湘东地区板杉铺加里东期埃达克质花岗闪长岩的成因及地质意义

The Caledonian Adakite-Like Granodiorites in Banshanpu Area, Eastern Hunan Province, South China: Petrogenesis and Geological Significance

湘东地区板杉铺加里东期岩体是一套具埃达克岩性质的花岗闪长岩。该岩石具有高的SiO2(~70%)和Al2O3(~16.5%)、中等高的Na2O/K2O比值(普遍大于1.0,可达1.4)和低但可变的Mg质指数(Mg#=37~59)。相对高的Sr(均值:520×10-6)和Ba(1052×10-6~1502×10-6)以及低的Y(<12×10-6)和Yb(<1×10-6)丰度,导致该岩石具有低的Rb/Sr(0.17~0.42)、高的Sr/Y(34~64)比值,以及LREE强烈富集[(La/Yb)N=27~71]而铕弱负异常(Eu/Eu*=0.76~0.91)的稀土配分模式。因此,板杉铺岩体源区岩可能为石榴角闪岩或石榴角闪岩-角闪榴辉岩过渡相。但低Nd和高Sr同位素组成[εNd(t)=-6.7^-8.4,(87Sr/86Sr)i=0.7078~0.7140]说明它们并不来源于俯冲的玄武质洋壳或亏损地幔;而高放射性成因Pb[(206Pb/204Pb)i=18.22~18.78]和高微量元素比值(如Ba/La=~15~28,Th/Ce=0.29~0.36)暗示深海沉积物及再循环的陆源沉积物可能已卷入这些岩石的成因。结合区域地质、地层和构造事件,认为早古生代可能的陆-弧-陆碰撞是导致研究区埃达克质花岗闪长岩产生的重要机制,而源于先前富集地幔的玄武质岩浆底侵则诱导了加厚的中酸性弧下地壳岩石部分熔融。部分熔融产生的岩浆可能经同化混染和分离结晶作用过程最终形成了板杉铺埃达克质花岗闪长岩。华南埃达克质花岗闪长岩的厘定不仅揭示早古生代时期华南地区经历了地壳增生事件,对正确理解华南、特别是扬子板块东南缘Au-Sb-W-Cu等多金属成矿作用还具有重要的意义。

The Caledonian granitoid rocks in Banshanpu area, eastern Hunan Province, South China are attributed to granodiorites with the affinities to typical adakites and Archean high Al-TTG suites. These rocks are characterized by high SiO2 （ -70% ） and Al2O3 （ - 16.5% ） contents, moderately high ratios of Na2O/K2O （generally 〉 1.0 and up to 1.4） and low but variable Mg^# values （37 -59）. The relatively high Sr （average： 520 ×10^-6） and Ba （ 1052 - 1502 ×10^-6） concentrations, and extremely low Y （ 〈 12 ×10^-6） and Yb （ 〈 1 ×10^-6） concentrations led to low Rb/Sr ratios （ ranging from 0.17 to 0.42 ） and high Sr/Y ratios （ ranging from 34 to 64 ）. The REE distribution patterns are strongly LREE-enriched [ （La/Yb） N = 27 - 71 ], with weakly negative Eu-anomalies （ Eu/Eu^ ＊ = 0.76 - 0.91 ）. Consequently, the source rocks for the studied rocks probably contain garnet-bearing amphibolites and/or transitional garnet-bearing amphibolites to amphibole-bearing ecolgites. The low values of εNd （400 Ma） （ -6.7 to -8.4） and high ratios of （ST Sr/S6 Sr）i（0. 7078 - 0. 7140） suggest that the Banshanpu granitoid rocks were impossible to have sourced from partial melting of subducted ocean crust or depleted mantle. However, the high radiogenic Pb [ （^206Pb/^204Pb） i = 18.22 - 18.78 ] and high ratios of Ba/La （15 -28） and Th/ Ce （0.29 -0.36） imply that the sea sediments and recycled terrestrial materials were involved into the provenance for these rocks. Combined with regional geological data, we suggest that the early Paleozoic continent-arc-continent collision in South China was a possible mechanism responsible for the generation of the Banshanpu adakite-like granodiorites. Underplating of basaltic magma, which was derived from previously enriched-mantle lithosphere, led to partial melting of the thickened, intermediate-acidic lower crust beneath an arc system. Thus assimilation and fractional crystallization （AFC） processes to produce the magma for the studied rocks were allowed to take place. The confirmation of the adakite-like granodiorites in Banshanpu area implies not only a new crustal growth of South China continent during early Paleozoic, but also a synchronous occurrence of gold and gold-polymetallic mineralization in South China, particularly in southeastern margin of the Yangtze Block.