Zircon U−Pb dating and geochemistry of basalt in Guaizhangshan Group from the Southern Danghe Mountain in South Qilian and its tectonic setting

This paper is the result of geological survey engineering.

Objective The Neoproterozoic Guaizhangshan Group was disintegrated from the Silurian Balonggonggaer Formation. The study of the petrogenesis and tectonic setting of the basalt in Guaizhangshan Group is crucial to clarify the tectonic evolution of South Qilian.

Methods Based on LA−ICP−MS zircon U−Pb datingand whole−rock geochemistry analysis, the formation age, geochemical characteristics and geological significance of the basalt in Guaizhangshan Group are constrained.

Results The dating results indicate the eruption age of the basalt is (786.6±5.8) Ma, suggesting that the Guaizhangshan Group formed during Early Neoproterozoic. Geochemical analyses of the basalt show that: SiO₂=48.09%–50.97%, TiO₂=1.34%–2.55%, MgO=5.78%–7.11%, displaying characteristics of subalkaline tholeiite; Significant differentiation between light rare earth element (LREE)and heavy rare earth elements (HREE) ((La/Yb)_N=3.76–4.51), and inconspicuous Eu anomalies (δEu=0.80–1.05); Enrichment of Ba, Th, U, Pb, and depletionin Nb, Ta, Ti.

Conclusions These geochemical characteristics indicate that the basalts were derived from mantle and contaminated by crustal materials. The basalt was formed in continental rift setting, which may be relevant to the break−up of the Rodinia supercontinent.