大别山北麓的奥陶纪岩浆弧:侵入岩年代学和地球化学证据

An Ordovician magmatic arc at the northern foot of Dabie mountains: Evidence from geochronology and geochemistry of intrusive rocks

采用颗粒级锆石U-Pb和角闪石的^(40)Ar-^(39)Ar定年方法,测得大别山北坡马畈闪长质杂岩体中石英闪长岩的结晶锆石年龄分别为(463.5±3.4)和(458.6±6.7)Ma,角闪石^(40)Ar-^(39)Ar坪年龄为(462.7±1.4)Ma。从镁铁质包体到闪长岩和花岗岩类,ε_(Nd)(t)介于4.3～1.8之间,且随着Mg~#值的减小和SiO_2及Nb含量增高,岩石的ε_(Nd)(t)值降低,结合低Nb/Zr比值和中等-高的Ba/Th比值等特点,认为有关岩石的形成与幔源岩浆受板片流体影响并发生地壳AFC(同化混染与分离结晶同时出现)作用有关,但它们亏损Nb等高场强元素的特征,并不是地壳物质影响所致,而反映了地幔岩浆源区的特点,表明岩浆形成于与俯冲有关的环境。这一研究进一步证明,在大别山北麓的北淮阳地区,曾经发育了奥陶纪岩浆弧,向西与北秦岭岩浆弧相接。从大别山北麓的马畈杂岩体到昆仑山的布青山英云闪长岩体,中央造山带内与俯冲有关的侵入岩侵位年龄有向西变新的趋势,可能意味着原特提斯秦岭洋在向华北地块南缘活动大陆边缘俯冲过程中,是从东向西逐渐关闭的。

With the metheds of single grain zircon U-Pb and hornblende ^(40)Ar-^(39)Ar dating, magmatic zircon ages of (463. 5± 3. 4 ) Ma and (458. 6±6. 7) Ma and a hornblende^(40)Ar-^(39)Ar plateau age of (462. 7±1.4) Ma are yielded in the quartz diorite of the Mafan dioritic complex at the northern foot of Dabie Mountains. Mafic microgranular enclaves(MME), and dioritic and granitoid rocks in the complex are characterized by low Nb/Zr and moderate-high Ba/Th ratios, high ε_(Nd) (460 Ma) values between 4.3 and - 1.8, a positive correlation between ε_(Nd) (460 Ma) and Mg~# values of rocks, and a negative correlation between ε_(Nd)(460 Ma) values and SiO_2 and Nb contents, implying an interaction of depleted mantle-derived magma and fluid from subducted slab, as well as crustal AFC (fractional crystallization paired with assimilation). However, the depletion of high field strength elements such as Nb in MME with ε_(Nd)(460 Ma) of 1.9 is chemical characteristics inherited from the mantle source, rather than the result of crustal contamination, indicating that mafic magma has formed in a subduction-related setting. It is suggested that there has been an once developed Ordovician magmatic arc, which joins the north Qinling are magmatic zone to the west. Emplacement ages of subduction-type intrusive rocks in Central China orogen show a tendency to become younger from Mafan complex at the northern foot of Dabie Mountains in the east to Buqingshan tonalite of Kunlun Mountain in the west, implying that the Proto-Tethyan Qinling ocean had been closed gradually from east to west in the course of slab subduction under the active southern continental margin of the North China Block.