溧水盆地晚中生代基性火山岩成因与深部动力学过程探讨

Petrogenesis and geodynamic implications of Late Mesozoic mafic volcanic rocks from the Lishui Basin of the Lower Yangtze region

下扬子地区溧水盆地晚中生代龙王山组基性火山岩属高钾钙碱性-钾玄岩系列,具有低MgO(3.16%～4.97%)和相容元素含量(如Cr11～34μg/g、Ni9～27μg/g),强烈富集大离子亲石元素(Ba/Nb=94～144)、轻稀土元素(La/Nb=3.7～4.6)和亏损Nb-Ta,稍高的Sr同位素组成(87Sr/86Sr)i=0.70573～0.70617和具弱负εNd(t)值(-3.0～-1.9)。龙王山组基性火山岩的元素-同位素地球化学特征反映其来源于富集轻稀土元素和大离子亲石元素的岩石圈地幔;它们在Sr-Nd同位素组成上与大别-苏鲁造山带同时代的基性火成岩具有明显的差别,暗示其熔融源区不大可能受到俯冲陆壳的改造富集作用,更可能是早期具高Rb/Sr、低Sm/Nd和亏损高场强元素的流体/熔体交代作用的结果,总体上具有俯冲大洋板片交代作用的特征。在岩浆熔融深部动力学方面,晚中生代高钾钙碱性-钾玄岩浆的起源主要归结于深部岩石圈地幔的热扰动作用和岩石圈伸展-减薄作用,在时空上匹配于同时代郯庐断裂带的巨大走滑活动。

Mafic volcanic rocks in Longwangshan Formation from the Lishui Bas in of the Lower Yangtze region are composed of high-K calc-alkaline to shosho nitic trachybasalts and basaltic trachyandesites. These rocks have relatively lo w MgO (3.16%～4.97%) and compatible elements (e.g., Cr 11～34 ìg/g, Ni 9 ～27 ìg/g), and are characterized by significant LILE and LREE enrichment and Nb-Ta depletion (e.g., Ba/Nb 94～144 and La/Nb 3.7～4.6) with insignificant Eu anomalies (Eu/Eu0.86～1.01), and moderate Sr isotope ratios [(87Sr/86Sr )i 0.705 73～0.706 17] and slightly negative ?Nd(t) values (-3.0～-1.9). They are isotopically different from the contemporaneous mafic counterparts in the Dabie-Sulu orogenic belt, which were interpreted as melts derived from an enriched lithospheric mantle modified by Triassic subduction of the Yangtze cont inental lithosphere. The major, trace element and Sr-Nd isotopic data of Longw angshan Formation volcanic rocks suggest that they were derived from an enriched lithospheric mantle that had undergone metasomatism by previous fluid/melt of h igh Rb/Sr, LILE/HFSE and low Sm/Nd ratios, a characteristic similar to that usua lly occurring under subduction of oceanic slabs. Generation of the Late Mesozoic high-K calc-alkaline to shoshonitic magmas in the Lower Yangtze region was attributed to lithospheric extension and asthenospheric upwelling, which was pro bably triggered by contemporaneous thermal perturbation of the large-scale str ike-slip movement of the Tan-Lu wrench fault system.