Ages and glass compositions for paired large-volume eruptions from the Acigöl volcanic complex, Cappadocia (Turkey)
Ages and glass compositions for paired large-volume eruptions from the Acigöl volcanic complex, Cappadocia (Turkey)
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卡帕多西亚（土耳其）阿西格尔火山群成对大量喷发的年龄和玻璃成分
DOI：
10.1007/s42990-019-00013-5
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发表时间：
2019
期刊：
影响因子：
--
通讯作者：
Wang K.-L.
中科院分区：
文献类型：
--
作者：
Schmitt;Friedrichs;Sparks;Danišík;Yurteri;Gündoğdu;Schindlbeck-Belo;Çobankaya;Wang K.-L.
Acigöl火山杂岩是位于土耳其安纳托利亚中部新近系—第四纪卡帕多奇亚火山省西部边缘的一个双峰火山场。它最大规模的喷发被保存为一对广泛分布的凝灰岩，称为下Acigöl凝灰岩（LAT）和上Acigöl凝灰岩（UAT）。两者均为高硅流纹岩，成分相同；然而，它们可以通过UAT中存在黑曜石岩石而LAT中不存在黑曜石岩石来区分。本文报道了这些凝灰岩在距其源60 ~ 65 km处的中间沉积的发现，并重新估计了LAT的体积在21.4 ~ 26.7 km3范围内（UAT缺乏类似的限制，但假设其体积小于等量）。LAT和UAT之间古土壤的发育为这两个事件之间的间隙提供了证据，并用U-Th和(U-Th)/He锆石年代学组合的放射性定年法进一步支持了这一证据。根据U-Th锆石结晶和(U-Th)/He锆石喷发年龄的序列模拟，修正了LAT的喷发年龄为190±11 ka，而新公布的(U-Th)/He锆石年龄表明UAT的喷发年龄为164±4 ka（不确定度一般为1σ）。以前从整个岩石分析中得知的两次喷发之间的成分相似性也延伸到玻璃的主要、次要和微量元素组成。此外，LAT和UAT在近端和中端位置的U-Th锆石结晶年龄也难以区分。长期平衡晶体的U-Pb锆石年龄表明，其物源来自典型的卡帕多西亚白垩纪花岗岩基底和中新世火山覆盖层，这种异种晶体在LAT地区比UAT地区略常见。LAT和UAT的成分相似，以及UAT事件期间未喷发的LAT岩浆中锆石的明显再循环表明，流纹岩熔体从岩浆系统中连续流出，在较长时间间隔内保持了热成分均匀性。这可以解释为流纹岩熔体从热缓冲的晶体糊状中提取出来，在几千年的时间里，它仍然可以产生大量的高硅喷发。随后Acigöl火山杂岩喷发活动自东向西的迁移也导致岩浆化学变化，引发新的锆石结晶脉冲，表明原有岩浆焦点的放弃，岩浆输入转移到新的位置。大规模的LAT和UAT中间沉积的发现表明，这两个tephra单元都是安纳托利亚及其周围海相同位素阶段（MIS） 6沉积序列的潜在有用的标志层位。最近在黑海沉积物岩心中发现的UAT支持了这一观点。
Acigöl volcanic complex is a bimodal volcanic field located at the western margin of the Neogene–Quaternary Cappadocian volcanic province in Central Anatolia (Turkey). Its most voluminous eruptions are preserved as a pair of widespread tuffs known as Lower Acigöl Tuff (LAT) and Upper Acigöl Tuff (UAT). Both are nearly aphyric high-silica rhyolites that are compositionally identical; they can be distinguished, however, by the presence of obsidian lithics in UAT and the absence thereof in LAT. Here, we report the discovery of medial deposits of these tuffs at ~ 60–65 km distance from their source, and re-estimate the volume for LAT in the range of 21.4–26.7 km3(similar constraints are lacking for UAT, but a subequal volume is assumed). Development of paleosol between LAT and UAT provides evidence for a hiatus between both events which is further supported by radiometric dating using combined U–Th and (U–Th)/He zircon geochronology. The revised age for LAT based on sequence modeling of U–Th zircon crystallization and (U–Th)/He zircon eruption ages is 190 ± 11 ka, whereas new and published (U–Th)/He zircon ages indicate eruption of UAT at 164 ± 4 ka (uncertainties generally stated at 1σ). The compositional similarity between both eruptions previously known from whole-rock analyses also extends to glass major, minor, and trace element compositions. Moreover, U–Th zircon crystallization ages are also indistinguishable between LAT and UAT in proximal and medial locations. U–Pb zircon ages for crystals in secular equilibrium indicate a provenance from Cretaceous granitic basement and Miocene volcanic overburden typical for Cappadocia, and such xenocrysts appear to be slightly more common in LAT compared to UAT. The compositional similarity between LAT and UAT along with the apparent recycling of zircon from the non-erupted LAT magma during the UAT event indicates sequential tapping of rhyolite melt from a magma system that maintained thermal and compositional uniformity over a protracted time interval. This can be explained by rhyolite melt extraction from a thermally buffered crystal mush that remained viable to produce voluminous high-silica eruptions for few 10s of ka. The subsequent migration of eruptive activity in the Acigöl volcanic complex from east to west also led to a change in magma chemistry and triggered a new pulse of zircon crystallization, indicating abandonment of the previous magmatic focus, and a shift of magma input to a new location. The identification of sizable medial deposits of LAT and UAT implies that both tephra units are potentially useful marker horizons for marine isotope stage (MIS) 6 sedimentary sequences in and around Anatolia. The recent discovery of UAT in a Black Sea sediment core supports this view.
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