华北北缘东段开原地区三叠纪侵入岩年代学及岩石地球化学研究

本文报道了位于华北北缘东段开原地区尖山子岩体、宝兴岩体和树德屯岩体的岩相学、岩石地球化学及年代学特征,探讨了上述岩体的形成时代、岩石成因及其构造环境。锆石U-Pb定年结果表明,尖山子岩体形成于251±1.3Ma,宝兴岩体形成于235±1.3Ma,树德屯岩体形成于224±1.9Ma,说明开原地区三叠纪存在早三叠世、中三叠世和晚三叠世三个时期的岩浆活动。尖山子岩体以二长花岗岩为主,具高硅低镁特征,属钙碱性系列;富集大离子亲石元素Rb、K、Ba、Th,相对亏损Ta、Nb、P、Hf、Zr等高强场元素;δEu=0.55~1.87,(La/Yb)N=6.23~47.9,轻重稀土分馏明显,富集轻稀土,亏损重稀土。宝兴岩体以花岗闪长岩为主,Si O2含量变化较大(52.36%~74.06%),Al2O3含量(14.5%~17.34%),Mg O含量(0.61%~3.66%),属钙碱性系列;富集大离子亲石元素Rb、K、Ba、Th,相对亏损Ta、Nb、P、Ti等高强场元素;无明显的Eu异常,(La/Yb)N=6.81~25.6,轻重稀土分馏较明显,富集轻稀土,亏损重稀土。树德屯岩体以闪长岩为主,岩石具有低硅高镁特征,K2O/Na2O比值为0.33~0.76,显示富K贫Na特征,属钙碱性系列;富集大离子亲石元素Rb、K、Ba、Th,相对亏损Ta、Nb、P、Ti、Hf、Zr等高强场元素;无明显的Eu异常,(La/Yb)N=3.87~10.2,轻重稀土分馏不明显。上述岩石地球化学特征表明,尖山子岩体和宝兴岩体的原始岩浆起源于下地壳基性物质的部分熔融,树德屯岩体的原始岩浆起源于亏损的地幔楔。研究区三叠纪岩浆岩形成于造山阶段挤压环境下。华北北缘东段的挤压碰撞作用一直持续到晚三叠世(224Ma),而造山阶段向造山后阶段的构造转换(挤压地壳加厚向伸展垮塌的环境转换)发生于晚三叠世-早侏罗世(224~180Ma)期间。华北北缘东段中生代岩石圈减薄或破坏始于晚三叠世-早侏罗世(224~180Ma)期间。

Geochronology and geochemistry of Triassic intrusive rocks in Kaiyuan area of the eastern section of the northern margin of North China

This paper has reported the petrography, geochemistry and geochronology of Jianshanzi, Baoxing and Shudetun plutons in the Kaiyuan area of the eastern section of the northern margin of North China, and discussed their formation age, rock genesis and tectonic environment. Zircon U-Pb dating results show that Jianshanzi pluton was formed in Early Triassic (251±1.3Ma), Baoxing pluton was formed in Middle Triassic (235±1.3Ma) and Shudetun pluton was formed in Late Triassic (224±1.9Ma). The Jianshanzi pluton is dominated by adamellite, with high Si and low Mg content, and belonged to calc-alkaline series. This unit is enriched in the light ion lithophile elements (e.g., Rb, K, Ba and Th) but is relatively depleted in the high field strength elements (e.g., Ta, Nb, Hf and Zr). They have (Eu values of 0.55~1.87 and high (La/Yb)_N values of 6.23~47.9. These characteristics indicate that the Jianshanzi adamellites have fractionated REE compositions that are enriched in the light REE (LREE) and depleted in the heavy REE (HREE). The Baoxing pluton is dominated by granodiorite. This granodiorite is calc-alkaline, with SiO₂ concentrations of 52.36%~74.06%, Al₂O₃ of 14.5%~17.34% and MgO of 0.61%~3.66%. In addition, this unit is enriched in the light ion lithophile elements (e.g., Rb, K, Ba and Th) and is relatively depleted in the high field strength elements (e.g., Ta, Nb, P and Ti). Furthermore, these samples have negligible Eu anomalies and (La/Yb)_N values of 6.81~25.6. These characteristics indicate that the Baoxing granodiorites have fractionated REE compositions that are enriched in the LREE and depleted in the HREE. The Shedetun pluton is dominated by diorite, with low Si and high Mg content. This diorite is enriched in K and depleted in Na, with K₂O/Na₂O ratios of 0.33~0.76, and belonged calc-alkaline series. In addition, the Baoxing pluton is enriched in the light ion lithophile elements (e.g., Rb, K, Ba and Th) and is relatively depleted in the high field strength elements (e.g., Ta, Nb, P, Ti, Hf and Zr). The Shudetun pluton has negligible Eu anomalies and (La/Yb)_N values of 3.87~10.2, with no obvious fractionation of LREE and HREE. According to the above research, the original magma of the Jianshanzi and Baoxing pluton are originated from partial melting of the lower crust basic rocks, but the Shudetun pluton are originated form depleted mantle wedge. The Triassic magmatites of study area are formed in compressional environment of orogenic stage. Meanwhile, extrusion and collision of the eastern section of the northern margin of North China was lasted to Late Triassic (224Ma). The tectonic transformation of orogenic stage and post orogenic stage (extrusion crustal thickening transition to extensional collapse environment) occurred between Late Triassic and Early Jurassic (224~180Ma), and also means that lithospheric thinning or destruction might begin in Late Triassic to Early Jurassic (224~180Ma).