溧水盆地两类晚中生代中酸性火山岩的岩石成因

The genesis of two types of Late Mesozoic intermediate-felsic volcanic rocks in Lishui Basin, Lower Yangtze valley

下扬子地区溧水盆地晚中生代中酸性火山岩的元素-同位素地球化学分析结果显示,区域上存在同期的两类准铝质高钾钙碱性中酸性火山岩。第1类以龙王山组粗面安山岩为代表,它们在空间上与玄武岩共生,SiO2含量为58.0%~58.9%,富集大离子亲石元素(如Rb=104×10-6~117×10-6,Ba=651×10-6~695×10-6)及LREE(如Ce=47.4×10-6~49.0×10-6),强烈亏损Nb_Ta,基本无Eu异常或弱异常(Eu/Eu=0.87~1.01,平均值为0.95),和同期玄武岩具有非常相似的Sr和Nd同位素比值[87Sr/86Sr(i)=0.70551~0.70557;εNd(t)=-3.6^-2.9],为同期基性岩浆经历了角闪石+斜长石+磷灰石分离结晶作用的产物。第2类火山岩由大王山组粗面岩组成,其SiO2含量为61.9%~66.4%,MgO变化在1.07%~2.56%之间,表现出与龙王山组中酸性火山岩类似的微量元素元素特征,但具更高的K2O、Rb、Th、REE、HFSE和低的FeOT、TiO2、P2O5、CaO、Sr和相容元素含量,较龙王山组基性火山岩具高Sr和低Nd同位素比值[87Sr/86Sr(i)=0.70663~0.70813;εNd(t)=-7.7^-4.8],其高钾、准铝质的英安岩成分特征难以通过地壳部分熔融作用来解释,而相对同期基性火山岩高Sr而低Nd的同位素组成反映它们为玄武质岩浆在上升过程中受到大陆上地壳物质同化混染作用(AFC)的产物。

Major element, trace element and Sr-Nd isotope data of two types of Late Mesozoic metaluminous and high-K calc-alkaline intermediate-felsic volcanic rocks from the Lishui Basin in Lower Yangtze valley are reported in this paper. The first type is represented by the trachyandesites occurring in the Longwangshan Formation. These rocks span a SiO2 range of 58.0 % - 58.9 %, characterized by significant LILE （e. g., Rb = 104 × 10^- 6～ 117 × 10^-6, Ba= 651 × 10^-6-695 × 10^-6） and LREE （e. g., Ce= 47.44 × 10^-6-48.96 × 10^-6） and Nd-Ta depletion with insignificant Eu anomalies （Eu/Eu^＊ = 0. 87-1.01, with an average of 0.95）, and similar Sr and Nd isotopic compositions [^87Sr/^86Sr（i） = 0.705 51 -0.705 57, εNd（ t ） = - 3.6- - 2.9 ] to the contemporaneous mafic lavas. These rocks are spatially coeval with the mafic volcanic rocks and are interpreted as the differentiates of the basaltic magmas after removal of a fractional assemblage of amphibole ＋ plagioclase ＋ apatite. The second type is composed of trachytes occurring in the Dawangshan Formation. These rocks span a SiO2 range of 61.9% -66.4% and MgO range of 1.07% -2.56%, showing similar trace element features to trachyandesites of the Longwangshan Formation. Compared with basalts and trachyandesites from the Longwangshan Formation, they have higher K2O, Rb, Th, REE, HFSE, lower FeOT, TiO2, P2O5, CaO, Sr and compatible element contents, and more radiogenic Sr and less radiogenic Nd isotopic compositions [ ^87Sr/^86Sr（i） = 0.706 63 -0.708 13; εNd（t） = - 7.7-- 4.8]. The unique characteristics （e. g., K-rich, metaluminous and dacitic compositions） in these rocks argue against an origin of crust-derived melts. It is thus considered that these melts as differentiates of the primary basaltic magmas were subjected to assimilation and fractional crystallization.