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首页> 外文期刊>Contributions to Mineralogy and Petrology >Shoshonite and sub-alkaline magmas from an ultrapotassic volcano: Sr-Nd-Pb isotope data on the Roccamonfina volcanic rocks, Roman Magmatic Province, Southern Italy
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Shoshonite and sub-alkaline magmas from an ultrapotassic volcano: Sr-Nd-Pb isotope data on the Roccamonfina volcanic rocks, Roman Magmatic Province, Southern Italy

机译：超钾质火山中的shoshonite和亚碱性岩浆：意大利南部罗马岩浆省Roccamonfina火山岩上的Sr-Nd-Pb同位素数据

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摘要

The Roccamonfina volcano is characterised by two stages of volcanic activity that are separated by volcano-tectonic caldera collapses. Ultrapotassic leucite bearing rocks are confined to the per-caldera stage and display geochemical characteristics similar to those of other volcanoes in the Roman Province. After the major sector collapse of the volcano, occurred at can. 400 ka, shoshonitic rocks erupt from cinder cones and domes both within the caldera and on the external flanks of the per-caldera Roccamonfina volcano. On the basis of new trace element and Sr-Nd-Pb isotope data, we show that the Roccamonfina shoshonitic rocks are distinct from shoshonites of the Northern Roman Province, but are very similar to those of the Neapolitan volcanoes. The last phases of volcanic activity erupt sub-alkaline magmas as enclaves in trachytic domes, and as lavas within the Monte Santa Croce dome. Ultrapotassic rocks of the per-caldera composite volcano are plagioclase-bearing leucitites characterised by high levels of incompatible trace elements with an orogenic signature having troughs at Ba, Ta, Nb, and Ti, and peaks at Cs, K, Th, U, and Pb. Initial values of ~87Sr/~86Sr range from 0.70926 to 0.70999, ~143Nd/~144Nd ranges from 0.51213 to 0.51217, while the lead isotope rations vary between 18.788-18.851 for ~206Pb/~204Pb, 15.685-15.701 for ~207Pb/~204Pb, and 39.048-39.076 for ~208Pb/~204Pb. Shoshonites show a similar pattern of trace element depletions and enrichments to the earlier ultrapotassic leucite-bearing rocks but have a larger degree of differentiation and lower concentrations of incompatible trace elements. On the other hand, shoshonitic rocks have Sr, Nd, and Pb isotopes consistently different than per caldera ultrapotassic leucite-bearing rocks. ~87Sr/~86Sr ranges from 0.70665 to 0.70745, ~143Nd/~144Nd ranges from 0.51234 to 0.51238, ~206Pb/~204Pb ranges from 18.924 to 19.153, ~207Pb/~204Pb ranges from 15.661 to 15.694, and ~208Pb/~204 Pb ranges from 39.084 to 39. 212. High-K calcalkaline samples have intermediate isotopic values between ultrapotassic plagioclase leucitites and shoshonites, but the lowest levels of incompatible trace element contents. It is argued that ultrapotassic magmas were generated in a modified lithospheric mantle after crustal-derived metasomatism. Interaction between the metasomatic agent and lithospheric upper mantle produced a low-melting point metasomatised veined network. The partial melting of the veins alone produced per-caldera leucite-bearing ultrapotassic magmas. It was possibly triggered by either post collisional isotherms relaxation or increasing T deg due increasing heat flow through slab tears. Shoshonitic magmas were generated by further melting, at higher temperature, of the same metasomatic assemblage with addition 10-20 percent of OIB-like astenospheric mantle material. We suggest that addition of astenospheric upper mantle material from foreland mantle, flowing through slab tearing after collision was achieved.
机译：罗卡莫非那火山的特征是火山活动的两个阶段，这些阶段被火山构造的破火山口塌陷分开。含超钾白云岩的岩石被限制在火山口阶段，并显示出与罗马省其他火山相似的地球化学特征。火山的主要部分坍塌后，就可以发生了。火山口内和火山口Roccamonfina火山外侧面的煤渣锥和圆顶喷出了约400 ka的火山岩。根据新的痕量元素和Sr-Nd-Pb同位素数据，我们显示Roccamonfina shoshonitic岩石与北罗马省的shoshonite不同，但与那不勒斯火山的岩石非常相似。火山活动的最后阶段喷出了亚碱性岩浆，形成了穿越的穹顶中的飞地和蒙特卡罗塞峰中的熔岩。火山口复合火山的超钾盐岩是斜长石的辉绿岩，其特征是高水平的不相容微量元素，其造山特征在Ba，Ta，Nb和Ti处具有波谷，在Cs，K，Th，U和铅〜87Sr /〜86Sr的初始值为0.70926至0.70999，〜143Nd /〜144Nd的范围为0.51213至0.51217，而〜206Pb /〜204Pb的铅同位素比在18.788-18.851之间，〜207Pb /〜的铅同位素比为15.685-15.701。 204Pb和〜208Pb /〜204Pb的39.048-39.076。 shoshonites与早期含钾的白钾矿岩相比，痕量元素的富集和富集模式相似，但分化程度较高，不相容元素的含量较低。另一方面，生铁质岩石中的Sr，Nd和Pb同位素与每个破火山口超钾质白云母岩石始终不同。 〜87Sr /〜86Sr的范围为0.70665至0.70745，〜143Nd /〜144Nd的范围为0.51234至0.51238，〜206Pb /〜204Pb的范围为18.924至19.153，〜207Pb /〜204Pb的范围为15.661至15.694，以及〜208Pb /〜204 Pb的范围从39.084到39.212。高K钙碱样品在超钾斜长石斜辉石和钠钾长石之间的同位素值中等，但不相容微量元素的含量最低。有人认为，在地壳衍生的交代作用之后，在改良的岩石圈地幔中产生了超钾质岩浆。交代剂与岩石圈上地幔之间的相互作用产生了一个低熔点的交代脉状网络。仅静脉的部分融化就产生了含全火山灰的白钾超浆岩浆。这可能是由于碰撞后的等温线松弛或由于流过平板裂缝的热量增加而导致T度升高所致。通过在更高的温度下进一步熔化相同的交代组合，再添加10-20％的OIB样的流星地幔材料，产生了火山岩浆。我们建议实现从前陆地幔中添加地幔层上地幔物质，并在碰撞后流过平板撕裂。

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