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Origin of the late Paleoproterozoic low-δ18O A-type granites on the southern margin of the North China Craton and their geodynamic mechanism
Precambrian Research ( IF 3.8 ) Pub Date : 2020-12-01 , DOI: 10.1016/j.precamres.2020.105960
Xiaoqin Deng , Touping Peng , Yanyan Zhou , Taiping Zhao , Zhili Qiu

Abstract A-type granites with 18O-depletion signatures are rather enigmatic as regards not only their typology and petrogenesis but also their rock-forming geodynamic mechanism. Here we report a suite of late Paleoproterozoic low-δ18O A-type granites from the Longwangzhuang pluton on the southern margin of the North China Craton. SIMS zircon U-Pb dating showed that the Longwangzhuang pluton was emplaced at 1613 ± 4 Ma. Combined with previous data, it is clear that the mineral components of Longwangzhuang granites are variable and consist of perthite, quartz, plagioclase, biotite and amphibole (calcic hastingsite, ferroedenite, and sodic ferrogedrite). Geochemically, they are metaluminous to peraluminous and have high alkali contents (K2O + Na2O > 8.76 wt%, K2O/Na2O > 1), FeOt/(FeOt + MgO) (>0.95) ratios, Zr + Nb + Ce + Y (>993 ppm) contents, and 10000 Ga/Al (>4.31) ratios, resembling A-type granites. In-situ oxygen isotope analysis displayed low zircon δ18O values between 4.0‰ and 4.9‰ with a weighted average of 4.4‰. Based on the Archean two-stage zircon Hf and whole-rock Nd model ages coupled with the low (La/Yb)CN ratios, negative anomalies of Eu and Sr, and high Zr saturation temperatures, and the presence of low pressures (3.3–5.7 kbar) amphiboles, the Longwangzhuang A-type granites were generated by “self-cannibalizing” pre-existing low-δ18O old rocks (i.e., the Taihua Complex) and emplaced at shallow depth. Combined with regional geological observations (i.e., coeval tuffites), a passive rift setting was the most plausible explanation for the generation of the late Paleoproterozoic low δ18O A-type granites on the southern margin of the North China Craton. Such a rifting could likely have lasted at least till 1.6 Ga.

中文翻译:

华北克拉通南缘晚古元古代低δ18O A型花岗岩的成因及其地球动力学机制

摘要 具有 18O 耗尽特征的 A 型花岗岩不仅在类型学和岩石成因方面,而且在其岩石形成地球动力学机制方面都相当神秘。在这里,我们报告了华北克拉通南缘龙王庄岩体的一套晚古元古代低δ18O A型花岗岩。SIMS锆石U-Pb定年表明龙王庄岩体在1613±4 Ma就位。结合前人资料可知,龙王庄花岗岩的矿物成分多变,主要由钙长石、石英、斜长石、黑云母和角闪石(钙辉石、铁闪石和钠铁闪石)组成。从地球化学上讲,它们是金属铝质至过铝质,并且具有高碱含量(K2O + Na2O > 8.76 wt%,K2O/Na2O > 1)、FeOt/(FeOt + MgO) (>0.95) 比率、Zr + Nb + Ce + Y (> 993 ppm) 含量和 10000 Ga/Al (>4.31) 比率,类似于 A 型花岗岩。原位氧同位素分析显示锆石 δ18O 值较低,介于 4.0‰ 和 4.9‰ 之间,加权平均值为 4.4‰。基于太古宙两阶段锆石 Hf 和全岩 Nd 模型年龄,加上低 (La/Yb)CN 比率、Eu 和 Sr 的负异常以及高 Zr 饱和温度以及低压 (3.3– 5.7 kbar)角闪石,龙王庄A型花岗岩是由先存的低δ18O古岩(即太华杂岩)“自噬”产生的,并在浅层就位。结合区域地质观测(即同时期凝灰岩),被动裂谷环境是华北克拉通南缘晚古元古代低δ18O A型花岗岩形成的最合理解释。
更新日期:2020-12-01
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