海南岛北部古生界时代:碎屑锆石U-Pb年代学约束

海南岛古生界主要分布于南部三亚地块,并在琼中地块零星出露。相对于三亚地块,琼中地块内的古生界由于缺乏古生物化石证据及精细年代学数据,其地层划分及时代归属问题一直广受争议,严重限制了对海南岛北部古生代构造属性的认识。目前,利用沉积岩碎屑锆石年龄限定地层的沉积时代已得到广泛应用。本文在野外地质调查基础上,对海南岛北部邦溪、儋州、琼海等地区原划属为古生界的相关样品进行了锆石激光U-Pb年代学研究。锆石U-Pb年代学分析表明:于邦溪地区原划为奥陶系南碧沟组中采集的变质砂岩和儋州、琼海地区原划为志留系陀烈组中采集的变质砂岩的碎屑锆石给出的最年轻206Pb/238 U表观年龄分别为373、352和369 Ma,其年龄谱系与屯昌地区二叠系砂岩相类似,总体上年龄峰值为370 Ma左右,另外在约447、980、1 969~1 520和2 681~2 328Ma呈现次要峰值。结合其他资料,本文认为前人在海南岛北部划属的下古生界中至少有部分岩石应划属石炭系—下二叠统。上述研究丰富了海南岛古生界的年代学资料,也为海南岛构造演化等问题提供了新的约束。

Age of Paleozoic Strata in Northern Hainan Island: Constraints from the Detrital Zircon U-Pb Geochronology

The Paleozoic sedimentary rocks in the Hainan Island are outcropped mainly in the southern Sanya and sporadically in Qiongzhong terranes.Unlike the Sanya terrane, the depositional period of the Paleozoic sedimentary rocks in the Qiongzhong terrane remains as a controversy due to the absence of fossils and geochronological data.This paper presents laser zircon U-Pb dating results for the representative sandstone samples from the Paleozoic sequences in the northern Hainan Island.The three samples collected from the previously-mapped Ordovician (11HN-23) and Silurian (11HN-07,-19) give the youngest 206Pb/238U apparent age-peaks at 373 Ma, 352 Ma and 369 Ma, respectively.Their detrital zircon age spectrum are similar to those of the sandstone samples taken from the Permian E'cha Formation (11HN-45).These samples yield a predominant age-peak at ~370 Ma and two subordinary age-peaks at ~980 Ma, ~447 Ma, along with two age spans of 1 969-1 520 Ma and 2 681-2 328 Ma.Our results suggest that part of the metasedimentary rocks, which were previously-mapped as Ordovician and Silurian in the northern Hainan Island, were most likely deposited in between Carboniferous and Early Permian.This study enriches the geochronological data for the Paleozoic strata and provides new constraints on the tectonic evolution of the Hainan Island.