Late Cenozoic evolution of epithermal gold metallogenic provinces in Kyushu, Japan
Late Cenozoic evolution of epithermal gold metallogenic provinces in Kyushu, Japan
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DOI：
10.1007/s00126-005-0025-7
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发表时间：
2005-10
影响因子：
4.8
通讯作者：
Yasushi Watanabe
中科院分区：
文献类型：
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作者：
Yasushi Watanabe
日本九州岛位于日本西南弧与琉球弧的交界处。九州有两个主要的晚新生代浅成热液金银省份，称为北部省份和南部省份。各省区的特点是：1）上新世火山活动以钙碱性安山岩为主，其次是第四纪火山活动，包括钙碱性和拉斑岩浆的挤压； 2）伸展地堑的形成； 3）上新世至更新世矿化，以丰富的低硫化度（LS）浅成热液矿床为主，也有少量高硫化度（HS）矿床。自大约 5 Ma 以来，这两个浅成热液金银区域的演化有所不同；从上新世到更新世，北部省份的热液活动减弱，而南部省份的热液活动主要在东和北方向增加。从上新世到更新世的构造环境变化导致了浅成热液金矿床形成的变化趋势。菲律宾海板块向西倾斜俯冲到西南日本弧之下，导致 Hohi 地堑和弧相关火山活动在大约 6 Ma 时发育。这与 Hohi 地堑及其周围广泛的 LS 矿化有关，以 Bajo 和 Taio 矿床为代表。上新世早期相对活跃的九州-帕劳山脊的俯冲加强了板片与上覆琉球岛弧之间的耦合，导致在南部省形成多成因安山岩火山活动以及相关的HS（春日、岩户和明石）和LS（串木野）矿床。上新世早期菲律宾海板块俯冲方向从西向北偏北方向的变化，增加了西南日本弧与菲律宾海板块之间的正交收敛速度，导致北部省河日地堑的火山和热液活动减少。菲律宾海板块向北的俯冲使九州-帕劳海岭俯冲的位置向北移动，导致南部省份较老（85-60 Ma）、负浮力的奄美盆地洋板片底侵，而不是继续俯冲年轻（27-15 Ma）、有浮力的四国盆地板片。这种更替导致了南部省的板片角度陡化和板片回滚，这与区域扩张、琉球火山锋向东移动以及鹿儿岛和岛原地堑以及冲绳海槽的发展有关。流纹岩和玄武岩火山作用，以及安山岩火山作用，自 2Ma 起就在琉球弧背地区发生；菱狩和大口同时发生的 LS 矿化与流纹岩火山活动有关。菲律宾海板块俯冲方向向西北方向的另一次变化发生在 2–1 Ma。由于向西收敛速度的增加，西南日本弧的前弧条向西移动，与沿中位构造线的右旋走滑断层有关。这一转变导致河西地区地堑发育的缩短和停止，将随后的火山活动和相关的热液活动限制在地堑中部。
Kyushu Island, Japan, is located at the junction of the Southwest Japan arc and the Ryukyu arc. There are two major late Cenozoic epithermal gold-silver provinces in Kyushu, which are termed the Northern and Southern provinces. The provinces are characterized by: 1) Pliocene volcanism dominated by calc-alkaline andesite, followed by Quaternary volcanism including extrusion of both calc-alkaline and tholeiitic magmas; 2) formation of extensional grabens; 3) Pliocene to Pleistocene mineralization, which was dominated by abundant low sulfidation (LS) epithermal deposits with a few high sulfidation (HS) examples. The two epithermal gold-silver provinces have evolved differently since about 5 Ma; the Northern province has exhibited diminished hydrothermal activity from the Pliocene to Pleistocene, whereas the Southern province has witnessed increased hydrothermal activity mainly in easterly and northerly directions. Changes of tectonic setting from the Pliocene to Pleistocene account for the variable trends in epithermal gold deposit formation. Westward oblique subduction of the Philippine Sea plate beneath the Southwest Japan arc caused development of the Hohi graben and arc-related volcanism at about 6 Ma. This was associated with widespread LS mineralization in and surrounding the Hohi graben, as is represented by the Bajo and Taio deposits. The subduction of the relatively buoyant Kyushu-Palau ridge during the early Pliocene strengthened the coupling between the slab and overriding Ryukyu arc, leading to polygenetic andesite volcanism with associated HS (Kasuga, Iwato, and Akeshi) and LS (Kushikino) mineral deposits forming in the Southern province. A change of the subduction direction of the Philippine Sea plate, from west to north-northwest in the early Pliocene, increased the orthogonal convergence rate between the Southwest Japan arc and the Philippine Sea plate, resulting in a decrease of volcanic and hydrothermal activity in the Hohi graben of the Northern province. The more northerly subduction of the Philippine Sea plate shifted the locus of the Kyushu-Palau ridge subduction northward, resulting in underplating of the older (85–60 Ma), negatively buoyant Amami basin oceanic slab in the Southern province, rather than continued subduction of the young (27–15 Ma), buoyant Shikoku basin slab. This replacement caused steepening of the slab angle and slab-rollback in the Southern province, which was associated with regional extension, an eastward shift of the Ryukyu volcanic front, and development of the Kagoshima and Shimabara grabens, as well as the Okinawa trough. Rhyolite and basalt volcanism, in addition to andesite volcanism, have occurred since 2 Ma in the area of the Ryukyu back arc; coincident LS mineralization at Hishikari and Ohkuchi was affiliated with the rhyolite volcanism. Another change of the subduction direction of the Philippine Sea plate to the northwest occurred at 2–1 Ma. The forearc sliver of the Southwest Japan arc shifted westward, in association with right-lateral strike-slip faulting along the Median tectonic line, due to the increase of the westward convergence rate. This shift resulted in shortening and cessation of graben development in the Hohi area, restricting the subsequent volcanism and related hydrothermal activity to the central part of the graben.