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《中国科学院地质与地球物理研究所2007学术论文汇编(第一卷)》 2008年
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Post-collisional,potassic monzonite-minette complex(Shahewan) in the Qinling Mountains(central China):~(40)Ar/~(39)Ar thermochronology,petrogenesis,and implications for the dynamic setting of the Qinling orogen

王非  
【摘要】:正The nondeformed,postcollisional Shahewan monzonite-minette complex in the North Qinling orogenic belt,central China,is composed of potassic monzonites and mafic minette dykes.The monzonites and minette dykes were coevally intruded as evidenced by a zircon U-Pb age of 211±2 Ma for the monzonites and a ~(40)Ar/~(39)Ar age of 209.0±1.4 Ma on the biotite from the minette dykes. These dates also indicate that the Shahewan complex postdated shortly the cessation of convergent deformation in the Qinling orogenic belt.The K-feldspar ~(40)Ar/~(39)Ar MDD modeling result combined with the zircon U-Pb age reveals a rapid average cooling history (11.5℃/Ma) for the complex from 211 to 150 Ma,implying a fast exhumation within a short period of time. The minettes take features of ultrapotassic rocks.Their geochemical characteristics,such as high MgO(Mg# up to 80),low TiO_2 (0.38-0.62 wt.%) and FeO(2.52-3.42 wt.%),enrichment in LILEs and LREEs,and depletion in HFSE,together with primitive initial ~(87)Sr/~(86)Sr ratios(0.70444-0.70562) and slightly negativeε_(Nd)(t)(-3.4 to -2.5),suggest that they were derived from an enriched refractory lithospheric mantle.The monzonites have the similar geochemical(incompatible element enrichment,Nb,Ta,Ti depletion) and isotopic (initial ~(87)Sr/~(86)Sr:0.70513-0.70646;ε_(Nd)(t):-0.5 to -3.6) imprints,indicating they were derived from a common source. The dynamic setting in which the Shahewan complex formed is discussed.The Shahewan complex formed most likely in a postcollisional extensional setting.Its formation requires a high enough temperature to partially melt an enriched,refractory mantle source. Considerable uplift is necessitated for the rapid exhumation.These may be explained by convective mantle thinning or slab-breakoff mechanisms.The enrichment features in the lithospheric mantle source can be interpreted by injection of small percentage(1%) partial melts from the convective mantle,or by addition of the metamorphic fluids from the subducted South China lithosphere.

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