玄武质岩石的单颗粒锆石U-Pb年龄谱

U-Pb age spectrums of single zircon from basaltic rocks

报道了托云、东浮山、羊角、雪花山和山旺5处新生代玄武岩的SHRIMP锆石U-Pb测年结果。位于西南天山造山带的托云新生代玄武岩的14个测点年龄值十分发散,最大值与最小值分别为857.1和203.4Ma,它们与其余4件玄武岩样品49个SHRIMP锆石测点年龄一同构筑了涵盖各个地质时期几乎贯穿整个地质时间的复杂年龄谱。位于华北克拉通中部区域南太行山造山带的东浮山、羊角和雪花山新生代玄武岩3件样品累计36个锆石测点形成的锆石年龄谱相对简单,其中35个测点的年龄集中在1719.9～2641.6Ma,唯一的古生代年龄(311.3Ma)出现在雪花山玄武岩7.1测点。位于华北克拉通东部裂谷带内山旺玄武岩6件样品27个测点的单颗粒锆石年龄构筑的锆石年龄谱形成3个集中时间段,分别为新太古代—古元古代(2595.4～1852.2Ma)、古生代(385.8～271.1Ma)和中生代(109.4Ma)。3个年龄谱中大部分单颗粒锆石U-Pb年龄均能在各自所处区域内发现与之对应的岩浆-热事件,部分单颗粒锆石年龄可能暗示所在区域至今未发现的岩浆热事件,托云、东浮山-羊角-雪花山、山旺新生代玄武岩的复杂单颗粒锆石年龄谱再造了各自所处区域的地质演化史。3个锆石年龄谱的复杂程度与各自区域地表出露岩浆岩的规模和期次复杂程度相关,天山造山带内岩浆岩发育,托云玄武岩锆石年龄普最复杂,记录了天山造山带的演化,而华北克拉通中部区域南太行山造山带地表零星出露岩浆岩,东浮山-羊角-雪花山玄武岩的锆石年龄普最简单。处于后期遭受破坏改造的华北克拉通东部裂谷带内的山旺玄武岩的单颗粒锆石年龄谱,其复杂程度明显低于托云玄武岩的年龄谱,而又高于东浮山-羊角-雪花山玄武岩的年龄谱。鉴于玄武质岩浆同化混染围岩过程中能量消耗和地球化学印记以及玄武质岩浆上升的耗时限制,认为托云、东浮山、羊角、雪花山和山旺新生代玄武岩中具有复杂年龄信息的锆石捕掳晶不是玄武质岩浆在快速上升过程中从围岩中捕获的,而是在岩石圈拆沉过程中进入软流圈地幔中,随着具原生岩浆性质的玄武质岩浆喷发到达地表。

SHRIMP U-Pb dating results of Cenozoic basalts from Tuoyun,Dongfushan,Yangjiao,Xuehuashan and Shanwang were reported. Fourteen spot ages of Tuoyun basalt,located in Southwest Tian Shan orogen,are more dispersive,and the age range is from 857.1 Ma to 203.4 Ma,which formed a complex age spectrum that nearly covers entire geologic time on the basis of combination with the other 49 single zircon SHRIMP ages in four samples. The SHRIMP age spectrum comprises a total of 36 spots from Dongfushan,Yangjiao and Xuehuashan basalts in South Taihang Mountains,which lies in the middle part of North China Craton,is simpler than Shanwang basalts＇,the 35 ages of the 36 spots is limited from 2641.6 Ma to 1719.9 Ma,the unique 311.3 Ma falls into Paleozoic was achieved at spot 7.1 in Xuehuashan basalt. The ages of 27 spots in total 6 basalt samples from Shanwang located in the rift belt of Eastern North China Craton are clustered in three time spans：late Archean-early Proterozoic （2595.4-1852.2 Ma）,Paleozoic （385.8-271.1 Ma） and Mesozoic （109.4 Ma）. Most of the single zircon ages in the age spectra can have found corresponding magma events in the region where the basalt occurred,respectively. A part of the ages,however,are not correspondent with any magma events on the surface,which probably indicates the existence of important magmatism that are not found till today in this region. These age spectra of Tuoyun,Dongfushan,Yangjiao,Xuehuashan and Shanwang basalts rebuilt the geologic evolution where they occurred. The complexities of 3 age spectra are related to size and times of magmatism where the basalt occurred,the most complex age spectrum is of the Tuoyun basalt＇s,which has recorded the evolution of Tian Shan belt,where a great diversity of magmatism are developed. On the contrary,there is small size magmatism in south Taihang Mountains belonging to the middle part of North China Craton,the simplest age spectrum is reflected by the 36 spots from Dongfushan,Yangjiao and Xuehuashan basalts. The age spectrum complexity composed by 27 spots from Shanwang basalt located in the eastern North-China Craton where underwent lithosphere destruction is higher than those of Dongfushan,Yangjiao and Xuehuashan basalts＇,and lower than Tuoyun basalt＇s. Considering the thermal energy consumption and composition change of basaltic magma in the process of magma-wall rock interaction,such as assimilation and fractional crystallization （AFC）,and the ascent time limitation of basaltic magma hosting peridotite xenoliths,the zircon xenocrysts in Tuoyun,Dongfushan,Yangjiao,Xuehuashan and Shanwang basalts were unlikely captured from the wallrocks during magma ascent. These zircon xenocrysts entering the asthenosphere resulted from lithosphere delamination where these basalts occurred,and reached the surface due to the eruption of the primary basaltic magma.