Large-volume and swift magmatic response to Late Cenozoic segmentation of the subducted Neotethyan oceanic slab: evidence from the Galatian Volcanic Province, northwestern Turkey

The Miocene Galatian Volcanic Province (GVP) is one of the largest volcanic provinces in central-western Anatolia, with an extent of ~ 8,900 km2. The volcanic activity is extended from 22.5 to 7.5 Ma. The volcanic compositions straddle the alkaline-subalkaline fields, from basic to acid compositions and mostly transitional to sodic affinity. Major oxides show good correlation with SiO2 indicating prolonged effects of fractional crystallization. Primitive mantle-normalized multi-element patterns indicate overall similarities among the different samples of the three geographic sectors, sharing strong negative anomalies in Nb–Ta–Ti, strong positive peaks at Cs and K, coupled with a common, albeit not always present, positive anomaly at Pb. Mineral-melt geothermobarometric estimates indicates ~1070–1235°C and ~7–19 kbar for melting conditions of basaltic compositions and ~1000–1150°C and ~3–12 kbar for andesitic-dacitic rocks. The absence of correlation between radiogenic isotopes and SiO2 and MgO is here interpreted as consequence of assimilation-fractional-crystalization processes involving lower continental crust as contaminant. The GVP parental magmas are generated from ~2% to 10% partial melting of a lherzolitic mantle with high spinel/garnet ratio based on intra-REE fractionation constraints. The subduction-related metasomatism inferred for the GVP mantle sources based on their chemistry is interpreted to be linked to the northward subduction of the northern branch of the Neo-Tethys slab. Successive slab retreat resulted in extension for the critical stress distribution through the Cyprus slab, favouring magma propagation for the GVP volcanic region. The eventual break-off of the slab after the continent-continent collision of Arabian and Eurasian plates could have caused a toroidal mantle flow, favouring the widely distributed 15–16 Ma alkaline magmatism in the eastern GVP, associated with passive hot asthenospheric upwelling imaged by teleseismic P-wave tomography.

中新世加拉塔火山省（Galatian Volcanic Province, GVP）是安纳托利亚中西部规模最大的火山省之一，分布面积约8900 km²。其火山活动时限为22.5至7.5 Ma。火山岩成分横跨碱性-亚碱性区间，涵盖基性至酸性岩石，且多数表现为向钠质亲和性过渡的特征。主量氧化物与SiO₂呈现良好相关性，指示分离结晶作用的长期持续影响。原始地幔标准化多元素配分模式显示，三个地理区域的不同样品整体特征相似：均存在Nb–Ta–Ti的显著负异常、Cs和K处的强正峰值，且普遍在Pb处存在正异常（不过并非所有样品均出现该特征）。矿物-熔体温压计量估算结果表明：玄武质岩石的熔融条件为~1070–1235℃、~7–19 kbar（千巴），安山岩-英安岩的熔融条件则为~1000–1150℃、~3–12 kbar。本次研究将放射成因同位素与SiO₂、MgO之间无相关性，归因于以下陆地壳作为混染端元的同化-分离结晶作用。加拉塔火山省的母岩浆源自尖晶石/石榴石比值较高的方辉橄榄岩型地幔的2%~10%部分熔融，该结论基于稀土元素分馏约束得到。基于地球化学特征推断，加拉塔火山省的地幔源区经历了俯冲相关的交代作用，该作用与新特提斯洋北支板片的北向俯冲存在关联。后续的板片回撤使得塞浦路斯板片内的临界应力分布发生伸展，有利于加拉塔火山省火山区域的岩浆运移。阿拉伯板块与欧亚板块发生陆陆碰撞后，板片最终发生断离，这一过程可能引发环形地幔流，进而促进了加拉塔火山省东部广泛分布的15–16 Ma碱性岩浆活动，该活动与远震P波层析成像所揭示的被动热软流圈上涌相关。