西藏日阿窄铅多金属矿区古新世岩浆岩岩石成因及意义

The genesis and significance of the Paleocene granitoids in the Riazhai Pb polymetallic deposit,southern Tibet

日阿窄Pb多金属矿床位于南冈底斯中新世斑岩Cu-Mo-Au成矿带,发育花岗闪长岩、斑状花岗岩以及石英斑岩等多种岩浆岩。为研究矿区内岩浆活动及其成因意义,对区内岩浆岩进行了SHRIMP年代学、主微量元素以及Sr-Nd同位素地球化学研究,结果表明:花岗闪长岩(62. 70±0. 78Ma)、镁铁质包体、斑状花岗岩(60. 43±0. 86Ma)以及石英斑岩(60. 30±1. 20Ma)是南冈底斯古新世岩浆活动的产物,显示从基性到中酸性的演化趋势(SiO2=53. 43%~76.46%)。岩浆岩Na2O+K2O含量介于4. 43%~6.88%之间,铝饱和指数A/CNK集中于0.90~1.06,显示准铝质到过铝质、钙碱性-高钾钙碱性的I型花岗岩特征。稀土总量变化于98.10×10^-6~100.8×10^-6之间,具轻稀土相对富集的特点,同时具有明显的Nb、Ta、P、Ti等异常,暗示了其岩浆形成过程中可能经历了金红石、钛铁矿和磷灰石等的结晶分异过程。相比花岗闪长岩(δEu=0. 80~0. 97),石英斑岩(δEu=0. 38)显示更加亏损Eu、P和Ti,暗示了岩浆演化过程中的进一步结晶分异。岩石(87Sr/86Sr)i比值(0. 7046~0. 7056)、εNd(t)值(-2. 9^-0. 2)揭示花岗闪长岩及其包体是由于新特提斯洋板片回撤时引起地幔软流圈上涌而诱发新生镁铁质下地壳部分熔融形成的岩浆,在经历了一定程度分离结晶作用后形成。石英斑岩具有相似的源区,但有更多的壳源组分的加入,推测为同一岩浆房进一步发生斜长石或钾长石结晶分异的产物。结合该带已报道的古新世Pb-Zn多金属矿床,提出可能是北部念青唐古拉多金属成矿带古新世-始新世岩浆成矿作用的向南延伸引起了日阿窄矿化,认为南部的中新世冈底斯斑岩Cu-Mo-Au成矿带可能具有发育与Fe-Cu相关的多金属矿床的潜力。

The Riazhai Pb polymetallic deposit is located in the Miocene porphyry Cu-Mo-Au metallogenic belt in the southern Gangdese and contains various intrusions including granodiorite,porphyritic granite and quartz porphyry. The SHRIMP U-Pb zircon geochronology,major and trace elements,and Sr-Nd isotopic geochemistry of these magmatic rocks were performed to study the magmatic activity in the mining area and its genetic significance. The results show that granodiorite( 62. 70 ± 0. 78 Ma),mafic microgranular enclave,porphyritic granite( 60. 43 ± 0. 86 Ma),and quartz porphyry( 60. 30 ± 1. 20 Ma) were the products of the Paleocene magmatism in the southern Gangdese belt,and they show the evolution from basic rocks to intermediate-acidic igneous rocks( Si O2= 53. 43% ~ 76. 46%). The contents of Na2O + K2O range from 4. 43% to 6. 88% and the values of A/CNK are restricted from0. 90 to 1. 06,which indicate the characteristics of metaluminous to peraluminous,calc-alkaline to high-K calc-alkaline I-type granite.These rocks have ∑REE varying from 98. 10×10^-6 to 100. 8×10^-6,with relative enrichment of light rare earth elements. They also show obvious depletion of Nb,Ta,P,and Ti,suggesting that they have probably experienced the crystallization differentiation of rutile,ilmenite,and apatite during the magma evolution. Compared with granodiorite( δEu = 0. 80 ~ 0. 97),quartz porphyry( δEu =0. 38) shows stronger negative Eu anomalies and depletion of P and Ti,suggesting further crystallization differentiation. The rocks have(87Sr/86Sr)ivalues of 0. 7046 ~ 0. 7056 and εNd( t) values of-2. 9 ^-0. 2,which reveal that the granodiorite and mafic microgranular enclave magma was mainly derived from the partial melting of juvenile lower crust( approximately 50% ~ 60%) during the collision and Neo-Tethyan subduction slab rollback,and then went through the crystallization differentiation. Quartz porphyry has similar a magmatic source,but more added crustal components,which are presumed to be the products of further plagioclase or Kfeldspar crystallization differentiation in the same magma chamber. Combined with the reported Paleocene Pb-Zn polymetallic deposits in the Gangdese belt,the southward extension of the Paleocene-Eocene magmatic mineralization in the northern Nyainqentanglha polymetallic metallogenic belt probably caused the Riazhai mineralization in the Miocene Gangdese porphyry Cu-Mo-Au metallogenic belt and thus we proposed the potential for Fe-Cu polymetallic mineralization in this belt.