Characteristics, zircon dating and tectonic significance of the gabbros along the north-central segments of the Alxa Block, Inner Mongolia

Detailed 1∶250000 field geological survey has revealed the distribution of mafic rocks in Wuliji area of Alxa Left Banner, which intruded the Upper Carboniferous Amushan Fomration and was also intruded by late acidic magma. The mafic rocks are composed of amphibolite gabbro and gabbro-diorite, with amphibolite gabbro being dominant. U-Th-Pb dating of zircons from amphibolite gabbro was conducted with LA-ICP-MS technique. The dating of zircons which have the typical characteristics of mafic magma yielded a weighted mean 206Pb/238U age of 248.0±0.9Ma, suggesting Early Triassic. The dating result in combination with the regional structure and an analysis of the relationship of lithology characters shows that the gabbro was formed obviously later than the closure of the Paleo-Asian Ocean. It is thus held that the gabbro did not result from early magmatic activity during orogeny in a subduction environment of Paleo-Asian oceanic crust beneath the North China plate in late Paleozoic, but was probably related to large-scale continental rifting. The gabbro of this region was generated in a regional continental extensional environment caused by the upwelling of mantle material.