Geochronology and geochemistry of gneissic metagranites in eastern Dabie Mountains: Implications for the Neoproterozoic tectono-magmatism along the northeastern margin of the Yangtze Block

Abstract

The gneissic metagranites with Neoproterozoic protolith ages are widely exposed along the eastern margin of the Dabie ultrahigh-pressure (UHP) metamorphic belt. In this paper, five representative plutons, including Huangzhen, Daba, Shima, Shuanghe, and Sanzusi, were selected for a detailed chronological and geochemical study aiming to identify the nature of the protoliths and to reveal their implications for the Neoproterozoic tectono-magmatic evolution along the northeastern margin of the Yangtze Block. These gneissic metagranites consist mainly of quartz, albite (oligoclase) and K-feldspar, and minor amphibole and biotite, as well as some metamorphic minerals due to the UHP metamorphism, such as phengite, epidote and minor kyanite and garnet. Mafic alkaline minerals (e.g., aegirine or aegirine-augite) have been observed in some of these plutons. Zircon LA-ICP-MS U-Pb dating results indicate that the protoliths of the gneissic metagranites have been generated in Mid-Neoproterozoic (770-780 Ma), and suffered both ultrahigh-pressure metamorphism in Early Mesozoic and subsequently high-pressure eclogite-facies recrystallization at about 215 Ma. Although the gneissic metagranites generally have high SiO 2 contents (70.23%-77.23%) and show metaluminous-weakly peraluminous signatures (ASI=0.90-1.05), there are still some geochemical variances between different plutons. Compared with the Sanzusi pluton, the metagranites from the Huangzhen, Daba, Shima, and Shuanghe have high K 2 O+Na 2 O contents (7.76%-9.45%), FeO total /(FeO total +MgO) ratios (0.82-0.96), HFSEs and Ga concentrations with an average 10 4 ×Ga/Al ratio up to 3.07. Combined with the features that the four plutons commonly contain aegirine-augite and have high zircon saturation temperatures (816-918°C), it is suggested that their protoliths belong to peralkaline A-type granites. The Sanzusi pluton generally contains biotite and epidote generated from metamorphic reaction of amphibole, and is chemically enriched in calcium and depleted in potassium with K 2 O/Na 2 O ratios ranging from 0.42 to 0.54, suggesting their protoliths should be ascribed to calc-alkaline granitoids (I-type). The gneissic metagranites in the eastern margin of Dabie Mountains show both arc-and rift-like geochemical signatures, and their protoliths likely have been generated by reworking of the preexisting arc-related crust under an extensional setting, suggesting that the northeastern margin of the Yangtze Block during the Middle Neoproterozoic likely have been under the initial stage of a passive rifting rather than in an active rifting setting typically induced by the upwelling of a mantle plume. gneissic metagranite, zircon U-Pb chronology, geochemistry, tectono-magmatism, eastern margin of Dabie Mountains Citation:Hu J, Qiu J S, Xu X S, et al. Geochronology and geochemistry of gneissic metagranites in eastern Dabie Mountains: Implications for the Neoproterozoic tectono-magmatism along the northeastern margin of the Yangtze Block.

U–Pb Dating of Fibrous Dolomite in the Hydrothermal Dolostone of the Dengying Formation, Central Sichuan Basin, and Its Response to Supercontinent Breakup

“…In this study, the dating results of multi-phase fibrous dolomite in the grape-like edges of Well Pengtan-1 show four age phases: the second phase, the fifth phase, the sixth phase, and the seventh phase, with ages of 781 ± 12, 683 ± 12, 562 ± 12, and 545.4 ± 6.9 Ma, respectively (Figure 9). In comparison with the regional tectonic evolution, the age of 781 Ma roughly corresponds to the regional Chengjiang Movement and aligns with the initial rifting of the global Rodinia supercontinent [70,71]. The 683 Ma event occurred during the process of the Rodinia supercontinent breakup, during which volcanic eruptions and the deposition of the Liantuo Formation, Datangpo Formation volcaniclastic rocks, and terrigenous clastic rocks occurred in the South China Plate, forming a suite of sedimentary assemblages in an extensional environment [6,[72][73][74].…”

A refined late-Cryogenian – Ediacaran earth history of South China: Phosphorous-rich marbles of the Dabie and Sulu orogens

“…In brief, U-Pb dating of magmatic zircon obtained from mid-upper crustal granitoid gneisses throughout the Dabie orogen produce ages that range mostly from 800-720 Ma (e.g. Ames et al, 1996;Hacker et al 1998;Hu et al 2010;Rowley et al 1997;Wu et al 2007;Xie et al 2004) with somewhat older ages of 860-760 Ma obtained for the cores of magmatic zircons derived from lower-crustal granulites in the UHP terrains in the northern part of the orogen (e.g. Ling et al 2003;Liu et al 2007Liu et al , 2011Wu et al 2012;Xu et al 2016).…”