Formation of the Late Cretaceous-Paleogene Sn-rich granites in the Tengchong block: The magma origin and differentiation conditions

Abstract

腾冲地块锡成矿作用主要与晚白垩世-古近纪岩浆活动相关，但仅矿区花岗岩体具有较高的锡含量（平均25×10^-6），区域上同时代的非成矿花岗岩体并未发生锡的富集。本文通过搜集分析现有研究数据，总结了富锡成矿岩体和区域非成矿岩体在岩浆源区、演化条件和结晶分异程度的异同：区域非成矿岩体锆石ε_Hf（t）值（-15.1~+3.39）表明，自东北向西南幔源物质加入有升高的趋势，但区域玄武岩中低含量的Sn （1.61×10^-6）表明幔源物质混入不利于岩浆中锡的富集。晚白垩世部分非成矿岩体与成矿岩体具有相同ε_Hf（t）（-9.7）值，表明其皆源于古老地壳物质的部分熔融，但岩体均表现为准铝质-弱过铝质特征，且锆石Hf同位素（t_DM2=1724Ma）和全岩Nd同位素（t_DM2=1836Ma）二阶段模式年龄基本一致，因此其源区可能并非富锡的高黎贡山群变质沉积岩，而可能是其中未经风化的变质花岗岩。根据腾冲地块地层厚度（28km）和莫霍面深度（47~35km）推断岩浆源区至少位于地下30km （8.4kbar），由于仅靠地温梯度（25℃/km）无法达到初始熔融温度（>1066℃），源区部分熔融过程很可能受地幔热的影响。根据Fe₂O₃/FeO比值，非成矿岩体（0.59）与成矿岩体（0.48）均具有较低的氧逸度，属钛铁矿系列，但成矿岩体的结晶分异程度明显高于非成矿岩体，且成矿岩体富含挥发分，高含量的挥发分降低了岩浆固结温度（650~550℃），延长了结晶分异时间，促进了锡在晚期岩浆中的富集。因此腾冲地块富锡花岗岩主要是普通岩浆在低氧逸度环境下发生高度结晶分异的结果。;Tin metallogenesis in Tengchong block is associated with the Late Cretaceous and Paleocene magmatism. However, only the ore-forming granitoids are characterized by high Sn content (25×10^-6 on average). The regional contemporary unmineralized granitoids hardly show Sn enrichment. By collecting and analyzing available research material, the differences between the Sn-rich ore-forming rocks and regional unmineralized rocks are summarized in terms of their magma origins, evolution conditions and differentiation degrees. Zircon ε_Hf(t) values (-15.1~+3.39) of the regional unmineralized rocks indicate that the magmas in this area are added with increasing mantle material from northeast to southwest, while the addition of the mantel material is disadvantageous to tin enrichment in the mixing magma according to the Sn content (1.61×10^-6) of the regional basalt. Part of the regional unmineralized granitoids formed in Late Cretaceous have the same ε_Hf(t) values of -9.7 with the Sn-rich ore-forming rocks, suggesting both of them were derived from partial melting of ancient crustal material. Considering these rocks are all metaluminous or weak peraluminous, and the crustal model age (t_DM2) calculated from zircon Hf isotopes (1724Ma) are nearly identical to that from the whole-rock Nd isotopes (1836Ma), the magma may not be derived from the Sn-rich Gaoligongshan Group consisting of metasedimentary rocks, but probably the inner metamorphic granitoids. The metamorphic granitoids are at least 30km underground (corresponding to 8.4kbar) based on the regional strata thickness (28km) and the Moho depth (47~35km). The initial melting temperature (1066℃) is unable to be reached according to geothermal gradient (25℃/km), therefore, the partial melting process is likely influenced by mantle heat. According to Fe₂O₃/FeO values, both the unmineralized granitoids (0.59) and the tin mineralized granitoids (0.48) have low oxygen fugacity, and belong to ilmenite series. While the fractionation of Sn-rich granitoids is much higher than that of unmineralized granitoids, moreover, the Sn-rich granitoids contain abundant volatiles, which decrease the magma consolidation temperature (650~550℃), prolong magma differentiation process, and promote Sn gathering in the late magma. In conclusion, the Sn-rich granitoids in the Tengchong block is mainly resulted from the high fractionation of normal magma under a low oxygen fugacity.