The temporal record of magmatism at Cerro Uturuncu, Bolivian Altiplano

Twenty-six new ⁴⁰Ar/³⁹Ar plateau ages for 23 lavas and domes from the Uturuncu volcano in the Altiplano of SW Bolivia reveal a protracted eruptive history from 1050±5 to 250±5 ka. Eruptions have been exclusively effusive, producing some 50 km³ of high-K dacites and silicic andesites. Bimodal mineral compositions, complex mineral textures, the presence of andesitic magmatic enclaves within dacites and linear chemical trends on binary element plots all indicate that magma mixing is an important petrogenetic process at Uturuncu. Post-458 ka, distinct high and low MgO–Cr magmas are resolved. These magmas erupt during similar times, suggesting that eruptions are tapping different parts of the magma system, albeit from the same vent system. Volcanic and petrological features are consistent with the existence of a vertically extensive magma mush column beneath Uturuncu, and calculated buoyancy forces are sufficient to drive effusive eruptions. Eruptive activity is episodic, with six eruptive periods separated by hiatuses of >50 kyr. Cumulative volume curves demonstrate that the majority of the edifice formed between 595 and 505 ka. The episodicity of eruptions is most likely to be related to fluctuations in the magma supply to the underlying Altiplano–Puno Magma Body.

对玻利维亚西南部阿尔蒂普拉诺高原（Altiplano）乌图伦古火山（Uturuncu volcano）的23处熔岩与穹丘，本研究获取了26组新的⁴⁰Ar/³⁹Ar坪年龄（⁴⁰Ar/³⁹Ar plateau ages），结果显示该火山的喷发历史跨度为1050±5 ka至250±5 ka，持续时间漫长。其喷发方式均为喷溢式，累计产出约50 km³的高钾英安岩（high-K dacites）与长英质安山岩（silicic andesites）。双峰式矿物组成（bimodal mineral compositions）、复杂的矿物结构（complex mineral textures）、英安岩中产出的安山岩质岩浆包体（andesitic magmatic enclaves），以及二元元素图解（binary element plots）上呈现的线性化学趋势，均表明岩浆混合是乌图伦古火山重要的成岩过程（petrogenetic process）。458 ka之后，可识别出两类差异显著的高MgO-Cr与低MgO-Cr岩浆。两类岩浆喷发时段相近，指示尽管喷发均源自同一喷口系统（vent system），但岩浆活动分别汲取了岩浆系统（magma system）内不同区域的岩浆。乌图伦古火山的火山学与岩石学特征（volcanic and petrological features），与该火山下方存在垂直延展的岩浆 mush 柱（magma mush column）的推论相符，且计算得到的浮力（buoyancy forces）足以驱动喷溢式喷发。喷发活动具有间歇性，共划分为6个喷发期，各期之间的喷发间隔均大于50 kyr。累积体积曲线（cumulative volume curves）显示，该火山锥（edifice）的主体形成于595 ka至505 ka之间。喷发的间歇性最可能与下方阿尔蒂普拉诺-普诺岩浆体（Altiplano–Puno Magma Body）的岩浆供给波动存在关联。