Geochemical Characteristics And Mesozoic Intermediate And Felsic Plutons In Malanyu Uplift, Estern Hebei Province, And It’s Tectonic Implications

The research area is located in the core of Malanyu uplift , eastern Hebei proince. Based on the investigations in the geological, petrological, geochemical characters of Mesozoic intermediate and felsic plutons in this area, combined with the estimation of the temperature-pressure conditions of these plutonic rocks, the geochemical classification and their tectonic significances as well as the uplift and unroofing amount are discussed and evaluated. Main achievements have been made as follows:1. The Mesozoic intermediate and felsic plutons in the core of Malanyu uplift can be classified into three groups based on the petrological geochemistry:①high-Sr granites : they exhibit high Na2O,K2O,Al2O3 contents in their bulk compositions, strongly-enriched Sr, strongly-depeted HREE and nearly absence of negative Eu anomalies in the trace elements.②Low-Sr granites: they exhibit high SiO2, K2O contents, and low Na2O,Al2O3,CaO contents in their bulk compositions, low Sr, strongly-depeted HREE and obvious negative Eu anomalies in the trace elements.③Strongly peraluminous leucogranites: they exhibit high Al2O3,ANKA>1.1,low Cao and MgO contents in their bulk compositions,low Sr,Ba,Yb,high Rb and strongly-depeted negative Eu anomalies in the trace elements.2.The High-Sr granites generated under high pressure(≥1.5Gpa) and high temperature (>850-9000C) , and were interpreted to be derived directly from an overthickened continental lower crusts. Their emplacement age is about 220~160Ma. The low-Sr were originated directly from the overthickened continental lower crusts,too(about 45km in depth).The emplacement age is about195~160Ma. Strongly peraluminous leucogranites were derived from dehydration melting of metapelites and petagreywadkes during uplife and decompression of the thickened crusts in the early stage of post-collisional phase. The emplacement age is about 166.3Ma.3. It is inferred that there had occurred an extensive uplift and unroofing process of plutons in the core of Malanyu uplife since middle Mesozoic age(about 220~179Ma) by calculating the temperature-pressure conditions, and the denuation amounts is about 6~10 km.4. The Malanyu uplift is inferred to be under an overthickened crust tectonic settings. Its uplift and unroofing was partially contributed to the plutonic magmatism during Mesozoic episode.