福建建瓯上房花岗岩热液锆石U-Pb年龄及地质学意义

The geological implications and hydrothermal zircon U-Pb dating in Shangfang granite,Jianou county,Fujian Province

建瓯上房白钨矿产于上房正长花岗岩体的外接触带上.上房正长花岗侵入时代为晚侏罗世.在上房矿区2线ZK201处正长花岗岩（样品SF201）中发现了一类不同于岩浆锆石的锆石类型,锆石中具有较多的包裹体,Th/U比值低（0.02～0.17,平均0.07）,稀土元素含量高,配分模式左倾平缓,Ce异常不明显,Eu负异常（Eu/Eu＊＜0.06）极显著.研究表明其属于热液锆石.利用LA-ICP-MS定年,热液锆石具2组谐和年龄,其中一组加权平均年龄为（232.6±2.2） Ma （MSWD=0.74）;另一组加权平均年龄为（218.8±6.9） Ma.前者代表了印支期岩浆侵入结晶年龄,后者可能代表了印支期后的热液叠加改造年龄.这些热液锆石是燕山早期（晚侏罗世）岩浆侵位过程中捕获的印支期岩浆锆石,暗示存在隐伏的印支期岩体,或是燕山早期花岗岩来自于深部印支期花岗岩的部分熔融（再造）.分析认为,福建省印支期岩浆活动强烈,花岗岩广泛存在,但不能仅从测年数据确定其形成时代,应着重分析和调查接触关系、岩相学特征等.利用锆石定年还要加强研究锆石成因类型.印支期花岗岩可能与华南钨多金属成矿有一定的关系,初步富集钨多金属的印支期花岗岩在燕山早期部分熔融形成含钨岩浆,为钨多金属成矿提供了成矿物质.

Shangfang scheelite deposit is localized along the outer contact zone of Shangfang syeni- tie granite. The emplacement age o{ Shangfang syenitie granite is late Jurassic. But one kind of zircon that unusual magmatic zircon is found in the syenitie granite, at end o{ borehole of ZK201 in No. 2 exploration line in Shangfang tungsten mine. The zircons have more inclu- sion, the average ratios of Th/U are low to 0.07 （0.02 - 0. 17）, they are enriched in REE with left-declined REE patterns , ace shows unconspicuous negative anomalies and ~Eu shows remarkable negative anomalies （the ratio of Eu/Eu ＊ is below 0.06） . It is founded that the zircons are belonged to hydrothermal zircon. LA-ICPMS zircon U-Pb ages shows that zircons have two-staged concordant age. the earlier with （232.6i2.2） Ma （MSWD= 0.74） of zircon U-Pb concordant age and the later with （218. 8±6.9） Ma （MSWD=2.7） of zircon U-Pb concordant age; the earlier zircon age represents the emplacement and crystal- lization age of Indosinian magma, and the later represents the post-Indosinian hydrothermal superimposition and reformation age. These zircons are captive Indosinian magmatic zircon during the emplacement of early Yanshanian （Late Jurassi） magma, which suggested there exits concealed Indosinian granite in this district, or deep Indosinian granite partial melting into Yanshanian granite. Indosinian magmatic activity is so strong that Indosinian granite is widespread in Fujian Province. So determining the granite formation age do not only from the dating data, but also certain by the analysis of contact relation and petrographic charac- teristics. And using zircon U-Pb ages need the analysis of the genetic types of zircon. It is though that Indosinian granite might have some relationship with the Southern China tung- sten polymetallic ore, which melt into tungsten magma in the early of Yanshanian and supply metallogenic materia of tungsten polymetallic.