宁芜盆地边界断裂带及邻区玄武岩特征和意义

Geological Characteristics and Significance of Cenozoic Basalts Along the Boundary Fault Zones of the Nanjing-Wuhu Volcanic Basin and Its Adjacent Regions

宁芜火山岩盆地边缘主要由北东和北西向边界断裂构成。西界为长江破碎带,发育早,属张扭性,切割较浅;东界为方山—小丹阳断裂,属压扭性,切割较深;南、北边界均属扭性,发育最晚。新生代玄武岩喷发受边界断裂性质及其构造演化控制,分三期:早期始于早第三纪,沿长江破碎带分布,属高铝拉斑玄武岩,生成较浅;中期喷发于晚第三纪,主要沿方山—小丹阳断裂分布,先后为响碧玄岩—碱性橄榄玄武岩,多幔源二辉橄榄岩包体、捕虏晶及下伏岩层碎块,生成很深;晚期喷发属早更新世,以富钛响碧空岩为主,受北西及东西向断裂交叉点控制。经计算比较,本区玄武岩不具备找金刚石矿产的前景,但从一些岩筒中深部地层碎块看,盆缘地带仍可能是寻找宁芜盆地本身深部铁矿的目标区。

The Nanjing-Wuhu volcanic basin is bounded by the faults stretchingmainly in nearly NE and NW directions. Its western boundary, a tension-shearing fault, is the Yangtze fragmented zone, developed prior to earlyTertiary and with relatively shallow dissection. The eastern boundary, a-compresion-shearing fault, is the Fangshan-Xiaodanyang fault, cutting deep-ly. As the youngest in age, the northern and southern limits belong to theshearing fault. The volcanic eruption of Cenozoic basalts is considered to be controlledby the characteristics and tectonic evolution of boundary faults. It may bedivided into 3 stages. The first eruption occurred during early Tertiary alongthe Yangtze fragmented zone with a shallower dissection, resulting in theformation of the high-aluminium basalt to the tholeiite. The second tookplace during late Tertiary along the Fangshan-Xiaodanyang fault, producingthe phonolitic tephrite followed by the alkali-olivine basalt, which are cha-racterized by the presence of the inclusions of mantle-peridotite and theblocks of underlying rocks, hence implying quite a deep magmatic source.The third eruption occurred in early Pleistocene and was controlled by inter-section of NW-and EW-trending faults, forming the titanium-rich phonoli-tic tephrite. According to the calculating balance of basalts, it is rather difficult tofind diamond in these basalts, but a few depth--derived blocks obtained frombasalt pipes here indicate that the boundary fault zones may be a possibletarget for prospecting iron ore-deposits at lower levels in the Nanjing-Wuhuvolcanic basin.