摘要
The Xiliu meta-gabbros are composed predominantly of orthopyroxene-clinopyroxene granulites and clinopyroxene amphibolites,and are found as irregular lenses within the Neoachean granitic and tonalitic gneisses in the Jiaobei terrane.On the basis of petrography,mineral transformation,mineral composition,temperature-pressure estimation and in situ U-Pb dating of zircons,three distinct mineral assemblages of the meta-gabbros have been identified by the characteristics of mineral reactions and evolutions.The relict ophitic and gabbro textures represent the early index igneous textures.High-augite core in clinopyroxene is regarded as one of the early igneous assemblage (M 0).The magmaitic zircons record a weighted mean 207 Pb /206 Pb age of 2102 ± 3Ma,which represents the magmatic crystallization timing of the meta-gabbros.The granulite-facies metamorphism (M 1) is defined by the mineral assemblage of orthopyroxene + clinopyroxene + plagioclase + quartz,which formed at 800 ~ 850℃ and less than 0.65 ~ 0.75GPa.The mineral assemblage of amphibole + plagioclase + quartz formed at a cooling retrograde stage (M 2) with P-T conditions of 675 ~700℃ and 0.40 ~ 0.55GPa.The metamorphic zircon records a younger 207 Pb /206 Pb age of 1907 ± 16Ma,which represent the timing of the granuliteamphibolite facies metamorphism of the meta-gabbros.These data indicate that the meta-gabbros record a clockwise P-T path involving near-isobaric cooling (IBC) process.Combined with other investigations of the Jiaobei terrane,it was suggested that the Xiliu metagabbros were formed by crustal extension before the deposition of Paleoproterozoic strata,and were involved in the Late Paleoproterozoic (1.95 ~1.80Ga) collisional orogenic process in the Jiaobei terrane.