青藏高原凯蒙蛇绿混杂岩中碱性火山岩的发现及意义

The discovery of alkaline volcanic rocks in the Kaimeng ophiolite mélange,Tibet,and its implications

班公湖-怒江洋的关闭时间直接制约青藏高原早期构造演化的认识。最近,在班公湖-怒江缝合带南侧凯蒙蛇绿混杂岩中发现一碱性火山岩,岩性主要是橄榄粗安岩,具粗面结构,斑晶主要是更长石和少量普通辉石,基质主要由更长石、普通辉石和少量填隙的碱性长石组成,有的具辉绿结构。岩石化学成分较一致,Si O2含量介于51.34%~53.91%之间,Ti O2含量为1.02%~1.55%,具有高Al2O3(17.06%~18.46%)和Na2O(4.90%~6.36%)、低K2O(0.05%~0.88%)含量特点,大多数Mg#大于60,最高68.62,里特曼指数(σ)介于3.65~4.47之间,为碱性系列火山岩;富集Sr、Rb、Ba等大离子亲石元素,亏损Nb、Y、Yb等高场强元素,相对富集Zr、Ti,Nb/U、Zr/Nb、La/Yb等比值稳定,分别为7.45~8.51、15.92~17.26和7.26~8.06;(87Sr/86Sr)i值变化范围较小,介于0.706~0.707之间,(143Nd/144Nd)t值在0.512 368~0.512 548之间,说明源区较为一致,结合Ce/Pb-Si O2图解判断结果,认为凯蒙碱性火山岩具有原始地幔、陆壳和深海沉积物源区混合特征。锆石U-Pb同位素定年结果表明该火山岩年龄为101.8±1.1 Ma,可能形成于洋壳俯冲阶段末期,由大陆边缘陆壳与俯冲洋壳板片断离导致软流圈地幔上涌诱发部分熔融所致,推测班公湖-怒江洋大约在早白垩世晚期关闭。

The closure time of the Bangong Co-Nujiang Tethys Ocean(BNTO)between Lhasa terrane and Qiangtang terrane directly constrains the early tectonic evolution of the Tibetan Plateau.Recently,alkaline volcanic rocks have been recognized from the Kaimeng ophiolitic mélange,a southern sector of Bangong Co-Nujiang suture zone.These rocks are mainly fresh mugearite with trachytic structure.The phenocrysts are mainly oligoclase with minor augite,and the groundmass is mainly composed of oligoclase,pyroxene and trace alkaline feldspar filling other minerals.The chemical compositions of the mugearite are fairly uniform,with the abundances of Si O2and Ti O2ranging from 51.34%to 53.91%and from 1.02%to 1.55%,respectively,suggesting alkaline series volcanic rocks with high Na2O content(4.90%~6.36%),low K2O content(0.05%~0.88%)and intermediat Rittman index(σ=3.65~4.47).The most of Mg#values are higher than 60,even up to 68.62.The trace elements of the mugearite are characterized by relative enrichment of large ion lithophile elements(LILE)Sr,Rb,Ba,Th and U and depletion of high field strength elements(HFSE),with negative Nb and Ta anomalies.The stable Nb/U,Zr/Nb and La/Yb ratios(7.45~8.51,15.92~17.26 and 7.26~8.06,respectively),the initial87Sr/86Sr ratios(0.706~0.707)and the(143Nd/144Nd)tratios(0.512 368~0.512 548)as well as the Ce/Pb-Si O2diagram suggest that the source of the mugearite was probably mixed primitive mantle(PM)and lower continental crust and abyssal sediments.The zircon U-Pb isotopic dating shows that the age of the mugearite is 101.8±1.1 Ma,indicating that it was likely produced by the partial melting of asthenospheric mantle upwelled by the slab break-off at the end of oceanic crust subduction,which implies that the BNTO was closed at the late stage of early Cretaceous period.