Search

The tectonic setting of the late Paleoproterozoic suite of anorthosite, mangerite, charnockite and alkali-feldspar granite (AMCG) on the northern margin of the North China Craton (NCC) is controversial. This paper reports on the petrology, geochronology and geochemistry of the Beibaiqi, Beipuzi and Shibalitai mangerites in the Longhua area and the Shinao mangerite in the Jianping area. Zircon U–Pb dating reveals that these mangerites formed 1768–1725 Ma, similar to the other lithologies (1753–1673 Ma) of the AMCG suite on the northern margin of the NCC. The mangerites have intermediate SiO 2 contents (57.2–66.8 wt.%) and high K 2 O (4.04–7.05 wt.%) and Na 2 O (3.75–5.50 wt.%) contents, which belong to alkaline series rocks. They are enriched in light rare-earth elements (REEs) and large-ion lithophile elements (LILEs) of Rb, Ba and K, and they are depleted in heavy REEs and the high field-strength elements (HFSEs) of Th, U, Nb, Ta, Ti and P. The zircon ɛ Hf ( t ) values of these mangerites are negative (−8.3 to −1.7), with depleted mantle model ages ( T DM ) and average crustal two-stage modal ages ( T 2DM ) from 2250 Ma to 2533 Ma and 2573 Ma to 2959 Ma, respectively. These geochemical characteristics suggest that these mangerites originated from the early Precambrian lower crust of the NCC. Compared with the mangerites, the other lithologies of the AMCG suite display different evolutionary trends of their major elements and different distribution patterns of their REEs and trace elements, indicating that those lithologies were not formed through fractional crystallization by the same primary magma. The anorthosites share similar distribution patterns of REEs and trace elements and zircon Hf isotopic composition with associated norites. The data suggest that the anorthosites were formed by noritic magmas through fractional crystallization at the base of the lower crust. The alkali-granites and rapakivi granites display the geochemical features of A-type granites, and might have originated from partial melting of the mid-upper crust. Combined with regional geological data, these findings suggest that the AMCG suite occurred in a post-collisional/post-orogenic extensional setting rather than an anorogenic continental rift setting. [ABSTRACT FROM AUTHOR]

Abstract: Zircon U–Pb dating and geochemical data are reported for the Jiefangyingzi and Yongshenghao granitic plutons from the Jiefangyingzi area to the northeast (NE) of Chifeng city, Inner Mongolia, with the objective of revealing the tectonic nature of the northern margin of the Sino-Korean paleoplate in early Mesozoic. Zircons from these granitic intrusions are euhedral and subeuhedral in shape with typical oscillatory zoning, indicating magmatic origin. The dating results indicate that these plutons formed in late middle Triassic (229±2Ma and 229±1Ma respectively), rather than in late Paleozoic, as previously thought. The Jiefangyingzi and Yongshegnhao granitic plutons are composed primarily of granodiorite and granite, with compositions of SiO2 =63.23–68.91wt.%, K2O=3.06–4.51wt.%, and A/CNK=0.94–0.99. Chemically, they belong to the high-K calc-alkaline series and meta-aluminous I-type granites. Zircon ε Hf(t) values of both plutons range from +2.1 to +5.0 and +1.4 to +3.9, respectively, whereas the whole-rock ε Nd(t) values of Yongshenghao pluton range from −1.2 to −0.7, indicating the participation of depleted mantle material in their magmatic source. The petrological and geochemical characteristics suggest that these plutons are post-collisional granitoids. Combined with regional geological data, we conclude that these late middle Triassic granitoids formed during the transitional period from the post-collision setting to the post-orogenic extension setting after the collision between the Siberian and Sino-Korean paleoplates. [Copyright &y& Elsevier]