深俯冲陆壳部分熔融初始熔体的厘定:来自苏鲁超高压地体混合岩中浅色体证据

摘要

深俯冲陆壳物质部分熔融产生的熔体,实验岩石学方面已有广泛报道,而天然初始熔体的组分却难以厘定.对此,本文从苏鲁超高压地体荣成混合岩中识别出了深俯冲花岗质陆壳部分熔融产生的天然初始熔体组成.野外露头显示,混合岩中主要矿物组成为钾长石+斜长石+石英的浅色熔体呈不连续的条带状与残余体互层产出,指示了原位或近源区的部分熔融特征.混合岩浅色体锆石CL图像呈明显的核-边结构,继承核部为扬子板块来源的岩浆锆石,形成时代为721±24Ma;新生边部CL图像具震荡环带结构,微量元素上REE呈明显左倾,具有Eu的负异常及Ce的正异常,低的Hf/Y和Th/U比值,具深熔锆石特征,指示形成于花岗质陆壳物质的部分熔融.边部U-Pb谐和年龄为225.9±2Ma,略晚于苏鲁超高压地体超高压峰期变质年龄,表明初始熔融发生在超高压地体折返早期.浅色熔体的全岩地球化学特征表明,主量元素上具有高SiO2、K2O及Na2O含量,低的Fe2O3T、MgO及CaO含量,A/CNK=1.02 ～ 1.04,呈弱过铝质亚碱性花岗岩的特征,这与实验岩石学中富硅陆壳物质部分熔融产生的熔体组分极为相近;微量元素上富集大离子亲石元素(如Rb、Ba、Pb等),亏损Nb、Ta、Ti等高场强元素,REE呈较为平坦的配分模式,具弱的Eu负异常并亏损Sr.本文通过上述对天然样品研究,厘定了深俯冲花岗质陆壳部分熔融及其初始熔体的组成,为理解大陆俯冲带壳幔相互作用提供了关键依据.%Partial melts from the continental crusts have been widely reported by experimental studies,however,components of natural incipient melts are hard to be determined.Here,we recognized the natural incipient melts of the deeply subducted granitic crust from the migmatite in Rongcheng,Sulu ultra-high pressure (UHP) terrane.Field outcrops show that the leucosomes,which are mainly composed of K-feldspar,plagioclase and quartz,are discontinuous interlayered with the melanosomes,indicating in situ or in source partial melting.CL images of zircons in the leucosomes reveal distinct core-rim texture.The inherited zircon domains are typical magmatic zircons and have a concordant age of 721 ± 24Ma.The new growth rims of zircons with oscillatory zoning in CL images show HREE enriched patterns with negative Eu anomalies,positive Ce anomalies and have low Hf/Y,Th/U ratios,these characteristics are generally similar to the core domains and indicate the recrystallization of partial melts of the granitic crust materials.The weight mean age of rim domains is 225.9 ± 2Ma,which is slightly younger than the peak metamorphism age of Sulu UHP terrane,indicating the partial melting event can date back to the early stage of exhumation.The leucosomes have high SiO2,K2O and Na2O content,low Fe2O3T,MgO and CaO content with alumimum saturation index (A/CNK) from 1.02 to 1.04,which is consistent with the melts generated by partial melting of Si-rich curst materials in experimental petrology studies;besides,the leucosomes are rich in LILE (e.g.,Rb,Ba,Pb),deplete in HFSE (e.g.,Nb,Ta,Ti) and show flat REE patterns with weak negative Eu anomalies.According to the study of these natural leucosomes,the incipient melts from partial melting of the deeply subducted granitic crust are recognized,and they can provide some inspiration to understand the crust-mantle interaction in the continental subduction zone.