Multiple mafic magmatic and high-grade metamorphic events revealed by zircons from meta-mafic rocks in the Daqingshan-Wulashan Complex of the Khondalite Belt, North China Craton

Meta-mafic rocks in the Daqingshan-Wulashan Complex, in the central segment of the Paleoproterozoic Khondalite Belt of the North China Craton, are composed predominantly of garnet-bearing mafic granulites, mafic granulites, and amphibolites. They occur mainly as irregular lenses and as a set of deformed dyke/sill swarms within dioritic-granitic orthogneisses and Al-rich paragneisses. An integrated study involving laser Raman and scanning electron microscope analyses of mineral inclusions from zircons, cathodoluminescence imaging, and LA-ICP-MS U-Pb dating of zircons, shows that magmatic zircons in these meta-mafic rocks record four age groups of 2500-2450, 2300-2100, 1970-1930, and ca. 1850 Ma, indicating multiple mafic magmatic events occurred within the Daqingshan-Wulashan Complex. In addition, four groups of metamorphic zircons with ages of ca. 2450, ca. 1950, ca. 1900, and ca. 1850 Ma are present within these meta-mafic rocks. Zircons of the first (oldest) group contain medium- to low-pressure (MLP) granulite fades mineral inclusions of orthopyroxene + clinopyoxene + amphibole + plagioclase + quartz, and yield ages of ca. 2450 Ma, representing the timing of the early MLP granulite facies metamorphic event within the Daqingshan-Wulashan reworked basement complex. In contrast, zircons of the second group are free of mineral inclusions, and yield ages of ca. 1950 Ma, which is considered to be the timing of the regional high-grade metamorphic and structural event probably caused by the collision of the Yinshan and Ordos Blocks. Zircons of the third group contain MLP granulite facies mineral inclusions of orthopyroxene + clinopyoxene + amphibole + plagioclase + quartz, similar to the zircons of the first group, but yield younger ages of ca. 1900 Ma, interpreted as the timing of the regional MLP granulite fades metamorphism that most likely occurred during the early stage of post-orogenic exhumation of the Daqingshan-Wulashan Complex. Zircons of the fourth group contain MLP amphibolite facies mineral inclusions of clinopyoxene + amphibole + plagioclase + quartz, and yield the youngest ages of ca. 1850 Ma, which probably represent the timing of the regional amphibolite facies metamorphism during the late stage of post-orogenic exhumation of the Daqingshan-Wulashan Complex. Combined with previous geological data of the Khondalite Belt, it is speculated that the multiple mafic magmatism and contemporaneous high-grade metamorphism of the Daqingshan-Wulashan Complex from the Khondalite Belt were most likely generated by an orogenic process involving early subduction-collision and late exhumation in an active continental margin arc. (C) 2014 Elsevier B.V. All rights reserved.