广东北部碎斑熔岩加里东期锆石SHRIMP年龄的首获及其地质意义

Caledonian Zircon SHRIMP U-Pb Age of Porphyroclastic Lava in Northern Guangdong Province and Its Geological Significance

赣南-粤北燕山期火山岩较为发育，并可分为下、中、上三个不同时期的火山岩系，但分布于粤北司前一赣南南迳一带的火山岩一直缺乏可靠的同位素年龄。河口破火山口构造碎斑熔岩的锆石具有清晰的震荡环带，Th／U比值高，属典型的岩浆成因锆石。锆石SHRIMPU—Pb测年结果显示，14颗锆石15个测点的年龄变化范围较小，为410～473Ma，加权平均年龄为（443．6±5．4）Ma，MSWD为1．3，代表了碎斑熔岩的成岩年龄。根据国际地层表，河口破火山口构造碎斑熔岩形成于晚奥陶世末期-早志留世初期，属加里东期火山活动的产物。河口破火山口构造碎斑熔岩加里东期锆石SHRIMPU—Pb年龄的首获，证明司前一南迳一带存在加里东期火山岩，这为赣南一粤北加里东期岩浆作用和花岗岩型铀矿成因的研究提供了新的窗口。

Yanshanian volcanic rocks are well developed in Southern Jiangxi-Northern Guangdong Provinces. They can be subdivided into three volcanic rock series： the upper, the middle, and the lower volcanic rock series. However, there are no reliable isotopic ages for volcanic rocks in the Hekou （Northern Guangdong） and Nanjing （Southern Jiangxi） areas. The zircons of the porphyroclastic lava from the Hekou caldera with clear oscillatory zoning and high Th/U ratios are typically magmatic. Our zircon SHRIMP U-Pb dating results show that the apparent ages of 15 zircon crystals range from 410 Ma to 473 Ma, with a weighted average of 443.6 ＋_ 5.4 Ma, which represents the emplacement age of the porphyroclastic lava. According to the international stratigraphic chart, the porphyroclastic lava in the Houkou caldera came into being in the Late Ordovician to Early Silurian time, belonging to a volcanic activity during the late Caledonian period. Our study attested existence of the late Caledonian volcanic rocks in the northern Guangdong region, which provided a new information for interpretation of magmatic processes of the Caledonian period and genesis of the granite-related uranium deposits in this region.