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Petrogenesis of Early Carboniferous Ultramafic–Mafic Volcanic Rocks in the Southern Changning–Menglian Belt, Southeastern Tibetan Plateau: Implications for the Evolution of the Paleo-Tethyan Ocean
Acta Geologica Sinica-English Edition ( IF 3.5 ) Pub Date : 2021-06-21 , DOI: 10.1111/1755-6724.14780 Zhipeng XIE 1 , Xuelu WU 1 , Tengfei WANG 1 , Chuandong XUE 1 , Tiannan YANG 2 , Wei WANG 1 , Binxi SUI 1 , Di XIN 2
Acta Geologica Sinica-English Edition ( IF 3.5 ) Pub Date : 2021-06-21 , DOI: 10.1111/1755-6724.14780 Zhipeng XIE 1 , Xuelu WU 1 , Tengfei WANG 1 , Chuandong XUE 1 , Tiannan YANG 2 , Wei WANG 1 , Binxi SUI 1 , Di XIN 2
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The Changning–Menglian Belt represents the main Paleo-Tethyan Suture in the southeastern Tibetan Plateau, which divides Gondwana- and Eurasia-derived continental fragments from each other. The belt contains ultramafic–mafic volcanic rocks that provide evidence of the tectonic processes which operated during the evolution of the Paleo-Tethyan Ocean. New geochemical data for Early Carboniferous volcanics in the southern Changning–Menglian Belt show that wehrlites have cumulate and poikilitic textures, and contain low-Fo (84.2–87.2) olivine, clinopyroxene with low Mg# values (79.4–85.6), and spinel with high Cr# values (67.8–72.4). Estimated equilibrium temperatures obtained using olivine-spinel Fe-Mg exchange geothermometry range from 978°C to 1373°C (mean = 1205°C; n = 3). These observations combined with a lack of reaction or melt impregnation textures indicate that these units represent magmatic cumulates rather than having formed as a result of mantle-melt reactions. Both wehrlites and basalts in the belt have subparallel rare earth element (REE)-and primitive-mantle-normalized multi-element patterns with slightly positive Nb-Ta anomalies, but negligible Eu and Zr-Hf anomalies. The volcanics have similar Sr-Nd-Pb isotopic compositions with εNd(t) values of 4.2–4.5 (mean = 4.3; n = 3) and 4.0–4.4 (mean 4.2; n = 4), respectively, and also have similar immobile element ratios, such as Nb/La, Nb/U, Th/La, Zr/Nb, Th/Ta, La/Yb, Nb/Th, Nb/Y, and Zr/Y. These characteristics indicate both units have ocean island basalt (OIB)-like geochemical affinities, consistent with the fact that the clinopyroxene in the wehrlites is compositionally similar to OIB-related cumulus clinopyroxene. This suggests that both the wehrlites and basalts were derived from similar parental magmas that underwent generally closed-system magmatic differentiation dominated by fractionation of the olivine and clinopyroxene. This parental magma was likely generated in an oceanic seamount setting from an OIB-type mantle source (i.e., asthenospheric mantle) containing garnet-spinel lherzolite material. Combing this new data with that from oceanic seamount volcano-sedimentary suites derived from previous research enables the identification of a mature late Paleozoic ocean basin between the passive northeastern Gondwanan margin and the northward-migrating microcontinent of Lanping–Simao.
