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首页> 外文期刊>Contributions to Mineralogy and Petrology >Evolution of the Taupo Volcanic Center, New Zealand: petrological and thermal constraints from the Omega dacite
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Evolution of the Taupo Volcanic Center, New Zealand: petrological and thermal constraints from the Omega dacite

机译：新西兰陶波火山中心的演变：欧米茄da石的岩石学和热学约束

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摘要

The 20 ka ~0.1 km3 Omega dacite, which erupted shortly after the 26.5 ka Oruanui super-eruption, compositionally stands out among Taupo Volcanic Zone (TVZ) magmas, which are overwhelmingly characterized by rhyolites (>90 % by volume). The previously reported presence of inherited zircons in this zircon-undersaturated magma has provided unequivocal evidence for the involvement of upper-crustal material in a 1–10 year timescale prior to the Omega eruption. However, whether this crustal involvement is characterized by wholesale, melting of preexisting crust or subordinate bulk assimilation into an already differentiated magma body remains unclear. To disentangle these processes, we describe the mineral chemistry of the major phases present in the Omega dacite and determine intensive parameters describing magma chamber conditions. Dominantly unimodal populations of plagioclase (An50–60), orthopyroxene (Mg# from 58 to 68), and clinopyroxene (Mg# from 65 to 73), along with coexisting equilibrium pairs of Fe–Ti oxides, constrain pre-eruptive temperatures to 850–950 °C, a pressure between ~3 and 7 kbars, and an oxygen fugacity of ~NNO. MELTS thermodynamic modeling suggests that this phase assemblage is in equilibrium with the bulk rock and glass compositions of the Omega dacite at these estimated P–T–fO2 pre-eruptive conditions. Combining these petrological observations with insights into conductive thermal models of magma–crust interactions, we argue that the Omega dacite more likely formed in the mid-to-lower crust via protracted processing through fractional crystallization coupled with some assimilation (AFC). Incorporation of crustal material is likely to have occurred at various stages, with the inherited zircons (and potentially parts of glomerocrysts) representing late and subordinate upper-crustal assimilants. This petrogenetic model is consistent with the presence of a differentiating crustal column, consisting of a polybaric fractional crystallization and assimilation history. On the basis of petrological, thermal, and geophysical considerations, upper-crustal reservoirs, which feed large-scale rhyolitic volcanism in the TVZ, most likely take the form of large, long-lived crystal mush zones. Following large eruptions, such as the Oruanui event, this mush is expected to crystallize significantly (up to 70–80 vol% crystals) due to syn-eruptive decompression. Hence, the Omega dacite, immediately post-dating the Oruanui event, potentially represents incoming deeper recharge of less-evolved magma that was able to penetrate the nearly solidified upper-crustal mush. Over the past 20,000 years, similar intermediate recharge magmas have incrementally reheated, reconstructed, and reactivated the upper-crustal mush zone, allowing a gradual return to rhyolitic volcanism at the Taupo Volcanic Center.
机译：20ka〜0.1 km3的欧米茄da榴石在26.5 ka Oruanui超级喷发后不久爆发，在陶波火山带（TVZ）岩浆中成分突出，其特征是流纹岩（> 90％）占绝大多数。先前报道的这种锆石欠饱和岩浆中存在继承的锆石，已经为欧米茄喷发之前的1-10年内涉及上地壳物质提供了明确的证据。但是，这种地壳活动的特征是大量地存在，已经存在的地壳融化还是从属的大量同化作用融化成已经分化的岩浆体，这一点尚不清楚。为了弄清这些过程，我们描述了欧米茄da石中主要相的矿物化学，并确定了描述岩浆室条件的密集参数。主要的单峰种群斜长石（An50–60），邻苯二茂（Mg＃从58到68）和斜ino（Mg＃从65到73），以及共存的Fe-Ti氧化物平衡对将喷发前温度限制在850 –950°C，约3至7 kbars的压力，和〜NNO的氧气逸度。 MELTS热力学模型表明，在这些估计的P–T–fO2喷发前条件下，该相组合与欧米茄mega石的块状岩石和玻璃成分平衡。将这些岩石学观测结果与对岩浆－地壳相互作用的传导热模型的见解相结合，我们认为，通过分步结晶和一定程度的同化作用（AFC）进行长时间的加工，欧米伽da榴石更有可能在中下部地壳中形成。地壳物质的合并可能在各个阶段发生，继承的锆石（以及潜在的部分珠状晶）代表了晚期和下级上地壳同化剂。该成岩模型与存在差异的地壳柱相一致，该地壳柱由多级分馏结晶和同化历史组成。基于岩石学，热学和地球物理方面的考虑，在TVZ上为大型流纹岩火山岩提供营养的上地壳储层很可能采取长寿命的大型晶体糊状带的形式。在大爆发之后，例如奥鲁阿努伊事件，由于协同降压作用，预计该糊状物会明显结晶（高达70-80％（体积）晶体）。因此，在奥鲁阿努伊事件发生后立即发生的欧米茄da榴石可能代表了演化较深的岩浆的更深层补给，它能够穿透几乎凝固的上地壳​​浆。在过去的20,000年中，类似的中间补给岩浆已逐渐重新加热，重建并重新激活了上地壳糊状区，从而使陶波火山中心逐渐恢复了流纹质火山作用。

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