Geochronology, geochemistry and petrogenesis of Miocene adakitic rocks in Milashan, Southern Tibet

Abstract

中新世是青藏高原隆升、增厚的重要时期，并且在这一时期内拉萨地块广泛发育碰撞后岩浆岩。本文对南拉萨地块米拉山地区的钙碱性钾质火山岩进行了锆石U-Pb年代学、Lu-Hf同位素和全岩主量、微量元素的测定与系统研究。米拉山中新世火山岩为粗面英安岩、英安岩和流纹岩（SiO₂=59.89%~71.78%）。锆石U-Pb定年结果为16.1±0.2Ma~20.4±0.3Ma，表明其喷发时代为中新世。岩石具有较高的Al₂O₃含量（13.54%~16.31%），低MgO（0.46%~1.95%）、高Sr（388×10^-6~804×10^-6）、低Y（6.55×10^-6~11.20×10^-6）和Yb（0.70×10^-6~1.07×10^-6）的特征，具有较高的Sr/Y值（51~80）、低相容元素（Cr=4.26×10^-6~32.53×10^-6，Ni=4.16×10^-6~25.75×10^-6）和弱Eu负异常。岩石具有轻稀土元素和Rb、Th、U、K等元素富集、重稀土元素和高场强元素Nb、Ta、Ti亏损的特征。米拉山中新世火山岩显示出埃达克质岩石的地球化学特征，可能来自于的镁铁质加厚下地壳的部分熔融，推测下地壳源区是石榴石角闪岩。锆石ε_Hf（t）值为+2.2~+7.8，表明源区为新生地壳物质，有俯冲板片熔体加入。米拉山中新世火山岩的喷发时代与米拉山断裂活动时间一致，二者可能同为拉萨地块岩石圈拆沉的结果。;Miocene is an important period for the uplifting and thickening of the Tibetan Plateau, and meanwhile, post-collisional igneous rocks are widely developed. The zircon U-Pb dating and Lu-Hf isotopes, and whole rock major and trace element compositions were analyzed on the calc-alkaline potassic volcanic rocks in the Milashan area, Southern Lhasa terrane. Milashan Miocene volcanic rocks are trachydacite, dacite and rhyolite, with intermediate to acid composition and SiO₂ ranging from 59.89% to 71.87%. The zircon U-Pb dating yielded ages from 16.1±0.2Ma to 20.4±0.3Ma. All samples exhibit high Al₂O₃ (13.54%~16.31%), low MgO (0.46%~1.95%), high Sr(388×10^-6~804×10^-6), low Y (6.55×10^-6~11.20×10^-6) and Yb (0.70×10^-6~1.07×10^-6), high Sr/Y ratios, low compatible element concentration (Cr=4.26×10^-6~32.53×10^-6, Ni=4.16×10^-6~25.75×10^-6) and slight negative Eu anomaly. The samples are characterized by enrichment of light rare earth elements and large ion lithophile element (LILEs, Rb, Th, U, K), and depletion of heavy rare earth elements and high field strength element (HFSEs, Nb, Ta, Ti). These Miocene volcanic rock samples show adakitic affinity, and may have been formed by partial melting of mafic thickened lower crust. The source region is probably garnet amphibolite. Lu-Hf isotopic determination yielded positive zircon ε_Hf(t) values ranging from +2.2 to +7.8, indicating that the source was juvenile crust with the addition of subducted slab melt. Milashan Miocene volcanic rocks were formed at the same time as the action of Milashan fault, which probably suggests that both Milashan Miocene volcanic rocks and Milashan fault are caused by the delamination of Lhasa terrane lithosphere.