【摘要】 广东省从化市石岭方钠石正长岩是南岭地区唯一已知的早白垩世硅酸不饱和碱性岩。岩石由方钠石、霞石、钠 钙角闪石、霓石 普通辉石、铁黑云母和条纹长石组成 ,不含独立相斜长石 ,是一种超溶线碱性正长岩。侵入于佛冈黑云母花岗岩中 (γ2 ( 3 )5) ,侵位年龄为 1 2 7.5Ma。据霞石、方钠石和钠 钙角闪石估算 ,结晶时温度分别为 70 0～ 80 0℃ ,660℃和 60 0～ 75 0℃。钠 钙角闪石反映其形成在中等氧逸度下 (lgfO2 =-2 0～ -2 5 )。液相线方钠石存在反映其形成时流体压力应不高于 1 60MPa,岩体定位深度约 5～ 6km。钕同位素具有接近于零的εtNd值 (-0 43～ -0 47) ,低的钕模式年龄 (t2DM=0 96Ga) ,指示岩浆可能起源于岩石圈地幔。推测在早白垩时期 ,南岭地区应力作用已转入拉张为主 ,大规模拉张作用 ,造成岩石圈减薄 ,有利于碱性岩浆形成和侵入。更多还原

【Abstract】 The Shiling sodalite syenite, Guangdong Province, is the only Early Cretaceous SiO 2 undersaturated alkali syenite known in the Nanling area, southeastern China. The rock mainly consists of sodalite, nepheline, Na Ca amphibole, aegirine augite, annite and perthite, without any separate phase of plagioclase, indicating that it is a hypersolvus alkali syenite. The syenite intruded into the Fugan biotite granitoids(γ 5 2(3) ), with an age of 127.5 Ma. The rock crystallized at a temperature range of 700～800℃, 660℃ and 750℃, deduced by compositions of nepheline, sodalite and Na Ca amphibole, respectively. The composition of Na Ca amphibole indicates that it is stable at a moderate oxygen fugacity ( lg f O 2 =-20～-25). Primary sodalite is stable at fluid pressure of 1600×10 5 Pa, suggesting that the emplaced depth of sodalite syenite is between 5 and 6 km. The rock is highly enriched in LILE and HFSE such as Rb, Nb, Ta, Zr, Ce, etc., with the trace element characteristics of Al subtype rocks of A type rock suites. In the diagrams of Na 2O+K 2O SiO 2, FeO */MgO SiO 2, (Zr+Nb+Ce+Y) Fe */MgO and (Zr+Nb+Ce+Y) (Na 2O+K 2O)/CaO, the data for the rock are all plotted in a field of A type granites, evidently distinguishable from fractionated I and S type granites. In the diagrams of Y/Nb Yb/Ta, Y/Nb Ce/Nb, Nb Y Ce and Nb Y 3Ga, the data points of the rock all fall in the Al subtype field of A type rock suites (by Eby, 1992), suggesting its affinity for OIB mantle source. In the field of Nd isotope composition, the rock has a ε t Nd value close to zero (-0.43～ -0.47 ), and a very younger Nd model age of t 2DM (0 96 Ga). These indicate the magma which formed sodalite syenite may be derived from lithosphere mantle, suggesting that the large scale lithopheric extension started from Early Cretaceous in the Nanling area of southeastern China, and resulted in the formation of alkali syenite.更多还原