Picritic porphyrites generated in a slab-window setting: Implications for the transition from Paleo-Tethyan to Neo-Tethyan tectonics
Picritic porphyrites generated in a slab-window setting: Implications for the transition from Paleo-Tethyan to Neo-Tethyan tectonics
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DOI：
10.1016/j.lithos.2012.09.013
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发表时间：
2012-12
期刊：
影响因子：
3.5
通讯作者：
C. Kou;Zhaochong Zhang;M. Santosh;He Huang;T. Hou;Baoli Liao;Hongbo Li
中科院分区：
文献类型：
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作者：
C. Kou;Zhaochong Zhang;M. Santosh;He Huang;T. Hou;Baoli Liao;Hongbo Li
本文报道了产于滇西剑川地区的苦橄玢岩，该地区位于三江-特提斯构造域，构成东特提斯构造域东南段的一部分，喜马拉雅-青藏造山带以东。斑岩中长石40 Ar 39 Ar坪年龄为201.7 ~ 190.6Ma。苦橄玢岩的地球化学特征是SiO_2含量较低（43.9-46.1wt.%）。MgO含量高（18.9- 26.7wt%），表现出洋岛玄武岩（OIB）特征，轻稀土元素（LREE）和大离子亲石元素（LILE）相对于高稀土元素富集（HREE：LaN/YbN=7.12-10.24）。这些岩石也没有明显的Eu异常和缺乏Nb，Ta和Ti槽。根据橄榄石-熔体平衡，推测形成斑岩的母岩浆为苦橄质岩浆，MgO含量为18.5-20.5wt.%。岩石显示出相对较高的初始（87 Sr/86 Sr）比值（0.705253至0.705640），负εNd（t）值放射成因Pb（206 Pb/204 Pb）t=18.165 ~ 18.343，（207 Pb/204 Pb）t =15.558 ~ 15.523，（208 Pb/204 Pb）t=38.547 ~ 38.741）。微量元素模拟表明，苦橄质岩浆是由尖晶石向石榴石转变的橄榄岩源的相对较低程度的部分熔融（4-6%）产生的，但在高温（~1580°C）和压力（~2.88Gpa）条件下更接近于石榴石相橄榄岩。结合区域地质特征，推测苦橄玢岩形成于古特提斯洋演化结束与新特提斯洋演化开始之间的过渡阶段，即晚三叠世-早侏罗世，向西俯冲于印度板块之下的金沙江洋片断裂，打开了软流圈上涌和部分熔融的洋片窗口。
We report picritic porphyrites from the Jianchuan area in western Yunnan, within the Sanjiang–Tethyan Tectonic Domain (STTD), forming part of the the southeastern segment of the East Tethyan tectonic domain, east of the Himalayan–Tibetan Orogen. Feldspar40Ar39Ar plateau ages from the porphyrites range from 201.7Ma to 190.6Ma. Geochemically, the picritic porphyrites are characterized by relatively low SiO2(43.9–46.1wt.%) and high MgO (18.9–26.7wt.%), and display ocean island basalt (OIB)-like signatures, characterized by the enrichment of light rare earth element (LREE) and large ion lithophile elements (LILE) relative to high rare earth element (HREE: LaN/YbN=7.12–10.24). These rocks also show the absence of prominent Eu anomalies and lack of Nb, Ta and Ti troughs. On the basis of the olivine-melt equilibrium, the parental magma that produced the porphyrites is estimated to be a picritic magma with MgO content of 18.5–20.5wt.%. The rocks show relatively high initial (87Sr/86Sr)iratios (0.705253 to 0.705640), negative εNd(t) values (−1.28 to −1.69) and radiogenic Pb ((206Pb/204Pb)t=18.165 to 18.343, (207Pb/204Pb)t=15.558 to 15.523, and (208Pb/204Pb)t=38.547 to 38.741). Trace element modeling suggests that the picritic magma was generated by a relatively low degree of partial melting (4–6%) of a peridotite source of the spinel to garnet transition, but closer to garnet-facies peridotite at high temperature (~1580°C) and pressure (~2.88Gpa) conditions. In combination with the regional geology, we infer that the picritic porphyrites were generated in the transitional phase between the termination of the Paleo-Tethys Ocean evolution and the onset of the Neo-Tethys Ocean evolution, when the break-off of the Jinshajiang oceanic slab that subducted westward underneath the India plate during the late Triassic to early Jurassic opened a slab window allowing asthenospheric upwelling and partial melting.