Initial subduction-related magmatism in southern Alaska identified by geochemistry and zircon Hf-O isotopes

初始俯冲岩浆的地球化学和锆石Hf-O同位素制约

Abundant arc-type magmatic and metamorphic rocks exist on Earth today,which provide insights into the equilibrium state of the subduction process.However,magmatic samples generated during the initial stage of subduction is largely unknown.This hinders our understanding of the subduction initiation process and by inference,the onset of plate tectonics as well as the history of crustal formation.To address this issue,we carried out a comprehensive geochemical-geochronological study of a suite of Late Triassic to mid-Jurassic plutonic rocks from southern Alaska that potentially represent magmas from the initial to mature stages of arc formation.While all studied samples show typical arc-type geochemical signatures,i.e.,enrichment of large ion lithophile elements(LILEs)and depletion of high field strength elements(HFSEs)relative to the heavy rare earth elements(HREEs),the Late Triassic trondhjemites show unique geochemical features such as strongly positiveε_(Hf)(t)andε_(Nd)(t)coupled with lowerδ^(18)O(average 4.77‰±0.09‰).These signatures,along with its higher zircon saturation temperatures compared with younger plutonic rocks,are best explained by shallow partial melting of subducting high-temperature hydrothermally altered lower oceanic crust(i.e.,gabbro).If true,these surprising findings would open up new ways to study subduction initiation which would have important bearing on future research on the onset of global plate tectonics and the formation of the continental crust.

板块俯冲起始是全球板块构造最不为人所知的关键过程之一.岛弧型岩浆岩和变质岩为研究成熟阶段的俯冲过程提供了有效制约,但俯冲初期或起始阶段形成的岩浆岩却知之甚少,严重制约了人类对全球板块构造如何启动的认知.针对这一问题,我们对长期经历大洋板块俯冲的阿拉斯加进行详细野外考察,选择有可能代表俯冲初期至成熟阶段的中生代岩体进行了系统同位素年代学和地球化学研究.结果表明,这些样品都具有典型的岛弧岩浆的地球化学特征,但晚三叠世岩石具有亏损Hf-Nd同位素、低于地幔值的O同位素和高的锆石饱和温度,明显区别于俯冲成熟阶段的岩浆岩,提出其来源于俯冲初期俯冲板块中高温热液蚀变辉长岩的部分熔融.这一发现为研究俯冲起始开辟了新的途径,将对全球板块构造的起始和大陆地壳的形成等研究产生重要影响.