河北平泉光头山碱性花岗岩的时代、Nd-Sr同位素特征及其对华北早中生代壳幔相互作用的意义  

Age and Nd-Sr isotopic geochemistry of the Guangtoushan alkaline granite, Hebei province, China: implications for early Mesozoic crust-mantle interaction in North China Block.

作　　者：韩宝福[1] 加加美宽雄[1] 李惠民[1] 

机构地区：[1]造山带与地壳演化教育部重点实验室(北京大学)北京大学地质学系,北京100871

出　　处：《岩石学报》2004年第6期1375-1388,共14页Acta Petrologica Sinica

基　　金：国家973项目(2001CB409802;教育部"跨世纪人才培养计划"和中俄国际合作基金项目(40211120647)

摘　　要：光头山碱性花岗岩产出在华北北部的前寒武纪基底变质岩系之中,造岩矿物组合为石英+碱性长石+纳铁闪石+霓辉石+钠铁非石±星叶石,副矿物有锆石、钛铁矿、硅钛铈铁矿等。晚期的伟晶岩囊状体由颗粒粗大的石英、碱性长石和纳铁闪石等组成,全岩Rb-Sr等时线年龄为T=200±16Ma,(^(87)Sr/^(86)Sr)_i=0.705±0.008,MSWD=11.2,代表冷却年龄,单颗粒锆石U-Pb谐和年龄为220±1Ma,代表岩体侵位时代。光头山碱性花岗岩以A/CNK<1和A/NK<1,Al_2O_3、MgO、CaO和Ba、Sr含量低,全碱含量、MnO和Rb、Ga等含量高,负Eu异常特别显著等为特征。矿物学和地球化学完全符合A型花岗岩的特征。光头山碱性花岗岩是华北地区早中生代后造山环境下岩浆活动的产物。光头山碱性花岗岩的ε_(Nd)(T=220Ma)平均值为-8.9,明显高于华北前寒武纪下地壳岩石的范围,而冀北地区前寒武纪高压麻粒岩地体虽具有大陆地幔的特征,但未经历过部分重熔,表明至少前寒武纪下地壳不可能是岩浆主要的或唯一的来源同样,现今华北下地壳由于时代较新,也不可能成为岩浆的源岩,对比时代相近的超镁铁岩和煌斑岩的Nd同位素特征,推测最可能的源区是1.8～1.9Ga形成的富集的岩石圈地幔。光头山碱性花岗岩和华北北缘早中生代侵入岩带规模很大,以富集地慢来源的岩浆为主,反映了当时的岩浆活动已经具有相当的规模和强度,如果130Ma前后中国东部大规模岩浆活动之时,是岩石圈减薄已经达到最大程度之际,那么,此前一定时间段内的幔源岩浆活动都有可能与岩石圈减薄从开始到鼎盛的过程有关,所以,华北北缘早中生代岩浆活动可能是华北中生代岩石圈减薄过程早期阶段的产物。与岩石圈减薄过程有关的早中生代岩浆活动还在中国东北地区东部和阿拉善北部形成了后造山A型花岗岩。与岩石圈减薄过程相关的早中生代侵入岩在一定范�The Guangtoushan alkaline granite occurs in the Precambrian metamorphic rocks of the North China block and contains the assemblage of quartz + alkaline feldspar + arfvedsonite + aegirine augite + aenigmatite +/- astrophyllite, accessory minerals are zircon, ilmenite, and chevkinite. Late pigmatitic pods in the granite are composed of coarse-grained quartz + alkaline feldspar + arfvedsonite. A whole-rock Rb-Sr isochron age is 200 +/- 16 Ma, with ((87) Sr/(86) Sr)(i) = 0.705 +/- 0.008 and MSWD = 11.2, in correspondence to a cooling age. A concordia age of 220 +/- 1 Ma yielded by the single-grain zircon U-Pb dating is considered to represent the emplacement age of the granite. Geochemically, the granite is characterized by A/CNK < 1 and A/NK < 1, low Al2O3, MgO, CaO, Ba, Sr and high total alkali, MnO, Rb, Ga contents and pronounced negative Eu anomalies. Mineralogy and geochemistry of the granites are consistent with those of post-orogenic A-type granites. Therefore, it is suggested that the Guangtoushan alkaline granite was produced by magmatism in a post-orogenic setting. The averaged epsilon(Nd) (T = 220 Ma) value is -8.9, higher than that for the lower crust of the North China block, implying that the parent magma for the granite could not be produced from the partial melting of the lower crust. It is also impossible that the parent magma was originated from the Precambrian high-pressure granulite terranes in northern Hebei province, because the granulites have no evidence for partial melting. Similarly, the magma could not be derived from the present-day lower crust beneath the North China block because of its young age of 120 to 140 Ma. An enriched mantle source in subcontinental lithosphere is suggested for the Guangtoushan alkaline granite, which was contaminated by the crustal materials subducted in early Proterozoic. The early Mesozoic intrusive zone in northern North China block, including the Guangtoushan alkaline granite, has been considered to be derived mainly from enriched mantle sources. The most in

关 键 词：锆石U-Pb年龄 同位素地球化学 A型花岗岩 壳幔相互作用 中生代 华北 

分 类 号：P597.3[天文地球—地球化学] P588.121[天文地球—地质学]