Geochronology and geochemistry of Early Devonian intrusions in the Qimantagh area, Northwest China: evidence for post-collisional slab break-off

摘要

The Qimantagh area of Northwest China lies in the western part of the East Kunlun Orogenic Belt and is characterized by extensive magmatism, particularly in the Triassic. However, recent research has shown that Devonian magmatism was also widespread in this area and has a genetic relationship with mineralization. This article presents a detailed study of three types of Early Devonian intrusions: high-K calc-alkaline granites, A-type granites, and mafic intrusions, all from the Qimantagh region. These rocks were subjected to precise zircon U-Pb dating, major and trace element analyses, and Sr-Nd isotope measurements, focusing on the Lalingzaohuo (eastern Qimantagh) and Yemaquan (central Qimantagh) monzogranites, as well as the coeval Tanbeixuefeng (western Qimantagh) mafic dike swarm. To better understand the Early Devonian igneous activity in the Eastern Kunlun, data for other coeval granitoids were compared with our data. The Yemaquan monzogranite yielded a mean zircon U-Pb age of 400.5 +/- 1.4Ma. These rocks are metaluminous to slightly peraluminous, with Al2O3 contents of 13.10-14.16wt.%, high alkali contents (total K2O + Na2O) of 6.89-7.68wt.%, relatively low Sr contents (79-192 ppm), and high (La/Yb)(N) ratios, all of which indicate an I-type granite affinity. The Lalingzaohuo monzogranites yielded mean zircon U-Pb ages of 396.2-402.2Ma. These rocks have higher SiO2 and alkali contents than the Yemaquan monzogranite, with high 10,000Ga/Al ratios, high Zr + Nb + Ce + Y contents, high Fe2O3T/MgO ratios, and high Y contents, indicating an A-type granite affinity. These two monzogranites have initial Sr-87/Sr-86 ratios of 0.703-0.706 and epsilon Nd(t) values of -0.1 to -0.7. The Sr-Nd isotopic data require a significant input of a mantle component in the petrogenesis of these granites. The Tanbeixuefeng diabase dikes formed at similar to 396Ma and have a continental tholeiitic affinity, as evident from small Ti-Nb-Ta anomalies and high contents of light rare earth and large-ion lithophile elements. We propose that post-collisional slab break-off was responsible for the generation of these Early Devonian intrusions in the Qimantagh area.