Geochemical and Environmental Characteristics of Hexipu Medium-acid Intrusive Rocks in the Southern Margin of North China Plate

The Hexipu medium-acid intrusive rock is located at the intersection area between the Qilian orogenic belt and the Longshou mountain of the southwestern margin of North China Carton, which is composed of fine grained quartz diorite, medium-fine grained granodiorite, medium-coarse grained porphyritic granodiorite and medium-coarse grained porphyritic monzogranite. The Hexipao granitic plutons are enriched in K and Si, but depleted in Al, Ca and Mg, having weakly negative Eu anomalies. Chondrite-normalized REE patterns show that all samples are enriched in LREEs. The δEu values of the ploutons are rising with the increasing of the SiO₂, ΣREE, ΣLREE/ΣHREE and La/Yb. The ⁸⁷Sr/⁸⁶Sr ratios of all samples vary from 0.706 0 to 0.707 8, these plutons have been transited from early I type to I-S type to S type, showing a complete process of transiting from plate compression to collisional orogeny. Combined with the regional geological data, the results show that the North Qilian was subducted into the North China craton in Early Caledonian to form a trench-arc-basin system, the compression was led to the mid-Caledonian collisional orogeny between the North Qilian and Longshou mountain, and this collisional stage was terminated in Late Caledonian, systematically reflecting the tectonic evolution of the Longshoushan orogenic belt.