Chemical Th-U-total Pb isochron ages of monazites and zircons from the Hikami Granite and “Siluro-Devonian” clastic rocks in the South Kitakami terrane
Chemical Th-U-total Pb isochron ages of monazites and zircons from the Hikami Granite and “Siluro-Devonian” clastic rocks in the South Kitakami terrane
复制标题
南北上地体日上花岗岩和“靳罗-泥盆系”碎屑岩独居石和锆石的化学 Th-U-总 Pb 等时线年龄
DOI：
10.2465/ganko.87.330
复制
发表时间：
1992
期刊：
影响因子：
--
通讯作者：
Hiroka Chiba
中科院分区：
文献类型：
--
作者：
Kazuhiro Suzuki;M. Adachi;Koji Sango;Hiroka Chiba
根据精确的电子探针分析，对南北上地体Hikami花岗岩中的独居石和锆石、泥盆纪Ono组的花岗岩碎屑和南北上地体的志留系川内组的砂岩中的独居石和锆石进行了CHIME(化学Th-U-全铅等时线)年龄测定。主要结果如下：(1)小野地区Hikami花岗岩中锆石的钟状年龄为340±20~360±20 Ma。(2)Higuchi-ZAwa“泥盆纪”小野组中花岗岩类碎屑岩可划分为250~260 Ma、420~450 Ma和610 Ma三个年龄群。碎屑的年龄数据和岩石学特征并不表明它们来自Hikami花岗岩。(3)库萨亚米-扎瓦“志留系”川内组砂岩中碎屑独居石的钟声年龄集中在约430 Ma，从属模式约为500 Ma，碎屑锆石的钟声年龄约为250 Ma~1800 Ma。(4)Kusayami-ZAwa砂岩覆盖的花岗岩类中的独居石和锆石获得了约250 Ma的钟状年龄，明显晚于小野地区Hikami花岗岩中的独居石和锆石。这些钟状独居石和锆石年龄不支持目前认为Hikami花岗岩是前志留纪基底的观点。相反，编钟年龄数据需要对小野组和川内组进行详细的地层学和沉积学重新检查。
CHIME (chemical Th-U-total Pb isochron) ages have been determined for monazites and zircons from the Hikami Granite, granitoid clasts of the “Devonian” Ono Formation and arkosic sandstones of the “Silurian” Kawauchi Formation in the South Kitakami terrane, on the basis of precise electron microprobe analyses. The main results are as follows: (1) The CHIME ages of zircons from the Hikami Granite in the Ono area are in the range from 340±20 to 360±20 Ma. (2) The granitoid clasts in the “Devonian” Ono Formation at Higuchi-zawa can be classified into three age groups: 250-260 Ma, 420-450 Ma and 610 Ma. The age data and petrographical features of the clasts do not show their derivation from the Hikami Granite. (3) The CHIME ages of detrital monazites from the arkosic sandstones of the “Silurian” Kawauchi Formation at Kusayami-zawa concentrate at ca. 430 Ma with a subordinate mode at ca. 500 Ma, and those of detrital zircons range from ca. 250 Ma to as old as ca. 1800 Ma. (4) Monazites and zircons from the granitoid overlain by the arkosic sandstone at Kusayami-zawa give ca. 250 Ma CHIME ages; they are significantly younger than those from the Hikami Granite in the Ono area. These CHIME monazite and, zircon ages do not support a current view that the Hikami Granite is pre-Silurian basement. Rather, the CHIME age data require the detailed stratigraphical and sedimentological re-examination of the Ono and Kawauchi Formations.