Petrogenesis and tectonic significance of Late Permian-Middle Triassic granitoids in Guobaoshan, eastern section of the eastern Tianshan mountains: Constraints from geochronology and geochemistry

Abstract

The eastern Tianshan-Beishan area is one of the important metallogenic belts in China, but the Late Permian-Middle Triassic tectonic evolution still remains a controversial issue. The Guobaoshan area is located in the eastern section of the eastern Tianshan mountains. Five kinds of Late Permian-Middle Triassic granitoids are exposed in this area, namely granodiorite, monzonitic granite, quartz syenite, syenogranite and amazonite granite. These granitoids are generally rich in silicon and alkali. Granodiorite, monzonitic granite and quartz syenite are characterized by the enrichment of potassium and magnesium, showing LREE enriched REE patterns (LREE/HREE=0.86) with weakly negative Eu anomaly, while syenogranite, being similar to sodic and ferrous amazonite granite, is characterized by a "sea-gull" REE pattern and significant negative Eu anomaly (δEu=0.03), as well as enrichment of Ta and Rb. The 206Pb/238U surface ages of zircons of granodiorite are concentrated in the range of 255~250 Ma, which are earlier than that of amazonite granite. Both syenogranite and amazonite are typical A-type granite and formed in intra-plate environment, but the other granitoids are Ⅰ-type granite and formed in post-collision tectonic setting. There is no genetic relation between two groups of granitoids and they belong to different magma series. The Guobaoshan area was still in the post-collision tectonic setting during 255~250 Ma, and switched to the intra-plate environment before 247 Ma.

Keywords

• eastern tianshan mountains
• granite
• permian
• triassic
• tectonic environment
• tectono-magmatic assemblages