摘要
弧安山岩/闪长岩与平均大陆地壳成分相似,破译其岩石成因对理解大陆地壳形成至关重要。本研究报道了青藏高原中部羌塘地块西南缘羌多地区雄巴日侵入岩的年代学、岩相学、全岩主量、微量元素和Sr-Nd同位素地球化学数据。雄巴日侵入岩包括闪长岩和花岗闪长岩,锆石U-Pb定年结果显示,两类岩石具有相似的结晶年龄,分别为107.3±1.0 Ma和106.0±1.0 Ma,均形成于早白垩世晚期。岩石属钙碱性系列,富集轻稀土元素,重稀土元素无明显分异,具有弱Eu负异常,富集大离子亲石元素(LILE)和Pb,亏损Nb、Ta、Ti等高场强元素(HFSE)。岩石具有富集的Sr-Nd同位素组成((^(87)Sr/^(86)Sr)_(i)=0.705747~0.709546,ε_(Nd)(t)=−4.50~−2.10)。结合南羌塘早白垩世晚期的闪长岩地球化学特征,认为包括雄巴日闪长岩在内的中性岩由俯冲混杂岩(洋中脊玄武岩(MORBs)、沉积物和地幔楔橄榄岩的机械混合物)部分熔融形成,而花岗闪长岩通过安山质岩浆分离结晶作用形成。早白垩世早期,北向俯冲的班公湖-怒江特提斯洋岩石圈板片发生平板俯冲,这一过程有利于俯冲隧道中混杂岩大量形成。早白垩世晚期,板片回卷引起软流圈地幔上升和热角流,进而触发底辟混杂岩熔融。南羌塘早白垩世中性岩浆岩总体上具有类似大陆地壳安山岩的成分,因此,混杂岩底辟部分熔融模型有力支撑陆壳生长的安山岩模型。原始安山质熔体或直接形成闪长岩/安山岩,或进一步结晶分异形成中酸性岩浆岩。
Continental crust is commonly considered to have an andesitic to dacitic composition,yet the question of when and how it formed remains the topic of considerable debate.Andesitic plutonic and volcanic rocks in subduction zones share striking compositional similarities with the bulk continental crust.Therefore,deciphering the petrogenesis of andesites and their plutonic equivalents is fundamental to understanding the origin of the andesitic signature of continental crust.In this study,we present a comprehensive investigation,including detailed field geological surveys,geochronological,mineralogical,whole-rock geochemical,and Sr-Nd isotopic studies of the Xiongbari diorites and granodiorites in the Southern Qiangtang,Central Qinghai-Xizang.LA-ICP-MS zircon U-Pb dating indicates that they crystallized during the late Early Cretaceous(ca.107 Ma).The Xiongbari intermediate-felsic intrusive rocks have calc-alkaline compositions,characterized by relative enrichment in light rare earth elements with slightly negative Eu anomalies,large ion lithophile elements,and relative depletion in high-field-strength elements.They exhibit relatively low initial ^(87)Sr/^(86)Sr ratios(0.705747-0.709546)and variably negativeε_(Nd)(t)values(−4.50 to−2.10).We suggest that the late Early Cretaceous dioritic rocks in the Southern Qiangtang(including the Xiongbari diorites)were derived from partial melting of the mélange(a physical mixture of oceanic crust,subducted sediments,and mantle-wedge peridotites).The Xiongbari granodiorites are interpreted to form via fractional crystallization of primary mantle-derived andesitic magmas.In the early Early Cretaceous,the Bangong-Nujiang Tethyan oceanic lithospheric slab underwent flat subduction,a process conducive to the abundant formation of mélanges in the subduction channel.In the late Early Cretaceous,upwelling asthenospheric mantle and hot corner flow during the slab roll-back heated the mélange diapir,whose partial melting then formed the andesitic magmas.Importantly,the Early Cretaceous intermediate magmatic rocks in the Southern Qiangtang area have compositions similar to the bulk continental crust.Therefore,the mélange partial melting model strongly supports the hypothesis of andesitic continental crust growth.The primary andesitic melts either formed diorites/andesites or underwent fractional crystallization of the andesitic magmas to form intermediate-acidic magmatic rocks.
作者
杨其济
郝露露
王强
刘懋锐
孙鹏
YANG Qiji;HAO Lulu;WANG Qiang;LIU Maorui;SUN Peng(State Key Laboratory of Deep Earth Processes and Resources,Guangzhou Institute of Geochemistry,Chinese Academy of Sciences,Guangzhou 510640,Guangdong,China;University of Chinese Academy of Sciences,Beijing 100049,China)
出处
《地球化学》
北大核心
2025年第5期695-713,I0009,I0010,共21页
Geochimica
基金
第二次青藏高原综合科学考察研究项目(2019QZKK0702)
中国科学院青年创新促进会项目(2022357)联合资助。
关键词
地壳生长
俯冲混杂岩
板片回卷
南羌塘地块
青藏高原中部
crustal growth
subduction mélange
slab roll-back
Southern Qiangtang
Central Qinghai-Xizang Plateau
