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Metamorphic P‒T paths and Zircon U–Pb ages of Archean ultra‐high temperature paragneisses from the Qian’an gneiss dome, East Hebei terrane, North China Craton
Journal of Metamorphic Geology ( IF 3.4 ) Pub Date : 2020-03-04 , DOI: 10.1111/jmg.12524
Ting Liu 1 , Chunjing Wei 1
Affiliation  

The East Hebei terrane of North China Craton is characterized by the dome‐and‐keel structure, a common feature in most Archean cratons, where supracrustal rocks of granulite facies commonly occur as enclaves or rafts in tonalite–trondhjemite–granodiorite (TTG) gneisses. The metamorphic P–T paths of the granulites are significant for addressing the Archean tectonic regimes. Two types of granulite facies paragneiss with pelitic and greywacke compositions from the western margin of Qian'an gneiss dome are documented for their petrography, mineral chemistry, phase equilibria modelling using thermocalc, and zircon dating. Anticlockwise P–T paths involving the pre‐peak pressure increase to the ultra‐high temperature peak conditions and post‐peak cooling and decompression processes were recognized. The pre‐peak pressure increase process was constrained for a pelitic granulite mainly based on the spinel and cordierite inclusions in garnet and rutile corona around ilmenite, where the transition from spinel to garnet is modelled at 6–7 kbar at a fixed T = 1,000°C. For greywacke granulite, the pre‐peak pressure increase evolution can be ascertained from the textural relation that orthopyroxene is surrounded by garnet, and the outwards increasing grossular (from 0.03 to 0.05) in the core of the atoll‐like garnet (Grt‐A), to occur from ~7 kbar at ~1,000°C. The peak P–T conditions for pelitic granulite are roughly limited to 7–11 kbar/890–1,050°C on the basis of the stability of the inferred peak assemblage involving garnet, perthite, sillimanite, rutile/ilmenite, and quartz. The peak P–T condition for greywacke granulite can be well constrained as 9–10 kbar/>1,000°C on the basis of the maximum grossular content (XGrs = 0.045–0.050) in the core of subhedral garnet (Grt‐B) and the mantle of Grt‐A together with an average re‐integrated anorthite content (XAn = 0.07) in K‐feldspar. The peak temperature condition is consistent with the ternary feldspar thermometer results mostly of 950–1,020°C for antiperthite and perthite in greywacke granulite, and in accordance with the development of oriented needle‐like exsolution of Ti±Fe oxides in garnet from pelitic granulite. The post‐peak cooling and decompression process was consistent with the decreasing XGrs in the mantle of Grt‐A and core of Grt‐B in greywacke sample, and the final‐stage cooling conditions can be well constrained from the stability of final assemblages marked by the later growth of biotite, as 8–9 kbar/820–880°C for pelitic granulite and 6–9 kbar/840–890°C for greywacke granulite. Zircon dating yields provenance ages from 3.34 to 2.57 Ga and metamorphic ages of c. 2.50 Ga for the two types of granulite. The metamorphic ages overlap the final pulse of the Neoarchean magmatic activity of TTGs that ranges from c. 2.56 to c. 2.48 Ga with a peak at c. 2.52 Ga. Combining the development of dome‐and‐keel structures, the penecontemporaneity between the metamorphism of supracrustal rocks and TTG magmatic activity, and also the unique anticlockwise P–T paths, we prefer a vertical sagduction regime to interpret the tectonic evolution of the East Hebei terrane, which may be also significant for other Archean cratons.

中文翻译:

华北克拉通东部迁安片麻岩穹顶太古宙超高温石质的变质P‒T路径和锆石U–Pb年龄

华北克拉通的河北东部地层以穹隆和龙骨结构为特征,这在大多数太古宙克拉通中都是普遍存在的特征,其中花岗石相的地壳上层岩石通常以斜方晶格或斜方晶镁质闪长岩(TTG)片麻岩中的飞地或木筏形式出现。变质对叔所述粒的路径是用于寻址太古构造体制显著。记录了迁安片麻岩穹顶西缘的两种类型的花岗石岩相,其具有胶粉质和格莱克瓦克岩相的岩相学,矿物化学,使用钙的相平衡模型和锆石测年。逆时针对叔确认了涉及峰前压力增加到超高温峰值条件以及峰后冷却和减压过程的路径。峰前压力的增加过程受制于主要基于石榴石中的尖晶石和堇青石夹杂物以及钛铁矿周围的金红石电晕的黄土粒状体,其中在固定的T  = 1,000°下,从尖晶石到石榴石的转变模型为6-7 kbar C。对于格雷瓦克花岗石,峰前压力升高的演变可以通过邻苯二酚被石榴石包围,而环礁状石榴石(Grt-A)的中心向外颗粒(从0.03到0.05)来确定。发生在〜1,000°C时〜7 kbar。峰值PT根据推断出的包括石榴石,蠕铁矿,硅线石,金红石/钛铁矿和石英的峰集合的稳定性,黄泥质花岗石的条件大致限于7-11 kbar / 890-1,050°C。峰值对叔为砂岩粒条件可以被很好的约束为9-10千巴/> 1000℃的最大钙铝榴石含量的基础上(X GRS 在半自石榴石(GRT-B)的核心= 0.045-0.050)和Grt‐A的地幔,以及平均重新整合的钙长石含量(X An = 0.07)在K-长石中。峰值温度条件与三元长石温度计的结果一致,主要是在950–1,020°C的灰泥瓦克花岗石中的防蠕虫和珠光体,并与针状粒状Ti±Fe氧化物从石榴石中的石榴石中的析出相一致。峰顶后的冷却和减压过程与格雷瓦克样品中Grt-A地幔和Grt-B岩心中X Grs的减少相一致,并且最终阶段的冷却条件可以通过标记的最终组件的稳定性得到很好的约束。后来黑云母的生长,例如,黄铁质花岗石为8–9 kbar / 820–880°C,灰瓦克花岗石为6–9 kbar / 840-890°C。锆石定年产生的物源年龄为3.34至2.57 Ga,而c的变质年龄两种类型的花岗石为2.50 Ga。变质年龄与TTG的新太古代岩浆活动的最终脉冲重叠,范围为c。2.56至c 2.48 Ga,峰值为c。2.52 Ga。结合穹顶和龙骨结构的发展,上壳岩石变质与TTG岩浆活动之间的准时变性,以及独特的逆时针P–T路径,我们更倾向于采用垂直陷落机制来解释该构造的演化。河北东部的地形,对于其他太古代克拉通也可能具有重要意义。
更新日期:2020-03-04
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