Late Paleozoic tectono-magmatic evolution in Bogda Orogenic Belt，Xinjiang: Evidence from geochemistry of volcanic rocks
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摘要: 最近,笔者在博格达山东段北部的西地-伊齐-小红柳峡一带的地质调查中发现,该区发育有大量晚石炭世柳树沟组双峰式火山岩及早二叠世卡拉岗组酸性火山岩建造,但双峰式火山岩性质及成因有别于其南侧七角井早石炭世双峰式火山岩。玄武岩富钠贫钾(K2O=0.18%~0.45%, Na2O=2.24%~3.63%), 属拉斑系列; TiO2=1.6%~1.7%, 略高于MORB,较高的Al (Al2O3=16.2％~16.7%)、高Mg(MgO= 8.12%~9.54%, Mg#＝61~64),以及低K2O/TiO2和K2O/P2O5比值(分别为0.1~0.27、0.63~1.68),反映了在岩浆演化过程中分离结晶作用不明显; Rb/Sr比值0.01~0.02,Zr/Nb=21.6~39.7,Zr/Y=5.38~7.47,以及不相容元素Ba、Zr、Hf相对略富集、Nb-Ta和Th相对亏损,显示岩石具有板内玄武岩的特点; 稀土元素球粒陨石标准化配分图上整体接近于平坦型,(La/Yb)N＝1.8~1.9,Eu无异常至轻微正异常 (δEu=1.07~1.12),正εNd(t)值(＋5.63~＋5.89),(143Nd/144Nd)I＝0.512927~0.512944,Th/Ybσ),具高Si (SiO2＝76%~80%), 富钾贫钠(K2O＝5.1%~5.7%, Na2O＝0.94%~2.03%); 低Al (Al2O3＝7.9%~10.4%); 低Ti、Ca和P含量,属高钾钙碱性系列; 微量元素Rb、Th、Zr、Hf、K相对富集,Ba、Sr、P、Ti、Nb、Ta为显著亏损; 轻稀土元素适度富集且轻、重稀土分馏程度低,(La/Yb)N＝5.1~7.1, (La/Sm)N＝2、(Gd/Yb)N＝1.6~2.2,以及强烈的负Eu异常(δEu＝0.17~0.2),(87Sr/86Sr)I＝0.7051~0.7052,δ18O=11.6‰,指示岩石源于地壳物质的部分熔融,源区存在有斜长石残留,形成于大陆裂谷环境。早二叠世末流纹岩 (Rb-Sr等时线年龄为278±2Ma) 具高Si(SiO2＝74%),富钾贫钠(K2O／Na2O > 2),低Al(Al2O3＝11.0%)以及较低的Ti和P含量的特征,岩石为高钾钙碱性系列; 微量元素PM标准化图解上表现为Rb、Ba、Th、U、K、La、Ce不相容元素相对富集,高场强元素Nb、Ta、P、Ti 以及Sr为明显的负异常; 轻稀土轻度富集,(La/Yb)N＝5~6,(La/Sm)N＝3,(Gd/Yb)N=1.3~1.4,以及强烈的负Eu异常 (δEu＝0.31~0.39),(87Sr/86Sr)I为0.7069,δ18O=11.97‰,指示源于地壳物质部分熔融的产物,形成于伸展垮塌的构造环境。综合研究结果表明,博格达山前身裂谷岩浆作用始于早石炭世,结束于晚石炭世末期,早二叠世末进入后造山伸展的演化阶段。Abstract: The Late Carboniferous bimodal volcanic rocks and Early Permian acidic volcanic rocks were revealed recently in Yiqi-Xiaohongxia of the northern margin of the eastern section of Bogda Mountain, Xinjiang, but in petrogenesis the former different from those in the Qijiaojing Region south to the studied area. The bimodal volcanic suite in the studied area is composed of basalts and a less amount of rhyolites (rhyolites/basalts=~1/4). The basalts have high-Al (Al2O3=16.2%~16.68%), -Mg (MgO=8.12%~9.54%, Mg#=61~64), and -Na (Na2O=2.24%~3.63%), but low-K contents (K2O=0.18%~0.45%), and low K2O/TiO2=0.1~0.27 and K2O/P2O5=0.63~1.68 ratios, belonging to tholeiitic series, and indicating no fractional crystallization of plagioclase during the basaltic magmatism; and have TiO2=1.66%~1.71%, slightly higher than MORB's, and low Rb/Sr ratio (0.01~0.02), Th/Yb (N=1.8~1.9, no Eu anormalies (δEu=1.07~1.12), but relatively depleted in Nb-Ta and Th, and positive εNd(t) value (+5.63~+5.89), (143Nd/144Nd)I =0.512927~0.512944, showing that the basalts have geochemical features indicative of basalts formed within continent and originated from depleted mantle, and that the continent crust became thinner and thinner due to extension for a long time. The rhyolites of bimodal volcanic rocks with Rb-Sr isochron age of 296±2Ma (1σ), have high-Si (SiO2=76%~79.76%), -K2O but low Na2O (K2O=5.14%~5.65%, Na2O=0.94%~2.03%), Al (Al2O3=7.90%~10.39%), Ti, CaO and P2O5 contents, showing they are high-K calc-alkaline series; and exhibit relatively rich in trace elements Rb, Th, Zr, Hf, K and LREE, (La/Yb)N=5.1~7.1, significantly depletd in Ba, Sr, P, Ti, Nb, Ta, and strongly negative Eu anomalies (δEu=0.17~0.2), (87Sr/86Sr)I =0.7051~0.7052, δ18O=11.6‰, suggesting that the rocks, similar to the geochemistry of the ryholites formed in continental rifts, originated from the melting of lower crust with plagioclase as an important relic mineral in the source. Early Permian rhyolites (Rb-Sr isochron age of 278±2Ma (1σ)) with high SiO2 (73.62%~74.24%), K-rich and Na-depleted, K2O/Na2O>2, low Al2O3 (11.06%~11.4%), as well as low P and Ti contents, belonging to high-K calc-alkaline series; enriched in incompatible elements Rb, Ba, Th, U, K, La, Ce and LREE, (La/Yb)N = 5~6, obviously depleted in HFSE (Nb, Ta, P, Ti) and Sr and HREE, as well as strongly negative Eu anomalies (δEu=0.31~0.39), (87Sr/86Sr) I =0.7069, and δ18O=11.97‰, resulted obviously from partial melting of lower crustal materials by underplating of basaltic magma or mantle diapirism in the tectonic setting of post-orogenic extension. The studied results indicate that magmatism of the Bogda rifting started in the Early Carboniferous, ended in the end of Late Carboniferous, and then transformed into the post-orogenic evolution of the extension in the end of Early Permian.
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Key words:
- Bogda Orogenic Belt /
- Late Paleozoic /
- Tectono-magmatism /
- Bimodal volcanic rocks /
- Continental rift /
- Post-orogeny
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