Metamorphic evolution and zircon ages of pelitic granulites in eastern Hebei, North China Craton: Insights into the regional Archean P–T–t history
Metamorphic evolution and zircon ages of pelitic granulites in eastern Hebei, North China Craton: Insights into the regional Archean P–T–t history
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DOI：
10.1016/j.precamres.2017.02.008
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发表时间：
2017-05
影响因子：
3.8
通讯作者：
Zhanzhan Duan;C. Wei;H. Rehman
中科院分区：
文献类型：
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作者：
Zhanzhan Duan;C. Wei;H. Rehman
太古宙上地壳麻粒岩以局部条状或片状的形式出现在花岗岩类中，这种特殊的上地壳麻粒岩从变质演化的角度来看并没有引起太多的关注。冀东泥质麻粒岩以条状产于TTG片麻岩和绿粒岩中。在太平寨和老李家地区采集了3个泥质麻粒岩样品，分别为直辉石-石榴石片麻岩（JD1334）、黑云母-石榴石片麻岩（JD1453）和硅线石-石榴石片麻岩（JD15126）。根据构造观察和矿物组成，确定了末期、峰期、包体期和套印期4代组合。观察到的最终组合屈服P-T条件为8.0-9.5 kbar/ 870-890°C。观察到的无基体黑云母峰组合特征记录了温度为T > ~ 950℃，固定P = 9 kbar的条件。从峰值到最终组合的等压冷却路径受到矿物等槽的限制。在低压和（或）高温场中，以高含黄含量的斜长石包裹体为特征的包裹体组合是稳定的。它们形成了一个逆时针的P-T路径。冠状石榴石（g2）与细粒矿物团聚体共存的叠印组合特征确定了10-12 kbar/ 830-880°C的P-T条件。泥质麻粒岩沉积原岩的沉积年龄为~ 2.5 Ga，与TTG片麻岩的结晶年龄一致。两者后来在~ 2.48 Ga发生麻粒岩相变质作用。太古宙的逆时针P-T路径表明了一种构造情景，可以解释太古宙上地壳麻粒岩的特殊发生。首先，由于高温TTGs的侵入，地壳上沉积经历了低压热事件，导致水相脱水。然后，干燥的变质沉积层间的含铁地层可能比周围岩性密度更大，它们倾向于落入TTG岩浆海，导致变质沉积层内压力升高。第三，沉降的变质沉积物在地壳下经历了冷却过程，并与周围TTG岩浆的结晶作用形成了观测到的最终组合。最后，它们在太古宙地体的穹隆作用下抬升。
Archean supracrustal granulites occur as localized slivers or rafts in granitoids, and this particular occurrence of supracrustal granulites has not been much concerned from the perspective of metamorphic evolution. The pelitic granulites in eastern Hebei occur as slivers in TTG gneisses and charnockite. Three pelitic granulite samples involving orthopyroxene–garnet gneiss (JD1334), biotite–garnet gneiss (JD1453) and sillimanite–garnet gneiss (JD15126) were collected from Taipingzhai and Laolijia areas. Based on textural observations and mineral compositions, four generations of assemblages involving final, peak, inclusion and overprinting stages are identified. The observed final assemblage yield P–T conditions of 8.0–9.5 kbar/870–890 °C. The observed peak assemblage characteristic of the absence of the matrix biotite record condition of T > 950 °C with fixed P = 9 kbar. An isobaric cooling path from peak to final assemblage is constrained by mineral isopleths. The inclusion assemblages characterized by plagioclase inclusions with high XAncontents are predicted to be stable at low pressure and/or high temperature field. They form an anticlockwise P–T path. The overprinting assemblage characteristic of coronary garnet (g2) coexisting with fine-grained mineral aggregates defines P–T condition of 10–12 kbar/830–880 °C. The deposition age of the sedimentary protolith of pelitic granulites was ∼2.5 Ga, coeval with the crystallization age of TTG gneisses. Both of them were later subjected to granulite facies metamorphism at ∼2.48 Ga. The Archean anticlockwise P–T paths suggest a tectonic scenario to explain the particular occurrence of the Archean supracrustal granulites. Firstly, supracrustal sediments experienced thermal event under low pressures attributed to the intrusion of high-temperature TTGs, resulting in dehydrations of hydrous phases. Then, the dried metasediments interlayered with iron-bearing formations potentially denser than the surrounding lithologies tend to drop into the TTG magma ocean, causing the pressure-increase in the metasediments. Thirdly, the dropped metasediments experienced cooling process under lower crust in coupled with the crystallization of the surrounding TTG magma to form the observed final assemblages. Finally, they uplifted as a consequence of the doming processes in Archean terranes.