Phase equilibrium modelling and SHRIMP zircon U-Pb dating of medium-pressure pelitic granulites in the Helanshan complex of the Khondalite Belt, North China Craton, and their tectonic implications
Phase equilibrium modelling and SHRIMP zircon U-Pb dating of medium-pressure pelitic granulites in the Helanshan complex of the Khondalite Belt, North China Craton, and their tectonic implications
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华北克拉通孔兹岩带贺兰山杂岩中压泥质麻粒岩相平衡模拟、SHRIMP锆石U-Pb定年及其构造意义
DOI：
10.1016/j.precamres.2018.05.030
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发表时间：
2018
影响因子：
3.8
通讯作者：
Zhao Jiao
中科院分区：
文献类型：
--
作者：
Xu Xiao-Fei;Gou Long-Long;Long Xiao-Ping;Dong Yun-Peng;Liu Xiao-Ming;Zi Jian-Wei;Li Zheng-Hui;Zhang Cheng-Li;Liu Liang;Zhao Jiao
华北克拉通孔兹岩带贺兰山杂岩的特征是同时发育中压（MP）和高压（HP）泥质麻粒岩。本文介绍了 MP 石榴石-硅线石麻粒岩的相平衡模型和 SHRIMP 锆石 U-Pb 定年的新结果，以便更好地了解 MP 和 HP 泥质麻粒岩之间的关系、MP 泥质麻粒岩对孔兹岩带形成的构造意义及其对古元古代造山作用的地球动力学意义。根据岩相观察，变质演化可分为四个阶段（M1～M4）。从露头的混合岩结构可以看出，M1-M3 阶段是上固相线。峰值变质阶段（M1）以石榴石原斑边缘及基质中硅线石+石英±黑云母、金红石残渣+斜长石+钾长石包裹体为代表； M2由部分取代金红石的钛铁矿定义； M3的特点是用堇青石代替石榴石。次固相线矿物组合为石榴石+黑云母+斜长石+钾长石+硅线石+堇青石+钛铁矿+石英（M4）。结合 Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O2 化学体系中的相平衡模型，MP 泥质麻粒岩记录了顺时针 P–T 路径，其特征是近等温减压 (ITD)，从 ∼830°C 的 ∼10.1kbar 到 ∼800°C 的 ∼5.8kbar 及后续接近等压冷却（IBC）。 SHRIMP锆石U-Pb测年得出的变质年龄为1931±21Ma，被解释为逆行冷却的时间。结合之前报道的结果，我们推测阴山-鄂尔多斯碰撞后板片的断裂发生在大约 100 年前。 1.95Ga 导致孔兹岩中的 MP 和 HP 麻粒岩在拉伸状态下抬升到中地壳水平，如 ITD 演化所示，随后在约 1.95Ga 处缓慢冷却。 1.93Ga 的近 IBC 演化证明了这一点。
The Helanshan complex in the Khondalite Belt, North China Craton (NCC) is characterized by the occurrence of both medium-pressure (MP) and high-pressure (HP) pelitic granulites. This paper presents new results from phase equilibrium modelling and SHRIMP zircon U–Pb dating on the MP garnet-sillimanite granulites, in order to better understand the relationship between the MP and HP pelitic granulites, the tectonic implications of the MP pelitic granulites for the formation of the Khondalite Belt, and their geodynamic implications for the Paleoproterozoic orogenesis. Based on petrographic observations, the metamorphic evolution can be divided into four stages (M1–M4). The M1–M3stages are suprasolidus as reflected by the migmatitic structure on the outcrop. The peak metamorphic stage (M1) is represented by the rim of garnet porphyroblasts along with inclusions of sillimanite + quartz ± biotite, and rutile relicts + plagioclase + K-feldspar in the matrix; the M2is defined by the ilmenite that have partially replaced rutile; and the M3is characterized by the replacement of garnet by cordierite. The subsolidus mineral assemblage is garnet + biotite + plagioclase + K-feldspar + sillimanite + cordierite + ilmenite + quartz (M4). Combined with the phase equilibrium modelling in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O2chemical system, the MP pelitic granulites record a clockwiseP–Tpath, which is characterized by near-isothermal decompression (ITD) from ∼10.1 kbar at ∼830 °C to ∼5.8 kbar at ∼800 °C and subsequent close-to-isobaric cooling (IBC). SHRIMP zircon U–Pb dating yielded a metamorphic age of 1931 ± 21 Ma, interpreted as the time of the retrograde cooling. Integrated with previously reported results, we propose that slab break-off subsequent to Yinshan-Ordos collision at ca. 1.95 Ga led to uplift of the MP and HP granulites in the Khondalite to the middle crustal level in an extensional regime as indicated by their ITD evolution, which was followed by slow cooling at ca. 1.93 Ga as evidenced by their nearly IBC evolution.