; Machang is695℃; Shangyaopu696℃. Description intrusive magma has a relatively low temperature, consistent with the presence of a large number of inherited zircon cores and the narrow zircon oscillatory zoning observed in CL image.Its high Sr, Sr/Y, low Y can be interpreted as partial melting of subducted oceanic crust and occurs fractional crystallization during ascent. High Mg#characteristics can be caused by both fractional crystallization and reacted with mantle jointly.Petrogenesis and mineralization mechanism could be explained by ridge subductions, during about125-140Ma, Pacific Plate subducted to southwest, as the Izanagi Plate subducted to NNW, and ridge between them is on the Low Yangtze River Block. As the westward movement component with two plates, it can lead to ridge subductions. Magma formed by partial melting of oceanic crust have a systemic high copper content, which is conducive to mineralization. Subducted oceanic slab with high oxygen fugacity can extract additional sulfur in the form of sulfate during partial melting, liberating more chalcophile elements such as Cu and Au, be further enriched for mineralization. |
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