斜斑玄武岩能较好地记录岩浆作用过程、动力学机制和构造环境.本文对华南南岭中段晚白垩世长城岭斜斑玄武岩的斜长石环带结构进行了详细研究,以制约其岩浆作用过程和岩石成因机制.长城岭斜斑玄武岩富集大离子亲石元素,亏损Nb-Ta,具强烈Pb正异常和轻微Ti正异常;其全岩87 Sr/86 Sr较高且较均匀(0.7088 ～ 0.7089),εNd(t)较低(-0.95 ～-0.94),而斜长石斑晶原位87 Sr/86 Sr变化相对较大(0.706 ～0.710).斜长石斑晶普遍显示反环带结构,从核部到边部的An组分以及MgO、FeO、TiO2含量明显升高而K2O含量降低;另外,反环带结构斜长石斑晶的核部An含量不均匀,但边部成分一致,指示岩浆补给作用.总体上,斜长石斑晶的边部相对较窄,指示补给岩浆在岩浆房中的驻留时间较短,补给岩浆的加入造成了玄武岩浆的快速喷发.斜长石斑晶的87 Sr/86 Sr比值与Sr含量呈负相关关系,表明岩浆房演化过程中存在地壳物质的同化混染作用.39 Ar-40 Ar定年结果显示,由基质斜长石获得的坪年龄(83.45±0.44Ma)略老于由斜长石斑晶获得的坪年龄(79.02±0.43Ma),为驻留时间较短的补给岩浆受地壳混染作用影响而具有偏低的初始39Ar/40Ar的结果.长城岭斜斑玄武岩在2Nb-Zr/4-Y图解中落在板内拉斑玄武岩区域内,并具有与板内拉斑玄武岩相似的斜长石An(60 ～70),应形成于板内拉张环境.华南地区晚白垩世基性岩浆活动,包括沿海地区的永泰玄武岩(～ 85Ma)和内陆地区的长城岭斜斑玄武岩(～ 79Ma)、衡阳玄武岩(～ 70Ma)及禾埠玄武岩(～ 63Ma)等,显示逐渐从岛弧玄武岩转变为板内碱性玄武岩的趋势,其Sr-Nd同位素也显示逐渐亏损的特征;它们的时空分布特征表明,华南内陆在晚白垩世期间经历了持续的拉张作用,可能与古太平洋板块后撤有关.%The plagioclase-phyric basalt may preserve the details of magmatism processes and related geodynamics and tectonic settings.In this paper,we present the details of zoning plagioclase of the Late Cretaceous plagioclase-phyric basalt in the Changchengling area in the central Nanling of South China Block,in order to constrain the magma processes and petrogenesis of this Mesozoic mafic rock.The Changchengling basalt is enriched in large ion lithophile elements,depleted in Nb-Ta with notable positive Pb anomaly and slightly positive Ti anomaly.The samples show high and uniform whole rock initial 87 Sr/86Sr (0.7088 ～ 0.7089) and slightly negative εNd(t) values (-0.95 ～-0.94),but in situ Sr isotopes of plagioclase phenocrysts are relatively variable (ca.0.706 ～ 0.710).Plagioclase phenocrysts usually show reverse zoning that MgO,FeO and TiO2 notably increase but K2O decreases with elevating An contents from core to rim.Reverse zoning plagioclases have different composition in cores but nearly uniform rims,suggesting the magma recharge.Overall,the plagioclase phenocrysts have relatively narrow rims,indicating relatively short-term storage of recharging magma in the chamber.Therefore,the magma recharge might have triggered the last eruption of the Changchengling basalt.The phenocryst's 87 Sr/86 Sr is negatively correlated to its Sr concentration,suggesting the crustal contamination in the evolution of magma chamber,and the residual melt in the chamber would have been contaminated by wall rock.The 39Ar-40Ar dating results reveal that the plateau age of the microcrystal plagioclase in the matrix (83.45 ±0.44Ma) is slightly older than that of plagioclase phenocrysts (79.02 ±0.43Ma),which would be due to the crust contamination that produced low initial 39Ar/40Ar in the recharging magma for its short-term storage in the chamber.The studied samples are plotted into the field of within plate tholeiite in the 2Nb-Zr/4-Y diagram,and plagioclase phenocrysts of the Changchengling basalt have An contents resembling those of intraplate tholeiite.Therefore,the Changchengling basalt should be primarily an intraplate magmatism.The Late Cretaceous mafic magmatismin South China Block,including the Yongtai basalt (～ 85Ma) in coastal area,and the Changchengling basalt (～ 79Ma),Hengyang basalt (～70Ma),and Hebu basalt (～63Ma) of inland area,changed gradually from island arc basalt to intraplate alkaline basalt with more and more depleted Sr-Nd isotopes.The spatial and temporal distribution of these basalts indicates that the inland area of South China Block was successively in an extension setting in the Late Cretaceous,which may be due to the roll back of subducting paleo-Pacific plate.
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