Petrological and isotope geochronological study on the uplift of the Hidaka Mountains, Hokkaido, Japan

Hidaka变质带(H。日本北海道的爱达克山脉（idaka Mountains）由岛弧陡倾逆冲板组成。向西至麻粒岩相及沿Hidaka主冲断向西的变质蛇绿岩杂岩上的逆冲，变质变质等级逐渐升高。为了考察山的隆升（侵蚀）过程和速率，在Hidaka变质带中部Kamuiekuutikaushi山横断面不同海拔位置采集了不同等级的闪长岩、片麻岩和片岩，分别对黑云母和角闪石进行了16个和2个K-Ar测年，并对锆石进行了5个裂变径迹测年。同位素年代学资料得出以下结论：所有K-Ar黑云母年龄基本一致（17.9 ~ 16.3 ma），而不考虑其变质等级和海拔高度。这一事实表明，变质带现今的东倾陡倾构造早在中新世中期就已形成。因此，在此之前，沿Hidaka主冲断构造的Hidaka岛弧就已经开始逆冲了。西部麻粒岩相岩石角闪石和黑云母的K-Ar测年表明，在18ma ~ 17ma期间，变质带西部隆升（侵蚀）速率高达6.7mm/a。锆石的裂变径迹年龄从15.7Ma到12.4Ma不等，只有10.9Ma发生在逆冲附近。这些年龄随着采样地点海拔的增加而变年轻。这一趋势与一般观察到的相反，表明中新世中期以来的隆升不是简单的旋转型隆升。K-Ar和裂变径迹年龄表明，中新世中期的隆升速率为3.5 ~ 0.8mm/a，中新世中期至今的隆升速率为0.5 ~ 0.3mm/a。因此，自早中新世结束以来，Hidaka山脉的隆升速率随时间而减小。

The Hidaka metamorphic belt(H. idaka Mountains), Hokkaido, Japan consist of easists -steeply dipping thrust sheet of island arc. The belt increases in metamorphic grade westward up to the granulite facies and thrust over the metamorphosed ophiolite complex on the west along the Hidaka Main Thrust. Sixteen and two K-Ar dates on biotites and hornblendes, respectively and five fission-track dates on zircons have been obtained from tonalites and gneisses and schists of various grades collected from localities with different altitudes on a traverse section through Mt. Kamuiekuutikaushi(1979m)in central part of the Hidaka metamorphic belt in order to examine the uplift(erosion)process and rate of the Mountains.The isotope geochronological data lead to the following conclusions :1. All of K-Ar biotite ages almost agree(17.9-16.3Ma)regardless of their metamorphic grade and altitudes. The fact means that the Present eastward-steeply dipping structure of the metamorphic belt had been already made up before the middle Miocene. Therefore, thrusting of the Hidaka island arc along the Hidaka Main Thrust had begun before that time.2. K-Ar dates on hornblendes and biotites from western granulite-facies rocks reveal the uplift(erosion)rate of the western part of the metamorphic belt reaches up to 6.7mm/a between l8Ma and 17Ma.3. Fission-track ages on zircons range from 15.7Ma Lo 12.4Ma with an exception of 10.9Ma which occurs near a thrust. These ages become younger with increase in altitude of sample locality. The tendency, which is the reverse of what observed generally, suggests that the uplift of the Mountains since middle Miocene age is not simple btit of rotational type.4. K-Ar and fission-track ages show uplift rates of the Mountains to be 3.5-0.8mm/a during the middle Miocene and 0.5-0.3mm/a during interval from the middle Miocene to the present. Thus, the uplift rate of the Hidaka Mountains decreases with time since the end of the early Miocene.