中文翻译：
青藏高原东南部长宁-孟连带南部早石炭世超镁铁质-镁铁质火山岩的岩石成因：对古特提斯洋演化的启示
长宁-孟连带代表了青藏高原东南部的主要古特提斯缝合带，它将冈瓦纳和欧亚大陆的大陆碎片彼此分开。该带包含超镁铁质-镁铁质火山岩，为古特提斯洋演化过程中的构造过程提供了证据。长宁-孟连带南部早石炭世火山岩的新地球化学数据表明，wehrlite具有堆积和多晶石结构，并含有低Fo（84.2-87.2）橄榄石、低Mg #值（79.4-85.6）的单斜辉石和尖晶石。高 Cr #值 (67.8–72.4)。使用橄榄石-尖晶石 Fe-Mg 交换地热测量法获得的估计平衡温度范围为 978°C 至 1373°C（平均值 = 1205°C；n= 3)。这些观察结果与缺乏反应或熔体浸渍纹理相结合，表明这些单元代表岩浆堆积物，而不是由于地幔熔融反应而形成。带中的wehrlites和玄武岩都具有亚平行稀土元素（REE）和原始地幔归一化的多元素模式，具有略为正的Nb-Ta异常，但可以忽略不计的Eu和Zr-Hf异常。火山岩具有相似的 Sr-Nd-Pb 同位素组成，ε Nd ( t ) 值为 4.2–4.5（平均值 = 4.3；n = 3）和 4.0–4.4（平均值 4.2；n= 4), 并且也有相似的固定元素比率, 如 Nb/La, Nb/U, Th/La, Zr/Nb, Th/Ta, La/Yb, Nb/Th, Nb/Y, and Zr /Y。这些特征表明这两个单元都具有类似海洋岛玄武岩 (OIB) 的地球化学亲和力，这与 Wehrlites 中的单斜辉石在组成上与 OIB 相关的积云单斜辉石相似的事实一致。这表明，wehrlites 和玄武岩都来自类似的母岩浆，这些母岩浆经历了以橄榄石和单斜辉石分馏为主的通常封闭系统的岩浆分化。这种母岩浆很可能是在海洋海山环境中由含有石榴石-尖晶石亮辉石材料的 OIB 型地幔源（即软流圈地幔）产生的。
更新日期：2021-06-21
中文翻译：
青藏高原东南部长宁-孟连带南部早石炭世超镁铁质-镁铁质火山岩的岩石成因：对古特提斯洋演化的启示
长宁-孟连带代表了青藏高原东南部的主要古特提斯缝合带，它将冈瓦纳和欧亚大陆的大陆碎片彼此分开。该带包含超镁铁质-镁铁质火山岩，为古特提斯洋演化过程中的构造过程提供了证据。长宁-孟连带南部早石炭世火山岩的新地球化学数据表明，wehrlite具有堆积和多晶石结构，并含有低Fo（84.2-87.2）橄榄石、低Mg #值（79.4-85.6）的单斜辉石和尖晶石。高 Cr #值 (67.8–72.4)。使用橄榄石-尖晶石 Fe-Mg 交换地热测量法获得的估计平衡温度范围为 978°C 至 1373°C（平均值 = 1205°C；n= 3)。这些观察结果与缺乏反应或熔体浸渍纹理相结合，表明这些单元代表岩浆堆积物，而不是由于地幔熔融反应而形成。带中的wehrlites和玄武岩都具有亚平行稀土元素（REE）和原始地幔归一化的多元素模式，具有略为正的Nb-Ta异常，但可以忽略不计的Eu和Zr-Hf异常。火山岩具有相似的 Sr-Nd-Pb 同位素组成，ε Nd ( t ) 值为 4.2–4.5（平均值 = 4.3；n = 3）和 4.0–4.4（平均值 4.2；n= 4), 并且也有相似的固定元素比率, 如 Nb/La, Nb/U, Th/La, Zr/Nb, Th/Ta, La/Yb, Nb/Th, Nb/Y, and Zr /Y。这些特征表明这两个单元都具有类似海洋岛玄武岩 (OIB) 的地球化学亲和力，这与 Wehrlites 中的单斜辉石在组成上与 OIB 相关的积云单斜辉石相似的事实一致。这表明，wehrlites 和玄武岩都来自类似的母岩浆，这些母岩浆经历了以橄榄石和单斜辉石分馏为主的通常封闭系统的岩浆分化。这种母岩浆很可能是在海洋海山环境中由含有石榴石-尖晶石亮辉石材料的 OIB 型地幔源（即软流圈地幔）产生的。
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