Sm-Nd age dating of high-pressure granulites and amphibolite from Sanggan area, North China craton

Abstract

The high pressure (HP) metamorphic age has been dated to HP rocks from the Sanggan area, North China craton. We have got garnet+whole rock isochron ages of (1 842±38) Ma for HP granulite, and (1 856 ± 26) Ma for HP amphibolite. The Sm-Nd whole rock isochron of HP granulites give out an age of (1 870±150) Ma with Nd depleted mantle model age of (2 402-2 482) Ma. Considering the Nd isotope homogenization during the peak metamorphism of the HP granulite, Sm-Nd closure temperature and the retention of Nd isotopic memory in garnets partially broken down during decompression, all these isochron ages are thought to be HP metamorphic age. Furthermore, we proposed that the HP metamorphism took place at the end of Paleoproterozoic during the large-scale collision and assembly of the North China craton.In recent years, most high pressure (HP) metamorphic rocks have been recognized from the Hengshan Mt. to Chicheng in the Sanggan area, North China craton [1,2] . The major rock types are HP basic granulites, amphibolites and retrograde eclogites. The HP granulites subjected to HP metamorphism at about 1.3-1.6 GPa and 800-850℃. All the HP rocks have a clockwise PT path with rapid isothermal decompression [3][4][5] . They are most probably the products of collisional orogen. However, it has not yet been determined when the significant Precambrian collisional orogenic event occurred.In general, Sm-Nd isochron of garnet+whole rock± other minerals is supposed to produce metamorphic age determination. These years, more understanding to Sm-Nd system in garnet has been achieved, especially about how the closure temperature is affected by garnet radius, initial temperature and cooling rate [6][7][8][9][10] . That made the garnet Sm-Nd method capable of dating metamorphic age. Particular advances are in age determination of multi-metamorphism [11][12][13][14] .In this note, we apply garnet+whole rock Sm-Nd isochron to determine the age of the HP metamorphism. A whole rock isochron of HP granulites is also measured. Then, the most possible age of HP metamorphism will be discussed, which will define the age of the collisional orogeny and assembly of the North China craton. Geological background and petrology of the HP metamorphic rocksThe major HP metamorphic rocks in the North China craton are HP basic granulite, HP amphibolite and retrograde eclogite [4,5,15] . In the Sanggan area, plenty of HP rocks were distributed northeast from the Hengshan Mt. to Chicheng as lenticular blocks or enclaves in highly deformed TTG gneiss, granitic gneiss, migmatite and granite. Therefore, the occurrences of HP rocks define the Sanggan strucure zone ( fig. 1). The samples of HP granulites and amphibolite of this note are from Xiwangshan and Womakeng respectively. These HP metamorphic rocks have been carefully investigated petrologically [3,16,17] , which will make the samples typical and representative. Fig. 1. Sketch map of Precambrian geology in the Sanggan area, North China craton.The HP granulites contain abundant garnet porphyroblasts, in which mi...

Derivation of Mesozoic adakitic magmas from ancient lower crust in the North China craton

“…Given that the lower crust in many areas of the world is mainly composed of granulite-facies rocks (Rudnick, 1992), we assume the ancient lower crust could be represented by the terrain granulites in the studied area. This is substantiated by the fact that the $2.5 and 1.8-1.9 Ga ages of the granulite terrains (Guo and Zhai, 2001;Zhao et al, 2001;Guo et al, 2005;Wu et al, 2005a) are also preserved in the Hannuoba granulite xenoliths . In addition, we assume that amphibolites are also likely components of the ancient lower crust as an amphibolite collected from the Zhangjiakou region yields pressure and temperature estimates of 12-14 kb and 700°C (Guo and Zhai, 2001), respectively, indicating lower crust conditions.…”

Origin of Reverse Zoned Cr-Spinels from the Paleoproterozoic Yanmenguan Mafic–Ultramafic Complex in the North China Craton

“…Consequently, the continent-arc-continent collision [88] introduced those rocks into lower crustal levels or the upper mantle [88], where granulite to eclogite facies [89][90][91][92] or greenschist to lower amphibolite facies [93][94][95][96] metamorphism occurred. This metamorphic history may provide long-lived dynamics and thermal status for elemental exchange in the Yanmenguan complex.…”

Origin and geological significance of the 1.81 Ga hyalophane-rich pegmatite veins from the high-pressure granulite terrain in the Central Zone of North China Craton

“…The widely distributed high-pressure granulites and amphibolites with clockwise P-T paths and decompressional textures strongly support the tectonic model of continental collision [3][4][5][6][7][8]. Recently, high-precision isotope chronological researches reveal that the peak age of high-pressure granulite facies metamorphism is 1.85 Ga [9][10][11][12][13][14][15].…”